US2113609A - Concentration - Google Patents

Description

April 12, 1938. c. E. wUENscH 2,113,609

' CONCENTRATION Original Filed April 8, 1935 2 Sheets-Shea*- l Medium Make a @alex/llo ATTORNEYS April 12, 1938- c. E. -wU'ENscH 2,113,609

CONCENTRATION Original Filed Apri1 8, 1935 2 She'ets-Sheet 2 `NVENTOR LQ BY lo @IM/l @MAI M ATTORNEYS Patented Apr. 12, 1938 UNITED STATES PATENT osi-lcs coscasmnos Original application April 8, 1935, Serial No.

Divided and this application February 8, 1936, Serial No. 63,020

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'I'his invention relates to the separation of heterogeneous mixtures of solid particles having different densities, and contemplates the provision of an improved method of and apparatus for effecting such separation. More specifically, the invention contemplates improvements in the separation of two or more minerals or other solids having different specific gravities by employing gravitational i'orces in the presence of a heavy uid.

This application is a division of my co-pending application Serial No. 15,217, illed April 8, 1935.

In the heretofore customary practice o! ore dressing and allied arts, mixtures of mineral particles having diierent densities have been separated by adding the mixture to a iluid having a density intermediate the densities oi the minerals to be separated. Under these conditions the mineral particles having a density greater than 20 that of the uid tend to sink. whereas mineral particles which are lighter than the iluid tend to float. The products are thus separated, and may be withdrawn separately from .different zones in the duid medium.

The term "iluid is used here to include true liquids, such as aqueous salt solutions oi high specific gravity and heavy organic liquids such as tetrabromethane (CnHzBn), as well as multiphased mixtures of finely divided solids with liquids, usually referred to as suspensions.

Mineral separating processes employing heavy fluids have presented attractive possibilities because of their apparent simplicity, and promising results have been obtained when some of these processes have been employed in the laboratory under carefully controlled conditions. However, duplication of these results in commercial practice has been impossible except in rare instances. Processes employing heavy organic liquids such as tetrabromethane have not come into commercial use because the relatively high cost of such liquids usually requires a high initial investment and excess replacement costs, for in commercial operations a substantial loss of the separating medium is almost inevitable. Commercial application of processes that employ heavy salt solutions for mineral separation has been handicapped by the presence of at least three outstanding disadvantages:

*i5 (l) The capital expenditure involved in employing large volumes of such solutions.

2) The cost of solution losses in the operation of the process.

3) The fact that such solutions become contaminated or diluted in commercial use. In

(ci. zes-11a) order to restore the solutions to the proper concentration, evaporation is necessary, and this entails large expense. Furthermore, removal of contaminants without removing the necessary salts which impart the necessary increase in density to the solutions is frequently impossible,

l and costly in any event.

Commercial'examples oi processes employing aqueous suspensions of finely divided solids in place oi true heavy liquids have met with some success. aqueous suspensions of ilnely ground clay, galeria, hematite, barytes, silica, and mixtures of these and other materials have been prepared. Such suspensions are not subject to the aforementioned objections in that they are less expensive. so that ilrst cost and the cost oi losses arerelatively less. Suspensions possess a further advantage over true liquids in that they may be restored from a diluted state by settling or filtration, which are less expensive than the evaporation required to remove excess liquid from true solutions.

Accompanying these advantages which the use of suspensions aii'ord there are, however, serious disadvantages. Unless the suspended solid particles are so tine as to be practically colloidal, they tend to settle out, so that the density through a column ot the suspension varies. Unless means are taken to prevent such settlement, the delicacy of the separation is' deleteriously aii'ected, not only by the variation in density from top to bottom of the column, but also by the fact that the particles crowd into the bottom of the column. where instead ot exerting an unimpaired and delicate buoyant eil'ect, they form a mat which prevents separation of the heterogeneous mixture. Lastly, for any given density, a suspension is usually considerably more viscous than an equivalent true liquid, so that hinderance to separation may be excessive.

