Anti-permeability Of Cathode For Low Temperature Aluminum Electrolysis In [k₃alf₆/na₃alf₆]-alf₃-al₂o₃ Melts

Low temperature electrolysis is an important means to achieve transformation of aluminum electrolysis technics, and penetration resistance of cathode is one of the key problems for low temperature electrolysis. Funded by the State Key Project of Fundamental Research (973 project:2005CB623703) and National High Technology Research and Development Project (863 project:2008AA030503) and based on [K3AlF6/Na3AlF6]-AlF3-Al2O3 system, with the aim to solve the problem related to penetration resistance of cathode. Interactivity of novel electrolyte including potassium and cathode is studied systematically considering the composition of electrolyte, technological parameter of electrolysis and composition of cathode. In addition, precipitations, penetrative and migratory behavior as well as the effects those conduct to the performance and structure of cathode during the aluminum electrolysis are also researched. The main results of the thesis can be summarized as follows:(1) The influence of CR,KR,ρCD and tS on electrolysis expansion of semigraphitic cathode in [K3AlF6/Na3AlF6]-AlF3-Al2O3 melts is revealed. When CR is 1.4～3.0, KR is 0.1～0.5,ρCD is 0.2～1.0A/cm2 and tS is 10～50℃, the regression equation showing the relationship between electrolysis expansion of semigraphitic cathode and melts composition,ρCD and tS is put forward. The ternary isotherm diagram refer to electrolysis expansion is also drawn. On these ground, a series of electrolyte composition and technological parameter of electrolysis are designed which include the typical character of "low temperature and low electrolysis expansion". When electrolysis temperature is 880℃～925℃, the electrolysis expansion can be controlled in 0.51%～1.10%.(2) The relationship between diffusion coefficient of alkali metal(K and Na) and species of cathode as well as melts composition is indicated. The diffusion coefficient of alkali metal in TiB2-C composite cathodes which take pitch, furan, phenolic aldehyde and epoxy as binder are 2.86×10-5 cm2/s,2.74×10-5 cm2/s,2.24×10-5 cm2/s,2.52×10-5 cm2/s respectively. However, the diffusion coefficient of alkali metal in carbon cathode is very large, which is 8.87×10-5cm2/s. The diffusion coefficient of alkali metal rest with the temperature of melts (surperheat temperature), not with the composition of melts (liquidus temperature). In the range of 893℃-990℃and under the same condition, the diffusion coefficient will increase 0.52×10-5cm2/s averagely, when the temperature of melts increases by 10℃.(3) Penetrative and migratory pathway of alkali metal(K,Na) in cathode during aluminum electrolysis is proposed. Under the condition of polarization, both K and Na will deposit and generate corresponding intercalation compound in [K3AlF6/Na3AlF6]-AlF3-Al2O3 melts at the same time. Alkali metal generated through discharge directly is the most important influencing factor on cathode expansion. After 10 minutes electrolysis, the cathode expansion in XAP,ANAP,NAP,KP,KNP and Np melts are 28.40%,23.27%,18.43%,13.56%.11.65%and 10.41% respectively. However, under the condition of non-polarization, the cathode expansion only occur in KAN,KNAN and NAN melts, and the expansion values are not large, which are 0.91%.0.43%and 0.22%. compared with the pure potassium salt, collapsing force of composite electrolyte to cathode reduces, such as KNAP＜KAP,KNP<KP and KNAN < KAN, the decreasing amplitude are 18.07%,14.1%和5.52% respectively. The penetration and migration path of alkali metals(K and Na) are similar. Firstly, alkali metal penetrate into the pore of cathode with electrolyte, and then penetrate into the binder burnt, with the proceeding of electrolysis, finally penetrate into the carbon aggregate. Alkali metal can not penetrate into TiB2, and alkali metal penetrated into binder burnt and carbon aggregate all provoke the expansion of cathode, which will reach equilibrium value when the concentration of alkali metal in cathode is saturated. By contrast, the stability of C-K intercalation compound is better than Na, which has stronger collapsing force to cathode.(4) The influence law of materials composition and preparation technology on electrolysis expansion and corrosion resistance of TiB2-C composite is revealed. The optimum addition of pitch, furan, phenolic aldehyde, and epoxy in TiB2-C composite cathode are 16wt%,18wt%, 14wt%and 12wt%, respectively. At this circumstance, electrolysis expansion of TiB2-C composite cathodes are 1.49%,1.26%,1.18%and 0.92%severally(CR=1.6, KR=0.3); At the same condition, using graphite can make the electrolysis expansion of corresponding composite cathode further decrease, the decreasing amplitude of which are 36.56%,35.89%,36.90%and 37.87%respectively. Meanwhile, increasing heat treating temperature from 1000℃to 1400℃can also make the electrolysis expansion of corresponding composite cathode decrease, he decreasing amplitude of which are 52.15%,53.94%.40.72%and 43.99%. When the binder content is 14wt.%, corrosion rate of composite cathode and penetration rate of alkali metal in it is smallest, which are 2.31mm·year-1 and 4.72 mm·h-1; aggregate type has no effect on the corrosion rate of composite cathode and penetration rate of alkali metal, but heat treating has great effect on them. Increasing heat treating temperature from 1000℃to 1400℃can make the corrosion rate of composite cathode and penetration rate of alkali metal decrease by 45.48%and 37.5%.(5) Effects of the binder coking on electrolysis expansion of TiB2-C composite cathode are posted. So far as binders be concerned, when only turbostratic structure exist, or the structure in a transition state from turbostratic structure to graphitic structure, the higher the graphitization degree is, the smaller the electrolysis expansion; But when materials transform from undefined structure or other complete non-graphited structure to turbostratic structure, the electrolysis expansion of which deviate contrarily. At this circumstance, the higher the thermolysis activation energy is, the smaller the electrolysis expansion is. So far as the four binder mentioned above be concerned, the thermolysis activation energy of epoxide resin is the highest, which is 780.32 kJ/mol, and its electrolysis expansion is the best.