MECHANICAL ELEMENTS OF TUBE MILLS

this is a part of ASEC Academy training Courses

SUMMARY

the mechanical elements of a tube mill could be separated into elements which have a direct function with the grinding process (i.e. grinding media, liners, diaphragms) and into elements which can be considered as individual units which are connected to each other to a specific tube mill. The latter group includes the feed and discharge arrangement, the mill shell, the mill heads, the mill bearings and the mill drive.

The feed devices have to be selected such as to provide a continuous flow of material to the grinding compartments and to allow the passage of either drying gases or cooling air.

The mill heads are either welded or bolted to the mill shell. Large mills are often equipped with concentrically divided mill heads. In cases where high volumes of gases have to be passed through the mill, the conventional trunnion bearings can be replaced by slide shoe bearings.

The mill drives are subdivided into girth gear drives, central drives and gearless drives. Girth gear drives are lower in the initial investment costs, they result however in higher operation costs (e.g. low gear efficiency and high lubrication costs). Better gear efficiencies are obtained with central drives which are built for driving powers of up to 10’000 kW. The gearless drive (ring motor) is only selected for Very large mills.

1.       INTRODUCTION

The mechanical elements of a tube mill can roughly be subdivided into internal and external parts. The mill internals have a direct function with regard to the grinding process and include principally the wear parts of a mill such as mill liners, diaphragms and grinding media.

The external parts of a standard tube mill consist of mill shell, mill heads, mill bearings, feed and discharge equipment and include also the mill drive.

In the following the external parts of the tube mill are described under special consideration of the process technological aspects. The mill internals are described in separate chapters.

2.       MECHANICAL COMPONENTS OF TUBE MILLS

The first tube mills were of a rather simple design, consisting basically of a drum equipped with some wear plates and filled with some balls.

In the course of increasing plant capacities and the development of new energy saving processes the design of the mechanical components of a tube mill were changed considerably.

Fig. 1 and Fig. 2 show the different design features of modern tube mills. The dimensions and design of the mill shell, the mill heads and the mill bearings are basically dictated by the required grinding capacity whereas the design of the feed and discharge devices are selected according to process considerations such as wet grinding or drying / grinding.

Fig. 1 indicates a typical arrangement of a tube mill for cement grinding, consisting of two grinding compartments, two trunnion bearings and a central drive

Fig. 2 shows an air swept mill equipped with an overhanging drying compartment and driven by a grith gear / pinion drive. On the inlet side the mill is provided with a slide shoe bearing whereas the outlet part of the mill is supported by a conventional trunnion bearing. The raw material is fed into the gas stream entering the mill and the ground material is swept out by the mill exhaust gas.

Two compartment mills as shown in Fig. 1 are also used for dry grinding of raw materials. The air swept mill as shown in Fig. 2 is principally used for drying/grinding of very wet raw materials (or coals).

3.       FEED AND DISCHARGE ARRANGEMENT

The feed devices have to fulfill the following functions:

• Continuous flow of material into mill without any blockages.
• No back-spillage of material.
• No false air intake in case of raw material drying inside the mill.
• Free area to permit the intake of cold ventilation air in case of large cement mills.

Fig. 3 shows the feed and discharge arrangement of a slurry mill. The raw material is often fed to the mill by means of spout feeders as shown in Fig. 4.

FLS provides their tube mills for wet and dry grinding with a drum feeder as shown in Fig. 5.

Tube mills for combined drying / grinding operation are either equipped with a feed arrangement according to Fig. 2 or with a “step-type” feeder according to Fig. 6. To prevent the intake of false air the feed inlet has to be sealed by pendulum flaps or by rotary valves.

The feed devices according to Fig. 3 and 4 should only be used for small mills due to their limited cross section for the intake of venting air. The feed arrangement as shown in Fig, 7 was especially developed for large cement mills. The arrangement of the feed chute permits an optimum intake of venting air. The screws in the mill trunnion avoid a back-spillage of material.

The design of the discharge device depends basically on the type of mill and type of process Wet grinding mills (Fig. 3) need no ventilation. The material (slurry) is discharged to the subsequent pump. To prevent entering of foreign matters (e.g. broken bails) the out-let is provided with a drum screen.

Two-compartment mills with end-discharge are used for dry grinding of raw material and cement. In general they are equipped with a discharge arrangement according to Fig. 1. The ground material is extracted to the subsequent bucket elevator. The venting air is drawn separately to the dedusting equipment. In case of a central drive the hollow mill shaft is provided with peripheral openings to allow the passage of material and air. The amount of air which can be drawn through this peripheral openings is limited.

The ground material and the mill exhaust gas have in case of an air swept mill a common outlet duct as shown in Fig. 2. Air swept mills are commonly driven by grith gear / pinion drives since a central drive would be rather problematic due to the arrangement of the discharge ducts.

Central discharge mills are in its central part provided with peripheral openings to allow a discharge of material of the two grinding compartment. The venting air (or exhaust gas air) is also drawn through these openings as shown in Fig. 8.

4.       DRYING COMPARTMENTS

Tube mills for raw material grinding are often equipped with drying compartments to evaporate the water contained in the raw material. The drying compartment is provided with lifters to obtain a dispersion of the material to be dried.

For small mills, the drying compartment is often an integrated part of the tube mill as shown in Fig. 8 and Fig. 9. The drying compartment is separated from the grinding compartment by means of a diaphragm. The drying gases have to pass the mill trunnion which on one hand limits the maximum hot gas volume and on the other hand causes

high pressure losses. To reduce the thermal load and allow higher hot gas rates an overhanging drying compartment as shown in Fig. 2 has been developed. The mill could in this case either be supported by slide show bearings or / and trunnion bearings.

