US3233731A - Method of separating coal - Google Patents

Description

Feb. 8, 1966 c. R. NAlLLER 3,233,731

METHOD OF SEPARATING COAL Filed Dec. 2'7, 1960 5 Sheets-Sheet 1 CHARLES R. NAILLER BY W k 4,; ATTORNEY Feb. 8, 1966 c, NAILLER 3,233,731

METHOD OF SEPARATING COAL Filed Dec. 27, 1960 5 Sheets-Sheet 2 FIG. 2

INVENTOR. CHARLES R. NAILLER BY J 77Z4 A; ATTORNEY Feb. 8, 1966 c. R. NAILLER 3,233,731

METHOD OF SEPARATING COAL Filed Dec. 27, 1960 3 Sheets-Sheet 5 INVENTOR.

CHARLES R. NAILLER BY 5' T; 4

A4 AT ORNEY United StatesPatent O ration of Pennsylvania Filed Dec. 27, 1960, Ser. No. 78,572 2 Claims. (Cl. 209-156) This invention relates to coal preparation, and more particularly to a method of thickening and reducing the ash content of a coal-liquid slurry.

Raw coal as it is mined contains inorganic impurities such as rock, slate, clay and other inorganic matter. The inorganic impurities are incombustible and have a higher specific gravity than coal and are conventionally separated from the coal by gravity separation processes. Modern coal preparation plants clean the coal by removing the high gravity ash impurities therefrom in order to produce high grade coal product, the very fine coal sizes are separated from the more coarse coal sizes early in the preparation process. The fine coal is cleaned separately and is, in certain instances, destined for different markets than the coarse coal particles.

One type of impurity that is found in raw coal and is considered an ash constituent is pyrite sulphur or pyrites. The pyrite (FeS is found both as layers or bands within the coal strata and also grains of pyrites are found dispersed throughout the coal strata. A substantial amount of pyrites that appear as grains in the coal strata are exposed or detached from the coal particles when the coal particles are comminuted to a relatively small size. Pyrites has a specific gravity of about 5.5, whereas clean relatively ash free coal has a specific gravity of between 1.25 and 1.35. The gravity of the admixture is indicative of the ash content. Where the particulate material has a low gravity, it contains little, if any, impurities or ash. Conversely, where the gravity of the material is relatively high, the admixture contains a substantial amount of impurities commonly called ash.

In the course of cleaning fine coal within modern preparation plants, the coal is handled in slurry form, that is, it is mixed with water for ease of movement and to facilitate certain of the cleaning processes. The fine coal, in slurry form, is cleaned on reciprocating coal cleaning tables and then dewatered on vibrating screens. The underflow from the vibrating screens is sent to a cyclone separator where a major portion of the water is separated from the fine coal particles as cyclone separator overflow. The overflow from the cyclone separator is a dilute slurry containing about 10% solids by weight. These solids are of an extremely fine size and have a relatively high ash content. In order to utilize the coal contained in the dilute slurry product from the cyclone separator, large accumulating vessels known as thickeners are employed to separate the solids from the Water carrier. These thickeners maintain the slurry in a quiescent state so that the solids settle out to the bottom of the vessel.

Large slurry thickeners are well known in the coal cleaning art. Primarily, the coal slurry is introduced to the large cylindrical tank of the thickener in a quiescent state at the center of the tank. The coal settles out of the slurry to the bottom of the tank and a substantially clear water overflow is removed from the periphery of the tank. The coal particles which settle to the bottom of the tank Patented Feb. 8, 1966 thickener tank is dewatered to the extent that the slurry removed from the coal discharge port has 50% or more by weight of solids.

The present invention is directed to a novel method for simultaneously reducing the overall ash content of the solids in the slurry leaving the thickener while the slurry is being thickened. The solids in the dilute slurry product introduced into the thickener are of an extremely fine size and have a relatively high ash content. For example, the ash content of the solids introduced to the thickener may be as high as 15% by weight of the solids. This high ash content can be attributed in part to the inefficient cleaning of fine sized solids by conventional coal cleaning apparatus and the colloidal properties of certain types of ash. Ash is the solid, noncombustible waste material which is mined with the coal and may consist of particles of rock, slate, clay, or other foreign material which is intermixed with the raw coal as it comes from the mine.

