Geochemistry and nano-mineralogy of feed coals, mine overburden, and coal-derived fly ashes from Assam (North-east India): a multi-faceted analytical approach

• Minerals are present in coals, overburden and fly ash as nano-particles in different forms.
• Significant proportion of Fe occurs in the coals in a non-mineral form.
• Possible recovery of rare earth elements from coal-derived fly ash.

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 (HR-TEM), 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.