Study On TiB₂ Inert Cathodes For Aluminium Electrolysis

Based on some previous research, following researches were studied in this paper on the preparation of TiB2 coating by molten salt electrolysis, such as influence of cathode current density on coating quality, corrosion resistance capability to electrolyte of TiB2 coating, some physical chemistry properties and electrochemical behavior of the electrolyte. Furthermore, penetration of sodium and electrolyte into different cathode materials during electrolysis and industrial testing of TiB2/C coating solidified at ambient temperature in 300kA prebaked aluminium reduction cells were also studied this paper.Cyclic voltammetry, linear sweep voltammetry, galvanostatic potentiometry (interval time<0.1s) and amperometry (interval time<0.1s) methods were used in the study of electrochemical behavior of electrolyte on tungsten electrode. The results showed that in KF-KCI-KBF4 molten salt system, electrochemical reduction of B(Ⅲ) was simple one-step reaction:B3++3e-→B. Cathode reduction process was reversible approximately at the scan rate of 0.1～2V·s-1, electrodeposition process was diffusion-control, and the reduction product of boron was soluble. In KF-KCl-K2TiF6 molten salt system, the reduction process of Ti(IV) was three-step:Ti4++e-←→Ti3+, Ti3++e-←→Ti2+, Ti2++2e-←→Ti. The electrochemical reduction of Ti(Ⅳ) on tungsten electrode was instantaneous nucleation. In KF-KCl-K2TiF6-KBF4 molten salt system, the reduction potential of boron and titanium and the cathode process of TiB2 were determined by the mole rate and concentration of K2TiF6 and KBF4. When mole rate of B/Ti<2, different reduction wave can be differentiated; when mole rate of B/Ti>2, the reduction wave of Ti(Ⅳ) and B(Ⅲ) merged one wave. The formation of TiB2 through interaction of boron and titanium and concentration polarization of ions may explain the potential alternation. These basic theory researches supplied warranty for electrodeposition of TiB2 by molten salt electrolysis.Some physical chemistry properties of electrolyte were tested, such as density, crystallizing point and conductivity. The mole rate of K2TiF6 and KBF4 in the electrolyte was different. Archimedes method was adapted to research the density of electrolyte; combination of digital universal meter and computer serial interface measuring technique was adopted, thermal analysis method was used, according to the step gradual cooling graph, crystallizing point of the electrolyte was determined; conductivity of the electrolyte was tested by CVCC technique. The results showed that, when temperature varied from 740℃to 800℃, the density of the two electrolyte systems decreased gradually with the increasing of temperature, crystallizing point of two electrolyte was 680℃and 678℃, the average conductivity of two electrolyte was 2.10772 and 2.02972. Conductivity of the electrolyte rose with the gradual increase of temperature. These researches can improve technical parameter of electrolysis, increase current efficiency and reduce consumption of energy.TiB2 coating was electrodeposited on graphite substrate by molten salt electrolysis. Influencing factor of TiB2 coating quality was studied, the component and element distribution were analyzed. The results showed that the quantity of coating was influenced by cathode current density, the component of the coating was TiB2 mostly and the distribution of Ti and B elements was well-proportioned, the thickness of the coating was about 0.2 mm. The penetration resistance capability of TiB2 coating used as cathode during electrolysis was studied. After 4hrs electrolysis, the analysis showed that TiB2 coating can slow up sodium penetration effectively compared with graphite cathode.Penetration resistance capability of different cathode materials to sodium and electrolyte was studied in this work. The results showed that the penetration resistance capability was enhanced with the increasing of graphite in the graphitic cathodes. Penetration resistance capability of vibratory compaction TiB2 cathode and graphited cathode was better than graphitic cathode, but the cost of them was too high which limited the large scale industrial application. Through improving of production technology and reducing of production cost, they may be used in aluminium reduction cells comprehensively.Introduced the technique of TiB2/C composite cathode coating solidified under ambient temperature and the industrial application of the coating in the 300kA aluminium reduction cells were studied in this paper. The bottom average voltage drop of two test cells was lessened by 9.2mV and 10.7mV after startup 5 months. The current efficiency of the two test cells was increased about 0.67% and 0.84%. The validity life of TiB2/C composite cathode coating in aluminium reduction cells can reach about 30 months. The TiB2/C coating can save energy and increase pot life obviously.