US3031074A - Process for cleaning coal by dense medium - Google Patents

Description

April 24, 1962 HIROSABURO OSAWA PROCESS FOR CLEANING COAL BY DENSE MEDIUM Filed Aug. 51, 1953 FIG.I

MATERIAL TO BE SEPARATED +2mm I RAW COAL DEDUSTER 2 Sheets-Sheet 1 WATER FINE DUST WATER D NS E E MEDIUM FLOATINGS I SINKINGS DENSE MEDIUM T SEPARATING DENSE MEDIUM TANK WATER 1 WATER (OVERSIZD WASHING DRAINAGE DRAINAGE WASHING ,(OVERSIZEI I SCREEN scREEN scREEN scREEN I h f I (UNDER) (UNDER) (OVERSIZE) an an (ovERsIzEI N I M lst A scREEN DE SE MED u lsl B SCREEN II COAL DILUTED DENSE MEDIUM h (UNDERSIZE) (UNDERSIZE) IOVERFLOW) CLASSIFIER FOR HEAVY MATERIAL SlNKlNGS (OVERFLOW) (TO REFUSE) CLASSIFIER HEAVY FOR SAND MATERIAL WATER a SAND WATER CLASSIFYING THICKENER FOR IovERFLowI SILT (UNDERFLOW) \J I i SUPER FINE CONDITIONER FOR WATER PARTICLES wATER DENSE MEDIUM (To REUSE) IIQK SAND DENSE MEDIUM J SEPARATOR FOR FINE PARTICLES WATER} FINE PARTICLES (TO REFUSE) WATER HIROSABURO (To REFUSE I INVENTOR OSAWA ATTORNEYS.

April 24, 1962 HIROSABURO osAwA 3,031,074

PROCESS FOR CLEANING COAL BY DENSE MEDIUM Filed Aug. 51. 1953 2 Sheets-Sheet 2 FIG. 2 Is? A IsI B SCREEN SCREEN DILUTED DENSE (UNDERFLOW) SANDIZYEQ) HEAVY MATERIAL 2nd SCREEN WATER (UNDERSIZE) (QVERFLOW) 2nd CLASSIFIER FOR HEAVY MATERIAL (UNDERFLOW) CLASSIFIER FOR SAND ovER I (UNDERMW, FLow FIG. 3 SAND +WATER WATER CLASSIFYING TST A lsI B 5 THICKER FOR SILT SCREEN SCREEN UNDERFLOW 1/5. (umslw WATER DILUTED DENSE h MEDIUM To SEFARATOR CONDITIONER FOR OVERFLOW FOR FINE DENSE MEDIUM I I CLASSIFIER FOR I PARTICLES v HEAVY MATERIAL M V R CLASSIFIER L w TIUNIERFLWI HEAVY MATERIAL WATER SAND T0 SEPARATING FOR SAND TAN K r( UNDERFLOW) SAND WATER OVERFLOW) CLEAN ER FOR HEAVY MATERIAL SAND WATER UNDERFLOWI HEAVY MATERIAL CLASSIFYING *WATER THICKER FOR SILT Fl G. 4 WATER SUPER 7 r FINE PARTICLES +SAND WATER mun-ED DENSE (UNDERFLOW) CONDITIONER FOR MEDIUM T0 SEPARATOR DENSE MEDIUM CLASSIFIER FOR DENSE MEDIUM T HE V TE To SEPARATING TANK (OVERFLOW) (UNDERFLOVI) and SCREEN HEAVY MATERIAL (OVERSIZE) (uunzamzz E,

CLASSIFYING THICKENER FOR SILT WATER (OVERFLOW) P u UNDERFLOVI v E A RETIOE ES INVENTOR.

