A green process for recovery of H2SO4 and Fe2O3 from FeSO4·7H2O by modeling phase equilibrium of the Fe(П)––H+–Cl– system

Ferrous sulfate heptahydrate FeSO4·7H2O is a major waste produced in titanium dioxide industry by the sulfate process and has caused heavy environmental problem. A new green process for the treatment of FeSO4·7H2O was proposed to make use of iron source and recycle sulfate source as H2SO4. It was found that by adding concentrated HCl to the FeSO4 solution, FeCl2·4H2O was crystallized out, which was subsequently calcined to produce Fe2O3 and HCl. Concentrated H2SO4 solution (about 65 wt %) was obtained by evaporating the FeCl2·4H2O‐saturated filtrate. To facilitate the process development and design, the solubilities of FeCl2·4H2O in HCl, H2SO4, and HCl + H2SO4 solutions were measured and the experimental data were regressed with both the mixed‐solvent electrolyte model and the electrolyte NRTL model. On the basis of the prediction of the optimum conditions for the crystallization of FeCl2·4H2O, material balance of the new process was calculated. FeCl2·4H2O and Fe2O3 were obtained from a laboratory‐scale test with about 70% recovery of ferrous source for a single cycle, indicating the feasibility of the process.