ball mill operation -grinding circuit startup & shutdown procedure
After the grinding circuit has been brought up to normal operating conditions, the operator must monitor the various process variables and alarms. Most of these variables are monitored in the mill control room, however, the operator is also required to sample and analyse process streams and read local indicators.
The ball mill is susceptible to variations in ore hardness resulting in various grinds at constant throughput, or alternatively, various tonnages at constant grind. The variation in grind is not directly determined. However, a changing cyclone overflow density, at a constant tonnage rate and feed density, would be indicative of changing ore hardness and in that case the tonnage fed to the ball mill should be changed accordingly.
The ore feed rate to the ball mill is controlled by the weightometer located on the mill feed conveyor which can be manually adjusted with in the control room to give a constant weight reading. The signal from the weightometer increases or decreases the belt feeder speed and adjusts the water addition to the ball mill (as a function of the actual weight reading). Both weight control and the proportion of water can be adjusted in the control room.
The water rationing controller must be adjusted programmatically to give the desired ball mill discharge density (normally 60-65% solids). The ball mill discharge density should be checked manually at regular intervals and adjustment made to water ratio controller setpoint to adjust the ball mill discharge density.
The grinding circuit operator must ensure that the ball mill runs properly loaded and gives the correct ore grind. A major practical indication of mill loading is the sound made by the mill. A properly loaded mill will have a deep rhythmic roar, while an under loaded mill will have a metallic rattling type noise and an overloaded mill will be quite silent.
The operator must manually measure the cyclone overflow and underflow densities regularly. An increase in overflowdensity is indicative of softer ore and will soon be accompanied by a lowering of power draw at the mill and a change of sound indicating that the mill is becoming under loaded. To compensate, feed tonnage must be increased. Similarity, a decrease in the cyclone overflow density is indicative of harder ore and this will be accompanied eventually by a coking of the mill. Feed to the ball mill must be reduced.
In the event of an emergency, the mill feed conveyor is shut down individually or by stopping the operating cyclone feed pump. The ball mill must be shut down separately. All equipmentshutdowns are performed locally or from the MCC located in the mill control room.
operating and troubleshooting a grinding circuit
In the operation of a grinding circuit you are managing several pieces of equipment as a single unit. If you make a change in the ore in the fine ore bin, that change will be reflected through the entire grinding process and beyond into the rest of the concentration system.
The first thing that an operator will have to learn is to be able to tell when the rod mill is, or isnt, grinding fine enough. With a little practice the operator will be able to simply look at the discharge of a rod mill and determine how well it is working. The variables that are being looked at are color and consistency of the slurry. If the rock is staying in suspension or is it classifying as it slides over the discharge trunnion? The amount of rejects that are being discharged. And the manner in which the slurry is beading on the side of the discharge trunnion as it is flowing across it. These will give an indication of any change in the density or the ore. To verify any change that you think that you see in the discharge, simply take a density. If it has changed then so has your circuit. Once you have finished weighing the density sample, pour it out over your gloved hand, you will be able to feel and see the difference in the grind.
Lets pretend that you have taken all of your densities, checked the grind and the number of rejects that are coming out of the mill. Prom this information you have decided that the circuit could use some more tonnage. So you go to your control panel and increase the through put, oh, ten tons per hour. What happens to your circuit? First the extra ore enters the rod mill the density begins to climb and the grind gets coarser. You start to get a few rejects. Then the heavier density with the courser grind is pumped to the cyclone. Both the overflow and the underflow from the cyclone become heavier. Remember you now have a higher amount of ore that will have to be ground further. This causes the density of the ball mill to climb. Which in turn is discharged back to the cyclone. If now, you go back through the circuit, and check your variables you will notice that the rod mill density has increased. The grind has gotten worse. The cyclone over flow density and the ball mill density has gotten heavier.
You think that the rod mill grind could be better, after all, the density is above the recommended range that your supervisor wants it run at. So you add a little water. The water decreases the retention time. It also improves the grinding action of the rods. The higher density was cushioning the rods. The movement in the core area of the rod load had been restricted due to the excess tonnage. Increasing the water improved the rate of fine material discharge. Maintaining the correct density will insure that the rate of discharge is correct. The lighter density may produce less coarse and marginally less fines but it will produce more of the middle sizes.
This size is called MIDDLINGS, which will have a large percentage of ore fine enough to be sent to the next stage of concentration. The water has caused both the amount of fines and the amount of coarse produced to drop. The amount of the middling sizes increased. The cyclone overflow would allow a lot of this middling size to escape the grinding circuit. This would decrease the amount of material that the ball mill would have to grind. Which in turn would lower the amount of ground ore returned to the cyclone for reclassification. This again would lower the resulting ball mill load even further.