Coal which leaves a modern preparation plant is graded according to the amount of ash content in the cleaned coal. The cleaner coal, that is, the coal having a reduced ash content, has a greater sale value than the coal having a greater ash content. Thus, if the ash content of coal can be reduced by one or two percent, it then becomes a more salable product.

The present invention, by reducing the overall ash content of the thickener product, increases its value to an extent that under certain instances the thickener product may be recombined with other cleaned coal to thereby increase the overall yield of cleaned coal issuing from the preparation plants.

To practice the present invention, a coal slurry within the thickener is separated by a strategically located slurry withdrawal means which separates a relatively low concentration slurry having a relatively high ash content from the thickener. Because the ash content of the slurry removed by the withdrawal means is substantially greater, percentagewise, than the average ash content of the slurry introduced to the thickener, the slurry leaving the thickener through the coal discharge port has a subsantially reduced ash content. It was discovered that the coal slurry within the thickener had varying solids concentration and varying ash content throughout the radial extent of the thickener. This varying coal concentration and ash content was charted and the slurry withdrawal means was positioned within a zone of the thickener where a slurry having a high ash concentration could be removed.

In the practice of the present invention, so much slurry can be removed through the slurry withdrawal means that substantially no clear water overflows the thickener tank at its periphery. Thus, the slurry level within the thickener tank is maintained only by the amount of slurry withdrawn through the withdrawal means of this invention. In other instances, it may bedesirable to with- 5 draw slurry through the withdrawal means of this invenare removed through a coal discharge port at the center 7 tion in reduced amounts. When so operated, clear water will overflow the thickener tank at the periphery thereof to maintain the slurry level within the tank.

With the foregoing considerations in mind, it is a pri mary object of the present invention to provide an improved method for simultaneously thickening and reducing the ash content of a coal slurry.

Another object of this invention is to provide an im proved method of simultaneously thickening and reducing the ash content of a coal slurry within a conventional slurry thickener.

These and other objects of this invention will become apparent as this description proceeds in conjunction with the accompanying drawings.

In the drawings: 7

FIGURE 1 is a sectional elevation of a slurry thickener embodying the present invention taken along line 1-1ot FIGURE 2.

FIGURE 2 is a partial plan view of the thickener of 5 FIGURE 1 taken along line 22 of FIGURE 1.

FIGURE 3 is an enlarged perspective view of a portion of the slurry withdrawal means shown in FIGURES 1 and 2. x

In the following description of the present invention, the conventional-slurry thickener will be described in some detail and the pattern of solids concentration and ash content throughout the conventionally operating slurry thickener will be considered. The structure of the present invention which modifies the thickener operation will then be described in detail.

Referring to FIGURES 1 and 2, a slurry thickener has a cylindrical tank 12. The tank 12 may be constructed above ground level or it may be positioned in a recessed portion of the topography so that substantially the whole thickener tank 12 is below ground level. The tank 12 has a central portion indicated generally at 14 and a peripheral portion indicated generally at 16.

As seen in FIGURE 1, a tank superstructure 18 extends diametrically across the tank 12 above the level of tank 12. superstructure 18 includes a walk way 20 which is maintained in suspension by truss supports 22. The superstructure 18 also supports a rake drive mechanism indicated generally at 24. The rake drive mechanism is located over the central portion 14 of tank 12 and drives 30 a vertically depending drive shaft 26.

entering through slurry inlet conduit 36 passes between the bottom of chamber 38 and the inlet baffle 49 and enters the tank 12 through the annular tank inlet port 42. By providing the baffle 40 and insuring that the inlet 42 is below the surface 44 of the slurry, the slurry enters the tank 12 with a minimum of turbulence so that the slurry within tank 12 is maintained in a quiescent state. The surface 44 of the slurry within the tank is maintained at its level by an overflow weir 46 which is formed about the periphery of the tank 12. Overflow weir 46 together with the wall of the tank 12 forms an annular launder 48 through which the overflow from tank 12 flows and from which the overflow utimately is removed.

