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Environment-friendly technology for recovering nonferrous metals from e-waste: Eddy current separation
Resources, Conservation and Recycling 87 (2014) 109–116
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Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec
Review
Environment-friendly technology for recovering nonferrous metals from e-waste: Eddy current separation Ruan Jujun a,b , Qian Yiming a , Xu Zhenming b,∗ a Jiangsu Key Laboratory of Environmental Material and Environmental Engineering, School of Environmental Science and Engineering, Yangzhou University, 196 West Huayang Road, Yangzhou, Jiangsu 225127, China b School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 November 2013 Received in revised form 16 February 2014 Accepted 28 March 2014 Keywords: e-Waste Nonferrous metals Recovery Eddy current separation
a b s t r a c t The current generation pattern of e-waste consisted of dead electronic and electrical equipments poses one of the world’s greatest pollution problem due to the lack of appropriate recovery technology. Crude recovery methods of resource materials (aluminum, zinc, copper, lead, gold) from e-waste caused serious pollution in China in the past years. Thus, environment-friendly technologies have been the pressing demand in e-waste recovering. Eddy current separation (ECS) was advised as the preferable technology for recovering nonferrous metals from e-waste. However, just a few reports focused on the application of ECS in e-waste recovering. This paper introduced the information about ECS including the models of eddy current force and movement behavior of nonferrous metallic particle in the separation process. Meanwhile, the developing process of eddy current separator was summarized. New industrial applications of ECS in e-waste (waste toner cartridges and refrigerator cabinets) recovering were also presented. Finally, for improving separation rate of ECS in industrial application of e-waste recovering, some suggestions were proposed related to crushing process, separator design, and separator operation. The aim of this paper is to demonstrate the effectiveness of ECS technology as practical and available tool for recovering non-ferrous metals from e-waste which is now being ignored. © 2014 Elsevier B.V. All rights reserved.
Contents 1. 2.
3.
4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eddy current force and eddy current separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Eddy current force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Eddy current separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Efforts for improving separation rate of ECS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Influencing of particle characteristics on separation rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Influence of operation conditions on separation rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Development of eddy current separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . New industrial application of ECS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measures for improving separation rate of ECS in its new industrial application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∗ Corresponding author. Tel.: +86 21 54747495; fax: +86 21 54747495. E-mail addresses: [email protected], [email protected] (X. Zhenming). http://dx.doi.org/10.1016/j.resconrec.2014.03.017 0921-3449/© 2014 Elsevier B.V. All rights reserved.
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Nomenclatures bn Bm
Fourier coefficient magnetic flux density of the magnetic drum surface (T) Bp induced magnetic flux density in the particle (T) Br (B, Ba ) magnetic flux density of the field (T) Fr repulsive force between the particle and the magnet (N) acceleration of gravity force (m/s2 ) g G gravity force of the aluminum flake (N) J (j) induced eddy current in particle (flakes) (A) k pairs of the magnets placed in the magnetic drum l height of the rectangle/triangle flake (m) circumference of the triangle flake (m) L r radius distance between the particle and the center of magnetic drum (m) radial distance between the particle and the inducr ing magnet (m) R (Rdrum ) radius of the magnetic drum (m) Sm per magnet side area which facing the flake (m3 ) Sp (d) maximal cross area of the flake in horizontal (m2 ) t time cost for the magnetic field rotation (s) thickness of the flake (m) T v (˝) feeding speed of the particle (flake) (m/s) v relative linear velocity between the flake and magnetic drum (m/s) volume of the particle (rectangle/triangle flake) V (m3 ) w width of the rectangle/triangle flake (m) ˝ resistance of circular/rectangle/triangle coil angle of the coordinate in the cylindrical coordinate ˛0 system (, ) conductivity of the flake (S/m) ı (s) oriental (shape) factor of the flake in eddy current separation induced emf in the circular/rectangle/triangle coil εi (V) relative magnetic permeability of iron (H/m) r 0 magnetic permeability of vacuum (H/m) ωm (ωdrum , ω) rotation velocity of the magnetic drum (rad/s) induced magnetic flux in the particle (Wb) Фp Фm magnetic flux of the permanent magnet (Wb) Ф variation of magnetic flux in the particle (Wb/s) w( ) width of pole
1. Introduction Quantities of e-waste are generating resulted from the use of electronic and electrical products. Computer accessories and mobile telephones are disproportionately abundant because of their short lifespan. The current global production of e-waste is estimated to be 20–25 million tons per year and about 95% useful materials were recovered (Robinson, 2009). About 2.5 million tons e-waste appeared in Chinese mainland including self-generated and imported from developed countries per year (Ongondo et al., 2011; Stone, 2009; Widmera et al., 2005). Fig. 1 shows samples of nonferrous metals found in e-waste. Waste PCB contains nearly 28% metals including copper, zinc, and other nonferrous metals (Li and Xu, 2010). Waste toner cartridge has 11.7% aluminum (Ruan et al., 2011). Waste refrigerator cabinet includes about 8.9% copper and aluminum (Ruan and Xu, 2011a,b). Additionally, purity of metals in e-waste is higher than that of rich-content minerals (Laner and
Rechberger, 2007; Ilgin and Gupta, 2010a,b). Thus, recovering ewaste can bring great economic benefits. Environmental sound technologies of recovering e-waste are developing challenges today. In the early stage, crude technologies (acid-washing or open incineration) were employed (Ruan and Xu, 2012a,b; Ilgin and Gupta, 2010a,b) and resulted to serious environmental pollution by the hazardous materials contained in e-waste (Duan et al., 2011; Huo et al., 2007; Leung et al., 2008). Then, for the sake of environmental protection and clean recovery of nonmetals from e-waste, it was proposed a procedure including crushing process and psychical separations of screen, shape sorting, jigging, magnetic separation, air current separation, corona-electrostatic separation, and eddy current separation (ECS) (Zhou and Xu, 2012; Cui and Forssberg, 2003). However, each technology has special limitation. Screen and shape sorting cannot separate the particles that have similar size and shape. Jigging brings waste water in separation process. Magnetic separation can only separate ferrous metals. Air current separation demands particles having great density difference when being in similar size. Corona-electrostatic separation and ECS are the preferable technologies for recovering nonferrous metals from e-waste. Corona-electrostatic is skilled in separating nonferrous metallic particles (NMP) less than 1 mm in size (Wu et al., 2008). ECS is adept in separating NMPs ranged from 2 to 50 mm in size. ECS may be the fittest technology for recovering nonferrous metals from large-scale (coarse crushing is enough for liberating the materials) e-waste (Zhang et al., 2002; Benaboua and Georgesa, 2008). ECS is an environment-friendly technology for separating nonferrous metals from solid waste. In separation process, eddy current is induced in nonferrous metal when meeting variable magnetic field. Interaction between eddy current and magnetic field changes the trajectory of nonferrous metal as well as separates them from others. No waste water, air pollution, and solid waste are generated in the separation process. Unfortunately, the public paid less attention on this environment-friendly technology. This paper discussed ECS technology from the models of eddy current force (ECF), models of particle movement behavior, developing of separator, and new industrial application standpoints for e-waste recovering. Furthermore, suggestions for improving separation rate of ECS are presented.
