Fluid-energy Mill And Process

Abstract

The kinetic energy of high-velocity jets of a gaseous fluid is employed to violently agitate and swirl solid particles. A gaseous fluid is introduced into the mill as plural jets, having supersonic velocity, which are directed at an angle around a circular section of the grinding chamber. Particulated solid is entrained within the jets to form a fluid mass which is swirled within the circular section of the chamber to effect attrition of the particles. The particles are attrited to smaller size by violent impact and shear resulting from rapidly moving particles striking one another while also striking stationary grinding surfaces within the chamber of the mill. Subsequently, the attrited particles are separated from the gaseous fluid and collected as product. The invention is especially suitable for grinding colloidal pigments in order to reduce agglomerates of the pigment particles to more discrete particles.

Description

This invention relates to size reduction of finely divided solids by means of fluid-energy grinding of the particles. More particularly, it relates to fluid-energy milling of essentially dry particles of a colloidal pigment to reduce agglomerates thereof to more discrete particles.

Fluid-energy grinding processes are known in which jets of a gaseous fluid, e.g. air, are injected at supersonic velocity into a circular grinding section of a milling chamber. The jets are directed to intersect a hypothetical tangent circle that is of significantly smaller diameter than the circumferential periphery of the grinding section of the chamber. Solid particles of the material to be ground are fed into the grinding section for entrainment within the jets of gaseous fluid. The velocity of the gaseous jets is imposed upon the solid particles causing them to swirl about the axis of the chamber so as to grind themselves by violent impact with one another.

In such processes, circular grinding chambers of sufficient diameter and cross-sectional area to permit considerable velocity gradient across the spiraling mass of gaseous fluid and entrained solid particles are employed since such operations involve size classification of particles during the grinding thereof. The coarse particles migrate to the circumferential periphery of the grinding chamber while the finer particles migrate toward the center of the spiral. By employing an axially positioned discharged outlet, the fine particles are removed from the milling chamber once they have migrated to the center of the spiral, while the coarser particles remain toward the outer edge of the spiral until they have been reduced to a size which permits their migration to the center of the chamber. By directing jets of gaseous fluid toward a hypothetical circle which is significantly smaller in diameter than the internal circumferential periphery of the grinding chamber, but larger than the diameter of the axial discharge outlet, the classifying operation is facilitated since the solid particles are not forcefully directed toward either the outer or inner regions of the spiral; i.e., the jets are only employed for imparting a spiraling motion to the mass of particles without impeding their autoclassification by centrifugal force. The object of this centrifugal classification of the particles is to prevent overgrinding of fine particles while permitting coarse particles to remain in the grinding chamber until they are reduced to fine particles. Thus, particles can be attrited to a relatively uniform size with reduced tendency for overgrinding of fine particles and undergrinding of coarse particles. This feature is particularly important when the particulate solid material is subject to progressive reduction in particle size as the attrition continues and when important physical characteristics of the material are strongly dependent upon its ultimate particle size, e.g. titanium dioxide pigments are attrited to reduce their grit content, but suffer a loss in whitening power if the pigment particles are overground.

It should be pointed out, however, that it is not important to prevent overgrinding of some solid materials. The ultimate particle size of certain colloidal pigments, e.g. fine carbon blacks, is not reduced to any significant extent even by severe grinding processes. It may be necessary, however, to subject friable aggregates or agglomerates of such pigment particles to severe attrition for disrupting of combined particles into more discrete particles. In such cases, classification of finer particles toward the central axis of the chamber for prompt removal from the mill will serve no useful purpose. If classification of the pigment particles were deliberately avoided, the kinetic energy of the jets of gaseous fluid could be more effectively utilized for grinding of the aggregated or agglomerated particles. Accordingly, the grinding operation could be carried out with greater speed and with less expenditure of the gaseous fluid that supplies the kinetic grinding energy.

It is an object of this invention to provide improved fluid-energy grinding process and apparatus whereby classification of particles during the grinding thereof is essentially avoided.

Another object of the invention is to provide an improved fluid-energy grinding process and apparatus that results in more efficient grinding of particulate solids.

Another object of the invention is to provide an improved fluid-energy grinding process and apparatus for reduction of friable aggregates or agglomerates of colloidal pigment particles in order to produce more discrete particles of the pigment.

These and other objects and advantages are obtained by means of the invention as hereinafter described, the novel features of which are set forth in appended claims.

