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IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 3, MAY 2002
Separation of Small Nonferrous Particles Using an Inclined Drum Eddy-Current Separator With Permanent Magnets Mihai Lungu and Peter Rem
Abstract—This paper presents a method for separating small metallic nonferrous particles from two-component nonferrous mixtures using a new type of dynamic eddy-current separator with permanent magnets. The so-called inclined drum eddy-current separator (IDECS) consists of an inclined vertical rotary drum covered with permanent magnets, alternately N–S and S–N oriented, directly fixed on the axis of an electric engine. The particles to be separated are brought into the field on an oblique trajectory, hit the drum, and are deflected in the variable field under electrodynamic and mechanical interactions. The strongly conducting and poorly conducting particles assume different trajectories depending on their electrical conductivities, which lead to their separation. The paper presents the results of grade and recovery of wastes consisting of small particles (under 5 mm) derived from Cu–Pb and Cu–Al mixtures. The advantages of IDECS are that its efficiency is close to that of the usual dynamic eddy-current separator and its equipment cost is lower. A disadvantage is that the intermediate product must be passed again through a separation process. Index Terms—Eddy-current separator, grade, nonferrous, poorly conducting particles, recovery, strongly conducting particles.
I. INTRODUCTION
E
DDY-CURRENT separation methods are used for the recovery of nonferrous metals (Cu, Al, Pb, Zn) from solid wastes and for separating various nonferrous metals from each other. These methods rely on the fact that a time-varying magnetic field induces electric currents in metallic particles and, hence, exerts forces on them. Eddy-current dynamic separators with permanent magnets represent a class of separators substantially improved in the past ten years, where the magnetic field is generated by machinery with moving permanent magnets. Modern eddy-current separators used at present for recovery of nonferrous metals [2], [3], [8] are the horizontal rotating drum (HRD) types, where the active part is a fast spinning drum covered with rows of permanent magnets of alternating polarity, mounted parallel with the drum axis. A conveyor belt takes the particles over the drum and the conductive particles are accelerated in order to follow the motion of the drum. Manuscript received March 7, 2001; revised December 13, 2001. M. Lungu is with Electricity, Magnetism and Materials Science Department, Faculty of Physics, West University Timisoara, Timisoara 1900, Romania (e-mail:
[email protected]). P. Rem is with Delft University of Technology, 2628 RX Delft, The Netherlands (e-mail:
[email protected]). Publisher Item Identifier S 0018-9464(02)03642-7.
Fig. 1.
Spinning drum.
Yet, the main problems associated with eddy-current separation are those referring to the selective separation of conductive nonferrous particles smaller than 5 mm from nonconductive ones or one from each other, while reducing the cost of the separation process. This paper describes a new type of eddy-current dynamic separator, namely the inclined drum eddy-current separator (IDECS). This separator consists of an inclined vertical spinning drum covered with NdFeB permanent magnets, alternately N–S and S–N oriented, directly fixed on the axis of an electric engine, as shown in Fig. 1. The purpose was to realize an eddy-current separator with a higher efficiency, a lower cost of the separation equipment, able to separate mixtures of small poorly conducting and strongly conducting metallic nonferrous particles. Unlike the HRD, where the length of the magnets is about tens of centimeters, equal to the active width of the conveyor belt, and the feeding of the particles is made on the whole breadth of the belt, in the case of the IDECS the magnets are only a few centimeters long. This is made possible by the vertical positioning of the drum and the fact that all particles have the same incident trajectory and hit the drum in a limited area. The particles to be separated are brought into the field on an oblique trajectory, in the horizontal plane as well as in the vertical one and hit the surface of the rotary drum. Under the combined effect of the deflection caused by collision and action of electrodynamic interactions in the variable magnetic field, the strongly conducting particles suffer a greater deflection than the poorly conducting particles. Consequently, they fall at a greater distance as the poorly conducting particles do. In this way, the poorly conducting and strongly conducting particles obtain different trajectories, which finally leads to their separation.
0018-9464/02$17.00 © 2002 IEEE
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The drum is inclined with respect to the vertical plane in order to obtain a greater deviation of the strongly conducting particles after they leave the separation zone.
TABLE I PARAMETERS OF EXPRESSIONS (4) FOR PARTICLES OF SEVERAL SHAPES AND PARALLEL ( ) OR PERPENDICULAR ( ) ORIENTATIONS OF THEIR AXIS OF SYMMETRY WITH RESPECT TO THE ROTATION AXIS OF THE MAGNET DRUM
k
?
