On the effect of formulation and porosity on cathode performance in modern aluminium reduction cells

Abstract

The use of graphitized and graphitic cathode blocks for aluminium smelting has been increasing as smelters strive to increase production. Both these cathode block types offer lower electrical resistivity and higher thermal conductivity and therefore can accommodate higher amperage without large increases in cathode voltage and temperature. However, along with the benefits of increased production there has been a decline in cell life due to higher cathode wear rates. Many theories have been proposed in regards to the cause of the increased wear, but it is becoming generally accepted that it is electrochemically driven and is determined by the rate at which aluminium carbide is formed and more importantly dissolved. The formation of aluminium carbide is widely accepted however the mechanisms which govern the formation and dissolution with different cathode blocks with varying properties is relatively unclear. The main aim of this work was to investigate the influence of key cathode formulation parameters such as filler material, granulometry and grain orientation on the porosity, the physical and mechanical properties and the electrochemical wear resistance of graphitized and graphitic cathode blocks. Laboratory scale graphitized isotropic coke and graphitic samples were developed with varying formulations. The resulting samples were found to have varying open porosities and pore size distributions which resulted in significant variations in physical and mechanical properties. It was found that filler material type and granulometry had a great effect on the porosity of the material with isotropic coke found to be highly sensitive to formulation which resulted in some samples being developed with greater than 30% open porosity and large pore size distributions. Graphite filler material was found to be less sensitive to formulation changes and ultimately produced dense samples with much narrower pores size distributions. Porosity which was a function of filler type and granulometry was found to have the greatest effect on the mechanical properties of the material with low flexural and compressive strengths, Young's modulus and high electrical resistivity found for high porosity samples. Samples were also tested to determine the effect of the aforementioned variables on the electrochemical wear resistance. This was carried out by a series of 96 hour electrolysis tests in an inverted configuration laboratory aluminium cell. The materials were tested under a range of current densities and bath compositions to determine the wear mechanism. It was found that filler type, granulometry, porosity, current density and bath chemistry had an impact on the cathode wear rate. High porosity samples (>25% open porosity) showed evidence of particle detachment and pitting through internal degradation. In some cases bath penetration was very limited which indicated that the critical pore size exists in which bath can penetrate into a material. This critical pore size was found to be somewhere between 10 and SOμm. In most cases it was found that the wear was predominately concentrated at the cathode surface and filler material and the heat treatment temperature had significant impacts on the overall wear resistance with graphitic samples exhibiting high wear rates despite superior physical and mechanical properties. In terms of electrochemical wear it was found that electrochemical aluminium carbide formation and dissolution was the dominate wear mechanism in the laboratory test cell. This mechanism of wear was found to be current density and bath chemistry dependant with higher current density and excess AlF 3 producing higher wear rates when compared to materials of similar type.