The Ball Mill Discharge Spiral Screen is designed to make a sized product, particularly for Sub-A Unit Flotation Cells and concentrating tables. It is also used to screen out and return to the ball mill any oversize in a ball mill-classifier circuit when using a (Selective) MineralJig.
This Ball Mill Discharge Spiral consists of a series of heavy steel bars made into one continuous, sturdy spiral. At the feed and discharge ends the spiral is supported by a steel flanged collar with heavy ribs to hold the flights as a single rugged member. The proper mesh screen cloth is placed around the spiral frame and held in position by bands which press against the outer surfaces of the flanges and spiral flights. This device eliminates short circuiting through the screen because the pulp is compelled to follow along the entire length of the steel spiral. A center screw can be furnished to automatically return the oversize into the mill.
The Ball Mill Discharge Trommel Screen has wide application in practically every type of placer work, in gravel plants, and in ore-dressing. This type of screen is particularly adapted for screening at coarse sizes, or at finer sizes if spray water can be used freely without interfering with screening.
The Trommel Screen consists of a shaft, bevel gears and bearings; spiders for supporting the screen, and bands for holding the screen cloth in place; a hopper with arrangements for removing the products, both undersize and oversize; and a cone on the feed end. It can be furnished with any type of punched plate or screen cloth specified by the customer. The 48 and 60 diameter Trommel Screens are furnished with steel tire and roller drive.These screens are well adapted for operation in series, with a range of screen openings to give graded products for use in stage treatment of ores.Let us make recommendations for your proposed trommel screen installation or redesign your present flowsheet for greater efficiency through use of a trommel screen.
The Spiral Classifier is available with spiral diameters up to 120. These classifiers are built in three models with 100%, 125% and 150% spiral submergence with straight side tanks or modified flared or full flared tanks. All sizes and models are available with single-, double- or triple-pitch spirals.
The tank is heavy plate with strong structural base. The extra heavy shaft has an improved submerged bearing. The greatest improvements, however, are found in the drive-unit which has been strengthened and improved over all other classifiers. A specially designed classifier reducer eliminates the over hang or cantilevered load normally found where the reducer shaft carries the pinion. The Classifier Reducer has an outboard bearing integral with the reducer base which provides positive alignment of the bevel gears.
The gears themselves are greatly improved as they are cast from metal patterns which have cut teeth. The accuracy of the patterns and the quality gear castings result in a cast-tooth gear of cut-tooth quality. The gears mesh smoothly, have greatly increased capacity and are noticeably more quiet than other spiral classifiers.
Spiral Classifiers are available in sizes up to 120 diameter, three tank styles, single, double and triple pitch spirals, three degrees of spiral submergence flexibility to provide a unit built for your job. Write for detailed recommendation on the correct size and type of Spiral Classifier to do your job economically and profitably.
This is the best method of determining the required pool area. However, if settling tests have not been made and it is inconvenient to make tests, the following procedure, which has been proved to be entirely satisfactory, may be used.
Example: Assume it is desired to overflow 100 tons of dry solids per 24 hours at 65 mesh in a pulp of 20 % solids. Table I shows that 6.43 tons will overflow per sq. ft. of effective pool area. Therefore, to overflow 100 tons in 24 hours, a classifier with 15.5 sq. ft. (100 divided by 6.43) effective pool area would be required.
For a 65 mesh separation it is preferable to set the classifier at 3 slope. Table II shows that at 3 slope the 30 Cross-Flow Classifier has an effective pool area from 12.9 to 16.4 sq. ft. depending on the height at which the weir is set. Therefore the 30 classifier would be the proper size for the overflow capacity desired.
Table III shows that at 65 mesh a peripheral speed of 44 ft. per min. is recommended, which on the 30 classifier corresponds to 5.6 R.P.M. At this speed the 30 classifier will convey 275 tons of solids per 24 hours, which then is ample for this job. Therefore, this size Cross-Flow Classifier will satisfy all the requirements of this problem.
But, suppose the circulating load required was 300% instead of 250% specified above. The amount of sand to be raked would be 300 tons per 24 hours. In this case it would be necessary to speed up the conveyor of the 30 classifier above the normal speed of 5.6 R.P.M. in order to handle the 300 tons of sand. In speeding up the conveyor more agitation is produced in the tank and settling is interfered with, resulting in a slightly coarser overflow. In this case it might be necessary to provide a 36 classifier.
If the classifier is to be used in open circuit it may be the shortest standard length made in that particular size. For installation in a closed grinding circuit the classifier length must be pre-determined to assure that it will close circuit with the ball mill. This is entirely a mechanical problem and the correct length is determined by making a ball mill-classifier layout to scale.
The pool area varies with the slope and since capacity at a required mesh depends on pool area, the slope cannot arbitrarily be changed to accomplish a closed circuit. With the classifier size and slope established it is necessary to make the classifier of sufficient length to close the circuit.
In general the classifier should be installed with a slope of from 3 to 4 in 1 ft. The steeper the slope the less the pool area of a given size classifier. The less the pool area the less the capacity. The maximum capacity for any mesh separation is obtained at a slope of about 3 in 1 ft. But, for very coarse separations it may be necessary to increase the slope and thus decrease the pool area so as not to cause overloading.
65 to 150 mesh3 per ft. 48 to 100 mesh3 per ft. 35 to 65 mesh..3 per ft. 28 to 48 mesh..3 per ft. 14 to 35 mesh4 per ft.
The launder from ball mill discharge to the classifier should have a slope of about 1 per ft. depending on fineness of grind and pulp density. The launder from sand discharge of the classifier to the ball mill scoop box should have a slope of from 4 to 6 per ft.