5.       MILL SHELL

The mill shell consists of several sections of welded plates as shown in Fig. 10.

The mill shells are made from boiler plates HI or from St 37-3 (DIN 17155) with a low sulphur content. In the USA the mill shells are manufactured with a steel quality ASIM A 283 Grade C. The carbon content should be smaller than 0.2 % to obtain adequate notch impact strength.

The thickness of the shell is determined by the size load and design of the mill. The shell thickness of long mills is graded, i.e. the thickness increases from both ends towards the mill center.

To allow a rough estimate of the stresses which act on the shell a simple beam effect can be assumed. The load on the beam (see Fig. 11) consists of the following weights:

QM = weight of material                            [N]

QB  = weight of grinding media                 [N]

QR =  weight of rotating parts                    [N]

The stresses are maximum at the center of the shell and minimum at the ends. The moment of inertia WM of the mill shell can be calculated as

WM = 0.8 (DM + s)2  . s                                         [m3]

where

DM = inside diameter of shell                               [m]

s      = shell thickness                                               [m]

The maximum bending moment (MB) is determined by:

MB = Q tot (2A -B)                                     [Nm]

where

Q tot =  total rotating weight          [N]

A       = center distance of bearing [m]

B       = length of mill shell              [m]

The maximum bending stress dB  can be calculated from the bending moment MB and the moment of inertia WM:

dB =    MB  . 10 – 6                           [N/mm²]

WM

For the before mentioned shell material qualities the following maximum admissible bending stresses for the different types of mills can be assumed.

Example:

Given:

– End discharge mill with trunnion bearings

– Mill dimension (air swept mill):                              Æ4.6 x 11.5 m

– Center distance of bearings:                                     14.68 m

– Rotating weight:                                                         251’000 kg

– Grinding media:                                                          162’000 kg                       = 431’000 kg

– Material:                                                                         18’000 kg

– Shell thickness:                                                            0.055 m

Solution:

WM    =          0.8 (4.6 + 0.055)2  .  0.055             =          0.953m3

MB      =          431000  .  9.8 (2  .  14.68 – 11.5)    =          9.43  .  106Nm

8

dB       =          9.43  .  106    .  10- 6                                    =          10N/mm2

0.953

During rotation of the mill the stresses reverse themselves. The beam effect also creates vertical shear stresses which are highest near the support. The recalculation of the bending stress represents therefore only a very rough check of the shell thickness. in practice the shells are designed according to sophisticated computer assisted calculations.

6.       MILL HEADS

Tube mills supported with trunnion bearings are provided with mill heads. The heads are bolted or welded to the mill shell. Small mills are often equipped with one piece mill heads (integral heads) bolted to the flange of the mill shell as shown in Fig. 12.

Mill heads can also be bolted to the trunnion as indicated in Fig. 13. The mill head in this example is directly welded onto the mill shell.

For large mills the mill head is mostly divided concentrically into an outer and an inner part. The outer part is bolted to the inner part and welded to the mill shell. The inner conical part of the head can either be casted together with the trunnion or it is jointed to each other by welding.

In order to entirely relieve the mill heads of large mills of the elevated bearing reaction forces, there is a certain trend to use slide shoe bearings in conjunction with a riding ring mounted to the mill shell. The mill head consists in this case of a welded construction of mild steel plates (Fig. 14).

The mill heads can be casted from modular cast iron. However a cast steel with a low sulphur content is often preferred to permit a proper welding of the mill heads to the mill shell.

7.       MILL BEARINGS

The hollow trunnions of tube mills are supported by plain bearings as illustrated in Fig. 15. The bearing comprises a welded or casted casing with the bearing liner, the supporting insert mounted on spherical surfaces, the white-metal lined bearing bush and the lubricating and cooling equipment.

The bearings are equipped with high pressure pumps for “floating” the mill during starting and shut-down period. During operation an internal lifter system or a low pressure pump furnishes oil from the sump for hydrodynamic lubrication. One or both trunnions can be replaced by slide shoe bearings (Fig. 2 and Fig. 15).

The slide ring (or riding ring) rests on a set of self adjusting sliding shoes as shown in Fig. 16. Hydrostatic lubrication is provided by low pressure pump during running for the start-up phase and hydrodynamic

Slide Shoe Bearings

FLS Design

The design of FLS slide shoe bearings is based on two babbited slide shoes in each end of the mill. Only one these 4 slide shoes are fixed in axial direction, whereas the other 3 are moveable, allowing expansion and contraction of the mill.

The slide shoe bearings are hydro dynamically lubricated during normal operation, but the system include supply of high pressure oil to the slide shoes before starting the mill and during barring.

As mentioned, FLSMIDTH is using only two slide shoes in each end of the mill, because we don’t believe it is possible to adjust 3 or more shoes in one bearing to obtain equal load.

The picture shows the fixed slide shoe in a bearing.  This slide shoe is bolted to the foundation and keeps the mill in a fixed axial position by means of bronze guide vanes, bolted to the slide shoe. The other slide shoes are also with bronze vanes toe move the slide shoe together with the mill during heat expansion and contraction.

All 4 slide shoes are placed in a ball socket to allow the slides shoe to follow smaller inaccuracies in the shape of the slide ring and to compensate for bending of the mill during stop.

The internal piping for low-and high pressure oil as well as for cooling water for the slide shoes is arranged inside the bearing casing and all external connections are positioned in the bottom. The external pipes between the bearing and the lubrication station are placed in hollows in the foundation, i.e. the piping is protected against damage from outside.

It should also the mentioned that the bearing casing is not used as oil reservoir, i.e. the oil is drained to a separate oil tank in the lubrication station.

