Int. J. Miner. Process. 93 (2009) 172–178

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Int. J. Miner. Process.

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Separation of small nonferrous particles using a two successive steps eddy-currentseparator with permanent magnets

Mihai Lungu ⁎Department of Physics, West University Timisoara, Blv. V. Parvan No. 4, Timisoara 300223, Romania

⁎ Tel.: +40 256592268; fax: +40 256592212.E-mail address: lmihai@physics.uvt.ro.

0301-7516/$ – see front matter © 2009 Elsevier B.V. Adoi:10.1016/j.minpro.2009.07.012

a b s t r a c t

a r t i c l e i n f o

Article history:Received 11 June 2008Received in revised form 17 April 2009Accepted 26 July 2009Available online 5 August 2009

Keywords:Eddy-current separatorNonferrousStrongly conducting particlesUndecided particlesPoorly conducting particlesSeparation efficiencyGradeRecovery

The paper presents a method for separating the small metallic nonferrous particles from two component metallicnonferrous mixtures using a new type of dynamic eddy-current separator with permanent magnets. The so-calledtwo successive steps eddy-current separator (TSECS) consists of a horizontal rotary drum covered with permanentmagnets, alternately N–S and S–N oriented. The separation process takes place in two stages, first the stronglyconducting particles are separated on the upper part of the drum, and then the remaining undecided and poorlyconducting particles are separated at the lower part of themagnetic drum. The experimental results and commentsregarding the values obtained for separation efficiency, gradeand recovery forwastes consisting inCu–PbandCu–Almixtures are given. The obtained results are in good agreement with the theoretical analysis.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

Eddy-current separation methods are used for both recovery andpurification of conductive nonferrous metals (Cu, Al, and Pb) and also forseparating various nonferrousmetals from each other (Rem 1999; Lungu2005).

In eddy-current separators, the eddy currents are induced in thenonferrousmetallic particles due to the changingmagnetic field in theactive zone of the separator. The interaction between these currentsand the magnetic field results into repulsive electrodynamic forces onthe metallic particles and so into their separation from nonconductiveones, or from each other (Rem et al., 1996; Schubert 1996).

Different devices can obtain the time dependent change of themagnetic field. Eddy-current dynamic separators with permanentmagnets represent a class of separators substantially improved in thelast few years, where the magnetic field is generated by machinery withmoving permanent magnets. By using permanent magnets, strong fieldscan be generated, therefore the operation costs of eddy-currentseparation are considerably reduced and the construction of theequipment can be less complex. The eddy-current separators have beendeveloped for the recovery of nonferrous metals frommixtures of wastematerials, i.e. shredded car scrap, granulated power cables andmunicipalsolidwastes (Vander Valk et al., 1986; Schubert 1996; Staude et al. 2002).

ll rights reserved.

Although eddy-current separation is used for a wide range ofparticle sizes and materials, different rotor designs are actually used,the specific choice depending on the feed characteristics and theseparation. Machines designed for small particles have many magnetpoles of small width, while machines for large particles have a fewlarge poles. Yet, the main problems associated with eddy-currentseparation are those referring to the selective separation of conduc-tive nonferrous particles smaller than 5 mm from nonconductive onesor one from each other, while reducing the cost of the separationprocess. These particles are difficult to recover with conventionaleddy-current separators of the rotary drum type, and are even harderto separate these fractions into the various alloys. The reason is that,for such particles eddy-current separation force (i.e. the tangentialforce that will be described in the theoretical part) producesacceleration less than the acceleration gravity. Therefore, the frictionalforces, which appear due to movement of the particles and acting inthe direction opposite to the tangential force, will tend to dominate.

Modern eddy-current separators used at present for the recoveryof nonferrous metals (Staude 1998; Rem 1999; Lungu and Schlett,2001) are the horizontal rotating drum (HDECS) types, where theactive part is a fast spinning drum covered with rows of permanentmagnets of alternating polarity, mounted parallel with the drum axis.A conveyor belt takes the particles over the drum and the conductiveparticles are accelerated so following themotion of the drum (Van derValk et al. 1986). The equipment of the HDECS is expensive; the mainproblems associated with these separators, and generally with eddy-current separation are those referring to the separation of conductive

Fig. 1. Active field (shaded zone in quadrant IV) used in the case of HDECS, a), activefield (shaded zone in quadrant III), used as supplementary in the case of TSECS, b).

