US2754963A

US2754963A - Coal washing process

Info

Publication number: US2754963A
Application number: US413562A
Authority: US
Inventors: Krijgsman Centinus; Freerk J Fontein; Jan N J Leeman
Original assignee: Stamicarbon BV
Current assignee: Stamicarbon BV
Priority date: 1954-03-02
Filing date: 1954-03-02
Publication date: 1956-07-17
Anticipated expiration: 1973-07-17

Description

July 17, 1956 Filed March 2, 1954 FIG. I

c. KRIJGSMAN ET AL 2,754,963

COAL WASHING PROCESS 2 Sheets-Sheet l July 17, 1956, V c, KRUGSMAN ET AL 2,754,963

com. WASHING PROCESS Filed March 2, 1954 k 2 Sheets-Sheet 2 United States Patent ()fifice COAL WASHING PROCESS Centinus Krijgsman, Hoensbroek, Freerk J. Fontein,

Heerlen, and Jan N. J. Leeman, Brunssurn, Netherlands, assignors to Stamicarbon N. V., Heerlen, Netherlands Application March 2, 1954, Serial No. 413,562

11 Claims. (Cl. 20912) This invention relates to coal washing. More in particular this invention relates to a heavy media process of coal washing by means of a suspension of fine shale particles and other fine waste particles derived from the raw coal.

It is the object of this invention to provide an improved process wherein the separation is sharp and easily adaptable to varying conditions, wherein the various machines are efliciently used and water consumption is maintained at a minimum. It is in particular the object of the present invention to provide a process in which the amount of liquid in the circuit and the level of the suspension in the various machines is readily kept constant, and wherein the specific gravity according to which the separation is made can easily be kept substantially constant at a desired value.

in summary these and other objects are obtained by feeding the coal to a washing tank which contains a suspension in water of fine shale particles or other fine waste particles derived from the raw coal; separately removing the floating and the settled particles from the suspension; feeding the particles thus removed to washing screens; spraying these particles first with clarified liquid from a suspension-thickener and thereafter with water; screening the underflow from the washing screens on a fine screen with apertures smaller than 0.6 mm. so as to remove coal and shale particles which are too coarse to be efficiently separated by a froth-flotation process; subjecting the undersize from the fine screen to a froth-flptation process in a froth-flotation machine; withdrawing underflow from the froth-flotation machine at a larger rate than is introduced therein as underflow from the fine screen and feeding such underflow to the said suspensionthickener; feeding thickened suspension from the suspension-thickener to the washing tank; feeding clarified liquid from the suspension-thickener to an overflow-vessel; feeding clarified liquid from the overflow-vessel to the frothflotation machine at a rate so as to maintain the level of the liquid in the froth-flotation machine substantially constant; and removing the balance of the feed to the overflow-vessel from the circuit.

The process and apparatus adapted to be used in this process will be further explained with reference to the drawings, of which Figure 1 schematically shows a flow-sheet of a coal washing plant according to the invention, and

Figure 2 schematically shows means for controlling the level and the specific gravity of the suspension in two washing tanks which are arranged in series.

In Figure l raw coal, larger than for instance mm. is fed to a washing tank 1, which contains a suspension convenient manner.

A frothing agent, such as pine oil, is introduced in the of fine waste particles derived from the coal, such as shale, pyrite and sand, hereinafter referred to as shale. The specific gravity of this suspension is such that coal floats thereon and that shale, which has a higher specific gravity than coal, sinks. This is the reason that this kind of separation is known as the sink-and-float process.

The Washing tank 1 is provided with a conventional device, such as an endless scraper, which removes the separated particles from the suspension and feeds the sink product, that is the shale fraction, to a Washingscreen 2, and the float product, that is the coal fraction, to a screen 3. Above screen 2 are sprayheads 4 which are fed via a pipe 5 with clarified liquid, and sprayheads 6 which are fed with water via a pipe 7. On the screen 2 suspension adhering to the shale particles is removed so that clean shale is discharged from the screen 2. The undersize from the screen 2 consists of diluted suspension which is fed via a pipe 8 to a screen 9.

