US3048271A - Particle classification - Google Patents

Description

Aug. 7, 1962 Filed Feb. 24, 1960 T. D. SHARPLES 3,048,271 PARTICLE CLASSIFICATION 5 Sheets-Sheet l INVENTOR. THOMAS D. SHARPLES ATTORNEY Aug. 1962 T. D. SHARPLES 3,048,271

PARTICLE CLASSIFICATION Fig. 2

Filed Feb. 24, 1960 5 Sheets-Sheet 2 Fig. 5 26 JNVENTOR. THOMAS D. SHARPLES ATTORNEY T. D. SHARPLES PARTICLE CLASSIFICATION Aug. 7, 1962 5 Sheets-Sheet 3 Filed Feb. 24, 1960 8 Km O P TR NA W H m D S A M O H T ATTORNEY United States Patent 33 18371 PARTICLE CLASSIFICATIUN Thomas D. Sharples, Plymouth Meeting, Pa, assignor to The Sharples Corporation, a corporation of Delaware Filed Feb. 24, 1960, Ser. No. 19,762 11 Claims. (Cl. 209-144) This invention pertains generally to the classification of finely divided solids on the basis of size and density to produce fractions of different degrees of fineness, and more particularly to an improved method and means for feeding the finely divided solids into the zone of classification with resulting increase in classification elficiency.

There are various types of classifiers that operate on the principle of subjecting finely divided solids, hereinafter referred to for convenience as powder, to opposing forces comprised of an outwardly directed centrifugal force and an inwardly directed drag force, the latter resulting from the inward fiow through the classification zone of a fluid, e.g. liquid or gas. In the operation of such classifiers, particles having a relatively lower ratio of surface area to mass, i.e. the larger particles, are thrown outwardly by centrifugal force, whereas particles having a relatively higher ratio of surface area to mass, i.e. the smaller particles, are carried inwardly by the liquid or gas, the fluid drag on the smaller particles offsetting and overcoming the centrifugal force imposed thereon.

The art is replete with all manner of designs and constructions that operate on the broad basic principle above set forth. Centrifugal force is applied to the powder by setting it in rotation such as by feeding it into an inwardly spiralling fluid, hereinafter referred to for convenience in description as air, or by setting the powder in rotation by means of a rotor, or otherwise. The powder may be fed into the classifying zone at its outer boundary, or at any desired point or area intermediate its inner and outer boundaries. The air may flow inwardly through the classifying zone in the form of an inwardly spiralling vortex, particularly if the classifying zone is free of obstructions, or the inward flow of air may be restricted to predetermined paths such as by the use of vanes which may take various shapes, flat, curved or otherwise. If desired, spaced annuli may be positioned transversely of the classifying zone to confine the inwardly flowing air to laminar paths. The classifying zone may have stationary end plates, such as given by way of example in U.S. Patent 2,616,563, or it may have rotating end plates such as given by Way of example in US. Patent 2,796,173, or the end plates may be in part stationary and in part rotating, such as given by way of example in co-pending application Serial No. 665,426, filed June 13, 1957, now Patent 2,943,735. Also the classifying zone may have various shapes, e.g. to produce new and unexpected results such as described and claimed in the foregoing patents and patent application.

While great advances have been made in the efliciency of classification of powders, to which the above-mentioned patents and patent application have made outstanding contributions, there still has remained room for improvement in sharpness of classification, in the degree of separation of fines from coarse, and in the capacity of a machine of a given size. It is to the improvement of these factors that this invention is directed.

In the practice of the present invention the powder is fed into the classifying zone in rotating condition and preferably without a large radially outward component of movement at the moment or instant of its entry into the classifying zone, although, broadly speaking, all lateral flow into the classifying zone in which the axial component of direction exceeds the radial component of direction is contemplated, and within the scope of the invention. Also the powder enters the classifying zone ice is an annularly shaped uniformly distributed deagglom erated condition. g

A preferred means for accomplishing these purposes is to effect relative movement between powder projecting mechanism and a surface against which the powder strikes, e.g. a rotor delivering the powder outwardly through a plurality of circumferentially distributed radial channels against a stationary ring positioned around and spaced from the outlets of said channels, the space between the radial channels and the stationary ring being open to the classification zone.

