Energy-efficient leaching process for preparation of aluminum sulfate and synergistic extraction of Li and Ga from circulating fluidized bed fly ash

Sequential leaching and extraction of precious metals from high aluminum (Al) circulating fluidized bed (CFB) fly ash in China, which associated with lithium (Li) and gallium (Ga), is one of the most significant ways for value-added utilization. However, industrial application has been limited due to serious corrosion of equipment, significant energy consumption, and complex purification processes. To sufficiently leach valuable metals from CFB fly ash, a novel energy-efficient sulfuric acid (H2SO4) leaching process using dilution heat was developed, and the effects of iron (Fe) amount in the raw materials on the extraction process of Al, Li, and Ga as well as the as-prepared Al-based products were studied to efficiently obtain the qualified aluminum sulfate (Al2(SO4)3) products. The leaching behavior of each target element was systematically investigated by the laboratory and pilot-scale experiments. The extraction efficiencies for Al, Li, and Ga were 95.2%, 94.2%, and 87.7%, respectively, when the acid leaching process was conducted at the temperature of 140°C for 2 h, with the H2SO4 concentration of 40% and the solid-to-liquid ratio of 1/3, In the Pilot-scale experiment, the temperature of the reaction system can reach 120°C, 140°C, and 160°C by regulating the amount of H2SO4 added to release different heat, and efficient leaching of the target elements was achieved by the energy-efficient sulfuric acid leaching process. To obtain qualified Al2(SO4)3 products from fly ash, carbothermal reduction magnetic separation, and acid leaching were used for Fe removal from raw fly ash, and the Fe content in the as-prepared Al2(SO4)3 products was only 0.45% and 0.29%, which reached the standards of Class II qualified products and Class II first-grade products, respectively. This paper might provide an energy-efficient process for efficient and high-value utilization of solid waste fly ash.