2. Eddy current force and eddy current separation 2.1. Eddy current force ECS is a physical method for separating nonferrous metals from inert materials. ECF is the cause of ECS. In general, eddy current separator is comprised of magnetic drum. The magnetic drum always consists of magnetic poles placed in N–S–N (see Fig. 2). A changing magnetic field will be induced by the rotation of magnetic drum (Peterson, 2003; Rolicz, 2009). There are two recognitions about the generation of ECF. Recognition (1): eddy current will appear in NMP when it experiences the changing magnetic field; a repulsive force will be generated between magnetic field and NMP possessing eddy current; this repulsive force is called ECF. Recognition (2): direction of the magnetic field changes constantly because of the running of magnetic drum; consequently, the direction of eddy current induced in NMP also changes continuously resulting in the generation of a new magnetic field in NMP; direction of the new magnetic field is also changed constantly; the two magnetic fields have the same directions and repulse each other; this repulsion is called ECF. Because of being the most important influencing factor of ECS, many models for computing ECF were constructed. The models of ECF established from recognition (1) including:
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Fig. 1. Nonferrous metals in e-waste: (a) waste PCBs, (b) waste toner cartridges, (c) waste refrigerator cabinet.
(i) Models of radial and tangential components of ECF (Rem et al., 1997; Zhang and Rem, 1999): Ft =
2 sV (kωdrum + ˝) Ba2 2 2 0 w 1 + (kω drum + ˝)
Ft =
2 sV (kωdrum + ˝) 2 Ba2 2 2 0 w 1 + (kω drum + ˝)
(1)
2
(2)
The models indicated that ECF was determined by the parameters of separator operation, particle characteristics, and separator design. The force increased with the increasing of particle volume, intensity of magnetic field, and decreased with the increasing of width of magnetic pole. Additionally, particle shape, pairs of magnetic poles, rotation speed of magnetic drum, and angular velocity of particle influenced the magnitude of ECF. Parameters of particle volume and shape can guide crushing process for liberating nonferrous metals. Parameters of intensity of magnetic field, pairs and width of
magnetic poles can be used to guide the manufacture of eddy current separator. Parameters of rotation speeds of magnetic drum and particle can guide the operation of ECS. (ii) The modified models of radial and tangential components of ECF (Lungu and Schlett, 2001): Fn = s0 (ω − ˝)R2 Ft Ft =
2 R T
R
(3) (4)
T=
r × fdV
(5)
V
f = j × BdV
(6)
These models introduced the parameter of radius distance between the particle and the center of magnetic drum as well as adding the parameter of eddy current induced in particle.
Fig. 2. NMP was separated from nonmetal because of eddy current force in ECS.
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(iii) Models for computing the moment of ECF (Maraspin et al., 2004): Fm
(k + 1)Bˆ 2 V = 0 r
R( )
(7)
I( )
Bˆ 2 V I 0 ( )
(8)
= 0 (kωm + ˝)d2
(9)
Tm = −
Besides the parameters contained in models (i) and (ii), maximal cross area of particle in horizontal was introduced in model (iii). This parameter could guide the crushing process more specifically and predicted that flake shape brought positive influence on ECF. The models of ECF established from recognition (2) (Ruan and Xu, 2011a,b): Model for circle flake: Fr C =
Br k(ωm R − v)VSp Bm Sm
1
16 3 R3
(sec˛0 − 1)2
(10)
Models for rectangle flake were: Fr R = ıR =
Br k(ωm R − v)VSp Bm Sm ıR
1
16 2 R3
(sec˛0 − 1)2
W 2(L + W )
(11)
Models for triangle flake were: Fr T =
Br k(ωm R − v)VSp Bm Sm ıT
1
16 2 R3
(sec˛0 − 1)2
L ıT = C
(12)
The parameters, contained in the above models of ECF, could be used to guide crushing, separator operation, and separator design. Compared the models established from recognitions (1) and (2), the models established from recognition (2) paid more attention to the influencing of shape of particle to ECF, and were more specific and accurate. Precisely, the application range was limited and smaller than the models established from recognition (1). 2.2. Eddy current separation ECF causes ECS. By neglecting air friction, NMP in ECS are subject to ECF and gravity force. Due to the absence of electrical conductivity, nonmetals are only subjects to gravity force. Thus, NMP are separated from nonmetals due to the different movement behaviors in ECS. The sketch map of the movement behavior of NMP and nonmetal are presented in Fig. 2. ECS also can be used to separate different nonferrous metals as long as they have different electrical conductivities. Different conductivities of nonferrous metals cause different magnitudes of ECF in order to bring distinguished movement behaviors of NMPs. For improving separation rate of ECS, the movement behavior of NMP was studied and the trajectory models were constructed. The models contained abundant influencing factors of ECS and could be used to predict separation results. Based on the models of (i), (ii), (ii) above, movement behavior of NMP in ECS was simulated by computer software (Maraspin et al., 2004; Zhang and Forssberg, 1999). Based on the models of ECF constructed from cognition (2), trajectory models of NMP in different position inside and outside the magnetic field (Fig. 3) of ECS were constructed (Ruan and Xu, 2012a,b).