A E CONDITIONER FOR HIROSABURO OSAWA DENSE MEDIUM To SEPARATER FOR FINE PARTICLES Foams: MEDIUM M To SEPARATING TANK ATTORNEYS 3,031,074 PROCESS FOR CLEANING COAL BY DENSE MEDIUM Hirosaburo Osawa, 698 Z-chome Kugayama, Suginami-ku, Tokyo, Japan Filed Aug. 31, 1953, Ser. No. 377,652 Claims priority, application Japan Aug. 30, 1952 4 Claims. (Cl. 209-12) The present invention relates to improvements in or relating to a method for cleaning coal by dense medium.

An object of the present invention is to provide a process of preparing a dense medium enabling high separating efficiency.

Another object of the present invention is the provision of a process of preparing a dense medium and of recovery of medium material enabling high recovery efficiency and adapted to be reused repeatedly.

A further object of the present invention is to provide a process of preparing a dense medium and cleaning coal enabling low cleaning and low installation costs.

Still another object is the provision of a process for cleaning coal or similar materials of a size finer than the smallest size of coal which can be cleaned by the usual heavy liquid separation process of this type.

Other objects, features and advantages of the present invention will be apparent from the hereinafter detailed illustration in the specification.

In order to prepare a stable dense medium from fine particles of chemical compound of iron or other heavy material for cleaning coal by dense medium, it will be necessary for the grain size to be extremely fine. However, in addition to the high pulverizing cost, the preparation of heavy material of a grain size of such an extreme fineness renders also difiicult the separation from the heavy material of the extreme fine matter mingled in the dense medium, in the separation treatment for recovering used dense medium, which results in lowering the recovery etficiency of the heavy material. The higher the fineness of the said extremely fine particles becomes, the more this phenomenon is made manifest. Hence, it has already been suggested to use the magnetic separator to .enhance the recovery efficiency of the heavy material. In

the said process, however, the available heavy material is limited only to the magnetizable material. In addition, the use of a magnetic separator, requires a magnetizer,

nited States ate nt .demagnetizer and so forth. Further, the dense medium is unstable and forms a hard precipitation while standing at rest. For avoiding these obstacles, specific devices and apparatus become necessary. Thus, such a process is not only unsuitable for the preparation of a stable dense medium, but there is also the disadvantage of increasing the installation cost for cleaning coal. A process is also well known, in which a coarse grain heavy material is used, with which some bentonite or other stabiliser is admixed to stabilise the density of the liquid. However, there is likewise a disadvantage in this process, in that if one intends to obtain a stable dense medium in accordance with this process, the viscosity of the dense medium becomes so high that even thecleaning of small lumps of coal will be difficult, the separating elficiency becomes low, and the recovery of the stabiliser is not satisfactory.

In an attempt to seek an improved coal cleaning process as a solution to the aforementioned deficiencies found in the usual processes, the inventor herein has investigated the cleaning of coal by a dense medium. It has been found from the results of such investigation that when a dense medium is produced by mixing in adequate proportions of pyrite cinder (sp. gr. 3.0-5.0), siderite cinder (sp. gr. 3.0-5.0), other iron-bearing slag (sp. gr. 3.0-4.0), limonite (sp. gr. 3-3.8), hematite (sp. gr. 3.5-5.2), magnetite (sp. gr. 3.5-5.2), pyrrhotite (sp. gr.

ice

3.2-4.6), galena (sp. gr. 5.0-7.5) or ferrosilicon (sp. gr. 5.0-7.0), having grain size ranging between approximately 300 and 60 micron, silt, finely divided material of shale, coaly shale or sandy shale, having grain size ranging between 50 and 5 micron, and water, the dense medium thus obtained indicates satisfactory stability in a specific gravity of 1.35 to 2.0, is never found to precipitate hard precipitates and is of low viscosity, thus enabling even the separation of fine coals, and that a dense medium of high cleaning efliciency can be obtained therefrom at relatively low cost, and moreover, when the dense medium is diluted with Water, the homogeneity of liquid density is lost immediately, and the heavy material and silt present distinct strata-forming phenomenon, which enables easy and perfect separation of the said two materials.

