Study Of Calcium And Magnesium Mineral Carbonation Under Supercritical CO₂ Condition

While mineral sequestration of supercritical carbon dioxide has been widely regarded of a leakage-free and long-term CCS（carbon dioxide capture and storage）option,the understanding about the thermodynamics and kinetics of the interaction between host rock and scCO₂ under actual sequestration is limited.Through the whole period of geological sequestration,the pH value will decrease temporarily and cause the rock dissolution in the area of injection.The ion will be transport to somewhere away from this area.With the consumption of the H+,the pH value will increase and mineral carbonation part will happen.As a result,the research is started from these two parts,anorthite batch dissolution experiments and carbonation of natural brucite in H2O were carried out.The dissolution experiments were done at 160℃、15MPa for 2、5、10、20、50 days.A sequence of secondary minerals including boehmite,pseudo-boehmite formed with increasing time.Pseudo-boehmite,probably a metastable phase,is basically similar to boehmite in structure,but their stacking orders of the A1 octahedral layers as well as morphologies and chemistries are different.The experiments indicate that in a supercritical CO₂-water-anorthite system the anorthite dissolution is incongruent under the above conditions and approximated by a two-stage process.The dissolution rate in second stage is slower than the first by approximately one order of magnitude because of the saturation state with respect to anorthite.And the dissolution rate in in a supercritical CO₂-water-anorthite system is faster than in atmospheric pressure by approximately two order of magnitude.On the carbonation part,we choose brucite and periclase as the raw material.It is widely recognized that the presence of water is vital for brucite carbonation in supercritical CO₂（scCO₂）based upon the observation that nesquehonite（MgCO3·3H2O）precipitated instantly in brucite-water-scCO₂ system while no carbonate was detected in brucite-scCO₂ medium.The importance of water is further emphasized by the result that shows higher water concentrations led to greater carbonation rates.However,the mechanism of water effect is not fully understood as two possible pathways exist for the carbonation reaction:1)the dissociation of H2O provides a)H+to release Mg2+ through associating with OH-in brucite,and b)OH-to combine with CO₂ to form HCO3-,leading to the interaction of magnesium cations with carbonate anions;2)H2O molecules polarizes CO₂ through hydrogen bonding to promote direct interactions between carbon dioxide and brucite followed by water release from Mg（OH）2（Churakov et al.,2004;Zhao et al.,2010）.Experiments were carried out in water-saturated scCO₂,and formamide-saturated scCO₂,at 50℃ temperature and 8MPa pressure.Results demonstrated that when water is a solvent in the system,it not only take effect by its polarization,but also works as a reactant.However,formamide just works as a catalyzer,and its effect is limited.