US2815176A - Apparatus for producing carbonaceous materials for electrodes - Google Patents

Description

D 1957 H. w. BODLAENDER 2,815,176

APPARATUS FOR PRODUCING CARBONACEOUS MATERIALS FOR ELECTRODES Filed April 6, 1955 5 Shets-Sheet 1 INVENTOR N Hons W. Bodluender BY I ATTS%%* Dec. 3, 1957 H. w. BODLAENDER 2,815,176

APPARATUS FOR PRODUCING CARBONACEOUS MATERIALS FOR ELECTRODES Filed April 6, l955 5 Sheets-Sheet 2 49 FIG. 2

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:MVENT R Hons W. Bodloender BY ,?(z,%wmmmr ATTORNEY Dec. 3, 1957 H. w. BODLAENDER 2,815,176

APPARATUS FOR PRODUCING CARBONACEOUS MATERIALS FOR ELECTRODES Filed April 6, 1955 5 Sheets-Sheet s INVENTOR Hons W. Bodloender BY Q AMAZW WM M ATTORNEYS Dec. 3, 1957 H. w. BODLAENDER 2,815,176

APPARATUS FOR PRODUCING CARBONACEOUS MATERIALS FOR ELECTRODES Filed April 6, 1955 R 5 Sheets-Sheet 4 INVENTOR Hons W. Bodloender BY gugwwz md q Dec. 3, 1957 H. w. BODLAENDER 2,815,176

Al PARATus FOR PRODUCING CARBONACEOUS MATERIALS FOR ELECTRODES Filed April a, 1955 E 5 Sheets-Sheet 5 Hons W. Bodloender BY WM,MFM M ATTOIRNE APPARATUS FDR PRODUCING CONACEOUS MATELS FOR ELECTRODES Hans W. Bodlaender, Long Island City, N. Y., assignor to Kennedy-Van Satin Mfg. & Eng. Corporation, New York, N. Y., a corporation of Delaware Application April 6, 1955, Serial No. 499,726

10 Claims. (Cl. 241-54) My invention relates to improvements in the method and apparatus for producing carbonaceous materials for electrodes, more particularly for producing a mixture of fine and coarse carbonaceous material for making electrodes for use in the smelting of aluminum ores and in the refining of aluminum.

At the present time, various carbonaceous materials are ground in rod mills and then subsequently finely ground in ball mills for the production of the necessary finely ground materials for use with coarse material in making electrodes.

The primary object of the present invention is to provide a method of grinding carbonaceous materials for electrodes in a single grinding operation, in which both the fine and coarse materials are simultaneously produced.

A further object of the invention is to provide an improved apparatus for simultaneously producing fine and coarse carbonaceous materials from raw carbonaceous stock.

I have discovered that fine and coarse carbonaceous material for forming electrodes can be simultaneously produced in a ball mill operated under air-sweeping conditions without the necessity of any pre-grinding of the raw material, such as calcined anthracite, petroleum coke, foundry coke or coke oven coke.

In the manufacture of electrodes of the type under consideration, that is, both for the cathode pot linings used in the aluminum and other industries and for prebaked and Soederberg anodes or any other kind of carbon electrodes, a mixture of fine and coarse aggregate carbonaceous materials is required. This mixture of ground carbonaceous material is mixed with a binder, such as tar or pitch, to produce a paste from which the cathode pot linings and anodes are formed. The electrodes thus produced are baked in the usual manner to bind the coarse and fine materials together.

It has been found extremely advantageous in the manufacture of such electrodes to use a considerable proportion of coarse aggregate along with the fine material.

According to the invention the improved apparatus comprises a ball mill provided with a screen partition adjacent its discharge end for holding the balls and coarse material in the mill away from the discharge outlet. The mill includes litters and chutes for removing the correct proportion of coarse material, while the fine material is removed in an air-sweeping stream. Means is also provided for regulating the proportion of coarse material removed from the mill in relation to the production of fines.

The invention includes other features, which are described in detail hereinafter in connection with the accompanying drawings forming a part of this application.

In the drawings:

Fig. 1 is a diagrammatic view illustrating the use of the improved air swept ball mill in the method of recovering the coarse and fine materials for making the electrode mixture;

Fig. 2 is an enlarged broken vertical sectional view of the discharge end of the ball mill shown in Pig. 1;

" ited States Patent Fig. 3 is a vertical sectional view taken on the line 3-3 of Fig. 2 with parts broken away;

Fig. 4 is a broken View showing the shape and construction of one of the elements of the partition screen shown in Fig. 2;

Fig. 5 is a broken sectional view taken on the line 55 of Fig. 4;

Fig. 6 is an enlarged broken view partly in section of a portion of the mechanism shown in Figs. 1, 2 and 3.