The present invention is characterized by the following facts: The required dense medium is prepared from a specific heavy material of a special grain size, silt and water. Raw coal is subjected to the separation by specific gravity, by using said medium. Then, the dense medium adhering to thus cleaned or separated product is Washed with Water to eliminate it. At the same time, in the recovery of heavy material and silt from diluted dense medium resulting from the Washing water, the heavy material is recovered on one hand from said diluted dense medium by using a screen and a classifier or similar machine, and the desired silt is recovered on the other hand by using a screen or a classifier for sand and classifying thickener for silt. By utilizing the heavy material and silt thus recovered, the required dense medium is prepared and adapted to be reused.

The required grain size of the heavy material in the dense medium varies in accordance with the behavior, type and so forth of the raw coal to be cleaned therewith. It has been ascertained, however, from the result of numerous experiments that the appropriate grain for the present purpose ranges from about 300 micron in maximum diameter to approximately. 60 micron in minimum diameter. It is, however, preferable to choose the difference as small as possible between the maximum and the minimum grain size of heavy material from the point of the recovery, when said heavy material is used for cleaning coal, because this serves for separating inutile fine particles mingled in the heavy material used, perfect recovering said dense material and for decreasing the installation cost for the cleaning of coal. The term silt as used herein means a material corresponding to one-of 50 to 5 micron in grain diameter and having specific gravity of approximately 2.1 to 2.7 which forms from the'natural or artificial pulverization of shale, coaly shale or sandy shale. The silt .as considered in ordinary applications results from the disintegration of shale in the raw coal to be cleaned. Thus, the dens medium to be used in the process of the present invention has a feature that both the specific gravity and the particle size of the heavy material contained therein are greater than those of the silt. Furthermore, the required composition of the said dense medium, though it varies remarkably depending upon the required specific gravity of the dense medium, the grain size and specific gravity of the heavy material, is found nearly satisfactory as referred to the result of the experiment, when the silt contained an amount of more than 10% by volume of the whole medium particles. In addition, the initial silt generally required is taken from any external source when starting to prepare the dense medium. Once the cleaning sets in, however, the external supply of silt is not needed as said silt will come out of coal to be cleaned or heavy material.

Now, when the dense medium is put in a container and diluted with water of several times of the volume, said dense medium loses immediately the homogeneity of liquid density and forms a settling zone of the heavy material in the lower portion and a liquid zone of silt in the upper portion. Then, the liquid zone of silt forms .gradually an upper clear layer and this upper clear layer increases in size with the passage of time, Whereas the resulting zone of silt particles underlying said upper clear layer decreases inversely with increase of said layer and the settling zone of heavy material does not show any substantial change in volume with the time elapsed. This phenomenon indicates that heavy material may be separated easily from the diluted dense medium, while the water added to the silt during washing operation may be separated easily from the silt.

iii

Therefore, according to the present invention, the desired cleaning of coal by a dense medium can be carried out exactly, the heavy material can be recovered simply and perfectly, and moreover the required recovery of silt can be also carried out.

Thus, the required cleaning of coal by the dense medium can be carried out with low cleaning and installation costs; and with high efliciency and high recovery of valuable dense medium material.

Further, in the present invention the dense medium is stable and of low viscosity, and in the recovery of dense medium inutile fine particles mingled in this liquid medium can also be separated.

In order that the present invention may be more clearly understood, several embodiments thereof will now be described by way of examples and with reference to the accompanying schematic flow sheets, in which:

FIG. 1 shows schematically a fundamental flow sheet illustrating one method of operating cleaning coal by dense medium according to the present invention.

FIGS. 2, 3 and 4 show respectively a schematic flow sheet of a modified embodiment of the present invention.

