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Chapter 4.1
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Finish Milling and Grinding
by Nile R. Strohman*
The finish mill system in cement manufacturing is used for reducing the size of the clinker from as large as several centimeters in diameter to a size that is a maximum of 100 micrometers across. This process is accomplished by grinding (milling) with the use of ball mill, roller mills, roll presses, or some combination of these processes. Out of 110 to 130 kWh/ton of electrical power consumed in making cement, between 30 and 50 kWh/ton are consumed by the finish milling operation. This is the largest single consumption point of electric power in the process of converting raw materials to finished cement. The main focus of this chapter is the ball mill, since this type of equipment produces most cement. However, the main features of this chapter apply regardless of the type of equipment. A typical ball mill for finish grinding at a cement plant is shown in Figure 4.1.1.
Figure 4.1.1. A ball mill used for finish grinding at a cement plant.
* Cement Additives, Grace Construction Products, W. R. Grace and Co. Conn., 11504 Long Meadow Drive, Glen Allen,
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Clinker may be harder or easier to grind depending on how it is burned, its chemical composition, and how it is cooled. One of the major causes of hard grinding is the amount of C
2 S (belite)
in the
clinker. One of the reasons for the formation of belite clusters in clinker is either the use or the inclusion of coarse silica such as quartz, in the raw mix. This is readily determined by the use of microscopic examination of the feed which is very useful in diagnosing burning as well as grinding problems. Regardless of the grindability of the clinker, it is very important that uniformity of the clinker be maintained so the grinding operation may also maintain a good degree of uniformity for optimum quality and production rates. The raw mix also needs to be maintained uniform in addition to being a mix that is not hard burning so that long retention times in the kiln are avoided. Long retention times in the kiln result in large C 3 S (alite) and/or large C
2 S (belite)
crystals. Both of these large crystals are hard to grind,
in addition to causing a dusty clinker with higher than desirable amounts of fines. It is well known that the clinker chemistry and burning conditions can have a great effect on the grinding rate as well as on the quality of the cement produced.
FINISH MILLING SYSTEM
In cement manufacturing plant, the finish milling system is comprised of four basic components namely, 1) feeders, 2) mill, 3) elevator, and 4) separator. A schematic of the finish milling systems is shown in Figure 4.1.2.
Cement Elevator Separator Ball mill
Feeder
Coarse material
Figure 4.1.2. Schematic of finish milling system in a cement plant.
The following sections will briefly describe the operations of these components of the milling system.
The Feeders
In too many instances, the weigh feeders give a nonrepresentative view of how much cement the mill is producing. These readings are subject to errors in their data from the calibration of the feeders. Regular calibrations should be performed as per the manufacturers recommended proce-
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dure. Normally this involves a known weight chain, which is placed on the feeder belt; the calibration is performed using this weight. Recently, it has become popular to calibrate weigh feeders electronically without weights; this procedure is rapid, but is regarded as less reliable than the one which uses live weights. A simple method of checking the accuracy of the feeder is to measure the amount of material on a known length of the feeder belt, while measuring the speed of the belt. This is accomplished by measuring the cross section of the material and timing the speed of the belt to give a volume of material. By using the bulk density of the plant-specific clinker, a reasonable feed rate can be determined as follows: FR = A x L x D / t where, FR = feed rate (mass of new feed to mill per minute) A = cross-sectional area L = length D = bulk density t = time in minutes (1)
The Mills
Many mills are operating with an impact scale in the reject (tails) stream. Although these can be calibrated electronically, it is ideal if this scale is calibrated by actual weight. Consideration should be given to inclusion of facilities to dump and weigh a timed portion of material from the impact scale to correct the calibration to a real weight for accurate real-time circulating load numbers. A ball mill is typically identified by connected horsepower (or kW) and internal dimen-
The liners.
sions, length, and diameter. The ball mill shell is protected by carefully designed wear-resistant liners, which provide lift to the ball charge. As depicted by the diagrams in Figure 4.1.3, there are straight lifting liners on the left, ripple liners in the center, and rarely used smooth liners on the right.
Figure 4.1.3. Three different lifting liners of a ball mill; straight lifting liners (left), ripple liners
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(center), and smooth liners (right).
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Figure 4.1.4 gives a view of a second compartment and liners in a ball mill. One can see the wear grooves in the liners. This compartment is typically used to reduce the size of the clinker down to less than 5 mm.
Figure 4.1.4. View of second compartment of a ball mill showing liners.
Some examples of classifying liners that are typically used in the second compartment of a ball mill are shown in Figure 4.1.5. These liners generate a classification of the ball charge and are used primarily in the second compartment to promote the classification of ball sizes, so the small balls are at the outlet of the mill and large balls at the division screen.
Figure 4.1.5. Classifying liners typically used in the second compartment of a ball mill.
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The ball charge.
