450 Question and answer Related to cement industry | complete reference

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Question 454:
Could you please to explain the possibility of steel slag in cement industry as raw material if it was added in the kiln inlet?
Answer:
Steel slag can certainly be added to the kiln inlet to boost the output from the kiln. This process has been patented by TXI in the USA and is known as Cemstar. The limiting factor is the high iron content of the steel slag, which effectively limits the possible addition rate. This subject is discussed in more detail in the February edition of International Cement Review.
Question 453:
It is known in all cement standards that the Initial Setting Time is 45 minutes minimum and the Final Setting Time is 10 hours maximum. Is there any definite time to be considered between end of initial setting and start of final setting to ensure cement quality and workability?
Answer:
There is no definite interval between the initial and the final set of cement that denotes good or bad quality or workability. You must bear in mind that the test for setting time is conducted at a constant temperature in the laboratory. This is not the case when the cement is used in practice. In cold countries the setting time extends in practical applications and therefore a short setting time is best in these conditions. In hot countries (such as the Sudan) the setting time is shortened in practice and therefore a longer set in the laboratory conditions would be desirable.

Question 452:
Our ESP on our long dry kiln (2520tpd) is out of order. A certain company has recommended instead of the ESP to install a water fogging system (droplet size = 10 micron) into the riser pipe of this kiln claiming that 80% of dust passing with exit gases before the ID fan can be suppressed that way. Can you advise us on the merits and/or disadvantages of such a system?
Answer:
I am afraid I cannot see many merits to this suggestion. It is common practice to condition (humidify and cool) the gases before entering the ESP by spraying droplets of water into the gas stream in a conditioning tower. This might indeed lead to 80% of the dust load dropping out in the conditioning tower. However in your kiln, the suggestion is not to spray into a tower that will act as an expansion chamber but into the gas riser. You will also cool and contract the volume of the gases, which will have the effect of increasing the draft at the kiln inlet leading to the possibility of higher dust losses from the kiln. I could not advise such a modification and believe that a much more detailed examination of the problem is required.
Question 451:
What could be the minimum velocity of gases in down-comer duct? Our idea is to reduce velocity by increasing duct diameter, as much as possible so as to get lowest pressure drop across down comer duct.
Answer:
I don’t think there is a minimum velocity in the down comer duct. You must draw the exhaust gases out of the kiln, but provided you do that adequately it does not matter at what velocity they flow down the down comer duct. However, I’m not sure what you are going to achieve by this modification. The pressure drop across the down comer duct is unlikely to be a limiting factor on your kiln. That limiting pressure drop will be elsewhere in the system.
Comment:
I agree that by expanding the down comer duct you won’t gain much pressure drop reduction. We are doing a lot of modifications to reduce the pressure drop of the complete pre-heater system to relieve the fan, but usually we have to modify the upper stage cyclones (sometimes only modification of inlet portion is sufficient) and riser ducts. Simple installation of a vortex finder guide vane in the upper stage cyclones reduces the pressure drop of each respective cyclone by minimum 30 per cent (this is guaranteed by the supplier). Michael Suppaner PMT-Asia.

Question 450:
We at our plant intend to utilise high sulphate resistant cement for oil well cement. To produce low viscosity and slow-setting slurry we need to know the type of most suitable retarder and friction reducing additive to be used with SRC.
Answer:
The key to producing oil well cement lies in the testing and meeting of the API performance specification. High sulphate resisting cement with low C3A content may well be a good starting point. Lignosulphonate is likely to be a useful retarder and friction reducing agent for this application.
Question 449:
What is the best method to charge a ball mill?
Answer:
The normal method to charge balls into the ball mill is to position the doors at the top of the mill and remove them. Then to pour the barrels of grinding balls into the mill using an overhead crane. The exact grading of media required is identified by performing an axial test taking samples along the axis of mill. You want the desired fineness to be reached at the exit of the mill and adjust the ball grading to achieve this. For a new mill you need to rely on the experience of the commissioning engineer to determine the initial ball grading. It is normal practice to begin commissioning with only 65% of the balls in the mill and bring this up to target by additions with monitoring between additions by axial testing.

Question 448:
We are analysing particle size distribution of raw meal, coal mill product & cement mill product through CELAS PSA having facility to determine 0.7 to 400 micron. Q1. What is the best particle distribution (on various micron sizes) of raw mill in terms of best mill performance, burnability & kiln performance & which plant in the world is producing? Q2. What is the best particle distribution (on various micron sizes) of coal mill in terms of best mill & kiln performance? Our coal ash is 28. Q3. What is the best particle distribution (on various micron sizes) of cement mill product to have optimum power consumption, best strength & good performance in concrete?
Answer:
There is no straightforward answer to your questions. For raw mix the optimum particle size distribution depends on the mineral composition of your raw materials. If you have a high content of quartz then you must have a very tight fine particle size distribution with low content of oversize particles where the quartz will concentrate. On the other hand with a homogenous raw mix with no quartz it is not beneficial to have too small a particle size distribution, as this will increase the dust losses from the top stage of the preheater. Similarly with coal there is no hard and fast rule. It is said that the 90-micron residue should not be more than 50% of the volatile content of the coal. Increasing the fineness beyond that is counter-productive. For cement the optimum particle size distribution depends on the cement performance characteristics that your customers have come to expect. Ring formation is caused by the formation of liquids in the kiln at a particular position. These penetrate the refractory lining and cause a localized thickening of the coating. Fe2O3 can be a cause of these problems and I am not surprised that raising the alumina modulus has solved the problem. Many cement factories operate with higher alumina modulus than 1.7 and I would recommend keeping the kiln feed mix design with the higher alumina modulus. A snowman is the formation of a large build-up on the first grate of the cooler where the clinker falls from the kiln rather than a build-up in the kiln.

Question 447:
We are facing a problem in bringing the whiteness of material that is close to cement composition in our R&D trials. We would like to know the effect of each oxide and cooling method on whiteness of the material.
Answer:
The principle colouring oxides are those of the first row of the transition metals. In cement the most likely to cause problems are iron, manganese and chromium in that order. The cooling of the clinker is absolutely fundamental to achieving the desired whiteness. You must (i) burn the clinker as close to the nose ring as possible to eliminate the possibility of oxidation of FeO to Fe2O3 in the kiln, and (ii) immediately quench cool the clinker in water.
Air-cooled blast furnace slag can be added to the inlet of the cement kiln in the patented Cemstar process developed by TXI cement in the USA. I know a number of US cement companies are doing this. This is in the manufacture of cement powder. Another use is as an aggregate in the manufacture of concrete.
Question 446:
Please advise us that are it possible to use the combination of Ca (OH) 2 and CO2 and waste of Molasses (in sugar and lump sugar factories) for correction of raw material, instead of CaCO3?
Answer:
It is possible to use Ca (OH) 2 as a corrective source of CaO. I do not believe you would want to use CO2, as this is one of the exhaust products from the cement manufacturing process rather than making a contribution to the final product. I have not heard of the use of molasses in cement manufacturing and can envisage two problems with this material: (i) it is likely to be sticky and present handling problems, and (ii) there will be some organic material in the molasses. This organic material will smoulder and burn in the upper stages of the preheater, or in the chain systems of long kilns and will give rise to CO and hydrocarbon emissions in the exhaust gases. To add this material safely to a cement kiln would require some means to add the molasses to the hotter sections of the process such as the precalciner or the kiln inlet of a preheater kiln or some mid-kiln firing system of a long kiln.

Question 445:
We are currently using blast furnace slag as the only additive to our new product of masonry cement but we are failing to control the compressive strengths within the desired levels. We are getting on average 26MPa at 7 days and we are adding 70% slag. What’s the way forward and please tell me more about masonry cement as this a new area for me.
Answer:
Masonry cement is usually made with Portland cement clinker, gypsum, up to 30% limestone addition and organic additives such as lignosulphonates to increase the workability of the mortar produced with the cement. High strength is not the property that is important with masonry cement. More important is high workability and adhesion to allow the mortar to bind masonry together or to be rendered onto the surface of masonry.
Question 444:
We are manufacturing white cement. Previously we used oil fuel, now gradually we have switch over to Pet coke & now a days we are using almost 85% of Pet coke & 15% of LSHS oil as a fuel. After this we are facing a problem that clinker is not coming out in a nodular form & it is almost in dusty condition. Kindly suggest that what corrective or preventive measure should be taken for getting the clinker in the nodular form from the Kiln.
Answer:
This problem of the clinker dusting with the conversion to pet coke firing has been reported from a number of Indian cement factories. I think you will find that your Grasim South factory at Reddipalayam is suffering similar problems. This has prompted me to research the problem and the Tech Forum article in the April issue of ICR will be based on this topic. From my research it seems that the excess sulphate in the clinker flux reduces the viscosity and surface tension of the liquid phases in the kiln. The sulphur content in the pet coke is causing the breakdown of the clinker nodules. With gray cement I would suggest increasing the melt content of the clinker, however with white clinker this is not possible. My only suggestion is to increase the alkali content of the clinker by addition of feldspar to the mix. his should combine the sulphur as alkali sulphate and solve the problem. Unfortunately nodulising the clinker before the cement mill will not solve your problem and will only increase your manufacturing costs.

Question 443:
What are the additional facilities required to use pet coke of sulphur content around 8% in a cement plant. Could you site some of the results of trial runs in cement plant using pet coke of 8% sulphur content.
Answer:
8% sulphur is very high and I don’t know of any cement companies using pet coke of that sulphur content. This is likely to result in over 2% sulphate in clinker and therefore the amount of gypsum that can be added in cement grinding will be limited. Possibly this coke could be used to produce mineralized clinker however extensive trials would be necessary.
Question 442:
We carryout refractory work with screw jacks, maximum 10m lining at a time (i.e. 5m maximum simultaneously at two different locations in same kiln). As ours is a small plant of 1000tpd, with single kiln of 64m length, deploying brick lining machine and dismantling machines are not viable in terms of cost for us. What therefore is the maximum length of refractory work can be done at a time, in how many parts, simultaneously, from the various aspects such as safety, life of refractory etc.
Answer:
I would recommend using the gluing method rather than screw jacks and then you can replace as long a section as you like without the concern of installing and removing the jacks. My understanding is that the gluing method will have no problems with a kiln of 3.95m diameter. However, you would be better to check this with suppliers of the refractory glue when placing your order.

Question 441:
We have ESP at the kiln system. It is also used for raw mill system. System is sent ESP’s feedback to kiln feeding silo. There are two homogeneous silos and two stockage silos. Material is sent from ESP to homogeneous silos. So, my question is that, I am thinking to feed the ESP’s feedback directly to preheater system but this line will be entered to system after weigh feeder. We have got line to feed this material before weigh feeder but it causes to reduce of fresh feeding. In this way I want to increase clinker production with same feeding amount. What do you think?
Answer:
Your idea is a very good one. There are plants that do exactly what you propose. This has the advantage that the chemistry of the kiln feed is more stable because it is not destabilized by slugs of ESP dust being added to the homogenization silo when the raw mill is not running. I am not sure that you will get more output but the kiln operation and clinker quality should be more stable.
Question 440:
How can the efficiency of clinker cooler can be increased without any modifications?
Answer:
Cooler efficiency is a function of the amount of air that is blown into the cooler for cooling the clinker. You might improve the efficiency by reducing the amount of cooling air. However, you need to develop a model of the cooler to ensure that you have the optimum balance between thermal efficiency and cooling performance.
Question 439:
What is function of preheater immersion pipe? What is the behaviour of material inside cyclone if immersion pipe is missing? If we have any immersion pipe missing it will effect on decarbonising degree or not?
Answer:
The function of the vortex finder is to improve the material separation efficiency in the preheater cyclones. If the vortex finder is not there then separation efficiency and preheater performance will decline. Yes, it will affect the decarbonation degree…some decarbonated material will pass back up the preheater and recarbonate. The biggest effects will be on fuel consumption and kiln output.

Question 438:
We would like to use phospho-gypsum in place of mineral gypsum in our cement plant. Please advice us of its suitability. Phospho-gypsum has 36 per cent moisture and pH is 2.3. The cost of using this material is very low but what are the drawbacks?
Answer:
I know of no reason why you should not use phosphogypsum. However, you will have to conduct trials to verify that no product quality problems are involved.
Question 437:
If air cooled slag is used as a raw material for kiln, replacing uncalcined calcium carbonate, will the calcium silicates, aluminates, etc. revert back to free state, so they can reform clinker crystals of cement design proportions? Will the existing silicates go to C2S state with the increased temperature and time? In other words could this slag combined with correction proportions of silica, iron, alumina, calcium be used as a primary raw material for a kiln feed? If there is a reversion of the state of the compound will there be a requirement for additional heat (as opposed to the exothermic heat released to compound formation)? Would there be any big colour changes if air-cooled slag was used as a primary raw material.
Answer:
The process of adding air-cooled slag to the kiln inlet has been patented by TXI under the name Cemstar. The minerals in the slag do not convert back to the free state. They are already intermediate states in the formation of Portland cement clinker and all that is required is the completion of exothermic combination to C3S. No additional heat is required, also there are no reported changes in the colour of the cement subsequently made from the clinker.

Question 436:
We have a two-chamber ball mill, for one quality we put 60 per cent cold clinker and synthetic gypsum (total H2O 25-30) in the mill and we add 40 per cent already milled blast furnace slag (4400 Blaine at 0.32O) in the separator or in the online mixer. We suffer from soft lumps in the finished product silo and of decreasing two days results after aging for four to six weeks in the silo (winter period). Where is the origin of the problem? Humidity of the gypsum (but we do not suffer it in the pure Portland quality), humidity of the slag? grinding? Any experience?
Answer (1):
I do not have specific experience of your lump formation problem, however it must be connected with amount of heat generated in the mill during the grinding process and the amount of water that must be dried from the gypsum and slag. You need to develop a heat balance model of the mill that allows you to test different scenarios with different rates of addition of the materials and points of addition of the slag. This will allow you to identify the limits of the drying capabilities in the process. You may be able to get round the problem by a process adjustment or you may need to add heat to the system.
Answer (2):
I would have thought it unlikely that the slag is the cause of the problem. Do you adjust gypsum addition level to the clinker/slag combination to take account of the 40 per cent slag addition? As the synthetic gypsum contains high free water content of up to 11 per cent (assuming all gypsum as .2H2O) any reduction in the gypsum addition may help avoid lump formation. No details have been provided of the clinker chemistry but if there is scope to increase clinker free lime level this can often act as a desiccant to remove small amounts of excess moisture from cement stored in silos and avoid air setting/lump formation.

Question 435:
We are about to lose our traditional sand supplies for our raw mix. Now we are using 78-80 per cent limestone, 15-17 per cent clay, 1-2 per cent iron ore and 2-4 per cent sand. Now we are thinking to use low-grade limestone and no sand. Our management says to use iron dust through kiln firing system in place of using in the raw mill grinding. Mix design is iron dust 0.5 per cent limestone 70 per cent, low-grade limestone 18.5 per cent and clay 11 per cent. My point is any raw mix is made to be through raw mill grinding and only fuel through firing system. Let me know your opinion and suggestion.
Answer (1):
As regards the enquiry, I have no experience of anyone firing iron ore through the burner. I presume they plan to use an insufflation pipe and not simply feed through the coal mill as this could lead to a number of problems. Many plants around the world insufflate kiln dust successfully without major quality problems but care has to be taken to ensure the dust can be properly absorbed into the raw mix and, of course, the chemistry of the dust is very similar to the kiln feed chemistry. I assume the plant is wet process and would suggest looking at back end insufflation as a possible alternative to ensure thorough mixing of the iron ore. If it is dry process then there is some comparable experience in Malaysia at the Rawang plant where they introduce their secondary raw material – oil shale – to get the benefit of the fuel value and thus rely on the precalciner and kiln to do the raw meal blending. To insufflate iron ore as a separate component will, in my view, be far from ideal and the following problems may be encountered:-
• the kiln feed mix would be deficient in flux and hence difficult to burn and less able to absorb the iron
• there would be the risk of chemical reduction of the iron ore in the flame to ferrous state which can increase the basicity of the mix by iron replacing calcium and hence artificially raising the LSF making the mix difficult to burn
• there would be the possibility of increased coating and build-up
Answer (2):
Many cement plants add high ballast and high ash fuels through the firing system. However, I do not know of any plant that is specifically doing that with iron additive. I also see no reason to do that. There are potential problems of in homogeneity in the clinker. Also you need the iron to form flux in the transition zone, not at the end of the burning zone.

Question 434:
What kind of changes are required in raw mix to increase the granulation with 50 per cent pet coke total CV 4.3per cent, LSF 93, SM-2.1 per cent, AM-1.0 per cent, in kiln feed?
Answer:
The granulation problems with pet coke firing arise because of the low viscosity of the flux caused by the sulphur in the pet coke. Therefore raising the silica and alumina modulii should help to improve the granulation.
Question 433:
We regulate our raw material pile at 105-110 lsf. However the lsf of the raw mill’s sample becomes 85-90. How can we have such a large difference?
Answer:
There might be a problem in the sampling station. More likely you have a problem of segregation of the material. This is most likely when you are reclaiming the end-cones of the pile or in the intermediate feed bin between the pile and the raw mill. One solution is to add the end-cone to the next pile rather than sending it to the raw mill. The intermediate bin should be kept at a high level by setting the reclaimer start signal to a high bin level.
Question 432:
I want to know about calculation methods of gas & material flow in preheater, pc, kiln, cooler. Further, can you advise why we are getting brown clinker problem in our 2000tpd off-line calciner kiln from which we are recently producing 2500tpd while having 90-91 LSF, 2.26 SM, 1.4 AM.& kiln feed residue up to 22 per cent. How we can improve our clinker quality?
Answer:
The calculation of gas and material flow in the preheater relies on the principles of stoichoimetry (to assess the combustion gas products from the fuel) and the conservation of mass. In principle what goes into the kiln must come out somewhere and will allow you to prepare a mass balance for the kiln. With regard to the brown clinker you are producing this is most probably caused by having reducing burning conditions in the burning zone of the kiln. This is very bad for clinker quality and fuel efficiency of the kiln. Almost certainly the problem is with the kiln burner and might be solved by adjustment. You must be sure that you maintain at least one per cent oxygen at the kiln inlet.
Question 431:
We are using synthesized calcium sulphate anhydrite from a nearby aluminium plant. This calcium sulphate (96 per cent purity) is fine in form and free from moisture. In addition to CaSO4 this material contains 2.0 per cent CaF2. Can Calcium sulphate be used as an alternative retarder for the Natural Gypsum? Your expert opinion on the solubility of this synthetic calcium sulphate anhydrite is appreciated.
Answer:
The solubility of anhydrite varies dependent on the temperature history to which it has been subjected. However, I am sure this will be a good material to use as a set-controlling additive in your cement. You may need to use it in combination with your natural gypsum or you may be able to replace your natural gypsum with 100 per cent of this material. The only way to find out will be to conduct trials with the synthetic CaSO4.
Question 430:
What is the current international standard of MTBF for kilns and mills?
Answer:
There is a difference between mean time between failures and mean time between stops. Best practice for kiln MTBF in the cement industry is in excess of 750 hours, while MTBS is somewhat shorter. The problem with MTBF is that it is easier to massage the data and cheat because some stops are planned and therefore do not count. In that situation it is easy to bring forward a planned stop when you have an unexpected unplanned stop.
Question 429:
What is the purpose of Triethanolamine in cement? Is it (triethanolamine) also in admixtures (water reducer, air entraining, superplasticizer, for example) for concrete? Could this chemical affect the final appearance of finished flatwork concrete? Could it cause uneven discolorations in the cured concrete?
Answer:
Triethanolamine is used as a set-controlling chemical in concrete admixtures. However, I am not aware of it causing differences in appearance of the surface of finished concrete. That phenomenon sounds more like efflorescence.

Question 428:
I would like information on the formation of “snowmen” in our kiln cooler. Our kiln produces 2000tpd clinker. The kiln uses 100 per cent solid fuel comprised 80 per cent coal and 20 per cent pet coke.
Answer:
Snowman formation is caused by some of the flux in the clinker remaining as a liquid after the clinker falls into the cooler. The root cause solution is therefore to achieve more cooling in the kiln itself. There are numerous solutions to the problem of the removing the snowman itself. Blasters are the most commonly used method. Alternatives are acoustic cleaners or Denko´s “pusher”.
Question 427:
We are using compressed air from Twin Lobe blowers for extracting fly ash from a 400t and 40t capacity hoppers equipped with open-air slides at the bottom floor of the hopper. Please let us know whether the compressed air from the twin lobe blowers need to heated since at times flow problem occurs due to clogging.
Answer:
If the fly ash is calcareous then there is the possibility of hydraulic reaction between the fly ash and any water present in the compressed air. My suggestion would be to install dryers on the compressed air supply
Question 426:
We have recently conducted some raw meal burnability testing on samples with varying residue and C3S. Results showed little dependence on these two variables. Following further investigation I was told burnability can be highly dependent on 45-micron insoluble residue. What are your thoughts on this? I am still reluctant to believe the low dependence on C3S because in the kiln I am still observing significant fuel variations with changing C3S.
Answer:
The classical burnability index takes into account both the chemical composition (i.e. LSF, C3S etc.) and also the fineness and mineral composition of the kiln feed. The theory is that large quartz (SiO2) grains and lime (CaO) grains are very difficult to combine in the kiln, and this is fairly well established. You can get the formula from the proceedings of the IEEE 2002 in Jacksonville.

Question 425:
Our plant has four cement mills (closed circuit with first generation separator) – we use about 20 percent slag , the output is 100tph ,the dimension of every mill is 15.5 x 4.4,the liners in the first chamber are lifting and in second chamber classifier. Are there any ways to increase mill production? How about for a raw mill?
Answer:
There are numerous ways to increase the production from your cement mills. In the short term use of grinding aids combined with optimization of the ball charge and drafting would be the best method. Longer term (and at greater cost) you could upgrade the separators to third generation or install a roll press to pre-crush the clinker ahead of the mills. With regard to raw mills you can again use grinding aids or install pre-grinding equipment.
Question 424:
I’ve been working with analysers for the cement plants for last nine years and have still a little doubt about the actual use of measurement of NOx in the kiln inlet. If you can kindly send me some literature about the firing in the kiln and the advantages of measuring various gases at the kiln inlet.
Answer:
The NOx level at the kiln inlet is a very good indication of the burning zone temperature in the kiln. In turn this is one of the most important considerations in the control and optimization of the kiln. NOx arises from two sources, (i) organic nitrogen in the fuel, and (ii) oxidation of the atmospheric nitrogen in the combustion air drawn into the kiln. This oxidation of the atmospheric nitrogen rises in direct proportion to the temperature in the flame and the burning zone.

Question 423:
We are producing slag cement with VRM technology (500t per day). Our cement setting time is coming down at the time of dam ring height reduced. Setting time is increased when the dam ring height was increased. Both above cases percentage of gypsum is same. Please explain why it is happening.
Answer:
We have received some suggestions regarding the possible effects of dam ring height from a major cement plant in North America as follows:
“I would suggest that the particle size distribution (PSD) be looked at for both instances. We have found that by changing internal settings in a VRM there are physical changes in the PSD, which will impact the performance of the slag. By changing the PSD as little as 2 microns there is a significant change in the performance while not showing up in a simple test of Blaine?” “If the dam ring height is increased the clinker and gypsum will stay on the grinding table longer and possibly be better ground when it exits the table and therefore the circulating load in the mill should be lower.
So the two things to look at are the amount of dehydration of the gypsum with lower dam ring height compared to the higher dam ring height and also the particle size distribution of the finished cement. With a lower dam ring there is the chance that there is more over grinding and a higher concentration of super fines.”
Please let us know if you find there is a relationship between dam ring height and particle size distribution or gypsum dehydration, as this would make an interesting case study for publication.
Question 422:
Why does false air infiltration from the bottom stage of a preheater tower increase tower temperature, but in top stage decrease the tower temperature. The preheater system is SLC-S supplied by Fuller. We understand this is a common phenomenon for any other system like ILC, SLC or SLC.
Answer:
I have not observed this phenomenon; however it is perhaps connected with the additional draft that is required to keep the kiln hood under suction when there is in leak in the bottom stage of the preheater. This would cause the thermal energy from the calciner to be drawn up the preheater. Conversely when the in leak is in the cyclones above the calciner then the in leak makes no difference to the draft on the calciner.

Question 421:
We are producing sulphate resistant cement clinker. We believe from the analysis of the clinker that we could also produce oil well cement as per API standard. Do you have any comments (analysis attached).

Secondly, our ALM has been reduced from 1.25 to 0.70 while keeping LSF as 0.90 and SIM 2.25 we are facing following problems:
1. Ring in inlet causing inlet spillage
2. Thick coating at 27-28m hampering production
How can we overcome these problems?

Answer:
Some classes of oil well cement have low C3A content and are coarsely ground. The key to producing oil well cements is the testing that is required to achieve the certification. You need to investigate the costs of installing the equipment to conduct this testing on a regular basis and then apply for certification to produce oil well cements. The ring and coating problems are caused by the high Fe2O3 content of the SR raw mix. I suggest you increase the silica modulus of the raw mix.
Question 420:
I wish to know if the test of consistency of cement pastes has any significance as regards the quality of cement, e.g. a cement of lower consistency (<25%) is better than a cement of higher consistency (> 25% water). Secondly: it is observed that most of international cement standards set limits only to mortar compressive strengths although in practice the main usage of cement is in concrete applications. May I know why is that?
Answer:
To calculate the velocity of the gases at the mill fan inlet you need the static pressure in addition to the dynamic pressure. If the mill fan has a capacity of 133 m3/s then you need to divide this by the cross-sectional area of the inlet ducting to arrive at the design gas velocity of the fan.

Question 419:
I have two questions: What is the effect of NOx & SOx in controlling the burning zone temp. And what other parameters like kiln torque, kiln inlet oxygen or if anything else affects the kiln operation. Secondly: How to improve the ventilation in ball mills, as we observe the coating formation on the balls is on the high side.
Answer:
NOx and kiln torque are both good indicators of the temperature in the burning zone of the kiln. This is key to ensuring stable operation and good combination of the clinker. Kiln inlet oxygen is a good indicator of the combustion conditions in the kiln where a small excess of oxygen is necessary to ensure burning in oxidizing conditions. SOx rises if there is reductive burning and CO present in the burning zone therefore this reinforces the kiln inlet oxygen measurement.
To improve the ventilation in the ball mill you need to increase the speed of the induced draft fan. However if you are suffering coating of the media this is more likely related to temperature control in the mill. This is a function of the clinker temperature, ventilation and the cooling with water injection.
Question 418:
Ours is ILC 5-stage pre-heater kiln with 3300t/day production capacity. Recently we have changed the kiln fuel from fuel oil to 100 per cent coal. The kiln torque in case of fuel oil firing used to vary between 28 and 38 on normal kiln running. After switching the kiln on coal, the torque figure remains between 6 to 11. The litre weight of the clinker remains between 1.35 where as on fuel oil the same varied between 1.22 to 1.28. Otherwise the operational parameters for kiln in both the cases are more or less same.
Answer:
The composition of the coal has affected the melt content of your clinker. This is affecting the kiln torque because stickiness in the charge in the kiln has changed and is not climbing the rotating walls of the same extent. This also explains the increased density of the clinker. However, you are not suffering any other operational problems, which is a good result with such a major change in operations. Congratulations!

Question 417:
We are 3100tpd cement plant situated in the Arabian Gulf. We have four cement silos 10,000 tons each at our plant, we are facing the problem of lump formation in silos, which is causing cement extraction delays and is also effecting our market reputation as well. Our cement mill is 136t/h rated capacity (OPC) closed circuit with water spray system (made in-house). The humidity reaches 80 to 90 per cent in summers, as it is a coastal area. Till now we have not come to any conclusion about the cause of lump formation in our plant. Can you please suggest us if grinding aid may help in solving the above said problem, how technically, and of course to what extent.
Answer:
Given your location and the humidity you suffer in the summer, the question you must answer is whether the lump formation is caused by the ambient humidity in the air in the free space in the silo or continued dehydration of gypsum in the silo. I suspect the problem will be continued dehydration of gypsum in the silo. To solve this you must carefully control the temperature of the cement entering the silo. The installation of a cement cooler may be necessary in your climate to bring the cement temperature below 80 degrees centigrade.
Question 416:
I want to know what is Roslin Ramler slope and characteristic value for cements? Do they have any relationships and how do you understand the fineness by using them.
Answer:
The Rosin-Rammler-Sperling-Bennett particle size distribution for cement is found by measuring the residue on various sieves (25,32,45,63,90 microns etc.) Alternatively you can use laser granulometry or some other method of measuring the residues. You then plot the natural log of the sieve sizes on the x axis (ln(sieve)), against the double natural log of 100 divided by the residue on the y axis (ln(ln(100/Residue))). The reason for using these axes is that for fine powders such as cement the plot approximates to a straight line. The slope of the line gives a measure of the “tightness” of the particle size distribution and varies from 0.8 to 1.1 for cements, dependent on the type of equipment used for grinding. The characteristic grain size is the sieve size where 36.8% of the cement would be retained. This can be calculated from the slope and intercept of the RRSB line and is typically between 20 and 30 microns for cement.

Question 415:
What in your opinion would be the effect, on clinker properties (reactivity, granulometry, hardness…) of having a high LSF (97-98) and silica module (SM) of 2.8 and a low alumina module (AM) around 1.3-1.4? Would the high LSF tend to rise the C3S content, improving early strengths? High MS would shift the relationship C3S/C2S towards more C3S? Also improving later strengths…if so, doing the opposite would rise the C2S and also the late strengths…but lowering the early ones?
What about granulometry? We have seen that low AM, but lower LSF and SM gives a better granulometry and a darker clinker … would you think that it will be the same under the conditions cited before (high LSF and high SM)? We produce pozzolanic cement (20% substitution) and would like to improve our late strengths. I believe it can only be done by working on the clinker, besides improving all the milling conditions, like Blaine and fineness, mill fill, gas flow, etc … Please let me know your opinion.
Answer:
High LSF will certainly increase the C3S content and the early strength of your cement. The high SM will increase the total silicates rather than specifically the C3S, and strengths at all ages. If you reduce the SM you can expect strengths to go down. The darker colour with low AM arises because of the increase in the C4AF content at the expense of the C3A and will apply at the high LSF and SM, although colour will lighten as the SM rises due to less total flux (C4AF & C3A). In terms of granulometry the effects are more difficult to predict and depends of your definition of good granulometry. Lower AM and therefore higher C4AF may increase the size of the clinker nodules due to earlier liquid formation in the kiln, however C3A can be a more effective flux than C4AF due to the higher relative volume compared to C4AF.
I certainly agree that improving clinker quality and mineralogy is the key to strength enhancement and control in all types of blended cement, including pozzolanic cements. Please look for an article in the February issue of International Cement Review, which talks about this topic as implemented at the Ladce factory of Cementaren Povazska in Slovakia.
Question 414:
Is there a formula for determining the correlation between kiln feed rate and kiln speed?
Answer:
No, there is no precise formula for the relationship between kiln speed and feed rate. This depends on the degree of preparation (calcination) of the feed at the kiln inlet and the progression of this feed preparation along the kiln length. However, there are “rules of thumb” and guidelines you can use for various kiln process types. What type of kiln are you operating?

Question 413:
I wish to know which are the problems we may face if we start using phosphogypsum as a set retarder in manufacturing OPC. Currently, we are adding 3.5 per cent of natural gypsum in the finish mill.
Answer:
I recommend that you conduct trials with the phosphogypsum replacing natural gypsum, however there is reported experience from India and South Africa that this can be done. I would recommend researching the work conducted by the University of Pretoria in conjunction with Pretoria Portland Cement in South Africa.
Question 412:
We have a problem related with high sand usage to make the consistent raw material mix. Sand addition for silica correction is roughly five per cent into raw material results roughly three per cent quartz content in the raw meal over 45 micron residue. Consequently the overall effect is frequent free lime formation in the clinker due to high quartz content in the raw meal, negatively effecting quality and high heat consumption in burning. In order to tackle with the problem following activities are already done
1. The burner pipe is replaced by a new design burner pipe with high momentum to improve burning and have stronger flame.
2. New coal dosing system is installed to have stable and accurate kiln fuel feed. Also with this system, we started to grind pet coke and other types of coal separately to facilitate burning. However the problem still persists.
Answer:
I agree with you that the high quartz content above three per cent over 45 micron is the root of the problem. The new burner and the improved coal dosing can only be advantages but they are unlikely to solve this problem of low reactivity of the kiln feed. There are many white cement factories that are operating with very high silica sand additions. These would be the best references. My suggestion of a remedy would be to introduce duplex grinding where the rejects from the raw mill classifier are separately ground and reintroduced to the raw mix. This is a radical solution and requires process modifications to introduce; however it will solve the problem.

Question 411:
I wish to know if the test of consistency of cement pastes has any significance as regards the quality of cement e.g. a cement of lower consistency (<25 per cent) is better than a cement of higher consistency (> 25 per cent water). Secondly: It is observed that most of international cement standards set limits only to Mortar compressive strengths although in practice the main usage of cement is in concrete applications. May I know why is that?
Answer:
The standard consistency test certainly has significance regarding the quality of the cement. You need to think in terms of the “concrete making potential” of the cement. In the test the amount of required to produce a paste of standard consistency is determined. This relates directly to the water demand and workability of concrete made from the cement. If more water must be added to achieve the desired consistency or workability then the strength development of the concrete will be impaired and more cement will need to be added to the concrete to compensate for the additional water. The reason for most standards adopting mortar testing of cement is to promote consistency and comparability between tests and laboratories over time. By using a tightly specified grading of testing sand the influences of the aggregates on the strength testing are eliminated or minimized. This means that the strength development potential of the cement is being measured by the standard mortar test.
Question 410:
I wish to investigate the differences in grinding of slag over clinker. We have a 100tph OPC cement ball mill. Could this mill grind slag? What modification would be needed? Is it always necessary to supply hot air when grinding slag? What sort percentage drop in production should be expected?
Answer:
Yes, your ball mill can grind slag. If it was to be permanently converted to slag grinding then a change to a finer ball grading would be necessary. Slag is normally ground finer than cement and has a smaller grain size to begin with. Certainly there is a limit to the drying capacity of a mill based on the heat generated in the mill alone. To determine whether you need to add hot air you need to develop a heat balance of the mill and test various scenarios with different moisture contents of slag and feed rates. Slag is much harder than cement clinker and therefore you could expect as much as a 50 per cent reduction in output from the mill.

Question 409:
Do you have an idea of each point decreased in the standard deviation of the kiln feed and how it affects to the specific heat consumption? (for example: 40 kj/kg of clinker for every additional 0.1 by which the standard deviation is raised in the usual range).
Answer:
The effect of kin feed standard deviation on unit energy consumption is complex and is likely to vary from kiln to kiln. For a precalciner kiln each 1 per cent increase in the standard deviation of the kiln feed lime saturation factor you might expect an increase in the energy consumption by 150 kJ/kg clinker. However, this relationship is unlikely to be linear and will vary around that figure from kiln to kiln.
Question 408:
We are intended to use the preheater exit gases for drying of coal having 10% moisture & dried coal production of 12 tons/hr in a dry/grinding ball mill. In respect of plant optimization following observation have recorded which I have used for calculating the Nm3 of preheater gas required to dry the coal as follows: For having 12 tons/hr dried coal qty of wet coal (having 10% moisture) will be 12 * 100/90 = 1.33 tonnes/hr =1333kg/hr.
Moisture to be removed = 1333 kg/hr. Specific heat of water = 0.482 Kcal/Kg./C o. Ambient temp. of coal = 40°C. Heat required to raise the temp. of moist water up to 100°C. = 0.482 * 60 = 28.92 Kcal/Kg.
Heat required for vaporization of moist water = 539.00 Kcal/Kg.
Total heat required = 567.92 Kcal/Kg.
Total heat required for 1333 kg./hr = 757037.36 Kcal/hr.
Would not some heat will be absorbed by coal. What percentage of heat losses during conveying of the gases to coal plant should be added?
Answer:
Your calculations seem to be correct, however I have the following comments: (i) the energy loss in the preheater exhaust seems too high at 328.22 kcal/kg clinker, (ii) the preheater exhaust volume also seems high at 2.39 Nm3/kg clinker. The coal will absorb some heat and you can use a specific heat of 0.262 kcal/kg/deg C. The dry coal and water vapour will be heated to more than 100 degrees C so you need to take this into consideration. The percentage heat loss between the preheater and the coal plant depends on the layout & insulation lagging of the ductwork.
Comment: In my opinion, to calculate heat required for heat drying coal some more information like preheater gas temperature, coal mill outlet temperature and final or residual moisture of coal are required. The specific heat of water 0.282 K cal/kg/degC seems high. The dry coal and water vapour will be heated upto PH gas temperature, so it needs to take this into consideration for calculating the latent heat of water vapour.

Question 407:
We are burning opc type 2 clinker in two long dry kilns (160mx4.4m dia. In one of the kilns the clinker is always in the shape of bigger balls (90mm dia) while the raw meal silo is same fuel is it operating parameters are same. Please guide me that what could be the possible reasons to solve this problem.
Answer:
If the size of the clinker in two identical kilns with the same feed and fuel is different it is very difficult to understand the reasons. The size of the nodules is linked with the clinkering range in the kiln and this is connected to the temperature profile through the kiln. I can only surmise that the temperature profile between the two kilns must be different. If the equipment is identical this might be related to different condition of the refractory lining between the two kilns.
Question 406:
I read an article advocating the use of acoustic pyrometers for monitoring kiln inlet gas temperature, and I am wondering if they are used much in the industry for this purpose and how successfully?
Answer:
The technology of using acoustic pyrometers is relatively new to the cement industry and therefore case studies are scarce. In particular the technique has attracted interest for measuring secondary air temperature where conventional means have proved unreliable.
Question 405:
We are in the process of converting from natural gas to coal, we are wondering if other plants just went through this transition and if any anybody is still running on 100% natural gas.
Answer:
Yes, there have been a number of gas and oil conversions to coal in the last year precipitated by the rising price of natural gas and fuel oil. However there are still a number of plants around the world that continue to fire with natural gas. Anyone else out there like to comment?

Question 404:
NOx control approaches applicable to the cement industry may be grouped in three categories: process modifications, combustion control, and NOx reduction controls. One process modification that can be quantified is the CemStar process, which is a small addition of steel slag to the raw kiln feed. Can you provide more information on CemStar?
Answer:
TXI have published various papers on their patented Cemstar process. The addition of steel slag or air-cooled blast furnace slag boosts the output of the kiln with virtually no increase if fuel consumption as the slags have already effectively been clinkered and contains trace elements that mineralise the combination of the kiln feed into the clinker in the kiln. More product with the same amount of fuel burnt inevitably dilutes the NOx emissions per tonne of clinker produced. The mineralising effect may also lead to lower burning zone temperature, and less thermal NOx formation in the flame. The real crux of the technology is adjusting the chemistry of the kiln feed to maintain the desired clinker mineralogy when the slags are being added t the kiln inlet. With steel slag the iron oxide input is increased and has to be compensated by increasing the alumina and silica modulii. With air cooled blast furnace slag the material is deficient in lime therefore the LSF of the kiln feed has be increased. This inevitably increases the demand for high CaCO3 content in limestone and is often the limiting factor.
Question 403:
What ratios can be suggested for ternary blending of cement with PFA and GGBS. Taking into account that the cement will be of Cem I quality and we would like to achieve a 32.5 MPa product.
Answer:
The ratios for blending GGBS or fly ash to achieve a 32.5 MPa product will depend on the hydraulic activity of the individual components. The only way to define these ratios is going to be by the process of producing trial mixes and physical testing of concrete made from the mixes. It may be possible to add up to 65% GGBS as this material is both cementitious and pozzolanic. The limit of PFA addition will be much lower, perhaps around 25%, as this is only pozzolanic.

Question 402:
We will appreciate your support clarifying the following points:
1-False air effect upon the (increase/decrease) of dust load in a closed system of air swept type ball mill (between the mill outlet duct-separator-cyclones-filter-stack). Also, will the dust resistivity increase/decrease?
Answer:
In an air-swept mill false air is usually introduced at the bottom of the ascension pipe in order to ensure there is sufficient volume, density and velocity of air to lift the ground material up the ascension pipe and through the separator. The amount of dust being carried up the ascension pipe is the same, however the volume of air is greater therefore the overall effect is a dilution of the dust load. In principle the false air will cause a reduction of the temperature and this will reduce the resistivity of the dust improving the electrostatic precipitation.
Comment: Depending on the degree of false air ingress, the flow velocity between the collector plates within the ESP may increase to a level, which negatively impacts on the EPS performance.

Question 401:
Where can I find results and data about sulphur or alkali by-pass system? How does one determine the percentage of bypass considering the feed material and the fuels?
Answer:
There is a lot of data published on alkali bypasses in cement kilns. A good source of information is “Technological problems in pyroprocessing cement clinker: cause and solution”, by S Sprung of the Research Institute of the German cement industry, and published by Beton-Verlag. The way to determine the percentage of bypass is to construct a computer model of the particular kiln process and use this model to evaluate scenarios with different percentages of bypass.

Question 400:
How can we solve the Nose Ring Formation at the Kiln? Any help much appreciated.
Answer:
The most likely solution will be to adjust the main burner of the kiln either by changing the position in the kiln by pushing the burner in or pulling it out a little. Alternatively adjustment of the axial and radial air settings might solve the problem. All these adjustments should be done cautiously and one at a time at sufficient intervals to fully evaluate the effects of the adjustments. There is potential to adversely affect the clinker quality or damage the refractory lining therefore only makes small changes before evaluating the effects.
Comment: I witnessed a phenomena at a plant here in Europe, which could not be explained by anybody there. At one plant I found ring formation at the upper transition zone in a rotary kiln at three different areas.
There is a refractory solution that solved this problem in a large Korean cement company in several kilns. They also apply similar fuels and faced these ring formations. RHI Refractories can offer a “tumbler lining” with custom tailored design to avoid these rings.

Question 399:
We wish to know about Oxygen enrichment in the Cement Kilns/ Precalciners. Please brief us about the expected merits and demerits. Are their any cement plants using this technique? If yes, then why is it not used commercially on regular basis in most of the cement plants?
Answer:
This is a big question! The merit is that the output of the kiln can be boosted by up to 15%; the demerit is that the oxygen is expensive and therefore the costs of producing the additional clinker are higher. If market demand exists with good prices for cement then oxygen enrichment can be a good option. There are a number of kilns in the USA using oxygen enrichment of either the main burner or the precalciner. It is worth noting that CEMEX USA consider oxygen enrichment of precalciner firing to be their technology for which they have lodged a patent. You can find papers on oxygen enrichment in the proceedings of the IEEE conference in Vancouver 2001.

Question 398:
Are there any recognised ways to increase raw mill production?
Answer:
The first stage is to maximise the output with the existing mill. You should conduct axial tests and recirculating load tests to make sure that the ball charge and lining are optimal. The next stage might be to conduct trials with grinding aids to try to boost the output of the mill by up to 15%. Beyond that you will need to consider modifications to the milling circuit by adding further equipment. There are two options to do that: (i) add a pregrinding facility such as a hammer mill or roll press, or (ii) add a regrind mill. Adding further equipment will be significantly more expensive than introducing grinding aids but has the potential to boost the output to higher levels.
Comment: An important factor top keep in mind is the drying capability of the system especially ball mills. Material flow within the mill can be limited if the moisture level is too high.
Sebastian Maibaum
Question 397:
I own a 400tpd cement plant. Because of a recent reshuffle in government, the state is now asking us proves our pollution free state of running. How can I prove the technology is pollution free one.
Answer:
You need to begin the process of obtaining ISO 14001 certification for environmental management systems. There are various consulting companies who should be able to help you with this process.

Question 396:
I would be very interested to hear any pointers you might have about where how best to get started on using microscopy as a further tool for examining clinker? Where training might be obtained? Anyone you might recommend that I could contact and who could possibly come on site and provide this training?
Answer:
An article describing the work we have been doing in Slovakia will appear in the February 2003 issue of International Cement Review. In part this describes how microscopy is used in combination with pattern recognition in hour-to-hour control of the real mineralogy of clinker.
Comment: You can get a good training in the Portland Cement Association (PCA, USA). Courses given at their Chicago facility. Mr. Don Campbell usually gives them and they have been proven very useful for me.
Comment: There’s a company in USA who teaches how to evaluate clinker minerals through microscopic analyses and how to apply the results to the burning process. It gives an annual seminar for the persons who are in charge of the burning sections and quality control sections of the cement plants. We’ve also participated in the meeting two times. Of course our kiln process and product quality have been improved very much beyond my expectations with its consultation and advice. The name of the company is ‘ Hamilton Technical Services.Inc.”. I’m recommending you to contact the company for better information by the fax 540-527-4875 or by the phone 888-262-9953.
Asia Cement, South Korea
Question 395:
Does better nodulisation mean more consistent size of nodules with less dust or with less large nodules ? What might be wrong if always getting a large portion of large nodules (4 – 6 inches in diameter), even at 25% liquid phase, which isn’t high, and AL of kiln feed ranging from 1.55 – 1.75. Kiln is typically run at 3.5 rpm. The large nodules are only visible on kiln stops when there is a chance to look into the cooler, before the clinker goes through the crusher.
Answer:
Your chemistry does not sound abnormal and we have discussed the possibility that the large nodules are caused by MgO variation before. A further remote possibility would be Mn2O3 variation where slight increases cause the formation of vary large balls and also take the coating out of the burning zone. However, that is a long shot.

