vertical drum eddy-current separator with permanent magnets

Ž .Int. J. Miner. Process. 63 2001 207–216www.elsevier.comrlocaterijminpro

Vertical drum eddy-current separator withpermanent magnets

Mihai Lungu), Zeno SchlettDepartment of Physics, West UniÕersity Timisoara, BÕ. V. ParÕan 4 Timisoara 1900, Romania

Received 24 July 2000; received in revised form 19 March 2001; accepted 3 April 2001

Abstract

This paper presents a new vertical type of dynamic eddy-current separator, intended to be usedfor separating small conductive non-ferrous particles, having dimensions of about 2–8 mm. This

Ž .separator, the so-called Õertical drum eddy-current separator VDECS consists of a verticalspinning drum covered with permanent magnets, alternatively N–S and S–N orientated, directlyfixed on the axis of an electric engine. The particles to be separated are brought into the field onan oblique trajectory, hit a shield which surrounds the drum and achieve a supplementarydeflection. To increase the separation efficiency, high values of the drum revolution are notneeded, but an appropriate arrangement of the separation parameters is. The results of grade andrecovery for some types of wastes consisting of mixtures of different small conductive non-ferrousand conductive–non-conductive particles are given. Comparative to these results, the values ofgrade and recovery obtained for the same types of wastes using a horizontal drum eddy-current

Ž .separator HDECS , designed to separate millimetric particles, are given. The advantages of theVDECS lie in fact that the efficiency is close to the one of the HDECS, and the cost of theequipment is lower. The disadvantages are that the intermediate product must be passed againthrough a separation process. q 2001 Elsevier Science B.V. All rights reserved.

Keywords: eddy current; spinning drum; non-ferrous; separation; grade; recovery

1. Introduction

ŽEddy-current separation methods are used for the recovery of non-ferrous metals Cu,.Al, Pb, Zn from solid wastes, and also for separating various non-ferrous metals one

from each other. These methods rely on the fact that eddy currents are induced in theconductive non-ferrous particles due to the changing of the magnetic field in such a

) Corresponding author. Fax: q40-56-190-333.Ž .E-mail address: [email protected] M. Lungu .

0301-7516r01r$ - see front matter q2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0301-7516 01 00047-3

( )M. Lungu, Z. Schlettr Int. J. Miner. Process. 63 2001 207–216208

separator. Interaction between eddy currents and the magnetic field results in electrody-namic forces upon the conductive particles, and hence different trajectories for theseparticles and the non-conductive ones. Eddy-current dynamic separators with permanentmagnets represent a class of separators substantially improved in the last 10 years, wherethe magnetic field is generated by machinery with moving permanent magnets.

Modern eddy-current separators used at present for recovery of non-ferrous metalsŽ .Rem et al., 1996; Rem, 1999; van der Valk et al., 1986 are the horizontal drum

Ž .eddy-current separator HDECS types, where the permanent magnets are placed,alternatively N–S and S–N orientated around the drum, parallel to its axis. A conveyorbelt takes the particles over the drum and the conductive particles are accelerated,

Žfollowing the motion of the drum Leest et al., 1995; Rem et al., 1996; van der Valk et.al., 1988 . The equipment of the HDECS is expensive and the main problems associated

with these separators, and generally with eddy-current separation, are those refering tothe separation of conductive non-ferrous particles smaller than 5 mm, from non-conduc-tive ones or one from each other. A solution to these problems might be a verticalorientation of the magnetic drum.

This paper describes a new vertical type of eddy-current dynamic separator, namelyŽ .the vertical drum eddy-current separator VDECS which consists of a vertical spinning

drum covered with NdFeB permanent magnets, alternatively N–S and S–N orientatedŽ .Schlett and Lungu, 1999 , as shown in Fig. 1.

The purpose was to realize an eddy-current separator with a higher efficiency, inorder to reduce the cost of the separation equipment, able to separate small non-ferrous

Ž .particles dimensions of about 2–8 mm from non-conductive or from other conductivenon-ferrous particles.

