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EVERYTHING YOU NEED TO KNOW ABOUT THE CHEMISTRY OF KILN FEED AND CLINKER

RAW MATERIALS

The basic ingredients for portland cement consist  of  limestone, sea shells, marl, or chalk, that provide the calcareous components; clay, shale, slate, or sand, to provide the silica and alumina; and iron ore, mill scale, or similar material to provide the iron components. The number of raw ma­terials required at any one plant depends upon the composition of these materials and the types of cement being produced. To effect the proper blend, raw materials are continually sampled and analyzed, and the pro­portions adjusted as they are blended together.

After being excavated in the quarry or mine, limestone is first passed through the primary crusher then to the secondary crusher where it is reduced to about 3/8 in. in size. At this point other raw materials are blended with the limestone and the blend is conveyed to raw storage piles. Samples, mentioned above, are obtained at this point and immediately analyzed. In a modem plant this sampling and testing is the source of data fed into a digital computer controlling composition of stored and blended raw feed.

In the dry process, the material is now removed from the blending piles and delivered to the raw grinding mills, where it is reduced in size until about 90% passes the 200-mesh screen.

In the wet process, the raw feed is transferred from raw storage piles to the grinding mills, which are substantially the same as the ball, tube, or compartment mills used for dry grinding. Introduction of water into the mill along with the feed results in the formation of a slurry. After grind­ing, dry kiln feed or slurry is drawn from storage and fed into the rotary kiln.

CHEMICAL AND PHYSICAL PROPERTIES

Usually, any one constituent of the blended kiln feed can  be found in more than one of the raw materials. For example, typical·raw materials might contain key oxides in the proportions shown in Table 9.1.

From such typical raw materials, a plant chemist tries to obtain a kiln­ feed mix that contains a predetermined oxide amount of calcium (CaO), silica  (Si02),  alumina  (AI203)  and  iron  (F 03).    In  some  locations,mixing  of  only  two  or  three  different  raw  materials  accomplishes  this,whereas  in  some other  plants  it might  need  up  to  four  or  five different materials to achieve the same results.

In addition to these basic oxides, r w materials also contain a certain percentage of so-called impurities which show up in the kiln-feed mix. Magnesia (MgO), in some plants can amount to levels of 4.2% in the mix which, if not properly controlled, can lead to unsound (expansion) cement. Magnesia acts as a flux at sintering temperatures which renders the burning slightly easier.  However, a magnesia-rich  kiln  feed tends  to .”ball” easily in the burning zone which, from an operator’s  viewpoint, is considered an undesirable property.

Clinker, made from magnesia-rich feed must be very rapidly cooled once it has been burned, to guard against production of unsound clinker. Plants faced with this problem usually locate the burning zone very close to the discharge end of the kiln and have quick quench compartments at the inlet to the cooler.

Oxides of potassium (K2O) and sodium (Na20), commonly referred  to as alkalies, are impurities that not only have a deleterious effect on cement quality but can pose considerable operating problems particularly  in a pre­heater kiln. Of the two alkalies, potassium is by far the predominant im­ purity that needs close attention from the plant chemist. During the burn­ ing process, alkalies vaporize in the lower part of the burning zone, travel with the kiln gases to the rear of the kiln, and condense again at a gas tem­ perature of around 900 C (1650 F).  These alkalies react in the colder part of the kiln with sulfur dioxide, carbon dioxide, and chlorides that are con­ tained in the kiln gases. Thus, an internal alkali cycle is created that can lead to troublesome buildup and ring formations in the kiln. In dry- and wet-process kilns, condensation of alkalies occurs in the lower end or just below the chain section whereas in  preheater,  semidry,  and precalciner kilns this condensation takes place in the lower stages  of  the preheater tower or grate preheater. Alkalies are quite an intriguing problem for any kiln manager. There must not be too much of them in the clinker; they should not be recycled and allowed to accumulate in the kiln; and yet they are found in great quantities in the raw materials. To combat these problems, various means are employed to keep these alkalies under con­trol. In wet- and dry-process plants, part or all of the kiln dust collected in the baghouse or electrostatic precipitator must be wasted. Some plants are fortunate in that they have to waste only the last section of these dust collectors, i.e., the very fine dust particles  that  are  richest  in  alkalies. Some of these plants sell this potassium-rich kiln dust as fertilizer. Pre­heater and precalciner kilns are equipped with alkali bypass systems at the preheater tower to control this internal alkali cycle.

