US2108290A - Method of cleaning coal and fluid separating medium therefor - Google Patents

Description

Feb. 15, 1938. N. D. LEVlN 21f AL.

METHOD OF CLEANING COAL AND FLUID SEPARATING MEDIUM THEREFOR Filed Oct. 11, 1934 2 Sheets-Sheet l ,x Z W ow A WLH w /B N a m N Mm Feb. 15, 1938. N. D. LEVIN ET AL METHOD OF CLEANING COAL AND FLUID SEPARATING MEDIUM THEREFOR 2 Sheets-Shet 2 Filed Oct; 11, 1934 VEN TOPS /Y/'/s D Lev/n,

jamue/ ff 3 05i ATT'Y Patented Feb. 15, 1938 PATENT LIETHOD F CLEANING COAL AND FLUm SEPARA'I'ING MEDIUM THEREFOR Nils D. Levin and Samuel H. Yost, Columbus, Ohio, assignors to The Jeffrey Manufacturing Company, a corporation of Ohio Application October 11, 1934, Serial No. 747,906

17 Claims.

Our invention relatesto the separation of ma terials according to their specific gravities and while it is particularly adapted to the separation of impurities from coal, it may have a general application.

One of the objects of our invention is the provision of coal cleaning equipment that can be so cheaply installed and operated, that coal seams that heretofore could not'be mined economically, can be mined profitably by using our coal cleaning apparatus.

. It isoften the case that in locations where there are severaliseams, only the cleanest ones have been mined, and the others are very often ruined by a subsidence due to a coal seam being mined underneath. The use of our invention greatly minimizes such waste of our natural resources.

Another object of our invention is to facilitate,

the supplying of better fuel to the consumer. At present only a small percentage of the mines in this country have coal cleaning equipment. This is due to the high first cost of the apparatus and also to the high cost of operation. One of the objects of our invention is to provide a cleaning apparatus so low in the first cost and capable of being operated so economically that mines that heretofore have attempted to have the impurities picked out manually, can afford to install this new coal cleaning equipment.

Our invention relates to methods of separating coal in which preliminary tests are made to predetermine the desired specific gravity of the fluid medium.' Such preliminary steps are dependent upon the principle of the float and sink test well known in practice. The medium used in such sink and float tests is usually zinc chloride and carbon tetrachloride. These chemicals are diluted to the desired specific gravity so that when raw coal or run of mine coal, including impurlties, is immersed in the medium, the coal to be separated will float and the refuse will sink.

It is evident that this would be an excellent way of separating materials of different specific gravities but the chemicals mentioned are too expenalve for commercial use. i

- A further object of our invention is the provision of a separating medium that will act in the same manner as the separating medium above referred to as being used in the preliminary sink and float test, comprising ingredients which may b0 obtained almost anywhere in this country and used at minimum cost and expense to effect max- 55 imum economy in cleaning coal by the separation of impurities therefrom and in accordance with the differences in their specific gravities.

Another object of the invention is the provision of a separating medium comprising granular insoluble materials which are available as found 5 in nature, one in a finely divided state such as argillaceous material, held in suspension in water by the action of an electrolyte for such a long period of time as to eliminate or greatly minimize the necessity of agitation while separating by flotation the coal from its impurities.

A further object of the invention is the provision of a separating medium comprising a mixture of clay, loam, marl or other argillaceous material, as found in nature, with water having a small amount of a deflocculating agent such as sodium aluminate or sodium silicate.

Another object of the invention is the provision of a separating medium for the cleaning of coal or other material, comprising a mixture of clay or other argillaceous material found in nature in a finely divided state, with sand (includ.. ing silica sand), or with crushed ganister, or with any other insoluble substance of suflicient specific gravity, with the granules of this latter:in-. soluble substance of appreciable size, and maintained in suspension in the water by the midi-- tion of a small amount of a deflocculating agent,

' for such a length of 'time as to minimize greatly or eliminate the necessity of agitation of the mixture during operation of the cleaning'of the coal.

More particularly it is the object of the present invention to provide a separating medium for use in a coal cleaning system, which will enable the use of clay and sand as found in nature without further treatment other than mixing with water having in solution an electrolyte to act as a suspension'agent on the granules of the clay and sand.