Although the cyclone overflow density may not change for a short period of time due to the extra water and the extra ore in the over flow more or less balancing each other out. It will as the decreased density of the underflow changes the grind of the ball mill. The density of the cyclone overflow will still be too high, but it has dropped a little from what it was. At this point, as the finished product, this is the MOST IMPORTANT DENSITY. To bring the density back to the operating perimeters, you add water going to the cyclone feed pump box. This will compensate for the higher density. Again there will be a reaction to the water in the underflow of the cyclone, but it will be a minor one, and a simple check of the overflow after a period of time will determine if any more adjustments are necessary. The other worry that the operator will have is the capacity of his circuit.
The maximum tonnage that can be processed will depend upon the capacity of the smallest piece of equipment that must handle the volume. This is another characteristic of each individual grinding circuit. It may be the rod mill that cant take the load, or perhaps the cyclone or pump box. Whatever it is, you may he guaranteed that one piece of equipment will reach its limit before any of the others. That will be the one that you will watch the most.
The reaction of our fictitious circuit is, by no means, meant to be taken as a standard reaction to the different variables. How the ore will behave in the mill will depend upon the grinding characteristics of each individual ore body.
The purpose of this exercise was to illustrate the DYNAMICS OF A CIRCUIT. How a change in one portion of the mill will affect the rest of the circuit. It was also to show that it is possible for an operator to get into trouble if he doesnt allow enough time for a change to completely go through the circuit before making another change. Checking the results of a change to soon can also lead to trouble. An example of this would be in the rod mill. If the operator had checked the rod mill load prior to the time it took for the extra volume of ore to fill up the mill, he could possibly feel that the resulting density was still within the perimeters set down by the supervisor. When in fact they hadnt finished climbing yet. The results could be an overloaded grinding circuit, and a very poor grind.
When a circuit becomes overloaded it is because the amount of ore that is going out of the overflow is less than the total volume of the ore coming into the circuit. What is known as a CIRCULATING LOAD builds up. This is the ore that is going to the classifier, back to the ball mill, back to the classifier, back to the mill again. The work index and the size of the cyclone feed will determine the ratio of ore that is returned to the ball mill for further grinding. A ball mill operates more with the grinding surface action of the balls than the impact form of grinding that the rod mill uses. Taking this into account you can see that if the rod mill doesnt reduce the ore to a size that the ball mill can reduce quickly, the circuit load will climb until its volume is greater than can be handled.
To maintain the maximum amount of through put that the grinding mills can handle it is necessary to keep what is termed as the MEDIA RUNNING LOAD to its maximum. This is the amount of grinding rods or balls that are in the mills. As the mills tumble and turn the grinding media wears away until the mill charge is too low to maintain the grind. The solution to this problem is to add more media. In the case of the rod mill the circuit will have to be shut down to accomplish this, the ball mill, however may have its media added while it is running. The standard method used to put the balls into the mill is to drop them into the feed end along with the feed. The rod mill will have a set of rollers put into the discharge end of the mill. The rods are then rolled in on top of these rollers to be dropped off onto the existing rod load. To know when to charge the mills you have to consult the RUNNING LOAD of the mill. This is simply the amperage load that the motor is working against. Every electric motor has a maximum amperage point, after which it kicks out. The running load is the percentage of the maximum amperage load that the electric motor can safely handle.
The grinding circuit is the most expensive circuit in the concentrator plant to run. To make the most profit from this equipment, it is necessary that it is run at the maximum amount of tonnage that the mills will handle. While maintaining the efficiency level that is set by management. There are times when it may seem that if you drop the tonnage five tons per hour you would save yourself a lot of work. The increased effectiveness would out-weight the lost tonnage. Most operators reason that the ore will not go away and it will still be here to be processed tomorrow. Unfortunately all of the mines finances will be set up on set of perimeters that have to be met. The farther that you can exceed them the bigger the safety margin you can have for emergency shut downs and unexpected expenses. That extra five tons per hour may be the gravy money. How the operator does his job goes a long way in the operation of a successful mine. There has been more than one mine that has been shut down by poor operating practices. It is the operators job to get every ton that he can out of his equipment. Keep down time to minimum. And to recover as much of the mineral as he or she can.
Before we get to a few of the safety aspects of grinding, I would like to REVIEW some areas of concern for the operator when CIRCUIT CHECKS are being done. Lets start at the feeders. Here the operator must watch for ore stoppages. Foreign objects in the feed that could damage belts, or block transfer chutes. With these chutes come lost and worn liners and ore blockages. The conveyors have to be watched for belt wander and mechanical damage and weightometer be kept clean.