The speed of the conveyor should be just sufficient to handle the sands to be removed. The slower the speed the less the agitation in the pool and the finer the overflow. The lower the speed the longer the life of the classifier and all wearing parts.
Most efficient grinding is effected by removal of material from the ball mill as soon as it has been reduced to the required size. This eliminates over-grinding and permits utilizing all of the power applied to the ball mill in actually grinding the oversize material. This may be accomplished by using the Cross-Flow Classifier in closed circuit. The entire ball mill discharge goes to the classifier which separates the material ground to the desired size; returning the oversize material to the ball mill.
The Cross-Flow Classifier is ideal for this closed-circuit work. Its exclusive Cross-Flow principle of operation results in an extremely accurate separation. Various lengths of this classifier and variation in slope make it possible to fit the classifier to the circuit without use of expensive, troublesome equipment such as elevators, pumps, etc.
In closed grinding circuit separations are easily and efficiently made at from 20 to 100 mesh sizes. Normally, it is considered best practice to use a Hydroclassifier for separations at 100 mesh and finer. Efficiency of separation in fine mesh range requires a very large pool area. Thus, the Hydroclassifier, with its large surface area gives more efficient classification, more economically, than is possible with a spiral classifier.
Usually such separations are made on dilute pulps with a relatively small amount of slimes. Under these conditions a mechanical classifier can make efficient separations at a much finer mesh than in a closed grinding circuit where there is a higher density pulp and larger percentage of fines. The Cross-Flow Classifier will efficiently handle sand- slime separations in the range from 150 to 325 mesh, with a minimum amount of dilution water.
The Cross-Flow Classifier provides an efficient means of dewatering sands and concentrates or other granular material. A common application in this work is when the granular material is difficult to handle in a thickener. Also in many cases, where tonnage is not large, classifiers are considerably more economical than a thickener-filter installation lower in first cost lower in operating and maintenance costs require practically no attention.
A very common application of classifiers is in washing granular material to remove reagents, liquors, etc. Classifiers have the same advantage on small tonnage as in the case of dewatering lower initial and operating costs and less attention required. The particles to be washed pass successively up the inclined tanks of several classifiers, while the wash passes through the classifiers in the opposite direction. In each classifier the pulp is diluted, mixed and rabbled, the particles washed, and the liquid removed resulting in a thoroughly washed and cleaned final product.
STAND: For convenience in installing, these smaller sizes are provided with steel legs. The stand is made to give the most commonly used slope of 3 inches per foot. SHAFT: Solid, square steel. FLIGHTS: Hard, cast iron; made in short segments which fit over the square shaft. Flights may be placed on shaft so that blades form a continuous spiral, or may be staggered to obtain an interrupted spiral. DRIVE: Enclosed worm-gear speed reducer driven by motor through V-belts; cone pulleys are used to permit speed variations desirable in experimental laboratory or pilot plant work.
SHAFT: Heavy steel pipe. FLIGHTS: Hard, cast iron; made in short sections; bolted to cast iron arms which are carried on the shaft. Easily replaceable without draining tank. DRIVE: Bevel gear driven by gearmotor through sprocket and chain. Speed of drive is determined by the requirements of each installation. A variable speed drive may be furnished, at extra cost, if desired.
SHAFT: Heavy steel pipe with steel reinforcing sleeve at the lower bearing. FLIGHTS: Steel plate; bolted to cage which is carried by steel pipe shaft. Hard, cast iron wearing shoes, made in short sections, are bolted to the steel flights and are easily replaceable without draining the tank. DRIVE: Cast steel bevel gear and bevel pinion driven from a countershaft through spur gears; gearmotor and V-belts to countershaft. Speed of drive is determined by requirements of each installation. Variable speed drive may be furnished, at extra cost,if desired.
SHAFTS: Same as for corresponding sizes of simplex classifiers. FLIGHTS: Same as for corresponding sizes of simplex classifiers. DRIVE: Heavy cast steel bevel gears and bevel pinions, driven from countershaft through heavy spur gears; gearmotor and V-belt or chain drive. LIFTING DEVICE: Same as for corresponding sizes of simplex classifiers. CONVEYOR ROTATION: The two helical conveyor flights rotate in opposite directions, thus conveying the sands up the center of the tank giving free drainage back along both sides of tank.
There are four types of classifiers, high weir type single and double spiral classifier, immersed single and double spiral classifier. Spiral classifiers are widely used for distributing ore in the close circuit with ball mill, grading ore and fine slit in the gravity mill, grading granularity in the flow of metal ore-dressing and desliming and dehydrating in the washing.
Hongxing's screw classifiers are designed to provide the most effective pool area and overflow velocity requirements. Our spiral classifier is widely used in the distribution of ore in closed circuits with ball mills, grading ore and fine slit in gravity mills, grading granularity and flow of metal ore-dressing, and de-sliming and dehydrating in washing.
Spiral classifieris made of U-steel, while the body is armor plated and the spiral axle is made of cast iron for durability. The machine's lifting equipment can be used manually or electronically. Spiral classifier consists of transmission part, spiral axis, tank, elevator mechanism, bearing, discharge valve. Spiral classifier is with simple structure, reliable working conditions and convenient operation.
China screw classifier can filter materials and send coarse materials towards the feeding mouth and discharge fine materials from your pipe. The machines lifting equipment can be utilized manually or electronically. Spiral classifier price produced by our company is reasonable. For this reason, we have large domestic and overseas market.
Combined with ball mill to form closed loop procedure, ore spiral classifier is widely applied in the ore beneficiation plant or gravity spiral classifier plant to separate mineral sand or fine sand. In addition, spiral concentrator is also applied in the metal ore beneficiation procedure to separate the ore pulp as well as desliming or dehydration operation in the ore washing process. Ore spiral classifier has the features of simple structure, reliable working and convenient operation and the like.
