The ore from the crushing section is delivered to the fine ore bin placed at the head of the grinding section, from which it is fed, together with water, to one or more grinding units consisting of a ball mill or rod mill in circuit with a classifier. One such unit with a conical ball mill is shown in Fig. 9. A ball mill consists essentially of a conical or cylindrical shell supported on hollow trunnion bearings on which it revolves ; the construction of the various types is described later. The ore enters through one trunnion and leaves by the other. The mill is kept about half-full of balls and is driven at the speed at which their cascading and rolling action has the maximum crushing and grinding effect on the ore as it passes through under pressure of incoming feed ; the cascading action crushes the ore by impact and the rolling action grinds it by attrition. In a rod mill steel rods replace the balls; the cascading action is less, and the rolling action is responsible for most of the work done.
The pulp is discharged into a classifier where it is further diluted with water. The classifier is adjusted so that the ore which has been crushed fine enough overflows with the water at its lower end, while all coarser particles are transported back to the feed end of the mill. Thus a circulating load is maintained round the circuit and any particles of ore that are too coarse to overflow pass through the mill again ; they continue to go round the circuit in this way until they have been crushed fine enough to be discharged. This circulating load may amount to five or six times the weight of new feed entering the circuit. However carefully the classifier is regulated, many particles are crushed so small that when they finally escape they are very much finer than is required. In practice it is always found that the discharge contains all sizes of particles from the maximum allowable size, as determined by the setting of the classifier, down to the finest slime. For example, if a classifier were set to give a discharge of 5% plus 48 mesh material, the proportion finer than 200 mesh would probably amount to 45% ; this would be termed “ 48 mesh grinding ”. The amount of minus 200 mesh material produced at a given “ mesh of grinding ” varies according to the physical condition of the ore, but it is usually between the following limits:
Fine grinding to 100 mesh or more is best done in two stages, and the present tendency is to extend this practice to grinding as coarse as 48 mesh. The original method was to pass the ore straight through the primary mill without classification (open circuit) to a secondary circuit similar to the arrangement of Fig. 9 (closed circuit). Greater efficiency in grinding is now obtained by adding a classifier to the first stage so as to convert it into a closed circuit. The overflow of the primary classifier passes to the second stage and the overflow of the secondary classifier is the finished product of the grinding section.
Ball Mill Grinding P80 Size
The mesh of grinding usually lies between 48 and 200 mesh. On account of their size in relation to that of the bubbles, particles larger than 48 mesh generally have too great a tendency to drop out of the froth and escape in the tailing, while the lower limit is determined by the fact that grinding finer than 200 mesh is usually too costly to be economical, rather than by any factors affecting flotation.
In theory the critical mesh of grinding should depend on the size of the smallest particle of mineral as it occurs in the ore. In order that the valuable minerals may be separated from the gangue in the flotation machines, they must have been more or less completely released from the rock-forming and other minerals with which they are associated, and for an ideal separation they would have to be entirely free, but in practice this is often undesirable and uneconomical on account of the fact that the operation of grinding is more costly than that of any other section of the flotation plant, for which reason the ore should not be crushed any finer than is necessary.
It has already been pointed out that a ball mill inevitably produces a considerable quantity of material that is too fine. In addition, the mineral is generally more friable than the rest of the ore, and is therefore ground proportionately finer than the gangue ; consequently the largest particles in the classifier overflow consist mainly of gangue with only a small proportion of mineral. For these reasons the mesh of grinding can generally be much coarser than the size of the smallest particle of mineral in the ore. If, however, the product is so coarse that the mineral is locked up in the gangue, it passes out with it in the tailing and the recovery suffers, but it must be remembered that it is often possible to float mineral which has not been completely liberated from the gangue and has only a portion of its surface exposed ; in this case the recovery of the mineral does not suffer, but the grade of the concentrate is reduced in proportion to the amount of gangue that enters it attached to the particles of mineral.
