Experiments to Support the Design of an Industrial Mineral Carbonation Process

Mineral carbonation is a potential method of carbon sequestration whereby CO2(g) reacts with an alkaline earth metal to form a thermodynamically favorable, inert form of carbon. Los Alamos National Laboratory (LANL) is investigating a process entailing carbonation of magnesium silicates with a focus on serpentine due to its natural abundance. Carbonation of serpentine is believed to require three unique steps: (1) CO2(g) must dissolve into the aqueous phase; (2) magnesium must be extracted from the serpentine and dissolve into the aqueous phase; and (3) magnesium carbonate must precipitate. As part of the ongoing work to elucidate the mechanism(s) controlling each step of this reaction process, a series of low temperature batch experiments have been conducted to determine how solution chemistry affects the dissolution rate of serpentine (considered to be the rate limiting step of the process) under various pH and buffer conditions, and a suite of calculations have been performed with the aid of a geochemical model to predict speciation of reactants at varied temperature, pH, and carbon dioxide fugacity. Both methods of analysis have been used to evaluate the effects of various catalytic chelating agents on the net reaction. Results from these analyses will be used to direct stirred reaction vessel experiments at elevated temperature and pressure. Batch dissolution experiments were performed in closed 1 L Teflon beakers that were placed in a heated 40 to 70°C circulating water bath and exposed to moisturized N2 or CO2 gas. Temperature, pH, and solution chemistry measurements (Mg, Si, Fe) were taken throughout each experiment to monitor reaction progress. Results indicate that local equilibrium has not been reached over the length of the reaction, which could be attributed to a leached-layer surface phenomena controlling dissolution. Geochemical calculations were performed using a modified thermodynamic database with Geochemist's Workbench. The results have determined dominant aqueous species and helped constrain conditions of competitive phases to magnesium carbonate generation. The calculations have also provided a framework for evaluation of potential reaction pathways.