Eddy Current Separator

What are Eddy Currents?

Eddy currents or Foucault currents are currents that are intentionally or unintentionally induced in a  conductor. It is a natural phenomenon that occurs when, for example, there are varying magnetic fields within a metal plate (it doesn't have to be ferro-magnetic). This can be induced by an alternating field of an electrical coil, but it could also occur when a plate is cut through by magnetic field lines. When a conductor cuts through magnetic field lines it creates electricity. This current is enclosed by the conductive plate.

The phenomenon was discovered in 1855 by French physicist Léon Foucault.

Application besides recycling 

Eddy currents are not only used in the recycling industry and they do not always occur when you want them to occur. Here a few examples of how we use eddy currents:

• An induction cooker makes use of eddy currents to produce heat.
  An induction cooker is a cooktop that uses the principle of induction heating. Within the induction cooker there are electrical coils, which generate a magnetic field. As a result, an eddy current is going to walk through the base of the pan (specially designed for induction cooking), through which the resistance of the bottom is converted into heat. In addition, heat is generated by magnetic hysteresis. Because the pan is heated directly, there is little energy loss, which makes this eco-friendly.
• Magnetic brakes also make use of eddy currents. The magnetic brake hangs a few millimeters above the track and provide a magnetic field in the rail. Due to the reaction of the magnetic field to the traveling direction of the train, the train slows down. This brake doesn't wear off but the tracks generate considerable heat. The heat generated is called hysteresis.
• Tachometer.
• Electricity meter.
• "The Meissner effect".
• Rollercoasters and other amusement park rides (applied for braking).
• In vending machines it is also used to detect counterfeit money.
• Metal detectors

A side effect - which I've mentioned already - is hysteresis. In all electric machines that make use of eddy currents, the side-effect you want to prevent is core loss. You can do this by covering the core with an insulating coating or to make the core out of non-conductive magnetic material.

How are eddy currents used in the recycling industry for nonferrous separation of (eg.) aluminium?

Eddy current separation is a sorting method in the recycling sector. Its application is mostly used to remove non-ferrous metals (such as aluminum, copper, brass, zinc and lead) from a diverse stream of waste materials (like wood, plastic, etc.). It is very important that all ferrous materials are removed BEFORE it goes through the eddy current separator, as this could damage your machine. Due to the heating and heavy vibrating of ferromagnetic particles, your machine might not last so long as it could. A possible solution to this problem is to put a suspended magnet above the conveyor belt before it leads to the eddy current separator. That way, you only have non ferrous and non metallic particles within your waste stream.

You can see that at the end of the conveyor belt, there is an eddy current rotor. Through Foucoult currents, the nonferrous rubbish (a good example would be aluminium cans) are thrown a bit further along the line in separate bin.

Designs, sizes, combinations and other ways to fine-tune your magnetic separator

Not every recycling plant is the same, so the manufacturers make sure that they can deliver the same kind of machine in various sizes. On my fancy paint graphic, you can see the machine from the side. What's inside shouldn't be visible. Therefore manufacturers design a frame, so the aluminium actually lands in the correct bin and not all over the place. Frames out of various alloys are offered by most manufacturers. The diagonal line you see on the picture is called a 'splitter'. It can be static or adjustable. When you have an adjustable splitter, there is also the possibility to adjust it using a control panel. Obviously, the control panel doesn't just have a splitter-adjust-function. It can operate nearly everything that can be adjusted (speed, intensity of electromagnet, etc.). That being said, there is also the possibility to have a mobile eddy current magnetic separator. Another way of tweaking your line is by adding or upgrading other recycling machines before or after the eddy current separation. It is possible to have vibrating conveyor placed before your eddy current separator, so the volume of the waste stream is better distributed. That way the distribution of waste is stable, which prevents unpleasantry like clogging up the waste stream. An uneven distrubtion of waste on a conveyor can also lead to less efficient sorting, which in its turn leads to less purity of the end product.

Applications of the eddy current separator

• The timber industry uses this device to dispose of metallic particles, so the recyclate is clean and pure. Getting the cleanest possible recyclate is basically always the ultimate goal of a sorting machine.
• This also applies for plastic recycling, where impurities can have grave consequences.
• The processing of construction waste also needs a decent eddy current separator.
• In the ashes produced by RDF plants, there can still be non ferrous particlesbetween the ashes.
• The eddy current sorting machine is indispensable device within a plant line.
• A municipal waste recycling plant also uses eddy current magnets.
• If rubber has a conductive element inside (and in some cases it has), the eddy current separator can indeed be used to divide it from other trash
• You name it. Every plant that somehow processes non ferrous / metallic trash needs an eddy current separator.