Above screen 3 are sprayheads 10 which are fed via a pipe 11 with clarified liquid and sprayheads 12 which are fed with water via a pipe 13. The oversize from screen 3 consists of clean coal which is removed from the circuit; the undersize consists of diluted suspension which is fed via pipe 8 to screen 9. i V

The undersize from screen 9 is fed to a froth-flotation machine 14 via a feed pipe 15 and the function of screen 9 is to prevent particles which are too coarse for frothflotation from entering the froth-flotation machine 14. Therefore screen 9 has apertures of at most 0.6 mm. and preferably of about 0.5 mm. Width. The oversize from screen 9 is removed from the circuit; it may be discarded or be further treated in a washing plant for fine coal or it can be mixed with clean coal or be used in any feed pipe 15. In the froth-flotation machine 14 shale particles settle but coal particles are caught in the froth which floats. The coalbearing froth is removed from the machine and from the circuit.

A diluted shale suspension is discharged from the frothflotation machine 14 through a discharge aperture 16 and is pumped by a pump 17 via a pipe 18 to a hydrocyclone-thickener 19. Thickened suspension is discharged therefrom through its apex aperture 2% and is fed via a pipe 21 and several devices which will be discussed later to washing tank 1.

23 and

pipes

5 and 11 to the sprayheads 4 and 10.

both coal and shale and which are hereinafter referred to as mixed, and shale particles. In broad terms it is the task of the washing plant to separate the coal from the shale, but it will be clear that at least mixed with a high coal content should also be separated from the shale and should be either obtained separately or as part of the coal fraction. Likewise mixed with a high ash content should be either obtained as a separate fraction or as part of the shale fraction. Often three fractions are made, called coal-, mixedand shale fraction, but

it should be understood that the coaland shale fractions 7 also contain mixed.

Mixed with a low coal content has a higher specific gravity than coal or mixed with a high coal content and Patented July 17, 1956 Clarified liquid is discharged through the overflow aperture 22 of the hydrocyclone-thickener 19 and flows via a flow divider it is therefore possible to separate the raw coal into fractions according to the coal content of the particles.

Figure 2 illustrates a variation of an installation of Figure 1, adapted for separation into three products and like numerals are employed therein to indicate like elements. According to Figure 2 there are two washing tanks 1 and 24. The suspension in washing tank 1 has a higher specific gravity than the suspension in washing tank 24; the specific gravity in washing tank 1 may for instance be 1.6 and in washing tank 24 1.4.

The shale fraction from washing tank 1 is fed to screen 2; the float product from washing tank 1, consisting of coal and mixed, is fed to washing tank 24 and separated therein. The coal fraction is discharged to washing screen 3 and the mixed fraction is discharged to a washing screen 25.

Above screen 25 are sprayheads 26 which are fed via a pipe 27 with clarified liquid from flow divider 23, and sprayheads 28 which are fed with water via a pipe 29. Clean mixed is discharged from screen 25 and is removed from the circuit. The undersize from screen 25 consists of diluted suspension which is united with the undersize from screens 2 and 3.

It is important that the specific gravity of the suspension in every washing tank is accurately controlled. In general the specific gravity should be kept constant, but it may be necessary to change the quality of the produced coal or mixed, or the quality of the raw coal may change and in such cases the specific gravity of the suspension in the washing tanks must be changed and maintained at the new level.

Figure 2 schematically shows how the specific gravity in the washing tanks 1 and 24 is controlled and also how the level of the suspension in the washing tanks is controlled.

