Investigation On The Interaction Of Metal And Molten Salts By High-Temperature In-Situ Spectroscopy Techniques

The interactions between metals and molten salts are common phenomena in the applications of molten salts,which will bring negative influence:The corrosion of molten salts to metals seriously affects the safe operation of MSR(Molten Salt Reactor)and CSP(Concentrating Solar Power)station;the dissolution of metals in molten salts is an important factor causing the reduction of current efficiency in metal production by molten salt electrolysis.Limited by the lack of high-temperature in-situ characterization methods for molten salt and the difficulty of theoretical simulation of complex molten salt systems,the research on the interaction mechanisms between metals and molten salts is not extensive and incisive.Therefore,this paper has developed a variety of high-temperature molten salt in-situ characterization techniques,including the self-built high-temperature UV-Visible absorption spectrometer,the customized high-temperature Nuclear Magnetic Resonance and the high-temperature X-ray absorption spectroscopy of Shanghai Synchrotron Radiation Facility.Three different types of interaction systems between metals and molten salts were investigated by these techniques,and the relevant reaction mechanisms were discussed deeply,respectively.The main research contents and achievements are as follows:(1)High-temperature UV-Visible absorption spectrometer was developed and High-temperature Nuclear Magnetic Resonance equipment was improved.The built-up high-temperature UV-Visible absorption spectrometer can measure the valence and concentration of metal ions in molten salts below 800?in the inert atmosphere.The customized Bruker 400 MHz Nuclear Magnetic Resonance has been improved for testing molten salts systems.Samples can be tested at 750?to determine the coordination structures and bonding properties of ions by the improved instrument.(2)The reaction mechanisms of metallic Cr and trivalent Cr(?)in molten FLiNaK (LiF:Na F:KF=46.5:11.5:42 mol%)and FLiBe(LiF:Be F₂=66.6:33.3 mol%)salts were investigated.The results showed that metallic Cr reacted with Cr(?)in the molten FLiBe salt to form Cr(II).However,the Cr(II)generated in molten FLiNaK decomposed to Cr and Cr(?),and Cr(?)continued to react with Cr.It is demonstrated that the difference in reaction mechanism was caused by the different concentration of free F^-ions in the two kinds of molten fluoride salts.(3)The interactions of metal Be with molten FLiBe and FLiNaK at 700?were investigated.The experimental results showed that metal Be would react with molten FLiNaK by substitution to form metal K,but could not reacted with molten FLiBe.The high-temperature volatilization of metal K and the covalent interaction of free F^-with Be were main reasons for the substitution reaction of Be in FLiNaK.Characterization experiments also detected the intermediate compound of Be F^-generated from F^-and metal Be.(4)Dissolution experiments of Cu,Ag,and Au in molten LiCl-KCl(LiCl:KCl=60:40 mol%)were designed to investigate the structures of dissolved matter and the dissolution mechanisms.The experimental results showed that the dissolution rate was Cu>Ag>Au(insoluble).The results of high-temperature in situ X-ray absorption spectra indicated that the dissolution of Cu in molten salt formed metal clusters.This research also demonstrated that the solubility of Cu,Ag and Au decreases with their increase of electronegativity.The work in this paper shows that high-temperature in situ spectroscopy is a powerful tool for studying the microstructure,bonding properties,and reaction mechanisms during metal-molten salt interactions.The work in this paper contributes to the understanding of mechanisms of metal-molten salt interactions.It provides a theoretical basis for the prevention or utilization of the processes of metal-molten salt interactions in industrial and scientific applications of molten salts.