Vertical drum eddy-current separator with permanent magnets

Int. J. Miner. Process. 63 Ž2001. 207–216 www.elsevier.comrlocaterijminpro

Vertical drum eddy-current separator with permanent magnets Mihai Lungu ) , Zeno Schlett Department 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 used for separating small conductive non-ferrous particles, having dimensions of about 2–8 mm. This separator, the so-called Õertical drum eddy-current separator ŽVDECS. consists of a vertical spinning drum covered with permanent magnets, alternatively N–S and S–N orientated, directly fixed on the axis of an electric engine. The particles to be separated are brought into the field on an oblique trajectory, hit a shield which surrounds the drum and achieve a supplementary deflection. To increase the separation efficiency, high values of the drum revolution are not needed, but an appropriate arrangement of the separation parameters is. The results of grade and recovery for some types of wastes consisting of mixtures of different small conductive non-ferrous and conductive–non-conductive particles are given. Comparative to these results, the values of grade and recovery obtained for the same types of wastes using a horizontal drum eddy-current separator ŽHDECS., designed to separate millimetric particles, are given. The advantages of the VDECS lie in fact that the efficiency is close to the one of the HDECS, and the cost of the equipment is lower. The disadvantages are that the intermediate product must be passed again through a separation process. q 2001 Elsevier Science B.V. All rights reserved. Keywords: eddy current; spinning drum; non-ferrous; separation; grade; recovery

1. Introduction Eddy-current separation methods are used for the recovery of non-ferrous metals ŽCu, 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 the conductive non-ferrous particles due to the changing of the magnetic field in such a )

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

0301-7516r01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 1 - 7 5 1 6 Ž 0 1 . 0 0 0 4 7 - 3

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separator. Interaction between eddy currents and the magnetic field results in electrodynamic forces upon the conductive particles, and hence different trajectories for these particles and the non-conductive ones. Eddy-current dynamic separators with permanent magnets represent a class of separators substantially improved in the last 10 years, where the magnetic field is generated by machinery with moving permanent magnets. Modern eddy-current separators used at present for recovery of non-ferrous metals ŽRem et al., 1996; Rem, 1999; van der Valk et al., 1986. are the horizontal drum eddy-current separator ŽHDECS. types, where the permanent magnets are placed, alternatively N–S and S–N orientated around the drum, parallel to its axis. A conveyor belt takes the particles over the drum and the conductive particles are accelerated, following the motion of the drum ŽLeest 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 to the separation of conductive non-ferrous particles smaller than 5 mm, from non-conductive ones or one from each other. A solution to these problems might be a vertical orientation of the magnetic drum. This paper describes a new vertical type of eddy-current dynamic separator, namely the vertical drum eddy-current separator ŽVDECS. which consists of a vertical spinning drum covered with NdFeB permanent magnets, alternatively N–S and S–N orientated ŽSchlett and Lungu, 1999., as shown in Fig. 1. The purpose was to realize an eddy-current separator with a higher efficiency, in order to reduce the cost of the separation equipment, able to separate small non-ferrous particles Ždimensions of about 2–8 mm. from non-conductive or from other conductive non-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 are only 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 an oblique trajectory in the horizontal plane, as well as in the vertical one. They hit the plastic surface which surrounds the drum, and thus a supplementary deflection is realized. The dielectric particles are reflected and fell. The metallic particles suffer the combined effect of the deflection caused by the collision, and the one resulting from the

Fig. 1. The vertical spinning drum.

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

2. Theoretical considerations As in the case of all dynamic eddy-current separators, for the VDECS, the fluctuating field of the spinning drum induces eddy currents in conductive non-ferrous particles moving close to the drum. Eddy currents in conductive non-ferrous particles moving through the inhomogeneous magnetic field are caused by Faraday’s induction law, and are induced in a particle as a response to a magnetic field which changes rapidly in time ŽSchloemann, 1975; van der Beek et al., 1995.. In fact, eddy currents develop as a reaction to the fluctuations of the field observed by the particle ŽRem, 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 the magnetic field acting on a particle moving close to the VDECS rotary drum are caused by two different motions: rotation of the drum and translational and rotational motion of the particle in the active zone of the magnetic field ŽRem 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 volume dV of a conductive particle carrying a current density j in a magnetic field B is ŽRem, 1999.: f s j = BdV .

