WO1994029411A1

WO1994029411A1 - Processes for desulfurizing coal

Info

Publication number: WO1994029411A1
Application number: PCT/US1993/012510
Authority: WO
Inventors: Sunggyu Lee; Henry H. Leehe; Conrad J. Kulik; Howard E. Lebowitz
Original assignee: Sunggyu Lee; Leehe Henry H; Kulik Conrad J; Lebowitz Howard E
Priority date: 1992-12-31
Filing date: 1993-12-22
Publication date: 1994-12-22
Also published as: AU5958794A

Abstract

A method of removing organic sulfur from coal in which the coal is dispersed in perchloroethylene in the presence of a catalyst which is effective to disrupt C-S bonds and thereby liberate sulfur species from the coal. The processed coal is recovered, and the perchloroethylene is separated from the liberated sulfur species and recycled.

Description

PROCESSES FOR DESULFURIZING COAL

TECHNICAL FIELD OF THE INVENTION

The present invention relates to novel, improved processes for removing organic sulfur from such native carbonaceous materials as coals.

BACKGROUND OF THE INVENTION

Existing and threatened scarcities of sweet crude oils and the increasing cost of these natural hydrocarbons has led, and will lead, to increasing reliance on crudes with significant sul- fur contents and to other fossil fuels such as coal and oil shales as fuels and as feedstocks for refin¬ eries and petrochemical processes. Like the lower quality crudes, remaining stocks of coal typically have significant concentrations of sulfur compounds. This sulfur is converted to the highly unacceptable pollutant sulfur dioxide when sulfur-containing coals are heated to combustion temperatures and to the temperatures typically required in processes employing coal as a feedstock. The Federal Clean Air Act and other rules and regulations severely limit the release of sulfur dioxide into the surrounding environment. Two ap¬ proaches for complying with such anti-pollution requirements have been employed. The traditional approach is to scrub the sulfur dioxide from the effluent gases of the com¬ bustion or other high temperature processes. This approach has the disadvantages that: large capital investments are required, operating costs are high, and large quantities of solid wastes are generated in power plants and other applications in which large quantities of the coal are consumed. An alternative approach is to remove the sulfur from the coal before it is supplied to the process.

In the case of coal, the sulfur is gener¬ ally present in two forms — inorganic, primarily pyritic, sulfur and organic sulfur. The organic sulfur may be in the form of thiols, sulfides, disu- lfides, thiophenes, and other aliphatic and hetero- cyclic sulfur compounds. It is believed that over 1600 patents in the United States alone deal with processes for removing these sulfur compounds from native carbonaceous materials.

Inorganic sulfur can be removed from coal with various degrees of efficiency by a variety of processes. Many of these involve coking, gasification and comparable high temperature or high temperature and pressure steps (See U.S. Patents Nos. 3,909,212 issued 30 September 1975 to Schroeder for REMOVAL OF SULFUR FROM CARBONACEOUS FUELS; 3,991,557 issued 16 November 1976 to Donath for PROCESS FOR CONVERTING HIGH SULFUR COAL TO LOW SUL¬ FUR POWER PLANT FUEL; 4,118,201 issued 3 October 1978 to Yan for PRODUCTION OF LOW SULFUR FUELS FROM COAL; 4,188,191 issued 12 February 1980 to Longanbach et al. for PROCESS FOR REDUCING THE SUL- FUR CONTENT OF COAL AND COAL CHAR AND THE IGNITION

TEMPERATURE OF COAL CHAR; and 4,268,358 issued 19 May 1981 to Schuster for METHOD OF REDUCING THE SULFUR CONTENT OF COAL REDUCED TO DUST) . Other processes for removing inorganic sulfur from coal require less extreme reaction conditions and employ such techniques as froth flotation (see U.S. Patent No. 4,211,642 issued 8 July 1980 to Vojislav for BENEFICIATION OF COAL AND METALLIC AND NON-METALLIC ORES BY FROTH FLOTATION PROCESS USING POLYHYDROXY ALKYL XANTHATE DEPRESSANTS) and selective agglomera¬ tion (see U.S. patent No. 4,770,766 issued 13 Sep¬ tember 1988 to Keller, Jr. for TIME-CONTROLLED PRO- CESSES FOR AGGLOMERATING COAL) .

Removal of aliphatic organic sulfur is a more difficult problem. However, a variety of pro¬ cesses for removing the sulfur have been proposed. These include flotation (see U.S. Patent No. 4,886,522 issued 12 December 1989 to Allred et al. for PROCESS FOR THE DESULFURIZATION OF COAL; PULVER¬ IZATION, REMOVING ORGANIC SULFUR MEMBRANE SEPARA¬ TION, SEDIMENTATION, FLOTATION, DEWATERING; and 4,211,642 issued 8 July 1980 to Petrovich for BENE- FICIATION OF COAL AND METALLIC AND NON-METALLIC ORES BY FROTH FLOTATION PROCESS USING POLYHYDROXY ALKYL- XANTHATE DEPRESSANTS; bacterial attack (see U.S. patents Nos. 4,808,535 issued 28 February 1989 to Isbister for ACINETOBACTER SPECIES AND ITS USE IN REMOVING ORGANIC SULFUR COMPOUNDS; 4,659,670 issued 21 April 1987 to Stevens, Jr. et al. for BIOLOGICAL DESULFURIZATION OF COAL; and 4,562,156 issued 31 December 1985 to Isbister et al. for MUTANT MICROOR¬ GANISM AND ITS USE IN REMOVING ORGANIC SULFUR COM- POUNDS; high temperatures (see U.S. patents Nos.