, Other objects of our invention will appear hereinafter, the novel features and combinations being set forth in the appended claims.

The accompanying drawings illustrate apparatus which may be used for carrying out our improved methods followed by the washing of the separated coal in a series of tanks, but the apparatus for washing the separated coal is claimed in the co-pending application of Nils D. Levin for an improvement in Apparatus for cleaning coal, Serial No. 747,907, filed October 11, 1934.

7 Since certain definite preparations must be made before using the apparatus shown in the accompanying drawings and since the actual separation of the impurities from the coal takes place "in a single separating tank before the steps 55 of washing the separated coal and reclaiming the solid constituents of the separating medium are carried out, we will first describe the separating medium and the object of apportioning the ingredients thereof in accordance with discoveries Which we have made from actual tests after giving careful consideration to the use of electrolytes or deflocculating agents in separating mediums found to be most emcient under the different conditions met with in practice,

In order to predetermine the specific gravity of the separating medium or mixture to be used in our improved method, the raw coal or run of mine coal to be cleaned is analyzed by methods of sampling, sink and float tests, chemical analysis, etc. Then, depending upon what grade of purity of coal is desired, the specific gravity at which separation should take place when following our improved methods, is predetermined. The separating medium or mixture is then prepared so that the marketable coal will fioat in the separating tank shown at the right-hand end of the series of tanks in the accompanying drawings.

In preparing the separating medium for the separating tank used in our improved method of cleaning coal, we prefer to follow certain definite formulas which from actual tests have given the best results but which may be varied in accordance with conditions. By using one part clay and four parts sand, by weight, adding water and stirring the mixture until the clay is entirely dispersed and a homogeneous mixture with the sand results, a medium of the desired specific gravity may be obtained with the assistance of well known tests of specific gravity of mixtures of solid granular matter with water. The sand may be any of the various kinds including silica sand, quartz sand, crushed ganister, etc.

The specific gravity of the sand is approximately the same as that of the clay, but if the mixture of the clay, sand and water is allowed to stand in a quiescent state for a short period of time the sand will settle in the bottom layer with a layer of clay above the sand, not by reason of the difference in specific gravity so much as by reason of the sand granules being of appreciable size while the clay is in a finely divided state and mostly colloidal.

The addition of a small amount of a. deflocculating agent such as sodium aluminate or sodium silicate to the mixture of natural clay and natural sand in the water, will cause the clay and sand after being stirred into the water, to remain in suspension for a relatively long period of time thereby rendering the fluid mixture very efficient in gravity separation of impurities from coal with little or no agitation of the separating medium.

In gravity separation of impurities from coal agitation of the liquid hampers the process and it is desirable therefore to reduce the agitation to a minimum. For instance, when the agitation of the liquid medium is too great, small and flat pieces of slate will follow upward currents and be mixed with the cleaned coal, thereby rendering perfect separation impossible. However, in our process as illustrated in Fig. 1, the operation of the conveyors l0 and I5 produces so little agitation of the liquid medium as not to disturb the desired separation of the impurities from the marketable coal.

If suflicient clay is mixed with water to produce a liquid medium of suficient specific gravity to float coal, it will be found to be too thick or viscous for practical use. However, by adding the proper amount of electrolyte such as sodium aluminate or sodium silicate, the mixture-will be rendered more fluid without afiecting the specific gravity. After testing various forms of clays we found one that could be used in practice without the addition of sand, but not very efficiently. We found one fireclay that could be used without the addition of sand provided the electrolyte is used in the mixture. In other words, while we have obtained satisfactory results by the use of mixture of certain clay and water without sand but including the addition of a small amount of sodium aluminate or sodium silicate, we obtained the best results by the inclusion of sand in the mixture. The use of clay alone in most cases decreases the fluidity of the mixture to such an extent as to slow down the separating process as compared with the speed of operation when sand is included in the mixture, because the inclusion of the sand increases the fluidity of the separating medium. (Ilay is an argillaceous material while sand is a non-argillaceous granular material. Both of these materials are available at low cost almost anywhere in this country and each may be used in its natural state without incurring the expense of grinding or pulverizing.