In FIG. 1, raw coal passes through the deduster where dust (particle size below about 1-2 mm.) is removed. An ordinary screen is used as the deduster. The screen is preferably washed with water to render the dedusting effective. The dedusted raw coal enters the separating tank containing the dense medium, where it is separated into floatings, i.e., coal, and sinkings. The fioatings and sinkings are removed separately from the separating tank, each being led to a washing machine wherein each is irrigated with water. Then, each is sent to its respective destination, as shown in the flow sheet. The dense medium from the drainage screen is usually sent directly to a conditioner for this particular medium. Furthermore, in order to ensure an efiective utilization of the plant said drainage and washing operations may be carried out in one and the same screen, and besides both coal and shale may be treated in one screen, by dividing the screen surface with a partition wall longitudinally. The resultant diluted dense medium is subjected to an operation for removing inutile fine particles mingled therein and then to a thickening operation. The total quantity of diluted dense medium is not always necessarily to be subjected to this operation. The treatment and the quantity to be treated depend on the amount and behavior of the inutile material mingled in this liquid as well as on the type and capacity of the machine employed in this procedure. The procedure may vary according to the machine used and the specific gravity, and grain size of the heavy material.

In either of the four embodiments shown in this specification, the diluted dense medium is first passed through the first A screen and first B screen to remove larger particles than dense medium material therefrom. These screens may be provided separately, as illustrated in the flow sheet, from the above mentioned washing screen. However, for the sake of eflicient utilization of the equipment, the screen may also be provided in the lower stage of the washing machine as a part thereof.

The liquid which passes through these screens is introduced to a classifier or similar machine for recovering heavy material and also for removing inutile fine powder contained therein. As such a classifier, a rising current hydraulic classifier or a classifying thickener is usually used. However, other machines such as cyclone separator and centrifugal separator may also be employed.

In the fundamental flow sheet of the embodiment of the present invention shown in FIG. 1, it is preferred that the ratio between the maximum diameter of grain (d of the fine particles passed through the first A screen and the first B screen and the minimum diameter of grain (d of the heavy material, i.e. (ti /d is equivalent or less than the equal falling ratio in the water (R), i.e.

Ai-t

between heavy material (sp. gr.=A in use and shale (sp. gr.=A in the raw coal.

In the flow sheets as shown in FIGS. 2 and 3 it is preferred that the said ratio (d /d is equivalent or larger than the said equal falling ratio, i.e. (R).

Now, the embodiments as shown in each figure will be described as follows:

In the case as illustrated in FIG. 1, the underflow from the classifier for heavy material (consisting of heavy material and water) enters the conditioner for dense medium, and the overflow from said classifier (consisting of all the fine particles plus water which have passed through the first A screen and first B screen and silt) enters the classifier for sand, in which the fine particles larger than the required silt are removed. The remaining silt then enters the thickener for silt. In the thickener fine particles finer than the required silt to be recovered are removed as overflow and the silt is led to the conditioner for dense medium after being thickened to the desired concentration. The said heavy material, silt and water led into this conditioner for dense medium are conditioned therein and the required dense medium thus obtained is transmitted to the separating tank by means of a pump and then used again as a dense medium.

In the case of FIG. 2, each of the underflows of the first and second classifiers for heavy material consisting respectively of heavy material and water enters the conditioner for dense medium, while the overflow of the second classifier for heavy material is treated as illustrated in the figure like the overflow of the classifier for heavy material shown in FIG. 1, and is conditioned to the required dense medium in the conditioner for dense medium and is used again as in the case of FIG. 1.

In the case of FIG. 3, the heavy material having been cleaned in the cleaner for heavy material enters the conditioner for dense medium, while the overflow from the classifier for sand is treated like the overflow in FIG. 1, and the dense medium is conditioned and then used again as in the case of FIG. 1.

FIG. 4 shows the case of using a screen in place of a. classifier for sand, therefore the undersize component passing this screen is fed to a classifying thickener for silt and the oversize component is fed to a separator for fine particles as described in FIG. 1.

The dense medium used is conditioned to the required specific gravity and behavior in the conditioner for dense medium and is transmitted to the separating tank through a pump. The supplementing of heavy material is usually made in this conditioner.

