Abstract
Al4C3 particles form during the primary production of aluminum via molten salt electrolysis due to the carbon solubility and direct contact between bath, metal, and carbon anodes. Additional Al4C3 may form during melt processing through direct contact between the melt and carbonaceous materials. As a result of their small size and similar density to aluminum, removal of aluminum carbide particles can be challenging. If not removed, carbides can produce inclusion defects or poor surface condition in aluminum products. The current work studies the removal and behavior of Al4C3 particles during flotation with different gas mixtures, as well as sedimentation. The interaction between carbide particles and Al2O3 films during the melt treatment processes was also studied and reported. Factsage thermochemical software was used to model the interactions at the interface of inclusions and bubbles covered by films. The highest degree of carbide removal was obtained after flotation with an H2O-containing argon gas mixture, where the carbide concentration dropped below the measured solubility limit of carbon at the corresponding temperature. Strong interaction between Al4C3 particles and Al2O3 films was observed during sedimentation which worked as an efficient removal method for the particles. Oxidation of carbides and formation of oxycarbides were suggested as the mechanisms promoting the attachment of carbides on oxide films.
Highlights
DURING aluminum production, different impurities contaminate the melt endo- or exogenously
Simensen[7] performed ten measurements of samples from an operating electrolysis cell in temperature range of 960 °C to 1000 °C and the results showed a good agreement with Oden and McCune‘results[4] after extrapolating the data to the relevant temperature range while Dorward[6] and Rødseth et al.[5] measured at least double the amount of aluminum carbides in the same temperature interval as Simensen.[7]
The first set of experiments tested the influence of different gas mixtures on the removal of aluminum carbide
Summary
DURING aluminum production, different impurities contaminate the melt endo- or exogenously. Sedimentation and filtration are used as melt treatment methods aimed at removing solid particles such as borides, carbides, nitrides, and oxides. Gas purging/fluxing is used to remove dissolved elements such as hydrogen and alkali elements. Gas purging and fluxing target the removal of dissolved elements, particles are to some extent removed by flotation, which is a positive side effect of the gas purging process.[1]. Manuscript submitted May 11, 2020; accepted December 3, 2020.
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