Extraction of alumina from coal fly ash by mixed-alkaline hydrothermal method

Alumina was leached from coal fly ash using a mixed alkali (NaOH + Ca(OH)2) through the hydrothermal method. The effects of leaching conditions on the alumina were studied, and the results showed that the alumina extraction ratio increased with increased reaction temperature, calcium–silicon ratio, and liquid–solid ratio. By optimising the leaching conditions, the alumina extraction ratio could reach 91.3%. The obtained leaching residues were analysed by X-ray diffraction (XRD) and scanning electron microscope (SEM) + energy dispersive spectrometer (EDS), and the reaction mechanism was also discussed. Results showed that amorphous silica and mullite were first dissolved and transformed into hydro-sodalite at relatively low temperatures. Hydro-sodalite reacted with calcium hydroxide to form sodium calcium silicate (NaCaHSiO4), which was the major component of the leaching residue. Zeolite was the equilibrium product, which accounted for most of the alumina leaching loss. The addition of Ca(OH)2 effectively inhibited the silicon leaching into the solution.