Separating processes employing suspensions have been at least partially successful in separating constituents having a marked diil'erence in density. Thus, such processes have been employed for the separation of coal from slate and pyrite, but have failed to accomplish an adequate separation in commercial practice when there was but a slight diiierence in the density of the minerals to be separated. This failure may be attributable to the following and other factors:

1. Improper average density of the suspension.

2. Variation in density throughout the suspension.

3. Excessive viscosity of the suspension.

4. Coarse particle size of the solids comprising the suspension.

5. Insuilicient size of the mineral particles undergoing separation.

6. Adhesion between the particles of the suspension and the particles of the mineral aggregate undergoing separation.

7. Crowding, i. e. a tendency of particles of the minerals aggregate or particles of the separating medium (suspension) to collect in zones in the separating chamber andract as a mat or screen through which heavy mineral particles fall to penetrate in their fall.

8. Contamination of the suspension with solids of improper particle size ordensity, soluble impurities, and oleaginous substances.

As a. result of my investigations, I have invented a method and apparatus for mineral -separation whereby the aforementioned dimculties heretofore involved in the use of suspensions are largely obviated or at least greatly reduced. As a result of my invention, it is now possible to eiIect a clean separation of mineral particles in commercial operation, even when the difference in specic gravity between two minerals is less than 0.10.

In accordance with my invention, the separating medium or suspension is conditioned prior to its introduction into the separating apparatus, and the proper density and consistency of the medium is maintained by controlling the size of the solid particles, the amount of liquid in the medium, or the ratio of the amounts of vario sizes of particles in the medium.`

During the separation operation the occurrence of excessive bottom densities" and crowding of particles within the separator is prevented by withdrawing a portion of the medium substantially continuously from the lower portion of the separator (together with sandy material from other sources) and varying the amount oi material so withdrawn in proportion to its density, thus maintaining the density and consistency of the contents oi the bottom of the separator substantially constant.

'Ihe cycle of now throughout the process of my invention is so arranged as to provide for a gradual removal of deleterious contaminants, both liquid and solid, so that at no time need there be a departure from optimum operating conditions due to these causes. Furthermore, the accumulation of coarse particles atpoints in the circuit where they would interfere-with proper separating conditions is eiectively prevented.

These and other features of my invention will be more readily understood if reference is made to the following description taken in conjunction with the accompanying drawings, in which:

Fig. 1 is a diagrammatic ow sheet of an ore concentration plant oi' my invention, and

Fig. 2 is a schematic representation of a modiiled mineral separating apparatus of my invention.

Referring to Fig. 1, it will be seen that the suspension employed (in the instance illustrated, an aqueous galena slime) is prepared by wet grinding in a ball mill operating in closed circuit with a classier, which may be of the rake type, the interrupted screw type or other appropriate design. A screw or bowl type classifier will probably be preferable in the grinding oi.' the solid particles to make upthe suspension or separating medium in most instances, in that the relatively quiescent state which exists in the overflow end oi classiers of these types permits the production of a very slimy material. 'Ihe ball mill circuit and classiiler preferably should be operated to produce a classifier overilow having a maximum particle size minus 100 to 150 mesh. Under these conditions the majority of the particles in the classier overiiow will be considerably finer than 100 mesh, and a large amount of impalpable slime will be present.

' 'I'he classifier overflow, together with the diluted medium returned from subsequent stages of the process is sent to a thickener of conventional design. The underflow or spigot product of the thickener is transported by a pump I to a surge tank, which is provided with an agitator 2 and with an overflow 3 back to the thickener. The density of the thickener underow should be greater than the required density of the conditioned medium sent to the separator so that density control may be effected by adding liquid to the material during the conditioning treatment as hereinafter described.

From the surge tank the dense medium ilows to a sump. Water and medium returned from later stages of the process are also added in the sump. The medium in the sump is pumped by l means of a pump l to the shallow end of a conditioner which comprises a sloping trough l, provided with bailles 6, and a rotatable screw 'I adapted to move sandy material upward to the vdischarge or shallow end of the trough, from whence the sandy material is returned to the ball mill circuit for further grinding. For convenience in operation, and to provide for surges in the delivery of gaiena slime from the pump, an overilow I is provided on the side of the conditioner at about the middle of the length of the trough. Slime overiiowing at this point returns to the thickener. In practice there is a tendency for relatively coarse material to build up in the deep end of the conditioner trough and to overilow in surges. If such an accumulation is allowed to occur there is, of course, a lack of uniformity in the density and ratio of particle sizes oi' the conditioned slime overflowing from the deep end of the apparatus, with consequent deleterious eiTect upon the subsequent mineral separating operation in which the slime is employed.