Unlike the HDECS, where the length of the magnets is about tens of centimeters,equal to the active width of the conveyor belt, in case of the VDECS, the magnets areonly a few centimeters long, this being possible by the vertical positioning of the drum.The particles to be separated are brought into the field with a certain velocity on anoblique trajectory in the horizontal plane, as well as in the vertical one. They hit theplastic surface which surrounds the drum, and thus a supplementary deflection isrealized. The dielectric particles are reflected and fell. The metallic particles suffer thecombined effect of the deflection caused by the collision, and the one resulting from the

Fig. 1. The vertical spinning drum.

( )M. Lungu, Z. Schlettr Int. J. Miner. Process. 63 2001 207–216 209

interaction between the eddy currents and the field. Consequently, they fell at a largerdistance as the dielectric particles do.

2. Theoretical considerations

As in the case of all dynamic eddy-current separators, for the VDECS, the fluctuatingfield of the spinning drum induces eddy currents in conductive non-ferrous particlesmoving close to the drum. Eddy currents in conductive non-ferrous particles movingthrough the inhomogeneous magnetic field are caused by Faraday’s induction law, andare induced in a particle as a response to a magnetic field which changes rapidly in timeŽ .Schloemann, 1975; van der Beek et al., 1995 . In fact, eddy currents develop as a

Ž .reaction to the fluctuations of the field observed by the particle Rem, 1999 .While a particle moves through the magnetic field, it experiences changes of size and

orientation of the field due to its translational and rotational motion. The changes of themagnetic field acting on a particle moving close to the VDECS rotary drum are causedby two different motions: rotation of the drum and translational and rotational motion of

Ž .the particle in the active zone of the magnetic field Rem et al., 1996; Rem, 1999 .Upon a conductive particle acts the Lorentz force, i.e. the electrodynamic force of the

magnetic field on eddy currents inside the particle. The Lorentz force on a small volumeŽdV of a conductive particle carrying a current density j in a magnetic field B is Rem,

.1999 :

fs j=BdV . 1Ž .For a known current density distribution inside the particle, the force upon it results by

Ž .integrating Eq. 1 :

Fs fdV . 2Ž .HV

Ž .In the meantime, the field exerts on a metallic particle the torque Rem, 1999 :

Ts r= fdV , 3Ž .HV

where r is the coordinate vector relative to the center of mass particle.If the particle is sufficiently small, it can be treated as a magnetic dipole and the

variations of the applied field within the particle are small. In this case, both the force Fand the torque T can be expressed in terms of the field gradient and the magnetic

Ž .moment M of the particle Rem et al., 1996; Rem, 1999 :

Fs M= BsM =B qM =B qM =B 4Ž . Ž .x x y y z z

TsM=B, 5Ž .where

1Ms r= jdV .H

2 V


The electrodynamic separation forces, i.e. the radial and tangential components ofŽforce F, as well as the torque upon a conductive particle near the drum are Braam et al.,

.1988; Rem, 1999; Schloemann, 1975; van der Valk et al., 1986 :

F symkÕr r

F smk vyV R"ÕŽ .t t

lTs F , 6Ž .t2p

with F —the radial component of the electrodynamic force F; F —the tangentialr t

component of the electrodynamic force F; m—the mass of the particle; Õ —the radialr

component of the velocity of the incident particle; Õ —the tangential component of thet

velocity of the incident particle; v—the angular drum velocity; V—the angular velocityŽ .of the particle; vyV —the angular velocity of the separator field; R—the radius of

Ž .the particle; l—the period of magnetization width of a pair poles ; and k a factor givenŽ .by van der Valk et al., 1988 :

1 s 2ks S =B . 7Ž . Ž .2 r

Ž .In Eq. 7 , srr is the separation factor, s and r are the electric conductivity andmass density of the particle, respectively, S is a shape factor depending on the shape anddimensions of the particle and =B is the flux intensity gradient in the active zone of thefield.

One can observe that the separation process depends strongly on the separation factorsrr. Values of this factor for some materials are given in Table 1.

With respect to the tangential component F , the radial component F is virtuallyt r

negligible because Õ is relatively small, and it changes sign after the particle hits ther

shield.The deviation of a particle depends on the tangential component F and torque Tt

Ž .given by Eq. 6 , as well as on the deflection caused by the collision with the shieldwhich surrounds the drum, and on different interactions between the particles. Thespinning of the metallic particles is responsable, in part, for the separation: in collisionwith the shield, a conductive particle having a larger separation factor srr bounces

Table 1The separation factor sr r for some materials

3 2Ž .Material sr r=10 m rV kg

Aluminium 13.1Copper 6.6Zinc 2.4Brass 1.7Lead 0.4


Fig. 2. Magnetic field lines and finite element mesh.

more violently than a non-conductive one or a particle having a smaller separationfactor.