There are also plants that could tolerate a slightly higher alkali content in the clinker but face the trouble of large, internal alkali cycles which call,for different solutions to inhibit the vaporization of alkalies.

Sulfur (S03) is introduced into  the kiln by the raw materials  and  the fuel. this impurity will also vaporize to form sulfur dioxide (S02) at a temperature of = 1000 C (1832 F) and condense in the form of sulfates within the kiln system. They readily combine with calcium to form cal­cium sulfates and potassium  to form potassium sulfate both of which  are the prime culprits for ring and buildup problems in the upper half of the kiln system. If there  is a Jack of aikalies present with which the sulfur dioxide would combine to form alkali-sulfates then much of the sulfur dioxide would leave the kiln system with the kiln gases.

Experience  gathered  by  many  plant  operators  has  shown  that  there should be a delicate balance between the alkali and sulfur contents in the raw mix.  If the molecular ratio of alkalies-to-sulfur is significantly below 1.0 this gives rise to calcium sulfate buildups near the kiln inlet in pre­ heater kilns.  By raising  this ratio to 1.0 (in the form of adding alkali-rich raw material) some plants have been successful in reducing  the frequency of calcium sulfate buildups  in the kiln.   Likewise,  the converse has been experienced, i.e., when this ratio exceeded  1.0 by a large margin.   In such cases, due to the excess  of alkalies,  alkali sulfate buildups  occurred.   In such cases, the solution would be to lower the alkali cycle within  the kiln by  wasting  kiln  dust or  letting  part of  the kiln  exit gas  bypass the pre­ heater vessels.  Another solution would be to add sulfur-bearing (S03) raw materials to the feed to balance the excess alkalies.

Buildup problems are usually attacked by first analyzing the material of the buildup, determining its predominant compounds (in  other  words trying to find out what compound caused the buildup) and finally selecting a solution that would reduce formation of these deleterious compounds.

Chlorides originate primarily from the raw materials and from the coal.

For proper kiln operation, plant chemists usually try to hold the total chloride content in the raw mix below  0.02%.  Chlorides,  too,  vaporize and react with alkalies to form alkali chloride. Alkali chlorides tend to remain in the internal kiln cycle for a long time and can lead to heavy coating and ring formation in the upper part of  the  rotary  kiln  and  the lower stages of the preheatcr. Chlorides, even in such small quantities as 0.02% in the kiln feed can become so troublesome on some preheater kilns that they are forced to operate with a bypass of up to 15% at the preheater tower.

Fluoride, although volatile like the alkalies, sulfur, and chlorides, does not participate as readily in the internal cycle as the above-mentioned com­pounds. Most of the fluoride leaves the kiln  with  the dust  in  the exit gases or in the clinker.

It is primarily  for these impurities that the preheater kiln didn’t find rapid acceptance when it was first invented 40 years ago. Buildup prob­ lems plagued these kilns to such an extent that many managers considered the almost daily kiln shutdowns for buildup removal a problem not worth dealing with. Today these problems  have been  predominantly  overcome and the preheater kiln has rightfully taken the leadership in preferred types of kilns when new plants are constructed.

