Does size matter? Particle size effects on Ni and Cr mobilization during ex situ mineral carbonation of ultramafic rocks.

  Ex situ mineral carbonation of ultramafic rocks (UMR) is a promising method for CO₂ removal from the atmosphere. UMR like The particle size is a one of key parameter affecting carbonation because the reduction of size increases surface area and consequently the availability of reactive divalent cations^[1].This research investigates the effect of initial particle size on the mobilization of Ni and Cr into carbonation fluid during ex situ carbonation of two types of UMR: (a) serpentinized peridotite, and (b) serpentinite.

          UMR used in this study were sourced from the Central Sudetic Ophiolite (SW Poland), representing late Devonian oceanic lithosphere. Serpentinized peridotite contained 2060 ppm of Ni and 2950 ppm of Cr, while serpentinite contained 2110 ppm of Ni, and 2240 ppm of Cr. In serpentinized peridotite the majority of Ni was hosted in forsterite, which contained up to 0,37 wt.% NiO and constituted ~45% of the rock's modal composition (based on Rietveld refinement). In serpentinite, Ni was primarily concentrated in serpentine subgroup minerals (containing up to 0.25 wt.% NiO), which constituted ~95% of the analyzed sample. In both studied rocks the highest Cr concentrations were measured in Fe-Cr spinels, and Cr₂O₃ content did not exceed 36 wt.%. Thus, in analysed UMR, Fe-Cr spinels may serve as the primary source of Cr, while the supply of Ni to the system may be controlled by forsterite and/or serpentine subgroup minerals.

          In the first stage of the project, samples were ground in an agate mortar using a planetary ball and sieved to obtain three fractions of different particle size distributions: 250 μm – 125 μm, 125 μm – 50 μm, and < 50μm. The carbonation experiments were performed in batch-type reactor for 24 hours, under controlled P-T conditions at 185°C and 100 bar of PCO₂, using 20 g of ultramafic feedstock and 200 ml of ultrapure water. Preliminary measurements of the specific surface area (SSA) of serpentinized peridotite before carbonation, determined using the CO₂ adsorption (DR) method, revealed minimal SSA variation between fractions, ranging from 12.61 to 14.61 m²×g-1. In the next stage, cation concentrations in post-experimental leachate will be analyzed using ICP-OES. Furthermore, the solid carbonation products will be studied by Electron probe microanalysis (EPMA) to identify secondary phases that could serve as sinks for Ni and Cr. We anticipate that this research approach will allow the identification of the optimal particle size of UMR for efficient CO₂ equestration and the controlled mobilization of potentially hazardous metallic elements during ex situ carbonation.

^[1] Santos, R. M., & Van Gerven, T. (2011). Process intensification routes for mineral carbonation. Greenhouse Gases: Science and Technology, 1(4), 287-293.