Experimental Study On The Removal Of Bismuth, Lead, Iron And Mercury From Crude Seleniu

As a typical rare dispersive element,selenium has been widely used in metallurgy,glass,chemical industry,electronics and other fields.With the development of science and technology,higher requirements are put forward for the preparation of high purity selenium.Vacuum distillation has the advantages of short flow,short production cycle,high direct metal yield and low cost.Compared with other crystallization refining methods such as zone melting,the efficiency of rotary crystallization is improved obviously.According to the characteristics of crude selenium impurities,this paper studied the purification of crude selenium from two aspects of vacuum distillation and rotary crystallization,which provided a new research idea for purifying crude selenium and preparing high purity selenium.The evaporation rates of Bi and Se in Bi-Se alloy at 953-993K and 1Pa were studied.The optimal experimental conditions of vacuum distillation and evaporation rate of elements in alloys were determined and calculated.The influence of component interaction on the kinetic behavior of element evaporation in alloys was discussed.Based on the nonrandom two-liquid（NRTL）equation,the activity coefficients of each element in the bismuth selenium series are calculated,and the results are in good agreement with the literature.The evaporation rates of Bi and Se in vacuum distillation process were quantitatively calculated by incorporating the component activity coefficient into Langmuir equation with the kinetic theory.The residue and volatiles were characterized and analyzed by SEM and EDS.The effect of compound Bi₂Se₃on reducing the evaporation rate of elements in Bi-Se alloy at 973K has been discussed.The activation energy of Bi in pure Bi and Bi-Se alloy has been calculated.The calculated results show that Bi is easier to evaporate in pure Bi,which is consistent with the activity coefficients in the system obtained by fitting that are all less than 1,indicating that the interaction force between molecules in the melt will hinder the volatilization of components in the distillation process.Based on Se-X（X=Pb,Fe,Hg）binary phase diagram,the polynomial function of the liquid phase was fitted,and the distribution coefficients of the main impurities Pb,Fe,Hg in selenium were calculated,which provided a theoretical reference for the separation and purification of crude selenium in the rotary internal air-cooled crystallizer.A single factor experiment was carried out on a rotating gas-cooled crystallizer,and experimental parameters such as crystallization temperature,crystallization time and rotation rate were analyzed and discussed.The results showed that the purity of selenium increased from 99.9%to more than99.997%after nine purification at 222℃crystallization temperature,120min crystallization time and 120 and 300 rpm respectively.The removal rates of Pb,Fe and Hg in Se were97.63%,96.28%and 28.70%,respectively,and the direct yield of purified Se was 92.5%.This study provides a new idea for further removing impurity from crude selenium,and has theoretical significance and reference value for the experimental preparation of high purity selenium.