As described thus far, the slurry thickener is conventional in all respects. In operation, the conventional slurry thickener receives a fine coal water slurry through conduit 36 and the slurry enters the tank 12 through inlet port 42. Upon entering the tank 12, the slurry radiates outwardly and the solid particles within the slurry begin to drop toward the bottom of tank 12. The rakes direct the solid particles toward the coal discharge sump 32 at the bottom center portion of tank 12. The water of the slurry overflows into the annular trough 48 extending around the periphery of tank 12 and is withdrawn. The water overflowing into trough 48 is substantially free of any particles.

During operation of a conventional slurry thickener, an analysis of the solids content and ash distribution throughout the thickener was made. The results of this analysis are tabulated in the following Table I.

TABLE I Analysis of solids and ash distribution four inches below the surface of a conventional thickener operating at 8100 g.p.m. inlet and 2000 g.p.m. overflow.

Distance from center Inlet 14 feet 28 feet 42 feet 56 feet 70 feet Solids concentration by weight 8.8% 0.45% 0.17% 0.10% 0.07% 0.07%

Size pcr- Percent Size per- Percent Size per- Percent Size per- Percent Size per- Percent Size per- Percent cent by ash by cent by ash by cent by ash by cent by ash by cent by ash by cent by ash by Size fractions weight weight of weight weight of weight weight of weight weight of weight weight of weight weight of of total fraction of total traction of total fraction of total traction of total traction of total fraction solids solids solids solids solids solids +28 mosh 28 x48 mesh 1.3 7.9 1.4 4.9 48 x 60 mesh 10.5 3. 6 0.9 3. 6 0.7 5. 2 1. 3 5.0 60 x 100 mesh 19. 2 4. 6 3. 9 3.7 2. 2 4.1 1. 3 5.0 0.6 4. 6 0.5 4. 9 100 x 200 mesh 14. 9 5. 5 '6. 3 4.1 4. 7 4.1 4. 4 5.0 4. 1 4. 6 3. 3 4. 9 200 x 0 mesh 54. 1 22. 4 87. 5 48. 6 92. 4 50. 8 93. 0 47. 0 95. 3 45. 5 96. 2 55. 3

Total 100. 0 14. 3 100. 0 43. 0 100. 0 47.3 100. 0 44. 1 100. 0 43. 6 100. 0 53. 4

Secured to drive shaft 26 and extending radially therefrom are rake arms 2-8. The rake arms 28 have a plurality of rakes 30 secured thereto. As viewed in FIGURE 2, the rake arms 28 turn in a clockwise direction so that rakes 30 drag the thickened product on the bottom of the tank 12 toward the central portion 14 of the tank.

At the bottom central portion of the tank 12, a coal discharge sump 32 is formed. The coal discharge sump 32 empties into a coal discharge conduit 34 through which the thickened coal slurry is withdrawn from the tank 12. This thickened coal slurry which is removed through conduit 34 is often termed the underflow from the thickener.

As seen in FIGURE 1, a slurry inlet conduit 36 extends radially inwardly from a source of fine coal slurry (not shown) into an annular inlet chamber 38 formed at the central portion 14 of the tank 12. Other inlet conduit means for the slurry may be provided such as a trough or sluiceway along the walk way 20. Inlet chamber 38 has an inlet baffle 40 which is suspended from the superstructure 18 and which extends radially in close proximity to the bottom wall of inlet chamber 38. The coal slurry As indicated in Table I, an analysis of the solids concentration and ash distribution was made four inches below the surface of the conventional thickener operating at a slurry inlet rate of 3100 gallons per minute and having 2000 gallons per minute overflow into the annular trough 48. Thus, 1100 gallons per minute were removed from the underflow or through the coal discharge port 34. In collecting the data presented in Table I, samples of the coal slurry within the thickener were taken from the inlet port, and at distances of 14, 28, 42, 56 and 70 feet from the center of the tank 12 four inches below the surface of the slurry. The radius of tank 12 was 70 feet, so that a sample taken at 70 feet was substantially clear water overflow from the conventionally operating slurry thickener.

