Impurities in Steel Slag during the CO₂ Mineralization: Optimization and Multicycle Operation

CO2 mineralization using steel slag presents a promising approach to reducing industrial carbon emissions while promoting the utilization of steel slag by converting CO2 and calcium into CaCO3. Among various reagents, NH4Cl has demonstrated superior efficiency in facilitating the carbonation process, owing to its high Ca2+ leaching and mineralization efficiencies. It is essential to assess the product properties, cyclic performance, and environmental implications, and therefore, we examined the leaching and carbonation behaviors of major and trace elements during process optimization and multicycle operations. Results revealed that NH4Cl achieved high selectivity in both the leaching and carbonation of Ca2+, leading to the formation of CaCO3. In the leaching phase, active minerals such as portlandite, dicalcium silicate, and srebrodolskite provided 99% efficiency in Ca2+ leaching. Elements with higher concentrations, including Fe, Mn, Al, and Cr, exhibited low leaching efficiencies, while more leachable elements, such as Sr, Ba, Mg, and Zn, appeared at low concentrations. The residues left after NH4Cl activation showed potential for enhanced hydration activity, suitable for forming C–S–H gel as supplementary cementitious materials. In the CO2 mineralization phase, although calcite-CaCO3 adsorbed some impurity metal ions, the coprecipitation of these impurities with CaCO3 had a negligible impact on the final product purity. The process achieved a CO2 sequestration capacity of 173.7 g/kg, a CaCO3 yield of 394.8 g/kg, and a purity of over 90% purity. Additionally, multicycle experiments demonstrated consistent concentrations of impurities in the leachates and mineralized solutions, with no accumulation of trace elements over successive cycles.