US3578418A - Sulfur extraction using halogenated hydrocarbons including washing the recovered sulfur with methanol,acetone,or ethylene glycol - Google Patents

Abstract

A SOLVENT-EXTRACTION PROCESS FOR RECOVERING SULFUR FROM SULFUR-BEARING ORES NOT AMENABLE TO TREATMENT BY THE FRASCH HOT-WATER PROCESS. SULFUR-BEARING ORE IS DRIED AND CONTACTED WITH A SOLVENT, SUCH AS TETRACHLOROETHYLENE, AT A TEMPERATURE ABOVE THE MELTING OF THE SULFUR. THE GANGUE IS SEPARATED FROM THE LOADED SOLVENT BY FILTRATION, AND THE LOADED SOLVENT IS COOLED TO A TEMPERATURE TO PERMIT CRYSTALLIZATION OF THE SULFUR. THE SOLID SULFUR IS SEPARATED FROM THE SOLVENT AND THE SULFUR CAKE IS PASSED THROUGH A COUNTERCURRENT WASHING SYSTEM TO REMOVE RESIDUAL SOLVENT AND TO PROVIDE SULFUR IN ESSENTIALLY PURE FORM.

Description

May 11., 1971 R. R. CANTRELL. ET AL 3,578,418

SULFUR EXTRACTION USING HALOGENATED HYDROCARBONS INCLUDING WASHING THE RECOVERED SULFUR WITH METHANOL ACETONE OR ETHYLENE GLYCOL Filed May 27, 1968 United States Patent O 3,578,418 SULFUR EXTRACTION USING HALOGENATED HYDROCARBONS INCLUDING WASHING THE RECOVERED SULFUR WITH METHANOL, ACE- TONE, OR ETHYLENE GLYCOL Robert R. Cantrell, Baltimore, and Vernon F. Swanson, Ellicott City, Md., assignors to W. R. Grace & Co., New York, N Y.

Filed May 27, 1968, Ser. No. 732,294 Int. Cl. C01b 17/.08, 17/14; B01d 9/02 U.S. Cl. 2.3-299 11 Claims ABSTRACT OF THE DISCLOSURE A solvent-extraction process for recovering sulfur from sulfur-bearing ores not amenable to treatment by the Frasch hot-water process. Sulfur-bearing ore is dried and contacted with a solvent, such as tetrachloroethylene, at a temperature above the melting of the sulfur. The gangue is separated from the loaded solvent by filtration, and the loaded solvent is cooled to a temperature to permit crystallization of the sulfur. The solid sulfur is separated from the solvent and the sulfur cake is passed through a countercurrent washing system to remove re- ;idual solvent and to provide sulfur in essentially pure orrn.

This invention relates to the recovery of elemental sulfur from sulfur-bearing ores or solid mineral matter, and more particularly to a solvent-extraction process for recovering sulfur from native ores not amenable to treatment by the Frasch hot-water process.

In recent years, there has been an ever increasing demand for elemental sulfur, which demand has resulted in part through the increased use of sulfur in the agricultural industry. As is well known in the art, sulfur-bearing ores exist in deposits either far below the surface of the earth in relatively massive bodies or in locations near the surface of the earth. Sulfur is recovered from deposits far below the surface of the earth by the well known Frasch process, wherein hot water is sent down into the earth to melt the sulfur which is then forced up to the surface. Sulfur ore located near the surface of the earth can be mined, for example, from opened pits. Separation of the sulfur from the ore in the latter type deposits can be accomplished, for example, by heating the ore to vaporize the sulfur, as by burning fuel in contact with the ore, or by the so called solvent-extraction processes wherein the sulfur is extracted from the ore by a selective solvent.

Due to the ever increasing demands for sulfur, as discussed above, prior art workers have recently focused much time and attention in efforts to develop processes for recovering sulfur from ores and deposits which heretofore have been considered nonprotable.

