POLYVALENT IMPURITIES AND CURRENT EFFICIENCY IN ALUMINIUM CELLS: A MODEL CONCERNING ELECTROCHEMICAL SHORT CIRCUITING

摘要

"Electrochemical short circuiting" describes the loss in currentrnefficiency (CE) due to polyvalent impurity elements that arernreduced at the cathode and re-oxidized at the anode. In aluminiumrncells; the oxidized form of the impurity reacts with dissolvedrnmetal near the metal-electrolyte interface, leading to a steeperrnconcentration gradient of dissolved metal. The phenomenon wasrnstudied in a model based on the Stefan-Maxwell equations forrndiffusion in multi component systems and a description of thernturbulent diffusion coefficient close to the cathode. The resultsrnindicate that the main factors for the slope of CE vs. impurityrnconcentration are the CE in absence of impurities and the changernin oxidation state during the reduction, while the diffusionrncoefficient of the impurity is of minor importance. The numericalrnresults could be represented by an analytical expression that fitsrnwell with experimental data.