US2893557A - Apparatus and method for centrifugal separation - Google Patents

Description

R. TEUTEBERG July 7, 1959 APPARATUS AND METHOD FOR CENTRIFUGAL SEPARATION 3 Sheets-Sheet l FIG. I

Filed Jan. 6, 1954 a W W y 7, 1959 R. TEUTEBERG 2,893,557

APPARATUS AND METHOD FOR CQENTRIFUGAL SEPARATION Filed Jan. 6, 1954 3 Shets-Sheet 2 INVENTOP ROAD \+\n TeuTebeRG- APPARATUS AND METHOD FOR CENTRIF'UGAL SEPARATION Filed Jan. 6, 1954 R. TEUTEBERG 7 July 7, 1959 3 Sheets-Sheet 3 United States Patent 0.

APPARATUS AND METHOD FOR CENTRIFUGAL SEPARATION Application .lanuary 6, 1954, Serial No. 402,449 :Claims priority,.applicati'on Germany January 10, 1953 17 Claims. 01. 209-1725 l This invention relates to improvements in the working up of minerals in heavy liquids. It more particularly relates to a method and apparatus for working up min erals, such as the cleaning of coal in heavy liquids with the use of centrifugal force.

The working up of minerals such as coal with heavy liquids is known. The heavy liquids used in addition to true solutions such as Zinc chloride include preferably suspensions of water and very finely ground weighting substances such as, for example, pyrites, magnetite and sand; The specific gravity of the heavy liquids used for the'working up should lie between the specific gravity of the mineral and that of the material to be separated. Thus, in the case of coal, the specific gravity of the heavy liquids should lie between the specific gravity of the coal and that of the rock, so that the former floats in the heavy liquid, while the rock will sink. Gravity was generally used as the force for effecting the separation in this working up with the use of heavy liquids.

The use of centrifugal force instead of gravity inthe working up and cleaning has also been proposed. Thus, for example, it was proposed 'to tangentially introduce a suspension of the material to be cleaned" in a heavy fluid mass into a cyclone, so as to utilize the centrifugal forces produced by the rapid eddying of the liquid in the cyclone to effect the separation of the components in accordance with their specific gravities. In this case, a portion of the heavy fluid will flow downward with a strong eddying-iaction together with the heavy constituents centrifuged outwardly-against the wall of the cyclone. These por tions will be discharged out of the discharge opening at the tip of the cyclone, while the rest of the liquid flows upwardly with violent eddying in the vicinity of the axis of the cyclone and discharges 'the light components with each other, particularly between the outer, downward-eddy and the inner upward eddy, and prevent the full action of the centrifugal forces on the solid com ponents. These eddy flows, which are unavoidable in cyclones, thus interfere in the operation and cause a substantially faulty discharge of the material, particularly in the case of material having large difierences in particle size.

It has furthermore been proposed to feed a suspension of material to be cleaned or worked up in heavy liquid to a jet centrifuge. In this jet centrifuge, the chamber is provided with discharge nozzles at the periphery for the heavy components which are centrifuged out. Nozzles are also provided in the vicinity of the axis of rotation for the discharge of the light components which are forced toward the center of rotation. With the use of the jet'centrifuge, however, substantial quantities of the heavy liquid are centrifuged out through the nozzles Patented July 7, 1959 ice so that there is a 'very strong flow of heavy liquid in the centrifuge chamber which prevents a dependable separa- Serial No. 308,630, filed September 9, 1952, I describe a method and apparatus for the working up and cleaning of minerals in heavy liquids with the use of centrifugal force without the above mentioned difiiculties.

One object of this invention is a further method and apparatus for the working up of minerals in heavy liquids which permits the use of centrifugal force without the previous difliculties encountered. This and still further objects will become apparent from the following description, read in conjunction with the drawings, in which:

Fig. 1 is a longitudinal section of the centrifugal heavy liquid separator showing the provision and connection of the auxiliary devices;

Fig. 2 is 'a radial section through a rotor of the centrifugal heavy liquid separator with graph showing the velocity of the inflow and the centrifugal rate of descent of the heavy material in the field of the rotation of the heavy liquid;

Fig. 3 shows a further embodiment of the centrifugal heavy liquid separator in section; and,

Fig. 4 is a section along the line II of Fig. 3.

