Eddy current separator: an essential technology for recycling

The eddy current separator, used in the recycling industry, efficiently sorts non-ferrous metals.

The eddy current separator is a machine used mainly in the recycling industry. Based on a principle discovered by a 19th-century French physicist, it performs extremely precise sorting between the various residues that make up the objects to be recycled and the non-ferrous metals they contain.

A tribute to Jean Bernard Léon Foucault

It is important to pay tribute to Jean Bernard Léon Foucault, who gave his name to the currents discussed in this article (known in French as “courants de Foucault”). Born in 1819 and deceased in 1868, this French physicist and astronomer was extremely prolific during his short life.

He left behind discoveries that had an impact on the entire world. In addition to the currents, he also gave his name to the Foucault pendulum, which demonstrates the rotation of the Earth about its axis; he was the first to determine the speed of light and he invented the gyroscope.

What are eddy currents?

Eddy currents – “courants de Foucault” in French – are the currents created by an alternating magnetic field in an electrically conductive material. Any variation of the magnetic flux passing through a metallic mass produces eddy currents. These electric currents can be induced intentionally or not.

In the case of a variable magnetic field in a metal plate – ferrous or non-ferrous – the phenomenon is natural. On the other hand, the magnetic field is induced artificially in the presence of the variable field of an electric coil.

These currents create a Joule effect and cause heating of the mass conducting the current. They can also be used as a braking force when they give rise to Laplace forces, if the flux variation results from the movement of the medium through a constant magnetic field.

Discover the Lenoir-Mec eddy current separators.

Eddy currents in everyday life

Eddy currents are part of our daily lives, and we use them in many applications.

Induction hobs work by subjecting a stationary metal part to a high-frequency magnetic field. The eddy currents generated in the part cause heating by the Joule effect.

To brake trucks, a copper disc is fixed to the wheel, next to an electromagnet supplied with current. The eddy currents induce Laplace forces that slow the movement, but this system does not replace conventional braking, as its effectiveness decreases as the wheel slows down.

On the negative side, eddy currents cause heating in transformers, as well as energy losses in electrical installations.

The eddy current separator

What is the eddy current separator used for?

The eddy current separator separates non-ferrous metals from other materials. The materials to be sorted are conveyed in bulk on a belt conveyor, which automatically separates and discharges the non-ferrous metals.

These machines are mainly used by recycling companies for waste treatment. The incoming flow is processed by the eddy current separator and, at the machine outlet, they obtain a clean separation that allows the different materials to be reused.

Overall, the non-ferrous metal separator is used to recycle a wide variety of materials such as glass, plastic, rubber, sand in aluminium foundries, shredded cables, automotive shredding lines, municipal solid waste, bottom ash (IBA)…

It can also remove ferrous or weakly magnetic particles, for example in stainless steel, wired glass, etc.

Here are some examples of applications:

• processing of manufactured wood and furniture: extraction of copper hinges, nails, screws, handles, etc.;
• recovery of door and window frames, handles and locking mechanisms;
• sorting of non-ferrous metals in residues after waste incineration;
• treatment of municipal solid waste and recovery of metal parts from electronic equipment and cans, among others;
• removal of non-ferrous elements from crushed glass, plastic, polyethylene, PVC, rubber, etc.;
• separation of aluminium capsules in the glass recycling industry or Nespresso capsule recycling;
• recycling of car bodies and recovery of aluminium and its derivatives from shredded aluminium;
• cleaning of aluminium foundry sand;
• recovery of copper, lead and aluminium from shredded cables;
• processing of all types of batteries…

How does the eddy current separator work?

The residues to be treated are first crushed. Most of the time, they still contain iron particles that must be removed before passing through the eddy current separator. For this purpose, they are processed by an overband magnet or a magnetic drum, which attracts the steel particles and discharges them.

The eddy current separator then comes into play, separating the non-ferrous metals from all other materials.

The function of the eddy current separator is to convey the materials to be treated in bulk on its conveyor belt. At the end, the machine is fitted with a multipolar rotating magnetic drum equipped with high-remanence neodymium permanent magnets. As the assembly is driven at high speed, the magnetic field induces eddy currents in the conductive non-ferrous metals.

These currents create their own magnetic field, which opposes that of the pole wheel. Thus, the inert residues continue along the cleated conveyor belt, while the non-ferrous residues are ejected by the repulsive force and collected in a separate receptacle. A splitter is placed at this point to separate the two trajectories. It is adjustable according to the parts and materials to be processed.

In other words, the non-ferrous metals, through which an electric current flows, become for a fraction of a second electromagnets with their own electromagnetic force. This is why the non-ferrous metals, thanks to the belt speed of the conveyor transporting them combined with the magnetic repulsion, follow a different trajectory from the inert residues.

What influences the accuracy of non-ferrous metal separation?

Ferrous metals are more or less easy to separate. This depends on their electrical conductivity, their density and the ratio between these two properties. The higher the electrical conductivity and the lower the density, the better the eddy currents will be able to separate a material.

Electrical conductivity

Electrical conductivity is measured in siemens per metre and symbolised by the Greek letter sigma: σ. In descending order, it ranges from silver at 62.1 σ to stainless steel at 1.4 σ, via copper, gold, zinc, aluminium, magnesium, brass, cadmium, platinum, tin, chromium, bronze, lead and titanium.

Density

Density determines how easily the metals can be ejected from the conveyor, as it involves the force of gravity. The eddy currents of the magnetic rotor must generate sufficient forces to overcome this force of gravity.

Optimise your recycling operations now!

Recycling plays an increasingly central role in our society.

With growing waste volumes, it is crucial for industrial companies to equip themselves
with high-performance eddy current separators.

At LENOIR-MEC, we have been designing and installing these technologies for years, equipping
more than 80% of sorting plants in France.

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