US2139789A - Method and apparatus for separating fragmentary minerals of different specific gravities in crushed ores - Google Patents

Description

Dec. 13, 1938. 3 E4 WUENSCH 2,139,789

METHOD AND APPARATUS FOR SEPARATING FRAGMENTARY MINERALS OF DIFFERENT SPECIFIC GRAVITIES IN.CRUSHED oREs Filed Feb. 4, 1953 2 Sheets-Sheet 1 H /N VE/VTOR.

g MW.

ATTORNEY.

Dec, 13, 1938. r Q E, WUENSCH 2,139,789

METHOD AND APPARATUS FOR SEPARATING FRAGMENTARY MINERALS OF DIFFERENT'SPEGIFIC GRAVITIBS IN QRUSHED ORES Filed Feb. 4, 1.953 2 Sheets-Sheet 2 //V l/EN 701?.

A TTORNEX Fatented Dec. 13, 1938 PATENT OFFICE METHOD AND APPARATUS FOR SEPARAT- ING FRAGMENTARY MINERALS OF DIF- FERENT CRUSHED ORES SPECIFIC GBAVITIES Charles Erb Wuensch, Berkeley, Calif., assignor to Wuensch Hetero Concentration Process Company, Wilmington, Del., a corporation of Delaware Application February 4, 1933, Serial No. 655,196

9 Claims.

This invention relates to a method and apparatus for separating fragmentary minerals of different specific gravity in crushed ores, and

more particularly to a novel method and apparatus for employing a heavy fluid, such as finely comminuted heavy minerals or solids suspended in water so as to form what may be termed a heavy fluid medium for gravitationally classifying or separating the several constituents of different specific gravities contained in ore or rock.

The use of chemicals of high specific gravity to effect the separation of mineral constituents of an ore or rock is well understood by those skilled in the art. In this practice, the specific gravity of the heavy chemical is varied with diluents so as to float one mineral and sink the other. Satisfactory separations can be made where there is as little as 0.05 of a point difference in specific gravity, whereas other methods of concentration, such as jigging or tabling, require at least one point or more difference to effect a satisfactory separation. However, the cost of such heavy.

density chemicals is so great that the inevitable small losses render their use prohibitive in large scale commercial operations.

Numerous attempts have been made to substitute finely comminuted heavy minerals or solids suspended in water to form a heavy fiuid for medium.of the expensive chemicals. The use of such solids suspended in water is nothing but a form of classification in which hindered settling is carried to the point where an endeavor is made to control the density of the hindered settling column so that the solid water suspension acts the same as a chemical of high specific gravity.

The difiiculties-inherent in attempting to control the density of this heavy fluid column so as to get the same effect as with a chemical of uniform density are familiar to anyone experienced in ore dressing. Uniform maintenance of the required density of the heavy solid fluid suspension composed of particles of different densities, sizes and shapes, even though finely comminuted, is obviously impossible because of the constant variations of conditions when the one feed is added and the contaminationof the medium itself by the constituents of the ore.

Heretofore, in practically all processes involving modifications in manipulations to make a solid Water suspension behave like a uniform chemical of heavy density, the ability to concentrate ores in general have been suggested. However, these processes have only been successful in washing coal, where the density of the (Cl. I 209-473) fluid mass column need not be controlled to such a degree as would be required for economic results I in ore dressing, because of the low specific gravity of the coal itself and because of the relatively wide gravity range between the various particles varying from coal at one end to refuse proper at the other. In addition to this, such operations are just the reverse to that required in ore concentration. In coal washing, the coal floats as a tailing and the refuse sinks as a concentrate; Therefore, the contamination of the medium in coal washing, where the refuse slimes to a large extent may constitute the separatory medium, presents a much simpler problem, than in ore concentration. Therefore, it is one object of this invention to provide an improved method and apparatus that shall properly utilize and coordinate collectively all of the underlying basic principals for effecting the delicate separations required in the application of heavy fluids to ore concentration.