Movement behavior of NMP in ECS is divided into three stages: (1) entering magnetic field, (2) detaching from conveyor belt surface, (3) exiting from magnetic field (see Fig. 3). At the beginning of stage (1), eddy current force increases as the particle moving close to magnetic drum, and the force is divided into vertical component and horizontal component. Horizontal component is counteracted by the friction force of conveyor belt and no relative motion happens between NMP and belt. As the increasing eddy current force, vertical component will be greater than the gravity force (G) of the flake, and NMP will have a verticalupward acceleration and move upward. Meanwhile, eddy current force will decrease along with the upward movement of NMP due to the decline of magnetic flux of magnetic drum. When the vertical component is equal to gravity force, NMP will be suspended and keep constant radial distance to the axis (O) of separator. The position of the flake is called point (x0 , y0 ). Point (x0 , y0 ) can be considered as the detachment point of NMP from separator surface. Meanwhile, point (x1 , y1 ) is supposed as the symmetry point of (x0 , y0 ) at the right of Y-axis (Fig. 3). The movement of NMP from point (x0 , y0 ) to point (x1 , y1 ) can be considered as rectilinear motion. At this rectilinear movement, magnetic fluxes of NMP and magnetic drum is considered to be parallel, horizontal component of eddy current force can be neglected, and repulsive force can be supposed as equal to its gravity force. When NMP passes over point (x1 , y1 ), vertical component of eddy current force will be less than gravity force. Horizontal component of repulsive force can no longer be neglected since the directions of the two magnetic fluxes will no longer been parallel. Horizontal component will accelerate NMP in horizontal direction until it passes through the boundary of the magnetic field. Furthermore, the movement of NMP in vertical direction is controlled by gravity force and vertical component of ECS. We suppose point (x2 , y2 ) as the exiting position of NMP from the magnetic field. As passing through point (x2 , y2 ), NMP only subjects to gravity force and the movement can be considered as horizontal projectile motion. Due to no response to magnetic field, movement behavior of plastic in ECS is considered as horizontal projectile motion. The trajectory models of NMP were given as Eq. (13). New influencing factors of magnetic field boundary and collection position were introduced in the trajectory models.
⎧
v 2 v 2 ⎪ ⎪ 1 2(x − x ) 1 ⎪ ⎪ + − + y1 y = − (g − ay ) ⎪ ⎪ 2 ax ax ax ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ xb2 + yb2 = (R + )2 ⎪ ⎪ ⎪ ⎪ ⎨ g sin a1 ax =
2
⎪ ⎪ g cos ˛1 ⎪ ⎪ ⎪ ay = ⎪ 2 ⎪ ⎪ ⎪ ⎪ = Rtg˛ x ⎪ 1 0 ⎪ ⎪ ⎪ ⎪ ⎪ ⎩ y1 = R
(13)
cos ˛0
3. Efforts for improving separation rate of ECS Experiments of ECS were performed to study the influence of particle characteristics and operation conditions (convenient to be controlled in ECS and crushing) on separation rate. 3.1. Influencing of particle characteristics on separation rate ECS of aluminum, copper, zinc, lead, and PVC sheets showed metal sheets that of differing in the ratio of electrical conductivity to density could be separated successfully by the condition of having
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Fig. 3. Analysis of the forces and movement behaviors of NMP and nonmetal in and out of the magnetic field.
the similar size and shape (Zhang and Forssberg, 1999). The metal sheets that differ in shape could be separated as long as having similar size and ratio of electrical conductivity to density. The metal sheets that differ in size could also be separated if they have similar shape and ratio of electrical conductivity to density. The metal sheets that differ in size, shape, and ratio of electrical conductivity to density could not be separated effectively. Particle size was of critical consideration. Particle size below 2 mm was not effectively responsive to the separator regardless of how the operating parameters were adjusted. Additionally, ECS was unable to recover nonferrous metallic foils. Particle’s shape was one of the most influential variables. Irregular shape of particle minimized ECF. ECF of plate particle was greater than those of ball-shaped particles and irregular shapes (Zhang et al., 2002). Additionally, moisture was another important influential factor. Wet fed would decrease the separation rate of ECS. Small metal particles would stick to the feed conveyor belt (Rahman and Bakker, 2013). 3.2. Influence of operation conditions on separation rate Feeding the particles with slight wet to traditionally eddy current separator could separate NMPs in the range from 2 to 6 mm ¨ et al., 2002). Rotor speeds should from waste streams (Kohnlechner be relatively low so as to keep ECF being used only to break the wet bond between NMPs and belt surface. Feeding system and feed speed significantly influenced separation rate. Multilayer feed stream could seriously deteriorate the separation process. NMPs were shielded from approaching the effective magnetic field by nonmetallic particles. Nonmetallic particles could flip away together with NMPs. Apparently, a monolayer feed stream could ensure effective separation (Zhang et al., 2002). Results of orthogonal experiment of ECS showed that difference between feeding speed and rotation speed of magnetic field (ωR − v) was the critical influencing factor on separation rate. Feeding speed (v) was general factor, and the collection position (H) was subordinate factor (Ruan and Xu, 2012a,b).
A splitter with appropriate position involved in eddy current separator during ECS was of great importance. Splitter could separate NMP timely when it deflected from nonmetals caused by ECF (Schlett and Lungu, 2002). 3.3. Development of eddy current separator The traditional eddy current separator was horizontal magnetic drum separator (see Fig. 4(a)). For improving the separation rate, vertical drum eddy current separator (Fig. 4(b)) was developed to separate small conductive nonferrous particles whose dimensions were within 2–8 mm (Lungu and Schlett, 2001; Lungu, 2005). Wet eddy current separator (Meier-Staude et al., 2002) was developed to separate the NMP whose particle size was from 2 to 5 mm. The sketch was given in Fig. 4(c), where: T, magnetic drum with 4 poles; I, dielectric chamber for liquid having two collectors C1 and C2; A, feeding system. The particles falling from the feeder in to the liquid are subjected to both Magnus force and translational force. The gravity and buoyancy force act upon the particles as well. The Magnus and the translational forces will make the trajectory of the particles deviate from the vertical. If the weight of the particles is higher than that of the other forces, the particles will continue to fall into the liquid being collected in the collector C2. The light particles continue to rotate and move to the left. The translational force will act afterwards, but the Magnus force will modify its direction having as an effect the removal of the particles from the magnetic drum. The effect of the Magnus force will rise up the particles. In this way, under the influence of the translational force, the light particles will be moved to the left, falling finally in the C1 collector. 4. New industrial application of ECS ECS is efficient skilled in separating NMPs whose size ranged from 5 to 10 mm. The materials contained in large-scale e-waste, such as waste computers, refrigerator cabinets, toner cartridges, can be completely liberated when crushed into 5–10 mm. Thus, ECS may be the most suitable technology for separating NMPs from the crushed large-scale e-waste.