Typical ball charges run from 30% to 35% of mill volume with some mills
slightly higher and some lower. If the ball charge is increased above about 36% with classifying liners, the ball charge will probably reverse classify, causing a reduction in grinding rate. Below 30%, the production rate will suffer. However, it is sometimes possible to reduce the kWh/ton by reducing the ball charge, but with a reduction in the production rate. The relationship between ball charge percentage loading, production, and kWh should be carefully watched. The decisions on how to charge the mill should be made with full knowledge of the costs and consequences generated by these decisions. Consideration must also be given to the fact that the use of classifying liners reduces the internal volume of the mill by up to 10%. A typical equation employed in calculating the ball charge is as follows: % volume loading = 0.0087278 x R [C (R-H)/2] ( R2 ) where, R = radius of the mill inside liners C = horizontal distance liner to liner at ball charge surface H = free vertical height, charge to liner D = diameter of the mill inside liners The definition of critical speed (CS) is the slowest speed at which an infinitely (2)
Critical speed.
small particle on the mill liner will centrifuge, i.e., the speed necessary to prevent this infinitely small particle from collapsing away from the mill shell at any point. The calculation for critical speed of a mill is: CS = 76 .63 / where, CS = critical speed D = internal diameter of mill in inches. The percent of critical speed a mill is running at is expressed as: % Critical speed = Actual rpm / CS, as a percentage Typical percent of critical speed is about 75% but may range from as low as 65% to 80%. The only way to change the critical speed is to change the motor speed or the gear reduction. This calculation should be done so that the critical speed is known. This knowledge may help explain operating parameters and help to determine ball sizing. The division head, or diaphragm, divides the mill into compartments. Typically, this is a double plate design with lifter plates to facilitate the flow of material through the plates. These plates are typically slotted with the slots comprising approximately 5% to 10% of the total area of the division head. Slot sizes are usually about 6mm with slot sizes in the discharge screen plate being (4) D (3)
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larger, at about 8 mm, to make sure no spitzers remain in the second compartment. Some plants use so-called flow control diaphragms to enable the plant operators to adjust the lifting action so the filling level of the first compartment can be controlled.
Grinding balls.
Grinding balls, also known as grinding media, are used in various sizes ranging
from as small as 12 mm to as large as 100 mm. in diameter (Figure 4.1.6). The various sizes are used in combinations to adjust the density of the charge, to regulate the speed the material flows through the mill, and to deal with various degrees of hardness of the materials to be ground. The first compartment (or primary compartment) will typically be charged with a mixture of balls from 50 mm up to 90 mm. Occasionally, with very hard to break materials, some 100 mm balls will be used.
Figure 4.1.6. Grinding balls in the first compartment of the ball mill.
The secondary or fine compartment may contain balls as small as 12 mm ranging up to 50 mm diameter balls. On the subject of ball size and ball charge in a grinding mill, Duda (1977) recommended that the first compartment of the mill, where the grinding is primarily done by impact alone, should contain ball size between 60 mm to 100 mm. In the second compartment where grinding is performed by a combination of impact and trituration, the balls sizes should range between 35 mm to 60 mm. In the third compartment, where the grinding is done mostly by the trituration effect, the ball size should range between 20 mm to 30 mm. The ball size and the corresponding ball charge in different compartment of ball mill is given in Table 4.1.1.
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Table 4.1.1. Recommended Ball Size and Total Charge in a Three-Compartment Ball Mill (Duda, 1977) Ball size, mm
60 100 35 60 20 30
Mill compartments and grinding mode
I II III Impact Impact and trituration Trituration
Ball charge, wt. %
30 27 24
Evaluation of Mill Operation
In order to evaluate the mill operation, a complete set of circulating load samples is needed. This includes mill discharge, separator feed, separator fines discharge, and separator coarse discharge (tails or rejects). These samples should be a composite of at least five samples taken over a 15 to 20 minute time period. (These samples should be taken close to the mill or separator.)
Mill Retention Time
An additional test for evaluating mill operation is a mill retention time test that can be conducted during the same period of time as the circulating load sample is being taken. This data is then all from the same period of time and operating conditions. The mill retention test consists of adding a dye (typically fluorescein) to the mill at a given time and taking samples from the discharge of the mill at least every 1 minute for 15 minutes. These samples are then tested to determine the time at which the maximum amount of dye was in the sample. This time is considered to be the mill retention time (MRT)*. The mill retention time procedure, as developed by W.R.
Mill retention time procedure.
Grace, consists of tagging cement clinker with fluorescein that has been put into a water solution and poured into a plastic bag of clinker. Typically this plastic bag would be at least doubled to preclude leaking of any of the fluorescein solution. The amount of fluorescein is about 15 grams for every 20 tons per hour of feed (total of fresh feed and rejects) to the mill. The fluorescein is put into solution with a minimum amount of hot tap water and poured into the doubled bag containing about 5 kilograms of clinker with a lot of fines in it. An ideal clinker is one that has been run through a jaw crusher. Precautions should be taken against breathing any of the dust or getting the solution on the skin, as with any chemical. (These tagged bags may be prepared ahead of time and even stored for several days if necessary.)
* CAUTION: Prior to running the circulating load tests and/or the MRT test or any other test, the milling system must be in equilibrium to have any meaning! If the system is not in equilibrium, the tests will reflect the instantaneous condi-
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tions existing at the time and will be meaningless with regard to normal operations.
