Determination and chemical modeling of the solubility of FeSO4·7H2O in the Ti(SO4)2−H2SO4−H2O system

• The solubility of the FeSO4·7H2O in concentrated Ti(SO4)2−H2SO4−H2O solutions was determined.
• A chemical model with new parameters of the mixed-solvent electrolyte (MSE) model was developed.
• The new chemical model was successfully applied to predict the speciation and crystallization of FeSO4·7H2O.

The phase equilibrium and thermodynamic properties of FeSO4·7H2O in the FeSO4−Ti(SO4)2−H2SO4−H2O system are of critical significance because iron removal is one of the most important stages in the sulfate process for titania production. The solubility of the FeSO4·7H2O in the Ti(SO4)2−H2SO4−H2O system was measured over the temperature range from 278 to 298 K using a dynamic method. The initial concentrations of Ti(SO4)2 and H2SO4 were 0.2–1.0 mol·kg−1 and 2.0–3.5 mol·kg−1, respectively. It was found that the solubility of FeSO4·7H2O significantly increased with the temperature throughout the entire concentration ranges of H2SO4 and Ti(SO4)2. A new chemical model was developed by obtaining the parameters of the mixed-solvent electrolyte (MSE) model via regression of the solubility data of FeSO4·7H2O in the Ti(SO4)2−H2SO4−H2O system. The model with the newly obtained parameters was successfully applied to predict the sulfate speciation in both FeSO4−H2SO4−H2O and FeSO4−Ti(SO4)2−H2SO4−H2O systems and the crystallization behavior of FeSO4·7H2O. The results will provide fundamental data and thermodynamic information for improving the existing sulfate process for the production of titanium dioxide pigment.