Modification of Desulfurization Ash via Thermal CO2 Treatment: Oxidation Behavior, Carbonation Characteristics, and Mineral Transformation Mechanisms

Desulfurization ash (DA) is a kind of industrial solid waste generated during flue gas desulfurization. To overcome the limited utilization of reactive calcium-bearing constituents and the insufficient mineralogical stability of DA, this study proposes a thermal CO2 modification approach based on the synergistic coupling of oxidation and carbonation, and systematically investigates the associated evolution of mineral phases and microstructural features. The effects of reaction temperature, CO2 flow rate, and reaction time are evaluated using TG-DSC, XRD, FT-IR, SEM-EDS, particle size analysis, and carbon-sulfur analysis to elucidate the reaction behavior of DA under a CO2 atmosphere. The results demonstrate that reaction temperature is the dominant factor governing the modification process. During heating, CaSO3 is preferentially oxidized to CaSO4 under the CO2 atmosphere, thereby stabilizing the sulfur-bearing components. Subsequently, Ca(OH)2 undergoes carbonation to form CaCO3, simultaneously contributing to CO2 sequestration. Under the optimized conditions of 450 °C, a CO2 flow rate of 120 mL/min, and a reaction time of 60 min, the resulting CaCO3 exhibits a pronounced needle-like CaCO3 morphology, while the CO2 uptake reaches a maximum of 16.71%. These findings clarify the coupled oxidation–carbonation mechanism and mineral transformation behavior of DA during thermal CO2 modification, providing a theoretical basis and technical reference for the low-carbon utilization of industrial solid wastes.