In general, such proposals include extraction of the sulfur with a selective solvent such as toluene, kerosene, carbon disulfide, chlorinated or brominate derivatives of aliphatic or aromatic hydrocarbon, e.g., tetrachloroethyylene or trichlorobenzene, etc. The gangue or insoluble material in the ore (which may contain siliceous material, limestone, gypsum, etc., depending upon the location of the ore deposits) is separated from the loaded solvent by conventional techniques and the hot liquor containing the dissolved sulfur is cooled to permit crystallization of the sulfur. The solid sulfur is then separated from the solvent by filtration, etc. An example of such a process is disclosed in U.S. Pat. 2,234,269 to McDonald, which issued Mar. 1l, 1941. This patent discloses a method for extracting sulfur from sulfur-bearing ores which comprises heating a mixture of the ore and solvent (tetrachloroice ethylene) to at least the melting point of sulfur and until substantially all of the sulfur has melted. The gangue is separated from thesulfur-bearing hot liquor which is agitated with a cooling medium, such as water, to crystallize the sulfur. The crystallized sulfur is separated from the solvent and any residual tetrachloroethylene is removed from the sulfur cake by vacuum distillation at a temperature below the melting point of the sulfur.

Other solvent-extraction processes for recovering sulfur are disclosed, for example, in U.S. Pats. 2,841,536 to Bert, which issued July 1, 1958, and U.S. Pat. 2,785,059 to McDonald, which issued Mar. 12, 1957. While processes disclosed by the above noted patents offer a number of advantages, such processes have been found to suffer from a number of serious disadvantages. In general, such disadvantages arise from the use of complex and somewhat detailed equipment and techniques employed in effecting the various steps evolved in extracting, crystallizing and purifying the recovered sulfur cake. Another serious diticulty involved in prior known processes and to which the present inention is particularly directed, is the difficulty of removing the residual solvent from the crystallized sulfur cake. In this regard, for example, while the use of a chlorinated hydrocarbon such as tetrachloroethylene is a particularly advantageous solvent, it has been found that processes employing this solvent are ineffective to remove the trace amounts of the chlorinated hydrocarbon in the crystallized sulfur to a point wherein the sulfur can be used in conventional sulfuric acid plants. In this regard, it has been determined that no more than l() parts chlorine/million could be left with the sulfur without contaminating the catalysts used in the sulfuric acid plants.

It is accordingly a general object of this invention to provide a unique and improved process for recovering sulfur from ores not amenable to the Frasch hot water process.

Another and more particular object of this invention is to provide a highly efficient, simplified and economical solvent-extraction process for recovering sulfur from sulfur-bearing ores.

Yet another object is to provide a method for the solvent extraction of sulfur from its ores wherein sulfur may be obtained in exceptionally pure form and at a cost which makes this process economically practical.

Still another object is to provide a continuous, economical method for recovering sulfur from its ores, the said process requiring relatively simple equipment and involving a minimum number of steps.

Another object is to provide a highly eicent and economical method for purifying and removing trace quantities of solvent remaining with sulfur cake that is recovered from sulfur bearing ores by extraction with a solvent followed by the crystallization of the sulfur from the hot solvent sulfur-bearing liquor.

The manner in which the foregoing and other objects are achieved in accordance with the present invention will be better understood in view of the following detailed descriptions and accompanying drawing which forms a part of the specification and herein:

The ligure is a diagrammatic illustration of a suitable arrangement of apparatus for carrying out a particularly advantageous method embodiment of the present invention.

Stated broadly, in accordance with the present invention sulfur-bearing ore, crushed and screened to a desired particle size, is leached with a solvent, which is preferably a halogenated aliphatic or aromatic hydrocarbon, such as tetrachloroethylene or p-dichlorobenzene, in a conventional liquid-solid leaching or extracting unit. Preferably, the extraction step is preceded by a premixing step wherein the ore and solvent are brought into intimate contact. The material from the extractor or leaching unit is charged into a filter to remove the gangue or insoluble solid material andthe hot liquor, which containsrapproximately 30-45 percent sulfur, is cooled to crystallize the sulfur. In general, this step preferably involves passing the hot liquor into a vertical column wherein it travels downwardly through a rising column of a cooling medium, such as water. The cooling medium cools the hot liquor causing crystallization of the sulfur. The crystallized sulfur then passed into a filter to effect a preliminary separation of the solid sulfur and residual solvent.

The sulfurcake from the filtration step contains from "3 to 15% by weight solvent. As briefly discussed above, investigations have shown that conventionally known methods of drying and/or vacuum distillation etc., are

ineffective to remove the chlorine content left with the sulfur to a point such that the sulfur may be used in conventional sulfuric acid plants without contaminating the catalysts.