In coal-fired kilns, a plant chemist must also consider the ash from the combustion of coal as an ingredient in the kiln-feed mix. If a kiln fires 25 tons/h of coal having an ash content of 17%, there will be 4250 kg of coal ash added hourly to the system.  Typical coal ash consists of:

From a plant chemist’s viewpoint, this is a potent material in matters of clinker chemistry modification and must therefore be closely monitored and considered in mix calculations. Problems are magnified when the kiln switches back and forth between different types of fuels burned, e.g., when natural gas is fired part of the day and coal fired for the rest. Since natural gas does not contain the clinker modifying ash, the plant chemist must find a kiln-feed blend that is suitable for both types of fuel burning. It should be suitable from a burning as well as a quality-control viewpoint. In such instances, the resultant kiln-feed mix is a compromise at best. But, it is far better to compromise than to design a mix for coal firing and then switch to natural gas without making a mix adjustment The re­percussions of such actions can be severe because the burnability of this kiln feed is drastically changed the moment the switch to natural gas is made. In this example, the kiln feed would be much harder to bum be­cause the fluxing alumina and iron components in the coal ash would suddenly be absent A plant chemist of a precalciner plant must also give the same attention to the amount of ash that enters the system at the flash furnace.

Now, it will be assumed that all these factors have been given due con­sideration by the chemist, and that the proper mix of different raw ma­terials to obtain a typical kiln-feed composition (Table 9.2) is found. (Note: The chemical analysis below is used as the basis for all remaining discussions in this chapter. This analysis is only an example for illus­trative purposes; every plant’s kiln feed will somehow deviate from the one shown below.)

The chemist now looks at the ash that enters the clinker and calculates what the theoretical clinker composition will be after the kiln feed has been burned and the ash included. Assume in this example that the clinker contains 3% coal ash:

This clinker composition is only a preliminary potential wherein the actual composition of the clinker can vary due to the volatility of the alkalies and the sulfur. The important thing to remember is that, at this stage, there is not a material that has cementitious (hydraulic) properties. That is where the kiln comes in for it is here that this kiln feed is trans­formed into clinker minerals to obtain the ultimate properties of what is known as cement. During burning, this kiln feed forms the four main clinker compounds:

This compound composition is calculated with  the help of the Bogue formulas, from the potential clinker analysis above (loss-free basis).

BOGUE FORMULAS  FOR  CLINKER AND  CEMENT  CONSTITUENTS

For a cement chemist, these formulas are the most important and fre­quently used indicators of the chemical properties of a cement or clinker. The constituents  calculated by these formulas, however, are only the po­tential compositions when the clinker has been burned and cooled at given conditions. Changes in cooling rate or burning temperature  can modify the true constituent composition to a considerable extent.

a)Bogue Formulas for  Cement

 B) Bogue  Formulas for  Clinker Constituents.

Having determined the appropriate values for the CaO and Fe203, one can then proceed to calculating the potential clinker constituents by using the previously given Bogue formulas. When the Bogue formulas are used for kiln feed compositions, keep in mind that the coal ash addition, dust losses, and alkali cycles can alter the final composition of the clinker. Also, it is necessary to use  the analysis on a “loss-free”  basis in the calculations of the constituents.

Tricalcium silicate is an important constituent as it is responsible main­ ly for early strength development of mortar and concrete. Regular portland cement kiln feed has usually a C3S potential of 52-62%.  Kiln feed with a potential  in excess of  65%  is extremely  difficult  to bum  and has  a poor coating characteristic.

Dicalcium silicate accounts for approximately 22% of  the clinker. Because a higher temperature is required to form C3S than CzS, under­ burning  could result  in  a higher  content  of  C2S and  a lower content  of c3s.

Tricalcium  aluminate  is responsible  for  the  workability  of  the mortar. The higher the C3A content, the higher the plasticity (workability) of the mortar. This explains why kiln feed for the so-called plastic cements has a higher  C3A  potential  than  that  for  regular  cement  in  which   the  C3A amounts to 6-8% of the clinker.   Concrete containing cement high in C3A is not as resistant to attack by sulfates in soil or water exposure as is concrete made with low C3A cement.

Tetracalcium  aluminoferrite  governs  the  color  of  the cement The higher the content of CA4F in the clinker, the darker the cement This is undesirable, as users almost unanimously prefer  a  light-colored  cement Iron has the desirable property of acting as a fluxing agent in the kiln, fa­cilitating formation of other compounds of the cement at somewhat lower temperature than would otherwise be possible.

