Recovery and preparation of battery-grade iron phosphate from lithium extraction slag via ammonium sulfite assisted acid leaching

A novel wet leaching process was proposed for recovering FePO 4 from lithium extraction slag. • H₂SO₄ and (NH₄)₂SO₃ were used as leaching and reduction reagents, respectively. • The leaching efficiency of FePO 4 reached up to 99.5% under optimized operation conditions. • The purity of final product met battery-grade standards with an overall recovery efficiency above 95%. • Hundred-kilogram-scale experiments demonstrated the feasibility and operational stability of the process A novel hydrometallurgical leaching process based on a H₂SO₄-(NH₄)₂SO₃ system for the efficient and low-cost recovery and regeneration of battery-grade iron phosphate (FePO₄) from lithium extraction slag (LES) was developed. Under optimized operating conditions (sulfuric acid concentration of 1 mol/L, ammonium sulfite dosage of 0.125 g/g LES, solid-to-liquid ratio of 160 g/L, reaction temperature of 50 °C and reaction time of 120 min), the process achieved highly efficient leaching of FePO₄ with a leaching efficiency exceeding 99.5%, significantly reducing alkali consumption in subsequent steps and the overall process cost. Through precise control of the precipitation pH, highly selective FePO₄ precipitation and effective impurity separation were realized. The purity of the final product met battery-grade standards, with an overall recovery efficiency above 95%. Through a comparative study of the leaching kinetics of Fe 2+ and Fe 3+ , it was revealed that the reductive leaching process is governed by a chemical reaction-controlled model, in which the acid-induced dissolution of the FePO₄ lattice (E a  = 51.29 kJ/mol) poses a higher kinetic barrier compared to the reduction reaction, serving as the rate-determining step for the overall process. Thermodynamic analysis and phase evolution tracking elucidated the mechanisms of efficient FePO₄ dissolution and selective precipitation, providing a theoretical basis for precise pH control to selectively recover Fe 3+ while suppressing the co-precipitation of Al, Mn, Ni, and Ca impurities. Crucially, the process enabled closed-loop circulation of the leaching medium by converting it into ammonium sulfate, thereby enhancing resource efficiency alongside environmental and economic benefits. Furthermore, hundred-kilogram-scale experiments demonstrated the feasibility and operational stability of the process. Compared with alternative routes, this approach offers a substantial advantage in raw material cost control. Its core unit operations align well with existing industrial FePO₄ production lines without requiring major retrofitting, highlighting its strong potential for industrial integration. In summary, the proposed recycling strategy provides a viable, efficient, and green technological route for FePO₄ valorization from lithium extraction slag.