Fig. 7 is a vertical sectional view taken on the line 7-7 of Fig. 6;

Fig. 8 is an enlarged broken View partly in vertical section of the control mechanism shown in Fig. 2; and

Fig. 9 is an enlarged view taken on the line 9-9 of Fig. 8.

The apparatus for producing the mixture of coarse and fine carbonaceous materials and the paste used for making electrodes is shown diagrammatically in Fig. l, and includes a cylindrical type ball or tube mill 12 having a large diameter, as for example a diameter of -10 feet. The mill 12 is supported by an inlet trunnion 14 and an outlet trunnion 16 on hearing supports 18 and 20, respectively. The outlet trunnion 16 extends through a gear casing 22 housing a set of driving gears of known type suitable for driving a heavy mill. The inlet end of the mill is provided with a chute 24 through which the carbonaceous material is supplied to the mill in lumps of any suitable size, along with air. The discharging end of the mill is provided with an upwardly inclined chute 26, through which a stream of air and fine pulverized carbonaceous material is drawn. Coarsely ground material is discharged from the discharge end of the mill through a downwardly inclined duct 28. The mill includes an internal structure shown in other figures of the drawings for selectively delivering coarsely ground material and operated by a reg ulator mechanism shown generally at 30.

The stream of air swept through the mill and containing the finely ground materials is drawn from the chute 26 of the mill by an exhauster fan 32, driven by a motor 33, and delivered therefrom through a duct 34 int-o a cyclone separator 35, in which a large part of the fine material is separated. The stream of air and remaining fine material passes from the cyclone separator 35 through a duct 36 into a second cyclone separator 37 where substantially all of the remainder of fine material is separated, the air being discharged through a duct 38. The fine material separated in the cyclone separators 35 and 37 are delivered through ducts 39 and 40 into a common duct 41 through which the fine material is delivered to a mixer 42.

The coarse material discharged through the duct 28 flows into a screw conveyor 43 driven by a motor 44 and is delivered into the mixer 42. The material delivered by the conveyor 43 may be screened to produce two or more coarse fractions. Hot pitch or tar from a container 45 is delivered in the right proportion, as for example 25% by weight, through a valved duct 46 into the mixer 42 in which the electrode paste is formed. This paste may be delivered to the electrode molds by means of a screw conveyor 47 operated by a motor 48.

Any type of separating equipment may be used instead of or in conjtmction with the cyclone separators 35 and 37, and the mixer 42 may be of the type presently used for mixing electrode paste, while the screw conveyor 47 may be used for extruding the paste into the forms for the pot and rod electrodes.

The mill 12 constitutes an important part of the invention since it includes means for simultaneously producing the required proportions of carbonaceous coarse aggregate and fines for making the electrode paste from various types of raw carbonaceous material. As shown in Fig. 2, the mill 12 includes a head 49 integral with the trunnion or trunnion bearing 16 to which is secured in spaced relation 21 screen type partition wall 50 adapted to hold the grinding balls 51 away from the discharge trunnion of the mill. The periphery of the partition 50 may be secured to the shell of the mill and thereby held in spaced relation to the head 49, but as in the construction illustrated, it is preferably supported in spaced relation to the head by means of webs 52 on certain sections, bolted to the inside of the head 49 by means of bolts 53, and by lifter webs 54 and cooperating webs 54 connected cross bars 55 bolted to the head 49 by means of bolts 55, on other sections of the partition.

The screen partition is advantageously made up of twelve pie-shaped segments or sections joined together in a known manner. For example, nine of these pieshaped segments may be generally of the screen type disclosed in Patent No. 2,620,987, while the other three segments or sections 56 have a structure as shown in Figs. 2, 4 and of the drawings and located at an angle of 120 with respect to each other. Where the partition is made up of 12 screen segments, there are three screen segments 57, each carrying a web 52, located in each of the spaces between the screen segments 56. Each of the three spaced segments includes a litter web 54 and a web 54 connected at their edges adjacent the head 49 by the integral cross bars 55.