It is quite obvious that it is necessary to have a continuing analysis of the material going into the kiln, if there is going to be adequate control of the product coming out of the other end of the kiln.  It is  the respon­ sibility of the plant chemist to determine the composition  of  these materials and to proportion them to produce a kiln feed that ensures a uniform, high-quality clinker, combined with good bumability.  Con­ tinuously uniform composition of the kiln feed is of greatest  importance for proper operation of the kiln.

Various systems are employed to introduce the feed into the kiln de­pending on whether  the  wet  or dry process  is to be  used.  These  systems all serve the same purpose: to feed the kiln  at a steady  and  uniform  rate with as little fluctuation as possible, which means that each raw material must  be carefully  metered  or measured.

Using our examples above and the appropriate Bogue formulas, the following potential clinker compound content would be obtained:

It must be again stressed here that the clinker compounds as calculated by the Bogue formulas are only potential in nature. Formation of these depends on the temperature, time of exposure, and cooling rate of the clinker in the burning zone and  in actuality are quite different than the calculated values.

Up until now, a plant chemist has laid the foundation  for the possible quality of the cement that will be produced from this clinker. But, his job doesn’t end here. It is his duty to reconcile this clinker with the burnability and coatability of this clinker. In other words, he not only has to concern himself with making a good quality cement  but  must  also give due attention to the ease at which this clinker can be burned. Later in this chapter the microstructures of cement clinkers will be discussed and it will be shown that a plant chemist, in today’s cement technology, should and must also concern himself with the burning-zone environments that will ultimately affect the quality of cement.

INFLUENCE  OF FEED  COMPOSITION ON BURNABILITY

The “burnability” of a kiln feed is the relative ease or difficulty with which the feed is changed into a clinker in the kiln; that is, it is an indi­ cation of the amount of fuel required to bum the kiln feed into a clinker of good quality. Although it is highly desirable to produce at all times  the same composition of kiln feed, this cannot readily be done.  One reason is that most cement plants manufacture different types of cement such as high-early-strength, block and sulfate-resistant; therefore, composition  of the kiln feed must change from time to time as different kinds of cement are being manufactured. Every time the feed composition changes, burnability in the kiln will also change.

A plant chemist, when calculating the kiln-feed  composition,  will em­ploy certain formulas to ensure that the finished product meets the specifi­ cations of the type of portland cement to be made. Kiln-feed compositions are identified by a multitude of factors and indexes which are also used to express burnability.  These are discussed briefly below.

Silica Ratio

The Silica Ratio is found by  dividing the silica content by  the sum of the contents of alumina and iron in the kiln-feed blend.  That is,

Increasing the silica ratio produces a clinker that is more difficult to burn; in other words, the clinker is “harder” to bum. It is mainly the content of alumina and ferric oxide that governs the combination of calcium and silica at lower sintering temperatures.

At this poin it is appropriate to define the terms “easy burning” and “hard burning.” In this text, an easy-burning kiln feed is one that requires less fuel to bum to a clinker than a hard-burning feed.

Alumina-Iron Ratio.

The Alumina-Iron Ratio is found by dividing the alumina content of the kiln feed by the iron content. That is,

The higher the ratio, the harder the burning. Iron has a favorable influence on the speed of reaction between lime and silica; therefore, one can also say: Other values remaining constan a higher iron content leads to easier burning.  Because both the numerator and denominator in the equation  are  expressions  of  fluxing  componenls,  however,  the  alumina alone is not used to express burnability.

This ratio indicates the quantity of initial liquid phase present during burning. It is generally accepted that an AIF ratio between 1.4-1.6 is a desirable optimum level and most beneficial to U1e burning of  U1c clinker. The higher this ratio, the harder the clinker will be to burn.

The Lime-Saturation Factor.

