Pyrite Oxidation Chemistry

TABLE 6.1. Concentrations of Environmentally Important Constituents (mg L 1) in Acid Mine Drainages in the United States and Canada. 

Substance Coal Mine Drainage Throughout the United States Acid Mine Drainage from Vancouver, Canada Waste Rock Seepage from Saskatoon, Canada Metal Mine Drainage from Colorado, U.S.A. Drinking Water Standard in the United States 

Source Data taken from Fyson et al., (1994) Rowley et al., (1994) and Wildeman (1991). 

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Evangelou, V. P. 1995. Pyrite Oxidation and its Control Solution Chemistry, Surface Chemistry, Acid Mine Drainage. CRC Press, Florida, 293 pp. 

Oxidation chemistry of major arsenic-bearing sulfides 3.7.4.1 Oxidation of pyrite and other Fe(II) sulfides... 

Huang, X. and V. P. Evangelou. 1994. Kinetics of pyrite oxidation and surface chemistry influences. In C. N. Alpers and D. W. Blowers, Eds, The Environmental Geochemistry of Sulfide Oxidation. American Chemical Society, Washington, DC, pp. 562-573. 

V. P. EVANGELOU, Ph.D., is Professor of Soil and Water Chemistry at the University of Kentucky. A recipient of the Marion L. and Chrystie M. Jackson Award from the Soil Science Society and the Senior Fulbright Award, he is the author of Pyrite Oxidation and Its Control. 

Some of fhe chemical processes that influence groundwater chemistry in coal-bearing strata include carbon dioxide production, silicate hydrolysis, pyrite oxidation, carbonate mineral dissolution, cation exchange, sulfate reduction, and the precipitation and dissolution of secondary minerals leading to contamination of the groundwater (Banaszak, 1980 Groenewold et al., 1981 Powell and Larson, 1985). 

Median leachate water chemistry during periods of sulfide oxidation was calculated for each parameter analyzed, by humidity cell. These data, for parameters that show distinct variation between cells, are presented as Table 1. HC2-SQFP, which contains more pyrite and very low carbonate content, rapidly generates acidic leachate causing its leachate to be higher in all parameters except Mo. 

Lowson, R. T., 1982. Aqueous oxidation of pyrite by molecular oxygen. Chemistry Review, 82(5) 461... 

Ash, as determined by the standard test method (ASTM D-3174), is the residue remaining after burning the coal and coke and differs in composition from the original inorganic constituents present in the coal. Incineration causes an expulsion of all water, the loss of carbon dioxide from carbonates, the conversion of iron pyrites into ferric oxide, and other chemical reactions. In addition, the ash, as determined by this test method, will differ in amount from ash produced in furnace operations and other firing systems because incineration conditions influence the chemistry and amount of the ash. 

Natural processes involving redox reactions are very frequent. Examples include the oxidation of aqueous ions [like Fe(II) to Fe(III)], oxidation of solids (like pyrite, FeS2 to SO2-), corrosion of metals, production of H2S by sulfate-reducing bacteria, photoredox processes in the atmosphere, water, and soil, etc. Therefore, it is important to understand the principles underlying redox chemistry and to organize them in the form of tables and diagrams such as those discussed below. 

A new approach for the chemical removal of pyritic sulfur from coal is described. The process is based on the discovery that aqueous ferric salts selectively oxidize the pyritic sulfur in coal to chemical forms which can be removed by vaporiza-tion, steam, or solvent extraction. Data for removal of the pyritic sulfur from four major coals (Lower Kittanning, Illu nois No. 5, Herrin No. 6 and Pittsburgh) are presented together with a discussion of the process chemistry. The effect of variables, such as coal particle size, acid and iron concern tration, reaction time, and temperature are discussed. The results show that near complete removal of pyritic sulfur can be obtained under mild conditions, resulting in a reduction of the total sulfur content of the coals from 40 to 80%, depending on the original pyritic sulfur content. 

An economically viable process for the chemical removal of pyrites from coal would require an oxidizing agent (most likely aqueous) which is (a) selective to pyrite, (b) regenerable, and (c) highly soluble in both oxidizing and reduced forms. Ferric sulfate and ferric chloride meet the above combination of requirements, and these reagents form the basis of the process chemistry described here. 

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