Abstract
Vacuum distillation represents an effective technique for the separation and purification of metals based on differences in vapor pressure. In this work, the separation behavior in Ag-Sn, Bi-Sn, Cu-Sn and Sb-Sn alloy systems was thermodynamically evaluated. Activities of liquid phase components were calculated using the CALPHAD method and COST 531 thermodynamic database for lead-free solders. Based on these data, separation coefficients were determined over a wide temperature and composition range. In addition, vapour-liquid equilibrium calculations were performed using Gibbs energy minimization implemented in HSC Chemistry software in order to estimate equilibrium distributions of components between vapor and liquid phases under reduced pressure. The results reveal pronounced differences in separation behavior among the investigated systems. Bi and Sb exhibit significantly higher volatility than Sn, with separation coefficients reaching values much higher than unity over a wide temperature range, enabling efficient removal from tin melts during vacuum distillation. In contrast, Ag and Cu show much lower relative volatility and separation coefficients close to unity, indicating limited separation efficiency. The calculated molar fluxes further represent the theoretical maximum evaporation rates under the considered conditions. These results provide a thermodynamic basis for assessing vacuum distillation as a refining method for Sn-based alloys.

Key words
vapor-liquid equilibrium, tin alloys, vacuum refining, separation coefficient

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