Phase transition and dissolution behavior of Ca/Mg-bearing silicates of steel slag in acidic solutions for integration with carbon sequestration

Steel slag (SS) and carbon dioxide are two major waste products of the iron- and steelmaking process. Indirect aqueous carbonation of SS using Ca/Mg is a promising way for carbon capture, utilization, and storage (CCUS). However, it is generally accepted that the Si-rich layer produced during the leaching of silicates in SS has serious implications for the Ca/Mg recovery. Therefore, understanding the phase transition and dissolution behavior of Ca/Mg-bearing silicates of SS in acidic solutions is of vital importance for SS-based CCUS. In this work, three synthetic Ca/Mg-bearing silicates (Ca₃MgSi₂O₈, Ca₂MgSi₂O₇, and γ-Ca₂SiO₄) of SS were prepared and studied by leaching tests. Experiments were carried out with leaching silicate powder in 0.1 mol· L⁻¹ HCl solution for various reaction time at room temperature. Results show γ-Ca₂SiO₄ and Ca₃MgSi₂O₈ dissolved rapidly during the early leaching stages, giving more silica gel towards the end. Phases of Ca₃MgSi₂O₈ and Ca₂MgSi₂O₇ dissolved to yield Ca²⁺ and Mg²⁺ in solution in a similar ratio as in the original mineral. The Ca₂MgSi₂O₇ phase was transformed into diopside (CaMgSi₂O₆) and silica gel during the acid leaching. Thus, the difference in phase transition of the studied Ca/Mg-bearing silicates is primarily related to the dissolution of the Si group in the crystal.