In accordance with the present invention, the feed material is brought by a stream of conveying liquid into the field of action of an annular heavy liquid rotating field which directly surrounds a rotating field of a liquid of lower specific gravity than the feed material, for instance, a rotating field consisting of water. This combined rotating field is maintained within a container which is filled with a heavy liquid, is under pressure and has passing therethrough a stream of heavy liquid and this combined rotating field communicates with the content of the container only at the outer periphery of said field and, therefore, at the periphery of its heavy liquid ring. The light central liquid rotating field is fed by a central stream of conveying liquid passing through said field and consisting preferably of water, it being possible to impart any desired radial extent to it by regulating the pressure in the container. This combined rotating field can be maintained with very low flow rates. It imposes its rotating motion on the feed material and separates the same into an outer ring consisting of heavy liquid by centrifugal forces exclusively on basis of the specific gravity, by forcing the light components towards the center of rotation into the central rotating field consisting of the liquid of light specific gravity and forcing the heavy constituent towards its outer periphery into the container.

In this connection, the central rotating field of light liquids is of special importance. This rotating field permits the entire feed material fed with the central conveying liquid stream of light specific gravity to fly with increasing centrifugal acceleration towards the outside into the surrounding rotating field of heavy liquid and,

therefore, into a region of higher centrifugal forces. Here the light components of the feed material are captured, while the heavy components are centrifuged with a high liquid, due to its large volume, reduces the dew rate of the central conveying liquid stream considerably so'l'that the feedmaterial remains exposed to the centrifugal forces for a relatively long time in the central rotating field of liquid of light specific gravity and, therefore, in a medium of low viscosity. In this way, also, the finely granular heavy components have sufficient opportunity to pass into the surrounding rotating field of heavy liquid so that a great sharpness of separation on basis of specific gravity is achieved even when there are large diiferences in the particle size of the feed material.

Inasmuch as the central liquid rotating field in practice always consists of water, it makes it possible to feed the feed material together with the stream of water feeding the central rotating field and for the light components of the feed material recovered to leave the rotating field together with the water stream.

In the case of the new method, therefore, only the heavy components are removed with heavy liquid, namely with the Stream of heavy liquid which passes through the container. This stream, in this connection, need only be sulficiently large for it to be able to remove the heavy components without disturbance from the container. Thus, the new method requires, for instance, in working up coal with a waste content of 25%, only about 25% of the heavy liquid circulation of the known processes, for instance the cyclone process, which must charge the entire feed material mixed with heavy liquid. In this way, there are obtained substantial simplifications in the recoveiy and regeneration of the heavy liquid.

The combined rotating field can be produced in any desired manner. In accordance with the present invention, it is produced in a hollow rotor filled with a heavy liquid the said rotor rotating in a container which is filled with heavy liquid and is under pressure and the hollow space of which is in communication with the contents of the container only at the periphery of the rotor. The pressure in the container prevents the rotor being emptied by pump action. Its contents then rotate with the rotor and thus form a heavy liquid rotating field. The central liquid rotating field is produced within the heavy liquid rotating field by means of a stream of conveying liquid of low specific gravity which traverses the hollow space of the rotor centrally, for instance, Water. The stream of conveying liquid washes out the core of the heavy liquid field and widens depending on the pressure prevailing in the container to a greater or lesser extent into the desired central field of liquid of low specific gravity.

Further features and advantages of the new method will become evident from the following description of several embodiments of the invention.

