Development of Grinding Media for Aqueous Mineral Carbonation Applications

Carbon capture utilisation and storage (CCUS) integrates a number of technologies with the overall aim of reducing the net emission of CO₂ into the receiving environment. Mineral carbonation converts CO₂ into stable mineral carbonates via its reaction with magnesium (Mg) or calcium rich minerals under aqueous conditions. Enormous reserves of peridotite and serpentinite rocks can be utilised as a feedstock for the safe and permanent sequestration of global CO₂ emissions via aqueous mineral carbonation.

The peridotite rock dunite was collected from a quarry located in The Great Serpentinite Belt, NSW, Australia. Mined olivine was imported from Aheim plant (Sibelco, Norway). The dunite and olivine samples were characterised using XRD, TGA-MS and ICP-OES analytical techniques. Dunite was found to be composed of 61% lizardite, 30% olivine, 8% brucite and 1% magnetite. The composition of the olivine sample was determined to be 94% (pure) olivine, with minor impurities of lizardite, clinochlore, enstatite and talc. Definite particle size “bins” of dunite and olivine were prepared via wet sieving. Carbonation experiments were performed at 180 °C, 130 bar CO₂ pressure, 15% solids slurry and using 0.64 M NaHCO₃ solution. Silica-rich layers were observed (SEM/EDS) to form around the reacted particles following aqueous mineral carbonation reaction. These layers were depleted in Mg and rich in silicon. Previous research has disclosed that during aqueous mineral carbonation, the reduction in the extent of Mg extraction is due to the formation of silica rich layers. The current research suggests that this process is counteracted through continuous exfoliation of the reacting mineral particles achieved through concurrent grinding, enabling continuous Mg extraction from the raw feedstock. In concurrent grinding, the grinding media is added during the reaction (in situ) to disrupt silica-rich layers.

Some feedstocks require ultrafine grinding, whereas others require heat activation, to engender reactivity in the rock. Both processes are energy intensive and are obstacles to the commercial application of mineral carbonation. Here we show that these limitations can be addressed, at least in part, through the application of an optimised concurrent grinding technique. Optimised concurrent grinding was shown to result in a significant increase in magnesite yields for non-heat activated dunite without fines, and the results obtained can, under some circumstances, achieve yields comparable to those produced from heat activated serpentine.

Using optimised concurrent grinding conditions, as determined from statistical experimental design, excellent magnesite yields of up to 62% have been attained with raw 20-45 µm dunite and without heat-activation. Qualitative and semi-quantitative XRD has shown that brucite and olivine are almost entirely consumed during the reaction whilst lizardite only partially so, indicating that raw feedstocks can be carbonated through application of concurrent grinding. Concurrent grinding also provided higher yields compared to non-simultaneous grinding (grinding and carbonation in separate steps) demonstrating that simultaneous grinding and leaching is preferable.

We propose that optimised concurrent grinding may be a suitable technique for the processing of alternative feedstocks (those containing significant proportions of forsterite and pyroxene that are therefore unsuitable for thermal activation) for aqueous mineral carbonation.