The Use of Carbonic Acid in an Ex Situ Mineral Carbonation Process
Open Access
- Author:
- Alexander, George Wilbur
- Graduate Program:
- Energy and Mineral Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- April 04, 2012
- Committee Members:
- Harold Harris Schobert, Dissertation Advisor/Co-Advisor
Harold Harris Schobert, Committee Chair/Co-Chair
Sharon Falcone Miller, Committee Member
Tanya Furman, Committee Member
Mark Stephen Klima, Committee Member - Keywords:
- Mineral Carbonation
Antigorite
Dissolution
Sequestration
CO2 Storage - Abstract:
- The reaction of common magnesium and calcium silicate minerals with carbon dioxide can provide benign and long-term carbon storage. The prevalence of antigorite, its ability to buffer against acidic conditions from the dissociation of carbonic acid, and the resultant extraction of magnesium show promise as an underlying strategy for the production of magnesium carbonates without the consumption of additional reagents. This process circumvents several challenges that are associated with the use of strongly acidic media, but is limited by the rate and extent of dissolution. A parametric study on the effects of CO2 partial pressure, particle size, reactor temperature, and solids concentration has been conducted to evaluate the extraction of magnesium. The presence of accessible and large-scale deposits of magnesium silicates owes to the recalcitrant nature of these minerals. As the dissolution of silicate minerals is critical to an ex situ mineral carbonation process, the quantification and characterization of this reaction is important. In this study, the evolution of the solution pH and release of magnesium from different particle size fractions of antigorite in the presence of carbonic acid provide insight into the potential for a large-scale, direct ex situ mineral carbonation process. Carbonic acid is shown to be capable of disrupting magnesium-oxygen bonds at reactive sites that were generated during the grinding process. However, the proportion and reactivity of these sites may be incapable of sustaining a dissolution rate that is commensurate with the scale of CO2 production for a coal-fired power plant without further technological advances. In addition to the challenges associated with magnesium silicate dissolution, the carbonation of magnesium ions once in solution is not trivial. The potential for the precipitation of magnesium carbonates was evaluated for a series of experiments that varied the partial pressure of CO2, particle size, reactor temperature, and solids concentration. This range of experimental conditions results in only modest improvements in the activity of magnesium in solution. However, these conditions lead to a range of bicarbonate activities that spans three orders of magnitude. Geochemical modeling indicates a high degree of supersaturation with respect to magnesite, although no precipitates have been observed. Kinetic limitations within the Mg-H2O-CO2 system favor the formation of the metastable magnesium carbonates hydromagnesite, Mg5(CO3)4(OH)2•4H2O, and nesquehonite, MgCO3•3H2O, under these reaction conditions. The saturation indexes for these phases indicate that the solutions in this study are likely undersaturated with respect to hydromagnesite and nesquehonite. The precipitation of magnesium carbonates can be facilitated by mitigating the strong Mg2+-H2O interactions. An initial investigation into the effects of methanol and monoethylene glycol indicates that the solubility of hydromagnesite and magnesite could be reduced without the consumption of additional reagents.