Parametric Study of Molten Aluminium Oxidation in Relation to Dross Formation at Laboratory and Industrial Scale

Abstract

Aluminium (Al) white dross, classified as a hazardous waste, is a heterogeneous mixture of metallic Al, oxides, and non-metallic compounds (NMC) originating from the primary production route of Al. Over the past few decades, a significant number of studies have been performed on topics related to molten Al oxidation. However, the formation mechanisms are still not fully understood, nor how to inhibit the oxidation.

The overall scientific outcome of the present work has therefore been to systematically determine the extent and rate of molten Al oxidation at the laboratory scale and dross formation at the industrial scale. Special attention was given to investigating how different parameters influenced the oxidation process and whether a correlation between laboratory and industrial-scale trials could be made. During the laboratory tests, variations in the Mg concentration in the AlMg and AlMgSi alloy systems were investigated in view of Al oxidation and inhibition, as well as the effect of the heat treatment atmosphere. For the industrial campaigns, the same AlMg alloys as those investigated in the laboratory tests were reproduced, and the influence of the general furnace operations and the location in the holding furnace on the characteristics of the dross investigated. Even protective cooling of the dross was addressed and evaluated in view of inhibition of further oxidation of the metallic Al content in the hot dross compared to cooling in ambient air.

From the laboratory activities, it was concluded that additions of as small amount as 4% CO2 into an oxidising atmosphere inhibited the oxidation rate of AlMg and AlMgSi alloys when heat treated at 750°C for 7h (normal temperature of the casthouse holding furnaces). The CO2 in the oxidising atmosphere was identified to have adsorbed onto a MgO layer, forming an amorphous C-C layer (never observed before) and thereby inhibiting further evaporation and oxidation of Mg and breakaway oxidation. This was proven to be the case for both high- and low Mg-containing Al alloys, but to a lesser extent for the low Mg-containing Al alloy.

To allow for a systematic study with reproducible results to be performed under industrial conditions, a sampling tool and a step-by-step procedure for collecting, pulverising, and quantitatively analysing the collected samples were designed, developed, and validated. For pulverising the dross samples, both ring milling and cryomilling were used, as well as XRD and EPMA combined with deterministic image analysis for analysing the dross. It was concluded from the industrial campaigns that the dross characteristics were influenced by the furnace operations and by where in the furnace the samples were collected from. Higher metallic Al concentrations were identified for the samples collected at Location 1 (closest to the injection point of primary produced Al), and the metallic Al concentration decreased as a function of distance from Location 1 towards Location 4 (furthest away from the injection point of primary produced Al) simultaneously as the oxide/NMC concentrations increased from Location 1 to Location 4. Protective cooling under a lid with additions of 5% CO2 proved to have an inhibiting effect on further oxidation of the dross during cooling, with lower metallic Al concentrations present in the samples cooled in ambient air. The inhibiting effect of small amounts of CO2 in the cooling atmosphere confirms the laboratory results at an industrial scale.