Question 394:
How can we reduce the clinker size -5mm >35% (existing), we want to reduce to under 20%. Our plant is a 1Mta capacity unit. Fuel is Pet Coke 70% balance Indian coal.
Answer:
The only way to change the clinker size will be to change the kiln feed chemistry or the operation of the kiln. To have fewer fines in the clinker you should try to increase the flux content by reducing the silica modulus. Alternatively adjust the rotational speed of the kiln by slowing slightly to increase the residence time.
Question 393:
Is there a set of Critical Success Factors (CSFs) that would generically apply to any player in the cement Industry such that if an organization does well on these indicators, then it can hope to gain a leadership position in the Cement industry?
Answer:
Big question! And the answers will depend on the markets the cement company is operating within. Current thinking is that a cement company should strive for a balanced scorecard with sound economic performance coupled with environmental and social responsibility. Achieving these requires continuous investment in people, equipment and innovation.

Question 392:
I am working in a grinding unit having a vertical roller mill for slag grinding (UBE VRM). Initially this mill is for only slag grinding but we are also able grind clinker. We are getting clinker from various sources. Because of this we are getting different output & power consumption. I want to ask you are this normal to get different output & lower consumption? How much influence does the wear rate of table & rollers have on output & power consumption. Also I want to know what are ways to increase the output of the vertical roller mill.
Answer:
Question I am working in a grinding unit having a vertical roller mill for slag grinding (UBE VRM). Initially this mill is for only slag grinding but we are also able grind clinker. We are getting clinker from various sources. Because of this we are getting different output & power consumption. I want to ask you is this normal to get different output & power consumption? How much influence does the wear rate of table & rollers have on output & power consumption. Also I want to know what are ways to increase the output of the vertical roller mill. It is normal to get different outputs from the mill with materials from different sources. This will depend on the hardness of individual materials. There are ways to increase the output from a vertical mill. These involve adjustments to the separator and the airflow through the mill. A number of case studies have been reported by Siam Cement where major increases in output were achieved by these means.
Question 391:
Please tell me, what are the standard norms of the gas velocity in kiln, at kiln inlet, riser pipes and cyclones at different stages of a preheater? Also, please give the formula or method to calculate the gas velocity in kiln, at kiln inlet and different stages of the preheater.
Answer:
The gas velocity at the kiln inlet should not be greater than 10 m/s and ideally significantly less than that. you can calculate the velocity by using the Ideal gas laws to convert the normal cubic metres of exhaust gas at the kiln inlet to the actual cubic metres at the inlet temperature. Division by the cross-sectional area of the kiln inside the refractory will then give you an estimate of the actual velocity.

Question 390:
I understand that flow of exit gases at preheater may be calculated by Q = velocity * cross sectional area of duct But I do not know the formula for velocity and density. Please explain the same. Secondly, please explain how an EP functions.
Answer:
You need to study the Ideal Gas Laws to understand the relationships between gas volume, density and velocity. An EP works by inducing a strong electrical field between discharge and collection electrodes. Dust carried in the gas stream passing between these oppositely charged electrodes itself becomes charged and is collected on the collection electrode from where it is periodically knocked off into the collection hopper at the bottom of the EP by the rapping devices.
Question 389:
What do you know about consuming SPL in cement kilns in North America. What are the environmental concerns with this. There are kilns burning SPL on North America. I believe that one of the plants at Harleyville was doing this. If there are any environmental concerns it is that the pot liners absorb some heavy metals in the electrolysis process and that this might lead to emissions of these materials. The SPL will also contain high levels of NaF and this will significantly affect clinker chemistry and kiln operations but not necessarily emissions. The sodium and fluoride should all be retained in the clinker.
Answer:
There are kilns burning SPL on North America. I believe that one of the plants at Harleyville was doing this. If there are any environmental concerns it is that the pot liners absorb some heavy metals in the electrolysis process and that this might lead to emissions of these materials. The SPL will also contain high levels of NaF and this will significantly affect clinker chemistry and kiln operations but not necessarily emissions. The sodium and fluoride should all be retained in the clinker.

Question 388:
I would like to ask if you are familiar with any national specific cement industry guidelines concerning production, amount of raw material consumption, energy consumption, emissions and etc regarding non-European countries. That is guidelines similar/corresponding to the European IPPC 8211;directive standard of Best Available Techniques. Especially for the following nations: Japan, Taiwan, Malaysia & Indonesia.
Answer:
Sorry, I am not aware of similar guidelines to the European IPPC for the countries you mention. Most of those countries are likely to follow the lead set by the EU or the USA.
Question 387:
I would like to ask about variable chloride values in clinker which can vary from 0.003 to 0.05. Also what will happen if the burner is operated in a reducing atmosphere and how to ensure that the main burner is always operating in an oxidising condition?
Answer:
The first thing you must do is tie down the source of the variable chloride input to your kiln. This must be in one of the raw materials or the fuel. If one of the raw materials perhaps you can solve the problem by selective quarrying. Operating the burner in a reducing manner will not affect the chloride recirculation but will greatly enhance the sulphate recirculation. If you have any incidence of CO at the kiln inlet then you are operating in a reducing manner and are likely to have sulphate based problems. The way to avoid this is to adjust the flow of fuel and primary air to the burner.
Question 386:
We would like to know the effect of removal of an ESP outside casing insulation if the prevailing ambient temperature is 40 degrees centigrade and rainfall is scarce i.e dry weather and is it possible to get a decrease in gas temperature inside the filter by this action or there are going to be some operational problems?
Answer:
There will certainly be operational problems if you remove the insulation from your ESP. The temperature of the inner walls of the ESP will then become equal to the ambient temperature of 40 degrees C. This will be below the acid dew point of the gases passing through the ESP and you will have condensation on the inner walls and internal fittings of the ESP. You will then suffer from severe corrosion problems and will have to replace the internal fittings of the ESP on a regular basis.

Question 385:
Our government is starting to limit NOx emissions from cement kilns and the regulation, as I heard, was more severe than in the European countries. I think the best way for reducing NOx emission with no further cost is to use fuels that contain the lower nitrogen content, especially coal and residual fuels. Am I correct?
Answer:
The NOx emissions arise from two sources: (i) the nitrogen in the fuel as you say, and (ii) “thermal” NOx from the breakdown of atmospheric nitrogen at the high temperatures in the burning zone. Usually the thermal NOx is much higher than the fuel NOx and the best way to reduce NOx emissions is to reduce this thermal NOx. Low NOx burners reduce the temperatures in the flame and produce less NOx by using less primary air. Low NOx calciners burn some of their fuel in a depleted oxygen environment to cause NOx degradation by the following reaction: NO + CO -> 0.5N2 + CO2. These are the best ways to reduce the NOx emissions of the kiln.
Question 384:
We would like to have your opinion and advice on following matters: Our unit is a FLS design latest unit with downdraft calciner with 100% Petcoke. We have SO3 and chloride problem in our process, can you advise? Secondly: how can we improve the clinker size as we are getting over-size clinker. For your reference we have included some sample data which we have collected from different places which may give you an idea about the material and our problems (data sheets as attachments).
Answer:
There are a number of concerns regarding the data you have provided: (i)The K2O and Cl in the 5th cyclone material are too high. I would expect you to suffer blockage problems with this level of KCl in the hot meal. The only way to solve this will be via installation of a bypass. (ii) The SO3 content is high at the kiln inlet probably originating from the petcoke. I would expect some problems of ring formation in the kiln inlet and spillage of feed through the kiln inlet seals. (iii) The burning zone material is high in KCl and K2SO4. I am surprised by the amount of KCl present at these temperatures in the process. You need to to tell me the CL in the clinker for me to assess the ability of the kiln to pass Cl out in the clinker. With these levels of K2SO4 you must ensure the main burner is never operating in a reducing environment. It is clear you face very unusual conditions. I believe you need to develop a computer model of the volatile cycles in the kiln and use this model to test scenarios with a bypass to break the internal and external cycles in the kiln.

Question 383:
What would be difference in terms of clinker burning and combinability of a raw meal containing sand of low quantity (below 3%) compared with a raw meal containing of high quantity (about 6%) sand. In both cases the raw meal residue was kept below 14% on +90mm.
Answer:
You really need to analyse the composition of the sieve fractions, i.e. +125 micron, 45 to 125 micron and sub 45 micron for SiO2 and Cao. I expect the silica is concentrating in the coarser fractions and that this becomes more severe as the amount of sand in the mix increases. So even if the raw mixes have the same +90 micron residue I expect that the burnability of the higher silica raw mix is more difficult.
Question 382:
Recently, we have changed the grinding balls in the second chamber of our mill to a lower size balls. The effect is a 10% of reducing in fineness at the discharge of the mill, therefore the efficiency of our separator is better, but we were waiting for an increase in the production, nevertheless it is the same than past periods with bigger balls. We don’t understand what the problem is.
Answer:
I don’t think you should necessarily expect an increase in production. I am confused. You now have smaller balls but the material at the mill exit is 10 per cent coarser? This is not the way it should be to get coarser material you should take out small balls.

Question 381:
We are going to install vertical cement mill (OK mill) in our plant. For hot gases to mill we shall be installing Hot Air Generator (HAG). Coal with 1.5 per cent sulphur will be used as fuel in HAG. The concentration of SO2 in the HAG flue gases works out to be 185 PPM at OK mill inlet. OK mill supplier has put a condition that the concentration of SO2 in mill inlet gases should not be more than 10 PPM. Supplier’s view is that SO2 will convert into sulphuric acid and since the dew point of sulphuric acid is 120 deg C, acid will condense and corrode the mill ducting and bag filter. We need your opinion about the required concentration of SO2 at mill inlet, whether 185 PPM concentration is sufficient. Please tell us the literature from where we can get information regarding dew point of Sulphuric acid.
Answer:
You do not need the dew point of sulphuric acid. The concern is that water will condense in the ductwork and that SO2 will then be absorbed in that water converting it into sulphuric acid. You must guard against the condensation of water by ensuring that the gases exiting the mill are above the dew point of water.
Question 380:
In our raw mix, iron ore is used as an additive. Its purity is around 94 per cent and it is in the proportion of 1.5 per cent in the raw meal. There is an iron rich material that is a collection of bag house fines from a steel plant. In this Fe2O3 content is around 50 per cent and also contains around 20 per cent Zinc Oxide. We want to know the effect of ZnO, on the Burning process, Brick life and on the quality of the finished product OPC, in case if use this material in our raw mix.
Answer:
You will need up to 3 per cent of the steel plant bag house fines to replace your iron oxide additive due to the relative Fe2O3 contents of the two materials. That means the ZnO content of the raw mix will be about 0.6 per cent and almost 1 per cent in clinker. The ZnO will mineralise the clinker formation, reducing the fuel consumption of the kiln and will be incorporated in the clinker minerals, C3S, C3A and C4AF. The lower burning zone temperatures should mean that the refractory brick life is improved. You can expect that the hydration of the final cement will be retarded therefore setting times will increase and early strengths will be lower. Perhaps the longer setting time will also be an advantage.
My recommendation would be to adjust the raw mix chemistry to raise the LSF and AM and compensate for these changes. This will reduce the amount of bag house fines required and therefore the ZnO additions. The mineralising effect of the ZnO will allow a higher LSF kiln feed to be combined in the kiln. At the same time C3S and C3A content in clinker and cement will be increased offsetting the retardation effects of the ZnO.
Question 379:
I witnessed phenomena at a plant here in Europe, which could not be explained by anybody there. At one plant I found ring formation at the upper transition zone in a rotary kiln at three different areas. The plant is burning alternative fuels like house trash, plastics and bone meal. I was told, that all actions taken with regards to the process have not been successful. My questions are: 1. Is there anything you could recommend apart of the standard procedures to avoid coating formation? 2. Do you know if there is any refractory manufacturer who does supply a refractory solution for this matter and what should be the technical datas for such a refractory?
Answer:
The rings you describe must be ash rings formed where the ash from the different alternative fuels combine with the charge in the kiln. It is not uncommon for coating and rings to form at that point in the kiln. As there are three distinct rings I suppose than the different alternative fuels have different aerodynamic properties in the kiln, or different delivery systems injecting them into the kiln and this is causes the ash to combine at different positions. If that were the case I would adjust the different fuel injection velocities to try to even out the coating. I am not aware of a refractory solution to this type of problem in the lower transition zone. I suggest you contact David Shepherd of Ceram Research who will know if such a solution exists, david.shepherd@ceram.co.uk.
Question 378:
Our preheater surging regularly. This is happening on a irregular basis. We suspect it is coming from cyclone No 2. We have investigated and have taken several actions. Unfortunately, we were unable to resolve it. Actions taken were as follows:
1. Damper adjustment
2. Check feeding system
3. Effects from other cyclones
4. Preheater riser duct build-up
5. Splash plate adjustment/build-up
We also include some technical data as an attachment.
Answer:
Please ask the laboratory to check the LOI in C1A, C1B and C2 again. The reported LOI in C2 is higher than in the C1 cyclones. This is difficult to explain unless you have some water injection into C2 cyclone or other special process feature. In order to calculate the cyclone efficiencies the LOI should reduce in each successive cyclone down the preheater. I also suggest that the LOI at 600 degrees C be measured in addition to the LOI at 1000 degrees C. This will tell us whether there is any hydrate or organic carbon present in the kiln feed. Finally we must have the dust LOI to calculate the efficiency of the C1 cyclones.
Question 377:
Please advise the chemical and physical properties of:
1. Oil well cement
2. Sulphate resistant cement
3. Slag cement
4. Fly ash cement
Can sulphate resistant cement is replaced with any of the other three cements mentioned above?
Answer:
1. Oil well cement comes in different grades (API Grades A to J inclusive). They are designed to be slow setting and therefore have very low C3A contents and are coarsely ground. Some grades also contain added retarders. As they have low C3A content they are sulphate resisting and could be substituted for SR cement, however the setting time would be long. 2. Sulphate resisting cement has a limit of 4% on the C3A content to prevent attack by sulphates on the hydrated C3A. 3. Slag cement has up to 65% granulated blast furnace slag added to the cement clinker and gypsum. The slag content undergoes the pozzolanic reaction and this enhances the impermeability and durability of concrete made from the cement. This makes the concrete resistant to sulphate attack and these cements can be substituted for SR cements. 4. Fly ash cement contains up to 35% fly ash added to the cement clinker and gypsum. The fly ash undergoes the pozzolanic reaction and this enhances the impermeability and durability of concrete made from the cement. This makes the concrete resistant to sulphate attack and these cements can be substituted for SR cements.
Question 376:
Can we use the slag of the steel plant as an additive during the cement grinding process, if so, in which percentage? The chemical composition of the slag is: SiO2 – 14.97%, Fe2O3 – 26.89%, Al2O3-8.43%, CaO – 21.55%, MgO – 8.48%, Na2O – 0.249%, K2O – 0.301%, MnO – 0.15%, Cr2O3 – 0.38%, P2O5 – 0.096%, LOI – 13.22%.
Answer:
Steel slag cannot be used in the cement grinding process. The slag, which can be used, is granulated blast furnace slag as it undergoes the pozzolanic reaction and therefore possesses hydraulic properties.

Question 375:
I’m working in a cement plant established at 1984, two lines with dry processing technology, the plant faces so many conflicts, major parts are not from the source company, for example the dust separator doesn’t work at all, so many efforts carried out with no result, also the computerized analysis system with daily production reports does not work either. The plant uses oil for heating up, there is a factor normally set at 1.64, which is a standard in cement production, but now it has been elevated to 1.90. After how many years how could the 1.64 will be increased to 1.90?
Answer:
The factor of 1.64 you refer to is the raw mix to clinker factor taking into consideration dust losses from the preheater. If you have very high dust losses then this factor might need to be higher. However to justify a factor of 1.90 the dust losses will need to be in the region of 30 per cent of the feed to the top of the preheater. If this is the case and your dust collector is not working the pollution must be terrible.
Question 374:
We are manufacturing white cement & facing problem of lump formation as well as coating on the wall of cement silo. We are keeping the temp of cement at inlet of silo at6 approx 80 degree and have installed a bag filter which runs round the clock to extract moisture from cement. Please suggest how to get rid of the problem and also that weather we can go for a Polymer liner or any other insulating paint to fix on the silo wall to prevent coatings.
Answer:
At 80 degrees C the cement is still hot enough for dehydration of the gypsum to continue in the silo. You need to cool the cement to less than 70 degrees if you are to be sure to prevent this. The other approach is to ensure that the gypsum dehydration takes place in the cement mill by lifting the mill exit temperature to 115 degrees.

Question 373:
Can we get the good quality clinker by using phosphogypsum as a raw material? Is any plant producing sulphuric acid and cement clinker as a by- product from waste phosphogypsum? If yes, kindly give the contact address of those plants.
Answer:
In Lea you can find reference to co-production of sulphuric acid and cement. Gypsum, clay, coke, iron oxide and sand are burnt together. The coke dissociates the gypsum producing lime and SO2. The lime combines with the other materials to produce clinker and the SO2 is captured to produce sulphuric acid. There would potentially be a problem with phosphogypsum due to the high P2O5 content of the resulting clinker. Sorry, I don’t know of any plants using this technology.
Question 372:
We have got two sulphate resistant cement types (CEM I and a CEM II using fly ash), and our clinker has got: C3S: 57-64%, C2S: 10-18%, C3A: 0-1%, free CaO less than 1%. Our fly ash has: SiO2: 48%, Al2O3: 19%, Fe2O3: 10%, CaO: 15-18% (free:2-4%), MgO: 4%, SO3:2%. How much (in percentage terms do you recommend to use of fly ash? How will it affect sulphate resistance? How should the fly ash be ground and what is the most important limit in being able to use the fly ash (I mean e.g. CaO, free CaO, Al2O3, residue on a 90 micron sieve or other else)?
Answer:
1. The percent fly ash addition depends on the cement standards in your country and also on the expectations of your customers. You must expect a drop in early strength and the maximum fly ash will be determined by the reduction in early strength the market will accept.
2. Addition of fly ash should increase the sulphate resistance of concrete made with the cement.
3. It is often best to add the fly ash to the separator of the cement mill circuit. The material that is fine enough already will pass straight to the finished product without grinding. This will preserve the spherical shapes of the ash and promote workability of the cement and reduce water demand.
4. The most important limit is the reduction in early compressive strength.

Question 371:
We have an automatic robo lab with pfaff divider, hertzog mills & press with Phillips 2400 XRF. We are seeing a drop in our fe2o3 in our raw meal & kiln feed samples of 0.2%, it occurs about one time in ten. It also occurs in a stock sample with similar frequency. If the low fe2o3 pellet is analysed ten times the same drop can be seen with a SD of 0.01. This suggests xrf is not the problem! The problem never occurred in fused beads. We have tried to contaminate the sample with cement in the dividing station to replicate the problem, this also failed. Contamination of sample with limestone going to press had no significant effect. Using more or less grinding aid had no significant effect on fe2o3. Using different weights of sample to both mill and press had no effect on the iron.
This problem left for 12 months and is now back. The problem never occurred in 12 years with a Hertzog manual mill, manual press and phillips 1606XRF. Fls Has looked at the problem for us with no answers. Has anybody else had a similar drop in fe2o3 in sample while other oxides remain constant?
Answer:
This question we are going to have to put on the web (anonymously), as I do not have an answer. It must be something systematic in the sample preparation or measurement, but you have tried everything obvious?
Question 370:
We operate two fine coal bin of cap. 200 tons each. Due to coal fluctuation we have provided compressed air at 2kg pressure but due to this sometimes we are having problem of fire & sintered formation in bin. We have already tried blower air but with this coal fluctuation problem. Can you suggest an alternate way to avoid this problem?
Answer:
I would not use air or gas of any kind to agitate the coal in the fine coal bins. I suggest using acoustic cleaners and sounding these at regular intervals to prevent bridging in the bins.

Question 369:
We have a problem in determining which residue is correct, either the raw meal or the kiln feed. In fact we do get about 13 per cent residue on 90 micron for raw meal and when it comes down to kiln feed it is about two per cent higher. Could you be kind enough to explain why this phenomena takes place and what actions could we attempt to resolve the differences.
Answer:
The kiln feed normally contains the dust from the precipitator in addition to the raw meal and therefore the two samples are quite distinct. The kiln feed is normally finer than the raw meal but this depends on the exact circuit used on the particular factory. Your situation is different and might be explained by drying the raw materials in the preheater exhaust prior to precipitation. In terms of quality control the raw meal is the residue you should be controlling. The kiln feed sample is within the external dust recirculation circuit. If you send more details of the circuit I can explain better.
Question 368:
Is there a published conversion formula or approximation from Blaine cement fineness values to sieve sizes? (I.e. 3800 Blaine is approximately 325 mesh)
Answer:
I’m afraid not. The two measures of fineness are quite distinct. Blaine is the total surface area, while sieve residues are the fractions above a certain size. So it would be possible for a 3800 Blaine cement to have a 45micron sieve residue of 10% or 20%. The cement with the coarser 45-micron residue would be compensated by a high proportion of super fine material to yield the same Blaine as the sample with the lower residue. This is why the best way to characterise the fineness of cement is the Rosin Rammler slope and characteristic value.

Question 367:
I am working on a project for my university studying the different technologies used in the cement production process from different companies, and I would like to know if any corporation has already constructed its process with a fluidized bed cement kiln for commercial operation and could reach acceptable efficiency. Also, I am researching for the system with most number of steps at the preheater, and the performance of the process compared to other ones.
Answer:
I am not aware of any commercial operations using fluidised bed for industrial cement clinker production. There are pilot and research installations but not of commercial capacity. There are number of kilns with six-stage preheaters. This brings the exhaust gas temperature down to less than 300 degrees Celcius. However, the most economic number of preheater stages is usually five, as increased capital costs for six stages cannot be recovered by the energy savings. A company might choose a six-stage preheater if there is a shortage of water for exhaust gas cooling and conditioning. Therefore most of the existing six-stage preheater kilns are in desert environments.
Question 366:
We’re suffering from brown clinker. As the results of microscopic analyses, there surely should be reducing condition in the burning of the kiln. So we increased the cooling airflow into the kiln to supply sufficient oxygen. It’s getting better but we still have brown clinker. We can’t increase the cooling air rate anymore because we already operated the exhaust fan with fully opened condition. So we’re considering reducing the coal flow rate. It means clinker production should be decreased. What are the next methods we can do except reduction of throughput production to avoid the brown clinker.
Answer:
It sounds to me that you must have combustion problems. I recommend that you should contact specialists and pay for a combustion audit. This will be the best way to find the solution. You might try increasing the primary air flow of the burner to maximum, or reducing the secondary air temperature by running the cooler grate with a lower under-grate pressure set point.

Question 365:
We intend to increase the strength of our pozzolan (from natural pozzolan) cement to 30MPa (28 days). But we could hardly reach it. What would the best thing to do?
Answer:
There are two possible options. First you need to maximise the strength potential of the cement clinker you are blending with the natural pozzolan to make the pozzolanic cement. This means stabilising the clinker mineralogy and boosting the alite content. You can do that by lifting LSF or SM dependent on your current clinker chemistry. Another alternative would be to add calcium fluoride to the raw mix. Secondly you could add a cement conditioner to activate the pozzolanic reaction in the cement. Possibly alkali carbonates or potassium sulphate.
Question 364:
Will an OPC concrete become less or more resistant to sulphates with addition of extra OPC.
Answer:
I believe it will become more susceptible to sulphate attack if more cement is present as there will be more hydrated C3A which can react with the sulphate. If you want to confer sulphate resistance to OPC then add ground granulated blast furnace slag.
Question 363:
Regarding silicon carbide blocks in cooler to ease the removal of snowman, would you kindly send installation’s name available in cement plant. Can you also please suggest LSF, S/R & A/F values to be maintained to avoid snowman. Kindly suggest any other parameters for raw mix & process to be maintained.

Answer:
I am unaware of the use of silicon carbide blocks to ease the removal of snowmen, and therefore cannot give any references. I also do not think there a value for LSF, SM or AM which will avoid the formation of snowmen. Snowman formation is caused by a complex interaction of the clinker chemistry and process conditions in the kiln & cooler. The chemistry cannot be considered in isolation. However you might expect that a higher silica modulus and alumina modulus will lead to less snowman formation as the amount of flux in the kiln will be reduced. Other factors to consider are the alkali and MgO content of the clinker and also the secondary air temperature and its stability.

Question 362:
I would like to know more about alkali silica reaction in the concrete. Kindly clarify the following. I am aware that alkalis in the cement will react with reactive silica of the aggregates and cause cracks. I would like to know how fast these cracks are likely to appear once the concrete is poured. Is it possible that within a day or two after pouring the concrete, cracks are likely to be surfaced due to ASR problem? Fly ash addition in cement is supposed to mitigate ASR problem in the concrete. Is there any reason due to which fly ash is likely to increase the ASR problem? What tests are to be conducted on fly ash, to find out the suitability for mixing in the cement? What tests are to be conducted on aggregates to understand the reactivity with alkalis present in the cement?
Answer:
No, it is not possible for the alkali-silica reaction to cause cracking of concrete within a day or two of pouring the concrete. The expansion reaction caused by the alkali -silica reaction causes cracks to appear a number of years after the concrete. Fly ash addition should not increase the susceptibility to alkali-silica reaction. The pozzolanic reaction of the fly ash fills the pores of the concrete and reduces the tendency for cracking. However, the fly ash should be of low alkali content. You must test the type of silica present in the aggregates, reactive forms are opal, chalcedony, some types of quartz, cristobalite, tridymite and various siliceous glasses.
Question 361:
What are the criteria for petcoke grinding? When I can “stop” petcoke grinding?
Answer:
The criteria for petcoke grinding should be the residue on a 90-micron sieve. You need to find the optimum by conducting trials however you can anticipate that you may need to grind the petcoke to less than 10% residue on the 90-micron sieve. The problem with petcoke is the low level of volatiles. With a coal particle the particle “explodes” when it enter the kiln atmosphere due to rapid evaporation of the volatiles in the coal. This causes very rapid combustion. With the petcoke there are no volatiles so the particle can fall into the bed of material before fully combusting. This can cause locally reducing conditions and badly affect the clinker quality. This is why petcoke is usually ground to a high fineness.

Question 360:
What is the optimal forced airflow on the kiln shell? How does this impact on the refractory life? What is the interval in the shell temperature when I should turn on and turn off a fan?
Answer:
I would say the optimum forced airflow on the kiln shell is zero. The higher the airflow then the higher is the loss of energy through the shell. This means greater heat requirements in the kiln and more penetration of salts into the refractory and therefore a negative impact on refractory life. However, as the refractory lining wears switching a fan onto the shell can be useful to try to build coating. I would suggest switching the fans on at above 350 degrees C.
Question 359:
Can I have the details about “patented TXI Cemstar addition of air cooled slag to cement kilns”? Do you have any information of any cement plant using Blast furnace slag as raw material for raw mix? What are the basic requirements and the precaution we must take to use GBFS as a raw mix component? Will it affect productivity of the mill as I heard that slag is very difficult to grind.
Answer:
The TXI Cemstar process involves the addition of air-cooled slag to the inlet of the rotary kiln with the aim of boosting the output by up to 20%. The distinction between “air cooled slag” and “granulated blast furnace slag” is important. I know of no factories using granulated slag as a raw mix addition while a number in the USA are using air-cooled slag and the Cemstar process. This air-cooled slag is not added to the raw mill, but added directly to the kiln inlet via a hopper and appropriate feeding device. This is the basis of the TXI patent from the Cemstar process.

Question 358:
I work at a plant 4500tpd, based on FLS design, with 5 stage preheater and precalciner, Our problem is repeated blockage in cyclone stage 5 (some four times at period of five months) We calculated the sulphate/alkali ratio before each blockage, we found it between 1.45 to 1.7. As we know the normal value should be less than 1.2. The LSF between 92 and 105, also the analysis result of a sample from the blockage material are: SiO2=19.5, Al2O3 =4.76, Fe2O3 =3.63, CaO=62.1, SO3=2.31, K2O=1.37, Na2O=0.93. Can you advise if the values of SO3/AlK ratio to cause the blockage in Cyclone 5 or is it another reason?
Answer:
From the information you have sent there seem to be two problems, (i) the SO3/Alkali ratio between 1.45 and 1.7 is very high and would be expected to cause very hard dense coatings, (ii) the variability of the LSF from 92 to 105 is much too high and would be expected to cause major process and quality variability. However. The samples from the blockage do not show very high SO3 levels and have a SO3/alkali ratio of 0.87, which should not cause any problems. The other cause could be premature calcination in the cyclone 5 due to variable LSF. This would happen with the change to low LSF, easy burning material.
Question 357:
Our plant is supplied by Polysius with inline calciner. The rated output of the plant was 3000tpd. The original system was designed based using Indian domestic coal. Presently we are operating the plant at 3900tpd and we are also using 65% petcoke and 35% imported coal. At this operating condition, we are facing the problem of very high generation of fine clinker during burning process, resulting in the high clinker temp. causing the problem in roller press operation. In view of above, we are interested to know the reason for the same, along with the corrective measure for the improvement in clinker granulometry.

Answer:
My only suggestion would be to adjust the kiln feed chemistry to achieve the same flux content as applied with the Indian coal. The other possibility is that the problem is caused by elevated sulphate in the clinker arising from the petroleum coke.

Question 356:
We are running a mini cement plant based on VSK technology in Africa and are having problems with initial strength. The plant produces Portland cement. The one day compressive strength is 10mpa using ENV 197-1 method. This increases to 17mpa after three days and becomes 28mpa in seven days. It reaches to about 42mpa in 28days. The clinker is ground with gypsum. We would like to improve initial strength (i.e. strength of 1-7 days) by at least 50%. Can you please suggest any admixtures that can be included while grinding gypsum and clinker and what percentage will have to be added?

Answer:
In order to boost the early strength you need to raise the lime saturation or silica modulus of the kiln feed and clinker. Unfortunately your VSK technology inhibits the possibilities to produce high early strength cement. You might approach Grace or Mapei and ask if any of their additives will solve your problem.

Question 355:
In our plant, we have a typical problem of high wear rate of mill liners. The basic reason identified is the percentage of free silica in silica (one of the components of raw meal) is very high about 50% and the consumption of this material is also very high (around 14%) due the very high quality of limestone (low grade limestone, clay or shale is not available in near vicinity) and thus making kiln feed free silica around 9 to 10%. We are of the opinion that the recirculation of the O-Sepa is causing a cumulative effect on the percentage of free silica inside the mill, which is the reason why we see that the total lifetime of the diaphragm plates is continually reducing (from 6 months to 4 months to 3 months). We would like to know, are you aware of any recent developments in design, which can provide an arrangements to remove this high density coarse grain material out of the mill to avoid high abrasion of mill liners and diaphragm plates?

Answer:
The silica will inevitably build up in the recirculating load of the mill as this is the hardest component of the raw mix. The limestone is preferentially ground leaving the silica in the rejects. Unfortunately I don’t know of any means to avoid this.

Question 354:
Our preheater surging. This is happening on an irregular basis. We suspect it is coming from cyclone No 2. We have investigated and have taken several actions. Unfortunately, we were unable to resolve it. Actions taken were as follows: 1 Damper adjustment. 2 Check feeding system. 3 Effects from other cyclones. 4 Preheater riser duct build-up. 5 Splash plate adjustment/build-up.

Answer:
You need to take samples from each cyclone and measure the loss on ignition. If you send the results along with the raw mill product, kiln feed and dust loss on ignitions then the preheater cyclone efficiencies can be calculated. This will tell us if the problem is originating in cyclone 2.

Question 353:
We have a small 3-chamber cement mill; size 2.4m dia. x 10m length, grinding ordinary Portland cement clinker. Mill diameter on liners is 2.30m. The mill is working in closed circuit with a mechanical separator, which has a diameter of 4.0m and is equipped with the main and auxiliary fans and distribution plate driven by a variable speed drive motor. The separator uses its own circulating air. The mill circulating load is 275%. The mill output is 12 ton per hour at 2900 cm2 /gm Blaine. Total grinding ball charge in the mill is 50t. Separator efficiency is 37%. We want to know if the mill output and Blaine can be increased or is it the maximum for mill of this size. Secondly, is the mill tube L/ D ratio at 4.16 too high for closed circuit operation? Will this mill give higher output if used as open circuit mill?

Answer:
I do not think you can achieve any more output with a mill of these dimensions. I also don’t think that operating it in open circuit will increase the output. In fact 12 tph is significantly better than I would expect for a mill of these dimensions.

Question 352:
We have to take gypsum out of a rectangular silo and we are not sure if a rotary valve is necessary between the silo and a screw conveyor. At the moment we have problem of rat holing and bridge as well, thanks in advance for any help.

Answer:
I would suggest installation of an acoustic cleaner and sounding the horn periodically to combat the rat holing problem.

Question 351:
I would like to ask what are the guidelines in selecting the best process and automation control system for cement plants.

Answer:
I am sure that your company must have guidelines for this. You need to draw up a list of the attributes you looking for in the system and their importance, i.e. price, capacity, local support capability, etc. Then rank each potential supplier against this list of attributes. The one, which scores the best aggregate against the list of attributes, will be the best choice.

Question 350:
We are running for the last one-year with a rapid build up at mix chamber, kiln housing and stage 4 riser and countering it with continuous manual cleaning. All the problems are due to the use of high sulphur coal at kiln head and at P.C. But we never experience the build-ups in P.C vessel. For the last 20 days we experience this new strange problem. Here I would like to point out that at stage 3 feed pipe has a double pendulum flap valve one of which is stuck in open condition for 2-3 weeks, through which feed enters in to the P.C and discharge of P.C goes to the mix chamber. Because this flapper is stuck, short-circuiting of air is expected. My question is that beside over-sulphatisation, could this short circuiting of air contribute to build-up in the P.C. Furthermore our quality chief has an opinion that over burning/high litreweight is also an important factor for increasing the sulphur cycle as some of sulphates break down into SO2/SO3 and CaO/Na2O/K2O and sulphur starts recycling. What is your opinion?

Answer:
Possibly the feed flap from stage 3 being stuck open is causing short-circuiting of gases from the calciner and the disturbance in the gas flow through the calciner is the cause of the build-ups. Over-burning will increase the volatility of the sulphates and therefore contribute to a heavier alkali cycle and increased build-ups. However the combustion conditions are a much more important factor. If there is any CO formation in the burning then the vapour pressure of SO3 will increase dramatically. You must be sure there is no reductive burning in your current operations.

Question 349:
I am a Chemical Engineer working in a company in the cement area in South America and the question is: What sort of fluorspar (acid, metallurgic) is used as fluxing agent in clinker process, what percentage of fluorspar and what quantity in the raw mix fed to the kiln. Does it imply savings in heat?

Answer:
In my experience acid grade fluorspar spar is used as a mineraliser, however there is no reason why other grades should not be used. Again in my experience addition rates have been up to 0.4% to kiln feed, achieving unit fuel consumption savings in excess of 200 kJ/kg clinker.
Question 348:
Is silicosis a potential hazard in cement manufacturing? If so, what kind of protective measures should be taken?
Answer:
Whether silicosis is a hazard of cement manufacturing depends on whether there is free silica in the raw materials. However, whether there is free silica or not workers should avoid breathing any dust into their lungs. Dust mask should be worn wherever there are high dust levels. There is no free silica in the cement final product.
Question 347:
I study about hexavalent chromium and its source in Portland cement production. Before I have studied about this and measured total chromium by ICP analysis method. Now I want to study Portland cement samples for determination and quantification of soluble and insoluble hexavalent chromium and find the main source of this pollutant and if possible to control such effects.

Answer:
The main source of hexavalent chromium in Portland cement and the dust associated with its manufacture are magnesite chrome refractories used to line the burning zone of cement rotary kilns. For this reason the use of these refractories has been discontinued in many parts of the world, but they are still used in some regions.

Question 346:
We are facing problem of poor fluidity of cement while extracting cement from silo even with fresh cement stock. We are having only Ordinary Portland Cement grinding with no water-cooling facility in mill. Cooling takes place only in O-sepa. Since the last two months we started water spray in mill inlet to control mill outlet product temperature of around 108 to 113 deg C. Product temp. entering to silo is around 85 deg.C and at outlet 72 to 75deg.C. Humidity is around 40 to 60 % in air because of coastal area. Please highlight some of the possible causes for such phenomenon and also give the method normally applied to determine moisture content in cement. As additional information we are using mineral gypsum 5 to 5.5 % in the cement.
Answer:
I suggest you should raise the mill outlet temperature set point of the PID loop controlling the water injection rate. This should allow more water to be driven off the gypsum in the cement mill. At the same time you should attempt to maximize the cooling in the O-Sepa.
Question 345:
During wintertime we have a problem with keeping the temperature high enough in the mill while grinding a blended type of cement. This problem is caused by false air that enters the system. Now we want to determine the amount of false air that enters do you know an accurate way to measure it? And do you have a solution to solve this problem?
Answer:
The way to determine the amount of false air entering the mill would be to construct and mass and energy balance for the mill based on gas flow measurements around the circuit. The problem with low temperatures when grinding blended cements is a common one. The only solution is applying some heat energy either to preheat the mill ventilation air or pre-dry the components of the blended cement. Alternatively produce a different grade of blended cement during the winter with a lower proportion of supplementary cementitious material.

Question 344:
What is the significance of axial sampling for a finished cement mill?
Answer:
The principle of axial testing is to crash stop the mill, then go inside and take samples at 1m intervals along the axis of the mill. You need to dig down into the media to take the samples as the mill ventilation fan will pull some of the cement off the top of the media while the mill cools sufficiently for you to enter. The size distribution of the samples is then measured in the laboratory. You are looking for progressively finer material along the axis of the mill, reaching the desired fineness at the outlet diaphragm.
Question 343:
We have a cement mill of 70t/h, but we run it nearly 80t/h.Main motor rating is 2925 kw. We have a Sepex seperator. Power consumption of main motor before ball charge was 3015kw (average) after 10-ton ball charge power consumption raises 3103kw (average). Ball charge calculation based on h/d ratio calculation, where h=empty height from charge to the mill centre and d=effective diameter of the mill. Blaine also increases. But after nearly 135 hours of operation Min motor power consumption rises to average 3200kw.

Answer:
The power drawn by the mill main motor is principally a function of the amount of media in the mill. Superimposed on that is some inverse variation due to the amount of material in the mill.

Question 342:
In order to optimise the performance of a ball mill, is there any suggested air velocity through the mill that we should use? Our cement plant has some very old ball mills. One in particular is a 4.572m x 13.9m two-compartment mill, closed circuit, with a Sepax separator 2S-375 separator. The separator and mill are vented by the same dust collector. We are currently running high mill outlet ventilation on the mill, greater than -4″ W.C. What is an acceptable mill ventilation static pressure range for this type mill circuit?

Answer:
1.5 to 2 m/s air velocity through the mill is normal for a closed circuit mill.

Question 341:
Recently we have successfully commissioned our project of conversion to coal from furnace oil. We have imported the coal stacking, reclaiming and grinding system from Hefei Cement Research and Design Institute, China and Dosing, Firing system from FLS Denmark. We are writing to you to seek your opinion regarding effects on thermal efficiency of our Kilns by use of coal in place of furnace oil. We have studied in Cement Data Book by Walter H. Duda, which says that coal generates less combustion gases compared with furnace oil and as a result, the heat consumption while using coal is always less compared to the furnace oil. The heat consumption, while using 100% furnace oil in our Kilns was found to be in the range of 770 to 800 K. Cal. per Kg of Clinker. We will like to have an idea that what K. Calories should we expect by use of 100% coal in place of furnace oil.

Answer:
I expect that you will see an increase in output and slight reduction unit thermal energy consumption when burning coal rather than furnace oil. This will arise for the reasons you have mentioned, i.e. a reduction in the volume of combustion product gases and the incorporation of the coal ash into the clinker. This coal ash does raise the possible problem of heterogeneity in the clinker mineralogy, however I would not expect it to be a major problem. I do not agree with the projected loss of 11% of the calorific value of the coal for the following reasons:
1. The coal should be dried by excess hot air from the grate cooler therefore additional energy does need to be supplied to dry the coal.
2. I consider coal handling losses of 2 to 3% to be excessive. The handling equipment must be designed and sealed to minimise these losses.
3. The coal transport air injected with the coal dosing supplements the primary air in the burner rather than being excess air. 4. The sensible heat input to the kiln in the fuel will be lower compared with furnace oil, however I expect the lower combustion product gases and coal ash incorporation to more than offset this.
Question 340:
We are looking for a non-contacting level measurement device that will work on finished cement storage silos, on homogenizing silos and on clinker storage bins. The largest silos are 38m high and 10m in diameter. It is required that the level is measured continuously and in all conditions, also during filling when a lot of dust is formed. In total we need 21 units and all will be connected to a PLC via 4 to 20 mA or field bus.
Answer:
I believe Endress & Hauser with their ultrasonic and time domain reflectometer technologies would be the people to help you out.

Question 339:
Is there any literature about the influence of triethanolamine addition in the cement milling process, related with the final characteristics of cement? Is there an optimum rate of triethanolamine addition in the cement milling process? Can I cause some damage in the final cement characteristics if we use a triethanolamine super dosage?
Answer:
There are many literature references to the effects of grinding aids and cement conditioners. The optimum rate of addition needs to be determined during a series of trials on the mill. The trials need to be designed to ensure there are no detrimental effects on the final cement characteristics.
Question 338:
We have a hot spot at 22m on our kiln, would it be possible to use a jet of water to cool the shell in this area?
Answer:
People sometimes do that. However, it causes a significant blemish on the shell and when the water is switched on a significant thermal shock and potential contraction. In general it is better to use fans and/or stop the kiln and repair the refractory lining. If you put a fan close to the shell you may induce the thin section of the lining to coat over and bring the shell temperature down.
Question 337:
I would like to know the influence of iron ore addition in raw mix i.e. in burnability, specific heat consumption, etc.
Answer:
The iron oxide additions in your raw mix increase the melt content at the clinkering temperature in the kiln. The iron oxide forms the mineral C4AF that is a liquid in the burning zone. This fluxes the combination into the clinker minerals increasing the burnability and reducing the fuel consumption.

Question 336:
We always have problems to get masonry cement with 95 % passing the 200 mesh in a mill with an O-sepa separator. We have high Blaine numbers but low in fineness. Do you have any solution please?
Answer:
I think you have a problem with different grindability for the clinker and limestone fractions of your masonry cement. The limestone is preferentially grinding and giving you the high Blaine, however the harder clinker remains coarse and is the reason for the high residue. The best solution would be to separately grind the limestone and blend with OPC to create the masonry cement.
Question 335:
We’re suffering from brown clinker. As the results of microscopic analyses, there surely should be reducing condition in the burning of the kiln. So we increased the cooling airflow into the kiln to supply sufficient oxygen. It’s getting better but we still have brown clinker. We can’t increase the cooling air rate anymore because we already operated the exhaust fan with fully opened condition. So we’re considering reducing the coal flow rate. It means clinker production should be decreased. What are the next methods we can do except reduction of throughput production to avoid the brown clinker.
Answer:
It sounds to me that you must have combustion problems. I recommend that you should contact specialists and pay for a combustion audit. This will be the best way to find the solution. You might try increasing the primary air flow of the burner to maximum, or reducing the secondary air temperature by running the cooler grate with a lower under-grate pressure set point.

Question 334:
We intend to increase the strength of our pozzolan (from natural pozzolan) cement to 30MPa (28 days). But we could hardly reach it. What would the best thing to do?
Answer:
There are two possible options. First you need to maximize the strength potential of the cement clinker you are blending with the natural pozzolan to make the pozzolanic cement. This means stabilizing the clinker mineralogy and boosting the alite content. You can do that by lifting LSF or SM dependent on your current clinker chemistry. Another alternative would be to add calcium fluoride to the raw mix. Secondly you could add a cement conditioner to activate the pozzolanic reaction in the cement. Possibly alkali carbonates or potassium sulphate.
Question 333:
Will an OPC concrete become less or more resistant to sulphates with addition of extra OPC.
Answer:
I believe it will become more susceptible to sulphate attack if more cement is present as there will be more hydrated C3A which can react with the sulphate. If you want to confer sulphate resistance to OPC then add ground granulated blast furnace slag.