To prevent this effect an outlet pipe 9 is provided about midway up the deep end of the conditioner trough through which the relatively coarse material is returned to the sump. The sandy material removed from the shallow end of the conditioner carries with it considerable material of the kind which tends to overflow in surges from the deep end of the conditioner, so that an excessive circulating load of this relatively coarse material (semi-sand) is prevented.

In order to insure that the density of the galena slime (to be used as the separating medium) is of a uniform and appropriate density when it overows from the conditioner at a lip Il a novel density regulator is employed. The density regulator comprises a hydrometer Il, equipped with a shield l2, positioned between a light Il and a photoelectric cell I4 at the discharge end of the conditioner, where the hydrometer floats in the relatively quiescent medium. The photoelectric cell is connected in series with a source of direct current I5 and a solenoid operated valve I8 which controls the amount ci water introduced into the sump. Should the density of the medium overflowing the lip of the conditioner become too high, the hydrometer rises and the shield between the light and the photoelectric cell is displaced, so that the light increases the conductivity of the conductor in the photoelectric cell permitting it to pass curum leaving the lip of the conditioner, no auto` matic means for increasing the amount of solids added to the conditioner feed is required. In the event that such a procedure were necessary the hydrometer could be provided with a second shield (not shown), movement of which would actuate a second photoelectric cell circuit (not shown) which would in turn control the amount of solids admitted to the sump.

'I'he requisite delicacy of the automatic density control will depend. of course. upon the difierence in specific gravity between the minerals to be separated. When this diiference is small the density regulator may be made correspondingly delicate by increasing the sensitivity of the hydrometer employed.

The mineral aggregate to be separated (designated on the iiow sheet as ore) is first screened and washed with water in an appropriate device such as a trommel, to remove the bulk of the fine material. 'I'he size of the material retained on E. the screen will depend in some measure upon the nature oi the mineral aggregate. In the treatment of Joplin `iig tailings containing about 1.5 to 2.0 percent of zinc in the form of sphalerite, it has been found that the material should range in size from V2 to 3/4 inch down to 20 mesh (Tyler scale). The fines produced by the trommel may be wasted, or if the practice is economically justiied they may be ground and treated by otation or other appropriate methods. The oversize from the trommels in a clean and slightly moist condition is transported to a separator of novel design as shown on Fig. 1.

Washing the aggregate and supplying it to the separator in a slightly moist condition is desirable for several reasons, viz.

(1) Soluble contaminants are thus removed.

(2) Materials are removed which are too fine to be separated by heavy fluid methods because of the excessive surface exposed per unit of volume.

(3) The wet surfaces of the aggregate reduce any deleterious effect of adhesion between particles of the mineral aggregate and the particles comprising the suspension or medium, at the same time facilitating removal of entrained medium from the products made in the separator.

(4) 'I'he presence of a slight amount o! moisture dilutes the medium in the top of. the separator, thus permitting light minerals to be immersed in the medium in the first instance so that they do not form a floating mat which would block the fall of heavy particles which follow.

When only two products are to be separated, I prefer to employ a separator of the type illustrated in Fig. 1, comprising an inverted conical chamber I1, having a peripheral tailing trough I8 tangential to its upper edge and a concentric rotatable agitator I8 equipped with horizontally adjustable blades 20 positioned within it. The lower end of the conical chamber terminates in a substantially cylindrical pipe 2l. A ne meshed screen 22 (say 25 mesh) is amxed on the side of the cylindrical pipe, and may be vibrated by an arm 22 which extends through a hutch 2Q. A drain pipe 25 is provided at the bottom of the hutch. 'Ihe bottom of the pipe 2l is connected to a housing 2i, which is inclined upwardly and extends to a point slightly above the top ot the conical chamber. A conveyorl 21 provided with scrapers 2l is positioned within the housing and serves to drag out coarse concentrates falling through the pipe 2i.

'Ihe drain pipe 2l is equipped with a solenoid operated valve 2l and drains into an agitator box I0 in which a hydrometer ll of an automatic density regulator is located. This density regulator is similar to that located at the conditioner outlet and comprises a light 32. a shield 32, a photoelectric cell ll, a current source 35 and the solenoid operated valve 2l. The agitator box should be so constructed as to provide a quiescent zone for the hydrometer without, however, providing a space in which substantial settlement of solids can occur.