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Fig. 4. (a) Horizontal drum ECS; (b) the sketch of vertical drum eddy current separator; (c) the sketch of the wet eddy current separator.
The lines of recovering waste toner cartridges and refrigerator cabinets had been constructed (Ruan et al., 2011; Ruan and Xu, 2011a,b) (Fig. 5). Waste toner cartridges recovering line was comprised of coarse crushing, magnetic separation, ECS, and bag-type dust collector (Fig. 5(a)). Waste toner cartridge was comprised of steels, toners, aluminum, and plastics (Table 1). Coarse crushing was employed to liberate the materials of waste toner cartridges. Magnetic separation was used to separate steels from the crushed materials. Bag-type dust collector was adopted to collect toners during the crushing process of toner cartridges. ECS was employed as crucial process for separating aluminum from crushed materials. In experiment, about 500 kg various waste toner cartridges with different sizes (5 cm × 5 cm × 18 cm to 10 cm × 12 cm × 40 cm) and types were fed into the recovery line. Aluminum were crushed into flake granule with sizes concentrating on 10 mm × 10 mm × 1 mm to 15 mm × 15 mm × 2 mm, and then separated from other particles by ECS. The recovery rates of steel (magnet), toner, aluminum, and plastic were 98.4%, 95%, 97.5%, and 98.8%, respectively. Waste refrigerator cabinet was comprised of steels, plastics, aluminum, copper, polyurethane foam, and CFC-11 (see Table 1). Thus, the recovering line of waste refrigerator cabinets was comprised of Table 1 Comprised materials of waste toner cartridge and refrigerator cabinet. Comprised materials Steels/iron Plastics Aluminum Copper Toner Magnet CFC-11 Polyurethane foam Total
Waste toner cartridge (wt.%) 39.3 34.6 11.7 – 7.5 6.9 – – 100
Waste refrigerator cabinet (wt.%) 53.5 22.4 6.3 2.5 – – 2.7 12.6 100
closed crushing, activated carbon adsorption tower, magnetic separation, air current separation, and ECS (Fig. 5(b)). Crushing was performed to liberate the materials. Activated carbon adsorption tower was used to collect CFC-11 emitted from polyurethane foam in the crushing process. Air current separation was designed to collect the crushed polyurethane foam powder. Magnetic separation was employed to separate steels from the crushed materials. ECS was adopted to separate aluminum and copper particles from plastic. In experiment, 50 waste refrigerators cabinets (1547.7 kg) with different sizes and types were fed into the automated production line. The comprised materials were liberated from the cabinets in shearing process, and the released CFC-11 gas was adsorbed by the activated carbon adsorption tower. Air current separation collected 196.5 kg polyurethane foam. 828.5 kg ferrous, 347.7 kg plastic, 97.7 kg aluminum, and 39.6 kg copper were obtained by the processes of magnetic/ECS. Recovery rate of cabinet reached 97.6% by the recovery line. Traditional eddy current separators were employed in the two lines. Traditional eddy current separator always has high separation rate (above 95%) in its applications for mineral processing and municipal solid waste treatment (Cui and Forssberg, 2003). Unfortunately, separation rates of ECS in the two recovering lines were less than 85%, far below the standard separation rate of 95%. The reasons of this gap may be: (1) crushing process of e-waste produces complex particles’ shapes (Fig. 5(a)); besides flake, large proportion of cuboid, triangular, and spherical particles, are presented in the liberated materials. The complex shape decrease the effects of ECF so that the separation rate is decreased; (2) nonferrous metals in e-waste are firmly tight combined to inert materials, greater degree of crushing is therefore needed to liberate the nonferrous materials completely; The size of NMPs in crushed e-waste are smaller than in mineral and municipal solid waste; Small size of particle decreases the separation rate; (3) kinds of nonferrous metals that are present in e-waste (Fig. 5(b)), ECS is not an efficient tool to separated different kinds of nonferrous metals as outlined
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Fig. 5. (a) Crushed materials of waste toner cartridges after magnetic separation and toner removed, (b) the flowcharts and the production line of recovering waste toner cartridges, (c) the flowcharts and the production line of recovering of waste refrigerator cabinets.
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(2012038-15) and Development Program of China (863 program 2012AA063206). The authors are grateful to the reviewers who help us improve the paper by many pertinent comments and suggestions. References
Fig. 6. Approaches to improve separation rate of ECS.
