Effects of electrolyte impurities on the current efficiency during aluminium electrolysis

Impurities enter the electrolyte mainly from the consumable carbon anode and aluminium oxide. Important impurity elements are iron, silicon, phosphorus and sulfur. Metallic impurities such as iron and silicon tend to codeposit at the cathode, while dissolved phosphorus and sulfur species also affect the current efficiency for aluminium production. In modern industrial cells more impurities end up in the produced metal due to the efficient recycling of impurities within the process. Laboratory experiments were carried out to determine the current efficiency for aluminium deposition and to study the electrochemical behaviour of dissolved phosphorus containing complexes as well as dissolved sulfate. Both additions of phosphate and sulfate to the electrolyte caused a significant reduction in the current efficiency, mainly due to cyclic redox reactions at the electrodes. The behaviour of phosphorus in industrial cells was studied by analyses of data for a large number of cells. Separate measurements were also conducted to study the distribution of impurities during industrial operation. Known amounts of impurity compounds were added to the electrolyte of industrial cells, and chemical analyses of samples of bath and metal as a function of time were carried out.