US2362811A - Ball mill liner - Google Patents

Description

Nov. 14, T944.

L. A. FITZGERALD BALL MILL LINER Filed Jan. 29, 1942 4 Sheets-Sheet l NOV. 14, 1944. A FlTzGERALD 2,362,811

BALL .MILL 'LINER Filed Jan. 29, 1942 4 Sheets-Sheet 2 mw ww ,vm ww NN muy .N

BALL MILL LINER Filed Jan. 29, 1942 4 Sheets-Sheet 3 Nov. 14, 1944 y L. A. FITZGERALD BALL MILL LINER Filed Jan. 29, 1942 4 sheets-sheet 4 f Um'lizpD-A STATE liner, the princpalmovement and` thus grindingl rforce of the balls beingy in verticalplanes atnon mals to the millv axisfthat is, up and down and horizontally.-

, of the mill. v

Patented Nov. 14, 1944 o BALLMllLLLINE'R'.` Louis A'. Fitzgerald Repub1ic,'Wash. o l Application January 29, 1942,y serial No. 428,639` 1 14 Claims.

Thisy invention relates to liners vfor ball-type grinding mills and more particularly concerns im provements in replaceable `breast'liners for such mills.

-In a mill of thistype the grindingvv action re' sults from the crushing and abrading of the maing bodies, and between the grinding bodies and ythe .liners of the'grinding chamber within the,

rotating cylinder of the mill. The grinding ybodies may be in the form of ksteel balls or slugs, pebbles,

or the like, depending upon the material to be vgronnd,7but for convenience willfbe referred to I generally as balls. f

In the conventional arrangement, the ballsare spready in a more or less uniformgrinding column throughout the length of [the horizontally mountl ed mill, and a large f proportion ofv the grinding takes place between Vthe'tumbling balls and the' to only a very limited extent axially,ithatis,

, A principal a'm of the presentinventionis to increase `beyond the usualextent thef grinding action of vball against ball, by causing theballs.y l to strike and grind againstone another positively and continuously in considerable horizontal components of'motion concurrently withbut in ad: dition to-the usual vertical tumbling movement v resulting from'` rotation ofl thegrinding cylinder Another object is to provide-the breast liner withnovel means `active-in the operation'ofthe' mill to concentrate the grinding ball load and mav glacier, against the liner surfaceflong enough at a timein any Vone givenl position to develop sub-,-

, y, terial such as ore by and between tumbling grind-1 stantial, flats on 'the balls.l Y

, Anothenadvantage of my liner. is that the slab sections of which they arecomposed may be Vof the Vsame Arectangular size andipattern "as ,that of the more or less standard ordinary slab sections, with the result/that'the standard retaining .bolts can 4be employed without changing either the boltsor vtheir locationsirrthe mill or drilling.v

any new bolt holes inthe mill.

,Another object 's to .provide an improved arrangement for efiectinga continuous circulation of the grinding balls longitudinally of the mill and thusjprevent channeling on racing of the breastv liner. Once such a channel; or race Vis started, the balls tend to stay in it to an increasing extent, wth'the result` thatl lthere -iscontnsously ,accelerated weer Causing theliners Ato vvearl through rather guiclly at thechannel or face,y .l c ,c .i VIf,.as in the present',l instance,vitis desired to promote longitudinal movement `of the balls and l material, any slich channeling acts something like atrifn'ch seriouslyimpeding. longitudinal y.movel n J Y V v f become' circumferential channels or" races in the terial in a grinding columnof` increased depth4 toward a medial plane of the ball chamber to the relief of wear on the end liner structure of the millg c Y A further object is to provide a ball mill breast liner which can bev manufactured at a cost comparable to conventional liners but which increases output through diverting grinding Wear to the lower cost grinding bodies and thusfprolongs the..