by the low separation rate between copper and aluminum in the recovering line of waste refrigerator cabinets. 5. Measures for improving separation rate of ECS in its new industrial application Approaches to improve separation rate of ECS were concluded in Fig. 6. Separation rate of ECS will increase with the increasing volume, cross area, and conductivity of particle. Flake shape particle always gets better separation rate than irregular shape particle. The increasing of intensity of magnetic field, number of magnetic poles, and radius of magnetic drum will improve separation rate. High rotation speed of magnetic field contributes to high separation rate. However, high feeding speed has negative effect on separation rate. In order to improve the separation rate of ECS in its industrial application for e-waste recovery, crushing should be controlled to keep NMP at great size (volume) and two-dimension area as long as the materials can be completely liberated. The shape of sieve mesh should have a circle-like design shape. On the premise of saving cost, eddy current separator should employ high intensity magnets, more pairs of magnetic poles, and small radius of magnetic drum. Rotation speed of magnetic drum should be increased. Meanwhile, feeding speed should keep be as low as possible. New eddy current separator, such as the innovative two-magnetic-drum separator, is the pressing demand for better application of ECS in e-waste recovering growing industry. 6. Conclusion The models of eddy current force and movement behavior of NMP in ECS were summarized in this paper. Meanwhile, the developing process of eddy current separator and its industrial applications in e-waste (waste toner cartridges and refrigerator cabinets) recovering were also presented. For improving separation rate of ECS in industrial application of e-waste recovering, some suggestions were made related to crushing process, separator design, and separator operation. The aim of this paper is to demonstrate the effectiveness of ECS technology as practical and available tool for recovering non-ferrous metals from e-waste which is now being ignored. Acknowledgments This work was supported by the National Natural Science Foundation of China (51308488), the Natural Science Foundation of Jiangsu province (BK20130449), the Science and Technology Cooperation Fund Program of Yangzhou City-Yangzhou University
Benaboua A, Georgesa S. Permanent magnet modeling for dynamic applications. J Magn Magn Mater 2008;320(6):830–5. Cui J, Forssberg E. Mechanical recycling of waste electric and electronic equipment: a review. J Hazard Mater 2003;99(3):243–63. Duan H, Li J, Liu Y, Yamazaki N, Jiang W. Characterization and inventory of PCDD/Fs and PBDD/Fs emissions from the incineration of waste printed circuit board. Environ Sci Technol 2011;45(15):6322–8. Huo X, Peng L, Xu X, Zheng L, Qiu B, Qi Z, et al. Elevated blood lead levels of children in Guiyu, an electronic waste recycling town in China. Environ Health Perspect 2007;115(7):1113–7. Ilgin M, Gupta S. Environmentally conscious manufacturing and product recovery: a review of the state of the art. J Environ Manage 2010a;91(3):563–91. Ilgin M, Gupta S. Comparison of economic benefits of sensor embedded products and conventional products in a multi-product disassembly line. Comput Ind Eng 2010b;59(4):748–63. ¨ Kohnlechner R, Schlett Z, Lungu M, Caizer C. A new wet Eddy-current separator. Resour Conserv Recy 2002;37:55–60. Laner D, Rechberger H. Treatment of cooling appliances: interrelations between environmental protection, resource conservation, and recovery rates. Resour Conserv Recy 2007;52(1):136–55. Leung A, Duzgoren-Aydin N, Cheung K, Wong M. Heavy metals concentrations of surface dust from e-waste recycling and its human health implications in southeast China. Environ Sci Technol 2008;42(7):2674–80. Li J, Xu Z. Environmental friendly automatic line for recovering metal from waste printed circuit boards. Environ Sci Technol 2010;44(4):1418–23. Lungu M, Schlett Z. Vertical drum eddy-current separator with permanent magnets. Int J Miner Process 2001;63:207–16. Lungu M. Separation of small nonferrous particles using an angular rotary drum eddy-current separator with permanent magnets. Int J Miner Process 2005;78:22–30. Maraspin F, Bevilacqua P, Rem P. Modeling the throw of metals and nonmetals in eddy current separations. Int J Miner Process 2004;73(1):1–11. Meier-Staude R, Schlett Z, Lungu M. A new possibility in eddy-current separation. Miner Eng 2002;15:287–91. Ongondo F, Williams I, Cherrett T. How are WEEE doing? A global review of the management of electrical and electronic wastes. Waste Manage 2011;31(4):714–30. Peterson W. Numerical solution of eddy current problems in ferromagnetic bodies travelling in a transverse magnetic field. Int J Numer Methods Eng 2003;58(12):1749–64. Rahman M, Bakker M. Sensor-based control in eddy current separation of incinerator bottom ash. Waste Manage 2013;33:1418–24. Rem P, Leest P, van den Akker A. A model for eddy current separation. Int J Miner Process 1997;49(3–4):193–200. Robinson B. E-waste: an assessment of global production and environmental impacts. Sci Total Environ 2009;408(2):183–91. Rolicz P. Eddy currents generated in a system of two cylindrical conductors by a transverse alternating magnetic field. Electr Power Syst Res 2009;79(2):295–300. Ruan J, Li J, Xu Z. An environmental friendly recovery production line of waste toner cartridges. J Hazard Mater 2011;185(2–3):696–702. Ruan J, Xu Z. A new model of repulsive force in eddy current separation for recovering waste toner cartridges. J Hazard Mater 2011a;192(1):307–13. Ruan J, Xu Z. Environmental friendly automated line for recovering the cabinet of waste refrigerator. Waste Manage 2011b;31(11):2319–26. Ruan J, Xu Z. Approaches to improve separation efficiency of eddy current separation for recovering aluminum from waste toner cartridges. Environ Sci Technol 2012a;46(11):6214–21. Ruan J, Xu Z. Risks in the physical recovery system of waste refrigerator cabinets and the controlling measure. Environ Sci Technol 2012b;46(24):13386–92. Schlett Z, Lungu M. Eddy-current separator with inclined magnetic disc. Miner Eng 2002;15(5):365–7. Stone R. Confronting a toxic blowback from the electronics trade. Science 2009;325:1055. Widmera R, Oswald-Krapf H, Sinha-Khetriwal D, Schnellmann M, Boni H. Global perspectives on e-waste. Environ Impact Assess Rev 2005;25:436–58. Wu J, Li J, Xu Z. Electrostatic separation for recovering metals and nonmetals from waste printed circuit board: problems and improvements. Environ Sci Technol 2008;42(14):5272–6. Zhang S, Forssberg E. Separation mechanisms and criteria of a rotating eddy-current separator operation. Resour Conserv Recy 1999;25:215–32. Zhang S, Forssberg E, Arvidson B, Moss W. Aluminum recovery from electronic scrap by High-Force eddy-current separator. Resour Conserv Recy 2002;37:55–60. Zhang S, Rem P. Particle trajectory simulation of two-drum eddy current separators. Resour Conserv Recy 1999;26:71–90. Zhou L, Xu Z. Response to waste electrical and electronic equipments in China: legislation, recycling system, and advanced integrated process. Environ Sci Technol 2012;46(9):4713–24.