In all the processes known tome suggesting methods of manipulation to create conditions so as to make the use of a solid water suspension approximate the separatory effects of a uniform chemical of high specific gravity, the processes have sought only the creation of. a perfect solid water suspension to produce the separatory effects of a--uniform, heavy chemical, and all sight was lost of the fact that laboratory sink and float.

tests have always been made as a batch operation and not for large scale continuous operations. In the latter, many other factors besides specific gravity enter into the problem, such as time, velocity, surface tension, crowding, purification of medium and size of particles.

All of the processes that have heretofore been proposed fall short of efiecting as delicate a separation as a heavy chemical. These processes include inherent factors that are not absolutely sound, and the apparatus fail to apply properly the basic principles, such as the influence of surface tension, resistance amongst various sized particles, velocity relatlonship, and soforth.

It is-well known that, in the finer sizes of ore particles, such as slimes, no gravity separation is possible, because of the large surface areas, as compared to volume and weight, whereby the surface tension forces are greater than the forces due to differences in specific gravity. In the heavy mediums, the surface tension and viscosity factors exert marked effects on even relatively coarse sand particles up to A" in size. The result is that, when material is fed to those apparatus employing a; cone, the smaller sizes of materials of the same specific gravity rise to the top of the cone more easily than the larger particles. The larger tailings particles must, therefore, break through the upper bed of fine tailings in order to be discharged over the top of the cone. This results in excessive crowding and the loss of some light middlings.

These factors also prevent the finer particles of concentrates from sinking to the bottom of the cone, because of the impossibility of 100% screening efliciency on the material that is fed to the cone to remove the finer sizes of particles, and also because of a small amount of attrition during treatment, these fine heavier sand particles cannot be kept out of the medium. This, therefore, creates an excessive denseand viscose zone at the bottom of the cone and makes it impossible to maintain a perfectly graduated differential density column where there is not over 0.1 or 0.2 of a point difference in specific gravity between the top and bottom of the cone. This combination also adds to the excessive crowding pointed out above, with a resultant loss of valuable middlings with the tailings. These processes recognize only density conditions and fail entirely to provide factors to ofiset the crowding effects, and, consequently, perfect separation is not obtained. The fundamental laws of viscous and turbulent resistances are ignored. The vertical component of this crowding force is less than the horizontal component, and, therefore, the lighter middlings move laterally more readily than downwardly, and as they spread out and reach the periphery of the cone, where the velocity, due to the rotation of the cone or an agitator therein, is greatest, these lighter middlings have a tendency to discharge with the tailings rather than to be crowded down and sink with the concentrates. I

Therefore, another object of this invention is to provide a novel method and apparatus for classifying or separating the mineral constituents of an ore in accordance with the specific gravity of such constituents, wherein the apparatus is adapted for continuous operation and provides a separatory effect that will be the resultant of density, time, velocity, crowding, surface tension and size of particles.

i The above and other objects will be made apparent throughout the further description of the invention, when taken in connection with the accompanying drawings. It is to be distinctly understood that the drawings are not a definition of the invention, but merely a: diagrammatic fiow sheets illustrating one manner in which the process may be carried out, the definition of the in-' vention being defined by the appended claims.

In the drawings:-

Fig. 1 is a diagrammatic flow sheet of an apparatus embodying the invention.

Fig. 2 is an enlarged detail view of a part of the apparatus illustrated in Fig. 1.

Fig. 3 is an enlarged detail view of the separatory trough embodying the invention, and

Fig. 4 is a sectional view taken on line IVIV of Fig. 2.

For the sake of simplicity, the apparatus will be described in accordance with the operation in carrying out the various steps of the process.

