Geochemistry and Nanomineralogy of Feed Coals and Their Coal Combustion Residues from Two Different Coal-Based Industries in Northeast India

The mineralogy, as well as the geochemistry of feed coals, are very important parameters for understanding the process conditions in a thermal plant. The present paper describes the nanomineralogical and geochemical aspects of a few feed coals and associated coal fly ashes collected from tea processing and brick-making industries from Northeast India. The optical microscopy, X-ray diffraction, field emission scanning electron microscopy-energy dispersive spectroscopy (FESEM-EDS), and chemical analysis techniques were applied to determine this mineralogical and geochemical aspect. Inductively coupled-mass spectroscopy was used to explore the occurrence and distribution of trace and rare earth elements (REEs), which are economically important, and are reported in this paper. The major minerals such as kaolinite, pyrite, quartz, and calcite were found to be present in the feed coals with variations with respect to the type of feed samples. The FESEM-EDS analysis revealed the occurrence of nanosphere consisiting of Al-Si-Fe-S and Al-Si-Mg-As compounds. The potentially hazardous elements (PHEs) including Hg, As, Pd, Cd, Sn, Ni, and Co were also found in fly ash samples. The REE abundances are observed to be considerably low in comparison to world average coal. A preliminary statistical analysis of the chemical parameters, trace and rare earth elements in the coals is performed to understand their mutual correlations.