PHASE EQUILIBRIA IN THE Hg–Se SYSTEM DURING VACUUM DISTILLATION

Abstract

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 ‒ΔGm E = 0.8–3.0 kJ/mol; ‒ΔHm E = 1.86–5.39 KJ/mol; ‒ΔSm E = 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.