Question 332:
I would like to know more about alkali silica reaction in the concrete. Kindly clarify the following. I am aware that alkalis in the cement will react with reactive silica of the aggregates and cause cracks. I would like to know how fast these cracks are likely to appear once the concrete is poured. Is it possible that within a day or two after pouring the concrete, cracks are likely to be surfaced due to ASR problem? Fly ash addition in cement is supposed to mitigate ASR problem in the concrete. Is there any reason due to which fly ash is likely to increase the ASR problem? What tests are to be conducted on fly ash, to find out the suitability for mixing in the cement? What tests are to be conducted on aggregates to understand the reactivity with alkalis present in the cement?
Answer:
No, it is not possible for the alkali-silica reaction to cause cracking of concrete within a day or two of pouring the concrete. The expansion reaction caused by the alkali -silica reaction causes cracks to appear a number of years after the concrete. Fly ash addition should not increase the susceptibility to alkali-silica reaction. The pozzolanic reaction of the fly ash fills the pores of the concrete and reduces the tendency for cracking. However, the fly ash should be of low alkali content. You must test the type of silica present in the aggregates, reactive forms are opal, chalcedony, some types of quartz, cristobalite, tridymite and various siliceous glasses.
Question 331:
What are the criteria for petcoke grinding? When I can “stop” petcoke grinding?
Answer:
The criteria for petcoke grinding should be the residue on a 90 micron sieve. You need to find the optimum by conducting trials however you can anticipate that you may need to grind the petcoke to less than 10% residue on the 90 micron sieve. The problem with petcoke is the low level of volatiles. With a coal particle the particle “explodes” when it enter the kiln atmosphere due to rapid evaporation of the volatiles in the coal. This causes very rapid combustion. With the petcoke there are no volatiles so the particle can fall into the bed of material before fully combusting. This can cause locally reducing conditions and badly affect the clinker quality. This is why petcoke is usually ground to a high fineness.

Question 330:
What is the optimal forced airflow on the kiln shell? How does this impact on the refractory life? What is the interval in the shell temperature when I should turn on and turn off a fan?
Answer:
I would say the optimum forced airflow on the kiln shell is zero. The higher the airflow then the higher is the loss of energy through the shell. This means greater heat requirements in the kiln and more penetration of salts into the refractory and therefore a negative impact on refractory life. However, as the refractory lining wears switching a fan onto the shell can be useful to try to build coating. I would suggest switching the fans on at above 350 degrees C.
Question 329:
Can I have the details about “patented TXI Cemstar addition of air cooled slag to cement kilns”? Do you have any information of any cement plant using Blast furnace slag as raw material for raw mix? What are the basic requirements and the precaution we must take to use GBFS as a raw mix component? Will it affect productivity of the mill as I heard that slag is very difficult to grind.
Answer:
The TXI Cemstar process involves the addition of air-cooled slag to the inlet of the rotary kiln with the aim of boosting the output by up to 20%. The distinction between “air cooled slag” and “granulated blast furnace slag” is important. I know of no factories using granulated slag as a raw mix addition while a number in the USA are using air-cooled slag and the Cemstar process. This air-cooled slag is not added to the raw mill, but added directly to the kiln inlet via a hopper and appropriate feeding device. This is the basis of the TXI patent from the Cemstar process.

Question 328:
I want to make a complete evaluation of the cement lines in my plant by doing a mass and heat balance, how do recommend for me to do and how to start, I’m still new in the field and don’t have much experience.

Answer:
Your idea is a good one to understand the process and construct the mass and heat balances. First you need to measure every flow into and out of the process. The raw feed and fuel should be metered and therefore easy. The dust loss from the top of the preheater is more difficult and requires that you use dilution techniques. The preheater exhaust gas flow is comprised of the combustion products from the fuel, plus the CO2 from calcination of the limestone, plus the excess air drawn into the kiln. Hopefully the airflow into cooler grate is measured, you can estimate the excess air from the changing oxygen contents through the process. The excess of the cooler air over the combustion and excess air is then the cooler exhaust. It is always worth crosschecking this by measuring the flow of the cooler exhaust. Having balanced the mass flow in this way you then need to calculate the energy flow associated with each of these mass flows and also the energy loss through the shell of the preheater, kiln and cooler. The difference in the balance is then the heat of clinker formation, which will be in the order of 1700 kJ/kg clinker.

Question 327:
One alternative we have considered to increase the strength of our pozzolan cement is to add grinding aid (strength-enhancer). But during our first run, the blank sample exhibited more strength in the lab test compared to cement with grinding aid. Can the grinding aid will really boost strength to a greater degree? What grinding aids you can suggest for this purpose? Currently we have 28-day strength of 23 MPa but our objective is to reach 30MPa.

Answer:
The interactions of grinding aids and quality conditioners are complex and you have to conduct trials to find the optimum additive and addition rate. The specialists in this area are Grace. If they cannot help you then perhaps Mapei? I think your wish to increase the 28 day strength by almost 40% is too ambitious and that no additive can do this. You must continue with your trials to find the best possible additive and addition rate.

Question 326:
We have a 1000 TPD FLS kiln with a 5 stage preheater. We are having a lot of clinker dust formation, which is causing great wear at the kiln hood. What would you recommend to get rid of the clinker dust?

Answer:
It is difficult to diagnose the problem over the internet, however these problems are often caused by inefficient combustion of the fuel in the main burner, long flames and poor heat flux in the kiln. I suggest you increase the axial primary to the maximum and see if that helps.

Question 325:
What are the possible causes of large snowballs occurring in a short precalciner kiln, which build up and eventually cross into the burning zone, and require a stoppage to break them up.

Answer:
I can’t recall any really good articles on the formation of snowballs but know they can be quite frightening. They are caused by variations in the clinkering range or flux content of the feed. Mostly they break up under the flame but I have seen monsters fall into the cooler, which is quite alarming and can cause major damage. This happened occasionally in one kiln where I was involved and we traced the problem to variations in the MgO content of the limestone and kiln feed. That might be the answer.

Question 324:
I would like to raise question with regard to the impact of Strontium (Sr) to EP efficiency. From our experience, there is a negative correlation between Sr concentration to EP efficiency. When Sr increased, dust emission increased too. Our Sr content in limestone rock varies from 183 to 1382mg/kg. Or, do you suspect from another elements such as organic matters as well as high dust resistivity, too?

Answer:
Your experience with strontium is very interesting and I have not heard of it before. The performance of your ESP will also be affected by organic matter and the resistivity of the dust. The key question then becomes are these variables independent of one-another? I know of no reason why strontium would increase the resistivity of the dust.

Question 323:
There are two kinds of alkalies contained in clinker-soluble and insoluble alkalies. Please explain about the formation processes in the kiln and the effects of two alkalies respectively on clinker quality, especially on setting and compressive strength.

Answer:
Alkalis in clinker will preferentially combine with chloride and sulphate, the alkali chloride and sulphate formed is volatile and leads to the build up of the alkali cycle in the kiln. Alkali sulphates are less volatile than chlorides and some pass out of the kiln in the clinker. These are the source of the soluble alkali in the clinker. They promote the hydration of the cement and lead to higher early strengths. If alkali is present in excess of the sulphates in clinker then it enters into the clinker minerals. These are the “insoluble” alkalis. They do affect the hydration behaviour of the cement as they affect the clinker mineralogy, however effects are not as clear as for the soluble alkalis.
Question 322:
I am working for a Ready mix concrete Co. in Saudi Arabia. I’ll appreciate to know of any procedure to cool cement while in silos. The ambient temperature is above 45 Degree centigrade.

Answer:
The only way to cool cement in silos would be to aerate the cement via air slides or pads in the bottom of the silo. The silo would have to be provided with these aeration devices and with an ambient temperature of 45 degrees C would not be particularly effective. I imagine you are having problems with rapid setting of the concrete. The most effective means at your disposal would be to use mixing water, which was just above freezing point. That will be much easier to accomplish than cooling the cement or aggregates.

Question 321:
We determinate free lime by X-ray, but when we make a program to determinate it. This program has a life period about one month, after that, the program starts to give wrong results and we can’t calibrate it, because the problem becomes bigger. What can we do to eliminate this problem and what can we to do to improve the program.

Answer:
I presume you are using X-ray diffraction to determine the free lime but the problem is that the calibration drifts with time. I suggest that you measure the free lime by traditional titration once per day. The program can then be modified to continuously update the relationship between the free lime and the XRD intensity using the last 20 results. In this way the calibration will be continuously updated and there will be no need to make a special recalibration once per month.

Question 320:
I am making studies for cement plant and would like to make a following question? Is possible to control the weight in the ball mill (cement or raw mill plus balls) in order to save energy? My viewpoint is if you have any chamber unbalanced in ball mill, the motor drive can be working in a forced way and spent much more energy than the necessary? Is possible to use the balls to correct that balance, because today we have in the cement plant a different weight in each chamber?

Answer:
The weight of grinding balls in the mill and the power drawn by the mill main motor are directly proportional to one another. Therefore it is possible to reduce the power drawn by the mill by reducing the ball charge. However, this will then result in the output of the mill falling due to less grinding balls being present to grind the clinker and generate surface area. As with all these things there is an optimum ball charge and output, which corresponds to the lowest unit energy consumption.
Question 319:
In our factory we have used iron ore as additive in raw material for production of Portland cement. What is your opinion about using of magnetite or hematite as additive in raw material?
Answer:
There is no problem with using magnetite or hematite as a raw material for cement manufacture. With all raw materials you must check for impurities, particularly chloride as this can cause operational problems in the preheater.
Question 318:
We are facing serious problem of build ups at stage 4 riser duct starting from kiln housing and at start of mix chamber it becomes to thick and remains thick through out the riser duct so that opening remains only 30-40 percent. Some coating has also observed at cylindrical part of cyclone and at its roof, but coating at riser duct becomes very tough and hard and similar to sintered material. My 1st.question: is there any special method to get rid off it during operation, as normal pocking do not results any way. Is there any chemical treatment, operational technique or physical methods? Secondly what could we do to prevent such things?

Answer:
It is no surprise that you have problems. Your instrumentation and control technology is inadequate. I am surprised that you can operate a large modern kiln without coal flow metering equipment to the main burner. I think your problems are caused by an excess of sulphate over alkalis. This results in the very hard coating in the riser duct. The best solution is to increase the alkali content and pass the alkali sulphate out in the clinker. However, with your inadequate coal flow control there is likely to be reductive burning and then there will be a heavy sulphate cycle in the kiln. This is also likely to be contributing to your current problems. You could try installing acoustic cleaners to try to prevent the sulphate build-up in the riser. The high temperature excursions in the precalciner and stage 4 cyclone are caused by interruption in feed down the preheater tower caused by the build up. Solve the build-up problem and you will solve the high temperature problem. For the oil compensation you need expert control, however it is doubtful if your general control can be the basis for expert control.
Question 317:
The conversion factor raw meal to clinker obviously depends on the raw meal’s LOI. Which temperature do you recommend to make the test of LOI to have the best estimation of this factor?

Answer:
It is normal to measure the LOI at 1000 degrees C. However the higher the better up to the sintering temperature of 1450 degrees if your furnace will reach those temperatures. In practice the conversion factor is also affected by the dust loss from the kiln.

Question 316:
I have a few questions on a o-sepa separator for a finish mill we are operating: (1) We have a mill draft fan that is not a frequency drive and with the damper completely closed the lowest mill exit presser we can run is about -1.8. The problem being controlling mill temps, there are four dampers on the separator that are being use to cool the mill temp. From what I can see this disrupts the airflow in the separator and causes problems with the cement quality. Here’s the question would it be better to introduce some false air in to the mill draft fans exit draft, lowering the draft on the mill, then opening the damper and controlling the mill draft from 0 to where ever we need it.

Answer:
I need to understand your milling circuit better to make an informed comment. However it sounds to me that the four separator dampers are being opened to cool the cement temperature. This will contribute to cooling the mill exit temperature but only because the reject returns from the O-Sepa will be at a lower temperature.

For sure there is a danger of affecting the efficiency of separation by this mode of operation. I don’t see the advantage of introducing false air after the mill draft fan except to control the temperature of the air entering the filter. You don’t want to reduce the ventilation and air flow through the mill, as this will make controlling temperatures more difficult. Key questions: (i) do you have water injection on the mill? and (ii) have you developed a heat balance model of the mill? The latter will allow you to run some what-if scenarios and identify the best way forward.

Question clarification
we do have a water injection system; we have one spray on the feed end of the mill. We have not done a heat balance on the mill. The reason for introducing false air to the mill draft fan is the fan is too big. The air is actually introduced after the dust collector. And is only introduced to lower the efficiently of the fan, the fan damper is always closed; this allows the operator some control with the damper. The way the damper is made when it is closed it still allows air though the damper, so what ever mill exit pressure you have with the damper closed, that’s what your stuck with. I don’t like the damper control and would like to have a frequency drive on the fan or a smaller fan to lower the power cost.

As an example of how the mill is being controlled. (This is just one scenario)…. the Blaine is high so the Lab tech will ask the mill operator to lower the separator speed. The operator then notices that the mill temperature is rising and opens one of the dampers on the separator this changes the airflow in the separator. It does begin to cool the mill off but the false air introduced to the separator lowers the differential pressure on the separator and allows more of the course material to return to the mill.

Further answer
I certainly agree that the separator drafting should not be the primary response to control mill temperature. That is the tail wagging the dog. The fresh air damper into the separator should be at a constant opening for a particular grade of cement. The amount open needs to be determined by trials but once the optimum is identified it should not be changed as this changes the separation characteristics.

I also agree that adjusting the mill drafting is a better response. You need to be careful of this as well as it affects the amount of material pulled out of the mill with the ventilation air and the particle size distribution of the cement. Again trials to identify the optimum are the only way forward.

Water injection is normally on the outlet of the mill first with injection into the inlet (feed) end when the outlet water injection reaches its maximum. The outlet water injection rate can be controlled in a PID loop with the mill outlet temperature and this loop works well on lots of cement mills. If you decide to go this route make sure you have good atomisation nozzles on the outlet water injection lance.

Question 315:
We have three 120tph cement mills. All of them are equipped with Sepax separators. We are looking at reducing the power consumption in cement grinding by installing roller presses. For market reason, any increase in capacity is not necessary. The downstream conveying equipment are also limiting factor for increase in capacity. Under such constraints, in order to use most of the existing equipment but without heavily investing into new ones, which roller press grinding process would be the best for us to achieve our objective?
Answer:
There is not a straightforward answer to this question. There are many possibilities and circuits for combinations of ball mills, roller presses and Sepax separators. You need to make a study of the performance of these various combinations and decide which one is best for your factory and particular constraints. A key questions is whether the circuits are only grinding clinker and cement or whether there are other requirements such as slag, fly ash or pozzolan? As a first suggestion a roll press might be used to pre-crush all the clinker into an intermediate storage, which then feeds to two of the existing three ball mills. With pre-crushed clinker the two remaining mills should be able to more than cope with your existing production rate.

Question 314:
I work in the US. Other companies pay us to use their waste as fuels in some of our processes. Essentially, we are then generating revenue for the use of those specific fuels. Currently, the revenue is being accounted for as a contra expense on our income statement. Some believe that all revenue is revenue and the alternative fuels payment should therefore be accounted for as revenue on our income statement. Others argue that the alternative fuels process is taking place within a cost centre, one of our manufacturing plants, and all revenues should role up into the costs of the of the plant, which is what we are doing now by treating the alternative fuels’ revenue as a contra expense. Is there is an industry standard or norm for the accounting of this process

Answer:
I do not think there is an industry standard. It would be more a case of a company standard or country standard requirements. I have heard a number of energy directors expressing the target or objective of achieving “negative fuel costs”. It seems to me that this corresponds with what you are doing by counting the revenue as “contra expense”. I cannot see how a company could achieve negative fuel costs without counting the revenues for burning the alternatives against the traditional fuel costs. Whether this is strictly correct in accounting practice I don’t know.

Question 313:
While our plant was designed in 1998, the necessity of preblending stacker and reclaimer system was not incorporated based on the ground that run of mine limestone quality is very consistent (CaO standard deviation 0.60%) and silica, iron or alumina oxide are very less in the range of 0.6,0.1,and 0.3% respectively. Our raw meal is four-component limestone, Silica rock (75% purity as SiO2), Iron ore (55% purity as Fe2O3) and Bauxite (48% purity as Al2O3). Now since the beginning of this year from the mines receipt limestone CaO is reduced from 54 to 51.5% and at the same time silica, Alumina and Iron oxide increased to the level of 3.5to 4.00,0.8to 1.1,and 0.25to 0.35% respectively. The standard deviation for L.S.F. on three random 8 hourly samples we are getting in the range 40 to 90. I would appreciate to have feedback from you which is the level of variation in run of mine limestone making it necessary for installation of preblending stacker and reclaimer system for a one million tonne dry process cement plant.

Answer:
I do not believe a stacker/reclaimer should be necessary in your situation. With a good software system proportioning the materials into the mill you should be able to achieve less than 2% standard deviation in kiln feed and clinker LSF. You have another big problem with your mix because it is heterogeneous and therefore of very low reactivity. Do you have problems with high fuel consumption or excursions of high free lime in the clinker? I would recommend strict control of the mineral composition of the clinker and possibly the introduction of Duplex grinding.

Question 312:
We’ve been faced with a problem that the collecting efficiency of the electrostatic precipitators lowers remarkably since having started to use sewage sludge as the secondary material. I’m not sure where the cause is from. Is there any possibility of causing such problem by using sewage sludge, and which kind of components that sewage contains can affect the performance of ESP? As the reference, we’ve been experienced in the same problem when we used TPA sludge (Telephtalic Acid) two years ago.

Answer:
I have not heard of these phenomena before. The resistivity of the dust exiting the preheater must be changed when burning these sludges. I hope all the sewage sludge is being burnt in the kiln and organic residues entering the ESP do not cause the problem. If so then the problem can only be associated with volatiles derived from the sludge entering the ESP with the dust. The most likely source is chlorides from the sludge. You should also check carefully that the problem is not caused by mercury from the sludge. If so then you have a big problem.

Question 311:
How does the content of chromium have influence in quality and performance of grinding balls? Is it better a low content of this metal to improve hardness in balls?

Answer:
If the grinding balls contain chromium then they are forged, hard grinding balls. If the balls don’t contain chrome then they are softer mild steel and the wear rate will be higher. Balls containing chrome cost more but last significantly longer and maintenance of the optimum media loading in the mill is easier. Many cement companies use soft media for raw grinding as the wear on the steel simply increases the iron content of the raw mix slightly. Most cement companies use hard media for cement grinding as the wear rate on soft media when grinding clinker is too high and the mills need continuous recharging.
Question 310:
Our facility operates two Humboldt air-swept ball mills for raw meal production. Combined capacity is 100 tph. We would like to optimise the raw meal circuit, in particular, mill loading. Unfortunately, there appears to be very little information on the optimisation of air-swept mills. How can mill throughput be improved, utilising existing equipment? The separator is a mechanical classifier, with adjustable vanes. Current reside set point is 15% on 90 micron. We currently use sonic ears to determine mill loading, with mill and fan current also available. There is also a proposal to implement a grit return weigh system to determine the recirculating load. Would this be worthwhile addition, with any positive process benefits? In summary, we wish to determine the most cost effective means of attaining optimal material levels in the milling circuit, with the intention of automating the operation sometime in the future.

Answer:
If you are adequately controlling the kiln feed residues to 15% I am not sure what the grit return weigher is going to tell you. Will you be able to remotely adjust the vanes of the mechanical separator from the control room while the mill is running? If not I would suggest periodic sampling and measuring residues around the circuit is all you need to do to determine the rejects rate. What do you mean by “optimise” the raw mills? Do you need more raw mix output? Is there sufficient drying capacity in the mills? I presume they are swept by preheater exhaust gases?

Question 309:
What are the relationships between clinker hard burning and grindability of clinker. Is hard burned clinker hard to grind? Finally what is the relation between silica modulus(SM), iron modulus, and time of storage of clinker, size of the clinker?
Answer:
When clinker is hard burnt the amount of liquid phase increases and the clinker becomes very dense and fused together. This is the reason why hard burnt clinker is hard to grind. High silica modulus means low liquid content therefore softer clinker. High iron content means more liquid so possibly denser and harder clinker. If clinker is stored for a long time then it can become hard to grind due to “relaxation” of the clinker. Large clinker implies hard burning and agglomeration into large balls so might well be hard to grind.

Question 308:
I recently had a new driveway poured and we have encountered a “blistering/popping” problem in several areas of the concrete. An engineer from the cement delivery plant says the defects are caused by lime chips that contaminated several tanker loads of cement mix. My questions are: Should I be concerned with structural integrity of the concrete because of the defective spots caused by the lime chips? Is there any way of knowing how long the blistering/popping may continue?

Answer:
This problem is a bit out of the ordinary nowadays, although it seems to have concerned people a century or so ago, when cement manufacturing methods were less sophisticated and coarse particles of various materials could end up in the product. The damage is caused by the contaminant material reacting with water and forming a hydration product, which is of larger volume than the starting material and therefore needs to create space in which to expand.

Finely ground high-calcium lime would not cause any problem, as it reacts rapidly with water and expands while the concrete is still plastic. Dolomitic lime (containing significant amounts of Magnesium) is well known to give problems because it reacts slowly (weeks rather than hours), and its acceptable level is therefore limited in national standards for cement. From what you describe, it appears that you have contamination from lime, which is both coarse and relatively unreactive (perhaps over burnt, dense and highly crystalline calcium lime?). You should ask the supplier to confirm that the lime is not dolomitic. If it turns out that it is dolomitic, then you can expect defects to continue to develop during the coming months and you should insist that they keep the matter open and monitor developments.

For the more likely case of high-calcium lime, there are two extreme scenarios. (a) If there is not a great deal of it in any region of the concrete and it has now more or less finished reacting, mainly affecting surface appearance, then that will be the end of the matter. You can complain to the supplier about the sub-standard appearance and demand that it is either put right or you are given a rebate. (b) On the other hand, if there is a good deal of this material in at least part of the bulk of the concrete, then its expansion may weaken the structure and shorten its working life – not that a domestic driveway is a very demanding structure in terms of its duty and of strength requirements (not like an airport runway, for example).

You should seek reassurance from the supplier that they know whether scenario (a) or (b) is correct. You can ask if they have test data or experience from which you can be assured that your driveway will be fit for normal use for as long a period of years as you had originally expected. They should be able either to replace or repair the driveway now or – because they are confident about their product – to give you a written promise that they will put it right in the future if it continues to deteriorate, perhaps with a commitment to inspect it with you in (say) a year’s time. Alternatively, you might get a rebate on the price due to the substandard quality.

(If you want to know more, you are best enquiring via a concrete information service, rather than through us, as our readership is mostly concerned with making and trading in cement, not concrete.)

Question 307:
Our cement plant has some very old ball mills. One in particular is a 13′ x 46′ two compartment mill, closed circuit, with a low-efficiency separator. It is not an air-swept mill. The same dust collector vents the separator and mill. We are currently running a high mill outlet ventilation on the mill, greater than -4″ W.C. What is an acceptable mill ventilation static pressure range for this type mill circuit?
Answer:
I don’t think it is a question of the static pressure. More important is the speed of the air exiting the mill. 1.5 to 2.0 m/sec (5 to 6.5 ft/sec) would be normal for a mill of the type you describe. If the air velocity is too high you risk pulling material out of the mill in ventilation air before it has been sufficiently ground. The particle size distribution and hence the hydraulic performance of the cement is influenced by the drafting to a surprising degree.

Question 306:
We are getting build-ups of material on the discharge flaps; can you advise how to stop the same?
Answer:
The only thing can suggest is that you carefully adjust the counter-weights on the cyclone discharge flaps so that they are continuously opening and shutting during normal operations. You do not want them to be permanently open, and equally you do not want them to remain shut while material builds up on the flap. You want them to be continuously opening and shutting.

Question 305:
I am confused by some of the terminology associated with clinkerisation and what55 is meant by the Bogue method etc. Can you simplify a little please?”
Answer:
C3S is the mineral providing the early strength of the cement. If it rises the 1, 2, 3 and 7 day strength will rise & vice versa. C2S is the mineral providing the later strength. If it rises then this should rise, however if it rises at the expense of C3S then you will lose early strength.

C3A causes the initial setting of cement. However hydrated C3A is susceptible to attack by sulphates in the ground water. Sulphate resisting cement therefore limits the C3A content to a maximum of around 4%.

C4AF is important for manufacturing cement clinker, as this is the first mineral to form a liquid in the kiln and therefore flux the clinker formation.

The Bogue method of calculating the content of these minerals is only another way of mathematically representing the oxide content of the clinker or cement. LSF, SM and AM are also just further methods of representing the oxide composition of the clinker or cement. High LSF means high C3S. High SM means high total C3S and C2S. Low AM means low C3A.

Question 304:
How can we determine the maximum capacity of a gas-conditioning tower?
Answer:
The sizing of the tower is determined by a heat balance which tells you the amount of water which would need to be evaporated to realise a certain fall in temperature of the gases entering the tower. The results of this heat balance are then used with the Ideal gas laws, which allow you to calculate the contraction in volume of the gases caused by the reduction in temperature. This contraction creates a negative pressure. This then needs to be compared with the negative pressure rating of the tower.
Question 303:
Is it possible to burn an alternative fuel in a white cement kiln, which would not reduce the whiteness of the actual cement.
Answer:
Yes it is possible to burn alternative fuels in a white cement kiln without affecting the whiteness. Animal fat or tallow would be a good example. It all depends on the trace metal content of the particular alternative fuel.
Question 302:
We have started using a new raw material as a silica source. This material contains a significant amount of titanium. Coincidentally, our free lime content has increased. Can the material be the cause? How stable is perovskite (CaTiO3) as a clinker mineral?
Answer:
TiO2 enters into solid solution in the clinker minerals until saturation is reached and then the mineral formed is 3CaO.2TiO2. Essentially the TiO2 substitutes for SiO2 in the clinker minerals therefore it should effectively reduce the lime saturation and so the free CaO content of the clinker. The opposite of the effect you are experiencing. There could be many explanations for that. Is the granulometry of the silica content in the new raw material source the same as in the old source? Are there any other minor components (alkalis, etc.) in the new source, which might be affecting the mineralogy and combination?

Question 301:
We are carrying out research on blended cements with a view to increase the utilisation of blast furnace slag/fly ash. In this context, we are exploring use of high-energy mills (Jet Mill, Vibratory Mill etc) together with traditional milling devices. I wish to ask:
[1] how important is the strength of cement? There is considerable amount of literature on high performance cement/energetically modified cement i.e. cement with compressive strength in the excess of 90 MPa and concrete with greater than 145 MPa.
[2] What could be the major limitations in using mills such as Jet Mill in cement industry? We understand these mills consume lots of energy but it is also true that the efficiency of traditional mills decreases as we approach the target size. Should not the combination work better?
[3] I am looking addresses of supplier for lab size jet mill and vibratory mill (2-10 kg/h) for the grinding of cement clinker, BF slag and fly ash.

Answer:
The strength of the cement is important, as the hydration of the cement is responsible for all the early strength development of blended cements. The hydration of the cement minerals also releases the calcium hydroxide, which activates the pozzolanic reaction of the fly ash or slag. Jet mills are not used in the cement industry due to the operating costs and the throughput capabilities of the mills. Large cement plants employ mills capable of producing tens per hundreds of cement per hour. I don’t think jet mills can reach that capacity? For a laboratory mill my first port of call would be ELE (Engineering Laboratory Equipment).

Question 300:
We have a V.S.K unit with 300TPD production. The raw mix contains limestone, clay, coke breeze. We are getting continuously high free lime. Please advice the procedure through which we can reduce the free lime.
Answer:
There are a number of reasons why you might be getting continuously high free lime in your clinker. The most likely ones are: (i) there is too much limestone in your raw mix, (ii) the raw mix is not ground to sufficient fineness, or (iii) the temperature in the kiln is not high enough to finally combine the free lime with the clay minerals. The problem might be a combination of all these factors.

Question 299:
In my plant there is a kiln of 62 m long, and 4.4 m diameter. Sometimes, when a ring begins to form before clinkerisation zone (aprox 40m), its a common practice adding a little amount of silica sand (90% SiO2) to raw feed, before the preheating system, to stop the ring growth. Tell me if this procedure is correct, and what is the chemical effect of SiO2 to avoid the increase of rings.
Answer:
Adding silica sand is not a common procedure to stop the growth of rings, however if it works on your kiln then it is a good procedure. The presence of the silica sand changes the clinker combination conditions in the kiln and the position in the kiln where phase changes and chemical reactions take place. Changing these positions interrupts the growth of the ring. The only potential problem is that the clinker mineral composition will be variable if you are changing the silica content.
Question 298:
Please kindly advise the specifications for the Portland cement (type I, II). What’s different with the ASTM and the BS systems?

Answer:
The BS equivalent of ASTM C-150 would be BS 12. More commonly used in the UK now is EN-197. All these standards cover the specification of Ordinary Portland Cement. The principle differences are in the testing methods associated with different standards. The cements they are regulating are essentially the same but the numbers in the specifications are different because the testing methods are different.
Question 297:
Is there any relation between Blaine and strength of cement?

Answer:
Yes there is a relationship between Blaine and the strength development of cement. Blaine measures the surface area of the cement in cm2/g. The higher the surface the more reactive surface of the cement clinker minerals is available for reaction with the hydration water in a mortar or concrete mix. If more reactive surface is available then hydration and strength growth are accelerated.

Question 296:
We have been studying the different cement standards in different countries and we found out that the IR content limit of other countries is higher than the ASTM’s C150 specification for Type I Portland Cement. Because of this, we have been asked to initiate the increase in Insoluble Residue content of cement from 0.75% to 1.5% maximum. However, we are meeting a lot of opposition from different quarters such as concrete practioners, and admixture manufacturers. My inquiry is that: Will IR content of 1.5% maximum affects the cement performance in concrete?
Answer:
IR is the % material insoluble in a certain strength hydrochloric acid under set conditions. The silicates, aluminates, etc. in Portland cement clinker all normally dissolve 100% in the test conditions.
There are 2 possible reasons for limiting IR in traditional (non-blended or non-composite) Portland cements: (1) to confirm the purity of the gypsum and the absence of other foreign matter; (2) to check the completeness of combination of acid-insoluble minerals in the clinker.
The first case is not necessarily relevant to composite cements or to cements with a minor additional component (as permitted by European Standards for some years now, up to the 5% level).
The second case is most relevant to clinker analysis (and to internal Works quality control investigations), where the IR may result from circumstances such as failure to combine coarse particles from the kiln feed or from contamination by kiln bricks. Here, if the bulk composition has been determined by (say) XRF and used to calculate percent of C3S, C2S, etc by the Bogue methods, then the values could be misleading results if the IR is large.
You can do calculations to see the effect for your own Works clinker by assuming that there is a certain IR and that it contains certain proportions of silica, lime, iron oxide or alumina. In the UK the composition of the residue varies, but is typically 70% silica, 20% lime and 10% iron oxide, I believe. Tests at some sites suggest that these proportions (whatever they are in your case) may change when the IR goes above a certain level – say 1%. It all depends on the origins of the uncombined material, and this could alter as material comes from different parts of a quarry or different items of grinding plant malfunction slightly in different ways. Microscopic examination of clinker samples might help to suggest what materials are most important to you.
According to Indian sources for example, although they’re national standards allow up to max. 4% IR, in practice the vast majority of cements come in below 2%, which is the level required by the Indian railways. USA standards are almost always out of line with other continents due to some historical domestic mishap or piece of ill-applied semi-academic research. The UK standards have applied a limit if 1.5% for decades.
Any study of IR effects on performance should be read in relation to the chemistry and the origin of the IR material. One recent paper was Cement & Concrete Research, Vol. 30, pp. 1209 – 1214, August 2000 – K. Kiattikomol, C. Jaturapitakkul, J. Tnagpagasit: “Effect of insoluble residue on properties of Portland cement”.
Question 295:
I am working in a cement plant. We want to use waste materials like waste tyres, waste plastic, etc. as the secondary fuels. So I presume it is required to install by pass system of chlorides. Which aspect do we study? And what we do examine in connection with chloride by pass system? Our plant has 81ppm chlorine in the raw materials, 35ppm chlorine in the clinker, 420ppm chlorine in the top cyclone, and 1350ppm chlorine in the bottom cyclone. Please inform me the materials and data about the chlorides by pass system.
Answer:
It is not always necessary to install a bypass when burning alternative fuels. Many kilns are burning waste tyres without a bypass. It might be necessary with some plastic wastes, which can contain chlorides. You need to determine the chloride contents and variability of the fuels you intend to burn. From the data you have you can construct a model of the current chloride cycles in the kiln. You must then use this model to predict the increased chloride content with the alternative fuels. In order to avoid preheater blockage problems the chloride content in the bottom cyclone needs to remain below 2000 ppm. If it will rise above this level then a bypass may be the answer. The model can be used to estimate the size of bypass, which will be required. From the top stage cyclone chloride content you will also have an external chloride cycle in the flue gas dust. Breaking this cycle by elimination of some of the dust from the kiln process may be sufficient to avoid the need to install a bypass. Your model needs to also incorporate this external cycle in order to explore this possibility.
Question 294:
We have ESP for raw meal dust collection after VRM, which takes preheater gases as per normal circuit. What is the safe limit of tripping of ESP with respect to CO % in preheater gases. We have tripping of ESP at 0.6% CO at ESP inlet . Is it safe ? Should we reduce the setting ? Kindly inform what is normal practice elsewhere.
Answer:
In my opinion 0.6 or 0.8% CO in the gases entering the ESP is normal for the trip level for the high tension on the ESP.
Question 293:
We have a cement kiln made by IHI: diameter=4.2 m, length=64 m, production rate=2300t/d. Recently we have a snowman into grate cooler inlet. Please advise on the cause of build up of it and methods of solving this problem.
Answer:
Snowmen are caused by the clinker retaining some melt content after entering the cooler. This causes it to be slightly sticky and to build up into the snowman. The situation is much more common with modern coolers with stationery first grates and higher temperatures of secondary air entering the kiln. With the fixed first grate clinker is supposed to build up on the grate and therefore snowman formation is much more likely. The higher secondary air temperature means the clinker is hotter falling from the kiln and therefore more likely to retain some phases in the molten state. The most common solution is to install big blasters. These might not be effective as they are uni-directional in their effect leading to the need to install more and more blasters. An alternative might be an acoustic cleaner. These will prevent build-up in the whole chamber within which they are sounded. If you decide on this route you must remember that they stop build-ups occurring rather than removing them after they have formed. The horn must be sounded continuously at intervals to prevent the build-up. The final method is to install a “pusher” as developed by Denka Cement Company in Japan. I can give you their contact details of this is interesting for you.
Question 292:
I am not sure how to calculate the reactive silicon and CaO contents in OPC from a given test report which states CaO content, Total Si02, IR (in KOH), > CO2 , SO3 etc. Please could you tell me a simple equation to calculate it as I am not a chemist.
Answer:
In OPC you want to know the amount of silica, which is bound with calcium oxide in the cement clinker minerals Ca3SiO5 and Ca2SiO4. These are the important hydraulic minerals conferring strength development properties to the cement when mixed in mortar or concrete. To calculate the amounts of these minerals from the oxide analysis you also need to know the Al2O3, Fe2O3, SO3 and Free CaO contents. Ca3SiO5 is then given by: 4.07*(CaO – Free CaO) – 2.85*SO3 – 7.6*SiO2 – 6.718*Al2O3 – 1.43*Fe2O3 Ca2SiO4 is given by: 2.867*SiO2 – 0.754*Ca3SiO5 Ca3SiO5 is the greater contributor to early strength development. Ca2SiO4 is the major contributor to strength at 28 days and beyond.

Question 291:
We have air separator ” D ” type designed by fuller for 65 tons ball mill for O.P. clinker grinding. Return material contains substantial amount of fines causing recycling of ground material and output of mill is reduced. Could you suggest some solutions for stopping the fines in return material.
Answer:
I am not completely familiar with “D” type separators from Fuller however the problem you describe is a common one. Modern high efficiency separators are designed to minimise the amount of fines returning to the mill in the rejects. There is no easy answer. You need to conduct some trials with different speeds and drafting of the separator. Take samples of the feed to the separator, rejects and fines and measure the residues. From this you should be able to find the optimum speeds and drafting rates for different feed rates to the separator.
Question 290:
I want to know that proper definition of static pressure.
Answer:
The static pressure is the pressure with no velocity component. In the kiln hood there will be a slightly negative static pressure. The secondary air will be drawn into the kiln by that negative static pressure. The secondary air has a velocity component. With a pitot tube you can measure the static pressure and the combination of the static pressure and the velocity component. With both measurements you are then able to calculate the velocity at which the secondary air enters the kiln.
Question 289:
The fineness of finished product in our cement mills is controlled through using PSD analysis. The aim in general is to look at the 3-30um %. If the <3um is increasing, but the 3-30 um stays within the set target, how can we change milling conditions to reduce the %<3um ?
Answer:
It is not easy to control these very fine fractions of the mill product. The most effective way will be to adjust the drafting of the mill, which will affect the rate at which fines are drawn out of the mill in the ventilation air.

Question 288:
We are producers of needle felts & filter bags in India. We have come across many cement plants wanting to know if a change from coal to petcoke will have any bearing on the type of filter bags they should use. From our own lab analysis we have come across no need to change over, as long as the petcoke is dried by exhaust gases through the coal mill, which should dry out the somewhat oily residue from pet coke. Can you advise us please?
Answer:
I am not aware of any need to change the filter bags when switching from coal to petcoke grinding.
Question 287:
Clinker quality is said by Mr. ONO to be controlled by:
1. Heating Rate (Alite Size) 2. Maximum burning temperature (Alite Birefringence) 3. Burning Time(Belite size) 4. Cooling Rate(Belite colour)
How do we control these four factors once we find from microscopic analysis that some factor is not good? For example, if our microscopic analysis reveals that a clinker has Belite size of only 10 micron and we intend to increase it to 30 micron what shall we do? In other words what we have to do to increase the heating rate, maximum burning time, burning time, and cooling rate to increase clinker quality? Can you also elaborate on factors such as increase kiln speed to feed ratio to increase heating and cooling rate and adjust burner in such a manner that air axial momentum reaches to a certain value.
Answer:
I confess that this is an area where I have particular interests and provide systems to control clinker mineralogy based on the techniques of Dr Chromy. What you need is a kiln control expert system that will adjust the burning process to achieve your desired mineralogy. This system should be able to adjust the heating rate (via kiln rotational speed), maximum burning temperature (quantity of feed and fuel), and time at maximum burning temperature (via burner adjustment) and cooling rate. With regard to your second point. If you increase the rotational speed of the kiln you will bring the material through to the burning zone more quickly. This will increase the rate of heating and lead to the development of smaller crystals. Adjusting the burner will change the heat flux profile in the kiln and therefore the time the clinker spends at the maximum burning temperature. The relative amounts of fuel and feed will adjust the maximum burning temperature. If you are going to do this then the adjustments must be made consistently and taking into consideration all the other process variables of the kiln. This is why you need an expert system to do the job.
Question 286:
I’m trying to substitute partially the main fuel that is Bunker C with petcoke. What are the problems if I have bad fineness in the petcoke?
Answer:
The problem with petcoke is the low level of volatiles. With a coal particle the particle “explodes” when it enter the kiln atmosphere due to rapid evaporation of the volatiles in the coal. This causes very rapid combustion. With the petcoke there are no volatiles so the particle can fall into the bed of material before fully combusting. This can cause locally reducing conditions and badly affect the clinker quality. This is why petcoke is usually ground to a high fineness.
Question 285:
We used a lot of small compressors to supply all the general plant air, but replaced them with a few of bigger compressors a few years ago for better ease of maintenance . According to the article titled as “Energy Saving in Wet Process” issued in ICR March 1999, operating smaller compressors is better for annual energy saving than operating bigger ones. If it’s true, I think we made a wrong policy for energy saving in spite of better easiness of maintenance. What’s the reason of increasing energy consumption when operating bigger compressors for the same production of air?
Answer:
It is not certain that you have made the wrong decision regarding compressors. Small compressors run for longer and are better suited to small applications. While a large compressor might be frequently stopping and starting. There is a trade-off between maintenance costs and energy costs. The best solution depends on the detail of your factory and the use of compressed air on your factory.
Question 284:
In order to optimise the performance of a ball mill with drying chamber, is there any suggested air velocity through the mill that we should use?

Answer: I really need some more information. I presume the mill is for raw material grinding as it has a drying chamber? Is the mill air-swept or does it have an elevator to lift the material from the end of the mill to the classifier? What is the moisture content of the raw material? As a rule of thumb for an end-discharge mill with elevator then the gas velocity through the mill should be less than 2 m/s. For an air-swept mill the velocity through the mill should be less than 6m/s.

Question 283:
Is it required to install a bypass for a new plant that has 0.04% chlorides and 0.8% So3, Also it may be helpful information to know that about 50% of the sulphur input is in the form of pyrite. If you recommend not installing the bypass, How far the process will be affected?
Answer:
I presume you mean 0.04% chloride in the raw materials. In that case you will need a bypass. Chlorides will concentrate to around 100 times the level in the inputs, which would mean that you would have more than 4% chloride in the hot meal entering the kiln. At this level you would have problems with coatings and blockages in the preheater. You do not need a bypass for 0.8% sulphate in the raw materials. If 50% of that sulphate is present as pyrites then a large proportion of that will be lost to the system as SO2 from the top stages of the preheater.

Question 282:
Have you seen or heard of any negative effects on clinker and/or cement properties when utilising a raw mix that has a high percentage of metallic in it (magnetic material)?

Answer:
No, I have not heard of any effects of magnetic material in the raw mix. All the magnetic material should be oxidised in the kiln and combined in the clinker minerals therefore I don’t think it should have any effects. Perhaps the magnetic particles would agglomerate into clusters making combination into the clinker minerals more difficult and causing some heterogeneity in the clinker.

Question 281:
I am working in a cement plant. We want to use the slag of a steel plant. The analysis of this slag is the following: Feo 22.77 per cent; Cao 36.77 per cent; Sio2 14.57 per cent; Mno 26.3 per cent; Mgo 14.13 per cent. How much per cent of this slag we can add to clinker?

Answer:
I don’t think that you should add any of this slag to the clinker. You can add it the raw mix or directly to the kiln inlet using the patented TXI Cemstar process. The amount you can add depends on the clinker mineralogy you are aiming to achieve. Do you want to maintain the same Bogue composition? If so then you need to proportion the kiln feed using this slag as an additional raw material. However, there are many complications to consider. The Bogue mineralogy bears no relation to the actual mineralogy and the MnO content is going to be much higher than your current clinker. This will affect the colour and the phase composition. You must do extensive trials monitoring the effects of the slag on the kiln operation and clinker mineralogy. The Tech Forum article in the June issue of International Cement Review is about exactly this topic.

Question 280:
What is the optimum height above the roof at which the material from the previous stage cyclone can enter the raiser duct ? I mean if it is less, it looks as if there is a possibility of material dropping into the cyclone and if it is more we lose a part of the raiser duct for heat transfer?

Answer:
There isn’t an optimum height above the cyclone roof. This would depend on the duct dimensions, gas flow and velocity through the ducts plus the kiln feed rates. I agree that too low increases the danger of drop-through and short-circuiting and that too high means less heat exchange. These are complex preheater design considerations. I would consult the manufacturers of preheaters such as KHD or PMT Zyclontechnik.

Question 279:
It has been suggested to install an automatic fine coal sampler before the coal is being fired into the kiln. The reasons of installing one are: 1. We have a poor coal blending. We are mixing four types of coal with significant Cv, ash and sulphur variations. To determine the blending performance and blended coal quality, a raw coal sampler collects the blended coal. But the collected sample is not representative due to the number of sampling increment. If, coal is properly blended and sampling procedure is already corrected, we still don’t know the coal quality being fired into the kiln.

2. A suggestion was made to install an automatic sampling device before the fine coal being fired to the kiln for quality monitoring because heating value varies due to introduction of clinker dust into the coal mill. Variations occurs when kiln is low and thus producing a dusty environment. Generally, we don’t have any data collected for the fired fine coal. My question is, what are the benefits of having an automatic composite sampler aside from quality monitoring? What can it do in terms of kiln process control?

Answer:
I can see the point of the fine coal sampler to investigate and quantify the variations being introduced into the kiln process by the coal. This will then provide the information necessary to justify sorting out the coal blending and the de-dusting of the air used for drying the coal in the mill. However, to use it to enhance kiln process control you would need some method of rapidly analysing for CV, ash, sulphur and clinker dust contamination. Unfortunately I don’t know an on-line device to do that. XRF could be calibrated to give you the ash, sulphur and clinker dust with reasonable accuracy. You could then probably infer the variation in CV from the ash and clinker dust contamination. The usefulness for kiln process control would hinge on how quickly you can perform the XRF analysis and get the results back to the kiln burner. Unfortunately the response of the burning zone temperature is likely to be faster than you can get the XRF result back. It sounds an interesting project and tailor made for some sort of low cost expert system.