In the practice of the invention there is no local overloading of the classifying zone, a condition which may exist in the case of a feed through a nozzle or nozzles. Due to the absence of local overloading, the capacity of a machine of a given size is very substantially increased. Also because of the absence of local overloading, the powder to air ratio may be very substantially increased, the result of which is reduced demand on air flow equipment with reduced capital cost. Elficiency is increased due to deagglomeration of powder just prior to entering the classifying zone. Efficiency is additionally increased by the preferred feeding of the powder into the classifying zone in a direction substantally perpendicular to the median surface, or at most in a direction defining a small angle (e.g. up to 15) with respect thereto, the median surface being defined as the imaginary surface of revolution generated by rotating about the axis of the classifier the line which is equidistant between the end walls or surfaces of the classifying zone.

Further features of the invention will become apparent to persons skilled in the art as the description proceeds in connection with the accompanying drawings which illustrate embodiments of the invention, and in which:

FIGURE 1 is an elevation partly in section taken on line l-l of FIGURE 3, and illustrating one embodiment of the invention;

FEGUREZ is an enlarged section shown broken of a portion of the right side of FIGURE 1;

FIGURE 3 is a plan view partly in section of the embodiment of FIGURE 1;

FIGURE 4 is a plan view partly in section and shown broken of the rotor and surrounding structure;

FIGURE. 5 is a sectional elevation shown :broken of another embodiment of the invention; and

FIGURE 6 is a section shown broken of a further embodiment.

Referring now more particularly to FIGURES 1 to 3, at lid is shown a base or support upon which is mounted a cylindrical casing 11 having an inwardly projecting upper rim 12 which supports and to which is attached, e.g. by welding, an annular ring 13 having a rounded inner circular edge 14 which functions as the inner edge of an annular classification zone 17. Resting upon and attached to ring 13, e.g. by bolts or screws, is an annular structural member 15 shown as having a fiat inner bottom surface which serves as the lower end surface or wall of classifying zone 17. The inner bottom and side Member 15 has an upwardly projecting annular portion 18 which supports annular member 19 attached thereto.

Projecting inwardly from member 19 are a plurality of circumferentially spaced pins 20 which project into and support an annular plate Z1, the spacing between 3 member 19 and plate 21 being such as to provide an annular inlet 22 to classifying zone 17, said inlet containing air directing vanes 23. Vanes 23 may be of any design andconstruction known in the art. If desired, they may take any of the forms described and claimed in copending application 657,213, filed May 6, 1957, now Patent 2,943,734. As an alternative, the inwardly flowing air may be controlled in the manner described and claimed in my copending application Serial No. 10,596, filed of even date herewith.

Resting upon and secured to annular plate 21 is a cylindrical support 24 for feed funnel 25 having a downwardly projectingfeed tube 26 surrounded by filler members 27 and 28.

Positioned within casing 11, and encased within an interior casing 30, is housing 31 in which is journaled, in bearings not shown, a spindle 32 having pulley 33 attached to its lower end. Mounted on the upper end of spindle 32 is a circular plate 34 shown shaped about its peripheral under portion in a manner to define a surface of revolution 35 of curved cross section, i.e. a surface generated by revolving a curved line, having a shape as illustrated, around the axis of the machine. Surface 35 which, as illustrated, is rotatable, comprises the inner part of the upper end surface or wall of the classifying zone. The rest of the upper end surtace or wall is supplied by surface 36 on ring 37, the latter being attached to the under surface of plate 21, as illustrated at 33. Surface 36 also is a surface of revolutions.

Positioned on top of plate 34 is an annular member 41 which is provided with a plurality of circumferentially spaced radial channels 42, preferably lined with wearresistant tubes, e.g. ceramic tubes, as illustrated at 43. Superimposed upon member 41 is an annular member 44 secured to plate 34 by any desired means, such as by circumferentially spaced screws as illustrated at 45, thus holding member 41 in position. Member 44 has an inwardly projecting inner circular edge which together with the upper central portion of plate 34 forms a powder feed chamber 47, plate 34, as shown, being provided with a central wear-resistant disc 48 resting upon a somewhat larger disc provided at the center of plate 34. It will be noted that feed tube 26 leads into feed chamber 47, and that radial channels 42 lead outwardly from feed chamber 47. It will also be noted that plate 34, members 41 and 44, spindle 32, and pulley 33 are joined together and comprise the rotor, whereas the rest of the parts remain stationary.

Spaced radially outwardly from the outer ends of channels 42 in a manner to form an annular gap 51 is a wear-ring 52 mounted on ring 37. Ring 52 is so positioned with respect to channels 42 as to intercept the powder fed outwardly through channels 42 upon rotation of the rotor.

