Crude Selenium Vacuum Distillation Process

Selenium, which is molten at temperature 221℃ and boiling at temperature 685℃, is a good volatile element. Based on easy volatility of selenium, extraction of metal selenium by vacuum distillation can avoid some defections in the old course of producing selenium. These defections are low recovery rate, badly environmental contamination, relative high cost of production and so on. Besides, extraction of metal selenium by vacuum distillation has short technological process, little erosion on equipment and so on. It is important part to enhance production efficiency of selenium and promote its application in the factory. The raw materials comes from industrial materials containing 95% Se in the process, which selenium purification is based on vacuum distillation theory and research a new process of vacuum distillation technology.Firstly, the present situation on the study of selenium and the recent development on preparation of selenium with vacuum distillation at home and abroad were reviewed in detail. The properties, uses and production situation of selenium were also reviewed.Secondly, thermodynamics analysis is carried through. According to different boiling point, we can predict that the selenium can be separated from the impurities. Then the vapour pressures of pure substance of selenium and impurities are compared under different temperature. The impurities in the raw material are classified through vapour pressures of pure substance. One kind is more volatilizable than selenium, the other is much more difficult to evaporate than selenium. According to different vapour pressures of pure substance the selenium can be separated from the impurities. The selenium is looked as chief body, phase diagram, and gas-liquid equilibrium composition diagram of selenium-based alloy are analysed. Finally, the kinetics study is carried through. There are four restriction processes which influence the process of vacuum distillation. The first process is the impurities diffusion the inside of the liquidselenium. The second is the selenium evaporation on the liquid surface. The third is the diffusion of gas in the vacuum furnace. The last is the condensation of selenium gas. The dynamics studies show that in order to increase the productivity of equipments and obtain low cost and achieve more economic benefit, a suitable temperature and reasonable vacuum degree is needed when vacuum distillation is adopted to purify selenium.According to the thermodynamics and kinetics analysis of vacuum distillation, the diminutive experimental research is carried out in vacuum furnace. The research shows that the optimal conditions are 330°C, 30min and lOPa in furnace. After 30g selenium is distillated under the above conditions, and selenium content is 99.9417%. Then the kilogram level experimental research is carried out in vacuum furnace. The optimal conditions are 350 °C, 30min and 40Pa in furnace. The 30g selenium content is 99.948%. Finally, the process of vacuum distillation meets our study is proposed on the basis of above studies.All these theoretical study results and experimental data will provide theoretical foundation and experiment guarantee on the industrialization of the technology of selenium purification by vacuum distillation.