Characteristics of the coal quality and elemental geochemistry in Permian coals from the Xinjier mine in the Huainan Coalfield, north China: Influence of terrigenous inputs

Fifty-six coals along with host rocks of Permian age (including samples from the Shanxi Formation, Lower Shihezi Formation and Upper Shihezi Formation) collected from the active Xinjier mine in the Huainan Coalfield, north China were studied in the present work. The overall object of this study was to characterize the coal quality and elucidate the possible genetic types for trace-element enrichment of Xinjier coals. Based on Chinese National Standards, all the coals can be classified as medium-high-volatile to high-volatile coal and ultra-low sulfur to low sulfur coal. The vitrinite-dominated Xinjier coals show phyiso-chemical properties difference along the coal-bearing strata. Compared to the coals of the Shanxi Formation, vitrinite contents are elevated in the coals of the Lower Shihezi and Upper Shihezi Formations, which indicates that they were probably exposed to a more reducing environment during peat accumulation. Most elements, such as Al2O3, K2O, Y, Se, and Sb, are enriched compared to those of Chinese and World coals. The major-element oxides Al2O3 and K2O as well as trace elements Th and Y increase from the lower to upper seams. Some elements are distinctly concentrated in host rocks (roof, floor, and parting) compared with adjacent coals. These geochemical anomalies and "increasing stratigraphically upward" ash yield trend are attributed to influence from terrigenous inputs. Elements in Xinjier coals were classified into three geochemical groups based on the statistical analysis. In particular, sequential extraction experiments of selected coals found that As, Se, and Sb predominantly occur as organic associations in coal.