life of the higher -cost liner, and results in subst'antal operating economies. y y n Another advantagelof my improved liner which I vnd fromtests is that when spherical balls are used as the grinding bodies, they tend to stay round for a longer time 'than with conventional liners. This seems to result from the actionioiv my liner in promoting a more continuous and variedmovementand/rotation ofthe ballsso that-no onev ball is scouredflike a'v boulderjembedded in ar insidesnrfaces of the'. liner". Suchfstarting 'deprssions'may be the end gaps between liner slab sections, or'bolt head holes,or iiaws, waviness, or

weak-spotsl This yphenomenon of channeling orv racing willi better be' understood tvlie'ri it is explained that the movementf.of the-mixture of balls and ore, or'other material, is snnethingoi` a combination dff'a-Sirlpllolins ofthe bans and liquid-,like ,Y How of the material, with-a `glacier-like scouring oi the liner, considering theballs to be embedded Mulden-like; in the 11n-ass 0f material` and Ato'a large extent heldagainst rotation-by the-pressure ofthe weight of the balls and material above the` balls and material which comeinscouringcontact with the liner. This is why the action of the balls-must be described in terms rof aid'li'dins or, scouring. ,Y Y

,- the past the jribs or `corrugations of .breast rdir1ar5v7 breast liners witlrplain surfaces or both 'rolling liner at the discharge end of the mill; an exces-r sive crowding of the material against the discharge ports such as to impede discharge; an insuiciency of the time the material remains in the mill, because of the urge toward the discharge end; and a faster wearing of one end half of the breast liner than of the other end half. Or, if

the spiraling is arranged the other way in reference to the direction of rotation so that the spirals tend to promote flow toward the feed end, then the results are: An excessive increase in the time which the material remains in the mill; a crowding toward the feed ports which impedes feed; and a faster wearing of one end half of the breast liner than of the other end half.

.In the past the obliquely arranged ribs or corrugations have sometimes been staggered or provided with interruptions or gaps. In such case, far from remedying the tendency to channeling, the tendency is greatly increased. This is because the gaps, or the ends of staggered ribs, as the case may be, provide very denite starting positions for the passage of successive balls which quickly develop channels or races in the breast liner.

By my invention the obliqued arrangement of the corrugations of the breast liner is such that the tendency to channeling is minimizedin fact, substantially eliminated-without promoting an increased ilow toward either end, without excessive wear upon the head liner at either end, without crowding the material against the feed inlet-or the discharge ports to impede their functions, and without causing one end half of the .breast liner to wear faster than the other half.

As a corrugated liner wears, the greater erosion on the leading slopes of the ridges than on their trailing slopes causes a gradual backward shift of the ridges. One of the impor-tant practical advantages of my liner is that despite this extensive shifting-even to the extent that the ridge nally shifts backwardly to a position entirely beyond the position of the original ridge-the pattern of the corrugations is maintained without tendency to develop channeling, and as a result the characteristic action of my liner, together with the beneiits iiowing from it, are maintained throughout the life of the liner and throughout the entire extreme range of backward ridge shift.

Other objects and advantages will become apparent from the following detailed description and from the accompanying drawings, in which:

Fig. 1 is a perspective view of a ball mill cylinder with the ends omitted, showing my improved liner therein;

Fig. 2 is a projected plan view of the liner;

Fig. 3 is an enlarged end view of one of the liner slabs;

Fig. "4 is a transverse vertical sectional view through the liner slab;

Fig. V5 is a vertical longitudinal sectional view taken on the diameter of the ball mill cylinder;

Fig. 6 is a schematic longitudinal sectional view taken along the center line of one of the grooves or valleys in the liner;

Fig. 7 is a transverse vertical sectional view through approximately the longitudinal center of the mill cylinder;

Fig. 8 is a schematic longitudinal sectional view, on a reduced scale, taken substantially along the line 8 8 of Fig. '7; and

Fig. 9 is a reduced schematic longitudinal sectional view, taken substantially along the line 9 9 of Fig. 7.

In carrying out my invention, the grinding balls are caused to have an improved ball-against-ball action in the operation of the mill. This I accomplish by imparting to the balls, in addition to the usual tumbling action, an impact action longitudinally of the mill axis, That is, while tumbling, the balls are urged constantly from opposite sides toward a common transverse plane of the grinding chamber to set up what may be likened to a riptide action.

The mass of balls may, for convenience, be considered as roughly divided into two opposing sections respectively urged toward a common transverse medial meeting plane where the opposing balls of the sections impinge against one another with considerable grinding pressure and also place the balls in the respective sections under similar, but, of course, gradually diminishing pressure as the distance increases from the meeting plane. I have found that as a result of this action the balls pile up in the region of the meeting plane in a substantially greater grinding column than in the case of simple conventional tumbling, and the heavier balls in particular tend to seek the meeting plane region throughout the grinding action. Thus, there is imparted toy the balls a definite, `purposeful grinding action which is in addition to and supplements the normal grinding action resulting from the usual tumbling of the balls by rotation of the mill.