In the present invention, plural jets of a gaseous fluid e.g. air, are injected into a circular grinding chamber a supersonic velocity, the projection paths of the jets being directed tangentially with respect to the outer circumferential periphery of the chamber and essentially transversally with respect to the axis thereof. The particulate solid material is fed into the chamber and mixed with the jets of gaseous fluid to form an annular mass which advances axially through the chamber while spiraling around the axis thereof. The constituents of this annular, spiraling mass are maintained away from the axis of the chamber and are confined within the outer region thereof into which the jets of gaseous fluid are injected tangentially. The solid particles within the annular spiraling mass thus remain essentially unclassified during the attritioning operation. It has been discovered that friable aggregates, e.g. pellets or agglomerates, of the material can be more quickly and effectively reduced to discrete particles than when a milling process involving classification of the particles is employed.

The annular, spiraling mass of particles and gaseous fluid is confined toward the internal surface of the circular wall of the grinding chamber by extreme centrifugal force resulting from the supersonic tangential injection of the jets of gaseous fluid. In other words, the centrifugal force exerted upon each of the particles in the grinding section of the chamber may be so great as to prevent migration of even the finest particles toward the central axis of the mill. In most instances, however, classification of the particles can be more reliably avoided by employing a grinding chamber having a structurally bounded, annular cross section so that migration of the smallest particles toward the axis of the grinding section of the chamber is prevented by an impervious, circular barrier that surrounds the axis of the grinding section.

FIG. 1 is a sketch in vertical section of an apparatus of the present invention; and

FIG. 2 is a horizontal cross section of the apparatus of FIG. 1 along line 2-2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIG. 1, a grinding chamber, represented at 1, is enclosed by a steel casing 2. The grinding chamber has three sections: a dome section 1a into which solid particles are fed through a coaxial inlet 3, a generally cylindrical section 1b in which jets of gaseous fluid and a flowing stream of solid particles are combined to form an annular, spiraling mass and wherein attritioning of the particles occurs, and a converging frustoconical section 1c wherein the spiraling annular mass is constricted and then passed out of the chamber through a coaxial discharge outlet 4. Jets of a gaseous fluid are fed into the section 1b at supersonic velocity through nozzles 5. As shown in the drawings, an impervious core member 6 is positioned centrally within the circular section 1b. The core member has an essentially circular cross section of significantly smaller diameter than section 1b, is coaxial with respect to that section, and has a surface 6a which is coextensive with wall section 2a to create an annular space 1b.sup.1 within the grinding chamber. The core member is rigidly suspended within the grinding chamber by means of struts 7.

In operation, a continuous flowing stream of solid particles is fed into the grinding chamber through inlet 3. The stream of flowing particles impinges upon dome 8 of the core member 6. The particles then flow outwardly over the surface of the dome and are distributed essentially uniformly radially into the annular space 1b.sup.1 where they become mixed with supersonic jets of gaseous fluid introduced into the chamber through the nozzles 5. The projection paths of nozzles, represented by the broken lines 5 a, are directed essentially tangentially with respect to the internal surface of the casing 2 which surrounds the annular space 1b.sup.1 and essentially transversely with respect to the axis of section 1 b of the grinding chamber. The spiral of gaseous fluid and solid particles formed in annular space 1b.sup.1 rotates at very high velocity, and the particles are attrited by impacting against one another and by striking the casing wall 2 a surrounding the annular space. As the operation proceeds, the annular, spiraling mass advances axially toward the discharge outlet and is gradually and progressively constricted within the frustoconical section 1c, prior to discharge from the outlet 4, without immediate fractionation of the ground particles and gaseous fluid.

It should be pointed out that in the apparatus of FIG. 1 attritioning of the particles occurs essentially within the annular space 1b.sup.1. Grinding of the particles is effected at least in part by the forceful propulsion thereof against the inner surface of the essentially cylindrical section of wall 2 a which surround the annular section 1b.sup.1 of the grinding chamber. It will also be appreciated that attritive impact between particles is considerably enhanced by the fact that they are not permitted to disperse across the full diameter of the chamber during grinding since they are crowded together for intimate contact with one another within annular space 1 b.sup.1, and are thus maintained away from the axis of the grinding section of the chamber by the core member 6. Accordingly, any fine particles which tend to migrate toward the axis of the chamber is in section 1 b are instead maintained within the violently turbulent region which exists in the annular space 1b.sup.1 by means of the essentially cylindrical surface 6 a of the core member which runs coextensively with the wall section 2a of the grinding chamber.

