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

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 Fe 2 O 3 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 Fe 2 O 3 , 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 Fe 2 O 3 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. • A synergistic removal process of calcium and iron impurities was proposed. • The optimal conditions for removal of calcium and iron impurities were obtained. • The removal efficiencies of CaO and Fe 2 O 3 are over 95% and 85% respectively. • The reaction kinetics of CaO and Fe 2 O 3 in HCl conform the layer diffusion control. • The minerals of gehlenite, anhydrite and hematite dissolve in the leaching process.