Carbonation is a carbon capture, utilization, and storage (CCUS) technology aimed at extracting Ca and Mg from basic waste, fixing CO2, and utilizing the resulting products (CaCO3 and MgCO3), while valorizing the unused resources. Steelmaking slag, with its high CaO concentration and large production volume, is a by-product with a high potential for carbonate recovery. In this study, we propose a cyclic indirect carbonation process consisting of (I) leaching of Ca and Mg from slag using HCl, (II) absorption of CO2 using an alkaline solution, (III) precipitation of carbonates from the leachate using Na2CO3, and (IV) regeneration of HCl and NaOH from the wastewater using bipolar membrane electrodialysis. (I) and (III) were investigated in detail. Slag leaching at a final pH of 2.46 resulted in a leaching rate of 81.4% for Ca. However, Mg, Si, P, Al, Fe, and Mn were also leached at approximately 30%–60%. When the slag was leached at a final pH of 11.83, the Ca leaching rate was only 3.0%, but other impurities were hardly leached. When carbonates were precipitated from these leachates using Na2CO3, ochre-colored CaCO3 powder with a purity of approximately 40% and containing many impurities in the slag was obtained from the leachate at a final pH of 2.46, while a white CaCO3 powder with a purity of 97.1% was obtained from the leachate at a final pH of 11.83. These carbonates are expected to be useful raw materials for cement and papermaking, respectively.
Tsuru et al. (Thu,) studied this question.