The sodium aluminate or sodium silicate may be a defiocculating agent so far as the clay suspension is concerned, but the sand suspension may perhaps be best explained by the fact that the sodium silicate or sodium aluminate acts as an electrolyte to produce electrical charges in the clay particles to hold in suspension for a relatively long period of time sand granules in intimate relation with the dispersed clay in a homogeneous separating medium. As above explained, clay in its natural state is composed of finely divided particles and these are uniformly dispersed when mixed with the water. The sand comprises granules of appreciable size, but because of the use of the electrolyte need not be pulverized or comminuted. When some insoluble substance of suificient specific gravity is crushed to form the sand the granules need be no smaller than those usually found in sand in nature. Screening to obtain only the finest sands is not necessary or desirable.

It is important to follow certain definite formulas according to conditions. It has been found from actual tests that under certain conditions the amount of sodium aluminate for the best results in maintaining the clay and sand in suspension for a relatively long period of time without agitation, should be approximately .8 percent of the weight of clay, and if this small amount is doubled, there will be a relatively rapid settling of the clay and sand in a mixed mass without layering. The critical ranges in the proportions of the ingredients are determined by tests.

In order to explain the procedure by which certain definite formulas may be arrived at for obtaining the best results in the use of a separating medium for the cleaning of coal, certain actual tests have been carried out and the results observed. By mixing one part Minford silt (argillaceous' material in its natural state) and four parts of white coarse sand (crushed ganister) by weight, and adding sufficient water to obtain a specific gravity of 1.55, after stirring into the mixture .8 percent of sodium aluminate,'no settlement of the solid insoluble ingredients was shown after a period-of twenty-four hours.

. um aluminate, so that there would be no appreci-v 75 When a mixture having a specific gravity oi 1.55 is made of one part Miniord silt and five parts sand and .8. percent ofsodium aluminate in water, a very slight settling was observed after a lapse of twenty-four-hours, but in similar mixtures having one part silt and six parts sand, and one part silt and seven parts sand, pronounced settling occurred after a lapse of twenty-tour hours, but the settled mass was a very soft intermixture of silt and sand such as not to interfere with the re -starting of the conveyor mechanism in the separating tank after a twelve hour. perind of idleness. Upon re-starting the conveyor mechanism through such settled mass only a slight mechanical agitation was necessary to recondition the separating medium but the homogeneous condition of the medium could be maintained only by suficient agitation. e water used in the foregoing tests was that from a city hydrant in Colbus, Ohio.

When Sandusky Foundry sand (10% clay) was used in the tests with Minford silt, there appeared to be no appreciable settling after a lapse of twenty-four hours, even when the ratio of the ingredients was one part of silt to eight parts oi sand, by weight.

Fireclay and white sand mixtures over a range of one part fireclay (four parts sand, to one part fireclay) six parts sand, all settled considerably faster than silt and white sand mixtures. However, fireclay and Sanduslry sand mixtures showed no appreciable settling after standing ior twenty-four hours at a ratio up to and including one part fireclay to six parts Sanduslzy sand.

Inasmuch as it consumes some time for the progressive defiocculating action of the sodium aluminate to take place, it is preferred to age the mixture for a period of at least twenty-four hours. When the Miniord silt is used with a mixture containing city. water; it was found that the minimum deflocculation was efiected by the use of sodium alurninate over a range oil .6' percent to .9 percent as to the cunt oi" sodium aluminate relative to the weight or the clay. After aging the mixture the maxim defiocculation is definitely indicated as .d percent with pronounced reaction over a range oi .t percent to one percent.

The percentage oi the solid ingredients in suspension in the separating medi is much great er for Miniord silt than for fireclay and the du ration of the suspension is also much longer for the ll/Iinford silt. This is probably due to the diflerence in the size of the particles in the two clays. Thelvlinford silt is the better suspending agent of the two clays tested and the Sanduslry sand, a relatively fine sand, is the one more readily maintained in suspension oi the two sands tested.

