Extraction of Valuable Metals and Preparation of Mesoporous Materials from Circulating Fluidized Bed-Derived Fly Ash via an Acid–Alkali-Based Alternate Method

Stringent leaching conditions including high pressure, temperature, and chemical consumption limit the extraction of valuable metals from circulating fluidized bed-derived high-alumina fly ash (CFB-HAFA) via the acid leaching method. In the present study, a complex utilization of CFB-HAFA, including the extraction of valuable metals (Al, Li, and Ga) and preparation of mesoporous material, is realized via a moderate acid–alkali-based alternate method. The results show that 82, 78, and 69% of Al, Li, and Ga, respectively, in CFB-HAFA are extracted by two treatments of acid leaching under moderate conditions of 15 wt % HCl concentration and 90 °C leaching temperature. The leaching behaviors of metals follow a shrinking core model, and the leaching process is first controlled by the surface chemical reaction at the initial stage and H+ diffusion thereafter. Numerous slit-shaped mesopores form in the residue during acid leaching. The final residue with a specific surface area of 273 m2/g can be used as an efficient adsorbent for removing methylene blue from dye wastewater. The maximum adsorption capacity is approximately 140.0 mg/g at room temperature. The Langmuir adsorption isotherm and pseudo second-order model can well describe the adsorption process and kinetics, implying that the adsorption is a monolayer and chemical adsorption.