Solid–Liquid Phase Equilibria of an Aqueous Quaternary Sulfate-type System Containing Fe 2+ , Mg 2+ , Mn 2+ at 298.2 K

The efficient recovery of iron from titanium ilmenite processing waste, which contains Fe 2+, Mg 2+, Mn 2+, is hindered by complex sulfate interactions. To address this, the stable phase equilibria of the quaternary system Fe 2+, Mg 2+, Mn 2+ //SO 4 2– –H 2 O and its constituent ternary subsystems were determined at 298.2 K using the isothermal dissolution method. The results reveal that the Mn–Mg and Fe–Mg systems are simple, enabling straightforward salt separation. In contrast, the Fe–Mn system is complex due to the formation of a continuous (Mn x Fe 1- x )SO 4 ·7H 2 O solid solution, which complicates Fe/Mn separation. The quaternary phase diagram, featuring three invariant points and five crystallization fields, provides a strategic separation pathway: the extensive crystallization field of FeSO 4 ·7H 2 O indicates that maintaining a high iron concentration allows for its preferential crystallization, facilitating subsequent purification of magnesium and manganese salts. This work delivers essential thermodynamic data and a practical crystallization-based strategy for recycling iron resources from complex sulfate wastes.