US3709359A

US3709359A - Ore classifier

Info

Publication number: US3709359A
Application number: US00005544A
Authority: US
Inventors: C Johnson
Original assignee: Individual
Current assignee: Individual
Priority date: 1970-01-26
Filing date: 1970-01-26
Publication date: 1973-01-09
Anticipated expiration: 1990-01-09

Abstract

An ore classifier in which a mass of reduced ore having a commingled gangue and mineral particles in it is circulated under blower pressure in a vertical plane in a somewhat circular chamber wherein the inlet for the commingled mass and the outlet for the gangue are tangential to the chamber and cross each other at the bottom through a plenum to scrub the gangue of any concentrate electrostatically held thereon. A classifier stack is disposed vertically at the top of the chamber for levitation of fines in relation to their size and weight for classification purposes and the classification means may be a plurality of spaced openings or an extendable member selectively adjustable for regulating the withdrawal of classified concentrates into a collection tank. A baffle system defining in part the primary path of flow of the incoming commingled mass operates to recirculate any uncollected concentrates still mixed in with outgoing gangue and return same to the stack.

Description

United States Patent [1 1 Street, Denver, Colo. 80220 [22] Filed: Jan. 26, 1970 [21] Appl. No.: 5,544

Related US. Application Data [63] Continuation-impart ofSer. No. 630,560, April 13,

1967, Pat. No. 3,511,446.

[52] US. Cl ..209/138, 209/143 [51] Int. Cl. ..B07b 7/10 [58] Field of Search ..209/133-l39,20, 209/477, 158, 132,143, 36, 37, 142, 144; 171/117; 55/461; 73/432 PS [56] References Cited UNITED STATES PATENTS 1,214,249 1/1917 Williams ..209/144 1,902,450 6/1934 Lykken... ....209/144 X 3,006,470 10/ l 961 Franken ..209/132 2,931,581 4/1960 Lykken et al... ....209/144 X 1,312,291 8/1919 Wicker ....209/l43 X 1,797,434 3/1931 McLaughlin ..209/138 X 2,091,514 8/1937 Weston ....209/139 R 2,204,032 6/1940 \Vhitlock ....209/139 R 2,668,330 2/1954 Gieszl ..209/138 3,010,576 11/1961 Marte ....209/133 T 3,397,780 8/1968 Beuzeval ..209/143 X Johnson 1 Jan. 9 1973 541 ORE CLASSIFIER v FOREIGN PATENTS OR APPLICATIONS lnventori Carl L- J0l1'n80nr1404 Hudson 491,563 7/1928 Germany .Q ..209 142 Primary Examiner-Frank W. Lutter Assistant Examiner-Robert Halper Attorney-Hatbaugh and Thomas ABSTRACT An ore classifier in which a mass of reduced ore having a commingled gangue and mineral particles in it is circulated under blower pressure in a vertical plane in a somewhat circular chamber wherein the inlet for the commingled mass and the outlet for the gangue are tangential to the chamber and cross each other at the bottom through a plenum to scrub the gangue of any concentrate electrostatically held thereon. A classifier stack is disposed vertically at the top of the chamber for levitation of fines in relation to" their size and weight for classification purposes and the classification means may be a plurality of spaced openings or an extendable member selectively adjustable for regulating the withdrawal of classified. concentrates into a collection tank. A baffle system defining in part the primary path of flow of the incoming commingled mass operates to recirculate any uncollected concen trates still mixed in with outgoing gangue and return same to the stack.

11 Claims, 4 Drawing; Figures PATENTEU JAN 9 I973 CARL L. JOHNSON ATT'Y.5.

ons CLASSIFIER CROSS REFERENCE This application is a continuation-in-part of my copending application, Ser. No. 630,560, now US. Pat. No. 3,511,446 filed Apr. 13, 1967. 4

BACKGROUND OF THE INVENTION Due to the high cost of transporting unprocessed ore to a processing station and the difficulty and extra cost of disposal of the gangue at that point, it is desirable, and highly economical, to process the ore at the site of a mine and then haul or handle the classified concentrates for final processing. In this way even low grade ore can be profitably processed both from large and small mines.

