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Physical sorting – The Eddy current separation method



How does an eddy current separator work? Eddy current separation (ECS) is a sorting technique that uses different electrical conductivity of substances, and its sorting principle is based on two important physical phenomena:

  • An alternating magnetic field that changes with time is always accompanied by an exchange Variable electric field (Electromagnetic induction law);

  • Current-carrying conductor generates magnetic field (Biot-Savart law).

The principle of eddy current separation introduced in this article is shown in the Figure below.


A rotating drum wrapped by a permanent magnet (High Performance NdFeB Magnets) produces a alternating magnetic field. When a metal with conductive properties passes through the magnetic field, an eddy current will be generated in the metal. The eddy current itself generates an alternating magnetic field, which is opposite to the direction of the magnetic field generated by the magnet rotor, that is, it generates a repulsive force (Lorentz force) on the metal, which separates the metal from the material flow and achieves the purpose of separation.

The repulsive force generated by eddy current on the conductor is related to the strength of the magnetic field change, the conductivity, density, area and shape of the conductor.

For different non-ferrous metal materials, the repulsion force is related to the composition characteristics, and the composition force can be expressed as:

F (compositional force) = mσ/ρs

In the equation:

  • "m" is the mass of the material, the unit is kg;

  • "σ" is the conductivity, the unit is S-1·m-1;

  • "ρ" is the density, the unit is kg·m-3;

  • "s" is the material shape factor, no unit.

The following table lists the specific conductance (σ/ρ) factor values for some metals. From the value of σ/ρ, the magnitude of the repulsion force and the difficulty of sorting can be judged. Generally, high conductivity and low density materials suffer the most repulsion.


Types Of Eddy Current Separation Equipments

In 1889, the American inventor Edison applied for a patent for an alternating magnetic field eddy current separator, which was used to separate nonferrous metals and non-metals. Early sorting equipment including:

  • Aluminum magnet separator;

  • Linear motor separator;

  • Rotary disc separator;

  • Sliding ramp separator.

With the development of permanent magnetic materials, the research & development of permanent magnetic roll eddy current separator have appeared one after another. From the late 1980s to the early 1990s, Germany and the United States successively developed the NdFeB permanent magnet roller eddy current separator (PMRECS), which was put on the market by the American Eriez magnetic company and the German Wagner company.