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

Abstract

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 FeCl 2 • 4H 2 O and to convert titanogypsum to saleable hemihydrate gypsum. For this purpose, we use HCl−FeCl 2 solution generated via the reaction of H 2 SO 4 −FeSO 4 with CaCl 2 to leach iron hydroxide from titanogypsum. The hemihydrate precipitation and phase-transition kinetics of the reaction of HCl−H 2 SO 4 −FeSO 4 with CaCl 2 are experimentally investigated at 353 K. The common ion effect of CaCl 2 in the FeCl 2 solution for FeCl 2 •4H 2 O crystallization is predicted by NRTL modeling of the solid−liquid equilibria of the FeCl 2 −CaCl 2 − H 2 O system. High-quality, saleable α-gypsum and FeCl 2 •4H 2 O solids obtained by treating titanogypsum samples provided by a commercial processing plant further shows that the new process is feasible.