The specific gravity of the suspension in washing tank 1 is automatically controlled by means of a pressure sensitive instrument 30, for instance a Foxboro M 40 Stabilog Controller of the Bell type with a reset. The instrument 30 is connected with bubble tubes 31 and 32 which have open ends terminating in vertically spaced relation in the suspension in washing tank 1. Compressed air is introduced through pipe 33 to instrument 30, through pipes 33 and 34 and reducing valve 35 to bubble tube 31 and through pipe 36 and reducing valve 37 to bubble tube 32. In bubble tube 31, which terminates not as deep in the suspension as bubble tube 32, there is a closed bubble vessel 38 which contains some liquid. The air delivered to the bubble tubes 31 and 32 will encounter different pressureresistance in washing tank 1 due to the vertical spacing of their ends. The pressure difference between the tubes 31 and 32 thus depends on the specific gravity of the suspension in washing tank 1. The instrument 30 responds to the pressure difference between the tubes 31 and 32 and consequently to the specific gravity in washing tank 1, so that if the specific gravity in washing tank 1 decreases, the pressure in the pipe 39, which is connected with the instrument 30, increases (This arrangement has been described in more detail in U. S. Patent 2,649,963 and in the Journal of the Institute of Fuel, April 1948, page 192.) The pipe 39 is connected through pipe 49 and valve 41 with a device 42 which controls the diameter of the apex aperture 20 of hydrocyclone 19. As has been described in more detail in U. S. Patents 2,654,479 and 2,649,963, the specific gravity of the thickened suspension from hydrocyclone-thickener 19 can be controlled in this manner: if the pressure in pipe increases the diameter of the apex aperture 20 decreases and the specific gravity of the apex fraction increases. Consequently any deviation from the specific gravity to which the instrument 30 is adjusted is automatically counteracted until the proper specific gravity is restored.

It is also possible to use a hydrocyclone-thickener 19 arranged as described in U. S. patent application Serial No. 360,464, filed June 9, 1953 by Ian N. I. Leeman and Freerk I. Fontein, wherein the discharge apertures of the hydrocyclone are provided with suction pipes discharging into receivers under the liquid level therein. Under such circumstances the rate of apex discharge is a function of the weight of the liquid columns in the two suction pipes. By properly selecting the length of those columns the hydrocyclone can be made to discharge a thickened suspension the specific gravity of which is substantially constant, even if the concentration of the feed varies. If such a hydrocyclone-thickener is used in the present process it should be regulated so as to discharge a suspension the specific gravity of which is somewhat higher than the desired specific gravity of the suspension in the washing tank 1. The proper specific gravity of the suspension in washing tank 1 may then be attained by continuously feeding a controlled amount of clarified liquid from flow divider 23 via pipe 43 to washing tank 1. The quantity is regulated by means of valve 44 in pipe 43, valve 44 being automatically adjusted by instrument 30 via pipe 39 and valve 45 therein. When the pressure in pipe 39 decreases the valve 44 is opened or opened further. To prevent obstructions valve 44 preferably is a needle valve which is directly connected to flow-divider 23. It is of course also possible to feed a controlled amount of water to washing tank 1 instead of clarified liquid.

It will be clear that either valve 41 or valve 45 should be opened. If the hydrocyclone-thickener 19 is of the type described in U. S. Patent 2,649,963 valve 45 is closed and the specific gravity is controlled by means of the device 42; if the hydrocyclone-thickener 19 is of the type described in U. S. application Serial No. 360,464 the specific gravity is controlled by means of valve 44.

A small amount of shale which is used in the separating suspension is continuously lost; shale adheres to the solids removed from the circuit and as will be discussed in more detail hereinafter, also shale containing liquid is removed from the circuit. Consequently new fine shale must be supplied. To this end a portion of the clean shale discharged from washing screen 2 is fed together with water or with clarified liquid from flow-divider 23 to a mill 46 (Figure l), for instance a ball mill. The ground shale is classified in a classifier 47, the coarse fraction from which is returned through pipe 48 to the mill 46, whereas the fines from classifier 47 are led through pipe 49 to screen 9 and from there to the froth-flotation machine 14, but may also be directed immediately from classifier 47 to the froth-flotation machine 14.