These notes are based on observations made while on a recent trip through the West, for the purpose of studying the practical operation of the ball-mill. The writer takes this opportunity to express his thanks for courtesies extended at the many plants visited as well as for the valuable data received.
While there are several types of ball-mill on the market, particular attention will here be given to the diaphragm type (Peripheral Discharge Ball Mills ), as the open-trunnion type, especially the conical mill, has been thoroughly discussed here.
There is a prevailing impression that the ball-mill is a recent development; however, ball-mills were used extensively in Montana and other western states for crushing ores for concentration. Its present prominence is due in part to its recent successful application by one of the large copper companies. Without any reference to dry grinding, the first successful ball-mill for wet crushing, which is still in operation, was built 10 years ago. This mill, designed by Erminio Ferraris for crushing Sardinian ores for concentration, is of more than passing interest. It embodies the peripheral discharge with grates, large forged-steel balls, and the principal features of the modern ball- mill. The results approach present-day practice, the chief differences being that the mechanical construction has been improved in the modern types.
The action of the balls and the principles of crushing have been studied by several investigators. Their conclusions are confirmed by results obtained by the writer in experimenting with a small machine built atthe Allis-Chalmers factory, and serve to explain the reasons for some of the results obtained in practice. A ball-mill may be revolved so fast that the balls will cling to the shell during the entire revolution, while at slow speeds they will be carried up only a short distance and roll back. On the other hand, at the critical speed, they will cascade as shown in Fig. 1. At the critical speed the balls ascending on the layer next to the shell start from rest at a point S and cling to the shell without revolving or rolling, which has often been ascribed to them. These balls are held at rest by centrifugal force until they reach a point G, the location of which is dependent on the speed of rotation. Beyond the point G, gravity overcomes centrifugal force and the balls fall with increasing velocity in a parabolic curve which is the resultant of the above two forces, striking at a point W, the force of the impact being expended in crushing the material.
The several layers of balls lying on top of those next to the shell follow a similar cycle except that, due to relative difference in the two forces, their paths become more nearly vertical. The outer layers, spreading more thanthe inner layers, increase the area in the zone of the falling balls. Within the circuit thus formed is a neutral axis or a sluggishly rotating kidney-shaped mass in which little actual work is performed.
The material being crushed is thoroughly distributed throughout the mass by filling the interstices between the balls, and follows in the same circuit. It is, therefore, evident that the material is crushed mainly by impact of the striking balls as the whole mass falls. There can be very little grinding by attrition due to the rotation of balls, except at the point S where the shell picks up the mass and accelerates it to the rotative speed of the shell. The argument has often been advanced that fine material cannot be produced by impact alone and that fine grinding is done entirely by attrition or rubbing of adjoining balls. It is only necessary to break up a few small pieces of rock on an anvil with a hammer to prove that fines are unavoidably produced by impact. Screen analyses of the discharges from tube-mills in open and in closed circuits lead to the conclusion that in many instances an ore fragment may pass through the mill six to eight times before it is crushed to the desired fineness. Quoting directly from the article by Hermann Fischer referred to above:
The grinding action, therefore, depends upon the height of the drop of the balls, i.e., the height of the curve vertex above the point where the ball strikes, the speed of the shell, the weight and number of balls.
The speed of the drum must be so determined that the curves can develop themselves properly. The weight of the balls and the height of drop are inter-related and their product must be sufficient to break the ore according to its size and hardness. Hard materials require heavier balls or greater height of drop than soft ones and steel balls in small diameter cylinders will do the same work as flint pebbles in large diameter cylinders.
The free fall of the balls is dependent upon the volume of ball load. With a charge equal to or greater than half the volume of the mill the free fall of the balls is decreased, the charge is held together, and the size of the inactive kidney-shaped mass is increased. When the charge is about one-third of the volume of the mill the size of the kidney-shaped mass is reduced and the balls fall from their maximum free height. Operating results bear out the above facts in that the greatest number of tons crushed to a certain mesh per kilowatt-hour are obtained with ball charges equal to approximately one-third the volume of the mill.
There is a general impression that the grate acts as a screen or sizer. This is true to a limited extent, but it is not of primary importance. The fineness of product delivered by a ball-mill, the size of feed, ball charge, and speed remaining constant, depends upon the tonnage fed, the density of the pulp (water to solids ratio), size of balls, and, when operating in closed circuit, on the efficiency of the external classifying apparatus. The screen analyses plotted in Fig. 2 show the effect of varying tonnages, other factors remaining constant. They are from actual results with a 6 by 4-ft. mill.
The screen analyses plotted in Fig. 3. show the difference in product when the initial charge included only 5-in. and 2-in. balls, and when the same charge contained a large percentage of 4, 3, and 2-in. balls. In some respects, these results do not agree with what would be expected, but I will not attempt to propound a theory to explain the deviations at this writing.
give a fine product and a large amount a coarse product. As the discharge is entirely at the periphery, and does not depend upon any classifying action to overflow the finished product, the greater the amount of water added the quicker the pulp will pass through the mill and the coarser the product.
In mills provided with means for raising the discharge or pulp level from the periphery to some intermediate height between the periphery and the trunnion, the fineness and the amount of oversize can be controlled within certain limits. No figures are available showing these differences, but from practical results in the field it appears that a wide variation can be obtained by this means.
