A novel synergistic impurity-removal mechanism mediated by in-situ CO2 activation and the formation of a Ca-Mg composite carbonate framework in crude magnesium melts

The selective and efficient removal of impurity elements remains a critical scientific challenge in the preparation of high-purity magnesium. To address the secondary impurity contamination and environmental burden associated with traditional flux refining, this study proposes a novel synergistic impurity removal mechanism involving in-situ activation of CO 2 in crude magnesium melt and mediation by Ca-Mg composite carbonate frameworks. In this process the electronic activation of CO 2 in high-temperature magnesium melt to generate highly reactive CO 3 2− . Through strong coordination between Ca 2+ , Mg 2+ , and CO 3 2− , stable Ca-Mg-CO 3 2− structural units are formed, which enable the selective capture and phase separation of K, Na, and Ca impurities. Thermodynamic calculations indicate that the composite carbonate phase remains stable only under conditions involving Ca participation, with its stability zone extends markedly toward lower CO 2 partial pressures. These results identify that Ca serves as the key medium for synergistic carbonation. First-principles molecular dynamics simulations further reveal that the Ca-Mg composite carbonate framework can promotes the peripheral incorporation of K + and Na + while maintaining structural integrity, gradually forming multicomponent composite carbonates. Experimental results further confirm that the formation of Ca-based composite carbonate slag phases is associated with impurity enrichment. This study systematically elucidates the green impurity removal pathway of CO 2 in molten magnesium: “in-situ activation—carbonate ion formation—Ca-Mg composite carbonate framework construction—synergistic incorporation of multiple impurities—stable formation of composite carbonates”. These findings provide a theoretical foundation for the development of a green, low-carbon, and highly selective magnesium melt refining technology system.