Characterization of Porous Structure and its Correlation to Sodium Expansion of Graphite Cathode Materials Using Image Analysis

Graphite cathode materials used in aluminum reduction cells are of porous structure. During aluminum electrolysis, the melt and sodium will penetrate into the cathode block leading to its expansion and deterioration. This work is aimed to give a better understanding of the relationship between the pore structure and the sodium expansion and to improve control of the cathode quality through quantitative image analysis.Experiments with graphite cathode materials were carried out using a modified Rapoport apparatus. Image analysis results indicate that the porosity, pore diameter and pore connectivity after electrolysis can be related to the sodium expansion, while the aspect ratio and the fractal dimension seem to be unchanged. X-ray Diffraction (XRD) measurement for interlayer spacing (d 002) was also found to correlate to the degree of such expansion. Quantitative information on the correlation between the sodium expansion and porous structures can serve both the cathode producer and the smelter for product selection and performance evaluation of cathode blocks.