II. THEORETICAL CONSIDERATIONS As for all dynamic eddy-current separators, the fluctuating field of the IDECS spinning drum induces eddy currents in conductive nonferrous particles moving close to the drum. Eddy currents in conductive nonferrous 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. The magnetic field of the drum then acts on the eddy currents through the Lorentz force and propels the particle. While a particle moves through the magnetic field, it experiences changes of size and orientation of the field due to its own translational and rotational motion as well as due to the rotation of the drum [2]. For magnet drums such as the IDECS, with poles and spinning with angular velocity , the field outside of the drum can be approximated by its funthe surface damental mode (i.e., its first Fourier component) (1) For a stationary particle in this field, it seems as if the magnetic field vector is rotating around at constant size, with angular ve. locity The particles in the separator acquire a significant angular velocity, , and as a result they perceive a field of constant size . On the other hand, the rotating at angular velocity linear velocity of the particles is typically an order of magnitude smaller than the surface of the drum, so the contribution of this effect to the particle motion can be ignored. If the particle is sufficiently small with respect to the pole , the variations of the width of the drum, where drum field within the particle are smooth and the eddy-current distribution can be treated as a magnetic dipole. In this case, both the force and the torque can be expressed in terms of of the particle the field (gradient) and the magnetic moment (2) (3) where
Earlier work on HRDs [3] has provided detailed expressions for the force and torque of a rotating drum field on conductive particles of simple shapes. The general form of these expressions is
(4)
D : Particle diameter. L: Length. : Thickness.
Here, and are the radial and the tangential component of the electromagnetic force (2), respectively. is the volume, the conductivity of the particle, and is a characteristic size. The functions and depend on the shape and orientation of the particle. Examples for simple shapes are listed in Table I. The torque makes the particle spin in the opposite direction of the magnetic field, which rotates counterclockwise, if the field is generated by a clockwise rotation of the eddy-current rotor (if the rotation of the rotor is forward, the nonferrous particle will roll backward). This has the effect, that, as the par, the field rotation as obticle rotation increases, i.e., served by the particle slows down. As a result, the forces acting on the particle also diminish as the particle approaches the rotor. For the experiments described in this paper, the dimensionis of order 1, so the -terms in the forless number mulas of Table I can be neglected (because their coefficients are smaller) and the tangential force is dominant over the repulsive radial force. Assuming that the particle approaches the rotor with a constant radial velocity , we can integrate the torque in (4) to get its speed of rotation at any point near the rotor
The shape and orientation factor depends on the ratio of the particle moment of inertia and its mass , as well as the linear term in (see Table I). For example, an aluminum sphere moving to a position near the experimental 18-pole rotor T, gives a of about 1 m/s, showing that the with . particle rotation can actually get close to Since the tangential force and the torque on the particle are strongly related according to (4), the result for also gives an approximate result for the tangential impulse on the particle as it approaches the rotor
The final deviation of a particle depends on the tangential and torque , as well as on the deflection caused by force the collision with the surface of the rotary drum and on different interactions between particles. The spinning of the strongly conducting particle is responsible, in part, for the separation: in collision with the drum surface, the particle bounces more violently than a poorly conducting particle and achieves a supplementary deflection. The
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TABLE II SEPARATION FACTOR = FOR Al, Cu, AND Pb
Fig. 3.
Fig. 2. Side view of the IDECS.
poorly conducting particles also hit the drum but fall at lower distances than the strongly conducting particles. So, in order to assure a separation as good as possible, the orientation of the incident trajectory of the particles must be correlated with the drum revolution. The separation process depends strongly on the material , the so-called separation factor. Values of the factor separation factor for Al, Cu, and Pb are given in Table II. III. ENGINEERING AND FUNCTIONING The principle outline of the IDECS is given in Fig. 2. Drum 1, made of weak magnetic steel, is covered with permanent magnets, alternately N–S and S–N oriented. The drum is directly fixed on axis 3 of electric engine 2, whose revolution can be modified from 0 to 4500 min . The material to be separated is brought into the active zone of with respect the field on a trajectory inclined with the angle is established to the horizontal plane. For each type of waste, in order to obtain a minimum deviation of the poorly conducting particles in the vertical plane with respect to their initial trajectory. The inclination of the rotary drum is established in order to obtain a maximum deviation of the strongly conducting parand are set after ticles in the vertical plane. The angles successive attempts for a certain type of waste. The horizontal angle between the incident trajectory of the particles and the surface of the drum (Fig. 3), and the drum revolution (min ) are the most important parameters in order to determine the best functioning conditions of the IDECS for a and have been set. certain type of waste after the values
Top view of the IDECS.
