The Steel Head Rod Mill(sometimes call a bar mill)gives the ore dressing engineer a very wide choice in grinding design. He can easily secure a standard Steel Head Rod Mill suited to his particular problem. The successful operation of any grinding unit is largely dependent on the method of removing the ground pulp. The Steel Head Rod Mill is available with five types of discharge trunnions and each type trunnion is available in small, medium, or large diameter. The types of Rod Mill discharge trunnions are:
The superiority of the Steel Head Rod Mill is due to the all-steel construction. The trunnions are an integral part of the cast steel heads and are machined with the axis of the mill. The mill heads are insured against breakage due to the high tensile strength of cast steel as compared to that of the cast iron head found on the ordinary rod mill. Trunnion Bearings are made of high-grade nickel babbitt, dovetailed into the casting. Ball and socket bearings can be furnished if desired.
Head and shell liners for Steel Head Rod Mills are available in Decolloy (a chrome-nickel alloy), hard iron, electric steel, molychrome steel, and manganese steel. The heads have a conical shaped head liner construction, both on the feed and discharge ends, so that there is ample room for the feed from the trunnion helical conveyor discharge to enter the mill betweenthe rods and head liners on the feed end of the mill. Drive gears are furnished either in cast tooth spur gear and pinion or cut tooth spur gear and pinion. The gears are furnished as standard on the discharge end of the mill, out of the way of the classifier return feed, but can be furnished at the mill feed end by request. Drives may be obtained according to the customers specifications.
The following table clearly illustrates why Steel Head Rod Mills have greater capacity than other mills. This is due to the fact that the diameters are measured inside the liners, while other mills measure their diameter inside the shell.
Rod Mills may be considered either fine crushers or coarse grinding equipment. They are capable of taking as large as 2 feed and making a product as fine as 35-48 mesh. Of particular advantage is their adaptability to handling wet sticky ores, which normally would cause difficulty in crushing operations. Under wet grinding conditions of course the problem of dust is eliminated.
The grinding action of a rod mill is line contact. As material travels from the feed end to the discharge end it is subjected to crushing forces inflicted by the grinding rods. The rods both tumble in essentially a parallel alignment and also spin, thus simulating the crushing and grinding action obtained from a series of roll crushers. The large feed tends to spread the rods at the feed end which imparts still an additional action which may be termed scissoring. As a result of this spreading the rods tend to work on the larger particles and thereby produce a minimum amount of extremely fine material.
The Rod Mill encourages the use of a thick pulp coating both the liners and the rods, thus minimizing steel consumption. Continuous movement of the pulp through the rod mass eliminates the possibility of short circuiting any material. The discharge end of the Rod Mill is virtually open and larger in diameter than the feed end, providing a steep gradient of material flow through the mill. This is described in more detail on pages 20 and 21.
Normally Rod Mills are furnished of the two trunnion design. For special applications they may be furnished of the tire trunnion or two- tire construction. These mills can be equipped with any type of feeder and type of drive, discussed separately in this catalog.
The above tables list some of the most common Open End Rod Mill sizes. Capacities are based on medium hard ore with mill operating in closed circuit under wet grinding conditions at speeds indicated. For dry grinding, speeds and power are reduced and capacities drop 30 to 50%.
The End Peripheral Discharge Rod Mill is designed to produce a minimum amount of fines when grinding either wet or dry. Material to be ground enters through a standard trunnion and is discharged through port openings equally spaced around the mill periphery. These ports are in a separate ring placed between the shell and the discharge head.
The construction of the end peripheral discharge mill emphasizes the principle of grinding. Due to the steep gradient between the point of entry and the point of discharge the pulp flows rapidly through the mill providing a fast change of mill content with a relatively small amount of pulp within the grinding chamber.
The sloping or conical shaped feed head proves ample space for a feed pocket to accommodate large quantities of material and assure their entrance into the grinding rods. Any type of feeder listed on pages 22 and 23 can be furnished for these mills; however, since the mills are not usually operated in closed circuit grinding, the drum or spout feeder is normally preferred.
