Chemical and Crystallographic Controls on Wollastonite Carbonation

During the natural processes of chemical weathering occurring on the Earth's surface, the interaction between atmospheric carbon dioxide and silicate rocks leads to the precipitation of carbonate phases, which represent a relatively stable and secure store for this CO₂ (Oelkers et al. 2008 and references cited there). Industrial carbonation reactions based on these natural processes are nowadays considered a potentially viable strategy to mitigate the increasingly higher levels of atmospheric CO₂ of anthropogenic origin.

However, before these processes can be industrially implemented, there are aspects that require of further study and understanding for their optimization, such as for example the parameters that control the distribution of the precipitated phases and the formation of porosity. These are critical aspects as they may determine the conditions that give rise to silicate surface passivation and, therefore, to block reaction. With this purpose, this paper presents an experimental study on the carbonation of wollastonite (CaSiO₃) in aqueous solutions under varying pH conditions. This system is considered a model of silicate carbonation reaction, and it has been selected since the dissolution rate of wollastonite is relatively fast, its behaviour in solution has been widely investigated for more than 20 years, and its chemical composition is simple.

Carbonation experiments were performed in aqueous phase (90 ºC) in closed Teflon® reactors using natural, high-purity wollastonite from Barberton District (province of Mpumalanga, South Africa). 30 mg of wollastonite were added to the reactors together with 1 mL of a 1M NaHCO₃solution at pHs ranging from 3 up to 10. Partially reacted samples were analyzed using scanning electron microscopy (FESEM), EDX microanalyses and micro X-ray computed tomography. In addition, sections of these samples were studied using electron backscattered diffraction (EBSD), QUEMSCAN and Raman spectrometry coupled to the SEM. The evolution of the fluid composition and the supersaturation with respect to the different phases was simulated using PHREEQC software (Parkhurst and Appelo, 2013).

The results of this preliminary study show how the simultaneous precipitation of calcite and amorphous silica as consequence of the dissolution of wollastonite in carbonated slightly alkaline solutions can passivate the wollastonite unreacted surface, what It would be the blocking of the carbonation reaction. The distribution of these phases is controlled by the evolution of the solution composition in contact with the wollastonite surface, which may significantly differ from that of the bulk solution. Additionally, we show how the presence of cracks represents an important pathway for the circulation of fluids and, therefore, for the progress of the reaction, even in those cases in which the mineral surface could be passivated as a result of this replacement reaction.

This work has been funded by the Spanish government under the grants CGL2015-70642-R and CGL2015-73103-EXP.

References

Oelkers, E.H., Gislason, S.R., Matter, J. (2008): Mineral carbonation of CO2. Elements 4, 333–337.

Parkhurst, D.L. & Appelo, C.A.J. (2013): Description of input and examples for PHREEQC version 3—A computer program for speciation, batch-reaction, one- dimensional transport, and inverse geochemical calculations. U.S. Geological Survey Techniques and Methods, 6, A43, 497 p.