Referring to the drawings, the crushed ore is fed upon a vibrating screen, represented in its entirety by H), where it is thoroughly washed by numerous water sprays II for removing the smaller particles of the material, such as the -20 mesh material, which material is preferably transferred direct to a classifier l2, in a closed circuit l3, with a ball mill I I. This product is then ground to about 60 mesh and the overflow goes directly to an ore pulp thickener I5, from which point the thickened ore pulp is sent to a flotation, cyanide or other finishing plant for producing a marketable concentrate. The purpose of this thorough washing of the crushed ore is first to eliminate the -20 mesh material, which on account of the viscosity of the heavy fluid and its surface tension effects on the finer sizes, renders this fine product unsatisfactory for separation by the present process. Also it is expedient to thoroughly wet the -1" to 1 plus 20 mesh material before delivering it to the separatory trough Hi. This film of water on the particles greatly facilitates the removal of the medium from the ore particles in the subsequent washing steps and also assists in diluting the top of the medium in the separatory trough so as to slightly reduce the specific gravity of the top zone in the flowing medium column in the trough.

As the ore feed enters the trough l6, by way of a suitable passageway or chute H, from the screen ID, the medium from the medium mixing trough 20 is introduced with it by way of a conduit l8. In the medium mixing trough, the density of the medium is automatically controlled by blending the return medium from the separatory trough with varying proportions of the thickened medium. This maintains the medium absolutely at a constant point. A float control I9 is provided in the medium mixing trough 20 for controlling the volume of the return medium entering the separatory trough Hi.

In the separatory trough there is considerable violence at the entrance point, indicated, generally at A, and due to the splash practically all of the feed is submerged to substantially the lower middle zone of the trough. On the rebound, the finer and lighter gangue particles rise faster than the middlings and heavier concentrate particles, respectively, because the finer and lighter particles do not penetrate the medium as deeply as the larger and heavier particles. This results in trapping the lighter middlings below the supernatant bed of tailings. ,The bottom of the separatory trough I6 is slightly inclined downwardly from the receiving end toward the tailing-discharge end, as shown. Within the separatory trough and extending horizontally thereof and positioned adjacent the bottom thereof, and underlying the feed conduit I1, is provided a conveyer, represented in its entirety by 2 I. This conveyer is preferably a spiral conveyer andoperates as an agitator in the lower zone of the trough, and also serves to move the heavier concentrates and middlings upwardly of the trough incline and to the right of the trough, as viewed in the drawings, to the concentrate trap, represented in its entirety by 22.

Within the trap 22 is provided a jigging compartment 26, for the purpose of producing two (a concentrate and heavy middlings) products. This is an important feature of the invention and can be employed in extending the utility of the apparatus and process as a cleaner instead of using it only as a, roughing device to eliminate the lighter gangue or low-value materials. The jigging compartment 26 operates to assemble the heavier larger concentrates to form a bed superimposed on the screen 23 within the trap 22, and as the bed of larger concentrates builds up, the heavier middlings within the trap 22 will be forced upwardly and into contact relation with the conveyer 2| and carried to a middling trap 21. Thecoarse concentrates, as they build up, are dischrged through the well, indicated by the arrow shown in Fig. 4, to join the finer product passing throughthe screen '23, where they are removed by a suitable elevating device 24 to washing screen 25. The middlings which build up in jigging compartment 26 and are conveyed by conveyor 2| -maybe removed by a scraper wheel 28 or any suitable type of elevator and transferred by way of a passageway 29 to the washing screen 25, or to a separate washing screen (not shown),

' these lighter middlings gradually work their way longitudinally of the trough (from right to left in the drawings) toward the tailing exit or discharge to a point above a light middling trap 3|. 3

At this point, a very small amount of water may be introduced into the trap 3|, as indicated at 32, in order to reduce the density of the medium at this point and thereby facilitates sinking and trapping even the lightest middling particles and finest sand particles. The elimination of these fine sand particles assist greatly in purifying the medium.