This factor has been used for kiln-feed control for many years in Europe and only recently  has  also found acceptance by  American  cement manu­facturers.  When the lime-saturation factor approaches unity, the clinker is difficult to burn  and often shows excessively high  free-lime contents.   A clinker, showing a lime-saturation  factor of 0.97 or higher approaches the· threshold of being “overlimed” wherein the free-lime content could remain at high levels regardless  of how much more fuel the kiln operator is feed­ing to the kiln.

This lime-saturation factor when viewed in context with other indicators is an excellent indicator of what the free-lime content will be in the clinker when  it has been  burned  at normal  temperatures.

There have been instances  in the past where a foreman, upon learning

from the lab results that the free-lime content was too high, approached  the kiln  bumer and  asked  him  to bum  the kiln hotter  to lower the free lime  It is true that lower burning zone temperatures deliver higher and, conversely, “hotter” kilns  resulting  in  lower  free  lime  in  the  clinker.  Bu this  is  not the: only factor. High free lime can  be  associatd  with  too  low  a burning­ zone temperature only when the feed mix and the fuel burned remain unchanged;  and. when the Iime saturation  factor of the feed is below  the so called saturation point. If for whatever reason the mix should suddenly show a lime-saturation factor of, e.g., 0.97 or higher, it would be very difficult for an operator to lower the resultant free lime by raising the burning-zone temperature. Such action most likely would do more harm to the coating and refractory than it would do any good to the clinker quality.

The opposite has also been observed where complaints were voiced to the kiln operator about burning the kiln too hot The reason given was that the free lime was consistently too low in the clinker. Again, a badly underlimed mix, having a lime saturation of less  than 0.88, tends to deliver clinker that is low on free lime.

The point to remember is that when the free lime in the clinker is not up to standards, a check with the laboratory should be made first to see if the mix (lime-saturation factor) has changed. If this factor is still within normal ranges, then and only then, is there an indication that the kiln operator might not have burned the clinker at the proper temperature.

The permissible range of variation for free-lime contents varies among different plants but the majority of the plants attempts to obtain values that are between 0.4 and 1.2%. Experience has shown that when the clinker is burned as close as possible to the 0.8% level of free lime, the mix is then within acceptable levels.

The Hydraulic Ratio.

The hydraulic ratio, developed by W. Michaelis over 100 years ago, is very seldom used any more in modern cement technology for kiln-feed control but is here included for plants that still regard this ratio as significant

Percent Liquid.

Clinker, when burned at 1450 C (2642 F) will be in a socalled semiliquid state. · This viscous appearance of the  clinker  bed is a very important control factor for a kiln operator  when  viewing  the burning zone.  This will be discussed in greater detail later on.  The percent   liquid is calculated by the Lea and Parker formula as follows:

In both  these formulas, the restriction applies that the MgO  content  is limited to a  maximum  of  2%.  In  other words,  a value of  not  more  than 2% MgO can be used in these formulas.

Most portland cement clinkers show a liquid content of 25-27.5%. Higher liquids produce stickier burning-zone clinker-bed appearances. Since the percent liquid as calculated by the above formulas applies to a temperature of 1450 C, higher temperatures give higher liquid and, con­versely, lower temperatures result in lower contents of liquid in the clin­ker. Also, since alumina, iron, magnesia, and alkalies are fluxes, higher liquid contents make a clinker easier to burn.

Burnabilty Index.

Kuehl’s bumability  index is based  on the potential clinker compounds C3S, C4AF, and C3A. The higher the content of C3S with corresponding lower contents in C4 AF or C3A, the harder the clinker is to burn.

The Burnability Factor.

This factor was first introduced by this auth0r in the original Rotary Cement Kiln book. It was brought to the attention  of  the  author  that several chemists and engineers have made further research on the applica­tion potential of combining the lime-saturation factor with the silica ratio to express bumability. As mentioned in the original writeup, this formula was developed based on pure empirical notions and observations and, hence, was suspect in its fundamental reasoning.