Referring to Fig. l, a hollow rotor 2, rotates in a stationary container 1 which is filled with heavy liquid. The hollow space of the impeller communicates with the contents of container 1 only at the periphery of the impeller by means of the annular slot 3. The shaft journals 4 and 5 of the impeller are tubular and form a central inlet and outlet for the hollow space of the rotor. They extend out of the container 1, [hermetically sealed by stuifing boxes 6 and 7. In the central inlet and outlet of the hollow space of the rotor, there are provided vanes 8 and 9 which are preferably curved in peripheral direction in the manner shown in Fig, 4. The rotor 2 has its shaft journals supported in the Ibearings 10 and 11 and is driven by means of the pulley 12 from an electric motor not shown in the drawing. In the cover of the container 1, there is an inlet connection 13; while at its lowest point, there is a discharge connection 14. By means of these connections 13 and 14, the container is connected in a heavy liquid circuit which starts from the hopper 15, passes through the container 1, over the drop part 16 of the screen 17, the collector tank '18 and the pump 19, back to the hopper 15. The hopper is at such a level above the container 1 that its natural feeding pressure exceeds the pressure which the hollow rotor 2 is capable ofproducing due to its pump action in the container. The discharge of the container 1 is throttled by a throttle valve 20 or a static tube 21 (gooseneck) opposite the inlet 13 so that a static pressure is produced in the container 1. This static pressure can be regulated as desired by means of a regulating valve 22 in the feed. The feed connection 13 can also be connected tangentially in the direction of rotation of the rotor 2 at the cylindrical part of the container 1.

When the apparatus is placed in operation, the rotor 2 is first of all caused to rotate. Thereupon, by opening the regulating valve 22, the container 1 and the hollow space of the rotor are filled with heavy liquid and the above described heavy liquid circuit is placed in operation by means of the pump 19. In this connection, the regulating valve 22 is so adjusted that the static pressure in container 1 is somewhat greater than the pressure produced by the rotor 2, filled with heavy liquid, as a result of its pumping action in the container. The excess pressure in container 1 then permits a quantity of heavy liquid to flow into the rotor through the annular slot 3 and emerge through the pipe connection 5. Inasmuch as the heavy liquid contained in the rotor takes part in the rotation of the latter and, inasmuch as no flows can be produced as a result of centrifugal action in the rotor due to the static pressure in the container 1, it forms a heavy liquid rotating field which is only encumhered by the slight inward flow which is precisely controlled by the regulating valve 22 and is in open com munication via the annular slot 3 with the heavy liquid contained in container 1. Now, a stream of water is fed to the rotor 2 via the hopper 23 and the hollow shaft journal 4, the said stream of water passing through the hollow space of the rotor in axial direction and emerging again from the hollow shaft journal 5. This stream of water, which is placed by the vanes 8 in rotation and is deflected into the peripheral direction, washes out the heavy liquid in the center of the heavy liquid rotating field and extends radially outward to the water rotating field a. The radial extent of the water rotating field is greater, the smaller the above described inward flow in the rotor 2. It can be precisely regulated and maintained in accordance with the existing requirements in every case by means of the regulating valve 22. Within the hollow space of the rotor, there is now a combined field of rotation which consists of the central water rotating field a and the annular heavy liquid rotating field b which directly surrounds the former. Mixing of the rotating fields a and b is impossible inasmuch as the water field a, which is of lower specific gravity, is not able to pass outward through the heavy liquid rotating field b and because no turbulent flow caused by centrifugal action can occur within the rotor. The contact surface x along and in which the rotating fields a and b touch each other, forms a paraboloid of rotation, the axis of rotation of which coincides with the axis of the rotor.

The feed material is fed to the central water stream by means of the feeding device 24. Upon entering the central water field a, it assumes the rotary motion of the later and is centrifuged outwards into the surrounding heavy liquid rotating field b with increasing centrifugal acceleration by the centrifugal forces acting in this connection. Inasmuch as the water field a is only of low viscosity, the feed material, particularly the heavy components contained therein, can assume high centrifugal velocities. Upon meeting the surrounding heavy liquid rotating field, the particles of low specific gravity are caught while the particles of high specific gravity are centrifuged at a high velocity through the heavy liquid field b into the container 1. They thus come into the above described circulating flow which passes through the container 1 and are discharged with the heavy liquid flow which leaves the container through the outlet con motion 14. The light components which are not able to pass through the heavy liquid field b remain within assess? d J the range of action of the central water field and are discharged from the hollow shaft journal 5, together with the stream of water passing through said water field and fed to the draining screen 25. Due to the inward flow in the rotor which has been set by means of valve 22, small quantities of heavy material and sludge pass into the central water stream. The water separated on the drainage screen 25 is, therefore, fed to a sump 26 from which it is pumped by pump 27 into the clarifier 28. The purified water flowing over, out of the clarifier Z8,- is, conducted in part to the sprays 29, 30 through the screens 17 and 25 and in part to the hopper 23.