As further shown in Table I, the inlet slurry had a solids concentration of 8.8% by weight and a total ash content of 14.3% of the solids concentration. The ash content by size fraction of the inlet slurry is broken down in the first two columns of Table 1. Thus, it may be seen that of the total solids in the inlet slurry, 1.3% pass through a 28 mesh Tyler Standard screen and remain on a 48 mesh Tyler Standard screen. Of this 28 x 48 mesh fraction, 7.9% was ash. Further, reading from columns 1 and 2, it will be seen that 54.1% of the total solids in the inlet slurry pass through a 200 mesh screen. Of this 200 x 0 fraction, 22.4% was ash.

As samples were taken progressively further out from the center of the tank, the overall solids content by weight of the slurry within the thickener became substantially reduced. Thus, at 14 feet, 0.45% of the slurry by weight was solids. At 56 feet, only 0.07% of the slurry was solids. By a review of Table I it can further be determined that at 14 feet Where the solids concentration was .045 by weight, 43% of those solids were ash. At 56 feet where the solids concentration was .07% by weight, 43.6% of those solids were ash.

By charting the solids and ash distribution as is done in Table I, zones may be selected where a substantial quantity of ash may be removed from the slurry. For example, the inlet slurry contains ash of only 14.3%. If slurry is removed from a zone at 14 feet, the ash content of the slurry removed is 43%. Thus, the overall percentage of ash content in the slurry remaining in tank 12 will be substantially reduced.

In addition to selecting a zone which contains a high percentage of ash, the overall solids concentration of the slurry should be considered. If the overall solids concentration of the slurry is relatively high, a greater absolute quantity of ash will be removed from the tank than if a removal zone is selected where the overall concentration of the slurry is less. Thus, for example, at 14 feet, 0.45% solids concentration, only 43% by weight of the solids are ash. At 28 feet, on the other hand, 47.3% of the solids are ash. But at 28 feet, the solids concentration of the slurry is only .17% by weight. Thus, although the percentage of solids that are ash is much higher at 28 feet, the total absolute quantity of ash which can be removed from the vessel is higher at 14 feet becaue the total solids removed at 14 feet in every gallon of slurry removed will be more than double the total solids removed at 28 feet in every gallon of slurry removed.

The data set forth in Table I is intended to represent only one set of conditions at a particular preparation plant. There may be installations where the dilute slurry product fed to the thickener is substantially higher in ash, especially clay type ash. Under these circumstances the solids concentration at the 14 foot point could be as high as 3% After considering the analysis of the solids and ash distribution within a conventional thickener, it will be seen that an annular zone may be selected where a quantity of slurry may be withdrawn from the thickener and disposed of in order to decrease the ash content of the thickened slurry leaving outlet port 34. An apparatus for withdrawing slurry from a thickener tank is indicated generally at 50 on FIGURES 1, 2 and 3. A slurry withdrawal zone 52 is selected from an analysis of the solids and ash distribution within the thickener so that an optimum amount of ash will be removed without undue sacrifice of the overall quantity of thickened slurry leaving the tank 12. A withdrawal zone 52 is indicated by dotted lines in FIGURE 2.

Within withdrawal zone 52, a series of pipe sections 54 are connected to each other to form a closed polygon. The pipe sections 54 are secured to a number of float drums 56 which float the apparatus 50 on the surface of the slurry so that the pipe sections 54 remain at a fixed distance below the surface of the slurry 44. For economy and simplicity of fabrication, the apparatus 50 is formed from a series of straight pipe sections. It will be appreciated that a continuous circular pipe section would be provided at greater expense and without the ease of fabrication.

As best seen in FIGURE 3, the pipe sections 54 have a series of holes 58 formed therein. Although the apertures 58 are indicated as positioned on the external pcriphery of the polygonal pipe sections, it should be understood it is within the scope of the invention to position the apertures on the inner periphery and thereby provide means to secure a depending polygonal curtain to the pipe sections 54 and provide a compartmentalized thickener.