While the surface of ring 52 facing gap 51 may be made parallel to the outer periphery of the rotor which in turn may be made parallel to the axis of rotation of the rotor, it is preferred that such inner surface 55 of ring 52, quite apart from the shape of the outer periphery of the rotor, define an angle a with respect to the axis of rotation of between 5 and 15 for highly efficient delivery of the powder into classifying zone 1.7, although values of angle a up to but less than are contemplated, i.e. directions of lateral flow into the classifying zone in which the axial component exceeds the radial component. Ring 52 is preferably of wear-resistant material which conveniently may be ceramic tile, e.g. of tungsten carbide, cemented in position on ring 37.

Any desired means may be employed for effecting the flow of air through classifying zone 17 inwardly through annular inlet 22, and outwardly through annular chamber 56 between casing 11 and casing 30, e.g. blower 57 positioned in conduit 58, the latter leading from bag filter 59 for collection of the fine fraction, filter 59 being connected to chamber 56 by conduit 60.

In the operation of the embodiment of the invention shown in FIGURES 1 to 4, upon the functioning of blower 57, fluid, such as air, flows into annular inlet 22 and acquires an inwardly spiralling fiow upon passing between circumferentially spaced vanes 23, said vanes being canted to atford the desired shape to thespiralling How, the tangential velocity being governed in large measure by the rate of air flow, all of which is well understood in the art. Vanes 23 may be made adjustable for adjustment of the angle of air flow into the classifying zone, all of which is also well understood in the art- The unitary structure comprised of spindle 32 and the parts mounted thereon being in rotation, the powder is fed into chamber 47 through funnel 25 and pipe 26. From chamber 47 the powder flows outwardly through channels 42, thereby acquiring rotary motion, and is projected against ring 52. The relative rotational movement between member 41 and ring 52 develops a relatively high degree of sheer in the powder to deagglornerate any agglomerated particles present, and also to uniformly distribute the powder circumferentially in gap 51 from which the powder enters annular classifying zone 17 laterally thereof, in the form of a rotating annulus comprised of substantially uniformly distributed powder.

The rotation of the annulus of powder entering classifying zone 17 is in the same direction as that of the spiralling air, and for best results the speed of rotation of powder and the speed of rotation of the air at the place or circular area of entry of powder laterally into the air are preferably substantially evenly matched at least within practical approximation.

The powder, upon laterally entering the inwardly spiralling flow of air, is acted upon by the centrifugal force resulting from its rotation, and also by the radial component of flow of fluid through the classifying zone. The result is that the larger particles, having a relatively lower ratio of surface area to mass, are thrown outwardly by centrifugal force, whereas the smaller particles having a relatively higher ratio of surface area to mass, are carried inwardly by the inwardly spiralling air.

The air, bearing the fine fraction, after passing around circular edge 14, enters annular chamber 56, and passes out through filter 59 wherein the fine fraction is collected, the air, free from fines, passing out through blower 57.

The coarse fraction, being thrown outwardly in rotat' ing condition, slides around the inner periphery of member 15 and passes out through tangential opening 61 leading into chamber 62 from which it is withdrawn through outlet 63. The rotation of the rotor, of air, of the annulus of feed powder, and of the coarse fraction deposited on the inner wall of member 15, is clockwise as seen in FIGURES 3 and 4. If desired, tangential opening 61 and chamber 62 may be duplicated as indicated at 65.

It is to be understood that the invention pertains to a new and improved process and apparatus for the feeding of powder into classifying zones generally, and particularly into those in which an outwarly directed centrifugal force on the particles is opposed by an inwardly directed drag force on the particles resulting from the inward flow of fluid, and while the invention has been more particularly described in connection with the feed of powder into a classifying zone intermediate its inner and outer boundaries, it is to be understood that such feed may be at or outwardly from the outer boundary thereof, if desired for any reason.

Also it is to be understood that classifying zone 17 may be of any desired shape with end walls stationary or rotating, or stationary in part and rotating in part, the stationary and/or rotating elements being equipped or not with vanes or other fluid directing means. The invention, however, is particularly suitable for use with classifying zones adapted for free vortex flow of inwardly spiralling fluid in which the axial distance, designated for convenience h, between the opposing end or wall r G) surfaces at any radial distance, designated for convenience r, from the axis of rotation bears within at least practical approximation the following relationship:

kin f T where h is the axial spacing between the end or wall surfaces at the outer boundary of the classifying zone, and r is the radius from the axis of rotation to the outer boundary of the classifying zone. Such classifying zones are more particularly described and claimed in the abovementioned patents and copending application, to the disclosures in which particular reference is made.

