US2283992A - Material classifier - Google Patents

Description

H. c. wRscl-l MATERIAL CLASSIFIEB Filed March 12, 1940 3 Sheets-Sheet 3 C HIRSCH MATERIAL CLASSIFIER Filed March 12, 1940 May 26, 1942.

' Z5 Sheets-Sheet 2 Home- 2 C. EQSCH,

May 26, 19 H. c. HIRSCH 2,233,992

MATERIAL CLASSIFIER Filed March 12, 1940- 3 Sheets-Sheet 3- HOMER C. HESCH,

Patented May 26, 1942 UNITED STATES PATENT OFFICE j 2,283,992 MATERIAL CLASSIFIER Homer C. Hirsch, El Paso, Tex. Application March-12, 1940, Serial No, 323,543

01mins;

The present invention is concerned generally with material classifiers of .the kind chiefly used in handling ores during metallurgical operations, and more especially with improvements in classifiers wherein solid particles are suspended in .a body of liquid, usually water, from which the particles settle out according to size and density.

improvements in Material classifier, an improved type of classifier in which the several functions of conveying the sands out of the classifier tank, agitating the pulp contained within the tank, and classifying the solids to determine the fines remaining in suspension in the overflow, are separated from each other. Not only are the functions carried out in different portions of the classifier, but each function is also carried out by separate and independent parts of the structure. In earlier types of classifiers these various functions were interrelated to a great extent that produced inherent difficulties and prevented flexibility and proper technical control of the product, Provision of separate and independent means for controlling the factors of agitation, size or particle separated, capacity measured by gross input, and density of overflow, produces a higher overall efliciency of the classifier and results in a classifier that is flexible and easily adjusted to meet any condition of ore, size or quantity of product, and rate of feed. For example, a change in the conveyor speed to accommodate a change in feed does not change the amount of agitation produced, and

' an increase in agitation can be accomplished to bring about an additional washing of the sands and agitation of the pulp without increasing the size of the fines in the overflow or changing the sharpness of separation.

, The Finney type of classifier accomplishes these advantages by providing a non-agitative 'conveyor, preferably of the'screw type; a separate liquid agitating means, such as a rotary paddle wheel, which may be driven at any desired speed independently of the conveyor, and which is preferably located at or near the liquid inlet; and a sorting column adjacent theoutlet launder through which all outgoing, liquid flows,

'ments in Material classifier.

carried out of the classifier'in the slimes. The

objects and advantages of this construction are set outin detail in the application above referred to, but the ultimate advantages may be summed up as being greater capacity, greater flexibility of operation to adapt the classifier to changing conditions, sharper separation, and cleaner coarse sands discharged from the conveyor.

Another embodiment of this type 0 fclassifier isdescribed in Patent 2,189,419 granted to John H, V. Finney, on February 6, 1940, for improve- This later application discloses certain advantageous constructions of the conveying means and liquid outlet means incorporated in a classifier of the general type described above. This improvement is described in greater detail hereinafter, but

and is shown asbeing in the nature of an immovement on this construction, but it is not limited thereto.

in such a classifier, the zone of classification f-isl'ocated inside the conveyor, that is, the ribbon the upward velocity of the liquid through the,

sorting column determining the size of particle flights of the screw conveyor move under and at both sides of the sorting column or columns. It has been found by. experience that the individual "ribbon flights of the conveyor tend to exert a pumping action'on the liquid in the tank because of the relatively close fit of the ribbons between the wall means'forming'the sorting columnsand the walls of the tank. While this pumping effect is negligible at relatively low speeds of the rotary conveyor, this action places a definite upper limit on the number of revolutions per minute that the conveyor may be turned, since at above a certain speed the pumping action causes pulsations in the entire body of ,liquid within the tank; and these pulsations adversely affect the sorting column action, since they create surges in the sorting column that carry particles of excessive size into theoverfiow. Since the rate of sand. discharge is proportional to the speed of the conveyor, any limit on. conveyor speed places a definite limit upon the rate at which sands can be removed from the classifier tank.

Thus it becomes a general object of my invenel r, .l.

tion to eliminate as far as possible any pulsations imparted to the liquid bodyby the conveyor, or to render such pulsations harmless as far as any effect on the classifying action of the sorting column is concerned.