Ž 1.

For a known current density distribution inside the particle, the force upon it results by integrating Eq. Ž1.: F s fdV .

HV

Ž 2.

In the meantime, the field exerts on a metallic particle the torque ŽRem, 1999.: T s r = fdV ,

HV

Ž 3.

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 F and the torque T can be expressed in terms of the field gradient and the magnetic moment M of the particle ŽRem et al., 1996; Rem, 1999.: F s Ž M= . B s M x=B x q M y=B y q M z=Bz

Ž 4.

T s M = B,

Ž 5.

where Ms

1 2

HVr = jdV .

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The electrodynamic separation forces, i.e. the radial and tangential components of force F, as well as the torque upon a conductive particle near the drum are ŽBraam et al., 1988; Rem, 1999; Schloemann, 1975; van der Valk et al., 1986.: Fr s ymkÕr Ft s mk Ž v y V . R " Õt

l Ts

2p

Ft ,

Ž 6.

with Fr —the radial component of the electrodynamic force F; Ft —the tangential component of the electrodynamic force F; m—the mass of the particle; Õr —the radial component of the velocity of the incident particle; Õt —the tangential component of the velocity of the incident particle; v —the angular drum velocity; V —the angular velocity of the particle; Ž v y V . —the angular velocity of the separator field; R—the radius of the particle; l —the period of magnetization Žwidth of a pair poles.; and k a factor given by Žvan der Valk et al., 1988.: ks

1 s 2 r

2

S Ž =B . .

Ž 7.

In Eq. Ž7., srr is the separation factor, s and r are the electric conductivity and mass density of the particle, respectively, S is a shape factor depending on the shape and dimensions of the particle and =B is the flux intensity gradient in the active zone of the field. One can observe that the separation process depends strongly on the separation factor srr . Values of this factor for some materials are given in Table 1. With respect to the tangential component Ft , the radial component Fr is virtually negligible because Õr is relatively small, and it changes sign after the particle hits the shield. The deviation of a particle depends on the tangential component Ft and torque T given by Eq. Ž6., as well as on the deflection caused by the collision with the shield which surrounds the drum, and on different interactions between the particles. The spinning of the metallic particles is responsable, in part, for the separation: in collision with the shield, a conductive particle having a larger separation factor srr bounces

Table 1 The separation factor s r r for some materials Material

s r r =10 3 Žm2 r V kg.

Aluminium Copper Zinc Brass Lead

13.1 6.6 2.4 1.7 0.4

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

more violently than a non-conductive one or a particle having a smaller separation factor. So, in order to assure a separation which is as good as possible, the values and orientation of the particles’ velocities must be correlated with the drum revolution. For a better understanding of the separation process, the optimal distribution of the magnetic field around the drum was computed using the finite element programme Quick 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.

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Fig. 4. Top view of the VDECS.

flux density Br s 1.08 T and the dimensions 40 = 20 = 10 mm, alternatively N–S and S–N orientated. The drum is directly fixed on the axis 7 of the electric engine 2. The revolution n of engine 2 can be modified from 0 to 4500 miny1 using the voltage supply 4. The drum and the electric engine are covered with a cylindrical plastic shield 6, 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. The pipe is inclined with the angle a with respect to the horizontal plane and its lower end is at the distance d from shield 6. The horizontal incident angle b of the feeding pipe with respect to the surface of the drum ŽFig. 4. can also be modified. The angles a and b and the distance d are set after successive tests for a certain type of waste. These separation parameters are responsable for the size and direction of the particle velocities near the drum in the separation zone Žthe 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 the VDECS for a certain type of waste after the values for angle a and distance d have been set. Table 2 n Žminy1 .