4,678,478 issued July 7, 1987 to Kelland for METHOD FOR DESULFURIZATION OF COAL; 4,270,928 issued June 2, 1981 to Frischmuth for DESULFURIZATION OF CARBO- NACEOUS MATERIALS; and 4,110,200 issued 29 August 1978 to Antos for HYDROCARBON CONVERSION WITH ACIDIC SULFUR-FREE MULTIMETALLIC CATALYTIC COMPOSITE) ; treatment with a variety of agents (see U.S. patents Nos. 4,491,454 issued 1 January 1985 to Lompa-Krzy- mien for SULFUR REMOVAL FROM COAL); 4,384,536 issued May 24, 1983 to Biswas for DESULFURIZATION AND IM¬ PROVEMENT OF COMBUSTION AND GASIFICATION CHARACTER¬ ISTICS OF COALS; 4,334,888 issued June 15, 1982 to Corcoran et al. for COAL DESULFURIZATION; 4,325,707 issued April 20, 1982 to Kalvinskas et al. for COAL DESULFURIZATION BY AQUEOUS CHLORINATION; 4,297,108 issued October 27, 1981 to Harowitz et al. for DE¬ SULFURIZATION OF COAL; 4,233,034 issued November 11, 1980 to Miller et al. for DESULFURIZATION OF COAL; 4,162,898 issued July 31, 1979 to Anthoney et al. for PROCESS FOR REMOVING SULFUR FROM COAL; 4,146,367 issued March 27, 1979 to Hsu for COAL DESULFURIZA¬ TION; 4,081,250 issued March 28, 1978 to Hsu et al. for COAL DESULFURIZATION PROCESS; and 4,018,572 issued April 19, 1977 for DESULFURIZATION OF FOSSIL FUELS) . Also proposed have been processes employing ultrasound (U.S. patent No. 4,391,608 issued July 5, 1983 to Dondelewski for PROCESS FOR THE BENEFICIA- TION OF CARBONACEOUS MATERIALS WITH THE AID OF UL¬ TRASOUND) ; electromagnetic radiation (U.S. patents Nos. 4,803,061 issued 7 February 1989 to Becker et al. for PARTIAL OXIDATION PROCESS WITH MAGNETIC SEPARATION OF THE GROUND SLAG; DESULFURIZATION, DEMETALLIZATION, 4,152,120 issued 1 May 1979 to

Bleiler et al. for COAL DESULFURIZATION USING ALKALI METAL OR ALKALINE EARTH COMPOUNDS AND ELECTROMAGNET¬ IC IRRADIATION; TO FORM SULFIDES AND POLYSULFIDES OF THE ALKALI AND ALKALINE EARTH METALS; and 4,076,607 issued 28 February 1978 to Bleiler et al. for PRO¬ CESS FOR COAL DESULFURIZATION; IRRADIATION WITH ELECTROMAGNETIC MICROWAVES TO INDUCE THERMOCHEMICAL, IN SITU REACTIONS) ; oxidation (e.g., U.S. patents Nos. 4,828,686 issued 16 April 1991 to Miller et al. for CHEMICAL CONDITIONING OF FINE COAL FOR IMPROVED FLOTATION AND PYRITE REJECTION; OXIDIZING WITH PER- OXY COMPOUNDS, PEROXIDES AND SUPEROXIDES TO DEPRESS SULFUR AND ASH CONTAMINANTS; DESULFURIZATION;

4,801,438 issued 15 May 1990 to Corbeels et al. for PARTIAL OXIDATION OF SULFUR-CONTAINING SOLID CARBO¬ NACEOUS FUEL; DESULFURIZATION TO CALCIUM SULFIDE; and 4,774,021 issued 27 September 1988 to Corbeels for PARTIAL OXIDATION OF SULFUR-CONTAINING SOLID

CARBONACEOUS FUEL; DESULFURIZATION BY COPPER OXYSUL- FIDE FORMATION) ; and solvent extraction (U.S. patent No. 4,008,054 issued February 15, 1977 to Clancey et al. for PROCESS FOR MAKING LOW-SULFUR AND LOW-ASH FUELS) .

The patented processes have a number of disadvantages including: high temperatures or high temperature and pressure requirements; expensive equipment; toxic and/or expensive reagents; low efficiency; high operating costs; and long reaction times.

SUMMARY OF THE INVENTION

Disclosed and claimed herein are novel desulfurization processes which do not have the above discussed disadvantages and drawbacks of those heretofore proposed. The processes disclosed herein are employed to remove organic sulfur and, in con¬ junction with other processes such as those identi¬ fied above, to remove inorganic sulfur as well. They can be employed to remove organic sulfur from coals of different ranks.

