Direct and indirect aqueous mineralization using red gypsum for carbon dioxide sequestration

Carbon capture and storage is gaining prominence as a means of combating climate change. Mineral carbonation is the only known form of permanent and leakage-free carbon storage. The aim of this research was to investigate the suitability and feasibility of utilizing red gypsum as the calcium source for the mineral carbonation process. The physico-chemical analysis of red gypsum showed that calcium and iron are its major constituents, which makes it a highly suitable and potential feedstock for mineral carbonation. The direct carbonation of red gypsum showed that both the purity of the product and the efficiency of the reaction were very low even at elevated reaction temperature and CO2 pressure. The maximum CaCO3 purity of 23.63% and carbonation efficiency of 41.04% were achieved during direct aqueous carbonation of red gypsum. The red gypsum dissolution studies showed that H2SO4 resulted in higher calcium extraction efficiency compared to HCl and HNO3. Increasing the reaction temperature from 30 °C to 70 °C and also increasing the reaction time from 5 to 120 minutes were found to be effective in enhancing the degree of extraction for all three types of acid used. The maximum of 100% and 84.6% extraction efficiency was achieved for Ca and Fe, respectively. Kinetic analysis found that the dissolution rate of red gypsum is controlled by the combination of product layer diffusion and chemical reaction control. The carbonation efficiency was found to be in direct relationship with CO2 pressure where the maximum carbonation efficiency of 100% was achieved at 8 bar CO2 pressure. The pH swing experiments resulted in CaCO3 with a maximum purity of 98%. The pH swing carbonation of red gypsum could be further investigated as a promising method for large scale CO2 sequestration