Mineral Phase Transformation-Mediated Integrated Separation for Calcium Carbide Slag Purification and CaCO3 Production

Calcium carbide slag (CS) mineralization of CO2 to produce CaCO3 is a promising strategy to solid waste utilization and carbon neutrality, yet inherent impurities seriously degrade product purity and whiteness, restricting high-value utilization. Herein, we propose an integrated impurity removal strategy based on high-temperature mineral phase transformation to achieve effective separation of critical impurities. FactSage thermodynamic calculations and experimental investigations revealed that during calcination, SiO2 in CS preferentially reacts with CaO to form Ca2SiO4, while carbon is eliminated as CO2. Effective CaO content and activity increased with calcination temperature and time, peaked at 900 °C and 180 min, then decreased slightly with further calcination. Magnetic FeSi impurities were efficiently removed via wet magnetic separation. Subsequently, the newly formed Ca2SiO4 was separated from the Ca(OH)2 matrix by gravitational settling, exploiting the density difference generated by the phase transformation, achieving a maximum Si removal efficiency of 64.2% under optimized conditions (solid–liquid ratio 1:10, 8 h). A mechanistic model linking thermodynamic driving forces, phase evolution, and separation principles is proposed. Ultimately, a calcite-phase CaCO3 of 98.2% purity and 94.0 whiteness with cubic morphology was produced. An elemental mass balance analysis demonstrates the technical viability of the integrated process. This work provides both mechanistic insights and an engineering basis for producing CaCO3 with improved purity from solid waste via CO2 mineralization.