Phase equilibria in the Hg-Se system during vacuum distillation

One of the possible ways to recover components from the Hg-Se alloy formed during the processing of copper-electrolyte sludge is vacuum distillation. Object of research: Hg-Se alloys of composition, mol %: 0.01-99.99 Hg; 99.99-0.01 Se, the formation of which is possible during the processing of copper-electrolyte sludge in the production of commercial selenium concentrate. The purpose of the work: calculation of the “gas - liquid” VLE (vapor liquid equilibrium) equilibrium states, including the dependence of the phase composition on temperature (T - x) and pressure (P - x) for the Hg-Se alloy during vacuum distillation. Methods and approaches used. The activity coefficients of the Hg-Se alloy components were calculated using a simplified version of the simple molecular interaction volume model (SMIVM). Phase diagrams of temperature (T - x) and pressure (P - x) are used to pre-select the system temperature and pressure, and to evaluate the efficiency of component separation during vacuum distillation. Novelty: calculation of activity coefficients using a simplified version of the SMIVM model. Main results. In the temperature range of 823-1073 K, saturated vapor pressures were calculated for Hg (pHg* = 1.418•106-1.046•107 Pa) and Se (pSe* = 1.42•104-3,66•105 Pa). High values of the ratio pHg*/pSe* = 100.2-28.6 and the separation coefficient lgβHg = 2.73-1.01 create theoretical prerequisites for the selective separation of these metals by vacuum distillation, when mercury is enriched in the gas phase (βHg > 1), and selenium - in the liquid phase. The molar fraction of selenium in the gas phase xSe = 0.553-1.43•10-12 decreases with a decrease in the temperature of 1073-823 K and the molar fraction of the element in the alloy xSe = 0.99-0.01. For the “liquid - gas” interface of the Hg-Se alloy, the values of changes in the excess Gibbs energy, enthalpy, and entropy are determined -ΔGmE = 0.8-3.0 kJ/mol; -ΔHmE = 1.86-5.39 KJ/mol; -ΔSmE = 0.99-2.94 J/mol.K. Practical significance: reducing the number of time-consuming and expensive installation experiments during the processing of Hg-Se compositions to optimize the temperature and pressure values of the vacuum treatment process distillation in order to obtain Se-containing products of a given composition.