Thegrate should, of course, retain some oversize, but this action can be carried to extremes, especially when a fine product is desired, as the consequent diminished capacity is not compensated by the reduction of oversize. In all cases when a fine product is desired, it is advisable to run themill in closed circuit with an efficient external classifier. The principal function of the grate is to retain the ball charge in the mill, while permitting a peripheral discharge. The efficiency of the classifier, when a ball-mill is run in closed circuit, directly affects both tonnage and fineness. This will be discussed under capacity.
Capacity of ball-mills depends upon the following factors: fineness of grinding, weight or volume of ball charge, hardness of material, size of grate openings, and size of balls, other factors remaining constant. Practically speaking, the most important limiting factors for capacity havebeen the size of the feed opening in the trunnion, the type of trunnion liner, and the type of feeder.
As previously shown, tonnage and fineness are inter-related and the capacity of a ball-mill should be figured on the following basis when sufficiently reliable figures have been collected. The kw.-hours required to crush a ton of ore from and to a certain mesh should be arrived at from average operating conditions. A ball-mill has a certain definite maximum power rating depending upon its ball load. Multiplying the kw.-hours per ton by the tons required to be crushed per hour, the product will represent the power required, and the mill nearest to that power rating should be selected. Fig. 4 is a preliminary power curve based on the recommended maximum ball charge, together with all available data at hand at the present time; however, 60 or more carefully taken power records would be needed for even an approximately correct curve.
Operating a mill at less than its maximum capacity for a given ball charge will result in excessive wear on fining and balls and produce a finer product than necessary. To crush a ton of ore of a certain hardness and size to a given fineness represents a definite amount of work; hence the capacity of a mill depends upon (a) the hardness, and (b) the ratio of reduction, the latter affecting capacity far more than the former.
It is useless to expect a large capacity from a mill operated with balls of a size too small to crush the ore, or when the balls are of a composition that will not withstand the shock of impact and shatter themselves to fragments.Hard ores, when fed direct from a crusher, require a proper percentage of 5-in. steel balls to do effective work. A 4-in. steel ball is often sufficient for some of the softer porphyry ores. Smaller steel balls may be used for regrinding work, but the charge should contain a percentage of 2-in. steel balls when working on hard ores. For regrinding soft ores, cast iron or composition balls may be used.
Where a fine product is desired together with a minimum amount of oversize, the grate opening should not be diminished. Smaller grate openings will reduce the amount of oversize but the decreased tonnage is not compensated. In such cases it is advisable to depend on an external classifier and operate the mill in closed circuit; the grate bars should be set with at least 1/8-in. opening. Where a coarse product is desired, for example for concentrating table work, the grate may be used as a sizer and an open-circuit scheme adopted.
When the mill is operated in closed circuit the efficiency of the. classifier directly affects the capacity and it is important that the classifier be of proper size and properly operated. In one case observed, a classifier of the mechanical drag type was set with the wrong slope; correcting the slope approximately doubled the capacity of the mill. Classifiers of the mechanical drag type, in order to make an efficient separation, must be operated with proper consistency of pulp in the classifying zone, the slope and length of thesand plane must be correct, and the speed of the drag must be suited to the material.
Power depends principally upon the weight of ball charge, an approximate figure being 9 to 10 hp. per ton. However, the power per ton of balls will vary according to the percentage of volume the ball charge occupies in the mill. An approximate curve from data at hand is given in Fig. 6, from which it will be seen that the power required per ton of balls is least when the mill is loaded half full and that the curve rises very rapidly as the ball load is reduced. A charge greater than half full causes a balancing effect until, when the mill is full, the power required is practically only that necessary to take care of friction after starting.
When the volume of ball charge is reduced, within certain limits, the power consumption per unit of ball charge is increased, because the center of gravity of the charge is further from the axis of the mill; but asthe mass of balls is more active and circulates more freely, the crushing efficiency is increased proportionately to the increase in power consumption per ton of ball load.
There are a number of ball-mill installations for fine crushing in the West. Most of these are arranged in two or more stages where a product finer than 100-mesh is desired, and there seems to be little difference of opinion as to the advantage of such an arrangement. Where coarser products are desired, say through 48-mesh, both single-reduction and stage-crushing installations are found. Stage crushing seems to have higher efficiency, but when first cost and simplicity are considered, the single-reduction installation seems to be more desirable, especially for small plants.
The curves (Fig. 7) plotted from recent tests show the power required per ton of material crushed under varying capacities. It can be seen that the power rises rapidly at the expense of capacity when a fine productis desired, and when compared with an average power curve it would make a saving to run a large tonnage through several stages.
The phrase single reduction as applied to ordinary ball-mill practice is misleading, because in the most common application of the ball-mill, running in closed circuit for preparing feed for flotation, a great deal of the material is returned from once to six or seven times before it is finally reduced. The most efficient installations in practice are undoubtedly those which have a large return circuit and the mill is crowded, making a small reduction at each pass through the mill, but handling a large tonnage at the same time.
The ball-mill is not to be recommended for all and sundry problems in the milling field. It is not suitable for concentration work where the ore contains a large amount of coarse mineral easily pulverized. Where crushing to 12-mesh and finer is necessary to release the mineral, the ball-mill makes a suitable product when properly operated, and is as good as any other regrinding machine.
The installation of concentrating tables within the mill circuit, as practised at Stoddard, Ariz., is a notable advance in this class of work. The special field of the ball-mill, however, is for products 20-mesh and finer.
The use of ball-mills for reducing crusher product to 85 per cent, below 200-mesh in two stages, as practised at the United Eastern, Tom Reed, and Montana mines, in Arizona, is a distinct advance in fine crushing. The simplicity, small floor space and large capacity of these installations are especially notable. While there is not such economy in power nor so small a number of repairs as compared with a stamp- battery and tube-mill plant of the same capacity, the operating troubles and attendance are much reduced.
