Mathematics will increase aluminium recycling

The objective of the five-year project is to develop technical production "recipes" which will enable aluminium scrap to re-emerge in the form of high quality consumer products -- everything from foil to window frames.

"The results are promising," says Yanjun Li, a project manager at SINTEF.

Accumulation of impurities

The recycling of aluminium requires only five per cent of the energy used in producing new aluminium. This means a potential 95 per cent reduction in greenhouse gases in connection with production.

However, every time aluminum is recycled, various alloy constituents such as iron, silicon and zinc, as well as trace elements such as sodium and lead, accumulate in the resulting material.

Until now this has placed clear limitations on what recycled aluminium can be used for, even when a high proportion of pure aluminium is added to dilute the concentration of undesirable elements.

However, these limitations are now to be eased.

Casting is the past

Recycled aluminium has so far mainly been used in cast products, which may be anything from car wheel rims to engine blocks.

However, in just a few years' time the casting market will probably be too small to absorb the rapidly increasing amount of recycled aluminium which will enter the material stream.

Rolling and extrusion are the future

If the world is to benefit from the increased availability of recycled aluminium, new routes must be opened to the market for such materials.

Put simply, this means that rolling and extruding plants must be adapted to accept far higher proportions of recycled material.

These are factories designed for aluminium-based mass production.

Sheets and mouldings

Rolling produces aluminium material in the form of sheets, foil and strips.

Aluminium which has been put through an extrusion plant ends up as mouldings or profiles. These can be found in the light fitting above your head, in the front panel of your radio, in the window frames of quality buildings or as heat exchangers in car radiators -- to mention just a few examples.

Specialist project

The desire to make recycled aluminium a raw material for rolling and extrusion mills is the background for the specialist project, MOREAL (2009-2013).

SINTEF and NTNU are running the project in collaboration with Hydro Aluminium and the Swedish company Sapa Technology, with partial financing from the Research Council of Norway.

Focus on tempering process

"The impurities which accumulate in aluminium through repeated recycling affect the mechanical properties of the recycled material. However, by changing the alloy composition and temperature conditions, and the speed of the homogenisation process -- the initial stage in a tempering process carried out in rolling and extrusion plants -- it is possible to compensate for this," says Yanjun Li.

According to the research scientist, by effecting such changes it will be entirely possible to make use of recycled aluminium in rolled products and aluminium mouldings which satisfy any required mechanical properties, such as strength and malleability.

The PC as a test lab

The SINTEF employee explains that it is both expensive and time-consuming to determine the right process changes by trial and error in physical experiments in the factories. Instead, the researchers prefer to use mathematical models -- in other words, tools which describe real conditions by means of mathematical formulae.

"In the MOREAL project we are developing advanced mathematical models as a supplement to laboratory experiments. These are powerful tools which make the development of recycling-friendly aluminium alloys cheaper and less time-consuming," says Yanjun Li.

The project will lead to three different models, all of which will show how the microstructure of recycled materials is affected by various modifications in homogenisation during the extrusion and rolling processes.

Promising results

"Using mathematical modelling as a guide, we have carried out physical laboratory experiments with an aluminium alloy in the so-called "3xxx family." This is a commonly used alloy group in which manganese is an additive, providing good malleability, increased tensile strength and high corrosion resistance. We demonstrated that the yield point of the alloy we investigated can be increased by 50 per cent by modifying the homogenisation process. In plain language this means that the material will tolerate far more bending before it breaks," says Li.

In 2009, around one fifth of the world's aluminium production originated from recycled materials. Most of the recycled raw material comes from the transport sector and packaging products, but in recent years aluminium from the construction industry has been increasingly recycled.

"The aim of our project is to enable our industrial partners to produce more tailored, recycling-friendly aluminium alloys. Expertise in this field will be increasingly important for sustainability and global competitiveness in the materials industry," says Yanjun Li.