Study On The New Technology Of Crude Selenium Oxidation Regulation-vacuum Volatilization Purification

90%of selenium in the world is extracted from copper anode slime(CAS).Crude selenium with many kinds of impurities and complex occurrences is produced after comprehensively treatment.How to clean and efficiently prepare high-purity selenium from crude selenium had become the focus of the industry.The traditional oxidation and reduction process for selenium purification has the disadvantages of low efficiency,low direct yield of refined selenium and serious pollution.In this work,a new ideas of"oxidation regulation combined with vacuum volatilization"for the preparation of pure selenium was proposed,which based on the occurrence and thermodynamic characteristics of elements in crude selenium.This method realized the clean and efficient purification of selenium and enriched high value-added metals such as silver and gold,it provides a new method for comprehensive utilization of crude selenium.The thesis focuses on the ideas of"oxidation regulation combined with vacuum volatilization",the theoretical and experimental studies on the occurrence characteristics of impurities in crude selenium,the oxidation regulation of impurities,the vacuum volatilization characteristics of selenium and the purification of selenium by vacuum volatilization were carried out,and the scal-up experiment was developed.The main conclusions are as follows:The occurrence characteristics of various elements in crude selenium were studied by means of SEM-EDS,XPS and EPMA.The thermodynamic studies of saturated vapor pressure,separation coefficient and thermal stability under vacuum were carried out to clarify the volatilization characteristics of impurities in the process of vacuum high temperature evaporation.The results show that the main phase of crude selenium is elemental selenium,the impurities of tellurium,copper,lead occur in the form of elemental tellurium,TeO₂,PbTe,Cu₂Se,PbSe.The saturated vapor pressure of these impurities are close to the Se,the separation coefficient of Se and Te is small,and it is difficult for PbTe,Cu₂Se and PbSe to decompose into elemental matter in vacuum.Therefore,it is difficult to purify selenium directly by vacuum distillation.The E-pH diagrams of Se-H₂O?Te-H₂O?Se-Cu-H₂O?Se-Pb-H₂O and Te-Pb-H₂O systems at 298K and 373K were drawn by thermodynamic calculation.The thermodynamic behavior of impurities in wet process system and the feasibility of impurities transformation by oxidation were discussed.The results show that the increase of redox potential can make the elemental tellurium,PbTe,Cu₂Se and PbSe oxidized into TeO₂,PbSeO₃ and CuSeO₃,which are difficult to volatilize,while the phase of elemental selenium remains unchanged.The hydrogen peroxide was used as an oxidant in the experimental study of impurity transformation.When complete the experiment of oxidation regulation under the liquid-solid ratio of hydrogen peroxide to selenium slag was 0.15:1,the chemical state of selenium remained unchanged.The direct yield of selenium was 90.01%.13.6%of tellurium,6.4%of copper and 0.9%of lead entered the solution in the form of ions,and the rest of tellurium,copper and lead were oxidized to high valence oxidation state,forming aggregates dispersed in selenium matrix,thus realizing the formation of volatile elemental tellurium,PbTe,PbSe,Cu₂Se to nonvolatile TeO₂,CuSeO₃ and PbSeO₃.The evaporation rates of selenium at 250?,300?,350?,400?,450?and 2.3?1500Pa were determined by vacuum differential gravimetric method.The results show that the logarithm of the evaporation rate of selenium has a linear relationship with the reciprocal of the temperature when the system pressure is constant;when the distillation temperature is constant,the evaporation rate of selenium has a maximum region,an acceleration region and a slow region with the change of the system pressure,and there is a Logistics nonlinear relationship between the evaporation rate and the pressure.At the same time,the critical pressures of selenium at 250?450?are 5.34 Pa,12.96 Pa,14.20 Pa,33.49 Pa and 37.06Pa,respectively.The relationship between critical pressure and temperature is lgcrit=-4751.3/T+2.6341,when the system pressure is lower than the critical pressure,selenium volatilizes at the maximum evaporation rate.The maximum evaporation rate of selenium increases with the increase of distillation temperature,and the relationship is lg?_e·m=-9385.6/+3.5887.The experimental maximum evaporation rate is always less than the theoretical maximum evaporation rate,and the condensation coefficient?is between 0.003 and 0.1.The purification experiment of vacuum distillation was carried out for the crude selenium after impurity oxidation regulated,and the effect of different distillation conditions on the purification effect was systematically studied.The 60g scale experimental study of vacuum distillation showed that the influence of each influencing factor on the volatilization rate of the material in descending order is:distillation temperature>distillation pressure>distillation time>height-diameter ratio.The influence factors on the total content of Te,Cu and Pb in volatiles were not significant.Under the conditions of distillation temperature260?,distillation time 50 min,system pressure 10 Pa and material height diameter 0.67,the volatilization rate of material is 92.8%.The volatiles contain 2.72 ppm of Te,0.83 ppm of Cu and 5.18 ppm of Pb.The purity of different materials after purification is greater than99.98%.The results show that under the condition of 460?,60min,and10Pa,the volatilization rate of material is 89.0%,the purity of refined selenium is 99.98%,and the direct yield of refined selenium is 97.44%.The contents of Se,Te,Cu and Pb in the residue were 19.5%,3.35%,3.29%and 10.63%,respectively.The main phases were Na₂SO₄,PbSeO₃ and CuSeO₃.Ag and Au in the residue increased from 0.27%and 103g/t in raw materials to 2.42%and 937 g/t,respectively,enriched by 8.96 times and 9.0 times;the recovery of Ag and Au was 99.7%and 99.8%,respectively.The second vacuum distillation of the volatiles from the first distillation can obtain refined selenium with a purity of99.995%.The purity of refined selenium was 99.993%,and four by-products including filtrate,condensate,slag and distilled residues were produced.The comprehensive direct yield of refined selenium is 83.80%,and the recovery of silver and gold is more than 99.4%.The estimated energy consumption for per ton of refined selenium output is about 1700k W·h.Through process comparison and analysis,this process has the characteristics of short process,high yield of refined selenium and low cost,which has a great application prospect.The results of sustainability assessment based on life cycle assessment show that this process has less impact on the environment than redox process,and it is a more sustainable and cleaner production process.To sum up,the new technology of oxidation regulation and vacuum volatilization purification of crude selenium developed in this research solves the problem of vacuum separation of selenium and tellurium from crude selenium with high tellurium content.High quality refined selenium with purity greater than 99.99%can be prepared from selenium residue containing about 70%selenium,and high value-added metals such as tellurium,silver and gold can be enriched at the same time.The clean and efficient purification of crude selenium is realized,which has a great application prospect.