Abstract:
It is practically a goal for all aluminium smelters to operate their aluminium reduction cells for as long as possible. However, some aluminium cells have to be shut down earlier than expected due to failure in the component(s) of the cell. Often, this component failure is associated with the cathode lining, resulting in the aluminium cell being taken out of electrical circuit for relining. There are numerous types of cathode lining failures of which one is known as "pothole formation". This thesis is a study of the cathode lining failures in aluminium reduction cells with the emphasis on pothole formation. The purpose of this thesis project is to further the current understanding of pothole formation, which may help in identifying or developing the appropriate measures required for preventing or reducing the occurrence of potholes and cell failure due to potholes. The pothole formation study is conducted based on three different cell technologies at two aluminium smelters. The investigation strategy employed was divided into two approaches; physical and computational. In the physical approach, pothole and cathode core samples from out-of-service aluminium cells were obtained and analysed using Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD). SEM microscopy studies found whiskers containing high concentrations of iron and aluminium elements inside potholes, but not on other cathode areas. The link between the whiskers and pothole formation is however not clear and understood. The degree of graphitisation of carbon as determined using XRD indicated that the pothole depth parameter may have an influence on the pothole graphitisation behaviour and at least 90% degree of graphitisation is required for potholing to occur. More data is however needed to confirm this. In the computational approach, the reported pothole distribution data for the three cell technologies considered were statistically analysed with respect to their mapped process variables and the different potholing regions. It was found that the calculated values of metal pad velocity, cathode surface current density and magnetic field, Bxy exhibited some degree of correlation with the pothole distribution frequency for all three cell technologies considered. When the three variables were appropriately combined, they showed an even stronger correlation. This led to the hypothesis that these variables may be working together to create an environment that promote the formation of potholes. Using the three identified variables, the neural network method was used to model the pothole distribution data. Good results were obtained in the neural network modelling and in the cross-testing between neural network models of different cell technologies. This method can therefore potentially be used as a pothole formation predictor. Based on the study conducted it can be concluded that the combined effects of metal pad velocity, cathode surface current density and magnetic field, Bxy play a significant role in pothole formation. This is a very important finding in view of the fact that many smelters worldwide are increasing their cell amperages above their design value. Increasing cell amperage will result in an increase in cathode surface current density, which effectively increases the variables combined effects on the pothole formation process. A consequence from the amperage increase would likely be a rise in the number of potholes occurring as well as the probability of more aluminium cell failures via this mode, reducing the cell life and thus impinging on the economics of the process.