In operation, moist oversize from the trommel and medium of substantially constant density and consistency are run continuously into the top of the separator near its center. The agitator 20 is revolved slowly by a chain or other appropriate means. The peripheral speed of the blade tips should not exceed 150 feet per minute. Separation begins immediately, the light minerals tending to rise and flow outwardly due to buoyancy exerted by the medium of galena slime and to thiso slight centrifugal force induced by the agita r. light minerals. designated on the ilow sheet as "tailings", pursue an ascending spiral path and overnow into the peripheral tailings launder accompanied by a portion of the medium. The heavier minerals or concentrates" sink through the cone into the pipe 2|, and come within the influence of the vibrating screen, the function of which is to prevent crowding or matting of the concentrates and of the galena particles.

Under the influence of these forces the4 Unless the washing of the material from the trommel has been perfect and the ore is unusually hard so that no attrition occurs in the separator, a small amount of relatively fine heavy mineral particles will tend to settle along with a portion of the galena in the medium, thus increasing the bottom density", aggravating the tendency for crowding, and thus interfering with the separation. The vibrating screen 22 is sumciently coarse to permit the ne mineral particles (due to attrition or imperfect washing) together with coarse galena particles to pass out into the hutch 2l and thence through the drain pipe 25 and' the valve 29 into the agitator box 3|. 'I'he valve 29 is so constructed that it is never completely closed. When the density of the material draining out of the hutch rises to a predetermined point which indicates that crowding or excessive bottom density is about to occur in the pipe 2l or in the bottom of the conical chamber, the density regulator operates to open the valve still further and permit the outflow of the material which is about to cause the crowding. When the density of the material leaving the hutch returns to normal the hydrometer drops; the circuit of the density regulator is broken; and the valve, under the influence of an appropriate spring or counterweight. resumes its normal opening. It will be apparent that the feature of the invention whereby a portion of the medium contaminated with ne concentrate is continuously withdrawn from the separator at a rate depending upon the density ofthe material withdrawn is also applicable to processes in which the separating medium or heavy fluid is a true heavy liquid instead of a suspension.

When the amount of ne concentrate drained out of the hutch warrants recovery, or when the contamination which would result from the presence of the concentrate in the galena slime returned to the separating circuit cannot be tolerated, outflow from Lhe agitator box is washed in a screen 36 of appropriate mesh (say 50 to 80). The concentrate is retained on the screen and the undersize, consisting chiefly of dilute galena slime is returned to the thickener. If contamination of the medium equal in amount to this fine concentrate can be tolerated, and if the amount of this fine concentrate does not warrant recovery, the screen 36 can be eliminated and the outflow from the agitator box may be returned to the ball mill for further grinding, or to the thickener.

The coarse concentrates falling through the conical chamber and the pipe 2| are picked up by the conveyor and dragged out above the level of the medium in the dragv housing. Some galena accompanies these coarse concentrates, so that drainage and washing on a screen are necessary. The coarse concentrates are removed from the upper surface of the screen and sent to further processing. 'I'he galeria is returned to the circuit as hereinafter described.

The tailings are removed from the peripheral trough at the top of the separator and subjected to screening and washing to remove galena. 'I'he galeria is returned to the separating circuit. The tailings (assuming that further treatment is not economical) are transported to the dump.

As shown on Fig. l, the draining and washing of the concentrates and tailings in order to produce clean products and to recover the galeria slimes are subjected to the following treatment:

The concentrate and tailings are transported respectively across screens 31 and 38 of appropriate mesh. Such medium as will drain off without the addition of water is permitted to pass through the screens into separate receptacles 39 and 40, and is returned in an undiluted condition to the sump. 'I'he drained concentrates and tailings next pass respectively over screens 4I and 42 under water sprays 43 and 44. The water for these sprays is the slightly turbid overflow from the thickener. The partially Washed concentrates and tailings are given a final washing under fresh water sprays 45 and 46, and are discharged respectively from the screens 4| and 42 in a slightly moist condition, i. e. containing about 3 to 5% of water. The dilute medium products resulting from the dual washing treatment of concentrates and tailings are passed respectively through flner screens 41 and 48 (say 50-80 mesh) in order to remove any coarse material which would contaminate the medium circuit, i. e. the circulating suspension of galeria. Because the screening of the ore is not perfect and because the screening and washing are advantageously conducted on moving or vibrating screens, some attrition occurs and a secondary screening of the dilute medium is usually necessary before it is returned to the circuit. In this way, the sandy material will not prevent the maintenance of optimum conditions in the separator. The sandy concentrates removed in this secondaryscreening operation are recovered. The sandy tailings are wasted, or subjected to further treatment. The dilute medium from which the sand has been removed is returned to the thickener.