These advantageous results are effected by a novel construction of the grinding surface of the breast liner with which the cylinder of the mill is equipped.

Referring now to the drawings, the mill identied generally as I5 (Figs. 1, 5 and 7) may be of standard or other suitable design including a horizontal cylinder I6 suitably mounted for power driven rotation about its longitudinal axis. In actual practice the cylinder I6 is equipped at its respective opposite ends with material inlet and discharge structures and end liners, which are not shown because they involve no feature of this invention but may be of any well known type.

Access into the grinding chamber within the cylinder I 6 may be had through one or more suitably located manholes I'I for charging the mill with or dumping therefrom a discrete mass of grinding balls I 8. The manhole Il has a conventional closure structure I9.

The manhole II is also of service for entry to equip the cylinder I6 with a replaceable breast liner 20. The latter is preferably in the form of relatively smallsize, easily handled, narrow segmental plates or slabs extending lengthwise of the cylinder` and shaped to conform to its inside wall. Removable attachment of the liner slabs is ef fected in any suitable manner by means such as bolts 2 I.

Each of the slabs of the liner 2@ is preferably subdivided for convenience in handling into a plurality of sections, and the joints between the sections of alternate slabs are staggered to prevent the channeling by the grinding bodies IB.

`|1, wherebylthe-peaksV 29itrail on the upswing side Thus, certain-rows 4of slabs may comprise two equal sections-22 (Fig. 2) while alternatel rows comprise three proportionately smaller equal sections f23.- At the manhole I1, there may be a slight alteration in size of the liner section to conform to the manhole'size, thus providing equal size end sections 24 and an intermediate manhole section 25 which is removably secured in place by the associated'manhole closure structure I9. A slight spacingr is left between all adjacent edges of the various liner sections to facilitate speedy mounting of the sections. However, the spacing about the manhole 'section 25'rnay be slightly greater to assure at all times adequate clearance for ready removal from the manhole I1, even l though there may be a certain amount of peening over or now ofthe metal of the section 25 or oi the immediatelyT adjacent'sections under the impact of the grinding bodies I8.

According to the present invention, the grindi ing surface of the liner 20 is formed over all with CTI two uniform and equally pitched (in this instance approximately 30) though' oppositely extending, series of oblique corrugations orxribs 21 and-28,

respectively, which in vthis instance extend equal distances inwardly from the respective'opposite Vends of the liner 28 andv meet substantially chevron fashion at the transverse center plane of the grinding chamber.4 The meeting ends of ribs 21 and 28 joinin a continuous curvingly peaked sweep, as indicated at 29. The over-all result isa herringbone-pattern. l

In the illustrated practicalv embodiment, there are as many of the ribs-in each series; as there l are slabs in the liner 20. The outer ends ofthe e ribshave their center radius on the longitudinal center. lines of, the associated slabs, The slopes v of the ribs-both leading. and trailing slopes--are of easy ogee transverse contour. The ribs project i v about two inches fromthe intervening valleys,

'that-is, from the adjacent cylindrical plane ofA the liner, and the liner slabs, whennew, are from a three to three and one-half inches thick, exclusive ofthe ribs.

The pitch of the ribs 21 and 28 may besoreof the slabs on which they start and .diagonally rlated to the width of theliner slabsas to run off l v across the respectively adjoining slabs toward the center,y with the peaksZS largely, if not. altogether, on the second adjacent slab. As shown,

each of the liner slabs yhas the outer starting end section of one of the ribs 21 at one end and that of one of the` ribs V28 at the other, end.` Inwardly 'space-d from therib end sections are rib extensions aligned with, and service .asy continuations of, Athe rib sections of theslab adjoining at the advancing side. The peaks `2.9 are in the form of convergences located on the slab adjoining on the" trailing side and co-operating with the inner ends of the rib extensions. As a result, each ofthe chevron-shaped rib units 21, 28 is nested between the adjacent rib units 'at both sides in a herringbone pattern. The several-' plate sections 22, 23 and 24 of the liner are, of course, made with aD- 4 prolpriately aligned sections ofthe various ribs conforming to the preconceived herringbonepattern. y v