It is also an object of my invention to improve the quality or sharpness of separation in a classifier of this type at relatively high speeds of the conveyor, and. thus more effectually separate the classifying action from any incidental efiects created by the conveying means.

Another object of my invention is to increase the capacity of a classifier of the type described, V by eliminating adverse effects resulting from rel-' atively high rotational speeds of the conveyor. j'

v though not necessary, to put an arcuate plate 120 r on the tank bed on the sand side of the conveyor 20. This quarter-round bottom extends from end wall H tothe lower edge of bay H.

A further object of the invention is to,strengthen the cantilever end section of the rotary conveying means. j 'Z These objects are attained according to my invention by providing thefrotary spiral conveyor of a classifier with a screen or trommel which is interposed between the central shaft of the conveyor and the ribbon flights at the lower end of the conveyor, the trommel thus being wholly or partly immersed in the body of liquid contained in the classifier tank. The trommel acts as a baflle and providesa quiet zone separated by the trommel from the agitation taking place in other parts of the liquid body, yet the pulp is able to enter the quiet zone maintained inside the screen because of the holes therein allowing free communication of the liquid with the spaces insideand outside the screen. The liquid outlet means of the overflow type is ordinarily placed at or adjacent the lower end of the tank, and thetrommel is so located that the quiet zone which it forms is located'adjacent the overflow launder in order. that the classifying action may be completed in the quiet zone and the liquid then passed from the quietdone to the overflow launder with a minimum of disturbance. reason, it is preferable to place the launder and sorting column within the quiet zone, and this n 'may be done by using'a longitudinallyextending launderwhich enters the trommel from the lower end, the launder being supported from the end wall of the' tank. ,Inithis type of construction, which is described in detail hereinafteiflthe conveyor has a cantilever end section'that permits rotation of the conveyor without interference with the launder; and the trommel. becomes a part of the supporting structure of'the conveyor th'atacts as a cantilever suport carrying the, helic'al'flights of the conveyor.

How the above and other advantages of my lugvention, as well as others not specifically mentioned, are attained will be more readily understood by reference to the following description and the annexed drawings, in which: I

Fig. 1 is a side view, partially in elevation and partly in median section, of a preferred form of my invention;

Fig. 2 is a plan view of the classifier of Fig, 1;

Fig. 3 is an end elevation of the lower end of the tank;'

Fig. 4 is a vertical cross-section taken at the end of the tank on line 4-4 of Fig. 1;

Fig.5 is a vertical cross-section at the upper end of the trommel, taken on line 5-5 of ,Fig. 1;

Fig. 6 is a fragmentary vertical median section of the lower end of a modified form of my invention; and f Fig. 7 is 'an end elevation of the classifier shown in-Fig.6.'

For this .includes an axial rotating shaft 2| on which are tank generally indicated at I0, which is adapted to contain a body of water or other liquid holding in suspension solid particles that are to be clasand beyond the liquid level to form a draining .deck. As the coarse particles or sands are moved -over this deck, the excess liquid draining from them runs back into the tank. It'is preferable,

A stream of liquid carrying suspended solid .particles is brought to the tank from. any suitable source by inlet Iaunder l5; and though this launder may empty into the tank in other ways, it is preferred that launder l5 have a short transverse section |5a that discharges the incoming fluid and suspended particles into a bay I! in one sidewall of the'tank. The reasons for this particularconstruction will be discussed later.

The coarse particles settle out of the liquid in the tank onto inclined bed l2, and suitable rotary conveying'means .20 are provided to move the sands out of the liquid and upbed 12 to discharge 'lip .l 8 at the upper end of the bed, over which the sands are discharged from the classifier. The construction so far described generally is disclosed in Finneyv Patent 2,189,419 referred to above but the principal one of the features constituting my present invention is the combination of a trommel with a rotary screw conveyor, as

will now be more particularly described.