Ore must initially be broken down, crushed or otherwise reduced to release the entrained mineral for recovery. Preferably, the processing of ore containing a high degree of sand includes breaking it down to particle size and passing it through an extracting device such as the scrubber disclosed in co-pending application Ser. No. 630,560, filed Apr. 13, 1967, to dislodge or separate the mineral from the ore. At this stage the particles of mineral concentrate are thoroughly commingled with the gangue and must be removed therefrom with the greatest degree of recovery possible commensurate with realized returns. The degree of recovery generally depends on one or more physical characteristics which distinguish the mineral from the gauge and the separation processis directed to augment and take advantage thereof with minimum time and expense.

Heretofore when a desired mineral, either because of size or actual weight, has a lesser effective weight factor than the gangue in moving air, a cyclone separator, such as shown in U.S. Pat. No. 2,252,581, can be used in which a spirally flowing stream of air centrifugates the heavier particles to the outer wall along which they slide down and out through a bottom funnel while the lighter particles are drawn toward the center for retrieval through the top. This provides a single pass but the recovery is not adequate and requires repetitions in a succession of like separators to impoverish the gangue. Accordingly, not only are high air velocities required but power requirements are great for the initial and -multi-stage repetitions, with substantial mineral loss occurring at each stage. Blower speed adj ustments and other considerations require close attention and careful regulation for optimum results. Furthermore, factors involving repair and shut-down time are multiplied along with increased cost of the handling of the ore and the skilled labor generally required.

The expense in time or money required for these operations tends to make them impractical for the processing plant as well as for the mining site processing of the ore and classification of the fines. The present invention solves these and other related problems with increased effectiveness and reduced expenses.

SUMMARY OF THE INVENTION The present invention provides an ore classifier which yields in one unit a high. degree separation of selectably different sizes of mineral particles including extremely fine sizes as well as a range of particle sizes smaller than a selected size.

The classifier contemplates a substantially circular compartment whose axis is disposed horizontally and at its bottom supports an inlet conduit having a width approximately the axial dimension. of the compartment. The inlet conduit is disposed tangentially to the compartment to receive commingled material from an air blower and direct the flow of the material upwardly and tangentially along and around the circular wall of the compartment. A gangue outlet passage operating as a gravity drain leads tangentially from the bottom of the compartment across the opening of the inlet conduit and a classification stack extends upwardly from the top of the chamber and has a concentrate outlet opening adjustable as to height, and size if desired, to allow the discharge therethrough of only those particles fine enough to be carried to that height by the upward flow of the air stream that discharges them into a collection tank. The outlet aperture is vertically adjustable with respect to the tank providing infinite adjustment for regulating the fineness of the particles to be collected. The heavier gangue particles remain behind and are generally returned gravitationally to the compartment when their upward movement either expires or cannot be sustained by the upward flow of air from the compartment in the stack to the outlet.

Smaller particles that may be captured by the large particles when falling may be carried back to the compartment and even accompany the large particle towards the gangue outlet. However, although the downward inertia of the falling particles maybe enough to carry them through to the gangue outlet they will be scrubbed by the incoming cross blast of the commingled particles, and any smaller particles clinging thereto will be freed and returned to the compartment. Accordingly, the inlet and outlet-pipes at the bottom of the tank are so disposed that the incoming and outgoing particle streams intersect to repeat conditions for classifying small particles. Otherwise, any waste material too heavy to be raised to the classification opening will go ultimately into the outlet pipe scrubbed of any small particles clinging thereto or affected thereby.

Furthermore, baffling elements in the compartment along the initial path of flow of incoming commingled DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of an ore classifierembodying the invention;

' FIG. 2 is an enlarged cross-sectional view of another classifier embodying the invention;

THE PREFERRED EMBODIMENT As more particularly described in said application, Ser. No. 630,560, reference to which is hereby made, low grade ore can be reduced to a basic particle size, preferably without crushing, and thereafter mineral coatings such as uranium oxide are released from the particles or grains of sand by scrubbing, abrading and impacting the particles. This provides a commingled mixture of mineral concentrate and gangue ready for classification.

In the present invention, classifiers are shown in FIGS. 1 and 3 which with the operation of the air blower indicated at 12 progressively forces the commingled mixture through a supply conduit 14 and inlet at the bottom of a classifier chamber 16 defined by a tank 18. A gangue outlet 22 is also located at the bottom of the tank 18 and drains the gangue through a discharge conduit 24, preferably by gravity alone, to a collection bin 26. A classifier stack 28 opens at the top of the compartment 16 to withdraw the mineral fines as levitated to different levels therein by the air driven by the blower and ejected through a classifier opening 30 (FIG. 3) or opening 30A (FIG. 4).