The permissible sand variation for a given specific gravity between 1.5 and 1.8; was found to be 25 percent impractical operations. With the less favorable sand, the permissible electrolyte variation plus or minus from the optim point .of .8 percent was found to be 12 percent. That is able settling after a lapse of twenty-four hours.

in suspension in a separating medium over a relatively long period of time without agitation.

Fireclay is composed of particles in a sly divided state but not as fine as either silt or Bentonite. However, fireclay was found to co-act with the electrolyte to hold in suspension six parts of Sandusky sand by weight in a suitable separating medium becauseno appreciable settling occurred after standing for twenty-four hours.

It will thus be seen that the tests which we have conducted showed that the selection of the finest argillaceous material in its natural state and the selection of the finest sand in its natural state enabled the electrolyte sodium aluminate to hold in suspension the greatest amount of solid ingredients in the separating medium over a long period of time without appreciable settling, thereby demonstarting the practicability of avoiding agitation although solid insoluble ingredients are used as found in nature without comminution or pulverization. In this manner a separating medium of relatively high specific gravity may be obtained. Such separating medium would have maximum buoyancy and would permit the addilng of more water to obtain the desired specific gravity of the mixture. Therefore the liner the solid materials the greater may bethe fluidity for a given specific gravity.

However, it was found from actualtests that the finer the solid ingredients in the separating medium the greater would be the carry-ofif of 'such materials with the separated coal and with the impurities separated and removed. For instance, the minim carry-od of solid ingredients from a separating medium composed of silt and Sanduslry sand (.1 part silt plus i parts Sanduslry send by weight) in water having in solution 3% sodium aluminate (the separating medium having the specific gravity of 1.53) was about as pounds I specific gravity of 1.53 and composed of 1 part fireclay and ii parts white sand by weight in water having in solution 3% sodium aiuminate the carry-ofi was found to be 18 lbs. of solid ingredients per ton of raw coal treated, whereas with a separating medium composed of 1 part fireclay and 6 parts Sandushy sand by weight in water having in solution .b% sodium alunate the carry-ed was found to be 20 lbs. of ingredients per ton of raw coal treated.

While therefore theoretically it may be desirable to have both or the ingredients in finely clivided condition, it is more practical and economical to have one relatively fine as found in nature and the other relatively coarse and also as found in nature.

Aging of the mixture of sodium aluminate, water; clay d 1:. ll: greatly reduces the aunt of carry-off in the treatment of coal to separate the impurities therefrom. The proper balance may readily be obtained in practice. Fireclaywhite sand mixtures over a range of to all settled considerably faster than silt-white sand mixtures. But fireclay-Sandusky sand mixtures showed no appreciable settling at ratios up to and including /6.

Inasmuch as the agrillaceous material and the sand vary in nature in various parts of the country the correct formula for the separating medium to suit conditions depends upon the clay suspension, the sand suspension, and the claysand adherence resulting from contact with smooth imporous surfaces. The related determinations to be made are (l) the concentration of the electrolyte such as sodium aluminate or sodium silicate to clay which produces the maximum deflocculation of the clay, (2) the effect of varying the sand-clay ratios upon the duration of the suspension and the homogeneity of the suspended mixture, (3) the effect of varying the sand-clay ratio upon the adherence value, and (4) the effect upon the foregoing of varying the type of clay and sand used in the mixtures. The most efficient formulas will be those which main. tain suspension of both ingredients over such a length of time as to renderagitation unnecessary, and which will result in a minimum carry-off of the solid ingredients-clay and sand-from the separating medium. The clay and sand should be available as found in nature without screening or comminution or pulverization, and therefore the process may be carried out anywhere at a very low cost in materials and maintenance. Such sand, of course, includes silica sand. Where solid materials are to be used as sand by crushing ganister, quartz, etc., the granules need be no smaller than those of ordinary silica sand.

The conclusions from the actual tests made are as to the solid insoluble ingredients, that Sandusky sand, a relatively fine sand, is not as favorable as coarser sands unless the matter of suspension becomes a controlling factor and requires its use at lower specific gravities. In other words the colloidal clay acted upon by the electrolyte has the property of keeping in suspension for a long period of time the finer sands. The tests also showed that silt gave more favorable results than fireclay. Silt in its natural state is composed of particles more finely divided than the fireclay. The aging of the mixture for a period of at least twenty-four'hours enabled better results to be obtained. The variation of the specific gravity of the mixture can be controlled by varying the proportion of the water. The fluidity of each of the mixtures tested was suflicient to permit free settling of the impurities during flotation of the cleaned coal, since the consistency or mobility of the mixture is not such as to interfere with the rapid operation of the separating process.

