Directional Control - Research On The Theory And Technology Of Purifying Selenium By Vacuum Distillatio

Selenium,a rare element,is a key material supporting the development of high-tech industries such as semiconductors and new energy,and is mainly extracted from copper anode slime.The traditional selenium extraction technology has a long process,large environmental pollution,and difficult impurity removal.Based on the differences in physical and chemical characteristics of impurities and selenium in crude selenium,this paper proposed a technical idea of directed regulation-vacuum distillation to purify selenium,systematically studied the vacuum volatilization and decomposition characteristics of impurities in crude selenium,and the redox behavior in aqueous solution system.The key technology and related mechanism of purification of 100g,kg and 100kg grades were studied.The main research contents and results are as follows:（1）The results of saturation vapour pressure,phase equilibrium components,Gibbs free energy of decomposition,and volatility characteristics of Cu₂Se,PbSe and PbTe in crude selenium show that typical impurities do not decompose at selenium volatility temperatures,and that the volatility rates of Cu₂Se,PbSe and PbTe are in the range of 523～623 K and 10～100 Pa,and the volatility rates are in the range of 10-1～10^-3 g·cm^-2·s^-1;CuSe is decomposed into Se and Cu₂Se at 523 K and 10 Pa.The R3 model is the mechanism function under the current experimental conditions,and the average values of the activation energy and the pre-exponential factor are 12.344 J·mol^-1 and 0.152 s^-1,respectively,and the kinetic equation is dα/dt=0.456×exp（-1.485/T）（1-α）^2/3,and the actual volatilisation rate is 5 × 10^-4 g·cm^-2·s^-1.It is revealed that the impurity content of tellurium,copper and lead in thevacuum distillation of selenium is high because Cu₂Se,PbSe and PbTe will volatilize together with selenium in the vacuum purification of crude selenium,resulting in poor quality of refined selenium obtained from volatilization.Therefore,in order to change the occurrence state of impurities in crude selenium and increase the saturated vapor pressure of selenium and impurities,the potential-pH diagrams of Se-Cu-H₂O,Se-Pb-H₂O and Te-Pb-H₂O systems at different temperatures were calculated and drawn.The results show that the typical impurities Cu₂Se,PbSe and PbTe can be transformed into CuSeO₃,PbSeO₃ and TeO₂ by controlling pH and potential in aqueous solution system.Compared with Cu₂Se,PbSe,PbTe and Se,these substances are more difficult to volatilize,which provides a theoretical basis for the efficient separation of these impurities and selenium.（2）The oxidation mechanism and evolution behavior of typical impurities in aqueous solutions were studied,and the optimal oxidation conditions of Cu₂Se,PbSe and PbTe were obtained,as well as the changes in pH and potential of the system during the reaction and the reaction path.The results show that Cu₂Se and H₂O₂ alone can only produce seleno-copper compound,and CuSeO₃ can be oxidized in the presence of H₂SeO₃.Only a small amount of PbSe and H₂O₂ are converted to PbSeO₃ at 298 K,and most of PbSe can be oxidized to PbSeO₃ after increasing the temperature.PbTe and H₂O₂ are oxidized separately the phase does not change,and are transformed into H₂TeO₃ and PbSeO₄ in the presence of selenium.（3）Based on the study results of volatilization and redox characteristics of impurities in crude selenium,a directional oxidation-vacuum distillation process was proposed to purify different crude selenium raw materials.The results showed that the purity of high tellurium crude selenium after purification could be increased from 89.24%to 99.988%.The removal rate of low mercury crude selenium was 97.9%after purification.In order to remove impurity mercury,the selective vulcanization-vacuum distillation process was proposed to reduce mercury content in selenium from 3200 ppm to 5.02 ppm,and the mercury removal rate was 99.84%.（4）The experimental results of directed oxidation and vacuum purification of crude selenium of 100g,kg and 100kg grades showed that the purity of crude selenium was 99.98%after one distillation,and the purity of fine selenium was 99.993%after two distillation.The impurities tellurium copper lead were reduced from 4100 ppm,4000 ppm and 12100 ppm to 13 ppm,1.75 ppm and 35 ppm respectively,and the removal rates were 99.68%,99.90%and 99.70%,respectively.