In this regard and in accordance with the present invention, the sulfurv cake is Washed with a Iwash liquid, such as methanol, in a countercurrent washing system wherein the sulfur cake is fed into one end of an elongated screw or rake classifier. Essentially pure wash liquid is spread over the solids at a discharge point or opposite end of the classifier. The wah liquid passed down through the classifier in a countercurrent direction to that of the solids, washing the residual chlorinated hydrocarbon or solvent from the sulfur. The washed solvent-free sulfur discharges into a filter for removal of residual wash liquid. The solvent-wash liquid mixture issuing from the lower portion of the classifier is fed to a distillation unit wherein the wash liquid is separated from the mixture. The sulfur cake from the filter is fed into a sulfur melter, and the molten sulfur from the melter, is pumped to a molten storage tank.

Turning now to the drawing in detail, there is shown a suitable arrangement of apparatus for carrying out a particularly advantageous method embodiment of the present invention. Ore, crushed and screened to a particle size in the range of approximately 1/2 or smaller, is preheated to a temperature of approximately 200 F. and dried to a moisture content of about 1% or less in a conventional drier (not shown). With reference to the drawing, the heated ore is transferred by a belt conveyor 1 and is charged through conduit 2 to a premixer, indicated generally at 3, wherein it is contacted with a solvent, introduced through conduit 4. The solvent may be fed from -a hold or surge tank (not shown) to which recovered solvent may be recycled, etc., as to be discussed in more detail hereinafter. The primary purpose of the premixing step is to intimately mix the ore and solvent to form a owable slurry. From the premixer the slurry is continuously fed through conduit into inlet or forward end of an extractor 6. The premixer 3 and the extractor 6 may consist of conventional liquid-solid containing units such as variable pitched screw conveyors, including conveying means or blades, mounted on shafts, extending through the length of the housings, etc., as is generally known in the art. The mixer 4 as well as the extractor 6 should be sealed from the atmosphere to avoid solvent and vapor losses. In the embodiment shown in the drawing, the premixer 3 acts as a vapor seal for the extracting unit 6. The extractor 6 is preferably provided with a steam jacket to which a suitable heating medium, such as steam, is applied through conduit 7 at such a pressure so as to maintain a temperature inside the extractor above the melting point of sulfur (.e., approximately 240 F.) but lower than the boiling point of the solvent at the existing pressure.

As indicated above, the present invention is particularly directed to solvent extraction process wherein a halogenated hydrocarbon, such as tetrachloroethylene or dibromoethane is employed. Chlorinated hydrocarbons are particularly advantageous solvents because of their thermal stability, relatively low cost, nonreactiveness toward sulfur, liquid range, etc. Thus, if tetrachloroethylene is employed as the solvent, the temperature in the extractor would normally be in the range of approximately 250- 260o F.

Again with reference to the drawing, the solvent-ore slurry passes from the premixer 3 into the extractor 6 wherein the sulfur content of the ore is progressively decreased, by solvent extraction, as the otre and solvent is advanced concurrently through the extractor.

The hot solvent, containing the sulfur in solution discharges from the exit end of the extractor through an insulated or heated pipe 8 into a conventional filter 9, such as a horizontal rotary filter. In the filter, three steps are'carried out: (l) the loaded solvent is separated from the gangue (2) the gangue is then washed vw'th fresh solvent and (3) hot gas is passed through the gangue to remove residual solvent.