The most desirable method of feeding coarse material is the arrangement as installed at the Tom Reed mill. The crusher product is fed direct from a bin to an apron feeder, the speed of which is controlled by a Reeves variable-speed transmission device, having a small hand crank, sprocket, and chain conveniently situated for the mill operator. This insures absolute control and allows quick changes.
When a ball-mill having a proper crushing load is rotated at the critical speed, the balls strike at a point on the periphery about 45 below horizontal, or S in Fig. 1. An experienced operator is able to judge by the sound whether a mill is crushing at maximum efficiency, or is being over- or under-fed. Excessive rattling denotes under-feeding; a sound of impact at W (Fig. 1) indicates overloading; while under proper conditions, the impact will be heard near S.
When a ball-mill fitted with a diaphragm is over-fed, the mill fills up to a certain level, then stops crushing and discharges any additional feed back through the feed trunnion. Once over-fed, it takes from 30 min. to 2 hr. to free itself. Ball-mills, therefore, should be provided with a central opening in the diaphragm connecting with the discharge trunnion, to prevent over-feeding and the delays incidental thereto.
The greatest difficulty in feeding most ball mills, when running on large tonnages and coarse feed, say, to 3 in., is due to the restricted area of the feed trunnion, which limits the quantity of coarse material that can be fed through it. A few simple calculations will show the velocity necessary to pass a given quantity feed through the trunnion, It can also be shown mathematically that the average spiral in the trunnion liner does not advance the feed rapidly enough; therefore, instead of aiding, it retards the feeding. These results are confirmed in practice. A smooth liner, tapering from the feeder into the mill, does not retard the flow of the feed, and is, therefore, more efficient than the spiral. Experiments with small models, as well as experiments in thefield, corroborate these conclusions. A short trunnion with large diameter is essential for feeding a large tonnage to a ball-mill.
The engineering department of the Allis-Chalmers Manufacturing Co. has recently conducted some experiments with feeders modeled after the various types in use, on a scale of 1 in. per foot. The feeders were operated at constant speed conformable with present practice, the material delivered in a given time being weighed. The following con-
clusions were drawn: The intake of a single-scoop feeder has far greater capacity than the throat or trunnion of the mill, and there is no good reason for using a double- or triple-scoop feeder, the capacity of the feeder not being controlled by the quantity it will pick up, but by the quantity that it can discharge through the throat or trunnion. These experiments further demonstrated that the capacity of a spiral feeder is in direct proportion to the length of the path of the spiral. In other words, a spiral feeder embodies all the principles of the Frenier sand pump, in which the long path of the spiral increases the pressure which forces the feed into the trunnion opening.
Fig. 10 shows a double-scoop feeder without a partition; Fig. 11 shows the same feeder with the two spirals connected across the center of the trunnion opening, making a partition so that the material taken up cannot drop from one scoop into the other. Fig. 12 shows a single-spiral feeder; Fig. 13 shows a triple-spiral feeder; and Fig. 14 shows a standard combination feeder which has a single spiral.
Disregarding the influence of the trunnion liner as determining the relative capacity of feeders, the experiments demonstrated that No. 12, the single-spiral feeder, has the greatest capacity; No. 11, double-spiral feeder with the partition across the trunnion opening, gave the next best capacity, which, however, was less than 50 per cent, that of No. 12. The capacity of No. 10 was only about 25 per cent, that of No. 12. The capacity of the triple-scoop feeder, Fig. 13, was but very little greater than that of No. 11. The results clearly demonstrate that increasing the number of spirals or scoops does not add to the capacity of a feeder.
The ratio of moisture to solids is important in ball-mill work. From actual operation it has been observed that fine grinding is best done when water constitutes 33 to 40 per cent, of the pulp, or the water-to- solids ratio is 1 :2 or 1 : 1. Where a minimum of fine material is desired, 50 per cent, and upward of water is desirable.
Ball consumption varies with the fineness of the product, hardness of the ore, quality of ball, and whether a mill is run in closed or open circuit. The ball consumption for mills delivering a coarse product, all passing 8-mesh and containing 10 to 20 per cent, below 200-mesh, the mill being run in open circuit, is about lb. per ton for steel balls and 1 lb. for cast composition balls.
The average ball consumption for mills in closed circuit has been plotted in Fig. 15 for steel balls and for cast composition balls. Enough data are not available to plot curves for hard and soft ores, and individual figures will vary considerably from the average of the curves, which are given merely a guide as to what may be expected and also to show the increased consumption with finer grinding. It should be noted that the curves apply to products practically all of which are finer than the meshes indicated, up to 65-mesh. Points on the curves representing finer products are for mills generally regrinding 10- to 20-mesh feed; hence corresponding amounts must be added to give the total ball consumption for reducing from crusher size to 100-mesh and finer.
Average consumption of shell liners, for both chrome and manganese steel, is 1/3 lb. per ton of ore crushed. The consumption of lining seems to be fairly constant regardless of the hardness of the ore, fineness of product, or other conditions. The greatest wear on the lining is probably caused by theimpact of the balls and by their slippage on the shell during the period of acceleration. If the mill is running below capacity the wear will increase.
is the general increase in weight and thickness. The proportion of scrap has been very high, and the consumption stated above may be reasonably expected to be diminished with heavier and thicker liners. Regarding the shape of liner, there is considerable difference of opinion. The smooth liner is probably as efficient as any of the others if run at slightly higher speed.
Hard-iron liners have not been found satisfactory when used with balls of 5 and 4-in. diameter, as they have invariably failed by cracking and breaking, but with balls of 2-in. diameter and smaller they are sufficiently durable. It is possible that a heavy hard-iron liner backed and set in cement mortar might be successful, but this has not yet been tried as far as we know.