With the mill themselves, the trunnion bearing oil ring has to be watched. Leaking liner bolts and other wear caused by the abrasion of the ore has to be reported or fixed. Bull gears and some types of bearings have to be lubricated. Running loads monitored, as well as the other standard readings that are required. And last but not least maintaining good housekeeping standards.
ball mill maintenance - the cement institute
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The Ball Mill Maintenance course is designed to engage in the effective use of hands-on learning methodology as a unique combination of theory and practical work section applied to the ball mill systems maintenance inspection. This course offers an in-depth understanding of the maintenance activities, providing the precise tools to achieve optimal levels of personal performance and accomplishment, obtaining tangible and positive impact in the ball mill areas performance and reliability.
Grinding is one of the cement industrys fundamental processes: (for the preparation of raw materials, coal grinding, and cement grinding). Cement manufacturing is a continuous process industry with very high requirements and performance rates, requiring high reliability in both the process and maintenance.
In the open circuit system, the mill product has the fineness required for the next stage. In the closed-circuit system, the mill product is classified in a separator in a fine fraction that is then taken to the next step and a thick fraction that is returned to the mill for subsequent milling.
It is necessary to grind clinker, gypsum, and sometimes other additives in the proportions required at a predetermined fineness in any cement grinding circuit. The fineness is usually defined by the cements specific surface area measured in m/kg or cm/g. High-efficiency separators are normal to grind cement in a closed-circuit system due to energy consumption savings.
Safety hazards in ball mills involve numerous situations; anywhere, we can observe conditions related to process, operation, and maintenance. Ball mills can be hazardous machines if they are not operated properly. Therefore, operators should follow the essential safety and maintenance advice; as part of the course, we will cover all the safety precautions to ensure safe operation and maintenance. Operators must take certain precautions before beginning to operate a ball mill. Here is a list of the most critical safety maintenance steps that all operators must follow when using a ball mill:
Cleaning the machine after use: A ball mill must be cleaned after each operation or at the end of the working day. Major components like the grinding tool, grinding roller, and grinding ring are prone to wear. Because of that, each part or component should be regularly lubricated and checked for damage.
Ball mill is generally used to grind material 1/4 inch and finer, down to the particle size of 20 to 75 microns. To achieve reasonable efficiency with ball mills, they must be operated in a closed system.
There is a specific operating speed for the most efficient grinding. At a certain point, controlled by the mill speed, the load nearest the cylinders wall breaks free, and other sections so quickly follow it in the top curves to form a cascading, sliding stream containing several layers of balls separated by the material of varying thickness. The top layers in the stream travel faster than the lower layers, causing a grinding action between them. There is also some action caused by the spinning of individual balls or pebbles and secondary movements having the nature of rubbing or rolling contacts inside the main contact line.
The mechanical elements of a tube mill could be separated into components that directly function with the grinding process (i.e., grinding media, liners, diaphragms) and into parts that can be considered individual units connected to a specific tube mill. The latter group includes:
The mechanical elements of a tube mill can roughly be subdivided into internal and external parts. The mill internals directly functions concerning the grinding process and include the wear parts of a mill, such as mill liners, diaphragms, and grinding media principally. We will explain how to inspect the mill liners, diaphragm, water spray system, and components as well as physical mill inspection with our checklist instruction:
This module will detail a regular sequential physical inspection of conveyors by operators and mechanical maintenance personnel. It will detail what items require inspection and what possible issues and problems the inspecting personnel should be trying to identify and rectify. It will cover each of the major components that make up a conveyor system and include the variations within different conveyor designs. The topic will cover such components as:
The turbomachines used for gas compression are classified into radial, axial, or mixed flow types according to the impellers flow. In a radial or centrifugal machine, the increase in pressure due to centrifugal action is an essential factor in its operation. Energy is transferred by dynamic means from the impeller to the fluid. The fluid due to centrifugal action is continuously thrown outward, allowing fresh fluid to enter due to reduced local pressure.
Another characteristic feature of the centrifugal impeller is that the angular momentum of the fluid flowing through the impeller increases because the impellers outer diameter is significantly larger than the inlet diameter. In axial flow machines, the rotating impeller sets in motion a large mass of gas and is advanced by the blades aerodynamic action.
Dust collection systems are the most widely used engineering control technique by cement processing plants to control dust and decrease workers exposure to respirable dust. A well-integrated ball mill dust collection system has multiple benefits, resulting in a dust-free environment that increases productivity and process operation. The most common dust control techniques in cement plants use local exhaust ventilation systems.
This course is primarily geared to all maintenance staff personnel with a focus on the preventive maintenance area. Future managerial persons, whether from maintenance or production, may also benefit significantly by participating.
Those with little or no prior experience with ball mill maintenance will learn to understand, interpret, and use the core concepts of equipment design and their limitations. Gain valuable skills that can be used immediately to analyze and implement preventive maintenance.
