Application Of Molecular Interaction Volume Model In Tin-based Alloys In Vacuum Distillation

Vacuum metallurgy, which has many advantages as high metalic recovery rate, simple technological process, little pollution to environment was proposed to investigate the vacuum distillation of binary and ternary Sn-based alloys in this work, because of the traditional pyrometallurgical process for refining of tin has many problems, such as long process flow, great consumption of reagent, low recovery rate of metals, severe environment pollution and so on.Firstly, the development of traditional treatment methods and vacuum distillation used in separation of Sn-based alloys were introduced. A technique which can be used to separate and purify Sn-based alloys for purifying Sn was proposed based on comparison of various methods. The activities and activity coefficients of Sn-Pb, Sn-Zn and Sn-Pb-Sb alloys were calculated with the molecular interaction volume model. At the same time, the separation coefficients and vapor-liquid phase equilibrium of the corresponding alloy systems, which can be used to predict the possibility and separating effect of purifying Sn-based alloys by vacuum distillation, were calculated using the activity coefficients. Finally, the vacuum distillation experimental research was carried out for Sn-Pb, Sn-Zn and Sn-Pb-Sb alloys, the volatilization behavior of each component of above alloys in vacuum distillation process was discussed, and a comparison between the predicted values and the experimental results was also executed.Thermodynamics research indicates that the saturated vapor pressure of pure substances Zn, Pb, Sb is far larger than that of Sn in the temperature range of900℃-1200℃, that is the volatility of Zn, Pb, Sb is far larger than that of Sn. The research results show that the predicted values of activity of Sn-Pb, Sn-Sb and Sn-Zn are in good agreement with the experimental results. The separation coefficients of Sn-Pb, Sn-Sb and Sn-Zn are far larger than1, which show that Pb, Sb, Zn can be separated from Sn by vacuum distillation, that is Pb, Sb, Zn will volatilizes into vapor phase while Sn still stay in liquid phase in vacuum distillation. The vapor-liquid phase equilibrium show that in the process of vacuum distillation of Sn-Pb alloy, the content of tin in vapor phase is0.008%while in liquid phase85%at900℃, which means that Sn and Pb can be separated by vacuum distillation. In the peocess of vacuum distillation Sn-Zn alloy, the content of tin in vapor phase is0.00001%while in liquid phase90%at800℃, which means that Zn volatilizes into vapor phase while Sn is still staying in liquid phase, that is Sn and Zn can be separated by vacuum distillation. In the peocess of vacuum distillation of Sn-Pb-Sb alloy, the content of tin in vapor phase is0.06%while in liquid phase94.59%at1100℃, which means that Sn and Pb volatilize into vapor phase while Sn stays in liquid phase, that is Pb and Sb can be separated from Sn by vacuum distillation.The experimental results of vacuum distillation of Sn-Pb alloy show that the content of lead decreased, while an increase was observed for tin in residue with an increasing of distillation temperature and time at the condition of distillation temperature of900℃and950℃corresponding to the distillation time of20min,40min,60min and80min. The purity of Pb in volatile decreased too. The content of tin in vapor phase is0.112%while in liquid phase85%at900℃, which is in good agreement with predicted values. The experimental results of vacuum distillation of Sn-Zn alloy show that the pressure and distillation temperature have a great influence on the volatilization of zinc at the condition of pressure15Pa-200Pa, distillation temperature of600℃-850℃and the distillation time of30-120min. The evaporation rate of zinc increased as the pressure was decreased which was99.87%at the pressure of15Pa, the evaporation rate of zinc increased and the content of tin in residue decreased as the distillation temperature was increased. However, the content of tin in residue remains unchanged at800℃.The experimental results of vacuum distillation of Sn-Pb-Sb alloy obtained at the distillation temperature of1100℃-1250℃, the distillation time of60min corresponding to the pressure of0.5Pa show that the content of tin in residue increased as the distillation temperature was increased, the content of tin in vapor phase is0.54%while in liquid phase94.59%at1100℃, which is in good agreement with predicted values. The above analysis indicates that Pb and Sb can be separated from Sn by vacuum distillation.