Precipitation–dissolution equilibrium of lithium sodium cryolite

A strong affinity is observed between fluorine and aluminum, which can both easily undergo sixth-level complexation to form \mathrmAlF_6^3- . Their coexistence with alkali metal ions (such as Li and Na) leads to the formation of a homogeneous solid solution, namely aluminum–sodium composite lithium salt, based on the stoichiometric ratio between the ions. The considerable affinity between fluorine–aluminum bonds and the insoluble nature of fluoroaluminate precipitation provide important guidance for the harmless treatment of fluorine-containing wastewater, aluminum removal from lithium battery positive electrode powder acid leaching solution, electrolytic aluminum smelting industry, and the geochemical and environmental chemical behavior research of aluminum and phosphorus in acidic fluorine-polluted soil. A current research hotspot focuses on the high-value recycling and reusing of iron and phosphorus components in waste lithium iron phosphate (LiFePO₄) batteries due to the rapid development of the new energy industry. However, the residual aluminum collector fluid transported by the positive electrode powder had high aluminum content in the recovered iron phosphate (FePO₄), thereby affecting the electrochemical performance of regenerated FePO₄ and reducing economic benefits. In this study, the synthesis of lithium sodium cryolite (Na₃Li₃Al₂F₁₂) by regulating the coordination of fluorine and aluminum was conducted using aluminum sulfate, lithium sulfate, sodium sulfate, and ammonium fluoride as raw materials. A basis for the fluorination coordination and removal of Al from waste LiFePO₄ powder acid leaching solutions is presented based on an investigation of the precipitation–dissolution equilibrium behavior of Na₃Li₃Al₂F₁₂. The effects of the F/Al, Na/Al, and Li/Al molar ratios, initial reaction pH, reaction temperature, and reaction time on the coordination precipitation process were also investigated. The following optimized precipitation conditions were obtained: F/Al molar ratio of 7, Na/Al molar ratio of 1.5, Li/Al molar ratio of 2, reaction temperature of 70 ℃, reaction pH of 4.5, and reaction time of 3 h. Under optimal process conditions, the concentrations of various ions in the filtrate tended to stabilize after coordination and precipitation of Al through the fluorinating agent, revealing an Al³⁺ concentration of 75.43 mg·L⁻¹. The solubility of Na₃Li₃Al₂F₁₂ in solutions with different values of pH was also investigated. At a temperature of 30 ℃ and a pH of less than 3, high acidity led to the high solubility of Na₃Li₃Al₂F₁₂. The pH had a minimal effect on the solubility of Na₃Li₃Al₂F₁₂ when the pH exceeded 3. Considering its guiding significance in practical applications, Al³⁺ reached a precipitation rate of 99.4% after NaF coordination precipitation of waste LiFePO₄ cathode material acid leaching solution, thereby realizing deep removal of Al.