Progress in research and application of indirect carbonation technology for steel slag carbon sequestration

In context of China's "dual carbon" goals, development of efficient CO₂ emission reduction technologies is critical to achieving carbon neutrality. Mineral carbonation has emerged as a promising CO₂ mitigation route, with steel slag—a highvolume alkaline solid waste rich in Ca and Fe—serving as an ideal feedstock. Recent advances and current application status of the indirect carbonation of steel slag are systematically reviewed. This process not only exhibits considerable CO₂ sequestration potential but also enables the efficient separation and highvalue utilization of both calcium and iron components, representing a critical technological pathway that synergizes carbon emission reduction with highvalue utilization of steel slag. Current research is primarily focused on the selection of leaching agents, reaction intensification strategies, mechanistic and kinetic analyses, and product quality control. Ammonium chloride is mainstream leaching agent owing to its high selectivity for calcium, albeit with a relatively low extraction efficiency. In contrast, acidbased systems achieve higher leaching rates but suffer from poor selectivity and are prone to silica gel encapsulation. Through various intensification measures, Ca²⁺ leaching efficiency can be raised to over 90%, while the carbonation conversion rate can be significantly improved from below 50% to above 85%. At the industrial demonstration level, noteworthy milestones include the world-first 100 000 t/y carbon mineralization demonstration plant commissioned by Baogang Group in 2023, the successful 100% high-value conversion of all steel slag components using an ammonium salt system by HBIS Group, the closed-loop leachant recycling and direct flue gas utilization realized at Guodian Datong's thousand-tonne-scale facility, and the production of precipitated calcium carbonate (PCC) with a purity exceeding 99.5% at Aalto University's pilot plant. Future research should focus on developing a universal process package adaptable to various raw materials, designing closedloop processes with low reagent loss, and establishing a directional regulation relationship between product crystal morphology and reaction conditions, so as to provide support for engineering promotion and overall economic optimization. Given its high CO₂ sequestration capacity and considerable economic benefits, the indirect carbonation of steel slag holds great promise for promoting the lowcarbon, green, and sustainable transformation of the steel industry.