High pressure oil for mill start and for barring of the mill is supplied to the center of each of the four slide shoes.

Low pressure oil for hydrodynamically lubricated during normal operation is supplied to each of the four slide shoes. Oil to the slide shoes in the ascending side is supplied from a shallow tray into which the slide ring dips, whereas the slide shoes in the descending side are lubricated form and oil pan, bolted to the slide shoes.

The slide shoe is with internal water cooling and the oil temperature is measured in the slide shoes close to the outlet end.

The slide shoes are lubricated from a separate lubrication unit, one for each bearing. The lubrication unit is supplied as a panel enclosed unit to be installed close to the bearing foundation. Each of the two lubrication unit are with oil tank, -pumps, -filter and –heaters.  Also all equipment for flow-, pressure- and temperature measurement is a part of the lubrication unit.

A local control cabinet, common for the two bearings, is supplied with the lubrication system. This control unit comprises complete supervision and safety interlocking for both mill bearings as well as programs for pre-conditioning of the oil and for filling/emptying of the two oil tanks. The local control unit may also be supplied with features for bearing lubrication during barring of the mill.

The bearing lubrication includes high pressure lubrication during start. When the mill is running the high pressure pumps are stopped and the slide shoes are hydrodynamic lubricated form the low pressure pumps.

The lubrication unit is also designed with a conditioning circuit with a separate pump to ensure optimal oil conditions before start and during normal operation in respect of viscosity (temperature) and cleanness.

The oil pressure is measured in each of the four slide shoes and of course a minimum pressure must be present to lubricate the bearings. However this pressure may deviate form one slide shoe to another due to smaller deviations in manufacture of the slide shoes. Also the pressure during a mill rotation will fluctuate because of the shape of the slide ring not is a perfect cylinder. Furthermore the temperature of the slide ring and the slide shoe will affect the geometry for the parts and consequently the oil pressure in the slide shoe. However, these pressure variations are not critical for the bearing performance as long as the pressure don’t drop below a certain level and in general the slide shoe bearing is very flexible in respect of pressure variations.

The main advantages of slide shoe bearings as compared with trunnion bearings are:

• No limitation in size and capacity of mill
• No delicate large size mill head castings are necessary
• Easy fixation of wear plates at the end diaphragm
• Simplification of feed and discharge devices
• Large feed and discharge openings allowing the passage of considerable amounts of gas
• No limitation in gas temperature influencing expansion of trunnion and trunnion bearing

8.       MILL DRIVE

8.1.    General

The mill drives can basically be subdivided into three groups:

• Girth gear / pinion drives
• Central drives
• Gearless drives

Each type can be applied for a certain range of drive power as indicated in Fig. 17.

The max. driving power is limited to about 11000 kW, since at higher values the dimension of the mill cylinder begins to create transport problems.

8.2     Girth Gear Drives

The girth gear (or spur rim) is bolted to the mill body and is driven either by one or two pinions as shown in Fig. 19 and Fig. 20 respectively. The reduction gear box arranged between pinion and mill motor is equipped with an auxiliary drive The auxiliary drive allows slow rotation of the mill (0.2 – 0.3 min.- 1 ) for inspection or to turn the mill to a certain position.

The girth gear is enclosed in an oil-tight casing and is spray or splash lubricated.
US suppliers often prefer a low speed synchronous motor instead of a reduction gear as shown in Fig. 21.

Girth gear drives are used for smaller mills or for mills which are designed for high drying capacities which due to the large outlet cross section do not permit the use of central drives (see also Fig. 2).

Girth gears are made from Cr-Mo heat treatable cast steel hardened to max. 900 N/mm2

tensile strength. The pinions are generally made from Cr-Ni-Mo heat treatable steel hardened to max. 1150 N/mm2 tensile strength.

An optimum designed high quality single pinion can be applied up to 4000 kW. The low investment cost of this drive has however to be compared with the potential risk of flank damages due to unfavorable running conditions and due to possible problems with the lubrication system. A double pinion drive for mills with an installed power of more than 5000 kW is nevertheless not always recommended due to the more complicated speed control and possible influences by oscillations and / or vibrations.

8.3.    Central Drives

The central drive consists of a compact gear unit which is coupled to the mill either by a hollow shaft or by a torsion shaft. The gear is operating in an oil-bath in a completely enclosed casing (Fig. 22).

For input powers up to 3000 kw a two-way spur gear drive equipped with one motor according to Fig. 23 or equipped with two motors as shown in Fig. 24 could be applied.

For a larger driving power either a multi-way spur gear (Fig. 25 or a planetary gear Fig . 26) could be used.

Multi-way spur gears (e.g. FLS “Symetro” gear (Fig. 27) or Flender “Polyflex” gear) are manufactured in sizes up to 10000 kw Single motor planetary gears are build up to 8500 kW. For higher power ratings up to 10000 kW dual planetary gears are available. The planetary gears are generally more expensive than multi-way spur gears, offer however a very high gear efficiency and are of a very compact design (see also paragraph 8.5).

8.4     Gearless Drive

Large tube mills can be driven by low frequency electrical motors; the so-called ring motors or wrap-around motors. The ring motor is directly arranged around the mill shell; i.e. there is no speed reducer required.

For the required low mill speed of 13 – 15 min.- 1 a speed adjustable AC drive with a frequency of 5 – 6 Hz is used.

The motor can be supported by an extended mill head as shown in Fig. 28 or it can be arranged on the mill shell according to Fig. 29.

The ring motor can be accelerated from standstill up to operation speed. For a limited time the mill can be turned with a very low speed to allow a positioning of the manholes in the mill shell.