A unique combination of theoretical and practical skills throughout this course will be learned, which will help you develop and execute the concepts and technical knowledge acquired in the daily maintenance activities. The following downloadable materials are part of the course to enhance and facilitate the participants learning experiences.
Work section book: Provide learning activities and hands-on practice case study and exercises. Solutions are included after each training is completed. Certification is achieved by completing a satisfactory level of exercises, quizzes, and final exams for each module.
The course is conducted online, allowing students flexibility (within four weeks) to complete all modules. Students should expect to spend more than 10 hours per week and some additional time for private reading/study. A computer with Internet access (broadband recommended) and email will be required.
ball mills - an overview | sciencedirect topics
A ball mill is a type of grinder used to grind and blend bulk material into QDs/nanosize using different sized balls. The working principle is simple; impact and attrition size reduction take place as the ball drops from near the top of a rotating hollow cylindrical shell. The nanostructure size can be varied by varying the number and size of balls, the material used for the balls, the material used for the surface of the cylinder, the rotation speed, and the choice of material to be milled. Ball mills are commonly used for crushing and grinding the materials into an extremely fine form. The ball mill contains a hollow cylindrical shell that rotates about its axis. This cylinder is filled with balls that are made of stainless steel or rubber to the material contained in it. Ball mills are classified as attritor, horizontal, planetary, high energy, or shaker.
Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles, as well as collision energy. These forces are derived from the rotational motion of the balls and movement of particles within the mill and contact zones of colliding balls.
By rotation of the mill body, due to friction between mill wall and balls, the latter rise in the direction of rotation till a helix angle does not exceed the angle of repose, whereupon, the balls roll down. Increasing of rotation rate leads to growth of the centrifugal force and the helix angle increases, correspondingly, till the component of weight strength of balls become larger than the centrifugal force. From this moment the balls are beginning to fall down, describing during falling certain parabolic curves (Figure 2.7). With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls are attached to the wall due to centrifugation:
where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 6580% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.
The degree of filling the mill with balls also influences productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 3035% of its volume.
The mill productivity also depends on many other factors: physical-chemical properties of feed material, filling of the mill by balls and their sizes, armor surface shape, speed of rotation, milling fineness and timely moving off of ground product.
where b.ap is the apparent density of the balls; l is the degree of filling of the mill by balls; n is revolutions per minute; 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.
A feature of ball mills is their high specific energy consumption; a mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, i.e. during grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.
Grinding elements in ball mills travel at different velocities. Therefore, collision force, direction, and kinetic energy between two or more elements vary greatly within the ball charge. Frictional wear or rubbing forces act on the particles as well as collision energy. These forces are derived from the rotational motion of the balls and the movement of particles within the mill and contact zones of colliding balls.
By the rotation of the mill body, due to friction between the mill wall and balls, the latter rise in the direction of rotation until a helix angle does not exceed the angle of repose, whereupon the balls roll down. Increasing the rotation rate leads to the growth of the centrifugal force and the helix angle increases, correspondingly, until the component of the weight strength of balls becomes larger than the centrifugal force. From this moment, the balls are beginning to fall down, describing certain parabolic curves during the fall (Fig. 2.10).
With the further increase of rotation rate, the centrifugal force may become so large that balls will turn together with the mill body without falling down. The critical speed n (rpm) when the balls remain attached to the wall with the aid of centrifugal force is:
where Dm is the mill diameter in meters. The optimum rotational speed is usually set at 65%80% of the critical speed. These data are approximate and may not be valid for metal particles that tend to agglomerate by welding.
where db.max is the maximum size of the feed (mm), is the compression strength (MPa), E is the modulus of elasticity (MPa), b is the density of material of balls (kg/m3), and D is the inner diameter of the mill body (m).
The degree of filling the mill with balls also influences the productivity of the mill and milling efficiency. With excessive filling, the rising balls collide with falling ones. Generally, filling the mill by balls must not exceed 30%35% of its volume.
The productivity of ball mills depends on the drum diameter and the relation of drum diameter and length. The optimum ratio between length L and diameter D, L:D, is usually accepted in the range 1.561.64. The mill productivity also depends on many other factors, including the physical-chemical properties of the feed material, the filling of the mill by balls and their sizes, the armor surface shape, the speed of rotation, the milling fineness, and the timely moving off of the ground product.
where D is the drum diameter, L is the drum length, b.ap is the apparent density of the balls, is the degree of filling of the mill by balls, n is the revolutions per minute, and 1, and 2 are coefficients of efficiency of electric engine and drive, respectively.
A feature of ball mills is their high specific energy consumption. A mill filled with balls, working idle, consumes approximately as much energy as at full-scale capacity, that is, during the grinding of material. Therefore, it is most disadvantageous to use a ball mill at less than full capacity.
