Synergistic removal of calcium and iron impurities from calcium-rich and high-alumina fly ash by acid leaching control

Abstract

Fly ash is an ultrafine solid waste generated during coal combustion in industry, and the impurities contained significantly inhibit its high-value utilization. Based on a novel acid leaching control process to comprehensively utilize the high-alumina fly ash, the synergistic removal of calcium and iron impurities and the reaction mechanism was systemically studied. The removal property of CaO and Fe2O3 using hydrochloric acid solution is better than other acid solutions. Increasing the acid concentration, leaching temperature, duration and liquid-solid ratio can promote the removal efficiencies of CaO and Fe2O3, but excessive acid concentration and leaching temperature decrease the removal efficiencies. The optimal conditions for the synergistic removal of calcium and iron impurities are leaching at 80 °C for 2.0 h under a liquid-solid ratio of 4.0 in 6.0 mol·L−1 hydrochloric acid, and both the residual contents of CaO and Fe2O3 in the leached product are less than 0.7%. The reaction kinetics of calcium and iron impurities in hydrochloric acid solution correspond to the layer diffusion control, and the apparent activation energies are 44.04 kJ·mol−1 and 50.68 kJ·mol−1 respectively. The minerals of fly ash after treatment are mainly mullite, quartz and amorphous alumina, while the minerals of gehlenite, anhydrite and hematite dissolve in the leaching process.