It has been found from actual tests that sodium silicate is preferable when the water is well water or hard water, while sodium aluminate is preferable for the hydrant water of the city of Columbus, Ohio, which water has been treated I chemically to purify the same. However, either Both sodium silicate and sodium aluminate can be obtained at very low cost. For instance, in the formula using one part natural clay and four parts natural sand with .8% of sodium aluminate, the cost of the latter in certain localities is only about 1/250ths of the cost of the clay and sand combined. That is to say, in a separating medium having one part clay, four parts sand and .8 percent sodium aluminate, the cost of the sodium aluminate required is only .4 percent of the cost of the clay and sand combined. This cost of the electrolyte is so low that no attempt need be made in practice to reclaim any of it that clings to the cleaned coal as it passes through the washing tanks shown in the accompanying drawings.

The formula for making up the mixture or separating medium used in our method of cleaning coal differs with the materials available to serve as the insoluble solid ingredients. The clays which are useful in our method, while having the general characteristics of kaolinite are not necessarily pure clays but would fall under the general classification of shale, surface clay, silt and fireclay. These types are Widely distributed and readily available in various parts of this country, and each is useful in our process in its natural state.

The essential characteristics of the argillaceous material in our method of separating coal from its impurities, are those having the property of being dispersed or defiocculated by the electrolyte and after being dispersed or defiocculated, having the property of maintaining in suspension the desired quantities of sand or other inert granular material.

Such properties in our separating medium used in our improved method of cleaning coal, may be readily determined by simple tests which may be carried out wherever clay and sand are found in various parts of the country or wherever it is desired to follow such method. We have set forth formulae for most efiicient operation in certain localities but such formulae may be varied in various parts of the country in accordance with the nature of the natural clays and the natural sands available. We will therefore set forth the procedure that should be adopted to arrive at the most efilcient results for various kinds of clays and sands.

The procedure is to mix the clay and water to be used, the. weight of the clay being about 25 percent of that of the water. This mixture is placed in a series of beakers and to each beaker is added a difierent amount of electrolyte such as sodium aluminate or sodium silicate. After the mixture in each beaker is stirred it is allowed to stand for forty-eight hours and the one that shows the best suspension of clay is the mixture that should be selected. If the clay cannot be maintained in suspension in any of the beakers it is not a suitable form of clay.

When a siiitable clay is found by following the tests with the electrolyte, the mixture of clay and electrolyte is introduced into a series of beakers, and sand in various proportions is added to the various beakers. The combined weight of the sand and clay should be the same in each beaker. One beaker may contain clay alone and the others one partclay and two parts sand, one part clay and three parts sand, etc. Some clays such as Bentonite, have the property of carrying ten parts of sand when the electrolyte sodium aluminate is used in a suitable separating medium in our method of cleaning coal. It should also be understood that sumcient water aioacco is added to the various beakers to obtain the specific gravity required for the desired coal separation.

After the mixtures in the beakers containing the sand have stood for twenty-four hours, observations are made and those beakers eliminated which show settling of the sand at the bottoms oi the beakers.

amount of separating medium is the most desir-' able to be used in practice. For'instance, in actual tests we found that for each one-tenth of a gram per square inch carried oh by the testing glass, there will be seventeen (17) pounds of mixture carried ed by a ton of separated coal.

These tests were made by dipping the glass plate into the beaker mixtures and weighing while wet. The weights were calculated as the weight of the mixture adhering to each square inch of surface of I the plate. Instead of using glass plates imporous card-board squares may be used for immersion in the beaker mixtures but it will be found that less solid material will adhere to the card-board than to the glass plate.

