Process for the Remediation of Titanogypsum (Red Gypsum) Using Weak Acid and CaCl2 to Produce Saleable α-Gypsum and FeCl2·4H2O

The disposal of titanogypsum, also known as red gypsum, which is generated during the titanium dioxide sulfate process, has become a significant environmental challenge. We propose a new process to recover iron impurities such as FeCl2·4H2O and to convert titanogypsum to saleable hemihydrate gypsum. For this purpose, we use HCl–FeCl2 solution generated via the reaction of H2SO4–FeSO4 with CaCl2 to leach iron hydroxide from titanogypsum. The hemihydrate precipitation and phase-transition kinetics of the reaction of HCl–H2SO4–FeSO4 with CaCl2 are experimentally investigated at 353 K. The common ion effect of CaCl2 in the FeCl2 solution for FeCl2·4H2O crystallization is predicted by NRTL modeling of the solid–liquid equilibria of the FeCl2–CaCl2–H2O system. High-quality, saleable α-gypsum and FeCl2·4H2O solids obtained by treating titanogypsum samples provided by a commercial processing plant further shows that the new process is feasible.