No other type of mill is so well adapted to dry grinding materials to -4 or -8 mesh in single pass with the production of a minimum amount of fines. A major factor in dry grinding is the rapid removal of finished material to prevent cushioning of the rods. This is accomplished in the End Peripheral Discharge Rod Mill.
The free discharge feature permits the grinding of material having a higher moisture content than with other types of rod or ball mills. Our Peripheral Discharge Mills have found wide application in grinding coke and friable non-metallics, material for glass, pyroborates, as well as gravel to produce sand. Another application is for grinding and mixing sand lime brick materials. The rod action gives a thorough mixture while grinding of the hydrated lime and sand.
For specifications of End Peripheral Discharge Rod Mills use table of standard open end rod mills given on pages 24 and 25. The capacity of the end peripheral discharge rod mill is slightly higher than shown for the Open End Rod Mills.
The CPD (Center Peripheral Discharge) Rod Mill has been developed to produce sand to meet U. S. Government or State specifications. It has also found application in grinding friable non-metallics, and industrial materials and ores which tend to slime excessively. Another application is in the field of abrasion milling on ores such as found on the Mesabi Iron Range. In this latter application true grinding is not desired, but more of a surface scrubbing of the individual particles.
Again with this construction grinding may be done either wet or dry. In this design, however, feed enters both ends by means of feeders and is discharged at the center through rectangular discharge ports equally spaced around the mill periphery. The center discharge openings are generally contained in a separate ring placed between shell halves. The ground material is discharged and directed to either side or directly under the mill by the use of a discharge ring housing.
In standard rod-milling it will be found that rods spread apart at the feed end in the amount of the maximum size of feed entering the mill. In the center peripheral discharge mill the rods are spread at both ends and parallel throughout the length of the mill. This feature results in more space between the rods and thereby lessens the amount of fines produced. Furthermore, fines are also diminished because the material moves rapidly through the mill due to the steep gradient of travel and the distance of travel is reduced by half. Similarly time of contact with the grinding media is reduced by half.
Another center peripheral discharge advantage is that a cubical shaped particle is produced. Maintenance is negligible and grinding media is relatively inexpensive. Other types of sand manufacturing equipment lose efficiency with wear and require excessive maintenance. This loss of efficiency increases rapidly as hardness of feed increases. The Center Peripheral Discharge Rod Mill can be easily maintained at peak operating efficiency by the periodical addition of rods. CPD Rod Mills give a wide range of flexibility to sand plant operation. By changing the rate of feed, pulp dilution (wet grinding), and discharge port area it is possible to produce and blend sand of virtually any fineness modulus and maintain it within Government specifications.
Unlike many crushers or grinders the CPD Mill can easily handle wet or sticky material. When grinding wet, the dust nuisance is completely eliminated. For dry grinding applications the mill is furnished with a dust proof discharge housing.
Various items must be considered in computing the cost of producing manufactured sand. These include wear on the constituent parts, power consumption, lubrication, labor and general maintenance. Maintenance of the center peripheral discharge mill is definitely much lower than that of any other sand manufacturing machine. The greater portion of the wear which takes place is on the inexpensive high carbon steel rods. Field installations show an average of less than 1 # per ton of sand ground as rod consumption, and from 0.08# to 0.10# per ton of sand ground as the steel liner wear. The overall cost of mill operation, exclusive of amortization, is generally less than 30c per ton (year 1958).
Every possible operating convenience has been incorporated in the center peripheral discharge mill design. On most sizes the trunnions are carried in large lead bronze bushed bearings. The interior of the mill is readily accessible through these large trunnion openings. The peripheral ring housing is furnished with a door for inspection and another lower door to facilitate sampling of the mill discharge. Covers for the discharge ports are furnished allowing any variation in discharge area which might be desired.
Given below are approximate capacities for several sizes of the center peripheral discharge mills. Such capacities are expressed in dry tons per hour, based on - x 4 mesh screened feed of medium hard gravel. Mill discharge is generally less than 5% + 4 mesh in wet open circuit operations, for dry grinding work reduce the capacities indicated by approximately 30% to 50%.