So, in order to assure a separation which is as good as possible, the values andorientation of the particles’ velocities must be correlated with the drum revolution.

For a better understanding of the separation process, the optimal distribution of themagnetic field around the drum was computed using the finite element programmeQuick Field. The finite element mesh and the magnetic field lines are shown in Fig. 2.For symmetry reasons, only a half of the drum is presented.

3. Engineering and functioning of the VDECS

The principal outline of the vertical drum eddy-current separator is given in Fig. 3.Drum 1 is made of weak-magnetic steel, covered with 18 NdFeB magnets with remanent

Fig. 3. Side view of the VDECS.


Fig. 4. Top view of the VDECS.

flux density B s1.08 T and the dimensions 40=20=10 mm, alternatively N–S andr

S–N orientated. The drum is directly fixed on the axis 7 of the electric engine 2. Therevolution n of engine 2 can be modified from 0 to 4500 miny1 using the voltagesupply 4. The drum and the electric engine are covered with a cylindrical plastic shield6, which has a minimal thickness in the active zone of the magnetic field.

The material to be separated is brought from feeder 5 through feeding pipe 3. Thepipe is inclined with the angle a with respect to the horizontal plane and its lower endis at the distance d from shield 6.

The horizontal incident angle b of the feeding pipe with respect to the surface of theŽ .drum Fig. 4 can also be modified.

The angles a and b and the distance d are set after successive tests for a certain typeof waste. These separation parameters are responsable for the size and direction of the

Ž .particle velocities near the drum in the separation zone the active zone of the field .As it has been found out experimentally, the angle b and the drum revolution n are

the most important parameters in order to determine the best working conditions of theVDECS for a certain type of waste after the values for angle a and distance d havebeen set.

Table 2y1Ž .n min bs158 bs228 bs308 bs458

( ) ( )A Grade of Cu %3000 82 87 85 803500 88 92 89 854000 90 93 92 894500 87 90 88 87

( ) ( )B RecoÕery of Cu %3000 75 82 79 683500 80 87 84 754000 84 87 86 794500 78 81 73 67



( ) ( )A Grade of Cu %3000 70 78 58 503500 76 83 62 574000 85 92 72 654500 82 88 68 62

( ) ( )B RecoÕery of Cu %3000 65 72 54 463500 70 77 57 534000 78 84 66 594500 76 81 63 57

Ž Ž ..In the active zone of the field, due to the electrodynamic forces Eq. 1 andcollisions with shield 6, the conductive particles which have a larger separation factor

Ž .srr fall into compartment III Fig. 4 of the collecting recipient, and the dielectricparticles or particles which have a lower separation factor fall into compartment I of thesame recipient. Compartment II was designed for the intermediate product, consisting ina compound containing both types of particles. The intermediate product is passed againthrough the separator.

The distances d and d are determined after successive tests, depending on the1 2

waste type to be separated, so that the material collected in compartment II shouldcontain particles in a proportion as close as possible to the one of the feed material.Thus, the intermediate product can be passed again through the separator, without a newadjustment of the separation parameters being necessary.

4. Experimental results

ŽIn the following, the obtained experimental values of grade G ratio between massfraction of a material in the product, i.e. the whole quantity of material collected in one


( ) ( )A Grade of Pb %3000 43 56 25 133500 52 62 30 254000 65 76 56 454500 60 70 45 35

( ) ( )B RecoÕery of Pb %3000 49 62 38 253500 59 75 35 274000 75 86 52 434500 70 82 50 40



( ) ( )A Grade of Cu %2300 37 52 25 222800 45 60 30 253300 60 82 42 353800 50 65 32 28

( ) ( )B RecoÕery of Cu %2300 30 43 20 182800 38 50 25 203300 50 68 35 303800 42 51 27 24

. Žof the useful compartments I or III, and the product and recovery R ratio between mass.fraction of a material in the product and mass fraction of the same material in the feed

are given for some types of electrotechnical wastes described below.Ž .A A mixture containing PVC material particles with an irregular shape and

dimensions between 4 and 6 mm, and copper wires with a diameter of 4 mm and lengthsbetween 2 and 6 mm. Both plastic and copper are in the same proportion, i.e. 50% Cuand 50% PVC. The geometry of the system has been adjusted in order to gather theplastic material in compartment I and copper in compartment II, which was designed forthe concentrate product. The values of grade and recovery for Cu collected in compart-ment II are given in Table 2A and B, respectively.

