Vapor–Liquid Equilibria in the Tin–Lead System in a Primary Vacuum

The boiling point method (isothermal version) was used to determine the partial pressure of saturated lead vapor over lead and tin solutions, the lead content of which (the rest is tin) was 96.43, 93.02, 89.55, 80.73, 64.18, and 43.80 wt % (93.93, 88.42, 83.08, 70.59, 50.65, and 30.87 at %, respectively). The partial pressures of tin were calculated by numerically integrating of the Duhem–Margules equation using the auxiliary function proposed by Darken. The tin and lead partial pressure values over their melts were approximated by temperature-concentration dependences. The total error of determination was calculated as the sum of errors of independent measurements: temperature, weighing, pressure, approximation of experimental data, equal to 7.78%. On the basis of the values of the partial pressure of the saturated lead and tin vapor, the boundaries of the fields of coexistence of liquid and vapor in the tin−lead system in a primary vacuum of 100 and 1 Pa were calculated and refined: the boiling temperature as the temperature at which the sum of the partial pressures of metals was equal to 100 and 1 Pa; the composition of vapor as the ratio of the partial pressure of vapor of metals at this temperature. It was found that the reason for the increased content of tin in lead condensate during the distillation of alloys with a lead content of less than 5 at % (8.41 wt %) and the accumulations of tin in the distillation residues are values of the partial vapor pressure of tin comparable to lead. In the distillation separation of lead and tin melts by evaporation of lead in a real process under nonequilibrium conditions, the accumulations of tin in the distillation residue should not exceed a concentration of ~50 wt %. Exceeding the specified concentration will be accompanied by the production of condensate, for which it is necessary to repeat the process of evaporation–condensation.