The laboratories of F.L. Smidth, Copenhagen, have recently presented their results of an investigation on this subject. Their findings  are sig­nificant since they used scientific methods to arrive at a direct indicator of percent free lime in the clinker when the clinker is burned to 1500 C. The effect of alkalies and magnesia on burnability have been assumed constant in their formula, but due consideration was given to the effect of kiln-feed fineness on burnability.  The FLS formula is:

Analysis of Burnability.

In addition to .the kiln-feed composition discussed above, the operator of a wet-process kiln has to consider also the moisture content of the kiln feed, as this indirectly affects burnability of the clinker. With unchanged composition of the kiln feed, a higher moisture content results in easier burning.   The reason for the change in burnability is the simple fact that less feed is available to be burned when the moisture content is higher.

Because changes in kiln-feed composition have a large influence on kiln operation, it is important that the kiln operator be advised by the labora­tory well in advance of any upcoming change in composition. Another good procedure is to note the chemical characteristics of the kiln feed every day in the kiln log.

So far only the influence of chemical properties on burnability in the kiln feed has been discussed.  Plant operators must also pay attention to  the kiln-feed fineness as these physical properties  of the feed can influence burnability and the stability of the kiln operation. Since each  plant pro­duces clinker by using different raw materials and various types of kilns are in use, there are no clear cut standards in matters of kiln feed fineness that would apply to all kilns. There is, however, a consensus among operators that

• The coarse fractions in the kiln feed tend to be more significant than the  finer  fractions  in  their  relationship  to   Hence, each plant needs to test for and  specify  the  maximum limits of allowable fractions retained on the 30- or 50-mesh sieve (300, 500J.1 sizes respectively).
• The kiln feed has to be ground consistently uniform and with a little variations in particle-size distribution as possible on a daily

It is essential to make a clear distinction between a feed blend that tends to give a better visibility in the burning zone and a blend that requires less fuel to burn. Certain feed compositions create “dirty” conditions in the burning zone. Here a kiln operator could come to the wrong conclusion that because of the poor visibility the burning zone has cooled down. The action of raising the fuel rate in order to clear 01e burning zone could result in an overburned clinker.

On the other hand, another kiln-feed blend could possibly improve visibility in the burning zone. In such a case, a kiln operator could wrongly reduce the fuel rate because he concluded that the burning zone was warming up. The final result of his action could then be an under­burned clinker.

In Tables 9.4-9.7  the original potential  clinker composition  and the

various ratios and factors are computed. The percent free lime at 1500 Cis calculated using the F.L. Smidth formula but assuming that the kiln-feed fineness (45 and 125J.1 respectively) remains constant at 0.04 and 0.11 percent respectively.

For the purpose of illustration, changes of identical magnitude have been made in all four main oxides to show the reader how changes affect these various factors.

Several of the computed compositions are completely unacceptable in terms of burnability and clinker quality but they serve the purpose of fa­miliarizing the reader with some of the intricate aspects of quality control.

Kiln-feed blends  for production  of special cement-clinker  types such as high-early, sulfate-resistan !ow-alkali, and oilwell cement will all have somehow .different properties and chemical compositions. Changing from one type of clinker to another bum always requires special attention from the kiln operator and advice of such a change should be given well before this new feed is being used in the kiln.

The Liter-Weight  Test.

One of the easiest tests a kiln operator can perform to learn if he has burned the clinker at the proper temperature is the liter-weight test. Free­ lime content also gives essentially the same information but analysis for free-lime content takes up to an hour until the results are reported. Since the sample is usually taken from the outlet of the cooler, the results tell what was done 1.5-2 h before which, from an operator’s viewpoin is not much help. In the liter-weight test, the sample is first passed through a 10- mm .screen then through a 5-mm screen.  The fraction retained on the 5-mm.screen is then allowed to fall through a prescribed distance into a 1000- ml.container which is in the shape of a frustrum of a cone with the small end up. The weight of the clinker in this container, called the liter weight, indicates how well the clinker has been burned, as a hard-burned clinker has a higher liter weight than a soft-burned clinker, provided there is no change in raw-mix composition. A well-burned clinker has a liter weight between 1250-1350 g. The liter weight can vary considerably, even though the clinker is well burned, between one feed composition and another. If the raw-feed ·COmposition remains constant, the best clinker will have the appropriate liter weight and lowest free-lime content. The time required to run a liter-weight test is approximately 5 min.