Ž .B Cu–Pb mixture containing Cu wires with diameters between 1 and 2 mm andlengths between 2 and 6 mm, and Pb particles of irregular shapes and dimensionsbetween 2 and 6 mm. The proportions are 60% Cu to 40% Pb. The values of grade andrecovery for Cu collected in compartment III are given in Table 3A and B, respectively.The values of grade and recovery for Pb collected in compartment I are given in Table4A and B, respectively.

Ž .C Cu–Al mixture containing Cu wires with diameters of 2 mm and lengths between6 and 8 mm, and Al particles of irregular shapes and dimensions between 2 and 8 mm.


( ) ( )A Grade of Al %2300 83 85 81 802800 85 88 82 813300 88 92 84 833800 86 89 82 81

( ) ( )B RecoÕery of Al %2300 87 90 84 832800 88 92 85 843300 91 96 88 863800 89 93 86 84


The proportions are 20% Cu to 80% Al. The values of grade and recovery for Cucollected in compartment I are given in Table 5A and B, respectively. The values ofgrade and recovery for Al collected in compartment III are given in Table 6A and B,respectively.

Ž . Ž .In cases B and C , the intermediate product collected in compartment II, consistingin mixtures of particles in a proportion close to the one of the feed material, was passedagain through a new separation process.

5. Conclusions

The experimental results show that for a given value of the incident angle b , themaximum separation grade is obtained at an intermediate value of the drum revolutionŽ y1 .e.g. ns4000 min for Cu–Pb mixture . This, because at high values of the drumrevolution, which imply high values of the electrodynamic force F and torque T , thet

strongly conducting particles are strongly repelled. They can collide with poorlyconducting particles and modify their trajectory. Thus, a fraction of the stronglyconducting, as well as poorly conducting, particles falls into compartment II. This effectturned out to be useful, because for increasing the efficiency of the separation process,high values of the drum revolution, which in fact can be very dangerous, are not needed;however, a good arrangement of the system geometry, especially of the incident angle b

is needed. By a proper arrangement of the distances d and d , according to the material1 2

to be separated, the intermediate product collected in compartment II contains particlesin a close proportion to the one of the feed material. This made possible the passingagain of the intermediate product through the separator, without being necessary a newarrangement of the system geometry.

Table 7Ž .Comparative results between the VDECS bs228 and the HDECS

Ž .A

Ž .Cu–Pb mixture case B VDECS HDECSy1 y1ns4000 min ns4500 min

Ž . Ž . Ž . Ž .G % R % G % R %

Cu 92 84 95 87Pb 76 86 80 88

Ž .B

Ž .Cu–Al mixture case C VDECS HDECSy1 y1ns3300 min ns4200 min

Ž . Ž . Ž . Ž .G % R % G % R %

Cu 82 68 85 70Al 92 96 95 92


The advantages of the VDECS compared to the HDECS are of economical andtechnical nature. The existence of a single electric engine, shorter magnets and theabsence of the conveyor belt ensure a lower cost of the separation equipment of theVDECS. In the meantime, the efficiency of the VDECS is close to the one obtained witha HDECS designed for the separation of millimetric particles. A comparison between thebest results obtained with both types of separators is given in Table 7A for Cu–Pb

Ž . Ž .mixture case B , and in Table 7B for Cu–Al mixture case C , respectively.Ž .Those values n, b in the case of the VDECS and n in the case of the HDECS have

been chosen, for which G and R are optimals.The disadvantages of the VDECS are referring to the intermediate product collected

in compartment II, which must be passed again through a separation process.

Acknowledgements

The authors wish to acknowledge helpfull discussions with Dr. P.C. Rem from theTU Delft, the Netherlands and Dr. I. Hrianca, from the Department of Physics, WestUniversity Timisoara, Romania.

References

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vertical drum eddy-current separator with permanent magnets

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