More particularly the tank portion 18 defining chamber 16 in both embodiments comprises spaced, vertically disposed side walls 32 joined by a substantially circular intermediate wall 34. The wall 34 at its lowest point receives in supporting relationship the upper ends of the inlet and outlet conduits 14 and 24. The inlet conduit particularly is disposed tangentially to the wall 34 and the two conduits are substantially at an acute angle to each other to define a plenum 36 between their

openings

20 and 22 and the compartment 16 through which incoming material and outgoing gangue must pass on collision courses.

Vertically spaced

inclined baffles

38, 40, and 42 interconnect the sides 32 bordering the path 44 of inflowing material as it is driven upwardly and around the wall 34 to the top of the chamber 16. These baffles serve several purposes including forming venturii openings between them which draw mixtures arriving in the center of the chamber back through and into the path 44 of the incoming material.

The lower baffle 38 in combination with the wall 34 is inclined to provide therewith on its lower side the lower portion 44A of passage 44 through which the incoming material is confined to flow. On its upper face is provided a slope 38A from which any particles tending to collect thereon will drop into the inlet opening 20 when free to do so if not carried upwardly by air movement.

The intermediate baffle 40 cooperates on its lower surface with the lower baffle 38 to provide a venturii throat at 46 and also in coaction with the lower edge of the lower baffle to provide a limited opening 48 which operates to permit the movement of gangue back to the inlet opening 20 by gravity yet deflecting any moving air to pass through the venturii 46. The upper baffle 42 serves with the second baffle to provide a venturii throat at 50 and has an upper portion 52 which cooperates with the wall 34 to direct the flow of material as indicated by arrows 54 close to the classifier stack,

opening 56 at the top of the compartment.

In FIGS. 2 and 4 the stack opening is closed with the stack 28A which has vertically spaced lateral openings adapted to be capped as by caps represented by cap 60.

A selected opening can be uncapped to determine the.

concentration of the mineral as the particle capable of attaining that level. In FIGS. 1 and 3 a modification for infinite adjustment of the level of the classification opening 30 is shown in which a tube 58 having the withdrawal opening 30 is telescoped on a stationary bottom tubular member 62 that is in communication with the stack opening 56. Screws 64 provide for the servicing of the assembled stack 28 and clamp screws 64A are provided in the tubular member 62 cooperating with the upper tube 58 within limits to locate the classification opening 30 at the desired level. Both stacks are narrowed towards the top as at 66 to accelerate the speed of flow of air if the opening 30 is at an upper level.

Although the

baffles

38, 40 and 42 may be curved rather than angularly formed as shown, the angular forming permits full length marginal flanges 68 along their laterally opposed edges for securement to the sides 32 in proper position as by welding or bolts.

When the classifier of this invention is in use, the commingled material is drawn and accelerated, preferably by the blower 12, into the inlet duct 14. The action of the blower 12 also serves to maintain the classifier compartment at a positive gauge pressure. The stream of air carrying the commingled material is directed into the compartment 16 throughthe inlet opening 20 and plenum 36 in a substantially tangential direction in the direction of the arrows 70 and is restricted by the

baffles

38, 40 and 42 and directed into the upwardly constricting peripheral passage 44. If some of the gangue particles may be too heavy to be carried by the air up to the opening 56 they may slide back down the wall 34 or go directly through the outlet opening 22 into the waste outlet pipe 24 for disposal in the bin 26. Thus, only the concentrate particles and finer gangue particles are carried to the upper outlet opening 56. Gangue collecting in the outlet conduit 24 serves to restrict the escape of air therethrough to hold the gauge pressure in the compartment to assure flow through the classification opening 56. This gauge pressure is assured and regulated by a weighted gate 71 in the outlet conduit 24.

The nozzle 72 (FIG. 1) or nozzle 72A (FIG. 2) defining the

classification openings

30 or 30A respectively are preferably connected by a hose 74 or some other suitable means to a collection tank (not shown) which is preferably unpressurized so that the pressure differential is maintained constant across the

opening

30 or 30A due to the pressurized condition of the compartment. The opening 30 thereby maintains a constant flow characteristic.

