Thermodynamics of Solutions and Azeotropy in Zinc–Calcium Melts

Abstract

The partial and integral thermodynamic functions of mixing and vaporization are calculated for Zn–Са alloys from the vapor pressures determined by the boiling point method. Alloy formation over the whole range of concentrations proceeds with heat release, with attendant considerable ordering in the system relative to an ideal solution up to ~70 at % Са in the alloy and with a minor increase in disorder when calcium concentration exceeds this value. The curves of integral functions of vaporization feature extremes at ~85 at % Ca: a minimum entropy of 85.48 J/(mol K) and a maximum enthalpy of 156.44 kJ/mol. The vapor pressures of alloy components were used to supplement the Zn–Са phase diagram with the liquid and vapor coexistence fields at the atmospheric pressure (101.33 kPa) and in a vacuum (1.33 and 0.7 kPa). The existence of a liquid containing 93.2 ± 6.7 at % Ca that boils unseparably at 1494°С was discovered. As the pressure (boiling temperature) decreases, the azeotrope composition shifts (in accordance with the Vrevsky law) toward the zinc edge of the phase diagram, where zinc has a lower enthalpy of vaporization compared to that of calcium (118.6 kJ/mol against 153.7 kJ/mol). The positions of boundaries of the vapor–liquid equilibrium fields indicate the impracticability of distillation separation of the Zn–Са system to the constituent metals.