Thermodynamics of Palladium Dissolution Behavior in Fe_tO–SiO₂–CaO–Al₂O₃–MgO Slag at 1873 K

Abstract

To establish the dissolution mechanism of palladium (Pd) in the Fe_tO–SiO₂–CaO–Al₂O₃–MgO_sat slag system, the Pd solubility was measured at 1873 K (1600 °C) and oxygen partial pressure ranging from ${{p}_{\text{O}_{2}}} = 1.0\times {10}^{-8}\,{\text{atm}}$ to ${{p}_{\text{O}_{2}}} = 1.0\times {10}^{-6}\,{\text{atm}}$. In the current slag system, the Pd solubility increases with increasing content of CaO and FeO, and decreases with increasing content of SiO₂ and Al₂O₃. This solubility behavior is quantified by employing the modified Vee ratio, defined as (CaO+FeO)/(SiO₂+Al₂O₃), the theoretical optical basicity and the activity of CaO in the slag. Thermodynamic analysis reveals that Pd stabilizes in the form of the (${{\text{PdO}}}_{2}^{3-}$) palladate complex ion in the present aluminosilicate melts. Based on this, the Pd dissolution reaction in the current slag system is proposed as follows:

${\text{Pd}}(l)+\frac{1}{4}{{\text{O}}}_{2}(g)+\frac{3}{2}\left({{\text{O}}}^{2-}\right)=({{\text{PdO}}}_{2}^{3-})$

Combining the results from the present experiment and previous findings suggests that a well-designed slag system can effectively minimize Pd loss during the pyrometallurgical processing of industrial wastes containing Pd. By adjusting the modified Vee ratio, in which CaO and FeO promote Pd solubility in the slag, whereas SiO₂ increases slag viscosity, it is possible to achieve optimal conditions. Therefore, we recommend implementing such an optimized slag system for pyrometallurgical treatment of Pd-containing industrial wastes, ensuring minimal Pd loss and maximizing resource recovery.