Removal of pyritic sulfur from coal using solutions containing ferric ions
United States Patent 3917465

Finely divided coal or solid coal derivatives containing pyrite are reacted with a ferric ion solution; FeCl3 is particularly suitable. The ferric ion is reduced to ferrous ion and free sulfur is formed. The solution is then filtered from the coal which is then washed and heat dried under low pressure. Most of the free sulfur is volatized from the coal due to the heat drying; additional free sulfur can be removed by additional washing and heat drying and/or solvent extraction techniques. At least 60% of the pyrite sulfur and pyrite iron is removed using the process of this invention. If desired, the ferrous chloride can be regenerated; this permits iron oxide to be recovered as a byproduct.

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International Classes:
C10L9/02; (IPC1-7): C10L5/24; C10L9/00
Field of Search:
44/1R,1B,4-6,2 208
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Primary Examiner:
Dees, Carl F.
Attorney, Agent or Firm:
Krawitz, Willie Anderson Daniel Akers Alan T. D.
Parent Case Data:

This application is a divisional application of application Ser. No. 163,893 filed July 17, 1971, now U.S. Pat. No. 3,768,988 which is a continuation-in-part of application Ser. No. 116,262 filed Feb. 17, 1971 and now abandoned.
What is claimed is

1. An apparatus for reducing the pyritic sulfur content of coal comprising in combination:


This invention relates to the removal of pyritic sulfur from coal and solid coal derivatives and more specifically to the solvent extraction of sulfur from pyrites in coal using a solution containing a ferric ion.

The present use of coal in the United States is primarily for the purpose of conversion into electrical energy and thermal generating plants. One of the principal drawbacks in the use of United States mined coal is due to their high sulfur contents which can range up to 5%.

Based on a 4% sulfur content, a one million kilowatt plant burns about 8500 tons per day of coal and consequently emits 6 tons per day of sulfur dioxide. If this sulfur could be removed and converted, it would produce 900 tons of H2 SO4 daily.

It has long been recognized that SO2 in the atmosphere will either retard growth or kill vegetation. In addition, the potential hazard to humans appears about the same as for the vegetable kingdom.

While it is possible to remove pyritic sulfur from coal by froth flotation or washing processes; these are not selective so that a large portion of the coal is discarded along with ash and pyrite. Hence, the solution so far has been to simply burn coal having a low sulfur content. However, many pollution control districts now prohibit the use of coal having an excess of 1% sulfur. The result has been to severely restrict the use of many United States coals, 90% of which average about 2.5% contained sulfur. This has lead to the importation of low sulfur content fuel oils for domestic and industrial use. The crude oil reserves, which are the sources of the residue, are expected to run out in 20 - 30 years while coal reserves are sufficient for several hundred years at a minimum.

It is, therefore, an object of this invention to provide a process for the reduction of sulfur, particularly pyritic sulfur in coal.

Another object is to provide a process for the recovery from coal of sulfur and sulfur compounds.

Another object is to provide a process for the recovery of iron values from coal containing pyrite.

Other objects of this invention will become apparent from the description and the diagrams to follow.

According to the invention, it has been found that it is possible to react the pyrite contained in the coal with a solution containing an effective amount of ferric ion to obtain a high yield of free sulfur. Fe+3 ion particularly as FeCl3 is preferred; other ferric salts such as acetate, nitrate, sulfate, citrate, oxide, ferrous ammonium sulfate, etc., may be employed. A typical reaction proceeds as follows:

2 FeCl3 + FeS2 ➝ 3FeCl2 + 2S. The solution containing some free sulfur, ferrous chloride and any unconsumed ferric chloride is removed from the coal by filtration.

The coal is then washed and dried, preferably by heating in a vacuum; this results in the major portion of free sulfur being volatized as follows: S.Coal ➝ S + coal. If desired, a further wash, filtration and heating will remove more of the sulfur and any residual ferrous ion. One or more extractions with a suitable organic sulfur solvent such as benzene, kerosene or para cresol may be employed to further reduce the sulfur content of the coal.

Regeneration of the unused ferric chloride and ferrous chloride solution may be accomplished by first concentrating the solution by evaporating most of the water. The concentrated solution is cooled, thereby precipitating the ferrous chloride from the ferric chloride, most of the latter still remaining in solution. The precipitated ferrous chloride is air oxidized to ferric chloride and iron oxide and finally the ferric chloride is recycled and the iron oxide recovered.

Typical treatment temperatures may vary from 50° - 110°C. Reflux times are typically 1/2 - 2 hours and higher. Typical cool particle sizes may vary from -200 mesh to 1/2-inch pieces. Atmospheric pressure may be employed, but higher pressures can also be used.

The effective amount of the ferric ion solution employed for extraction depends on the amount of coal treated and its pyritic sulfur content, the amount of sulfur desired to be extracted, extraction times, extraction temperatures, concentration of the ferric ion in the solution, etc.