In general, the novel desulfurization processes disclosed herein employ catalyzed, low temperature cleavage of the C-S bonds in those ali¬ phatic sulfur compounds present in the material being treated to liberate S_n and (SH)_n species.

These species are solubilized in a sol¬ vent/swelling agent which is relatively insoluble relative to the material being desulfurized and removed from the reaction zone. Thereafter, the solvent is separated from the sulfur and sulfur species and recycled.

Important advantages of the novel desul¬ furization process just described are that: it is efficient, can be conducted under mild process con- ditions, and can be employed to remove organic sul¬ fur from a wide variety of coals.

Other advantages as well as significant features and the objects of the invention will be apparent to the reader from the foregoing, the ap- pended claims, and the ensuing detailed description and discussion of the invention. DETAILED DESCRIPTION OF THE INVENTION

To expand on what was pointed out above, the present invention is a process for removing organic sulfur from coal in which the material, in particle form, is: (1) dispersed in a liquid medium which functions as a swelling agent and a solvent, and (2) then maintained in a reaction zone in the presence of a sulfur extraction catalyst which pro- motes the cleavage of aliphatic C-S bonds. This releases sulfur species from the coal to the liquid, and the sulfur can subsequently be separated from the liquid by any conventional or other technique.

The aliphatic sulfur compounds referred to in this specification are those in which sulfur atoms are attached to carbon atoms of aliphatic hydrocarbons. The C-S bonds of such sulfur com¬ pounds can be broken to liberate sulfur from the host material at mild temperatures (as low as 90-100^*C) and in relatively short — 3 to 60 minute — periods of time.

Sulfur extraction catalysts that have proven effective include FeS0₄, Fe₂(S0₄)₃, Fe₂0₃, and Fe₃0₄. Also considered suitable is ferric stearate. Although cobalt and nickel are in many ways similar to iron, compounds of the metals are not suitable for the purposes disclosed in this application. Cobalt sulfate, for example, was found by actual experimentation to be ineffective. Iron sulfates and oxides occur naturally in carbonaceous materials which can be desulfurized by the processes disclosed herein. If present in sufficient amounts, these naturally occurring com- pounds are typically particularly effective because of the patterns in which they are distributed in the host material.

Ohio No. 5 and No. 6 coals, for example, contain enough appropriately distributed magnetite (Fe₂0₃) and hematite (Fe₃0₄) that very high organic sulfur extraction efficiencies of 45-70 percent can be obtained without adding a catalyst. Aliphatic sulfur can also be removed from gob pile coal to a remarkable degree without catalyst additions because of the high concentrations of FeS0₄ and Fe₂(S0₄)₃ in those coals. The same high desulfurization rates are obtained when weathered coals are desulfurized as disclosed herein. Other candidate materials such as Austra¬ lian brown coal and lignite contain little, if any, iron. Therefore, to remove organic sulfur from materials of that character, one or more catalysts with the properties identified above is added. Moisture is removed from the candidate material before that material is desulfurized in those instances in which in situ or added iron sul- fates are relied on to promote the cleavage of the C-S bonds and the consequent liberation of the S^* and SH^* species. This is because iron sulfates are solu¬ ble in warm water and would accordingly dissolve and be lost to the process if moisture were present. For effective removal of organic sulfur compounds, it is preferred that the feedstock coal have a mois- ture content of not more than 2.0 percent with a moisture content in the range of 1.5 to 2.0 percent typically being acceptable. Major process requirements are that the solvent be brought into intimate contact with the organic sulfur and that this be done without causing crosslinking between the organic sulfur and the host material.

These goals are realized by employing a solvent which will also make the organic sulfur swell and become mobile. Mobility ensures that there is that intimate contact between the aliphatic sulfur compounds and the catalyst needed to effec¬ tively promote those reactions which liberate the S^* and SH^* species.

Other requirements of the solvent are:

(a) the maximum sulfur solubility for the solvent must coincide with the maximum activity of the catalyst in terms of temperature;

(b) the solvent must facilitate contact between the catalyst and the organic sulfur;

(c) the solvent must be chemically stable and reusable;

(d) the solvent should have a high sulfur solubility but very low solubility toward hydrocar¬ bons.

It is also a requirement of the solvent that it dissolve the liberated S^* and HS^* species very quickly. Otherwise, these species will react at active sites in the host material to produce cross-linked molecules. Once cross-linking occurs, the involved sulfur cannot be removed by the process disclosed herein.

A solvent which meets the above require¬ ments is perchloroethylene. This compound boils at approximately 120°C, which is well above the pre- ferred reaction temperature range. Therefore, loss of the solvent by boiling is not a problem.

