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

Copper slag, which contains Fe-rich fayalite (FeSiO), 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 FeSiO to obtain detailed understanding of the role of CaO in enhancing iron recovery. The presence of CaO was found to accelerate the decomposition of FeSiO via an ion-exchange-like process. Specifically, CaO dissociated into Ca(II) and a Ca-deficient CaO species at high temperatures. The Fe(II) ion at the M2 site of FeSiO was substituted by the released Ca(II) ion, resulting in the formation of [(FeCa)SiO]∙xFe(II). Subsequently, the substituted Fe(II) occupied the Ca vacancy in CaO to form (CaFe(II))O. The disproportionation of Fe(II) and the combination reaction between CaO and the SiO separated from FeSiO led to the generation of the final products, viz. FeO, FeO, and CaSiO. This study explains the specific role of CaO in decomposing FeSiO. 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).