The conveying means generally indicated at 20 mounted spider 22 and perforated disk 23. At-

tached to the outer end of the arms of spider 22 and to suitable lugs at or near the periphery of disk 23, are four longitudinally extending braces 1 24 which are spaced apart around central shaft 2|, though ofcourse a larger or smaller number of arms and braces'may be used if desired. Disk 23 is attached to shaft 2| at a point removed from the end of the conveyor, and beftween the disk and the lower endof the conveyor is located trommel 25, which is coaxial with axial shaft 2| and attached at its upper end to disk 23,

as may be seen in Fig. 2. Trommel 25 is a screen or foraminous bafile that breaks up currents in the liquidwithout exercisingany sizing operation on the solid particles; and the trommel is preferably cylindrical, or substantially, so, though it may be given one of other shapes that are annular or approximately so when viewed in transverse cross-section. Thus, the trommel may be frusto-conical as shown in Fig; 6,

The flights of heIicaLribbonlB pass around and are secured to the outer faces of longitudinal braces 24and trommel 25, the ribbon flights thus being spaced from the center shaft 2|. As will be noted from Fig. 2, it may be desirable that the flights 26 attached to braces 24. have agreater radial depththanthose at the-lower end of the conveyor in order to provide greater conveying capacity. I p

Braces 24and'trommel 25, together with ribbon flights 26, form a rigid trussed structure The classifier illustrated. inthe drawings'has a Zymountedon shaft 2t to be rotated thereby; This '34 extending at right anglesto shaft 28. sprocket 36 is fastened to shaft 34 and is driven "launder at or near end wall H.

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the lower end of the tank and is closely spaced from end wall it,

Several advantages result from this trussed construction with the cantilever end sections, and among them'is the ability to support the central rotating member 2! 'atpoints inward of the end of the conveyor. This type of arrangement is illustrated at the upper end of conveyor '20 where shaft 2| is drivingly mounted in onehalf of universal joint 21, and ribbons '26 extend upwardly around'the universal joint. The other half of universal joint 2'? isfastened to the end 'of a short shaft 28, rotatably mounted in bearings 29 and 39, which are mounted in fixed positions relative to each other upon the framework of tank I5. Bevel gear 32 is keyed to shaft 28 intermediate bearings 29 and 35 and meshes with driving pinion 33 which is secured to drive shaft 34 rotatably mounted in bearings29 and 35, shaft by chain 37 from motor.38 mounted upon platform 36 erected upon tank '50: Conveying means 26 are thus driven from motor 38 or other source of power, and suitable means may be added,

either electrical control 'means or mechanical transmission, for varying the speed of rotation of the conveying means.

Advantage is also taken at the other end of the conveyor of the overhanging end section to support the center shaft and bearings above the end of the conveyor, but the construction and arrangement of partswill be more "easily understood if there is first described a preferred form of liquid outlet means for discharging liquid and suspended particles from the tank. The reasons for preferring the subsequently described liquid "outlet means will become apparent; but it will also be seen that my invention may also be used in conjunction with a transverse overflow As will be seen particularly from Fig. 3, end wall H of the tank is formed with a vertically extending slot 42 which is preferably reinforced around its margin by the channel and angle construction indicated at i3. In this vertical slot 42 is placed overflow launder 45. The twov spaced vertical side walls st of launder 15 are the shape of a trapezium, as illustrated in Fig. l, with the top edge'of the side walls horizontal. The two side walls are joined along their bottom edges by a "rectangular bottom wall which is, preferably substantially parallel to tank bed [2, and are joined at their back edges by a rectangular back wall which extends upwardly between the side walls "from the bottom wall to the top of the side walls.

The lower end of launder 45 is open and discharges liquid and suspended fines from the classifier into any suitable receptacle. The entire overflow launder is supported from tank end wall ii by attaching it to the flange structure 43 about the margin of slot 42, the width of slot 42 being determined by the dimensions of launder 35. The horizontal top edges of thetwo side walls "4% form a pair of spaced parallel weirs over which liquid inside the tank flows into launder '45, and the height of the liquid level within the,

Drive t'a nkis determined by'the iocation'of these weir-$.

thelower-end of the shaft bearing, even though 'the-endof sleeve "48 is left open. Sleeve 48 retatably supports the lower end'of the rotary conveyor shaft 2!, which extends downwardly to a :point below the liquid level of the tank, 'andthe sleeve has one or more internal liners 49 (see Fig; '1') that provide journal bearings for shaft -21. Sleeve 48 extends upwardly from the lower end'of and around'shaft 2i to a point where the.

upper end of the sleeve is above the liquid level in the tank, and consequently, even though open, is out of contact with the liquid so that solid par- 'tioles cannot enter the bearings inside the sleeve. Disk 23 is sufficiently spaced from the end of shaft 25 that bearing sleeve 48 is long enough for this purpose, and the cantilever end section formed by trommel 25 is long enough to carry the ribbcn'fiights 25 substantially to the end wall H.

