Thermodynamic Analysis of Silicon–Platinum Group Metal (Pt, Pd, and Rh) Interactions in Iron-Based Solutions

Platinum group metals (Pt, Pd, Rh), valuable secondary resources from spent automotive three-way catalysts, are generally recovered by high-temperature pyrometallurgical iron-collection processes. The cordierite support (2MgO·2Al₂O₃·5SiO₂) releases Si into the iron melt, yet how Si affects Pt, Pd and Rh capture by liquid Fe remains poorly understood due to insufficient thermodynamic data. This work is the first to systematically calculate the temperature-dependent activity interaction coefficients between Si and PGMs in the Fe–Pt/Pd/Rh–Si ternary system. Using the Molecular Interaction Volume Model (MIVM) and the Wagner model, the activity interaction coefficients between Si and the PGMs were calculated over the temperature range of 1773–1973 K. The results indicate that, within this temperature range, the activity interaction coefficient between Si and Pd (${\epsilon }_{\mathrm{Pd}}^{\mathrm{Si}}$) is negative, indicating that the addition of Si reduces the activity coefficient of Pd; the activity interaction coefficients between Si and Pt and Rh (${\epsilon }_{\mathrm{Pt}}^{\mathrm{Si}}$ and ${\epsilon }_{\mathrm{Rh}}^{\mathrm{Si}}$) are positive, indicating that the addition of Si increases the activity coefficients of Pt and Rh. Furthermore, the temperature rose from 1773 K to 1973 K, and ${\epsilon }_{\mathrm{Pd}}^{\mathrm{Si}}$ and ${\epsilon }_{\mathrm{Rh}}^{\mathrm{Si}}$ increase with rising temperature, with ${\epsilon }_{\mathrm{Pd}}^{\mathrm{Si}}$ranging from −1.016 to −0.880; ${\epsilon }_{\mathrm{Rh}}^{\mathrm{Si}}$ increases from 0.051 to 0.075, while ${\epsilon }_{\mathrm{Pt}}^{\mathrm{Si}}$ decreases with rising temperature, with ${\epsilon }_{\mathrm{Pt}}^{\mathrm{Si}}$ decreasing from 0.536 to 0.497. Electron probe microanalysis (EPMA) results confirmed the above calculations: Pd forms an enriched phase of an intermetallic compound with Si, while Pt and Rh are uniformly solid-solved in the iron matrix. These findings provide theoretical guidance for optimizing pyrometallurgical recovery processes of platinum group metals from silicon-containing secondary resources (particularly waste automotive catalysts).