Sequestration of CO2 after reaction with alkaline earth metal oxides CaO and MgO

The uptake of CO2 by CaO and MgO in aqueous solution has been studied in batch experiments at different temperatures (15–75 °C), stirring rates (300–600 rpm) and solid liquid ratios (0.0125–0.125 mol L−1) in order to identify the kinetic controls and limitations. Both minerals are constituents in many combustion residues and their carbonation is considered as an option for CO2 sequestration. Generally the uptake of CO2 by metal (hydr)oxide suspension increased with higher turbulence due to better gas/water exchange and higher temperature due to enhanced rates of mineral dissolution and carbonate precipitation. The reaction of CaO and MgO with CO2 in solution occurred at distinctly different pH values. While in the CaO systems a pH between 12.8 (0.1 mol L−1) and 11.7 (0.0125 mol L−1) was established, suspensions with MgO only reached maximum pH values of ∼10.3 even though the mineral surface area was larger in these assays (140 m2 L−1 for MgO compared to 7.5 m2 L−1 for CaO). This difference in reaction pH affected the transfer of CO2 from the gas into the liquid phase as well as dissolution rates of the minerals themselves.Maximum CO2 uptake rates were 0.027 mmol L−1 s−1 for the CaO containing experiments at pH 12.8. Dissolution of CaO was fast and calcite precipitation occurred instantaneously at all tested temperatures and concentrations. Thus, the CO2 dissolution step was limiting the overall carbonation reaction. Under otherwise similar conditions maximum CO2 uptake rates were lower for the MgO system. Magnesium carbonate precipitation was slower than calcite precipitation and depended strongly on the different boundary conditions, in particular temperature and the degree of supersaturation. Temperatures greater than 50 °C were required for assays with low dissolved Mg2+ and CO32- concentrations in order to precipitate Mg carbonate in the form of hydromagnesite (Mg5(CO3)4(OH)2·4H2O). Below 50 °C the use of high concentrations of Mg and C species were necessary to initialise precipitation of Mg carbonate as nesquehonite (MgCO3·3H2O). This has implications for the application of mineral carbonation in an aqueous system as a means of C capture and storage. Calcium-rich materials can be utilized efficiently and almost independently of reaction regime if an optimum supply of CO2 is provided. In contrast specific conditions are required to use the CO2 binding potential of alkaline Mg minerals, as T > 50 °C or high concentrations are required for Mg carbonate precipitation.

► Gas phase CO2 transfer into the aqueous phase was the rate limiting step for CaO carbonation. ► MgO carbonation is a slower and much more complex process. ► Temperatures >50 °C are favourable for precipitation of CaCO3 and Mg carbonates. ► Ca rich materials should be utilized preferentially for aqueous mineral carbonation. ► CaO and MgO can be used as proxies to estimate most common alkaline waste reaction with CO2.