The bearing construction shown is preferred; but my invention is not limited thereto. A sleeve of the same general type may be used, but supported in any other suitable manner; or a fully submerged bearing'of any conventional type may be used.

Bearing sleeve 48 is stationary with respect to the conveying means and is supported in bearing block 50 at a point beyond the lower end of ribbon 26. Bearing block 55 is guided vertically and held against transverse horizontal movement by the faces of launder walls 48 where they pass between flanges '43 around opening 42, and is hung by threaded bolt 5i from cross-bar 52 resting on the side walls of tank ill. By turning adjusting nut 53 on rod 5!, block 58 in the lower end of the conveying means may be raised or lowered, for vertical adjustment of the conveying means with respect to the inclined tank bed I2. Guides 54 and walls 46 hold the block and sleeve against longitudinal movement on shaft 2!. This adjustable support on the lower end of the conveyor is optional, but is of particular advantage in starting the conveyor after the classifier has been temporarily shut down, when the sands have had an opportunity to settle out of the liquid and are tightly packed around the lower portions of the conveying means. The articulate character of central member 2| imparted by universal joint '21 permits'angular movement of the conveyor to free it from the sands without disturbing the alinement of driving gears 32 and '33. V

Bearing sleeve 48, "being non-rotatable, is a convenient support also for b'afiles 55, placedone on either side of launder -45. Baflles 55 are mounted on the opposite ends of two lead screws 56 rotatably mounted in bosses on collars 51 attached to sleeve '48. Each collar '51 has two bosses andacollar'58 is placed on the screw 56 between the two bosses to keep the screw 56 from moving endwise. Toprovide simultaneous adjustment of bafiles 55 toward or, away from side walls 46 of launder 4'5, th'e'baffles haveattached to them threaded nuts engaging lead screws 56, each of "which has both right and 'left hand threads.

both baflles simultaneously'an equal amount toward or away from the side walls of the launder. Obviously,- any other suitable construction may be employed to move baflles 55 either individually or simultaneously. a

Side walls 46 of, overflow launder'45 are preferably 'vertical, and baflles 55 are also vertical and parallel to the side walls. Each side wall 45 r and its associated baflle 55 form vertically extending wall means which define between them a vertically extending spacetll which is termed a sorting column. Though it may be suflicient to have but one such sorting column, it is preferred to have two, as shown. At the top of each sorting column 60 is the overflow weir defined by the top edge of a launder wall 46. The velocity of the liquid flowing vertically throughvsorting columns 60 determines the size of particle carried in suspension by the liquid over the "weirs and into the overflow launder. Baflies 55 extend from a point above the normal liquid level to apoint below that liquid level in order to prevent liquid from reaching and overflowing the weirs except by passing through the sorting columns 60. The sorting columns are preferably of By this construction, rotation of screws 55 moves considerable'vertical extent, as shown, in order that relatively stable conditions, free from eddy.

currents outside the sorting column, may be' established and an accurate separation-of the particles accomplished. Obviously, the size of separation between particles settled out and the particles carried over insuspension is determined by the liquid velocity, and this in turn is determined by the cross-sectional area of the sorting columns.

For a further discussion of the advantages of typical variations in the construction of sorting columns, reference should be made to the patents of John H. V. Finney mentioned above.

Transverse baffle 62 is positioned on sleeve 48 as close as practical to the top edge of the back wall of launder 45, and prevents liquid in the tank from overflowing the rear wall of thelaunder, except in negligible amounts. 'Baflle 62 extends transversely to the baffles 55, so that the ends of the sorting columns are defined by tank wall II and baffle 62. v

Brackets 64 extend upwardly from collars 51 and support at their upper ends arcuate shield 65, asseen in Figs, 1 and4. Shield 65 extends completely over launder and both sorting columns 60 to prevent any liquid or solid material from falling intothese parts of the liquid outlet meansfrom therotary conveyor as it passes over the sorting columns and launder.