In the first step, the hot dissolved sulfur-solvent liquor passes through outlet pipe 10 and is fed to the upper portion of the vertically mounted crystallization column, indicated generally at 14. Preferably, and as shown in the drawing, a portion of the hot liquor is recycled (through conduit 10') to the solvent inlet pipe 4 in order to insure wetting of the solids and to maintain a flowable slurry in the premixer. In the second step, the gangue is washed with fresh solvent which is introduced through conduit 12 to effect a rst -wash of the gangue. In this step, the solvent discharges through conduit 12 and may be recycled to the solvent inlet pipe 4, if required or to a solvent hold tank (not shown), etc. In the third step, a hot gas, such as hot ue gas from a steam boiler, is passed through the gangue to remove residual solvent. The hot gas may be introduced through a suitable conduit such as indicated at 11. The hot gas, from the filter is removed frorn the upper portion thereof through conduit 11 wherein it is further processed to separate the solvent from the gas, In this regard the hot flue gas may, for example, be changed into a water-filled column (not shown) wherein the solvent is scrubbed from the gas and falls to the bottom of the column (a chlorinated hydrocarbon solvent is heavier than water and essentially insoluble in Water). The recovered solvent may then be pumped into a surge or hold tank, feeding the premixer. The dried essentially solvent free gangue passes from the filter through an air lock 13 onto a suitable conveying means (not shown) which takes the gangue to a waste pile. While any conventional known methods of filtering, washing, and drying may be employed to effect the above described steps, as indicated above, a particularly advantageous filtration unit is a horizontal rotary lter. To carry out the above described sequence of steps in a continuous manner, the filter would be divided into four compartments, that is, in the first compartment, the gangue would be separaed from the hot solvent, the former being retained on a filter cloth rotating in a horizontal plane. As the cloth is rotated it enters the second compartment, wherein the gangue is washed to recover entrained sulfurladen solvent. The washed gangue is then dried as the filter cloth passes into the third compartment by passing hot gas through the cloth, etc., as described above. The dried gangue on the rotating filter cloth is then removed or scraped from the said cloth in a fourth compartment by a scoll or doctor blade.

In crystallization tower 14, the hot liquor descends countercurrently through `an upwardly flowing cool stream of water introduced through 17. The water is maintained at a temperature sufficient to cool the contents of the lower part of the tower to cause precipitation of substantially all of the sulfur. The water, having a lower specific gravity than the solvent, rises through the tower and is continuously Withdrawn through an overliow pipe 15. Preferably, the lwater lwhich picks up heat in the tower, passes through a heat exchanger 16, which may be cooled with air or other suitable cooling medium, and is recycled or returned to the bottom of the column. The sulfur is heavier than the hydrocarbon solvent and will settle to the bottom of the column. The crystallized sulfur and solvent mixture is continuously withdrawn from the bottom of the wash tower through pipe 18 and is passed to a filter 19 to effect a preliminary separation of the solid sulfur cake and solvent. Depending upon the design, size, etc., of the filter, the sulfur cake at this point contains approximately from 315% by weight solvent. The filtrate is discharged through conduit 21 and preferably is recycled to the solvent inlet conduit 4.

The sulfur cake from the filter 19 discharges through conduit 20 into an enclosed, inclined screw or rake classifier, indicated generally at 22. The classifier may be of any conventional design and should generally be sealed from the atmosphere to avoid solvent vapor losses. The housing or casing of the classifier is preferably inclined, as shown, so as to drain towards the end at which the ore is introduced through conduit 20. In the classifier, essentially pure wash liquid, such as methanol at ambient temperature, is spread over the solids at a discharge point or opposite end of the classifier 22. The wash liquid passes down through the classifier in a countercurrent direction to that of the solids, 'washing the residual solvent from the sulfur. The solvent-free sulfur discharges into a filter 23 for removal of residual methanol. The solvent-wash liquid mixture, issuing from the lower portion of the classifier, is fed through conduit 26 to a distillation column 30 `wherein the wash liquid is separated from the mixture. The distillation unit, may be of any conventional type or design and having a steam reboiler, etc. The essential pure wash liquid and solvent, separated in the distillation column, may be returned to the system through conduits 31 and 32 respectively, for reuse.

In general, the :Wash liquid may be selected from any number of liquids, such as methanol, acetone, benzene, ethylene glycol, etc., which possess the following characteristics: (l) miscibility with the solvent employed for extraction, (2) low sulfur solubility (at essentially ambient temperature), and (3) no azeotrope between the wash liquid and solvent used for extraction.

The latter characteristic is desirable since an azeotrope would require a more elaborate distillation or separation procedure over that of a simple binary nonazeotropic distillation unit shown in the drawing. The fwash liquid from the filter may be recycled and introduced into the upper portion of the classifier, The sulfur cake from the filter 23 is fed by way of a screw conveyor 27 into a conventional sulfur melter, indicated generally at 28 and the molten sulfur, from the melter, is pumped to a molten storage tank 29. In the sulfur melter, any residual wash liquid is vaporized or distilled from the solid sulfur and may be recycled to the classifier, etc.