,The spacing between the b ases ofthe adjacent ribs 21 and 28 may be about thesame as the width of the 4ribs and is designed to receive Athe balls `I-B and provide flat bottom grooves 30 within which the balls are guided by the ribs inwardlyr away from the ends of ther cylinder I6 toward the meeting' points 29, as the cylinder is rotatedY in the direction shown by the'arrows in Figs. l and of the grinding chamber. At the peaks 29, the grooves 30 may `widen out slightly to afford greater clearance.`

` As'observed in operation, there is superimposed upon'the usual tumbling laction of the balls I8 withinthe 'mill amovement of the balls away from opposite ends and toward a meeting plane, indicated generally by the line 3| in Figs, 8 and 9, coinciding with the -center line of the peaks 29. This effect is apparently produced by deflection of the liner-contacting balls I8 toward the meeting plane 3I by the ribs-21 and 28 as the cylinder I8 rotates to tumble the ball mass.

Thus, referring to Figs. 7, 8 and 9, frictional contactbetwe'enthe liner 2l) and the contacting balls I8 carries such balls in the direction of rotation of the cylinder, but gravity and the inertia and friction of superimposed balls cause the contacting balls to seek the low point in the grinding chamber by sliding and rolling down the uptraveling surface -of the liner. rolling balls I8 encounter the Arespective ribs 21 and 28, they seek to follow the advancing sides of the ribs down the channels 30 and crowd toward the meeting plane 3|. Because of opposition from surrounding balls, this lateral deflection of the balls by any given one of the ribs 21, 28 is only more or less momentary and of limited extent, but the accumulative effect after engagement with several successive ribs is appreciable.Y

In addition', as the balls ride over the unbroken rib humps, they jostle the superimposed balls and agitato the mass which appears to give enough relief from the weight of the mass so that rl`as the balls movedown the trailing sides of the ribs, they more or less follow the inclination of such sides in the direction of deection. This is strikingly evident from 'an examination ofthe liner after it is worn away to the point of necessitating replacement, lwhich reveals the rib ridges still prominent and retaining their identilty in the pattern though worn down from the original arcuate form to relatively sharp peaks and shifted, by the brunt of wear on the advancing sides of the ribs, over adjacent to where the trailing side had been. (See the dot-dash line-W in Figs. 3 and 4 which is representative of the ultimate wear surface found in the slabs of a replaced linerJ) Further support for this conclusion is in the worn condition of the grooves '38 which are now revealed asl valleys of substanj ticable to the top ofthe grinding chamber beforethey have traveled to any great extent toward the meeting plane 3l. At the top the balls fall away from the liner by force of gravity and tumble down thesloping topof the ball mass. 'As a result the balls I8 travel continuously in a cyclical path whichv takes them recurrently under the ball mass up to the top and then down again.

Because of the inward deflection ofthe engaged balls I8 by the herringbone rib structure, there is a general movement or gradual working As the sliding, f

over of the balls progressively from the ends of the mill so that the cyelialrath traveled nro.- ceeds in substantially helical `fashion toward the meeting plane 3l. As shown diagrammatically in Figs. c and 9, the balls in the two sections of the mass, designated A and B, at opposite sides of the meeting plane 3|, work gradually inwardly in respective generally deformedhelical paths 32 and 33, the helices of which gradually flatten out and describe progressively larger arcs adjacent the plane 3l as the pressure increases and the grinding column deepens from the crowding .together of the balls of the two sections A and B i the mass. At the plllle 3l, the balls reach the peak of the grinding column and fall back, as indicated, to the startillgpoints of the respective helical paths 3,2 and 33 ,for

` repetition of the cycle.

It is to be understood, of course, that the 13.1"??- ceding description of theball action is directed simply to the total effect which has been observed and that due to the vigorous agitation and hard crowding of the balls i8 effected by movement over the ribs 21, 28 added to the turbulence incident to tumbling there islnuch erratic travel ci individual balls. All of this results in substantial improvement in the grinding effectiveness of the balls one against the other and an increase in the total productive capacity of the mill between necessary breast liner replacements.

Since the ball-against-ball grinding is increased and the breast liner is to that extent relieved from the burden of the grinding, a resilient material such as rubber may be used for the liner instead of the usual manganese or other alloy steel. Such resilient material must be .capable of resisting the cutting action of broken balls, tramp iron or the like. Its advantage over metal lies in the lreedom from Wear, but at the same time the rate of production is reduced to the extent that results from action of the balls against the metal liners.