Liquid agitating means is provided to obtain independent agitation of the liquid. Without being limited thereto, this means may comprise a paddle wheel 68 rotatably mounted on the side wall of tank It to turn about a horizontal axis. Sprocket 69, attached to the paddle wheel shaft, is driven by chain 10 from a smaller sprocket attachedto shaft 34. Agitator 68 is preferably located in bay l1 formed in one side wallof the tank, and launder l5a is adjacent the periphery of wheel 68 in order that theincoming feed is preferably discharged directly 'into'the path of v the agitator blades to secure thorough agitation of the suspended particles near the point of'inlet. The open construction of the rotary conveyor permits currents in the liquid to flow through the conveyor away from the agitator near the upper :end of the liquid body and return through'the conveyor to the agitator. These currents wash the sands which have accumulated on'the tank bottom opposite the agitator, and carry fine particles into suspension in the deeper liquid. As alread mentioned, bed plate [2a stops at the lower edge of Day II in order not to interfere with the freecirculation of currents set up by agitator 68. Whi1e the spiral conveyor is a substantially non-agitative conveyor, yet the flights 26 create surges or pulsations in the liquid body as they pass downwardly around bafiles 55 and 62. This is a result of the relatively limited clearance betweenthe spiral flights and the baffles on one side and the tank walls on the other side. Consequently, flights 26 and braces 24 -(if the latter are carried to the lower end of the tank) tend to push the liquid ahead of them at higher conveyor speeds, rather than moving through it free- .ly; and this tendency creates surges that travel come by placing a screen or foraminous baflle be tween the conveyor flights and the liquid outlet means. This baflle is foraminous to allow fluid to flow easily through the baflle, yet it is a barrier to the surges which are created outside the baflie since they are, in effect, screened out. The

screen openings break up surges and eddy currents to the point that they have no appreciable efiect on the liquid body inside the screen, which thus establishes within itself a quiet zone in which the sorting column and overflow weirs are placed and carry out their functions without interference from the conveyor.

Trommel 25 affords another advantage in that it breaks up currents in the liquid that flow directly through the classifier from inlet to outlet, thus insuring that the liquid is retained in the tank for sufiicient time to settle out coarse particles. In this way short circuiting currents that carry oversize particles through the classifier are prevented and the sharpness. of separation is improved.

A further advantage is that the trommel provides a stronger cantilever end section for the conveyor than would the four braces used on the remainder of the conveyor, and at the same time the braces are eliminated at the lower end.

Tied in with the substantial elimination of any disturbance in the sorting columns is the increased capacity of the conveyor. Since it '.can be revolved at" greater speeds because of the charged at lip I8 per unit time without any loss 'of separation efiiiciency. This means a greater 1 capacity for a given classifier.

out any increase in capacity, the higher speed trommel, a greater amountof sands can be dis- But even withof rotation of the conveyor removes coarse sands faster as 'they settle and so prevents the pulp -from building up to a density that interferes with. proper classifying action, as usually is the case when a thick bed of sands accumulates. In a preferred form, trommel 25 is open at the jlower end and covered at the upper end by a foraminous disk 23. Disk 23 can, under some circumstances, be imperforate; also it could be omitted if the trommel is extended above the liquid level to prevent surges from entering the open upper end to any material extent. Although the trommel is shown as perforated for substantially its entire length, this is not necessary, but it is preferable to have the perforations extend at least for the full length of sorting columns 60 when they are used and arranged as shown. If the total length of the overflow weir is decreased, as by placing it transversely of the tank at the end wall ll, either with or without a sorting column, the perforated length can be shortened in accord withthe decreased volume of overflow into the outlet launder.

Some of these possible modifications in the trommel are shown in Figs. 6 and 7 which illustrate a modified form of classifier embodying the principles of my invention and constructed as described above except for the various features mentioned below. In this form of the invention, outlet launder 45a is arranged transversely of trative of, rather than limitative upon, the invention is defined by the appended claims.