These middlings within the moved by a suitable lift or pump, by way of a passageway 33, onto a screen, represented in its entirety by 34, where the medium is drained off of the middlings and returned to the trough by way of a conduit 35, preferably intermediate the middling trap 3| and the adjacent overflow outlet,..andthus rethicken the medium after it has been diluted with water in the zone overlying middling trap 3|. The middlings and fine sandy impurities collected by the screen 34 are transferred to the washing screen 25, or to a, separate washingscreen (not shown), by. way of a passageway 36, where the medium is removed and recovered in a medium thickener 31.1

The natant tailings overflowing at the lower end (left end in the drawings) of the separating trough l6 are conveyed by way of a' chute or passageway 38 to a vibrating screen 39 which partially removes the medium adhering to the tailings. The medium that filters through the screen 39, without any washing since it is the low gravity surface medium from trough I6, is returned to the medium mixing trough 2|! by way of a conduit 4|, with the assistance of a suitable pump 42. The tailings then pass onto another screen 43, where the remaining medium adhering thereto is removed and eventually recovered in the 1 same medium thickener 31 as that removed from the concentrates and middlings. The tailings go to waste and are sent to a point of distribution by means of a conveyer or the like.

The wash water from the several washing screens pass over a vibrating screen 44, the latter being of substantia ly 150 mesh, before go ng to the medium thickener. The 150 mesh vibrating screen operates to eliminate the l or 2% of plus 20 mesh material and fine sandy material,

which was not removed from the feed because of slight inefficiency. in the initial screening plant I. These impurities are transferred through a trap 3| are re-- suitable passageway from the 150 mesh vibrating screen to a concentrate bin 48 containing the concentrates and middling products, where they are subsequently conveyed to their original 20 mesh material in the ball mill classifier circuit, above mentioned.

Additional medium is prepared in a small auxiliary ball mill classifier circuit, represented in its entirety by 41. The medium from this ball mill classifier circuit is supplied to the medium thickener 31 for building up the diluted medium from the several washing screens to compensate for the small losses of medium during the process. This may be done periodically.

Above the medium thickener 31 there is provided two reagent feeders 48 and 49. One of these feeders 48 may be employed for adding a reagent, such as lime, or, in some instances, acids may be required to control the p. H. in the thickener. This reagent must be'varied for each particular ore in order to get the optimum settling conditions. The other reagent feeder 49 may be employed for adding a reagent, such as sodium silicate, alum, or some other suitable dispersion or fiocculating reagent to effect a selective dispersion of the small amount of gangue slimes that may remain in the diluted medium, as well as to flocculate the medium slimes. The overflow of the medium thickener 31 containing the gangue slimes with a very small quantity of the medium slimes are transferred to the ore pulp thickener, by way of a passageway 5|, where the valuable constituents are recovered in the subsequent finishing steps. It is here pointed out that experiments have shown that the over-all medium loss isf less than one pound of medium solids per ton o ore. 1

Novel means are provided for automatically regulating the amount of feed to the separatory trough l6, which means may comprise a suitable control, such as a Pitot tube 52 or viscometer, adapted to actuate a rheostat 53, the latter being ooeratably connected to a motor driven ore feeder 54, the ore feeder 54 being positioned to receive ore from a supply source 55 and transfer the same to a secondary crusher 56, where the ore is conditioned for delivery to the original vibrating screen ID; the Pitot tube 52 being responsive to 'ings uniform.

After the diluted medium has been reconditioned in the medium thickener 31, it is transferred by way of a passageway 51, with the assistance of a suitable pump 58, to a surge tank 59. The surge tank 59 is preferably provided with a suitable agitator 6| for maintaining the proper suspension of the solids in the medium. As above stated, the medium separated from the tailings, by the screen 39, is returned by way of Else passageway 4| to the medium mixing trough Since it is necessary to maintain a constant medium level, as shown at 62,. in the medium trough 20, a suitable by-pa s 63 within the passageway 4| is provided for by-passing the medium, by way of a passageway 64, into the diluted medium passageway 66, leading to the I mesh screen 44, where it is thereafter transferred to the medium thickener 31. medium is preferably controlled by blending in the mixing trough 20 the light density return The density of the medium from the passageway 4| with the thickened heavy medium from the surge tank 59.