The heavy liquid emerging from the heavy components of the feed material from connection 14 is deposited on the screen 17 above the drip portion 16. The mine waste is,.-thereupon, washed away above the shallow portion 31 by means of the showers 29. The shower water which drops into shower portion 31 and contains the washedaway heavy material, flows to the sump 26 and is pumped by pump 27 into the clarifier 28. The thickened suspension of heavy material which is present at the apex of the clarifier is fed to the collector tank 18. In the hopper 15, there is provided a known device 32 for regulating the specific gravity of the heavy liquid, which device regulates the quantity of suspension flowing back from the clarifier 28 to the collector tank 18 by means of throttle valve 33. The quantity of suspension which has.

not been permitted to pass by the throttle valve 33, is discharged in the overflow 34 to the sump 26 and again charged into the clarifier 28. Any possible losses of heavy material are replaced in the manner known per se by the introduction of fresh heavy material into container 18.

The above described adjustment of the specific gravity is stable because the heavy liquid (heavy material suspension flowing through the container 1) experiences a certain concentration due to the centrifugal action of the rotor 2 in combination with the inward flow adjusted for by the valve 22. The specific gravity of the heavy liquid in' container 15, therefore, continuously has a tendency to increase. Therefore, it is only necessary to provide a sufliciently large discharge nozzle 35 in the apex of the clarifier 28 in order to insure that the thickened suspension has a lower specific gravity than the heavy liquid in container 1.

When operating the device with true solutions, such as, for instance, calcium chloride as a heavy liquid, the shape of the rotor is only of minor importance. When using heavy material suspensions, on the other hand, the shape of the rotor is of importance.

The finely ground, heavy material, which is suspended in the heavy material suspension, is subject in precisely the samev manner as the preparation material to the centrifugal force and if special measures were not taken, would be centrifuged out of the heavy liquid rotating field b. This can be prevented, however, by the above described inward flow which passes through the rotor 2 from the container 1, provided the velocity of this inward flow is equal to or greater than the velocity at which the heavy material moves under the influence of the centrifugal force in the opposite direction, i.e. towards the outside. This velocity of the heavy material is termed hereinafter the centrifugal rate of velocity of descent." By the imparting of a suitable shape to the rotor, any desired velocity of. the inward flow can be produced' Thus, it is possible to cause the inward flow from the outside to the center of rotation to increase or decrease or to maintain it constant.

.By experimentation, it has been discovered that a velocity of the inward flow which decreases in the field of action of the heavy liquid rotating field b is to be preferred. This, however, does not mean that the other possibilities are to be entirely discarded.

