US 2678168 A
Description (OCR text may contain errors)
May 11. 1954 E. D. PHILLIPS FLUID SWEPT BALL MILL WITH BALL MOVING ROTOR AND STATIONARY DRUM 2 Sheets-Sheet 1 Filed Feb. 4, 1952 SOUND 2 PROOFING 6 9 MATERIAL INVENTOR EVERETT' D. PHILLIPS BY I ATTORNEY May 11. 1 E. D. PHILLIPS FLUID SWEPT BALL. MILL WITH BALL MOVING ROTOR AND STATIONARY DRUM 2 Sheets-Sheet 2 Filed Feb. 4, 1952 SOUND PROOFING MATERIAL FIG.
INVENTQR EVERETT D. PHILLIPS BY ATTQRNEY Patented May 11, 1954 FLUID sWEPT BALL MILL WITH BALL Mov- ING ROTOR AND STATIONARY DRUM Everett D. Phillips, Danville, Pa.; Rubye W. Phillips, executrix of said Everett D. Phillips,
deceased Application February 4, 1952, Serial N0. 269,820
. 1 My present invention broadly relates to means, primarily mechanical, for reducing, even to superfine form, certainfragmentary materials such as coals (including extremely difiicult to reduce anthracite), cement clinkers, iron pyrites, coke breezes, petroleum cokes, pigments, grains and many other vegetable matters. This application is a continuation-in-part of my co-pending application entitled Fluid Swept Ball Mill With Ball Moving Rotor and Stationary Drum, Serial No. 123,245, filed October 24, 1949, now Patent 2,621,- 859 of December 16, 1952, which application, in turn, is a continuation-in-part of my abandoned application entitled Ball Mill, Serial No. 531,951, filed April 20, 1944.
An important object of my invention is to so design the particular arrangement of mechanical means that it is readily adaptable to having associated therewith an active fluid medium by which the reduced material can be most effectively collected and continuously removed from the region of the reducing means. I
I am fully aware of the fact that numerous efforts have been made to create and design means having the same objectives as my invention. These designs have produced only moderate degrees of success. The generalobjective of my invention is to materially expand the success obtainable over anything heretofore accomplished in theparticular field. In this connection, I cite in particular a number of fairly Wellknown so-called. drum and ball or ball mill commercial products, also commonly termed pulverizer." Each of these products, I know from first hand experience, have been criticized by commercial users and condemned by competition because of their relatively large power and space requirements. There are several major deficiencies contributing to the present inefficiency of the drum and ball type pulverizers. I
First, and perhaps the greatest deficiency is the inability of these mills to separate the finely ground materials from the almost solid mass of material and balls rotating in the revolving drum before the reduction of these materials to a superfine product. This lack of separating capacity partially explains the relatively large power re-. quirements of such mills. ,This excessive power requirement makes it difllcult for this type of mill to successfully compete against high speed impact or hammer mill pulverizers, ball race type pulverizers, roller bearing mill type pulverizers and others whose power requirements are relatively. lower. but producinga coarsely ground product. I
2 Second, an excessive amount of energy is expended in rotating the drum or its equivalent element beyond that which is actually expended in reducing the material.
Third, all of these methods and machines employ excessive weights of machinery and occupy excessive mounting spaces with consequential high initial costs and inconveniences. Also, they require a large amount of plant space for dismantling and repairing.
All of the various types of pulverizers, both separately and as a group, because of the lack of control of the quality of their product, complicated operation, high power costs and over-all inefficiency are being displaced commercially by spreader stokers, particularly in the small and medium size installations. Until my invention, nothing was being done to eliminate these deficiencies of the drum and ball type mill, despite both the possibility and necessity for such improvement.
. In the conventional drum and ball mill pulverizer, movement of the balls is dependent solely upon the balls being dragged into and maintained in action by the slight frictional and point contact existing between the balls and the wall of the rotating drum. This frictional contact is limited to those few of the balls that manage to make individual respective contact with the inner sur-j face of the rotating drums. The total area of frictional contact is but. a minute fraction of either the balls or the inner surface area of the drum. Thus, these conventional ball mills must be operated over a longer period of time to effect r the same amount of reduction as will occur in a of the necessity for moving the mass and weight Thus,.
of the drum or grinding chamber shell. although the power. consumption, per revolution, of my invention and of ball mills of conventional design are approximately equal, appreciable efliciencyis obtained by my invention since the total reducing action obtained by each revolution of my invention is several times that of conventional ball mills.