A sensitive inverted submerged pressure tube or density gauge or a hydrometer ii is provided within the medium mixing trough, and below the medium level, the gauge 1| being operatively connected to a rheostat control 12 for automatically operating a valve 13 for supplying the proper amount of thickened medium to the medium mixing trough by way of a passageway 14 for maintaining the' density within the medium trough 20 at a predetermined point. A high speed agitator 15 is preferably provided adjacent the bottom of the mixing trough 20, and extending longitudinally thereof, for maintaining the proper suspension and blending of the solids in the medium.

The fioat 9 operates to control the medium level in the trough 20, andis operatively connected to a icy-pass 63 for varying the amount of the return medium to the medium mixing trough and by-passing the' remainder to the medium thickener circuit 66. This float I9 also controls the volume of medium that is supplied to the separatory trough l5, as above mentioned.

The medium within the separatory trough I6 inherently possesses a slight density differential. In general, the top gravity should be 0.1 to 0.2 of a point lighter than the bottom gravity and 0.05 lighter than the lightest tailing particle. This permits the wetted feed to sink below the surface of the medium as it falls into the trough It. This compensates for the turbulence set up by the incoming feed, and also for the surface tension effect on the finer sizes of gangue. Initially, the feed, regardless of the specific gravity of the particles, tends to sink tothe middle zone of the separatory trough. However, on the rebound, the finer sizes of the gangue particles, due to surface tension are lifted ahead of the coarser gangue particles. The middlings rise slower and are consequently trapped below the tailing bed instead of having to break through from above the tailing bed, as in. the prior process.

Particular attention is directed to the novel construction and operation of the conveyor 2| within the separatory trough. The axis of the conveyor 2| is inclined substantially the same amount as the bottom of the separatory trough l6, and that portion of the conveyer 2| to the right of the light medium trap 3| is of considerable larger diameter than that portion of the conveyer at the left of the middling trap 3| and the conveyer blade is preferably omitted overlying trap 3| The purpose of the larger portion at the right of the middling trap 3|, as viewed in the drawings, is to provide an undertow for drawing the middlings suspended just above the conveyer downwardly and into conveying relation with the conveyer, to convey the concentrates and middlings to their respective traps, and to induce a counter-current below the surface of the medium, which current operates to carry the suspended lighter middlings down the trough to a point above the middling trap 3|, at which point the current within the trough forms a zone of unagitated quiescence, permitting the lighter middlings to sink in the trap 3|, facilitated, if desired, by the minute dilution of the medium at this point by the means 32. This point of quiescence is also partially due to the particular shape of the mixing trough is, which trough gradually increase in cross sectional area from the upper or right end to the lower or left the separatory the control of veyer blades at the left of the trap 3| preferably taper in crest to crest diameter from the trap 3| toward the discharge end. This'diametral decrease in the conveyer permits a decrease in the inclination of the bottom of the trough adjacent this portion of the conveyer, which assists in checking the current and forming the zone of quiescence above the trap 3|. Also it will be noted that the heavy bottom medium returned from the bottom of trap 3| to the light middling screen 34, by way of passageway 35, is delivered within the trough l6 at a point adjacent 'the discharge end, the density of which medium will assist in the discharge of the tailings over the discharge end of the separatory trough, as well as decrease the differential of density in the medium column from the top to the bottom of trough at the discharge end.

