A green approach to prepare polymorph CaCO3 for clean utilization of salt gypsum residue and CO2 mineralization

Salt gypsum residue, an industrial solid waste, is the by-product of vacuum evaporation in the salt-making industry. This study aimed to recover salt gypsum and prepare CaCO3 via NaOH leaching and pressure strengthening with CO2; meanwhile, CO2 mineralization was realized. The method can not only produce CaCO3 with controllable morphology, but also efficient realize CO2 solidification using metal cations in the salt gypsum residue and utilization of the industrial solid waste, so as to zero discharge of waste residue in the salt-making industry. The thermodynamic calculation results showed that the method is feasible. The experimental results showed that the phase transformation efficiency of salt gypsum residue was 99.94 % under the optimal experimental conditions at NaOH concentration of 3 mol/L, liquid-solid ratio of 8 mL/g, time of 10 min, temperature of 50 celcius, stirring at 300 rpm, and CO2 flow rate of 1.0 L/min. The diffusion of CO2 is the rate-limiting step of carbonation; it was disrupted by CO2 pressure strengthening, and the conversion efficiency of salt gypsum improved. Moreover, the migration of CaCO3 molecules to the crystal kink, nucleation, and growth rate of CaCO3 crystals were improved via increasing CO2 partial pressure, affecting the morphology of CaCO3 crystals. Therefore, Calcite and aragonite CaCO3 crystals were obtained when the CO2 pressure was 0.0 MPa and 0.2 MPa, respectively. Moreover, the CO2 mineralization efficiency and CaCO3 yield were 635.3 kg CO2/1000 kg salt gypsum residue and 975.0 kg CaCO3/1000 kg salt gypsum residue under optimal experimental conditions. These new observations are expected to provide further understanding and guidance for the clean utilization of salt gypsum residue and CO2 mineralization in the salt-making industry.