Sorting of aluminium alloy

For decades, the consumption of aluminium has been on the increase, also in Europe and in spite of protracted phases of economic stagnation during the later stages of the 20th century. The fact that the production of primary aluminium fails to meet the growing demand for the light metal underscores the importance of recycling. Yet, although only 5 % of the energy needed for the manufacture of the base material is required in order to remelt scrap aluminium, the recycling of aluminium is far from uncomplicated and frictionless. We take a look at various procedures for sorting and preparation of aluminium which are applied prior to remelting.

Sorting with magnet and eddy current separators

The accumulating scrap metal originates from a sheer endless and growing array of industrial applications. It ranges from window frames made from aluminium with foam cores, profiles with plastic sealings, automobile parts to laminates from the aerospace industry and also includes cans, tubes, foils, foil dishes and tablet blister.

In automated sorting, metal and eddy current separators have traditionally occupied an important position prior to the remelting process proper. Both procedures rely on fundamental findings in electrophysics. The operation of magnet separators exploits the observation that different materials are induced to exhibit distinct trajectories depending on their respective magnetic properties. The quality of sorting is essentially determined by the conveyor speed of the magnet separator, pole clearance, and the size of the magnet wheel. According to ferromagnetic content of the scrap mixture and the dimensions of individual scrap pieces different devices may be deployed.

Once ferrous metals have been removed, further separation can be undertaken by means of an eddy current separator. In principle, this is a special form of a magnetic separator. The electrical conductivity and density of the material which is to be sorted are crucial in this application: A heteropolar arrangement of magnets in a magnet wheel produces a rapidly alternating magnetic field, generating an electrical current or eddy current in metallic particles. The metals’ own magnetic field is oppositely charged to that of the magnetic wheel, which in turn causes a mutual repulsion. Again, the metallic pieces chose a different trajectory, correspondent to their conductivity. The favourable ratio of conductivity to density renders this method effective at sorting out aluminium from a mix of metal scraps.

Automated sorting of aluminium alloys by means of high-tech sensors

Such separation processes appear, just as heavy media separation or air classification, like procedures from some other era compared to modern sensor-based sorting techniques that have increasingly found their way into recycling since the 1980s. In recycling of aluminium, the problem arises that more often than not so called cast alloys of high silicon content are inevitably remelted together with wrought alloys. The mixture obtained in this way can only be used as base material for cast alloys.

Therefore, researchers and enterprises have been attempting to tackle the problem of downcycling and to conserve/retain the original quality of primary aluminium. For a long time, optical, x-ray fluorescence spectroscopy and chemical analysis were either to inaccurate or slow in order to swiftly determine the alloy of piece of metal. Today, various manufacturers supply capable equipment for optical emission spectroscopy. These devices trigger a spark, establish the specific light spectrum and assign it to the corresponding alloy.

Computing power plays a crucial role in these processes, and accordingly, a considerable evolutionary potential remains to be exploited. This is exemplified by the year-long research on laser-induced emission spectroscopy (LISB) undertaken by Metallgesellschaft AG in Frankfurt, Germany, and, by Huron Valley Steel Corporation during the early 1980s. The latter successfully applied for a patent and is said to have utilised LISB for its own operations. LISB makes use of a high-energy laser impulse to condensate a part of the sample’s surface. The plasma is subsequently tested for spectral distribution of its light emission. Finally, pulsed jets of airs separate the desired aluminium alloys from the rest of the scrap.

At the time, however, said efforts “met with certain technological barriers, in particular with respect to the available laser technology and computing power”, Dr. Joachim Makowe, former project manager with the Fraunhofer-Institut für Lasertechnik and now managing director of LSA Systems GmbH explains. “Strictly speaking, it was not the procedure itself which failed but rather its industrial application with the available technical means.”

Consequently, the project launched in 2002 by the Fraunhofer ILT and the RTWH Aachen as well as five private companies, focussed “in particular on the economical implementation of the latest technologies”, says Makowe. In practice, this means that the chemical concentration of all chemical elements within a moving object can be measured within milliseconds. Though even today and two years after the conclusion of SILAS LIBS is not applied on a broad scale, the Fraunhofer ILT, together with LSA GmbH, is working to complete an industrial prototype. Commissioning of the installation is scheduled to occur in January 2009, with a projected 40 measurements per second, a conveyor belt speed of 3 m/s and a sorting band width of 600 mm.

