Difference of Ca extraction characteristics of steel slag in various acidic solutions

Steel slag is a complex heterogeneous material composed of multiple components and coexisting mineral phases. The low efficiency of selective Ca extraction severely restricts the large-scale application of steel slag in CO₂ mineralization technology. Basic oxygen furnace（BOF） slag from a steel plant was selected as the research mate‑rial. Leaching experiments were carried out in three representative solutions, HCl（a strong acid）, CH₃COOH（a weak acid）, and NH₄Cl（a salt with acidic cations）. The leaching behaviors of major elements（Ca, Mg, Fe, and Si） in BOF slag were systematically analyzed. Furthermore, the mineral phase transformations and morphological evolution of BOF slag before and after leaching were investigated using X-ray diffraction（XRD） and scanning elec‑tron microscopy（SEM）. This analysis aimed to establish a structure-activity relationship among the solution envi‑ronment, mineral dissolution, and elemental release. The results show that Ca in BOF slag mainly exists in trical‑cium silicate（Ca₃SiO₅）, dicalcium ferrite（Ca₂Fe₂O₅）, and free calcium oxide（CaO） phases. The major compo‑nents（Ca, Mg, Fe, and Si） in BOF slag exhibit a high leaching yield（>90%） in HCl solution, while NH₄Cl solu‑tion shows excellent selective Ca leaching performance for BOF slag, with a Ca²⁺ leaching yield of 54%. The Ca²⁺ leaching yield of BOF slag is determined by both the solution environment and the type of Ca-bearing mineral phase. In HCl（strong acid） solution, the Ca-bearing mineral phases in BOF slag undergoes significant dissolution, in con‑trast, in CH₃COOH and NH₄Cl solutions, Ca₃SiO₅ and CaO show high solubility, while Ca₂Fe₂O₅ exhibit leaching inertness. Additionally, the solubility of Ca-bearing phases in BOF slag is associated with their crystal structures, Ca₃SiO₅ is composed of SiO₄^4- tetrahedral monomers, which are easily soluble in the tested solution environ‑ments, whereas Ca₂Fe₂O₅ crystals have high Fe—O bond energy and complex anionic chain structures, making it dif‑ficult to decompose the Ca₂Fe₂O₅ mineral phase under weak acid and ammonium salt conditions. Future research should focus on the targeted regulation of molten slag mineral phases to enrich calcium components predominantly in the forms of CaO and Ca₃SiO₅. This approach will be conducive to improving the efficiency of selective Ca extraction from steel slag.