Inert anodes for aluminium electrowinning: nickel ferrite based cermets

摘要

Greenhouse gases including carbon dioxide are formed by the consumption of carbon anodes during aluminium production, making it a major contributor to global emissions. This consumption necessitates replacement of the anodes in electrolysis cells every 2-3 weeks.A solution to the environmental and economic problems posed may be found in an inert anode which facilitates direct decomposition of alumina to aluminium and oxygen. Finding a material which is stable in the aggressive high temperature electrolyte poses a major materials engineering challenge.In this study, apparatus was designed and constructed to allow cermets to be manufactured in the laboratory, and a method of establishing electrical contact developed. Additionally, apparatus was designed to perform high temperature conductivity measurements on the cermets.Nickel ferrite-nickel oxide-copper-silver cermets were prepared and conductivity measured. No significant change in the activation energy of the conduction process was observed for cermets with 40wt% excess NiO compared to those with no excess. No significant difference in conductivity was observed between the compositions at cell operating temperatures.Voltammetric techniques were used to identify anode processes. High anodic currents associated with oxidation of anode constituents were observed repeatedly, the magnitude of which could not simply explained by oxidation of the metal phase. This suggested the formation of other reduced species during sintering (confirmed by thermodynamic analysis).Gaseous oxidation products were confirmed at the anode at potentials expected for oxygen evolution, and the application of high potentials (>4V vs Al/A13+) was found to passivate the cermets.Voltammetry and chemical microanalysis (using scanning electron microscopy (SEM) with energy dispersive x-ray spectrometry (EDS)) showed that copper in the cermets was depleted at the anode surface, apparently by oxidation then dissolution into the electrolyte.The inclusion of silver powder into the cermets was not found to improve the corrosion resistance of the cermets, existing almost entirely as a discrete phase. Preliminary SEM and EDS results highlighted several areas for further investigation regarding the compounds formed during sintering and electrolysis and the anode corrosion mechanisms. Of particular interest were a copper nickel oxide formed during sintering and complex oxyfluorides containing anode and bath constituents, formed during electrolysis.