Dual benefits of indirect mineral carbonation of titanium gypsum: waste gypsum recycling and carbon sequestration.

Titanium gypsum, a typical industrial solid waste primarily composed of calcium sulfate, poses significant environmental challenges. This work proposes an innovative indirect mineral carbonation process to recycle titanium gypsum, combining ammonium chloride leaching and carbonation for carbon sequestration. Effective leaching Ca2+ from titanium gypsum is obtained, and process parameters such as NH4Cl concentration and liquid-to-solid ratio play significant roles in the selective leaching Ca. Under optimal conditions of NH4Cl concentration 2.5 mol/L, L/S 200:3 mL/g, and temperature 25 °C, the leaching ratio reaches 87.55 % at 10 min. The carbonation stage is predominantly influenced by the solution pH, with temperature and CO2 flow rate inducing slight improvement. Under optimum conditions of pH 9.00, temperature 45 °C, and CO2 flow rate 200 mL/min, carbonation efficiency reaches 96.52 % at 5 min. Vaterite-type calcium carbonate with high purity of 99.38 % is successfully produced. Additionally, the feasibility of NH4Cl recycling is confirmed through five consecutive cycles, followed by crystallization recovery. This study offers a sustainable solution for titanium gypsum recycling, contributing to CO2 mitigation and the production of valuable by-products.