Question 278:
I asked you how we could get wider particle size distribution of cement through tube mills a couple of months ago. Can you be more specific again about operating methods of tube mills and roller mills to get higher residue value under the same Blaine value?

Answer:
In a tube mill you need to operate with a lower recalculating load to achieve a higher residue at the same Blaine. Slow the separator down to generate less rejects and control the Blaine via the amount of fresh feed to the mill. It is more difficult with a roller mill. You need to reduce the grinding pressure, the airflow through the mill and the speed of the separator.

Question 277:
We are running a 1Mta gas-fired plant. Whenever we measure the main stack dust emission it is always below 40 mg/Nm3 but we always see some dust coming out. Some say it is “blue haze”. What is this?
Answer:
Blue haze is associated with traces of ammonia in the raw materials or fuel. This combines with chloride in the kiln and passes out of the chimney as an ammonium chloride gas. When it hits the ambient air it condenses causing the blue haze plume. Is this plume you see detached from the chimney? If so then that would be further evidence that this is the cause.
Question 276:
We have 136t/h cement mill. We have started water spray in cement mill 1st chamber; we do not have any problem with mill exhaust filter bags (polyester) until now. Is it true that polyester bags react with water and the strength of cloth is decreased, do we have to change the bags, which would be very costly?

Answer:
Changing the bags would indeed be a costly affair. You should have no problems provided that the gas temperature remains below the dew point throughout the system.

Question 275:
I am interested in reclaiming a 60Mt slag pile that originated in early 1900’s. Pile consists of both open hearth and blast furnace slag and it is located in Northeast USA. Preliminary observations/tests show granulated in nature with metallic content in excess of 30% for 3/8- sizes. Can this material be processed and re-introduced within the cement manufacturing process? If so, what parameters should I be testing for to determine feasibility and ultimate marketing strategy?

Answer:
You need to test the hydraulic index of the slag and also conduct an extensive sampling exercise to determine the variability. If it is indeed granulated then it is potentially valuable as a cement extender. If air-cooled then it will still be useful as an aggregate or possible a raw material for cement manufacture using the Cemstar process, although the prices would not be so good.

Question 274:
If the material that remains in a quarry consists of mainly higher silica limestone can it still be used by the use of PFA, and if so what are the market implications for the resultant cement. Do you know if anyone has been successful with levels as high as 15 per cent PFA?

Answer:
It might well be possible to use the higher silica limestone remaining in your quarry. We are aware of factories using limestone that has typical silica content of 10 per cent. Fly ash may be the secondary material to use with this. You need to run some raw mix proportioning algorithms. There should be no adverse aspects for the resultant cement provided that the raw mix chemistry targets can be achieved with the raw materials. The proportioning algorithms will tell you this. At 15 per cent you are approaching (or are at) the maximum amount of silica in raw mix used to produce modern cements. In that case the material might need sweetening with high-grade limestone, alumina or iron additive.

Question 273:
Recently, we are having inquiry from an admixture company regarding incompatibility of cement with synthetic gypsum used as retarder. He is asking is it all right to use synthetic gypsum in the manufacture of cement?

Answer:
Many cement companies around the world are using synthetic gypsum in the manufacture of cement. I have not heard of any incompatibility with concrete admixtures and do not know of any literature discussing the subject. The key question will be the purity of the synthetic gypsum you are using. The gypsum itself will have no adverse reaction with admixtures, however if the gypsum is contaminated then it is possible.

Question 272:
How do we calculate the amount of moisture in limestone? Let’s say we received 50,000t of limestone. Moisture is five per cent. We calculate as dry is 47,500t and thus moisture is content 2500t. But supplier does not agree with us. Could you clarify?

Answer:
Your calculation of percentage moisture is correct. However, whether your supplier agrees with you depends on the terms your contract with them. Are they contracted to supply on a dry-basis corrected for any moisture in the as received material? The important thing is to get the contract and specification right in the first place.
Question 271:
I am interested in other methods to determine the pozzolanic index of pozzolans, I usually employ the compressive strength at 28 days test, but it’s too slow for me. Could you tell me other ways to measure pozzolanic activity?

Answer:
I suggest you try accelerated strength temperature where you cure the cubes in a heated water bath. This will give you the compressive strength result more quickly.

Question 270:
Are there any know correlations that affect vicat set time? C3A, kiln feed fineness, free lime, etc. Sometimes the vicat is erratic from day to day – yet no noticeable other changes are observed.

Answer:
You would expect a correlation with the C3A content. I have also seen a clear inverse relationship with free CaO. The most striking correlation is with traces of fluoride where the relationship is direct. A change of 0.1% in fluoride being enough to increase the setting time by 60 minutes. You don’t need much variation in fluoride to have the erratic setting times you describe.
Question 269:
We want to add the coal fly ash into the cement. We learn that it will be better add it direct into the Separator instead of the ball mill. We want to know how to add it into the O-Sepa N-500 model.

Answer:
I would add the fly ash to the air slide taking the material from the end of the mill and feeding it to the O-Sepa separator.

Question 268:
We have a 140t/h cement mill and for controlling the outlet temperature we have decided to spray water in mill. We tried to make water spray nozzles in workshop but all failed. The water pressure is 5-6 bars. Can anybody help me by sending comments and nozzle designs.

Answer:
Water injection is a good way to control the outlet temperature of the cement mill but adequate atomisation of the water is essential. I think you will have to purchase nozzles to give adequate atomisation and cooling in your cement mill via water injection.

Question 267:
Does synthetic gypsum require the same mill temperatures to create adequate soluble sulphate for setting control. Might there be a situation that if we were adding synthetic gypsum instead of natural gypsum, that we could afford to mill at lower temperatures, and therefore alleviate slightly the possible problems of high final cement temperatures?
Answer:
There is no reason that I am aware of why synthetic gypsum should behave differently to natural gypsum. I don’t think this will allow you to grind at lower temperatures. However, I don’t think this should be an issue on your factory. With a new mill, adequate cooling and temperature control should easily be achieved through ventilation and water injection.

Question 266:
We operate a 150tph cement mill, we have about 5.0 per cent residue on 45-micron sieve in product. Is it possible to increase residue to 10 per cent by adjusting the O-Sepa unit?

Answer:
Yes, you can reduce the speed of the O-Sepa separator to coarsen the product from the mill. You should also be able to adjust the drafting around the separator. To achieve the level of coarsening you are wanting you will also need to increase the feed rate to the mill and possibly adjust the ball charge grading.

Question 265:
What is the preferred granulametric profile of clinker to have a best cooling with a grate cooler having two grate drives? Please also let me know about various factors effecting the granulametric profile of clinker such as chemistry of clinker, burning conditions and length of cooling zone in kiln.

Answer:
I do not know that there is a figure I can give you for the preferred clinker granulometry, however you need to avoid too large lumps or excessive dust. You need an even grading of small nodules. This is a factor of the “clinkering range” of the raw mix and is associated with the flux content at the burning temperature. You want a short clinkering range, which means that when liquid is first formed it rapidly builds up to its maximum value causing the mix to ball up into the clinker nodules. This is primarily a function of the iron and alumina content of the raw mix. The optimum is that which gives you the even grading of clinker nodules. Too wide a clinkering range can give problems of ring formation in the kiln. After the burning zone you want as short a cooling zone in the kiln as possible.
Question 264:
I would like to know if we can gain energy savings in using thermal insulate in the precalciner. Is it possible to use special additives in cement and raw milling machine to save energy? If yes , what kind of additives can be used and what the amount of energy we can save ? Finally, what is the maximum temperature of clinker we can take, in order to reduce the airflow of clinker cooler to save energy and maintain the quality product and safety conditions?

Answer:
The refractory linings used in the precalciner of a cement kiln are usually an insulating grade. Greater thickness of refractory lining increases the insulation and saves energy, however the cross-section of the precalciner is reduced and this can restrict the gas flow. Grinding aids are regularly used in ball mills for raw mix and cement grinding. Ethylene glycol has been used, also amine derivatives such as triethanolamine. Unit energy savings of up to 25% have been reported. I am not sure that reducing airflow in the cooler at the expense of higher clinker exit temperature will save energy. It is critical to recover the heat energy from the clinker to preheat the combustion air for the kiln. Higher clinker exit temperature means a higher energy loss to the system.

Question 263:
What is the effect of roller wear on vertical roller mill output rate and power consumption. How do we calculate /quantify this. We have started using the mill scale waste in place of iron ore as an additive. This material is having 3-5mm pieces of mill scale. Will this increase the wear in the vertical roller mill body. Secondly, what will be the possible impact on the refractory in the kiln.
Answer:
Eventually wear of the rollers will lead to reduced output and increased unit power consumption. The only way I know to monitor this is via periodic inspection. It is possible that mill scale will increase wear rate due to the presence of tramp metal. In the kiln there should be no effect on the refractory lining provided that no additional trace materials are involved.

Question 262:
As I understand, the alite size decreases as the kiln speed increases. So we’ve increased the kiln speed and now it’s about 240-250 rph. But what I want to know of the maximum kiln speed we can increase. Can you advise me how to decide the optimum speed and the maximum speed based on the crystal size of silicates.

Answer:
Many people are increasing kiln speed and achieving increases in output. The only way to find the limits and the optimum is to conduct controlled experiments.

Question 261:
We intend to use furnace slag to replace sandstone for raw material preparation, and slag ratio is about 23% to produce clinker of KH=0.90, SM=2.6, IM=1.7. How about the heat consumption and kiln capacity compared with sandstone proportion for a 2000t/d NSF system?

Answer:
I have recently been in discussions in connection with the patented TXI Cemstar addition of air-cooled slag to cement kilns. 23% slag addition sounds high……You should be looking for over 20% increase in output and at least a 7% reduction in unit fuel consumption.

Question 260:
I am working in a cement plant with a kiln size 4.75m * 75m, running at present at 6750tpd. The shell temperature from 25 to 30m length is running high in the range of 425 to 455 degree C for last 6-7 weeks. We are waiting for the red spot to come but because of the market pressure we have kept the kiln running. What could be the possible damage if we continue to run like this. What shall we do to reduce the temperature. The normal temperature is 350 to 370 deg C in the above zone. The tyre is situated at 35m. Coating is very unstable after 25mn. What should be the minimum velocity in the tertiary air duct for SLC system for designing the duct diameter.

Answer:
The danger is that the kiln shell will soften and possibly blister or become deformed. At worst you may need to replace a section, which will cost some months loss of output. That will be disastrous under your current market pressure. To reduce the temperature you might apply shell-cooling fans. Possibly an adjustment of the burner might help, however this would have to be done very cautiously.

Question 259:
I’ve heard that the reducing condition in the calciner and kiln feed end area affects the size of belite crystal and the growth of C2S crystals is hindered by the reducing condition in such areas. Would you explain about the affecting mechanism of reducing condition on formation of smaller C2S? And what’s the effect of such smaller C2S on the burning process and clinker quality ?

Answer:
I have not heard of reducing conditions in the precalciner affecting belite crystallisation. Belite formation should not occur in the precalciner as the temperature is not high enough. Reducing conditions in the precalciner are used for NOx reduction. Reducing conditions in the burning zone can lead to solid solution of Fe2+ in the belite and inversion to the gamma form with dusting of the clinker and loss of hydraulic activity.
Question 258:
What is the optimum cement-to-air ratio and conveying air velocity in the pipeline when conveying cement from silo to ship by F K Pump? The conveying distance is about 1500m and includes three ninety-degree bends.
Answer:
There are several more factors involved than just overall convey distance and number of bends. Capacity and pipeline diameter for example are extremely important. The exact pipeline configuration is required to make a proper calculation including horizontal and vertical lengths, bends, appendages, etc,etc. Please note that an optimum cement to air ratio is extremely difficult to obtain as in reality you have to make a selection out of standard sizes of FK pumps, compressors, pipeline diameters, etc. Pneumatic calculations are fairly complex and the best (and easiest) thing to do in your situation is to contact Fuller Bulk Handling in the USA, provide them with your project information and they will advise the correct equipment and pipeline configuration. Please note that a conveying distance of 1500 metres is about the maximum that an FK pump can do. The energy consumption will be very high on this distance.

Question 257:
We are planning to go for coal fuel in our cement plant. At present we are using gas and oil. What could be the best combination fuel percentage (coal & oil, coal & gas, coal & gas & oil), which would give the best results. What could be the maximum coal dust residue we can go without affecting our process. If we use more residue coal dust is it will effect in our process. Kindly suggest us is there any reasons will effect in our process.

Answer:
I would retain the capability to burn all three fuels, coal, oil and gas. The best is to burn a single fuel at any one time to overcome any difference in the volatility and burnout profile of the fuels. The best results will be given by the cheapest fuel at the particular time, however in general the output of the kilns should be higher on coal or oil rather than gas. There is no absolute answer to the fineness of coal you should target. This depends on the volatility and the ash content as well. The potential impact on the process is that the coal does not completely burn in the flame and falls into the bed of clinker and continues to burn there. This can cause local reducing conditions and adversely affect the clinker quality.
Question 256:
How can we avoid blocking problems in inlet chute vertical roller mill. Our mill is Fuller Loesche Mill, LM 59.42, air lock system using triple gate. The Inlet chute and triple gate is equipped with hot gas box. Any advice?

Answer:
I think your raw mill inlet blocking problem must be related to the material rather than the raw mill inlet itself. The solution will be found by changing the characteristics of the material feed to the mill. I suspect it is probably too wet. Somehow you need to pre-mix the materials so that the sticky material (clay?) is mixed with limestone or other harder material.

Question 255:
Our plant is facing snowmen problems in the CIS cooler (fixed inlet system). The problem happened when we modified the cooler from grate cooler to CIS cooler. Our lab crew has changed the kiln feed specially the SM from 2.2 to 2.3 – 2.4 and also the liquid phase from 27 to 24 – 25, even change the burner position 37 cm into the kiln. But we still have snowmen on the seventh rows of the CIS cooler. Any solution?
Answer:
This is quite a common problem. The first option is to install big blasters but you have probably already done that. You could try installing extra ones but it will probably not solve the problem. The second option is to fit acoustic cleaners and continually sound these to prevent the build-up of the snowman. If you do that remember that the cleaner will stop the snowman growing rather than removing one that is already there. You must sound the cleaner continuously to prevent formation of the snowman. The final option would be to install a hydraulic pusher as developed by Denka Cement in Japan.

Question 254 :
How we can cool the discharge of cement from open circuit mill.

Answer:
In open circuit mills the cooling of the cement is usually via the ventilation of the mill. Sometimes water is also sprayed onto the mill shell. High temperatures can persist and it is possible to pass the cement through a cement cooler. However, this would be a large investment and the money might be better spent introducing a closed circuit with cooling in the separator.

Question 253:
What are the factors that should be considered while determining the silo storage capacity requirement for a 3.8Mta capacity plant, what should be the optimum silo storage requirement?

Answer:
The finished product storage at a cement plant is a buffer between the (steady) output of the kiln and the (sometimes irregular) cement deliveries from the plant. Please note that it is possible to use the clinker storage as such a buffer as well. Clinker storage space in general is cheaper than cement storage. The grinding capacity is then adapted to the daily deliveries of the plant and the finished cement silos can be relatively small.

The determination of the size of the clinker and finished cement storage is depending on the following factors.
• Output of kiln(s)
• Maximum downtime of kiln(s) (annual maintenance /breakdown) Required storage time of finished product (sometimes required for testing)
• Ratio between bulk and bag deliveries (bagged cement can provide additional storage of finished cement)
• Fluctuations in daily deliveries (effect of Saturdays, Sundays, etc.)
• Seasonal influences effect of peak deliveries (loading a large ship for export, etc)
•Ratio of grinding capacity to kiln capacity

Based on above factors a logistical calculation can be made that provides the required storage size(s) and the basics for inventory management for clinker, bulk cement and bagged cement storage.

Response There is no hard and fast rules for silo storage capacity. There has been a recent trend towards minimising silo capacity to reduce capital cost of new plants and to encourage efficient operating practices by reducing buffer stocks. A 3.8Mta plant is going to consume 15,000tpd of raw mix therefore I would expect a minimum of 30,000 tonnes of raw mix silo capacity to allow for raw mill maintenance and repairs. Clinker and cement storage capacity are dependent on the seasonality of the market to some extent. I would expect clinker storage of at least one months production therefore that indicates 300,000 tonnes. Cement storage should cover at least 1 weeks despatches so 75,000t minimum.

Question 252:
Regarding the uses of slag cements, what are the associated risks related to heavy elements or oxides within the cement matrix and in the final product? Do you know specifically of any disadvantage of using slag in cement or concrete?

Answer:
Concretes produced with blast furnace slag are denser than those produced with ordinary cement and therefore any heavy metals will be more tightly bound into the concrete matrix. With newly placed concrete there may be some leaching from concrete but there is no reason why the heavy metal leaching should be higher with slag cements than with ordinary cements.

I have found a paper available from the HMSO in London: “International review of the composition of cement pastes, mortars, concretes and aggregates likely to be used in water retaining structures”, by C.D. Lawrence of the University of Sheffield Centre for Cement and Concrete. On page 126 “The solubility of Te was reduced …with slag cement and the effective diffusivity for blastfurnace slag grouts was 5 orders of magnitude lower than for Type 1 Portland cement grouts…”

In principle I think that concretes made with slag cements are denser than those made with Ordinary Portland cement and therefore the likelihood of leaching of heavy metals will be lower. However, this always depends on the heavy metal content of the cement and the particular slag used.

The only disadvantages I know of slag cements are the lower early strength development of concretes made with these cements. This makes them unattractive for the pre-cast concrete products industry due to the need for the pre-cast pieces to be retained in their moulds for longer than would be the case with pure Portland cement.

Question 251:
What do we do for our magnesite-chrome refractory brick’s waste for protection of environment? Can we use these waste materials in raw material?
Answer:
The European cement industry no longer uses magnesite-chrome refractories due to concerns regarding the disposal of hexavalent chrome in the environment. You should not use the waste materials in the raw materials for cement production. They should be sent back to the refractory supplier for processing into further refractory materials.
The European cement industry no longer uses magnesite-chrome refractories due to concerns regarding the disposal of hexavalent chrome in the environment. You should not use the waste materials in the raw materials for cement production. They should be sent back to the refractory supplier for processing into further refractory materials.” I know that magnesia spinel refractories are more expensive, however I think you should pay the additional cost and raise your cement prices to reflect the contribution you are making to the environment in your country.
Question 250:
We are facing some problems to operate the quarry in the good way and we spend a lot of money for the quarry equipment like dump trucks, bulldozer, wheel-loaders, excavators. Please advice us the best method for quarries management and the best way to maintain the quarry equipment.
Answer:
I can imagine that managing your large fleet of quarry machines is a challenge. You need a combination of mine planning, chemistry control, truck despatching and machine conditioning monitoring systems to manage the operation in the optimum way. We shall write to you separately in more details of the way to approach this problem.
Question 249:
Please let me know about ceramic anchors when using monolithic refractory lining instead of metal anchors.
Answer:
Ceramic anchors can be an advantage if you have corrosion of the metallic anchors in your monolithic refractory. However, they are usually more costly and time consuming to install. If you have no problems with metallic anchors then that is probably the best solution.
Question 248:
We have some retractable bulk loading system of cement in order to deal with a closed container driven by trucks. The capacity of the containers is 30 tons and each loading system can give 300tph, which means, theoretically, that we could fill a container in six minutes. However, because the air mixed on the cement we have to fill a container on 2 or 3 steps in order to search the correct weight, since the truck has to drive around to compact the cement and extract the air making space to receive more cement. All together the operation is taking 20 minutes. Do you know any system to reduce the loading time?
Answer:
I would recommend the use of an acoustic horn in the loading head. This horn is then sounded as the truck is filled with cement and the sound waves cause the cement to be compacted in the tank. This should avoid the need to drive round to compact the cement. I suggest you contact sound@primasonics.com and I am sure they will tell you their local agent.

Question 247:
I want to know about the major parameters to convert a long dry kiln to a one or two stage preheater kiln. We have a long dry kiln with grate cooler with the following specifications: Diameter; 3.2m
Length: 110m
Specific heat consumption: 1280 kcal/kg
Grate cooler chambers: Two chambers
Cooler area: ch 1=4.8m2, ch 2=10.8m2
The capacity of kiln at present: 320t/d
Feed rate: 28t/h Kiln feed to clinker: 2.1
Brick thickness: 180mm
Specific power consumption: 35 kwh/ton

I would be so grateful if you kindly tell me how much can the output be increased by installing a one (or two) stage preheater without any change in kiln length and removing the chains out of the kiln. Your recommendation about the number of stages of preheater would be appreciated. In addition, I would like to know which of these alternatives are more economical: 1. The above suggestion or 2. Conversion of this kiln to a 4 stage preheater-calciner kiln.

Answer:
The major parameter, which I can foresee, is establishing that there is a market for the additional cement, which the modified kiln will produce. Based on the dimensions you have given I estimate that the clinker output could be raised to 570tpd by the installation of a 2-stage preheater. In order to increase the production beyond this level you must install a precalciner and shorten the kiln. This will allow the burning zone to be relieved of some of the thermal load by firing some of the fuel in the precalciner. By such a conversion you should be able to raise the output to around 1000tpd. However, again I must emphasise that much more detailed calculations and considerations are necessary to confirm these numbers.

Question 246:
What are the advantages / disadvantages of fitting a low Nox burner to a gas fired rotary kiln, Does this affect efficiency, throughput, refractory life?
Answer:
The advantages are the obvious one of lower NOx emissions. On cement kiln there is also the advantage that the low NOx is achieved by a lower proportion of primary air. This means more secondary air to compensate and therefore more heat recovery from the clinker cooler and better thermal efficiency of the process. I am not so familiar with rotary limekilns – do you recover heat from the product to preheat the combustion air? If so the same would apply. Against this potential advantage is the disadvantage that the combustion of the fuel can be degraded by the lower proportion of primary air. This can erode the thermal efficiency benefits and adversely impact on throughput. These potential problems can be overcome by combustion audits and good design of the burner system.
Question 245:
What is your opinion on use petcoke in cement industry? Is it good for the product? What about emissions to atmosphere?
Answer:
Petcoke is widely used in the cement industry to fire the cement kilns. Some cement companies fire their kilns with 100% pet coke. There are no detrimental effects on the product. The high sulphur content can cause some process problems but these can normally
Question 244:
Please inform me of the formula for the calculation of the charge of ball and power in a raw mill (Tandem mill}.

Answer:
The power drawn by a ball mill is related to the media charge (t), mill internal diameter (m) and rotational speed of the mill (rpm), by the following formula: Power Drawn (kW) = 0.225 x Media Weight (t) x Diameter (m) x Rotational Speed (rpm)

Question 243:
what is the effect of temperature variation in formation of rings. Does recarbonation play a major role in ring build-ups?

Answer:
For sure temperature variation has a role to play in the formation of rings. At particular points in the kiln melting and condensing of materials is taking place and then latent heat effects affect the temperature profile. The melting or condensing of materials is intimately connected with the formation of rings. In a modern kiln with preheater I would not expect recarbonation to take place in the rotary section of the kiln and therefore I don’t think it plays a role in ring formation.
Question 242:
We currently run at around 1000 ppm on NOx however we have been asked to look at reducing this amount, we do not recover heat to preheat the primary air as we have a short rotary section combined with a preheater which recovers the majority of the heat, I have concerns in regard to increasing secondary air through our cooler as this is of a vertical shaft type which if the air is forced through to quickly causes the entrainment of dust into the burner zone which has a detrimental effect on flame stability and heat transfer.

We also control our flame efficiency to some degree on NOx as we have and Expert system installed. My main concern however is that I have heard conflicting reports on the benefits of low NOx gyrotherm type burners in lime kilns, which I believe is in part due to the low heat transfer to the product (we currently have burning zone temp of about 1800 deg C.)
Answer:
My expertise is in cement kilns, however I think you are probably right. A low NOx burner is likely to result in a longer heat flux profile with lower peak temperature. From what I know of lime burning this is likely to reduce the reactivity of your product. I know the Linkman system you have installed well (was applications manager there for 6 years). Presumably you could lower the NOx target in the Linkman, but that would probably result in the same problem of lower peak temperature and lower reactivity of lime. Is there no opportunity for staged combustion the addition of some secondary fuel into the preheater? That would degrade the NOx.

Question 241:
How much synthetic gypsum products is used in the cement industry? Any problems in relation to setting time control? Are there any pitfalls in using a synthetic gypsum only? If using natural gypsum, is there an optimum ratio of anhydrite to dihydrite? For example, could all anhydrite be used? What is the optimum and minimum cement milling temperature? Is there any problem if storing cement at 110 – 120 deg? For example, could material flow or lump formation be more problematic?

Answer:
Synthetic gypsum is used quite extensively to replace natural gypsum as setting control agents. There are some countries (Denmark) where 100% replacement is the normal practice. This is particularly those countries where flue gas desulphurisation has been widely deployed creating vast amounts of synthetic gypsum. There are no pitfalls with regard to synthetic gypsum controlling the setting of cement. There can be some handling problems as it can have a high moisture content if derived from wet scrubbers. It is sometimes beneficial to mix with some natural gypsum to alleviate problems of the material hanging up in bunkers and feeders, etc.

With natural gypsum any anhydrite is essential an inert filler. Only the dihydrate dehydrates to become a source of soluble sulphate in the cement and control the setting. The reason anhydrite is popular is that international specifications allow its addition. This allows a greater level of total additions to cement and therefore the overall amount of production, sales and profits is swelled.

Temperature control in the cement mill is vital. You must dehydrate the dihydrate to produce soluble sulphate for setting control. In practice you need a temperature in excess of 100 degrees C to ensure that this reaction proceeds sufficiently during the time the cement is in the mill. On the other hand you must guard against too high temperature as this will reduce mill efficiency and can lead to conversion of the gypsum into the insoluble natural anhydrite.

Storing cement at 110-120 degrees C is very likely to cause lump formation and flow problems in the cement. The problem is that gypsum dehydration will continue in the silo at these temperatures. When the dehydration takes place in the mill the liberated water is drawn out of the mill in the mill exhaust gases. There is no such venting in the bulk cement standing in the silo. The liberated water will migrate to the cooler sides of the silo where it will condense and cause the cement to hydrate and build up into massive coatings stuck to the silo sides (I have seen these 2m thick). The capacity of the silo is significantly reduced and the lumps periodically fall off blocking the extraction equipment. To avoid this you need to cool the cement below the temperature of gypsum dehydration before blowing into the silo. This can be done with a cement cooler, or often by cooling in the closed circuit of the cement mill.
Question 240:
Please kindly provide me the detailed information on suppliers for grab unloading equipment for clinker and slag.

Answer:
In respect to grabs the following suppliers could be of use www.landers.de www.mrs-greifer.de www.nemag.com www.orts-greifer.de www.kroeger-greifertechnik.de www.peiner.de it is important to understand however that when discharging dusty materials it is not only the grab that matters but also a combination of grab design, hopper design and operating procedures. For dusty materials grabs should be fully enclosed, have special lip seals and should be able to be opened slowly in a controlled matter. Hoppers should be designed for that specific grab and should have a dust containment and dust collection system. Operating procedures should specify speeds, conditions and specific actions to prevent large dust emissions.

Question 239:
In one cement plant, we use a vertical roller mill (MPS type) for coal grinding. For normal coal, it runs very well. Recently we’d like to change normal coal to anthracite, but the mill rollers did not run while the table was rotating, so the grinding bed could not be stable. We tried adding some water in the raw coal and low the grinding press to minimizing its fluidising, but failed, no product could be got, all anthracite piling up on the table. When we changed back to normal coal, everything is ok. The table is 2.2m, table speed is 24.2r/min. Any advice please?

Answer:
I have heard of a number of problems when converting from coal grinding to anthracite. However, I have not heard of it being impossible to grind. You must expect significant loss of output due to the hardness of the anthracite. A number of plants in Vietnam are successfully grinding and firing their kilns with anthracite. I think you should consult the supplier of your MPS mill for advice.

Question 238:
We are facing frequent brick failure in takeoff duct of grate cooler. We have increased our production rate from 4500tpd to 5300tpd by increasing p/h capacity and recently we have increased the cooler hood area. We have replaced cooler first grate with static grate and increased the cooler fan flow. TAD temperature has increased from 800 to 1000 deg and velocity in take off has also increased in takeoff from 10.5m/sec to 15.02m/sec. Please advise and tell me the effect of velocity on brick failure.

Answer:
These problems seem to be very common with the conversion of grate coolers to static first grates. As the temperature of the secondary and tertiary air increase so does its volume and therefore the velocity to draw the same amount of stoichiometric combustion air into the kiln. The problem with refractory failures is caused by the higher temperatures rather than the velocity of the secondary and tertiary air. A higher refrcatoriness is required in the materials and the alumina content has to be increased. Some cement companies have even had to resort to lining their kiln hoods with basic bricks. SIFCA blocks have also worked well in some kilns.
Question 237:
One day our cooler grate dc drive was taking high current and very high voltage around 570v. At this time cooler grates were loaded high with clinker. Suddenly the cooler grate speed increased very high and by techo feed back and voltage feedback it was showing normal speed according to set point, but physically speed was very high, almost to the maximum speed. After some time, when load decreased to normal load, speed become normal automatically with out doing anything. How it is possible with increase in load of a dc drive physical speed automatically increase yet voltage and techo feedback is showing normal speed.
Answer:
Complex electrical questions of this type are beyond my capability to answer. You could ask Reinhold Errath at ABB who is the most knowledgeable person I know about drives in the cement industry. reinhold.errath@ch.abb.com

Question 236:
I would like to know how can we can do an economic cost analysis of two or three alternatives for the modification of an old cement production line.
Answer:
There are numerous references and articles regarding the conversion of long kilns to modern dry process kilns with preheaters and precalciners. However, every case is different. Reference to other projects is very useful but a full economic evaluation of the particular project must be carried out. I would recommend employing a consultant to help you with this.
Question 235:
We have a problem of raw meal composition change with vertical raw mill stopping. We don’t have “Dust bin” for EP dust. Normally we are blending the EP dust with raw meal. We are thinking this problem caused by storage silo segregation and EP dust mix. Please teach me most effective improvement method for this problem. Our plant: 4500t/d 4-stage NSP kiln, LM-41.40 Vertical raw mill, 4800t Air Blending silo, 2 storage silo 4000t.

Answer:
The problem is really segregation of the kiln feed in the top stage of the preheater. Most of the feed goes to the kiln but some (~5%) escapes as dust and is captured in the ESP. The finer particles tend to escape in the dust and these are usually higher in CaO. Blending the high CaO material with the raw mill product is fine until the raw mill has to stop for maintenance. The best solution is to add the ESP dust directly to the kiln feed rather than to the mill product. The effect on kiln feed chemistry is then constant and unaffected by the starting and stopping of the raw mill. The external dust cycle is constantly added back into the kilns feed whenever the kiln is running. When the kiln is stopped there is no ESP dust anyway.

Question 234:
We are spraying water on clinker bad in the grate cooler for reducing the temperature of clinker. Last month we have found a number of plate cracks near wall and big coating formation at cooler outlet. Please suggest the better method of water spray in the cooler?

Answer:
Spraying water into the cooler is not usually done to cool the clinker. To cool the clinker you need to increase the airflow through the bed or the residence time of the clinker in the cooler. Water sprays are used to condition the cooler exhaust gases prior to electrostatic precipitation. The water should completely evaporate before it reaches the bed of the clinker. From your description I suspect some water is reaching the bed. You need to improve or replace the atomisation nozzles on your water spraying equipment.

Question 233:
We have two DC Variable Speed Drives AEG Type (Maxisem), which drive the Preheater Fan and Kiln Main Drive, which constantly trip whenever we have power fluctuations. Attempts to contact AEG have not been successful. How can we alter the sensitivity of the drives to voltage fluctuations?

Answer:
I am not an expert in this area but it sounds as if you have harmonic feedback interactions between your variable speed drives and the electrical network on your factory. I am sure you need to contact specialist electrical engineering companies to solve your problems. It might be possible by the installation of some filters etc. ABB should be able to help you, however I suspect that any solution is likely to be expensive.

Question 232:
What is the process that is used to reduce the bypass in the rotary kiln with preheater. Is Ca (OH) 2 used for this purpose to react with SO3, Na2SO4… and what is the ratio of Ca (OH) 2 we must use in kiln feed.

Answer:
Ca (OH) 2 is added to reduce emissions of SO2 from cement kilns. The mechanism is to combine the SO2 with the Ca (OH) 2 in CaSO4. This can lead to increased recirculation of sulphates, ring formation and build-ups in the kiln. There are various techniques to reduce the proportion of bypass in kilns with preheaters. In order to find the way it is necessary to construct a model of the cycles within the kiln. This can then be used to test the impact of various changes in chemistry and to find the best way to reduce the bypass. I can help you with this if required.
Question 231:
We are having some corrosion problem in kiln’s shell at upper transition zone. Can you recommend any ceramic coating or refractory to prevent this failure?
Answer:
First you will have to clean the shell to remove the scaling by grinding or grit blasting. Then apply phosphoric acid, followed by plasma coating with aluminium oxide. There are several steps to prevent shell corrosion: > 1. Reduce volatiles (K2O, Na2O, SO3, Cl) in kiln feed if possible. 2. Use dense high conductivity refractories in affected area.
Question 230:
Can use river sand to manufacture cement. I will be turning river sand into condensed silica fume. Can I use dryers to remove moisture from river sand and use electric furnace to turn river sand into condensed silica fume.
Answer:
Silica fume is a very good pozzolan to make extended cement. This type of cement is definitely marketable and produces very high strength and durability. Silica fume is very fine and it is not necessary to grind with the cement. Normally it is blended. I do not know whether you can use river sand. The normal source of silica fume is the dust collected in the filters from a furnace producing silicon or ferro-silicon. I do not think that drying river sand and using an electric furnace to produce silica fume alone will be an economically attractive proposition. The energy costs will be too high and the silica fume will not command a sufficiently high price to recover the costs. It is only attractive as a by-product of the much more valuable silicon or ferro-silicon industry.
Question 229:
We had installed nose ring segment plates about 20 days ago on 1500tpd PC plant having a grate cooler. The metallurgy of segment plates is Ni-ch steel and fitted with single counter sink bolts on each plate. After operating for 20 days it was noticed that bricks from nose ring and adjacent four rings are missing. Hence plant was stopped. On inspection it was find that about 22 segment plates out of 28 had developed a hair crack on its seat resting on kiln shell. Each cracks is at the same place and starting from hole for bolting and is propagating towards kiln inlet. During this 20 days operation kiln was stopped several times for various reasons and duration of stoppages were from 4–12 hours. A red spot at one meter was also being observed for the last ten days. In fact few bricks were missing from there initially and later on 4–5 red spot further appeared in the same ring. Are such cracks due to operating of kiln with out brick lining at 1 meter (5 rings), which might have not recorded/noticed due to some reasons. Or it may be a metal casting failure or due to poor metallurgy of segment plates?

Answer:
It sounds as if you have suffered a very difficult period of operations on your factory. I don’t think it is possible to diagnose the causes of the cracks in the segment plates in these circumstances. The fact that the cracks are all at the same place might indicate a design or metallurgical problem, however the service conditions have been extreme. Your factory must focus on achieving more stable and sustained kiln operations if you are going to avoid such problems. Kiln operational reliability and avoidance of regular stops seem to be the biggest challenge facing your factory. Improving these aspects will take a determined and sustained effort by the factory management.
Question 228:
Will you please let me know about the percentage rise in Kcal/kg of clinker: (1) if we will operate our kiln with 0.3 per cent CO instead of 0.2 per cent CO? (2) If we will operate with 10°C higher temperature at before kiln ID fan?
Answer:
These answers can only be indications, as you need a heat balance model specific to your kiln to make a proper evaluation. You also need to specify where the increase in CO content will be measured – the kiln inlet or the preheater exit. With regard to the increase in temperature of the exhaust gases at the ID fan you might expect 10 degrees to result in an increase in fuel consumption of 7 kcal/kg clinker on a precalciner kiln

Question 227:
In your experience, what is the best technique for measuring the air temperature entering the kiln from the cooler? In plants I have seen, this is a very difficult measurement. Optical methods are confused by dusting and are very expensive. Direct measurements, such as thermocouples, can’t endure the high temperature, abrasive environment. Yet, this is a key variable for kiln and cooler stability. Have you had any experience with inferred measurements?

Answer:
You are right that secondary air temperature measurement is tricky. I have heard recently of people advocating acoustic pyrometers measuring the temperature across the cooler throat.
Question 226:
We have high P2O5 content, up to 1.7 per cent in the raw mix. Please suggest the impact of P2O5 on clinker.

Answer:
The P2O5 in the clinker is likely to reduce the C3S content of the clinker as P2O5 decomposes the C3S to produce C2S and free lime. This means that you must be careful of potential unsoundness in the cement due to elevated free lime content. Sound cements can be produced but the lime saturation must be lowered with the inevitable penalty of lower early strengths. The problems can be overcome to some extent by addition of fluoride to the raw mix and also rapid burning and cooling. > This is an area where there is ongoing research resulting from the increased burning of bone meal in cement kilns. I am currently involved with a project, which has this as one of its main focuses.

Question 225:
We have white kiln (56m length, 3.2m diam) three stations. As part of planning, the kiln was stopped for two hours and during that time, the live ring No 2 greased, but then a small fire happened around this area, but quickly put out by the use of chemical dry powder fire extinguisher. Upon starting the kiln, and running on main drive, we found high torque of main motor because the live ring No 2 not in contact with its supporting rollers. The decision was then made to run the kiln on auxiliary drive about 15 minutes continuously and then after 15 minutes the live ring again became in contact with the two rollers. Can you explain this?

Answer:
This is very difficult to answer. I can only presume that the temperature generated by the fire at the No.2 live ring caused the ring to lift off the support rollers. The kiln drive was then trying to turn the weight of the unsupported kiln. I cannot foresee that the dry chemical powder was a factor as the live ring was not in contact with the rollers. Possibly the grease was compromised by the dry chemical powder?
Question 224:
We are cement manufacturers in Africa using a 1680 tpd dry process kiln plant. We have been using four primary raw materials to make our raw limestone, kunkur (a low-grade limestone used as clay, shale or low-grade marl) plus iron ore & alumina clay. But of late we started using alumina dust by blending it in the ratio of 1:1 with the alumina clay). We thought this could improve both the kiln efficiency and clinker burnability, but on the contrary. This mix only helped in increasing kunkur usage by 10 per cent and lowering both the limestone and alumina clay percentage ratios. Thus we kindly inquire from you on the suitability of using alumina dust in raw meal preparation.

Answer:
The only explanation for not achieving the expected improvement in burnability must be connected with some adverse minor component in the iron oxide or the alumina dust. Both have increased in the new mix design. Both contain increased alkali contents and the raw mix alkalis have increased with the new mix design. Alkalis are inhibiting agents and this might be the cause of the problem. An alternative might be that there is some beneficial component in the alumina clay or limestone (both have reduced in the new mix design). Your experience is that mixes with higher proportions of kunkur suggest that the beneficial component is probably not in the limestone therefore this point towards the alumina clay. Is there any fluoride in the alumina clay?

Question 223:
Do you know of any references, which illustrate how the benefits resulting from the application of advanced control techniques for, kiln control and optimisation have been calculated?

Answer:
It is relatively simple to calculate the saving in reduced fuel or electrical energy consumption if you know the amount consumed, the cost and the percentage saving the advanced control technique has achieved. Unfortunately the savings are not normally large and therefore do not usually generate an attractive payback. The real payback comes from additional output (provided that additional output can be sold at an attractive price). In this case you need to calculate the marginal value of the additional output. Provided the market is good then this marginal value is very high and very short payback periods are provided by only minor increases in output.
Question 222:
Generally it is believed that due to use of pet coke, life of metallic parts (like Dip Tube, Nose Ring Sectors etc) can be affected. I would like to know your opinion in this regard.

Answer:
Pet cokes are generally high in sulphur content and this will result in higher sulphur dioxide content in the kiln gases. Sulphur dioxide is an acidic gas and therefore more corrosion of metal components in the kiln might be expected. It seems reasonable to expect this to be worse in the preheater components such as the dip tubes than the nose ring castings.

Question 221:
What are the possible areas in closed circuit raw grinding system (ball mill-seperator-cyclones-esp) to optimise the production output of the mill system?

Answer:
There are possibilities throughout the raw milling system for optimisation. The raw mill usually must carry out two tasks: grinding and drying the raw materials. If you want to increase the output then you must make sure both these tasks can be carried out adequately. The ball charge and grading must be optimised using axial testing of the material in the mill. The mill liners should be classifying to ensure the grading of the charge is optimum along the axis of the mill. A high efficiency separator will give a sharper cut point and reduce the amount of recirculating material which is already ground to sufficient fineness. A hot air generator can be used to boost the drying capacity of the mill. When these aspects have been optimised the output can be increased in the short term by reducing the fineness of the kiln feed. In the longer term it may be necessary to add additional equipment to the circuit such as hammer mills for pre-grinding.
Question 220:
We have two ball mills used for limestone and laterite grinding: diameter 3.4m and 7.1m length, speed 16.7rpm, and single compartment. The quality target is 16% 90 microns. Please advise what is the optimum ball charge and why most of sorted out balls are elongated (not circular shape) and what are the possible reasons for this unshaped ball?
Answer:
It is normal for the ball charge in raw mills to occupy 27% of the internal volume of the mill. The most likely explanation from the elongated shape of the media is that the mill was initially charged with cyclopebs. These are cylinders of grinding media rather than round media balls. On wear they might well attain the shape you describe.

Question 219:
Please explain the different types of coal mills that are used in the cement industry and their advantages and the disadvantages in terms of the process and activity. Finally, what precautions do we have to observe when handling the pulverised coal?

Answer:
Ball mills or vertical mills are used for coal grinding and drying the cement industry. The advantage of a vertical mill is lower unit power consumption in the same way as it is in raw mix preparation. This is because compression forces are brought to bear directly on the material being ground. When handling pulverised coal the main concern is that mixtures of fine coal and air are potentially explosive. Explosions avoidance and protection are therefore the main concerns. A secondary consideration is the potential for segregation in the pneumatic conveying of the fine coal. Pipe runs must be carefully designed to avoid this and combustion problems in the kiln.
Question 218:
In our plant the conversion factor for raw meal to clinker is within the range 1.65 to 1.7 (clinker yield/raw meal consumption). What are the factors that do influence this factor? Does the equipment design and efficiency have an impact on this?

Answer:
The raw mix to clinker factor is related to the lime saturation factor of the kiln feed. It is a function of the amount of CO2 present in the kiln feed which will be driven off during the process of calcination. This CO2 is present as CaCO3, calcium carbonate and this determines the lime saturation factor. The conversion factor is normally 1.53 to 1.55. There is a further consideration. Your factor of 1.65 to 1.70 almost certainly relates to the actual amount of feed to the kiln, which is converted into clinker. This takes into account the amount of kiln feed lost to the kiln in dust in addition to the CO2 lost in calcination. The collection efficiency of the cyclones at the top of the preheater determines what this combined conversion factor should be. Question what negative effects does Zirconium (0.5%) have on the clinker if used in the raw mix? Also, what kiln effects and cement effects, if any? Answer Zirconium is reported to change the size and shape of the alite crystals and to modify the type of the belite crystals. This in turn leading to significant retardation in setting and subsequent reduction in strength growth. A noticeable colour change is also reported. No significant change in burning and cooling conditions are reported.

Question 217:
Please advise me, how shall I calculate (theoretically) the percentage of slag to be used for manufacturing the portland slag cement?

Answer:
Blast furnace slag cements contain variable amounts of slag dependent on the application for the cement. Some of these cements contain up to 65% slag, however a more typical amount would be 35 to 40% slag.
Question 216:
My plant produces special pozzolanic cement with a fineness about 4000cm2/g, this cement is stored in a silo, and its height is 7 m. In some recent batches that we discharge, there are pieces of compacted cement (10cm approx) and our customers think that this is caused by contact with water, I think is a consequence of high fineness and the storage time. Any comments?

