Commentary: Ex Situ Aqueous Mineral Carbonation

CO2 conversion to calcium and magnesium carbonates has garnered considerable attention since it is a thermodynamically downhill pathway to safely and permanently sequester large quantities of CO2. This seminal work performed at The National Energy Technology Laboratory in Albany (NETL-Albany) reports the conversion of calcium-and magnesium-bearing silicate minerals, such as olivine [(Mg, Fe)2SiO4], wollastonite (CaSiO3), and serpentine [Mg3Si2O5(OH)4], as they are reacted with CO2 in an aqueous environment to form magnesium or calcium carbonates. This paper discusses various pretreatment methods of the starting materials, such as grinding or heat treatment of hydroxylated Mg silicates, to enhance the reaction kinetics. The effects of various chemical additives (e.g., NaCl and NaHCO3), and reaction parameters, such as temperature, pressure, and reaction time, on the conversion are investigated. Feasibility assessments and energy and economic analyses of the direct carbonation of calcium-and magnesium-bearing minerals are presented. The key contributions of this study are the identification of the optimal conditions for the carbonation of olivine (185°C, P 15 atm CO 2 0 = , 1.0M⋅NaCl + 0.64M⋅NaHCO3), wollastonite (100°C, P 4 atm CO 2 0 = , distilled water), and heat-treated serpentine (155°C, P 15 atm CO 2 0 = , 1.0M⋅NaCl + 0.64M⋅NaHCO3). High extents of carbonation of 49.5, 81.8, and 73.5% of olivine, wollastonite, and heat-treated serpentine, respectively, achieved within an hour of reacting with CO2 in an aqueous environment are reported. The identification of the optimal reaction conditions to achieve the rapid conversion of calcium-and magnesium-bearing silicate minerals to carbonates has spurred a significant global scientific interest in mineral carbonation as a promising technology for CO2 conversion and storage with the potential reuse of carbonates. In particular, this work has found unique relevance for ex situ and in situ carbon mineralization. In ex situ mineral carbonation, CO2 is converted to carbonates in engineered processes where there is considerable control over the reaction conditions. In in situ mineral carbonation, CO2 is injected into geological formations containing calcium-and magnesium-bearing minerals and rocks with the aim toward natural carbon mineralization over time. Predicting the fate of CO2 injected into geological formations containing these rocks and minerals and developing chemical processes for converting CO2 to carbonates require a fundamental understanding of the kinetics of mineral carbonation and the corresponding morphological changes in materials. Thus, more recent studies have focused on understanding the carbon mineralization behavior via direct carbonation (Bonfils et al.