b s158

b s 228

b s 308

b s 458

(A) Grade of Cu (%) 3000 3500 4000 4500

82 88 90 87

87 92 93 90

85 89 92 88

80 85 89 87

(B) RecoÕery of Cu (%) 3000 75 3500 80 4000 84 4500 78

82 87 87 81

79 84 86 73

68 75 79 67

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Table 3 n Žminy1 .

b s158

b s 228

b s 308

b s 458

(A) Grade of Cu (%) 3000 3500 4000 4500

70 76 85 82

78 83 92 88

58 62 72 68

50 57 65 62

(B) RecoÕery of Cu (%) 3000 65 3500 70 4000 78 4500 76

72 77 84 81

54 57 66 63

46 53 59 57

In the active zone of the field, due to the electrodynamic forces ŽEq. Ž1.. and collisions with shield 6, the conductive particles which have a larger separation factor srr fall into compartment III ŽFig. 4. of the collecting recipient, and the dielectric particles or particles which have a lower separation factor fall into compartment I of the same recipient. Compartment II was designed for the intermediate product, consisting in a compound containing both types of particles. The intermediate product is passed again through the separator. The distances d1 and d 2 are determined after successive tests, depending on the waste type to be separated, so that the material collected in compartment II should contain 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 new adjustment of the separation parameters being necessary. 4. Experimental results In the following, the obtained experimental values of grade G Žratio between mass fraction of a material in the product, i.e. the whole quantity of material collected in one

Table 4 n Žminy1 .

b s158

b s 228

b s 308

b s 458

(A) Grade of Pb (%) 3000 3500 4000 4500

43 52 65 60

56 62 76 70

25 30 56 45

13 25 45 35

(B) RecoÕery of Pb (%) 3000 49 3500 59 4000 75 4500 70

62 75 86 82

38 35 52 50

25 27 43 40

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Table 5 n Žminy1 .

b s158

b s 228

b s 308

b s 458

(A) Grade of Cu (%) 2300 2800 3300 3800

37 45 60 50

52 60 82 65

25 30 42 32

22 25 35 28

(B) RecoÕery of Cu (%) 2300 30 2800 38 3300 50 3800 42

43 50 68 51

20 25 35 27

18 20 30 24

of the useful compartments I or III, and the product. and recovery R Žratio 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. ŽA. 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 lengths between 2 and 6 mm. Both plastic and copper are in the same proportion, i.e. 50% Cu and 50% PVC. The geometry of the system has been adjusted in order to gather the plastic material in compartment I and copper in compartment II, which was designed for the concentrate product. The values of grade and recovery for Cu collected in compartment II are given in Table 2A and B, respectively. ŽB. Cu–Pb mixture containing Cu wires with diameters between 1 and 2 mm and lengths between 2 and 6 mm, and Pb particles of irregular shapes and dimensions between 2 and 6 mm. The proportions are 60% Cu to 40% Pb. The values of grade and recovery 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 Table 4A and B, respectively. ŽC. Cu–Al mixture containing Cu wires with diameters of 2 mm and lengths between 6 and 8 mm, and Al particles of irregular shapes and dimensions between 2 and 8 mm. Table 6 n Žminy1 .

b s158

b s 228

b s 308

b s 458

(A) Grade of Al (%) 2300 2800 3300 3800

83 85 88 86

85 88 92 89

81 82 84 82

80 81 83 81

(B) RecoÕery of Al (%) 2300 2800 3300 3800

87 88 91 89

90 92 96 93

84 85 88 86

83 84 86 84

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The proportions are 20% Cu to 80% Al. The values of grade and recovery for Cu collected in compartment I are given in Table 5A and B, respectively. The values of grade and recovery for Al collected in compartment III are given in Table 6A and B, respectively. In cases ŽB. and ŽC., the intermediate product collected in compartment II, consisting in mixtures of particles in a proportion close to the one of the feed material, was passed again through a new separation process.