Appropriate reagents to promote settling, such as lime or alum, may be added to the pulp ln the thickener. In commercial operation, a small amount of oil almost inevitably leaks into the The thickener overflow is run into a trap 49 from the top of which any froth may be skimmed. After appropriate treatment to r'emove oleaginous substances, the solid constituents of the froth may be returned to the medium or suspension. Water for the sprays employed in the first washing of the concentrate and tailings is withdrawn from the bottom of the trap. The controlled excess water overflows from the trap and is sent to settling ponds (not shown) or to other appropriate treatment, if the amount of suspended galena or other valuable solid is such as to warrant recovery. The bleeding off of this controlled excess water at this point prevents an undue accumulation of deleterious soluble contaminants in the circuit.

In the event that the mineral aggregate undergoing treatment is such that more than two classes of products should be separated from each other, I prefer to employ a novel separating apparatus of my invention as illustrated in Fig. 2. This apparatus comprises a trough 50 having a gently sloping bottom 5I, and adapted to be partially filled with the separating medium or suspension employed. Underneath the deep end of the trough is located a jig chamber 52 equipped with a screen 53 of such mesh that a jig bed of concentrates will be retained thereon. A movable piston 54 operating in a box 55 which communicates with the jig chamber serves to pulsate the bed on the screen. Two middling traps 58 and 51 are located in the bottom of the trough intermediate the ends thereof. A screw conveyor 58 extends lengthwise along the bottom of the trough and isv provided with a helix 59 on its lower portion and a helix 60 on its upper portion. The helix 59 is adapted to move aggregate which settles to the bottomI of the trough following the jig upwardly to middling traps 55 and 51. 'I'he helix 60 is adapted to move material settling in the upper portion of the trough downwardly to the middling trap 51. An overow 5| is provided at the shallow end of the trough. A drag conveyor 62 supportedby pulleys 63, 64, 65, and 56 and provided with blades 61 is positioned within and above the trough. The blades on the lower portion of the drag conveyor are adapted to be partially immersed in the contents of the trough and are adapted to move continuously'and slowly toward the shallow end thereof.

Mineral aggregate is supplied to the lower end of the trough through a chute 68 located at a point such that the aggregate will tend to fall into and not beyond the pulsating current induced by the jig. The separating medium (an aqueous suspension of galeria, for example) is supplied to the deep end of the trough through a conduit 69 in amount sufficient to maintain the level of the contents of the trough approximately constant. Concentrates fall into and through the jig chamber and the screen into a concentrate trap 10, from which they are withdrawn together with some of the separating medium or suspension.- Middlings fall to the bottom of the trough and are moved to the middling traps by the screw conveyor. Tailings overow into the shallow end of the trough together with some oi the medium.

The function of the drag conveyor is to supplement the natural current in the shallow end of the trough and to prevent crowding.

In the eventthat only three products are de sired, the middling trap 56 may be omitted. Should more than three products be desired, additional product traps may be provided in the bottom of the trough.

'Ihe separating apparatus illustrated in Fig. 2 is adapted to be substituted for the conical separator shown in Fig. 1, when a multiple separation is desired. The automatic density regulator associated with the hutch ofthe conical separator may also be connected tothe hutch of the jig in the apparatus of Fig. 2 or to the other traps.

It will be understood that my invention is not limited to the use of a separating medium of suspended galeria. Any appropriate solid may be substituted for galena. Thus, an aqueous suspension of hematite, barytes. pyrite, silica, clay, etc. or combinations thereof may be employed depending upon the respective densities of the minerals to be separated.

It will also be understood that the application of my invention is' not limited to a case in which the speciilc gravity of the medium is intermediate the specific gravity of two minerals which are to be separated. By appropriate control, a ,Y

plurality of minerals which are either heavier or lighter than the medium may be separated from each other for the reason that these minerals will rise or fall in the medium with different velocities and thus become separated. 'Ihe apparatus illustrated in Fig. 2 is particularly adapted to such practice.