It will thus become apparent that the essence of the invention is formation of an annular, spiraling mass of solid particles and gaseous fluid wherein the particles are ground in an unclassified state while being confined to the outer wall of the annular section of the grinding chamber while the mass is violently agitated and caused to swirl by tangential injection of the gaseous fluid at supersonic velocity. After attrition of the particles is completed, the annular, spiraling mass may conveniently be constricted to a smaller cross-sectional area as, for example, by means of the convergent frustoconical section as shown in the drawing or by means of a flat orifice plate. This feature serves primarily to reduce the diameter of the off-take conduit from the grinding chamber when the mill is relatively large. When smaller diameter mills are employed, no reduction in the cross section of the mill chamber is necessarily required. Furthermore, the ground particles may be at least partially fractionated from the gaseous chamber before discharge from the grinding chamber, but from the standpoint of compactness and simplicity of design, the chamber may be provided with only one discharge outlet for removal of solids and gas without any intermediate fractionation thereof. Accordingly, the gas-solids mixture may be passed to an external separator and collector, e.g. a cyclone, bag filter or a combination thereof, for fractionation of the mixture after removal from the grinding chamber.

Any suitable means may be employed for feeding a stream of solid particles into the grinding chamber. The particles, may for instance, be premixed with the gaseous fluid which is injected tangentially into the chamber for formation of the annular and spiral therein or it may be introduced separately and axially as shown in the drawings. In FIG. 1, an eductor, generally represented at 9, may be employed for propelling the solid particles into the chamber. Thus, a jet of gaseous fluid is blasted from nozzle 10 into the throat 11 of the eductor so that the solid particles are aspirated from a feed conduit 12 and propelled into the chamber through inlet 3. However, a series of axially extending vanes 13 may be installed proximal to the discharge outlet of the chamber to disrupt the annular spiraling flow of the gas-solids mass and thus convert the flow to a more linear pattern. When this is done, the resistance to axial flow of the mass through the grinding chamber may be relieved to the extent that a negative pressure is created at the solids inlet of the chamber. The mill can thus be made self-feeding with respect to the solid particles.

The invention may be utilized for the grinding of any suitable frangible solid particles which are subject to size reduction of the ultimate particles by means of the fluid-energy principle, e.g. ores, coal, clay, cement, sandstone or the like. It may also be used to particular advantage for grinding colloidal pigments such as carbon black in order to reduce friable aggregates or agglomerates of the particles to more discrete particles. The particles may be fed to the grinding chamber as free flowing granules or a powder. Fine particles which have been formed into macroscopic pellets may also be used as the feed provided the pellets are sufficiently friable for rapid redispersion of the particles by means of the grid grinding action of the invention. Thus, powders of very fine colloidal pigment particles may be treated in the grinding chamber for reduction of agglomerates of the particles to more discrete particles, and where preferable and practical, the powder may be previously pelletized since both pellets and agglomerates may be reduced to unusually discrete particulation.

The gaseous fluid which is injected supersonically and tangentially into the grinding chamber may be any gas or vapor which is not detrimentally reactive with the solid being ground. The choice thereof will otherwise be influenced by availability and cost. Thus, air or steam may frequently be used to particular advantage since they are most economical and readily available in a highly compressed state. Other gaseous fluids may also be employed where preferable and practical. Furthermore, a chemical treating agent may be introduced into the grinding chamber along with the gaseous fluid and the solid particles when it is desirable to combine the particles with a gas or vapor for further improving the properties of the material being ground. The chemical agent may, therefore, be combined with the solid particles by reaction, absorption or adsorption while the particles are being ground within the milling chamber. The type of chemical treating agent which is employed in such cases will, of course, depend upon the material being ground and the nature of the property alteration sought, but it will be readily apparent that a large variety of treating agents such as oxidants, surfactants and the like may be added to the milling chamber in order to achieve high efficacious contact between the treating agent and the solid particles.

The proportion of gaseous fluid to solid particles which are fed into the mill and the minimum rotational velocity necessary to effect grinding within the milling chamber cannot be stated in general terms since the essential dynamic conditions vary within such factors as changes in the type of materials fed to the mill, the desired end result, the design and size of the mill, and to some extent the severity of grinding that is required to achieve a particular fineness level in the finished product.