A Rod Mill has for Working Principle its inside filledgrinding media, in this case STEEL RODS. These rods run the length of the machine, which is most commonly between eight and sixteen feet in length. The diameter of these rods will range from, when new, between two and four inches. The rods arefree inside the mill. When the mill is turned, the rods tumble against one another grinding all the ore that is between them to aid in the grinding, water is added with the ore as it enters the mill.So from that you can see why it is called a wet tumbling mill. The ore is ground wet and the mill revolves. This causes the grinding media inside of it to tumble grinding the ore.
Historically there has been three basic ways of grinding ore, hammer mills, rolls, or wet tumbling mills. Hammer mills and rolls are not used that often and then usually only for special applications as in lab work or chemical preparation.
The type of mill that is used for grinding ore in a modern concentrator is the wet tumbling mill. These mills may be divided into three types ROD MILLS, BALL MILLS andAUTOGENOUS MILLS. In the first type, the ROD MILL, the ore is introduced into the mill.
From the trunnion liner out wards first we will come to the FACE PLATE. It is slightly concave to create the POOLING AREA for the rock to collect in before entry to the ROD-LOAD. On the outside attached to the face plate is the BULL GEAR. This gear completely circles the mill and provides the interface between the motor and the mill. The bull gear and drive line may be at the other end of the mill instead. There are advantages and disadvantages to either end this will be explained later when we are discussing the motor and drive line. But for now back to the face plate, attached to the other side of the face plate is the SHELL. The shell is the body of the mill. On the inside of the mill there are two layers of material, the first layer is the BACKING for the liners. This is customarily constructed from rubber but wood may be used as well. The purpose of this backing is two-fold, one to absorb the shock that is transmitted through the liners from normal running. And to provide the shell with a protective covering to eliminate the abrasion that is produced by the finely ground rock and water. Without this rubber or wood backing, the life of the mill is drastically reduced due to metal fatigue and simply being worn away.For those of you arent familiar with METAL FATIGUE I will explain. When metal is continually pounded or vibrated, the molecular structure of the metal begins to change, it is said to CRYSTALLIZE, and the metal becomes hard and finally loses all ability to give with the vibration. Thousands of microscopic cracks will begin to appear, as the fatigue of the metal continues, these cracks will grow to become major problems.
Later for interest sake we will explain the difference in some of them, but for now lets stay with identifying the parts of the mill. We have already mentioned the trunnion liner so let start from there.
The trunnion liner may also be referred to as the THROAT LINER. You will find that many of these parts will be called two or even sometimes three names, All I can say is try not to let it confuse you, The name isnt as important as the job that it does. As long as everybody that you work with agree on which name to use, it doesnt matter that much.
Next to this liner is the END LINERS, or to some, the PACE PLATE LINERS.The FILLER RING which is next is not standard in all mills, some mills have them, and some dont. Their job is to fill the corner of the mill up so the shell will not wear at that point. They dont provide any lift to the media, in fact quite often the media will not come into contact with them at all, but what they do is make changing liners that much easier. With different liner designs the replacement of a single liner may be quite difficult and to change one could become a lengthy project.
The liner that butts into the filler liner is known as a BELLY LINER or SHELL LINER, and in some designs LIFTER BARS. These liners and/or lifters give the media its CASCADING action and also receive the most wear. They cover the complete body of the mill and have the largest selection of types to choose from.
As the two ends of the mill are the same there isnt any reason to go over the other face plate. The discharge trunnion assembly is very much like the feed trunnion except that, it wont have a worm as part of the liner. Instead of a feed seal bolted to it, it may have a screen.
This is called a TRUMMEL SCREEN and its purpose is to screen out any rock that didnt get ground as well as any TRAMP METAL or REJECT STEEL that may be coming out of the mill. Reject steel is the old grinding media that has been worn so small that it comes out of the mill. If this tramp metal and steel is allowed to get into pumps and classifiers damage and plug- ups may be caused.