The correct mesh of grinding for any particular ore can be determined within certain limits by small-scale tests, but it can only be finally settled by experiment in the plant itself. It is always a compromise between the completeness of the liberation of the valuable minerals and the cost of grinding, and it depends to a certain extent on the physical characteristics of the ore as they affect grinding and of the minerals as they affect flotation.
Cylindrical Ball Mills
The only cylindrical ball mills in common use are those of the grate discharge type, the most important being the Marcy and the Allis-Chalmers mills.
Fig. 10 illustrates the construction of the Marcy Ball Mill. The cylindrical shell and the feed and discharge heads are made of cast semi-steel. The hollow trunnions at each end run in self-aligning bearings and are fitted on the inside with manganese steel liners to protect them from wear. The feed-end liner is made with a deep spiral screw on the inside to facilitate the entrance of the feed, but the liner at the discharge-end is smooth. A scoop feeder is bolted to the trunnion at the feed end. As most of the wear occurs at the end of the scoop when it digs into the material in the feed-box at every revolution, this point is provided with a renewable lip of manganese steel.
The shell liners are of the rectangular overlapping type and are made of manganese steel. They vary from 3 to 5 in. in thickness at the deepest part of the section according to the size of mill for which they are intended. The feed-end liners are also made of manganese steel and vary in thickness at the deepest part from 2 to 4½ in.
The discharge head and trunnion are shut off from the interior of the mill by a grate assembly which is shown in Fig. 11. It consists of a number of grate sections held in place by clamp bars which fit over the junctions between the sections and are bolted to the head. Radial spacing and lifting ribs, cast in one piece with the head and arranged parallel to the clamp bars, keep the whole assembly several inches clear of the end of the mill. The space so formed is thus divided by the ribs into the same number of compartments as there are grates, each compartment being open at its apex to the discharge-end trunnion.
All the material that passes through a grate when it is in its lowest position collects in the compartment behind and is carried up by the revolution of the mill until it can flow out by gravity through the opening at the apex. This is shaped so as to deflect the pulp into the trunnion, through which it is discharged from the mill.
The bars composing the grates are made of high-carbon tempered tool steel with a certain proportion of chromium and are welded up into sections. They are tapered slightly towards the discharge side to prevent choking, and the spacing between them varies from 3/16 to 3/8 in. according to the work required of the mill. The direction of the slots is designed to present an almost continuous opening to the pulp as the mill revolves, and the arrangement provides an efficient and rapid method of discharge into the lifting compartments behind. The radial clamp bars assist the passage of the pulp through the grates to some extent by keeping them clear of balls and large pieces of ore, protecting them at the same time from excessive wear.
The mill is filled with balls to a level several inches below the axis. The ball load, speed, etc., are regulated so that under normal running conditions most of the ore is small enough to pass through the grates by the time that it reaches the discharge end. At the feed end the pulp enters at the level of the trunnion, but its level gradually falls along the length of the mill until at the discharge end it passes freely through the grates at a point only a few inches above the periphery of the shell. Thus the intensity of grinding is greatest at the feed end where it is needed, but diminishes towards the discharge end owing to the gradually increasing rate of travel of the particles of ore, a condition which tends to reduce overgrinding. With a similar object in view this type of mill is designed with its diameter greater than its length so as to achieve the maximum grinding effect in the minimum time.
The capacities, power requirements, etc., of the various sizes of Marcy Ball Mills are given in Table 11. It should be noted that the first figure of the number of a mill represents its diameter, and the second one its length.
The Allis-Chalmers Ball Granulator is made on the same principles as the Marcy Ball Mill, but differs considerably in detail. One of the main points of difference is in the design of the grate assembly. A diaphragm, perforated with round holes, shuts the discharge compartments off from the interior of the mill. The grates are mounted on the front of the diaphragm, the openings between the bars being arranged radially instead of circumferentially as in the Marcy Mill. Any material that gets through the grates has also to pass through the holes of the diaphragm, the purpose of which is to enable the pulp level to be raised if necessary. This adjustment is effected by plugging up one or more of the outermost rows of holes, and the level can in this way be brought up to a point halfway between the periphery and the axis of the mill. For grinding to 65 mesh or coarser the lowest possible discharge is generally found to give the best results, the ability to raise the pulp level being a doubtful advantage ; the adjustment is useful, however, for finer grinding. While it is possible in both types of machine to bring up the level by increasing the amount of water added to the pulp, this does not always give satisfactory results. The Allis-Chalmers design has the advantage that the level can be altered without interfering with the W/S ratio of the pulp. Its grate area, however, is not as large as that of a Marcy Mill of the same diameter and the radial arrangement of the bars does not give such a free discharge as is obtained when they are in a circumferential direction. Thus the Marcy Mill has the advantage in cases where the lowest possible discharge level is required.