Perchloroethylene-to-carbonaceous material ratios ranging from 1.0 to 10 parts of perchloroethylene per one part coal are useful with ratios ranging from 2.5 to 7.5 parts of perchloro¬ ethylene per part of coal being preferred. A liquid medium-to-coal ratio of 5:1 will commonly prove to be optimal. It is preferred that the perchloroethylene be preheated to the selected process temperature and then brought into contact with the material to be treated. This reduces the likelihood of unwanted polymerization reactions occurring in the coal. Before it is dispersed in the liquid medi¬ um, the carbonaceous material is comminuted if and to the extent necessary to reduce the material to particles with an appropriate top size of, typical¬ ly, the above-mentioned 35 U.S. Standard Sieve. This: ensures that the host material is swelled to the extent needed to make in situ sulfur species mobile; promotes intimate contact between the cata¬ lyst and the host material resulting in the optimal catalytic extraction of sulfur from the host materi- al; and also promotes the movement of released sul¬ fur species from the host material to the liquid material in which that material is dispersed.

In perchloroethylene, aliphatic sulfur compounds swell slightly at temperatures higher than 90°C. If the coal naturally contains an adequate amount of one or more of the catalytically active compounds identified above, the swollen, mobile aliphatic sulfur will come in contact with the ran- domly distributed, naturally occurring catalyst(s) , thereby inducing the rupture of C-C bonds (i.e., catalytic depolymerization) as shown by the follow¬ ing reactions:

Aliphat Solvent _t Swollen Aliphatic

Sulfur

R C-SH Catalyst → R- C^* + S^* (or HS^*) ,

H ά where R is an aliphatic radical and the reaction takes place primarily on the C-S bonds, thus liber¬ ating active species of aliphatic hydrocarbons as well as elemental sulfur and HS^*.

Perchloroethylene immediately dissolves the highly reactive elemental sulfur and HS^*, separating these sulfur species from the coal being treated before unwanted chemical reactions involving these chemical species can occur. The treated material is then removed— for example, by passing the disper¬ sion over an appropriate sieve or set of sieves. Then, the sulfur is removed from the liquid medium (or mother liquor) ; and the liquid medium can then be recycled. If the process is carried out correct¬ ly, the liberated sulfur and sulfur species in the solvent phase are as follows: _{4 S}' _„— s₄

6 S^* —- S₆ 8 S^* S₈

HS^* + ^*SH liquid phase , H₂S₂ liquid Phase . H₂ + S₂

"^ > S , S₆, S_g

However, if the sulfur and sulfur species are not recovered in the solvent fast enough or if the temperature in the reaction zone becomes too high, unwanted cross-linking (including the forma¬ tion of interpenetrating networks) will take place among the host material, the sulfur and sulfur spe- cies, and hydrocarbons undesirably extracted from the coal by overly prolonged exposure to the sol¬ vent. The following reactions, which result in the formation of an interpenetrating network are typi¬ cal: Coal or hydrocarbons with unsaturated bonds

(IPN: Interpenetrating Net-

This makes the temporarily extracted sul- fur go back into the coal structure, more or less permanently.

The sulfur extraction process just de¬ scribed can be carried out at ambient pressures and, as discussed above, at temperatures as low as 90^*C. Commonly preferred are reaction temperatures in the range of 115-121^*C. Temperatures in this range provide optimal reaction rates, yet allow the pro¬ cess to be carried out at ambient pressure when the preferred perchloroethylene medium is employed. It is, in any event, important that the temperature be kept constant — preferably within one degree Celsius — until the extracted sulfur is separated from the liquid medium or mother liquor. Isothermal conditions insure that the liberated labile sulfur liberated from the coal is dissolved by perchloroethylene. The solvation potential of sulfur in perchloroethylene is the maximum-most at the process conditions (normal boiling point of perchloroethylene) . If the operating temperature is reduced in any way, the sulfur solubility in per¬ chloroethylene drops drastically. Thus, perchloro- ethylene is incapable of dissolving the liberated sulfur. Under such non-isothermal conditions, the liberated labile sulfur, which cannot be dissolved by perchloroethylene, re-enters the coal matrix. On re-entering the coal organic matter, it forms an interpenetrating polymer network (IPN) , which makes this sulfur very hard to liberate and extract. Hence, to prevent such unwanted interpenetration reactions, an isothermal condition is essential during extraction and separation of coal from per- chloroethylene. Tables 1 and 2 shown the signifi¬ cant drop in organosulfur extractability when the system loses isothermality during coal-perchloro- ethylene separation.

Table 1

Coal Type Pretreated* Extracted Extn,

Org. Pyr. Sulf. Org. Pyr. Sulf. CC)

Sulf. Sulf. Sulf. Sulf. Sulf. Sulf.

Freeport (PA) 0.98 1.09 1.26 0.61 0.92 1.20 119.5

P-3-1 (PA) 1.02 2.49 0.08 0.69 2.54 0.07 119.3

N. Dakota 1.57 0.86 0.34 0.95 0.94 0.31 119.8

10 (Lignite)

Indiana 5 2.42 3.67 1.67 1.61 3.70 1.57 119.3

(Petersburg)

Note: *Coal Samples were dried at 90°C under 20 inch Hg vacuum. 15 #: All mass concentrations are shown on dry basis.

Table 2

Non-Isothermal Filtration

(Dry Basis)

5 Coal Type Untreated, Dry Basis Treated, Dry Basis

Org. Pyr. Sulf. Org. Pyr. Sulf.

