Inorganic constituents in American lignites

Abstract

Both the discrete mineral phases and the ion-exchangeable inorganic constituents of lignites from Texas, Montana, and North Dakota have been studied. The ion-exchangeable cations and the carboxyl groups with which they are associated were characterized by ion exchange methods utilizing ammonium acetate and barium acetate respectively. Na, K, Mg, Ca, Sr, and Ba were found to be present in all three coals, but significant variations in the relative and absolute concentrations of all the cations were observed. It was found that Ca and Mg were the most abundant cations and that 40–60% of the carboxyl groups in the raw coals were exchanged with cations. The discrete mineral phases in these lignites were studied by semi-quantitative X-ray diffraction and infrared spectroscopy. The importance of the cations in this analysis was shown when the mineralogical analyses of the low-temperature ash (LTA) of the coals with the cations removed and the raw coals were compared. Results show that up to 50% of the LTA of these raw coals can be attributed to the existence of metal cations and that fixation of sulphur, carbon, and oxygen to form carbonates and sulphates is the major reason for this contribution.