Conical Ball Mills
The only conical ball mills on the market are those made by the Metso. Fig. 12 shows the construction of this type of mill. It consists essentially of a short cylindrical section with conical feed and discharge ends. The shell is made up of steel plates riveted or welded together. The hollow feed and discharge trunnions are riveted or welded to the conical ends and run in bearings of ample size ; they are protected from wear on the inside by cast-iron liners. The feed trunnion carries a scoop feeder of the type shown in Fig. 16. The shell is lined with specially shaped sections of chrome or manganese steel, each of which is held in position by two bolts ; these are illustrated in Fig. 13. A cheaper lining, shown in Fig. 14, consists of cast-iron plates clamped in place by wedge bars, the bars being secured to the shell by two bolts each. The bars are of chrome steel and project above the plates ; the thickness of each is such that they are both worn out at about the same time.
The cone at the feed end of the mill is steep in order that the incoming material may pass as rapidly as possible into the cylindrical section which constitutes the zone where the grinding action is most intense. The length of the cylindrical part is only about half its diameter, the proportions being designed to keep overgrinding at a minimum while still reducing as much of the ore as possible to a size suitable for the “ finishing ” stage in the conical section at the discharge end. The slope of the cone at this end is not as steep as at the feed end and a decided classification of the material takes place inside it. It is most noticeable in the case of the balls, the smallest sizes of which segregate at the smallest diameter of the cone and vice versa. The segregation of the ore is similar but is not so marked ; this is chiefly due to the irregularity in the shape of the pieces and to the continual surging motion inside the mill, which interferes with the proper classification of the comparatively light particles of ore in the cone while scarcely affecting the much heavier balls. The general tendency is for the largest pieces of ore to remain with the largest balls at the bottom of the cone where the intensity of grinding is greatest, and for the finest particles to segregate at the smallest diameter with the smallest balls where the intensity of grinding is least.
Overgrinding is therefore reduced to a minimum and the smallest particles rise in preference to the larger sizes to the apex of the cone where they are discharged. In order to get the most economical consumption of power, the ball load in a Hardinge Mill should be kept up to the level of the trunnions.
The capacities, power requirements, etc., of the various sizes of Hardinge Ball Mills are given in Table 12. It should be noted that the two figures describing the size of a mill represent the diameter of the cylindrical section in feet by its length in inches.
A rod mill is a long cylindrical mill without grates using rods as a grinding medium. Its length is usually twice its diameter, the most popular size being 6 ft. in diameter by 12 ft. in length; the largest made up to the present is 9 ft by 12 ft.
The first two are fitted with trunnions at both ends, the opening at the discharge end being rather larger than at the feed end. The Marcy Mill, however, has a trunnion at the feed end only ; round the shell at the other end is bolted a tyre which runs on two supporting rollers. This construction makes it possible to provide a very large opening in the discharge head, which enables a correspondingly low pulp discharge level to be maintained and so helps to reduce overgrinding in much the same way as in a ball mill with discharge grates ; it also gives very convenient access to the interior. The discharge opening is provided with a splash door which swings on an independently supported hinge ; in the closed position it is held a few inches clear of the discharge head, leaving an annular opening through which the pulp can pass without undue splash. The Hardinge Mill has conical ends.
The mills are charged with high-carbon steel rods cut a few inches shorter than the interior length of the mill. As a rule 3-in. diameter rods are employed for primary and 2-in. for secondary grinding. The shell liners are usually of chrome steel of the ordinary rectangular pattern.