Most of the air stream will move upward into the stack 28 because of this pressure drop across the opening 30, depending on the amount of pressure differential maintained across the opening. This involves also the power of the blower and the size of the opening 30, both absolutely and in relation to the area of the lower outlet'opening 22 and any obstructions therein.

The lighter, finer concentrate and gangue particles are levitated by this air stream into the stack where they must travel vertically upwardly to reach the level of the outlet opening 30.

As particle dimensions decrease by any given scaling factor, N. the air resistance ordrag upon the particle, which is dependent on its cross-sectional area, decreases by a factor of N. The weight of the particle, which depends on its volume, decreases by a factor of N Therefore, the ratio or air resistance to weight actually increases as particle size decreases so that a finer particle has a greater rate of fall in still air and a greater rate of rise and distance in an upwardly surging air column. The latter is the situation within the stack 28. The lighter or finer concentrate particles are levitated faster and higher by the column of air than the heavier, large particles. In this way, an ordered hierarchy of particles is established within the stack 28 with the finest particles at the top and the coarsest at the bottom and with the mid-range particles distributed according to fineness between these levels. The relation of height to which the particles of different weights ascend is uniform so that, by adjusting the level of the opening 30 a maximum particle size can be separated, or, as in FIG. 1, the slidable portion can be adjusted to whatever height is necessary to collect the coarsest concentrate particle desired. All finer particles will also be drawn into the fines collection tank by the pressure drop across the aperture 30 and the coarser particles will not ascend high enough to reach the aperture 30 but will be dropped by the preponderance of gravity back into the compartment 16. The inwardly and downwardly sloping portion of the wall 34 proximate to the inlet opening aids in returning air and mixture to the baffles and will slide unentrained particles to the inlet opening 20.

Material that may not be collected immediately includes some concentrate fines which are electrostatically or otherwise held by the gangue or were unable to reach the aperture due to collisions or flow conditions in the air stream. This material falls into the compartment or is circulated therein with the gangue particles. The baffle system 36 is constructed of more than one baffle plate so that the openings 46 and 50 are provided between the baffle plates as previously indicated. The constricting effect of the baffle system on the incoming air stream causes higher velocity air fiow along the path 44 and draws uncollected fine particles (primarily concentrate) through the openings 46 and 50 back into the upward moving air stream.

The heavy gangue particles are not drawn through the openings 46 and 50 but fall or are inertially circulated to the bottom of the tank 18 and if present on the left side of the compartment as viewed in the drawing they pass through the outlet opening 22 to the gangue outlet duct 24. The baffle plates 46 and 50, as mentioned, are extended to cover the inlet opening 20 and serve as deflectors for particles that would otherwise fall directly into the inlet conduit 14. Instead, the heavies travel downwardly to contact the wall 34 portion proximate to the opening 20 and develop sufficient momentum to reach the gangue outlet opening 22. However, these particles must pass through the incoming air-particle stream and be scrubbed thereby. Any remaining loose fines are picked up by the airstream and carried upwardly again. The inlet air stream also serves as a bridge providing upward force to carry the tailings over the inlet opening 20 so they may reach the outlet opening and go the bin 26.

Although the classifier of this invention has been described in relation to the problem of separating fine mineral particles from coarse, heavy waste it is understood that its utility extends to separating desired heavier particles from lighter waste material and to classification of materials other than ore such as grain or other classifiable particles. Waste can be blown off as fines and heavies collected at the lower outlet 22.

It is possible for certain basic structural or opera-. tional alterations to be made without departing from the scope or spirit of the present invention although some of these alterations could result in lost efficiency or the sacrifice of some features of the classifier. The

number of components of the baffle system could be increased or decreased even to the extent of eliminating the venturi effecLThis would provide the same effect as having separate upward bound inlet and downward bound outlet regions or even separate ducts joined at and with the concentrate outlet duct 14.

Instead of a blower driving air, compressed gas could be utilized to operate the classifier of this invention if desired. Such operation would simply require that the commingled particles be introduced into the gas stream before or at the inlet opening 20.

Although the preferred embodiment of the classifier of this invention has been shown to have a substantially cylindrical tank portion 12 with the vertically disposed concentrate outlet duct extending radially at the top and the inlet opening 20 and lower outlet opening 22 adjacent to each other at the bottom, it should be understood that other shapes, locations and orientations operating in a vertical plane are possible and those shown are intended to be illustrative of the preferred operation of the present invention.