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The tagged material is added to the finish mill by dropping the bag and all its contents into the feed chute of the mill. The stopwatch is started at this point. A sample of at least a 50 g is taken exactly every 1 minute from the mill discharge. In the case of very short mills or the ones in which the retention time may be very short, sampling should be done every 30 seconds for the first 5 minutes to improve the accuracy of the testing. These samples are analyzed by taking the same quantity (typically 3 to 5 g) and adding 25 mL of water to each sample. This sample is then stirred, allowed to set for at least 30 seconds, and filtered. The liquid is then analyzed in a photometer set at 484 nm wavelength. This is plotted, absorbance vs. time, and the peak in minutes is used as the mill retention time. Mill retention time (MRT) is very helpful in determining the operating conditions of the finish mill. One of the calculations where it is used is determining the % void fill of the mill without the need for shutting the mill down in a crash stop manner. The calculation is done as follows: ICC = MRT/60 x TPH x 2000 x ({100 + CL}/100) Void Capacity = (BC/285) x 0.45 x (avg. ft % Void Fill = (ICC/Void Capacity) x 100 where, ICC = instant clinker charge MRT = mill retention time TPH = tons per hour mill production CL = circulating load BC = total ball charge in mill in pounds A depiction of the filling of a ball mill in a static condition is shown in Figure 4.1.7.
3
(5) (6) (7)
cement in mill)
Height above powder Height above charge High void filling Mill diameter
Volume loading
Low void filling
Figure 4.1.7. Filling of ball mill in a static condition.
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This calculation will give a very accurate determination of the operating condition of the mill as far as void fill is concerned, provided the measurements, testing, and determinations are conducted accurately. The best operating conditions are when the mill has between 85% and 100% void fill. The mill retention time for a closed circuit mill will be between 6 and 9 minutes. Short stubby mills may have a very short mill retention time, down as low as 3 or 4 minutes. It is difficult to change this, due to the configuration of the mill. However on a normal mill grinding closed circuit, there are several things that can be done to change the MRT. If the mill has an adjustable diaphragm, adjustments to it may change the MRT. Changes to the composition of the ball charge have the largest effect on the MRT. A denser ball charge will increase the MRT, and conversely, a less dense ball charge will shorten the MRT. The airflow through the mill will affect the MRT to some extent, as when the airflow is increased more material will be pulled through the mill with the air into the dust collector. If the airflow is too high, this will hurt the performance of the mill by not allowing it to grind all the material sufficiently for optimum operation, but it will also shorten the MRT.
Mill Efficiency
To help determine the efficiency of the mill, by itself, in a closed circuit system, a method that has proven very beneficial is the 325-mesh improvement through the mill. The calculation is as follows: 325 increase through mill = MD where, MD = mill discharge passing 325 T = tails (rejects) passing 325 CL = circulating load of the mill system This increase in the 325 should be between 20% and 30%, if the mill itself is grinding well. If it is below this value the mill is not preparing the cement well enough for the separator and is reducing the output of the mill system by overloading the separator and sending an excessive amount of rejects (tails) back to the mill for regrind. If the increase is above 30%, then the mill is overgrinding and not allowing the separator to do its share of the work. This again reduces the mill system production rate as the mill is operating more like an open circuit mill.
(1+CL )
T CL
(8)
Mill Throughput
The throughput of a ball mill is another parameter for gauging the mill efficiency. The throughput is often estimated from a grindability curve that relates the throughput as function of product size in Blaine numbers (Duda, 1977); the grindability curve is shown in Figure 4.1.8.
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70
60 Wh / 50 k 40
30
20 Specific throughput, a, kg 10
1000
2000
3000 Blaine number, cm
2
4000 /g
5000
6000
Figure 4.1.8. Grindability curve showing relationship between specific throughput and fineness of finished product in grinding mill (Duda, 1985).
To estimate the specific throughput in kg/kWh from the curve, the fineness of the finished product from the mill in Blaine number (cm
2 /g)
is used. From this value the actual mill throughput is
calculated in tons/hour using Jacobs throughput formula as follows: L = (M g x Tp x D where, L = mill through put (t/hr) M g = mill capacity value Tp = specific throughput (kg/kWh) taken from the curve using the Blaine number D m = mean grinding path diameter, m V = volume of grinding space, m
3 m
x V)/20,000 x D
Mb
(9)
D Mb = reference grinding path diameter, 1.0m
Particle Size Distribution
Particle size determinations (PSDs) are very useful in helping determine the operating efficiency of a mill system. These values of the separator product (fines), separator feed, and rejects (tails) can be used to develop a Tromp curve for the separator. The size selectivity curve or Tromp curve describes
Size selectivity curve or Tromp curve.
classifier performance for all particle sizes in the feed to the classifier. The curve is based on particle
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Document Outline
HOME PAGE Section 4 Page FINISH MILLING AND GRINDING Finish Milling System The Feeders The Mills Evaluation of Mill Operation Mill Retention Time Mill Efficiency Mill Throughput Particle Size Distribution Separator Performance Grinding Aids Theory of Grinding Aids Grinding Aids Function Application of Grinding Aid Methods of Addition References NEXT CHAPTER
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