Solid–Liquid Phase Diagram of the FeSO4–MgSO4–MnSO4–H2O System at 60 °C

The separation of iron, magnesium, and manganese from sulfate-rich industrial byproducts is challenging due to the formation of solid solutions and temperature-sensitive hydrate transitions. In this work, the stable phase equilibria of the FeSO4–MgSO4–MnSO4–H2O quaternary system were determined at 60 °C using the isothermal dissolution method, with particular attention to preventing Fe2+ oxidation under an inert atmosphere. The ternary subsystems MnSO4–MgSO4–H2O and FeSO4–MgSO4–H2O exhibit simple eutectic behavior, whereas the FeSO4–MnSO4–H2O system forms a continuous (Fe,Mn)SO4·H2O solid solution with complete miscibility. The quaternary phase diagram at 60 °C contains two invariant points and four crystallization fields. Notably, FeSO4·4H2O dominates the crystallization landscape, enabling a selective iron removal strategy that bypasses the solid solution region and yields pure ferrous sulfate. After iron recovery, the remaining Mg–Mn solution allows sequential crystallization of MgSO4·6H2O and MnSO4·H2O. The elevated temperature accelerates crystallization kinetics and provides a cleaner separation route. This work supplies fundamental thermodynamic data and a practical crystallization framework for recovering Fe, Mg, and Mn from complex sulfate wastes.