THE INVESTIGATION OF SEPARABILITY OF PARTICLES SMALLER THAN 5 mm BY EDDY CURRENT SEPARATION TECHNOLOGY . PART I : ROTATING TYPE EDDY CURRENT SEPARATORS SHUNLI ZHANGa

Owing to the growing emergence of the end-of-life electrical and electronic products with complex material structures and an ever-diminishing particle size of the valuable metals involved, development of eddy current separators (ECS) has been targeting selective separation of small non-ferrous metal particles smaller than 5 mm. Separability of various materials smaller than 5 ram, including fine copper wires, has been investigated using ECS with various design concepts. The present research work is divided into two parts, with Part focusing on the rotating type ECS which are today common in practice, and with Part II dedicated to the ECS with novel concepts such as wet ECS technology. In Part I, three rotating belted-drum ECS were employed, which are manufactured by Bakker Magnetics, the Netherlands, Huron Valley Steel Co., US, and Eriez Magnetics, UK respectively. It is found that the belted-drum ECS are effective for separating materials below 5 mm if the magnetic drum rotates in opposite direction to the conveyor belt. The separation principle, particularly the "backward phenomenon" of the rotating type ECS for small particles has been unravelled in the present study. Moreover, separation of AI from the 0-10 mm fraction of electronic scrap has been conducted. The results obtained demonstrate that the belted-drum ECS with appropriate design may be applicable for separation of small aluminum particles from electronic scrap.


INTRODUCTION
The design and performance of the rotating belted-drum type eddy current separators have been improved substantially in the last ten years, due primarily to the advance of the magnet materials and magnet configurations, and to a better understanding of the separation mechanisms [1][2][3]. Yet, problems associated with separation of small particles, remain in particular, those smaller than 5 mm. Therefore, the current development ofeddy current separators (ECS) has been directed to selective separation of small non-ferrous metal particles.
In the present study, separability of various materials of 3 mm size, including fine copper wires, has been investigated using different ECS with various design concepts. On the basis of the present investigation and of the critical comparison of different ECS, each with a unique design, it can be anticipated that a novel ECS capable of separating metal particles smaller than 5 mm with both a high selectivity and a large capacity will be developed.
This research work is divided into two parts, with Part I focusing on the rotating type ECS which are presently common in practice and with Part II dedicated to ECS with novel concepts such as wet eddy current separation technology. In Part I, three rotating belted-drum ECS were employed, which are manufactured by Bakker Magnetics, the Netherlands, Huron Valley Steel Co., US, and Eriez Magnetics, UK respectively. The separation results obtained from those three devices are analyzed, in terms of various separation forces and the separation mechanisms for small particles are discussed in the present study. Further, the aluminum recovery from the 0-10mm fraction of electronic scrap has been conducted using two of the three devices. Additionally, the present results are evaluated to demonstrate that modern ECS may well be progressed to separate fine metal particles like copper wires effectively and efficiently. semi-automatic cutting machine. A 3 kg electronic scrap sample was obtained in the way shown in Fig. 1. The light fraction of the air jig separation, consisting essentially of aluminum and various plastics, was used in the present study. Its particle size distribution is presented in

Equipment to be Used
The rotating belted-drum eddy current separators (RBECS) used are illustrated in Fig. 3  respectively, were employed. Several major characteristics of the equipment, that are relevant to their performance, are compared in Table I.

The Separability Investigation
The separability of the materials was characterized by their distribution in an array of the collection bins which were placed in front of the conveyor belt pulley, as shown in Fig. 3. Twelve collection bins, each with dimensions of 400 x 40 x 90 (length x width x height)mm, were used. The material distribution was analyzed by its percent weight in each collection bin such that: where (P W)ij is the percent weight of the ith material in the.jth collection bin, and W;i the weight of the ith material in the.jth collection bin.   The separability of small particles with Eriez ECS was carried out by adjusting its splitters, instead of using the collection bins.

