Several methods are used to physically sort aluminum scrap., which complement each other.
Detailed information on methods for sorting and recycling aluminum scrap:
Automotive scrap shredding
In the US, the whole car scrap - the most numerous - before sending it for processing are subjected to grinding – shredding. Companies large special machines used for grinding automotive scrap with hammer and anvil – shredders. Before you load the cars in the car they are subjected to dismantling major units, for example, engine block, gearbox, body. Naturally, remove the wheel centers and rubber tires. These powerful monsters break all, that gets them into pieces no larger than 100 mm (figure 1).
The magnetic sorting method consists of separating ferrous metal scrap from other metal scrap, stainless steels and non-ferrous metals. When mixed scrap passes on a conveyor past the magnet, the ferrous metal, carbon and alloy steel, as well as cast iron, attracted by the magnet and dropped to another conveyor, and non-magnetic pieces of scrap fall into the container (Figure 2).
When air classification using various methods. A popular method is an absorption method, to capture light materials, that are in particulate mass, car, scrap. Such materials are, for example, plastic, tires and high foam. The differences in the density of different materials, which may be in the unsorted scrap aluminum, are shown in Table.
The disadvantage of air separation aluminum scrap is, it may not work for light beer cans and small particles of crushed aluminum scrap.
Current heat method
Sorting using eddy current effective, when mixed in the crushed aluminum scrap has a lot of different metals, other than aluminum and its alloys. The rotor with magnets mounted on it excites the magnetic field, which repels non-magnetic electrically conductive metals with varying strengths and leaves non-metallic materials intact (Figure 3).
The disadvantage of this method is, that some of the components of scrap metal, such as wire and foil, often do not lend themselves to eddy current separation, as they appear too small eddy currents.
Gravitational sorting technology uses aqueous compositions of materials of known density, to separate non-magnetic materials with different densities. for instance, in the case of the crushed road aluminum scrap many of its components have different density values.
A typical process for this gravity sorting involves three steps:
- bath with plain water (density equal to one), wherein the separated non-metallic materials such, as light plastics, foam and wood;
- Bath with a solution density 2,5 grams per milliliter, which separates magnesium and denser plastics;
- Bath with a solution density 3,5 grams per milliliter, which separates the cast and wrought aluminum and leaves at the bottom of a heavy metal, such as copper, zinc and lead.
A disadvantage of gravitational sorting method is the high cost of maintaining a constant density of the solutions, and poor separation and floating hollow, for example, in the form of "boat", scrap components.
Sorting of scrap aluminum color often produce manually. This is especially true of developing countries, such as China, with very cheap labor (figure 4).
It is believed, that the possibility of such a hand - literally hands - branches of non-ferrous metals aluminum up 99 %! Besides, wrought and cast aluminum alloys also have differences in texture, color and other characteristics of the surface, which makes it possible to separate them.
Chemical etching method
For the separation of aluminum alloys are sometimes used between chemical etching method. This technology makes it possible to separate the aluminum alloys of various series. for instance, etching solution of copper sulphate in hydrochloric acid gives the possibility to distinguish between spots of color in the spot etching 5xxx series alloys 6xxx and. sulfuric acid solution stains the alloys with a high content of silicon and manganese in the light gray color, and alloys with a high content of zinc and copper - dark gray. This makes it possible to distinguish the 2xxx series aluminum alloys, 3xxx and 7xxx.
Currently, a popular practical spectroscopic method for sorting aluminum alloys - cast and wrought - is laser induced breakdown spectroscopy – LIBS).
In this method, the laser pulse hits the spectroscope in the metal surface, It produces emission of atoms and receiving chemical information about the material their spectral sensor. If the spectrometer is installed in the sorting line of aluminum scrap, then it generates a signal to a mechanical device to direct the investigated scrap component to the desired container (Figure 5).
A similar method is used on portable laser spectrometers for sorting of bulky aluminum scrap.
The main limitation to the use of laser spectrometers for sorting aluminum scrap is, that the aluminum scrap must be free of contaminants, grease, as well as paint and other coatings. The thing is, that the laser pulse penetrates only to a depth of approximately 30 angstrem. In such a case better suited X-ray sorting aluminum alloys. See also:. more here.
The thermomechanical method is intended for separation of wrought and cast aluminum alloys. It is based on a low eutectic melting point of the casting aluminum alloy with high silicon content. This method is, that mixed aluminum scrap is kept at a temperature slightly below the eutectic temperature (about 550 ºS). This leads to a softening of aluminum castings at their grain boundaries. Subsequent grinding or mechanical flattening leads to destruction of these alloys into small pieces. Then these small pieces are separated from the rest of the scrap mass by methods of sorting scrap components according to their size (Figure 6).
1) Gabrielle G. Gaustad, Ph.D Thesis, Massachusetts Institute of Technology, 2009
2) Sorting and Classification of Aluminum, Report No. 2003-22(CF), Canada, 2003