When the grinding chamber is provided with an impervious core member as shown in FIG. 1, the diameter of the core member should be sufficiently large to significantly reduce the unobstructed cross-sectional area of the chamber. Advantageously the outside diameter of the impervious core member, at the grinding section of the mill chamber, will be at least about one-half the diameter of the grinding section.

Since grinding of the solid particles is effected to a considerable extent by violent impact of the particles with the section of the grinding chamber wall which surrounds the annular space 1b, the inside surface of the wall section 2a should be abrasion resistant, e.g. provided with a coating or liner of extremely hard material. The remainder of the mill can be fabricated entirely from easily workable and readily available materials such as mild steel or stainless steel.

The injector nozzles 5 need not have any special internal configuration such as tapered inlet and outlet bores which intercommunicate through a cylindrical neck. A straight cylindrical bore throughout the length of the nozzle is satisfactory in the present invention for conveying the gaseous grinding fluid into the milling chamber at supersonic velocity, but as previously indicated, the projection paths of the nozzles should be directed to intersect the inside surface of the grinding chamber wall essentially tangentially, i.e. the angle of intersection should not vary from tangential by more than about 10.degree. and preferably no more than 5.degree. or less. In addition, the projection paths of the nozzles should be directed essentially transversally with respect to the axis of the milling chamber, i.e. no more than .+-.45.degree. from perpendicular with respect to the axis and preferably no more than .+-.20.degree. or less.

When employing the invention for the grinding of carbon blacks in order to reduce friable agglomerates of the particles into more discrete particles, it has been found that a mill constructed substantially in accordance with FIG. 1 will perform best with air when injected at a velocity within the range of about mach 2 to mach 3, the proportion of air to carbon black which is fed tangentially into the grinding chamber is within the range of about 50 to 80 SCF (cubic feet measured at standard conditions) per pound of black which is fed to the chamber, and the rotational velocity of the annular spiraling mass within the grinding section of the chamber is within the range of about 475 to about 625 feet per second.

As has been previously pointed out, the present invention may be used to particular advantage in the fluid energy grinding of pigments such as carbon black since classification of the particles during grinding --as occurs in prior mills --is deliberately avoided and the kinetic energy of the gaseous grinding fluid is, therefore, much more efficiently utilized for attrition of the agglomerates. More specifically, carbon blacks having a 20-- 30percent by weight content of agglomerates in excess of about 1 micron diameter can be treated to reduce the content of agglomerates above about 1 micron diameter to less than 10 percent by weight.

In carrying out the following experiments, a fluid energy mill constructed essentially in accordance with FIG. 1 was employed. The internal diameter of the grinding section 1b was 13 inches and the outside diameter of the cylindrical surface 6a of the core member was 85/8 inches. The length of cylindrical surface 6a was 4 inches and the overall length of the grinding chamber was 185/8 inches. The inlet 3 had a diameter of 2 inches and the outlet 4 had a diameter of 6 inches. The grinding chamber was provided with four vanes 13, as shown, each measuring 1/4 .times. 2 .times. 6 inches. Eight nozzles were equispaced around the circumference of the grinding chamber, each having a straight bore of 1/2 inches diameter .times. 3 inches length. The projection path of each nozzle was tangential with respect to the internal surface 2a of section 1b of the grinding chamber and transversal with respect to the axis of that section. No eductor 9 was employed for feeding the carbon black into the mill, since it fed itself by aspiration through inlet 3.

In the first example, two commercial grades of carbon black were attrited in the fluid-energy mill of the present invention and two prior-art mills constructed and operated generally in accordance with processes described in reference to FIG. 8-51 (Process A) and FIG. 8-52 (Process B) in Perry's Chemical Engineering Handbook, Fourth Edition. The carbon blacks which were employed in the example are marketed under the proprietary trade names Raven 40 and Peerless 155, the former being a medium-flow, low-structure furnace black and the latter a long-flow, low-structure furnace black. Both blacks were ground into each mill, using compressed air as the grinding fluid, to effect a specific degree of grinding as indicated by the dispersing characteristics of the attrited products in an ink vehicle. Operating conditions and results are shown in Table I. ##SPC1##

In order to obtain the Comparative Dispersion Rating of a black, the following tests are run in accordance with well known PC and NPIRI Production Grindometer procedures: 1. PC Microns 2. Sand 25.0--10. Microns 3. Sand 10.0-- 0 Microns 4. Scratches 4(in microns) 5. Scratches 10 (in microns) The Comparative Dispersion Rating is then determined by means of the following formula: CDR No. percent = 250 - (the sum of the numerical values obtained is tests 1,2, 3, 4, and 5, above) .times. 0.4. Higher CDR numbers are associated with better dispersion; hence, any improvement in the dispersability of a black in indicated by an increase in its CDR value.