Separability of 3 mm Particles
Binary mixtures of A1/Cu and PVC with 3 mm particle size were processed after carefully optimizing all process parameters except for the employing Bakker ECS with a forward or clockwise rotation of the drum. It can be seen that small conducting particles like A1 and Cu are either mixed up with the non-conducting ones or distributed in the collection bins that are closer to the magnetic drum. Analysis of the material distribution indicates that it is difficult to separate small metal particles from non-metal ones selectively, when the magnetic drum rotates in the "Forward" mode. It has been found, though, that if the magnetic drum rotates in the "Backward" or counterclockwise manner, separation of small conducting particles from non-conducting ones is improved drastically. It is shown in Figs. 6 and 7 that more than 80% of A1 and Cu of 3 mm size is distributed in the collection bins which are farther away from the magnetic drum. In addition, as was anticipated, non-conducting PVC particles have approximately consistent distribution in Figs. 4-7, since the oscillating direction of the magnetic field has no effect on non-conductors. Similar results were obtained by using Eriez ECS which permits both "Forward" and "Backward" rotation of the magnetic drum.      FIGURE 7 Material distribution in the collection bins for Cu and PVC by Bakker ECS (particle size--3mm, belt speed= 1.6m/s, drum speed=3000rpm, backward rotation). Figure 8 shows that approxiately 85 % Al can be separated from a mixture of A1 and PVC of 3 mm size with almost 100% purity, as the magnetic drum spins in "Backward" direction. However, with the magnetic drum spinning in the "Forward" mode, less than 50% of A1 can be recovered.
Unfortunately Huron ECS can be operated only in the "Forward" mode. The material distribution of the mixtures of A1-PVC and Cu-PVC is presented in Figs. 9 and 10. It appears that when compared to Bakker ECS in the "Forward" mode, less conducting particles are mixed up with non-conducting ones. Yet, selective separation of small metal particles from non-metal ones is still difficult.
Based on a comparative investigation on three belted-drum ECS, it is found that a selective separation of metal particles below 5 mm may be established by spinning the magnetic drum in the "Backward" mode. This is referred to as the "backward phenomenon" in the present study. Therefore, the rotating belted-drum ECS tentatively designed for processing small particles below 5ram shall incorporate a "Backward" mode like the Bakker and Eriez ECS, enabling the magnetic drum to rotate backwards.

