US2680570A - Material reduction mill employing ball charges - Google Patents

Description

June 8, 1954 D. WESTON MATERIAL REDUCTION MILL- E MPLOYING BALL CHARGES 4 Sheets-Sheet 1 Filed Dec. 9 1953 FALSE TOE' CRUSHING ZO INVENTOR DAVID WESTON BV-J ATTORNEYS June 8, 1954 WESTON 2,680,570

MATERIAL REDUCTION MILL EMPLOYING BALL CHARGES Filed Dec. 9, 1953 4 Sheets-Sheet 2 GRAVITATIONRL FORCE TE-i- INVENTOR DAVID WESTON BY-JWA Q. -ATTORN vs MATERIAL REDUCTION MILL- EMPLOY Filed Dec. 9, 1953 June 8, 1954 WESTON 2,680,570

ING BALL CHARGES 4 Sheets-Sheet 3 FEED MATERIAL ZONE INVENTOR DAVID WESTON BY-Jdlm k ATTORN YS June 8, 1954 WESTON 2,680,570

MATERIAL REDUC'IYI'ON MILL EMPLOYING BALL CHARGES Filed D60. 9, 1953 4 Sheets-Sheet 4 CAPACITY LBS/HOURS FJ 4. %CRITICAL SPEED 5 g 4 Q 2- o. 40 E as v S co m 30 25 o a v E 2 2o Lu v G U E Q 30 2a 26 24 22 20 I8 I6 INVENTOR F ILL t-" 7 I RPM 0 M DAVID wssrou I ammaa m ATTOR NE YS Patented June 8, 1954 MATERIAL REDUCTION MILL EMPLOYING BALL CHARGES David Weston,

2 Claims.

This invention relates to the comminution of materials in material reduction mills employing a ball charge and more particularly it relates to upstanding circumferentially crusher bars as will be described in more detail hereinafter. This application is a continuationin-part of my copending application Serial No. 234,782, filed July 2, 1951.

Heretofore in the operation of ball mills it has been the general practice to employ what has been referred to as a seasoned ball charge; that is to say, a ball charge which is made up of individual balls of varying diameters. tion, the balls are gradually reduced in size due Large balls are operation of the seasoned been used of 35% and the volume occupied by the ball charge is frequently as high as 50% increase in volume of the ball charge up to this According to this current practice, the material undergoing comminution and the ball charge is of the order of from about 45% to about 50% of the mill volume and the proportion of balls in of Mineral Dressing 3d edit. 1948).

In contradistinction to the above described prior practices, my invention contemplates the use of a relatively small ball charge composed of relaballs, the ball charge occupying a maximum of 3% of the volume of the mill exclusive of voids and the combined volume of the ball charge and material charge being not more than a maximum of about 32% of the volume of the mill.

This novel type of ball charge is used according Toronto, Ontario, Canada Application December 9, 1953, Serial No.

to the invention in combination with a mill which is provided with highly upstanding crusher bars which are fairly widely alfords particularly With combined dry crushing and grinding mills of the kind described in my copending applications Numbers 175,353, filed July 22d, 1950, now abandoned, and 749,131 filed May 20, 1947, now Patent 2,555,771, and will for convenience be described in conjunction with this type of mill. Thus, although the present invention has a broader application, it is preferred to use it in connection with mills of the type described in my said prior applications, in which mills the diameter of the drum is at least twice the length thereof.

The type of mill to which this invention has particular application generally comprises a drum arranged for rotation in a substantially vertical plane, said drum having two end walls and a cylindrical wall, which latter is provided with a plurality of highly upstanding circumferentially spaced apart crusher bars mounted around the interior thereof. Inlet and outlet ports for the charging of feed material and withdrawing of comminuted material are provided in false trunnions about which the drum is rotated. Feed is generally fed to the mill through the inlet port by gravity and the mill is kept clean by a current of air which is drawn across the mill through the inlet port and out the outlet port, which current of air on passing through the mill entrains the finely divided product of the mill and carries it through the outlet port where it is passed through a classifier which separates the heavier oversize particles from the stream, the oversize being allowed to flow back into the mill through the outlet port. The above type of mill is becoming well known and is used in various embodiments fairly widely throughout the art. For convenience of reference such type of mill will hereinafter be referred to as a mill of the type described.