5. Conclusions The experimental results show that for a given value of the incident angle b , the maximum separation grade is obtained at an intermediate value of the drum revolution Že.g. n s 4000 miny1 for Cu–Pb mixture.. This, because at high values of the drum revolution, which imply high values of the electrodynamic force Ft and torque T, the strongly conducting particles are strongly repelled. They can collide with poorly conducting particles and modify their trajectory. Thus, a fraction of the strongly conducting, as well as poorly conducting, particles falls into compartment II. This effect turned 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 d1 and d 2 , according to the material to be separated, the intermediate product collected in compartment II contains particles in a close proportion to the one of the feed material. This made possible the passing again of the intermediate product through the separator, without being necessary a new arrangement of the system geometry.

Table 7 Comparative results between the VDECS Ž b s 228. and the HDECS ŽA. Cu–Pb mixture Žcase B.

VDECS

HDECS y1

ns 4500 miny1

ns 4000 min

Cu Pb

GŽ%.

RŽ%.

GŽ%.

RŽ%.

92 76

84 86

95 80

87 88

ŽB. Cu–Al mixture Žcase C.

VDECS

HDECS y1

ns 4200 miny1

ns 3300 min

Cu Al

GŽ%.

RŽ%.

GŽ%.

RŽ%.

82 92

68 96

85 95

70 92

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

The advantages of the VDECS compared to the HDECS are of economical and technical nature. The existence of a single electric engine, shorter magnets and the absence of the conveyor belt ensure a lower cost of the separation equipment of the VDECS. In the meantime, the efficiency of the VDECS is close to the one obtained with a HDECS designed for the separation of millimetric particles. A comparison between the best results obtained with both types of separators is given in Table 7A for Cu–Pb mixture Žcase B., and in Table 7B for Cu–Al mixture Žcase C., respectively. Those values Ž 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 the TU Delft, the Netherlands and Dr. I. Hrianca, from the Department of Physics, West University Timis¸oara, Romania.

References Braam, B.C., van der Valk, H.J.L., Dalmijn, W.L., 1988. Eddy-current separation by permanent magnets Part II: Rotating disc separators. Resour., Conserv. Recycl. 1, 3. Leest, P.A., Rem, P.C., Dalmijn, W.L., 1995. Analytical approach for custom designing of eddy-current separators. Proc. XLVI. Berg-und Huttenmannischer Tag, Technische Universitat ¨ ¨ ¨ Bergakademie Freiberg. TU Bergakademie Freiberg, Germany, V 18r1. Rem, P.C., 1999. Eddy-current Separation. Eburon Delft, The Netherlands. Rem, P.C., Leest, P.A., van den Akker, A.J., 1996. A model for eddy-current separation. Int. J. Miner. Process. 49, 193. Schlett, Z., Lungu, M., 1999. Vertical eddy-currents separator for electronic wastes. Forschungshefte Kolloquium 2: Sortierung von Abfallen und mineralischen Rohstoffen, 50. Berg- und Huttenmannischer Tag, TU ¨ ¨ ¨ Bergakademie FreibergrSachsen, Germany, June, pp. 395–400. Schloemann, E., 1975. Separation of nonmagnetic metals from solid waste by permanent magnets. J. Appl. Phys. 46 Ž11., 5012. van der Beek, A., Buch, R., Dillmann, J., 1995. Sicheres Trennen von NE-Metallen mit Wirbelstromscheidern. Proc. XLVI Berg-und Huttenmannischer Tag, Technische Universitat ¨ ¨ ¨ Bergakademie Freiberg, Germany, V 20r1. van der Valk, H.J.L., Braam, B.C., Dalmijn, W.L., 1986. Eddy-current separation by permanent magnets Part I: theory. Resour. Conserv. 12, 233. van der Valk, H.J.L., Dalmijn, H.L., Duyvesteyn, W.P.C., 1988. Eddy-current separation methods with permanent magnets for the recovery of non-ferrous metals and alloys. Erzmetall 41 Ž5., 266.