In aluminum recycling, thermal de-coating pre-treatments remove moisture and organic contamination before re-melting. If the scrap is compacted into bales or briquettes before the thermal treatment and re-melting processes, less surface area is exposed to oxidation in contact with air. However, compaction may also limit the efficiency of the de-coating process. In this study, coated sheets of aluminum were thermally de-coated at varied temperatures and durations. Observations of changes in coating thickness, mass, color, and composition revealed a maximum de-coating efficiency of close to 75% wt due to remaining oxide residues. The relationship between de-coating and compaction was investigated by thermally treating loose shreddings (chips) and briquettes of various densities. The briquettes were compacted by three methods: uniaxial, moderate-pressure torsion (MPT), and MPT at 450 °C (Hot MPT); and the de-coating efficiency was calculated from the mass loss. Subsequently, the samples were re-melted under salt-flux and compared with another set of samples which were re-melted without thermal pre-treatment. The results showed that thermal de-coating significantly promotes the coalescence of loose chips and briquettes compacted uniaxially, up to similar coalescences than initially uncoated aluminum samples. Thermally treating the MPT briquettes, which were more densely compacted, led to less de-coating, and subsequently lower coalescences. The analysis of re-melted material revealed that the coating residues did not significantly affect the composition, while the compaction prevented Mg loss for the uncoated materials.
Graphical Abstract
One of the problems when recycling aluminium is its oxidation and consequent metal loss. This is especially critical for the thin sheet/foil materials used for food packaging applications. Compacting the scrap into briquettes may partly reduce such losses in addition to facilitate transport and storage. Shredded aluminium materials of different thicknesses (15-300 µm) were compacted into cylindrical briquettes of 4 cm diameter, each weighting 20 g by uniaxial pressure or moderate-pressuretorsion. A sub-set of briquettes and chips was subsequently oxidized at 650 • C, while a sub-set was left untreated. Finally, all samples were re-melted under molten protective salt-flux. Compacting reduced the specific oxidation during the heat-treatment and promoted the coalescence and yield for the heat-treated materials. Both effects were most significant for the thinnest foil in the study (15 µm). The material thickness influenced the porosity and surface roughness of the resultant briquette, as well as the pressure required to reach a given bulk density.
Scrap pre-treatments, such as compaction and thermal de-coating, are standard industrial practices for recycling aluminium post-consumer scrap. This study compares the recyclability of a coated and uncoated 8111 alloy under the application of compaction and/or thermal de-coating pre-treatments. Sheets of 600 μm thickness were shredded into chips and compacted by uniaxial pressure, moderate pressure torsion (MPT) or MPT at 450 °C (Hot MPT) into briquettes of 4 cm diameter. A subset of briquettes and loose chips was subsequently heat-treated for 1 hour at 550 °C, while the other set was left untreated. The effectiveness of the heat-treatment for the different compaction methods was examined by mass balance and the internal porosity of the briquettes by computed tomography. Re-melting the samples under molten salt-flux showed that the coalescence of the coated material significantly improves with the thermal de-coating pre-treatment, especially for the loose chips and briquettes compacted uniaxially. Lower coalescences were obtained for the de-coated MPT briquettes, as a result of an incomplete de-coating.
The original version of this chapter was published with incorrect values. On page no. 736. The line "68.6% KCl, 29.4% NaCl" has been updated as "68.6% NaCl, 29.4% KCl". The erratum chapter has been updated with the changes.
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