Critical Role of Water during CO2 Sequestration in Nickel Mining Residues Under Ambient Conditions

One of the main environmental issues associated with industrial mining activities is the production and above-ground storage of large amounts of coarse-grained mining wastes, or fine-grained residues produced during mineral processing. Some of the minerals present in these wastes, especially magnesium silicates and brucite, are known to exhibit reactivity with carbon dioxide to form stable and environmentally benign carbonates. Milling process residues from the Dumont Nickel Project (DNP, Québec) contain considerable amounts of chrysotile, lizardite and brucite. A differential diffusion carbonation cell was built to monitor the kinetics of mineral carbonation under ambient conditions and concurrent evolution of pore water saturation across a differential (or zero-length) bed consisting of DNP nickel milling residues. The kinetic measurements reveal the complex role of water both as reacting medium and moiety in the carbonation pathway, especially with regard to brucite in the residues. Mg²⁺ and OH^- ions, produced by brucite dissolution, are essential components in regulating medium alkalinity and formation of carbonated product. Instead of forming anhydrous MgCO₃, the highly hydrated nature of Mg²⁺ rather favors the formation of metastable hydrous carbonates. The pH drop of almost one unit in the early reaction stages is followed by a gradual recovery of basicity suggesting that rate-controlling CO₂ dissolution is slower than magnesium and hydroxyl production rates. In the pH drop phase, carbon dioxide depletion from the cell headspace was mirrored by a sharp increase in water saturation (as monitored with a total diffuse reflectance probe buried in the bed) providing evidence of formation of hydrated carbonate species. Nesquehonite (Mg(HCO₃)(OH)•2H₂O) was identified by XRD analysis as the neoformed carbonate mineral. In this study emphasis will be placed on the formation and consumption of water during ambient mineral carbonation and possible carbonation reaction pathways will be discussed. The results of this study can bring insights on the carbonation mechanisms occurring within mining residue piles in contact with ambient air.