The Separation Results of Electronic Scrap by Belted-Drum ECS
The traditional "burning and smelting" recycling technique for handling electronic scrap is, to a great extent, applied in practice, which is aimed at recovering precious metals and copper. It is known that efficient removal ofAI by using modern ECS is able to facilitate the metallurgical process. Furthermore, an extra revenue, due to the high value of the recovered A1, can be obtained. It is for those reasons that the eddy current separation technology is of a great interest for the electronic scrap recycling industry. A point of concern is that the liberation size of AI particles present is in general small. Our study [4] shows that, after a two-stage liberation about 50% of A1 in the printed circuit boards is distributed in the -7 mm fraction. With a view towards implementing an effective ECS for this specific material stream, two modern belteddrum ECS were attempted, with the results given in Table II  It was indeed observed that a substantial proportion of plastics particles with Cu laminates report to the A1 fraction. On the contrary, it can be seen that Bakker ECS is capable of producing a high quality A1 product (about 95% purity) in a single pass, but the recovery is approximately 50% in the "Backward" operation mode. Also, Table II indicates that both the recovery and selectivity in the "Backward" mode is much better than in the "Forward" mode. It is noteworthy that the splitter of Bakker ECS can be adjusted so as to recover almost all A1 particles in a single pass as does the Huron ECS, but the product quality will then deteriorate significantly. As a result, these two ECS will have a similar performance in terms of both recovery and grade of the product. The performance of Bakker ECS shall be enhanced, if the feed material is screened into +5 mm and -5 mm fractions, as a consequence of the "backward phenomenon". It is also expected that effective recovery of small A1 particles will be realized by means of a multi-pass circuit configuration. Separation mechanisms of small particles in eddy current separation are different from those of large particles. The radial and tangential eddy current forces (F,. and Ft) as well as the force derived from the electromagnetic torque (Fa-) are given by the authors in [3]. Therefore, radial and tangential accelerations (ar and at) as well as the acceleration where s is the shape factor related to the magnetization of a particle, s' is the shape factor related to the characteristic decay time ofeddy currents, B a is the amplitude of the magnetic field, k is the number of pairs of the magnets in a drum, COdrum is the angular velocity of the magnetic drum, R is the characteristic particle size, w is the width of one pair of the magnets, cr is the electrical conductivity, p is the mass density and #0 47r x 10 -7 Tm/A.
For simplicity, the following analysis is done for a spherical particle, due mainly to the fact that the spherical particle is orientation-independent. In the case of Bakker ECS (k 9 and C0drum 1007rrad/s), accelerations of aluminum particles, which are computed by using Eqs.
(2)-(4), are given in Fig. 11, as a function of #0kcoarumcrR -(in which R is variable from 1.0 to 25.0mm). It is clear from Fig. 11 that, for a small particle, the dominant acceleration is the one from the electromagnetic torque and the radial acceleration is negligible, whereas for a large particle the predominant acceleration is in the radial direction. Thus, it follows, in terms of Fig. 1, that: aT )) at > ar 0 (R < 5 mln) a,. >> at < aT (R > 20 ram).
Evaluation of Eq. (2) shows that the radial force acting on a small particle is so feeble that it is unable to lift up the particle, which instead rotates with the field due to a strong electromagnetic torque. Figure 12 illustrates the rotation of a small conducting particle with the field and the forces involved. It is argued that, if the magnetic drum rotates clockwise or in the "Forward" mode, the dynamic frictional force (Ffric) as a result of the electromagnetic torque will be anti-parallel (backward) to the tangential force (Ft, forward) as indicated by the solid line shown in Fig. 12. On the other hand, if the magnetic drum rotates counterclockwise or in the "Backward" mode, the tangential force is directed backwards and the frictional force is directed forwards. Consequently, selective separation of small particles will be determined by a competition between the tangential eddy current force and the dynamic frictional force. In effect, this concept can be quantified as: Ft > F fric =: the magnetic drum shall rotate clockwise or forwards (7) Ft < F fric = the magnetic drum shall rotate counterclockwise or backwards. Since Fr is negligible, we have F fric =famg where fa is the dynamic frictional coefficient, rn is the mass of the particle and g is the acceleration due to gravity. Equations (7) and (8) are also expressed as: at > fag = the magnetic drum shall rotate clockwise or forwards (9) at < fdg = the magnetic drum shall rotate counterclockwise or backwards. (10) For the rubber belt of Bakker ECS that is used in the present study, fd is about 1.1 on average, so that fag is about 10 m/s 2. It must also be pointed out that the belt is used for such a long time that it is covered with dirt, thereby rendering the belt surface even rougher. The average tangential acceleration of small particles as a function of the drum speed and particle size are shown in Fig. 13. It is clear that, at 50 rps drum speed which is the operating condition for Figs. 4-7, the average tangential acceleration of 3 mm AI particles is much smaller than 10m/s 2 whilst the average tangential acceleration of 5mm Al particles is larger than 10 m/s 2. Therefore, it is predicted that in order to separate out A1 particles of 5 ram, the magnetic drum shall rotate in the ROTATING TYPE EDDY CURRENT SEPARATORS   247   3O   25   0  20  40  60  80  100 drum speed, rps FIGURE 13 Tangential acceleration as a function of drum speed and particle size.
"Forward" mode. This is in line with the experimental work, as shown in Figs. 14 and 5. Alternatively, the above argument can be verified further by calculating the average horizontal displacements of 3 mm A1 particles for the present experiments. It follows that: S+/-1 / 2 gAt 2 (11) where S_L is the vertical displacement of a particle, and At is the time needed for that vertical displacement.
Since S_ =0.6 m, we have At--0.35 s. If the magnetic drum rotates backwards, the particle rolls forwards on the belt. The time for the particle from starting to roll to leaving the belt (Atroll) is estimated as: Sroll troll FIGURE 15 Material distribution in the collection bins for AI and Pb by Bakker ECS (particle size 5 ram, belt speed 1.6 m/s, drum speed 3000 rpm, backward rotation).