15' In Fig. 2, there is showna radial section of a.rotor section'which has proven satisfactory. Inasmuch as the ,6 heavy liquid enters the rotor from the surrounding cori= tainer 1, over the entire peripheral cross-section F=dwk of. the annular slot 3 in a uniform manner and flows radially (as shown by the arrows) towards the axis of rotation of the rotor, there is obtained, corresponding to the continuously changing peripheral cross-sections in this direction, the course of the velocity V of the inward flow shown within the rotor. In connection with the inward flowing quantity of heavy liquid of 490 liters per hour taken as basis in connection with the representation in the figure, the inward flow in the annular slot 3 has a velocity of 18 mm. per second, but decreases greatly within the rotor reaching its minimum value of 2.5 mm. per second at the rotor diameter d millimeters and then increasing again. Upon changing the quantity of inward flowing heavy liquid by the valve 22 in Fig. 1, only the absolute velocity values V of the inward fiow change. The minimum velocity value will, therefore, always be located at the rotor diameter 0! =160 mm., inasmuch as the rotor has the greatest peripheral crosssection there, namely F -max=d,,'1r-h In accordance with the present invention, by adjusting the pressure in the container 1 by means of the valve 22 in Fig. l and thus, by adjusting the quantity of heavy liquid flowing inwards in the rotor, the limit X between the central liquid rotating field a and the surrounding heavy liquid field b is displaced into the region of the said minimum velocity of the inward flow, as shown in Fig. 2. In this way, it is achieved that the separation of the feed material into its components of lighter and heavier specific gravity takes place in an area of flow velocity and, therefore, exclusively on basis of the specific gravity. A further advantage is that the inward flowing heavy liquid becomes mixed with the water of the liquid field a in the zone of minimum flow velocity. The quantity of inward flowing heavy liquid is, in this connection, strongly diluted so that the velocity of descent of the weighting increases considerably due to the viscosity being strongly decreased by the dilution. This has the result that a substantial part of the heavy material carried together with the inward flow into the water field a is again thrown back by centrifugal force into the heavy liquid rotating field b so that practically only the water content and the finest particles of heavy material and sludge of the inward flowing quantity of heavy liquid will flow towards the axis of rotation of the rotor and mix with the central stream of conveying water which enters through the inlet connection 4 into the rotor and leaves through the outlet connection 5. The central liquid field a, therefore, retains the heavy material predominantly in the heavy liquid rotating field b and causes the lighter constituents of the feed material to be discharged out through the discharge connection 5 with a liquid which contains only a small amount of heavy material.

This property of the central water rotating field a makes it possible to select the particle size of the heavy material in such a manner, that due to the fact that its velocity of descent has been increased by the above described dilution in the water rotating field a, it is retained practically completely in the heavy liquid rotating field b. If its centrifugal velocity of descen in the heavy liquid rotating field b is too great with respect to the specific gravity required for the separation of the feed material, fine tailings must be added to the heavy liquid in order to maintain the heavy liquid rotating field b. These tailings increase the viscosity of the liquid and thus sufiiciently reduce the velocity of descent of the heavy material in the heavy liquid rotating field 11. Only small quantities of the worthless fine tailing will enter the water field from the heavy liquid brought into the central water field by the inward flow. These can be removed without any difiiculty by separating the water from the components of low specific gravity recovered from the feed material with the use of screens, centrifuges or in drip towers. In working up mineral coal, tailings which sufficiently increase the viscosity of the heavy liquid are generally obtained from the extremely fine rocks contained in the crude coal and which tend to separate from the water, so that the addition of outside tailings can be dispensed with. In most cases, this natural sludge formation and the increase in the viscosity of the heavy liquid inherent therein is so great, that the sharpness of the separation of the feed material is impaired. It is, therefore, desirable to maintain the viscosity of the heavy liquid approximately constant. In the case of the apparatus, in accordance with the invention, for all practical purposes, only the sludges which increase the viscosity of the heavy liquid enter the Water field a and from there, the stream of conveying liquid which discharges out of the rotor 2 through the connection 5. With increasing viscosity of the heavy liquid, the quantity of sludge passing into the stream of conveying liquid and thus the specific gravity of the stream will increase. This specific gravity is, thus, an exact measure of the viscosity of the heavy liquid in the rotating field b and in the container 1.

The regulation of the viscosity of the heavy liquid is effected in accordance with Fig. l, in accordance with the invention, by a specific gravity regulator 36 of known type, which measures, in an overflow vessel 3-7, the specific gravity of the stream of conveying liquid emerging ,from connection 5 and feeds water to the collector tank 18 for the heavy liquid, by means of the throttle valve 38 controlled by it. In this connection, the arrangement is such that the quantity of water fed to the collector tank 18 is increased with increasing specific gravity or viscosity and vice versa. The addition of water to the collector tank 18 has the result that the specific gravity regulator 32 moves in the hopper 15, the throttle valve 33 in the direction of closure so that less fine sludge is fed the collector tank 18 from the clarifier 28, the larger the quantity of water fed to the collector tank 18 by the regulator 36.

