Selenium in coal: A review

Abstract

Abstract The World average Se content in coals (coal Clarke of Se) for hard coals and brown coals are respectively 1.6 ± 0.1 and 1.0 ± 0.15 ppm. On an ash basis, these contents are greatly increased and are 9.9 ± 0.7 and 7.6 ± 0.6 ppm, respectively. Therefore, Se is a very coalphile element: it has strong affinity to coal matter — organic and (or) inorganic but is certainly authigenic. The “coal affinity” of Se is like that for Ge and S. Both organic (Seorg) and inorganic selenium (Semin) can exist in coal. In addition, Se can occur not only as a chemical-bound form, but also in sorbed (acid leachable) selenate form in the oxidized coals. The sulfidic form of Se is represented by isomorph Se in pyrite and some more rare sulfides, and the selenidic form by clausthalite PbSe. “Organic” Se may be present as both Se-organic compounds and elemental Se° dissiminated through organic matter. There are some preliminary data that cleaning of high-sulfur coals, very effective for S, is less effective for Se due to enhanced contribution of the Seorg form (vs. Sorg). There are two genetic types of the Se-accumulations in coal: “reducing” and “oxidizing”. In the first type, Se is enriched in high-sulfur coals, concentrating in sulfide phases. These accumulations are, in general, syngenetic, and may be epigenetic only if there are abundant hydrothermal sulfides of Fe, Cu, As, Pb. In coals of the second type, Se is enriched in the bed oxidation zones. Such coals are (or were) located in the areas with arid climate and enhanced Se content in water. Se concentrates in coal as a reduction or sorption geochemical barrier, probably, mostly as Se° in oxidized organic matter and partly pyrite. This type is characterized by oxidizing zonality and close paragenesis of Se with U, Fe, Mo, V, and Pb. These Se accumulations are mostly epigenetic. A study of Se in coal is evidently insufficient, and further detailed studies are needed, especially regarding Se forms (modes of occurrence).