Carbon dioxide sequestration by mineral carbonation

Aqueous mineral carbonation is a technology that is being studied and developed within the framework of the carbon dioxide capture and storage (CCS) system [6]. It aims at binding CO₂ in stable carbonates using metal oxides via a high-pressure and high-temperature ex situ process, which mimics natural rock weathering. For a large scale implementation of this process, the most suitable source of metal oxides are magnesium silicate minerals, such as olivine and serpentine, due to their large worldwide availability. Aqueous mineral carbonation of olivine involves CO₂ dissolution into water, the dissolution of the silicate minerals, followed by the precipitation of Mg-carbonates. In the case of the carbonation of the olivine mineral forsterite, the magnesium end-member of olivine, the chemical reaction in CO₂-saturated solution is:

In particular, the dissolution of the silicate minerals is considered to be the rate-limiting step of the entire aqueous mineral carbonation process. As the slowness of the dissolution kinetics hinders the overall carbonation reaction, to make the process technically and economically feasible, such a reaction should be sped up by finding the optimum operating conditions. At this aim, we performed an extensive experimental series to study the dissolution kinetics of natural olivine (Mg_1.82Fe_0.18SiO₄) in a system highly undersaturated with respect to the dissolving phase. We used a flow-through reactor at 90-120-150°C, adjusting the pH by adding either acids (e.g., HCl) or LiOH, and by changing P_CO2 [1,2,4]. The salinity was changed by adding NaCl and NaNO₃ and furthermore, organic salts, such as sodium oxalate and sodium citrate, were added to enhance the kinetics [5]. The dissolution rate was estimated from the experimental data by using a population balance equation (PBE) model, assuming surface-controlled reaction kinetics, coupled with a mass balance. The obtained values were regressed with the linear model, logr = -npH - B, where r is the specific dissolution rate (mol s^-1 cm^-2). The dissolution kinetics was found to depend primarily on pH and it can be accelerated by the addition of organic salts.

With the final aim to investigate the feasibility of a single-step mineral carbonation process in which both the dissolution of silicate and the precipitation of carbonate can occur successively, we also studied Mg-carbonate crystallization kinetics. We carried out experiments in batch with the H₂O-CO₂-Na₂CO₃-MgCl₂ system at 90, 120, and 150°C and at 100 bar of CO₂, using Raman spectroscopy for on-line monitoring. Within the pH range of 5 to 6, we observed two mechanisms; direct precipitation of magnesite when the system was slightly supersaturated with respect to hydromagnesite and magnesite and simultaneous precipitation of both crystals followed by the transformation of the latter into the former at higher supersaturation ratio values [3]. Keeping the temperature constant, the process slowed down at lower P_CO2.

Given these results, it appears that the combination of olivine dissolution and magnesite precipitation is thermodynamically and kinetically feasible at the investigated temperature range, at high P_CO2, and at pH between 5 and 6. However, the combined process could present various features still to be explored. Since a lower undersaturation with respect to the dissolving olivine is expected, the dissolution kinetics could change. Moreover, the precipitation of other phases, which is likely to happen, could hinder the overall carbonation kinetics. A deep understanding of the phenomena is the topic of our actual research.

 

Publications

[1] Prigiobbe, V., Costa, G., Hänchen, M., Baciocchi, R., Mazzotti, M. - submitted. The effect of CO₂ and salinity on olivine dissolution kinetics at 120°C. Chem. Eng. Sci.

[2] Hänchen, M., Prigiobbe, V., Baciocchi, R., Mazzotti, M. - 2008. Precipitation in the Mg-carbonate system - effects of temperature and CO₂ pressure. Chem. Eng. Sci. 63, 1012-1028.

[3] Hänchen, M., Krevor, S., Lackner, K., Mazzotti, M. - 2007. Validation of a population balance model for olivine dissolution. Chem. Eng. Sci. 62, 6412-6422.

[4] Hänchen, M., Prigiobbe, V., Storti, G., Seward, T.M., Mazzotti, M. - 2006. Dissolution kinetics of fosteritic olivine at 90-150°C including effects of the presence of CO₂ . Geochim. et Cosmochim. Acta 70, 4403-4416.

Other references

[5] Olsen, A. A., Rimstidt, J. D. - 2008. Oxalate-promoted forsterite dissolution at low pH. Geochim. et Cosmochim. Acta 72, 1758-1766.

[6] IPCC - 2005. Special Report on CCS. Metz, B., Davidson, O., de Coninck, H., Loos, M., Meyer, L. (Eds.) Cambridge University Press, UK. pp. 431.