Answer:
It is difficult to diagnose the problem without knowing the storage time. I would suspect that there is some continued dehydration of gypsum or the pozzolan after the cement has been blown into the silo. What is the temperature of the material when you blow it into the silo? Do you have a cement cooler? Is the cement ground on a closed circuit mill?
Question 215:
I am in charge on a project to use tyres as a alternative energy source. The idea is to feed tyres in the calcinator at the entrance of the kiln. We are using petcoke in the calcinator and flourite in the raw meal. So, I kindly request you about the disturbances that this change can cause on the process.
Answer:
Many cement companies are adding tyres at the calciner with no problems. The most likely disturbance is an increase in the frequency of CO formation in the calciner due to the large size of the tyre pieces. This can cause problems if you have electrostatic de-dusting of the kiln exhaust gas. A number of cement companies have installed a separate tyre-burning chamber alongside their calciner. This produces a stream of lean gas, which is then burnt in the calciner with less danger of CO formation. In this way the companies have been able to increase the rate of tyre burning in their kilns. There are also a number of cement companies who have replaced their electrostatic precipitators with bag-houses to allow them to burn more tyres.

Question 214:
I would like to know more about rubber as an alternative fuel in the cement industry. I would like especially like to know about experiments that have been done in this field.
Answer:
Waste tyres are a major alternative fuel used in many cement kilns around the world. Trials have been carried out on hundreds of cement factories and many burn tyres on a regular basis. This is now recognised to be one of the best ways to dispose of old tyres rather than land filling them. For the cement process rubber is a good source of energy. A large tyre is a discrete source of energy rather than a continuous supplies such a fuel oil, gas or fine coal. This can cause some formation of CO spikes due to the difficulty of providing sufficient oxygen for combustion of a large tyre. There is also the issue of the metal reinforcement in the tyres. Many kilns have solved these issues and burn tyres continuously.
Question 213:
I would like to know the way to determine the feeding ratios for raw materials to mill. We have four materials: limestone, clay, iron mineral and silica mineral.
Answer:
You determine the feeding ratios of raw materials to the raw mill dependent on the chemistry targets of your kiln feed. These targets might be the lime saturation, silica modulus, alumina modulus or an individual oxide or a potential clinker mineral content. If you know the chemical composition of the individual raw materials then the correct proportions for feeding to the raw mill can be found by solving simultaneous equations. In practice the chemical composition of the individual raw materials usually varies a little. In order to compensate for this you need to use control charts and vary the feeding ratios to maintain your kiln feed chemistry targets.

Question 212:
I would like to know the relation between feeding ratios, feed in tonnes and percentage of every material. Is there a general relation or does it vary with the balances and its specific calibration?

Answer:
There is no general relationship between the feeding ratios, feed in tonnes and percentage of each raw material. The ratios vary with the composition of the raw materials and also with the calibration of the weigh feeders. This is why you need to use techniques such as control charts to keep the kiln feed on target.
Question 211:
How do low nox burners work?
Answer:
Low NOx burners work on the principle of reducing the amount of combustion air delivered through the burner along with the fuel. In the cement industry we call this the primary air. It is the oxidation of the nitrogen in this primary air, which causes the formation of thermal NOx in the flame. By reducing the amount of primary air there is less potential for nitrogen oxidation.

Question 210:
I need to know the advantages of the grate coolers over the rotary coolers.

Answer:
With a rotary cooler all the cooling air is drawn into the kiln. With a grate cooler only the amount required for combustion is drawn into the kiln and the excess is separately exhausted. The air requirements for cooling are greater than the requirements for combustion and therefore the grate cooler better matches the requirements of the kiln for combustion. This allows the secondary air to be at a higher temperature and leads to the kiln being more fuel efficient and productive.

Question 209:
Presently we are using average 60 per cent high sulphur petcoke in a white kiln. Now, we are unable to increase the high sulphur petcoke percentage due to operational problems that causes ring formation at calcining zone of the kiln. The mainly difficulty is high sulphur in petcoke which results in an increase of the recirculation of the volatile constituent inside the calcining zone. We are looking forward to increase the percentage of petcoke in the kiln. Thus, if we mix the mix petcoke with a certain proportion of raw meal, we think that a considerable fraction of the sulphur will react with CaCO3 content of the raw meal, thus forming the CaSO4 (Calcium sulphates) that will come out with clinker mass, preventing the large fraction of sulphur load of the fuel from entering the burning system. Can we do this?

Answer:
The formation of sulphate rings in the calcining zone of kilns fired with high sulphur pet coke is a common problem. I do not think mixing the coke with the raw feed is a good idea. You are likely to get incomplete combustion of the coke in the upper stages of the preheater and increased CO and hydrocarbon emissions from the kiln. If you have electrostatic precipitators you will be unable to run them due to the high CO content of the exhaust gases. My information is the CIMSA plant at Mersin in Turkey, and the Bunol plant of Valenciana in Spain is burning 100% pet coke. I suggest that you need to balance the sulphate input to the kilns with alkalis. You might be able to do this by increasing the alkali content of the raw mix with feldspar or soda ash.

Question 208:
How far is P2O5 content responsible for ring formations in kiln? Other than alkalies, are chlorides chiefly responsible for ring formations? How will P2O5 will reduce the C3S content? Can you please explain?
Answer:
I can find no specific reference to P2O5 being responsible for ring formations in the kiln. However there are a number of references to P2O5 changing the course and products of mineral formation in the kiln. It is entirely possible that in changing the reaction sequence a ring could be formed in a particular area of the kiln. The most common cause of ring formations in kilns is sulphate in my experience. These rings tend to form at the inlet end of kilns fitted with preheaters. The second most common cause is formation of liquid phase at a particular point in the kiln. (This is the way that P2O5 might cause ring formation). I have seen these types of problems occur with increases in Fe2O3 and MgO content of the raw mix. The explanation given for the reduction of C3S with P2O5 content is that the P2O5 decomposes the C3S to produce C2S and free lime. Sound cements can be produced but the lime saturation must be lowered with the inevitable penalty of lower early strengths. The problems can be overcome to some extent by addition of fluoride to the raw mix and also rapid burning and cooling. This is an area where there is ongoing research resulting from the increased burning of bone meal in cement kilns. I am currently involved with a project, which has this as one of its main focuses.

Question 207:
I come from a different industry. Recently, a company we were working with used a titration method for the determination of calcium carbonate-calcium hydroxide mixtures and referred to it as a cement industry standard. What is the cement industry standard titration method for determination of calcium carbonate /calcium hydroxide content?
Answer:
You are correct that the content of calcium carbonate in cement raw mix used to be determined by an acid alkali titration using phenol pthalein as indicator. My recollection is that a gram of raw mix was dissolved in 100 cm3 of standard strength hydrochloric acid. The calcium carbonate dissolves and neutralises part of the hydrochloric acid. The residual acid was then back titrated with standard strength sodium hydroxide allowing the calcium carbonate content to be estimated. Normally this would now be done via X-ray fluorescence spectrometry. There is also a standard method for the estimation of free lime. This method involves dissolving the uncombined lime in the cement or clinker in ethylene glycol and titrating with standard strength hydrochloric acid. Calcium hydroxide will also dissolve in the ethylene glycol and therefore the titration gives the sum of the uncombined lime and the calcium hydroxide. In conclusion I think you are referring to two standard titrations in the cement industry. However, I do not think they can be used in series to estimate calcium carbonate and hydroxide mixtures, as they will not distinguish between the species. Do any practicing cement analytical chemists have any suggestions?
Question 206:
What is the advantage/disadvantage in using chemical gypsum.
Answer:
The advantage of chemical gypsum is that you are making use of the by-product from another industry and therefore making a contribution to the life cycle cost of that industry. I do not know of any disadvantages in using chemical gypsum compared to natural gypsum.

Question 205:
During an inspection of a 160 tph ball mill, I noticed a lot of agglomeration in the fine chamber. The separator tromp curve increases dramatically in the fine particle range, again indicating that fine particles are sticking together. One of the causes of agglomeration is static electricity, and I have come across a device that claims to increase production by approx 10% by reducing the static electricity. Do you have any experience or opinions on such a device?
Answer:
I have come across the ECOFOR device and in fact conducted a remote investigation into its effectiveness on behalf of a cement company a couple of years ago. At that time the main advocates of the device were Vassiliko Cement Company in Cyprus who had achieved considerable improvements with the device on their cement mills. The device had achieved this success on mills with first generation separators, however preliminary results on a mill with a high efficiency separator showed lower benefits. At the time other cement companies had taken the device on a trial basis but I could find no-one who had actually installed the device long-term other than Vassiliko (I think there are references in Egypt but I could not obtain any references from the factories). Another company who had conducted a trial confirmed that all the cement adhering to the shell of mill was cleaned off due to grounding the static charges and that the media was completely clear of agglomeration. I certainly think that a trial with the ECOFOR device may solve your problems of agglomeration.

Question 204:
Ours is a cement manufacturing unit producing mainly fly ash based portland pozzolana cement. We have a some questions regarding quality: 1. We are facing problem concerning the hardness of our clinker and relatively low early strength of cement even with lsf – 90 to 92 and C3S – 45 per cent in clinker. 2. Can we improve the strength by adding 5 to 7% limestone during clinker grinding? 3. We have got a zub separator supplied by KHD for cement mill, efficiency of which is only 50 to 60%. How we can improve the efficiency of our separator?
Answer:
Unfortunately low early strengths are characteristic of Portland pozzolanic cements. I do not think that addition of 5 to 7% limestone at the cement grinding stage will improve the early strength; in fact I think that this is likely to reduce the strengths. The hydraulic properties of Portland pozzolanic cements is a complex area with difference between the concrete making potential and the standard testing. The fly ash should improve the workability of the cement and hence the concrete making potential even though the standard tests do not show imporved quality. Limestone additions can also help concrete making potential by boosting the consistency of the concrete. One important consideration is the point at which you are adding the fly ash. This is much better at the separator than to the mill feed. Unfortunately I do not think there is any simple way to improve the efficiency of your Zub separator. However, KHD should be able to convert this to a high efficiency separator by the insertion of a cage within the separator body.
Question 203:
I want to devise a test to that can show from the final cement what its composition is. This is highlighted in table 4 of EN 197 NOTE d. This relates back to table 1 EN 197 CEM 1,CEM 2 ect. We use limestone filler in one cement type, and pfa in another. The additions are controlled by calibrated feeders. Is there any standard for reference on a test method.
Answer:
I am not aware of any standard reference method for what you want to do. You can find various quantitative methods for clinker phase estimation in the literature. These would make a start by identifying what was clinker and what was not. X-ray diffraction would also be a possibility, although probably expensive. If I find any further references I will pass them along.

Question 202:
Please can you tell me about the limitation of SO3 in clinker? What is the disadvantages of its increase? Is this SO3 able to control setting as the hydrated gypsum does?
Answer:
The limit on sulphate in clinker is that it restricts the amount of sulphate that can be added as gypsum during cement grinding. International specifications for cement limit the amount of sulphate therefore if the clinker contains significant amounts then the gypsum addition must be reduced to remain within the specification. This can be a disadvantage as the ability to adequately retard the cement can be restricted. There can also be process problems associated with higher sulphates in the kiln. The amount of sulphate relative to the alkalis in the clinker is also important. Sulphate will preferentially be bound as alkali sulphates and these provide soluble alkalis, which influence the hydration reactions of the cement and will control the setting. Sulphate in excess of alkali is likely to be bound as dead-burnt anhydrite and this is essentially inert and therefore will not control the setting. As you can see there are a number of factors to take into consideration.
Question 201:
I am working in a cement plant. I would like to ask one question regarding cooler to calciner tertiary air flow measurement. Could you please tell us what is the general method adopting in all cement plants? Is it required continuous measurement or when ever it is required we can measure with some pitot tube. Could you please suggest any instrument which can measure continuous tertiary duct air ( gas )flow. Temperature of gas is around 850 to 950 Degree C.
Answer:
The air flow in the tertiary air duct between cooler and precalciner is not normally measured. The tertiary air damper is opened to ensure that there is no CO formation in the precalciner. When there is no CO formation the main induced draft fan of the preheater is pulling sufficient air through the tertiary air duct. A pitot tube can be used to measure the flow through the duct when you are conducting your heat balances and process investigations. There is a company Promecon that is providing equipment for in-process flow measurement. This might be worth investigating.

Question 200:
We can always see red-rivers in the grate cooler of a smaller kiln we run. It’s a precalciner kiln with 5 stages of cyclones, and daily capacity is 2,000 metric tons. I think red-river is caused by the wide width and lower height of clinker bed in the cooler. So I’m considering narrowing the cooler width to get higher clinker bed. The effective kiln diameter inside the refractory is 3.26m and the effective width of the existing cooler is 1.96m. As I think, 1.2-1.6m of the width would be sufficient for our kiln specifications. What do you think about that? And please advise me about any methods or principles to design the optimum width considering kiln capacity.
Answer:
Your idea of reducing the width of the cooler seems the right thing to do. The way people often do this is to insert blank grate plates along the sides of the cooler. Clinker then builds up on these blank plates along the sides of the cooler and effectively reduces the active width of the bed. The principles depend on the type of aeration you have in your cooler – is it aerated through air beams or into chambers beneath the bed? I suspect it must be chambers if you have red river problems. The principle to design the active area of your cooler is to calculate the tonnes of clinker produced per day per active square metre of your cooler. With chamber aeration you should not raise this factor much above 30 tpd/m2.

Question 199:
Our cement plant is suffering from clay handling problems. The clay in quarried off-site, then crushed to approximately 50 mm, before being stored undercover for approximately 5 days. The clay is then dispatched to the plant via road haulage. The clay moisture content is up to 17%, causing conveying problems at the plant (lump formation and hence blockages). The clay doesn’t pass through a secondary crusher onsite, as blockages have previously been experienced. I would imagine this is a common problem in many cement plants and any input as to a way to solve this problem would be very much appreciated.
Answer:
Clay handling is indeed a problem on many cement plants. A key factor is not to store the material for too long after it is crushed. This gives it time to settle and re-agglomerate. On one factory where I worked we used to only crush for the immediate requirements during the winter. This had the added benefit that if it did set into a lump there was only 1 days material to dig out of the store. There were also sandstone bands in the quarry and we could selectively take the rougher more sandy material in the winter. Perhaps you could pre-blend some limestone into the clay? On another factory I know they would suffer the blades being ripped off the reclaimer after the mill maintenance. This was due to wet clay clinging together when it was not being continually reclaimed. I would try to work on a much more just-in-time basis, reduce the amount of clay which you store and cut the 5 days under cover storage. Maybe other readers can come up with suggestions?
Question 198:
We believe Japan has now started burning bone meal from infected BSE cows If every bone has a phosphorus content, won’t this have an effect on the quality of the clinker?
Answer:
There are a number of factories in Europe where they are burning bone meal and animal fat in their kilns. You are correct that the phosphorous in the bone meal is an issue. The normal practice is to limit the amount of bone meal burnt to ensure that the clinker P2O5 content is not greater than 0.5%. However, by careful control of the kiln and particularly the clinker combination and mineralogy, it is possible to combine the P2O5 in the belite and increase amount of bone meal, which can be burnt. There is one factory where they are conducting extensive trials on this control of the clinker mineralogy with bone meal burning.

Question 197:
We have cement silo height 33.6m and the radius 15m and the cone height 8m. We need a formula to know exactly what inside the silo if we are using the height (silo pilot) of the cement inside the silo to calculate and we need a trend of relation between the height and density of the cement.
Answer:
For a silo with an inverse cone bottom it is simple to calculate the volume.

Volume cylinder = H * 0.25 * pi *d” = 33.5 * 0.25 * 3.14 * 225 = 5917 m“
Volume cone = 0.333 * h * 0.25 * pi * d”= 0.333 * 8 * 0.25 * 3.14 * 225 = 471 m“.
Total volume silo is therefore 5446 m“ minus the volume of the concrete fill below the silo outlets. This will probably be approx. 150 m“ leaving a silo volume of approx. 5296 m“. If you can send us a cross section drawing of the silo we can prepare a table of the volume of the cement in relation to the height of the silo.
For silos of this size a specific weight of the cement is approximately 1.4 tonne/cubic metre. This is not a firm figure but is dependent on the quality and type of the cement, the height of the cement in the silo and the time that the cement has been stored. When the cement is loaded into the silo it is fully aerated and has a specific weight of approx. 1.0 tonne/cubic metre. The air escapes quickly and within a few hours the specific weight will be about 1.3 tonne/cubic metre. Under pressure of the cement load above it, in combination with time, the cement will compact further. Heavily compact cement at the bottom of a silo can have a specific weight exceeding 1, ton/cubic metre.

Please note that in vertical silos the cement flow is not “first in – first out” but to a substantial degree “last in – first out”. When an outlet valve at the bottom of the silo opens, a downward flow through a narrow vertical tunnel is achieved. Cement from the top of the silo actually flows downward through these tunnels and is extracted earlier from the silo than the cement stored below it. The fluidisation pads at the bottom of the silo only become effective when the cement height is less than 6-7 metres and are used for the cleanout of the silo only.

The funnel flow phenomenon in silos makes that there is not a homogenous specific weight of the cement related to the storage height. However, assuming an average specific weight of 1.4 tonne/cubic metre should provide a reasonably close estimate of the contents of the silo.

Question 196:
We are experiencing a ring formation problem in our kiln. The kiln is 65m long with a diameter of 4.2m. The ring forms at about 22m. Both alumina and ferric oxide contents have increased slightly resulting into slightly low silica ratio. What could be the cause of the ring formation?
Answer:
It is quite possible that the increase in the alumina and ferric oxide contents of the kiln feed have caused the ring formation. These changes will affect the amount of liquid phase in the kiln charge and the position in the kiln where this liquid phase first appears. There are many factors to take into consideration to address this problem. What is the refractory lining at the 22m point? At what point does the insulating lining in the calcining zone finish and the semi-insulating upper transition zone lining start?

Question 195:
Gentleman could you please explain the ‘soundness’ of cement in terms of chemical composition and the definition of the soundness.

Answer:
Soundness of cement refers to the long term durability of concrete or mortar made from the cement, and particularly its susceptibility to expansive forces after the cement has set. The main components, which can cause unsoundness or expansion, are excess free lime, CaO, or magnesia, MgO. Both of these hydrate much more slowly than the clinker minerals and the hydrated reaction products occupy a larger volume than the lime or magnesia. This means they react after the hydrated cement matrix has formed and introduce expansive forces, which can crack the mortar or concrete. This is why the amount of free lime or magnesia is limited by international specifications for cement.

Question 194:
What are the positives, negatives and gains in using a vertical roller mill for grinding cement?
Answer:
The positives and gains from using a vertical mill for grinding cement are that the power consumption per unit of cement produced is lower by around 30%. The negative aspect is that a much tighter particle size distribution is generated in the cement. There are less coarse particles but also less super-fines. This changes the hydration and strength growth characteristics of the cement, which can be unpopular with customers.

Question 193:
What could be the direct effect on the refractory bricks for a 40% utilisation of petcoke fuel in cement kiln operation? Could you give me literature studies for my references and further information on this subject?

Answer:
The effects of 40% pet coke firing on the refractory lining should be minimal. The coke contains virtually no ash therefore in homogeneity in the burning zone should be reduced. The coke is likely to be high in sulphur however. This might lead to a heavier alkali cycle in the kiln and potential for alkali ingress into the refractory. The two effects will balance out to some effect – I would not expect major problems with the refractory lining.
Follow up
Thank you very much for your informative response. However, I am very much concerned with the effect of petcoke on refractory lining because we have observed that cement kilns using petcoke have moved their burning zone towards the back end, and will not this create a redox atmosphere?
Response
Yes, I agree that the burning zone may be moved back in the kiln. I do not think this will cause redox problems, however the cooling zone in the kiln would be lengthened and this might be detrimental to clinker reactivity. This must be countered by appropriate burner modifications. Alkalis can attack the lining throughout the kiln. With high alumina refractories in the upper transition zone you might suffer from alkali bursting, however I would not expect the alkalis to be condensing in that zone. In the burning zone you also get penetration of the basic lining by
the alkalis vapours and crystallisation within the lining. This can cause problems, however the inhomogenity arising from combination of the coal ash at the burning zone can also cause problems. These were the failure mechanisms I suggested might cancel one another out with introduction of 40% petcoke.

Question 192:
A am developing an academic project about the cement industry. Could you tell me, what is the average composition of gypsum and calcareous filler employed in the cement grinding on this process?

Answer:
Normally high purity gypsum containing more than 95% CaSO4.2H2O is used in cement grinding. The international standards fro cement limit the SO3 content to 3 or 3.5% therefore the maximum gypsum addition is around 4%. Limestone additions are allowed in some countries (I presume this is the calcareous filler you refer to). This limestone would also normally be high purity and more than 95% CaCO3. Under European standards up to 5% limestone can be added to all cements. This addition rate can be increased in Portland limestone cements and masonary cements.
Question 191:
I am doing research about “Fuzzy Cement Kiln”. Can you advise me how the fuzzy logic could be implemented on cement kiln operation?
Answer:
Fuzzy Logic has been used in expert control systems on cement kilns for more than 20 years now. In fact the system supplied by FL Smidth of Denmark is called FLS Fuzzy. However, Fuzzy Logic is a concept and is also used in other expert systems used to control cement kilns and other processes. Fuzzy Logic is a way of mathematically simulating the decision processes of a person. In these expert control systems the decisions of the best kiln operator are being simulated.

Question 190:
We run a dry kiln made by FLS in 1968. For years we used water to cool the kiln surface in the burning zone. FLS gave us the advice to change the water for air. Since then we use fans for the cooling. However they produce so much noise that we get problems with our environment. About a year ago I read an article from a company that makes insulation what is put between the refractory and the kiln to reduce the temperature from the kiln surface. Is this product in use somewhere, and what the experience with this product?
Answer:
I thought all refractory suppliers were able to supply insulating backing materials for refractories. The Technical Manager of Hoganas-Bjuf UK, Mr Brian Boon, can probably help you. bboon@hoganas-bjuf.co.uk. I should say that I have some experience of this and it can cause problems with the burning zone refractories. In a white cement kiln we had to convert from two layer linings with insulating backing to single layer alumina spinal refractories. The first problem is that the working refractory relies on the cooling of the shell. If it is insulated from the shell then the penetration of vapours and liquid from the kiln charge is much deeper with associated wear and failure problems. There is also the problem of relative movement between the basic working lining and the insulating backing. The basic lining tends to grind away the backing and then the whole lining falls out. I am sure there are solutions to these problems but they need to be considered.
Question 189:
Is possible introduce in the clinker kiln waste of glass as silica corrector instead of another usual silicate? Is there any experience that you could inform to me about?

Answer:
I do not know of any experience of using waste glass as a silica correction agent. Usually silica sand is used which brings its own problems for the process. The sand is abrasive and causes wear in the raw grinding and blending equipment. Also the combination of the quartz in silica sand into the clinker minerals is much more difficult than the combination of the silicates in clay minerals. I am sure this would also be the case with waste glass. Sorry I cannot be of more help.

Question 188:
What is the reason for high free lime as well as high litre weights at the same time? Are there any additives to counter the expansion problems in cement?

Answer:
It is entirely possible to have high free lime and high litre weights if the lime saturation of the clinker exceeds 100%. In this case the maximum combinable lime is exceeded and the excess lime is present as free lime. I do not know of any additive to offset expansion problems. Correct proportioning, blending and quality control of the kiln feed is the way to avoid these problems.
Question 187:
I would like to know the work of C3A in cement and how much C3A to adjust in cement accordance to ASTM type one in minimum and maximum and what is the procedure of making compressive strength cubes of opc and src accordance to ASTM?

Answer:
The C3A in cement is principally responsible for the setting of the cement. ASTM C150 does not impose any limits on the amount of C3A in Type 1 cement. ASTM standard C109 sets out the procedures for making cubes for compressive strength testing.
Question 186:
In case coal and petcoke are ground (to respective fineness) and stored separately will there be any requirement for installing a blender of any kind after extraction of fine coal and petcoke from individual bins by metering screws before being conveyed pneumatically to kiln burner/calciner. If so please suggest type of blender.
Answer:
I don’t think there is an absolute requirement for a blender if you store your fine coal and petcoke separately. However, the potential for process disturbance would be reduced by a blender. I would suggest a mixing screw between the metering screws and the pneumatic pumps delivering the fine coal and petcoke to the burners.

Question 185:
We are facing the same problem every day. We have high temperature product, which is caused from the increase of the cement mill temperature. How we can control the temperature of the mill? We even tried to put the clinker out side the silos to reduce its temperature but no use…so please help me to figure something out so we can get the mill temperature down.

Answer:
There are many possibilities to control your mill temperature problem. What kind of cooler do you have? If it is a grate then you should be able to increase the air flow to the back fans of the cooler and achieve further cooling. If satellites or a rotary cooler then water sprays into the cooler can help.

Then there is in the cement mill itself. Is the ventilation of the mill adequate? Are you using water injection? Are you cooling the cement via the cold air bleeds in the closed circuit of the mill? As you can see there are many options. You need to develop a heat balance of the mill and this will guide you on how to solve your problem.
Question 184:
We are having problems with snowmen. Apart from the method of removal what changes in the burning profile can you recommend to alleviate this problem? What other fluctuations in meal chemistry should we look out for?

Answer:
Certainly a change in the burner position might help your snowman problem. The problem is caused by the clinker being too “sticky” when it falls into the cooler. If you push the burner in then there is more time for the clinker to cool and solidify in the outlet section of the kiln before falling into the cooler. Changing the position of the burner is not to be undertaken lightly however. Do it in a planned way and by small steps, observing the effects for some hours between steps. In the raw mix chemistry you are looking for anything which might affect the
amount of flux/liquid in the clinker in the burning zone. So alumina, iron, alkalis, fluorides, sulphates, magnesia. My experience was with large snowballs coming through the kiln when magnesia in the limestone changed from 1.3 to 1.6% for a short period. The key thing is stability of the raw mix composition for the main ratios and for the minor oxides and species. Do you have any raw mix proportioning/blending control systems?

Question 183:
I am undertaking a study of our cement milling (closed) circuits with particular emphasis on milling process parameters and their effect on setting times. The major contributor to setting time is the state of gypsum in the cement. However, I am more interested in what you consider to be the key milling parameters that affect setting time. Secondly and perhaps more importantly what would you consider as acceptable variations (deviations) for these parameters to achieve a stable milling system? For example:
• Inlet clinker temperature
• Discharge cement temperature
• The preferred mode of water injection, i.e., mill inlet, mill discharge or both
• Circulating load
• Use of synthetic and natural gypsum or a blend of the two
• Fineness (Residue and Surface Area)
and any others you might suggest.
Answer:
You have a big project to undertake! The state of the gypsum is the major controlling factor with regard to setting time, but this state is determined by the mineralogy of the raw gypsum and the milling conditions. Milling conditions are therefore fundamental in controlling setting.

Achieving a temperature in excess of 85 degrees C is necessary to ensure the gypsum is dehydrated to one of the soluble forms, hemi hydrate or soluble anhydrite. The temperature of the cement leaving the mill should be controlled to less than 120 degrees C to avoid problems of coating of the
media and conversion of the gypsum to the insoluble natural anhydrite.
The first step in achieving this temperature control is the ventilation of the mill. If temperature can be controlled in this way it is preferable to water injection, however with high capacity, large diameter mills you cannot get enough air flow without having unacceptably high velocity. This is why water injection becomes necessary. (High velocity is a problem because it will pull the fines out of the mill and adversely affect the particle size distribution).
If water injection is required then mill outlet is preferable to inlet as it is the outlet temperature you need to control, and also the majority of the heat is generated in the second chamber. Sometimes outlet water injection is insufficient however, and then you need a combination of both outlet and inlet injection.
The key to all these considerations is a heat balance of the mill. This will give you the thresholds where successive methods of cooling become necessary. Clinker input temperature is a key consideration in these balances. The lower and more stable you can get this the better you are able to adequately control the temperature in the mill. Control of cement discharge temperature via a PID loop with the water injection is normally not a problem. A set point of 105 to 115 degrees C is the normal range.
There is no fundamental reason why a synthetic gypsum should require a different approach to natural gypsum. A number of cement companies use of blend of synthetic and natural gypsum. Fineness (Blaine and particle size distribution) is a function of the recirculating load and the strengths you wish to achieve, rather than the setting time.
A final consideration – cooling of the cement before storage in the silos. If you blow 110 degrees cement into the silos you risk continued gypsum dehydration in the silos and lump/coating formation. I suggest you investigate cooling of the cement via the cold air bleed in the separator/closed circuit system.
Question 182:
Do you have a recommendation on the best method to determine chlorides in raw materials, CKD, clinker, etc Do you recommend X-ray, potentiometric titration, wet classical, etc?

Answer:
I would recommend XRF because it then becomes part of your hour to hour quality control testing. However this has to be backed up by another method to allow you to calibrate the XRF. Personally I would favour a potentiometric method for this back-up.
Question 181:
We wish to use coal from two different mines with different specification to reduce sulphur percentage. Can such coals be mixed and used and what could be potential problems?
Answer:
There is no real problem in mixing coal from two mines. Many cement companies do this around the world. Actually many cement companies use a mixture of different solid fuels (coal, petcoke etc.) and mixtures of solid, liquid and gaseous fuels. The only potential problem is consistency. You should make sure you bend the two coals and use the same blend to avoid process instability.

Question 180:
Can some one help me out? If a cement silo hold 1000 tons of normal portland cement at 3800 Blaine fineness, would be the capacity for the same silo if held ground slag at a 4000 Blaine and a 5500 Blaine?

Answer:
Sorry, I can’t answer this one. I do not know and have not been able to find a formula for the variation of cement density with fineness. I suspect that there is no absolute formula and the variation depends on the dimensions of the silo and the age of the cement. All I can suggest is that you make assumptions as to the reduction in density, and err on the conservative side, i.e. use larger reductions in density and under-estimate the capacity of the silo at 4000 and 5500 Blaine.
Question 179:
We are trying to obtain a darker clinker. Aside from the iron, what other element has influence on the colour of clinker?
Answer:
All the transition metal ions have colouring effects on the clinker, however they are only trace contaminants in normal cement raw materials. Specifically for darkening the clinker manganese oxides would be the most effective, but again normally only present as traces.

Question 178:
I want to know how kiln exit gas oxygen percentage affects on kiln production capacity, specific fuel consumption and secondary air temperature and so on for calciner kilns with grate cooler. Of course, the higher percentage of oxygen should result in the higher fuel consumption, lower production and the lower secondary air temperature. I heard one percent of increase in oxygen content resulted in about 2-3% increase of heat consumption for wet kiln with planetary cooler. But for calciner kiln with grate cooler?
Answer:
It is not straightforward to predict the effect of a 1% increase in kiln inlet oxygen for a precalciner kiln. This depends whether the calciner is in line or separate line. If in line then maybe no difference in fuel consumption because this can be compensated by less air drawn through the tertiary air duct. The same complications apply to the secondary air temperature and kiln inlet temperature. I have used a computer heat balance model of a precalciner kiln to see that a 1% increase in preheater exit oxygen results in a 1% increase in fuel consumption. You need a model like this for your kiln in order to investigate these effects. This will also allow you to explore the impact of your various multiple fuel scenarios and oxygen enrichment projects.
Question 177:
I want to measure the density of any ore in a pile (for example, iron ore, gypsum, raw mix, etc) and I want to have a standardised procedure for density determination as used in cement industry.
Answer:
This is more complicated than it sounds. The problem you face is compaction of the material in a pile. You could take a sample and fill and weigh a container of known volume and calculate the density in that way, but it is unlikely that the compaction you achieve in the container will be the same as in the pile. The only real way is to weigh the materials onto the pile as it is constructed and then measure the physical dimensions of the pile. Not an easy process and impossible if the pile is already in existence!

Question 176:
Our cement ball mill always has high residues. Can you give me a solution for that and also how to increase the output from the mill?
Answer:
Is your cement mill open circuit? Or does it operate in closed circuit with a classifier. If open circuit then the only thing you can do is to regrade the ball charge to produce a finer cement. If in closed circuit with a classifier then you should increase the feed rate and the amount of returns to the mill. This will achieve your joint aims of lower residue and higher output.
Question 175:
Could you please inform me what are the parameters, which affect the resistivity of gas dust? We have a strange problem in the EP filter of raw mills. Some time the resistivity of gas dust is very low although the distance between the electrodes are in the allowable range (11-15cm) inside the EP. This filter is normalised negative pressure but some time we have a little bit positive pressure and buffing in the inlet of EP filter and the temperature of the EP inlet of around 110 – 140¾C).
Answer:
Many factors affect resistivity: temperature: humidity: chemical composition. From the description of your problem you have given I would inspect and modify the gas flow distribution plates at the inlet of the EP.
Question 174:
My question is about the charging of grinding media in the ball mill. We are now following the instruction given by the manufacturer of the ball mill (FL Smidth) to charge new grinding media. We follow the following formula: Ratio h1/D1 where:
h1= height of grinding media from the centre of the ball mill
D1= inner dia of the first chamber.
Then from table we measure the required quantity of grinding media.
My question is what other thing we should check and follow to add new grinding media. Are there different methods for measuring this process?

Answer:
The easiest way to track the wear of the media in your ball mill is to monitor the kW drawn by the main motor. As the media wears the kW will fall. When it reaches a certain level you should stop and recharge. There is a formula relating power drawn with media weight and mill dimensions. I suggest that when you are next intending to recharge the mill you note the power drawn before and after and the media weight loaded. You will then be able to work out how extra kW you draw for each tonne of media you have installed.
Question 173:
I have few questions on TDF, these are: –
1. Who is best in making TDF manufacturing plant, from where I can get this plant and how much its going to cost me in US $.
2.What I know is that the recommended proportion in which TDF is mixed with coal or furnace oil is max 25%, why it is so and why it cannot be mixed in more quantity?
3. Do you have to change the flow equipment normally used in the kiln for coal or furnace oil?
4. Does the use of TDF changes the chemical composition of clinker both in negative or positive sense?
5. Is it possible to use two nozzles in the kiln, one for firing coal or furnace oil and one for TDF?
6. What are the effects/ pollution increase in using TDF in kiln?
Answer:
1. There are lots of suppliers of TDF plants and lots of different types of equipment. I suggest you contact ERAtech who specialise in this area.
2. The 25% refers to TDF entering the kiln via the gas riser at the bottom of the preheater. With tyre shreds or chips in the precalciner you may be able to achieve a higher proportion, but processing costs will rise. Even tyre shreds or chips are not going to burn out as quickly as pulverised coal or fuel oil. There are always compromises to be made.
3. Yes, the flow measuring equipment for pulverised coal or fuel oil will not measure the flow of TDF.
4. TDF does not contain much ash. There is the possibility of increased Fe2O3 and ZnO content from the reinforcement in the tyres, but this is not detrimental to the quality of the clinker.
5. TDF will not go through a nozzle. You will definitely need a different feeding and injection system for the TDF.
6. There are no increases in the emissions from the kiln with tyre burning. In fact the cement kiln is the best way to dispose of used tyres. However, storage and handling of tyres on your cement factory will raise some challenges.

Question 172:
We run two kilns, and one has ILC-calciner, another has SLC-calciner. We started to burn TDF as shredded in the meal chutes of the lowest cyclones in 1998. Even though we had a lot of problems like build-ups and decrease of compressive strength in the beginning of burning TDF, but solved those problems by improving process conditions like firing system, coal fineness, cooler operation etc. In addition, we started to burn plastics and rubber as the secondary fuel a month ago. We have two shredders and put those secondary fuel into the inlet of tertiary duct to the calciners. The problem we have is increase of CO content at the calciner outlets. Sometimes, the maximum CO content was more than 1.0 percent. Of course such increase of CO content results in build-ups and quality problems. <BR><BR> We’re reviewing several ways of decreasing the size of the secondary fuels and of oxygen enrichment, but it needs more time to be reviewed completely. And I think about moving the introduction point of plastics and rubber into the calciner from the inlet of the tertiary duct to other points. If you have any standards to select the best points for the most efficient combustion, please let me know that.
Answer:
Oxygen enrichment is a good idea to overcome your CO formation problem. This will intensify the combustion and consume the CO. You should also achieve an increase in output from the kiln. A number of cement companies in the USA are doing this. Another possibility would be to introduce a swirl chamber in your calciner, like the KHD Pyrotop. Many cement companies are installing a separate combustion chamber, with a “hot spot” or zone, to allow them to increase the amount of tyres they can burn without meeting the problems you facing. As to identify the best points for introducing the shreds I would
recommend acid alkali modelling of your precalciner.

Question 171:
I am a regulator for the local cement plant. The 35tph plant uses high sulphur coke from a heavy oil upgrader for about 50% of the fuel requirements. As part of the emissions permit, they are required to maintain a 24 hour stack SOX content of 300 ppm. The plant has had continuing problems with the accuracy of the existing continuous monitor, due to difficulty in drying the stack gas sample prior to the analyser. The analyser in use is infrared, and the plant is considering changing to a UV based analyser. However this is an expensive proposition, and I would appreciate advice on a reliable gas conditioning system.

Answer:
There are many potential suppliers and I can’t say if one is better than the other. I would approach them and ask for references. I would then take up the references with the cement companies where they have installed equipment to obtain first hand information on the reliability of their equipment. I
have heard good reports of Orbital, who represent Siemens gas analysis probes in some countries. You might try Universal Dynamics Technologies. Another company might be IMR Environmental Equipment.
Question 170:
I work at a plant in South Korea. Recently I read your article about fluorine in May 2001. I am very interested in your opinion because we want to produce more clinker than before, so your recommendation is very useful for me. I heard another way to producing more clinker more is for example, by oxygen enrichment, but that is very expensive and dangerous to try. I want to get more information such as setting time, strength, etc.

Answer:
Yes addition of calcium fluoride is one way for you to increase the output of your kiln. You must expect that this addition will increase the setting time of your cement. Any changes in strength growth will be slight and not the main issue. I have experience of the use of fluorspar in both grey and white cement production. I also know that it is being used by manufacturers in Europe.

Question 169:
I have two inquiries for you. We have one old wet process kiln of 800tpd supplied by FLS with no preheater or calciner system and planetary cooler. We want to convert this grey cement kiln into white cement, what basic step route of conversion should we follow.
Answer:
This is a big question and the detail is too much for the forum, however the following are the main steps you must take:
1. Ensure that you can produce raw materials with a sufficiently low Fe2O3 content.
2. Ensure that your raw milling has sufficient capacity to grind to the fineness required for white clinker production.
3. Replace any refractories in the kiln, which contain chromium.
4. Replace the existing coolers with water quenchers.
5. Investigate means to preheat the combustion air as no heat will be recovered from the clinker.
Question 168:
I have a quick general question. I am attempting to obtain indicative energy consumption data per tonne of clinker and cement produced in dry process portland cement manufacture in Australia. I am interested in trying to obtain overall electrical and overall gas and/or coal energy consumption per tonne of product for a plant of approximate capacity of 1200 kt/year.
Answer:
There is a wide range of electrical and thermal energy consumption in the dry process cement kilns in Australia. Indicative consumptions would be 110kWh/t cement produced and 3600 kJ/kg clinker produced.
Reader’s comment:
Typical figures for new plants could be 85 units/ton of cement and 710kcal/kg clinker for plants built in India. I believe it should be similar in Australia too as the principle cement plant equipments have reached almost parity across the continents.

Question 167:
I would like to know about the raw mix calculation of portland cement and sulphate resistant cement.
Answer:
Cement raw mix chemistry is characterised by ratios of the oxides present in the raw mix. These ratios relate to the combination of the oxides in the cement clinker minerals after burning in the cement kiln. The important ratios are:

The lime saturation factor – there is a maximum amount of basic lime, CaO, which can be combined with the acidic oxides SiO2, Al2O3 and Fe2O3 in the cement clinker minerals. The higher this lime saturation the more lime will be combined up to maximum of 100%, and the more C3S mineral will be present and the higher the potential strength of the cement. Above 100% lime
saturation the lime is present as free lime and detrimental to the quality of the final cement.

Silica modulus – this is the ratio of the amount of SiO2 to the sum of the Al2O3 and the Fe2O3. This controls the amount of strength developing silicate minerals in the clinker. Higher silica modulus means higher strength.

Alumina modulus – this is the ratio of the Al2O3 to the Fe2O3. This controls the amounts of calcium alumino ferrite and tri-calcium aluminate present in the clinker. This is the important factor for making SRC cement. This cement must have a low tri-calcium aluminate content and this is achieved by reducing the alumina modulus to below 1.
Question 166:
We are using raw mix with heterogeneity problems. Can we add fluorspar to improve burnability and notarisation characteristics?

Answer:
You can certainly add fluorspar to improve burnability. I do not think it will assist nodulisation however. The key to nodulisation lies in the alumina modulus – if you can increase the amount of alumina in the mix you should achieve better nodulisation.

Question 165:
I am facing problem of high residue (9% on 90u sieve) of cement. The blaine of cement remains between 2700 – 2800cm2/gram. What could be the possible causes and its solution?
Answer:
Provided the test results are correct you have a problem of insufficient recirculating load to the mill. You need a higher returns and can achieve this by adjusting the separator.
A US reader writes:
The best way to regulate the residue and blaine is to consider them two different regulations. We have a Fuller O’Sepa separator and we use the separator damper to regulate the residue and we use the separator speed to regulate the surface area. This works quite well.
Question 164:
Normally we change the dipper pipes of preheater cyclones every year, but more recently, the life-time of those pipes becomes shorter than before. We’ve had the problems of dipper damages like being transformed and of clogging in the meal chutes caused by broken pieces of dippers. We started to burn the secondary fuels in the meal chutes of the lowest
cyclones and in the calciners like waste tires and plastics, rubber, etc. I think it can be a reason that dipper pipes should be damaged and eroded with higher temperature and the higher content of corrosives like sulfur and chloride. What are the ways to avoid such problems?
Answer:
This is quite a common problem. I suggest you try stainless steel dip tubes.

Question 163:
In choosing between a direct, semi-direct and indirect petcoke firing system could you please comment on the following. Except for the simplicity and the evident gain in capex, what can justify the choice of a direct/semi-direct system knowing/assuming that (and please comment if any of these assumptions are unjustified):
-The fine grain size of petcoke reduces the efficiency of a cyclone separator battery. The clean gas flow will consequently contain a significant dust-load.
As kiln gases are not available for drying in this case, the recirculating gas must be heated by an HGG at a temperature of 300-350 degree C, which is considered critical for petcoke dust.
-There is a potentially higher chance of material clogging in the recirculating duct especially at the mill inlet.
-The use of wet drying/grinding air as primary air reduces flame temperature and may cause problems especially with low-reactivity petcoke.
-The burner diameter and weight is significantly increased.
-The diameter of the coal/petcoke conveying duct will be significantly increased which may cause difficulty in installation in a satellite kiln, as is the case.
-To keep sufficient excess air rate to the burner, the airflow through the satellites must be limited causing insufficient clinker cooling. This may however be compensated by upgrading the kiln ID-fan. <BR><BR>Could you furthermore comment on the choice of mill-type (ball mill or roller mill), when grinding low HGI petcoke (though not fluidised). It would also be interesting to have some information regarding the correlation between HGI and kWh/t consumption (which is difficult to establish in general terms), and more particularly which kind of increase you would expect when moving from HGI 50-55 to HGI35-40 at a 90micron sieve-residue of 5%.

Answer:
You raise a number of potential problems with indirect firing systems for your kiln. I am sure your concerns are justified; however NOx emissions might be an over-riding consideration. With a direct firing system and a high proportion of primary air then high NOx emissions are almost inevitable. I take you have no precalciner as the kiln has satellite coolers. Perhaps you have secondary fuel firing behind the burning zone somewhere? If not then a direct firing system will probably mean too high NOx emissions.

You are also considering a ball mill or vertical mill for grinding the petcoke. I can say that the vertical mill will certainly be the most efficient but I will need to try to find some data to provide a correlation between HGI & kWh/t power consumption.

Follow-up:
Thank you for your response. I omitted some pertinent data and these details are as follows: The kiln in question is a 1000tpd wet-process kiln so evidently there is no precalciner. Currently this is being fired with 100% heavy fuel oil, as there is no fuel grinding installation presently installed. There is no secondary fuel firing. Our intention is to install either a semi-direct or indirect firing system, but for financial reasons and also simplicity we are looking into the possibility of a direct system. The concerns I outlined are potential drawbacks of direct firing, and there is little doubt in my mind that an indirect system is by far the best alternative.

Regarding the mill type, our original idea was to install a vertical mill. However suppliers we have contacted seem reluctant to guarantee the output of a vertical mill when grinding low Hardgrove petcoke without seriously over-dimensioning the mill (i.e. they propose 20-25tph mills when the necessary production is 12tph), and are therefore suggesting ball mills as an alternative (the reason we are not considering coal is the raw material chemistry which simply does not permit coal firing).

Question 162:
I would like to know what is the normal number of people working on new cement plants. I understand there is a relation with the capacity of the plant so a relation of workers per million ton could also be useful.

Answer:
The most efficient plants have manpower productivity of more than 10,000 tonnes per man-year. So a 1 million tonne per year factory would have 100 people. There is a vast range however, and many factors to be taken into consideration; number of kilns, age of equipment, skills and training of workforce, availability of support services.

