Electrochemical behaviour of dissolved titanium oxides during aluminium deposition from molten fluoride electrolytes

Abstract

A small but significant amount of dissolved titanium is always present in the molten cryolite based electrolyte used during electrowinning of aluminium. The fact that titanium can appear in several oxidation states may be a cause for reduced current efficiency for aluminium, although codepositon of titanium should be thermodynamically favoured. Voltammetric studies were carried out in molten Na3AlF6 - Al2O3 (sat) containing TiO2 at 1020℃. The results suggested that titanium was reduced in two steps from Ti (IV) to Ti (III) followed by reduction of Ti (III) to Ti metal.

Experiments were also carried out in industrial aluminium cells. Large quantities of TiO2 were added to the electrolyte during electrolysis, and samples of electrolyte and produced metal were taken and analysed as a function of time after addition. The results suggested that titanium was codeposited at the aluminium cathode by a diffusion controlled reaction. The apparent current efficiency for titanium deposition was estimated to be higher than 90%.

The valency problem seems to be less challenging when depositing liquid titanium alloys from molten fluoride electrolytes. This approach may give rise to the development of production of a valuable titanium containing alloy TiAl3.