173M. Lungu / Int. J. Miner. Process. 93 (2009) 172–178

nonferrous particles smaller than 5 mm, from nonconductive ones orone from each other. The conventional HDECS uses only a reducedzone of the active field near the rotor, the shaded zone placed inquadrant IV, as in Fig. 1a).

A possible solution to these problems might be the introduction ofthe tailing (undecided and the poorly conducting particles) again inthe separator at the lower part of the drum, in order to use also theactive separation zone, corresponding to the shaded zone placed inquadrant III, as in Fig. 1b).

This paper describes a new type of eddy-current separator withpermanent magnets, namely, the two-steps eddy-current separator(TSECS), designed for separation with a higher efficiency the mixturesof conductive nonferrous and nonconductive particles, or strongly con-ducting and poorly conducting nonferrous particles, smaller than 5 mm.

In this separator, the strongly conducting particles leave the separatorat the upper part of the drum (first step) and the tailing introduced at thelower part of the drum is subjected to a new separation process (secondstep), leading finally to an efficient separation.

Table 1Parameters of expressions (4), (5) and (6) for particles of several shapes and parallel (||) orperpendicular (⊥) orientations of their axis of symmetry with respect to the rotation axis ofthe magnet drum.

Shape c d γ

Sphere 140

D14

Cylinder ||116

D12

Cylinder ⊥ 364

D9D2

16L2

Disk ||112

δ2δ2

3D2

Disk ⊥ 164

D14

D stands for particle diameter, L for length and δ for thickness.

2. Theoretical considerations

Eddy currents are induced in a conductive nonferrous particle placedin the active zone of the separator as a response to the magnetic field,which change rapidly in time due to the rotation of the drum (Rem 1999;Lungu 2005). They are caused by Faraday's induction law and are devel-oping as a reaction to the fluctuations of the field observed by theparticle. The complex interactions between the magnetic field and theinduced eddy currents lead to the appearance of electrodynamic actionsupon conductive nonferrous particles and are responsible for the sepa-ration process.

When a particle moves through the magnetic field, it experienceschanges of size and orientation of the field due to its own translationaland rotational motions as well as due to the rotation of the drum. Formagnet drums with k poles and spinning with angular velocity ω, the

field outside the surface r=Rd of the drum can be approximated by itsfundamental mode (i.e., its first Fourier component) (Rem 1999):

B =BrBϕ

� �≅ b

Rd

r

� �k + 1 cos kðϕ−ωtÞsin kðϕ−ωtÞ

� �: ð1Þ

If the particle is sufficiently small with respect to the pole width λof the drum, where 2πRd=kλ, the variations of the drum field withinthe particle are smooth and the eddy-current distribution can betreated as a magnetic dipole. In this case, both the force F and thetorque T can be expressed in terms of the field gradient and themagnetic moment M of the particle (Rem et al., 1996):

F = ðM∇ÞB = Mx∇Bx + My∇By + Mz∇Bz ð2Þ

T = M × B; ð3Þ

where: M = 12∫V

r × jdV with j the current density of the eddy cur-

rent in the particle.If the global behavior of the particle'smagneticmoment is known, the

force and torque upon a conductive nonferrous particle can be estimatedby standard magnetic theory. Because these expressions have similarterms, there can bewritten relations between the two components of theelectrodynamic force and the torque. So, the component of the forcetangential to the magnet surface, Ft, and the component away from thesurface (i.e. the radial component), Fr, are (Rem 1999):

Ft =2πdλ

� �Td

Fr = sμ0ðω−ωpÞσd2Ft ; ð4Þ

withd the characteristic size of the particle,λ thewidth of a pair of poles, sa shape factor,ω the angular velocity of themagnetic field,ωp the angularvelocity of the particle, σ the electric conductivity of the particle, and µ0the magnetic permeability of vacuum. For the experiments described inthis paper, the dimensionless number sµ0(ω−ωp)σd2bb1, so thetangential force is dominant over the repulsive radial force (with aminimum computational effort resulting that Ft≈10Fr).

The torque T makes the particle spins in the same direction as themagneticfield,which rotates counterclockwise, if thefield is generatedbya clockwise rotation of the eddy-current rotor (if the rotation of the rotoris forward, the nonferrous particle will roll backward). This has the effectthat, as the particle rotation increases, i.e.ω→kωp, the field rotation asobserved by the particle slows down. As a result, also the forces acting onthe particle diminish as the particle approaches the rotor.