The capacity of the mill 46 and the classifier 47 which form a closed circuit, is so large that normally they need not operate continuously. That is, if the mill-classifier unit is in operation, the amount of fine shale in the circuit increases, whereas otherwise this amount decreases. This is one of the reasons why a storage vessel 50 may be provided in the circuit. As best shown in Figure 2, the thickened suspension from hydrocyclone-thickener 19 is received in a pivoting funnel 51 which, under normal operating conditions, directs the suspension to a compartment 52 of receiver 53 from where the suspension fiOWs to a flow-divider 54. From flow-divider 54 one portion of the thickened suspension goes via a pipe 55 directly to washing tank 1, whereas another portion is fed via a pipe 56 to storage vessel 50. The quantity fed to washing tank 1 via pipe 55 is made smaller than the quantity which must be continuously supplied to washing tank 1, and the balance is supplied from storage tank 50 via a pipe 57. The quantity supplied from storage tank 50 to washing tank 1 is regulated by means of valve 58, preferably a needle valve at the infced end of pipe 57 so as to prevent clogging. Valve 58 is pneumatically operated by means of a pressure sensitive instrument 59, for instance a Foxboro M 40 Stabilog Controller of the Bellow type with a reset. This instrument 59 is connected with a bubble tube 60 which has an open end terminating in the suspension in washing tank 1. Compressed air is introduced through

pipes

61 and 62 to instrument 59 and through pipe 61 and reducing valve 63 to bubble tube 60. The pressure in bubble tube 60 depends on the level of suspension in washing tank 1. When this level drops too low the pressure in bubble tube 60 becomes too low and the instrument 59 reduces the air pressure in the pipe 64, which is connected with the instrument, as a result of which the valve 58 is opened or is opened further, so that the level of the suspension in the washing tank 1 rises.

When the mill-

classifier circuits

46 and 47 is in operation the level of the liquid in storage vessel 50 rises. When storage vessel 50 is filled the mill-

classifier circuits

46 and 47 is stopped until the storage vessel 50 is almost empty.

To prevent settling of the suspension in storage vessel 5% compressed air is introduced therein near the bottom via a pipe 65.

There is no reason for the level of the suspension in washing tank 1 to become too high. However, if accidentally too much liquid is introduced into washing tank 1 the excess flows off through an overflow aperture 66 and flows via a pipe 67 to pipe 8.

The above explains how the specific gravity and the level of the suspension in the washing tank 1 are kept substantially constant. The specific gravity of the suspension can be controlled by selecting the level of the liquid in bubble vessel 38. Furthermore the instrument 30 may be provided with means for regulating the relation between the pressure difference between the bubble tubes 31 and 32 on the one hand and the pressure in pipe 39 on the other hand. A Foxboro M 40 Stabilog Controller is provided with such means.

As has been mentioned already, the coal and the mixed are separated in washing tank 24 and the suspension therein consequently has a lower specific gravity than the suspension in washing tank 1. Washing tank 24 receives from washing tank 1 coal and mixed with adhering suspension. To maintain the desired specific gravity of the suspension in washing tank 24 clarified liquid is supplied from flow-divider 23 via pipe 68 and valve 69, preferably a needle valve. To control the specific gravity of the suspension in washing tank 24 the amount of clarified liquid fed to washing tank 24 is regulated. To this end there is a pressure sensitive instrument '70 which controls the specific gravity of the suspension in washing tank 24 in substantially the same way wherein the instrument 30 controls the specific gravity of the suspension in washing tank 1. There are bubble tubes '71 and 72 which have open ends terminating in vertically spaced relation in the suspension in washing tank 24, reducing valves 73 and 74, pipes 75, 76, 77 and 78 and a bubble vessel 79; instrument 70 regulates the pressure in pipe 78 in dependence on the pressure difference between the bubble tubes 71 and 72. When this pressure difference increases the pressure in pipe 78 decreases and valve 69 is opened further.

Generally it is not necessary to feed thickened suspension into washing tank 24. If it is desired, however, to increase the specific gravity of the suspension in washing tank 24 thickened suspension may be withdrawn from storage vessel 5% via pipe 80 and valve 81 therein. Valve 81 may be hand operated.

The amount of liquid introduced into washing tank 24 generally is larger than the quantity of liquid which adheres to the particles discharged from the washing tank. The excess liquid is removed through overflow aperture 85 and flows via pipe 86 to pipe 8.

