Main precautions in the design of aluminium parts
Aluminium alloys are widely recycled materials. Recycling reduces by a factor of seven energy consumption in production of one kilogram of aluminium compared to the production of one kilogram of primary aluminium.
Environmental impacts are also reduced:
Source European Aluminium Association
In general, there are two main types of aluminium, wrought alloys and foundries alloys. Wrought alloys are alloys with high mechanical performance which are widely used in aircraft construction, automotive or other applications that require substantial performance and lightness.
Their recycling is complex because it requires a pre-sorting by type of alloy to give to the material, a performance equivalent to the initial alloy.
Cast alloys are often made â€‹â€‹with aluminium alloys from manufacturing scraps or from shredding of product at end of life. The automotive sector is a major consumer of casting alloys from car at end of life!
The high value of aluminium alloys in end of life often leads to dismantling of large parts and manual sorting (engine cylinder head, or profiled sheet metal from the buildings). Recognition of the alloys can be facilitated using portable analysis based on X-ray fluorescence of alloying elements.
The recovery of aluminium from small parts is usually done after products grinding. The fraction of non-ferrous metals is generally recovered after grinding of the product and magnetic sorting of scrap. Aluminium fractions are then extracted using dense medium flotation methods of non-ferrous fractions with heavy liquids (water and ferrosilicon). The density of aluminium alloys in the range of 2.6 to 2.9 covers the many non-ferrous fractions: the concrete (2.7), magnesium alloys (1.75), and plastics / elastomers (0.9 to 1.5).
The sorting process by flotation will often be performed in three density separation steps:
Note that aluminium will be found in two places depending to it nature:
- Fraction 2.5 < d < 3.5 with magnesium, rubber and plastic, if aluminium is in the form of foamed or cellular structure. Manual sorting is required.
- In the fraction d < 3.5 (floating) mixed with concrete and often capacitors from washing machines.
The presence of concrete mixed with aluminium alloys leads to an additional step of sorting, often by eddy current system and lead to extra costs. This concrete mainly come from ballasts of washing machines, it would be advisable to replace this material by cast iron to provide this function.
The capacitors are difficult to extract aluminium flows. The presence of PCBs leads inexorably to pollution problems during the recycling of aluminium. Therefore it needs to avoid the use of PCB capacitors and the removing at the end of life of household products.
Rules of association for alloying elements with aluminium
Source Techniques de l’ingÃ©nieur
As the table above shows, many base metals diminish the properties of aluminium alloys after recycling. It will be necessary to avoid the association of aluminium alloys with these metals too closely and to ensure that materials are released during grinding (Y. Xiao and M.A. Reuter (2002) et M.A. Reuter & al. (2005)).
The main design action will to avoid associating closely aluminium alloys with metals listed (see example parts below).
Heat exchanger before and after passage through a grinder. The honeycomb aluminium encloses the copper tube.
Bi-material design of this heat exchanger will lead to the lost of copper and the degradation of aluminium alloy. A single-material design (all copper or all aluminium) would be benefit from a recycling perspective.
Given the value of wrought alloys and the poor performance of recycled aluminium obtained by mixing severals types of alloys, a visible marking on the back of the part (indicating the quality of the alloy) would be required for large pieces made of wrought aluminium.
– Y. Xiao and M.A. Reuter (2002): Recycling of different aluminium scraps. Minerals Engineering 2002, Vol. 15, Issue IIS1, pp. 763-970.
– M.A. Reuter, K. Heiskanen, U. Boin, A. van Schaik, E. Verhoef and Y. Yang (2005) : The Metrics of Material and Metal Ecology, Harmonizing the resource, technology and environmental cycles Elsevier BV, Amsterdam, 706p. (ISBN: 13 978-0-444-51137-9, ISBN: 10: 0-444-51137-7, ISSN: 0167-4528).