Milling time in tumbler mills is longer to accomplish the same level of blending achieved in the attrition or vibratory mill, but the overall productivity is substantially greater. Tumbler mills usually are used to pulverize or flake metals, using a grinding aid or lubricant to prevent cold welding agglomeration and to minimize oxidation .
Cylindrical Ball Mills differ usually in steel drum design (Fig. 2.11), which is lined inside by armor slabs that have dissimilar sizes and form a rough inside surface. Due to such juts, the impact force of falling balls is strengthened. The initial material is fed into the mill by a screw feeder located in a hollow trunnion; the ground product is discharged through the opposite hollow trunnion.
Cylindrical screen ball mills have a drum with spiral curved plates with longitudinal slits between them. The ground product passes into these slits and then through a cylindrical sieve and is discharged via the unloading funnel of the mill body.
Conical Ball Mills differ in mill body construction, which is composed of two cones and a short cylindrical part located between them (Fig. 2.12). Such a ball mill body is expedient because efficiency is appreciably increased. Peripheral velocity along the conical drum scales down in the direction from the cylindrical part to the discharge outlet; the helix angle of balls is decreased and, consequently, so is their kinetic energy. The size of the disintegrated particles also decreases as the discharge outlet is approached and the energy used decreases. In a conical mill, most big balls take up a position in the deeper, cylindrical part of the body; thus, the size of the balls scales down in the direction of the discharge outlet.
For emptying, the conical mill is installed with a slope from bearing to one. In wet grinding, emptying is realized by the decantation principle, that is, by means of unloading through one of two trunnions.
With dry grinding, these mills often work in a closed cycle. A scheme of the conical ball mill supplied with an air separator is shown in Fig. 2.13. Air is fed to the mill by means of a fan. Carried off by air currents, the product arrives at the air separator, from which the coarse particles are returned by gravity via a tube into the mill. The finished product is trapped in a cyclone while the air is returned in the fan.
The ball mill is a tumbling mill that uses steel balls as the grinding media. The length of the cylindrical shell is usually 11.5 times the shell diameter (Figure 8.11). The feed can be dry, with less than 3% moisture to minimize ball coating, or slurry containing 2040% water by weight. Ball mills are employed in either primary or secondary grinding applications. In primary applications, they receive their feed from crushers, and in secondary applications, they receive their feed from rod mills, AG mills, or SAG mills.
Ball mills are filled up to 40% with steel balls (with 3080mm diameter), which effectively grind the ore. The material that is to be ground fills the voids between the balls. The tumbling balls capture the particles in ball/ball or ball/liner events and load them to the point of fracture.
When hard pebbles rather than steel balls are used for the grinding media, the mills are known as pebble mills. As mentioned earlier, pebble mills are widely used in the North American taconite iron ore operations. Since the weight of pebbles per unit volume is 3555% of that of steel balls, and as the power input is directly proportional to the volume weight of the grinding medium, the power input and capacity of pebble mills are correspondingly lower. Thus, in a given grinding circuit, for a certain feed rate, a pebble mill would be much larger than a ball mill, with correspondingly a higher capital cost. However, the increase in capital cost is justified economically by a reduction in operating cost attributed to the elimination of steel grinding media.
In general, ball mills can be operated either wet or dry and are capable of producing products in the order of 100m. This represents reduction ratios of as great as 100. Very large tonnages can be ground with these ball mills because they are very effective material handling devices. Ball mills are rated by power rather than capacity. Today, the largest ball mill in operation is 8.53m diameter and 13.41m long with a corresponding motor power of 22MW (Toromocho, private communications).
Modern ball mills consist of two chambers separated by a diaphragm. In the first chamber the steel-alloy balls (also described as charge balls or media) are about 90mm diameter. The mill liners are designed to lift the media as the mill rotates, so the comminution process in the first chamber is dominated by crushing. In the second chamber the ball diameters are of smaller diameter, between 60 and 15mm. In this chamber the lining is typically a classifying lining which sorts the media so that ball size reduces towards the discharge end of the mill. Here, comminution takes place in the rolling point-contact zone between each charge ball. An example of a two chamber ball mill is illustrated in Fig. 2.22.15
Much of the energy consumed by a ball mill generates heat. Water is injected into the second chamber of the mill to provide evaporative cooling. Air flow through the mill is one medium for cement transport but also removes water vapour and makes some contribution to cooling.
Grinding is an energy intensive process and grinding more finely than necessary wastes energy. Cement consists of clinker, gypsum and other components mostly more easily ground than clinker. To minimise over-grinding modern ball mills are fitted with dynamic separators (otherwise described as classifiers or more simply as separators). The working principle is that cement is removed from the mill before over-grinding has taken place. The cement is then separated into a fine fraction, which meets finished product requirements, and a coarse fraction which is returned to mill inlet. Recirculation factor, that is, the ratio of mill throughput to fresh feed is up to three. Beyond this, efficiency gains are minimal.
