Saikia, and 9 co-authors, reported in 2015 that in order to address the scarcity of information on the nature and mode of occurrence of minerals and elements in coal, coal mine overburden and coal ashes from North-east India, and also the relations between coal mineralogy and ash chemistry, the petrology, mineralogy and nano-mineralogy, and ash chemistry of some industrially important high-sulfur coals, mine overburden materials and fly ash samples from Assam (India) have been evaluated. A combined approach, using petrography, low-temperature ashing plus X-ray diffraction (LTA-XRD), field emission scanning electron microscopy (FE-SEM), high resolution transmission electron microscope (HRTEM),
Raman spectroscopy, and inductively coupled plasma mass spectrometry (ICP-MS), has provided new information on the mineralogy and nano-mineralogy of these sub bituminous coals and associated mine overburden. The presence of rare earth elements and yttrium (REY) in Northeast Indian coal, mine overburden and fly ash samples is also reported for the first time. The identification of these components may be significant from an economic point of view. Some Northeast Indian fly ashes, with REE oxides up to 1580 ppm on an ash basis, might possibly represent sources for recovery of rare earth elements.
The Makum coals are vitrinite-rich and of high volatile bituminous rank,with medium to high total sulfur and most of the sulfur being organically bound. The crystalline mineral matter in the coals consists mainly of quartz, kaolinite, and illite, with minor proportions of pyrite and pyrite oxidation products such as jarosite. Similar mineral assemblages occur in the overburden sedimentary rocks, suggesting that the minerals in the coal are mainly detrital in origin. Although only representing a small overall proportion, the organic matter in the overburden rocks is also vitrinite-rich; however, it contains a higher proportion of liptinite macerals, on a mineral-free basis, than the mineral-free organic matter of the coal samples.
In addition to these silicate and sulphide minerals, the low-temperature oxygen-plasma ash (LTA) of the coals contains small but significant proportions of gypsum and bassanite. Gypsum is also present in the overburden materials. Although some of the bassanite, and possibly also some of the gypsum, in the coal LTAs may have been derived from interaction of non-mineral Ca in the coal with organic sulfur during ashing, the occurrence of gypsum also in the overburden strata suggests that crystalline gypsum also occurs in the coal themselves, precipitated perhaps from dissolved Ca and SO4 in the pore water in mine exposures, or during sample air-drying.
Comparison of XRD and chemical analysis data suggests that a significant proportion of the Fe also occurs in the coal in a non-mineral form. This may include Fe in solution in the pore water, amorphous Fe oxides or hydroxides within the mineral matter, or Fe related in some way (e.g. by adsorption) to the organic matter. The presence of sabeite (NH4Fe(SO4)2) in the LTA of one of the coal samples may reflect interaction of such Fe with organic nitrogen and sulfur during the plasma-ashing process. Although pyrite itself was not identified in the mine overburden samples, significant proportions of jarosite and, in one case, melanterite may indicate that any pyrite in those materials has oxidized with exposure to the atmosphere, either at the mine site or during sample storage.
These phases suggest that pyrite oxidation has occurred or is occurring in the strata, and may contribute to the formation of acid mine drainage in the region. Small proportions of many other minerals were identified the coal, overburden and fly-ash samples by electron beam methods, including anatase, anhydrite, barite, calcite, galena, goethite, gypsum, hematite, illite, jarosite, kaolinite, magnetite, melanterite, mullite, pyrite, pyrophyllite, pyrrhotite, quartz, rutile, schwertmannite, siderite, and sphalerite. Some minerals, e.g. calcite, goethite, gypsum, hematite, jarosite, kaolinite, magnetite, pyrite, rutile, schwertmannite, and siderite, are present as nanoparticles in many different forms, often with associated trace element components.
The concentrations of most trace elements in the coals are relatively low; however, higher concentrations are present in the overburden and fly ash samples. The coals contain significant amounts of V, Cr, Ni, Zn, Sr, Zr, and Ba,with lesser amounts of Li, Sc, Co, Cu, Ga, Ge, Rb, Pb, Th, and Y. Although the sources and relative abundance of the various elements require further evaluation, some of the coal fly ashes examined in the present study could possibly be used for recovery of economically valuable rare earth elements.
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