The loosening of liners may be avoided by using deeply countersunk bolts of large diameter with double nuts. When the liners are first put in place, after running the mill for several hours the bolts should be gone over again and the nuts tightened with a short wrench and hammer. Later, after the feed is on, they should be gone over once more. Leakage around bolt holes is caused entirely by lossening of the bolts due to lack of tightening or a worn-out lining. If candle-wicking is used as packing around a bolt, between the shell and the washer, and the nut is kept tight, no leakage will occur until the liners are worn out.
In Mineral Processing, the SPIRAL Classifier on the other hand is rotated through the ore. It doesnt lift out of the slurry but is revolved through it. The direction of rotation causes the slurry to be pulled up the inclined bed of the classifier in much the same manner as the rakes do. As it is revolved in the slurry the spiral is constantly moving the coarse backwards the fine material will flow over the top and be travelling fast enough to be able to work its way downwards to escape. The Variables of these two types of classifiers are The ANGLE of the inclined bed, this is normally a fixed angle the operator will not be able to adjust it.
The SPEED of the rakes or spirals, the DENSITY of the slurry, the TONNAGE throughput and finally the SETTLING RATE of the ore itself.To be effective all of these variables must be balanced. If the incline is too steep the flow of slurry will be too fast for the rakes or spirals to separate the ore. If the angle is too flat the settling rate will be too high and the classifier will over load. The discharge rate will be lower than the feed rate, in this case. The load on the rakes will continue to build until the weight is greater than the rake or spiral mechanism is able to move. This will cause the classifier to stop and is known as being SANDED UP. If the speed of the rakes or spirals are too fast, too much will be pulled, out the top. This will increase the feed to the mill and result in an overload in either the mill or classifier as the circuit tries to process the increased CIRCULATING LOAD.
The DENSITY of the slurry is very important, too high the settling will be hampered by too many solids. Each particle will support each other preventing the heavier material from quickly reaching the bottom of the slurry. This will not allow a separation to take place quickly. The speed at which the slurry will be travelling will be slow and that will hamper effective classification. Another variable is the TONNAGE. All equipment has a limit on the throughput that anyone is able to process, classifiers are no different. This and the other factors will have to be adjusted to compensate for the last variable, the ore itself. Every ore type has a different rate of settling. To be effective each of the previous variables will have to be adjusted to conform to each ones settling characteristics.
The design of these classifiers (rake, spiral, screw) have inherent problems, First, they are very susceptible to wear, caused by the scrubbing action of the ore, that plus all of the mechanical moving parts create many worn areas to contend with. The other problem that these classifiers have is that they are easily overloaded. An overloaded classifier can quickly deteriorate into a sanded-up classifier. Once that happens the results are lost operating time, spillage and a period of poor Mineral Processing and Separation performance.
Another mechanical classifier is the spiral classifier. The spiral classifier such as the Akins classifier consists of a semi-cylindrical trough (a trough that is semicircular in cross-section) inclined to the horizontal. The trough is provided with a slow-rotating spiral conveyor and a liquid overflow at the lower end. The spiral conveyor moves the solids which settle to the bottom upward toward the top of the trough.
The slurry is fed continuously near the middle of the trough. The slurry feed rate is so adjusted that fines do not have time to settle and are carried out with the overflow .liquid. Heavy particles have time to settle, they settle to the bottom of the trough and the spiral conveyor moves the settled solids upward along the floor of the trough toward the top of the trough/the sand product discharge chute.
Allow metocompare:Ball mills can be of the overflow or of the grate discharge type. Overflow discharge mills are used when a product with high specific surface is wanted, without any respect to the particle size distribution curve. Overflow discharge mills give a final product in an open circuit.
Grate discharge mills are used when the grinding energy shall be concentrated to the coarse particles without production of slimes. In order to get a steep particle size distribution curve, the mill is used in closed circuit with some kind of classifier and the coarse particles-known as classifier underflow-are recycled.
In the past, I had worked with +10% as an expected increase created by the conversion. How much capacity you gain by using grate discharge over overflow discharge on a mill =The +20% data comes from an old paper.
BallMills have a very large discharge opening or area and smaller area for incoming feed. The gradient between the incoming feed opening and the discharge near the periphery of the shell provides a faster migration of the fines than the oversize particles. In deep pulp level mills commonly known as overflow mills this migration can not occur since material enters and leaves at the same level by displacement only. Independent tests have shown that regardless of mill shape or design, the discharge product of an overflow mill will be the same no matter at which end the feed enters.
Grate Discharge Ball mills withlow pulp levels benefit from the full impact of the grinding media acting on the ore particles, as it falls into the shallow pulp. With a deep pulp level the grinding media is cushioned in the pulp, thus losing its energy and reducing its grinding ability. Grate Discharge Ball Mills have shown 25% to 45% more tonnage ground and a substantial reduction in power consumed per ton of material compared size for size with overflow mills.
A general statement can be made that the closer the discharge is to the periphery of the shell, the quicker the material will pass through and less overgrinding will take place. This is important in both rod mill and ball mill grinding. First, regardless of how fine a grind is required, overgrinding is costly and undesirable. The ideal condition is to remove the particles as soon as they have reached the optimum size. Secondly, in grinding applications where a minimum amount of fine material is preferred, again a rapid flow through the grinding mill is required. These can be accomplished with the grate for ball mill operations, or the various Grate Discharge discharge arrangements for the rod mill.
