With rapid technological innovations and market expansion, the amounts of electronic wastes and spent catalysts increase rapidly in the past decades. Electronic wastes and spent catalysts contain large amounts of precious metals (Ag, Au, Pd, etc.). Therefore, such wastes may be considered as a secondary source of valuable metals. Due to the conflicts of limited natural resources and increasing demands, as well as their economic value, it is essential to recover precious metals from secondary resources [1,2,3].

In this study, the solubility of palladium and gold were measured at 1600oC, p(O2)=10-7atm (for Pd solubility); and 1500oC, p(O2)=10-8 atm (for Au solubility) conditions, by employing the FeO-SiO2-CaO-Al2O3-FetO-MgOsat slags obtained by mixing printed circuit board (PCB), petrochemical waste catalyst, and electroplating copper sludge. The major findings can be summarized as follows;

First, when the FeO content in the slags is high, magnesiowüstitie intermediate layer is formed at the hot face of MgO refractory, protecting the MgO crucible from further corrosion. As the Al2O3 content increases, MgO dissolution increases and hence MgAl2O4 spinel particles are formed in the slag phase.

Second, the solubility of Pd and Au in the FeO-SiO2-CaO-Al2O3-FetO-MgOsat slags increases with increasing slag basicity, which can be represented by several indices, viz. vee ratio (=CaO/SiO2), optical basicity, and the activity of CaO. That is, the excess addition of basic oxides such as CaO and MgO is not recommended because of higher chemical dissolution of Pd and Au in the slags. Alternatively, the high-SiO2 slags are highly viscous, resulting in the slow kinetics of the slag-metal reaction as well as the slow falling rate of metallic particles during the process. Therfore, the physicochemical properties of slag should be carefully designed for the pyrometallurgical processing of industrial wastes.

Finally, the ionic chemical reactions for the palladium and gold dissolution into the slags has been confirmed as follows;

pd(l)+0.25O₂(g)+1.5(O^2–)=(PdO₂^3-) and  Au(l)+0.25O₂(g)+0.5(O^2-)=(AuO^-).