In my said prior application Serial No. 234,782 filed July 2, 1951, there is described and claimed a method of operating a mill of the type described which comprises; maintaining within the drum a ball charge consisting essentially of large balls occupying at least about 0.35% but not more than about 3% of the total mill volume exclusive of voids; maintaining the total charge in the mill at from about 20% to about 32% of the mill volume where R. is the internal radius of the drum in feed between opposing crusher bar faces. It is the speed at which a particle of material of 3" diameter or smaller within the mill will be carried completely around the mill without falling away from the face of the crusher bars, or for all intents and purposes from the periphery, the centrifugal force at speeds greater than the critical speed being sufiicient to hold these particles firmly against the cylindrical wall of the drum at all times.

When operating at a range of speeds as above set forth, the ball charge of the present invention produces a greatl enhanced crushing action within the mill which results in a considerable increase in capacity in some instances and in other instances in improved metallurgical results. In addition, however, I have found that the ball charge of the present invention may be used in a mill of the type described operated within other ranges of speed to produce improved results of considerable value in connection with the milling of certain types of material.

The ball charge of the invention which consists of at least ten balls which occupy at least about 0.35% but not more than about 3% of the tota mill volume exclusive of voids and consisting essentially of balls weighing more than four pounds and having a diameter of more than about three inches (preferably about five inches) will produce an entirely different type of action within the mill at speeds of from say about 40-60% of critical speed than are produced if the mill is operated within the range of from about 84-90% of critical speed in accordance with the method claimed in the parent application.

The invention and the theory upon which it is based will be described in greater detail in the following specification in which reference is had to the accompanying drawings, in which:

Figure 1 is a vertical cross-section of a charged drum in a mill of the type described to which feed is being supplied, which will be followed by balls of varying diameters when the drum is rotated at a speed within the range of from about 84% to 90% of critical;

Figure 2 is a cross-section similar to that shown in Figure 1 illustrating the path balls of a ball charge according to the invention when the mill is operated according to the method of the present invention;

Figure 3 is a further cross-section similar to that shown in Figure l but illustrating what takes place when the total charge within the drum is permitted to exceed the maximum according to the method of the present invention;

Figure 4 is a graph showing the relation between critical speed and capacity obtained when using a ball charge according to the present invention on sillimanite-corundum; and

Figure 5 is a graph showing variation in size range of product in relation to mill speed on the same base as the graph of Figure 4.

Referring now more particularly to the drawings, it will be seen from Figure 1 that generally speaking the smaller the diameter of the balls in the ball charge, the further they will be carried upwardly by the drum before the force of illustrating the paths followed by the eter of about 5", all of the centrifugal force which is holding them against the periphery thereof. The reason for this is that the centre of gravity of a smal ball will be closer to the cylindrical wall of the drum than will be the centre of gravity of a large ball and, accordingly, the moment arm of the small balls will be greater and centrifugal force holding them to the periphery of the drum will be greater. Accordingly, the interior of the drum may be theoretically divided into zones; there will be the cataracting zone in which balls of, for instance, ,4 to 11" in diameter, will be falling freely from point near the top of the drum to strike the descending side of the cylindrical wall of the drum near the bottom with free fall impact; there will be a cascading zone where balls of 3" diameter and larger will be rolling down or cascading over the top of the charge eventually to move vertically towards the periphery of the drum near the bottom thereof; and there will be an abrasion zone within the charge near the upwardly moving side of the cylindrical wall of the drum.

If the mill is operated at between 84% to of the critical speed with total charge of 20% to 32% of the mill volume including voids, the charge itself will assume a position against the upwardly moving side of the cylindrical wall of the drum with the toe of the charge being situated approximately astride a line drawn verti cally through the axis of the drum, with a false toe of feed material extending slightly past said line. It will be seen moreover, that the path followed by the balls of generally 3" in diameter and over will as they cascade down through the cascade zone cause them to approach the cylindrical wall of the drum substantially vertically substantially between the true toe and the false toe of the charge.

