A new sustainable process was developed to recycle FeSO4 center dot 7H(2)O produced from the sulfate process for titanium dioxide production. In this process, FeSO4 center dot H2O was crystallized out from a subsequent cooling crystallization operation rather than FeSO4 center dot 7H(2)O. In order to better develop the new process and provide a theoretical basis for ferrous sulfate dehydration, the study of the phase equilibrium of the FeSO4-H2SO4-HCl-H2O system is necessary. For this purpose, the solubility of FeSO4 center dot 7H(2)O in the H2SO4-HCl-H2O system was determined at a temperature range from 278.15 to 313.15 K using a dynamic method. By raising temperature and the concentration of HCl, more FeSO4 center dot 7H(2)O can be dissolved in this system. However, with the addition of H2SO4, the solubility of FeSO4 center dot 7H(2)O is found to decrease due to common-ion effect. A speciation-based model equipped with middle-range parameters was established by regressing the experimental and reported solubility of FeSO4 center dot 7H(2)O under varying conditions. The phase transition temperature of FeSO4 center dot 7H(2)O to FeSO4 center dot 7H(2)O was successfully predicted by the new model, which was found to decrease with increasing concentration of both H2SO4 and HCl.