The discharge end of the conventional Open End Rod Mill is virtually open as the name implies. As a means of controlling splash and to prevent unruly rods from moving out of the mill a discharge plug or plug door arrangement is furnished. The use of this construction permits pulp to discharge freely around the annular opening between the plug and the discharge trunnion liner. By simple removal of the plug the full large area of the discharge end may be used for re-rodding, inspection of the mill when in operation, and an easy access to the mill interior for relining. This large opening does away with the necessity of manholes for mill entrance as commonly employed in the overflow type mill. The plug door arrangement is a great time saver during re-rodding and re-lining operations.
On smaller diameter Grate Discharge Rod Mills a discharge plug is furnished mounted on the trunnion liner and extending through to line up with the discharge head liners. The larger diameter Grate Discharge Mills are furnished with a discharge housing arrangement independent of the mill. A hinged door is mounted in this housing and easily swings in or out of the discharge trunnion liner. These housings are also used to control the direction of discharge pulp flow leaving the mill. Such flow may be directed to the left, right, or directly below the mill centerline.
The discharge housing is of very heavy construction for strength and rigidly. Maintenance of this housing is kept at a minimum, the only wearing part being the replaceable Manganese Steel plug door liner.
The discharge end of a Grate Discharge Ball Mill is fitted with grate sections approximately 3 thick, made of special heat treated alloy steel developed for this particular application. The grate sections have tapered openings between and 7/8 dependent upon the specific grinding application. These are selected to provide the greatest efficiency for any particular job. The grate sections are held in place by tapered Manganese Steel side clamp bars, a center discharge liner, and end clamp bars. The discharge grates are very simple to install and require no attention during operation. The overall life of the discharge end parts generally is greater than that of the feed head liners or shell liners. The discharge end of the Grate Discharge Ball Mill has at least ten times the discharge opening area, through the grate slots, compared to the common trunnion overflow type mill. The discharge grates are designed to run clean and free of any blinding or choking. The pulp level in the mill may be varied by merely changing the pulp dilution. There is no complicated mechanical arrangement to compensate for pulp level changes. The side clamp bars and center discharge liner besides holding the grate sections in place, act as a means of stirring up the ball charge and reduces the amount of wear on the grate sections. The pulp discharges through the grate slots into a lifter compartment in the discharge head, lined with replaceable wearing parts. This lifting compartment elevates the discharge pulp up to the level of the discharge trunnion liner opening and spills this against a deflecting cone which directs it out through the trunnion liner.
The above is a Grate Discharge Mill head with discharge grates, side clamp bars, end damp bars, and center discharge liners in place. The grates and side clamp bars are accurately ground to fit the machined surfaces of the discharge head lifters.
We have already discussed grinding in a general way and have referred numerous times to the grate dischargeprinciple of grinding. To illustrate roughly this principle, take a certain weight of crushed ore and grind it with a mortar and pestle until all of the ore particles will pass through a 65-mesh screen. Then take a similar sample but this time grind for a few minutes and screen at 65-mesh removing the finished material, then return the oversize particles and grind for another short period of time and repeat the screening operation. You would find that the actual net grinding time required for the second sample is about half the time required under the first condition. This same process takes place in the Grate Discharge Ball Mill. It must be borne in mind that it is the classifier which determines the size of the finished product, not the grinding mill itself. The Grate Discharge Mill permits a quick discharge of the finished material into the classifier which makes the desired mesh size separation and returns the oversize particles to the mill for another pass.Contrary to the usual belief, material does not discharge through the grates at the bottom. In fact it is carried up in the ball load so that the greater portion passes out from the ball load on the upturning side of the mill, in the grate area from about half way below the centerline of the mill, on up to the point where the balls start to leave the shell on their downward paths. This indicates then that the thick pulp carried in the mill is well within the ball mass where the actual grinding is taking place. The discharge grates are not to control the size of particle discharged, but merely to retain the grinding balls within the mill, provide the full discharge area required, and form the steep gradient between the feed entrance and product discharge.
To illustrate the comparison of the grate discharge Ball Mill to an overflow type of mill we are showing on page 31 several actual case histories of installations where the performance of grate discharge mills versus overflow mills have been proven. In each such test, run for long periods of time, the ore characteristics and size of feed were maintained identical so that the tests could be compared under like conditions. It will be noted that in each case the grate discharge Mill provided a high increase in tonnage with a lesser increase in power consumption so that the actual KWH per ton consumed was reduced. From these field examples you can verify the previous statement that an overflow type of mill has somewhere near 70% the capacity of the grate mill. These tests were conducted independently by the actual operating companies involved.
The above tables list some of the most common Grate Discharge Ball 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 are reduced and capacities drop between 30% to 50% .
The above dimensions are approximate and for preliminary use only. Right hand mills are shown. For left hand mills put drive on opposite side. Drive may also be located at feed end. but clearance of scoop must be considered.
The above dimensions are approximate and for preliminary use only. Right hand mills are shown. For left hand mills put drive on opposite side. Drive may also be located at feed end, but clearance of scoop must be considered.
Ball mills are also classified by the nature of the discharge. They may be simple trunnion over-flow mills, operated in open or closed circuit, or grate discharge (low-level discharge) mills. The latter type is fitted with discharge grates between the cylindrical mill body and the discharge trunnion. The pulp can flow freely through the openings in the grate and is then lifted up to the level of the discharge trunnion. These mills have a lower pulp level than overflow mills, thus reducing the dwell time of particles in the mill. Very little over grinding takes place and the product contains a large fraction of coarse material, which is returned to the mill by some form of classifying device. Closed-circuit grinding, with high circulating loads, produces a closely sized end product and a high output per unit volume compared with open circuit grinding. Grate discharge mills usually take a coarser feed than overflow mills and are not required to grind so finely, the main reason being that with many small balls forming the charge the grate open area plugs very quickly. The trunnion overflow mill is the simplest to operate and is used for most ball mill applications, especially for fine grinding and regrinding. Energy consumption is said to be about 15% less than that of a grate discharge mill of the same size, although the grinding efficiencies of the two mills are the same.
