Crystallization of Carbonates with a Core–Shell Structure: Insights into Cobalt–Iron Separation for Enhanced Critical Element Recovery

Carbonation of mafic and ultramafic silicates holds great potential for carbon removal by the formation of stable carbonate minerals. Cobalt (Co), a critical element alongside minor cations, such as iron (Fe) and calcium (Ca), often coexists in these silicates. Understanding carbonate coprecipitation dynamics in multi-ion systems is essential to optimize carbon mineralization and critical element recovery. In this study, coprecipitation experiments were conducted with dominant Mg and minor cations ( i.e., Co, Fe, or Ca) for a range of concentrations at 95 °C and 30 bar CO 2 . Characterization of solids revealed the formation of carbonate particles with a core–shell structure, with minor cations encapsulated within the core and covered by a shell of magnesite (MgCO 3 ). MgCO 3 crystallization was accelerated in the presence of these minor cations, with Co having the strongest effect, followed by Fe and then Ca. Conversely, minor cation carbonate precipitation was slowed by the presence of Mg. Additional coprecipitation experiments in the Co–Fe–Mg triple-cation system revealed that under aerobic conditions Co and Fe became spatially and phase-isolated due to MgCO 3 formation, preferably trapping Co inside carbonates, while Fe precipitated separately as iron hydroxides. These findings illuminate kinetics changes and heterogeneity evolution of the carbonation process in multi-ion systems and provide scientific foundations for Co–Fe isolation in coprecipitation processes for better Co recovery.