It is here pointed out that in actual practice, gravity of the medium is obtained by grinding the solid particles in the medium not greater. than approximately 200 mesh, but when an endeavor is made to grind the solid constituents of the medium to substantially 200 mesh or finer, that actually only about 10% will be of this size and the balance vary in proportions from 200 mesh down to 500 mesh, or even finer. Therefore, when the medium flows toward the tailing discharge end of the separatory trough, there will be a stratification and classification with the finer slime particles, and hence a lighter density medium near the surface. It is also pointed out that the velocity factor of the present process involves many details and components. The mean velocity is controlled by the slope of the trough, and the induced cross-currents set up by the conveyer. The velocity-is least nearest the sides and bottom of the trough and greatest at a point slightly below the surface of the medium. When the feed is dropped into the separatory trough, stratification takes place, due to specific gravity of the particles, and, to a limited extent, due to the size and shape of the particles. The velocity of flow being almost uniform, has little effect on various sizes and shapes of the gangue particles, but it greatly accentuates even minute difference between the particles of difierent specific gravity, by virtue of their suspension in the medium.

The rotation of the spiral conveyer in the separatory trough counter to the flow of the medium creates an undertow and induces a current in the direction of the normal stream flow just below the zone of maximum flow. The concentrates and heavier middlings are conveyed upstream while the finer gangue particles rise nearest the surface in the top part of the zone of maximum flow, and the larger gangue particles rise to a point just below the zone of maximum velocity. The intermediate sizes of gangue collect along the upper sides of the trough. The lighter middlings which are too light-to be conveyed upstream with the concentrates, arrange themselves amazes below the tailings. Therefore, inasmuch as the crowding effect has a greater component laterally than upwardly or downwardly, as above pointed out, any crowding causes lateral readjustment in the upper tailing zone, so as to mix and distribute more or less uniform particles of different sizes of the same gravity. This tends to make the flow velocity more or less uniform throughout the channel and provides an ideal condition for preventing the formation of zones of excessive velocities which are inherent in other types of apparatus.

The-lighter middlings are removed continuously by way of the trap 3| and never reach the tailing end or discharge point. As the tailings reach the discharge point, they flow more rapidly than at any point in the trough, due to the velocity of approach. This also provides a lateral flow differential with the smallest velocity at the point of feeding, a substantially uniform flow to middlings trap 3| and progressively increasing towards the discharge end. Because of the fact that all along the trough the volume of material is reduced by the continuous removal of concentrates and middlings, no harm results when the velocity increases at the discharge point, since the valuable constituents of the on have already'been removed.

Other important features of the present process and apparatus includes the complete clarifying of the effect of the different sizes of particles, greatly minimizing the crowding effect, compensation for the surface tension effects on the finer sizes of particles, the automatic feed for supplying the ore to the separatory trough, the novel jigging compartment on the concentrate end so as to make two or more concentrated prodnets, and, thereby extending the utility of the process and apparatus as a cleaner instead of using it only as a roughing device to eliminate the lighter gangue material. The continuous removing of the heavy fine sand particles of concentrates will accumulate in the medium, thereby preventing excessive specific gravity of the medium at the bottom of the separatory trough; the introduction of water in the light medium trap and the returning of this heavy bottom medium near the tailing end of the separatory trough, and causing a dimunition of the differential in the differential density column near the tailing discharge; the processive removal of concentrates and middlings; together with the harmonius functioning of these various factors for bringing about a condition vital for savin the lightest middling particles.

While I have illustrated and described one form of an apparatus for carrying out the present method, it will be apparent to those skilled in the art that various changes, modifications, substitutions, additions and omissions may be made in the method and apparatus without departing from the spirit and scope of the invention as defined by the appended claims.

I claim:

1. The method of separating fragmentary materials of differing specific gravities, which includes the steps of feeding a supply of such material into a fluid medium composed of liquid and comminuted particles of solid' matter, inducing a flow of medium and fed material in opposite directions at different levels toward separate outlets, agitating the medium in the lowest zone while maintaining a zone thereabove more quiescent, and providing within the lowest zone a zone intermediate said outlets more quiescent than in the other portions of the lower zone.

