Physical sorting – The Eddy current separation method
The most obvious example of sorting is that of using magnets to attract ferrous scrap. Magnetism occurs in iron due to unpaired electrons in the d-orbital giving each iron atom a magnetic moment. All these moments are aligned due to the interaction of the d-orbitals, giving an overall magnetic orientation. Magnetic materials can therefore be separated easily.
A large number of materials are not magnetic - aluminium, for example. They still need to be separated before recycling.
The eddy current separation method usually sorts this non-ferrous scrap. Eddy current separation takes the principles of electromagnetic induction in conducting materials, to separate non-ferrous metals by their different electric conductivities.
The main principle is that ‘an electrical charge is induced into a conductor by changes in magnetic flux cutting through it’. Moving permanent magnets passing a conductor generates the change in magnetic flux.
Electromagnetic induction and Eddy current generation will not be explored further here (although there are links in the Going Further section if you wish to find out more about this subject). Faraday’s law (electromagnetic) describes the generation of swirling currents in conductors, such as the non-ferrous metals in this example. Swirling currents create a magnetic field in accordance with Lenz’s law that will act to oppose the change in magnetic field being applied.
The basic set up is to have the non-ferrous scrap on a conveyor belt. The conveyor passes a rotating drum, inside of which is a much faster rotating magnet block (up to 4000 rpm). The magnet block causes the changing magnetic flux. Try the interactive demonstration below!
When the conducting particles move through this changing flux on the conveyor, a spiralling current and resulting magnetic field are induced. This magnetic field of the metal particles interacts with the magnetic field of the rotating drum. The interaction gives the particles kinetic energy. The scrap particles are thrown off the end of the conveyor with varying energies, causing different trajectories depending on the conductivity of the particle.
The size of the particle and the direction of rotation of the drum can be changed to vary the degree of separation. Small particles (10–50mm) can be separated owing to the degree of electrical conductivity . The most conductive materials interact the most with the magnetic field and have the longest trajectories. Aluminium has the highest conductivity for a given weight at ambient temperature than any other element. Non-metallic elements such as plastic labels and paint do not interact with the magnetic field at all. They simply fall off the end of the conveyor belt with no change in energy.
The eddy current separator is another excellent example of how knowledge of materials properties (electrical conductivity and density ) has improved recycling technology.
However, further processing is still needed to remove coatings and some alloys before re-melting, for example the tin coating on tin cans.