As should be readily appreciated by those skilled in the art, the uniquel process of the present invention provides a highly simplified, economical method for producing essentially pure sulfure. By preheating the ore and solvent in the premixer and maintaining the resultant slurry at a temperature in the range of approximately 200 F., the requirement for a complicated multistage extraction system is eliminated. The extractor may be of simple design and need be the only sized to provide the desired retention time to extract or dissolve the sulfur from the ore. The hot liquor from the extractor is thereafter passed into a continuously operated filter `wherein the gangue is separated from the hot liquid and is dried to recover residual solvent adhering thereto for reuse in the system. By introducing the hot liquor containing the dissolved sulfur into a vertical sulfur crystallizer in direct contact with water, large crystals are formed which settle to the bottom of the column for easy removal thereof.

Further, and as discussed above, the present invention is particularly directed to a process for extracting sulfur from ores with a halogenated aliphatic or aromatic hydrocarbon liquid solvent. However, prior known processes employing these solvents have proved to be ineffective for removing the residual solvent from the sulfur cake to a point such that the sulfur may be used in conventional sulfuric acid plants. Any chlorine, fluorine, etc., present in the sulfur will damage the catalysts used in the sulfuric acid plants as well as cause corrosion of the plant equipment. In accordance with the present invention however, there is provided a direct, highly effective method for removing the residual solvent in the sulfur product. By washing the sulfur cake containing the residual solvent, with a wash liquid, such as methanol, in the countercurrent fashion as described in detail above, an essentially pure sulfur product is obtained. As further, should be appreciated to those skilled in the art, in accordance with the present invention, there is provided an improved method for producing relatively pure sulfur wherein the amount of make up solvent is greatly minimized. In this regard, for example, the solvent clinging to the gangue in the filtration step, is essentially entirely recovered by the direct solvent 'washing step, followed by drying of the gangue `with the hot gases.

The following examples serve to illustrate the present invention but is not intended to limit it thereto.

EXAMPLE I The equipment used in this example was substantially as shown in the figure. Elemental-sulfur bearing ore, containing approximately 25% sulfur, was crushed to a particle size of approximately minus 1/2 mesh and introduced into a conventional rotary drier wherein the ore was preheated to about 200 F. and dried -to a moisture content of less than 1%. The heated ore was transferred by a belt conveyor to a l2 I D. x 20 long variable pitch screw premixer wherein the heated ore and tetrachloroethylene were intimately mixed and preblended to form a fiowable slurry. The slurry from the premixer and at a temperature of approximately 210 F. was next continuously introduced into a 4 I.D. x 35 long hollow fiight, jacketed screw extraction unit with steam acting a-s the heat sOurce and supplied by conventional regulating means at a pressure to maintain the temperature of the slurry within the extractor at a temperature of approximately 258 F. The hot liquor from the discharge end of the extractor discharged by gravity onto a totally enclosed l5 diameter horizontal rotary vacuum filter wherein the gauge or insoluble solid material was separated from the loaded solvent containing the dissolved sulfur. The gangue retained on the filter cloth of the filter was washed with fresh solvent and the gangue was dried by passing hot flue gas through the filter cloth, in accordance with lthe techniques described in detail above. The loaded solvent from the filter was split into two streams, with one stream being recycled to a solvent hold tank feeding the premixer. The second lstream was pumped to a polishing filter to remove traces of gangue and the hot liquor from the polishing filter was pumped to the top of a 8 I.D. x 30 tall sulfur crystallizing column wherein the hot -liquor was contacted with an upwardly owing stream of water introduced at the bottom of the column at a temperature of approximately 70 F. The cooled solvent and sulfur crystals were continuously withdrawn from the bottom of the column and pumped to a disc-type filter to effect a primary separation between the solid sulfur and solvent. The filtrate was recycled to the solvent hold tank feeding the premixer. In the crysta'llizing column, the water, having a lower specific gravity than the liquor, was continuously withdrawn from an overflow pipe at the top of the column and was passed through a conventional aircooled heat exchanger and then returned to the water inlet conduit.