Fig. 2. Electrodynamic actions upon a metallic nonferrous particle.

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The expression of the torque is finally given by (Lungu and Rem,2002):

T = −c jB j2Vðω−ωpÞσd2 ð5Þ

where B is themagnetic induction at the position of the particle, V is thevolume of the particle and c is a factor depending on the shape andorientation of the particle, which is documented in Table 1 (Rem 1999).

For small to medium-sized particles the force T/d, which canpotentially be derived from the eddy-current torque, is larger than thetangential force that is conventionally used for the separation on aconventional rotary drum separator (i.e. the HDECS). Such a separator,the conversionof the torque toa linear force through frictionwitha solidsurface (i.e. the conveyor belt) is generally not efficient because it islimitedby the friction factor,whichbesides being small, is also subject torandom effects. The tangential force, which is always counteracting thefriction,maybe in caseof small particles, of the sameorder ofmagnitudeas the frictional force. In this case, the expressions (4) for the force showthat a substantial spin of theparticlewill reduce the eddy-current forces.This means that for small particles, in case of conventional rotary drumseparators, the spinning motion may limit the separation force. Thestrongly conducting particles will jump if the torque is sufficientlystrong. This is the so-called jump effect. In case of HDECS this effect is adisadvantage: the T/d factor makes the small particles jump up to earlyand fall close over the rotor into the reject, becoming an undecidedparticle, which leads to a decrease of the product purity. In the case ofthe TSECS this disadvantagewas avoided, due to the second stage of theseparation, realized at the lower part of the drum, in which the tailingintroduced at the lower part of the drum is subjected to a newseparation process, leading finally to an efficient separation.

All the electrodynamic actions upon ametallic nonferrous particlelying on the upper (quadrant IV) and lower (quadrant III) part of theseparation device (the TSECS) near the rotating magnets are shownin Fig. 2.

Therefore, wemust take into account the two steps of the separationprocess:

Step 1, consisting in a conventional separation, similar to HDECS(quadrant IV): assuming that the particle approaches the rotorwith a

Table 2Density, conductivity and separation factors for materials used in the experimentsdescribed in this paper.

Material ρ [kg/m3] σ [1/m Ohm] σ/ρ

Aluminum 2700 27·106 10000Copper 8900 56·106 6300Lead 11400 5·106 440

constant radial velocity ur, we can integrate the torque in Eq. (4) toget its speed of rotation at any point near the rotor (Lungu and Rem,2002):

ωp = −kω½1− expð−u0 = urÞ�; where : u0 =γ jB j2d2k + 1

σρ

ð6Þ

with the shape and orientation factor γ documented in Table 1. Sincethe tangential force and the torque on the particle are strongly relatedaccording to Eq. (4), the result forωp also gives an approximate resultfor the tangential forces on the particle as it approaches the rotor:

Ft = −kðk + 1ÞmωjB j2r

½1− expð−u0 = urÞ�: ð7Þ

The final deviation of a particle depends on the tangential force Ftand torqueT, aswell as on thedeflection causedbydifferent interactionsbetween particles. The spinning of the strongly conducting particle isresponsible, in part, for the separation, as explained before, they canbecome undecided particles and fall at the lower part of the drum.

Step two, which refers to the separation of undecided and poorlyconducting particles at the lower part of the separator (quadrant III).The resulting tangential force is perpendicular to the flux anddirection of induced eddy currents. The magnitude of the resulting

Fig. 3. Computed values of separation force Ft in quadrant III versus drum revolution nfor Al, Cu and Pb functions of particles properties.

Fig. 4. Magnetization curve for a NdFeB magnet.

Fig. 6. Particle's trajectory in TSECS.

175M. Lungu / Int. J. Miner. Process. 93 (2009) 172–178

force (separation force) depends on several variables summarized inEq. (8) (Lungu 2005; FEMP, 2008):

Ft =12mðω−ωpÞ

jB jr

2cσρ

Q ð8Þ

with Q the quality factor, in this case takes the values between 0.35and 0.5.

Fig. 5. The TSECS: photo with a front view a), outline with lateral view b).

