Electrochemical Behavior And Coordination Mechanism Of Tantalum Ion In Molten Salt

As a refractory metal,tantalum has many excellent properties,such as strong corrosion resistance,high chemical stability,low thermal expansion coefficient,good cold workability,etc.It is widely used in aerospace,electronic industry,weapon systems,medicine and other fields.There are two main methods of smelting tantalum.Before 1903,tantalum was mostly prepared by thermal reduction.After 1922,molten salt electrolysis was successfully used to produce tantalum powder,which laid the foundation for industrial production of tantalum.The extraction and purification of metal tantalum by molten salt electrolysis is a short-process and green metallurgical technology with great development potential and application prospects.The influence of the composition of the electrolyte in the electrolysis elements on the current efficiency and electrolysis quality is a key issue that needs to be studied urgently in the electrolysis process.Based on this,this paper takes the composition of the electrolyte in the electrochemical metallurgy of metal tantalum as the research object,and analyzes the interaction between tantalum ions and anions（chloride ions,fluoride ions）and their effects on the reduction mechanism of tantalum ions.In this paper,the redox steps of tantalum ions in chloride and fluorochloride molten salts and the chemical coordination mechanism of tantalum ions in the above molten salts are revealed through electrochemical tests and chemical analysis methods.The choice of electrolyte in the medium provides a theoretical basis.The work of this paper mainly includes the following three parts:first,the electrochemical reduction mechanism and kinetic parameters of tantalum ions in Na Cl-KCl molten salt are analyzed;second,the effect of fluoride ion concentration on the reduction step of tantalum ion and its role are clarified Mechanism;third,the principle of the volatilization inhibition of tantalum ions in molten salts by fluorine-tantalum complex ions is elucidated.The following research results were obtained:1.Using electrochemical methods including cyclic voltammetry,square wave voltammetry,chronopotentiometry and chronoamperometry,it is clear that the reduction steps of tantalum ions in Na Cl-KCl molten salt are:Ta（V）→Ta（II）→Ta,the diffusion coefficient of Ta（V）was calculated,and the reduction of Ta（V）was determined to be a reversible process controlled by diffusion.The nucleation mode of cathodic deposition of tantalum ions was analyzed by chronoamperometry as instantaneous nucleation.2.With the continuous increase of the fluoride ion concentration in the molten salt,the peak potential of the reduction peak gradually moves negatively,and when the molar ratio of the fluoride ion concentration to the tantalum ion concentration is greater than or equal to 8.0,the electrode reduction step changes from Ta（V）→Ta（II）→Ta transforms into Ta（V）→Ta;it is found that with the continuous increase of fluoride ion concentration,the peak current density of Ta（II）→Ta reduction peak gradually decreases,and the concentration of Ta（II）keeps increasing.decreases,and the grain size continues to be refined.The research shows that due to the existence of fluoride ion,it can form a more stable high-valence complex with tantalum ion,so that Ta（V）can be reduced to tantalum metal in one step.3.The ICP-OES analysis revealed the volatilization inhibition effect of fluoride ion on tantalum ion.The results show that the concentration of tantalum ions in the Na Cl-KCl molten salt decreases rapidly with time,and the volatilization of tantalum ions is inhibited by the addition of fluoride ions.When the molar ratio of fluoride ion concentration to tantalum ion concentration is equal to 8.0,the volatilization rate of tantalum ion drops to 67.11%.The coordination behavior of fluoride ion and tantalum ion was analyzed by XPS and Raman spectroscopy.The results show that with the increase of the fluoride ion concentration ratio,Ta-Cl is gradually replaced by Ta-F bonds to form relatively stable Ta F_xCl_y^n-complexes.When the fluoride ion concentration is large enough,tantalum ions are converted to Ta F_x^n-.This explains the reduction of reduction steps of tantalum ions,the negative shift of reduction peak potential and the reduction of the grain size of electrolysis products in the above electrochemical tests.In this paper,through a series of electrochemical tests and chemical analysis,the reduction steps of tantalum ions in Na Cl-KCl molten salt were determined,and the influence of electrolyte composition on the reduction steps was revealed,and the selection of electrolytes in the production of tantalum metal by molten salt electrolysis and identify new ways and ideas.