Abstract
The basic theory for the behaviour of impurity elements in aluminium electrolysis cells is reviewed, with emphasis on the difference between noble and non-noble elements. Noble elements are transferred from bath to metal by diffusion and reaction at the metal-bath interface, while non-noble elements in bath and metal are at equilibrium. A general expression for the co-variation between the concentration of sodium and other trace elements in the produced metal was derived. This co-variation can be used for control purposes, provided that the methods for sampling and analysis are good enough. Non-noble elements can be removed by oxidation, but this is not possible with noble elements. Noble impurities are expected to become an increasing challenge in the future, since the concentrations in the raw materials are increasing, and because the ratio between recycled metal (alloys) and primary metal is increasing. Therefore, new methods are needed for the removal of noble impurities. It may be possible to reduce impurities by removing the fine fraction from the alumina and by electrolytic separation of aluminium and noble elements in scrap.