Further, since the balls in my inven-, tion are positively pushed and lifted into cascade:
ing position rather than raised into cascading position by a combination of frictional and centrifugal forces, my mill may be efficiently operated at less revolutions per minute. This speed reduction permits a power source of lesser horse power to apply thesame torque to the shaft driving the mill. These are but two of the ways in which my mill obtains a substantial increase in efficiency over ball mills of conventional design.
Another excessively expensive, operational handicap in these prior products arises out of the Weight distribution opposing rotation. The weight is normally concentrated at an unnecessarily excessive radial distance from the center of rotation without provision of any counterbalance means, thereby creating an excessive, counter to rotation, torque plus a decidedly lop-sided wear and tear on the supporting journals and bearings. These objectionable factors are also overcome by the instant invention. By rotating only the rotor within the drum rather than the drum itself, the moment arm of the operating load is reduced, thus, reducing the torque necessary to operate the mill.
Another object of the instant invention is that of materially facilitating the making of repairs andthe replacement of parts.
A further object of my invention is th elimination of certain of the operational hazards common to ball mills of conventional design. These hazards and their elimination are specifically brought out in connection with the description of the structure relating to them.
Other important objects of the instant invention will become readily apparent to those skilled in the particular'art as its description in connection with the figures of the accompanying drawings is hereinafter carried out, which figures may be briefly identified as follows;
Figure l is a mid portion, cross-sectional side View elevation of the instant invention.
Figure 2 is a cross-sectional view along the line II--II of Figure .1.
'Like reference symbols in the same figur and in the various figures of the drawings represent like elements and parts in the same. Referring first to Figure l, numeral I indicates a horizontally mounted, preferably metallic, cylinder,
stub shafts I5 and I5 leaves the major part of the internal, horizontal housing space unobstructed. The purpose of leaving the area between the spider leg elements I3 unobstructed will appear more fully hereinafter.
To each of the spider legs I3 (four being shown in Figure 2) is afiixed, by bolts M, a radially extending plate element I2. Extending between V the plate lements I2 of one spider element I3 are pusher bars or partitions II. The ends of each of the partitions II are bent at a right angle to the main body of the partition by which affixation to the respective plates I2 may be accomplished by installation of the bolts 24. Although Figure 1 indicates bolted construction, they may 7 bewelded. Thus,when shafts I5 and I5,'or either,
partially closed at eachend by preferably metallic, ellipsoidal heads 2, bolted, as indicated,
r partially closed at each end by ellipsoidal heads Like,
8 bolted or welded to cylindrical liner I. the heads 2, the heads 8 may vary in shape somewhat from that of an ellipsoid without departing from the principle of my invention. The second cylinder together with the heads 3 define the grinding chamber which is also stationary and. cannot rotate; The elements I5 and I5 indicate cylindrical stub shafts extending from outside'internally of the said cylinders through close fitting'openin gs centrally located in the I respective heads 2 and 8; Each of thestub shafts I5 and I5 is supported in and by one of:
are rotated, the elements I3 being aifixed thereto, transmit the rotary motion tothe hereinafter described balls I9 through the plates I2 and partitions iI. The plane of each of the partitions II is parallel to the axis of the stub shafts l5 and It. The plate elements, spider leg elements I3, and partitions I I together form a cage or rotor occupying a .major portion of the internal area of the second cylinder 1.. Th ends of the rotor may each be made. as a single part, but such part should have openings or apertures to permit material to pass intothe rotor. The rotor operates as an agitator for actuating the grinding balls. 7
Referring to Figure 1, the inner wall of the inner cylinder 1 is shown to be constructed as to hav a longitudinal series of closely spaced channels or grooves 23 therein each of which completely circles the'said inner wall. The grooves are so shaped as to substantially fit that part of the arc of each of those ones of the larger balls I9 shown to be positioned therein. Referring to Figure 2, the larger balls I9 are also'shown to V be positioned in what may be termed pockets formed by th radially positioned pusher bars or partitions II, Il H H H H II, II", H H and Il which pockets so taper, radially inwardly, that the balls I9 cannot escape therefrom by way of'the inner opening in each pocket. The balls in each pocket are as closely spaced as their size will permit.
In order to take advantage of the full reduction potential of my modified ball mill, a charge of smaller size balls indicated in part by the symbol I9, is used to fill what would otherwise be an idle, potential reduction area within the rotor.