The following examples illustrate the way in which the process according to the present invention may be carried out in practice.

Example 1 59 kg. of powdered pyrite cinder produced in sulfuric acid plant, having specific gravity 4.3 and grain size from to micron, 25.3 kg. of silt formed by the disintegration of coal seam shale, having specific gravity 2.4 and a grain size from 10 to 50 micron and 75.7 kg. of water were mixed with each other. Thereby, 100 liter of dense medium having specific gravity'of 1.6 was produced. The viscosity of this dense liquid medium was lower than that of aqueous solution of calcium chloride having specific gravity of 1.4, and the liquid density had required stability. A bituminous coal of a grain diameter of about 60-2 mm. containing 42.27 percent ash was subjected to cleaning as a sample, the result of cleaning being as follows:

Further, above floatings and sinkings were subjected to the sink and float test, using aqueous solution of zinc chloride having specific gravity of 1.6, the result being as follows:

Then, the dense medium adhered to the separated product was washed with water of a quantity five times that of the dense medium adhered to the separated product; the diluted dense medium thus obtained was screened through a screen having meshes of approximately 150 micron; the undersize was introduced to the hydraulic classifier, i.e. a recovering machine for heavy material, the upward current velocity was adjusted to 70 cm. per minute. By the inspection of the overflow, the presence of heavy material was not observed. In order to recover silt from the overflow, it was screened in a screen of about 50 micron and the undersize was introduced to the classifying thickener, where an upward current of about 0.3 cm. per minute was given, and its overflow was rejected. Then, the underflow from the said classifier for recovering heavy material and the underflow from the thickener for recovering silt were suitably mixed and conditioned, so that the composition consisting of heavy material, silt and water was made equal to that of said used dense medium. Thus, a dense medium entirely equal to the original dense medium in the specific gravity, viscosity, the stability of liquid density and the like was obtained. The whole operations from the cleaning to recovery were carried out repeatedly by using the dense medium thus obtained. The behavior of this dense medium showed the same result as in the above mentioned case.

Example 2 70.8 kg. of galena having specific gravity of 6.4 and grain size of 61-74 micron, 35.9 kg. of silt having specific gravity of 2.3 and grain size of 5 to 50 micron and 73.7 kg. of water were mixed, and 100 liter of dense medium of specific gravity of 1.8 was obtained. The viscosity of the dense medium was lower than that of aqueous zinc chloride solution having specific gravity of 1.8 and its liquid density possessed the stability as required. The cleaning coal by dense medium was carried out on a high bituminous coal specimen having a size of about 2-60 mm. and ash content of 38.05 percent, the result thereof being as follows:

The floatings (clean coal) and sinkings were subjected to the sinking test, using aqueous solution of zinc chloride with sp. gr. 1.8, the result being as follows:

Specific Specific gravity less gravity Grade than 1.8, higher than percent 1.8, percent Floatings (clean coal) 99. 5 0.5 sinkings 0.2 99. 8

Then, the dense medium adhered to the cleaned product Was washed with water to remove it, and as in the case of Example 1, the underflow from the classifier for recovering dense medium and that from the thickener for recovering silt were mixed properly for conditioning the composition. Thereby the required dense medium was obtained. With this dense medium, another cleaning coal operation was carried out repeatedly, the result of which was entirely the same as that had been obtained in the cleaning with above mentioned dense medium.

The above examples are illustrated with respect to two modes of embodiments of the present invention by way of examples. Further, with siderite cinder, iron-bearing slag, limonite, hematite, magnetite, pyrrhotite, ferosilicon other than the above mentioned pyrite cinder and galena, similar result may be obtained.

The dense medium above described is composed of approximately: to 65 percent of the heavy material, 3 to 32 percent of silt, and 17 to 3 percent of water by volume.