The segments 56 and 57 terminate short of the axis of the mill and the segments 56 are bolted respectively to angularly spaced extensions 58 on a heavy metal plate 58 by means of bolts 59, as shown in Figs. 2, 3, 6 and 7. The partition screen 50, therefore, constitutes a rigid structure supported against the head of the mill for holding back the grinding balls and permitting coarse aggregate stock to pass through the screen into an adequate space between it and the head 49. Furthermore, the screen includes ample flow capacity above the charge of balls and raw material to permit the flow of an adequate stream of air, even when the mill is half filled with grinding balls and carbonaceous material.

The outlet trunnion 16 of the mill 12 is provided with a flange 60 to which a large ring driving gear is bolted, in accordance with known types of construction. The trunnion 16 is provided with a sleeve lining 61 having a flange 62 by which it is bolted to the rear face of the trunnion. The sleeve 61 extends somewhat beyond the end of the trunnion and is provided with interior spiral fins 63 by which coarse material separated out in the delivery chute 26 is returned to the drum of the mill. In the showing in Fig. 2 the drum of the mill rotates in a clockwise direction as viewed from the outlet end, as shown by the arrows in Figs. 2 and 3, and the fins 63 are so directed that any material collected therebetween moves back into the drum of the mill behind the screen partition 50.

Coarse carbonaceous aggregate selectively obtained in the discharge end of the mill is delivered into the outlet duct 28 by means of a cylindrical member 64 located axially in spaced relation to the fin 63 of the sleeve 61 and supported by brackets 64' attached to the fins 63 as shown in Fig. 3. The member 64 is provided with internal helical fins 65, which extend in a direction opposite to that of the fins 63 so that any coarse aggregate carbonaceous material which is delivered into the inner end of the cylinder 64 is moved therethrough and discharged into the inlet of the duct 28.

The shovel or lifter 54 and the cooperating web 54' attached to each of the screen partition segments 56 are preferably cast integral with that segment and comprise web sections having the shape shown in Figs. 4 and 5. Fig. 5 shows the contour of the rear edges of the elements 54 and 54, which fit along the head and carry the cross bars 55 through which the bolts 55 extend. As seen in Fig. 4, the shovel or lifter 54 extends along one side of the pie-shaped section 56 and then is curved forward at 67 in the direction of rotation of the mill so that it is adapted to pick up coarsely ground aggregate in the lower part of the drum of the mill as the mill rotates. The web 54 has the same rear-edge contour as the lifter 54, but it extends straight along its edge of the screen segment 56 and only out to the point 66, substantially spaced from the scoop edge of the litter or shovel 54.

As the aggregate is lifted by the shovel or lifter 54, it slides toward the axis of the mill into the pocket between elements 54 and 54 and along the portion of the lifter extending along the edge of the partition screen section 56 until it engages a movable chute 68, shown in Fig. 2, with its upper beveled end resting against the rear face of the partition screen section 56. The chute 68 fits between the elements 54 and 54 and delivers the coarsely ground carbonaceous aggregate into the cylindrical member 64 so that it is fed into the duct 28. Each of the partition screen units 56 is provided with a pivoted movable chute 68 so that as each section moves upwardly by the rotation of the drum of the mill, coarse carbonaceous material is lifted by the shovel 54 and delivered onto its chute 68.

The web of each lifter 54 includes a rearwardly-extending portion 69, which cooperates with its chute 68 to aid in delivering the coarse aggregate into the feed cylinder 64. Furthermore, the portion 69 serves as one side of a pivot support for the chute 68, which is pivoted on a pin 70 extending through a hole in the portion 69 and through a hole in a parallel portion 71, shaped like the portion 69 and comprising a rearwardly-extending part of the web 54', as shown in Figs. 2 to 5. Fig. 2

shows only one of the three units comprising elements 54, 54' "and 68.

The chutes 68 are adapted to be adjusted at any position between the full and dotted line positions shown in Fig. 2, in order to regulate the quantity of coarse aggregate removed from the mill in accordance with the production of fine carbonaceous material as delivered through the duct 41 in Fig. 1. All three of the chutes 68 are simultaneously adjusted and set, and the means for accomplishing this by the control 30 comprises a longitudinally-movable rod or pipe member 72 extending axially through the trunnion 16 and cylinder 64 and carrying three brackets 73 (Figs. 2, 3 and 6), which abut against the inner end of the pipe 72 and are fixed to a smaller pipe 74 by means of set screws 75. The brackets 73 are arranged at 120 apart and each comprises a pair of leaves through which a bolt 75' extends. Each of the movable chutes 68 carries a bracket 76 on the under portion of the apron below the pivot 70, the bracket 76 having a downwardly-extending slot 77 engaging the bolt 75'.

