The association of major, minor and trace inorganic elements with lignites. I. Experimental approach and study of a North Dakota lignite

Lignites resemble peats, the precursors of coals, in containing many carboxylic acid and other functional groups. Consequently much of the relatively small amount of inorganic matter in lignites is present as cations in carboxylates and in chelated coordination complexes, and not only as distinct mineral phases. Consequently the distribution of inorganic matter in lignites will be influenced by the structure of the organic matter, as well as by microbial processes in peats and the geochemical processes involving erosion of rocks and transport of mineral grains and cations in solution. The objective of this study was to seek information on the distribution of major, minor and trace elements in different forms of combination, and in particular to document organic/inorganic interactions in coal formation. Study of the first of five lignites is reported here. The coal (from the Hagel seam in North Dakota) was separated into five fractions by float/ sink methods, and the fractions were further separated into an ammonium acetate extract, an HCl extract and an insoluble residue. Analysis of the fractions (by atomic absorption, plasma arc emission, emission spectroscopy and neutron activation) was found to give much information on how elements were combined in the coals. Results of the fractionation indicate that Ca, Mg, Na, K, Sr, Ba and Mn were present largely or partly in ion-exchangeable form; appreciable amounts of K (illite), Ba (sulfate, carbonate) and Mn were also present in mineral phases. Some Al appeared to be present in organic association. Ti often occurs in sediments by substitution in clays, but we infer that substantial amounts are present here in both acidsoluble and acid-insoluble organic chelates. The considerable enrichment of a number of elements in the fractions of lowest specific gravity suggests that Be, Sc, Cr, Y, Yb, V, Ni, Cu and Zn are associated primarily or partly with the organic matter. The extent to which these elements are associated with the organic matter in this lignite is much greater than it is with the bituminous coals studied by others.