Accelerated Carbonation of Brucite in Mine Tailings for Carbon Sequestration

Atmospheric CO2 is sequestered within ultramafic mine tailings via carbonation of Mg-bearing minerals. The rate of carbon sequestration at some mine sites appears to be limited by the rate of CO2 supply. If carbonation of bulk tailings were accelerated, large mines may have the capacity to sequester millions of tonnes of CO2 annually, offsetting mine emissions. The effect of supplying elevated partial pressures of CO2 (pCO(2)) at 1 atm total pressure, on the carbonation rate of brucite [Mg(OH)(2)], a tailings mineral, was investigated experimentally with conditions emulating those at Mount Keith Nickel Mine (MKM), Western Australia. Brucite was carbonated to form nesquehonite [MgCO3 center dot 3H(2)O] at a rate that increased linearly with pCO(2). Geochemical modeling indicated that HCO3- promoted dissolution accelerated brucite carbonation. Isotopic and aqueous chemistry data indicated that equilibrium between CO2 in the gas and aqueous phases was not attained during carbonation, yet nesquehonite precipitation occurred at equilibrium. This implies CO2 uptake into solution remains rate-limiting for brucite carbonation at elevated pCO(2), providing potential for further acceleration. Accelerated brucite carbonation at MKM offers the potential to offset annual mine emissions by similar to 22-57%. Recognition of mechanisms for brucite carbonation will guide ongoing work to accelerate Mg-silicate carbonation in tailings.