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. Graphical Abstract

Solid Salt Fluxes for Molten Aluminum Processing—A Review

“…The aluminum pieces are then recovered from the crucible by leaching the salt in water and sieving. The coalesced pieces are then observed to assess the extent of coalescence, usually according to [62] Coalescence (%) = Mass of Coalesced Pieces Mass of Scrap Input (4)…”

“…The evaluation of the mass of coalesced pieces varies to a great extent in the literature. For example, Vallejo-Olivares et al [62] used the mass of the largest piece recovered, while Capuzzi et al [63] considered the fraction of the coalesced droplets and their average diameters to account for their roundness. Besson et al [18] considered the size of a single molten aluminum chip and chose coalesced pieces that were larger than a certain size limit.…”

Recycling of Magnesium Alloy Scrap by Remelting and Chemical De-coating Process

“…In this treatment, the scrap is heated up to temperatures that allow thermal and chemical decompositions of the coating substrates, through which organic compounds that composed the outermost layer could be removed from the scrap surface through volatilization [9]. The earlier works [10][11][12][13], for instance, showed the importance of thermal de-coating as a pre-treatment during the recycling of Mg-and Al-based materials. Despite simple, this thermalbased treatment could not eliminate inorganic compounds [12].…”

Effects of Compaction and Thermal Pre-treatments on Generation of Dross and Off-Gases in Aluminium Recycling

“…The thermal treatment also reduces the risks of health and safety hazards during re-melting, such as the generation of toxic, poisonous, or combustible gases (H 2 S, PH 3 , H 2 and CH 4 ) [9,18]. Moreover, in salt-recycling processes, de-coating promotes the coalescence of the metal droplets, potentially reducing the amount of metal entrapped in the salt slag residue [19][20][21]. However, the volatile organic compounds (VOC's) and other gases generated during the thermal de-coating pre-treatments can also pose health, safety and environmental risks and must be handled adequately, e.g.…”

“…A previous study showed that pressing coated aluminium chips into briquettes of low densities did not affect its recycling in salts. However, compacting to higher densities by the moderate-pressure torsion (MPT) method limited the de-coating efficiency and metal coalescence [21]. Steglich [25] re-melted used beverage cans (UBCs) without salt-flux and concluded that a thermal de-coating would reduce dross if conditions for stoichiometric thermolysis were applied for at least 30 min at 550-570 °C.…”