Solid state recycling of aluminium scrap and dross characterization
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The global demand for aluminium and aluminium products is progressively increasing. However, the production of primary aluminium is an energy intensive process and emits at the world average ~12 tonnes of carbon dioxide equivalent (CDE) per tonne metal. However, aluminium recycling is attractive because one can save up to 90% of the energy, as well as 95% of the greenhouse gas (GHG) emissions. The aluminium recycling industry is facing several challenges, such as treatment and disposal of by-products, emissions control and requirements related to energy efficiency. A state of the art remelting furnace requires 2.5 GJ/tonne metal, which is more than twice the theoretical energy consumption. Also, remelting of fine scrap, such as turnings and chips leads to significant metal loss. However, solid state recycling methods not involving remelting might offer higher metal yield and lower energy consumption than conventional recycling based on remelting. In the present study, solid state recycling of aluminium turnings and sawing scrap was investigated by cold compaction and subsequent hot extrusion and Cyclic Extrusion and Compression (CEC), respectively. Degreasing of aluminium scrap by thermal treatment or acetone treatment was compared with respect to the recycled metal products. In addition, ECAP of hot extruded materials was used for evaluating the internal bonding and deformation behaviour during severe plastic deformation. Mechanical properties and microstructures of the extruded and ECAPed materials were carefully investigated. Also, characterization of aluminium scraps, e.g. automotive aluminium scrap and aluminium primary dross was performed using state of the art techniques, such as EPMA and XRD with Rietveld refinement. Finally, the energy requirement for solid state recycling processing was calculated and discussed. Characterization of aluminium scraps revealed that impurities were quite common in industry scraps. Also contaminations, e.g. metallic iron and its oxides, lead and TiO2 were observed in the automotive scrap collected from a commercial shredder. Aluminium dross has a complicated composition, containing metallic aluminium particles, corundum, spinel etc. The EPMA results showed that iron and manganese and their oxides were widely distributed in the dross fines. Metallic aluminium in dross tended to solidify as large size particles. The leachate from aluminium dross water leaching exhibited a high content of fluoride and H2 gas bubbles after leaching for 72 hours. The results from the TG/DTA-MS analysis indicated an optimal thermal degreasing temperature, e.g. ~428 C. High temperature thermal treatment of turnings resulted in a significant increase of scrap surface oxide films. However, the combustion peak for the lubricants was ~374 C, corresponding to the second DTG peak at ~350 C. In addition, the thermal treatment time should be optimized so as to complete the decomposition and combustion of the lubricants. CEC processed aluminium sawing scrap turned out to be fully dense. However, room temperature tensile properties after CEC revealed high variability, indicating the presence of material inhomogeneity. Besides, microstructure observations showed that the scrap from sawing contained a mixture of different aluminium alloys. This could be part of the explanation versus the inhomogeneity. Also, a fully dense material was obtained for the aluminium turnings processed by hot extrusion at 500 C at an extrusion ratio of 15:1. The corresponding hardness and room temperature tensile strength values constituted 95-98% of the properties of conventional reference material. To be noticed, the thermal treatment of feedstock turnings at 350 C for 30 minutes did not introduce obvious differences in mechanical properties of the recycled extruded materials as compared to acetone treated feedstock. Post extrusion ECAP showed improved mechanical properties and enhanced internal interface bonding. Oxides produced at the feedstock surfaces could be broken and redistributed upon hot extrusion and especially after ECAP applying a high number of passes at elevated temperatures, e.g. 8 passes at 100 C or 200 C. As a consequence, the key mechanical properties, such as the fracture elongation and the UTS, could be nearly as good as the conventional material counterparts. Energy requirement estimations for solid state recycling revealed that the precompaction process uses less than 7% of the energy required for the world’s best practice conventional remelting, and resulted in a 93% carbon dioxide equivalent emission reduction.