What I claim is:

1. A process for cleaning coal comprising preparing a dense cleaning medium, said medium comprising finely divided inorganic material having a particle size of from 60 to 300 microns, and a specific gravity of from 3.0 to 7.5; finely divided inorganic material having a particle size of from 5 to 50 microns and a specific gravity of from 2.1 to 2.7 and water in proportions of approximately 8065 percent of the first mentioned material, 3 to 32 percent of the second mentioned material and 17 to 3 percent of water by volume to provide said medium with a specific gravity of from 1.35 to 2.0; mixing coal with said cleaning medium, to provide sinkings and floatings each admixed with medium; separating said sinkings and floatings; washing and screening the fioatings to recover the floatings; screening the diluted medium to recover additional fioatings of a size larger than the particle size of said medium; diluting the medium to break the homogeneity thereof; classifying the medium received from said screenings to separate the heavy and light portions of said medium and reconstituting said separated heavy and light portions of said medium with water to provide said dense cleaning medium.

2. A process as in claim 1, wherein the separated sinkings admixed with medium is washed and screened to recover diluted medium; and combining the said diluted medium with the diluted medium recovered from the washing and screening of said floatings.

3. A process as in claim 2, wherein the overflow from said classifying step is passed to a sand classifier, the overflow of said sand classifier being passed to a second classifier, separating lines from the light portions of said medium in said second classifier and passing the separated light portion of said medium for reconstitution with the heavy portion of said medium.

4. A process for cleaning coal comprising preparing a dense cleaning medium having a specific gravity of from 1.35 to 2.0, said medium comprising a mixture of heavy particles having a specific gravity of from 3.0 to 7.5 and a particle size from 60 to 300 microns, light particles having a specific gravity of from 2.1 to 2.7 and a particle size from 5 to 50 microns, and water, in proportions of approximately 80-65 percent of heavy particles, 3 to 32 percent of light particles, and 17 to 3 percent of water by volume; mixing said medium with the coal; separating the :mixture into fioatings associated with medium and sinkings associated with medium; washing and screening the fioatings to recover fioatings; Washing and screening the sinkings to recover the medium associated therewith, combining medium recovered from said fioatings and sinkings, diluting the combined medium to break the homogeneity thereof, classifying the combined medium to recover the heavy portion of said medium; screening the overflow of said classifying step; classifying the underfiow of said last screening step to recover as underflow the light portion of said medium, and recombining the recovered heavy and light portions of said medium for recycling.

UNITED STATES PATENTS Rakowsky Feb. 13, 1940 Pearson July 2, 1940 Davis Dec. 14, 1943 Marsh Feb. 7, 1950 Driessen Oct. 6, 1953 Vogel Aug. 17, 1954 Maust Sept. 28, 1954 FOREIGN PATENTS Great Britain Jan. 20, 1949

Claims (1)

1. A PROCESS FOR CLEANING COAL COMPRISING PREPARING A DENSE CLEANING MEDIUM, SAID MEDIUM COMPRISING FINELY DIVIDED INORGANIC MATERIAL HAVING A PARTICLE SIZE OF FROM 60 TO 300 MICRONS, AND A SPECIFIC GRAVITY OF FROM 3.0 TO 7.5; FINELY DIVIDED INORGANIC MATERIAL HAVING A PARTICLE SIZE OF FROM 5 TO 50 MICRONS AND A SPECIFIC GRAVITY OF FROM 2.1 TO 2.7 AND WATER IN PROPORTIONS OF APPROXIMATELY 80-65 PERCENT OF THE FIRST MENTIONED MATERIAL, 3 TO 32 PERCENT OF THE SECOND MENTIONED MATERIAL AND 17 TO 3 PERCENT OF WATER BY VOLUME TO PROVIDE SAID MEDIUM WITH A SPECIFIC GRAVITY OF FROM 1.35 TO 2.0; MIXING COAL WITH SAID CLEANING MEDIUM, TO PROVIDE SINKINGS AND FLOATINGS EACH AD-

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