For more than 50years vertical mills have been the mill of choice for grinding raw materials into raw meal. More recently they have become widely used for cement production. They have lower specific energy consumption than ball mills and the separator, as in raw mills, is integral with the mill body.
In the Loesche mill, Fig. 2.23,16 two pairs of rollers are used. In each pair the first, smaller diameter, roller stabilises the bed prior to grinding which takes place under the larger roller. Manufacturers use different technologies for bed stabilisation.
Comminution in ball mills and vertical mills differs fundamentally. In a ball mill, size reduction takes place by impact and attrition. In a vertical mill the bed of material is subject to such a high pressure that individual particles within the bed are fractured, even though the particles are very much smaller than the bed thickness.
Early issues with vertical mills, such as narrower PSD and modified cement hydration characteristics compared with ball mills, have been resolved. One modification has been to install a hot gas generator so the gas temperature is high enough to partially dehydrate the gypsum.
For many decades the two-compartment ball mill in closed circuit with a high-efficiency separator has been the mill of choice. In the last decade vertical mills have taken an increasing share of the cement milling market, not least because the specific power consumption of vertical mills is about 30% less than that of ball mills and for finely ground cement less still. The vertical mill has a proven track record in grinding blastfurnace slag, where it has the additional advantage of being a much more effective drier of wet feedstock than a ball mill.
The vertical mill is more complex but its installation is more compact. The relative installed capital costs tend to be site specific. Historically the installed cost has tended to be slightly higher for the vertical mill.
Special graph paper is used with lglg(1/R(x)) on the abscissa and lg(x) on the ordinate axes. The higher the value of n, the narrower the particle size distribution. The position parameter is the particle size with the highest mass density distribution, the peak of the mass density distribution curve.
Vertical mills tend to produce cement with a higher value of n. Values of n normally lie between 0.8 and 1.2, dependent particularly on cement fineness. The position parameter is, of course, lower for more finely ground cements.
Separator efficiency is defined as specific power consumption reduction of the mill open-to-closed-circuit with the actual separator, compared with specific power consumption reduction of the mill open-to-closed-circuit with an ideal separator.
As shown in Fig. 2.24, circulating factor is defined as mill mass flow, that is, fresh feed plus separator returns. The maximum power reduction arising from use of an ideal separator increases non-linearly with circulation factor and is dependent on Rf, normally based on residues in the interval 3245m. The value of the comminution index, W, is also a function of Rf. The finer the cement, the lower Rf and the greater the maximum power reduction. At C = 2 most of maximum power reduction is achieved, but beyond C = 3 there is very little further reduction.
Separator particle separation performance is assessed using the Tromp curve, a graph of percentage separator feed to rejects against particle size range. An example is shown in Fig. 2.25. Data required is the PSD of separator feed material and of rejects and finished product streams. The bypass and slope provide a measure of separator performance.
The particle size is plotted on a logarithmic scale on the ordinate axis. The percentage is plotted on the abscissa either on a linear (as shown here) or on a Gaussian scale. The advantage of using the Gaussian scale is that the two parts of the graph can be approximated by two straight lines.
The measurement of PSD of a sample of cement is carried out using laser-based methodologies. It requires a skilled operator to achieve consistent results. Agglomeration will vary dependent on whether grinding aid is used. Different laser analysis methods may not give the same results, so for comparative purposes the same method must be used.
The ball mill is a cylindrical drum (or cylindrical conical) turning around its horizontal axis. It is partially filled with grinding bodies: cast iron or steel balls, or even flint (silica) or porcelain bearings. Spaces between balls or bearings are occupied by the load to be milled.
Following drum rotation, balls or bearings rise by rolling along the cylindrical wall and descending again in a cascade or cataract from a certain height. The output is then milled between two grinding bodies.
Ball mills could operate dry or even process a water suspension (almost always for ores). Dry, it is fed through a chute or a screw through the units opening. In a wet path, a system of scoops that turn with the mill is used and it plunges into a stationary tank.
Mechanochemical synthesis involves high-energy milling techniques and is generally carried out under controlled atmospheres. Nanocomposite powders of oxide, nonoxide, and mixed oxide/nonoxide materials can be prepared using this method. The major drawbacks of this synthesis method are: (1) discrete nanoparticles in the finest size range cannot be prepared; and (2) contamination of the product by the milling media.