According to Figure 2 the raw coal is first separated according to a higher specific gravity, for instance 1.6, in washing tank 1 and thereafter according to a lower specific gravity, for instance 1.4, in washing tank 24. This system is used when the raw coal is substantially dry. If the raw coal contains an appreciable amount of water, it is generally preferred to feed the raw coal to the washing tank in which the suspension has the lower specific gravity (1.4) and to feed the shale and mixed discharged therefrom to the washing tank in which the suspension has the higher specific gravity (1.6). In that case both washing tanks must continuously receive thickened suspension, the first washing tank in which the specific gravity is lowest for counteracting dilution by the water in the raw product and the second washing tank which holds the suspension with the higher specific gravity so as to counteract dilution by the suspension adhering to the discharge from the first washing tank. Control of the level of the suspension in the washing tanks is unnecessary, as both tanks will continuously overflow.

Thus in case of wet feed instrument 59 can be deleted and

instruments

30 and 70 are either made to control the quantities of thickened suspension introduced into the washing tank, or one of the

instruments

30 and 70 is made to control the apex aperture of the hydrocyclone thickener 16, the other instrument controlling the quantity of thickened suspension introduced into the appertaining washing tank.

Thus the system shown in Figure 2 can be readily adapted to the handling of wet feed.

When there is but a single washing tank as in Figure l and the feed is wet, no clarified liquid has to be added to the washing tank 1. The quantity of thickened suspension fed to the washing tank is then controlled so as to maintain the desired specific gravity in the washing tank 1, which should continuously overflow. In that case the specific gravity of the thickened suspension discharged from suspension thickener 19 should of course be higher than the specific gravity of the suspension in washing tank 1. 1

During operation water is continuously introduced into the system, viz. on the washing screens 2 and 3 or 2, 3 and 25. This makes it necessary also to withdraw liquid from the system continuously. To this end a quantity of clarified liquid is fed from flow-divider 23 to a pipe 87 via a valve 88. The quantity of liquid fed to pipe 8'7 is made larger than the quantity to be withdrawn from the circuit.

There is of course liquid adhering to the solids withdrawn from the circuit, viz. the cleaned shale, mixed, coal, oversize from screen 9 and froth from froth-flotation machine 14; on the other hand the raw coal may be wet. By the quantity of liquid to be withdrawn from the circuit is therefore understood the quantity of liquid which must be removed from the circuit as a separate stream for maintaining a substantially constant quantity of liquid in the circuit. Generally, the quantity of liquid to be removed from the circuit is smaller than the quantity of spraying water added on screens 2, 3 and 25. Furthermore it will make some difference whether the

millclassifier circuit

46 and 47 is in operation or not.

Referring again to Figure l, the quantity of liquid fed to pipe 87 is made larger than the quantity to be withdrawn from the circuit. Pipe 87 discharges into an overflow vessel 89 which has an adjustable overflow rim 90. The overflow vessel 89 communicates with froth-flotation machine 14 via a pipe 91, and the overflow rim 90 is so adjusted that the liquid in froth-flotation machine 14 is at the proper level, that is so high, that froth can be readily removed therefrom but not so high that a significant amount of liquid is removed with the froth.

The capacity of pump 17 is such that the quantity of diluted suspension which is discharged from the frothflotation machine 14 through discharge aperture 16 is larger than the amount of liquid fed to the froth-flotation machine through pipe 15 minus the liquid in the froth. As a consequence a portion of the liquid fed to overflow vessel 89 flows through pipe 91 to froth-flotation machine 14 and the balance is discharged over overflow rim 90. Consequently the excess of liquid discharged through discharge aperture 16 is returned to the froth-flotation machine through pipe 91 whereas the proper amount of liquid is withdrawn from the circuit.

One advantage of this system is that as a consequence of the downward current which is maintained in pipe 91 only clarified liquid is withdrawn from the circuit, and not suspension from froth-flotation machine 14, which contains more shale particles than the clarified liquid. (It will be clear that these shale particles must be kept in the system, since all shale particles which are lost must be replaced by new shale particles from the mill-

classifier circuit

46 and 47.)

Another advantage is that the liquid level in frothfiotation machine 14 is automatically kept constant.

These two problems which here are solved together, give rise to difiiculties in other washing systems of the same kind, that is in old systems only one of these problems can be adequately solved.

