Study On Deterioration Mechanism Of Physical And Chemical Properties Of NaCl-KCL Molten Salt System For Purification Of Crude ZrCl₄

Crude ZrCl₄obtained from electrofused zirconium desilicate and carbon（petroleum coke）chlorination was purified with molten salt Na Cl-KCl.During the purification process,impurities Fe Cl₃and Al Cl₃in crude Zr Cl₄and Zr O₂and C in raw materials will be brought into the purification furnace along with gaseous Zr Cl₄,resulting in the deterioration of the physicochemical properties of the molten salt system,and the purification reaction can not proceed smoothly.In response to this situation,industrial measures are taken to drain part of the degraded waste salt and add new molten salt to it,but this method not only causes a large amount of zirconium loss,but also pollutes the environment.Therefore,the research on the deterioration mechanism of physical and chemical properties of molten salt,waste salt treatment and application can not only reduce waste salt emission from the source,but also contribute to the comprehensive utilization of waste zirconium-containing salt.In this thesis,SEM,XRF and XRD were used to study the existing state and distribution of inclusions in molten salt during the purification of crude Zr Cl₄.Rotation method,Archimedes principle and tubular method were used to measure and study the influence of main inclusions on the viscosity（η）,density（r）and surface tension（s）of Na Cl-KCl high temperature molten salt.At the same time,the wettability of Zr O₂,petroleum coke and molten salt was investigated with wettability tester,so as to explore the deterioration mechanism of physical and chemical properties of molten salt system.The treatment and application of waste salt containing zirconium were systematically studied.The specific content is as follows:（1）The physical and chemical properties of crude ZrCl₄and zirconium-containing waste salt were determined.It was found that Fe and Al were mainly enriched in waste salt and reacted with molten salt.In addition,there were a lot of Zr O₂and petroleum coke in the waste salt.Zr O₂is mainly deposited in the bottom of molten salt,petroleum coke is floating in the surface of molten salt,Fe Cl₃and Al Cl₃are dispersed in the molten salt.（2）The influence of main inclusions on physicochemical properties of Na Cl-KCl molten salt system was studied.With the increase of Zr O₂content,theηof the molten salt system increases gradually,and therandschange irregularly.The effect of temperature onrandηis small,andsis inversely proportional to temperature.The wettability of Zr O₂and molten salt system is good.With the increase of petroleum coke content,thesof the molten salt system decreases,but therandηincreases gradually.The effect of temperature onrandηis small,andsis inversely proportional to temperature.The petroleum coke and the molten salt basically do not wetting.Adding a small amount of Fe Cl₃and Al Cl₃to the molten salt will increase theηof the molten salt system,which is not conducive to the volatilization of Zr Cl₄.（3）ZrO₂and C do not participate in the reaction in the molten salt,but the enrichment in the molten salt affects the purification of Zr Cl₄.Fe Cl₃and Al Cl₃react with the molten salt to generate a relatively large volume of double salt,which increases the viscosity of the molten salt and prevents the volatilization of Zr Cl₄.（4）Water leaching-roasting process is mainly used to treat zirconium-containing waste salt.Response surface optimization method is used to optimize the water leaching process.When the liquid-solid ratio is 8:1 m L/g,the stirring time is 60 min,and the leaching times are 3 times,the content of zirconia is 95.2%.After roasting,nanometer Zr O₂with purity of 96%was obtained,and the decarburization effect was obvious.The extracted nanometer Zr O₂was applied to the flux to increase the toughness of the welded joint effectively.