What is claimed is:

1. In an ore processing apparatus, a classifier for classifying mineral fines according to size, said classifier comprising a generally circular housing oriented with its axis disposed horizontally with curved laterally spaced walls and including a lower outlet opening adjacent to an upwardly inclined wall portion of said housing and a lower inlet opening adjacent to a downwardly inclined wall portion of said housing, said openings crossing each other at an acute angle to define a plenum, means to introduce air entrained reduced ore under pressure thru said inlet and across said outlet, said air entrained ore moving upwardly and perpherally adjacent said outlet :and downwardly adjacent said inlet and an upper outlet opening;

inclined baffle means disposed. within said housing bordering the path of incoming material including a plurality of vertically spaced baffle plates inclined towards said inlet opening and disposed between said spaced walls and whose lower faces define in cooperation with said upwardly directed wall portion of the housing a restricting peripheral passage for upward flow from the inlet of air entrained mineral .fines therethrough and directly to the upper outlet opening; a classifier stack extending upwardly from the outlet opening and having a classifier opening at a predetermined level for removal of the mineral fines by movement of air through said classifier opening.

2. In an apparatus according to claim 1 in which said lower outlet opening is a gangue outlet opening disposed adjacent to the inlet opening to discharge heavy particles after they have passed through the flow of air entrained minerals for scrubbing mineral values from the falling heavy particles.

3. In an apparatus according to claim 1 in which said classifier stack comprises two telescoping tubular members, the upper one of which has the classifier opening in it for removal of the mineral values from the duct according to size.

4. A particle classifier comprising:

a housing having a wall and an upper outlet opening and lower inlet and outlet openings;

means to direct particles to be classified from said lower inlet opening in an upwardly directed peripheral path in said housing separate from a return path of gangue to the lower outlet opening;

a classifier stack extending upwardly from said upper outlet opening;

a source of a pressurized gas to propel said particles from the inlet opening along said upward path and toward said classifier stack to maintain a positive gauge pressure within said classifying device;

vertically adjustable outlet means on said classifier stack to control the size of particle emitted whereby the smaller of said particles are emitted through said outlet means and the larger of said particles fall back and through said lower outlet opening; and

baffle means provided between the two paths to coact with said wall and direct said particles from said lower inlet opening in an upwardly oriented peripheral path:

said baffle means including a plurality of spaced baffle plates inclined towards said inlet opening.

5. A classifier in accordance with claim 4 wherein said housing is essentially circular with closed ends and is oriented with its axis disposed substantially horizontal. r

6. A classifier in accordance with claim 4 wherein said baffle means extends over said lower inlet opening to direct descending gangue particles falling against it back into said upwardly oriented peripheral path.

7. A classifier in accordance with claim 4' wherein said paths cross above said lower inlet and outlet openings.

8. A classifier in accordance with claim 4 wherein vertically spaced aperture means are provided along said classifier stack.

9. A classifier in accordance with claim 4 wherein gangue particles pass through the incoming stream of air entrained particles before reaching the lower outlet opening.

10. A particle classifier comprising;

means to direct particles to be classified from said lower inlet opening in an upwardly directed peripheral path in said housing separate from a return path of gangue tothe lower outlet opening;

a classifier stack extending upwardly from said upper outlet opening; I a source of a pressurized gas to propel said particles from the inlet opening along said upward path and toward said classifier stack to maintain a positive gauge pressure within said classifying device;

vertically adjustable outlet means on said classifier stack to control the size of particle emitted whereby the smaller of said particles are emitted through said outlet means and the larger of said particles fall back and through said lower outlet openings;

said classifier stack comprising two telescoping conduit members, the lower one of which has major cross-sectional dimensions throughout and the upper conduit member has major cross-section dimensions on its lower end telescoping in said lower conduit member and tapering to minor cross-sectional dimensions at its upper end;

said upper conduit member having a discharge opening in its upper end.

1 l. A particle classifier comprising:

a circular housing defining a circular compartment with vertical disposed spaced side walls with an upper outlet and lower inlet and outlet openings;

baffle means cooperating with a circular wall portion of said housing to direct a stream of gas entrained particles from said lower inlet opening in an upwardly peripheral path in said compartment, said baffle means including a plurality of inclined, vertically spaced baffle plates and extending over said lower inlet opening at one end and converging towards each other at the other to provide at least one venturii throat between them;

a classifier stack extending vertically upward from said upper outlet opening;

a source of a pressurized gaseous medium to propel said entrained particles along said upward path and toward said classifier stack and to maintain a positive gauge pressure within said classifying device;

vertically adjustable outlet means on said classifier duct to control the size of particle emitted from the stack;

whereby the smaller of said particles are transmitted through said outlet means on said classifier stack and the larger of said particles pass through said stream of gas entrained particles before discharge through said lower outlet opening.