The froth-flotation machine 1"! is preferably of the Kleinbentink type as described in more detail in U. S. Patent 2,569,141, but other types may also be used.

In the above a few times reference has been made to normal operating conditions. The meaning of this term is that all machines operate satisfactorily and that there is a continuous supply of raw coal and water. A plant must of course be able to cope with small interruptions. In case of serious breakdowns it will always be necessary to stop the plant, but in that case it still is important that operation can be easily started again as soon as the necessary repairs have been made. Also, when the plant is shut-down for the night or for holidays, starting must be simple.

The separating suspension for instance tends to settle during a shut-down, thus clogging pipes and making starting of the washing tank 1 hazardous. To cope with these problems there is a storage tank 92, see Figure 1. During a shut-down all suspension is stored therein, being fed thereinto via washing tank 1, valve 93 and pipe 94. Compressed air is admitted continuously during a shutdown or before restarting the plant to storage tank 92 at the bottom, so that when the plant is started again the suspension is ready for use. When starting again the suspension is pumped via valve 95, pipe 96, pump 97 and pipe 98 to washing tank 1. The suspension which remains in pipes 96 and 98, pump 97 and storage tank 92 is drained over valve 99. If desired this drained suspension can be collected in a bucket and be reintroduced into the circuit or it can be discarded.

If there are two washing tanks 1 and 24 there is a received on the screens 2 and 3 (and screen 25 of Figure l 2), so that the feed to hydrocyclone-thickencr 19 becomes very diluted. Under such circumstances hydrocyclonethickener 19 cannot discharge a suspension of the desire l specific gravity. To prevent dilution of the suspension in the washing tanks pivoting funnel 51 is under such circumstances set for discharging into compartment 166 of receiver 53 and the apex discharge from hydrocyclonethickener 19 is then returned from compartment Hi l via. pipe 101 to pipe 3, screen 9 and froth-flotation machine 14. It may be advisable, however, to direct suspension received in compartment 100 directly to froth-flotation machine 14.

Pivoting funnel 51 may be hand operated or be automatically controlled, for instance as described in U. S. Patent 2,649,963.

Furthermore it will be clear that there may be spare pumps and spare hydrocyclones available. It further is obvious that any number of machines may be employed in parallel to have a suthcient capacity, but in the drawings but one unit of each machine has been shown. A single hydrocyclone-thickener for instance will in general be insufiicient.

By way of example the following figures are given for a plant according to Figure 1 with a capacity of 160 metric tons of raw coal per hour, the size of the coal being between A1" and 4". The D. S. M. washing tank 1 is 6'6 wide. Screens 2, 3 and 9 are Allis- Chalmers Low-Head vibrating screens which are 5 wide and 10 long. The slotted apertures of screens 2 and 3 nr /s" wide, the slotted apertures of screen 9 are 0.5 1112 1. wide. The Kleinbcntink froth-tlotation machine 14 has a diameter of 9'. There are 3 hydrocyclonethickener-s 19 operated at a feed pressure of 19 p. s. i. These hydroeyclones have a diameter of 14''; the diameter of the feed aperture and of the overflow aperture is 2.; the diameter of the apex aperture can be varied between and 1% in the manner described in U. 5. Patent 2,649,963; the cone angle is 20; the hydrocyclones have a cylindrical portion which is 4" high, and a vortex finder which is 4 long. Ball mill 46 has a capacity of metric tons of shale per hour. The overflow from classifier 47 is for 95% finer than 60 mesh. Storage vessel has a capacity of 1500 U. S. gallons; storage tank 92 has a capacity of 5500 U. S. gallons. The capacity of pump 17 is 450 U. 5. gallons per minute, at the necessary pressure. The apex discharge from the hydrocyclone amounts to approximately 50 U. S. gallons per minute.

We claim:

1. A continuous process of washing coal comprising the steps of separating raw coal into coal and shale fractions by scans of a separating suspension of fine shale particles in water, spraying each of said separated fractions on washing screens first with clarified liquid and then with water, subjecting the underfiow from said washing screens to froth-flotation, withdrawing undertlow from the froth-flotation operation at a rate exceeding the rate of washing screen underflow feed thereto, subjecting said froth-flotation underfiow to a thickening operation, feeding thickened suspension from the thickening operation to said sink-and-fioat suspension, utilizing part of the clarified liquid from said thickening operation for spraying said coal and shale fractions, and feeding part of said clarified liquid to said froth-flotation operation at a rate effective to maintain the liquid level therein substantially constant.