The sulfur cake containing approximately 7% by weight solvent was discharged from the filter into the lower portion or end of a 54 diameter x 50 long inclined screw classifier having a sectional ribbon fiight conveyor mounted on a shaft extended through the unit. Methanol at F.

was introduced into the sulfur discharge end or upper portion of the inclined washing unit and the conveyor was rotated, causing the sulfur cake to advance through the unit in countercurrent relation to the downwardly ilowing pool of methanol. The screw was rotated at a rate such that the methanol filtered down through the sulfur faster than the said sulfur was moved upwardly through the classier. The methanol was fed into the unit such that the concentration of the solvent-methanol mixture discharging from the lower end of the washing unit was approximately 50-50. This mixture was pumped to a 21/2 LD. X 33 tall-20 plate-distillation column of conventional design wherein the methanol was separated from the mixture, recovered overhead, condensed and recycled to a methanol hold Itank feeding the wash unit. The still bottoms consisting of essentially pure solvent was pumped back to the hold tank feeding the premixer.

The solvent-free sulfur cake was continuously discharged from the upper portion of the washing unit and passed to a disc lter to remove residual methanol. The methanol was recycled to the classifier. The sulfur cake from the lter was continuously fed by a screw conveyor to a 15' diameter x 10 deep concrete sulfur melter of conventional design and equipped with an agitator and steam coils. Residual methanol in the sulfur cake was vaporized and withdrawn from the top of the sulfur melter and recycled to the methanol hold tank. The molten sulfur from the melter was pumped to a molten storage tank.

A sample of the sulfur from the sulfur melter was analyzed by conventional chromatographic analytical procedures capable of detecting chlorine at one part per billion. Neither tetrachloroethylene or methanol was detected. Sulfur recovery Was approximately 95% of the sulfur in the ore being fed the premixer.

EXAMPLE II The procedure of Example I was repeated except that trichloroethylene was substituted for the tetrachloroethylene. An analysis of a sample of the sulfur cake from the melter detected neither trichloroethylene nor methanol.

EXAMPLE III The procedure of Example I was repeated except that the wash liquid used was acetone. Neither acetone or tetrachloroethylene was detected in the analysis of a sample of the sulfur cake from the sulfur melter.

While particularly advantageous embodiments of the invention have been described and illustrated, it will be recognized by those skilled in the art that various changes and modifications can be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

1. In a continuous process for extracting sulfur from sulfur-bearing ores comprising: introducing crushed ore and a water-immiscible chlorinated hydrocarbon liquid solvent selected from the group consisting of trichloroethane, tetrachloroethane, tetrachloroethylene, trichloroethylene, p-dichlorobenzene, and o-dichlorobenzene into an extraction zone, progressively moving the ore and solvent through the extraction zone for continued extraction of sulfur from said ore, continuously withdrawing hot liquor from said extraction zone, said hot liquor comprising dissolved sulfur-solvent and insoluble solid material, continuously separating said insoluble solid material from said dissolved sulfur-solvent hot liquor, lowering the temperature of the dissolved sulfur-solvent hot liquor to precipitate the sulfur in solid form, and continuously separating said precipitated sulfur from the said solvent, the improvement comprising washing said solid sulfur to remove solvent down to less than l part per billion chlorine with a wash liquid selected from the group consisting of methanol, acetone, and ethylene glycol.

2. A method according to claim 1 wherein said dissolved sulfur-solvent hot liquor is cooled to precipitate said sulfur by direct admixture with cool water at a temperature sufficient to precipitate essentially all of said sulfur present in said solvent.