I claim:

1. In a material classifier having a tank with an inclined bed and adapted to contain a body of liquid and solid particles therein to be classified, and a rotary screw-type conveyor for movingsettled coarse solids along the tank bed to a place of discharge: a screen of substantially annular transverse cross-section carried by the conveyor and at least partly immersed in the liquid body, the screen establishing therewith a quiet zone in I the liquid body adjacent the deep end of the tank tank Ill adjacent end wall II, but preferably spaced therefrom in order that the two transverse sides of the launder'may each act as an overflow weir. In this simplified construction, the sorting columns are omitted. Overflow launder 45a rests directly upon bearing sleeve 48 and is held in place by two outlet pipes 15 which pass through end wall H of the tank in order to conduct the dischargedliquid from the tank to any suitable receptacle. I

The bearing for the lower end of the conveyor shaft 2| is of the same general type previously described; but here the lower end of sleeve 48 is closed by means of a transverse sealing plate 16 attached to a flanged ring around the lower end of the bearing sleeve to prevent entry of liquid at the lower end of the bearing. The

and inside the periphery of the conveyor; liquid. inlet means introducing liquid and solid particles into the liquid body at a point outside the screen; and liquid outlet means located adjacent the deep end of the tank in a position to receive liquid substantially only from the quiet zone.

2. The combination in a classifier as set out in claim 1, in which the screen'is open at its lower end and is covered at its upper end by a transverse screen.

plate and flange are bolted to end wall I I in order spider 22, disk 23a closing the upper end of trommel 25a. Trommel 25a, is tapered upwardly to a smaller diameter at its upper end than it has at its open lower end, the screen thus being of frusto-conical shape. The annular shape of the trommel in transverse cross-section has been retained. However, it will be noted that in addition to disk 23a, a portion at the upper end of trommel 25a is imperforate since an adequate 3. The combination of a classifier as set out in claim 1, .in which the screen is open at its lower end, and a transverse fluid impervious member covers the upper end of the screen.

4. In a material classifier having a tank with an inclined bed and adapted to' contain a body of liquid and solid particles therein to be classified, liquid inlet means, and a rotary screw type conveyor for moving settled coarse solids along the tank bed to a place of discharge, the combination of: a screen of substantially annular transverse cross-section carried by the conveyor and at least partly immersed in the liquid body, the screen establishing a quiet zone adjacent the deep end of the tank and inside the periphery of the conveyor, and overflow type liquid outlet means located adjacent the deepend of the tank and at least in part inside the screen to receive liquid from the quiet zone.

amount of fluid with suspended solid particles passes through openings in the lower end of the foraminous baffle to reach transverse outlet I launder a which is located within the rotat- One-additional feature not previously described is cylindrical guard 11, which is mounted upon disk 23a concentrically of shaft 2| in order to protect the open upper end of bearing sleeve 48 against the entry of liquid or solid particleswhich might drop onto the shaft from the rotating trommel as it passes over the shaft. In this way any falling particles drop upon cylindrical guard 11 and are deflected back into the tank without danger of entering the bearings inside sleeve 48.

From the foregoing disclosure of my invention, it will be clear that various changes in the shape and disposition of the parts of the classifier can be made without departing from the spirit and scope of my invention; and it will be understood 5. In a material classifier having a tank with an inclined bed and adapted to contain a body of liquid and solid particles therein to be classified, liquid inlet means, and a rotary screw type conveyor for moving settled coarse solidsalong the tank bed to a place'ofdischarge'the conibination of: a screen of substantially annular transverse cross-section carried by the conveyor and at least partly immersed in thev liquid body. the screen establishing a quiet zone adjacent the deep end of the tank and inside the periphery of the conveyor, and an overflow type liquid outlet launder extending longitudinally of the tank adjacent the deep end thereof and within the quiet zone. rl

6. In a material classifier having ,a tank with an inclined bed and adapted to contain a body of liquid and solid particles therein to be classified, liquid inlet means, and a rotary screw type conveyor for moving settled coarse solids along the tank bed to a' place of discharge; the combina tion of: an annularscreen carried by thebonveyor and at least partly immersed in the liquid body, the screen establishing a'quiet zone adja-r 7 7. In a material classified having a tank with an inclined bed and adapted to contain a body of liquid and solid particles therein to be classified, liquid 'inlet means, and arptary screw type conveyor for movingisettled -coarse'solids alongthe' jacent the deep end thereof and within the quiet zone; and wall means within the quiet zone associated with but spaced from the launder to form in conjunction therewith a sorting column through which liquid fiows vertically to determine the size of fine particles carried into the launder by the liquid. l

8. Ina material classifier having a tank with an inclined bed and 'adapted to contain a body of liquid and solid particles therein to be classified: a rotatable shaft; an annular screen secured to'but spaced "from the shaft by means supporting the screenatthe upper end only to form a"cantilever sectionysaid screen being'at least partly immersed in the liquid body, and establishing therewith afquiet zone in the liquid body adjacent the deep endfof the tank'and inside the periphery of the screen; helical ribbon flights'mounted on the external surface of the the screen; and an overflow type liquid outlet launder extending longitudinally of the tank through the open end of the screen into the can-.

tilevered section and withinthe quiet zone.