2. A process as defined in claim 1, wherein shale scparatcd from the raw coal is subjected to closed circuit grinding and classification, and the overflow from the classification step is fed to the froth-flotation operation.

3. A process as defined in claim 2, wherein at least part of the thickened suspension from the thickening operation is stored prior to feeding to said sink-and-fiout suspension, and said grinding operation is discontinuous, being started and stopped in accordance with the quantity of thickened suspension in storage.

4. A process as defined in claim 1, wherein the underflow from said washing screens, prior to froth-flotation, is finely screened to remove therefrom particles too course for etficient froth-flotation separation.

5. A process as defined in claim 1, wherein the thickening operation is carried out in a hydrocyclonc adjusted to discharge a. thickened suspension of specific gravity higher than th desired specific gravity of said separating suspension.

6. A process as defined in claim 1, wherein the specific gravity of said separating suspension is maintained constant by feeding thereto an automatically controlled quantity of clarified liquid from said thickening operation.

7. A process as defined in claim 1, wherein the specific gravity of said separating suspension is maintained constant by automatically controlling the specific gravity of thickened suspension from the thickening operation.

8. A process as defined in claim 1, wherein the raw coal is wet and the specific gravity of said separating suspension is maintained constant by feeding thereto an automatically controlled quantity of thickened suspension from the thickening operation.

9. A process as defined in claim 1, wherein the level of said separating suspension is maintained constant by I 9 automatically controlling the feed thereto of thickened suspension from the thickening operation.

10. A process as defined in claim 1, wherein the coal fraction is separated into fractions in a second separating suspension of lower specific gravity than the first separat ing suspension.

11. A process as defined in claim 1, wherein the coal fraction is separated into fractions in a second separating suspension of lower specific gravity than the first sinkand-float suspension, and the specific gravity of said second separating suspension is maintained constant by feed- 10 ing thereto an automatically controlled quantity of clarified liquid from said thickening operation.

References Cited in the file of this patent UNITED STATES PATENTS 1,656,270 Downs et a1 Jan. 17, 1928 2,113,609 Wuensch Apr. 12, 1938 2,497,790 Pauvrasseau Feb. 14, 1950 2,569,141 Bakels Sept. 25, 1951 2,649,963 Fontein Aug. 25, 1953

Claims

1. A CONTINUOUS PROCESS OF WASHING COAL COMPRISING THE STEPS OF SEPARATING RAW COAL INTO COAL AND SHALE FRACTIONS BY MEANS OF A SEPARATING SUSPENSION OF FINE SHALE PARTICLES IN WATER, SPRAYING EACH OF SAID SEPARATED FRACTIONS ON WASHING SCREENS FIRST WITH CLARIFIED LIQUID AND THEN WITH WATER, SUBJECTING THE UNDERFLOW FROM SAID WASHING SCREENS TO FROTH-FLOTATION, WITHDRAWING UNDERFLOW FROM THE FROTH-FLOTATION OPERATION AT A RATE EXCEEDING THE RATE OF WASHING SCREEN UNDERFLOW FEED THERETO, SUBJECTING SAID FROTH-FLOTATION UNDERFLOW FEED THERETO, SUBJECTING SAID ING THICKENED SUSPENSION FROM THE THICKENING OPERATION TO SAID SINK-AND-FLOAT SUSPENSION, UTILIZING PART OF THE CLARIFIED LIQUID FROM SAID THICKENING OPERATION FOR SPRAYING SAID COAL AND SHALE FRACTIONS, AND FEEDING PART OF SAID CLARIFIED LIQUID TO SAID FROTH-FLOTATION OPERATION AT A RATE EFFECTIVE TO MAINTAIN THE LIQUID LEVEL THEREIN SUBSTNATIALLY CONSTANT.