Characterization of common lignite, xylitic lignite and pyropissite varieties of low-rank coals

Abstract

The chemical characteristics of the organic and mineral constituents of common lignite, xylitic lignite and pyropissite varieties of Spanish low-rank coals were studied. Chemical analyses showed distinct differences in the H and O contents of xylitic lignite and pyropissite: the former is particularly rich in O and poor in H, whereas the reverse is true for the latter. FT-i.r. spectroscopy showed that the organic matter of xylitic lignite contains abundant hydroxyl and ether groups whereas pyropissite is rich in acid/ester and methylene groups. Mineral matter was characterized by X-ray diffraction and FT-i.r. spectroscopy of low-temperature ash. The main minerals found were kaolinite and quartz; illite was detected only in pyropissite. Ca is present in form of carboxylate salts which react during low-temperature ashing with sulfur and nitrogen oxides, leading to formation of sulfate and nitrate artefacts; these species produced the strongest peaks in the X-ray diffractograms and FT-i.r. spectra of all varieties. Aluminium, relatively abundant in xylitic lignite, is probably organic in origin. In addition to nine major and minor inorganic elements, 13 trace elements were analysed by X-ray fluorescence spectrometry. These trace elements do not exhibit a specific association with any of the lignite varieties and their concentrations cannot be used as distinguishing characteristics. In contrast, the chemical characteristics of the organic component allow the three coal varieties to be differentiated clearly.