In such carry-0d tests'oi beaker am: a curve may be. plotted from the results obtained. For instance, in connection with actual tests made with a series of beaker mixtures in which the amount of sodium silicate ranged from zero to 3.6 percent by weight of clay in the fluid, the plotted curve will show a gradual decrease in the amount of clay carried out on a card until the percentage of sodium silicate is about 1.3 or 1.4, after which the increase of the percentage oi sodium silicate increases the amount oi clay carried out on a card. The clay used was a form of fireclay having a pH value of 5. After per-'- mitting the beaker mixtures to stand for twentyiour hours after adding the electrolyte and stirring the mixture, we found that the clay remained in suspension the longest in the beaker having 1% of sodium silicate by weight of clay in the mixture, and the tests further showed that the range for best results would be between 1% and 1.4% of electrolyte. The carry-oh tests of clay mixtures also enabled the plotted curve toshow pounds of clay per ton of coal that would be carried out of the medium during the coal separating process. For instance, in the tests just described where the best results would be obtained with the electrolyte 1.3% by weight of clay in the mixture, the least amount of clay would be carried out on the card and would represent about 30 pounds of clay per ton of coal.

As above explained, there is a permissible range in the use of the proportions of the ingredients in the, water for maintenance of the clay and sand in suspension over relatively long periods of time. In the beaker tests there may therefore be various mixtures which would be suitable so far as maintenance of the clay and sand in suspension is concerned, but by following the polished glass test the best separating medium may be obtained so that a minimum amount of the separating-medium will be carried away with the separated coal and a minimum amount will be carried away when the impurities at the bottom of the separating tank are conveyed out of the tank. It should be understood that during the operation of the separating apparatus the intermixture of the ingredients is so thorough that when the cleaned coal and impurities are removed from the separation tank, the medium that clings to the removed materials is about the same in its proportions of constituents as the medium that remains in the tank. The medium in the tank therefore remains as to specific gravity and efiiciency, the same as when first introduced and the replenishment of the medium in the separating tank is by means of the same mixture as that selected by means of the foregoing tests. v

In the accompanying drawings,

Figs. l and 2, placed end to end, illustrate apparatus including a separating tank-for cleaning coal according to our improved process after which the cleaned coal may be washed successively in a series of washing tanks.

In Figs. 1 and 2 we have shown a supportin frame 3 on which are mounted the separating tank t and a series of washing tanks 5, t, and l. The raw or run of mine coal may be introduced into the hopper It as shown in Fig. 1, from the bottom of which the discharge of the coal is regulated by the vertically slidable gate 9. The separating medium, comprising the naturally finely divided clay, the relative coarse sand and the electrolyte, may be varied in depth in the tank it according to conditions. The impurities which sink in the separating medium are received by the conveyor it comprising the flights 'M, ii. These flights are adapted to travel in the direction of the arrow it to convey the impurities such as slate to the discharge chute it from which such impurities are delivered to the endless belt conveyor it.

The coal which is capable of floating on the separating medium in the separating tank t is crowdm toward the left as viewed in Fig. i, by the incoming raw coal, and such separated coal is received by the conveyor l5 comprising the flights it, it. The cleaned coal is conveyed upwardly along an incline in the direction of the arrow ill and discharged into the washing tank 5 in which a deflector plate it directs the coal to the conveyor it comprising the flights 2d, 2d.

masmuch as the washing tank 5 is filled withwater the cleaned coal will tend to sink therein and consequently it is desirable to provide an inclined plate ii for the flights it so that coal which reaches the plate ill may be scooped from the semi-circular receiving portion 22 and moved upwardly along the incline in the direction of the arrow 23 for discharge into the next washing tank dotted lines the position that the operating electricmotor it occupies on that side of the frame it remote from. the sprocket chain gearing 3b which is connected to simultaneously drive all of the conveyorsshown in .Figs. 1 and 2. The motor it is connected by reduction gearing it and lit the right-hand end of Fig. 1 is shown inill the sprocket gearing 32 to that end of the shaft 33 remote from the sprocket chain 30. That is to say, the driving connection from the motor 29 is to one end of the shaft 33, and to the opposite end of the latter is connected the endless sprocket chain 30 which is connected to all of the traveling conveyors to drive them simultaneously.