More or less any ceramic composite powder can be synthesized by mechanical mixing of the constituent phases. The main factors that determine the properties of the resultant nanocomposite products are the type of raw materials, purity, the particle size, size distribution, and degree of agglomeration. Maintaining purity of the powders is essential for avoiding the formation of a secondary phase during sintering. Wet ball or attrition milling techniques can be used for the synthesis of homogeneous powder mixture. Al2O3/SiC composites are widely prepared by this conventional powder mixing route by using ball milling . However, the disadvantage in the milling step is that it may induce certain pollution derived from the milling media.
In this mechanical method of production of nanomaterials, which works on the principle of impact, the size reduction is achieved through the impact caused when the balls drop from the top of the chamber containing the source material.
A ball mill consists of a hollow cylindrical chamber (Fig. 6.2) which rotates about a horizontal axis, and the chamber is partially filled with small balls made of steel, tungsten carbide, zirconia, agate, alumina, or silicon nitride having diameter generally 10mm. The inner surface area of the chamber is lined with an abrasion-resistant material like manganese, steel, or rubber. The magnet, placed outside the chamber, provides the pulling force to the grinding material, and by changing the magnetic force, the milling energy can be varied as desired. The ball milling process is carried out for approximately 100150h to obtain uniform-sized fine powder. In high-energy ball milling, vacuum or a specific gaseous atmosphere is maintained inside the chamber. High-energy mills are classified into attrition ball mills, planetary ball mills, vibrating ball mills, and low-energy tumbling mills. In high-energy ball milling, formation of ceramic nano-reinforcement by in situ reaction is possible.
It is an inexpensive and easy process which enables industrial scale productivity. As grinding is done in a closed chamber, dust, or contamination from the surroundings is avoided. This technique can be used to prepare dry as well as wet nanopowders. Composition of the grinding material can be varied as desired. Even though this method has several advantages, there are some disadvantages. The major disadvantage is that the shape of the produced nanoparticles is not regular. Moreover, energy consumption is relatively high, which reduces the production efficiency. This technique is suitable for the fabrication of several nanocomposites, which include Co- and Cu-based nanomaterials, Ni-NiO nanocomposites, and nanocomposites of Ti,C .
Planetary ball mill was used to synthesize iron nanoparticles. The synthesized nanoparticles were subjected to the characterization studies by X-ray diffraction (XRD), and scanning electron microscopy (SEM) techniques using a SIEMENS-D5000 diffractometer and Hitachi S-4800. For the synthesis of iron nanoparticles, commercial iron powder having particles size of 10m was used. The iron powder was subjected to planetary ball milling for various period of time. The optimum time period for the synthesis of nanoparticles was observed to be 10h because after that time period, chances of contamination inclined and the particles size became almost constant so the powder was ball milled for 10h to synthesize nanoparticles . Fig. 12 shows the SEM image of the iron nanoparticles.
The vibratory ball mill is another kind of high-energy ball mill that is used mainly for preparing amorphous alloys. The vials capacities in the vibratory mills are smaller (about 10 ml in volume) compared to the previous types of mills. In this mill, the charge of the powder and milling tools are agitated in three perpendicular directions (Fig. 1.6) at very high speed, as high as 1200 rpm.
Another type of the vibratory ball mill, which is used at the van der Waals-Zeeman Laboratory, consists of a stainless steel vial with a hardened steel bottom, and a single hardened steel ball of 6 cm in diameter (Fig. 1.7).
The mill is evacuated during milling to a pressure of 106 Torr, in order to avoid reactions with a gas atmosphere. Subsequently, this mill is suitable for mechanical alloying of some special systems that are highly reactive with the surrounding atmosphere, such as rare earth elements.
In spite of the traditional approaches used for gas-solid reaction at relatively high temperature, Calka etal. and El-Eskandarany etal. proposed a solid-state approach, the so-called reactive ball milling (RBM), used for preparations different families of meal nitrides and hydrides at ambient temperature. This mechanically induced gas-solid reaction can be successfully achieved, using either high- or low-energy ball-milling methods, as shown in Fig.9.5. However, high-energy ball mill is an efficient process for synthesizing nanocrystalline MgH2 powders using RBM technique, it may be difficult to scale up for matching the mass production required by industrial sector. Therefore, from a practical point of view, high-capacity low-energy milling, which can be easily scaled-up to produce large amount of MgH2 fine powders, may be more suitable for industrial mass production.
In both approaches but with different scale of time and milling efficiency, the starting Mg metal powders milled under hydrogen gas atmosphere are practicing to dramatic lattice imperfections such as twinning and dislocations. These defects are caused by plastics deformation coupled with shear and impact forces generated by the ball-milling media. The powders are, therefore, disintegrated into smaller particles with large surface area, where very clean or fresh oxygen-free active surfaces of the powders are created. Moreover, these defects, which are intensively located at the grain boundaries, lead to separate micro-scaled Mg grains into finer grains capable to getter hydrogen by the first atomically clean surfaces to form MgH2 nanopowders.
