Enhancing Sustainable Resource Circulation through pH Swing-Assisted Carbon Mineralization: Integrating CO₂ storage, Utilization, and valuable Metal Recovery

Abstract

Ex-situ carbon mineralization has emerged as a promising method for permanently sequestering carbon dioxide (CO₂) by transforming it into stable solid carbonates (CaCO₃, MgCO₃) through reactions with alkaline metals (Ca, Mg). Industrial by-products from iron and steelmaking processes have been evaluated as a suitable feedstock due to their high CO₂ storage potential and ability to reduce landfill burdens while recovering valuable resources. However, the unpredictable physicochemical properties of these by-products, low valuable metal recovery efficiency, and limited value of the final products pose commercialization challenges. Low leaching efficiency further reduces carbonation performance, making it difficult to implement. To address these issues, this dissertation proposes a pH swing-assisted carbon mineralization process to recover valuable metals and store CO₂, evaluating its feasibility and offering specific solutions for overcoming commercialization challenges. In chapter 1, the background of ex-situ carbon mineralization technology and its potential application to iron and steelmaking by-products were introduced. Key challenges such as improving metal recovery, enhancing the value of final products, and boosting leaching efficiency for better carbonation were discussed, with proposed strategies outlined in Chapters 2, 3, and 4. In chapter 2, pH swing-based carbon mineralization was applied to six domestic by-products from various generation processes, and their physicochemical characteristics were investigated. The results revealed significant variations in leaching, precipitation, and carbonation behaviors depending on the generation process. The simultaneous application of a reductant and organic acid to steel slag demonstrated a synergistic effect, improving both leaching efficiency and metal recovery. In chapter 3, the formation conditions and mechanisms of CaCO₃ polymorphs, the final product, were explored to enhance its value. Spectroscopy and transmission electron microscopy (TEM) were employed to characterize the crystallographic properties of each polymorph. The findings indicated that Nuclear Magnetic Resonance (NMR) bulk analysis could offer greater efficiency for commercial applications, and two distinct crystal structures within vaterite were thoroughly confirmed. In chapter 4, CaO/CaCO₃ composites were developed for high-value applications as thermochemical storage sorbents. Doping with transition metal additives provided excellent photo-to-thermal and thermo-to-chemical energy conversion performance, which was further enhanced by co-doping with CaCl₂ to improve reactivity. Overall, this dissertation demonstrated the feasibility of ex-situ carbon mineralization for CO₂ storage and metal recovery, providing a foundation for the conversion of by-products into high-value products. These results could significantly contribute to the commercialization of carbon mineralization and resource circularity.