One of the pipe sections 54 has a T fitting 60 secured thereto so that the interior of the pipe sections 54 may communicate with a flexible conduit 62 extending radially from the polygonal structure formed from pipe sections 54.

Communicating with the end of a flexible conduit 62 is a pump 64 shown in FIGURE 2. Pump 64 is designed to withdraw slurry through flexible conduit 62, pipe sections 54, and out of the tank 12.

In order to promote uniform flow from the tank 12 into the pipe sections 54, the holes 58 formed in pipe sections 54 vary in size at progressively greater distances from the T connection 60 which joins the flexible conduit 62 to pipe sections 54. The holes 58 formed in the pipe section 54 in which T connection 60 is located, are of relatively small size. The holes 58 in the pipe sections 54 next adjacent to the pipe section containing T fitting 60 are slightly larger. The holes 58 in the pipe sections 54 at greater distances from T fitting 60 are even larger. Thus, the pressure drop through the polygonal structure formed from pipe sections 54 does not materially affect the flow of slurry from tank 12 into the polygonal structure since there is a greater area open to the interior of the structure at the farthermost points where the force drawing the slurry into the structure is least.

A slurry thickener equipped with the structure of the present invention may be operated to maintain an amount of overflow around the periphery of tank 12 into annular trough 48, or the slurry withdrawal apparatus 50 of the present invention may be operated to withdraw all excess slurry from the tank 12 so that there is no overflow about the periphery of tank 12. For optimum de-ashing, the apparatus of the present invention should be operated so that there is no overflow over the launder 46 into trough 48. However, in some instances it is desirable to reduce the overall de-ashing in order to obtain a higher quantity of usable material from the outlet port 34.

A slurry leaving the tank 12 through flexible conduit 62 is a mixture of fine coal particles and water. This slurry has a very high ash content (well over 40%) so that the solid material is not recoverable as a useful product. Accordingly, the slurry leaving tank 12 through flexible conduit 62 may be disposed of in any manner conventionally utilized for disposing of Waste slurry products.

With this invention, the overall percentage of ash in the solids recovered from a coal-water slurry may be reduced as much as 2%. This 2% reduction in ash content often means the difference between an unmarketable coal product and a useful coal product which is highly marketable. A reduction in ash can be obtained with a very simple modification to the structure of a conventional thickening unit.

According to the provisions of the patent statutes, I have explained the principle, preferred construction and mode of operation of my invention and have illustrated and described what I now consider to represent its best embodiment. However, I desire to have it understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically illustrated and described.

I claim:

1. A method of separating high gravity solids from a mixture containing different gravity solids, said solids having a spectrum of sizes and a substantial portion of fine high gravity solids, said method comprising introducing said solids in the form of a slurry of solids and water into the central portion of a tank, said tank having a central portion and a peripheral portion, maintaining said slurry under substantially quiescent conditions in said tank so that certain of said solids in said slurry settle to the bottom of said tank, determining the percent by Weight concentration of solids in said slurry at predetermined radial locations from the center of said tank, determining the relative proportion of high gravity particulate material at each preselected location, Withdrawing a dilute slurry having a decreased solids concentration relative to said slurry introduced into said tank from a zone adjacent the upper level of said slurry in said tank and at a selected one of said predetermined radial locations from the center of said tank, said solids in said dilute slurry having an increased concentration of finesized high gravity solids relative to said solids introduced into said tank, and Withdrawing a concentrated slurry from the bottom of said tank, said concentrated slurry having an increased solids concentration relative to said slurry introduced into said tank, said solids in said concentrated slurry having decreased concentration of high gravity solids and an increased concentration of larger sized solids relative to said solids introduced into said tank.

