Supplementary material from Extraction of alumina from alumina rich coal gangue by a hydro-chemical process

Vast quantities of gangue from coal mining and processing have accumulated over the years and caused significant economic and environmental problems in China. For high-added value utilization of alumina rich coal gangue (ARCG), a mild hydro-chemical process was investigated to extract alumina. The influences of NaOH concentration, mass ratio of alkali to gangue, reaction temperature and reaction time were systematically studied. An alumina extraction rate of 94.68% was achieved at the condition of NaOH concentration 47.5%, alkali to gangue ratio of 6, reaction temperature of 260°C and reaction time of 120 min. The obtained leaching residues were characterized through X-ray diffraction, scanning electron microscopy and energy-dispersive spectrometer. Research confirmed that kaolinite the main alumina-bearing phase of ARCG can be decomposed and transformed to Na₈Al₆Si₆O₂₄(OH)₂(H₂O)₂ and Ca₂Al₂SiO₆(OH)₂ at relatively low temperature and short reaction time. Additionally, Na₈Al₆Si₆O₂₄(OH)₂(H₂O)₂ and Ca₂Al₂SiO₆(OH)₂ are unstable and will transform to alumina-free phase NaCaHSiO₄ under the optimal conditions, which is the major reason for high alumina extraction rates.