Question 161:
In a new 2.5 metre dia.white cement kiln being installed can we use 100% combustion air from the primary air fan. What should be the capacity of Primary air fan in air flow rate/hour and suitable primary air. The total combustion air requirement is close to 12 Nm3/kg of fuel. How much extra air
flow capacity the fan should have over and above the theoretical combustion air requirement of 12 Nm3/kg fuel. Clinker cooling will be by water spray in rotary cooler. The fuel is crude oil having calorific value 10,400kcal/kg. Will it be better to preheat the primary air?
Answer:
This is quite complicated without process diagrams. The designs depend on whether it is Onoda, BSH, FLS or someone else. You should deliver all the combustion air through the firing fan as you do not want to pull any steam into the kiln from the quenching water sprays. I would also recommend preheating
the combustion air to reduce fuel consumption and/or
increase output.

If you are having problems you really need to do a full combustion audit. It is almost impossible to advise on these things without much more information. White cement production is of special interest to me and many of our readers. We would be pleased to help further if necessary.

Question 160:
What is the heat flux emitted from the surface of a 11.5ft diameter wet process kiln with a kiln surface temperature of between 620F on the fire end and 350F on the cooler end (100 ft span of a 450 ft kiln). The kiln in question is being considered for a waste heat recovery system utilizing stationary half-moon arrangement parrell coils along the upper half of the kiln raised inches from the rotating surface with a cover above the tubes. I am having trouble understanding if this is a free convection, radiative or conductive heat transfer problem?
Answer:
I think you have to use a combined radiation and convection heat transfer coefficient, but there are complications. Your stationary half moon parallel coils mounted inches from the kiln surface mean you do not have radiation to ambient, i.e. the presence of the coils will tend to reduce the radiation heat loss from the kiln. You also don’t have convection to ambient, which will have the same effect of reducing the convection heat loss from the kiln.
Convection heat transfer arises because air at different temperatures has different densities. The density of air is also affected by elevation therefore that has to be taken into account.
I would use a spreadsheet model to estimate the heat transfer in 1 metre sections along the length of the kiln where you will install the coils. Dependent on the arc of the half moon you then have an area of the shell radiating heat for each metre section. A shell temperature can also be assigned to each section. The heat flow in watts for each section would then be:5.67 x 0.9 x Area x ((Shell Temp + 273/100)4 – (Ambient + 273/100)4)The temperatures/100 are raised to the power 4. The temperatures are also centigrade, and the area metres squared. (We have spreadsheets available to better explain this)In fact, this is going to overestimate the heat transfer. The coefficient 5.67 needs to be lower because of not radiating to ambient. The spreadsheet can also cover convection loss in each 1 metre section.1.4 x Area x (Air density ambient x Air density shell temperature x (Shell temperature – ambient) 1/3 Area in metres squared. Air density in kg/metre cubed. Density and temperature expression raised to the power one third.

Question 159:
Could you tell me what’s the side effect of the alkalis in the E.P. filters, I’m an electrical engineer filter manager in one international cement companies and I would like please know what’s the optimum chemical composition of raw mix % gas of by pass ,the kiln feeding is 270t/hr the speed of by-pass fan is 15-30 %,gas flow 250,000 m3/hr,temperture inlet of condition tower about 400-500, outlet temp. about 110- 150 after we open windows in the duct between outlet C.T. and E.S.P.filter
Answer:
Alkalis have two side effects on the operation of an ESP. Alkali dusts tend to be very fine and therefore are difficult to precipitate. If you analyse the composition of the dust collected in the successive banks of the precipitator you will probably find that the alkalis concentrate in the final banks. The second effect is that they are hygroscopic and can tend to be sticky and difficult to rap from the collection electrodes. This is likely to be the case with your esp as it is dedusting a bypass system where the chloride content of the dust is likely to be high. You must be careful to ensure that the temperature of the gas exiting the precipitator is above the dew point. 110 degrees C at the conditioning tower outlet seems to be very low.

Question 158:
We don’t have the conditioning tower and we quench hot gas by air cooling. I think most of all process conditions are good and stable, but the only problem is clogging in the pneumatic transportation tube. Now we don’t have hydration problem anymore after some modifications. If you have any design standards for transportation fine dust contains chloride, please let me know. Maybe, is there any possibility that fine dust containing volatile matters becomes sticky?
Answer:
Dust containing alkali chlorides certainly does become sticky. In electrostatic precipitators it can be difficult to rap the collected dust off the electrodes for this reason. Even though you do not have a conditioning tower there is significant water content in the combustion product gases and also in the atmospheric air you are using for quenching. To prevent clogging you could try the installation of sonic cleaners in the pneumatic feed hopper.
You have done very well with your grinding aids! There are many complications in answering your question. There is the difference between apparent surface area and true surface area when the cement is used in concrete. When you measure the Blaine the cement tends to agglomerate due to inter-particle charges and give a lower than true surface area. When mixed in concrete these interparticle charges might be negated by the polar water molecule. Grinding aids reduce inter-particle charges, preventing agglomeration and making the apparent surface area closer to the true surface area. The real question is are you getting the same strengths with the 25% higher mill output after introducing grinding aids?
Question 157:
In our cement plant we use grinding aid materials in the cement mills. These materials (in small volume) have increased the cement mill output for about %25 with blaine 3500. My query is: When the output with relatively low cost materials can be increased, Why the high efficiency separators are introduced to cement plants? I would like to know your suggestion about the advantages of each method.
Answer:
High efficiency separators increase the output of the mill because they reduce the fines recirculating in the mill. There is less over-grinding and therefore less agglomeration. The two technologies of high efficiency separators and grinding aids used together will push up the output of the mill even more. However, the combined effect will not be completely cumulative, and on balance I probably agree with you that grinding aids are the more cost effective solution for an existing cement mill.

Question 156:
We are planning to produce pozzalanic cement (as a new cement product),could you please indicate the following information: How to handle the high moisture content in pozzolana as it is ca.10%. How to handle the high fineness. Is it necessary to introduce a drier.
Answer:
There are a number of options to handle the high moisture content:
1: You could install a dryer with hot gas generator, or utilise some exhaust air from the cooler or preheater.
2. You can dry the pozzolan in the mill by installing a hot air generator or sweeping the mill with exhaust air from cooler or preheater.
3. You can add the material to the mill with the clinker and gypsum and rely on the heat generated in the mill to dry the pozzolan. The addition rate of pozzolan will then depend on the ambient temperature and the clinker temperature. This will be the lowest cost option but temperature control in the mill will become a critical factor. One option to produce the high fineness required would be to separately grind the pozzolan and then mix or blend with the cement. You should also consider the cement quality. Pozzolanic cements usually suffer high water demand and poor workability. This can be improved by the addition of functional additives in the grinding mill.
Question 155:
In the recent past lump formation and resultant silo blockages was experienced by one of our cement industries in Southern part of India. Though the cement production parameters were not changed in any of the sections of cement plant, including raw materials and grinding aids they use, this phenomenon occurred. After going through all the possible reasons for this problem, I personally feel that some incompatible started
developing between the clinker and the grinding aid.

Answer:
It is conceivable that your problem concerns an adverse reaction between grinding aid and clinker, but I doubt if that is the root of the problem. Lump formation is quite common in silos and it is nearly always caused by the dehydration of the gypsum, added in the cement mill, continuing after the cement is blown into the silo. The solution is to improve temperature control in the cement mill and cooling of the cement in the separator.

Question 154:
In addition to furnace oil, coal and gas what else do you suggest can be used in kiln for heating purposes as fuel?. What do you say about TDF (Tyre Derived fuel) being used as a fuel?

Answer:
Many alternatives to traditional fossil fuels are being used to fire cement kilns, tyres, waste plastics, household waste, bonemeal and animal fat, sewage sludge, paper sludge. waste wood, rice husks, spent pot liner, etc. The advantages are that they reduce the overall energy costs of cement manufacture, while at the same time providing a disposal service for society. The disadvantage is that they can contain trace materials not present in traditional fuels, and can also be of variable composition,
leading to process disturbances and possibly the need to install by-passes and other special equipment.
Question 153:
We are currently investigating the effects of curing temperature on strengths of locally manufactured cements. The local practice of testing is following the old BS4550 practice, we want to adopt an higher temperature testing regime due to the local climate. Do you know of any studies performed on cement strength testing at higher temperatures than 20 deg C?
Answer:
The US Corps of Engineers conducted an extensive study of elevated temperature in mixing water and aggregates and concluded that this provided a reliable means of accelerated strength testing for quality control of concrete. Ref: J.F.Lamond, J.ACI, Proc., Vol 76, 4, pp 499-512, 1979.

Question 152:
We are proposing to make Portland Pozzalana Cement (PPC) by mixing ordinary Portland cement (OPC) and fly ash. We wish to add 65-70% OPC with 35-30% fly ash. We are receiving fly ash at about 2000 blaine and we would like to grind it to about 4000 blaine in the ball mill. What will be the power consumption required for grinding fly ash from 2000 blaine to 4000 blaine?

Further when we add 30-35% fly ash (OPC containing 5% gypsum) do we have to add gypsum in the blender for PPC?

Secondly In one of the cement plants there was a severe problem of castable/refractory lining failure in the kiln outlet. In order to solve the problem the conical portion of the kiln has been removed and replaced with straight portion. Because of this change as well as modification of the cooler with new generation grate plates there is problem of “snowman” formation in the cooler. What could be the reason for the same?
Answer:
Why do you want to grind the fly ash? One of the primary benefits of fly ash as a pozzolanic additive to cement is that it is composed of glassy spheres. These spheres increase the workability of the blended cement and reduce water demand, thereby offsetting loss of strength incurred by adding fly ash.
If you grind the fly ash you will destroy the spheres and lose this benefit. If you must grind the fly ash then you do not need to add gypsum. The gypsum is added to retard the setting of the C3A in the cement clinker. There is no C3A in fly ash.

Snowman formation is a common problem with static first grates at the front of coolers. These plates rely on the formation of a layer of cooled clinker to protect the static plates, and this layer of clinker can become too thick and lead to formation of snowmen. Also the higher secondary air temperature leads to more liquid phase in the clinker entering the cooler and a greater
stickiness of that clinker and tendency to form snowmen.

Big blasters or sonic cleaners are sometimes installed to combat the formation of snowmen. In Japan one cement company have developed a “pusher” to hydraulically push the snowmen onto the second grate.

Question 151:
I did a study in the finish grinding in the plant that I work and I calculated the air swept that we actually have. I need to have a reference to confirm the best or optimal air swept that we must have inside the mill. A process engineer told me a rule of thumb and that there are some of them, one of them is that we need to have 3 changes of air inside the mill per minute, and the other is that the air velocity must be between 0.5 and 1 m/s inside the mill. The system is closed circuit with bucket elevator.
Do you have any guidance?
Answer:
The objective of ventilating the mill is to keep the outlet temperature below 120 degrees C and avoid complete dehydration of gypsum and false setting of the cement. Problems of coating of the media might also be encountered at higher temperatures. You need to develop a heat balance of the mill to find the volume of air required to maintain this temperature. Important considerations are the temperature of the clinker feed to the mill, the dimensions of the mill, media weight and installed drive power. Because of these variables it is not really possible to have a “rule of thumb”.

In large diameter mills the volume of air required might be so high that the air velocity required through the mill exceeds 2 m/s. This is unacceptable as coarse particles will be swept through the mill without grinding. FLS recommend a maximum air velocity of 0.8 m/s in the mill. Other sources say a higher velocity is acceptable but this depends on whether there is a static separator in the air circuit returning these coarse particles to the elevator and separator feed. When the air velocity through the mill is too high then it is necessary to introduce additional cooling such as internal water sprays in the mill.

The heat balance is the key to allow you to make these decisions and properly control the mill.
Question 150:
Our kiln is wet process with 300tpd clinker production. We feed slurry equal to or more than 35 per cent moisture using warman centrifugal pumps. Are there pumps or systems that can feed slurry of less than 35 per cent down to 30 per cent moisture?
Answer:
Yes there are slurry feeder, which can deliver slurry of 30% moisture. Many cement companies operating wet process kilns are using water reducers to improve their fuel consumption. If you add lignosulphonate to your slurry you will be able to reduce the water content to 30% and your Warman pumps will still be able to deliver the slurry to the kiln.
Question 149:
We run a 20tph open circuit finish ball mill, which does three different types of cement. Our mill has a history of filling up both in the first and second chamber, generally attributed to changes in grindability. We normally monitor the mill ‘filling’ by monitoring the mill kilowatts on-line. However, usually by the time a change is apparent in the mill kilowatts, it’s usually too later to take corrective actions. We know that other plants use a sonic ‘ear’/device to monitor the decibels versus the filling level. The questions I have are: Is the ‘sonic’ ear superior to monitoring the kilowatts? Is it more sensitive and can it give earlier warning of changes in filling levels? Is the equipment identical to standard sound meters or does it monitor a particular frequency etc? Are there any preferred suppliers for this equipment in the cement industry?
Answer:
Does your open circuit mill have only 2 chambers? Often these mills have 3 or 4 chambers and this makes a difference to my answer. An electronic ear is superior to the kW drawn by the mill main motor because it allows you to monitor the grinding conditions in a particular chamber of the mill. Whereas the kW drawn by the mill main motor tell you the overall grinding conditions in the whole mill rather than specific chambers.

The equipment is not the same as normal sound meters. It has to be mounted right up against the mill shell and the stability of that mounting is critical. I have seen installations, which are useless due to incorrect positioning. Another drawback is that they are expensive. I believe Schenck supply these electronic ears and that there are other suppliers.
Question 148:
What are the specifications of pozzolana oil well cement?
Answer:
I am unaware of a particular specification for pozzolana oilwell cement. The addition of pozzolans can produce some of the retardation of the setting and hardening of cement, which is demanded by API classes A-C and G,H oil-well cements and therefore meet the specifications. This can be an alternative to
producing a low C3A clinker.

Question 147:
We wish to understand from your expertise that a mix fuel of following composition is possible to be ground in close circuit ball mill(with high efficiency separator) of air swept design?
Fuel Ratio South African Coal: Pet coke: Lignite is at 50:25:25.
Moisture: 12% of total feed.
Product fineness: 7% Residue retained on 90 micron sieve.
The hot gases are from cooler (fresh air).
Please indicate what precaution for the above is to be taken?
Answer:
The first question is whether the petroleum coke you intend to use is “green delayed” or “fluid”. If it is green delayed you can grind it with your coal and lignite, and should expect about 20% reduction in the output of the coal mill. All safety and explosion protection as per conventional coal milling should be followed. Fluid petroleum coke is too hard to grind in your mill and should be blown into the kiln as received.
Question 146:
We run 1650 t/d dry process plant and have been having problems with 4 stage preheater tower blockages on start up. All of the stages are checked for blocks by doing a ball test but we are still having problems particularly in stage 2. How can we prevent this? Is there other ways to check for partially blocked cyclone? What do other plants do before a start up?
Answer:
There does not sound to be any problem with your pre-start procedure. Other factories are doing the same sort of thing. The problem is more likely connected with the time when feed is introduced to the kiln. The conditions and procedure for heating up and feed on should be reviewed. Stage 2 is very high in the tower to experience these problems. Are new materials or fuels being used? Do these contain any chlorides?

Question 145:
Could you please inform me what is the side effect of the poor gas distribution in the E.P. filter ,the filter normalized negative pressure, the speed of E.P. FAN IS 750rpm, the flow rate 400,000 m3/hr,the gas temperature 110-115 degree C when the mill is running, 140-160 degree C when the mill is stopped.
Answer:
The effect of poor gas distribution in an ESP is higher dust emissions. The cross-sectional loading is uneven and some parts of the filter are overloaded. If you have problems when the mill is stopped I do not think this is the cause. Poor gas distribution will cause higher emissions irrespective of whether the mill is running.
Question 144:
We have to control the quality of cement within certain range. Say for instance:
Residue: 8%
Blaine :3500
SO3 : 2.6
The result of SO3 is ok.

But the main problem is how to control residue and blaine simultaneously. When we get blaine 3480 or 3467 then residue is 6.7% or 6.3%. Again when we receive the residue 7.8% or 8.2% , then the Blaine is 3206 or 3146. Normally we run the ball mill with a separator speed of 90%-86%, New feed 75tph,and return 120tph. Sieve Size for fineness test:
45 micron, 325 mesh
Our plant is running through PLC. Air volume through air separator 105,500 m3/hr. Air volume through Mill 1365 m3/hr. The product residue we want to aim at for best cement quality : 8% Can you help how I can control with my desired level of residue with exact ratio of Blaine so that I can control
quality of Cement.

Answer:
In order to independently control the Blaine and residue you need to vary the recirculating load, i.e. the amount of returns to the mill. You do this via the separator speed or drafting, with compensating changes in the feed rate. An expert system is the best way to do this.

Question 143:
My question is how Br, contained in kiln feed as minor component, affects kiln process and clinker quality. As the results of analysing the lump of coating drawn from SLC calciner, the content of Br is about 3%. But I can’t find any articles on the effect of Br. Can you let me know about the effects of Br and the possible paths to be introduced into kiln process.
Answer:
Bromides will recirculate and build up in the same way as chlorides. Therefore only a small input to the kiln will concentrate up to the 3% you have experienced. Bromides are rare in cement raw materials. Are you adding any waste fuels, recycled oil or wastes from the photographic industry? These are the most likely sources of the bromide.
Question 142:
We are a group of students who have been assigned the project of trying to find out solutions for a particular problem the cement industry we work with has been facing. The production dept. wants to reduce the double handling of the raw material that comes in from the mines. They want to optimise the money spent in the production of cement. They want to reduce the time and money spent in unloading and reloading the raw material and then take it to the necessary place like the crusher or wherever. They also would like to reduce the idle running hours of the above machine and the mills. Kindly suggest a solution for this problem.
Answer:
Reducing double handling of raw materials at the crusher is a matter of tipping directly into the crusher rather than to stockpiles. In order to be able to do this the operations of the quarry must be matched with the requirements of the factory. This involves integrating the management and control systems of the quarry and factory. Reducing idle running time of crushers and mills is again a question of “balancing” the line. The capacities and running hours of the successive stages of the process need to be brought into line.

Question 141:
What is the composition of copper slag used in the production of cement and at what ratio it can be used?
Answer:
A typical copper slag composition would be: SiO2 34%, Al2O3 6%, Fe2O3 40%, CaO 2%, MgO 1%.

This would material would be used as an iron additive to adjust the alumina modulus in the raw mix and the flux content in the kiln. The amounts used would depend on the composition of the other raw materials and target for the alumina modulus, however I would expect the addition rate to be around 1% of raw mix.
Question 140:
I am working at 3300tpd OPC plant. Our company has starting coal feeding at calciner due to higher cost of furnace oil. This modification has led reduction in furnace oil from 8.0cum/hr to 6cum/hr. We are facing small problem of build up in kiln inlet smoke chamber, which we have rectified through raw meal composition. Our management wants to increase the coal feeding, so my question is how much coal we can fire maximum through one inlet port safely. Currently we are feeding nearly 4t/h.
Answer:
This is a difficult question. Is the coal ground to a powder or being added as lumps? If lumps what is the size distribution? The safe amount that you can add in this way is the maximum amount, which does not lead to process problems, so I suggest you increase the amount until you start to experience problems. However, as you have already experienced a problem with build-up in the smoke chamber, I suspect that you are reaching the limits of what is possible already.

Question 139:
I’m working on a project to evaluate the costs of some major equipment when they experience shut downs, In other words how much does it cost me to shut down a raw mill system per hour, as an example. What are the most important issues that I have to take care of, in your opinion, besides energy consumption? Thanks.
Answer:
This is a little difficult. Raw mills usually have excess capacity over the kiln in order to allow shutdown for periodic maintenance while the kiln keeps running. In this situation there are no major costs associated with the shutdown. Probably the lubrication and hydraulic pumps need to run on for a period. Also more power is drawn when the mill first starts. There is then the consideration of the conditioning of the kiln exhaust gases by passing through the raw mill in compound operation. There are many factories where direct operation when the mill is stopped increases emissions and can lead to an output penalty on the kiln. These factors are very kiln and factory dependent. Certainly you would want to minimise unnecessary stops for stable operation of the factory.
Question 138:
We installed the chlorine bypass system in the riser duct of the ILC calciner-kiln system, and it is under test operation now. We normally put the collected dust from the bypass system into the end of the clinker cooler by using pneumatic blower. As the results of XRD analyses, those dust contains about 30 % of Cl, and CaO, K2O,etc. But we have a problem of clogging in the inclined area of collecting hopper just before pneumatic blower and in the inclined area of the plastic transportation tube between the blower and the clinker cooler. XRD analysis of agglomerated dust shows it contains Ca(OH)2 instead of CaO. We also see the electrical sparks inside the inclined tubes because the tubes are transparent. I think those sparks were caused by static electricity as in the impellers of the preheater fan. Please advise me of the possible causes and the countermeasures we can do.
Answer:
Regarding your bypass dust it is clear that the free CaO in the dust is being hydrated. This might occur in the conditioning tower ahead of the bypass precipitator. Do you have a conditioning tower and electrostatic precipitator on the bypass exhaust gas stream? If so then you should look at the temperature control of that conditioning tower. Perhaps there is excessive water spray? Or the atomisation of the spray is insufficient? Do you suffer wet bottoms in that conditioning tower? If no conditioning tower then maybe the hydration is taking place in the bypass quenching chamber. Again the amount of water spray and the atomisation of the water should be investigated.
Question 137:
I am interested to know how can we get the best flame in a white clinker kiln with natural gas. Our system is in a long wet kiln, we don’t have a preheater nor precalciner. Our kiln is FL Smidth. We are designing a natural gas burner and we are thinking to supply the total combustion air through the burner. The air distribution is: Radial air: 61% of total combustion air at ambient temperature. Axial air: 35%. Internal air: 4%. We quench the clinker after the kiln, in a water screw conveyor.
Answer:
Supplying all the combustion air through the burner is a good idea. I would recommend that you use a variable speed fan and increase the amount of combustion air with the amount of fuel. It would be good if you can preheat the combustion air to 200 degrees C in some way to avoid cooling the flame. I have experience of quench cooling clinker in a water bath with screw conveyor. It can be a good idea to arrange two baths in series with a screen between the two baths to allow fine material to bypass the second bath. This ensures that any larger nodules of clinker are adequately quenched. You will need to think about the safe discharge of the steam generated from the water baths. Also arrangements for drying of the clinker after the quenching. You also need to avoid steam from the water bath entering the kiln through the kiln discharge throat. A refractory lined flap or curtain of chains across the discharge throat can be used. An alternative would be to use water sprays in the kiln discharge throat to quench the clinker to below 800 degrees C. This semi-quenched clinker can then be cooled in a rotary cooler with no necessity to dry the clinker. The exhaust gases from the cooler can be used for firing the kiln. The engineering is more complex but the thermal efficiency will be better.
Question 136:
I have a question on the recycling of spent catalyst from petroleum refinery Fluid Catalytic Crackers in cement manufacture. As you may be aware, spent FCC catalysts may contain heavy metals in concentrations ranging from 300 ppmw to 10,000 ppmw.<BR>How widespread is this practise and is there a limit on the heavy metal content of spent FCC catalyst recycled/accepted in cement units? Does the ISO product specification for cement have anything to say on its heavy metal content? Could you point me to published papers or information on the net on this matter.
Answer:
Cement companies would certainly impose limits in the heavy metal content of any alternative fuels. This is the requirement of the permits to burn such fuels. Product specifications for cement do not define heavy metal contents. As you are based in the US you should be aware that The PCA have published a report on heavy metal contents.
Question 135:
I work in a 1500 tpd cement plant. We are interested in burning tdf. We want to know the necessary equipment to produce tdf and to introduce it into the kiln.
Answer:
There are many ways to introduce tyre derived fuel into your kiln, and many suppliers who can provide you with the equipment to do this. I suggest you look at www.recycle.net. This then will lead you through to equipment suppliers in South America.
Question 134:
In our cement plant, we are planning to add a bypass system for the burning of alternative fuel, which contains chloride. We would like to ask for your advise on the dust loading figure of the gas stream coming out from the kiln. The dust loading figure we have now range from 250g/Nm3 to 750g/Nm3. Our plant is a 2 streams 4 stages preheater with RSP Precalciner.
Answer:
The dust loading of the gas exiting the kiln will depend on the feed rate to the kiln and the condition of the refractory on the kiln feed shelf. Also whether there are any build-up or rings in the kiln inlet. This will be why you have a range of dust loadings. How have the dust loadings you quote been measured? Or calculated? Anyway the dust loading of the gas entering the bypass will be the same as that exiting the kiln.
Question 133:
Can you comment on the effect of high alkali in the raw materials for analysis using Cross belt analysers ? If so to what extent and what is the solution ?
Answer:
We need more information. Who is the supplier of your cross belt analyser? What do they say about the effect of high alkalis? Possibly an analyser manufacturer may wish to comment?

Question 132:
My question concerns clinker cooling. I know about water sprays and the effects they can cause. However I have also read about the use of high pressure fogging systems that are used on the intakes of gas turbines to cool the air. They also talk about fogging which raises the humidity to over 100 per cent. It appears that fogging can help but it is unclear how. Possibly a second flash evaporation occurs as the temperature rises. Do you think that this technology could be beneficial to cool clinker if it was used on the inlets of the cooler fans, particularly so in the colder sections of the cooler.
Answer:
I have not read about high pressure fogging of the inlet to gas turbine engines. From your question I think you have a grate cooler? I would be worried about fogging on the inlet to the cooler fans of a grate cooler. The ambient air temperature in the chamber beneath the grate is not high enough to prevent condensation. Water might run down into the cooler drag conveyor and react with the clinker dust there, causing build-up and possible damage to the drag and a stoppage of the kiln and cooler.
Water sprays are sometimes used on the inlet of planetary and rotary coolers. This is because all the cooling air is drawn into the kiln as combustion air. There can be a tendency for high clinker exit temperature with these types of coolers as the volume of air required to cool the clinker is in excess of that required for combustion (i.e. not enough cooling air so add some water). This should not be the case with a grate cooler where the excess of cooling air is separately vented through the cooler exhaust fan. Sometimes water is sprayed above the clinker bed and grate to condition the exhaust gases before electrostatic precipitation.
Are you suffering with hot clinker from your cooler? If so please send more details of the cooler and I will try to help. (PS Your email was corrupted, we do not have your full address – please resend to receive further more specific guidance).

Question 131(1)
We face a peculiar problem in the Raw Mill and Cement mill where the vibration on the free end (or) the non drive end bearing is high on the axial direction and sometimes on the vertical direction. Can you please highlight the reason/ cause, which gives this vibration, and the remedy, considering all factor like foundation, alignment are OK.
Question 131(2)
There is also an argument that the spray system for lubricating the girth gears are costlier both on installation and also on running. Can the Dip system be introduced like olden days. What is the main reason for big cement machinery, manufacturers to recommend the spray for lubricating the girth gears?
Answers:
I am not a mechanical engineer so not really able to answer your questions. I suspect your non-drive mill end bearing vibration requires a specialist inspection to identify the problem. My understanding is that sump lubrication for girth gears has the following drawbacks: (i) limited to low rotational speeds, (ii) debris builds up in the sump and can lead to increased abrasive wear. The later problem can be alleviated by side stream filtration. Modern, enclosed spray systems for lubrication are also preferred for health, safety and environmental reasons.
Question 130:
What are the basic and essential works to check before we start a cement mill without any load on the main motors.
Answer:
This is very difficult to answer without more information about the mill, really you should refer to the mill suppliers instructions. It is normally necessary to ensure that all the hydraulics and lubrication systems are fully operational before loading the main drive.

Question 129:
I would like to ask you a question regarding fly ash(dry) feeding into cement mill. I want to feed dry fly ash into cement mill. I have one old cement silo ,which can be converted into fly ash silo. The distance between this silo and cement mill is 35 meters. The existing extraction system: one manual gate, one pneumatic gate and one flow control gate can be used for extraction. After extraction we are planning to install vertical conveying system to convey fly ash(15 TPH) up to 15 meters, from discharge point of vertical conveyor a small air slide shall feed the material into aerated calibration bin mounted on load cells with one set of extraction gates below which the solid flow meter shall be installed , from which the material shall be conveyed through 30 meters air slide into cement mill directly. Our main reason to choose air slide system is to reduce power consumption ,when compared to FK pump, dense phase or lean phase conveying. Now I would like to have advise regarding vertical conveying system. Can I use Belt bucket elevator or vertical screw conveyor for fly ash vertical conveying without any problem? What design precautions should be taken ? will I face maintenance problems? Should I use some other conveying system?
Answer:
You should not face any problems if you use a combination of bucket elevators and screw conveyors to transport your fly ash. No special precautions, other than those demanded by good engineering principles, should be required.
Question 128:
Could you please comment on the effect of burning waste rubber tyres in cement kilns has on heavy metal and dioxin emissions? There seems to be some contradiction as to whether an increase or decrease in levels is observed.
Answer:
We do not think there is any increased heavy metal or dioxin emission when burning waste tyres in a cement kiln. This practice is widely adopted in Europe and the USA where emissions regulations are particularly severe. We cannot believe that permission would have been granted if there was any possibility of heavy metal or dioxin emissions. These emissions are rare from cement kilns. The few instances of dioxin emissions are usually traced to organic materials in the raw materials. Instances of increased heavy metal emissions have been traced to the burning of household waste in cement kilns.

Question 127:
We have seen higher than expected emissions of volatile organic compounds at a precalciner kiln. The problem is believed to originate in mill scale used as raw material. It enters as part of the raw meal into the preheater. Are there types (origin, processes, etc.) of mill scale to stay away to improve VOC emissions? Might feeding the mill scale directly into the kiln be preferable. After all, tires, which are a good source of iron, can be rolled in at the cool side of the kiln? Obviously some energy penalty would be expected.
Answer:
Mill scale is added to cement raw mix to elevate the iron content of the mix and adjust the chemistry. It is entirely possible to find sources of iron oxide, which do not contain this volatile organic contamination. The best route is to specify and select sources, which do not suffer this problem.
If you have to take this contaminated mill scale then feeding it directly to the kiln would certainly eliminate the THC emission problem. As you say this would have adverse effects of the kiln operation, mainly due to the iron not been homogenously integrated into the raw mix. You will be addressing the symptom rather than the problem. Better to eliminate the problem we think!

Question 126:
For control of inventories and material balance purposes in a cement grinding plant, we need to know the norms and figures for clinker and cement loss or wastage in different steps of cement production, storage & dispatching(specially in clinker yard, clinker grinding plant, cement storage, bulk loading & packing plant). We know that these norms vary from plant to plant and mostly depend on design and existing condition of plant. Of course in a new and modern plant with dust free material transportation, storage, bulk loading, packing facilities, etc. It can be very little and minimum. On the other hand, in an old factory with old machinery and material spillages everywhere, it should be very high or maximum. For this reason we describe the situation in the referred cement grinding plant as follows:
Clinker storage: Open yard without concrete or asphalt pavement.
Clinker and gypsum yard transport: Un-covered conveyor belt. Cement transport from mill to silos: Continuous pneumatic conveyor(Fuller pump).
Cement transport from silos to packing plant: Covered conveyor belt.
Bulk loading facilities: Bulk loading spout underneath the silos, without dedusting facilities.
Cement silos:4 cement silos consisting of 2 x 500 tons and 2 x 2000 tons
Dedusting facilities: Cement Mill, cement silos and packing plant are equipped with bag filters and performing in a acceptable condition.
Answer:
You ask a very difficult question! Certainly material losses can be almost eliminated on a modern cement factory. Dependent on the type and condition of the equipment this might rise to as high as 10% on an older plant. From your description of your plant the main problem areas are likely to be the open storage of clinker and the open transport of clinker from this storage to the cement mills. Inventory control must be a question of weighing the materials into and through the process and cross-checking the inventories by physical stocktaking on a monthly basis. We will be happy to help you further, but need more information to do so.

Question 125:
Our project is to transform our kiln electrostatic precipitator into a bag filter. We will apply for a financial help from the European Union. In order to document my report, I would like to know the status of EP filter in Europe. How many have been transformed or changed yet? How many are still to be transformed. What will be the possible future legislation in Europe about dust emissions.
Answer:
Many EP filters remain in service in Europe, and some new kilns continue to be fitted with EP filters as the method of de-ducting the exhaust gases. The new kiln of Rugby Cement commissioned at the end of 1999 has an EP filter and is located in the centre of the town.
However, your are correct. Many cement companies are having to upgrade their EP filters to meet the latest emission limits. There are two routes which can be chosen: (i) improve the internal layout of the EP and modernise the high voltage control and EP filters are capable of meeting the most stringent emission limits, or (ii) many cement factories are choosing to replace their EP filters with bag houses. An important consideration is the stability of combustion in your kiln. If you are burning alternative fuels, particularly tyres, then there is an increased risk of spikes of CO entering
the filter. In this case with an EP the high tension must be grounded to protect the filter from explosion, leading to an incidence of high dust emissions. This has been one of the main drivers for the conversion of EP filters to bag houses. Many examples can be quoted from the literature to support your application, however we are not aware of any Europe-wide statistics, which you could use.

Question 124:
What is effect of using caF2 in raw material either longdry or kilns with preheaters. Of course the amount of quartz in red soil with 20% consumption is about 30-35%
Answer:
The effects of using CaF2 as an additive to the raw mix will be to mineralise the clinker formation in the long dry and preheater kilns. This will lead to lower fuel consumption and/or higher output from the kiln. The lower temperature of the burning zone will also reduce the cycle of volatile
components and may lead to less restriction in the preheater. CaF2 is therefore a good additive for cement manufacture.
However, the quartz content of the red soil will increase the mechanical wear rate in the raw mix grinding equipment. This quartz will also increase the combinability temperature in the kiln, partially offsetting the benefits of the CaF2.
The CaF2 will also affect the hydraulic properties of the cement. The most pronounced effect will be to lengthen the setting time.
Question 123:
We are trying to find out which would be the optimal temperature and pressure parameters in a ball cement mill to ensure the best cooling process with the use of air sprayed water. And if a psychometric chart would be able to help find this optimum balance point.
Answer:
The optimum temperature in a cement grinding ball mill is quite complex to find. What is important is to control the dehydration of the gypsum to give the optimum cement hydraulic properties (principally the setting time and water demand or workability). You need to find the
temperature, which gives you the optimum of these two properties. The problem is complicated if there are variations in temperature or moisture content of the feed to the mill (i.e. the clinker, gypsum and extenders). There can be particular problems if you are adding pozzolans or granulated blast furnace slag to the cement.

I am not sure that you can develop a chart to help with this. Most cement companies maximise the volume of air sweeping the mill as the first method of cooling. Water injection is then used with a PID loop to control to a mill exit air temperature of between 105 and 115 degrees C. This ensures that you have dehydration of the gypsum.

There is then a potential problem with the gypsum continuing to dehydrate in the silo and forming lumps in the silo or coatings on the silo wall. This can be avoided by cooling the cement in the separator, or alternatively passing the cement through a cement cooler.
Question 122:
In My Country Egypt, by-pass cement causes many pollution problems where its amount 10-13% of total cement production because Our raw materials contain high percentage of alkalis, thus by-pass in some dry cement plant sometimes 100%. By-pass cannot be recycled to avoid decreasing the cement quality and kiln efficiency. What the ideal reuses of these large amount of
by-pass dust do you suggest. Thank you very much.
Answer:
The best solution to dispose of alkali bypass dust is to grind the dust with the clinker in the final cement. This acts in the same way as the limestone, which ENV 197 allows to be added in the grinding of cement. There are some other issues to be addressed if this solution is followed. The alkalis in the cement are likely to accelerate the hydration of the cement. This can be controlled by the addition of retarders such as some modifications of lignosulfonate.
Question 121:
I am in charge of a new project regarding to use fluorite in the raw mix in order to reduce heat consumption, there are many questions about this issue.
Question No. 1, Which is the impact in the kiln operation ?
Question No. 2 Which are the most common percentage of use ?
Question No. 3? Where has been used
Any information regarding to the use of this mineral flux, where can I find more information
Answer:
Fluoride mineralises the clinker formation in the kiln by reducing the combination temperature. This reduction in the burning zone temperature has a number of effects on the kiln operation:
1. Heat consumption of the kiln will be reduced.
2. Output of the kiln can be increased.
3. Nodulisation of the clinker will be improved.
4. Circulation phenomena (alkali cycle) will be reduced, possibly reducing the need for preheater cleaning.
Most common % of use is 0.2% as CaF2. Higher additions can be beneficial but expensive. Has been used at Aalborg (Denmark), Ras Al Khaimah (UAE), Hope (UK).

Question 120:
We produces lime from an impure limestone (96% to 99% CaCO3), which contains the micaeceous mineral, sericite – a very fine grained talc-like mineral. The limestone is burned in a rotary kiln using low ash coal (14%). The kiln is equipped with an electrostatic precipitator. We have concluded that the fine material (100% minus 45 micron) collected in the ESP consists of unburned limestone, burnt lime, and coal fly ash. We have also noted that this material “sets” like cement, but very slowly, when mixed with aggregate at 5% ESP dust addition – 1MPa UCS 7-day accelerated cure, 4MPa 28 day UCS, and 9MPa 120-day UCS. We would be pleased if you could advise us on which minerals you would expect to be forming to give us the pozzolanic set.
Answer:
We think the ESP dust is a form of hydraulic lime. When water is added to the mixture of quicklime / fly ash / sericite, the calcium hydroxide reacts with the silica and alumina in the other components to produce hydrated calcium silicates (di- and mono-) and aluminates. The silicates are mainly responsible for generating the strength.
Question 119:
We are having some corrosion problem in kiln’s shell at upper transition zone. Can you recommend any ceramic coating or refractory to prevent this failure?
Answer:
First you will have to clean the shell to remove the scaling by grinding or grit blasting. Then apply phosphoric acid, followed by plasma coating with aluminium oxide. There are several steps to prevent shell corrosion:
1. Reduce volatiles (K2O, Na2O, SO3, Cl) in kiln feed if possible.
2. Use dense high conductivity refractories in affected area.

Question 118:
It is pure R & D question. I would like to know whether it is possible to activate the alumina portion of the fly ash with gypsum. if so what is the stringent procedure to be adopted. how much extra dosage of gypsum is required to activate the fly ash. As our experience shows that even with 20 % replacement of cement with fly ash we were unable to activate the fly ash with anhydrite gypsum to get very good early strength. What type of gypsum has to be added or is there any organic chemical to activate the fly ash effectively.
Answer:
We cannot see that you will activate the alumina in fly ash with gypsum. In extended cements it is the calcium hydroxide liberated by from the cement minerals, which activates the hydraulic minerals in the extender. There are organic functional additives, which will assist in maintaining the strength development of extended cements. These organic chemicals are lignosulfonates, however there is a potential problem with residual carbon in the fly ash absorbing the chemical and reducing their effectiveness.
Question 117:
We run a 350tpd kiln with a rotary cooler. We sometimes experience brown centred clinker. We have tested chemically for reducing condition. Some of the brown clinker tests positive but at other times not. Are there other conditions or mechanisms that give rise to brown centred clinker aside from reducing conditions, given our circumstances?
Answer:
Brown centred clinker is very common and is not necessarily detrimental to cement quality. It is not necessarily caused by reductive burning – this is better indicated by the CO content of the kiln exhaust gases. At the temperature in the burning zone some of the iron is always in the reduced 2+ state. As the clinker cools this Fe2+ oxidises to Fe3+ however if you have large clinker nodules or fast cooling then the cores may not be fully reoxidised. Fast cooling of the clinker is good for cement quality. On balance it is better to have rapid cooling and some brown cores in the clinker rather than having slow cooling, provided that you do not have CO in the kiln exit gases.

Question 116:
Calling from Ukraine: My question is about an energy tax in cement industry?
Have the Member States introduce this tax (as was promised that it would be introduced by the year 2000? If “not”, what is situation with the energy tax and tax on CO2- emission? Thank you.
Answer:
Our information is that climate change levy will apply to cement companies in the EU from April this year.
Question 115:
What is output and method of spraying water inside of the cooler in a long dry kiln system?
Answer:
We presume your long dry kiln has planetary coolers or a rotary cooler? You need a pump, which delivers a variable volume of water and a water injection lance with atomising nozzles. The amount of water sprayed into the inlet of the cooler is varied to help in reducing clinker discharge temperature. Beware of the effects of the increased moisture content of the secondary air on the flame and emissions. There is a danger of increased CO formation and intensifying the alkali cycle in the kiln.
Question 114:
I am a manufacturer of wear resistant castings in India. I have manufactured and supplied mill liner plates for a cement mill 4.0m in diameter. The material used for the plates is as per ASTM A 532 containing 24%Cr and 2.5%C. After using for about 8 weeks a few plates have cracked at the centre i.e.:- near the bolt hole. I have no knowledge about mill plates design parameter but I am confident about the quality of castings. The failure of plates could be caused by wrong fitment, design of bolt hole etc. Please guide me to arrive at a proper solution .If there is anyone who could provide professional guidance I shall be glad to obtain the same.
Answer:
Design of mill liner plates is a very specialist area. I suggest you contact one of the main mill liner manufacturers. Perhaps somebody may like to add any further comment below?

Question 113:
I am involved in the preliminary stages of a study on the costs of capturing carbon dioxide from large point sources. Design of the (theoretical) capture facility requires a comprehensive understanding of the composition of the stack gases from the kiln. The plant we are designing for is a coal-fired cement plant with pre-heating/pre-calcinating facilities. Is there any available data on the physical properties (temperature and pressure) and composition (particulate levels, CO, Co2, O2, NOx, SOx, etc.) of the stack gases of a representative cement plant? Thank you.
Answer:
A key question is where in the process will the stack gases be extracted? The kiln stack gases are usually used for drying raw materials therefore the extraction would have to be after the raw mill. Dust burden depends on whether the extraction is before or after the filters. Typically the oxygen content after filters would be 10% plus due to in-leak of air, temperature about 150 centigrade. A very approximate composition would be as follows: CO2 17%, N2 70%, O2 10%, H2O 3%.
Question 112:
What is the main causes for the coating formation and clogging in the preheaters and rotary kiln? and how we can control clogging ad coating formation thickness?
Answer:
Coating and clogging in cement kiln preheaters can have a number of causes but the most common is the presence of chlorides in the raw materials or fuels. These build up within the kiln in an alkali cycle, volatilising in the hotter sections of the kiln and recondensing on the feed and on the walls of the preheater. The low temperatures melts formed by these alkali chlorides can lead to clogging of the preheater. There are a number of solutions: (i) selection of raw materials and fuels with no chloride contamination, (ii) close control of raw mix chemistry, (iii) installation of an alkali bypass to bleed the alkali chlorides out of the kiln.
Preheater Clogging: The use of engineered two-component refractory linings can also help minimize the problem. Silicon carbide and zirconia castables have both been used with a good degree of success.

Question 111:
Please let me know the reason of lumps formation in cement silo sidewall. We have seen that this seems to be a common problem in cement plants in India. Even using lump crushers also does not solve this problem. please inform the reason behind this. Please note that cement temperature is normally below 100 degree centigrade & silos are constructed in a manner that moisture can not enter silo. Please inform the solution for this. Secondly it has been noticed that this problem is more common in inverted cone silos compared to flat bottom silos.
Answer:
Lump formation on the side-walls of cement silos can have a number of causes, but the most common cause is continued dehydration of gypsum after the cement has been blown into the silo. After being liberated the moisture from the gypsum migrates to the side walls of the silo, which are slightly cooled from the outside by the ambient air. The moisture condenses at the side walls and reacts with the cement to form lumps. These lumps can grow to 2m think from the walls of the silo. The solution is adequate control of the gypsum dehydration in the cement mill via ventilation and water injection, followed by cooling of the cement in the separator or cement cooler.
Question 110:
We are adding water at the cooler discharge in a quest to try and reduce clinker temperatures (in a rotary cooler).The major problem is that of carbon monoxide build ups. How can one easily strike a balance in the amount of water addition and acceptable clinker temperatures?
Answer:
High clinker temperatures of 200 to 250 degrees C are one of the problems, which are inevitable with a rotary cooler. The first means to lower this temperature is through the lifters in the cooler, and also efficient seals at the kiln outlet and cooler inlet. In leaking air must be minimised around the kiln hood and cooler inlet. Provided these seals are well maintained then and your response of spraying water into the outlet is the only one you can try. The CO formation must then be controlled by adjusting the water spray to the highest level, which can be permitted without formation of CO – not an easy thing for the operators to do. You really need an expert system or self tuning control loop.
Water Spray in rotary Coolers: Another efficient way to reduce clinker temperature is to pre-cool it in the kiln using refractory tumblers properly designed, manufactured and installed. Cam linings and high-alumina castables have proven inefficient because they spell off. Another good way is to further increase the residence time of the clinker in the cooler by installing dams coupled with tumblers.
Question 109:
Which is the most suitable equipment for crushing, grinding and micronising of white cement, without compromising on quality and whiteness.
Answer:
Are you interested in grinding the raw materials for white cement production or the white cement clinker? In either case you should have no problems of colour contamination if you use hard forged grinding media in a ball mill. Alternatively you can use a vertical mill or roll press with no likelihood of colour contamination.
Question 108:
I am making a study on the use of copper slag in our cement plant. However, I have limited material on how it will be fed into a dry kiln. I hope you can give me an idea and some reference materials on the subject.
Answer (1):
There are a number of ways to feed the copper slag into your kiln. There are also some key questions: What type of kiln do you operate? Is the copper slag contaminated with toxic materials? What is the size distribution of the copper slag? If you can send more information then we will try to answer. Response 18/2/01: The copper slag does not contain toxic materials – See attached files re the kilns and material specification
Answer (2):
Thank you for the information regarding your kilns. The easiest method to feed the copper slag to your kilns will be with the kiln feed. we suggest that you add the material to the raw mills as an additional component of the raw mix. This could be either at a fixed addition rate, or as a variable additional component of the raw mix, allowing you to control an additional chemistry modulus. From the composition of the slag it would allow you to control the alumina modulus of your kiln feed.