Therefore, in order to assure a separation as good as possible must

correlate the drum revolution n which gives the factor (ω−ωp) withrespect of the particles properties, namely the material factor σ/ρ (theseparation factor). In this sensewe computed Eq. (8) considering that theparticles are spherical with the same diameter d=2.5 mm, B=1 T,r=0.5 cm, factor c documented in Table 1 and separation factorσ/ρ fromTable 2 (Lungu2008). Fig. 3 represents the separation forces forAl, Cu andPb versus the drum revolution as percentages reported to the maximumvalue of separation force for aluminum. For all threematerials on observethat separation force increaseswith thedrumrevolution, aswasexpected,obtain a maximum, then decreases. A higher vale of drum revolutionimplies also an increased rotation of the particles, so that a maximumvalueof separation force results at an intermediatevalueof rotor speed fora type of material.

Values of the separation factor for Al, Cu and Pb are given in Table 2(Lungu 2005).

3. Engineering and operation

In order to increase the separation efficiency we have built a newdevice. Unlike the traditional solution, in our device the feeding oftailing (undecided and poorly conducting particles) takes place as asecond stage of the separation process at the lower part of the drum.

Fig. 7. Cu–Pb mixture.

176 M. Lungu / Int. J. Miner. Process. 93 (2009) 172–178

The separator consists of a horizontal spinning drum, made of softmagnetic steel and covered with rows of permanent NdFeB magnetswith a remanent induction Br=1.08 T, and magnetization curve as inFig. 4, alternately N–S and S–N oriented. The length of the magnets is4 cm and the drum diameter is 18 cm, which revolution can be variedfrom 0 to 3500 rot/min.

Fig. 5a) presents a photo with the separator (front view) and theprinciple outline of TSECS is presented in Fig. 5b), as a lateral view.

The electrical DC engine rotates the magnetic drum through amechanical device (gear) at the same values of rotation speed betweenhis axis and the drum, the values established by modifying the outputvoltage of the power supply (0–80 V, 50 A). The tachogeneratormounted directly on the axis of the DC engine measures and thetachometer displays the values of the rotor speed in rot/min.

The separation process takes place at room temperature in rather dryconditions, at the atmospheric relative humidity and depends on thephysical properties of theparticles subjected to the separationprocess, i.e.,

Fig. 8. Separation parameters versus drum revolution n diagrams for Cu, considered asstrongly conductor, for only one a) and two successive steps b).

the separation factorσ = ρ, shape, dimensions and their proportions in thewaste.

Under the combined actions of electromagnetic interactions, gravita-tional and frictional forces, the particles of the feed material describevarious trajectories, depending on their physical properties. The feedmaterial is introduced into the active zone of the field by a feedingvibratory plane over the rotary drum and the separation process takesplace in two successive steps. Thus, the strongly conducting particles areseparated by tailing (the undecided and poorly conducting particles) onthe upper part of the drum and leave the separator in first step. Then theremaining undecided and poorly conducting particles, introduced againby another vibratory feeding plane at the lower part of the magneticdrum, in a second step, leave the lower part of the separator on differentways, leading finally to an efficient separation, as in Fig. 6.

The drum revolution n is an important factor, which determines theseparation quality. Therefore, that parameter must correlate for eachmixture in order to obtain a maximum efficiency of separation process.

Fig. 9. Separation parameters versus drum revolution n diagrams for Pb, considered aspoorly conductor, for only one a) and two successive steps b).

177M. Lungu / Int. J. Miner. Process. 93 (2009) 172–178

4. Experimental results

After each separation, the collected products were measured using adigital balance (precision=0.01 g). The performance of the experimentaldevice was estimated using three separation parameters:

1. Grade G or purity, defined as ratio of the mass of a material i in theproduct and the product as a whole:

G ð%Þ = mic

mtc

:100 ð9Þ

with mic: mass of the material i collected in the compartmentreserved for it,

mtc: total mass of the product collected in that compartment.

2. Recovery R, defined as ratio of the mass of the material i in theproduct and the mass of that same material in the feed:

R ð%Þ = mic

mit

:100 ð10Þ

with mit as the total mass of material i at the inlet of the separator(feed).

3. Separation efficiency SE that provides more flexibility to evidence theefficiencyof separation stages, defined for this separator as adifferencebetween recovery and product impurity I (Kawatra and EiseleTimothy, 2001; Fourie, 2007):

SEð%Þ = Rð%Þ−Ið%Þ

Ið%Þ = mjc

mtc

:100ð11Þ

with mjc=mtc−mic mass of unwanted material j in the product(compartment designed for component i). Finally, to obtain that:

SEð%Þ = Rð%Þ−½100−Gð%Þ� ð12Þ

Experimentally, these quantities have been determined for twotypes of electro technical wastes, namely:

Fig. 10. Cu–Al mixture.