The main advantages of the ring motor as compared with conventional mill drives are:

• Adjustable mill speed
• No wearing part on gearless mill drive
• No costly lubrication required
• Low mechanical maintenance costs
• No limitations for larger tube mills
• More flexible layout of mill room.

The disadvantages include mainly:

• Low efficiency ( 91 – 92 %)
• Higher investment cost (for small units)
• Extra load ( 15 %) on mill bearing due to weight of motor)
• Sensitive for electrical failures

The adjustable mill speeds allow theoretically an optimization of the specific power consumption and mill output. Practical application has however shown that the adjustable speed is not used in its original sense but is often “misused” to adjust the mill output with regard to storage capacity of the cement silos or to maintain a maximum mill output at peak sales demands. The higher mill output has in those cases to be paid with a higher specific power consumption.

8.5     Selection Criteria

The investment costs of a mill drive amounts often up to 50 % of the total cost of a tube mill. The proper selection of the type of mill drive is therefore of utmost importance.
A comparison of the investment costs of four different mill drives for a driving power of

5000 kw is shown in Fig. 30.

The type of mill drives should however, not only be selected according to the investment cost but also the following facts have to be considered:

• Operation costs (e.g. gear efficiency)
• Maintenance costs (e.g. lubrication)
• Process requirements (e.g. mill discharge design)
• Availability and reliability
• Space requirement.

The most important features of the various type of mill drives are summarized in Table 1.

L : Low                                   1) Drive inclusive tube mill.

M: Medium                           2) Electrical energy lubrication

H: High                                  3) Man power

4) Spare Parts

WEAR PARTS AND WEAR RESISTANT MATERIALS IN MILLS

1.       INTRODUCTION

A cement plant handles large quantities of many different materials, which to a large extent, are hard and abrasive. Consequently, wear may be a very serious problem. This is even more the case for the grinding installations where the materials are exposed to heavy forces and movements. The equipment in the grinding installations therefore will have to be protected with sufficient wear resistant materials for two different reasons:

1. In order to protect the equipment from damage and decay.
2. In order to maintain maximum output at optimum operating conditions.

2.       Nature Of Wear In Ball Mills

Grinding in ball mills takes place by lifting the grinding media charge and the material to be ground followed by a return to the bottom part of the mill tube In this continuous cascading movement.

The material quality of the wear parts in a ball mill must therefore partly be resistant to over hammering without cracking, and partly withstand abrasive wear.

3.       Influence On Lifetime for Wear Parts

Aside from the material quality for the lining parts, several other circumstances result in different wear rates in a grinding mill.

3.1     Influence from Material to be Ground

The wear rate due to chemical composition of the grinding material differs from plant to plant, and in general, the wear rate has to be determined individually.

In a two compartment dry grinding ball mill, the highest wear occurs in the coarse grinding compartment due to coarser material, but of course also due to the larger grinding media than in the fine grinding compartment.

For grinding in ball mills, special attention must be given to the amount of material in the mill. Running the mill with a too low feed rate or no feed at all, for any length of time, can be extremely dangerous for the lining parts, especially in the coarse grinding compartment.

Therefore, the operator should always run the mill with a certain amount of material in the mill monitored by the noise level from the coarse grinding compartment.

3.2     Influence from Lining Thickness

It is obvious that at a thicker lining will have a longer lifetime than a thin one. But in this connection.

By selecting a thicker shell lining to obtain a longer lining lifetime, the load on mill shell and on mill bearings will increase, due to a higher weight of the thicker lining and due to the bigger ball charge. Also, the bigger ball charge will contain a higher amount of material to be ground.

The mill shell and the bearings must be dimensioned for this extra load before changing to a thicker lining.

3.3     Influence from Lining Shapes and Surfaces

Many different types of linings in ball mills have been used to optimize mill performance and grinding efficiency. Basically, the shape and surface have an effect on the grinding efficiency.

On the other hand, very fancy designs of the lining are not needed. The lining must be able to lift the ball charge to give appropriate impact grinding without crushing the shell lining and the lining must minimize sliding movement between ball charge and shell lining

3.4     Influence from Mill Diameter

The diameter of a ball mill can have a detrimental effect on the mill lining. The impact from the grinding media in large diameter mills will be greatly increased by the higher fall of the cascading ball charge.

Apart from the impact, the wear from proper attrition will also reduce life in large diameter mills.

3.5     Influence From Ball Size

Particularly for large diameter ball mills, use of oversized grinding balls will increase the wear rate and the risk for crushing the shell lining, especially in the coarse grinding compartment. It is very important to determine the largest ball size in such a way that it is just able to crush the coarse part of the grinding material in the mill .If the grain sizes in front of the diaphragm are appropriate when using 90 mm balls as the largest size, then 100 mm balls should not be used. A 100 mm ball has 37 % higher energy when it hits the lining than a 90 mm ball. This higher energy level may have a detrimental effect on the lining.
At regular intervals, the ball charge, especially in the coarse grinding compartment, should be emptied out for classification. In multi-compartment mills, grinding balls less than about half the diameter of the biggest grinding ball in the compartment are normally not useful and have to be scrapped. In one-compartment mills with classifying lining, the ball charge often is a combination between a coarse and a fine grinding charge. For a 50/50 ration between the two ball charges, the scrapping size is normally about half the diameter of the biggest ball size in the fine grinding charge. Too many undersized grinding media in a ball mill result in; a poor grinding efficiency, reduction in the transport capacity in the compartment and blockage of the slots in the diaphragm and in the outlet grate. When recharging the mill, it is important to protect the mill shell lining by placing sufficient material in the mill to prevent the heavy balls from hitting the shell lining directly.