Sulfur Sulfur Sulfur Sulfur Sulfur Sulfur

10 Freeport (PA) 0.98 1.09 1.26 0.93 1.10 1.17

P-3-1 (PA) 1.02 2.49 0.08 0.92 2.44 0.02

N.Dakota Lignite 1.57 0.86 0.34 1.27 0.60 0.45

Indiana 5 2.42 3.67 1.67 2.32 3.03 1.48

(Petersburg)

15

Note: All numbers are in mass percentages,

The time for which the carbonaceous mate¬ rial remains in the perchloroethylene or other liq¬ uid medium can range between 3 and 60 minutes with a period of 30 minutes perhaps most often proving to be optimal. Reaction times below the stated minimum may result in extractable organics sulfur being left in the host material. Reaction times of more than one hour are avoided due to the risk of interpene- tration reactions. These can result in the extract- ed sulfur being reabsorbed into the host material in a more-or-less permanent form by redispersion and cross-linking.

The minimum contact time required for desulfurization depends upon the type of coal, the size of the coal particles, the amount of natural catalytic ingredients, and the distribution pattern of those ingredients.

Generally:

(1) Indiana 5 coal as one example re- quires a shorter contact time than

Illinois 6 and Ohio 5/6 coals;

(2) The smaller the size of particle, the shorter the time required;

(3) The higher the sulfatic and pyritic sulfur level, the shorter the time required. Even with a 3-5 minute contact time, a substantial amount of reaction occurs. In many cases, however, fifteen minutes is the lower bound for practical operation; and 45-60 minutes is the upper limit. As is shown in Tables 3, 4, and 5, an appreciable amount of organic sulfur is removed with even a relatively short contact time of 10 minutes in treating Ohio 5/6 and Illinois 6 coals; and an almost 12% organosulfur extraction efficiency was obtained in treating Indiana 5 coal for that length of time.

Table 3

Illinois 6 Coal

Time Extraction Efficiency

5 11.38 10 33.66 15 35.84 30 29.39 45 22.94 60 19.35

Table 4 Ohio 5/6 Coal

Time Extraction Efficiency

10 30.00 20 35.00 30 45.00 60 45.00 75 45.00 Table 5 Indiana 5 Coal

Time Extraction Efficiency

5 11.38 10 33.66 15 35.84 30 29.39 4455 22.94 60 19.35

Sulfur is separated from the mother liquor formed in the course of the extraction process by crystallizing it or by distilling the solvent. Perchloroethylene boils at 121°C at ambient pres¬ sure. This makes the distillation very easy and economical. In this process, perchloroethylene is evaporated, and the sulfur residue is collected from the bottom of the distillation vessel. The distil- lation apparatus can be operated in a semi-batch mode with the sulfur-rich perchloroethylene being continuously evaporated. The sulfur residue can be collected periodically.

The use of perchloroethylene as a "sol- vent" in a coal desulfurization process is proposed in U.S. Reissue Patent No. 32,454 issued 7 July 1987 to Starbuck for PROCESS FOR THE DESULFURIZATION OF COALS. However, there is no suggestion in the Star¬ buck patent that any organic sulfur — let alone aliphatic sulfur — would be extracted in the poten¬ tial process. Nor is there any suggestion in the Starbuck patent of using an extraction catalyst to promote the removal of sulfur from the feedstocks intended to be treated by the potential process.

The following examples illustrate how the principles of the present invention can be effec- tively applied to the desulfurization of such di¬ verse carbonaceous materials as unoxidized coals, weathered (oxidized) coals, and coals recovered from gob piles.

EXAMPLE I Selected coals were ground to an appropri¬ ate size which is dependent on the distribution of catalyzing components in the coal being treated. For the runs described in the examples, a coal size of - 35 + 65 Tyler mesh was selected. Coals with high catalytic content were chosen.

In each case the coal was dried to reduce its moisture content to the level identified above. The dried coal was then contacted with perchloro- ethylene at its boiling point. A contact time of 30 minutes was employed. After extraction, the coal and perchloroethylene were separated; and the per¬ chloroethylene was distilled for reuse.

The coal was dried under mild conditions to prevent compositional and thermal degradation and to remove the residual perchloroethylene. The anal¬ yses of thus processed "high-catalytic content" raw and extracted coals are shown in Table 6. To show the difference between the organosulfur extractabil- ity of high and low catalytic content coals, Table 6 also presents data on a low catalytic content coal; viz.. North Dakota lignite. Table 6 shows the "before and after" sul¬ fur contents of the specified coals. The total sulfur can be calculated as the sum of organic sul¬ fur, pyritic sulfur and sulfatic sulfur.

Table 6 PCE Batch Extraction Results

Coal Type Untreated, Dry Basis Treated, Dry Basis

Org. Pyr. Sulf. Org. Pyr. Sulf. Sulfur Sulfur Sulfur Sulfur Sulfur Sulfur

Indiana 5 2.12 3.13 1.52 1.46 3.05 1.47 10 (Petersburg) Indiana Gob 2.98 2.00 1.85 1.31 2.36 1.96 N. Dakota 1.28 0.76 0.14 1.27 0.16 0.60

Note: The data shows the organodesulfurization stage only.