Mixture A:

Cu and Pb mixture containing particles of irregular shapes anddimensions between2.5–4 mm, as in Fig. 7. Theproportions are 70%Cu to30% Pb.

Fig. 8 shows the separation parameters versus drum revolution ndiagram for Cu, considered as strongly conductor, for only one a) andtwo successive steps b) of separation process, respectively. Fig. 9shows the diagrams for Pb, considered as poor conductor in thismixture, for the same situations.

Mixture B:

Cu and Al mixture containing particles of irregular shapes and dimen-sions between2–5 mm, as in Fig. 10. Theproportions are 40%Cu to60%Al.

Fig. 11 shows the separation parameters versus drum revolution ndiagram for Al, considered as strongly conductor, for only one a) andtwo successive steps b) of separation processes, respectively. Fig. 12shows the diagrams for Cu, considered as poor conductor in thismixture, in the same situations.

Fig. 11. Separation parameters versus drum revolution n diagrams for Al, considered asstrongly conductor, for only one a) and two successive steps b).

Fig. 12. Separation parameters versus drum revolution n diagrams for Cu, considered aspoorly conductor, for only one a) and two successive steps b).

178 M. Lungu / Int. J. Miner. Process. 93 (2009) 172–178

5. Discussion

The separation experimental diagrams show that in all casesseparation efficiency increases with the increasing rotor speed, achievea maximum approximately at the intersection of Grade–Recoverydiagrams, then decreases with the rotor speed. As was expected, all thethree parameters achieve higher values in the case of the two-stepseparation processes and a good correlation between experimental andtheoretical results was obtained.

In the case of the Cu–Pbmixture, with the increasing rotor speed anincreasing amount of copper particles are able to jump to the level of theupper product while the lead particles remain on the tailing. At arotation speed of 3000 rpm, the lead particles start to contaminate theupper product, while the recovery of copper in the upper product is at amaximum at a somewhat higher speed of 3500 rpm. It is clear that atstill higher speeds, the lead particles reach a maximum grade as well. Asimilar behavior is apparent from the data for the aluminum–copper

mixture, with aluminum reaching the upper product level first. A 100%grade means that the product contains only the useful material (i.e.copper in the case of Mixture A, or aluminum in the case of Mixture B),and a 100% recovery means that all of that material ends up in theproduct. Therefore, a compromisemust be struck between the purity oftheproduct,whichwill set the price/ton, and the amount of the product,compromiseput into evidenceby themaximumof separation efficiency.

The data show that the order inwhich thematerials reach the upperproduct level was well predicted by the theoretical qualitative analysisgiven above. Using the separation diagram of a separation process, toselect the economic optimum, the experimental results show that themaximum separation accuracy is obtained at an intermediate value ofthe drum revolution (i.e. at n=2500–3000 r/min for Cu in the case ofMixture A, Fig. 8). At high values of the drum revolution, which implyhigh values of the electrodynamic force Ft and torque T, the stronglyconducting particles are strongly repelled. They can hit the poorlyconducting particles and modify their trajectory, thus, a fraction of thestrongly conducting as well as poorly conducting particles become asundecided particles and must passed again to a separation process (thesecond step). This effect turned out to be useful, because for increasingthe efficiency of the separation process one does not need high values ofthe drum revolution, which in fact can be very dangerous.

6. Conclusions

The TSECS successfully separates wastes containing conductivenonferrous and nonconductive particles or strongly conducting andpoorly conducting nonferrous particles, respectively. Themain advantageof this solution is that the separation takes place in two successive stages,a fact which increases the separation accuracy. Another advantage is thatthe feed can be brought closer to the rotor, so that less of the high fieldregion is lost. Besides its high efficiency is close to the one of the usualdynamic eddy-current separators (i.e. the HDECS), the TSECS has also theadvantage of a low cost due to the short length of the magnets andabsences of the conveyor belt and the pulley. A disadvantage is that byusing this design, the strongly and poorly conducting particles can bedifficult to collect in the different regions, and the mix product must bepassed again to a new separation process, also the productivity is lowerthan in the case of HDECS. On the other hand, a good correlation betweenexperimental and theoretical results was obtained.

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