2. A method of separating fine particulate material having a high gravity from a mixture containing difierent gravity particulate material, said particulate material having a spectrum of sizes including said high gravity fine particulate material, said method comprising introducing said particulate material in the form of a slurry of particulate material and Water into the central portion of a tank, maintaining said slurry under substantially quiescent conditions in said tank so that certain of said particulate material in said slurry settles to the bottom of said tank and certain of said high gravity fine particulate material remains suspended in a zone adjacent theupper surface of said slurry in said tank and at a predetermined radial location from the center of said tank, determining the percent by Weight concentration of solids in said slurry at predetermined radial locations from the center of said tank, determining the relative proportion of high gravity particulate material at each preselected location, withdrawing a dilute slurry from said zone at a selected one of said predetermined radial locations from the center of said tank, said dilute slurry having a decreased solids concentration relative to said slurry introduced into said tank, said dilute slurry having an increased concentration of high gravity fine particulate material relative to said slurry introduced into said tank, and Withdrawing a con centrated slurry from the bottom of said tank, said concentrated slurry having an increased concentration of particulate material relative to said slurry introduced into said tank, said particulate material in said concentrated slurry having a decreased concentration of fine high gravity particulate material relative to said particulate material introduced into said tank.

References Cited by the Examiner UNITED STATES PATENTS 558,018 4/1896 Goldsmith 210.242 841,176 1/1907 Powers 210-242 XR 2,593,036 4/1952 Koch 210+525 2,681,151 6/1954 Coulter Q 210-528 2,744,632 5/1956 Gardes 210-323 HARRY B. THORNTON, Primary Examiner.

RICHARD A. OLEARY, HERBERT L. MARTIN,

Examiners.

Claims (1)

1. A METHOD OF SEPARATING HIGH GRAVITY SOLIDS FROM A MIXTURE CONTAINING DIFFERENT GRAVITY SOLIDS, SAID SOLIDS HAVING A SPECTRUM OF SIZES AND A SUBSTANTIAL PORTION OF FINE HIGH GRAVITY SOLIDS, SAID METHOD COMPRISING INTRODUCING SAID SOLIDS IN THE FORM OF A SLURRY OF SOLIDS AND WATER INTO THE CENTRAL PORTION OF A TANK, SAID TANK HAVING A CENTRAL PORTION AND A PERIPHERAL PORTION, MAINTAINING SAID SLURRY UNDER SUBSTANTIALLY QUIESCENT CONDITIONS IN SAID TANK SO THAT CERTAIN OF SAID SOLIDS IN SAID SLURRY SETTLE TO THE BOTTOM OF SAID TANK, DETERMINING THE PERCENT BY WEIGHT CONCENTRATION OF SOLIDS IN SAID SLURRY AT PREDETERMINED RADIAL LOCATIONS FROM THE CENTER OF SAID TANK, DETERMINING THE RELATIVE PROPORTION OF HIGH GRAVITY PARTICULATE MATERIAL AT EACH PRESELECTED LOCATION, WITHDRAWING A DILUTE SLURRY HAVING A DECREASED SOLIDS CONCENTRATION RELATIVE TO SAID SLURRY INTRODUCED INTO SAID TANK FROM A ZONE ADJACENT THE UPPER LEVEL OF SAID SLURRY IN SAID TANK AND AT A SELECTED ONE OF SAID PREDETERMINED RADIAL LOCATIONS FROM THE CENTER OF SAID TANK SAID SOLIDS IN SAID DILUTE SLURRY HAVING AN INCREASED CONCENTRATION OF FINESIZE HIGH GRAVITY SOLIDS CONCENTRATION RELATIVE TO SAID INTO SAID TANK, AND WITHDRAWING A CONCENTRATED SLURRY FROM THE BOTTOM OF SAID TANK, SAID CONCENTRATED SLURRY HAVING AN INCREASED SOLIDS CONCENTRATION RELATIVE TO SAID SLURRY INTRODUCED INTO SAID TANK, SAID SOLIDS IN SAID CONCENTRATED SLURRY HAVING DECREASED CONCENTRATION OF HIGH GRAVITY SOLIDS AND AN INCREASED CONCENTRATION OF LARGER SIZED SOLIDS RELATIVE TO SAID SOLIDS INTRODUCED INTO SAID TANK.

Images