Phase equilibria in Hg-Al system during vacuum distillation

Abstract

   In this work, gas-liquid equilibrium states are calculated, including phase composition dependences on temperature (T-x) and pressure (P-x) for the Hg-Al alloy during vacuum distillation. The objects of research comprised Hg-Al alloys having the following composition, mole %: 20–80 Hg; 80–20 Al, whose formation may occur during the processing of copper anode slime upon producing commercial selenium concentrate. A simplified molecular interaction volume model was used to calculate the activity coefficients of the components in the Hg-Al alloy. Phase diagrams of temperature (T-x) and pressure (P-x) are used for the preliminary selection of temperature and pressure in the system, as well as for the evaluation of the separation efficiency of components. The novelty of the research stems from calculating activity coefficients using the selected simplified model. Saturated vapour pressures for Hg (p*Hg) and Al (p*Al) were calculated in the temperature range of 823–1073 K. The high values of the p*Hg / p*Al ≥ 3 . 1010 ratio and separation coefficient llogβHg ≥ 10 provide theoretical premises for selective extraction of these metals by vacuum distillation, where mercury is concentrated in the gas phase (βHg > 1) and aluminium in the liquid phase. The values of excess Gibb’s energy, enthalpy and entropy changes for the liquid-gas interface of Hg-Al alloy were determined: ΔGE/m = 1–3 кJ/mol;  = 1-3 kJ/mol; +-ΔGE/m = 0,03–0,17 kJ/mol. The practical significance of the research lies in minimising the number of initial experiments during the processing of Hg-Al compositions for optimising the temperature and pressure in the vacuum distillation process.