At the upper left-hand end of Fig. 2 is shown an inlet port 34 and at the bottoms of the washing tanks 5, 6, and I are shown drain ports 35, 36, and 31. Inasmuch as the separating medium in the separating tank 4 contains the naturally finely divided clay and relatively coarse sand mixed with water and a very small quantity of the electrolyte, a small quantity of the medium will adhere to the cleaned coal which is tranierred by the conveyor I5 to the tank 5. When the conveyor 99 operates to transfer the coal to the next tank ii, most of the clay and sand is- Washed from the coal and dispersed in the water in the tank 5 where it settles on the conical bottom so that it may flow through the outlet port If any clay and sand still remain on the coal when introduced into'the tank 6 further washing occurs and the clay and sand may be withdrawn through the port 36. Still further Washing may occur in the tank l and the clay and sand eliminated through the port M. The clean water may be introduced intermittently or continuously through the port 341 for flow into the tank i and thence into the tank 65 and thence into the tank 5, the upper edges tit and 39 of the walls or separating partitions 39 and iii are at lower elevations than the upper edges M and d2 of the end walls 613 and 4t. Consequently when the tank i is filled it may overflow the edge 39 into the tank 5 and when the latter is filled it may overflow the edge 38 into the tank 5. Regulation of the flow from the ports 3t and 37 may be varied as desired. However, the apparatus for washing the coal after leaving the separating tank t is claimed in the co-pending application of Nils D. Levin, filed Qctober ll, 1934, for an improvement in Apparatus for cleaning coal, the present application being directed to the method of cleaning coal, the medium used in such method and the apparatus for containing the medium and removing the separated material therefrom.

Obviously those skilled in the art may make various changes in the details and arrangement of parts without departing from the spirit and scope of the invention asdefined by the claims hereto appended, and we therefore wish not to be restricted to the precise construction herein disclosed.

Having thus described and shown an embodiment of our invention, what we desire to secure by Letters Patent of the United States is:

1. The method of separating coal from heavier impurities which comprises introducing raw coal into a separating medium composed oi! naturally finely divided argillaceous material held in substantially quiescent suspension in water by a relatively small quantity of a deflocculating agent, and removing the cleaned coal from the separating medium.

2. The method of separating coal from heavier impurities which comprises introducing raw coal into a separating medium composed of finely divided argillaceous material thoroughly mixed with water and a small quantity of a deflocculating agent to augment the colloidal particles in such 35 ,argillaceous material and thereby maintain the suspension with minimum agitation, and removing the cleaned coal while imparting to the separating medium such small agitation suificient to coact with said agent to maintain by homogeneity a predetermined specific gravity of the separating medium.

3. The method of separating coal from heavier impurities which comprises introducing raw coal into a separating medium composed of mixture of clay and sand in water having in the mixture a deflocculating or dispersion agent serving to maintain said clay and sand in suspension during separation of the impurities with minimum agitation of the separating medium, and removing from the separating medium the cleaned coal.

4. The method of separating coal from heavier impurities which consists in immersing the coal and impurities in a separating medium comprising finely divided argillaceous material thoroughly mixed with water having in the mixture a relatively small quantity of a deflocculating agent to maintain such finely divided argillaceous material in suspension over a relatively long period of time without agitation and thereby maintain sub stantially constant the density of the medium during separation of the impurities from the coal, and removing the coal and impuritiesindependently from the medium from upper and lower portions thereof respectively.

5. The method of producing a medium of sufficient fluidity to effect rapid separation of materials of different specific gravities by flotation and gravity, which consists in intermixing in a liquid a plurality of difierent kinds of granular solids insoluble in such liquid and of approximately the same specific gravities but relatively fine and coarse, and introducing into the liquid a deflocculating or dispersion agent for maintaining over a relatively long period of time uniform density of the separating medium at a predetermined specific gravity.

6. The method of producing a separating medium having a predetermined specific gravity, which consists in thoroughly mixing colloidal solids with granular solids in a liquid in which saidsolids are insoluble and proportioning the amounts of such liquid and said solids to obtain such predetermined specific gravity, and introducing into the liquid without materially changing the specific gravity of the mixture a deflocculating or dispersion agent to maintain uniform the density of the separating medium to adapt the same for continuous separating operations of materials immersed in the medium for separation by flotation and gravitation, without any more agitation than that incident to removal of the separated materials.

