Mineralogy and geochemistry of the magnetic fraction of combustion ash from Talcher coal (Angul district, Odisha, India)

Abstract

India generates huge amounts of coal ash. The state of Odisha alone generates approximately 30 million tons of coal fly ash (CFA). About 80% of this material is used, and 6 to 7 million tons are landfilled. However, metals recovery is yet to be fully exploited. In this study, Rare Earth Elements (REE) and gallium potential, and Fe-rich magnetic morphotypes were assessed via a multi-technique approach, including X-ray fluorescence, Inductively coupled plasma mass spectrometry, X-ray diffraction, Reflected light microscopy, Scanning electron microscopy with X-ray micro analysis, Raman microspectroscopy, Magnetic Susceptibility, Isothermal Remanent Magnetization, and Mo & uml;ssbauer spectroscopy. The magnetic fractions of fly ash and bottom ash are composed of approximately 25 wt% of iron oxides. The results indicate that the potential for REE recovery from the bulk ash samples is low, and the magnetic fractions are depleted in REE. However, Ga is enriched in the magnetic fractions, which contradicts the main global trends, probably due to the combination of the feed coal mineralogy composed of kaolinite and siderite as forming minerals, and the combustion conditions. A ferrite magnet allows the recovery of almost all discrete ferrospheres, while a neodymium magnet concentrates the glassy aluminosilicate particles with embedded submicrometric magnetic crystallites. The main iron mineral identified in the coal was siderite, and the main Fe-mineral in the resulting ferrospheres is magnetite. However, one characteristic of these ferrospheres is the aluminosilicate glass dispersed or even covering most of the surface, which is not mentioned in pyrite-derived ferrospheres.