If desired, the liner 2D may be reinforced, for increased wear at its inner or central region where grinding is concentrated by the action of the converging ribs 21, 28. Thus, as seen inFg. 6, the liner 2l) is uniformly thickened from points at the inner ends of relatively short opposite end portions C of the liner slabs throughout the central region indicated at D and inclusive throughout ofthe ribs 21, 28 as Well as the grooves 30. This difference in thickness is also'shown comparatively in Figs. 3 and 4.

This central thickening need not be as great as might at rst be supposed. Despite the central concentration of load, the wear is not so excessive therebeneath in my liner because the overlying weight of the concentrated load tends to restrain ball-on-liner impact. Also, the end regions ol'` the breast liner are subject to more forcei'ul ball impact, although by fewer balls, and this tends to result in end region Wear more closely approximating the central region Wear of the breast liner. Another limitation on the central thickening of the liner is that the outward slope of the liner face away from the central thickening tends to urge the load toward the ends of the mill, and this tendency should not be permitted materially to defeat the function of my herringbone corrugation pattern in urging the load toward the central meeting plane.

In the specic illustrative embodiment here shown, the mill diameter is about 61/2 feet, its length about 81/2y feet, the balls, where new, are

estasi;

about 31/ 2 inches in diameter, and the mill is rotated at a sneed of about ,2D R. P. M.

While I have illustrated and described a prefered embodiment of my invention, modifications may be made Without departing from the spirit of the invention, and I do not Wish to be limited to the precise details of construction as set forth, but desire to avail myself of all changes within the scope of the appended claims.

Having thus described my invention, what I claim as new and desire to secure by Letters Patent of the United States is:

l. In a breast liner providing a protective grinding surface for the inner wall of a ball mill cylinder, a uniform series of continuous ribs extending spirally inwardly from the opposite ends of the inner surface of the liner and meeting chevron-fashion to form a herringbone pattern, the ribs being adapted, when the cylinder is rotated with the meeting ends of the ribs trailing, to deect the grinding balls of the mill toward a meeting zone substantially coinciding with said meeting ends, whereby to impart a component of positive longitudinal ball-against-.ball grinding motion t0 the balls in addition to the tumbling motion effected by rotation of the cylinder.

2. A breast liner as defined in claim 1, which is replaceably secured to the grinding cylinder and is constructed of a set of co-operating slabs carrying predetermined aligned sections of the rib pattern.

3. A liner as dened in claim 1, in which the entire central area ofthe liner is appreciably thickened to withstand greater Wear.

4. A ball mill breast liner comprising a grinding surface formed with a continuous series of alternate continuous ribs and gropves extending inwardly from the liner ends in a herringbone pattern, the convergences of the ribs being at substantially the longitudinal center of the liner, said ribs and grooves being adapted for guiding the grinding balls of the mill toward said center as an incident to rotation of the liner with said convergences trailing.

5. A ball mill breast liner comprising a surface adapted to cooperate grindingly With a mass of balls, and a continuous series of spaced substantially chevron-shaped continuous rib units in internested relation on the grinding surface, the peaks of the rib units being alined circumferentially of the grinding surface, and the spaces between adjacent rib units providing grooves within which the balls are received and urged by the rib units toward the peaks as an incident to operation of the mill.

6. A liner as specied in claim 5 in which the rib units are of rounded transverse surface so that the balls may roll and slide thereover relatively smoothly.

7. A breast liner for ball mills comprising a surface adapted to cooperate grindingly with a tumbling mass of grinding balls in the operation of the mill, and a series of continuous ribs extending spirally from opposite sides toward a common plane and designed to deect the balls toward such plane when the liner rotates in the direction in which the inner ends of the ribs trail in movement, the trailing sides cf the ribs extending in the same spiral direction as the advancing sides of the ribs so as to cooperate in the ball-deflecting action with the opposed advancing sides of the immediately adjacentribs.

8, A' ball mill breast liner comprising a grinding surface supporting the mass of balls in the mill, and continuous ribs onsaid surface extending inwardly from opposite ends of the surface at approximately 30 pitch into convergence for deflecting the balls toward the convergences during v rotation of the mill. V

9. A replaceable cylindrical ball mill breast liner comprising a set of cooperating longitudinally extending slabs subdivided into shorter more readily manipulable sections, and integrally formed portions of ribs on the slab sections arranged to cooperate with the rib portions of contiguous slab sections to form continuous ribs units respectively extending inwardly from both of the respective opposite ends of the liner and biased in opposition to the direction of grinding rotation of the liner for deecting the grinding balls into a zone of maximum grinding concentration spaced inwardly from said ends.

