HOW DO LITHOPHILE ELEMENTS OCCUR IN ORGANIC ASSOCIATION IN BITUMINOUS COALS?

Abstract

In some bituminous coals, typically of high-volatile bituminous rank, certain lithophile trace elements (e.g. Ti, V, Cr, Zr, etc.) often have surprisingly high organic affinities, defined as the fraction of the element reporting to the predominantly organic product (float fraction) in coal-cleaning tests. Examination of these elements in float fractions using XAFS spectroscopy reveals a spectral signature that is incompatible with the mineralogical occurrences of the element that are typically observed for the element in tailings fractions. The spectral signatures for Cr and Zr in float fractions appear similar to those obtained from precipitates from aqueous solution that have been heated at relatively low-temperatures. Such precipitates tend to be poorly crystallized oxides or hydroxides and to have small particle sizes, typically in the 2–10 nm range. However, the corresponding XANES signature for Ti appears more like that from organo-Ti4+ complexes. Further, it has been observed that the XANES spectra for Cr in a float fraction and in the corresponding low-temperature ash (LTA) are not significantly different, whereas the XANES spectra for Ti in the float fraction and the LTA are different as a result of ashing. We interpret these observations as indicating that these lithophile elements are at different stages in the transition from carboxyl-bound ions to small particle oxide or hydroxide minerals, as a consequence of maceral ion-exchange sites that have undergone decarboxylation during coalification. Closed or isolated porosity of the macerals then prevents significant migration of the liberated inorganic species and their incorporation into larger mineral grains, leaving them as basically poorly crystalline nano-sized oxides or hydroxides.