Mechanistic study on calcium ion diffusion into fayalite: A step toward sustainable management of copper slag

Copper slag, which contains Fe-rich fayalite (Fe₂SiO₄), is a valuable solid waste that warrants further research in order to recover iron. Calcium oxide (CaO) can significantly enhance iron recovery from copper slag; however, the associated mechanism has not yet been explored. In this study, we investigated the interaction between CaO and Fe₂SiO₄ to obtain detailed understanding of the role of CaO in enhancing iron recovery. The presence of CaO was found to accelerate the decomposition of Fe₂SiO₄ via an ion-exchange-like process. Specifically, CaO dissociated into Ca(II) and a Ca-deficient Ca_1-yO species at high temperatures. The Fe(II) ion at the M2 site of Fe₂SiO₄ was substituted by the released Ca(II) ion, resulting in the formation of [(Fe_(2-x)Ca_x)SiO₄]∙xFe(II). Subsequently, the substituted Fe(II) occupied the Ca vacancy in Ca_1-yO to form (Ca_(1-y)Fe(II)_y)O. The disproportionation of Fe(II) and the combination reaction between CaO and the SiO₂ separated from Fe₂SiO₄ led to the generation of the final products, viz. Fe₂O₃, Fe₃O₄, and CaSiO₃. This study explains the specific role of CaO in decomposing Fe₂SiO₄. It would not only provide theoretical guidance for iron recovery from copper slag but also present a new perspective on the recycling of valuable resources from many other smelting slags (e.g., iron slag, lead slag, and nickel slag).