From sorting to remelting: The removal of organic adhesions

A frequent obstacle to the efficient recovery of aluminium arises in the shape of organic, i. e. carbonic materials. For instance, many window profiles from aluminium include a foam core while countless other items, namely from the packaging industry feature plastic sealings, remainders of glass, finishings or other organic coatings. Whenever those materials are melted with aluminium, the oxygen-affine light metal reacts with the carbonic substances and a significant proportion of it oxidises. A further difficulty consists in the ensuing contamination of the melt.

Scraps Organic content (%)
mixed alloy old sheet 5
extrusions with thermal break inserts 5-10
yoghurt lids 20
heavily contaminated used beverage cans 20
cable wrap 20
foam filled extrusion 23
bottle caps with plastic liners 25
Litographic sheet with plastic contamination 30
plastic-filled sandwich panels 55
toothpaste tubes 70
food pouches 85

Data from: C. Schmitz, Handbook of Aluminium Recycling, Essen 2006, p. 65

In addressing the issue, scientists and engineers availed themselves of a thoroughly understood thermal process known by the euphonic greek idiom pyrolysis. The term describes a procedure whereby organic substances are decomposed at temperatures between 100 and 1500° C and from which oxygen is largely excluded. The mechanism is extensively used in various industries, for example to produce coke from charcoal in the steel industry or to convert wood into methanol. Unlike in waste pyrolysis, which seeks to harness the thermal energy in the form of gas and coke contained in organic material, the pyrolysis of aluminium scrap intends to recover the predominant base material, acdording to authorised energy expert Dipl. Ing. Robert Jaspers.

Under the impact of heat the organic components disintegrate and pyrolysis gas develops. In the case of contaminated aluminium scrap, the process takes place at temperatures above 350° C. In multi-chamber melting furnaces with integrated pyrolysis, the carbonisation gas which forms in the pyrolysis chamber is subsequently diverted to heat the installation. In a second step, the now purified and preheated metal is melted.

Potential and limitations of pyrolysis

Pyrolysis cannot work wonders, rather the method contributes to optimising the process of remelting. Even after careful pretreatment of aluminium, such action incurs smelting losses that are retroactively separated as skimmings. In fact, these skimmings comprise up to 30% aluminium, ready to be recovered during yet another procedure. To carry out pyrolysis separately from remelting facilitates the regulation of both the temperature and the oxygen content in the pyrolysis atmosphere. That said, a complete separation of carbonic substances is rarely possible, not even by means of this highly effective technique.

For this reason rotary drum furnaces are commonly used to melt contaminated scrap aluminium. A layer of salt binds the remaining contaminations and likewise protects the liquid metal surface from oxidation and smelting losses, Dipl. Ing. Henrik Rahms M. Sc. from the Essener Gaswärmeinstitut e. V. outlines. When the scrap is heavily contaminated 500 kg of salt dross accrue for each ton of aluminium. Scrap aluminium frequently finds its way directly into the rotary furnace. “You need to consider that the required energy effort for the pretreatment of aluminium for may be too high and therefore inefficient when you are simultaneously dealing with mixed scraps and a very high organic proportion”, Rahms says.

Based on the results of study which was completed 18 months ago, the Essener Gaswärmeinstitut e. V. recommends to perform pyrolysis separately. Thereby, melting losses may be reduced as much as 5.5%, depending on the type of scrap. Looking into the future, Rahms is convinced that "the potential of pyrolysis is nowhere near to being exhausted.”

Microwave-induced pyrolysis

This view is shared by a group of researchers led by the Cambridge professors Howard Chase and Carlos Ludlow-Palafox. For almost 10 years, the team has been concerned with the possibility of pyrolysis induced by microwaves that utilises charcoal as conductor of heat. Compared to conventional techniques, this method offers several advantages: For a start it enables a more constant distribution of heat and simpler handling of the temperature. The scientists circumscribe the procedure as “very gentle” and suitable for the pretreatment of aluminium used in tetrapacks and even toothpaste tubes and food pouches. On top of that, microwave induced pyrolysis is a cheap alternative, the Britons write, promising a recovery rate of 100% in laminates made from plastic and aluminium.
Whether and when pyrolysis will be achieved through the discrete workings of a microwave reactor appears unclear. What is clear, however, is that the demand for high grade secondary aluminium remains as high as ever, and that therefore recyclers and users of aluminium alike are interested in seeing innovative and capable solutions.