Nesquehonite as a carbon sink in ambient mineral carbonation of ultramafic mining wastes

X-ray diffraction of solid products combined with attenuated total reflection-Fourier transform infrared spectroscopy analysis reveals that during mineral carbonation of brucite-rich nickel mining tailings, parallel to brucite dissolution, hydrated magnesium carbonates such as nesquehonite are being formed. Wetting/drying cycles of the carbonation products revealed the important impact of temperature (25-70 °C) oscillations on the stability of the primary carbonates. While nesquehonite was observed to transform into an amorphous phase during hot dry episodes, evidences are presented on nesquehonite transformation into hydromagnesite and dypingite on the surface of already carbonated layers during long-term contacts under occasional wetting and drying episodes. Such observations, made at controlled laboratory temperatures, suggest that even under environmental CO2 partial pressures and over long periods, the hydrated magnesium carbonates act as precursors for the formation of more stable carbonate products. Moreover, it was observed that drying and freeze/thaw cycles were at the origin of a thermomechanical “peel-off” effect which inflicted micro-fractures to the carbonate product layers enabling water and gas to engulf beneath and react with freshly unearthed Mg donor sites. Results of an experimental campaign designed to evaluate the nature and stability of the ambient carbonation products are also discussed.