Inter-relating aluminium smelting carbon cathodes formulations to cell operations and wear mechanism

Abstract

Aluminium electrolysis cell Iife is of great importance. The smelting process is a continuous process and cannot be stopped, interrupted and restarted effortlessly. As the process is stopped, the electrolyte and formed aluminium solidify. The interruption to the process would require expensive reconstruction of the cell. To achieve longer cell lives, selecting and careful testing of the cell materials in terms of their properties is very important. This thesis focuses on the quality of the cathode materials, formulations and impact of different variables on physical, chemical and electrochemical properties of the cathodes. Carbon cathodes consist of carbon Tiller aggregate (e.g. graphite) and a binder (usually coal tar pitch). Cathode blocks made with similar filler aggregate materials and similar physical properties have been known to exhibit quite different performances in smelting cells. Accordingly, an investigation has been undertaken to ascertain whether the coal tar pitch used can play a role by exhibiting different behaviours during the baking process. The first part of the study was to develop a test to differentiate coal tar pitches with similar physical properties but from different sources. The pitches were subjected to slow heat treatment and differentiated by analysis of volatile decomposition products released. In the initial heat-up, because of both volatilisation and decomposition, polyaromatic hydrocarbons are released. These products were collected by Solid Phase Micro-Extraction (SPME) and analysed by Gas Chromatography/Mass Spectrometry (GCdVlS). It was found that the pitches fell into distinctly different groups. In the second part, a factorial cathode matrix was developed with five important variables, which were postulated to be influential on the quality of the cathode. These variables were filler material, granulometry, binder pitch, baking temperature and flour material. This matrix made up to 32 cathode samples and all were tested for their classic physical properties such as density, open porosity, electrical resistivity and young's modulus. Influence of these variables on the outcome was analysed using statistical analysis software, STATGRAPHICS Plus-version 3.0. It was found that all the properties were influenced largely by granulometry, filler material or an interaction effect between these two. However, from the plots of these properties versus the samples, binder pitch appeared to have impact on density and open porosity. Baking temperature definitely impacted the porosity and electrical resistivity of the cathodes. The influence of baking temperature on electrical resistivity demonstrated the effect of the heattreatment temperature on the structural order of carbon materials. There are two generally accepted electrochemical processes involving the carbon of cathode blocks, these being sodium uptake and aluminium carbide formation. Both processes occur at or below the potential required for aluminium deposition, and therefore can occur at all times in blocks permeated with electrolyte - as does occur with modern cell designs and operation. Then, since the cathode carbon is at a more cathodic potential than the molten aluminium metal pad, the electrolyte filled material can sustain the electrochemical reactions, even though the rate may be very low. A special laboratory test was developed to identify and ascertain the extent of the carbide forming reaction within the carbon matrix oflaboratory cathode specimens. Results showed that these reactions were dependent on the carbon binder pitch, the carbon filler material and the cathode heat treatment. This electrochemical reaction was expected to become more prevalent at higher current densities and the growth of the carbide would potentially contribute to accelerated wear in cathode blocks. In addition, an alternative mechanism is proposed based on in-plant observations and laboratory experiments, to explain the formation of carbide within the pores of a cathode. Internal carbide formation results in accelerated wear via particulate detachment and explains the difference in wear performance of amorphous cathode blocks and graphitic or graphitised blocks. During operation, cathode pores can become filled with electrolyte, thus providing a parallel, Iow current density path for carbide formation. The electrochemical formation of aluminium carbide is thermodynamically more favoured for disordered carbons, therefore the binder carbon can be partly electrochemically dissolved by formation of the carbide, resulting in a weaker Iink between particles and aggregates. These stresses will cause micro or macro weakening, enabling particulate detachment. This latter mechanism is more favoured at high current densities.