The reaction of ferric chloride and ferrous persulfide to produce free sulfur is known. However, it was unexpected that the reaction with ferric ion (e.g., FeCl3) and pyrite could be carried out in a coal medium since pyrite is dispersed very finely throughout the coal matrix, and penetration of such an organic matrix with water is known to be difficult. Furthermore, the volatization of sulfur from coal is unusual since it well might be expected that the free sulfur would recombine either with iron or with the coal upon heating. It is also well known that iron pyrites may be oxidatively dissolved from the coal matrix with strong aqueous oxidizing agents such as NHO3, H2 O2 or HOCl. This will convert the sulfur content to sulfate, but not to free sulfur. This is the basis for chemical analysis of the pyritic sulfur content of coal; however, such strong oxidizing agents also extensively oxidize the organic coal matrix. By contrast, ferric salts are almost totally selective in the sense that the organic coal matrix is undisturbed. Hence, ferric salts, but not HNO3, H2 O2 or HOCl, provide an economical route to the removal of pyrites from coal.

Coals which may be employed in this invention include those which are considered as coals in the popular or commercial sense, such as anthracites, charcoal, coke, bituminous coals, lignites, etc. In addition, chars from hydrocracked coals and middlings are all capable of being refined by the extraction process of this invention.

The invention will be understood by reference to FIGS. 1 and 2 in which ferric chloride make-up solution and coal are fed into a pyrite reactor 10 maintained at atmospheric pressure and about 212°F. Pyrite (FeS2) is extracted from the coal, and the slurry containing unreacted ferric chloride, ferrous chloride, sulfur, ferrous persulfide, and the treated coal are fed to a coal filtration unit 11. Vacuum disk filters in the coal filtration unit are used to separate the bulk of the iron chloride solution from the treated coal.

In the coal washing sections 12, 13, 14, and 15, four stages of countercurrent washing with intermediate filtration steps are used to reduce the residual chloride content of the coal to less than about 100 ppm. A suitable residence time of the coal in each of the washing stages is about 15 minutes; rotary vacuum disk filters are used to separate the coal and wash the solution between washing stages.

The washed coal is then fed to a coal drying unit 16 where rotary steam tube dryers are employed to remove the residual water from the washed coal, this operation being carried out at atmospheric pressure and about 212°F. The heated dry coal is then forwarded to a sulfur vaporization unit 17 where free sulfur, which was produced in the extraction reaction in reactor 10, is vaporized at atmospheric pressure and a temperature of about 450°F or under reduced pressure (30 min.) and at 250° - 350°F. The vaporized sulfur is removed by nitrogen gas into a sulfur condensation unit 18 and cooled to about 225°F causing it to condense. The sulfur vapor is then passed to a recovery unit as bright sulfur. The treated coal was reduced pyrite content is then forwarded for use.

In the ferric chloride regeneration stage, the filtrate from the coal filtration unit 11 is passed to a thickener unit 10 where water is vaporized from the solution at atmospheric pressure and about 212°F. The concentrated solution is then passed to a precipitation unit 20 where ferrous chloride is precipitated by cooling the solution to 155°F at atmospheric pressure. Unreacted ferric chloride solution from the precipitation unit 20 is heated in a reheater 21 and then combined with ferric chloride make-up for feeding to the reactor 10.

The ferrous chloride precipitate, which has been separated by filtration from the ferric chloride solution in reactor 20, is transferred to an air oxidation furnace 22 where it is reoxidized back to ferric chloride and iron oxide, the reaction equation being FeCl2 + 3/2 O2 ➝ 4FeCl3 + Fe2 O3. The reaction employs air and is carried out at atmospheric pressure at about 480°F.

The oxidized precipitate (ferric chloride and iron oxide) is then transferred to a solution-filtration unit 23 where the soluble ferric chloride is then separated from the insoluble iron oxide by dissolving in water. The ferric chloride solution is recycled to the ferric chloride make-up solution for use in reactor 10. The iron oxide is filtered from the ferric chloride solution and may be recovered as a byproduct of the process.

Typical coals which may be employed in the process include Missour, Lower Freeport, Bevier, Indiana No. V, and Pittsburgh. These coals contain sulfur forms as shown in Table 1 when freshly mined. As they stand exposed to air, small amounts of sulfate sulfur are formed from the pyrite content.

TABLE 1 __________________________________________________________________________ SULFUR COMPOUNDS IN COAL Lower Bevier Freeport Indiana No. V Pittsburgh __________________________________________________________________________ Pyritic S % 1.7 - 2.3 2.2 - 3.8 1.5 - 1.8 0.5 - 1.7 Organic S % 1.7 - 2.3 0.4 - 0.8 1.5 - 1.8 0.5 - 0.7 Total S % 3.5 - 4.5 3.0 - 4.2 3.0 - 3.5 1.2 - 2.2 __________________________________________________________________________

Table 2 shows the original pyritic sulfur content of the Missouri and Lower Freeport coals and the reduction in sulfur content due to treatment of FeCl3.