When the mill is operating in accordance with the method claimed in the aforesaid parent application and the ball charge (in accordance with the present invention) consists essentially of balls of relatively large diameter, preferably of a diamthe balls in the charge will follow the same general path which will be generally curved, substantially as illustrated in Figure 2. In fact, it might be considered that the ball charge behaves as if groups of the balls were strung together in a number of chains lying parallel to each other along the length of the mill and continuously rotating in the direction of the arrows around the path illustrated in Figure 2. It will be seen, moreover, from Figure 2 that the balls descend into the toe of the charge at a point adjacent the bottom of the mill at which point the radial centrifugal force produced by rot-tion of the mill and the force of gravity are acting together. Each ball thus entering the toe of the charge will, accordingly, have a high degree of inertia. As each crusher bar enters the false toe of the charge, the material in the toe between the crusher bar and a ball of the ball charge which is entering the toe in the manner aforesaid will be crushed between the crusher bar and the ball. The crushing action is similar to the action of a jaw crusher and the crusher bar may be considered the moving jaw and the ball of the ball charge may be considered to be the stationary jaw During periods when no feed is being supplied to the mill, and there is therefore no false toe, the crushing action results from the balls being driven into the true toe of the charge .by the crusher bars.

gravity overcomes ameter of about 3" tremendous proportions.

In accordance with the foregoing, it will be appreciated that theoretically there should be a suihcient number of balls present in the charge to form a series of chains around which the balls are spaced with suilici nt density that series of balls are arriving in the toe of the charge at about the same rate as crusher bars are entering the toe of the charge. It will further be apof the balls as short as possible. This can be accomplished very simply by arranging to have the charge consist essentially of large balls. the balls become reduced in size due to wear during operation of the mill, they will progressively travel further and further in their generally curved course and it is desirable at intervals to eliminate from the mill balls of a diameter of less than about 3". It may, however, in some instances be desirable to retain a small number of balls of under one inch in diameter because of the work which may be done by these balls in free fall impact on the particles of charge which are also falling through the cataracting zone of the mill.

Although the specific gravity of the balls apparently has no appreciable effect upon the path that a ball of any given diameter will follow during operation of the mill, it will, of course, be appreciated that the efiectiveness of a ball as an inertia body in the crushing zone will be increased. if the ball is made heavier. It is, accordingly, preferred according to the invention to form the balls of the ball charge from a material having a high specific gravity. Tungsten carbide compositions, which have a specific gravity of approximately 14 and are immensely hard, are ideal substances from which to form ball charges according to the invention. nately, due to material shortages and high cost, tungsten carbide is not always available at a cost which would enable its economical use and, consequently, the balls will generally be formed from alloy steel of one sort or another.

Generally speaking, any ball which has a dior more and a weight of about four pounds or more will be useful in a cordance with the invention as an inertia body and, accordingly, in carrying out the method of the invention it is essential to ensure that the charge consist essentially of balls meeting these It has been found, moreover, that .a minimum of ten balls is generally required to assure the formation of the so-called chains and effectively produce a continual crushing action of the character described above. In mills of from five to nine feet in diameter, I have found that about five inches in diameter is a practical.

maximum limit for the size of individual balls making up the charge. In larger mills, larger balls may be used. It should be remembered, however, that as the size of the mill increases .the crushing force exerted by each ball also increases and the forces exerted by the balls reach A six inch iron ball which would weight about 32 pounds in an 18- foot mill where the peripheral speed would be about 14 feet per second,

when operating within the cascade zone do not the range of speeds prescribed according to the invention, would place a force on the crusher bars capable of exceeding the permissible stress material from which these are made, unless special precautions are taken to increase their strength. There does not appear, however, in

dicated by the results of the examples hereinfater set forth.

The above-described principle of operation involves three critical factors. First of all, the mill must be rotated at a speed of from 84% to 90% of the critical speed in order The third critical factor in the method of the present in- It has been found that the drops off markedly if the total volume of charge exceeds 32%. The reason for this is apparent from a consideration of Figure 3. If the total volume of charge becomes too large then, even at the high speed of operation according to the pres ent invention, the toe of the charge extends over past the vertical line drawn through the axis of rotation of the drum in the manner illustrated in Figure 3 and balls which are cascading through approach the periphery of the drum in a substantially vertical direction and, consequently, are incapable of with the crusher bars to act as efiicient inertia bodies in the manner set forth. Generally speal 2- eificiency of the mill vention the charge volume including voids is diameter. The data in the following examples is illustrative of the application the reduction of three general namely friable material, medium-tough material and tough material. The results achieved illustrate the advantages of the invention and demonstrate the increase in effectiveness of the crushing action obtained when operating in accordance with the invention. In the case of friable material, for instance, which is very readily "broken down, as would be expected, the use of the invention does not increase the capacity of the mill. In that case the advantages derived arise as a result oi the quick initial breakdown of the feed material in the crushing zone permitting more work to be done on the individual broken down particles in the abrasion zone to produce a final product within the desired range of particle size which has improved metallurgical characteristics. As would be expected, however, in the reduction of medium-tough material and tough material, the increase in efiiciency of the crushing action obtained results in a pronounced increase in the capacity of the mill.

