Study On Reduction Mechanism And Current Efficiency Optimization Of Electrolysis Of Titanium Dioxide In Molten Salt

Because of the high strength,corrosion resistance,good biocompatibility and other outstanding characteristics,metallic titanium is considered to be the most competitive structural materials and functional materials in the 21st century.But at present,the preparation of titanium metal has a long process and high cost,so the application of titanium and its alloys limited in the aerospace and other high-tech fields.Therefore,the development of low-cost titanium preparation process is imminent.Metallic titanium prepared by molten salt electrolysis has many advantages,which is one of the most likely ways to realize industrialized production of titanium at low cost,and which has been paid close attention by scientists and researchers.In recent years,the method of direct preparation of titanium by molten salt electrolysis of solid TiO₂?FFC process?has attracted the attention of researchers at home and abroad for its simple process,short process and low cost.At present,the study of deoxidation mechanism,the formation of cathode mesophase were ambiguous,and the low current efficiency problem in the electrolysis process of titanium dioxide,has not been fundamentally solved.This paper,based on the FFC process,currently faces the bottleneck problem of low valence titanium oxides,unclear cathode mesophase formation mechanism and low current efficiency.Firstly,it calculates the difficulty of the theory deoxidation in various stages of electrochemical reduction of titanium dioxide by thermodynamics,and analyzes the reason of the deep deoxidation of Ti-O solid solution from the thermodynamic angle.Secondly,the electrochemical deoxidation process of titanium dioxide was investigated by the intermittent experiment of titanium dioxide cathode,deoxidation model was analyzed and deoxidation mechanism was clarified.Finally,the wettability between molten salt and graphite anode and its effect on deoxidation rate were discussed,and the basic theory of hydrogen assisted Ti-O solid solution deep deoxidation was studied,which provides a basis for optimizing the current efficiency of the process.The main conclusions are as follows:?1?The thermodynamic calculation results show that the deoxidation of titanium dioxide in electrolysis process is progressive,and with the electrolysis time increasing,the lower is the oxygen potential of the cathode,the greater is the difficulty of deoxidation,which is the fundamental reason of low current efficiency.?2?The analysis of the phase and morphology of cathode products after discontinuous electrolysis of titanium dioxide at different time shows that the deoxidation process of titanium dioxide electrolysis is:TiO₂?CaTiO₃?TiO_x?TiO?Ti^[O].The cathode deoxidation mainly includes three stages:partial oxygen is removed from the cathode TiO₂ and reacts with calcium ions from the molten salt to form CaO,and the formed CaO reacts with the initial TiO₂ to generate perovskite phases CaTiO₃.The newly generated CaTiO₃ is gradually electrolyzed to titanium suboxides.The titanium suboxides will gradually be electrolyzed into metallic titanium.At the beginning of electrolysis,the current efficiency is high,but deoxidation of titanium suboxides and Ti-O solid solution is slow,and the current efficiency of the whole electrolysis process is about 24.07%.?3?The electrolysis deoxidation of titanium dioxide is a multiphase and multi interface transformation process,including TiO₂/CaTiO₃/molten salt three-phase interface,CaTiO₃/TixO_y/molten salt three-phase interface,Tix O_y/TiO/molten salt three-phase interface and TiO/Ti/molten salt three-phase interface.The traditional three-phase boundary model is not completely suitable for titanium oxide electrolysis deoxidation process.?4?For CaCl₂,NaCl,KCl three kinds of molten salts,KC l molten salt has good wettability with graphite anode,CaCl₂ and NaCl shows poor wettability with graphite.For CaCl₂,the wettability between CaCl₂-KCl with graphite is best,and the wettability of CaCl₂-NaCl-KCl mixed molten salt is better,and the wettability of CaCl₂-NaCl is poor.In other words,the wettability between mixed molten salt with graphite anode can be improved by mixing KCl.Moreover,the more quality of KCl in the CaCl₂-KCl mixed molten salt,the better the wettability improved.Metallic titanium was obtained from 6 h with CaCl₂-KCl?mass ratio CaCl₂:KCl=9:1?,and the current efficiency was51.97%,which was nearly one times higher than that of single CaCl₂ molten salt electrolysis.It also shows that the better wettability of molten salt and graphite anode,the faster the deoxidation rate of titanium dioxide.?5?After absorpting hydrogen,the formation of Gibbs free energy of high oxygen content of Ti-O solid solution?such as oxygen 2%and 1%of the Ti-O solid solution?become larger.In other words,the Ti-H-O system solid solution(such as TiH_x O_0.03)produced after absorpting hydrogen is more unstable than the original Ti-O system solid solution(for example,TiO_0.03)and is more prone to occur chemical changes.In other words,when the oxygen content in the solid solution of Ti-O system is high,the hydrogen absorption has a greater influence on the auxiliary deoxidation.The formation of Gibbs free energy is related to the amount of hydrogen in Ti-O,the higher the content of hydrogen,the smaller the Gibbs energy is,and the lower the oxygen content in the same amount of hydrogen absorption,the smaller the Gibbs free energy is.When the oxygen content in Ti-O solid solution is low,the increased amount of free energy is small after absorpting hydrogen.From the thermodynamic view,only control the amount of hydrogen absorption at a small value,will achieve hydrogen assisted further deoxidation.When oxygen content is extremely low?such as 0.02%oxygen in solid solution?,Ti-O solid solution cannot be further deoxidation by hydrogen.