A load of rods has more of a rolling and less of a cascading motion than a load of balls, and the shattering effect of the rods on the ore is consequently not as great as that of balls. While it is possible, if not advisable, to allow pieces of 2″ or even 3″ size in a ball mill, it is essential that the feed of a rod mill should be no larger than 1-in. and preferably smaller. For the same reason the ore must be friable ; rods cannot compete with balls in grinding hard or tough material.
Since the surface area of a load of rods is much less than that of a load of balls occupying the same space, the ore is not ground so fine in a rod mill as it would be in a ball mill of the same size. Moreover, a given feed of ore passes more quickly through a rod mill, since the percentage of voids in a rod load is less than in a ball load. A rod mill generally works best with a thick pulp, which should be viscous enough to stick to and coat the rods to some extent. The larger pieces of ore keep the rods apart so that the grinding action is concentrated on the larger sizes and the smaller particles are not overground to the same extent as in a ball mill.
For these reasons the shell is made about 50% longer than that of a ball mill of the same diameter and still gives less undersize when grinding the same tonnage to the same mesh. In fact, the characteristic of the rod mill is the unusually granular nature of its product but, because the larger pieces keep the rods apart, a rod mill will not grind efficiently below 65 mesh and it works best at a comparatively small reduction ratio. Its most efficient range is usually from about ½ in. to 20 mesh, and, when grinding finer, it is common practice to employ two stages, each in closed circuit with a classifier.
Although rod mill installations are thus restricted in normal practice to the coarse crushing of soft and friable ores, it is sometimes found that the most economical method of grinding an ore of not more than average hardness is to use a rod mill in the primary and a ball mill in the secondary stage. Such a circuit is not uncommon in large plants, but it is seldom adopted in small installations, for which it is generally found preferable to provide each stage with the same type of mill in order to facilitate maintenance and repairs.
On the class of work for which it is especially suitable the power consumption of a rod mill is about three-quarters that of a ball mill with the production of less undersize. The rod and liner consumption is also about 75% of that of a ball mill, and rods cost less than balls because they are more easily manufactured. Outside of this special sphere of work a rod mill cannot compete with a ball mill.
Ball Mill and Rod Mills Drives
In all cases, except in a few laboratory sizes, ball and rod mills are driven by a pinion which is mounted on a counter-shaft and engages with a gear-wheel bolted to the shell. For small machines the simplest way of driving the countershaft is by belt and pulley to a motor, Tex-ropes being preferable to a flat belt because the distance between pulley centres can be much shorter, making a more compact layout, and there is less danger of accident in the event of breakage. Ordinary spur gearing is used for the gear-wheel and pinion.
A belt drive is unsuitable for large mills and present practice is to connect the counter-shaft directly to a slow-speed motor through a flexible coupling, using double helical gearing for the gear-wheel and pinion. This method is illustrated in Fig. 15. Fig. 16 shows a single helical drive with thrust bearing, which is a recent development of the direct drive and is somewhat simpler and easier to maintain in adjustment than the double helical gear. The slow-speed motor and helical gearing, however, make the direct drive rather expensive ; a cheaper way is to use ordinary spur gearing and to drive the counter-shaft by a high-speed motor through a train of gears and a flexible coupling. This method is preferable to a belt-drive, but a direct drive gives less trouble on the whole than any other.
Grinding Media & Ball Mill Liners
Balls are made of cast-iron, high-carbon, and chrome steel. Cast-iron balls are only used as a rule when they can be cheaply produced locally with no charge for freight. Ordinarily high- carbon or chrome forged steel balls give a lower cost per ton of ore ground on account of their better wearing qualities, although they are more expensive in first cost.
Table 13 is a rough guide to the wear that may be expected from the three classes of balls. Their average relative grinding ball consumption is approximately as follows:
Chrome: high-carbon: cast-iron = 100 : 140 : 170.
Their average factory cost is:
Chrome: high-carbon: cast-iron = 100 : 95 : 75.