It was pointed out above and apparent from Table 6 that there are carbonaceous materials which contain extractable aliphatic sulfur plus components that can be readily converted into sulfur extraction catalysts. For example, coals containing pyrites can be oxidized under process condition simulating weathering in order to convert the pyrites into iron sulfate extraction catalysts. This is a slow pro¬ cess of oxidizing coals carried out under conditions more mild than the subsequent process conditions.

Simulated weathering is preferably effected by trea¬ ting the coal in a mixture of steam and air at tem¬ peratures below 105 degrees Celsius. Air/steam ratios ranging from 10:0.1 to 10:5 can be employed. During the artificial oxidation of coal, the pyrites or marcasites in the coal are oxidized to sulfates. This reaction is favorable at higher temperatures and occurs in the presence of moisture. Hence, in the air-steam treatment used to artificially oxidize the coal, wet steam is used to ensure the presence of moisture. Temperatures above 105"C are prefera¬ bly avoided to prevent thermal degradation of the coal.

After the coal has been artificially weat- hered, it is processed to extract aliphatic sulfur from it in the manner described above in Example I.

As will be apparent to the reader from the foregoing, there will be many instances in which an appropriate sulfur extraction catalyst does not occur naturally in the carbonaceous material in adequate amounts, if at all, and can not be produced by natural or artificial weathering or other practi¬ cal treatment of existing compounds. In an application of the character just described, the selected catalyst is incorporated into the carbonaceous material before that material is dispersed in the perchloroethylene or other liq- uid medium. This can be done by treating the carbo¬ naceous material with a hot aqueous solution of the selected catalyst or catalysts.

Representative of carbonaceous materials that do not contain naturally occurring sulfur ex- traction catalysts or precursors of such catalysts are Australian brown coal and North Dakota lignite. The following example shows how the pro¬ cess of the present invention as just described was carried out in one representative instance.

EXAMPLE II An eight percent (by weight) , aqueous, ferrous sulfate solution was prepared. A 2:1 solu¬ tion-to-coal mixture was made, using this solution and coal with a 35 U.S. Standard Sieve top size.

Pennsylvania (P-3-1) , Harworth, Silverdale (British Coals) , and North Dakota (Lignite) were some of the coal samples used in the test described in this example. The mixture was maintained at 80 degrees Celsius. After three minutes, the temperature was quickly reduced to room temperature with a stream of cold water; and the solid and the liquid phases were separated. This produced coal loaded with ferrous sulfate catalyst. Subsequently, aliphatic sulfur was ex¬ tracted from the loaded coal, using the protocol described in EXAMPLE I. Analysis of the extracted coal showed a 35 percent reduction in the aliphatic sulfur content of the pretreated coal. A control processed in the same manner without the catalyst loading pretreat- ment was unaffected by the catalytic extraction step.

A solution containing a mixture of ferrous and ferric sulfates can often be substituted to advantage for the ferrous sulfate catalyst solution just described. Any ferrous/ferric sulfate ratio equal to or greater than 1 can be employed.

In those embodiments of the invention involving impregnation of the coal with a catalytic iron sulfate solution, a catalyst concentration of 0.1 to 0.6 percent by mass in the solution can be employed with a concentration of 0.3 to 0.6 percent being preferred. A catalytic solution-to-coal ratio ranging between 1:1 and 10:1 can be used with a concentration in the range of 2.5:1 to 7.5:1 being the most effective. A contact time in the range of 5 to 45 minutes is effective.

At 80^°C, the solubility of the sulfates in water is the best. The catalyst solution has the highest diffusive power as well as good mass trans- fer capability. This enables the catalytic solution to enter these micropores of coal where the organo¬ sulfur is located. Preheating the catalytic solu¬ tion to a temperature in the range of 20 to 95°C promotes the effectiveness of the subsequent extrac- tion of organic sulfur from the coal.

Natural weathering can also be employed to promote oraganosulfur extraction. Table 7 shows that superior results are obtained in processing the same coal in a weathered as opposed to unweathered condi¬ tion. The coals were processed as described in EXAM¬ PLE I.

Table 7

Coal Type Untreated Extracted

Org Pyr Sulf Org Pyr Sulf

Illinois 6 1.56 2.54 0.00 1.17 2.53 0.03

(MOPC) t σv

Illinois 6* 1.61 2.06 0.41 1.12 1.60 0.41

(MOPC)

10 Illinois 6* 2.15 1.28 1.25 1.22 1.29 1.25

(PERC)

Indiana 5 2.35 2.52 0.14 1.68 2.56 0.14

Indiana 5* 1.82 1.27 1.83 1.21 1.28 1.88

Freeport PA 1.83 2.05 0.13 1.09 2.00 0.16

15 Freeport PA* 0.93 1.03 1.59 0.44 1.14 1.66

N. Dakota 1.28 0.76 0.14 1.00 0.74 0.12

N. Dakota* 1.30 1.01 0.47 0.78 1.03 0.41

Pennsylvania 0.75 2.84 0.00 0.51 1.95 0.00

(P-3-1)