With regards to Rod Mills, let us start by identifying the different portions of the rod load as it goes through one revolution, as you will see, each of these areas will hold interest for the Grinding operator.
As the rod mill turns, the rods are carried by the lifting portion of the liners. The height that they are lifted is referred to as the lift of the liners. As they roll off of the liners, the rods enter the cascade zone. The rods roll through the cascade zone until they come to the toe of the load. At this point the rods come to rest in relation to the shell of the mill. The liners lift the rods back to begin the cascade again. You will notice, that as you go deeper into the rod load, the rod movement becomes less and less until the movement is very slight at the deepest part. This area is called the core of the load. As a description of the normal grinding action, the rods and the ore react together like this. The ore enters-the mill and is deposited in the pooling area directly under the feed trunnion.
This pooling area allows the large rock to fall towards the outside portion of the load, the TOE area. This is the zone with the greatest movement in it, which means the area that will have the highest impact on the ore.
The rock will be carried up by the rods as they go through the CASCADE ZONE reducing the size of the rock. As each particle of ore becomes smaller it will work towards the CORE ZONE while travelling the length of the mill. That makes for a rather neat arrangement doesnt it. The larger rock is deposited in the area where the maximum impact from the rod load occurs and then as each particle gets smaller it slowly travels inwards towards the centre of the load.
This is where the maximum surface contact takes place, producing the finer grind. When the ore has travelled from one end of the mill to the other end it will have completed its grinding cycle in this mill. As it exits the rod load it will be deposited in another POOLING AREA prior to leaving the mill by way of the DISCHARGE TRUNNION. Prom that you can see how a mill will become over loaded. If for some reason the rock begins to separate the rods over their entire length, the larger rock will prevent the intermediate rock from being ground. Which in turn will begin to invade the area that the fine material is being ground in. As the rods become separated through the entire load, the grind will get progressively worse until the unground rock is in the discharge pooling area. At this point, the operator will notice, that large rock is being discharged from the discharge trunnion.
During normal operations there is usually a certain amount of this larger rock that wont get ground. These are known as REJECTS and they serve as one of the tattle tales as to how the mill is grinding. If there is an increase of these rejects then the mill isnt grinding that well and the operator will have to do something about it. If he doesnt the mill load will continue to climb, until the rods in the lifting zone are completely separated. When this happens those rods will have quit grinding.
There is a visual warning of this happening that the operator can take advantage of. The lift on the rods will get higher and higher until they are being carried to the very top of the mill before cascading. I think falling would be a better word for it though. As this is happening, the core of the load will be slowly moving away from the shell towards the center of the mill. This is because the volume of the mill is being filled with unground rock. This will continue until the load hits a critical volume and a critical density. The rock still coming in to the mill will have to have some where to go so it tries pushing the rods out of the mill. Unfortunately they wont make it, the first hunch of rods that get far enough into the discharge trunnion will be- hit by the rest of the load bending and twisting them until they look like SPAGHETTI. This usually shuts the mill down for a couple of days while the millwrights cut the bent rods out of the mill.
On the other end of the scale, if the density is to light, the rod load will become too active, not having the solids in the mill to cushion the impact of rod on rod and rod on liner. As the rods enter the cascade zone, the pattern of the movement of the rods will be different. Instead of having a tightly tumbling mass of rods, the rods will be separated. The lift will be higher and the cascade will form more of an arc. The impact of the rods on the rock will be less because there will be more give in the rod load, with high amount of steel on steel causing the rods to bounce.
Letslook at how these Rod mills work, as I mentioned earlier there are steel rods inside the mill, it is their job to do the actual grinding. If you look at the mill in a cross section of an end view. You will get a very good illustration of the grinding action, of the mill.
The LINERS provide the tumbling action of the rods. When the mill rotates the rods are lifted until they roll off of the liners, this is known as CASCADING. The ore enters the mill at the feed end, as the rods cascade and tumble, the rock is caught between the rods and is ground. The size that the rock will be ground to is dependent on the amount of time the ore is in the mill, how many rods there are in the mill V and the size of the incoming ore.
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).Get in Touch with Mechanic