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

• The feasibility of salt gypsum residue utilization and CO 2 mineralization were verified by thermodynamic calculation. • Efficient converting salt gypsum to portlandite was realized by NaOH leaching without ammonia/ ammonium reagent. • Different CaCO 3 crystal forms were prepared via regulating the CO 2 partial pressure. • Salt gypsum residue is a kind of raw material for preparing CaCO 3 and CO 2 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 CaCO 3 via NaOH leaching and pressure strengthening with CO 2 ; meanwhile, CO 2 mineralization was realized. The method can not only produce CaCO 3 with controllable morphology, but also efficient realize CO 2 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 ℃, stirring at 300 rpm, and CO 2 flow rate of 1.0 L/min. The diffusion of CO 2 is the rate-limiting step of carbonation; it was disrupted by CO 2 pressure strengthening, and the conversion efficiency of salt gypsum improved. Moreover, the migration of CaCO 3 molecules to the crystal kink, nucleation, and growth rate of CaCO 3 crystals were improved via increasing CO 2 partial pressure, affecting the morphology of CaCO 3 crystals. Therefore, Calcite and aragonite CaCO 3 crystals were obtained when the CO 2 pressure was 0.0 MPa and 0.2 MPa, respectively. Moreover, the CO 2 mineralization efficiency and CaCO 3 yield were 635.3 kg CO 2 /1000 kg salt gypsum residue and 975.0 kg CaCO 3 /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 CO 2 mineralization in the salt-making industry.