4.       Grinding Media Quality

Cast grinding media made of Cr-alloyed steel is extremely wear resistant and can be supplied from several suppliers. For dry grinding, the wear rate may be reduced as much as 6 to 8 times compared with the grinding media.

Wear on grinding media increases drastically if the mill operates with too low levels of grinding material.

If a poor quality of grinding media is being used, a high quantity of small worn out or broken grinding media will be formed. An excessive amount of small pieces from grinding media will reduce the grinding efficiency and reduce the transport capacity in

the mill. Furthermore, they may gradually block the grates, causing a reduction in the free flow of material and air through the grates. In this situation, the grate will often have to be cleaned by hand.

5.       Mill Lining Materials

The Cr-Mo steel will, by heat treatment, give a material with a combination of a certain resistance to wear and impact.

The service life of Cr-Mo grates is inferior to what can be obtained with modern diaphragms.

6.       Lining types

6.1     Shell Lining in Ball Mills

In addition to the material quality for the lining, the shape and the surface also influences wear in ball mills. Sliding motion between the ball mill liners and the charge of grinding media and material is, to a very great extent, responsible for wear on lining plates on the mill shell. The lining plates should be designed to ensure a minimum of sliding movement, and at the same time, the profile of the linings should lift the charge of grinding media sufficiently to transfer a high power up take while maintaining the impact forces at an acceptable level.

6.2     Vertical Lining Parts in Ball Mills

The lining parts and the grates for inlet head, diaphragm and outlet head always pose a special problem due to the relative speed between the grinding media charge and the mill. The movement of the ball charge It is noted that the outermost part of the grinding media charge is drawn by the lining and moves at the same speed as the mill with only sliding movement in the acceleration zone. At the top of the charge, the balance between rotational force and gravity causes the grinding balls to leave the lining and accelerate downwards in the inner zone of the charge. This downward movement relative to the rotational movement of the vertical parts, explains why higher wear normally is found on these parts. The wear is far higher in the first compartment due to the combination of large grinding media and coarse material to be ground.

To face this problem, the vertical lining parts are normally split in several rings. In this design, it is possible to replace only the worn-out rings in the high-wear zonw . But also the smaller plates offer advantage in respect to manufacturing, handling and erection.

As mentioned, it is normal that grates for diaphragm in the coarse grinding compartment are made of Cr-Mo alloy. Because of the slots, the material quality has to be rather ductile to prevent early breakage of the grate sectors.

6.3 Mill Diaphragm:

Type:

A-The intermediate Diaphragm:

Intermediate diaphragm fulfils the most important, it divides the tube mill into two grinding chambers with different grinding ball sizes and maintains a material level in the first chamber for optimal pre-grinding. Additional safety is created limiting the largest grain size by allowing only the fine material to pass. This way the material to be ground is crushed in energy-saving manner in two steps: first it is crushed, then it is ground fine.

The open lifter diaphragm is installed between the drying and the first grinding chamber. It is prone to high thermal strain from one side and to large axial pressure from the other.

C- The Double Outlet Diaphragm:

The double outlet diaphragm allows the material ground to pass from both head sides of the mill through the centre of the mill length. The special retention rings ensure sufficient material load in each of the grinding chambers. The plate division in two to four rings pays off because such division allows for the exchange of only the linings within the main wear zone possible.

Discharge diaphragm is individually adapted to the existing mill and fixed on the inside of the mill head. The ground material is pemitted to pass to the separator while the grinding balls are retained in the chamber. Thanks to the exchangeable centre piece, the open centre of the diaphragm can be adapted to the individual ball filling rate (volume load can vary between 25% and 35%)

7.       Mill Inspection/Spare Parts

The wear rate on the different lining parts in the mill should be reflected in the necessary stock for each item. Already during  the first year of operation, it is possible to estimate

the necessary stock by regular measurements and check in the mill. A very careful record should be kept for each and every item in the entire mill to optimize the necessary amount of spare parts.

Normally a wear plate has to be changed when about 50 %of its weight have been worn away. Through regular mill inspection, the wear pattern should be followed and the possible risk of cracks or other damages, making an earlier replacement necessary, should be recorded. By carefully checking and by use of previous records, the necessary stop of the mill for replacements, and consequently stop of production from the actual grinding installation can be planned well in advance. This means optimum utilization of resources and minimized disturbance of production flow.

An often seen conflict between production people and maintenance people is the demand of continuous production from the production staff and a certain number of mill stops from the maintenance staff. By using a well-planned preventive maintenance system this conflict should never occur.

In extreme cases without preventive maintenance, the situation may result in a totally damaged ball mill lining and may be damage of the mill body too. If for example, the demand for production has over-ruled a stop for changing worn or broken lining plates, these lining plates may become loose and the heavy pieces will start the so-called “domino effect”. Other lining plates will be cracked by the first ones and so on, ending up with the entire lining being destroy in the grinding compartment. Instead of a short stop for 8 hours for changing a few lining plates, the mill installation may be stopped for months until a completely new lining is delivered and installed.

Further, damages to the mill shell, heads, trunnions or slide rings may result in future cracks in these heavily loaded components. For this reason, it is very important to remove cracked lining plates and other heavy metal pieces from the grinding media charge.

In order to avoid the above-mentioned problems, regular mill inspections should comprise:

• Check for broken lining plates, especially in coarse grinding chambers. Change lining plates if required. Larger lining fragments in the ball charge must be found and removed.
• Check for loose lining plates, both bolted-on and bolt-less (Liners).
• Measure actual thickness of lining plates for estimate of remaining service life and planning of purchase and installing new lining plates.
• Clean grates incase they are blocked with nibs or fragments of grinding media.
• Check grinding media charge.