The -material to be reduced in size or pulverized is introduced into the grinding chamber through the passage 20 indicated as entering into and at the right-hand portion of the upper portion of the grinding. chamber. The fines produced in the grinding chamber are removed through thepassage 22 at the opposite end'of the grinding chamber from the passage 20; Al-- though the passages 20" and '22 are shown and described as at the extreme ends of the grinding chamber, a preferred'arrangement; it is possible to relocate them closer togetherwithout departing from the principle of my invention.
As a result of the description of the mechanism up to this point, th reducing operation maybe stated by way of illustration as follows: as is mechanically apparent, any suitable source of driving power, such as an electric motor, may be used to drive the rotor. Power may be delivered to the rotor by either or both of the stub shafts l and 15 Rotation of the stub shafts causes similar mo tion in the rotor forcing the partitions l l around the grinding chamber. The movement of the partitions l l urges the balls 19 along in their respective grooves until cascading of the balls it takes place due to gravity overcoming their centrifugal force. The cascading action of the balls I9 is limited by the partitions I l since the inward taper of the pockets between the partitions prevents inward escape of the balls [9. The taper of the pockets also acts to increase the pressure applied by the balls to the materiallodged in the grooves. This accelerates the reduction of the material. The rotation of the rotor or cage elevates smaller balls I9. The smaller balls l9 are raised by the rotor, until cascading of these balls takes place due to gravity overcoming their centrifugal force.
The larger balls l9, being exposed as they are to the activities of the smaller balls l9, receive from them supplementary power to act as crushers.. The pockets between the partitions H act as channels for. introducing the material to the grooves 23 after it has been subjected to prelim,- inary reduction by the action of the smaller balls 19. The material, once having entered the grooves 23 may be ground to a fine powder by the action of thelarger balls [9. The pockets also serve effectively as elevators for raising the reduced material into the upper portion of the grinding chamber where the fines will be entrained by the sweeping fluid for removal.
The inner shell 1 and outer shell 2, comprising the cylindrical drum remain stationary at all times, thus, all of the motion for generating the grinding or reducing action of my invention is imparted to the grinding balls 19 and I9 by the rotor. This design limits the energy requirements of my invention to that which is necessary to move the rotor against the resistance of the grinding balls and the charge of material being reduced. The energy requirements of previous devices of this type for moving the mass ofthe drum are eliminated. a
As is readily apparent, the movement of the partitions II, the preferred direction of which is clockwise as viewed in Figure 2, will forcibly maintain all of the balls in action with respect to each other and all other surfaces contacted by these balls. The rotor is operated at a relatively low angular velocity. The exact revolutions per minute will vary according to that which will produce the most efficient results with the particular material and the size of the charge of grinding balls involved.
The balls 19 will be carried into the upper region of the grinding chamber from which point the raised grinding balls l9 will cascade downwardly with considerable impact momentum. The height to which these balls [8' will be elevated will depend, in part, upon the angular. velocity of the rotor. By eliminating the central shaft through the rotor, the cascading balls are enabled to fall upon the material without interference. Thus, all of the velocity obtained by the falling balls is available for reducing .the
material. This feature alone adds materially to the efficiency of my invention.
All of the principles of reducing or pulverizing utilized in isolated cases, namely, crushing, impact and attrition, not heretofore combined in a single unit, are simultaneously and most effectively brought to bear in the instant invention. By way of explanation: reduction by crushing occurs between those of the balls adjacent the walls of the grinding chamberbeing forced to roll on surfaces with the material being crushed between the moving balls and the stationary wall surface; reduction by impact is effected by the cascading balls landing with appreciable impact on fragments of the material; and reduction by1 attrition occurs through the action of any two or more moving balls competing with each otherin forcefully rubbing the same material.
Rotation of the partitions ll not only elevates the balls l9 and I9 but also both the heavies and fines of the material being reduced. A stream of sweeping or transport fluid is passed through the grinding chamber by means of the passages 2 i and 22. The fines of the material elevated by the rotor will be entrained by this fluid and removed from the grinding chamber through passage 22. The size of the fines which will be thus removed may be controlled by regulating the volume of fluid being passed through the grinding chamber. The fluid may be either forced through or sucked through the grinding chamber. By placing the input passage 21 and output passage 22 at the extreme opposite ends of the grinding chamber, sweeping of the entire grinding chamber by the fluid is assured.
