Oxidation of molten aluminium

Oxidation is a significant source of material loss in the production of primary aluminium, with estimates ranging up to ~1% of metal loss due to oxidation. An understanding of the fundamental oxidation mechanisms is necessary to develop a kinetic model of oxidation behaviour. Studies of aluminium oxidation have traditionally been thermogravimetric and carried out in a controlled atmosphere furnace. However, it is known that this technique is sensitive to the initial condition of the oxide present on the feedstock. This has led to significant variations in data reported in the literature, making modelling problematic. An experimental technique has been developed to address this limitation, based on earlier work by Freti et al, in which the oxide is 'skimmed' off of the surface of an open melt to give an effectively pure aluminium surface. The melt surface was skimmed at regular intervals and the skimmed material was processed in a molten salt flux at 750 degrees Celsius to separate the metal from the oxide. The recovered aluminium metal was weighed and the amount of oxide was then calculated by difference. Experiments were carried out on a melt of commercial purity aluminium exposed to ambient air. The melt surface area was ~0.05 m^2 (approximately the size of an A4 piece of paper). Oxide samples were taken at melt temperatures of 750, 850, and 900 degrees Celsius after holding times ranging from 30 seconds to 8 hours. The amount of oxide formed increased with both time and temperature, and the oxidation rate at 850 degrees Celsius was greater than at 750 degrees Celsius. The data at 750 and 850 degrees Celsius can be described reasonably well by a power law relationship, within experimental error, indicating a continuous reduction in oxidation rate over time, although the 900 degrees Celsius data is currently too limited to make a similar observation. Overall, very little oxide formed on the aluminium melt, significantly less than what forms in an industrial setting. This suggests that there are other factors more significant than time and temperature that influence the oxidation behaviour under such conditions.