Copper Stabilization in Glass-Ceramics: Effect of CaO and SiO₂ on CuAl_xFe_2-xO₄ Formation

Industrial activities discharge heavy metal-laden wastes; and copper is one of these metals that can be stabilized after ceramic sintering by Al- or Fe-containing precursors. Besides traditional precursors, Al- and Fe-containing solid wastes (e.g., sewage sludge incineration ash, coal fly ash) were found equally capable of copper stabilization when they were adopted in the reaction matrices. However, the effect of CaO and SiO₂, the predominant components in most of the inorganic solid wastes, on metal transformation and stabilization has not been addressed. Therefore, a CaO-SiO₂-CuO-Al₂O₃-Fe₂O₃ reaction matrix was proposed in this study, focusing on the effect of CaO and SiO₂ on copper transformation and immobilization. Results showed that a single-phase CuAl_xFe_2-xO₄ spinel solution was developed in the system without CaO and SiO₂, whereas the addition of CaO led to the generation of two separate spinel phases: CuAl₂O₄ and CuFe₂O₄. Compared with CaO, the existence of SiO₂ showed less impact on the phase transformation. The effect of sintering temperature and additive dosage was also illustrated in this study and the results provided useful guidance for the optimization of reaction systems to achieve beneficial utilization of solid wastes for heavy metal stabilization.