As to refinements and special features, reference is made to Figures 1 and 2 jointly.
To facilitate assembly, making repairs and replacing parts, the lower halves of cylinders l and. I, together with their corresponding heads 2 and 8, are indicated to be separate in construction from their respective upper halves. As more fully shown in Figure 2, the upper halves of these same cylinders and their respective heads are indicated to each be constructed in two equal quarter sections. The flanges 3 3 and 3 t0 gether with the bolts and nuts 3& 3& and 35 respectively, are the means necessary. for properly assembled external and internal containers and for securing them in place during operations. 7
Since it is also elementary that the wear and tear on the inner surface of the lower half of the cylinder 1 will likewise be severe, even though its material may be the most durable available and it is protected in part by the layer of balls contacting it, it stands to reason that the cost of renewing this part is greatly minimized by constructing it in the form of a separate, replaceable unit.
Of course, it is all important that the balls 19 and I9 should also be made of the most durable material obtainable. The additional expense of such material will be more than recovered by the savings effected in replacement, time and labor.
By eliminating the necessity for rotating the entire cylinder portion or the drum of the mechanism, it is possible to utilize insulation plus the inner and outer cylinder construction gasses by filling of the. space intervening between the inner and outer cylinders, as indicated by the hatchings of this particular space in the drawings,-with anyone of the many sound proofing materials now available. Because of the high temperature prolonged reducing can create, which may be further increased by using a preheated gaseous medium for sweeping the grinding' chamber, a sound proofing material composed primarily of asbestos would be among those preferable insuch a case. 7
'Where the matter ofrotating a heavy cylinder and its contents is involved, as it is in all of the V drumand ballreducing-mi1ls with which I am acquainted; the matter of wear of the journals and bearings becomesa very' serious problem due tothe'excessive weight supported by these elements. The problem of rapid wear is aggravated by-the unbalanced torque loads generated by the mill, plus the exposure of the journals and bearings to the material being reducedand the frequently preheated, 'material saturated, pressurized, fluid for sweeping the grinding chamber. The arrangementof Figure 1 indicates thatno such aggravated journal and bearing problem exists inthe case of the instant invention. In my invention the cylinder or drum is stationary, thus-relieving the journals and bearings of this 'weight. The torque radius'is reduced, and the design offsets, at least partially, the load attendant-lifting the cascading balls by the weight of the balls actingdownwardly on those of the pusher bars moving downwardly through the body of grinding balls in the lower portion of the grinding chamber; Furthermore, my design permits the journals and bearings to be completely isolated-from'the operating area of my mill. Since the drum or cylinder is stationary, the stub shafts l5 and I5 may pass through the drum or cylinders without contact with any parts other thana dust and-pressure seal. rhe bearings -may be -mounted outside the drum and sealedfrom the wear and corrosion promoting materials within the drum.
All of the drum andballf mechanisms, with which I am acquainted,introduce'the collection and removal fluid into the grinding chamber through an opening-in the center of one end head and remove the fluid through a similar opening opening in the center of the other head. To avoid having balls enter these openings, the total number of grinding balls is chosen-with a margin of safety to have a top level somewhat lower than the lowest points of'these openings. Because of the circular nature of the conventional grinding chamber this necessarilylimits the number of grinding balls to somewhat less than one-fourth of the number of balls the grinding chamber could hold if completely filled. The design of my invention makes feasible using enough balls to fill at least one-half of the available cylinder volume. Although such a quantity of grinding balls-is frequently neither necessary nor desirable, the capacity is available when needed. Conventional ball mills are incapable'of accompish ing this. I
an ineiiicient 'reducing' action results as comi will automaticallyserve to accelerate reduction of the softer materials.
From the foregoing, in addition to it being readily apparent that the instant invention permits substantial power savings and a substantial increase in operatingefiioiency over conventional drum and ball mechanisms, it is equally apparent that substantial savings in weight of materials and mounting spaces are effected. When it comes to efiiciency in the matter of collection and removal of the-reduced portion of the material'involved, it is clear'that the action of the partitions H, of the instant invention, serves to expose all of the material charge to the grinding chamber sweeping fiuidi In conven tion ball mills much of the material charge remains submerged in the ball charge. Thusefliciency, inthis respect, is necessarily greatly in V favor of the instant invention.