Care must be taken to assure that the entire test procedure  is carried out in the same consistent manner. From personal experience it better serves the operator to determine at what time the liter weight  should be per­formed. In a well-running kiln, there is not much use in wasting time run­ning a liter-weight test every hour. Liter-weight tests should be done when the operator or foreman is not quite sure if the clinker is being burned at the proper temperature.

Here, too, as with the free-lime tes there is the disadvantage of the time lag to consider since most samples are extracted from the cooler dis­charge. Some plants have overcome this by extracting the sample directly from the kiln  hood before it falls into the cooler using careful cooling procedures to guard against burns.

How many and what types of  clinker  indexes,  factors,  etc.,  are  to  be used for effective quality control is a matter that has to be decided by the operator and plant chemist. This author has used  a backward  approach  to arrive at  an optimum mix that theoretically would successfully produce a desired clinker. First, certain fixed desired properties like the lime-sat­uration factor, silica ratio, and AIF ratio are set and using these as con­stants the needed oxide composition is dived at. this method of opti­ \mum-mix design is very tedious when done by hand since it involves repeated trial-and-error calculations until the right  suitable  mix  is reached. But, with the help  of a computer program,  this  work  is greatly simplified and quickly accomplished. An example using this  procedure  is  shown  in Table  9.8 In the preceding pages tile chemistry of the kiln feed and clinker have been extensively discussed.   The novice reader should now have a fairly broad knowledge of the many problems and factors that are associated with making cement clinker in a kiln.  For new kiln operators there will come a time when the realization  that something is not quite right with tile mix occurs.  It might suddenly happen while burning  a problematic  kiln feed. Sometimes these problems can persist for several days and a kiln operator can become frustrated.  It is common for kiln operators, whenever the kiln doesn’t operate and handle properly, to blame these problems  mistakenly on ti1e laboratory staff.  In most instances this is not justified.   It must be realized  that  the plant  chemists  face just  as  many  obstacles  as the kiln operator. Most of the time a chemist is aware that the mix is not up to his  liking  but  he can do nothing  about  it because  of  many  factors  not  · directly under his control.  Raw materials might not be available to make appropriate corrections, kiln-feed reserves could be at low levels, or the raw­ grinding department might face it’s own disturbances.

In  other  instances  the  laboratory  might  get  right  back  at  the  kiln operator and tell him that there is nothing wrong with the mix. The best way to guard against this type of stalemate is to establish a good dialogue between the laboratory and the kiln control room. A kiln operator can accept problems when they are explained. Being made aware of potential upcoming disturbances in the kiln is a more tolerable situation.

Assuming that the point has been reached where a shining clinker of superior quality has discharged from the cooler, the stage would be set for a good finished product. However, the cement as yet has not been produced. If just clinker alone is ground, the final product would be an inferior, unworkable cement. To control setting time, gypsum has to be added to the clinker during the finish-grinding process. Typical portland cement contains approximately 5% of gypsum.

At this stage, the production  of  cement,  the  binder  that  holds  the  sand and the a.ggreg::Hes in the concrete and mortar together, is completed. 1his is the most durable construction  material that is known.  No wood, glass, or steel construction will ever have the life span of concrete. The many things thnt were made of clinker serve as tributes to the kiln operator. An old, now retired kiln operator,  told  this  author a  few years  ago that one of the most memorable things he did in life was to be involved in making the cement   used   for   the   construction   of   the   Hoover   Dam   in   Nevada .

. Considered  at that  time to be one of the eight technical  wonders of the world, it is still around and doing well. What a monument  to  a  kiln operator that had no automatic controller, no computers, no air-conditioned control room, and operated a kiln that was driven by a leather belt around the shell.

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