Aluminum smelting residue upcycling via targeted fluoride extraction and alkaline-driven cryolite synthesis

Abstract

Spent carbon anode, a byproduct of the aluminum electrolytic process, are characterized by a significant concentration of fluorides. It is crucial to manage their disposal with the highest level of care, as improper handling may lead to adverse environmental consequences. Chemical leaching is widely utilized in solid waste disposal due to its efficiency and scalability; however, it presents challenges such as process complexity and the necessity for secondary recovery of substantial filtrate volumes. This study introduces an optimized chemical leaching and recovery process aimed at efficient fluoride extraction. The alkaline leaching recovery method was systematically re-evaluated based on the various types and forms of fluorides present in spent carbon anode. Cryolite-type fluorides exhibit enhanced reactivity and separability within an alkaline environment. Under optimal conditions with a leaching duration of 30min, the fluoride leaching rate achieved 95.50%. The residual calcium fluoride and aluminum oxide found in the leaching residue were determined to be insoluble. A single-step alkaline leaching process for environmentally friendly treatment of spent carbon anode has been demonstrated as feasible. The recovery mechanism for F–, Al3+, and Na+ in an alkaline environment is straightforward. In solutions with a low F/Al mole ratio, the precipitate consists primarily of aluminum hydroxide and cryolite; conversely, as the F/Al ratio increases, cryolite becomes the predominant precipitate. With optimized processing conditions, the fluoride recovery rate exceeds 97%. In summary, the proposed alkaline leaching strategy for fluoride extraction and cryolite synthesis from spent carbon anode offers significant advantages regarding simplicity, efficiency, and scalability—providing new insights into industrial disposal practices.