Study On Electrolysis Reduction Of Titanium Dioxide To Titanium In Molten Salt And Its Reaction Mechanism

Titanium is a kind of functional material with superior performance. But there are still problems existing in the fabrication titanium process by traditional Kroll method, such as, complicated technology, long production cycle and intermittent operation, etc. Since 2000, sponge titanium prepared by electrochemical reduction of TiO₂ has attracted much attention in the world. Among all kinds of methods, a typical metallurgical process of TiO₂ deoxidation, named FFC, is the best and promising technique considered by scientists.In this research, based on FFC process, titanium were obtained successfully by a series of electrolysis experiments, including the designing and improving the experimental device and experimental scheme. The influences of different sintering temperatures on the composition and microstructure of TiO₂ cathode and cathodic products electrolysed, the impacts of different electrolysis temperatures on TiO₂ cathode and the effects of different scale samples on electric current were investigated. Conductivity of TiO₂ cathode and the dishydrolysis of CaCl₂ molten salt system were explored. Electrolytic reduction mechanism was analysed thoroughly. The factors of dynamic process and the controlling steps of molten salt electrolysis were examined. The main contents of this disquisition are summarized as following:After research on TiO₂ sintering and electrolytic process, the result of appropriate sintering temperature and electrolysis temperature was both at 900?. Sintering temperature affected on the composition and microstructure of TiO₂ cathode and cathodic electrolysis products. In the range of 600?1000?, with the increasing of the sintering temperature, the grain size and compact extent of the sintering sample gradually increased, while the open holes gradually reduced, the open pore rate,pore surface area,pore average diameter, pore volume and effective reaction area became small gradually. On the same electrolysis condition, the content of titanium was impacted by the conductivity and effective reaction area of the cathode. The titanium content became greater and the oxygen content was lesser with sintering temperature increasing. At the same sintering temperature, higher electrolysis temperature was helpful for the generation of titanium. The difference between inner and outer electrolysis products illuminated that TiO₂ was electrolysed from outer to inner. The electric current became larger with the increasing weight. Through research and thermodynamic calculation, the key parameters of TiO₂ electrolysis in molten salt were obtained.Based on the measurement of XRD, XPS and specific conduction of sintered TiO₂, conductivity of TiO₂ electrode was analyzed. It showed that sintering at high temperature may change the energy band structure of TiO₂, forming conductive vacancies, generate O^2-; the form of electric double layer in molten salt of TiO₂ made the oxygen atoms ionized, forming ionic structure of TiO₂-X; The ion in molten salt penetrate internally to the TiO₂ electrode and improved the number of conductive ion in TiO₂ electrode. It can explain reasonable why the TiO₂ cathode could be made effective electrode, and why the more sample weight is, the more electric current is. Experiment and theoretical calculation both indicated that, on the condition of experiment, CaCl2 molten salt could not hydrolysis.The results of cyclic voltammetry and chronocoulometry showed the electrolytic reduction process included TiO₂?TiO and TiO?Ti. According to the experiment of reduction mechanism, the electrolytic reduction mechanism is not the calcium heat reaction but the oxide ionization. TiO₂ reduction process is carried out gradually from high valence to low, finally generate the metal, oxide of low valence titanium and perovskite are necessary intermediate products. The result of theoretical calculation by quantum chemistry method of TiO₂ crystal structure proved that there were two kinds of oxide atom, which had different activities; one of the most kinetic oxide atoms can make departure easily. This can well explain the formation of sample sintered oxide vacancy, and the electrolytic reduction process.Dynamic study of molten salt electrolytic showed that, O^2- migration in the solid cathode and cathodic reaction are the controlling steps of the whole process. The generation of CaTiO3 impact ion migration, and the dense layer of titanium will hinder internal O^2- from migrating to the surface. The result of electrochemical impedance spectroscopy indicates that, among the gradual step of the cathode, the step from TiO₂ to TiO and TiO to Ti are both controlling steps, the step from TiO to Ti is more difficult than TiO₂ to TiO.