Inconventional drum and ball products, as well as in the instant invention, simplification of design calls for introducing the material to be reduced at one end of the cylinder or-grind-. ing chamberp Since operating speed or efficiency also depend, in part, upon having the material undergoing reduction equally distributed over the length of the reducing region, which region beexplosion attendant on too small a charge, when 7 When it comes to reducing the harder ma terials that offer greater resistance thereto, as-in the case of anthracite coals as compared to bituminous coals, it is elementary thatmore can be accomplished with such materials if greater and the driving force is-employed a's is the case in conventional drum and ball -n1edhani'sms,
the material charge is a combustible material, such as coal, is reduced by my'invention since it permits, as a normal operatingcondition, a
Thinking in terms of long lives for the parts directly involved in the various reducing activi-. ties, it is important to use the most durable, suitable materials obtainable; for these parts. Thus, the fact that the hatching used in the drawings with respect to these parts is that of the standard indication for steel (without qualification) is not to be taken as an intended limitation to steel per se.
The life of the grinding chamber liner or inner 7 the safety and efiieiency of my invention. These heads are so shaped that the charge of free or cascading balls, balls i9 and i9, cannot become jammed between the heads andrthe rotor; The heads, as they extend'outwardly from. the straight portion of the grinding chamber, are designed to curve on a radius at least equal to the radius By eliminating all sharp corners and permitting thegrinding balls freedom of travel throughout the entire area of the grinding chamber the formation of pockets of material is avoided. These pockets, when the material is combustible, create a serious explosion hazard and their elimination is an appreciable improvement in safety.
My invention has heretofore been described as primarily used for grinding dry, solid fuel. Because of its positive and continuous agitation of both the material being rocessed and the balls and its adaptability to operation with a pressurized sweeping fluid, it is capable of handling fuels containing high percentages of total moisture. The construction of existing mills of this type has prevented their use with high moisture content materials. For fuels containing inherent moisture only, preheated sweeping fluids are not required for satisfactory operation of my pulverizer. Since a high percentage of surface moisture is acquired in the transportation of solid fuels between the mines and their ultimate. point of use, it is necessary to preheat the sweeping fluid to a high enough temperature to vaporize the moisture and dry the fuel as it is pulverized. The design of my invention, permitting the grinding chamber to be insulated and the bearings to be isolated from the grinding chamber, ideally adapts my invention to the use of a heated sweeping medium.
The pulverizer mill described has successfully pulverized and delivered to a burner coals having total moisture contentsbetween 7 and 12 percent without employing a preheated sweepingfluid. Although this type, of operation is not desirable from the standpoint of furnace efficiency, it illustrates the capacity of my mill.
My invention will operate with either the dry or wet process and for that reason is readily adaptable and suitable forefilciently grinding and dispersing other materials requiring the wet process such as pigments, for example.
Since my invention, by its design and construction, is suitable tofabrication from steel plate rather than the conventional cast iron castings, an expensive type of fabrication at best, it is structurally suitable for safe operation under pressures of 160 pounds per square inch or more.
The recent development of gas turbines, at present in the experimental stage, requires pulverizers capable of operation under pressures as high as 75 pounds per square inch. Gas turbines require pulverizers capable of continuous and consistent superfine pulverization at low cost. The circumstances of installation of many gas turbines, such as locomotives, make a compact pulverizer an absolute essential. Conventional pulverizer designs, whether or not they are of the drum and ball type, are incapable of meeting the specifications of gas turbines. My invention fulfills and meets these requirements.
Numerous modifications of my invention may be made Without departing from the principle of my invention. Each of these modifications is to be considered as included in the hereinafter appended claims unless these claims by their language expressly provide otherwise.
1. A ball mill, the combination comprising: a stationary, horizontal, cylindrical drum; heads closing each of the ends of said drum; a pair of end members having elongated pusher bars mounted therebetween for forming a rotatable, hollow, cylindrical cage within and extending substantially the full length of said cylindrical drum; said end members defining openings for the passage therethrough of material to be ground; a stud shaft mounted to each of said end members and rotatably carried by said closing means; the inner surface of said drum having a plurality of spaced channelsparallel to the direction of rotation of said cage and of segmental, arcuate cross-section; a plurality of balls seated within each of said channels and having a radius substantially equal to the cross-sectional radius of said channels; said balls aligned in rows parallel to the axis of said cage and each row thereof separated from each adjacent row by one of said pusher bars and held by said bars against radially inward movement relative to said cage; said drum. having a carrying fluid inlet at one end and a carrying fluid outlet at the other end whereby a fluid may fiow through said drum for removing ground material.
