Resource utilization of strongly acidic wastewater and red gypsum by a harmless self-treatment process

Abstract

The acidic wastewater and red gypsum generated by the titanium dioxide (TiO2) industry pose a great threat to humans and the environment. In order to achieve low-cost and high-benefit resource utilization of acidic wastewater and red gypsum, a harmless process for the self-treatment of acidic wastewater and red gypsum as well as for the recovery of valuable resources such as Ti, Fe, Ca, and S is proposed in this study. Acidic wastewater was innovatively used to purify red gypsum while reducing its acidity. The results showed that Fe, Ti, Al, Mn, and Mg impurities in red gypsum could be leached efficiently by the acidic wastewater, resulting in the calcium sulfate products, which can be used as a substitute for natural gypsums in industrial production. Subsequently, phosphoric acid was directly added to the Ti-rich filtrate and stirred at 85 ℃ for 45 min, so that the titanium was converted into titanium phosphate precipitation, with an optimal titanium recovery ratio of 94.86%. After recovering Ti, the filtrate was used again to purify red gypsum. Finally, the pH of the low acidity solution was treated by oxidation and precipitation, and filtered to obtain the Fe-rich filter residue and ammonium sulfate solution. The Fe-rich filter residue could be used in the metallurgical industry, and the ammonium sulfate solution was evaporated and crystallized to obtain the (NH4)2SO4 product. The harmless self-treatment process not only facilitated the resource utilisation of acidic wastewater and red gypsum, but also solved the pollution problems caused by wastewater and solid waste in the TiO2 industry.