20

Coal Type Untreated Extracted

Org Pyr Sulf Org Pyr Sulf 5

Pennsylvania* 0.50 2.73 0.02 0.28 2.61 0.06 (P-3-1)

Indiana 5 3.24 3.14 1.04 2.42 3.60 1.09 (Petersburg) 10 Indiana 5* 2.79 3.84 1.52 1.79 3.32 1.49 --J (Petersburg)

Note: # naturally weathered coal under laboratory conditions

* gravitational separation of pyrites was not done in order to 15 demonstrate the organic sulfur removal.

org = organic sulfur

Pyr = pyritic sulfur

Sulf = sulfatic sulfur

20

The catalyst can also be provided to im¬ prove extractability by impregnating a low sulfatic content coal with a mineral matter-rich coal such as Indiana gob. Gob is the residue left after washing the original coal. This coal (gob) is very high in catalytic content.

As can be seen from Table 8, the organo¬ sulfur extraction of a low catalytic content coal (North Dakota lignite) was significantly improved by impregnating it with a coal of high mineral matter content. In this example, the lignite is a low catalytic content coal with a organosulfur extract¬ ability of barely 10%. After addition of gob pile coal in a proportion of 50:50 to the lignite,, the organosulfur extractability of the lignite went up to nearly 30%. For the other coals identified in EXAMPLE I, extraction efficiencies of 70-80 percent were achieved.

Table 8 Cobeneficiation

Mixture Type: Indiana Gob and North Dakota Lignite

22Sample Untreated, Extracted, Dry Basis (%)

Organic S Suullffatic Pyritic Organic Sulfatic Pyritic Sulfur Sulf Sulfur Sulfur Sulfur Sulfur

M-101 1.49 0.47 0.75 1.37 0.45 0.66

M-102 2.01 0.84 0.93 1.79 0.87 0.86

10 M-103 2.31 1.20 1.25 1.57 1.24 0.97

M-104 3.19 1.74 1.60 1.93 1.82 0.87

M-105 2.98 1.85 2.00 1.31 1.96 2.36

M-106 1.28 0.14 0.76 1.27 0.16 0.60

15

Catalytic sulfur extraction is carried out in those applications of the invention discussed above at a temperature at least slightly below the boiling point of the liquid medium in which the coal is dispersed (121^*C for the preferred perchloro¬ ethylene) . This is not required, however; and above boiling point temperatures can be employed in cir¬ cumstances where such higher temperatures will in¬ crease extraction efficiency or otherwise optimize the process to an extent justifying the additional cost of maintaining the reaction under the slight above-atmospheric pressure needed to keep the liquid medium from boiling off.

Temperatures as high as 132"C. can be employed with higher temperatures being avoided because of the above discussed likelihood that this will result in thermal degradation of the coal. Also, shorter residence times (3 to 45 minutes de¬ pending upon the type of coal) are employed to mini- mize the risk of unwanted crosslinking reactions.

Subsequent unit operations such as separation of the extracted sulfur may be carried out at lower temper¬ atures, but not lower than 115"C.

Coal can in many cases be magnetite-treat- ed to advantage before the extraction of organic sulfur. In this pretreatment, the coal is washed with magnetite to remove its mineral matter content. In particular, a magnetite slurry is made with water as the carrier; and the coal is subjected to heavy- medium classification using this slurry as a sink- float or gravity separation medium. The magnetite in the slurry helps the mineral matter sink to the bottom. Owing to its lighter specific gravity, clean coal floats at the top. Clean coal extracted with perchloroethylene gives a much higher organo¬ sulfur extraction than the unwashed coal. As men¬ tioned earlier, the organosulfur tends to crosslink and polymerize in the coal matrix. This makes its extraction more difficult. A coal which is subject¬ ed to magnetite washing; tends to extract more or¬ ganosulfur. Thus magnetite washing depolymerizes the organosulfur in coal for easy removal. The magnetite treatment is more effective when it is carried out after the removal of other mineral matter from the coal.

The invention may be embodied in many forms without departing from the spirit or essential characteristics of the invention. The present em¬ bodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the ap¬ pended claims rather than by the foregoing descrip- tion; and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.

Claims

CLAIMSWhat is claimed is:

1. A method of desulfurizing coal, said method comprising the steps of: dispersing the material in perchloro¬ ethylene and maintaining the dispersion in a reac¬ tion environment having therein a sulfur extraction catalyst which is effective to promote the reactions

R C-SH → R C- + S or HS i i for a time sufficient to produce a disruption of C-S bonds and a consequent liberation of sulfur species from the coal.

2. A method as defined in claim 1 which is carried out at a temperature of at least 90°C.

3. A method as defined in claim 1 which is carried out at a temperature in the range of 115-

121°C.

4. A method as defined in claim 1 in which the catalyst is an in situ constituent of the coal.

5. A method as defined in claim 1 in which the catalyst is selected from the group con¬ sisting of FeS0₄, Fe₂(S0₄)₃, Fe₃0₄, Fe₂0₅, and Fe(C₁₇H₃₅COO)₃.