If, therefore, the freight charge is not taken into consideration, chrome steel balls show a lower cost per ton of ore ground than those of high-carbon steel, and the latter show a lower cost than those of cast- iron. The freight charge per pound of metal is the same for all classes of balls, but the freight cost per ton of ore ground is less for chrome steel than for high-carbon steel because it is spread over a larger tonnage of ore ; similarly, the cost per ton of ore for high-carbon steel is less than for cast-iron. It follows that, when freight charges are high, it is cheapest to use the steel that wears longest, even though it is the most expensive in first cost, but if balls can be made or bought locally at a reasonably cheap rate, the cost of freight is eliminated, and it may pay to use them in preference to a more expensive class of ball that has to be imported. This is the reason in most cases for the employment of cast-iron balls, which will generally be found to come from a local foundry. They are not satisfactory, however, when their diameter exceeds 2½ in., since they break up too quickly and give rise to an excessive consumption of iron.
The above considerations apply also to the linings of ball and rod mills. Liners of cast-iron can be substituted for chrome or manganese steel, but, as a general rule, they only show a lower cost per ton of ore ground when they can be produced locally and when steel liners would have to bear a heavy freight charge.
For rough calculations the wear of liners is usually reckoned to be 25% of the ball consumption when both are made of the same material.
The size of the balls used for grinding depends on the size of the material entering the mill; they should be just large enough to break the biggest lump of ore likely to be encountered. Balls of 5-in. diameter are the largest in common use, but they are only suitable for coarse feeds of 1-in. size or over. Balls from 3 to 4 in. in diameter are ordinarily employed in mills when the feed consists of the discharge of a cone crusher, while 2 to 2½-in. balls are necessary for secondary grinding. They are not made in diameters smaller than 2 in. The first charge of balls is graded to give the minimum percentage of voids, but the largest size only is added subsequently to make up for wear, as the balls themselves wear down more or less automatically in such a way as to give the minimum volume of voids.
Cylindrical VS Conical Ball Mills
For coarse grinding the cylindrical grate mill will, as a rule, give a lower consumption of power per ton of ore than a conical mill. The reason for its better performance lies in the low pulp discharge level which reduces overgrinding at the point where it would do most harm—i.e., at the discharge end of the mill. Thus the least possible amount of power is wasted in grinding material that is already too fine and more power is therefore available to do useful work in grinding ore that is not fine enough.
For fine grinding, on the other hand, it is usually found that the conical mill is the more economical. A fine product demands the use of smaller balls than a coarse one, since the smaller particles of ore can be broken with a lighter blow, and, as has already been explained, in this type of mill the balls tend to grade themselves automatically in the conical discharge end according to size, the smallest balls lying in the smallest diameter. The mill feed, therefore, is reduced in the cylindrical section to a size suitable for the final stage, overgrinding being kept to a minimum by making the length of this section short compared with its diameter, and it then enters the finishing zone in the conical discharge end. Here, as the size of the balls progressively decreases, there is a corresponding increase in the area of grinding surface per unit of ball charge volume and so in the efficiency of grinding, and the particles, passing through the cone from smaller to smaller balls, receive successively lighter and lighter blows, with the result that a more uniformly ground product is made than is possible in a cylindrical grate mill, in which the composition of the ball charge at the discharge end is the same as at any other point.
The above explanation is largely conjecture, but it can be stated as a fact that an analysis of flotation mill construction during the last few years shows that the Marcy type of ball mill is preferred for coarse and the Hardinge type for fine grinding.
For medium grinding between 65 and 100 mesh some difference of opinion exists as to which type of mill is the better. Other conditions being equal, the Hardinge Mill possesses the advantage that it can be relined without lifting the shell off the trunnion bearings, whereas a grate mill has to be taken off its bearings and dismantled for some of the routine replacements.
This advantage may be worth considering in a small plant, but it is negligible in a large installation where maintenance work can be better organized. The decision to install a particular make of mill for medium grinding has often depended on the fact that the type has previously proved satisfactory in an existing plant or in the locality. For a small company that cannot afford to experiment there is a good deal to be said for such a policy.