It has already been mentioned that the heavy material in the rotor is subjected to oppositely acting forces, namely, the centrifugal force which tends to throw it out of the rotor and the thrust of the inward flow which tends, on the other hand, to push it towards the center of rotation of the rotor. It can be assumed that the centrifugal acceleration z increases, by as many time as it is greater than g, the normal velocity of descent S, which the heavy material in the heavy material suspension assumes under the action of gravitational acceleration g. Accordingly, the centrifugal velocity of descent 8,, which the heavy material assumes in the rotating field 1s:

in which connection:

There can thus be calculated, at a given speed of rotation, n of the rotor, the corresponding centrifugal velocity of descent S of the heavy material for each rotor diameter d=2r. If there are plotted the values of S with respect to the diameter of the rotor, there is obtained a straight line which passes through the point of origin. In Fig. 2, there are shown two such lines representing the centrifugal velocities of descent S and S' The lower line is the tangent to the V curve (V =velocity of the inward flow) while the upper line S' intersects the V curve at the point which corresponds to the inward velocity at the outer diameter of the rotor. Assuming a speed of rotation of the rotor of n=545 rpm, the line S represents the centrifugal velocity of descent of a heavy material which, under the action of gravitational acceleration, has a normal velocity of descent S=5.26 mm. per minute, while in case of the upper line S',, the normal velocity of descent is 8:21] mm. per minute. A. heavy material suspension consists always of a mixture of finer and coarser particles of heavy material, the fine particles have a low rate of descent in the suspension and the coarse particles a high rate. In the embodiment,

according to Fig. 2, the particle of heavy material, the normal velocity of descent of which, is greater than 21.7 mm. per minute (upper line S is not able to enter the rotor or, therefore, the rotating field b at all, inasmuch as its centrifugal velocity of descent S is greater than the inward fiow V at the periphery of the rotor. The heavy material particle, the velocity of descent of which is equal to 5.26 mm. per minute (lower line 8 only reaches the rotor diameter of 178 mm. because at that place, its centrifugal velocity of descent S is equal to the inward flow V =2.6 mm. per second. The heavy material granule finally, the velocity of descent of which is smaller than 5.26 mm. per minute is carried by the inward flow into the water field a on the other side of limit x. The heavy liquid rotating field b thus retains the heavy material granules, the normal velocity of descent of which is within the region between 5.26 and 21.7 mm. per minute and only permits the finer granules ofheavy material, i.e. a fraction of the total heavy material of the inward flowing heavy material suspension, to pass into the central water field a. A considerable part of the fraction entering the water field will then, furthermore, be retained by the fact that, as already described, its velocity of descent is suddenly increased upon mixing with the water.

This property of the heavy liquid rotating field, namely, to retain certain heavy material particle sizes, is achieved, in accordance with the present invention, as can be noted immediately from the velocity graph of Fig. 2, due to the fact that the inward flow V decreases more strongly in the region of the heavy liquid rotating field b than, seen in the same direction, the centrifugal velocity of descent S of the heavy material determined by the centrifugal forces.

In the case of the embodiment according to Fig. 1, the feed material is added tothe stream of convey liquid of low specific gravity, for instance water, which passes centrally through the rotor 2 and, in this manner, it is fed to the region of the heavy liquid rotating field b. The feed material can, however, also be admixed to the stream of heavy liquid passing through the container 1. However, in such case, the container 1 must have the shape shown in Figs. 3 and 4. In accordance with Figs. 3 and 4, the container 1 is adapted to the shape of the rotor 2 and has an annular space 39 in the region of the annular slot 3 in the rotor, into which space, the rotor 2 extends with a slight amount of play. Into the annular space 39, there open tangentially two connections 40 and 41. Connection 40 corresponds to connection 13 of Fig. 1 and serves as inlet connection for the heavy liquid, while the connection 41 corresponds to connection 14 of Fig. 1 and permits the heavy liquid to discharge again tangentially from the container 1 after the largest possible peripheral path u seen in the direction of flow. The feed material admixed to the entering heavy liquid is fed in the region of the annular slot 3 to the heavy liquid rotating field b maintained within the rotor 2, this feed being effected at the outer periphery of said rotating field, the feed material being offered along the peripheral path It. In this connection, the light components are displaced through the rotating field b all the way to the central liquid rotating field a of lower specific gravity and from there discharged with the central stream of conveying liquid of light specific gravity entering through the inlet connection 4, via the central outlet connection 5 while the heavy constituents remain in the stream of heavy liquid and leave the container 1 together with the latter through the connection 41.