Question 107:
How can one establish a relationship between kiln feed rate and coal dosing, disregarding the kiln speed and ratios between the precalciner and main burner.
Answer:
You need a certain amount of coal on the main burner before you start the firing in the precalciner. After starting the precalciner firing then the increase in coal with feed should mostly be on the precalciner. The front coal should be virtually constant irrespective of feed. In practice you usually need to slightly increase the front coal with the feed. There is not a direct relationship between feed and coal however. As the chemistry of the feed changes so the amount of coal required will vary. This is the type of thing which expert systems are very good at doing.
Question 106:
What are the causes of material ring formation? and if in case ring formation exist what are the possible solution to have it removed or knocked down? (1) at about 12 to 20 meters from kiln discharge end and (2) at about 40 to 50 meters from kiln feed end.
Answer:
There can be a number of causes of ring formations. They are associated with chemical reactions or phase changes in material in the kiln, i.e. formation of a particular mineral, or melting or vaporisation of a compound. Sometimes you can adjust the chemistry of the kiln feed to eliminate the ring formation problem. Good control of the kiln feed chemistry and the kiln operation using an expert system is the best way to avoid rings. 1. Sometimes you can solve a ring problem by adjusting the flame and changing the heat profile in the kiln. 2. Sometimes it is necessary to stop the kiln and shoot the ring down using an industrial gun. 3. Some factories fit hollow tubes to the kiln shell, with holes drilled through the shell and refractory. The kiln is stopped and an explosive charge is fired down the tube to blow off the ring and coating. 4. Sometimes it is necessary to stop the kiln, cool down and then break away the ring manually. The rings formed at the different positions you mention depends on what process your kiln is. The ring at 12 to 20 metres from discharge is a clinker ring and you might solve by adjusting the burner. The ring at 40 to 50 metres is more likely a sulphur ring, which might be solved by adjusting the kiln, feed chemistry.
Calcining zone rings, mostly comprised of spurrite or sulfated spurrite, can be eliminated by installing carbide tumblers in the area. I have done so in several kilns in the US and Mexico. Changing the kiln feed is not always possible or economically feasible. As to the clinker rings, also consider changing the refractory coatability pattern in the region, besides working on the flame.
Question 105:
Dear Sirs, I have recently being contacted by a cement company concerning the incineration of hazardous wastes in their kiln. They are concerned if the heavy metals contained in the hazardous wastes would not be: a) released to the atmosphere; b) be leached from the cement; c) could be concentrated in parts of the clinker; d)weaken or change the quality of the clinker. I would very much appreciate any information you could provide regarding the points mentioned above.
Answer:
Your cement company client should be concerned about all the factors you raise. The potential problems depend on which heavy metal species are present in the hazardous waste. The majority of heavy metals (V, Cr, Mn, Cu, Zn, Cd) will pass out of the kiln in the clinker. It is unlikely that the hydraulic properties of the cement would be affected, however there is a slight chance of leaching of the metals from concrete into the environment. Particular problems might be encountered with Hg and Tl which will volatise in the hotter sections of the kiln and condense in the cooler sections. In this way they build up to a high concentration. Any process disturbance can then lead to an escape to atmosphere. Before burning these hazardous wastes you client should conduct a detailed environmental impact analysis.

Question 104:
We have installed multi-channelled low NOX burners in two of our cement plants. At about the same time, we began experiencing longer setting times (Vicats) in our cement. We have also realized that the Bogue C3A is much higher than the actual C3A found in the clinker. The new burner pipes seem to be the only similarity between the two plants. Could there be a relationship between the burner pipes and the long setting time problem in the cement? Have you heard of any other plants experiencing this problem?
Answer:
Your problem with long setting time after installation of low NOx burners in the kilns is very interesting. We have searched and been unable to find a similar reference to date. We have been trying to think of how this might lead to a drop in the actual C3A content, while the Bogue C3A remains constant. The only suggestion we can come up with is that the solid solution series of the C4AF has been shifted towards C6A2F, leaving less alumina available to combine in C3A. Before deciding that this is the explanation we need to be sure that the lower actual C3A content actually corresponds with the installation of the low NOx burners. If anything we would have expected more C3A rather than less due to possible reduction of Fe3+ to Fe2+. If the low C3A compared to Bogue proves to be coincidental, then we would suspect that the long setting is due to changes in the sulphatisation of the alkalis and the binding of the K2O in the clinker.

Question 103:
I am doing some research on energy consumption in the cement industry. What are the major techniques/technologies that need to be examined in order to further reduce specific energy consumption, both of fuel and electricity. Are there any publicly available studies or reports on using Organis Rankine cycle heat recovery to power systems and oxygen enrichment technologies?
Answer:
Energy consumption in the cement industry and the techniques, which need to be examined, is a wide area. It is also closely tied up with IPPC legislation, carbon taxation and climate change levies. This drives the interest in technologies such as the Organic Rankine cycle, oxygen enrichment and CO2 sequestration.
In kiln fuel consumption the general belief is that technology has gone as far as it can in driving down energy consumption. The losses in the exhaust gases and shell radiation cannot be brought down much further economically. The only remaining avenue is to reduce the overall endothermic heat of clinker formation by the use of mineralisers. This is being pursued by some companies.
In electrical energy consumption there is the possibility to further reduce via the introduction of compression grinding. Unfortunately other trends in the industry, such as the increased replacement of clinker by extenders, are pushing up electrical energy consumption in grinding.
Power generation equipment utilising the Organic Rankine cycle has been implemented at the Lengfurt factory of Heidelberger in Germany for over 1 year now. It is currently under evaluation but all reports to date are that it achieves what was expected of it. Power costs to the industry are coming down in general, and this makes it difficult to generate a return on investment for this technology. Climate change levies may affect this calculation. There are also some unresolved process engineering barriers.
References are ICR 08/99, 09/2000. References on oxygen enrichment are more difficult to find and older. The technology is being used quite extensively in the USA to boost kiln output. The main driving force for the implementation elsewhere is likely to come from tightening IPPC regulation and increased use of alternative fuels.

Question 102:
Due to bell mouth (trumpet) effect we are facing problem in installation of tip casting brick refractory lining. Please give your comments on following points:- 1.What measures to be taken to avoid bell mouth (trumpet) effect in the kiln? 2.If bell mouth effect occurred then what steps to be taken to avoid further destruction of kiln tip ? 3.Other then replacement of kiln shell in the area what best can be done to achieve normal refractory life ?
Answer:
Trumpet shaped deformation of the kiln outlet section is commonly caused by insufficient cooling of the nose ring. The high secondary air temperatures with direct aerated coolers can make this problem more likely. Avoiding further distortion requires a combination of material selection, mechanical and operational improvements. These would be selecting nose ring casting of sufficient thermal resistance, improved nose ring cooling, and close control of the kiln hood and secondary air temperature. Replacement of the outlet section of the kiln including nose ring and retaining ring might be the only solution. In the short term, or if the distortion is not too large then the use of high grade castable refractory in the outlet section might improve the service of refractories in this section.
Bell-Shaped Kiln Shell at the Nose Ring:
(1)Increase the amount and velocity of the cooling air at the cowl.
(2) Weld hex-mesh metal anchors on the face of the castings facing the
cooler and fill it with refractory plastic. 50 mm are sufficient.
(3) Install a coarse-grained, 60% alumina low cement castable in the area,
using floating metal anchors in a busy pattern.
(4) Decrease the overall lining thickness to 200 mm.
(5) Re-design the brick retainer to 25 mm x 100 mm steel plate segments and
cantilever them against the shell every 230 mm. along the shell perimeter.

Question 101:
Dear Dr.Clark: (From Korea). You stated (in your earlier response – see below) that weshould suffer from higher Nox emissions with straight pipe burners even though we can make short hot flame and short length of burning zone in the kilns. But, in my opinion, there’s a possibility of lower Nox with straight burner because of shorter burning zone. Even though burning temperature is higher with straight pipe burners, the period of passing the high temperature zone is too short to form Nox so much. I think it’s possible to get short hot flame and lower Nox with straight pipe burners in spite of higher burning temperature. If it’s possible, we can be also released from clinker quality problems. What do you think about that? This site is really helpful to cement manufacturers like us.
Answer:
NOx emissions from cement kilns arise from two causes: (1) the nitrogen in the fuel (fuel NOx), and (2) breakdown of nitrogen in the combustion air at the high temperatures in the flame of the main burner (thermal NOx). The amount of thermal NOx depends on the peak temperature in the flame, rather than the length of the burning zone. So a straight pipe burner will give you higher NOx because the peak temperatures in the burning zone are higher. There are many ways to have a short hot flame, good clinker quality and low NOx emissions. Try speeding up the rotation of the kiln to bring the clinker through the burning zone more quickly. Try stepped combustion in the precalciner to reduce the NOx from the burning zone.
Question 100:
Our company new to the cement business, and we are unsure if there is a serious need to measure gas temperature from 100F to 3500F, using sound waves. We currently sell to power plants and waste to energy boilers and just recently to a US cement plant and are measuring under the burner in the hood. What type of benefits can our customers obtain by measuring precisely this temperature? Thanks, for a response.
Answer:
There is certainly a serious need for cement companies to monitor temperatures in the range 100 to 3500F. The temperatures in the burning zone of a cement kiln must be above 2500F for product quality reasons. The temperatures in the flame of the kiln are significantly higher. Another important factor is the temperature of the combustion air entering the kiln from the cooler. This can be in the range 1250 to 2000F and maintaining stability of this air temperature is very important for operational stability.

Question 99:
We believe you have published articles on problems with kiln live ring cracks. We have found some defect indications in live rings of one of our kilns. We have detected these defects by doing ultrasonic testing. Details are attached in enclosed file. You are kindly requested to send the article published material on kiln live rings. We will be thankful to you for this.
Answer:
We think the article to which you are referring appeared in the September 1997 issue of International Cement Review on page 81. This is principally about welding repair techniques in the unfortunate event of a kiln tyre crack. There is nothing about measures to prevent the formation of cracks. These would be as follows:
1. Ultra-sonic testing and inspection of tyres prior to installation to detect any casting inclusions.
2. Regular inspection of tyres with dye penetrant or ultra sonic testing.
3. Maintain correct alignment of kiln, tyres and rollers.
4. Maintain clean surface of tyres and rollers.
5. Graphite or similar lubrication of surfaces of tyres and rollers.
6. Grind out any burrs on edges of rollers.
Question 98:
I am making a study on the use of copper slag in our cement plant. However, I have limited material on how it will be fed into a dry kiln. I hope you can give me an idea and some reference materials on the subject. By the way, we are a regular subscriber to ICR here in the Philippines.
Answer:
There a number of ways to feed the copper slag into your kiln. There are also some key questions: What type of kiln do you operate? Is the copper slag contaminated with toxic materials? What is the size distribution of the copper slag? If you can send more information then we will try to answer.

Question 97:
We are experiencing a recurrent Snowman formation problem in the kiln and we are looking for to get information on how we could avoid it. We also heard that one can use up to 55% of natural pozzolana as an additive and produce good quality of portland pozzolana cement. Would any one recommend something about this and we would like to be updated if there are such studies about cement additives.
Answer:
Persistent snowman formation is usually associated with the control of the raw mix chemistry. The other alternative is to install some device to knock down the snowman before it falls. Many cement companies use air blasters. I think it is better to use an acoustic cleaner or sonic horn.
Snowman in the Cooler: We have been able to destabilize these build-ups using pre-cast, pre-fired silicon carbide blocks heavily anchored through the cooler shell. Please The carbide surface does not bind to clinker-melt, thus facilitating snowman removal with the air blasters. The life of such linings
varies between 2 years and 10 years.
Question 96:
How can one establish a relationship between kiln feed rate and coal dosing, disregarding the kiln speed and ratios between the precalciner and main burner.
Answer:
You need a certain amount of coal on the main burner before you start the firing in the precalciner. After starting the precalciner firing then the increase in coal with feed should mostly be on the precalciner. The front coal should be virtually constant irrespective of feed. In practice you usually need to slightly increase the front coal with the feed. There is not a direct relationship between feed and coal however. As the chemistry of the feed changes so the amount of coal required will vary. This is the type of thing which expert systems are very good at doing.

Question 95:
I am chemical engineer and I work in one of the industries of the largest cement group of Brazil. One of our largest concerns today is with the enormous passive environmental concerns relating to the use of used refractory bricks. Therefore I seek information about the solutions cement companies are using to combat this problem.
Answer:
We agree that chrome containing refractory waste materials are a major passive environmental concern for the cement industry around the world. This has lead to materials containing chrome ores to be phased out of use in Europe and the USA. These chrome containing refractories have been replaced by magnesite spinel and dolomite refractories. The problem remains of old refractory materials containing chrome. One solution is disposal in bunded landfills with adequate protection against chromium compounds leaching into the ground water.
There was a paper referring to the problems of scrap magnesite chrome refractory waste in the Environmental Yearbook produced by ICR in 1997. According to this the only safe means of disposal of these refractory wastes is by recylcing to the refractory supplier. Before return to the supplier the waste should be stored under cover to prevent any leaching of hexavalent chrome by rain water. This is the main reason for the switch over to dolomite. magnesia spinel and magnesia zirconia refractories. These can be land filled with no danger to the environment.

Question 94:
I am attempting to collate information on the use of waste tyres as an alternative fuel source. Please could you let me have any information on the following: Percentage of tyre shred/chip to conventional fuel. Efficiency of tyre shred/chip compared to conventional fuels. Total current tonnage used in UK cement production. Projected tonnage per year in five years. Advantages of TDF Disadvantages of TDF. This information will greatly assist my research. Many thanks.
Answer:
Cement companies are moving towards 25% replacement of convention fuels with tyre derived fuels, although the majority of UK cement companies are well short of this. Burning whole tyres is far more common due to lower costs compared to chips or shreds. Tyres are a richer thermal energy source than most coals therefore there is no energy efficiency penalty. Currently around 35,000 tonnes per year of tyres are burnt in UK cement kilns with this expected to rise to 135,000 in the next 5 years. The advantages are:
(i) lowering operating costs,
(ii) environmentally attractive method of disposing of waste tyres, possibly offsetting climate change levies.
The disadvantages are in the handling of the tyres in any form, and the disturbances, which whole tyres can cause to the cement, kiln process. These disturbances can lead to loss of output or increased dust emissions where electro-static precipitators are used for de-dusting the kiln exhaust.

Question 93:
Calling from South Korea. I am confused about merit of multi-channel burner, because we hope to make short flame in kiln (according to literature) and someone who is expert in cement industries said straight pipe line burner is more useful to make short flame in kiln than a multi-channel burner. If you have heard about that, I want hear your opinion and if so, how can we get the data of that? By the way, we think this service is great.
Answer:
Thanks, we are pleased our responses are of use. It is true that a straight pipe single channel burner will produce a short hot flame. This type of burner delivers purely axial, primary combustion air into the kiln. In order to entrain the secondary air for combustion of the fuel it is necessary to use a high percentage of primary air (maybe more than 20%). Your NOx emissions will rise, and thermal efficiency fall, due to using a lower proportion of preheated secondary air.
With a multi-channel burner some of the primary air is delivered radically into the kiln to induce a swirl in the flame. This helps entrain the secondary air into the flame and means that a much lower proportion of primary air is required. NOx emissions are lower and kiln thermal efficiency increases.
However, these combustion issues are very complex and can be modelled and optimised by expert companies.

Question 92:
We’re planning to burn plastic derived fuel as the secondary fuel in the kiln burner and in the calciner burner. I think there will be a lot of changes in temperature profile in the kiln and in clinker quality. For example, the temperature in the kiln inlet and the length of burning zone will increase. The reducing atmosphere will be severer. It means the clinker quality can be lowered. I met a maker who had supplied the crusher to a cement plant in your country for PDF size reduction. He said the crusher had been supposed to make raw plastic under 20mm. What I am asking is what size of PDF should be needed for both of kiln burning and calciner burning? And what kind of effects on kiln process and clinker quality do you expect by burning PDF ? As a reference, we’re burning only coal in the kiln, and coal and petcoke in the calciner, and waste tire in the kiln inlet(= at the meal chute of the lowest cyclones.
Answer:
You are correct when you say that adding plastic derived fuel might have many effects on your kiln process. In order to predict the effect on kiln inlet temperature and length of the burning zone you need to consider the calorific value and volatility or ignition temperature of the plastics. You should aim for as low particle size as possible for the main burner. The plastics will be low density and large particles can be carried out of the flame down the kiln while still burning. Probably the better place for addition is the precalciner. Be careful of the chloride content of the plastics – this might cause you severe preheater choking problems.
Question 91:
In our clinker cooler the first grate, is to be modified from plate direct aeration back to chamber aeration. The existing fans will be adapted for the new situation and that includes chamber #2 to be serviced by three fans and chamber #3 by two fans. My question is: can you please comment on problems that can occur when operating two or more fans into one chamber, process wise or otherwise.
Answer:
The potential problem you will face is that the fans will be working against the pressure of one-another with more than one fan operating into the same chamber. In the extreme case you could have reversing of the cooling air back through some fans. This will cause the fans to run in reverse – does the design of the gear box allow this?

Question 90:
We are regular subscriber to your esteemed journal “International Cement Review”. We understand that you have published an article on Kiln Live ring cracks sometime back. We have found some defect indications in live rings of one of our kilns. We have detected these defects by doing ultrasonic testing.
Answer:
We think the article to which you are referring appeared in the September 1997 issue of International Cement Review on page 81. This is principally about welding repair techniques in the unfortunate event of a kiln tyre crack. There is nothing about measures to prevent the formation of cracks. These would be as follows: 1. Sonic-sonic testing and inspection of tyres prior to installation to detect any casting inclusions. 2. Regular inspection of tyres with dye penetrant or ultra sonic testing. 3. Maintain correct alignment of kiln, tyres and rollers. 4. Maintain clean surface of tyres and rollers. 5. Graphite or similar lubrication of surfaces of tyres and rollers. 6. Grind out any burrs on edges of rollers.
Question 89:
Are there any recent studies or research break through in utilizing this mostly wasted material in cement or mortar blends for certain suitable applications? could you also direct me to such recent research publications?
Answer:
The major methods for handling of cement kiln dust (CKD) are to recycle the material into the process. With modern dry process plants the material is normally added to the kiln feed blending silo or directly to the kiln feed. On long kilns the dust is blown into the front of the kiln, or added to the kiln through dust scoops after the chain section. The main drawback with these methods is that all the alkalis are retained within the product, which can be detrimental in concrete produced with the cement. In research undertaken by Queensland Cement in Australia CKD has been used to ameliorate acid sulphates in soil rather than agricultural lime.
In December 1993 the US Environmental Protection Agency published the report of a 7 year study into CKD.

Question 88:
When designing a precalciner with swirl flow, how do I scale up or down the model? Eg. How does the residence time vary with the flow rate of the air or meal? What are the main equation governing the system?
Answer:
I’m sorry but this is a bit beyond the technical forum. I suggest you contact Con Manias at FCT in Australia, Professor Clemente of Greco in Brazil, or Richard Manning at Kiln Flame Systems. Any of them might be able to help you. If you get an answer please send it to us!
Question 87:
Calling from Mexico: My question is related to the petcoke handling. What is the experience on handling ground petcoke 200 mesh ?, What are the alternatives for it’s handling in order to assure that no petcoke dust contaminates the environment?
Answer:
The considerations for handling and containment of ground petroleum coke would be the same as for pulverised coal. Should be stored and conveyed in enclosed hoppers and pipelines. Free space above ground fuel should be minimised and inerted with nitrogen.
Question 86:
What are the latest advances in the utilization of cement kiln dust? are there any recent studies or research break throughs in utilizing this mostly wasted material in cement or mortar blends for certain suitable applications? could you also direct me to such recent research publications? Saudi Arabian subscriber
Answer:
The major methods for handling of cement kiln dust (CKD) are to recycle the material into the process. With modern dry process plants the material is normally added to the kiln feed blending silo or directly to the kiln feed. On long kilns the dust is blown into the front of the kiln, or added to the kiln through dust scoops after the chain section. The main drawback with these methods is that all the alkalis are retained within the product, which can be detrimental in concrete produced with the cement. In research undertaken by Queensland Cement in Australia CKD has been used to ameliorate acid sulphates in soil rather than agricultural lime. In December 1993 the US Environmental Protection Agency published the report of a 7 year study into CKD.

Question 85:
Please advise me which is the best way to burn tires in the main kiln burner. What effects there are going to happen within in the clinker and stack emissions. What is the maximum size for the tire chips, the maximum replacement of fuel in the main burner. Does the heat consumption increase, what are the causes. It is recommendable to inject the chips by a separate pipe under the burner, How about the steel reinforcements in the tire?
Answer:
The best way to burn tyres in the main burner would be with injection of chips or shreds together with the main fuel through a burner designed for the task. Provided that the shreds or chips are small enough then there should be no effect on the emissions or the clinker quality. Maximum size of the chips should be 2×2 cm. Replacement levels of 25% should be achievable. Heat consumption should not increase. No, it is not recommended to inject with a separate pipe – see above. Iron wire reinforcement is not a problem for the cement process but might present problems in the handling equipment or burner. When tyres are chipped the wires do not break regularly and tend to leave hooks causing the chips to be caught on the handling equipment. Steel content is normally 10 to 12% by weight.
Question 84:
I wonder what relation between alkali(K2O, Na2O) and grindability (44 residue) because recently we consider fluidity of cement, we couldn’t ignore grindability.
Answer:
The relationship between alkalis and grindability is complex. The principle effect of alkalis is on the combinability of the clinker in the kiln. Alkalis make the clinker more difficult to combine due to their inhibiting effect on the formation of C3S is the kiln. They also cause there to be a greater proportion of flux in the kiln. Both these factors will lead to the clinker being more dense and therefore more difficult to grind. So the relationship between alkalis and grindability is that the clinker becomes more difficult to grind as the amount of alkalis increases.

Question 83:
Why does a reducing atmosphere in the kiln cause brown centered clinker?
And what are the practical effects of this on the final concrete strengths?
Answer:
A reducing atmosphere in the burning zone of the kiln leads to the partial reduction of ferric iron, Fe3+, to ferrous iron, Fe2+. These ferrous ions are incorporated in the C3S mineral imparting a brown colouration. It is necessary for the clinker to be slowly cooled in addition to being burnt in reducing conditions for this to occur. This explains why only the cores the nodules are coloured brown. The cooling of the outer layers of the nodules is too rapid for this to occur. The practical effects are that the compressive strength of the cement are substantially reduced due to impaired hydraulic reactivity of the modified C3S. Setting times are also significantly shortened due to higher C3A mineral content as less iron enters into the alumino-ferrite mineral.
Question 82:
what is the normal limit of Nox at kiln inlet and preheater outlet in ILC kiln of 3300 TPD production?
Answer:
The NOx levels at kiln inlet and preheater exit will depend on many factors. Do you have a low primary air burner?
Do you have staged combustion in the in-line calciner?
I would expect anything from 300 to 1500 ppm at the kiln inlet. If there is CO present at the kiln inlet the level of NOx might be even lower. I would expect the level at the preheater exit to 50% lower than the kiln inlet due to some degradation of the NOx from the burning zone in the in-line calciner.

Question 81:
My query relates to petcoke grinding and burning and ways and means to overcome the same. Does the burner need to be modified? In the existing burner, what change is needed in axial / radial air. What kind of residue is to be maintained compared to that for normal South African / Australian coal / Chinese coal of typical calorific value 6500 KCal / Kg Coal.
What is generally the HGI and Bond Index of Petcoke. Please give comparison of typical values for coal and Petcoke. What is the expected power consumption of grinding mill compared to that for coal mentioned for it’s grinding and corresponding residues.
Answer:
As your question acknowledges there are many things to consider. These depend on the source, type and composition of the coke. Higher sulphur input to the kiln is the most common cause of problems, with potential for heavy recirculation of sulphates, preheater choking and ring formations at the kiln inlet. Rigorous control of the combustion conditions in the kiln is a pre-requisite to avoid these problems. Burner adjustment or replacement might be necessary on a case by case basis. There is also the issue of higher sulphate content in clinker and constraints on the amount of gypsum, which can be added in finish grinding. Hardgrove index and whether the coke can be inter-ground with coal again depend on the source and type of the coal. Green coke can be inter-ground but fluid coke is too hard for inter-grinding. Volatile content is less than for coals therefore the need for finer grinding can be anticipated.
Question 80:
We want to know how coarse quartz particles in kiln feed can affect clinker quality, especially clinker’s strength. We had read some articles about this subject, and it would be interesting to know more, especially if there is any correlation between quantity of quartz in coarse fractions and clinker’s strength
Answer:
Coarse quartz crystals in kiln feed cause clinker quality problems due to heterogeneity in the clinker. The reaction of CaO with SiO2 to form C3S and C2S minerals in clinker is a solid state reaction. If there are coarse crystals of quartz (SiO2) then the centres of the crystals remain as unreacted cores of SiO2. There is a secondary problem that as some of the SiO2 is unreacted then a corresponding amount of CaO will be uncombined as free lime.
As the SiO2 is not fully combined into the strength developing calcium silicates C3S and C2S, the clinker strength development is reduced. The uncombined free lime can also lead to long term expansion of concrete. This can cause concrete failures and is known as causing durability problems.
Question 79:
We are building a new plant in a sensitive environmental region and are concerned about NOx emission levels. Furthermore, to satisfy environmental concerns about NOx, it may be necessary to inject ammonia (SNCR). Is this incompatible with the principles of multi-stage precalciners?
Answer:
Staged combustion is usually sufficient to allay NOx emission worries. It is also much more widely used than SNCR. SNCR can cause additional problems due to ammonia emissions and blue haze if the process is not tightly controlled. The ammonia must be injected into the process at a precise temperature. Any deviation leads to significant emissions. We know of no installations where both methods are used in tandem. Further research would be necessary to identify the optimal configuration for such an installation.
A further comment from another contributor…
I might add that I have been researching this matter and that there are many opinions about the location of the temperature window for ammonia injection. Following is one from Rugby Cement that was included in the House of Lords European Committee – 11th Report: “SNCR is a technique that involves the injection of ammonia or urea into the combustion process at an appropriate point. The hydrogen in the ammonia combines with the oxygen in the NOx to form water vapour, leaving the nitrogen atoms from the ammonia and NOx to from N2. The temperature range within which the reactions will take place is very limited, around 800 to 900deg.C. The exact point of ammonia injection is critical. If the ammonia is injected at too high a temperature it is burnt by excess air and does not achieve a reduction in NOx. If it is injected at too low a temperature then little ammonia reacts, and the unreacted gas is discharged to atmosphere, highly undesirable from the environmental point of view. Source: evidence from Rugby Cement (p 179); see also evidence from SELCHP (p 184).”
The temperature range is an important point because the 800-900oC window is also the position of the people who wrote the IPPC BREF. CEMBUREAU claims the window is 900-1000deg.C. There are those who believe that the window shifts based on CO concentration and excess air. In all fairness, your summary ought to say that the LOWER limits of the temperature window are debatable and that estimates range from 800 to 900deg.C

Question 78:
My question is in relation to statistical process control and its application to the cement industry. I am working on a project investigating the relationship between kiln conditions and optimal kiln operation. I am experiencing difficulty in locating relevant literature and would appreciate any recommendations or suggestions.
Answer:
Statistical Process Control (SPC) This is an extensive subject but ideally suited to investigating the relationship between kiln conditions and operating technique. Some of the variation in conditions is just process noise and the operators should not respond to this noise or this might be the cause of instability. So SPC says you should have upper and lower control limits, which define the ranges of normal variation. Operators should respond to variation outside these limits. There is then the option to use custom techniques to address the situation where the kiln is stable but just above the optimum. Finally long term process improvement is achieved by eliminating the causes of variation and reducing the range of normal variation. ICR has published a number of relevant papers in the past on such topics.
Question 77:
Raw mill feed extraction
Our factory has been able to increase total raw mill production to 150tph from 125tph by changing the loop on the LS storage bin, so that it is now only a feed through chute direct to the mill feed system. The feed rate of the reclaimer feeding the bin is regulated to match the mill production. The problem is when using the bin as a storage bin, the fines and rock separate when extracting . This causes flushing in the Mill thus an accumulation of Pebbles in the recirculation system limiting production. This solution places high wear and tear on the reclaimer. What suggestions do you have to stop the separation of the rock sizes in the extraction of the bin.
Answer:
Unfortunately segregation of raw materials in intermediate storage bins is a common problem, which is usually solved in the way you have done, i.e. put the bin low and high levels in the plc very close to one-another so that the bin level is held constant. As you say the drawback is that the reclaimer runs all the time and you might as well not have an intermediate storage bin. A solution might be to fit an acoustic cleaner in the top of the bin to encourage even discharge of the limestone and eliminate the segregation.

Question 76:
Analysis methods
For quality control we need to decide which type of On-line analyser to install. We have had various types presented to us. We feel correcting before the mill with a cross belt analyser is the best method. Suppliers of mill controls we have dealt with advise after the mill due to limiting performances of the analysers. Where and what make would you recommend?
Answer:
Around the world in the cement industry there are many more cross-the-belt analysers installed before the mill, than in-process analysers installed after the mill. Cross-the-belt is therefore a proven solution while in-process would be a higher risk choice. An alternative would be auto-sampling and an auto-lab. This would probably be the best solution but the most expensive.
Question 75:
Can you advise on suppliers of cross-the-belt analysers
Answer:
Different cement companies around the world have selected analysers from all three established suppliers. All use the same principle so there is not that much to choose between them. The deciding factor is likely to be the ability to provide after sales support in a remote location. There are some horror stories around!
Question 74:
We have a Linkman control system on the kiln, which is very successful in doing what it is meant to do. Based on this, we want to have the mill controlled on LM. Can the system improve performance when we feel the mill is fully optimised as is and when would be the best timing for installation.
Answer:
As the kiln is successfully controlled using an expert system it makes sense to extend this to the raw mill. If nothing else it will standardise the control response and you will learn more about the dynamics of the mill and circuit. It will also place a template on the operation to allow you to track long term drift in the performance of the mill. If the objective is to optimise the output as well as the quality then it makes sense to do it immediately. If optimising quality is the primary objective then it would be better to wait for an analyser to be fitted (see above).

Question 73:
Producer/Chile:
Why would a separator require a 4 quadrant motor, with its attendant expense?
Answer:
Only some suppliers of separators require this. The reason is there are process occasions when the motor needs to apply braking to the separator cage in order to slow the speed.
Question 72:
Producer/Germany:
Is there any means of continuously monitoring the degree of calcination in the material ex the precalciner?
Answer:
Consider installing a CO2 meter at the kiln inlet. The signal is noisy because of CO2 generated from the main burner, but step achnges can be seen dependent on the residual CaCO3 content of the material entering the rotary section of the kiln.
Question 71:
Producer: United Kingdom:
What are the causes of blue haze from a cement factory chimney?
Answer:
Any ammonium salts in the raw material will evaporate in the top stage of the preheater and pass out of the kiln in the exhaust gases. They can then condense again as ammonium chloride or sulphate when exhausted to atmosphere from the kiln stack. The distance from the top of the chimney depends on the exhaust gas temperature. This can be seen as the blue haze problem.

Question 70:
Producer Slovakia:
How can a cement factory meet Kyoto obligations without totally changing the equipment installed?
Answer:
Many ways: (i) Increased substitution of clinker in cement by extenders, (ii) substitute fossil fuels with alternatives which would otherwise be incinerated, (iii) switch to a less carbon rich fuel, e.g. coal to natural gas, (iv) generate power from waste heat gas flows, (v) use oxygen enrichment of combustion to reduce waste heat gas volumes.
Question 69:
In our cement plant (two lines), we have two kind of limestone reserves: high grade limestone (52% CaO) and low grade limestone (napalan=43% CaO). After five years operation there is problem that if we use low grade limestone (mixing with high grade) there is heavy levels of dust in the EP. Until now we cannot use this reserve, which totals some about 40 % of total reserve. We have two plants with same capacity. For new plant (1997 start operation) this type of material is no problem. We do not know what is the problem, raw material or raw mill, separator, EP ? To day we are studying about: mineral composition and X ray diffraction of raw materials. Is there relation between mineral composition with the resistivity ?
Answer:
For sure there is a relationship between the mineral composition of the raw mix and the resistivity. The variation in resistivity of dusts is a very complex question and dependent on many factors. Temperature and moisture content of the gases are key factors, however I presume this is the same whether you are using the high grade or the low grade limestone? I suspect the problem is associated with build-up of static charges in the dynamic separator. This might be affected by the mineral composition and reduce the efficiency of the ESP. Two possible solutions:
1. Upgrade the ESP high voltage controls to switched integrated rectifiers and pulse generation.
2. Conduct a trial with the ECOFOR device coupled to the dynamic separator. This device grounds static electrical charges and may solve the problem.

Question 68:
In a new 2.5m diameter white cement kiln being installed can we use 100% combustion air from the primary air fan. What should be the capacity of primary air fan in air flow rate/hour and suitable primary air. The total combustion air requirement is close to 12 Nm3/kg of fuel. How much extra air flow capacity the fan should have over and above the theoretical combustion air requirement of 12 Nm3/kg fuel. Clinker cooling will be by water spray in rotary cooler. The fuel is crude oil having calorific value 10,400 kcal/kg. Will it be better to preheat the primary air?
Answer:
This is quite complicated without process diagrams. The designs depend on whether it is Onoda, BSH, FLS or someone else. You should deliver all the combustion air through the firing fan as you do not want to pull any steam into the kiln from the quenching water sprays. I would also recommend preheating the combustion air to reduce fuel consumption and/or increase output. If you are having problems you really need to do a full combustion audit. It is almost impossible to advise on these things without much more information. Anyone else like to comment?
Question 67:
I find that there are tremendous advantages in production increases and reduction in specific power consumption by cooling preheater exhaust gases by water spray. OEM’s rarely recommend such a philosophy but for what reasons? Can you highlight advantages and disadvantages of evaporative cooling of preheater exhaust gases and care to be taken when burning high sulphur petcoke.
Answer:
The advantage is that you cool the gases and therefore pull a much greater mass of exhaust gas through the preheater fan. The danger to my understanding is the increased suction through the preheater and down comer and possible danger of collapsing the ducts. Many kilns are firing 100% pet coke therefore it is perfectly possible
Among the potential problems are the low volatility of the pet coke and the sulphur content. The low volatility can lead to combustion problems. If these lead to reductive burning then the sulphate cycle in the kiln will be enhanced. The high sulphur content then couples into this cycle and you may well suffer preheater blockages. The key is to optimise the main burner and precalciner firing for pet coke firing and avoid reductive burning and CO formation.

Question 66:
Recently we are occasionally finding Br (bromide) in the coating of our kiln. I think that it is affected by using waste materials so I want to know about Br’s effect on the kiln and our products (cement, clinker). I heard little arbour Br, Br proceed like Cl ion, but that’s all and then, I expect to know more about Br. Can you help?
Answer:
There is very little information about the effects of bromide beyond what you already know. From a process standpoint it acts in the same way as chloride and recirculates in the kiln. This could conceivably lead to build up and blockage problems in kilns with preheaters, however the amounts involved are usually too small to cause problems. You are also correct that the few reported cases are associated with burning alternative fuels and wastes. Particularly photographic processing water and waste. Sorry there is no more information readily available – it is a very rare situation.

Question 65:
We are considering replacing solid fuels in a 5300 t/day precalcination kiln with natural gas. What will be the impacts on:
(a) Clinker quality
(b) Production
(c) Heat specific consumption
(d) Dust and CO2, SOx, NOx emissions
(e) CO trips.
In addition, what specific measures must be applied in order to eliminate explosion risks?
Answer:
You are correct to be concerned about replacing solid fuels on your 5300 tpd kiln with natural gas. Last year one company suffered a 15% loss in output and significant deterioration in clinker quality after making this change. A small reduction in output of about 3% can be expected due to the increased volume of combustion products involved in the combustion of gas compared to solid fuels. The biggest problem comes with the burner and whether it is correctly designed for the combustion of gas. If not you can suffer a long flame, poor heat flux and transfer to the material, increased CO formation, dusty clinker, etc. The effects on dust, NOx and SO2 emissions depend on whether you suffer any of these problems. I would strongly recommend a combustion survey by specialist companies to avoid these problems. One benefit is that you will have reduced CO2 emissions. Gas (CH4) contains relatively more hydrogen and less carbon than solid or liquid fuels, therefore there is more H2O and less CO2 in the combustion product exhaust gases. For avoiding explosions you need to avoid CO formation. Again optimum combustion is the key and a survey will identify any changes required. This must be backed up by redundant CO monitoring on the preheater exit and precipitator inlet ducts. It is normal to trip the high tension from the ESP at 0.6% CO in the exhaust gases to allow for time lags in the sampling and measurement. However, do not rely on this for protection. You need to ensure you have complete combustion.

Question 64:
At our plant we have a 180m long dry process kiln from FLS. Maximum production level is around 130 tons per hour. We also use a later added preheater where the material is preheated till around 750 degrees C before the material flows into the kiln. As fuel we use natural gas, lignite, different kind of coal and all kinds of industrial waste products. We have a big cycle problem with our kiln. The actual kiln torque cycles up and down. It all starts with a little cycle in the CO2, after a short time this cycle goes over into the torque, the oxygen, the temperature and even in the kiln production. The kiln production cycles at those moments between 50 and 240 tph. As strategy against the cycle we keep the oxygen at a constant level of 1.3% at kiln inlet by regulating the fuel in the burning zone. After around two hours the cycle gets less and oft it is complete gone after four to five hours. What we would like to know is what the course of this cycling can be and if there is a way to provide this cycling.

Answer:
Your cycle sounds like a very difficult situation. It is very difficult to diagnose the problem without more information. It really needs to be studied on the site. The kiln torque cycling starts with the CO2? This points towards instability in the preheater, perhaps with material flushing through one of the cyclones. I would investigate that first.
Holding oxygen steady solves the problem? Do you see any variation in CO formation at the start of the cycle? That could cause variations in the alkali cycle and lead to the cycling torque as the amount of liquid phase varies and flushes through the kiln. I am concerned about the feed rate cycling between 50 and 240 tph. Is this in an automatic control loop? If so the loop may be contributing to the problem.
Question 63:
I am facing the problem of clinker quality in that the core of the clinker is very often brown in colour. What is the solution?

Answer:
The normal cause of brown cores in clinker is reduction of the Fe3+ to Fe2+. This is caused by insufficient oxygen available to fully combust the fuel in the kiln. You need to improve the control of the combustion.

Question 62:
We want to make use of dust being generated at Cement Plants. Please let us know the nature and composition of dust being generated at such works. Also please indicate any usage of such waste made in the past and expected in the future. For example, is it possible to use this dust to treat acidic water?
Answer:
The dust from a cement kiln will be primarily calcium carbonate, with some clay minerals or other oxides of silicon, aluminium and iron. There can also be traces of lime, and sodium and potassium chlorides and sulphates. In rare cases there are other heavy metals present. Disposal of cement kiln dust is normally a problem for cement companies. It has been used as a source of agricultural lime and spread on fields, but this is not possible in many parts of the world. It is also sometimes interground with the cement as an extender. It can certainly be used to treat acidic water as it is primarily alkaline. However, it is also very fine and contains the clay minerals. This might lead to problems of suspension in the water and blinding of any filters.

Question 61:
In our cement mill plant we have a false set problem. We have already made improvements by decreasing temperature of discharge mill down to about 118 deg C. Unfortunately, sometimes cement false set is still happening. I understand that with discharge mill temperature at about 118 deg C, the silo cement, cement temperature is still high (above 95 deg C)and causing gypsum dehydration and cement to be false set. In my laboratory research, the safe temperature of cement in silo is below 70 deg C for over one week storage residence time. My questions : (1) What is maximum temperature of discharge mill permitted in order to prevent false set problem ? (2). Do I need to install cement cooler in order to reduce cement temperature in silo? (3) What is the desired maximum temperature of cement in silo cement ?
Answer:
False setting, pack setting in silos and workability are all inter-related. I agree with your research that below 70 degrees C is a safe storage temperature for cement in silos. Many cement mills operate with discharge temperatures above 110 deg C. It is necessary to reach these temperatures in order to dehydrate the gypsum and make soluble sulphate available to control the setting of the cement.
The answers to your questions in order:
1.There is no problem with a mill discharge temperature up to 118 degrees C.
2.Installing a cement cooler is one option. However you should be able to cool the cement in the closed circuit of the mill. This can be done in the separator by drawing ambient air into the circuit. 3.70 degrees in the silo should be sufficient to prevent further dehydration in the silo.

Question 60:
I am interested to know the kind of cost implications (savings) of using alternative fuels and extenders.
Answer:
It’s difficult to be specific but here are some indications: The cost of kiln fuel is likely to be in the order of US$6 per tonne of clinker produced. Some cement companies are achieving 80% replacement of traditional fossil fuels with alternatives, therefore would be saving almost US$5 per tonne of costs. Some cement companies can also be paid for burning some alternatives, which might mean that they have negative fuel costs. This is rare however. A more normal situation would be to save up to 25% of fossil fuel costs, so US$ 1.5 per tonne of clinker produced. Extenders cost a fraction of the manufacturing costs of cement clinker. The benefits then depend on the amount of extender addition, which can be achieved. Some slag cement contain up to 80% slag. Again that is rare. Fly ash cements regularly contain 20 to 30% fly ash. Pozzolanic cements regularly contain 15% pozzolan. Portland limestone cements contain 5 to 20% limestone. The other key factor is whether the increased product provided can be sold, i.e. the market situation. If so then the marginal value of the additional product is very high.
Question 59:
This is my problem: In my kiln of capacity 2000tpd, KHD supply, over recent days I am facing snowman problems in the cooler. May I get a check list to find out the reasons for the above problem.
Answer:
Snowman problems are usually caused by irregular kiln feed chemistry, or by unstable operation of the kiln. There are ways to eliminate both these causes by improved training, operations or automation. Alternatively you can address the symptoms by installation of blasters or acoustic cleaners. I have even heard of companies installing “pushers” to clear the build-up. These things can be very difficult to solve but I would recommend eliminating the causes rather than responding to the symptoms. For example, how stable is the MgO content of your raw mix?

Question 58:
We are working in a project related to the safety of the ESP. I want to ask what is (are) the main cause(s) for the ESP explosion in cement plant? How can we avoid the explosion to happen and what is the safety standard?
Answer:
The main cause of explosions in the ESP is incomplete combustion in the main burner or precalciner. This leads to formation of CO and unburnt hydrocarbons in the kiln exit gases. When these encounter the high tension in the ESP the mixture with air can be explosive. The best way to avoid the explosion is to optimise the combustion so there is not formation of CO. If you have combustion problems this can be complex to solve and there are specialist companies who can help you. You should also have CO detection in the preheater exhaust gases and trip the high tension on the ESP is the CO content is more than 0.6%. This of course means you have a period of high dust emissions. You should also not rely on this protection alone. A failure of the gas analysis can then lead to the explosion. You must optimise the combustion to avoid the formation of CO and unburnt hydrocarbons.
Question 57:
According to reference articles, it’s possible to get a shorter and more intense flame by preheating primary air or by increasing the primary air temperature. It may make faster ignition velocity of fuel without increasing tip velocity. As I think, if we preheat primary air or if we use cooler vent gas as the primary air without increasing the primary fan capacity, there may be a possibility of CO formation caused by insufficient oxygen content. Under the same fan capacity of volume, temperature increase of primary air means that oxygen rate supplied into the kiln decreases. If so, we can’t avoid reducing condition, long flame, build-up problems, and worse quality of clinker. Can I get your opinion about that?
Answer:
Certainly preheating of primary air will intensify the flame, but also expand the air meaning a greater volume is required to provide the same oxygen content. Cooler vent gas is regularly used as primary air. These things need to be very carefully considered. there is not only the danger of incomplete combustion and CO formation, but also safety issues connected with hot air and finely ground or atomised fuels. I recommend that you consult specialist combustion companies before making any experiments.

