Efficient removal of iron from red gypsum via synergistic regulation of gypsum phase transformation and iron speciation.

Abstract Red gypsum is a type of iron-rich gypsum residue originated from industrial titanium dioxide process using Ilmenite. Currently, it has a low rate of comprehensive utilization about 20%, and deep removal of iron impurity is the crucial factor that directly limits its multipurpose utilization. In this study, the iron was efficiently removed from red gypsum residue by synergistic controlling the phase transformation of gypsum and the iron speciation under hydrothermal conditions. The iron removal efficiency was more than 99% under the optimized treatment condition (i.e. liquid-solid ratio of 10, with 1.5 M HCl as mineralizer, heating at 140 °C for 6 h). The X-ray powder diffraction (XRD) and scanning electron microscopy (SEM) analyses showed that the phase transformation and crystal growth of gypsum accelerated by H+ was the essential reasons to fully remove iron. Moreover, H+ also provided acidic conditions (pH