Fig.9.5 illustrates common lab scale procedure for preparing MgH2 powders, starting from pure Mg powders, using RBM via (1) high-energy and (2) low-energy ball milling. The starting material can be Mg-rods, in which they are processed via sever plastic deformation, using for example cold-rolling approach, as illustrated in Fig.9.5. The heavily deformed Mg-rods obtained after certain cold rolling passes can be snipped into small chips and then ball-milled under hydrogen gas to produce MgH2 powders.
Planetary ball mills are the most popular mills used in scientific research for synthesizing MgH2 nanopowders. In this type of mill, the ball-milling media have considerably high energy, because milling stock and balls come off the inner wall of the vial and the effective centrifugal force reaches up to 20 times gravitational acceleration. The centrifugal forces caused by the rotation of the supporting disc and autonomous turning of the vial act on the milling charge (balls and powders). Since the turning directions of the supporting disc and the vial are opposite, the centrifugal forces alternately are synchronized and opposite. Therefore, the milling media and the charged powders alternatively roll on the inner wall of the vial, and are lifted and thrown off across the bowl at high speed.
In the typical experimental procedure, a certain amount of the Mg (usually in the range between 3 and 10g based on the vials volume) is balanced inside an inert gas atmosphere (argon or helium) in a glove box and sealed together with certain number of balls (e.g., 2050 hardened steel balls) into a hardened steel vial (Fig.9.5A and B), using, for example, a gas-temperature-monitoring system (GST). With the GST system, it becomes possible to monitor the progress of the gas-solid reaction taking place during the RBM process, as shown in Fig.9.5C and D. The temperature and pressure changes in the system during milling can be also used to realize the completion of the reaction and the expected end product during the different stages of milling (Fig.9.5D). The ball-to-powder weight ratio is usually selected to be in the range between 10:1 and 50:1. The vial is then evacuated to the level of 103bar before introducing H2 gas to fill the vial with a pressure of 550bar (Fig.9.5B). The milling process is started by mounting the vial on a high-energy ball mill operated at ambient temperature (Fig.9.5C).
Tumbling mill is cylindrical shell (Fig.9.6AC) that rotates about a horizontal axis (Fig.9.6D). Hydrogen gas is pressurized into the vial (Fig.9.6C) together with Mg powders and ball-milling media, using ball-to-powder weight ratio in the range between 30:1 and 100:1. Mg powder particles meet the abrasive and impacting force (Fig.9.6E), which reduce the particle size and create fresh-powder surfaces (Fig.9.6F) ready to react with hydrogen milling atmosphere.
Figure 9.6. Photographs taken from KISR-EBRC/NAM Lab, Kuwait, show (A) the vial and milling media (balls) and (B) the setup performed to charge the vial with 50bar of hydrogen gas. The photograph in (C) presents the complete setup of GST (supplied by Evico-magnetic, Germany) system prior to start the RBM experiment for preparing of MgH2 powders, using Planetary Ball Mill P400 (provided by Retsch, Germany). GST system allows us to monitor the progress of RBM process, as indexed by temperature and pressure versus milling time (D).
The useful kinetic energy in tumbling mill can be applied to the Mg powder particles (Fig.9.7E) by the following means: (1) collision between the balls and the powders; (2) pressure loading of powders pinned between milling media or between the milling media and the liner; (3) impact of the falling milling media; (4) shear and abrasion caused by dragging of particles between moving milling media; and (5) shock-wave transmitted through crop load by falling milling media. One advantage of this type of mill is that large amount of the powders (100500g or more based on the mill capacity) can be fabricated for each milling run. Thus, it is suitable for pilot and/or industrial scale of MgH2 production. In addition, low-energy ball mill produces homogeneous and uniform powders when compared with the high-energy ball mill. Furthermore, such tumbling mills are cheaper than high-energy mills and operated simply with low-maintenance requirements. However, this kind of low-energy mill requires long-term milling time (more than 300h) to complete the gas-solid reaction and to obtain nanocrystalline MgH2 powders.
Figure 9.7. Photos taken from KISR-EBRC/NAM Lab, Kuwait, display setup of a lab-scale roller mill (1000m in volume) showing (A) the milling tools including the balls (milling media and vial), (B) charging Mg powders in the vial inside inert gas atmosphere glove box, (C) evacuation setup and pressurizing hydrogen gas in the vial, and (D) ball milling processed, using a roller mill. Schematic presentations show the ball positions and movement inside the vial of a tumbler mall mill at a dynamic mode is shown in (E), where a typical ball-powder-ball collusion for a low energy tumbling ball mill is presented in (F).