Distribution of trace elements in selected pulverized coals as a function of particle size and density

Trace elements in coal have diverse modes of occurrence that will greatly i nfluence their behavior in many coal utilization processes. Mode of occurre nce is important in determining the partitioning during coal cleaning by co nventional processes, the susceptibility to oxidation upon exposure to air, as well as the changes in physical properties upon heating. In this study, three complementary methods were used to determine the concentrations and chemical states of trace elements in pulverized samples of four US coals: P ittsburgh, Illinois No. 6, Elkhorn and Hazard, and Wyodak coals. Neutron Ac tivation Analysis (NAA) was used to measure the absolute concentration of e lements in the parent coals and in the size- and density-fractionated sampl es. Chemical leaching and X-ray absorption fine structure (XAFS) spectrosco py were used to provide information on the form of occurrence of an element in the parent coals. The composition differences between size-segregated c oal samples of different density mainly reflect the large density differenc e between minerals, especially pyrite, and the organic portion of the coal. The heavy density fractions are therefore enriched in pyrite and the eleme nts associated with pyrite, as also shown by the leaching and XAFS methods. Nearly all the As is associated with pyrite in the three bituminous coals studied. The sub-bituminous coal has a very low content of pyrite and arsen ic; in this coal arsenic appears to be primarily organically associated. Se lenium is mainly associated with pyrite in the bituminous coal samples. In two bituminous coal samples, zinc is mostly in the form of ZnS or associate d with pyrite, whereas it appears to be associated with other minerals in t he other two coals. Zinc is also the only trace element studied that is sig nificantly more concentrated in the smaller (45 to 63 mu m) coal particles. (C) 2000 Elsevier Science B.V. All rights reserved.