EXAMPLE 1 Friable material The following table of results was compiled from the operation of the pilot mill using as feed loosely bonded standstone:

. Screen Am i gg M111 ialysis Prod- Chemical Analysis Nora-Percent volume of mill occupied by ball charge 1.0%.

In this case the feed material being loosely bonded sandstone, is very friable and it is accordingly to be noted that there is no overall increase in capacity brought about by the use of a ball charge in accordance with the invention. a metallurgical point oi view, however, it is noteworthy that there is a considerable reduction of iron oxide content of the product of requisite grain size representing in all about :1. 28 92; improvement. In the material treated. the iron oxid occurred chiefly as a finely bonded coating to the silica particles. In breaking down the feed material. on :kly by means of the enhanced crushing action using a ball charge according to the invention the abrasion action in the abrasion zone is permitted to work on the individual particles for a longer length of time and the result, as exemplified above, is that a cleaner product is produced. The sandstone used as a feed in this example was intended for use in the manufacture of glass and, as is well known, the presence of iron oxide in glass sand is exceedingly deleterious and each 0.61% of iron oxide in a product lowers the grade oi the glass sand and materially increases the cost of purification in the subsequent glass manufacturing process.

EXAMPLE 2 M radium-tough material A medium-tough quartzite was used as feed From material to the pilot mill, giving the following results:

Conditions of Pilot Mill Run Screen Analysis Max. Feed lllanometer Product.

Ball Charge l Rate lbs.l Reading, 20+100 hr. ins. None 636 9. 4 56. 5 12 5" Dia. Steel Balls 340 l 9.4 51.5

Nora-Percent of mill volume occupied by inertia media charge Gold-hearing porphyry ore was used as feed to the pilot mill. and the following results were obtained:

Percent l Screen 82 6 3 Mar. Msnom- Occupie-d 'bylnertia Ball Charge Feed Rate cter N lbs/hr. Reading None 440 7. 0 39. 3 0. 00 10 5 De. Steel Balls,

Total Weight Risk. 850 7.0 31. 9 0.83 45 3 Dia. Steel Balls,

Total Weight 180 lbs. 720 7. 0 26. i] 0. 82 12 3.7" Die. Tungsten Carbide Balls, Total wt. 168 lbs 985 7. 0 43. 0 0. 41

Nora: Lbs. Weight oi 5-inch dia. steel ball 18.

This material is considered as one of the toughest materials to crush and grind. It is noteworthy that the use of inertia bodies according to the invention does not necessarily result in a greater proportion of the product being within the 200 screen analysis size range. The reason for this is that, using no hall charge the crushing action obtained has a low eiliciency so that a large amount of grinding takes place by abrasion between comparatively large solid surfaces. This accounts for the fineness of grind obtained without the use of a call charge.

The great increase in the crushing efficiency brought about by the presence of the ball charge according to the invention quickly reduces the large size particles of feed and produces a much greater range of particle sizes within the range where the air stream will remove them, the result being a much higher overall capacity with a smaller percentage of the product being within the -2l0 screen analysis size range. Measured in terms of surface area produced per unit of time the increase in the capacity is remarkable.

It will be noted, moreover, in comparing the use of 5 inch steel balls with the use of 3 inch steel balls, that the 5 inch balls produce a better result both as regards capacity of the mill and as regards screen analysis of the products.

lurgical result, the improvement in this respect depending upon the nature of the particular material being reduced and the characteristics of the product considered desirable in the subsequent treatment of such material.

If a mill of the type described is operated at the relatively low rate or speed of from about 40 to about 65% of critical speed, a situation occurs within the mill which involves the charge being successively carried around to a point on the upwardly moving side of the drum at which sudden slippage occurs causing the charge to fall back to a position in the bottom of the drum. This action repeats itself continuously at intervals of from one to about five seconds depending upon the diameter of the mill drum. If a ball charge in accordance with the present invention is present in the charge, the individual balls, acting as inertia bodies, produce a shattering effect as they meet the oncoming crusher bars as the charge slips back towards the bottom of the mill. The result with some materials is a greatly increased capacity in the coarse size range in the product produced and an appreciable increase in overall capacity or the mill compared to operation in accordance with the method claimed in the parent application. In the reduction of certain materials where a relatively coarse product and a relatively fine product is desired with a minimum of intermediate sizes (1. e. a typical differential grind problem) operation of a mill of the type described containing a ball charge according to the invention is capable in many cases of producing very valuable results. This will be apparent from the following example where a greatly increased proportion of the product was obtained in the +28 4 mesh size range and an overall increase in capacity was obtained at the expense of the middle size ranges.