I claim:

1. In a process oi' separating coarse heavy solid 'particles from coarse light solid particles in a mixture thereof in the presence of a medium comprising a suspension of ne solid particles in liquid in a chamber in a circuit involving the withdrawal of the separated coarse particles from the chamber, the removal of medium from the coarse particles thus withdrawn and the return of the medium thus removed to the chamber, the improvement which comprises washing the coarse particles withdrawn from the chamber to remove the medium therefrom in a diluted condition, settling the diluted medium in a thickener, withdrawing irom the top portion of the thickener and removing from the circuit liquid containing finely divided solids, withdrawing thickened medium from the lower portion of the thickener, then classifying the thickened medium and removing sandy and semi-sandy material therefrom, returning the semi-sandy material to the classiiication step and returning the medium from which the sand has been removed to the chamber.

2. A process according to claim 1 in which a portion of the medium is drained from the coarse particles withdrawn from the chamber prior to washing said coarse particles, the medium thus drained being returned to the chamber without thickening, and the coarse particles withdrawn from the chamber are countercurrentlywashed first with a portion of the liquid containing finely divided solids withdrawn from the top portion of the thickener and then with fresh water.

3. A process accordingI to claim 1 in which solids and liquids in the form of froth are skimmed off the liquid Withdrawn from the top portionV of the thickener and removed from the circuit.

4. In a process of separating coarse heavy solid particles from coarse light particles in a mixture thereof in the presence of a medium comprising a suspension of nely divided solid particles in liquid in a chamber. in a circuit, said process involving the withdrawal of coarse heavy particles from a lower portion of the chamber and of coarse light particles from an upper portion of the chamber and the removal of medium from the coarse particles thus withdrawn and the return of the medium removed to the chamber, the improvement which comprises washing the coarse particles Withdrawn from the chamber to remove the medium therefrom in a dilutecondition, settling the dilute medium in a vessel, withdrawing from the top portion of the vessel liquid containing nely divided solids, returning a portion of the liquid containing finely divided solids to the washing step, removing another portion of" the liquid containing finely divided solids from the circuit, withdrawing the thickened medium from a lower portion of the thickener, adjusting the density of the thickened medium to a predetermined point by the addition of liquid thereto, and

thereafter returning the medium to the chamber.

5. In a process of separating coarse heavy solid particles from coarse light solid particles in a mixture thereof in the presence of a medium comprising a suspension of nely divided solid particles in liquid in a chamber in a circuit, said process involving the Withdrawal of coarse heavy particles from a lower portion of the chamber and of coarse light particles from an upper portion of the chamber and the removal of medium from the coarse particles thus withdrawn and the return of the medium removed to the chamber, the improvement which comprises washing the coarse particles withdrawn from the chamber with turbid liquid and thereafter with clear liquid to remove the medium from the coarse particles in a dilute condition, settling the diluted medium in a vessel, withdrawing from the top portion of the vessel turbid liquid, removing a portion of the turbid liquid from the circuit, returning a portion of the turbid liquid to the washing step, withdrawing thickened medium from a lower portion of the vessel, diluting the thickened medium to a. predetermined density and removing sandy material therefrom, and returning the medium of predetermined density from which the sand has been removed to the chamber.

6. In a process of separating coarse heavy solid particles from coarse light solid particles in a mixture thereof in the presence of a medium comprising a suspension of iinely divided solid particles in liquid in a chamber in a circuit, said process involving the withdrawal of coarse heavy particles from a lower portion of the chamber and of coarse light particles from an upper portion of the chamber and the removal of medium from the coarse particles thus withdrawn and the return of the medium removed to the chamber,

the improvement which comprises draining the coarse particles withdrawn from the chamber to remove a portion of the medium therefrom in an undiluted condition, thereafter Washing the coarse particles withdrawn from the chamber to remove the rest of the medium in a diluted condition, settling the diluted portion of the medium in a vessel, withdrawing from the top portion'of the vessel and removing from the circuit liquid containing finely divided solids, mixing the undiluted portion of the medium with tbe thickened portion of .the medium, adjusting the density of the resulting mixture to a. predetermined point and removing sandy material therefrom, and returning the mixture of predetermined density from which the sand has been removed to the chamber.

7. Process according to claim 6 in which froth containing iinely divided particles is skimmed from the liquid from the upper portion of the 5 vessel and removed from the circuit.

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