8.       Handling/Erection Of Lining Parts

The lining materials with excellent characteristics in terms of wear resistance and hardness, Whereas the toughness characteristics are limited. Because of the relative brittleness of these materials, it is essential that lining parts manufactured from such materials are handled with care during transportation and handling at site. These lining parts must never be thrown, tumbled inside the mill, or hurled casually into a pile on top of one another, since impact and pressure loads may cause the lining parts to crack.

However, correctly installed, lining parts will have a very long service life.

Where adapting of parts made of high Cr-alloys are required, this must be done only by grinding, and the operation must be performed in a manner preventing local heating which may lead to crack formation.

Operations involving welding or usage of cutting torches never be allowed.

9.       Fixation Of Lining Parts

When lining plates in ball mills are to be installed, they must be supported in such a manner that major bending stresses are avoided when the bolts are tightened, since such stresses will often result in fractures. It is therefore essential to ensure that the contact faces of the lining parts around the bolt holes lie directly against the support. This is especially important for the relatively thin lining parts and grates in chamber 2 of cement mills.

The fastening bolts for the lining plates must be tightened to the specified torque level and re- tightened after the start of the mill until the torque remains constant.

            Note : Any superfluous bolt holes in the mill shell, heads, trurnions or slide rings, must be closed with glued-in rubber plugs. The holes should never be closed by welding

Since such welding leaves stress concentrations so substantial that destructive crack formation may develop.

In dry process ball mills, the lining parts are normally fitted directly on the mill body. Only when larger spacing exist between liners and mill body, these spacing must be filled with sponge rubber, concrete or similar.

All bolts which penetrate the lining, both at the mill heads and on mill shell must be sealed in order to ensure tightness of the mill body. Besides contamination of the mill building leaking materials can cause wear on the mill body. The bolt seals consist of steel washers with a rubber grommet in between.

The liner bolts must be tightened to the specific torque by means of a torque spanner. To avoid loose lining plates, it is important to re-tighten the bolts after 2 hours of operation and then every 12 hours until the torque remains constant. Dismantling of worn lining plates is often done by melting the bolt head or cutting the nut off by means of a cutting torch. The utmost care should be taken when doing this to prevent damage to the mill body. Cutting scars in the dynamically loaded mill body will in most cases, develop cracks. These scars must be removed by careful grinding to obtain a smooth surface. Scars must not be repaired by welding.

CONCLUSION

Effective preventive maintenance, including regular mill inspection, combined with good mill operation will result in maximum output from the installation at minimum production cost.

Maintenance of wear parts and grinding media is necessary in order to keep the grinding installation in good working condition

Careful records and good planning through a preventive maintenance system, including regular mill inspections, can help to protect the installation against damage and decay, and optimize an adequate stock at the lowest level of investment.

Further more, proper mill operation without blocking the grates, running the mill without feed, etc., will give the lowest number of stop and the longest life time for the wear parts

In other words: “Maximum output at the lowest production cost.”

WEAR PARTS IN BALL MILLS MUST BE USED TO:

PROTECT THE EQUIPMENT PROPER FROM DAMAGE AND DECAY

MAINTAIN MAXIMUM OUTPUT AT OPTIMUM OPERATING CONDITIONS

INTERNAL WATER COOLING

1.       GENERAL

Internal water cooling in a cement mill serves to maintain the operating temperatures constant at preset values. This is on account of the cement properties and mill operation and mill operation.

A cement temperature in excess of approx. 125OC may cause dry-clogging in the mill and dehydration of the crystal water of the gypsum, which involves the risk of false setting and poor storage resistance.

Conversely, if the internal water cooling causes a temperature below approx. 110OC, there is a risk of wet-clogging in the mill and beginning hydration and consequently a reduction of cement strength.

Water cooling a cement mill is accomplished by injecting and evaporating water. Either at both mill ends or solely at the outlet end of the mill. The water vapour is removed from the mill together with the ventilation air which must be so ample that the temperature does not drop below the dew point anywhere in pipes, dedusting filter, etc., after the mill.

(Fig. 1) shows the importance in a cement mill. The figure shows different combinations of feed temperature and water injection. The specified temperatures are for guidance only.

Curve a: cold clinker – no water cooling.

Curve b: cold clinker – water cooling in outlet.

Curve c: hot clinker – water cooling in inlet and outlet.

2.       STRUCTURE

The meaning of the symbols used in the attached

2.1     Types

The equipment described is used for mills with central drive  unit. The equipment is either used for water injection at the mill outlet end or at both mill ends. The two types are shown in enclosures I and II, respectively.

Water injection takes place without the use of compressed air for atomization. The required spreading of water inside the mill is accomplished by means of the water pressure and the shape of the nozzles.

Compressed air must, however, be used to keep the nozzles clean when there is no water injection. The equipment at either mill end consists mainly of identical components. At the outlet end the equipment has been supplement by a water-lubricated stuffing box with lubricating water pump to match. The other differences appear from the text.

2.2     Water tank

The water tank (WT) for the two types holds approx. 500 and 800 liters, respectively. The water is fed through float (FO). The water supply can be stopped with stop valve (SVS), and the tank, which is provided with overflow, can be emptied though stop valve (SVD).

2.3     Outlet end

The cooling water to the outlet end of the mill is pumped from the tank (WT) by means of pump (WPWO) through motorized valve (MRWO) to a stuffing box around the torsion shaft between the mill and the gear unit. From here, the water is forced through a nozzle which is positioned in the center axis of the outlet end of the last mill compartment.

The cooling water is sprayed upstream the ventilation air, and is partly given a rotary movement and consequently spread in the mill when leaving the nozzle.

Between pump (WPWO) and the motorized regulating valve (MRWO) there is return flow to water tank through pressure control valve (PRWO). Same maintains a constant a pressure head of regulating valve, and hence favorable regulation characteristics.

