Aluminium recycling

Automotive aluminium scrap

In the automotive industry, aluminium is used to make many parts. Therefore, automotive aluminum scrap plays an important role in the production of secondary aluminium.

Automotive aluminium scrap

In the automotive industry, aluminium is used to make many car, lorry and truck parts. The distribution of cast and wrought aluminum in a typical car is shown in Figure 1 and Table 1. From point of view remelting aluminum scrap the differences between cast and wrought alloys are very important. The thing is, that only pure wrought aluminum scrap can be directly melted back into wrought aluminum. At present, the only way to reduce the levels of alloying elements in the remelting of scrap wrought alloys is to dilute. A mixture of wrought and foundry scrap can only be smelted into foundry alloy.

Figure 1 – Typical application of aluminium in a passenger car [3]

Table 1 – The shares of cast and wrought aluminium in cars

The share of various wrought aluminium alloys in a modern car is on average [1]:

  • 35 % – 6060 (0,45 % silicon – 0,2 % iron – 0,5 % magnesium);
  • 11 % – 6082 (1,0 % silicon – 0,3 % iron – 0,7 % manganese – 0,9 % magnesium);
  • 10 % – 3003 (0,5 % silicon – 0,5 % iron – 1,3 % manganese);
  • 9 % – 5182 (0,1 % silicon – 0,2 % iron – 0,4 % manganese – 4,5 % magnesium);
  • 14 % – 5754 (0,3 % silicon – 0,2 % iron – 0,4 % manganese – 3,2 % magnesium);
  • 15 % – 6016 (1,25 % silicon – 0,3 % iron – 0,5 % magnesium);
  • 6 % – 7020 (0,2 % silicon – 0,3 % iron – 0,3 % manganese – 1,2 % magnesium – 4,5 % zinc).

If this mixture directly, without sorting, melted, we get the “alloy”, which will contain 0,57 % silicon – 0,26 % gland – 0,32 % manganese – 1,27 % magnesium – 0,27 % zinc.

This zinc content is too high for all alloys except alloys of the 7xxx series. However, the silicon content is too high for 7xxx alloys. Therefore, for the manufacture of any of the remelted scrap alloy it is necessary to dilute the primary aluminium. If the scrap contains aluminium waste and scrap cast aluminium alloys, then the problem becomes even more acute.

Preparing automotive aluminium scrap for sorting

Figure 2 –  Modern End-of-Life Vehicle Dismantling and
Aluminium Recycling Process [4]

Dismantling of automobile parts and assemblies

Near 95 % cars are dismantled before being processed in a shredder. Capturing details, which can be restored and sold. Be sure to remove hazardous materials and products, such batteries, fuel, liquids, air conditioning. After such dismantling, only 50 %.

Shredders

A typical shredder uses millstones, that work with power 2000-6000 horsepower. They are able to turn a full load over 45 seconds into chunks no larger than 100 mm. The principle of operation of the shredder is shown in Figure 3.

Figure 3 – A typical automotive aluminium scrap shredder [2]

Methods for sorting automotive aluminium scrap

At present, after processing in a shredder for sorting materials, containing aluminum, In industry, four main technologies are used:

  • magnetic separation
  • air separation
  • separation by eddy electric currents and
  • gravity separation in liquid solutions.

Magnetic separation

After magnetic separation of the fraction, which were obtained on the shredder, divided into magnetic and non-magnetic. The most commonly used magnetic drum separator, shown in Figure 4.

 Figure 4 – Magnetic separation of automotive aluminum scrap [2]

Air separation

Non-magnetic particles, sorted by magnetic separator, repeatedly processed by air separation. Suction nozzles work on top of the conveyor, which separate light non-metals from metals, such as plastic, rubber, foam and fibers. Another method is the elutriator – material is passed through a stream of air, upward. Both of these methods are used to remove low density materials..

Separation by eddy currents

Separation by eddy electric currents is usually the third and last stage, used for materials, past shredder. This method treats the material with an external magnetic field, which repels non-magnetic electrically conductive metallic materials. When this type of particle enters a magnetic field, they generate an opposing electric current, due to which they are pushed out of the magnetic field (Fig. 5).

