Electrodeposition of aluminium containing alloys from molten fluoride electrolytes containing metal oxides

Abstract

Aluminium is produced by the Hall-Heroult process by electrowinning in a molten fluoride electrolyte with dissolved alumina at ~955oC. Several metallic impurities typically end up in the produced metal due to the fact that most metals are more noble than aluminium. Such co-deposition of metals may be utilized to form aluminium alloys. Another advantage is that many metal oxides are readily soluble in molten cryolite based electrolytes. The more noble metals will deposit at the limiting current density. The alloy composition is essentially determined by the bath concentration of the dissolved cations of the alloying element. Therefore it is possible to produce alloys of certain compositions by controlled addition of metal oxides. Interesting candidate alloying elements are silicon, iron, titanium, and manganese, but also exotic elements such as scandium are possible. Electrochemical studies were carried out in laboratory experiments and the behaviour of dissolved metal oxides was studied in industrial cells. High current efficiencies for the codeposition of the alloying elements Si, Fe, Ti, and Mn were confirmed. Significant savings for producing alloys by this method are expected.