Vapor-liquid equilibrium in the tin–lead system in primary vacuum

The boiling point method (isothermal version) was used to determine the partial pressure of saturated lead vapor over lead-tin solutions with the following lead content (the rest is tin), wt.%: 96.43, 93.02, 89.55, 80.73, 64.18, and 43.80 (93.93, 88.42, 83.08, 70.59, 50.65, and 30.87 at.%, respectively). The partial pressures of tin were calculated by the numerical integration 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 temperatureconcentration dependences. The total determination error was calculated as a sum of independent measurement errors: temperature, mass, pressure, approximation of experimental data, equal to 7.78 %. Based on the values of saturated lead and tin vapor partial pressures, the boundaries of liquid and vapor coexistence fields in the tin-lead system in a primary vacuum of 100 and 1 Pa were calculated and specified: boiling temperature – as a temperature at which the sum of metal partial pressures is equal to 100 and 1 Pa, vapor composition – as the ratio of metal vapor partial pressures 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 tin accumulation in the distillation residue is partial pressure values of tin vapor comparable to that of lead. Tin accumulation in the distillation residue should not exceed a concentration of ~ 50 wt.% during the distillation separation of lead-tin melts by lead evaporation in a real process under non-equilibrium conditions. If the specified concentration is exceeded, the condensate obtained will require repeating the evaporation-condensation process.

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