Mineral changes and trace element releases during extraction of alumina from high aluminum fly ash in Inner Mongolia, China

The mineralogy and trace element redistribution in the processes of extracting alumina from high aluminum fly ash in an industrial-scale production line (ISPL) in Inner Mongolia, China, are systematically investigated. Three samples were collected from the Togtoh power plant and ten samples from different processing sections in the ISPL. The mineralogy, chemical composition and morphology of the samples were characterized respectively by X-ray diffraction (XRD), X-ray fluorescence (XRF) spectrometry, and field scanning electron microscopy combined with energy dispersive X-ray spectrometry (FSEM-EDX). The trace element contents were determined by atomic fluorescence spectroscopy (AFS) and inductively coupled plasma-mass spectrometry (ICP-MS). The results show that the minerals in the pre-desilication fly ash (PDFA) include mullite, corundum, quartz, and nosean, and that amorphous SiO2 substantially disappears after the high aluminum fly ash (HAFA) is desiliconized. The minerals in the sintered fly ash (SFA) are sodium aluminate and larnite; most of the mullite reacts with Na2CO3 in the sintering process. Compared with trace element data for global hard coal ashes, elements that are slightly enriched elements in the HAFA are Li, Ga, Zr, Nb, Hf, Pb, and Th, and elements that are depleted include Ni, Ge, Rb, Sb, Cs, and Bi. The by-product silicon-calcium residue (SCR) is slightly enriched in Li, Zr, Nb, Hf, Ta, and Th, and depleted in Cr, Co, Ni, Cu, Ge, As, Rb, Mo, Cd, Sb, Cs, Ba, W, Hg, TI, and Bi. Gallium shows a close affinity with Al2O3, and is strongly enriched (97 ppm) in the aluminum hydroxide (AH) product. For the whole ISPL, most of the trace elements are redistributed into the main SCR by-product, including Li, Be, Sc, V, Cr, Co, Ni, Cu, Sr, Zr, Nb, Cd, In, Hf, Ta, Bi, Th, U, and REY (rare earth elements and Y) (relative enrichment factor >0.6; relative enrichment factor is used to describe the distribution of trace elements that end up in the extraction process). Some of the trace elements are released to atmosphere at high temperature, such as Mo, Hg and TI, and only small amounts of trace elements become redistributed into the product or other by-products. (C) 2016 Elsevier B.V. All rights reserved.