New tools for stimulating dissolution and carbonation of ultramafic mining residues

The carbonation of chrysotile and nickel mining residues was studied under ambient laboratory conditions to assess their response under various potential field conditions (pore saturation, watering episodes, temperature, CO 2 diffusion, and dissolved oxygen) and to a variety of natural and chemical enhancers (sulfide minerals, brucite, chelate ligands, ionic liquids, and carbonic anhydrase enzyme). Watering of the residues to achieve partial pore saturation was critical for optimal ambient CO 2 sequestration by mediating magnesium leaching and CO 2 absorption, and by favouring diffusion of gaseous CO 2 in pores deep inside the residues. Increasing temperature stimulated CO 2 uptake whereas dissolved oxygen triggered undesirable oxidative precipitation and passivation by iron (III) hydroxides. The latter effect was attenuated through addition of depassivation chelates, which impeded iron precipitation and enhanced carbonation under ambient conditions. Pyrite and pyrrhotite, as natural acid generators, failed to improve ambient carbonation by fostering iron passivation to the detriment of mining residue dissolution whereas the use of carbonic anhydrase inhibited formation of magnesium carbonates. Ionic liquids were found suitable for magnesium extraction under alkaline conditions but inefficient for inducing carbonate precipitation. The lower rate of carbonation with hydrophobic ionic liquid‐water mixtures was ascribed to a slower gas‐liquid mass transfer of CO 2 to the liquid, which primarily prompted a loss of gas‐liquid interfacial area due to viscous effects and bubble coalescence.