Cobalt substitution slows forsterite carbonation in low-water supercritical carbon dioxide.

Cobalt recovery from low-grade mafic and ultramafic ores could be economically viable if combined with CO ₂ storage under low-water conditions, but the impact of Co on metal silicate carbonation and the fate of Co during the carbonation reaction must be understood. In this study, in situ infrared spectroscopy was used to investigate the carbonation of Co-doped forsterite ((Mg,Co) ₂ SiO ₄ ) in thin water films in humidified supercritical CO ₂ at 50 °C and 90 bar. Rates of carbonation of Co-doped forsterite to Co-rich magnesite ((Mg,Co)CO ₃ ) increased with water film thickness but were at least 10 times smaller than previously measured for pure forsterite at similar conditions. We suggest that the smaller rates are due to thermodynamic drivers that cause water films on Co-doped forsterite to be much less oversaturated with respect to Co-doped magnesite, compared to the pure minerals.