Solid–Liquid Phase Equilibria of Ternary System Li₂SO₄‐Na₂SO₄‐H₂O in a Wide Range of Temperatures: Measurement and Application

Abstract Due to the ever‐increasing demand for lithium in various fields, recycling lithium from used batteries is crucial. However, the insufficient thermodynamic phase equilibrium study limited the development of lithium recovery processes. To provide a fundamental basis for the recycling of Li and Na sources from lithium battery leachate, solid–liquid equilibrium data of the Li 2 SO 4 ‐Na 2 SO 4 ‐H 2 O system at different temperatures are measured based on the isothermal solution saturation method and Schreinemaker's wet residue method, which are also successfully fitted by the Pitzer model. It is found that, Na 2 SO 4 zone is highly sensitive to temperature, while the zone of Li 2 SO 4 remained almost unchanged when the temperature changed. Furthermore, the crystallization region of the double salt changed from one to two and then back to one as the temperature increased. Based on the thermodynamic characteristics of Li 2 SO 4 and Na 2 SO 4 , a novel crystallization process for separating Na 2 SO 4 from the mixed solution of Li 2 SO 4 ‐Na 2 SO 4 is proposed, which not only enables the effective separation of high‐purity sodium sulfate crystals but also enriches lithium sulfate in the solution, thereby enhancing the overall separation efficiency.