An H3O-alunite method for production of smelter grade alumina from an ammonium alum solution after high-pressure acid leaching of coal fly ash

Precipitation of H₃O-alunite from an ammonium alum solution using boehmite as a seed was studied. The ammonium alum solution was obtained after high-pressure leaching of mullite-type coal fly ash (CFA) with a mixture of 7.5 M H₂SO₄ + 40% NH₄HSO₄ at 200 °C. Fe(III) was extracted from the ammonium alum solution by ion exchange sorption using the Purolite S957 resin. Effects of temperature (70–90 °C), seed boehmite (100–400 g/l), ammonium alum (60–240 g/l), and precipitation duration (2–8 h) on the Al precipitation degree were evaluated. Calcination of the precipitate at 300–950 °C to obtain alumina powder was studied next. Concentration of impurities, specific surface area, phase composition, morphology, and the average particle size distribution of alumina powders were investigated. The results show that smelter grade alumina (SGA) can be obtained at optimized parameters of precipitation and calcination.