6. A method as defined in claim 1 in which the sulfur is present in aliphatic sulfur compounds.

7. A method as defined in claim 1 in which the ratio of perchloroethylene to coal in the dispersion is in the range of from 1:1 to 10:1.

8. A method as defined in claim 1 in which the coal is comminuted as necessary to reduce it to a top size of 35 U.S. Standard sieve before the coal dispersed in the perchloroethylene.

9. A method as defined in claim 1 in which contact between the coal and the perchloro¬ ethylene is limited to a period of not more than one hour.

10. A method as defined in claim 1 in which contact between the coal and the perchloro¬ ethylene is limited to a period of about 15 to 45 minutes.

11. A method as defined in claim 1 in which the perchloroethylene is preheated to the process temperature before the coal is dispersed in it.

12. A method as defined in claim 1 which is carried out under conditions which are effective to inhibit cross-linking among hydrocarbons, sulfur species extracted from the coal, and organic moi¬ eties of said coal.

13. A method as defined in claim 1 which the reaction zone is maintained at a temperature of not more than 132°C and said zone is maintained under a pressure which is high enough to keep the perchloroethylene from boiling.

14. A method as defined in claim 1 in which the coal is a naturally weathered coal.

15. A method as defined in claim 1 which includes the step of so mixing a gob pile coal with the coal being treated prior to the dispersion of the coal in the perchloroethylene as to increase the concentration of desulfurization catalyst in the dispersion.

16. A method of desulfurizing coal, said method comprising the steps of: dispersing the coal in perchloroethylene, providing a reaction zone; Supplying to the reaction zone the dis¬ persed coal, the perchloroethylene in which the coal is dispersed, and a catalyst which did not occur naturally in the coal and which is effective to promote the reactions

R C-SH „ R C- + S^* or HS^*; and k A maintaining the dispersion and desulfur¬ ization catalyst in the reaction zone under condi¬ tions and for a period of time sufficient to produce a disruption of C-S bonds and a consequent libera¬ tion of sulfur species from the coal.

17. A method as defined in claim 16 in which the catalyst is incorporated in the coal by: immersing the coal in a preheated aqueous solution of the catalyst, reducing the temperature of the coal and catalyst solution, and removing the coal from the solution.

18. A method as defined in claim 17 in which the catalyst solution is preheated to a tem¬ perature in the range of 20 to 95°C.

19. A method as defined in claim 17 in which the coal is kept in contact with the impreg¬ nating catalytic solution for a period in the range of 5 to 45 minutes.

20. A method as defined in claim 16 in which the catalyst solution contains a mixture of ferrous and ferric sulfates in a concentration rang¬ ing between 0.1 and 0.6 mass percent and the ratio of ferrous to ferric sulfates is equal to or greater than one.

21. A method as defined in claim 20 in which the concentration of sulfates in the catalytic solution is in the range of 0.3 to 0.6 percent.

22. A method as defined in claim 17 in which the catalyst is ferrous sulfate in a concen- tration of ca. 8 weight percent.

23. A method of desulfurizing coal, said method comprising the steps of: removing moisture from the coal as and if necessary to reduce the moisture content of the coal to a level of not more than 2.0 percent; dispersing the coal in perchloroethylene; and keeping the coal in the perchloroethylene for a time sufficient to produce a disruption of C-S bonds and a consequent liberation of sulfur species from coal.

24. A method as defined in claim 23 in which moisture is removed from the coal at a temper¬ ature which does not exceed that maintained in the reaction zone.

25. A method of desulfurizing coal, said method comprising the steps of: subjecting the coal to heavy medium clas¬ sification, thereafter dispersing the coal in per¬ chloroethylene; and maintaining the dispersion in a reaction environment having therein a sulfur extraction cata- lyst which is effective to promote the reactions

R C -SH „ R C - + S^* or HS^*

H H for a time sufficient to produce a disruption of C-S bonds and a consequent liberation of sulfur species from the carbonaceous material.

26. A method as defined in claim 25 in which: the classification medium is an aqueous slurry of magnetite.

27. In a method of desulfurizing a coal which contains both pyrite and organic sulfur and comprising the steps of: dispersing the coal in perchloroethylene; and maintaining the dispersion in a reaction environment having therein a sulfur extraction cata¬ lyst which is effective to promote the reactions R C-SH „ R C- + S^* or HS^*

for a time sufficient to produce a disruption of C-S bonds and a consequent liberation of sulfur species from the coal: the step of, prior to dispersing it in the perchloroethylene, subjecting the coal to conditions which are effective to convert pyrite in the coal to iron sulfate(s) and thereby increase the concentra¬ tion of sulfur extraction catalyst in the coal prior to the removal of sulfur therefrom.

28. A method as defined in claim 27 in which the pyrite containing coal is subjected to oxidizing conditions by treating it with a mixture of steam and air at a temperature of not more than 105^*C.

29. A method as defined in claim 28 in which the air/steam ratio in the mixture is in the range of 10:01 to 10:5.