Applied materials: in the metal beneficiation process including gravity concentration and ore washing of the materials such as quartz, gold ore, iron ore, copper ore, ore, ore pulp, fine mud, cement clinker, magnetite, etc.
Coarse particles sink to the bottom of the tank and are conveyed by the screw to the discharge port to be discharged. Generally, the coarse sand is returned to the ball mill for secondary grinding, that is, the spiral classifier and the ball mill form a closed circuit.
It is composed of the hollow shaft, bracket, spiral blade, lining iron, etc. The hollow shaft is welded by seamless steel pipe, journal and flange. Wear-resistant life is the key in use, so the lining iron is made of wear-resistant materials.
It is easy to wear, for its long-term immersion in the slurry. Therefore, frequent maintenance and replacement are its main characteristics. For this, the spiral classifier adopts the structure of movable shaft sleeves and nylon bearing bushes.
When the shaft sleeve and bearing bush are worn out, a new set of sleeve and bush can be replaced ten minutes after the screw is lifted by the lifting mechanism, which is convenient for maintenance and replacement.
It is welded by steel plates and various section steels. The foundation construction of the tank is very important because it contains all the slurry and bears the weight of the whole body and all the load.
In order to discharge slurry from the water tank when necessary, a water drain valve is installed in the lower part of the tank, which can discharge the slurry at any time and can be closed during normal production.
The settlement area of this kind of classifier is large and its weir height can be adjusted within a certain range, that is, the area of the settlement area can be adjusted and changed within a certain range.
It has a small settlement area and low overflow production capacity, so it is mainly suitable for ore classification with an overflow particle size of 0.15-0.07 mm. It is also used to wash ore for desliming.
Model description: Take "2FG-15" as an example, "2" means double spiral, the single spiral is not marked; "F" means spiral classifier, "G" means high weir type and "C" means submerged type; "15" indicates the spiral diameter of the classifier, in dm.
If the grinding fineness is required to be finer, angle irons of a certain height can be welded on both sides of the classifier, and the level of the classifier overflow weir can be adjusted by the method of inserting wood. Sometimes the overflow weir can be naturally increased after a long-term accumulation of ore mud.
To obtain coarse overflow, the speed of the spiral with a diameter of 2 m should not exceed 6 r/min, and with a diameter of 1 m or more should be controlled at 2-8 r/min. For example, for a spiral with a diameter of 0.3 m, in order to obtain a coarse overflow, the spiral speed can be increased, but it cannot exceed 25 r/min.
If the screw pitch is short at the overflow end and long at the sand return end, the amount of sand return can be increased and stable agitation at the overflow end can be ensured, which is convenient for improving the overflowing quality and processing capacity.
The width of the grading tank has little effect on the grading effect, but is closely related to the processing capacity of the grading machine. The larger the tank width, the larger the processing capacity. On the contrary, the processing power is small.
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The spiral classifier working principle: Fine ore pulps are fed into a water tank through feeding opening located in the center of settling zone. Beneath the inclined a water tank is the ore pulp classification zone where ore pulps are stirred by low-speed impellers. Fine ore particles are lifted up and then spilled out the overflow opening; coarse ore particles precipitate down to the tank bottom and are discharged via discharge opening.
The Spiral Classifier is available with spiral diameters up to 120. These classifiers are built in three models with 100%, 125% and 150% spiral submergence with straight side tanks or modified flared or full flared tanks. The spiral classifier is one of the size classifying equipment for the mining industry. It is a kind of equipment for mineral classification based on the principle that the specific gravity of solid particles is different and the speed of precipitation in the liquid is different. It can filter the material powder from the mill, then screw the coarse material into the mill inlet with the spiral slice, and discharge the filtered fine material from the overflow pipe. Spiral classifier short for the classifier. The classification machine mainly has the high weir type single screw and the double screw, the sinking type single screw and the double screw four kinds of classification machines. The classifier is mainly composed of a transmission device, a spiral body, a trough body, a lifting mechanism, a lower bearing (Bush) and a discharge valve. Spiral classifier is widely used in mineral processing plants with a ball mill as a closed-circuit circuit to separate the flow of ore sand, or used in gravity concentrator to grade ore sand and fine mud, and metal beneficiation processes to grade the size of ore pulp and washing operations in the desliming hopper, dehydration and other operations.
Spiral classifier features: 1. Low power consumption; 2. Heavy-duty, long working life; 3. Powerful self-contained spiral lifting device; 4. Continuous spiral raking; 5. High classifying efficiency; 6. Wide choice of weir height; 7. Rigid tank and substructure; 8. Wide choice of tank design; 9. A wide range of industries serviced.
Spiral classifier is by means of solid particles of different sizes, the proportion of different, thus settling velocity in liquids of different principles, fine mineral particles floating in the water to overflow out of coarse mineral particles sink to the bottom. Discharged from the screw into the upper part, to a hierarchical classification of mechanical equipment, can mill to grind the material powder level in the filter, and then use the course material helical screw rotary vane into the mill feed, the filter out the fine material is discharged from the overflow pipe. The machine base is made of a channel steel body with steel plates welded together. Into the head of the screw shaft, shaft, using pig iron, wear-resistant and durable, lifting devices of electric and manual. The main types of the spiral classifier are high Weir single screw and double screw, low weir single screw and double screw, sinking single screw and double screw Mainly High Weir type and sinking type and XL spiral classifier.