Presently, mitigating global warming tops the ‘to do’ list of environmental sustainability agenda. This has been triggered by the current realization of the adverse environmental effects being witnessed today, such as: decreasing number of glaciers, increasing ocean acidification and strengthening seasonal variations. Excess dumping of carbon dioxide (CO2), the culprit associated with the aforementioned effects, has since become a global concern. As a remedy, CO2 sequestration using various techniques, such as: mineral carbonation (MC), use of saline aquifers – among other carbon sinks, has been proposed. Compared to other methods, the MC has the special advantages of permanent safe storage and abundant mineral resources. Better still, majority of materials suitable for the MC are Mg-/Fe-/Ca- silicates minerals which are major minerals of the Earth’s crust.
Unfortunately, the MC process is limited by kinetics, mainly by one of the three possible steps: the dissolution of CO2 into aqueous solution, the dissolution of Mg-silicates or the formation of solid products under diffusion control. Worse off, at present, the detailed kinetics and mechanism of how carbonation proceeds, is more of a mystery, thereby hindering the successful application of this process.
To this note, University of British Columbia researchers Fei Wang (PhD candidate) and Professor David Dreisinger in collaboration with Giga Metals Corporation scientists: Dr. Mark Jarvis and Tony Hitchins presented a study where they systematically assessed how various factors of mineral carbonation (e.g. temperature, particle size, addition of salts and additives, carbon dioxide overpressure) affect the kinetics and mechanism of the MC process. Specifically, they investigated MC using narrowly sized particles of olivine and lower CO2 pressure, less than 45bar, so as to avoid high pressure equipment that would have high capital costs, while still incorporating sodium bicarbonate and sodium chloride, as they are the most classical additives for the MC thus far. Their work is currently published in the research journal, Minerals Engineering.
The experimental procedure involved the use of narrow-size particles of olivine throughout the fundamental research. Firstly, the factor of particle size was studied. All things considered, the comprehensive effects of all factors on MC of olivine were investigated. The materials used for the study were: high grade natural olivine, high purity CO2, sodium bicarbonate and sodium chloride. Several high-end equipment was also used in the course of the study.
The authors observed that, with low addition of sodium salts, the MC process was controlled by diffusion through the Si-rich layer. Alternatively, with high addition of sodium salts coupled with high CO2 partial pressure, the researchers noted that the chemical reaction between H+ and non-reacted olivine took control of the MC process. As such, the functions of the sodium bicarbonate introduced to the MC were seen to not only include continuous supply of sufficient dissociated H+ and CO32− from CO2 gas, but also to increase the ionic strength, which enhanced the dissolution of Mg/Fe and the precipitation of bivalent metal carbonates and the diffusion of temporarily aqueous silica respectively.
In summary, the study presented a comprehensive kinetic research on the effects of various factors on the MC of olivine for CO2 sequestration. Generally, the variation of the mechanism of MC of olivine was dependent on the usage of sodium salts and the CO2 partial pressure. Altogether, the improvement of the MC theory provides the opportunity to economically enhance the process, which would translate low CO2 levels hence better environmental mitigation.
Fei Wang, David Dreisinger, Mark Jarvis, Tony Hitchins. Kinetics and mechanism of mineral carbonation of olivine for CO2 sequestration. Minerals Engineering, volume 131 (2019) page 185–197.Go To Minerals Engineering