Contents lists available at ScienceDirect
Resources, Conservation and Recycling journal homepage: www.elsevier.com/locate/resconrec
Review
Environment-friendly technology for recovering nonferrous metals from e-waste: Eddy current separation Ruan Jujun a,b , Qian Yiming a , Xu Zhenming b,∗ a Jiangsu Key Laboratory of Environmental Material and Environmental Engineering, School of Environmental Science and Engineering, Yangzhou University, 196 West Huayang Road, Yangzhou, Jiangsu 225127, China b School of Environmental Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 20 November 2013 Received in revised form 16 February 2014 Accepted 28 March 2014 Keywords: e-Waste Nonferrous metals Recovery Eddy current separation
a b s t r a c t The current generation pattern of e-waste consisted of dead electronic and electrical equipments poses one of the world’s greatest pollution problem due to the lack of appropriate recovery technology. Crude recovery methods of resource materials (aluminum, zinc, copper, lead, gold) from e-waste caused serious pollution in China in the past years. Thus, environment-friendly technologies have been the pressing demand in e-waste recovering. Eddy current separation (ECS) was advised as the preferable technology for recovering nonferrous metals from e-waste. However, just a few reports focused on the application of ECS in e-waste recovering. This paper introduced the information about ECS including the models of eddy current force and movement behavior of nonferrous metallic particle in the separation process. Meanwhile, the developing process of eddy current separator was summarized. New industrial applications of ECS in e-waste (waste toner cartridges and refrigerator cabinets) recovering were also presented. Finally, for improving separation rate of ECS in industrial application of e-waste recovering, some suggestions were proposed related to crushing process, separator design, and separator operation. The aim of this paper is to demonstrate the effectiveness of ECS technology as practical and available tool for recovering non-ferrous metals from e-waste which is now being ignored. © 2014 Elsevier B.V. All rights reserved.
Contents 1. 2.
3.
4. 5. 6.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Eddy current force and eddy current separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Eddy current force . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Eddy current separation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Efforts for improving separation rate of ECS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Influencing of particle characteristics on separation rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Influence of operation conditions on separation rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Development of eddy current separator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . New industrial application of ECS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measures for improving separation rate of ECS in its new industrial application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
∗ Corresponding author. Tel.: +86 21 54747495; fax: +86 21 54747495. E-mail addresses: [email protected], [email protected] (X. Zhenming). http://dx.doi.org/10.1016/j.resconrec.2014.03.017 0921-3449/© 2014 Elsevier B.V. All rights reserved.
110 110 110 112 112 112 113 113 113 116 116 116 116
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Nomenclatures bn Bm
Fourier coefficient magnetic flux density of the magnetic drum surface (T) Bp induced magnetic flux density in the particle (T) Br (B, Ba ) magnetic flux density of the field (T) Fr repulsive force between the particle and the magnet (N) acceleration of gravity force (m/s2 ) g G gravity force of the aluminum flake (N) J (j) induced eddy current in particle (flakes) (A) k pairs of the magnets placed in the magnetic drum l height of the rectangle/triangle flake (m) circumference of the triangle flake (m) L r radius distance between the particle and the center of magnetic drum (m) radial distance between the particle and the inducr ing magnet (m) R (Rdrum ) radius of the magnetic drum (m) Sm per magnet side area which facing the flake (m3 ) Sp (d) maximal cross area of the flake in horizontal (m2 ) t time cost for the magnetic field rotation (s) thickness of the flake (m) T v (˝) feeding speed of the particle (flake) (m/s) v relative linear velocity between the flake and magnetic drum (m/s) volume of the particle (rectangle/triangle flake) V (m3 ) w width of the rectangle/triangle flake (m) ˝ resistance of circular/rectangle/triangle coil angle of the coordinate in the cylindrical coordinate ˛0 system (, ) conductivity of the flake (S/m) ı (s) oriental (shape) factor of the flake in eddy current separation induced emf in the circular/rectangle/triangle coil εi (V) relative magnetic permeability of iron (H/m) r 0 magnetic permeability of vacuum (H/m) ωm (ωdrum , ω) rotation velocity of the magnetic drum (rad/s) induced magnetic flux in the particle (Wb) Фp Фm magnetic flux of the permanent magnet (Wb) Ф variation of magnetic flux in the particle (Wb/s) w( ) width of pole
1. Introduction Quantities of e-waste are generating resulted from the use of electronic and electrical products. Computer accessories and mobile telephones are disproportionately abundant because of their short lifespan. The current global production of e-waste is estimated to be 20–25 million tons per year and about 95% useful materials were recovered (Robinson, 2009). About 2.5 million tons e-waste appeared in Chinese mainland including self-generated and imported from developed countries per year (Ongondo et al., 2011; Stone, 2009; Widmera et al., 2005). Fig. 1 shows samples of nonferrous metals found in e-waste. Waste PCB contains nearly 28% metals including copper, zinc, and other nonferrous metals (Li and Xu, 2010). Waste toner cartridge has 11.7% aluminum (Ruan et al., 2011). Waste refrigerator cabinet includes about 8.9% copper and aluminum (Ruan and Xu, 2011a,b). Additionally, purity of metals in e-waste is higher than that of rich-content minerals (Laner and
Rechberger, 2007; Ilgin and Gupta, 2010a,b). Thus, recovering ewaste can bring great economic benefits. Environmental sound technologies of recovering e-waste are developing challenges today. In the early stage, crude technologies (acid-washing or open incineration) were employed (Ruan and Xu, 2012a,b; Ilgin and Gupta, 2010a,b) and resulted to serious environmental pollution by the hazardous materials contained in e-waste (Duan et al., 2011; Huo et al., 2007; Leung et al., 2008). Then, for the sake of environmental protection and clean recovery of nonmetals from e-waste, it was proposed a procedure including crushing process and psychical separations of screen, shape sorting, jigging, magnetic separation, air current separation, corona-electrostatic separation, and eddy current separation (ECS) (Zhou and Xu, 2012; Cui and Forssberg, 2003). However, each technology has special limitation. Screen and shape sorting cannot separate the particles that have similar size and shape. Jigging brings waste water in separation process. Magnetic separation can only separate ferrous metals. Air current separation demands particles having great density difference when being in similar size. Corona-electrostatic separation and ECS are the preferable technologies for recovering nonferrous metals from e-waste. Corona-electrostatic is skilled in separating nonferrous metallic particles (NMP) less than 1 mm in size (Wu et al., 2008). ECS is adept in separating NMPs ranged from 2 to 50 mm in size. ECS may be the fittest technology for recovering nonferrous metals from large-scale (coarse crushing is enough for liberating the materials) e-waste (Zhang et al., 2002; Benaboua and Georgesa, 2008). ECS is an environment-friendly technology for separating nonferrous metals from solid waste. In separation process, eddy current is induced in nonferrous metal when meeting variable magnetic field. Interaction between eddy current and magnetic field changes the trajectory of nonferrous metal as well as separates them from others. No waste water, air pollution, and solid waste are generated in the separation process. Unfortunately, the public paid less attention on this environment-friendly technology. This paper discussed ECS technology from the models of eddy current force (ECF), models of particle movement behavior, developing of separator, and new industrial application standpoints for e-waste recovering. Furthermore, suggestions for improving separation rate of ECS are presented.
