Phase equilibria in the system Pb-Ag in the pyrometallurgical fuming

In the processing of lead a silver foam (SP) is formed, which requires complex processing, with commercial production of single products. One of the possible ways of SP recovery is vacuum distillation, which is considered one of the most effective and environmentally friendly methods for the separation and purification, processing and refining of various metals. To pre-select the temperature and pressure of the system, evaluate the effectiveness of component separation in a vacuum distillation one uses phase diagrams. The aim of this work consists in calculating the equilibrium “gas-liquid” (VLE, vapor-liquid equilibrium), including the dependence of phase composition on temperature (T-x) and pressure (P-x) for Pb-Ag alloy during vacuum distillation based on the molecular interaction volume model (MIVM). In the temperature range of 1073-1773 K the pressure of saturated steam is calculated for Pb (1.5•10-1-4.86•102) and Ag (5.1•10-5-6.6). High values of the ratio R*Pb / R*Ag = 2.9•103-74) and separation factor βPb = 1.8-3.9 provide the theoretical preconditions for the selective separation of these metals by vacuum distillation, when the lead is enriched in the gas phase (βPb > 1), and silver in the liquid. The mole fraction of lead in the gas phase yPb = 0.878-0.999 increases with increasing temperature 1073-1773 K and the molar fraction of the metal in the alloy xPb = 0.1-0.9. Using the MIVM model activity coefficients of lead γPb= 0.474-0.999 and silver γAg= 0.331-0.999 were calculated for Pb-Ag alloy under different composition in the investigated temperature range. For VLE phase diagrams the lever rule can be used to help predict quantities of substances, residues and sublimates at a predetermined temperature. For the phase boundary “liquid -gas” of Pb-Ag alloy the values of the excess Gibbs energy, enthalpy and entropy are determined: GmE = 0.01-1.34 kJ/mol; -HmE = 0.32-1.53 kJ/mol; SmE = 0.52-1.65 kJ/mol.K. VLE phase diagrams of alloys provide the necessary information for the design of technological parameters in industrial production, vacuum metallurgy, as well as for predicting the necessary composition of Pb-Ag alloy, depending on temperature and pressure during vacuum distillation.