Due to the electromagnetic interactions with the rotating field of the drum, the trajectory of the particles is slightly deviated. As a result, they will not hit the drum in point A, but in point B (see Fig. 3). In the active zone of the field, due to the electrodynamic force , the torque and collisions with the surface of the spinning drum, the strongly conducting particles (characterized by a high separation factor) fall into compartment III of the collecting recipient and the poorly conducting particles (characterized by a lower separation factor) fall into compartment I of the same recipient. Compartment II was designed for the intermediate product, consisting of a compound containing both types of particles. The intermediate product is passed again through the separator. The distances and (see Fig. 2) are determined after successive attempts, 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, even the intermediate product must be passed again through the separator; this is possible without a new arrangement of the separation parameters. IV. EXPERIMENTAL RESULTS Experimentally, the quantities grade (ratio between mass of a material in the product—the whole quantity collected in one of the useful compartments—and the product) and recovery (ratio between mass of a material in the product and mass of the same material in the feed) have been determined for two types of electrotechnical wastes, namely: 1) Cu–Pb mixture containing Cu wires with diameters between 1–2 mm and lengths between 2–6 mm, and Pb particles of irregular shapes and dimensions between 2–6 mm. The proportions are 60% Cu to 40% Pb. Figs. 4 and 5 show the dependence of and on the drum revolution for different values of the incident angle for Cu collected in compartment III and Pb collected in compartment I, respectively. 2) Cu–Al mixture containing Cu wires (diameter 2 mm, lengths 6–8 mm) and Al particles of irregular shapes and dimensions between 2–8 mm. The propor-
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different values of the incident angle for Cu collected in compartment I and Al collected in compartment III, respectively. In both cases, the intermediate product collected in compartment II, consisting of mixtures of particles in proportions close to the one of the feed material was passed again through a new separation process.
V. CONCLUSION Fig. 4.
Grade and recovery for Cu collected in compartment III.
Fig. 5.
Grade and recovery for Pb collected in compartment I.
Fig. 6.
Grade and recovery for Cu collected in compartment I.
The experimental results show that for a given value of the incident angle , the maximum separation grade is obtained at an intermediate value of the drum revolution (i.e., at min ). This is because at high values of the drum revolution, and which imply high values of the electrodynamic force torque , 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 are not needed, which in fact can be very dangerous, but a good arrangement of the system geometry, especially of the incident angle . By a proper arrangement of the distances and , 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 a new arrangement of the system geometry. The advantages of IDECS are that its efficiency is close to that of the usual dynamic eddy-current separators (i.e., the HDS) in conditions of a lower cost of the equipment assured by shorter magnets, a single electric engine, and the absence of the conveyor belt. A disadvantage is that the intermediate product collected in compartment II must be passed again through a separation process.
REFERENCES
Fig. 7.
Grade and recovery for Al collected in compartment III.
tions are 20% Cu to 80% Al. Figs. 6 and 7 show the dependence of and on the drum revolution for
[1] P. C. Rem, P. A. Leest, and A. J. van den Akker, “A model for eddycurrent separation,” Int. J. Mineral Process., vol. 49, p. 193, 1996. [2] P. C. Rem, E. M. Beunder, and A. J. van den Akker, “Simulation of eddy current separators,” IEEE Trans. Magn., vol. 34, pp. 2280–2286, July 1998. [3] P. C. Rem, Eddy Current Separation, Delft, The Netherlands: Eburon, 1999, p. 97. [4] E. Schloemann, “Separation of nonmagnetic metals from solid waste by permanent magnets,” J. Appl. Phys., vol. 46, no. 11, p. 5012, 1975. [5] A. van der Beek, R. Buch, and J. Dillmann, “Sicheres Trennen von NE-Metallen mit Wirbelstromscheidern,” in Proc. XLVI. Berg-und Hüttenmännischer Tag, Germany, 1995. V 20/1. [6] H. J. L. van der Valk, B. C. Braam, and W. L. Dalmijn, “Eddy-current separation by permanent magnets part I: Theory,” Resources Conserv., vol. 12, p. 233, 1986. [7] B. C. Braam, H. J. L. van der Valk, and W. L. Dalmijn, “Eddy-current separation by permanent magnets part II: Rotating disc separators,” Resources, Conserv. Recycl., vol. 1, p. 3, 1988. [8] H. J. L. van der Valk, H. L. Dalmijn, and W. P. C. Duyvesteyn, “Eddycurrent separation methods with permanent magnets for the recovery of nonferrous metals and alloys,” Erzmetall, vol. 41, no. 5, p. 266, 1988.
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Mihai Lungu was born in 1962. He graduated from the Faculty of Physics, Solid State Physics Department of the University of Timisoara, Romania, in 1986 and received the Ph.D. degree in condensed matter physics in 1998. From 1991 to 1995, he was associated with the Physics Faculty, West University Timisoara, as a Professor Assistant and in 1995 he was appointed a lecturer. Since 1992, he has been working on recovery of useful components from mineral and industrial wastes. His fields of interest are solid state devices, recovery of useful components from mineral and industrial wastes, and plasma physics.
IEEE TRANSACTIONS ON MAGNETICS, VOL. 38, NO. 3, MAY 2002
Peter Rem was born in 1959. He graduated from the Faculty of Physics at the University of Leiden, the Netherlands, in 1982 and received the Ph.D. degree in superconductivity from the University of Twente, the Netherlands. In 1986, he started at Shell Research on fluid dynamics of two-phase flows. Since 1994, he has worked as a research coordinator in the Raw Materials section of Delft University of Technology, the Netherlands. His particular field of interest is physical separation technology.
