Synergistic Extraction of Valuable Elements from High-Alumina Fly Ash via Carbochlorination

Carbochlorination was employed to synergistically extract valuable components (Al and Si) and critical metals (Li, Ga, and Sc) from high-alumina fly ash (HAFA). The effects of gas flow, chlorination time, oxygen content, coking coal addition amount, and chlorination temperature on HAFA carbochlorination were experimentally investigated. Then, the phase transformation of HAFA was systemically investigated via XRD, SEM/EDS, and FT-IR analysis to determine the carbochlorination mechanism. Experimental investigation shows that under the optimal experimental conditions (gas flow, 10 L/min; oxygen concentration, 15%; C/O molar ratio, 1.379; chlorination temperature, 1100 °C; and chlorination time, 60 min), the chlorination rates of Al₂O₃, SiO₂, Li₂O, Ga₂O₃, and Sc₂O₃ reach 89.04%, 72.02%, 96.15%, 97.02%, and 95.30%, respectively. Chlorination residue characterizations show that the main phase mullite in HAFA is involved in carbochlorination, the aluminum in mullite is the first to complete chlorination, and the unreacted silicon is transformed into the cristobalite phase. Part of the aluminum and silicon in mullite participate in carbochlorination, resulting in the defects of mullite structure and transformation into mullite mesophase (Al_1.69Si_1.22O_4.85). Finally, SiO₂ participated in carbochlorination to produce SiCl₄. Since Li, Ga, and Sc are coated in aluminum–silicon glass, they all participate in the carbochlorination after the mullite structure is broken, transforming into the corresponding metal chlorides. AlCl₃, SiCl₄, GaCl₃, and ScCl₃ are collected in the condensing tubes, while LiCl and CaCl₂ remain in the chlorination residues.

Graphical Abstract