The basic idea of carbon dioxide mineral sequestration is to transform minerals (mostly calcium or magnesium silicates) with CO2 to (Ca or Mg-) carbonates. The most promising feedstock minerals are Olivine, Serpentine and Wollastonite. These silicates of Magnesium and Calcium react with CO2 to form the corresponding carbonates and SiO2. The decisive fact for these reactions is that all of them are exothermal which means that theoretically no energy is required for the reactions to proceed but could potentially even be gained. The resulting mineral carbonates are stable and abundant in nature anyways and could hence be disposed off without any considerable consequences.
The separated stream of carbon dioxide is transported to a mineral mining site where a chemical process scheme will be erected to convert the carbon dioxide with a mineral to the carbonates which then can be disposed off back in the mine. A variety of process pathways are described in literature for the mineral carbonation process.
An overview of the proposed pathways is shown in the below figure. These processes can roughly be divided into direct and indirect routes. In the direct routes the dissolution and the carbonation proceed in one step, whereas in the indirect routes the reactive compound is first extracted from the mineral and then carbonated in a second step.
Investigated CO2 mineral carbonation process pathways
The indirect process routes can be further subcategorized into strong acid and weak acid approaches.
The carbonation and solvent recovery proceed in one step. The solvent (HX) has to be more acidic than silicic acid (pKa = 9.61) thus being able to dissolve the mineral. At the same time the solvent has to be less acidic than carbonic acid (pKa = 3.6) as the acid shall be replaced by carbonic acid in the crystallization step. The strong acid approaches can be split up in three process steps:
In a first step, the mineral is dissolved in a strong acid. The carbonation proceeds in a second step if the pH is raised. With energy input (ΔH>0) the salt can then be recovered to the original acid and base. Further variations of the strong acid approach are described with a split up carbonation stage
This approach is favorable as the exothermic energy from the carbonation step is available on a fairly high temperature level and can possibly be utilized for e.g. the acid recovery step.