Solid–Liquid Phase Diagram of the FeSO ₄ –MgSO ₄ –MnSO ₄ –H ₂ O Quaternary Water-Salt System and Its Subsystems at 40 °C

The efficient recovery of Fe, Mg, and Mn from various ore processing wastes is often hindered by complex sulfate interactions and temperature-dependent behaviors. To address this challenge, the stable phase equilibria of the quaternary system FeSO 4 –MgSO 4 –MnSO 4 –H 2 O and its constituent ternary subsystems were systematically investigated at 40 °C using the isothermal dissolution method. The results reveal that the MnSO 4 –MgSO 4 –H 2 O and FeSO 4 –MgSO 4 –H 2 O systems remain simple eutectic type, facilitating straightforward separation of magnesium from iron and manganese. In contrast, the FeSO 4 –MnSO 4 –H 2 O system exhibits complex behavior due to the formation of a continuous (Fe,Mn)SO 4 ·4H 2 O solid solution. The quaternary phase diagram at 40 °C features 4 invariant points and 6 crystallization fields. Within this framework, FeSO 4 ·4H 2 O emerges as the dominant crystalline phase, providing a direct thermodynamic basis for a priority iron removal strategy. This dominance enables the selective crystallization of pure FeSO 4 ·4H 2 O, allowing the solution to effectively bypass the narrow (Fe,Mn)SO 4 ·4H 2 O solid solution region. After iron removal, the residual system enables sequential recovery of pure manganese and magnesium salts. This work delivers essential thermodynamic data and a practical crystallization-based separation pathway for the efficient recovery of metal resources from complex sulfate wastes.