2. Eddy current force and eddy current separation 2.1. Eddy current force ECS is a physical method for separating nonferrous metals from inert materials. ECF is the cause of ECS. In general, eddy current separator is comprised of magnetic drum. The magnetic drum always consists of magnetic poles placed in N–S–N (see Fig. 2). A changing magnetic field will be induced by the rotation of magnetic drum (Peterson, 2003; Rolicz, 2009). There are two recognitions about the generation of ECF. Recognition (1): eddy current will appear in NMP when it experiences the changing magnetic field; a repulsive force will be generated between magnetic field and NMP possessing eddy current; this repulsive force is called ECF. Recognition (2): direction of the magnetic field changes constantly because of the running of magnetic drum; consequently, the direction of eddy current induced in NMP also changes continuously resulting in the generation of a new magnetic field in NMP; direction of the new magnetic field is also changed constantly; the two magnetic fields have the same directions and repulse each other; this repulsion is called ECF. Because of being the most important influencing factor of ECS, many models for computing ECF were constructed. The models of ECF established from recognition (1) including:
R. Jujun et al. / Resources, Conservation and Recycling 87 (2014) 109–116
111
Fig. 1. Nonferrous metals in e-waste: (a) waste PCBs, (b) waste toner cartridges, (c) waste refrigerator cabinet.
(i) Models of radial and tangential components of ECF (Rem et al., 1997; Zhang and Rem, 1999): Ft =
2 sV (kωdrum + ˝) Ba2 2 2 0 w 1 + (kω drum + ˝)
Ft =
2 sV (kωdrum + ˝) 2 Ba2 2 2 0 w 1 + (kω drum + ˝)
(1)
2
(2)
The models indicated that ECF was determined by the parameters of separator operation, particle characteristics, and separator design. The force increased with the increasing of particle volume, intensity of magnetic field, and decreased with the increasing of width of magnetic pole. Additionally, particle shape, pairs of magnetic poles, rotation speed of magnetic drum, and angular velocity of particle influenced the magnitude of ECF. Parameters of particle volume and shape can guide crushing process for liberating nonferrous metals. Parameters of intensity of magnetic field, pairs and width of
magnetic poles can be used to guide the manufacture of eddy current separator. Parameters of rotation speeds of magnetic drum and particle can guide the operation of ECS. (ii) The modified models of radial and tangential components of ECF (Lungu and Schlett, 2001): Fn = s0 (ω − ˝)R2 Ft Ft =
2 R T
R
(3) (4)
T=
r × fdV
(5)
V
f = j × BdV
(6)
These models introduced the parameter of radius distance between the particle and the center of magnetic drum as well as adding the parameter of eddy current induced in particle.
Fig. 2. NMP was separated from nonmetal because of eddy current force in ECS.
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R. Jujun et al. / Resources, Conservation and Recycling 87 (2014) 109–116
(iii) Models for computing the moment of ECF (Maraspin et al., 2004): Fm
(k + 1)Bˆ 2 V = 0 r
R( )
(7)
I( )
Bˆ 2 V I 0 ( )
(8)
= 0 (kωm + ˝)d2
(9)
Tm = −
Besides the parameters contained in models (i) and (ii), maximal cross area of particle in horizontal was introduced in model (iii). This parameter could guide the crushing process more specifically and predicted that flake shape brought positive influence on ECF. The models of ECF established from recognition (2) (Ruan and Xu, 2011a,b): Model for circle flake: Fr C =
Br k(ωm R − v)VSp Bm Sm
1
16 3 R3
(sec˛0 − 1)2
(10)
Models for rectangle flake were: Fr R = ıR =
Br k(ωm R − v)VSp Bm Sm ıR
1
16 2 R3
(sec˛0 − 1)2
W 2(L + W )
(11)
Models for triangle flake were: Fr T =
Br k(ωm R − v)VSp Bm Sm ıT
1
16 2 R3
(sec˛0 − 1)2
L ıT = C
(12)
The parameters, contained in the above models of ECF, could be used to guide crushing, separator operation, and separator design. Compared the models established from recognitions (1) and (2), the models established from recognition (2) paid more attention to the influencing of shape of particle to ECF, and were more specific and accurate. Precisely, the application range was limited and smaller than the models established from recognition (1). 2.2. Eddy current separation ECF causes ECS. By neglecting air friction, NMP in ECS are subject to ECF and gravity force. Due to the absence of electrical conductivity, nonmetals are only subjects to gravity force. Thus, NMP are separated from nonmetals due to the different movement behaviors in ECS. The sketch map of the movement behavior of NMP and nonmetal are presented in Fig. 2. ECS also can be used to separate different nonferrous metals as long as they have different electrical conductivities. Different conductivities of nonferrous metals cause different magnitudes of ECF in order to bring distinguished movement behaviors of NMPs. For improving separation rate of ECS, the movement behavior of NMP was studied and the trajectory models were constructed. The models contained abundant influencing factors of ECS and could be used to predict separation results. Based on the models of (i), (ii), (ii) above, movement behavior of NMP in ECS was simulated by computer software (Maraspin et al., 2004; Zhang and Forssberg, 1999). Based on the models of ECF constructed from cognition (2), trajectory models of NMP in different position inside and outside the magnetic field (Fig. 3) of ECS were constructed (Ruan and Xu, 2012a,b).
