Preparation of calcium sulfate from recycled red gypsum to neutralize acidic wastewater and application of high silica residue

Red gypsum (RG) is an industrial waste generated from the titanium dioxide process produced. For the low-cost and high-benefit utilization of RG, calcium sulfate (CaS) was prepared from recycled RG. Rod-like gypsum crystals with a rough surface, uniform size, 1 µm in diameter and 5 µm in length, were obtained, when the as-prepared CaS was divided into three equal portions and one portion was added at each interval of 2 h to neutralize the simulated acidic wastewater. CaS was prepared from RG to neutralize acidic wastewater to produce silicon-enriched RG for the next cycle. High silica residue (HSR) was obtained after 6 cycles. The effect of calcined temperature on the desulfurization of HSR was investigated. The sulfur content in the residue decreased by 97% after calcined at 1200 ℃ for 2 h. According to GB/T 12957-2005 “Industrial waste residue activity test method for cement admixture”, the activity index of desulfurized HSR was 90%, which it can be used as a cementing material.