Carbonate is the lowest energy-state of carbon. The carbonation reactions of magnesium and calcium oxide are strongly exothermic (Lackner et al., 1995)
CaO (s) + CO2 (g) Τ CaCO3 (s) (ΔHr = -179kJ/mol)
MgO (s) + CO2 (g) Τ MgCO3 (s) (ΔHr = -118kJ/mol)
As mentioned before, magnesium and calcium occur typically as calcium and magnesium silicates. The silicates of Mg and Ca react with CO2 to give the respective carbonates, SiO2 and water. These reactions are still exothermic, but to a lesser extent than the carbonation of pure oxides.
The carbonation reaction with gaseous CO2 proceeds very slowly at room temperature and pressure. Increasing the temperature increases the reaction rate. However, because of entropy effects the chemical equilibrium favors gaseous CO2 over solid-bound CO2 at high temperatures (calcination reaction). The highest temperature at which the carbonation occurs spontaneously depends on the CO2 pressure and the type of mineral. Some examples are given below.
Different process routes for mineral CO2 sequestration have been postulated in the literature. Most of them are a combination of a pre-treatment and a sequestration process.
The pretreatment options are discussed first, followed by a description of the sequestration routes.