Investigation of Carbon Block/Cast Iron/Collector Bar Interfaces in Aluminum Electrolysis
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The chemical events occurring at the carbon/cast iron/collector bar interface that might have an impact on the cathodic voltage drop in aluminum electrolysis is elucidated in this thesis. The primary aluminum production is an energy intensive process which constantly requires to bring as high an energy efficient development as possible in order to be competitive. The cathodic voltage drop represents ~10% of the overall energy consumption of the process. Among the measures to reduce the voltage drop, the collector bar and its surroundings need to be investigated as the literature on this part of the assembly is very narrow. In this thesis, the microstructural and compositional evolution of the cast irons and the collector bars taken from the cells that were shut down after 7 days and 2190 days (6 years) in operation is presented using the analytical tools such as SEM, EDX and optic microscope. The effect of this evolution on the voltage drop is then evaluated by referring to the electrical conductivities of the observed phases and microstructures. The penetration of the bath components, especially aluminum (as high as 14wt.% in the cast iron and 0.44wt.% in the steel) and sodium (1.38wt.% in the cast iron and 0.02wt.% in the steel), into the cast iron and the steel was found partly linked to the morphology of the graphite in the cast iron. The kinetics of the development of the morphology of the graphite was suspected to be determined by the presence of the magnetic field or the magnetic field gradient. It was also shown that the magnetic field might cause a shift in the equilibrium Fe-C diagram. In this respect, stabilization of the ferrite over the austenite is noteworthy as its resistivity is ~one third the resistivity of the austenite (0.1µΩm vs 0.34 µΩm at 300K and 313K respectively). High phosphorous cast iron was found to form liquid eutectics that preferentially settle at the grain boundaries. This phase is regarded to have a negative effect both on the contact pressure and on the electrical conductivity. Therefore it is suggested to employ lower P containing cast iron grades as the rodding material.