10. A replaceable cylindrical ball mill breast liner comprising a set of relatively narrow cooperating longitudinally extending slabs; and integral ball-deflecting continuous ribs formed on the grinding surfaces of the slabs; said ribs commencing at, and extending diagonally inwardly from, the opposite ends of the slabs and being of such length as to extend across a plurality of contiguous slabs into substantially chevron-shape `more readily manipulable shorter sections with the sections of alternate slabs staggered to avoid channeling the end joints between sections by the grinding balls riding thereover in the rotation of the mill, and chevron-shaped continuous balldeiiecting ribs on said slabs, said ribs having their convergences trailing with respect to the direction of rotation of the mill and being formed'to extend across a plurality of contiguous slabs, the convergences of the ribs being located at the joints between adjoining slab sections on alternate slabs and on the staggered slab sections of the remaining slabs extending across the latter joints.

12. In a ball mill, the combination of a substantially cylindrical supporting and enclosing housing, having an inlet and outlet; and having means for its-support for rotation on the axis of said housing, with a liner for the wall of said housing, said rliner .being ofr resilient, non-metallic material, and a multiplicity of substantially spherical grinding balls of predetermined size, in said mill, engaging said liner, said liner being formed with :a multiplicity of substantially parallel and continuous' ridges and grooves, each ridge and groove having two end portions extending longitudinally and inwardly from the ends of said housing and in a rotary direction inside said housing, and the end portions of each ridge and groove converging at an apex located between the ends of the mill, said mill being constructed and arranged for rotation in such direction that the being lifted by the ridges and impelled inwardly from both ends of the mill, to effect an improved grinding action of the balls against each other, while maintaining their shape substantially spherical and driving .the :balls and the charge inwardly from both ends of the mill to prevent clogging at the inlet or outlet.y

13.l In a lloa-ll mill, the combination of a sulbstantially cylindrical supporting and enclosing housing, having an inlet and outlet and having means for its support for rotation on .the axis of said housing, with a liner for the wall of said housing, said liner being of resilient non-metallic material; and a multiplicity of substantially spherical grinding balls of `predetermined size, in said mill, engaging said liner, said liner .being formed with a multiplicity of substantially 'parallel and continuous ridges and grooves, each ridge and groove having two end portions extending longitudinally and inwardly from the ends of said housing and" in a rotary direction inside said housing, and the end portions of each ridge and groove converging at an apex located between the ends of the mul, said mili being constructed and arranged for rotation in such direction -that the apices of the ridges ,trail aflter the end portions of the ridges, the balls being caused to rotate and being lifted by the ridges `and impelled inwardly `from ,both ends of the mill, to eiect an improved grinding action of the balls against each other, while maintaining their shape substantially spherical and driving the balls and the charge inwardly from both ends of the mill to prevent `clogging at the inlet or outlet, the said ribs .being of substantially uni-form cross sectional shape,

14. In a ball mill, the combination of a substantially cylindrical supporting and enclosing housing, having an inlet and outlet and having means for its support for rotation on .the axis of said housing, with a liner for ythe wall of said housing, said liner being of resilient, non-metallic material, and a multiplicity of substantially spherical grinding `balls of predetermined size, in said mill, engaging said liner, said liner being formed with a multiplicity of substantially parallel and continuous ridges and grooves, each ridge and groove having two end portions extending longitudinally and inwardly from the ends of said housing and in a rotary direction inside said housing, and the end portions of each ridge and groove converging at an apex located between the ends of the mill, said mill being constructed and arranged for rotation in such direction that the apices of the ridges trail after @the end portions of the ridges, -the balls being caused to rotate and being lifted `by the ridges and impelled inwardly from both ends of the mill, to effect an improved grinding action of the balls against each other, while maintaining their shape substantially spherical and driving the balls and the charge inwardly from both ends of 'the mill to prevent clogging at the inlet or outlet, the said ribs being of substantially uniform cross sectional shape, and the ribs and grooves being formed with transversely curved surfaces of gradual curvature.

LOUIS A. FITZGERALD.

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