In obtaining CDR values shown in Table I, each black was milled for one pass at 22 percent loading in Litho 3 vehicle on a 4 inch, three-roll laboratory mill set at 350 p.s.i. In commercial practice, an ink mix is subjected to multiple passes on a three-roll mill to achieve an optimum dispersion level. Multiple-pass mixing tends to negate any differences in dispersion which can be determined from the aforementioned tests, and it is, therefore, more meaningful to determine the test values after one pass when the purpose of the tests is to compare the grinding efficiency to one fluid-energy milling method against another. It will be appreciated, however, than when higher CDR values are obtained on the first pass, less total milling of the mix is required to achieve the specific level of dispersion required in the finished ink compound.

As can be seen from Table I, the Raven 40 carbon black was attrited to equivalent dispersion levels by all three grinding methods, but it is especially significant that the black could be ground at a much higher rate with the present invention while feeding the mill a substantially lower proportion of air to carbon black. The same type of results were obtained with the Peerless 155 black, and even though an insufficient ratio of air to black was used with Process B to obtain an equivalent level of dispersion as with Process A and the present process, it may be assumed that use of a lower ratio would not have provided equivalent dispersion, since in all trial cases with both Processes A and B, progressive reduction in the air-to-black ratio resulted in proportional reduction of the CDR values of the attrited black.

In another example, the present invention was employed for grinding Peerless 155 carbon black which had been converted into pellets by a dry pelletizing process, intending to demonstrate that the pellets could be efficiently reduced back to a powder while at the same time reducing agglomerates, which existed in the original powder, to more discrete particles. Using the mill previously described, the pellets were fed to the grinding chamber at the rate of 1,200 lbs. per hour while air was introduced at the rate of 1,600 SCFM. The resultant milled product is shown in Table II compared to an attrited Peerless 155 carbon black which had been fed to the mill as a powder rather than pellets. The same feed rate of black and air was employed with both powder and pellets. ##SPC2## Note: Both samples 22percent loading in Litho 3, one pass on three-roll mill.

Thus, as can be seen from Table II, equivalent dispersion was obtained for the Peerless 155 black, when milled in accordance with the present invention, whether fed to the mill in the form of dry pellets or powder. Additional tests showed that these two milled products also had equivalent ABC color value and wetting time. These are most unexpected results in view of the fact that it was necessary in the milling process to reduce the pellets (macroscopic, i.e. 35 to 60mesh, U.S. Standard) back to powder before the agglomerates (microscopic, i.e. 1 to 20Microns) could be reduced to a more discrete particulation which assured excellent dispersibility in an ink compound.

It will be understood that various changes and modifications in the details and arrangement of parts and materials, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

Claims

We claim:

1. A fluid-energy process for grinding particles of frangible solid materials comprising: a. feeding a stream of said particles into an elongated circular grinding chamber; and b. injecting a gaseous fluid at supersonic velocity into the grinding chamber essentially tangentially with respect to the axis thereof, thus forming an agitated mixture of solid particles and gaseous fluid in which attritioning of the solid particles occurs, said mixture advancing axially through the chamber as an annular mass while spiraling around the axis thereof, the constituents of said annular, spiraling mass being maintained away from the axis of the chamber and confined within the peripheral region of said chamber into which the gaseous fluid is injected tangentially, the solid particles of the annular, spiraling mass remaining essentially unclassified during attrition of said solid particles therein, and forcefully propelling said particles against the wall of the grinding chamber by said injection of the gaseous fluid and thus grinding said particles by violent impact with said wall and by attritive impact between the particles.

2. The process of claim 1 wherein the gaseous fluid is projected into the elongated circular grinding chamber as a plurality of jets from injection points spaced around the circumferential periphery of the chamber.

3. The process of claim 2 in which the mixture of solid particles and gaseous fluid is removed from the grinding chamber without intermediate fractionation of the constituents of said mixture.

