Design of a Hybrid Leaching Process for Mineral Carbonation of Magnesium Silicates: Learnings and Issues Raised from Combined Experimental and Geochemical Modelling Approaches

The formation of passivation layers is the major obstacle to the development of direct aqueous mineral carbonation of magnesium silicates, one of the most promising routes for large-scale CO₂ mitigation. This observation has led to considerable research efforts to prevent the formation of these layers and/or to exfoliate them continuously during the reaction. This keynote gives an account of different solutions investigated by a French research consortium initiated within the framework of several collaborative projects, including the methodology developed to optimize process efficiency and cost. Chemical-based solutions using chelating agents were first examined, but the variety of minerals and also the complex nature of leached surface layers for a given ore demonstrated the need for highly versatile and tunable chemical systems (able to modulate the precipitation of silica or phyllosilicates without sequestrating alkaline metals) and thus the need for a more robust solution. Moving to mechanical exfoliation, it was found that concomitant attrition and leaching could drastically enhance the conversion of silicate ores under mild conditions (20 bar of CO₂, 180°C). This was achieved inside the favourable environment of a stirred ball mill. Without the need for thermal pre-treatment, carbonation yields in excess of 80% were obtained after 24 hours starting with variously serpentinised ores or mining residues of minus 100 µm size fraction. Under these conditions, the performance of the attrition-leaching process is driven by the dissolution rate of continuously refreshed ore surfaces and thermodynamic equilibria. Coupled geochemical modelling and experimentation, including thorough product characterization proved the soundness of this concept. Geochemical modelling also guided the selection of suitable operating conditions and process inputs, including the choice of grinding media and the influence of minor elements (Al, M, Ca) on carbonation yield. In particular, the model predicted the existence of a CO₂ partial pressure dependent threshold temperature above which precipitation of talc-like phases is favoured at the expense of carbonates. Current research developments are exploring the possibility of combining the attrition-leaching process with CO₂-transfering agents, such as polyamines. These additives are expected to expand the process operating window to diluted flue gases (without pre-capture step) and milder temperatures, by (reversibly) concentrating CO₂ in solution and by catalysing mineralisation. Other focus points of the work include process scale-up and operation in continuous mode, valorisation of the mineralisation products into construction materials and selective co-extraction of valuable metals.