When there is no water injection in the mill, the nozzle is kept clean by compressed air which is blown through the nozzle.

Injection of compressed air is started an stopped by solenoid valve (SOAO). The air pressure in the nozzle is regulated by reduction valve (RVAO). On a pressurestat (PMH) in the pneumatic pipe giving alarm and interlocking at too high air pressure. At the outlet and there is also a separate lubricating water pump (WPLO) for the lubrication of the seals of the stuffing box when there is no water in the mill.

In the pipes for water injection and compressed air respectively, non-return valves (CVWO) and (CVAO) ensure that water and air, respectively, are not forced back into the systems when the equipment changes between water injection and air injection. The non-return valves (CVLO) in the pipe for lubricating water fulfill the same function.

Start and stop of water injection and the volume of the injected water are controlled by material temperature (T03) in the mill discharge casing where the material flows over the temperature sensor.

If temperature controller or motorized control valve is out of operation, water injection can be controlled manually by means of manual regulation valve (HRWO). But this should be avoided to extent possible.

2.4     Inlet end

The water injection equipment at the inlet end is designed in the same manner as the equipment at outlet end, (see sub-section 2.3) with the following exceptions:

The nozzle, which does not rotate with the mill, is designed so as the water is partly given a rotary movement, and hence is spread as a cone-shaped spout after the nozzle, and partly sprayed down into the mill charge in the ascending side of the mill at an angle of 60O to vertical.

Operating temperature (T01) is recorded directly in the diaphragm of the mill. From here, the signal is transferred to the temperature system,

There is no stuffing box with matching equipment for lubrication of seals or monitoring of air pressure.

See also sub-section 2.3, in which the 0 character (for outlet) in the used symbols, is replaced with I (for inlet).

2.5     Stuffing box

The level indicator in the stuffing box starts the lubricating-water pump (WPLO) so that there will always be sufficient lubricating water in the packing bush.

The pressure in the packing bush determines the service life of the packing. The cleaning air pressure is recorded on monometer (PMLO) and is set on reduction valve (RVAO).

3.       OPERATING PRINCIPLE

Cooling is based on injection and evaporation of so much water that the required operating temperatures in the mill diaphragm and outlet are kept constant. Water injection takes place in the outlet and/or inlet end when the measured temperature is higher than the set minimum value in the matching controller. However, water injection cannot start before the temperature has reached a reference value which is set in the controller.

The injection water volume depends on the deviation between measured temperature and reference temperature.

Deviation entails up or down regulation of the motorized regulating valve.

If the measured temperature increases above the set max. value, an alarm is tripped. If the measured temperature drops below the set min. value, the water pump stops and solenoid valve opens, thus starting air injection through the nozzle.

On account of the regulating characteristics for motorized regulating valve, the pressure ahead of same is kept relatively constant by the pressure control valve, irrespective of the opening degree of the motorized valve.

As long as there is no water injection, compressed air is blow through the nozzle to keep it clean. Compressed air is blow through the nozzle during start and when the mill temperature is so low during operation that cooling is unnecessary. Irrespective of the temperature during a stopping situation, there is always compressed air injection through the nozzle for one hour after mill stoppage.

4.       STARTING AND OPERATION

4.1     Adjustment

4.1.1   Nozzles

The nozzles are supplied bored to a diameter corresponding to 50% of the foreseen water volume. With new mills, the supplied nozzles may normally be used as they are during the running-in period. It is recommended not to weld the nozzles right away so that impurities (packing yarn, etc.) which are collected in the nozzles, can be removed. When the mill operates at full charge, and the normal cooling water demand is known, bore the nozzles as specified in the table.

For measuring of water volume, see sub-section 4.3.4.

4.1.2   Motorized regulating valve

(See separate instruction manual)

Motorized regulating valve (MRW_) is supplied with valve seat and valve spindle adapted to the foreseen water volume. The limit switch of the motorized valve shown in Fig. 3, has been preset by the supplier. Contracts B and C serve as limit stops for spindle movement. Contact A is adjusted to be used as starting condition for the water pump. In PLC controlled systems contact A is normally not used, and contact D is normally not used, irrespective of the control system.

4.1.3   Water tank

When filling water tank (WT), check float valve (FO) operation and overflow with all tank bottom valves closed.

4.1.4   Pumps

Pumps (WPW_ and WPLO) are primed by removing the filling plugs. SeeFig. 4. then open the matching shut-off valves (SVW_ and SVLO) gradually. When the water flows evenly out of the filling holes, close them again and open the shut-off valves entirely.

Check the direction of rotation by starting the pumps for short while.

Note:

If the electrical interlocking are tripped the lubricating-water pump (WPLO) will be able to run for 5 minutes injecting thereby water in the outlet end of the mill.

With closed motorized valve (MRW_), water pump (WPW_) will correspondingly pump the water back to water tank (WT). Part of the water is pumped into the mill when the motorized valve is open during testing.

4.1.5   Stuffing box

Check the float chamber of the stuffing box. See Fig. 5. The float chamber must have been adjusted to vertical position so that the water level is correct inside the stuffing box, that is, the water level is level with the bore in the stationary part (U-ring) of the stuffing box. Adjustment, if any, to be made with screws, pos. 1.

Fill the inside water chamber to correct height (X). either by means of pump (WPLO) (see sub-section 4.1.4) or manually through one of the hose connections.

The mill must not operate without water in the stuffing box.

Fill the outside chamber of the stuffing box manually until water flows out the overflow, pos. 2.

4.1.6   Air pressure

For outlet end, adjust reduction valve (RVAO) in the compressed air pipe so that the pressure (PMLO) in the stuffing box is 0.5 – 0.7 bar.

