Coal desulfurization with aqueous chlorine
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I.
INTRODUCTION
MUCH of the coal produced in the United States contains a sulfur content greater than 2 pct, although it must be less than 0.7 pct to meet the SO: emission standards set by the Clean Air Act, which has been effective since 1976. This high-sulfur coal has a rather poor market value, which is a serious problem of the coal industries in this country. Since coal pyrite cannot always be effectively rem o v e d by physical cleaning processes and it is almost impossible to remove organic sulfur by these processes, chemical cleaning methods should be employed to remove sulfur content from the coal. Various methods of chemical desulfurization are well described in the literature.ill Among chemical desulfurization methods, chlorinolysis [E] s e e m s to be the only method that can remove organic sulfur at ambient pressure and temperatures below, 100 ~ In this method, chlorine gas and methyl chloroform are utilized to leach pyrite and organic sulfur at 74 ~ Methyl chloroform was used to solubilize bubbled chlorine gas. The coal matrix, which is chlorinated during the leaching, can be effectively dechlorinated by treating with steam at a temperature up to 500 ~ This method can remove up to 70 pct organic sulfur, 90 pct pyritic sulfur, and 76 pct total sulfur. The leachability of organic sulfur at low temperatures might be the most significant merit of the desulfurization method with aqueous chlorine. Organic sulfur is considered to be a highly refractory species to leach, and severe leaching conditions are employed to leach it. The Gravimelt process [3] utilizes a 340 ~ to 390 ~ molten bath of sodium hydroxide and potassium hydroxide to leach more than 90 pct of the total sulfur. The method of oxidation, [4] by contacting an aqueous slurry of coal with air at pressures up to 1000 psi (70 k g / c m E) and temperatures of 140 ~ to 200 ~ can remove up to 40 pct of the organic sulfur. Hsu et al. [2] proposed a reaction mechanism in which organic sulfur is leached with aqueous chlorine. First, EUNG HA CHO, Professor, is with the Mineral Processing Engineering Department, College of Mineral and Energy Resources, West Virginia University, Morgantown, WV 26506-6070. Manuscript submitted December 8, 1988. METALLURGICAL TRANSACTIONS B
each carbon sulfide and disulfide in coal reacts with chlorine as shown: R - S - R' + C1: = RSC1 + R ' C 1
[1]
RS - SR' + C12 = RSCI + R'SC1
[2]
where R and R' represent hydrocarbon groups and S stands for sulfur. The sulfenyl chloride (RSC1) so produced is oxidized to sulfonate or sulfate according to the following reactions: RSC1 + 2C12 + 3H20 = RSO3H + 5HCI RSC1 + 3C1E + 4H20 = RC1 + H2SO4 + 6HC1
[3] [4]
Pyrite is leached by aqueous chlorine at a fast rate even at room temperature. [5,61 The proposed leaching reaction is given by [5,61 FeS2 + 7.5CIE + 8H20 = F e 3+ + 2 5 0 4 2 - + 16H ยง + 15C1-
[5]
Note that 7.5 moles of chlorine are needed to leach one mole of pyrite, and that the reaction produces sulfate rather than elemental sulfur. When coal is chlorinated in an aqueous m
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