It will be observed that a marked reduction in pyritic sulfur occurs after only a single treatment with FeCl3 followed by a water washing and drying.

Table 3 shows the effect of employing an organic solvent to remove the free sulfur which remains following the FeCl3 and water washing treatment.

TABLE 2 __________________________________________________________________________ FeCl3 Extraction Data Wt Loss Reflux After Sam- Wt Mol- (90°C) Washing Sulfur1 Pyritic ple Coal Vol. arity 2Fe+3 Time & Drying Eschka Wt % Sulfur Coal No. Grms FeCl3 FeCl3 Pyritic Fe (hrs.) Wt %5 Wt % in Coal2 Removed __________________________________________________________________________ Missouri Untreated -- -- -- -- -- 4.75 1.65 -- (Mesh Size -200) Missouri 1 40 200 0.5 3.9/1 16 -- 0.22 -- (Mesh Size -200) 0.17 Missouri 2 50 500 0.3 5/1 2 -9.1 4.18 0.86 -31% (Mesh Size -200) 4.17 0.84 Missouri 3 30 500 0.5 13/1 20 +2.0 3.67 0.19 -62% (Mesh Size -200) 3.65 0.19 Lower Freeport U. -- -- -- -- -- 3.54 3.16 -- (Mesh Size -14) 3.99 3.52 Lower Freeport 4 50 730 0.5 6/1 23 -14.14 1.99 1.10 -70% (Mesh Size -14) __________________________________________________________________________ 1 ASTM D271 2 Bureau of Mines procedure and Standard Methods of Chemical Analysis, Furman, Volume 1, page 542. 3 Remarks: Residue removed from condenser was analyzed by electron microprobe as follows: Fe, S, Si, O, C were major constituents; Ca, Cl, A were trace. 4 Yellow crystals formed; Hg spot test for free S was positive. 5 Sample 2 was washed once with 250 cc hot water and dried 24 hours in a 90°C vacuum oven. Samples 3 and 4 were washed twice with 250 cc hot water and dried 72 hours at 90°C in a vacuum oven.

TABLE 3 __________________________________________________________________________ SOLVENT EXTRACTION DATA FOLLOWING FeCl3 TREATMENT AND WATER WASH Wt. Sulfur Concentration Sample Extraction Loss, % Extrac- No. Solvent Procedure Gain Eschka tion __________________________________________________________________________ 1 Benzene 5-10 3 hot (80°C) -- 3.22 84% min. 100 cc washings 3.21 2 Benzene 5-10 3 hot (80°C) 0 3.62 60% min. 100 cc washings 3.61 3a Benzene 5-10 3 hot (80°C) +2 3.06 89% min. 100 cc washings 3.03 3b p-Cresol 5-10 1/2 hr (200°C) -13.6% 2.81 110% min. reflux 2.18 4 Benzene 5-10 3 hot (80°C) -- 2.23 74% min. 100 cc washings 1.77 __________________________________________________________________________

From the data in Table 3, it appears that the organic solvent treatment causes a major portion of the pyritic sulfur to be extracted; also, use of para cresol appears to result in extraction of organic as well as pyritic bound sulfur. While the efficiencies shown range from 60% to at least 89%, this efficiency range can be changed by altering such factors as wash times, particle size, amounts and concentrations of FeCl3 and solvents, ferric salt treatment, reflux temperature, etc.

Table 4 shows the effect of FeCl3 extraction on various coals employing reaction conditions similar to Table 2. The table shows that 72 - 93% of the pyritic sulfur content may be removed in two hours by 0.5 M FeCl3 solution from a wide variety of coals. Further, the process was applicable to all the coals, and in the case of Indiana No. V, the extraction efficiency was excellent.

TABLE 4 ______________________________________ PYRITIC SULFUR REMOVAL DATA Pyritic Total Total Total Sulfur Sulfur Sulfur Sulfur Coala Removed Removed Before After % % % % ______________________________________ Lower Freeport 48 75 3.87 2.01 Lower Freeport 64 72 3.40 1.23 Bevier 36 72 4.60 2.94 Indiana No. V 51 93 3.28 1.67 Pittsburgh 39 78 1.81 1.10 ______________________________________ a All coals were -14 mesh except Bevier which was -200 mesh

The process of this invention is extremely efficient in that at least 60% of the pyrite sulfur is extracted and the iron employed for extraction is easily recovered (about 85-90%) and may be reused. Furthermore, iron removal is facilitated since the iron contained in the FeCl3 extraction solution and the iron in the pyrite are indistinguishable; hence, no special techniques are required to separate different metals from the wash-extraction operation if metal recycling is desired.

In addition, the process is simple in that no high temperatures, pressures or catalysts are required.

Finally, the extraction with FeCl3 does not produce an interaction with the organic coal matrix; this permits substantially all of the coal to be utilized as low sulfur fuel.