Distribution and affinity of trace elements in Samaleswari coal, Eastern India

Abstract

Coal samples from a borehole in the Samaleswari open cast coal block (S-OCB) have been collected. A chemical data set (n = 17) (proximate parameters, sulfur contents, mineral composition, trace and major element oxide concentrations) has been generated to evaluate the origin of trace elements in a vertical sequence through the stratigraphic column (similar to 300 m). The variations of volatile matter and fixed carbon indicate that the rank of the coal in the stratigraphic section studied increases from lignite (similar to 50% VMdaf) at the top to high-volatile bituminous (similar to 25% VMdaf) at the base, which reflects a more or less progressive increase in metamorphism from younger to older coal horizons. The major minerals present in the samples are quartz and kaolinite, followed by illite; the minor constituents are siderite, apatite and rutile. The mineralogical variations are primarily related to depth, with depositional environment having a significant role in the succession. In the studied borehole concentrations of the three major element oxides, SiO2, Al2O3 and Fe2O3, reflect the abundance of the minerals. The study indicates a systematic increase in the concentration of some trace elements (Se, Be, Sb) with depth. These elements have no similarity in trend to ash or pyritic sulfur but have strong association with fixed carbon (dry, ash-free). Two other trace elements, Co and Mn, more or less decrease in concentration in the older coal horizons, and have a significant association with ash. Another group of trace elements (As, Pb, Cd, Cu, Ni and Zn) has a sudden increase in concentration at a particular depth. These trace elements also show a similar vertical trend to pyritic sulfur. The work is further supported by the use of statistical tools (correlation coefficient, hierarchical cluster and linear multiple regression) which suggest several possible affinities of the trace elements such as inorganic, organic and multiple. Among the analyzed trace elements Co, Mn, Cd, Pb, As, Ni, Cu, Zn display principally an inorganic affinity, whereas Be, Se and Sb show an organic affinity. The variation in concentration of some trace elements (Be, Se, Sb) appears from the study to be a consequence of rank advance. (C) 2016 Elsevier Ltd. All rights reserved.