Direct carbonation or direct sequestration of a mineral can be conducted in two ways: as a direct dry gas-solid reaction or in an aqueous solution.
Direct gas-solid carbonation with CO2
The most straightforward process route is the direct gas-solid carbonation. This was first studied by Lackner. Various reactions depending on the feedstock are possible. As an example, the direct gas-solid reaction of olivine is given:
Mg2SiO4 (s) + 2CO2 (g) 2MgCO3 (s) + SiO2 (s)
High CO2 pressures are necessary in order to obtain reasonable reaction rates. The reaction rate can further be improved by the use of supercritical CO2. The produced water dissolves in supercritical CO2.
From natural rock weathering it is known that water greatly improves the reaction rate. A process developed on the basis of this principle is the carbonic acid route (O'Connor et al.,1999; O'Connor et al., 2000b), in which CO2 reacts at high pressure in an aqueous suspension of forsterite or serpentine. First, CO2 dissolves in the water and dissociates to bicarbonate and H+ resulting in a pH of about 5.0 to 5.5 at high CO2 pressure:
Mg2SiO4 (s) + 2HCO3- (aq) Τ 2MgCO3 (s) + SiO2 (s) + 2OH- (aq)
A bicarbonate/salt mixture (NaHCO3/NaCl) can be used to accelerate the reaction (O'Connor et al., 2000b). The sodium bicarbonate increases the HCO3 - concentration and thus accelerates the carbonation reaction.
The solution chemistry can be further improved by adding alkali metal hydroxides. Thus the pH of the solution is elevated and the absorption of CO2 is further improved. The hydroxide is also not consumed. Both HCO3- and OH- act as a catalyst. Probably an optimum pH-value exists, depending on which route is preferred: reaction of magnesium with bicarbonate or carbonate ions. Another option that could improve the dissolution of serpentine and simultaneously elevate the pH is the addition of Na 2CO3
Process flow diagram of the aqueous direct carbonation route using serpentine and olivine (O'Connor et al., 2000).