 

Figure 5 – Non-magnetic metal particle in a rotary magnetic field [2]

Separation “drowned-surfaced”

  • In this method, aqueous solutions with different densities are used., in which materials with different densities (Table 2) can float or sink.
  • A three-step process is usually used with rasters of density 1; 2,5 и 3,5.

Table 2 – Density of various automotive materials [2]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Laser spectroscopy

For, to get high quality aluminum scrap, necessary system, which separates cast and wrought aluminium alloys into specific alloys. Such a system must be able to quickly and reliably determine the actual chemical composition of each piece of scrap.. This led to the development of laser spectroscopy (LIBS) to accurately sort aluminium scrap into individual alloys (Figure 6).


Figure 6 – Sorting of aluminum scrap using laser spectroscopy [2]

Removal of coatings from aluminum scrap

Several industrial technologies are known, which are used to remove coatings from automotive aluminium scrap. All of them involve the cleaning of aluminium scrap after the shredder by treatment with hot air., for example, in a rotary kiln or in a bed of fluidized sand. Processing in fluidized sand gives the best results – the cleanest aluminium surface in the shortest time [2]. Before processing aluminum scrap to remove paint, it is sorted into painted and unpainted scrap (Figure 7).

Figure 7 – Sorting of painted and unpainted aluminium scrap [2]

Separation of cast and wrought aluminium

Incompatibility of wrought and cast aluminium alloys

Cast and wrought alloys of various series have very limited compatibility in terms of chemical composition.

1xxx series alloys, 2xxx and 7xxx alloys are compatible only with their own series.

  • 3xxx series alloys have limited compatibility with a series of casting alloys 5hh.h, which contains a lot of magnesium, and Mn.
  • 5xxx series alloys are highly compatible with alloys casting 5hh.h series.
  • 6xxx alloys have limited compatibility with casting alloys series 3hh.h, 5xx.x and 4hh.h.

To increase the efficiency of separation of aluminium scrap into individual aluminium alloys, it is very important to pre-separate them into cast and wrought alloys.. These two categories of alloys differ significantly in several ways: surface texture, scrap particle shape and chemical composition. Although cast and wrought alloys contain the same alloying elements, their number in cast alloys is much greater. The maximum concentration of alloying elements in wrought alloys is approximately 5 %, while in cast alloys it can reach 15 %.

Thermomechanical separation of cast and wrought alloys

To directly remelt wrought aluminium alloys without costly chemical refining, they must not be mixed with casting alloys. Due to the physical differences between these two alloy groups, they can only be sorted by hand, which is a very laborious operation (Fig. 8).

 
Figure 8 – Manual sorting of cast and wrought aluminum alloys

Among the many developments to solve this problem, the method of thermomechanical separation deserves the most attention.. This method uses the fact, that all casting alloys have a relatively low eutectic temperature. When casting alloys are heated above the eutectic temperature (520-560 ºC), they become brittle due to intergranular melting in places of the eutectic chemical composition. This leads to a significant decrease in the mechanical properties of cast alloys.. Cast alloys at such temperatures during machining, for example, in the millstones are greatly crushed. On the other hand, wrought alloy does not experience such grinding when heated to 600 ºC and retain their shape during machining. The difference in size between cast and wrought alloys can be seen in Figure 9. The separation of small pieces of cast alloys from large parts of wrought alloys is carried out, for example, by screening [2].

Figure 8 – Wrought and cast aluminum alloys after thermomechanical separation [2]

Currently in the world are developing aluminium alloys, which had a wide compatibility for future recycling. This will make the aluminium alloys are more environmentally friendly and commercially attractive.

Sources:

  1.  Mark E. Schlesinger, Aluminum Recycling, 2011
  2. https://www.nrcan.gc.ca/sites/www.nrcan.gc.ca/files/mineralsmetals/pdf/mms-smm/busi-indu/rad-rad/pdf/2003-22(cf)cc-eng.pdf
  3. https://aluminiuminsider.com/steel-cannot-compete-with-aluminium-in-vehicle-lightweighting/
  4. Aluminium Recycling in Europe The Road to High Quality Products – EAA/OEA Recycling Division