Distribution of As, Hg and other trace elements in different size and density fractions of the Reshuihe high-sulfur coal, Yunnan Province, China

Abstract

A Lopingian coal from the Reshuihe Mine in southwestern China contains high pyritic sulfur (5.2%) and trace elements including As (10.8μg/g), Hg (0.81μg/g), V (98.9μg/g), Co (15.7μg/g), Cu (47.5μg/g), Se (4.8μg/g) and Th (7.2μg/g). The variation of trace elements in coal samples of different size and density were investigated. The results showed that the vertical variation of As and Hg through the seam section is the result of hydrothermal activity. Arsenic and Hg are highly concentrated in coal layers far from the parting, roof and floor in vertical section. For example, the highest concentrations of As (44.3μg/g) and Hg (2.8μg/g) occur in ply 6, which is 40-cm vertically away from partings. Elements As, Hg, Co, Ni, Se, Sb and Tl occur mainly in pyrite; Be, F, Cs, Th and U are evenly distributed in clay minerals and organic components; V and Cr mainly occur in the organic portions of the coal. The As and Hg content generally increases with greater particle size and density, with content in the light fractions as low as 1.1 and 0.14μg/g, respectively, and as high as 75.8 and 3.8μg/g, respectively, in heavy fractions. Co, Ni, Se, Sb and Tl exhibit similar density dependence. Vanadium and Cr are more concentrated in lower-density fractions.The genetic type and mode of occurrence of elements in coal both exert a great impact on their removability; epigenetic and coarse minerals may be readily liberated by gravity separation. Elements associated with pyrite (e.g., As, Hg, Co, Ni, Cu, Se, Sb and Tl) demonstrate high removability; F, Th, U, V and Cr, which mainly occur in clay minerals and/or organic components of the coal, show a relatively low removability. Trace elements in particle size fractions 6–13mm and <0.5mm are most easily removed by gravity separation. In order to fully remove toxic elements from coal, it is suggested fractions with particle sizes of 3–6mm and 0.5–3mm be reduced in size to <0.5mm prior to gravity separation.