Blast furnace slag (BFS) was selected as the source of Ca for CO₂ mineralization purposes to store CO₂ as CaCO₃. BFS was dissolved using aqua regia (AR) for leaching metal ions for CO₂ mineralization and rejecting metal ions that were not useful to obtain pure CaCO₃ (as confirmed by XRD analysis). The AR concentration, as well as the weight of BFS in an AR solution, was varied. Increasing the AR concentration resulted in increased metal ion leaching efficiencies. An optimum concentration of 20% AR was required for completely leaching Ca and Mg for a chemical reaction with CO₂ and for suppressing the leaching of impurities for the production of high-purity carbonate minerals. Increasing the liquid-to-solid ratio (L/S) resulted in the increased leaching of all metal ions. An optimum L/S of 0.3/0.03 (=10) was required for completely leaching alkaline-earth metal ions for CO₂ mineralization and for retaining other metal ions in the filtered residue. Moreover, the filtrate obtained using 20% AR and an L/S of 0.3/0.03 was utilized as Ca sources for forming carbonate minerals by CO₂ mineralization, affording CaCO₃. The results obtained herein demonstrated the feasibility of the use of AR, as well as increasing pH, for the storage of CO₂ as high-purity CaCO₃.

Direct mineral carbonation (MC) is used to mitigate carbon dioxide (CO₂) emissions. This method has the great advantages of reducing the amount of industrial residues and creating valuable materials by incorporating CO₂. Waste gypsum, industrial waste including flue gas desulfurization (FGD) gypsum (25.27–53.40 wt% of CaO), and phosphogypsum (30.50–39.06 wt% of CaO) can be used for direct MC (conversion rate up to 96%). Mineral carbonation converts waste gypsum into calcium carbonate (CaCO₃), which can be recycled during desulfurization. Furthermore, ammonium sulfate ((NH₄)₂SO₄), which is used as a fertilizer, can be prepared as a by-product when the carbonation reaction is performed using ammonia (NH₃) as a base. In this study, recent progress in the carbonation kinetics and preparation of CaCO₃ using FGD gypsum and phosphogypsum with NH₃ was investigated. Temperature, CO₂ partial pressure, CO₂ flow rate, and NH₃ concentration were reviewed as factors affecting carbonation kinetics and efficiency. The factors influencing the polymorphs of the prepared CaCO₃ were also reviewed and summarized. A state-of-the-art bench-scale plant study was also proposed. In addition, economic feasibility was investigated based on a bench-scale study to analyze the future applicability of this technology.

The South Korean government is pursuing a national project to use the complex carbonates found in coal ash to capture CO₂ and promote coal ash recycling. One possible approach is the use of coal ash as fill material in mine reclamation, but environmental concerns have so far blocked the implementation of this procedure, and no relevant regulations or guidelines exist. In this study, we review international approaches to the environmental management of coal ash recycling and consider how the lessons learned can be applied to South Korea. Each studied country was proactively using coal ash for beneficial uses under locally suitable conditions. The United States, European Union, United Kingdom, Australia, and Japan are all putting coal ash to beneficial use following thorough analyses of the environmental impact based on several considerations, including bulk concentration, coal ash leachate concentration, field inspections, and water quality monitoring. Our findings can contribute to the development of proper regulations and policies to encourage the use of recycled coal ash in South Korea as an approach to managing carbon emissions and climate changes. There are currently no relevant regulations in South Korea, so we consider the adoption of the strictest standards at each stage of the other cases at the time of introduction. Based on our findings, detailed and appropriate management guidelines can be developed in the future. Establishing management plans for complex carbonates, verifying their environmental stability, and using them as fill material will provide clear benefits for South Korea in the future.

With rising of harmful algae blooming and toxin exposure, practical utilization of harmful algae has been developed. This work aimed to magnetically harvest Microcystis aeruginosa (MA) using iron oxides and investigate the feasibility of algae/iron oxides mixture as feedstock in pyrolytic platform to produce syngas and metal biochar. Carbon dioxide (CO₂) was used as a feeding gas to enhance the production efficiency of syngas and also functioned pH controller for better MA harvesting and toxin removal. CO₂ support brought multiple benefits: magnetite (Fe₃O₄) and maghemite (γ-Fe₂O₃) recovered MA in a relatively short period of time (∼1 min), the recovered biomass generated 34-fold increased carbon monoxide, and metal biochar adsorbed higher amount of toxin from MA (2.8-fold). Pyrolytic utilization of harmful algae supported by CO₂ and iron oxides could be one of promising techniques for evolution of metal biochar to remove toxin, while efficiently recover biomass and enhance syngas production.

In this study, we evaluated a two-step process for detoxification of waste roofing slate, involving cement hydrate removal and low temperature detoxification using oxalic acid. These treatments were conducted on raw material and intermediate product, respectively. Cement hydrate removal effectively eliminated most Ca-containing cement hydrate components from the raw material under the following conditions: HCl to solid ratio: 0.456 g/g, reaction time: 2 h, and solid to liquid ratio: 0.124 g/mL. Following low temperature (~100 °C) detoxification of intermediate product obtained after cement hydrate removal, chrysotile in waste roofing slate was effectively transformed to Mg-oxalate under conditions of oxalic acid to solid ratio of >0.67 g/g.