'7. A separating medium adapted for use in separating coal from impurities, comprising the combination with a mixture by weight of one partclay, at least two parts of sand and a small quantity of a deflocculating or dispersion agent of the class consisting of sodium silicate and sodium aluminate in suspension in water of Suificient volume to obtain such specific gravity of the medium as to cause coal to float and impurities to sink.

8. A separating medium for use in separating coal from impurities, comprising the combination with a mixture by weight of one part of clay and at least two partsof sand in suspension in water of suflicient volume to obtain such specific gravity of the medium as to cause coal to float and impurities to sink, such water having in solution a deflocculating or dispersion agent amounting to approximately eight-tenths of one per' cent, of the weight of the clay.

' the impurities to sink, such mixture also containing in solution a small quantity of a reagent to act as a deflocculating or dispersion agent.

10. A separating medium adapted for use in separating coal from impurities comprising the combination with a colloid with water in such proportion that if used alone it would be too viscous for practical use, a small amount of a dispersion or deflocculating agent adapted to reduce the viscosity thereof without appreciably changing its density, and a sand in such proportion as to provide a medium having a specific gravity higher than that of coal and lower than that of \the impurities found in coal.

11. A fluid medium having a predetermined specific gravity greater than the specific gravity of coal and less than the specific gravity of impurities found in coal comprising a mixture of water, clay, sand and a deflocculating or dispersion agent.

12. The method of cleaning coal which comprises testing run of mine coal to predetermine the specific gravity of the coal to be separated from the impurities with which it is associated in the mine, preparing a separating medium of a higher specific gravity than coal and lower than the impurities and composed of clay and sand mixed thoroughly with water having in the mixture a deflocculating or dispersion agent, introducing the run of the mine coal into such separating medium having such higher specific gravity to eifect separation of the impurities as predetermined, and finally removing the-cleaned coal from the separating medium.

13. The method of producing a medium of suincient fluidity to separate by flotation and gravity materials of different specific gravities, which consists in intermixing in a liquid difierent kinds of granular materials relatively fine and coarse, and introducing into the liquid a deflocculating or dispersion agent to maintain such granular materials in suspension a relatively long period without any more agitation than that incident to the separation and removal of the first-named.

materials from the said medium.

14. The method of cleaning coal" which comprises testing run of mine coal to predetermine the specific gravity of the coal to be separated from the impurities with which it is associated in the mine, preparing a separating medium of a higher specific gravity than coal and lower than the impurities and composed of clay and sand mixed thoroughly with water having in the mixture a suspension agent the, weight of which is less than one and one-half per cent of the weight of the clay, introducing the run of the mine coal into such separating medium having such higher specific gravity to efiect separation of the impurities as predetermined, and finally removing the cleaned coal from the separating medium. 15. A separating medium consisting of a mixture of colloidal material and granular material in a liquid in which such materials are insoluble and proportioning the volumes of such materials and said water to 'obtain a stirred mixture having a predetermined specific gravity, and a suspension agent in said liquid in an amount insufiicient to change appreciably the specific gravity of the liquid but serving to keep said colloidal and granular materials in suspension to maintain the said specific gravity of the medium over such a long period of time as to enable the medium to be used for continuous separating operation by flotation and gravitation on materials having physical characteristics difierent from those of the colloidal and granular materials.

16. The method of separating coal from heavier impurities which comprises introducing raw coal into a separating medium composed of a mixture of clay and sand in water having in the mixture a deflocculating or dispersion agent comprising a small amount of sodium silicate and serving to maintain said clay and sand in suspension, and removing the cleaned coal from the separating medium.

17. The method of separating coal from heavier impurities which comprises introducing raw coal into a separating medium composed of a mixture of clay and sand in water having in the mixture a deflocculating or dispersion agent comprising a small amount of sodium aluminate and serving to maintain said clay and sand in suspension, and removing the cleaned coal from the separating medium.

Mrs D. SAMUEL H. YOST.

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