Claims

1. In an ore processing apparatus, a classifier for classifying mineral fines according to size, said classifier comprising a generally circular housing oriented with its axis disposed horizontally with curved laterally spaced walls and including a lower outlet opening adjacent to an upwardly inclined wall portion of said housing and a lower inlet opening adjacent to a downwardly inclined wall portion of said housing, said openings crossing each other at an acute angle to define a plenum, means to introduce air entrained reduced ore under pressure thru said inlet and across said outlet, said air entrained ore moving upwardly and perpherally adjacent said outlet and downwardly adjacent said inlet and an upper outlet opening; inclined baffle means disposed within said housing bordering the path of incoming material including a plurality of vertically spaced baffle plates inclined towards said inlet opening and disposed between said spaced walls and whose lower faces define in cooperation with said upwardly directed wall portion of the housing a restricting peripheral passage for upward flow from the inlet of air entrained mineral fines therethrough and directly to the upper outlet opening; a classifier stack extending upwardly from the outlet opening and having a classifier opening at a predetermined level for removal of the mineral fines by movement of air through said classifier opening.

4. A particle classifier comprising: a housing having a wall and an upper outlet opening and lower inlet and outlet openings; means to direct particles to be classified from said lower inlet opening in an upwardly directed peripheral path in said housing separate from a return path of gangue to the lower outlet opening; a classifier stack extending upwardly from said upper outlet opening; a source of a pressurized gas to propel said particles from the inlet opening along said upward path and toward said classifier stack to maintain a positive gauge pressure within said classifying device; vertically adjustable outlet means on said classifier stack to control the size of particle emitted whereby the smaller of said particles are emitted through said outlet means and the larger of said particles fall back and through said lower outlet opening; and baffle means provided between the two paths to coact with said wall and direct said particles from said lower inlet opening in an upwardly oriented peripheral path: said baffle means including a plurality of spaced baffle plates inclined towards said inlet opening.

5. A classifier in accordance with claim 4 wherein said housing is essentially circular with closed ends and is oriented with its axis disposed substantially horizontal.

7. A classifier in accordance with claim 4 wherein said paths cross above said lower inlet and outlet openings.

10. A particle classifier comprising; a housing having a wall and an upper outlet opening and lower inlet and outlet openings; means to direct particles to be classified from said lower inlet opening in an upwardly directed peripheral path in said housing separate from a return path of gangue to the lower outlet opening; a classifier stack extending upwardly from said upper outlet opening; a source of a pressurized gas to propel said particles from the inlet opening along said upward path and toward said classifier stack to maintain a positive gauge pressure within said classifying device; vertically adjustable outlet means on said classifier stack to control the size of particle emitted whereby the smaller of said particles are emitted through said outlet means and the larger of said particles fall back and through said lower outlet openings; said classifier stack comprising two telescoping conduit members, the lower one of which has major cross-sectional dimensions throughout and the upper conduit member has major cross-section dimensions on its lower end telescoping in said lower conduit member and tapering to minor cross-sectional dimensions at its upper end; said upper conduit member having a discharge opening in its upper end.

11. A particle classifier comprising: a circular housing defining a circular compartment with vertical disposed spaced side walls with an upper outlet and lower inlet and outlet openings; baffle means cooperating with a circular wall portion of said housing to direct a stream of gas entrained particles from said lower inlet opening in an upwardly peripheral path in said compartment, said baffle means including a plurality of inclined, vertically spaced baffle plates and extending over said lower inlet opening at one end and converging towards each other at the other to provide at least one venturii throat between them; a classifier stack extending vertically upward from said upper outlet opening; a source of a pressurized gaseous medium to propel said entrained particles along said upward path and toward said classifier stack and to maintain a positive gauge pressure within said classifying device; vertically adjustable outlet means on said classifier duct to control the size of particle emitted from the stack; whereby the smaller of said particles are transmitted through said outlet means on said classifier stack and the larger of said particles pass through said stream of gas entrained particles before discharge through said lower outlet opening.