Technical NoteA new static separator for metallic particles from metal–plastic mixtures, using eddy currents
Introduction
We shall study the interaction between an alternating magnetic field of decreasing amplitude along a conductive loop in the particular case of axial symmetry, having the Oz axis as symmetry axis (Fig. 1). The conductive loop represents the simplified case of a conductive particle, as the currents induced into this have a circular form.
The magnetic field is produced by a coil by the discharging through it of a capacitor charged previously at high voltage. The produced magnetic induction is given by (Timotin , 1970) the expression:where is a value dependent on the spatial coordinates but independent of time, ω is the angular frequency of free oscillations which occur subsequent to the discharge of the capacitor and β is the damping factor of the oscillations. The loop is run through by a magnetic flow Φ having the expression:where the relation (1) was used, and Σ is the surface of the loop. This variable magnetic flow induces along the loop an electric current of intensity i(t) given by the relation:where R is the electric resistance of the loop, the sense of the current being given by Lenz's rule (in Fig. 1, i(t)>0 for an increasing magnetic flow).
The interaction force between the magnetic field and the loop iswhere C is the outline of the loop and is the length element. By using the relations , the relation (4) is writtenwhere we have denoted
The axial symmetry requires that the direction of this force should coincide with the symmetry axis Oz. As it results that in the expression of the force there will be a term FC which does not depend on sin(ωt) and cos(ωt)which acts in the positive sense of the Oz axis. This force will reject the loop, driving it away from the coil.
We specify that the cause which explains the nature of the force which is exerted upon the conductive particles in the non-uniform magnetic field of the coil is the same as the one shown in the simplified case of the conductive loop previously presented.
We have to point out the fact that the force exerted upon the particles (along the loop, respectively) occurs only when the particles are situated in a magnetic field with a gradient.
Section snippets
Experimental
The device uses a flat rectangular coil (Fig. 2) through which a capacitor charged at high voltage (2–5 kV) is discharged. This discharge leads to the emerging of a very intense variable magnetic field. It induces high intensity eddy currents in the metallic particles which are moving in the immediate neighborhood of the coil were the magnetic field is strongly non-uniform and acts upon them with rejection forces. The advantage of the installation consists in the absence of moving parts (Rem,
Results
The recovery of the particles from the mixture can reach high value (∼90%) as can be seen in Fig. 5 where the graphs R=R(G) are presented for two kinds of particles (a) copper particles of 2–3 mm in diameter, and (b) iron particles of 1 mm in diameter in mixtures with plastic particles of the same size as the metallic particles, the machine parameter being the charging voltage of the capacitor.
Conclusions
Our results confirm that the new method for separating of the metallic particles from a mixture containing metallic and dielectric particles, is very efficient, the grade and recovery being very close to 100%. It is also possible to separate both ferro and non-ferro magnetic particles from dielectrics.
At the same time, the new device presents many advantages in comparison with the separators having moving parts, as with the drum E.C. separators.
References (3)
- Schubert, H., 1996. Aufbereitung fester Stoffe, Band II Sortierprozesse, DUG...
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