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Ex-situ carbon mineralization utilizing iron and steelmaking by-products are a promising approach for the circular economy by enabling by-product recycling, resource recovery, substantial CO2sequestration, and the production of value-added solid carbonates (e.g., CaCO3, MgCO3). However, the variability in the physicochemical properties of these by-products poses challenges in predicting leaching, precipitation, and carbonation behavior, which is generally influenced by their generation processes. In this context, this study focuses on the recovery of manganese (Mn) and CO2sequestration utilizing blast furnace slags (air-cooled, IS-A; water-cooled, IS-W), and electric arc furnace slag (EAF) and its dust (EAF-D) via a pH swing-assisted carbonation process. Leaching behavior for magnesium (Mg), aluminum (Al), silicon (Si), calcium (Ca), iron (Fe), and manganese (Mn) in IS-A and IS-W was identical, though differences emerged in the precipitation behavior of major elements due to co-precipitation. The IS-A and IS-W included higher Ca content (IS-A, 21.4wt%; IS-W, 24.3wt%) compared to EAF and EAF-D, resulting in impressive CO₂ storage capacities (IS-A, 93kg CO2/ton slag; IS-W, 128kg CO2/ton slag). In contrast, EAF and EAF-D had higher Mn content (EAF, 19.4wt%; EAF-D, 34.7wt%), with concentrations achieving up to 27.4wt% for EAF and 46.7wt% for EAF-D, alongside notable recovery efficiencies (EAF, 29%; EAF-D, 68%). The carbonation process produced high purity CaCO3(92-98%). These findings underscore the potential of iron and steelmaking by-products as feedstock for carbon mineralization and resource recovery, thereby contributing to a more sustainable society.

The CO2capture process, aimed at mitigation of anthropogenic emission, has conventionally adopted aqueous amine-based absorption. However, amine scrubbing faces some critical barriers such as high energy consumption and chemical degradation due to thermal regeneration. On the other hand, a more promising option for CO2storage and utilization is mineral carbonation, which not only is the most stable way to store CO2but also has a variety of applications. To reduce the regeneration of amines and to produce more valuable materials, the question arises whether we can combine the CO2capture and storage processes in a single unit. This study performed the precipitation-assisted amine regeneration of monoethanolamine (MEA), diethanolamine (DEA), methyl diethanolamine (MDEA), and 2-amino-2-methyl-1-propanol (AMP) solutions and then investigated their CaCO3polymorphs. The recovery yield of CaCO3reached about 100% in the MEA solution due to pKarelated to CO2solubility. Using spectroscopic analyses, we revealed the types of amine groups that, when adjacent to the CaCO3surface, inhibit crystal growth. They can influence crystal growth in ways that lead to the formation of vaterite and/or aragonite, rather than calcite. Moreover, the reaction temperature and supersaturation affected the particle size of CaCO3.

Carbon mineralization is recognized as a promising strategy for large-scale CO2storage and utilization. To practically employ the carbon mineralization technology on a large scale, producing value-added solid carbonates is the key to reduce the overall costs. In this paper, we propose a novel concept not only for CO2fixation but also recovery of rare earth elements (REEs) via the pH swing-assisted carbonation process. The blast furnace slag was selected as a feedstock, and NaOH and oxalic acid were adopted as the precipitation agent. The results revealed that the maximum REEs can be obtained in NaOH and oxalic acid-assisted pH swing carbonation and were 1986.1 and 9349.2 ppm, respectively. The CO2storage capacities for NaOH and oxalic acid-assisted pH swing carbonation were also calculated to 107.38 and 117.89 kg CO2/ton-blast furnace slag, respectively, and both cases produced calcite of high purity. Conclusively, oxalic acid-assisted pH swing carbonation was advantageous over the case of NaOH for both simultaneous CO2storage and REE recovery. These findings confirm that the pH swing carbonation process can contribute to building the waste-to resource supply chain as well as to improving CO2storage and utilization.