Beyond the brackets 73, the pipe 74 is closed at the end and movably extends into a length of pipe 78 (Fig. 6) located axially of the drum and welded to the back face of the plate 58. The pipe section 78 is rigidly held in position by radially-extending flanges 79, which are welded to the pipe and to the plate 58. The chutes 68 are readily movable from their closed to open positions as in Fig. 2, by moving the rod 72 forward to pivot the chutes respectively on the pivots 70.

The outer end of the pipes 72 and 74 are welded together and to a rod 80 having an end portion of reduced diameter fitting in the pipe 74. The rod 80 is slidably mounted in a sleeve 81 which extends through the casing of the outlet 26 in the manner shown in Fig. 8. The casing 81 extends through and is welded to a plate 82 of a manhole cover unit attached to the sloping side of the outlet chute 26 by means of studs 83. The manhole cover unit includes a rectangular-shaped metal frame 84, through which the studs 83 extend, as shown in Figs. 8 and 9. This frame is sealed by means of a gasket 85 and is welded to an annular spacer 86 which in turn is welded to the manhole plate 82. Brackets 87 welded to the plate 82 and sleeve 81 serve to support a pair of horizontal arms 88 carrying an A-shaped control frame 89, the arms 88 being bolted to the brackets 87 by means of bolts 90. A control arm or lever 91 is pivoted on a bolt 92 at the top of the control frame and to a forwardly-extending link 93 by means of a pin 94, the forward end of the link 93 being in turn pivoted in a coupling 95 by means of a pin 96.

Since the pipes 72 and 74 and the rod 80 must rotate with the mill, the bracket 95 includes means for actuating the rod 811 longitudinally, while at the same time permitting its relative rotation. Accordingly, the bracket 95 includes spaced forwardly-extending arms 97 carrying a bearing casing 98 containing a ball bearing unit 99, through which a reduced end portion of the rod 80 extends. The rod 88 is secured against longitudinal movement by means of an end nut 100 set over a washer 1111 engaging the inner race of the ball bearing unit 99. The outer race of the ball bearing unit is held in place by an annular plate 102 fastened to the rear face of the casing 98 by screws 103. Relative longitudinal movement between the rod 80 and the bearing casing 98 is therefore prevented while permitting free rotation of the rod :88. The casing 98 is welded to a pipe section 104 extending in spaced relation around the rod 80 and slidably mounted in the casing 81 provided with a surrounding packing or hearing sleeve 1115. The inner end of the casing 81 is provided with a packing seal 106 fitting around the rod 80.

During the operation of the mill, the movable chutes 68 may be shifted to any desired position with respect to the partition screen 56 by moving the lever 91 in or out, and the movable chutes 68 may be set in a predetermined position to deliver the required amount of coarse material and maintained there by moving the lever 91 to the corresponding position and locking it to the frame 89. The lever 91 is locked in the desired position by means of a handle 107 fixed to a nut 108 threaded onto a bolt slidable in the slot of an arcuate member 109. When the nut 108 is turned up by the handle 107, it clamps the arm or lever 91 to the member 109 in a position according to the desired setting for the movable chutes 68. Regardless of this setting, the pipes 72 and 74 and the rod 80 continue to rotate with the brackets 73 and other elements carried by the screen partition. The pipe 72 provides a thrust shoulder engaging mounting for the brackets 73. Fig. 3 shows the shape of the chute 68 with its widened upper portion fitting the diverging portions of elements 54 and 54.

The plate 82 of the manhole cover unit carries a pair of inspection tubes 110 which are normally covered by caps 111 locked to the tubes 110 respectively by a bayonet joint. Each of the caps 111 carries a handle 112 to facilitate rotation of the cap. By removing a cap 111, it is possible to look directly through the cylinder 64 and inspect the movable chutes 68 and their associated mechanisms. If the nut 1110 is removed the whole control assembly 31 is removable with the manhole cover unit.

