Thermal De‑coating Pre‑treatment for Loose or Compacted Aluminum Scrap and Consequences for Salt‑Flux Recycling

Abstract

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.