Research On Preparation Process Of High-purity Vanadium Pentoxide By Chlorination Method

With the situation that the demand of high purity vanadium oxides used in the emerging fields like all-vanadium redox flow battery?VRFB?and high-performance vanadium alloys is increasing year by year.It is therefore urgently needed to manufacture high purity V₂O₅ with a low cost and a high efficiency.Traditionally,high purity V₂O₅ is most commonly manufactured through processes such as purification of the leaching vanadium solution by chemical selective precipitation or solvent extraction or ion exchange and which generally will take a high cost obtaining V₂O₅ with purity of 99.99%.Meanwhile,these processes are facing a series of challenges like ammonia nitrogen wastewater generation,lengthy operating procedures,small production capacity,etc.The chlorination procedure is one of the most potential methods for the preparation of high purity V₂O₅ with a low cost and a high efficiency.VOCl₃ intermediate can be obtained via carbochlorination using industrial grade V₂O₅ as the raw material.Most of the impurities in the VOCl₃ can be removed by means of distillation,making chlorination procedure advantageous in purification and impurity removal.High purity V₂O₅ can be obtained in a single step via VOCl₃ vapor hydrolysis without generation of ammonia nitrogen wastewater.Efficient chlorination of industrial V₂O₅ and the conversion of VOCl₃ intermediate to high purity V₂O₅ are of great significance for improving chlorination efficiency and lowering the cost.Therefore,the processes of industrial grade flake V₂O₅ chlorination and vapor hydrolysis of VOCl₃ were systematically studied.The main findings of this thesis are as follows.?1?Thermodynamic equilibrium analysis of carbochlorination of V₂O₅ with Cl2 shows that there exists a chlorination predominance area obtaining VOCl₃.The chlorination conditions were then determined,which were charcoal ratio of 20%,fluidization gas velocity of 0.6L/min?0.072?0.085m/s at the experimental temperature?,chlorine partial pressure of 0.5 and reaction time of 15min.The effect of reaction temperature on the chlorination efficiency in the range of 400?550? was investigated.It indicated that with the increase of temperature in the range of 400?525?,the material weight loss rate and V₂O₅ chlorination rate continuously increase.From 525? to 550?,the rate of weight loss and V₂O₅ chlorination have slowed down significantly.V₂O₅ chlorination rate of above 93%was obtained under the conditions that chlorination temperature of 525? and time of 60min.?2?The impurities existing in industrial grade flake V₂O₅ are mainly Fe₂O₃,MnO,SiO₂,etc.The chlorination thermodynamic calculations show that in the range of 400?550?,the chlorination reaction Gibbs free energy is:AG0?K2O?<AG0?CaO?<AG0?MnO?<?G0?MgO?<AG0?V₂O₅?<AG0?Fe₂O₃?<AG0?TiO₂?<AG0?SiO₂?<AG0?Al₂O₃?.According to the calculations,K2O,CaO,MnO and MgO are more easily chlorinated than V₂O₅ to form KCl,CaCl2,MnCl2 and MgCl2.In the range of 400?550?,these chlorides would not affect the normal fluidization by forming melt,nor affect the purity of the product VOCl₃ in gaseous form.Though Fe₂O₃,TiO₂,SiO₂,and Al₂O₃ have no thermodynamic preference for chlorination than V₂O₅,but FeCl₃,TiCl4,SiCl4 and AICl₃ exist as gas in the range of 400?550?.Subsequent difficulty in removing impurities and cost would increase once these chlorides generated adulterating into VOCl₃ condensate.Therefore,this article focuses on the analysis of the chlorination behavior of these impurity components.The results indicated that the chlorination volatilization rate of Fe₂O₃ increases with increasing temperature.At 550?,the chlorination volatilization rate reaches to above 40%.The chlorination temperature should be controlled below 550? avoiding the excessive introduction of FeCl₃ into VOCl₃.The chlorination volatilization rates of Ti02,SiO₂,and Al₂O₃ maintained below 10%at 550?.V2O4 and V₂O₃ were not detected in the chlorinated slags,indicating that V₂O₅ did not undergo reduction.With the reaction carrying on,the original compact vanadium surface became rough,products of impurity chlorides with low saturated vapor pressure and unreacted impurity oxides concentrated in the chlorinated slags.?3?In order to investigate the behavior of VOCl₃ vapor hydrolysis,the reaction thermodynamic and thermodynamic equilibrium of the hydrolysis system were calculated.The results shown that the reaction system is very complicated and there is a possibility that multiple reactions occur at the same time.Products including V₂O₅,VO₂CI,VO₂,HCl,Cl2 and O₂ get the potential to be formed.Within the temperature range of 0?600?,increasing the temperature is of benefit to the formation of V₂O₅ and reduction of VO₂Cl.Above 200?,H2O/VOCl₃ ratio should be greater than theoretical value of 5 to guarantee a high conversion of VOCL3 to V₂O₅.?4?Experiments of fluidized gas phase VOCl₃ hydrolysis were carried out to investigate the effect of reaction parameters on the conversion of VOCl₃ and Cl content of the product.The experimental results shown that VOCl₃ conversion markedly increases and Cl content of V₂O₅ dramatically decreases with increasing temperature from 200? to 450? or increasing H2O/VOCl₃ molar ratio from 1.5 to 12.6.VOCl₃ conversion slowly increases and Cl content of V₂O₅ slowly decreases with the further increase of temperature to 500? or H2O/VOCl₃ molar ratio to 18.The Cl content of V₂O₅ below 0.05wt%and VOCl₃ conversion above 85%were obtained under the optimal conditions with temperature of 500?and H2O/VOCl₃ molar ratio of 18.The purity of product reaches to 99.95%.?5?Through the characterization and analysis of the products' microstructure,it was found that the V₂O₅ product formed by nanoscale?ca.100nm?particle aggregation.Cl occurrence form in V₂O₅ was characterized by XPS.It shown that the introduction of Cl into V₂O₅ is mainly because of existence of O-V-CI,not HCl or CI2.