I claim:

1. Method for the working up of granular materials n. heavy liquid with centrifugal force, which comprises,

establishing a substantially volume-confined, pressurerestricted zone of heavy liquid, rotating and maintaining an annular field of heavy liquid within said zone only in peripheral communication with the rest of the heavy liquid in said volume-confined zone, rotating and maintaining within said annular field of heavy liquid a central field of light liquid of lower specific gravity than said heavy liquid, conveying granular material in a convey ing liquid stream to said field, and centrifugally separating said material into at least two specifically differing fractions by the centrifugal action of said field and the passage of one of said fractions through said field.

2. Method in accordance with claim 1, which includes passing a fraction of lighter liquid of lower specific gravity than said heavy liquid centrally through the field of light liquid in an axial direction, and discharging such a lighter fraction from said central field.

3. Method in accordance with claim 2, in which said granular material is fed to said field in said stream of light liquid.

4. Method in accordance with claim 1, in which heavy liquid is maintained and passed through said zone at a pressure suflicient to cause a flow in said annular field in the direction of said central field.

5. Method in accordance with claim 4, in which the flow velocity decreases in said annular field towards the interphase between the annular field and the central field.

6. Method in accordance with claim 5, in which said annular field possesses an increasing axial cross-section from its periphery towards the interphase between it and the central field.

7. Method in accordance with claim 6, in which the granular material to be worked up is fed into the central field.

8. Method in accordance with claim 1, in which said granular material to be worked up is fed tangentially to said annular field.

9. Method in accordance with claim 8, in which the lighter of the two specifically differing fractions is removed from said central field and the heavier of said fractions is removed tangentially from said annular field.

10. Method in accordance with claim 1, which includes adding a viscosity-increasing agent to said annular field of heavy liquid.

11. Method in accordance with claim 1, in which the specific gravity of said annular field of heavy liquid is regulated and maintained at a predetermined value.

12. Method in accordance with claim 1, in which the light liquid in said central field is water.

13. A centrifugal heavy liquid separator for the working up of granular materials, which comprises, a stationary, substantially enclosed, heavy liquid container, a hollow impeller rotatably positioned 'in said container, said impeller having at least two spaced-apart opposed surfaces defining a space therebetween in free peripheral communication with said container, means defining a fluid inlet to one axial side of the central portion of the interior of said hollow impeller, means defining a fluid outlet from the central portion of the opposite axial side of the hollow interior of said impeller, means defining a fluid outlet from said container, means for retaining fluid in said container at a predetermined pressure, and means for rotating at least one of said opposed surfaces of said impeller.

14. A separator in accordance with claim 13, in which the peripheral cross-section of said hollow impeller increases from its periphery towards its axis of rotation.

15. A separator in accordance with claim 13, which includes guide vanes positioned in the fluid inlet and the fluid outlet of said hollow impeller.

16. A separator in accordance with claim 13, in which said heavy liquid container is of substantially the same shape as said hollow impeller, and which includes an annular space substantially surrounding the space defined between the two spaced-apart opposed surfaces of said impeller, with means defining a tangential fiuid inlet and a tangential fluid outlet from said annular space.

17. A separator in accordance with claim 16, in which the peripheral cross-section of said hollow impeller increases from its periphery towards its axis of rotation.

References Cited in the file of this patent UNITED STATES PATENTS 978,238 Trent Dec. 13, 1910 1,158,959 Beach Nov. 2, 1915 1,373,219 Beach Mar. 29, 1921 1,640,707 Laughlin Aug. 30, 1927 2,263,095 Lieberman Nov. 18, 1941 2,495,950 Van Der Werfi Jan. 31, 1950 FOREIGN PATENTS 667,128 Great Britain Feb. 27, 1952

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