Referring to FIGURE 2 of the drawings, classifying zone 17 is considered as having an outer periphery beginning at an imaginary circular line 64 on surface 36 and an inner periphery at circular edge 14, and even though the lower end surface or wall may be flat, its spacing with respect to surfaces 35 and 36, in the preferred embodiment, conforms within practical approximation to the above equation, the feed of powder into the classifying zone being intermediate its inner and outer boundaries. Classifying zones, such as illustrated in FIGURES l to 4, and which have end surfaces or Walls in part stationary and in part rotatnig, are more particularly described and claimed in the above-mentioned co-pending application.

An embodiment of the invention in which the end surfaces or walls of the classifying zone are stationary is illustrated in FIGURE wherein the structure illustrated is similar to that described in connection with FIGURES 1 to 4, except as hereinafter indicated.

The rotor in FIGURE 5 is comprised of a spindle 74 which is shown as being integral with a laterally extending annulus 75, annulus 75 taking the form of members 41 and 44 of FIGURES l to 4 combined into a single unit. Annulus 75 is provided with a plurality of circumferentially spaced radial channels 76 corresponding to radial channels 42 of FIGURES 1 to 4. Channels 76 may be similarly lined with wear-resistant ceramic tubes corresponding tubes 43, if desired.

In the structure of FIGURE 5 feed tube 26 projects into a chamber 47 the same as in the embodiment of the invention previously described.

The feed of the powder into the classifying zone 77 of FIGURE 5 is in all respects similar to that described in connection with the embodiment previously described, the powder being intercepted in its outward flow, upon rotation of the rotor, by ring 52, and deposited laterally into classifying zone 77 in the form of an annular ring of substantially evenly distributed deagglomerated particles through gap 51 intermediate the inner and outer boundaries thereof.

Classifying zone 77 may take any desired shape the same as in the previously described embodiment, and preferably conforms within at least practical approximation to the equation above given.

The operation of the invention described in connection with FIGURE 5 will be obvious to persons skilled in the art in view of the description of the operation of the previously described embodiment, the difference being in that, whereas in the embodiment first described the Walls of the annular classifying zone are in part rotating, the walls of the annular classifying zone in the embodiment of "FIGURE 5 are stationary.

As is customary in the design of particle classifiers, seals may be provided at various points between stationary and rotating parts.

It will be understood that various points throughout the classifier may be lined with wear-resistant material, if desired. In addition to such provisions already referred to, it will be noted that wear-resistant ceramic tile are illustrated at 81 in coarse fraction collecting chamber 62 in both embodiments, and at 82 in chamber 56. Their use is, of course, optional. -An excellent ceramic for use as wear-resistant material at any point of wear herein I is aluminum oxide. Tungsten carbide which in a sense is ceramic-like is also very useful.

Referring now to FIGURE 6, it will be noted that wear ring 83 is provided with an annular recess 84 opposite the delivery ends of channels 42. Otherwise stated, wear ring 83 is provided with an inwardly extending annular lip 35 at the entrance of powder into the classifying zone. The stepped configuration of wear ring 83 shown in FIG- URE 6 is found to increase somewhat the efiiciency of Powdered Mississippi limestone, a diflicult material to classify, was fed to a particle classifier of the type shown in FIGURES l to 4 of the drawings, the feed of powder being outwardly against a stationary wear ring as illustrated. The feed rate of powder was 280* pounds per hour, the air flow was 135 cubic feet per minute, and the separation point was 12 microns. The fine fraction thus obtained contained 74% of the fines of the particular separation point characteristics available in the feed.

The term separation point as used in this and other examples means the micron size which is not exceeded by the size of 99% of the particles in the fine fraction collected.

Example 2 This run was carried out in a classifier very similar to the one employed in Example 1, exceptthat the powder was fed directly into the classifying zone near the periphery of the rotor instead of against a wear ring. The powder was of Mississippi limestone, the powder feed rate was 242 pounds per hour, the air flow was 140 cubic feet per minute, and the separation point was 13 microns. The fine fraction thus obtained contained 65% of the fiines of the particular separation point characteristics available in the feed.

