Chemistry of aqueous mineral carbonation for carbon sequestration and explanation of experimental results

In aqueous mineral carbonation for carbon sequestration, high-pressure CO, is injected into water or sodium bicarbonate solution mixed with olivine or serpentine to produce magnesium carbonate. Thus, CO2 gas is fixed in a solid carbonate for sequestration. Such reactions are generally slow, and a significant amount of research was conducted to increase the reaction rate. This article is an initial effort using basic thermodynamic analysis to understand this complicated heterogeneous chemical process, and to explain some experimental results. The approach started with decomposing the process into two basic steps: magnesium ion dissolution from the olivine or serpentine, and magnesite precipitation. Thermodynamic calculation then is used to assess two important parameters for each of these two steps: the pH and carbonic ion concentration. The calculations explain the roles of increased CO2 pressure, elevated temperature, and adding sodium bicarbonate for enhancing the carbonation reaction, and these results agreed well with the experimental data. The analysis also indicates that for reaction routes in which leaching magnesium silicate and forming magnesium carbonate occur in the same reactor, lowering the pH helps dissolve magnesium ions from silicate, but it lowers carbonic ion concentration and limits precipitation of magnesite; careful balance of these two steps is critical. Further improvement may be built upon this model. (c) 2006 American Institute of Chemical Engineers Environ Prog.