Adjust pressure stat (PMH), having a fixed difference of 0.4 bar, for a break pressure (P41) of 1.2 bar read on manometer (PMAO).

For inlet end, adjust reduction valve (RVAI) so the pressure (PMAI) is approx. 1.0 bar.

Activate solenoid valve (SOA_) and check that air is blown through the nozzle, and that pressure (PMLO) is 0.5 – 0.7 bar and (PMAI) is 0.8 – 1.2 bar. If not, adjust the pressure in reduction valve (RVA_)

4.1.7   Water pressure

Close motorized valve (MRW_) manually and start pump (WPW_) then adjust pressure control valve (PRW_) that pressure (PMW_) ahead of the motorized reduction valve is approx. 3 bar. Then stop the pump.

4.1.8   Controller

Adjust the desired reference temperature on temperature controller (TRAH) for inlet end and outlet end respectively. Recommended temperatures are approx. 110OC in mill diaphragm and approx. 115OC in outlet.

Set the max. and min. temperatures used for  max. alarm and start/stop of pump for water injection, respectively, in the control system. For guidance, it is recommended to adjust the max. temperatures to the reference value plus 10OC, and the minimum temperatures at reference value minus 10OC. For controllers with percentage reading convert temperatures to percentages based on the set scale value of the controller. For example, 100% = 150OC.

4.2     Starting and operating conditions

4.2.1   Water pump (WPW-) for water injection

Motor M2 for pump (WPWO) for outlet starts, when:

– Mill motor runs

– And when temperature T03 is higher than the set point

– And when water valve M1 is more than 10% open.

During operation with water spraying, the pump stops when operating temperature T03 becomes lower than the minimum temperature set in the control system.

Motor M5 for pump WPWI for inlet starts and stops as for the outlet, but with T as starting condition.

The water injection pumps must be stopped in all other situations.

4.2.2   Water pump (WPLO) for lubricating water to stuffing box

Motor M3 for lubricating-water pump WPLO starts when the water level in the floater chamber of the stuffing box is below minimum value. The pump operates during 3 minutes and stops when the water level is above minimum. If the water level is still below minimum, after 3 minutes, the pump continues operation for an additional period of 2 minutes. If the level is still minimum, an alarm is tripped and the mill motor is cut out.

The alarm indicates that either the stuffing box is too leaky or the level switch in the floater chamber of the stuffing box is defective and water is injected into the mill.

During operation with water injection, the water level in the packing bush will be sufficient for the lubrication of the seals.

4.2.3   Solenoid valve (SOAO) for cleaning air

Coil Y1 in the solenoid valve (SOAO) opens and compressed air is injected when:

• Mill motor operates and when
• Pump motor M2 is stopped, i.e. during operation without water injection.

Coil Y2 in solenoid valve (SOAI) for inlet opens the valve as for inlet, by with pump motor M5 being the starting condition.

In addition, both solenoid valves are open 1 hour after mill stoppage.

4.2.4   Mill motor

Mill motor cannot start if

– level switch in stuffing box indicates a too low water level in the floater chamber of the stuffing box,

or if:

– Air pressure P41 exceeds 1.2 bar set on the pressure stat PMH.

During operation, the mill motor stops if:

– the water level in the stuffing box is below minimum for more than 3 minutes, or if:

the air pressure in the stuffing box becomes higher than 1.2 bar.

4.3     Operation

The water injection equipment should always run with controller in AUTOMATIC mode, which ensures the safest mill operation.

When water injection starts or stops, minor changes may occur in the material volume in the mill, particularly at the outlet end. This is recorded by changes in the folaphone signal, elevator load or under-pressure.

Minor changes will normally be of on importance. Defective regulation of water injection should be avoided, because excessive regulations may cause unsteady mill operation, obstructions in diaphragm or outlet and irregular product quality.

4.3.1   Reference values

The values indicated in sub-section 4.1.8 for reference max. and min. temperatures are for guidance only for the running-in of the mill.

The current operation conditions and the type of controller can lead to the necessity of adjusting these values. Adjustment should preferably be made when mill operating condition, and temperatures T01 and T03 are stable.

In general, reference values should be set individually at site so that wet as well as dry-clogging are prevented.

On the other hand, adjustment should be made with due account taken of surface hydration, dehydration of gypsum and cement storage resistance.

4.3.2   Alarm limits

The equipment has alarm indication without interlocking, if the temperature T01 or T03 is higher than the maximum set in the control system.

NO alarm is given in case the water or compressed air supply fails.

4.3.3   Dew point

When the water injection is in operation, it is important that the temperature nowhere in mill dedusting becomes lower than the dew point.

If so, the cement dust will cake and set.

It is recommended to determine the dew point at the mill fan (for example by measuring the dry and wet temperature) when injecting maximum water volume. If the temperature is less than approx. 15OC from dew point, step up mill ventilation.

4.3.4   Measuring of water volume

Normally there is no need to measure the injected water volumes. However, in connection with the determination of nozzle size, this may be required.

Measurement may be made with the mill in stable operation by probing in the water tank while valve (SVS) is closed.

For water tanks holding respectively 500 and 800 liters, the emptying speeds are 25.0 and 15.7 mm per minute at a flow rate of 1000 liters/hour. The effective measuring height is approx. 500 mm. If there is water injection at either end of the mill at the same time, interrupt one of them temporarily by stopping the matching water pump while making the measurement at the opposite end.

The interrupt must be so brief that the mill operation if not seriously disturbed. Do not stop the water injection by closing valve (SVW_).

5.       SYMBOLS

Reference : http://www.metso.com/products/iron-ore-pelletizing/

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