Optimized fluorine recovery and regulated synthesis of aluminum fluoride hydroxy hydrate from waste cryolite via chemical equilibrium modeling and DFT analysis.

Waste cryolite generated during industrial aluminum smelting is rich in fluorine, posing significant risks to the environment. In this study, we propose a novel two-step method for efficiently synthesizing aluminum fluoride hydroxy hydrate (AFHH) from this waste, guided by chemical equilibrium modeling and density functional theory (DFT) calculations. This process involves the leaching of waste cryolite with Al(NO) and the subsequent alkalization of the leachate with NaOH. Optimal leaching conditions-F/Al molar ratio of 0.3, liquid-to-solid ratio of 20 ml/g, 70°C for 4 hours-achieved high extraction rates of fluorine (85.6 %) and lithium (83.6 %), with an F/Al ratio of 1.61 in the leachate. Chemical equilibrium modeling indicated that the Al-F and Al-F-OH complexes in the leaching solution vary with pH. A lower F/Al ratio not only inhibits the co-precipitation of cryolite but also facilitates the formation of amorphous gibbsite. Additionally, DFT calculations identified AlF and AlFas the dominant Al-F complex in the system's lowest energy configuration in the leachate. Neutralization at lower temperature (pH ≤ 5) selectively precipitated AFHH, preserving lithium in the filtrate for future use. This study provides detailed research and theoretical guidance for the conversion of toxic fluorine from waste cryolite to AFHH.