2. A ball mill, the combination comprising: a stationary, horizontal, cylindrical drum; heads closing each of the ends of said drum; a pair of perforate supporting ends having elongated pusher bars therebetween for forming a rotatable, hollow, cylindrical rotor within and extending substantially the full length of the cylindrical drum; a stud shaft mounted to each of said sup:
porting ends and rotatably carried by said close ing means; the inner surface of said drum having a plurality of spaced channels parallel to the direction of rotation of said rotor and of segmental, arcuate cross-section; a plurality of balls seated within each of said channels and having a radius substantially equal to the cross-sectional radius of said channels; said balls aligned in rows parallel to the axis of said rotor and each row thereof separated from each adjacent row by one of said pusher bars and held by said pusher bars against radially inward movement relative to said rotor; a plurality of grinding balls within said rotor for cascading movement therein; said drum having a carrying fluid inlet at one end and a carrying fluid outlet at the other end whereby a fluid may flow through said drum for removing ground material.
3. A ball mill, the combination comprising: a
, stationary, horizontal, cylindrical drum; heads closing each of the ends of said drum; a pair of perforate supporting ends having rigid, elongated pusher bars therebetween for forming a rotatable, hollow, cylindrical rotor within and extending substantially the full length of said cylindrical drum; said pusher bars being of rectangular cross-section and having their greater cross-sectional dimension extending radially of said rotor; a stud shaft mounted to each of said supporting ends and rotatably carried by said closing means; the inner surface of said drum having a plurality of spaced channels parallel to the direction of rotation of said rotor and of segmental, arcuate cross-section; a plurality of balls seated Within each of said channels and having a radius substantially equal to the crossseotional radius of said channels; said balls aligned in rows parallel to the axis of said rotor and each row thereof separated from each adjacent row by one of said pusher bars; said pusher bars at their radially innermost extremity spaced apart less than the diameter of said balls whereby said balls are held by said pusher bars against radially inward movement relative to said rotor; a plurality of grinding balls within said rotor for cascading movement therein; said drum having a carrying fluid inlet at one end and a carrying fluid outlet at the other end whereby a fluid may flow through said drum for removing ground material.
.4. A ball mill as described in claim 3 wherein said heads are convex and the inner surface of the drum is curved at the ends of the cylindrical surface on a radius of curvature greater than the radius of said balls whereby to avoid lodging of material to be crushed at that point.
5. A ball mill, the combination comprising: a stationary, horizontal, cylindrical drum; heads closingeach of the ends of said drum; a pair of end memberes having elongated pusher bars mounted therebetween for forming a rotatable; hollow rotor within and extending substantially the full length of said cylindrical drum; said end members defining openings for the passage therethrough of material tobe ground; a stud shaft mounted to each of said end members and rotatably carried by said closing means; the inner surface of said drum having a plurality of spaced channels parallel to the direction of rotation of said rotor and of segmental, arcuate cross sectioma plurality of balls seated within each of said channels and having a radius substantially equal to'the cross-sectional radius of said channels; said balls aligned in rows parallel to the axis of said rotor and each roW thereof separated from each adjacent row by one of said pusher bars and held by said pusher bars against radially inward movement relative to said rotor; conduits communicating with the interior of said drum whereby a carrying fluid may flow through said drum for removing ground material.
6. A ball mill comprising: a stationary, elongated, horizontal drum; heads for closing the ends of said drum, said drum and said heads defining a grinding chamber; a pair of stud shafts, one rotatablymounted in each of said headsconcentrically of said grinding chamber; a journal exterior of said drum for supporting each of said stud shafts: a rotor mounted onsaid stud shafts, including a plurality of pusher .bars, mounted within said grinding "chamber; power transmitting means on said stud shafts for driv ing said rotor; a plurality of channels, each of arcuate cross-section, in the inner wall of said drum and'surrounding said grinding chamber in a direction parallel to the rotation of said rotor; a first plurality of grinding balls, seated insaid channels arranged in rows parallel to said channels and parallel to said pusher bars; said channels, longitudinally of said drum, spaced apart the diameter of one of said balls whereby the balls in each of said rows parallel to said pusher bars are in contact with each other; a second plurality of grinding balls within said rotor.
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