Sustainable treatment of solid titanium-gypsum-waste using acidic titanium-white-wastewater to produce high-value α-hemihydrate gypsum

Titanium gypsum is a solid waste produced when titanium dioxide manufacturers use sulfuric acid and neutralize the waste acid with lime. The global production of titanium gypsum waste has reached 10 million tons annually. To address the environmental hazards, this study proposes a cost-effective ($2.39/ton), emission-free method to sustainably produce high-value α-hemihydrate gypsum by consuming large quantities of titanium gypsum. The primary iron impurity in titanium gypsum was separated (> 80 %) using titanium white waste acid, yielding high-value red iron oxide (Fe₂O₃) and green vitriol, i.e., hydrated iron(II) sulfate (FeSO₄·7H₂O). Additionally, the thermodynamic process of in-situ dehydration of titanium gypsum at atmospheric pressure in a chlorine-free salt solution was studied, producing large α-hemihydrate gypsum particles of 20 μm size through hydration-recrystallization. This work enhances resource utilization efficiency, reduces environmental contamination from titanium dioxide waste acid and titanium gypsum, and exemplifies an environmentally responsible process with significant implications for the sustainable development of the titanium dioxide industry.