Recovery elemental sulfur from calcium sulfide prepared by red gypsum in sulfuric acid wastewater treatment

Red gypsum comes from TiO 2 production industry by sulfate method, which comes from the low acidic liquid stream treatment using lime or limestone. At present, RG main is treated by outdoor stacking, which induces additional costs and raises environmental concerns. It will be of interest to see the transformations of RG to obtain elemental sulfur and calcium cyclic utilization in the acidic liquid stream treatment. Calcium sulfide was prepared by calcining RG at 900 °C for 10 min and pure gypsum at 900 °C for 20 min, respectively. The decomposition of RG was easy than pure gypsum because of Fe 2 O 3 catalysis. And srebrodolskite was observed in the samples from RG calcined at 800 to 1100 °C, respectively. Hydrogen sulfide was produced when CaS reacted with sulphuric acid wastewater and rod-like gypsum crystals were observed. And then, pellets of elemental sulfur, with 1.5–5 μm in diameter, were obtained by the H 2 S oxidation in Fe(III) chelate solution, which achieved elemental sulfur recovery and calcium cyclic utilization.