2. The method of separating fragmentary materials of diflering specific gravities which includes the steps of feeding a supply of such material into a fluid medium composed of a liquid and comminuted particles of solid matter, inducing a flow of medium and fed material in opposite directions at different levels toward separate outlets, agitating the medium in the lowest zone while maintaining a zone thereabove more quiescent, providing within the lowest zone a zone intermediate said outlets more quiescent than in the other portions of the lower zone, and increasing the velocity of flow of the medium in the upper quiescent zone adjacent the lower more quiescent zone.

3'. The method of separating fragmentary maproviding withinthe lowest zone a zone intermediate said outlets more quiescent than in the other portions of the lower zone, and separately removing particles of the fed material from the respective zones of the different levels and separately removing other particles of the fed material from the more quiescent zone within the lower zone.

4. The method of separating fragmentary materials of differing specific gravities which includes the steps of feeding a supply of such material into a fluid medium composed of a liquid and comminuted particles of solid material, agitating the medium in the lower zone thereof while maintaining a zone'thereabove more quiescent, inducing a counter-current fiow in the respective upper and lower zones, and providing a zone intermediate the length of flow wherein the medium is maintained more quiescent from top to bottom than in the lower agitatedvzone.

5. The method of separating fragmentary materials of differing specific gravities which includes the steps of-feeding a supply of such material into a fluid medium composed of a liquid and comminuted particles of solid matter, agitating the medium in the lower zone thereof while maintaining a zone thereabove more quiescent,

inducing counter-current flow in the respective upper and lower zones, providing a zone intermediate the length of flow wherein the medium is maintained more quiescent from top to bottom than in the lower agitated zone, and separately removing particles of the fed material from the respective zones of the different levels, and separately removing other particles of the fed material from the zone wherein the medium agitated portions of the lower zone, and separately removing ore and medium from difl'erent levels and separately removing another portion of ore and medium from the more quiescent portion 01 the lower zone, and feeding additional medium to the fiowing mixture of ore and medium at a point intermediate the more quiescent portion of the lower zone and the outlet of the less agitated upper zone.

7. An apparatus for separating fragmentary materials of different specific gravities including an elongated trough, said trough having an outlet at an upper portion and being adapted to contain a compound mass of fragmentary material to be separated and a heavy fluid medium consisting of finely comminuted matter and a liquid, conveyor means in the lower portion of said trough adapted for conveying portions 01' the compound mass in the lowest zone of the trough and simultaneously agitating the compound mass in the trough, means to feed a supply of fragmentary material to said trough above the zone of the conveyor and agitator means, said conveyor having intermediate the feed means and the said upper outlet of the trough, a portion in which the agitative and conveying capacity of the conveyor is reduced thereby providing a zone of quiescence intermediate said feed and said upper outlet, a trap underlying the zone of quiescence, a trap for receiving material from the conveyor, and a circuit for receiving medium from respective outlets of the trough, said circuit including a thickener, a purification screen for medium and wash water ahead of the thickener, and a mixing trough for receiving diluted medium and thickened medium, said mixing trough including means operatively responsive to the viscosity oithe medium in the mixing trough for regulating the inflow to said trough of thickened medium and diluent.

8. A method of separating heterogeneous mixtures of solid particles having different densities which comprises washing and draining the mixture, introducing the mixture while still moist into a body of heavy fluid medium, withdrawing relatively heavy solid particles contaminated with medium from a lower portion of the body, with-- troducing the mixture of predetermined density' into the body of medium.

9. A method for separating a mixture 01' light and heavy ore particles which comprises introducing the mixture into a body of heavy fluid medium, withdrawing the light particles contaminated with medium from an upper portion of the body, withdrawing the heavy particles contaminated with medium from a lower portion of the body, washing the ore particles thus withdrawn to cleanse them of medium and dilute the medium, settling the diluted medium in a thickening chamber, withdrawing the thickened medium from a lower portion of the thickening chamber, withdrawing liquid and ore slimes from an upper portion of the thickening chamber thus removing ore slimes from the circuit, adjusting the density of the thickened medium to a predetermined point and then returning it to the body.

C. ERB WUENSCH.

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