Phase Equilibrium in the Pb–Sn System During Pyrometallurgical Sublimation

Vacuum distillation is one of the ways to recover components of the Pb--Sn alloy created during lead bullion processing. We used phase diagrams for preliminary selection of system temperature and pressure as well as for estimation of component separation efficiency during vacuum distillation. We computed parameters of vapour-liquid equilibrium states, including phase composition as function of temperature and pressure for the Pb--Sn alloy undergoing vacuum distillation. We computed saturated vapour pressure in the temperature range of 823--1073 K for Pb (2.63·10^-4--1.49·10^-1) and Sn (3.32·10^-9--8.12·10^-5). The values of the ratio р*_Pb/р*_Sn = 7.91·10⁴--1.84·10³ and the separation index logß_Pb = 3.19--5.07 are high, which provides a theoretical justification for selective recovery of these metals by means of vacuum distillation, concentrating the lead in the gaseous phase (Pb>1) and the tin in the liquid phase. The mole fraction of lead in the gaseous phase у_Pb = 0.9955--0.9999 increases with increases in temperature (from 823 to 1073 K) and the mole fraction of metal in the alloy х_Pb = 0.1--0.9. We computed the following parameters for the vapour-gas phase boundary in the Pb--Sn alloy: excess Gibbs energy G^E_m=0.18--0.72 kJ/mole; excess enthalpy H^E_m=0.056--0.40 kJ/mole; excess entropy S^E_m=0.07--0.35 J / (mole·K)

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