Aluminium Recycling Pre-treatments: Compaction and Thermal De-coating

Abstract

Due to their light weight and versatility, aluminium alloys are found everywhere around us, from food packaging to aerospace applications. Aluminium recycling saves substantial amounts of energy, resources, and waste compared to the primary production route. In addition, the processes for collecting and recycling aluminium are some of the most effective in the market, so aluminium is becoming a popular choice as a sustainable material by producers and consumers. Aluminium packaging products have particularly short lifetimes and are made of valuable wrought alloys with high purity requirements. As the demand for these products is expected to keep growing while the industrial emissions and environmental impacts need to be cut, it is imperative to increase their circularity and keep optimising recycling processes so that the metal lost in each cycle loop is minimal. This thesis examines how conditioning aluminium materials by shredding, briquetting, and thermal de-coating pre-treatments affects the main mechanisms behind the metal losses during recycling: oxidation and metal entrainment by saltslag or dross residues. Several types of aluminium foils, sheets, and incinerator bottom ash (IBA), with diverse contents of oxides and organics, were recycled via laboratory experiments to understand the relationship between their properties (size/thickness, coated or bare) and their recyclability. Subsets of the foils and sheet materials were shredded into chips, pre-treated by different routes, and finally re-melted by one of two methods: using molten salts or an aluminium heel. Emphasis was placed on understanding the interaction between compaction and thermal de-coating. This was investigated by thermally treating and subsequently re-melting materials in various compaction states: loose chips and briquettes of multiple densities, compacted by applying uniaxial pressure, moderate-pressuretorsion (MPT), or MPT at elevated temperature (450 °C). The efficiency of the thermal de-coating was evaluated by measuring the weight losses and the off-gas emissions. A final study assessed the environmental impacts of recycling a tonne of aluminium-containing side/waste streams (dross, IBA, shavings) in a rotary furnace with salt-flux, a standard industrial route to recycle partially oxidised and contaminated scrap. The LCA analysis discussed the main environmental impacts of the process, with detail on those linked to the usage of salts and the consequent generation of salt-slag residues, by comparing two end-of-life alternatives: valorisation or landfill. Compacting thin, clean aluminium proved an effective way to reduce their susceptibility to oxidation and promote the coalescence of the aluminium droplets when re-melting in salt flux. This was especially beneficial for the thinnest materials with large amounts of exposed surface area, such as aluminium household foil. The metal yields of IBA also showed a direct dependence on their specific surface area. Since the thickness of the oxide layer was comparable across sizes (68 μm in average), smaller particles have a lower specific metallic content, hence lower yields, which ranged from 76 to 93 % for sizes between 2-30 mm. The observations from recycling coated and bare aluminium sheets in salt-flux highlighted that organic contamination, e.g. from coatings, increases the remelting losses by hindering the coalescence of the metallic droplets when remelting in molten salts. This can be tackled by applying a thermal de-coating pretreatment, which promotes the degree of coalescence up to similar levels to those obtained when re-melting bare materials. The re-melting experiments in molten heel showed that the char residues from the incomplete combustion of organics is the main factor impacting the metal losses due to dross formation. Furthermore, the high densification of the briquettes by the MPT method limited the de-coating efficiency, thus increasing the re-melting losses for both re-melting processes. The LCA study outlined the environmental benefits of recycling aluminium, all of which increase if the salt-slag residue is treated for recovery instead of landfilling it. The most critical parameters for improved sustainability were the metal recovery, process emissions, energy consumption and recovery of byproducts from the salt-slag. A sensitivity analysis illustrated how the metal yield could affect the global warming contributions: from -3.5 to -17 tonnes CO2- eq for treating Al-containing streams of metal yields of 20 wt% or 95 %, respectively.