In the operation of the apparatus to carry out the improved method, the rate of production of fine material as delivered through the duct 41 is readily determined, from which it is a comparatively easy matter to set the lever 91 and the movable chutes 68 to give the required proportion of coarse aggregate. Particular mixtures of coarse and fine materials for the production of electrodes is known in advance in accordance with known practice, and it is a very simple matter to produce those proportions simultaneously on the present apparatus. In general the finely ground material produced by the apparatus of the present invention by an air sweeping mill comprises from 35 to 85% which would pass through a 200 mesh screen. In this connection, the screen partition of the mill has a free flow area which is at least equal to the free flow area of the outlet of the mill through the outlet trunnion. may contain a small proportion of finely ground material but usually does not include more than from 9 to 13% which would pass through a 200 mesh screen. Since the coarse aggregate delivered by the chutes 68 flows directly into the air stream flowing through the cylinder 64, most The coarse aggregate produced by the mill of the finely ground material will be picked up by the air stream. On the other hand, a slight amount of the fine material may settle out in the outlet chute 26 and may flow into the duct 28 along with the coarse material delivered from the cylinder 64. The body of coarse material in the conveyor 43 prevents the in-flow of air through duct 28.

In a particular operation carried out by the apparatus, in which the fine and coarse materials were produced simultaneously in the desired proportions for the electrode mixture, approximately 44.6% of the mixture comprised coarse material delivered through the duct 28 and the screw conveyor 43. 42% of the mixture was obtained from the cyclone separator 35, while the remaining 12.4% was obtained mostly as dust from the cyclone separator 37. The material obtained from the cyclone separator 37 was found to contain a small amount of 48 mesh material, 100 mesh, 200 mesh and 200 mesh material. Material of these meshes or sizes was also obtained from the cyclone separator 35, including some 28 mesh material.

Material delivered through the duct 28 was screened into coarse and intermediate fractions respectively comprising about 21.2% coarse and 23.4% intermediate. The coarse fraction contained material of 4, 8 and 14 mesh sizes, while the intermediate fraction contained material of 14, 28 and 48 mesh sizes.

The method and apparatus of the present invention provides means by which the proper proportions of coarse and fine carbonaceous materials are simultaneously obtainable ready for use in the manufacture of electrodes. Furthermore, this improved result is accomplished without the use of two or more pieces of crushing and grinding equipment. The method and apparatus provides an economical system for producing carbonaceous mixtures suitable for the production of electrodes and one in which large quantities of electrode materials can be produced to supply the enormous metal industries for the production of aluminum and magnesium.

I claim:

1. An apparatus for simultaneously producing separate fractions respectively of coarse and fine carbonaceous materials from raw unground carbonaceous stock for mixing to produce a mixture suitable for making electrodes for use in aluminum production, comprising a rotary grinding mill adapted to simultaneously produce the fine and coarse materials from said raw stock, said mill being provided with an inlet for supplying the carbonaceous stock to be ground and an outlet for the discharge of the ground material, a partition screen in said mill between the inlet and outlet thereof dividing the mill into a grinding chamber at the inlet end portion of the mill and into a receiving chamber for coarsely ground material at its outlet end portion, loose grinding elements in the grinding chamber, means for air-sweeping the grinding mill with a stream of air flowing from the inlet through the outlet of the mill for collecting the fine material as produced in the mill, means for separating the fine material from the air stream to provide the fine fraction, means for conveying coarse material from the receiving chamber through the outlet of the mill, and adjustable means in the receiving chamber for selectively delivering coarse material in the receiving chamber into said conveying means.

2. An apparatus as claimed in claim 1, in which said adjustable means comprises a chute movable to ditferent positions for receiving varying quantities of coarse material as the mill is rotated.

3. An apparatus as claimed in claim 1, including lifter means for coarse material located in said receiving chamber for lifting coarse material as the mill rotates, said lifter means being adapted to deposit the lifted coarse material onto said adjustable means.

4. An apparatus as claimed in claim 1, including at least one lifter unit located in said receiving chamber and rotatable with the mill for elevating coarse material therein into the upper portion of the mill as the mill rotates, said t adjustable means comprising a chute cooperating with said lifter unit, and means for setting said chute to different positions with respect to said lifter means for controlling the rate at which coarse material is delivered into said conveying means.

5. A generally horizontal rotary tube mill provided with hollow inlet and outlet trunnions through which the raw material to be ground and the ground material are respectively conducted, a transverse screen partition located in the outlet end of said mill in spaced relation to the end portion of the mill providing a collection chamber for coarsely ground material between said partition and the portion of the mill, at least one lifter unit in the collection chamber rotatable with the mill and adapted to lift coarse material into the upper portion of the collection chamber as the mill rotates, a conveyor for coarse material located in said outlet trunnion and rotatable with the mill, an adjustable chute operatively associated with said lifter unit for directing coarse material lifted by said unit into said conveyor, and means for adjusting said adjustable chute for controlling the rate at which coarse material lifted by the lifter unit is directed into said conveyor, said adjustable chute being rotatable with the mill.