Example 3 This run was carried out in the classifier employed in tics available in the feed.

Example 4 Example 5 This run was carried out with hydrated alumina and in a classifier similar to that employed in Example 1, except that it was considerably larger, having a classifying zone approximately 27" in diameter, and a rotor approximately 23 /2 in diameter. The speed of rotation was approximately 2275 rpm. The feed rate was 3 /2 I tons per hour, the air flow 3500 cubic feet per minute,

and the separation point was 20 microns. The fine fraction contained 95 %'of the fines of the particular separation point characteristics available in the feed.

7 Example 6 Comparative runs were made under substantially the same conditions with the same source of powder, the first with a wear ring of the shape shown at 52 in FIGURE 2, and the second with a stepped wear ring as shown at 83 in FIGURE 6. The fine fraction contained in the first case 71.5% and in the second case 75.2% of the fines of the particular separation point characteristics available in the feed.

The classifier employed in Examples 1, 3 and 6 had a classifying zone approximately 8" in diameter, the rotor had a diameter of approximately 7", and the speeds of rotation were approximately 7800 r.p.m., 1800 r.p.m., and 7800 r.p.m., respectively In Examples 2 and 4 the classifying zone also was approximately 8" in diameter, the rotor was approximately 6%" in diameter, and the speeds of rotation were approximately 9000 rpm. and 1570 r.p.m., respectively.

As brought out in the above-mentioned patents and pending patent application, vortex classifiers having classification zones or chambers conforming to the above equation operate on the principle of balancing, for a particle of given size, centrifugal force with drag force, at any given point in the classification zone, so that theoretically any particle of such size should move neither outwardly nor inwardly, and accumulation of particles of such size should eventually clog the classifying zone. It is found in practice, however, that the operation of such classifiers departs sufiiciently from the theoretical to preclude the occurrence of such condition.

Moreover, it is found that, for practical purposes, the ideal shape as given by the above equation may be departed from somewhat Without seriously significant loss in advantage, so that practical approximation to the ideal shape is sufficient.

Furthermore, While the best mode contemplated by me of carrying out my invention is in combination or association with a vortex classifier having a classifying zone of a shape conforming within practical approximation to the above equation, outstanding advantages of the invention may be realized with classifiers generally, irrespective of construction or mode of operation, wherein classification is effected by subjecting the particles to the opposing action thereon of centrifugal force (which is outwardly directed) and of the inwardly directed drag force of an inwardly flowing fluid.

The powder is fed into the classifying zone laterally thereof in rotating condition, the direction of rotation and preferably, at least within practical approximation, the axis of rotation being the same as that of the fluid. For best results, the speed of rotation of powder is, at least within practical approximation, the same as that of the fluid around the area of entry of powder into the fluid. The powder enters the classifying zone in diffused condition and in annular form in which the particles are for practical purposes uniformly distributed, the annulus taking any of a number of shapes such as that of a hollow cylinder or frustocone, or otherwise, such entry at the instant thereof having a greater axial component of flow than radial.

Thus for convenience in the claims, and unless otherwise modified, the terms lateral andlaterally, or their equivalent, when used in connection with the direction of entry of powder into the classifying zone, are intended to contemplate an angle a up to but less than 45, or a radial component of flow of powder less than that obtainable with such angle. When the surface of the ring against which the powder strikes by impact or collision upon rotation of the rotor is other than bounded by a straight line falling in a plane passing through the axis of rotation, as shown in FIGURES l, 2 and 5, e.g. as illustrated in FIGURE 6, then it is the imaginary straight line, falling in a plane passing through the axis of rotation and which is brought as close as possible to such surface, that is to be used in the determination of angle a. 7

While in the above particular description the feed channels on the rotor for powder have been illustrated and described as radial, it will be understood that any other arrangement, disposition and/ or shape of such channels on the rotor may be substituted in effecting the outward flow of powder for relative rotary movement with respect to a surface intercepting the outward flow of such powder, and while the intercepting surface has been more particularly described as a wear ring, it is to be understood that this is for purposes of illustration and not of limitation, for such surface may be provided in any other manner. Likewise, any other method or means for effecting the relative rotary movement between outwardly projected powder and a surface for intercepting such powder may be substituted.