3. A method according to claim 1 wherein said wash liquid is methanol.

4. A method according to claim I wherein said chlorinated hydrocarbon is tetrachloroethylene.

5. In a continuous process for extracting sulfur from :sulfur-bearing ores which comprises continuously extracting sulfur from the ore with a water-immiscible chlorinated hydrocarbon liquid solvent of the group consisting selected from trichloroethane, tetrachloroethane, tetrachloroethylene, trichloroethylene, p-dichlorobenzene, and o-dichlorobenzene, said extraction being carried out at a temperature above the melting point of sulfur and for a time suliicient to dissolve substantially all the sulfur in said ore, separating the resulting dissolved sulfur-solvent mixture from insoluble solid material, cooling the hot liquor to a sulfur crystallizing temperature by directly mixing cool water therewith, and separating the crystallized sulfur from the solvent, the improvement comprising washing the resulting sulfur cake to remove residual solvent down to less than 1 part per billion chlorine with a wash liquid selected from the group consisting of methanol, acetone, and ethylene glycol, said washing step comprising introducing the sulfur cake containing the said residual solvent into the lower portion of a wash zone, conveying said sulfur cake in a direction toward said wash liquid and in countercurrent relationship therewith and at a rate such that the wash liquid `ilows downwardly through said wash zone faster than the said sulfur cake is being moved upwardly, and discharging the solventfree sulfur cake from the upper portion of said wash zone.

6. The method according to claim 5 and further comprising discharging the wash liquid-solvent mixture from the lower portion of said wash zone and distilling said mixture to recover essentially pure solvent and wash liquid.

7. In a continuous process for extracting sulfur of high purity from sulfur-bearing ores, said process comprising the steps of: mixing particles of crushed ore with a waterimmiscible chlorinated aliphatic or aromatic hydrocarbon liquid solvent selected from the group consisting of trichloroethane, tetrachloroethane, tetrachloroethylene, trichloroethylene, p-dichlorobenzene, and o-dichlorobenzene to form a flowable slurry, passing said slurry into one end of an extraction zone maintained at a temperature above the melting point of sulfur, progressively moving the ore and solvent through the extraction zone for continued extraction of sulfur from the ore, continuously withdrawing the dissolved sulfur-solvent slurry from another end of said extraction zone, continuously separating insoluble solid material from the dissolved sulfur-solvent hot liquor, introducing said hot liquor into the upper portion of the crystallizing zone and in direct contact with an upwardly flowing stream of cool water so as to crystallize essentially all of the sulfur contained in said hot solvent, gravitationally separating and withdrawing the crystallized sulfur and solvent from the said crystallizing zone, and ltering the solid sulfur to remove said solvent, the improvement comprising countercurrently washing the solid sulfur cake to remove residual quantities of solvent down to less than 1 part per `billion chlorine with a wash liquid selected from the group consisting of methanol, acetone, and ethylene glycol, said countercurrent washing comprising the steps of introducing the sulfur cake to the lower portion of a wash zone within an inclined screw conveyor means, introducing wash liquid into the upper portion of said wash zone, rotating said screw conveyor means in a direction to move the sulfur cake upwardly through the wash zone, maintaining the speed of said screw means so that the wash liquid filters down through the wash zone faster than the sulfur cake is moved upwardly, discharging the solvent-free sulfur cake from the upper portion of said wash zone, discharging the wash liquid-solvent solution from the lower portion of said wash zone ,and passing said solvent-free sulfur into a sulfur melter to remove by vaporization any entrained wash liquid.

8. A method according to claim 7 wherein said dissolved sulfur-solvent hot liquor is separated from said insoluble solid material by passing the hot liquor to a filtration zone wherein the dissolved sulfur-solvent liquor is separated from the solid material and further cornprising continuously washing the said solid material with fresh solvent and passing a stream of hot gas through said washed solid material in said filtration zone to remove residual solvent and passing the stream of hot gas containing said residual solvent into a scrubbing zone so as to separate and recover the solvent from said hot gas.

9. A method according to claim 8 wherein at least a portion of the hot liquor recovered from the ltration step, is recycled to the premixing zone so as to maintain a ilowable slurry therein.

10. A method according to claim 7 wherein said solvent is tetrachloroethylene and said wash liquid is methanol.

11. A method according to claim 9 wherein said solvent is tetrachloroethylene and said wash liquid is methanol.

References Cited UNITED STATES PATENTS 4/ 1934 Prins 23-224X 3/ 1941 McDonald 23-312X 1/ 1944 Syers 23--312 4/ 1960 Bradley 23-312 8/ 1967 Kuster 23-312 FOREIGN PATENTS 1868 Great Britain 23-312 1902 Great Britain 23-312 1902 Great Britain 23--312 1932 Great Britain 23-312 1957 Australia 23--312 NORMAN YUDKOFF, Primary Examiner S. J. EMERY, Assistant Examiner U.S. C1. X.R.

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