Analytical Modeling of Current and Potential Distribution over Carbon and Low-Consumable Anodes during Aluminum Reduction Process

Abstract

Based on the theory of current and potential distribution for aqueous solution systems and existing analytical models, tertiary current and potential distributions over carbon and non-consumable anodes in cryolite-alumina melt are calculated. By varying different electrolysis parameters and anode surface geometry (rectangular smooth anode and section of anode sinusoidal profile), their functional relationship with current and potential distribution is established. The influence of resulting distribution pattern and the values of local current density and local anode potential on anode process stability together with related probable causes of the increased consumption of carbon and non-consumable anodes are discussed. The main factors influencing on current and potential distribution are anode overpotential and diffusion layer thickness, which represents the electrolyte properties related to electrolyte mixing conditions. Local current density on the edges of a rectangular anode is 2 times higher than in the central part. Current distribution over rough anode surface is uneven—local current density at peaks of surface defects is higher by 40% to 60%. When limiting current density is approached at these preferential spots, the values of standard potentials for perfluorocarbons evolution on carbon anodes and metal fluorides formation on inert anodes are reached. The theoretically established relationship between the anode surface structure and current and potential distribution could be the primary cause for increased anode consumption and passivation with subsequent destabilization of electrolysis in standard and low-melting electrolytes. The possible ways to equalize the current and potential distribution across the anode surface are presented.