Gap 51 may be of any desired width. Preferably it is of suificient width to accommodate the feed of powder into the classifying zone at the desired rate. Smaller width gaps result in greater sheer, whereas larger width gaps allow greater rates of feed. In any case, substantially all, or in other words, at least approximately all, of the outwardly projected powder strikes and is intercepted by the wear ring or other similar surface before entering the classifying zone with a directional component of flow parallel to the axis of rotation greater than the directional component of flow radial with respect to said axis.

Gap 51 may be of any desired cross-sectional configuration including shapes or end walls which slope inwardly in cross-section toward the classifying zone instead of outwardly as illustrated, or both of which slope inwardly toward the axis of rotation or outwardly from it. Thus it is conceivable to impart a radial inward movement to the powder entering the classifying zone by an appropriate design of gap 51 in which case the axial component of flow is still to predominate, and preferably the resultant radially inward direction of flow of powder is held to a maximum angle of 10 with respect to the axis of rotation.

Other departures from the above particular description may be made. It is therefore to be understood that the above particular description is by way of illustration and not of limitation, and that changes, omissions, additions, substitutions and/or other modifications may be made without departing from the spirit of the invention. Accordingly it is intended that the patent shall cover, by suitable expression in the claims, the various features of patentable novelty that reside in the invention.

I claim:

1. The method of classifying finely divided material on the basis of size and density which comprises directing a fluid toward an annular classifying zone positioned between spaced walls to bring the body of fluid between said Walls into an inwardly spiraling flow, feeding said finely divided material into said classifying zone intermediate its inner and outer boundaries under shear stresses and in the form of an annulus rapidly rotating in the same general direction as said fluid, said material being substantially uniformly distributed throughout said annulus, withdrawing from the inner boundary of said zone a fine fraction of said finely divided material, and withdrawing from the periphery of said zone a coarse fraction of said finely divided material.

2. The method of claim 1 in which said annulus rotates within practical approximation at the same speed as said fluid.

3. The method of classifying finely divided material on the basis of size and density which comprises directing a fluid toward an annular classifying zone positioned between spaced walls to bring the body of fluid between said walls into an inwardly spiralling flow, feeding said finely divided material into said classifying zone intermediate its inner and outer boundaries under shear stresses and in the form of an annulus rapidly rotating in the same general direction as said fluid, said finely divided material being substantially uniformly distributed throughout said annulus, creating said rotating annulus by projecting said particles outwardly against an intercepting surf-ace between which and said particles relative rotary movement is maintained, withdrawing from the inner boundary of said zone a fine fraction of said finely divided material, and withdrawing from the periphery of said zone a coarse fraction of said finely divided material.

4. The process of claim 3 in which the ends of the classifying zone are in part rotating, and in which the annulus rotates within practical approximation at the same speed as the fluid.

5. The method of claim 1 in which the air flows inwardly of the classifying zone in the form of a substantially free vortex.

6. A classifier for finely divided material comprising boundary means including first and second opposing wall structures forming between them an annular classifying zone, inlet means including fluid directing means surrounding the outer boundary of said zone for producing within said zone an inwardly spiralling vortex, an annular inlet for said finely divided material positioned between the inner and outer limits of said classifying zone, said annular inlet being bounded inwardly by rotating means for feeding said finely divided material outwardly to said annular inlet and outwardly by a stationary surface surrounding and spaced from the outer limit of said lastmentioned rotating means, said surface being positioned to intercept said material projected outwardly by said rotating means and being of a character to facilitate relative rotation of said material with respect thereto, outlet means communicating with the inner boundary of said classifying zone, means for producing a differential in pressure between said first-mentioned inlet means and said outlet means for the flow of fluid into said classifying zone directed by said fluid directing means and out of said classifying zone together with a fine fraction through said outlet means, and outlet means for a coarse fraction communicating with the outer boundary of said classifying zone.

7. The classifier of claim 6 in which the classifying chamber is adapted for vortex flow of the fluid inwardly therethrough.

8. The classified of claim 7 having first and second opposing wall structures forming between them said annular classifying chamber, and in which the axial distance h between the opposing wall structures at any radial distance r from the axis of rotation bears within practical approximation the following relationship:

r m where h is the axial spacing between the wall structures at the outer boundary of the classifying zone and r is the radius from the axis of rotation to the outer boundary of the classifying zone.

9. The combination of claim 6 wherein the stationary surface is frusto-conical.

10. The combination of claim 6 in which the stationary surface is irregular.

11. The combination of claim 10in which the stationary surface is stepped.

References Cited in the file of this patent UNITED STATES PATENTS

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