SPECIFICITY OF ANODIC PROCESSES IN CYCLIC VOLTAMMETRY TO THE TYPE OF CARBON USED IN ELECTROLYSIS OF CRYOLITE ALUMINA MELTS

Anodic processes associated with oxidation of carbon anodes used in electrolysis of cryolite-alumina melts, simulating the Hall-Heroult process, were studied by means of cyclic voltammetry in a comparative way at four graphitic carbon materials and at glassy carbon. Conditions were sought that give a current response function characteristic of diffusion-controlled oxidation of the anode by O2- or oxyfluoride complex anions. Only at glassy carbon anodes are such conditions realized with a linear relation between response current in cyclic voltammetry and Al2O3 content in the melt. At the graphitic materials, complex mixed activation and diffusion controlled processes arise that are also relatively irreproducible from one experiment to the next, probably due to irreversible changes of the graphite surfaces. The effects of aluminium metal dissolved in the melt, to simulate practical smelter cell conditions, were also evaluated.