Question 56:
We’re using converter slag which contains C2S mineral as an iron additive instead of copper slag and natural iron ore. I think those substitution results in the same effect as heating velocity lowers like burning under the condition of long flame. I mean the size of clinker minerals can be increased. Please advise.
Answer:
Certainly there is the possibility of larger crystals if you are adding slags to the raw mix. With metallurgical slags there is also the possibility that you are adding flux or mineralisers to the kilns. This could also lead to premature mineral formation and large crystal formation. To solve both problems you need to reduce the fuel supply to the kiln. This is very difficult for operators to do. I suggest you use an expert system.
Question 55:
We have dry kiln of 5700 tpd. We have a problem of feed material that goes from the kiln inlet chamber up to cyclone No 5 in that about 20% goes up instead of going to the kiln. How can we solve this problem?
Answer:
It is normal to have some recirculation of the material from the kiln inlet to the bottom stage cyclone, however 20% is too high. I would expect around 10%. You must make sure there are no obstructions on the feed shelf or kiln hearth that can throw the feed material into the gas stream entering the preheater from the kiln. It may be that you have to change the profile of
the refractory in that area to reduce the pick up of material in the gas stream.
Question 54:
I am working in one of the cement plants in the southern part of the Philippines. Sometimes we will experience snowman formation inside the clinker cooler. Can you recommend any equipment supplier who sells an equipment used in declogging the snowman formation?

Answer:
You can try big blasters or acoustic cleaners. if you install an acoustic cleaner remember they are good at preventing the snowman formation rather than removing it when it is formed. So you must sound the horn regularly when there is no snowman to stop one forming.

Question 53:
As new engineer I want to know about :
(1) basic design calculation of preheater
(2) effect of design with inner tube and without inner tube
(3) flow of heat transfer inside cyclone included in the preheater
(4) flow if any alkaline circulation

Answer:
There are various papers about the design of preheaters but these are quite complex – I suggest you read up in the literature. The purpose of the dip tube or vortex finder is to create a vortex in the cyclones and improve the collection efficiency of the cyclones which collect the preheater raw mix and pass it down to the next stage of the preheater. You can construct a heat balance around each cyclone stage. There are a number of considerations including the collection efficiency of the cyclones to determine the mass balances and also the calcination and any recarbonation which might be occurring. Alkali recirculation and condensation also adds to the complexity. I am sure you could spend many months studying these fascinating
topics. They can also be very useful for improving the efficiency of your kiln.
Question 52:
I would like to ask what makes the difference if:
(1) the Insoluble residue will be changed from 0.75% to 1.5% and (2) If the Loss of Ignition will also be changed from 3% to 5%?
Answer:
The significance of the insoluble residue and loss on ignition is that both measure the amounts of material in the cement which does not have hydraulic properties. The insoluble residue is inert filler which makes no contribution to the cementing properties of the cement. International standards limit this residue to 1.5% for this reason. The loss on ignition
may arise from a number of sources. the most likely would be limestone in the cement, pre-hydrated clinker minerals, or atmospheric carbonation of the clinker minerals. However, I would not expect that the changes in the insoluble residue and loss on ignition you have quoted would make much difference to the cementing qualities of the cement.

Question 51:
Gentlemen, please tell me how raw mix design play a major role in clinker quality?

Answer:
Raw mix design is fundamental to clinker quality. The clinker mineral composition is determined by the raw mix chemistry taking into consideration the incorporation of any ash from the fuels. The clinker mineral composition is the content of the minerals C3S, C2S, C3A and C4AF in the clinker and this determines the strength characteristics and the setting characteristics of the cement product. There are other important considerations of the raw mix design. The contents of minor oxides such as MgO, Na2O and K2O also affect the strength and setting characteristics, but also the soundness of the cement. The amount of CaO as a proportion of the maximum combinable lime also impacts directly on the soundness of the cement. The amount of Fe2O3 directly affects the colour of the clinker and the fineness of the cement. Finally raw mix design is not only about the chemistry of the raw mix. Raw mix mineralogy is also important with marls and clay minerals being easier to combine into the clinker minerals than quartz.
Question 50:
Is the variable speed necessary for the reclaimer rake movement? If it is what are advantages or disadvantages and how can we make variable speed in hydraulic system?

Answer:
Variable speed for the reclaimer rake is not necessary. To my knowledge most of these are fixed speed. The reclaimer normally runs to maintain the level in a premix bin and automatically starts and stops based on level sensors in that bin. In this case there is no need for variable speed. Having said that there is a common problem of segregation in the premix bin and this can only be overcome by running the bin at a constant level. Effectively this means the reclaimer is constantly stopping and starting and in this case a variable speed capability would be desirable. You can make variable speed in a hydraulic drive – most coolers have hydraulic drive and some kilns. These are both variable speed. It is done by a series of pumps which vary the pressure in the hydraulic fluid.

Question 49:
Calling from Mexico: please comment and send references about the use of raw bauxite (60% alumina max) as alumina source into formulation of portland clinker. I learned it has advantages because of its low alkali content.

Answer:
Yes, there are quite a few cement factories that use bauxite to supplement the alumina in their raw mix. That would be the main reason to use bauxite, although if the alkalis are lower than in local clays then that would be an added benefit. However, there would have to be some special circumstances to justify replacing clay with bauxite as the costs would be higher. Bauxite is also used as a raw material for production of calcium aluminate cements.

Question 48:
Can you tell me how a chloride bypass is different from alkali bypass and sulphur bypass? We have recently started the utilisation of coal as fuel along with furnace oil at our kiln having 1500t/day capacity. The coal has substituted 40 to 45 per cent of the fuel oil. Coal is being fired at kiln head in pulverized form along with fuel oil & being fed at kiln inlet in crushed form @ 3t/h. The raw meal has 0.3 per cent Na2O and 0.6 per cent k2O. A seven per cent chloride bypass was already in practice, which has been increased to about 12 per cent. The coal has 5-7 per cent sulphur and about 25 per cent ash content. My question is that why we are getting the same SO3 in clinker as before without coal firing about 0.7 per cent. Although we have increased the bypass volume we are still suffering with build up in kiln housing and in lower part of mix chamber. Can you suggest some remedial actions to over come this problem.
Answer:
A bypass is much more effective in reducing chloride cycles compared with sulphate cycles due to the difference in volatility of the two species. Alkali chlorides are very volatile and build up into a heavy alkali cycle. This means they can concentrate to 100 times the level of the input in the fuel and raw materials. In this case bleeding off a very small percentage is sufficient to break the cycle and avoid problems, because the amount bled off in the bypass is equal to the level in the inputs.

Sulphates are much less volatile. They only concentrate to 4 or 5 times the level in the fuel and raw material inputs to the kiln. A much higher bypass level is required to bleed off the amount entering the system in the fuel and raw materials. I do not think it will be practical to increase bypass levels to reduce sulphate related problems. I suspect that the addition of crushed coal to the kiln inlet may be part of the problem as this will inevitably lead to local reducing conditions. It is very important to avoid reducing conditions as this promotes the cycling of sulphates in the kiln. You should also optimise the combustion in the main burner to ensure there is no local reducing in the burning zone.

Question 47:
Instead of converting to a dry process, a US plant has recently converted a wet kiln to a semi-dry. The reason was apparently the high potential for SO2, THC and CO emissions, as the raw meal composition indicated. The dry process would impose a SO2 scrubber and a thermal CO oxidizer. I would like to know if there are any empirical limits for the organic carbon, reduced and/or organic sulphur in the raw meal, in order to be adequate to a dry process (with a calciner) to fulfil European or USA environmental standards.

Answer:
We are aware of such project. The carbon content of the raw materials are particularly high at greater than 0.5 per cent. The pyritic sulphur is not particularly high however at 0.25 per cent. Many European plants have higher levels of pyritic sulphur than this. The principal problem is that the carbon in the raw materials and pyritic sulphur volatilise in the top stages of the preheater on a dry process plant. This is overcome in the semi-wet process by feeding the raw material direct to the precalciner. This could also be done in the dry process with the raw material component containing the carbon and pyritic sulphur (usually the clay) being fed directly to the precalciner. In order to forecast the emission levels it is necessary to determine the levels of organic carbon and pyritic sulphur in the raw materials. In my experience all the carbon content and 33 per cent of the pyritic sulphur is
lost from the preheater. However, these are only rules of thumb and are likely to vary from plant to plant.
Question 46:
Can we use a reduction agent to reduce the oxidation of iron in order to increase the whiteness of the white cement? If yes what kind of reduction agent can we use? Are there other associated problems?

Answer:
It is well known to use a bleaching flame to reduce the iron in white cement clinker. This is achieved by a small auxiliary flame directed at the clinker as it falls over the nose ring into the quencher. Alternatively you need to adjust the main burner to combine the clinker as close to the nose ring as possible. Your fears regarding the affects on the calcium silicate minerals are justified. Reducing conditions can cause inversion of the belite and loss of hydraulic properties. These adjustments of the burner are specialist tasks. I would encourage you to seek the assistance of a specialist company.

Question 45:
We have a problem on our vertical coal mill(Loesche) since we switched to grinding anthracite. The capacity has reduced by more than 50 per cent. The mill used to grind and is designed to grind Australian and Indian coal. We have already tried to change parameters with our grinding pressures and dam ring height but still got minimum effect. Our mill model is LM 21.22D. Can you give us some tips on improving our grinding capacity or refer us to other companies with the similar situation but have overcome such problems.

Answer:
We have checked with another company grinding anthracite for firing on a cement kiln. They confirm that very low mill outputs are to be expected as the anthracite is hard and must be ground very fine. They have not made any adjustments to their mill but simply put up with the low outputs.
Question 44:
What could be the direct effect on the refractory bricks for a 40 per cent utilisation of petcoke fuel in cement kiln operation.
Answer:
The effects of 40 per cent pet coke firing on the refractory lining should be minimal. The coke contains virtually no ash therefore in homogeneity in the burning zone should be reduced. The coke is likely to be high in sulphur however. This might lead to a heavier alkali cycle in the kiln and potential for alkali ingress into the refractory. The two effects will balance out to some effect – I would not expect major problems with the refractory lining.

Question 43:
We are wondering if there is any risk to use a kind of spray water system in order to control hot spots in a cement kiln? Could it lead to kiln shell cracks, etc.

Answer: There is risk in using water sprays to control hot spots on the kiln. Intermittent spraying of water will result in a large thermal shock to the kiln shell and lining when the water is turned on and off. I have seen water sprays used for continuous cooling, but these again cause problems if there is any interruption in the supply of water. Once off you dare not switch the sprays on again! I would not recommend doing this.
Response:
I’ve read your comment regarding water spraying on kiln shell. I agree with the principle you gave. But I’m a little bit confused by other information. We were informed that the system works in many kilns in Japan without encountering any problem. Also our kiln here in the Philippines is not roofed and rain water occasionally cools our shell which can be similar in effect as off-on water supply. So far we haven’t experienced problems due to thermal shock. The reason why we are studying the use of water cooling is the power savings that we can derive from it and the quick cooling effect which enables to stabilise the kiln coating. From our discussion, there seems to be a way to eliminate the risk, perhaps a water flow control system. Could there be other reasons associated with water spraying that can lead to problems in other kilns?

Comment:
I agree that many kilns have used water cooling of the shell for many years with no problems. Also that many kilns in the tropics are open to the atmosphere and therefore exposed to heavy rainfall. There are no problems as long as you have regular operations. It can be a big problem if you have heavy rain on a static hot kiln – in this case you must keep the kiln turning during any stoppage, particularly during heavy rain. This is one reason why you must have a reliable auxiliary drive for the kiln. I agree that rain fall is equivalent to the on-off situation. With shell cooling the volumes involved are usually higher but the situations are analogous. We would like to have a wider exchange of knowledge and experience through the Forum

Question 42:
At this moment I am working in research for a very large cement group. I am not very sure if this is the right place for my question but I know here are the right persons technicians and suppliers) with whom I want to talk. I would like to ask an open question and invite both, expert technicians and scientists to participate in a forum that has relation with the developing their ideas on how making the cement industry more profitable through developing to new equipment or materials and how a cement company could support to put in practice such ideas. The vision is to integrate us like a single business. Any advice?
Answer:
I think your idea is a very good one. For sure there are many suppliers and specialists who have ideas on how to improve cement manufacturing but don’t have the resources or the contacts. Also they don’t have a cement company where their ideas can be developed. I think the only way to test your approach is to pose your open question to the Cemnet Technical Forum and we will see what responses we get.
Question 41:
We are intended to put up a small cement grinding mill of 100,000 tons per year capacity. Our electricity cost is very high, about USD 0.12 per kWh. What kind of ball mill will be more effective, open or closed circuit.

Answer:
A closed circuit ball mill will be more effective than an open circuit, however if power cost is your main concern then you should be thinking of a vertical mill, or a roll press and ball mill for finish grinding. The other concern is the capital cost, of course, and this may dictate your eventual choice.
Question 40:
I am calling from China. Recently I am facing a problem in that our cement plant wants to use a four-channel-burner in our burning system, but they don’t know the detail craft parameters of this kind of burner and the demands of using this product. do you have any advice?
Answer:
Sorry but this is too difficult – there are many things to take into account. The types of fuel to be injected through the channels, the volume and velocity of the swirl and axial air, the dimensions of the kiln hood, the diameter of the kiln, the secondary air temperature, whether there is a precalciner. I think questions regarding multi-channel burners should be left to combustion experts.
Question 39:
I am interested to understand about slag reactivity, if there is such a reaction and whether it needs cement for such reactions to take place?
Answer:
Granulated blast furnace slag is itself cementitious therefore it does not need the presence of cement to have cementing properties. The hydration and strength growth would be slow however. Slag is also pozzolanic therefore the Ca(OH)2 liberated when cement hydrates also activates the slag. This is why slag is usually mixed with cement which activates the pozzolanic reaction of the slag.
Question 38:
Our Company is intending to do some research on auxiliary burning materials in order to reduce energy costs as well as to help environment matters. For This purpose we are thinking of using waste or used auto-tyres. Please therefore guide us whether or not the case is advisable since the fuel (natural gas) is quiet cheap in this country? If so what is the technology to be used? Do we have to take other steps to modify the existing ESP in order to purify the polluted and unwanted particals and so on.
Answer:
The primary reason to burn waste tyres on your kiln would be economic. If gas is cheap in your country then it will make the economics harder to justify. The technology to be used depends on your kiln type, i.e. long kiln or dry kiln with preheater and/or precalciner. You should not need to modify your ESP, however there is a greater danger of CO formation when burning whole tyres therefore you must ensure that your explosion protection circuits and CO detection systems are fully functional. There are of course other alternative fuels you could use. This depends on availability in your country and the other industries which operate there.

Question 37:
Can you advise: our plant is a 4000tpd capacity unit producing OPC type 2, KHD double string preheater with pyroclone and shortest kiln 52mx4.8m diameter Each string has five stages. There is by pass system for kiln gases to control volatiles. Nowadays we are running kiln with 60 to 70% kiln gas by pass, but even then there is always ring formation at 15 to 25 metres from outlet due to sulphur internal cycle. Now we have been asked by management to start heavy fuel oil burning that will increase sulphur more in our system. Presently we are using crude oil as fuel both at kiln and pyroclone. Please advise if it will be possible to use HFO partially or directly. Once we tried 30% mixing of HFO in crude oil but we got a 9.0 m long ring in upper sintering zone.

Answer:
As the sulphur input to the kiln will increase when you switch from crude to heavy oil then you can expect your sulphate ring problems to increase, as you found in your trials. I suggest there are two things you need to do,
(i) conduct a combustion survey as incomplete combustion will increase sulphate cycles in the kiln and reduce the amount which passes out in the clinker, and
(ii) develop a model of the alkali cycles in your kiln in order to
optimise the bypass.
Question 36:
Recently, our government set up a new rule about the test requirement for imported cement. All imported cement will now have to be subject to a 28 day test before it is allowed to be sold – even if it first passes all the appropriate tests at 7 days. Is this acceptable or is it against the general rules shipping and trade in cement.
Answer:
The 28 day test requirement before it is permitted for imported cement to be sold is very uncommon in world-wide cement trade. Many countries have quality requirements for imported cement. These quality requirements in almost all cases are that the imported cement must comply to the same quality standards that are imposed on domestically produced cement. Testing in most cases will have to be done in a test facility approved by that countries government. Imposing a 28 day test requirement before sales on imported cement is a quality requirement that exceeds the quality requirements for domestic cement in virtually all countries. This measure is clearly protectionist and most probably could be legally challenged. If the cement complies with the quality standards that are in place for the domestic cement industry, no additional testing should be required.

Question 35:
A few of our customers have noted that after using our cement, green and black spots appear on the plaster after drying. This complaint comes after using a new brand of cement introduced by us, which has a lighter colour. The problem is not universal. A house is constructed and plastered with this cement but the green spots appear only at some places. In another case, several houses are constructed with same cement but only one or two face this problem. The houses are normally built with clay bricks, which are plastered with cement using a ratio of 1:4 (by volume) for cement and sand and water ratio of 0.45 ~ 0.50 to cement. We shall appreciate if you can let us have your views about possible reasons of this problem.
Answer:
I do not think the problem of the green spots on render using your cement is connected with the cement. Based on the information given I think this is a problem of efflorescence. When concrete or mortar/render is in contact with soft water (water containing very little dissolved calcium) then some of the
calcium hydroxide formed on hydration dissolves in the water. At the surface of the concrete or render the leached calcium hydroxide reacts with atmospheric CO2 and produces blooms on the surface of the concrete/render. I suspect this is the problem you are suffering. It would explain why it does not always happen. The problem is caused by variable hardness in the mixing water used with your cement.

Question 34:
In cement up to what percentage we can add the fly ash to make the composite cement? By adding fly ash in cement what will be the effect on its physical and chemical properties : 1- Strength, 2- setting time? What action we can take to increase the strength further in composite cement? How are the properties of the fly ash affecting the cement properties ? What chemical change can we make in clinker for further enhancement of the cement strength after fly ash addition?
Answer:
In the ASTM standard C595 Portland Pozzolanic cement are defined with up to 40% pozzolan additions. This is the class your fly ash cement would fall into. You can expect the setting time to increase and the early strength to be reduced. Effects on workability depend on how you add the fly ash to the cement. These changes do not mean the cement is inferior to Portland cement. In many concrete applications Portland pozzolanic cements provide distinct advantages – higher long term strength, lower heat of hydration and improved durability. You can increase the early strength by activating the cement and the pozzolanic reaction. Cement conditioners containing lignosulphonate and
alkali carbonate and sulphate would be the way to do that. The properties of the fly do impact on the ultimate strength and workability. The main distinction is between low and high calcium fly ashes. High calcium fly ashes are cementitious as well as pozzolanic therefore give higher ultimate strength. Unfortunately they also contain more alkali and sulphate and tend to reduce workability. You can offset the reduction in early strength by increasing the lime saturation and silica modulus of the clinker.
Question 33:
At present we are using diesel firing in kiln at the time of start up of the kiln after shut down. The diesel runs for 24 hours approximately, and consumption is 20,000 to 25,000 litres. The cost of diesel is quite high. We want to use the furnace oil in place of diesel. Will it be feasible and economical to use it. What necessary arrangements, we have to made to use furnace oil in place of diesel? Can we use the mixture of diesel and furnace oil at the time of start up? If yes, in what ratio we have to mix, so we can use it without changing the present firing arrangements.
Answer:
We know of a few cement kiln which are always started on heavy fuel oil. In fact, arguing against what you propose, we recommended that such plants switch to diesel for light-ups as they only maintain the steam driven heavy fuel oil heating system for light-ups. The rest of the time they operate on coal. With regard to using mixtures of diesel and furnace oil, I think you need to consult specialist combustion engineers.
Question 32:
We are facing problems of coating on preheater fan impeller. What may be the possible causes of coating formation? How we can check the chloride cycle in preheater? At what temp does it evaporate or condensate? At present we are using 50% pet coke in totality. Coating formed on impeller some times it is hard and some times it is soft? If this coating is due to chloride what actions should be taken to avoid coating formation? If coating is due to So3 what actions should be taken to avoid coating formation? This coating formation occurs when we run the fan at 100% speed. if we reduce the fan rpm slightly then we can run the fan for longer period but still coating formation is there. Pet coke is having 6% sulfur. In our Raw mix there is 0.02% to 0.04% chloride.
Answer:
Build up of coating on the i.d. fan impellor is quite a common problem and there are a number of causes and solutions. It may be due to chlorides passing out of the preheater in the dust, however it is more likely to be caused by static electrical charges on the dust causing the material to cling to the impellor. Solutions which have been reported are
(i) changing the shape of the impellor,
(ii) installation of acoustic cleaners on the fan casing,
(iii) injection of sand into the down comers entering the fan,
(iv) injection of water into the down comers to neutralise
the static charges.

You can check the chloride cycle in the preheater by sampling the esp dust, material in each cyclone, the kiln hearth and clinker and measuring the chloride content. You can then build a model of the alkali cycle in the kiln – I can help you with this. Chlorides are big problems and the cycle can only be properly checked by installation of a bypass. You might be successful by rejection of the GCT or ESP dust from the system but then have the problem of disposal of the dust. If the problem is sulphate based then
you might be able to solve it by adjustments of the raw mix chemistry.

Question 31:
I would like to find out the differences in performance characteristics between triethanolamine-acetate and diethylene glycol as cement grinding aids in terms of (1) mill efficiency, (2) dispersion, (3) reduction of agglomeration, (4) improving flowability, set time and pack-set properties of the cement and (5) price. Also, are there any other good substitutes for TEA acetates for a grinding aid? Thank you.
Answer:
Triethanolamine (TEA) acetate grinding aids are produced by the reaction of TEA and acetic acid and these were the first generation of modern grinding aids. Later research then lead to the introduction of phenol amine products and then the glycol based products. Ethylene glycol grinding aids are therefore a later generation than the TEA grinding aids. These later products were introduced because of their superior price-performance characteristics. I would suggest that you approach the specialists, Grace or Mapei. There are many developments taking place in this field. The focus is as much on cement conditioners as on grinding aids. This is being driven by the increasing trend to produce extended cements with supplementary cementitious materials replacing a significant proportion of the cement clinker. This means that the workability, setting time and strength development of the cement all become critical factors and various accelerators and water reducing agents are used to condition the cement performance. There are many overlaps with concrete admixture chemistry. The materials being introduced are lignosulphonates, phthalates, tri-sopropanolamine and various chlorides and alkali carbonates and sulphates.
Reader’s view:
The influence of triethanolamine acetate versus diethylene glycol can be cement and mill system dependent to some extent and needs to be assessed in a field trials. But in general we can expect triethanolamine acetate compared to diethylene glycol to have:
1. Equal or better grinding efficiency
2. Better effect on dry dispersion
3. Greater reduction in agglomeration
4. Improved flowability and lower packset
5. Shorter set time (at low dosages)
6. Better early strength
7. Equal or higher price.
The first three points can also mean a better cement particle size
distribution, which in itself can lead to better strength development. We supply a range of formulated alternatives, the choice of which will be a function of:
1. Cement Type and composition
2. Cement physical and chemical properties
3. Mill system design and operation
4. Cement performance requirements
Question 30:
I want produce coloured cement using different pigments and clinker. What the different pigments which I can use?

Answer:
There are various pigments you can use: chromic oxide green, red iron oxide, carbon black, black iron oxide, yellow iron oxide and iron hydroxide, brown iron-manganese oxide, titanium dioxide white, phthalocyanine blue and phthaloncyanine green. All these will be more effective if you use a white cement or cement clinker as the base. They may also affect the hydraulic
properties of the cement.
Question 29:
I am working in a cement plant. The kiln has the 4.7m diameter length with a 5000tpd capacity. Recently, we are facing the formation of coating at 45m which we have never seen before. As a result, the kiln feed has to be reduced or even stopped due to the feed build-up.

The length of the kiln is 74m. The ring is close the inlet of the kiln. The K2O for clinker is 0.7 and Na2O is 0.06. The SO3 of the clinker is 0.7. Also: The thickness of the coating can be as high as 2metres height. What’s the proper way to remove the ring if there is any? Would the addition of alternative fuel contribute to the formation of the ring?
Answer:
Firstly, you do not seem to have a serious imbalance between alkali and sulphate in clinker. However from the position of the ring I would expect it be composed of sulphate spurrite, or maybe spurrite. Do you have any information on the alkali and sulphate levels in the hot meal entering the kiln?

Heavy sulphate build-ups can be caused by formation of CO in the burning zone. Do you suffer from CO at the kiln inlet? If none of these then you must try to change the temperature profile in the kiln so that the spurrite forms further into the kiln. To do this I would suggest increasing the percentage of the fuel burnt at the main burner and reducing the percentage burnt at the precalciner. Alternative fuel could certainly be a cause of the ring. It depends where the fuel is added and the sulphur content of the fuel. There are various devices available to remove ring build-ups. Have you made contact with Winchester in the US?

Question 28:
We run a white plant. Our clinker whiteness is about 83-86 and the gypsum whiteness is about 83. Grinding is achieved by a close circuit grinding in tube mill that consists of two-compartment, the first one includes steel balls and the second includes ceramic balls. The whiteness of the finished cement is about 80-81.
(1) what is the reasons of the reduction of the cement whiteness. How can we increase the percentage of the clinker whiteness? What kind of additives can we use in raw meal to increase the whiteness?
(2) How can the iron oxides (Fe2O3, Fe3O4, and FeO) effect in different ways in the whiteness percentage.
Answer:
The clinker whiteness must be measured on a finely ground sample of clinker. The reason you are seeing a reduction in the cement whiteness is that the cement is not ground as finely as the clinker in your test. The way to improve the cement whiteness with your clinker is the increase the grit returns and recirculating load of the cement mill. There is no additive which you can economically use to increase the clinker whiteness. The way to maximise the clinker whiteness is to minimise the Fe content of the raw materials and fuel and to ensure that the Fe is present in the ferrous (FeO) oxidation state rather than the ferric (Fe2O3) oxidations state. This control of the oxidation state of the Fe is achieved through adjustment of the kiln burner and the combustion in the kiln.
Question 27:
We are a small grinding unit producing OPC cement of high blaine around 400 m2/kg We have observed that there is lump formation in cement bags after storing for around 15 days. The lump formation is occasional and not in all bags. We are maintaining cement mill outlet temperature around 100 to 105
deg C with no water spray inside the mill. The alkali content in cement is around 0.8 per cent and chlorides around 0.05 per cent. Does this have any effect on lump formation if their quantity increases occasionally?

Furthermore, We do have lump formation problems occasionally in silo also. We already have a closed circuit mill.
Answer:
Variable alkali content can cause lump formation via formation of syngenite (alkali carbonate), however I doubt if this is the problem. I suspect there is continued dehydration of gypsum in the cement silo and bags. I would recommend cooling the cement in the closed circuit of the mill (via the cold air bleed) before pumping to the cement silos.

Question 26:
I was wondering if I can have some information about freelime and the decarbonation which happens in kiln and how do they effect the whole operation. What about lime saturation factor (LSF) and its effects?
Answer:
The raw mix feed to the cement kiln contains about 80 per cent calcium carbonate. As the feed is heated to 900°C centigrade the CO2 is driven off the calcium carbonate to liberate free lime. This process is known as decarbonation and is endothermic, consuming the greatest part of the energy supplied to the kiln.

The liberated free lime then combines with the clay minerals to form the cement clinker minerals. There is a maximum amount of lime which can be combined in this way. When this maximum is combined the clinker is 100% lime saturated. This is the basis for the LSF – it is the lime content as a proportion of the maximum combinable lime. The higher the LSF the more energy the kiln consumes. Above 100 per cent LSF there will be residual free lime present in the clinker.
Question 25:
Our kiln has the 4.7m diameter, with a 5000tpd output. Recently, we are facing the formation of coating at 45m which we have never experienced before. As a result, the kiln feed has to be reduced or even stopped due to such build-up problems What’s your opinion?
Answer:
What is the length of the kiln? I presume the ring is close to the inlet of the kiln as your are suffering from feed spillage over the back of the kiln. The most likely cause is formation of a sulphate based ring close to the inlet. What is the alkali content of the clinker and the sulphate content? An alternative would be over-calcination in the precalciner. What is the loss on ignition of the material entering the rotary section of the kiln?

Question 24:
We are a small grinding unit producing OPC cement of high blaine around 400m2/kg We have observed that there is lump formation in cement bags after storing for around 15 days. The lump formation is occasional and not in all bags. We are maintaining cement mill outlet temperature around 100-105¾C with no water spray inside the mill. Kindly let us know what could be the possible reason and its remedy. The alkali content in cement is around 0.8 % and chlorides around 0.05 %. Does this have any effect on lump formation if their quantity increases occasionally?
Answer:
Variable alkali content can cause lump formation via formation of syngenite (alkali carbonate), however I doubt if this is the problem. I suspect there is continued dehydration of gypsum in the cement silo and bags. Do you suffer lump formation in the silos? I would recommend cooling the cement in the closed circuit of the mill before pumping to the cement silos.
Question 23:
Our Kiln is designed of 3300tpd (double string with calciner), supplied by Krupp. At what extent we can increase clinker production. At present we are operating on 5400tpd. The details of other auxiliary equipment are as follows:
• Raw mill: 360tph
• Coal Mill: 40tph
• Grate Cooler: 90.3m2.
Answer:
From the information provided you seem to have pushed the kiln output significantly above the design capacity. It is hard to imagine there is much scope for obtaining more from the kiln. To properly evaluate this would require much more data on the size and design of the kiln. However there are two possibilities:
(i) you might consider enriching the combustion air with oxygen – this is being used to boost output by up to 20% on several US kilns,
(ii) you might consider mineralising the kiln feed with calcium fluorspar – this can certainly boost production. Both options do involve other impacts on the process and need careful consideration.

Question 22:
I would like to know the reaction mechanisms and kinetics between the raw materials in the different stages of a 5-stage preheater with a calciner and a 4.67x60m cement kiln in order to get clinker, or the global reaction kinetics between the CaO, SiO2, Al2O3 and Fe2O3 to get C3S, C2S,C4AF and C3A
Answer:
The raw mix undergoes a sequence of reactions in the preheater and kiln. First there is the evaporation of any residual moisture, followed by dehydration of the clay minerals, calcination of the limestone, reaction of the liberated lime with the clay minerals to form the first liquid phases (C4AF & C3A) and solid C2S, and then the final combination of C2S with the residual free CaO to form C3S. The dehydration and calcination reactions are essentially sublimation reactions. The reactions between lime and the clay minerals are solid state reactions and diffusion controlled, however the presence of the liquid phases (flux) promotes these reactions and increases the reaction kinetics. A further important consideration is the presence of trace minerals which can act as inhibitors (principally alkalis) or mineralisers (principally fluorides).
Question 21:
I am working on a 4 stage SP kiln with grate cooler Which was installed initially with a capacity of 1000tpd and was optimised to 1500tpd by providing a rsp calciner. Now by providing a high pressure fan at cooler compartment No.1, so that we may keep under grate pressure at 65-70 m.bar instead of 45 m.bar, could you expect a rise in production by 150tpd. While
the kiln I.D is already being operated at 90-100% damper opening. Will you please let me have the standards or criteria to asses the capacity of this plant with respect to various equipment installed.
Answer:
I do not think that increasing the pressure in the first chamber of the cooler will be sufficient to give you a further 10% increase in output. The air will simply find the path of least resistance and blow a hole in the clinker bed. The basis of direct aerated coolers is to distribute the cooling air beneath the bed via aeration beams. With regard to assess your plant you need to benchmark against similar operations.

Question 20:
Please inform us, what environmental change you expect by using rubber tyres in kiln. What are the European standards for ambient air or stack conditions. What are the ways and means to control them, if one of the parameters is beyond prescribed limits.
Answer:
The only problem you might experience are increased CO emissions. Because of burning lumps of rubber tyres there is a greater chance of incomplete combustion and spikes of CO in the exhaust gases. This can the cause you problems if you have an electrostatic precipitator as you will have to switch off the high tension when you experience these spikes of CO. This then leads to increased dust emissions. A number of factories in Europe have converted to bag filters on their kilns for this reason.
Question 19:
Our smoke chamber, by-pass conditioning tower, inlet duct and conditioning tower inlet deflector’s build-up (first) with sticky materials. Then the build-up changes to hard materials.
The kiln condition is as follows:
• 5200tpd kiln, speed 3.2rpm
• ID fans speed 1100rpm, by-pass speed 500rpm
• heavy oil percentage main burner 6650kg/h, prolong 12450kg/h
• smoke chamber temp.420°C
• alkali ratio% ~1.5%-by pass c.t.outlet temp.~150c.
The above mentioned problem leads to last stage cyclones clogging. Could you explain why the bypass system rapidly builds up?

Answer:
I presume you mean the bypass quenching chamber when you refer to the smoke chamber? The problem sound sto be build-up in this quench chamber leading to restriction in the bypass gas flow. This then leads to insufficient bleed of alkali from the system and blocking of the lower of the preheater. There is a need for much more information on the configuration of the bypass and the alkali, sulphate and chloride content before a real diagnosis can be made. My suggestion is that the build-up in the smoke chamber must be eliminated. Are blasters fitted? Have you tried sonic cleaners?

Question 18:
We are a cement factory which produces mineral clinker with SO3 (2.5%) by burning 100% petroleum coke containing 6% sulphur. We have two questions regarding our clinker:

1 – What is the possible form of SO3 in the clinker since we found the percentage of CaSO4 very low.
2 – We are facing a problem concerning the hardness of our clinker and relatively low early strength even with LSF = 100. What are the possible solutions in you opinion to solve this problem?

Answer:
The SO3 in your clinker can be in the form of sodium or potassium sulphate or possibly the double salt of potassium and calcium, langbeinite. Please let me know the alkali content of your clinker. This might well be the cause of your problems. Alkalis act as inhibitors and result in the need to burn the clinker hard. Also the presence of high flux contents of alkalies and sulphates can lead to the growth of large unreacive crystals in the clinker.

One possible solution would be to add calcium fluoride to the raw mix. This will lower the burning temperature resulting in softer clinker and less flux formation. Please let me know if you want more information on this.
Question 17:
How can I form coating in a cement kiln? The coating in my kiln is very unstable. What’s the most important factor?
Answer:
Kiln feed chemistry is the most likely cause of unstable coating. Firstly you need to have stable chemistry. The next most important thing is the flux content. This is mainly affected by the silica modulus of the kiln feed. Other important factors are alumina modulus, magnesia content and alkali content.
Follow up: I have now studied the clinker information you sent to me. I do not think this is the cause of your problems. In fact the variation in the main moduli is very low and reflects well on your quality control.

There is some variability in the clinker SO3 and free CaO content however. This indicates some variance in the kiln burning intensity and combination of the clinker. The silica modulus is relatively high, but I would not expect this to cause major problems with the coating.

The next area to investigate would be the variability of the fuels and particularly the positioning and set up of the burner in the kiln.
Question 16:
We have a white cement plant having kiln size 3.2 m x 48 M. We are using kiln and calciner air heating through heat exchanger provided with preheater duct going to exhaust fan. At present, the air temperature going to exchanger is ambient temperature. We like to propose to the management to recover the heat from kiln shell by providing a circular cover lined with refractory between tyre–I (outlet end) and tyre-II. The gap between kiln shell and this circular cover will be considered 530 mm to maintain 2.5 m / second velocity with 23,700 Nm3/hr air to pass through the opening at both side.

The length of covered area will be approximately 20m and air will be tapped at top or bottom centre point of this circular duct and will be connected to the heat exchanger inlet. We expect to increase the inlet temperature of air at heat exchanger and ultimately the temperature of air being used for kiln and calciner firing. Is this feasible?
Answer:
Your proposal is feasible, but you must be careful as the loss of heat from the shell is a fundamental requirement for the integrity of the refractory lining in your kiln. It is also vital for coating formation in the burning zone. Definitely you should not enclose the entire shell – only the top half of the shell. By this means you will capture radiation from the upper half and all the convected heat. This means that any tapping must be from the top.

Quantifying the heat recovery and temperature rise requires the development of a heat balance model. There are other cement companies doing this, or investigating it. We would be happy to help if you wish.
Question 15:
We are planning to produce pozzalanic cement (as a new cement product),could you please indicate the following information:
• How to handle the high moisture content in pozzolana as it is ca.10%; how to handle the high fineness in order to avoid mill clogging. Also we may think to produce natural pozzolana is it necessary to install a dryer?

Answer:
The pozzolan addition will be limited by the drying capacity of your mill. You need to develop a heat balance of the mill to assess the maximum addition rates which you can use with your mills. The key point is limit the pozzolan addition such that the exhaust gas temperature is not below the dew point. If you add more you will indeed have clogging. I would certainly recommend a dryer. Cost will depend on whether exhaust gases from your coolers or preheaters can be used in this dryer. How long it will take will depend on whether you have existing equipment which can be converted. To go further really requires much more information to evaluate the options.
Question 14:
I want ask the effect of adding iron ore which contains 21% of iron metal for the process in kiln.
Answer:
I do not think 21% iron metal will be any problem in your kiln. The iron ore addition will be low (perhaps 1% of the kiln feed), and therefore the total amount of iron metal will be low. This is similar to the reinforcement in tyres when they are burnt in the kiln.
Question 13:
Normally slag powder has been used as an additive in making concrete in our region, but the only unfavourable problem is the lower compressive strength in early age. Of course we can increase the early strength of concrete by increasing the Superplasticizer dosage or using silica fume, or by heating the concrete, etc. I’ve heard that the several components and material like aluminate, alkalies, and gypsum act as activators to activate the reaction of slag powder, and many manufacturers of slag powder add gypsum as an activator to compensate for lower early strength while producing slag powder. There’re many forms of gypsum like gypsum, hemi hydrate, soluble anhydrite and insoluble natural gypsum. And there’re also several kinds of gypsum depending on their sources like synthetic gypsum, flue gas desulfuric gypsum, fluoro gypsum, titano-gypsum, etc. Which forms or kinds of gypsum would be more effective to increase the early strength of concrete?
Answer:
The gypsum must be a source of soluble sulphate, therefore hemi hydrate or soluble anhydrite. It doesn’t matter whether this is derived from natural or synthetic gypsum.

Question 12:
I’ve have a problem in the junction from smoke chamber to conditioning tower of the bypass where there is accumulation of clogged materials which lead to decrease of suction from the kiln inlet and many other problem in consequence. Please tell me how could I solve this problem.
Answer:
It seems to me that the temperatures must be too low in the region where you are experiencing these build-ups. With a bypass the intention is that the volatile materials should remain in the gaseous phase until they are taken into the bypass where they are then predicated by the sudden reduction in temperature. If you are experiencing condensation before the bypass take-off then the temperature must be too low. Perhaps your bypass is designed for chloride and the build-up is sulphate based? In that case you might try to adjust the alkali to sulphate ration.
Question 11:
I’ve tried to get the wider size distribution of cement through the tube mills. I mean I want to increase the blaine value and the residue on 44 micron sieve. Normally I decrease the circulating load of separators by closing the mill fan dampers and by decreasing the separator speed. That is, I think the wider size distribution can be done by making the mill retention time shorter. Please let me know if you have other easier ways for that. Reading “technical forum” in the morning becomes my first daily routine.
Answer:
Your strategy seems to be the correct one. To independently control the Blaine and 45 micron residue you need to independently vary the feed to the mill and the recirculating load. You could also try adjusting the drafting around the separator in addition to the separator speed.

Question 10:
Dam ring break lining breaking, a satellite cooler kiln 80meters long, dam ring linings when new start breaking every 15 days abut 50 pieces of bricks break causing kiln stoppage. after two or three times, the breaking stops and it works for six to 12 month nothing happens to this part until next new lining then it happens again?
Answer:
It is difficult to diagnose the problem over the internet, but it sounds as if there may be insufficient expansion compensation in the dam ring linings when they are first installed. This would cause the initial failures when first put into service, and then when the refractory has been in service for some time the inelastic expansion would explain the subsequent 6 to 12 months service.
Question 9:
Normally it is specified that the ball mills should operate 68-75% critical speed for optimum solution. one of our raw mills (a ball mill) is operating at 70% of critical speed. Our people feel that by increasing to 75% critical speed, we would get improvement in grinding efficiency and improved specific energy consumption. Is it possible to apriorily predict the mill performance with increased speed? Is there any published articles on this subject?
Answer:
We do not recall any recent articles but will look for you. Personally I would doubt that an increase of 5% from 70 to 75% would make that much difference. Also the costs of changing the motor or gearing might be quite high. We should be very interested to hear of the results your experiment.

Question 8:
I thought that when a mill is charged correctly (filled to 30% of the mill volume) it should draw 95% of full load current of the mill motor. By using the FLS calculation h/D to determine the amount of charge required to provide 30% I have found that the draw is less than 95% after charging. A local consultant has a different model. They suggest that you should determine 30% of cross sectional area rather than using the h/D model. By using this alternative model I have found that after charging the mill I am closer to 95% of maximum power draw.

As a side note, I have also monitored kw draw vs tonnes of media charged and have found that each tonne of media draws an extra 8kW. This however cannot be used in the field to determine the amount of charge required because it is a dual chamber mil and this model does not specify the amount of charge required in each chamber.
Answer:
I completely agree with you. The h/d approach is a simplification. It is more accurate to use basic geometry and work out the true volume fill of the mill. If you are not going over 95% of the installed motor capacity then I cannot see that you are over-charging the mill. I also agree that monitoring the kW drawn does not tell you which chamber to put the media in – but it does tell you when it is time to go inside and do a physical measurement.
Question 7:
How does variation in LSF affect fuel consumption (ie. for a 0.01 reduction in LSF does BTU usage drop by a set amount)?
Answer:
Relationships are quoted by FLS and other companies. For example, A one per cent LSF reducing fuel consumption on a 4 stage preheater kiln by about 15 kcal.kg clinker. But there are many other factors to take into consideration. Use these numbers with caution.

Question 6:
Does anyone know whether there exists a study on the influence on organic emissions when burning old tyres in a cement kiln? Especially I would be interested if there is a difference in using shredded tyres or whole tyres.
Answer:
There are many papers about burning tyres in a cement kiln. The temperatures are such that there are no organic emissions, irrespective of whether the tyres are added as shreds or whole tyres. I suggest you look at the ERATECH web site, or www.tyrederivedfuel for references.
Question 5:
How I can form coating in a cement kiln. The coating in my kiln is very unstable. What is the most important factor?
Answer:
Kiln feed chemistry is the most likely cause of unstable coating. Firstly you need to have stable chemistry. The next most important thing is the flux content. This is mainly affected by the silica modulus of the kiln feed. Other important factors are alumina modulus, magnesia content and alkali content.
Question 4:
I am launching a study on alternative raw materials to produce raw meal. To what degree is it possible to substitute natural raw materials with the alternative ones?
Answer:
There are a number of factories which use fly ash to replace the clay component in raw mix. Also a number are using by-products from the iron industry as mill scale to increase the iron oxide content of raw mix. You can find references to the use of synthetic gypsum in raw mix and also calcium silicofluoride as a mineralising agent. I do not know of any examples where the calcium carbonate content of the raw mix has been replaced however. This means that the limestone, which is the major component, is not replaced.

Question 3:
We run three big cement plants with capacity 7800tph each. We have problems with the quality pile of mixed limestone and clay (20% of clay). We build the piles with stacking equipment using chevron method. Our problems are that at the end of the pile and the early on in the pile, the LSF (quality) is fluctuating. Could you help me to eliminate the conical effect in our pile?
Answer:
Problems of variation of end-cones of chevron blending beds are very common. Many cement companies just “ride through” the variation. Another alternative is to recirculate the end-cones onto the next blending bed. The problem with this is that it means double handling the material, but does eliminate the variability problem.
Question 2:
I just want to know if we used petroleum coke instead of crude oil in cement production how it would effect the climate and on the human health of the people living close by?

Answer:
Provided you install the correct equipment for storing, grinding and feeding the pet coke to your kiln there would be no health implications for the people living around your cement factory.
Question 1:
If I have SRC cement, and the analysis showed 5% of C3A and the expansion is reaching 0.04%, will this be ok and safe to be used in construction work?
Answer:
5% C3A is the upper limit for Type 5 sulphate resisting cement. Is the expansion you are referring to that measured in a sulphate expansion test or the normal Le Chatellier expansion? These are only some of the considerations in deciding whether the cement will be suitable for use in construction. What is the application? Foundations or super-structure? What are the ground water conditions? In the Gulf these might well be susceptible to sulphate attack. What concrete mix design will be used? Pozzolanic or blast furnace slag mineral admixtures can confer excellent sulphate resistance to concrete.