11. In a material classifier having a tank with an inclined bed and adapted to contain a body of liquid and solid particles therein to be classified, liquid inlet means, and a rotary screw type conveyor for moving settled coarse solids along the tank bed-to a place of discharge, the combination of: a frusto-conical screen carried by the conveyor and at least partly immersed in the liquid body with-the end of larger diameter near the deep end of the tank, the screenestablishing a quiet zone adjacent the deep end of the tank and inside the periphery of the conveyor, and an overflow type liquid outlet launder adjacent the deep end of the tank and within the quiet zone. I

12. In a material classifier having a tank with an inclined bed and adapted to contain a body screen to engage and move settled coarse solids along the tank bed to aplace; of discharge; liquid inlet means introducing liquid and solid particles into the liquid body at a point outside the screen; and liquid Toutlet means located adjacent the deep end of thetank in a position to receive liquid substantially only from the quietzone.

9. In a material classifier having a tank with an inclined bed and adapted to contain a body of liquid and solid particles therein to be classified:

a rotatable shaft; an annular screen'secured to but spaced from the shaft by a transverse'screen supporting. the annular screen at the upper end only to form a cantileversection, said annular screen being at least partlyimmersed in the liquid body and establishing therewith a quiet zone in the liquid body adjacent the deep end of the tank and inside the periphery of the. screen; helical ribbon flights mounted on the externalsurfaceiof the annular screen to engage and move settled coarse solids along the tank bed to a place of discharge; liquid inlet means introducing liquid and solid particles into the liquid body at a point outside the annular screen; and liquid outlet means located adjacent the deep end of the'tank in a position to receive liquid substantially only from the quiet zone.

10. In a material classifier having a tank with an inclined bed and adapted to contain a body of liquid and solid particles therein to be classified,

. a rotary screw-type conveyor for moving settled coarse solids along the tank bed to a place of discharge: a screen of substantially annular cross-section carried by the conveyor and forming therewith a'rigid trussed structure having a particlesinto the liquid body at. a pointoutside 75') of liquid and solid particles therein to be classified, liquid inlet means, and'a rotary screw type conveyor for-moving settled coarse solids along the tank bed to a place of dischargethe combination of: an annular screen carried by the conveyor andat'least partly immersed in the liquid body with the end of larger diameter near the deep end of the tank, the screen establishing a, quiet zone adjacent the deep end of the tank and inside the periphery of the conveyor, and an overflow type 'liquid outlet launder extending transversely o'f'the tank'adjacent the deep end of the tank and within the quiet zone.

13. In a material classifier'having a tank with an inclined bed and adapted to contain a body of liquid and solid particles therein to be classified, the combination of: a rotary screw type con veyor for moving settled coarse solids along the tank bed to a. place of discharge; a screen of substantially annular transverse cross-section carried by the conveyor and at least partly immersed in the liquid body, the screen establishing a quiet zone in the liquid body adjacent the deep end of the tank and inside, the screerpliquid inlet means introducing liquid and solidparticles into the liquid body at a point outside the screen; and overflow type liquid outlet means located adjacent the deep end of the tank in a position to receive liquid substantially only from the quiet zoneinside the screen.

. V 14. The combination in a classifier as. set out in claim 13, in which the screen is open 'at its lower end, and a transverse fiuid impervious member closes the upper end of the screen.

15. The combination with, a material classifier having a tank with an inclined bed and adapted to contain a body of liquid and solid particles therein to be classified, inlet means for. liquid and solid particles, liquid outletmeans adjacent the deep end of the tank, rotary conveying means for moving settled coarse particles along the tank bed toa place of discharge, and means associated with the liquid outlet means establishing a sorting column in whichis determined the size of fine particles carried into the outlet by the liquid, of a rotating screen of substantially annular cross-section interposed between the liquid inlet and outlet and surrounding the sorting column to shield the outlet and sorting column from 10- calized currents set up by the conveyor and to maintain substantially uniform conditions of liquid fiow through the sorting column.

-' V .HOMER C. HIRSCH.

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