Recovery mission - Recycling light metal alloys
Geoff Scamans from Innoval Technology Ltd and Zhongyun Fan of Brunel University, both in the UK, put forward the case for light alloy recycling and overcoming the divide between cast and wrought light alloys.
Primary metal-based aluminium and magnesium alloys produce high emissions from the energy and carbon required for their mining, processing and manufacture. On average every kilogramme of primary aluminium produced is responsible for 10kg of CO2e (carbon dioxide equivalence) and half of this is from power generation (it can be higher from old pot-lines and where electricity production is coal based).
Therefore, once primary aluminium has been alloyed, processed and manufactured into components every effort should be made to recycle the aluminium post-consumer scrap at end-of-life.
Annual primary magnesium production is only 800kt compared to 37Mt of aluminium. The Pidgeon process accounts for 78% of magnesium production while 22% comes from electrolytic reduction. For electrolytic production each kilogramme of magnesium can represent up to 44kg CO2e embodied, although this reduces to 21kg CO2e if SF6 (sulphur hexafluoride) is not used as a covering gas, and to six kilogrammes if the electricity for reduction is hydro-generated. For the Pidgeon process, the embodied carbon is 47kg CO2e/kg for most Chinese production (660kt/year in 2007), though process improvements, such as the change from coal to natural gas, could reduce this to 13kg CO2e/kg. In primary form, the carbon burden is high, but if this is amortised over their first product use, then the drive to recover and recycle these light alloys should be overwhelming as the embodied energy in recycled metal is substantially reduced.
Aluminium recycling of post-consumer scrap is a challenge compared to obtaining production and process scrap from producers or fabricators. For an aluminium-intensive carmaker such as Jaguar Land Rover or Audi, recycling of process scrap is relatively straightforward. Recovery of process scrap is important, but it is mainly recycling of primary alloy or the mix of primary and secondary metal used to form the alloy initially. Both aluminium and magnesium have the potential for high level recycling where the old scrap is recovered and recycled into the same high performance product.
For aluminium, a major example of successful recovery and high level recycling of post-consumer scrap are used beverage cans (UBCs). In the UK, 6.7bln (90kt) drinks cans are sold each year. The recovery rate of UBCs in the UK has improved progressively from 30% in 2001 50%, meaning only 45kt of UBCs are returned and most of the remainder goes to landfill.
However, of the returned UBCs only 30kt are directly recycled into can body stock, the remainder are used in other products mainly castings. For most aluminium wrought products, particularly in Europe, recovery of post-consumer scrap and its recycling back into the same product is not significant, except for a low volume of architectural sheet made by continuous casting.
A barrier to the recycling of light alloys is the increase of inclusions and impurity elements in re-melted post-consumer scrap. These can result in severe losses in ductility and strength, and certain impurity elements can significantly reduce corrosion resistance.
Conventional wisdom states that inclusions and impurities must be reduced by chemical refinement, a high cost and low efficiency process. In contrast to refining, melt conditioning (MC) technologies are being developed to process light alloy scrap using a physical approach. This eliminates the detrimental effects of both the inclusions and impurities, so that higher grade light alloy products can be produced directly from their scrap. This could enable the recycling of castings into wrought products.
Melt conditioning by advanced shear (MCAST) processing, a technology developed recently at Brunel University, UK, for conditioning liquid metal prior to solidification processing, can be used to reprocess light alloy scrap into cast engineering components or feedstock materials, with equivalent or improved properties compared to currently available primary alloys. This has been called ‘upcycling’, in contrast to the current concept of recycling where post-consumer scrap is converted either into a grade of alloy with inferior quality or is used as a more impurity tolerant casting alloy.
Upcycling is achieved using a twin-screw melt conditioner, which consists of a pair of co-rotating, fully intermeshing and self-wiping screws rotating inside a barrel. The screws have specially designed profiles to achieve a high shear rate and intensity of turbulence for 10-20 seconds, resulting in the break up of oxide films and agglomerates, and their dispersion throughout the melt as fine nanoscale oxide particles. These dispersed oxides in both aluminium and magnesium alloys are potent sites for the nucleation of solidification once the processed melt is transferred to a mould or a casting station.
Solidification after MC results in a fine and uniform microstructure, a uniform chemical composition throughout the entire casting (either components or billets), and a much reduced (or eliminated) presence of cast defects, resulting in a substantial improvement in mechanical properties. Large inclusions are eliminated and the cast microstructure is much more tolerant to impurities because the intermetallics are refined.
Research at Brunel University has established that once a melt has been conditioned and cast by the MCAST process, it can be re-melted and cast repeatedly without loss of the refined microstructure for magnesium alloy AZ91D. The series of micrographs were obtained from the alloy cast and re-cast at 650°C with and without MC. Melt conditioning enables the alloy to be formulated entirely from recycled high pressure diecasting process scrap without any loss of mechanical properties.
If an MCAST processed melt is passed through a ceramic filter and the filtrate solidified and sectioned for metallographic examination, the magnesium oxide disperses and is responsible for the nucleation of Al8Mn5 intermetalics. After MC the Al8Mn5 intermetallics are smaller and more uniformly dispersed, although they are probably not responsible for the direct nucleation of magnesium. However, the Brunel team has demonstrated that magnesium oxide can directly nucleate magnesium, as excellent lattice matching is seen in the high resolution lattice images. Recycling magnesium casting process scrap is therefore achieved by turning oxide films from defects into potent nucleating sites by MC.
In addition to recycling casting scrap, the MCAST process should be suitable for the direct recycling of magnesium scrap from end-of-life vehicles, although this has not been evaluated so far. However, by working with George Thompson at The University of Manchester, UK, BCAST has demonstrated that MC of aluminium alloys using the MCAST process also disperses oxides and refines primary intermetallic phases and grain structure.
The process is especially useful when combined with twin roll casting (TRC) strip can be cast with significantly reduced centre line segregation and at low roll force. Continuous casting of aluminium alloy strip by roll, belt and block casting inherently increases tolerance to impurities due to the higher solidification rate. This type of process is already used to make high iron foil alloys and architectural sheet from recycled scrap.
To date, these processes have not produced aluminium automotive sheet used for commercial vehicle production. The combination of MC and TRC offers the possibility of manufacturing aluminium automotive sheet directly from recycled automotive scrap, making it cost competitive for low carbon vehicles for the mass market.
It is time to blur the distinction between cast and wrought alloys and develop MC and casting processes for the economic production of castings and preforms from melts made from post-consumer scrap. Presently about 8.3Mt of aluminium scrap from used products are available globally each year and there is a significant amount in landfill sites waiting to be reclaimed.
Rather than continuing to increase primary production of aluminium and magnesium each year, especially where this is based on carbon intensive electricity generation and the use of carbon in the reduction process, more wrought and high performance cast products should be made from upcycled post-consumer scrap.
Further information: Innoval Technology