EXAMPLE i Sillimanite-corundum from surface deposits in South West Africa was used as a feed material. This material was desired in a final product in two size ranges, firstly a size range of approximately +28 mesh, and secondly as a fine product of 100% l mesh, the idea being to produce approximately 50% of the product in each of the above-mentioned size ranges while at the same time producing as little as possible in the immediate range to reduce the amount of regrinding necessary.

A mill of the type described having nominal dimensions of in diameter and 2' in length was charged with about eight hundred pounds of the feed material together with ten 5" manganese steel balls as inertia bodies. The mill was run at a speed of about 85% of critical speed until generally balanced conditions between the feed and products had been reached, the load in the mill being maintained at approximately 27% of total mill volume inclusive of voids. The mill was then run successively at a number of lower speeds and the following results were tabulated:

- operation can be Screen Analysis of Cyclone Product Mill Speed, Percent of g fiyj fgg g g per Hour Percent Percent jf fi fgg pee +4 mesh +28 mesh mesh mesh 313 2. 8 l6. 2 l7. 5 63. 4 312 2. 8 18. 3 22. O 56. 9 351 2. 9 21. 3 24. 8 51. 0 387 4. 8 33. l 20. 5 41. 9

The aboveresults are tabulated in Figures 5 and 4, Figure 5 illustrating the increase in perspeed of the mill is varied and Figure 4 illustrating the increase in overall capacity as the speed of the mill is reduced. It will be observed that in this case at about 53% is an optimum grind having regard to capacity and coarseness of the product. At this point, slightly over 40% was in the -'-l +28 mesh range of the +4 mesh for regrind. At there is approximately 40% of the product in the mesh range. At the same time, the capacity of the mill is increased from about 313 pounds per hour to 377 pounds per hour.

Examination of the products produced leads to the conclusion that operating at these slower speeds with a ball charge according to the present invention greatly increases the impact crushing factor while practically eliminating the grindthe same time,

Furthermore, it will be observed from a com parison of the results achieved at higher speeds tribution in the product.

It is obvious, of course, that the shape of the capacity curve will vary widely with the physical the feed material. With matewhich is of relatively uniform hardness and in particular with very hard, tough materials, a distinct peak is obtained in the capacity curve within a range of critical speeds of from about 84-90% of critical over which range of speeds the action of the ball charge of the invention is as previously described and claimed in the parent application, while on the other hand with materials such as sand stone which break down rapidly, the capacity curve will tend to be rather flat as would be expected having regard to the results indicated in Example 1, and in general it has been found that the most satisfactory achieved within the 84-90% of critical speed range. It is desired to point out, however, that the ball charge of the present invention in combination with a mill of the type described can also be most useful in the solution of differential grinding problems on particular materials in which type of operation the action of the ball charge is very different than it is at the higher operating speeds.

It will be appreciated, therefore, that the combination of a ball charge as hereinbefore de- 11 fined with a mill of the type described afiords in itself a new means for the reduction of materials capable of affording a wide variety of useful results under a wide variety of operating conditions.

While the above tests were conducted using a ball charge consisting of balls which were essentially spherical in shape, these being the preferred form of reduction media, it will be appreciated that the present invention contemplates the use of any of the well known shapes of reduc tion media and accordingly it is to be understood that Whenever I refer herein to a bal or a0 a ball charge, I intend to include any of the usual reduction media.

What I claim as my invention is:

1. In combination with a combined dry crushing and grinding material reduction mill of the type comprising a drum having a diameter which is at least twice the length thereof arranged for rotation about a horizontal axis and having a cylindrical wall and two end walls, said cylindrical wall having mounted thereon a plurality of circumferentially spaced apart highly upstanding crusher bars, a ball charge consisting, during normal operation of the mill, of at least ten balls and occupying at least about 0.35 but not more than about 3% of the mill volume exclusive of voids, said ball charg consisting essentially of balls weighing more than four pounds, and having a diameter of more than about 3 inches.

2. The combination defined in claim in which the ball charge consists essentially of balls of about 5" diameter.

No references cited.

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