Augmenting the Magnesite Yield Produced during Aqueous Mineral Carbonation of Dunite Rock

The capture, storage and utilisation of CO₂ emissions from large volume point sources is challenging. Given the volume of CO₂ to be sequestered from these sources, it is necessary to consider all magnesium-rich (ultramafic) rocks as potential feedstocks when developing a mitigation process based on mineral carbonation. This research explores serpentinised dunite (which comprises 61% lizardite) as a potential feedstock for mineral carbonation. Dunite rock for this study was collected from a dunite quarry (located near Bingara), The Great Serpentinite Belt, NSW, Australia. The dunite was characterised using XRD, TGA-MS and ICP-OES techniques and this showed the presence of olivine and brucite, more reactive components compared to the predominate lizardite mineral.

Dunite was first heat-activated (630 °C, 4 h), similar to the procedure adopted for serpentinite rocks to increase their chemical reactivity. Heat-activation converts crystalline lizardite mineral into an amorphous, reactive phase and the use of this heat-activated material resulted in a magnesite yield of 44% compared to 20% obtained with raw dunite under similar reaction conditions. Carbonation experiments were performed using a sub 75 µm fraction of these materials at 180 °C, 130 bar CO₂ pressure, 15% solids slurry and using 0.64 M NaHCO₃ solution. Samples of dunite were also heat-transformed at high temperature (heat treated at 800 °C, 4 h) to convert lizardite to forsterite and these samples were also studied as feedstock for carbonation experiments. Heat-activated dunite was found to engender much higher magnesite yields compared to heat-transformed dunite (forsterite rich) and raw dunite. Magnesite yields from heat-transformed dunite (64% forsterite) were slightly higher compared to the yield obtained from raw dunite.

The fraction of magnesium, silicon and iron extracted (through acid dissolution) from the heat-activated dunite was higher compared to the heat-transformed dunite and raw dunite. Temperature programmed desorption experiments disclosed a higher number of magnesium sights in the heat-activated dunite compared to raw dunite and heat-transformed dunite, further confirming that amorphous magnesium silicate rich materials are more reactive. This study suggests that during heat-activation of dunite, harzburgite, antigorite or lizardite, conditions should be maintained to avoid forsterite formation.

The order of reactivity was found to be; heat-activated dunite > raw dunite with regrinding > heat-transformed dunite > raw dunite > raw lizardite. Forsterite-rich materials (Twin Sisters Mountain dunite, raw dunite and heat-transformed dunite) displayed incongruent dissolution while amorphous magnesium silicates rich (heat-activated dunite) showed congruent dissolution.

The OH stretch present around 3690 cm^-1 wavenumber (FTIR spectra) was revealed to be due to presence of OH stretches in lizardite, which is major constituent of raw dunite. Heat-activation removes chemically bound water, so the OH bands disappear in heat-activated dunite. Following the carbonation of heat-activated dunite, the OH stretch band reappears in the carbonated samples, centred around 3680 cm^-1 wavenumbers. We postulate that vicinal silanol formation has occurred during the carbonation reaction as a result of the amorphous phase reaction with water. Formation of silanol nests has been observed through FTIR and is consistent with TGA-MS analysis.