The oxidation reactions of heterogeneous carbon cathodes used in the electrolytic production of aluminium

Abstract

A technique has been developed that allows the gasification reaction rates of representative samples of carbon-carbon composite materials to be examined. The technique involves heating the sample in a controlled and monitored environment; the product gases of the reaction are then analysed by a mass spectrometer, allowing their identification and quantification. The technique was used to characterise the oxidation reactions of cathode carbons. These materials are composites and so the oxidation reactions of their constituent raw materials were also examined. Surface area was determined for each sample, allowing specific rates of reaction to be determined, normalising surface area effects. The anodes, cathodes and sidewalls of aluminium smelting cells are made of composite carbon materials comprising filler materials (such as coke, anthracite and graphite) and a binder (almost exclusively coal tar pitch). Whilst the oxidation of anode carbons has received extensive study the oxidation reactions of cathodes have been neglected largely because they have not been a cause of smelting cell failure. However, with the longer lives now being achieved from smelting cells the long term degradation reactions, such as oxidation, will have to be considered. Oxidation of cathodes in the area of the collector bar will increase resistance and affect the heat balance of the cell. Gasification reactions of carbon materials are frequently characterised using techniques such as thermal gravimetric analysis (TGA). These techniques are accurate for examining such reactions when the sample is of small size and a single carbon type. To characterise composite carbons correlations have been made between the overall oxidation resistance (determined by weight loss) and the ignition temperature of one of the constituent materials (determined by TGA). The results obtained using the new technique of product gas analysis (PGA) revealed an exponential dependence of oxidation rate on temperature for the carbons examined. At higher rates the limiting condition appeared to be mass transfer through the pores of the sample. Arrhenius plots of reaction rates allowed the activation energy of oxidation to be determined for each material. When the rate was controlled by the chemical reactivity of the material the activation energies determined agreed well with values obtained from the literature. The two graphites examined had activation energies of 164 and 183 kJ.mol-1, Ea of graphite has been measured in the range 175-281 kJ.mol-1, the latter figure being for a highly pure graphite. For the two anthracites Ea was ll3 and 118 kJ.mol-1, literature values have it between 100 and l5l kJ.mol-1. The pitches, used as binders of cathode carbons, had Ea equal to 112 and 123 kJ.mol-1, values from the literature range from 121-165 kJ.mol-1. Activation energies were determined for the cathode materials, and were clearly influenced by the reactivity of the constituent materials. An amorphous cathode carbon, having nominally 30% graphite, had an activation energy of 121 kJ.mol-1. A semigraphitic cathode material comprising 100% graphite in a pitch binder had an activation energy of 123 kJ.mol-1. The similarity of these values to those for Ea of the pitch and anthracite indicates that the binder phase is having a strong influence on cathode reactivity. These values of Ea accord well with values determined for similar samples, reported in the literature ranging from 114 to 138 kJ.mol-1. A semigraphitised cathode material had an activation energy of 176 kJ.mol-1 in the same range as that of graphite. This sample oxidised significantly less rapidly at all temperatures. The variation in reactivity of the constituent materials of cathode carbons accounts for the highly selective oxidation behaviour observed in these materials. Porosity development is rapid as binder matrix is preferentially oxidised, leading to an acceleration of oxidation rate with increasing burnoff. The rate begins to decelerate once all the binder matrix has been oxidised, the residue being less reactive than the starting material. The structure of the materials was quantified using X-ray diffraction (XRD). A peak ratio method was employed, comparing the intensify of the 4oz peaks of cathode carbons and a standard electrographite. Once effects of cathode porosity had been normalised an excellent correlation between increasing peak intensity ratio and increasing oxidation resistance was found.