Ultramafic mine waste - potential material for CO2 storage and metallic elements immobilization

Carbon dioxide, which is a direct product of the combustion of fossil fuels is the main component that enhances the greenhouse effect on Earth. For years, solutions have been developed to reduce the carbon dioxide present in the atmosphere. One of the methods of CO₂ reduction is the mineral carbonation of rocks and anthropogenic materials. As a result of the reaction of silicates with CO₂, new carbonate minerals that are stable under surface conditions are formed. Ultramafic rocks are considered one of the best substrates for mineral carbonation with the experiments showing relatively high carbonation efficiency. This is because they contain abundant Mg-rich minerals (olivine, serpentine) that readily react with CO₂-rich fluids to form Mg-carbonates and silica. However these rocks contain also high abundances of metallic elements, in particular Ni, Cr, and Co, that may be mobilized during the carbonation experiments. This presentation aims at the characterization of selected ultramafic mine waste, in terms of the metallic element content. The goal of our research is to answer the question of whether it is possible to simultaneously bind both CO₂ and metallic elements using mineral carbonation experiments.
We have chosen three types of ultramafic rocks for our study, two are from abandoned quarries and the third is a mine waste accompanying magnesite exploitation. Their chemical composition is characteristic of ultramafic rocks with high contents of silica and magnesia (both up to 40 wt %), with minor Fe₂O₃ and alumina (up to 10 and 3 wt % respectively). Metallic element content reaches values of up to 3,400 mg kg^-1 for Cr, 2,500 mg kg^-1 for Ni, and 125 mg kg^-1 for Co. Two rocks represent partially serpentinized peridotites, whose main minerals are olivine and serpentine. The third rock is serpentinite, composed almost exclusively of serpentine group minerals. The minor phases in all the rocks are chlorite and spinel group minerals, with variable chemical compositions ranging from magnesiochromite through Cr-magnetite to magnetite. Peridotites contain amphiboles as minor components whereas carbonates (dolomite and magnesite) and sulfides (mainly Ni-Fe sulfides) were identified as accessory minerals in all the samples. The main Cr-bearing phases are the spinel group minerals, in the case of Ni these are serpentines and olivines, while Co is mainly concentrated in sulfides. Bulk chemical analyses of magnesite veins, naturally occurring in one of the quarries, revealed up to 250 mg kg^-1 of Ni, and up to 3 and 5 mg kg^-1 of Cr and Co, respectively. Although this indicates that magnesite has the potential to structurally incorporate Ni, further investigations are required to constrain the incorporation mechanism and the potential for the immobilization of Cr and Co in carbonate minerals.
Acknowledgment
The work is funded by the National Science Centre, research project No. 2021/43/B/ST10/01594.