Influence of Current Density on the Microstructure of Carbon-Based Cathode Materials during Aluminum Electrolysis

Sodium expansion plays an important role in cathode deterioration during aluminum electrolysis. In this work, the sodium expansion of semigraphitic cathode material has been measured at various cathodic current densities using a modified Rapoport apparatus. We have studied the microstructural changes of carbon cathodes after aluminum electrolysis using high-resolution transmission electron microscopy (HRTEM). Because of an increasing trend toward higher amperage in retrofitted aluminum reduction cells, an investigation is conducted both at a representative cathode current density (0.45 A/cm2) and at a high cathodic current density (0.7 A/cm2). The results indicate that the microstructures of carbon cathodes can be modified by Joule heating and electrostatic charging with higher current densities during aluminum electrolysis. With the penetration of the sodium and melt, zigzag and armchair edges, disordered carbon, and exfoliation of the surface layers may appear in the interior of the carbon cathode. The penetration of the sodium and melt causes remarkable stresses and strains in the carbon cathodes, that gradually result in performance degradation. This shows that increasing the amperage in aluminum reduction cells may exacerbate the material deterioration of the cathodes.

Experimental Research and Numerical Simulation Analysis of Sodium Expansion in Tib2-Carbon Cathodes during Aluminum Electrolysis

Experimental investigation of sodium expansion in TiB2-carbon with different TiB2 content during aluminum electrolysis was carried out in a laboratory cell. The model based on the swelling due to the sodium penetration has been discussed, and relationship between the sodium expansion and time has been proposed that has been calculated by MATLAB. The proposed model was extended to the three-dimensional distribution of sodium concentration, stress and first principal stress that was implemented by ANSYS. Validation of this numerical tool was achieved through the Rapoport tests. The results show good agreement between analytical solution and numerical modeling as well as between experimental and numerical results. This means that the chosen numerical model is suitable to predict sodium expansion as well as induced stresses in TiB2-carbon cathodes and is great interest in the work to extend aluminum reduction cell lifetime.