6. A mill as claimed in claim 5, in which the lifter unit includes a plate between the partition and the outlet end of the mill extending toward the periphery of the mill with its outer end curving in the direction of rotation of the mill, said chute being located on the concave side of said plate, and means to the rear of said partition for pivoting said chute.

7. A mill as claimed in claim 6, in which the chute includes a movable portion movable toward and away from said partition and normally resting against the rear face of said partition when fully open for receiving coarsely ground material.

8. A mill as claimed in claim 7, including an operating rod extending axially through the outlet opening of the mill and connected to said chute, and means outside the mill for moving the rod longitudinally for pivoting the chute to a selected position.

9. A mill as claimed in claim 8, in which said rod is rotatable with the mill, and means outside the mill for locking the rod against longitudinal movement while permitting its rotation with the mill.

10. An apparatus for simultaneously producing separate fractions of coarse and fine carbonaceous materials from unground carbonaceous stock for use in producing a mixture suitable for making electrodes, comprising a rotary grinding mill adapted to simultaneously produce the fine and coarse materials from said stock, the grinding mill being provided with a partition screen dividing it into a grinding chamber and a collecton chamber for coarsely ground materal adapted to make up the coarse fraction, means for air sweeping the grinding mill with an air stream to pick up the fine material as produced therein, means for separating the fine material from the air stream to provide the fine fraction, means for conveying the recovered coarse fraction from the collection chamber, means in said collection chamber for delivering coarsely ground material into the conveying means, and means for adjusting said delivering means to vary the rate at which the coarsely ground material is delivered to said conveying means, whereby the coarsely ground material delivered to the conveying means may be proportioned in accordance with the rate of production of the fine fraction.

References Cited in the file of this patent UNITED STATES PATENTS 448,844 Burnham Mar. 24, 1891 1,200,104 Haskell Oct. 3, 1916 1,698,758 Knittel Jan. 15, 1929 2,527,595 Swallen et al. Oct. 31, 1950 2,563,285 Shea Aug. 7, 1951 2,620,987 Kennedy Dec. 9, 1952 2,664,248 Martin Dec. 29, 1953 FOREIGN PATENTS 14,234 Great Britain June 12, 1914

Claims (1)

1. AN APPARATUS FOR SIMULTANEOUSLY PRODUCING SEPARATE FRACTIONS RESPECTIVELY OF COARSE AND FINE CARBONACEOUS MATERIALS FROM RAW UNGROUND CARBONACEOUS STOCK FOR MIXING TO PRODUCE A MIXTURE SUITABLE FOR MAKING ELECTRODES FOR USE IN ALUMINUM PRODUCTION, COMPRISING A ROTARY GRINDING MILL ADAPTED TO SIMULTANEOUSLY PRODUCE THE FINE AND COARSE MATERIALS FROM SAID RAW STOCK, SAID MILL BEING PROVIDED WITH AN INLET FOR SUPPLYING THE CARBONACEOUS STOCK TO BE GROUND AND AN OUTLET FOR THE DISCHARGE OF THE GROUND MATERIAL, A PARTITION SCREEN IN SAID MILL BETWEEN THE INLET AND OUTLET THEREOF DIVIDING THE MILL INTO S GRINDING CHAMBER AT THE INLET END PORTION OF THE MILL AND INTO A RECEIVING CHAMBER FOR COARSELY GROUND MATERIAL AT ITS OUTLET END PORTION LOOSE GRINDING ELEMENTS IN THE GRINDING CHAMBER, MEANS FOR AIR-SWEEPING THE GRINDING MILL WITH A STREAM OF AIR FLOWING FROM THE INLET THROUGH THE OUTLET OF THE MILL FOR COLLECTING THE FINE MATERIAL AS PRODUCED IN THE MILL, MEANS FOR SEPARATING THE FINE MATERIAL FROM THE AIR STREAM TO PROVIDE THE FINE FRACTION, MEANS FOR CONVEYING COARSE MATERIAL FROM THE RECEIVING CHAMBER THROUGH THE OUTLET OF THE MILL AND ADJUSTABLE MEANS IN THE RECEIVING CHAMBER FOR SELECTIVELY DELIVERING COARSE MATERIAL IN THE RECEIVING CHAMBER INTO SAID CONVEYING MEANS.

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