Effect of (5%) CO2 on the Oxidation Rate During Cooling of Industrial Aluminum White Dross

Abstract

Aluminum (Al) white dross is classified as hazardous waste and originates from the primary production of Al as a result of molten metal oxidation at an industrial scale. As the produced dross reduces the annual metallic Al production by up to 10%, finding a way to reduce the dross formation has always been of interest. Several laboratory studies have shown that exposing molten Al to small amounts of CO2 has an inhibiting effect on the oxidation rate of the metal. Still, few studies have described this effect at an industrial scale. The present work has therefore studied the impact of protective cooling of industrial white dross samples on the oxidation rate of the dross to shed some light on the issue. Samples were collected from the holding furnace in a casthouse during the production of two different charges of Al alloys with varying amounts of magnesium (Mg), i.e., Charge 1 (AlMg1.0Mn0.4) and Charge 2 (AlMg1.2Mn0.4). The samples were cooled in ambient air and under a lid with 5% CO2 mixed with 95% synthetic air in two parallels and later characterized by X-Ray Diffraction and Electron Probe MicroAnalysis combined with deterministic image analysis. Both methods confirm that all samples cooled under the protective atmosphere with 5% CO2 had a higher content of metallic Al (89.4 wt%) than those cooled in ambient air (86.0 wt%), indicating that small amounts of CO2 also inhibit the oxidation rate of dross at an industrial scale.