Direct aqueous mineral carbonation has been investigated as a process to convert gaseous CO_2 into a geologically stable, solid final form. The process utilizes a solution of sodium bicarbonate (NaHCO_3), sodium chloride (NaCl), and water, mixed with a mineral reactant, such as olivine (Mg_2SiO_4) or serpentine [Mg_3Si_2O_5(OH)_4]. Carbon dioxide is dissolved into this slurry, by diffusion through the surface and gas dispersion within the aqueous phase. The process includes dissolution of the mineral and precipitation of the magnesium carbonate mineral magnesite (MgCO_3) in a single unit operation. Activation of the silicate minerals has been achieved by thermal and mechanical means, resulting in up to 80% stoichiometric conversion of the silicate to the carbonate within 30 minutes. Heat treatment of the serpentine, or attrition grinding of the olivine and/or serpentine, appear to activate the minerals by the generation of a non-crystalline phase. Successful conversion to the carbonate has been demonstrated at ambient temperature and relatively low (10 atm) partial pressure of CO_2 (P_(CO2))- However, optimum results have been achieved using the bicarbonate-bearing solution, and high P_(CO2). Specific conditions include: 185°C; P_(CO2)=150 atm; 30% solids. Studies suggest that the mineral dissolution rate is not solely surface controlled, while the carbonate precipitation rate is primarily dependent on the bicarbonate concentration of the slurry. Current and future activities include further examination of the reaction pathways and pretreatment options, the development of a continuous flow reactor, and an evaluation of the economic feasibility of the process.
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