4. The process of claim 2 in which the annular, spiraling mass is constricted to a substantially smaller cross-sectional dimension after attrition of the solid particles therein.

5. The process of claim 4 in which the mass is constricted gradually and progressively as the mass advances axially through the grinding chamber.

6. The process of claim 2 in which the annular, spiraling mass is formed within an annular section of the grinding chamber.

7. The process of claim 6 in which the annular, spiraling mass is formed to the inside diameter of at least about one-half the outside diameter thereof.

8. The process of claim 4 in which the spiraling flow of said annular mass is disrupted during the constriction thereof and the flow pattern of said mass is converted to predominately linear flow prior to discharge of the mass from the grinding chamber.

9. The process of claim 2 in which the solid particles are fed into the grinding chamber in the form of powder.

10. The process of claim 2 in which the solid particles are fed into the grinding chamber in the form of granules.

11. The process of claim 2 in which the solid particles are fed into the grinding chamber in the form of friable pellets formed from finely powdered solids.

12. The process of claim 2 in which the particulate solid material is distributed substantially uniformly axially into the region of the grinding chamber in which said jets of gaseous fluid are injected tangentially.

13. The process of claim 2 in which the solid particles are essentially colloidal pigment particles.

14. The process of claim 13 in which the colloidal pigment particles are carbon black.

15. The process of claim 2 in which said gaseous fluid is predominately air.

16. The process of claim 2 in which the solid particles are carbon black particles and the rotational velocity of said annular, spiraling mass is at least 475 feet per second.

17. The process of Claim 16 in which the gaseous fluid is predominately air, the proportion of said jetted gaseous fluid and carbon black which are fed to said grinding chamber is within the range of about 50 SCF to about 80 SCF of fluid per pound of carbon black and the rotational velocity of said annular, spiraling mass is within the range of about 475 feet per second and about 625 feet per second.

18. A fluid energy grinder comprising; a. an elongated energy grinder chamber having a circular cross section bounded by a peripheral wall; b. an inlet opening for feeding particulate solids into said circular section of the grinding chamber; c. a plurality of jet nozzles spaced around the periphery of said grinding chamber for the injection of a gaseous fluid into said circular section of the grinding chamber at supersonic velocity, the jet nozzles being positioned so as to direct said gaseous fluid essentially tangentially with respect to the peripheral wall of said circular section of the grinding chamber and also to direct said fluid essentially transversally with respect to the axis of said circular section said nozzles being positioned to forcefully propel said particulate solids against said wall of the grinding chamber by said injection of the gaseous fluid and thus grind said particulate solids by violent impact with the wall and by attritive impact between the particles; and d. discharge means for recovering a particulate solid and gaseous fluid mixture from the grinding chamber.

19. The apparatus of claim 18 in which said discharge means comprises a discharge outlet having a significantly smaller diameter than said circular section of the chamber into which the gaseous fluid is injected.

20. The apparatus of claim 19 in which the discharge outlet is located coaxially with respect to said circular section of the chamber into which the gaseous fluid is injected tangentially.

21. The apparatus of claim 19 in which the surface of the grinding chamber tapers gradually and progressively inward to surround said discharge outlet.

22. The apparatus of claim 18 and including an impervious core member positioned within said circular section of the grinding chamber into which the jets of gaseous fluid are injected, said core having a circular cross section of significantly smaller outside diameter than the inside diameter of said circular section, said core being in coaxial relationship with said circular section and coextensive therewith.

23. The apparatus of claim 22 in which the inlet opening for feeding particulate solid into the grinding chamber is oriented coaxially with respect to the grinding chamber.

24. The apparatus of claim 22 in which the inlet opening is located on the axial center line of the grinding chamber and the impervious core includes a dome that is axially displaced downstream from said opening and is adapted to distribute a flow of solid particles radially uniformly as they flow out of said inlet opening and into the circular section of the grinding chamber into which the jets of gaseous fluid is injected and said chamber discharge outlet means is located downstream with respect to said circular section of the chamber.

25. The apparatus of claim 22 in which said circular section of the grinding chamber is essentially a cylindrical section and said core has an essentially cylindrical outer surface which is coextensive with the inner wall surface of said cylindrical section.

26. The apparatus of claim 21 including one or more axially extending vanes which are affixed to the inside wall of the grinding chamber proximal to said discharge outlet.