Degradation of Cathode Lining in Hall-Héroult Cells
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The aluminium industry is continually facing the problem to lower the energy consumption and to increase the productivity by improving the current high amperage cell technology. Another challenge is to eliminate cell failures, which cause disturbances or premature cell shutdowns. The degradation of the cathode lining and cathode heave are two of the key factors which influence the performance and lifetime of the cathode, and the present study has focused on these two phenomena. Autopsies of cathodes after pot failures or shutdowns of cells have frequently been used to identify the cause of pot failures and the degradation of the cathode lining. The sequence of materials observed from the carbon cathode to the non-reacted refractory lining has been considered to reflect the situation in the lining before the pot was taken out of service. In this work it was demonstrated that this is not necessary the case based on annealing experiments of spent pot lining (SPL) samples and X-ray diffraction analysis and electron microscopy of the corresponding specimens. The annealing experiments demonstrated that the thermal gradient in the lining is reversed during cooling and that the physical appearance of the lining reflects a combination of the cooling and operation of the cell. A revised view of the analysis of SPL is presented taking into consideration the presence of a molten phase below the carbon cathode. This molten phase consists mainly of bath components and is therefore non-viscous in nature. Solid particles originating from the pristine material or precipitated reaction products are also possibly present. Since the cell is well insulated in the bottom the cathode lining is cooled from the top to the bottom when the cell is shut down. Thus the molten phase solidifies from top to the bottom in the lining, which was supported by the experimental findings. Sodium was identified to govern the degradation of the refractory lining based on electron microprobe analysis of the reaction front in the SPL. Since only sodium was found in the first few millimeters of the reaction front, this region was termed the "first reaction front". Furthermore, fluorides originating from the penetrating bath were detected in a certain distance and this was denoted the "second reaction front". The strong reductive nature of sodium was demonstrated by Si(s) droplets found in the SPL down to the first reaction front. The presence of Si also permitted a possible explanation for the nature of the transport of sodium from the carbon cathode to the reaction front. Inside and below the carbon cathode Na(g) is dissolved and transported through the bath. Below the build-up layer oxidation of Na(g) to Na+ takes place accompanied by the reduction of SiO2 to Si(s). Reported data on the mobility of the different ions in relevant viscous melts revealed that in contrast to diffusion of sodium, the mobility of oxygen is strongly dependent on the viscosity of the molten phase formed at the reaction front. Furthermore it was proposed that the degradation reaction with sodium is faster than the diffusion. Thus the rate limiting step for the degradation of refractory linings is the mobility of O2− ions. The viscous barrier was therefore proposed as a barrier for the diffusion of O2− and F− and not Na+ ions as previously proposed. The chemical reactions caused by sodium infiltration were qualitatively explained by the construction of a chemical degradation map. The degradation map corresponds to a predominance phase diagram showing the stable phases present as a function of SiO2/Al2O3 ratio in the refractory lining and the amount of sodium infiltrated in the lining. Through experimental observations it was demonstrated that the degradation map is a useful tool for the evaluation of autopsies of SPL and the prediction of the mineralogical composition of SPL. The thermal conductivity of SPL samples was determined by the Laser Flash method. The results demonstrated that the thermal conductivity decreases with increasing temperature. Compared to the pristine materials the SPL exhibited an increase in thermal conductivity, which depends on the respective density and chemical composition. It was possible to distinguish between crystalline and amorphous materials. This was also reflected by the measurements of the top and bottom part of the lens build up. Simulations of the heat transfer in cathode bottom linings were performed to study the consequences of the increase in thermal conductivity due the chemical degradation. The simulations showed that an overall increase in heat loss occurred, but was damped by the decrease of cathode block thickness and the growth of the build-up layer. Moreover, the importance of an the insulation layer with respect to a stable thermal balance was addressed. It has been shown in this study that a significant amount of liquid phase is present below the carbon block during operation. The buoyancy force in the liquid phase has previously been suggested to contribute to the cathode heave. Estimation of the forces demonstrated that the buoyancy force alone is not enough to lift a cathode block and cause the observed cathode heave. Computer simulations in 2D using finite element method were performed to identify the stress level acting on the collector bar and to identify possible forces leading to the cathode heave phenomenon. The influence of sodium expansion was also taken into consideration. The simulation demonstrated that the forces acting upwards are indeed caused by thermal expansion and by the sodium infiltration of the carbon block. The thermal expansion of the materials was proposed to be the main reason for the cathode heave and that the chemical expansion due to sodium infiltration gives an additional contribution to the stress build up. The "weak part" of the cell lining was identified, which corresponds to the region in the lining with the strongest temperature gradient. The deformations obtained by the simulations were qualitatively consistent with typical observation of cathode heave.