Geochemistry and mineralogy of Tongue River member coal from three Montana coal mines

Abstract

The geochemistry and mineralogy of coal from the Absaloka, Rosebud, and Decker mines located in the Powder River Basin in Montana have been characterized to determine the bulk composition of inorganic constituents in the coal; mineralogy of the coal, including the identity, morphology, composition and distribution of minerals present; occurrence and distribution of potentially hazardous trace elements; and chemical state of selected elements (N, O, S). These data were acquired using instrumental neutron activation analysis (INAA), X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and time of flight mass spectrometry (ToF-SIMS). In aggregate, these analytical techniques provide complementary information and also allow for cross-technique comparison of results. Overall the Montana coals contain mineral assemblages that are typical of coals described from across the United States and the world. Comparing Montana Powder River basin coal versus the rest of the United States coals, Montana Power River has, overall, a remarkably lower abundance of potentially hazardous elements. XRD analysis revealed kaolinite in all samples, variably present are quartz, illite, calcite, dolomite, gypsum, and pyrite. SEM/EDS imaging and analysis confirmed the occurrence of these mineral phases and the dominance of kaolinite in ash layers. Pyrite is the primary sulfide mineral that occurs in a variety of crystal forms that could affect its solubility, and therefore, potential for acid release. Pyrite occurs as a pure compound with no As nor other potentially hazardous element as part of a solid solution. However XPS analysis of sulfide rich areas reveal a concentration of Se and Co suggesting these elements are sorbed onto the pyrite surfaces. This study provided a reconnaissance overview of the geochemistry and mineralogy of these Montana coal mines. Future work could include a more detailed chemical stratigraphy of the coal-producing layers to better characterize the distribution of minerals and elemental components in the coal, and to determine the processes responsible for their occurrence and distribution. The results of this study are applicable to the future development of clean coal technologies and to address the potential environmental and health impacts of coal combustion.