Abstract:

The Canadian industrial sector accounts for about 40% of the nation’s greenhouse gas emissions, making decarbonization vital for reaching net-zero targets. Mineral carbonation offers a method for CO₂ removal and permanent storage using industrial solid wastes like mine tailings and steel slags, which are more reactive and reduce pre-treatment costs. Aqueous mineral carbonation, characterized by higher reaction rates and lower energy needs, involves leaching minerals and subsequently carbonating them. Optimizing these reactions through a multistage approach allows for increased purity of carbonates, potentially generating sellable products to offset costs.

Based on the above considerations, this study was aimed at optimizing the extraction of calcium and magnesium from steel slag, as the rate limiting reaction in mineralization. The electric arc furnace (EAF) slag was used as a source of minerals. The slag utilized in this study was rich in CaO (37.5 wt.%) and MgO (9.2 wt.%). These results imply a potentially high CO₂ sequestration capacity of the slag via formation of calcium/magnesium carbonates of a high purity.

The effect of process parameters such as, type of leaching agents (e.g., strong acids and organic acids) and their concentration (i.e., 0.5-5M) as well as, leaching time, solid/liquid ratio and process temperature was investigated. The process conditions were optimized using response surface methodology (RSM) based on central composite design. More than 90% of calcium and magnesium were recovered under the optimum experimental conditions. The proposed model equation using RSM has shown excellent agreement with the experimental data, with a correlation coefficient R² of > 0.95. The samples before and after leaching were characterized for the mineralogical compositions using XRD technique, as well as the chemical compositions using WDXRF and ICP-AES techniques.

A relatively high cost of the aqueous mineralization is the major obstacle towards the commercialization of this approach. In order to offset the cost of the CO₂ mineralization, this work also addressed the potential recovery of marketable minerals prior to carbonation. The advantage of indirect CO₂ mineralization is the ease of incorporating intermediate steps for the recovery of valuable minerals present in EAF slag. The EAF slag utilized in this work had a relatively high concentrations of Fe₂O₃ (23.5 wt.%), Al₂O₃ (6.0 wt.%) and Mn₃O₄ (3.8 wt.%), all of which might be recovered prior to the carbonation stage.

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