Movement behavior of NMP in ECS is divided into three stages: (1) entering magnetic field, (2) detaching from conveyor belt surface, (3) exiting from magnetic field (see Fig. 3). At the beginning of stage (1), eddy current force increases as the particle moving close to magnetic drum, and the force is divided into vertical component and horizontal component. Horizontal component is counteracted by the friction force of conveyor belt and no relative motion happens between NMP and belt. As the increasing eddy current force, vertical component will be greater than the gravity force (G) of the flake, and NMP will have a verticalupward acceleration and move upward. Meanwhile, eddy current force will decrease along with the upward movement of NMP due to the decline of magnetic flux of magnetic drum. When the vertical component is equal to gravity force, NMP will be suspended and keep constant radial distance to the axis (O) of separator. The position of the flake is called point (x0 , y0 ). Point (x0 , y0 ) can be considered as the detachment point of NMP from separator surface. Meanwhile, point (x1 , y1 ) is supposed as the symmetry point of (x0 , y0 ) at the right of Y-axis (Fig. 3). The movement of NMP from point (x0 , y0 ) to point (x1 , y1 ) can be considered as rectilinear motion. At this rectilinear movement, magnetic fluxes of NMP and magnetic drum is considered to be parallel, horizontal component of eddy current force can be neglected, and repulsive force can be supposed as equal to its gravity force. When NMP passes over point (x1 , y1 ), vertical component of eddy current force will be less than gravity force. Horizontal component of repulsive force can no longer be neglected since the directions of the two magnetic fluxes will no longer been parallel. Horizontal component will accelerate NMP in horizontal direction until it passes through the boundary of the magnetic field. Furthermore, the movement of NMP in vertical direction is controlled by gravity force and vertical component of ECS. We suppose point (x2 , y2 ) as the exiting position of NMP from the magnetic field. As passing through point (x2 , y2 ), NMP only subjects to gravity force and the movement can be considered as horizontal projectile motion. Due to no response to magnetic field, movement behavior of plastic in ECS is considered as horizontal projectile motion. The trajectory models of NMP were given as Eq. (13). New influencing factors of magnetic field boundary and collection position were introduced in the trajectory models.
⎧
v 2 v 2 ⎪ ⎪ 1 2(x − x ) 1 ⎪ ⎪ + − + y1 y = − (g − ay ) ⎪ ⎪ 2 ax ax ax ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ xb2 + yb2 = (R + )2 ⎪ ⎪ ⎪ ⎪ ⎨ g sin a1 ax =
2
⎪ ⎪ g cos ˛1 ⎪ ⎪ ⎪ ay = ⎪ 2 ⎪ ⎪ ⎪ ⎪ = Rtg˛ x ⎪ 1 0 ⎪ ⎪ ⎪ ⎪ ⎪ ⎩ y1 = R
(13)
cos ˛0
3. Efforts for improving separation rate of ECS Experiments of ECS were performed to study the influence of particle characteristics and operation conditions (convenient to be controlled in ECS and crushing) on separation rate. 3.1. Influencing of particle characteristics on separation rate ECS of aluminum, copper, zinc, lead, and PVC sheets showed metal sheets that of differing in the ratio of electrical conductivity to density could be separated successfully by the condition of having
R. Jujun et al. / Resources, Conservation and Recycling 87 (2014) 109–116
113
Fig. 3. Analysis of the forces and movement behaviors of NMP and nonmetal in and out of the magnetic field.
the similar size and shape (Zhang and Forssberg, 1999). The metal sheets that differ in shape could be separated as long as having similar size and ratio of electrical conductivity to density. The metal sheets that differ in size could also be separated if they have similar shape and ratio of electrical conductivity to density. The metal sheets that differ in size, shape, and ratio of electrical conductivity to density could not be separated effectively. Particle size was of critical consideration. Particle size below 2 mm was not effectively responsive to the separator regardless of how the operating parameters were adjusted. Additionally, ECS was unable to recover nonferrous metallic foils. Particle’s shape was one of the most influential variables. Irregular shape of particle minimized ECF. ECF of plate particle was greater than those of ball-shaped particles and irregular shapes (Zhang et al., 2002). Additionally, moisture was another important influential factor. Wet fed would decrease the separation rate of ECS. Small metal particles would stick to the feed conveyor belt (Rahman and Bakker, 2013). 3.2. Influence of operation conditions on separation rate Feeding the particles with slight wet to traditionally eddy current separator could separate NMPs in the range from 2 to 6 mm ¨ et al., 2002). Rotor speeds should from waste streams (Kohnlechner be relatively low so as to keep ECF being used only to break the wet bond between NMPs and belt surface. Feeding system and feed speed significantly influenced separation rate. Multilayer feed stream could seriously deteriorate the separation process. NMPs were shielded from approaching the effective magnetic field by nonmetallic particles. Nonmetallic particles could flip away together with NMPs. Apparently, a monolayer feed stream could ensure effective separation (Zhang et al., 2002). Results of orthogonal experiment of ECS showed that difference between feeding speed and rotation speed of magnetic field (ωR − v) was the critical influencing factor on separation rate. Feeding speed (v) was general factor, and the collection position (H) was subordinate factor (Ruan and Xu, 2012a,b).
A splitter with appropriate position involved in eddy current separator during ECS was of great importance. Splitter could separate NMP timely when it deflected from nonmetals caused by ECF (Schlett and Lungu, 2002). 3.3. Development of eddy current separator The traditional eddy current separator was horizontal magnetic drum separator (see Fig. 4(a)). For improving the separation rate, vertical drum eddy current separator (Fig. 4(b)) was developed to separate small conductive nonferrous particles whose dimensions were within 2–8 mm (Lungu and Schlett, 2001; Lungu, 2005). Wet eddy current separator (Meier-Staude et al., 2002) was developed to separate the NMP whose particle size was from 2 to 5 mm. The sketch was given in Fig. 4(c), where: T, magnetic drum with 4 poles; I, dielectric chamber for liquid having two collectors C1 and C2; A, feeding system. The particles falling from the feeder in to the liquid are subjected to both Magnus force and translational force. The gravity and buoyancy force act upon the particles as well. The Magnus and the translational forces will make the trajectory of the particles deviate from the vertical. If the weight of the particles is higher than that of the other forces, the particles will continue to fall into the liquid being collected in the collector C2. The light particles continue to rotate and move to the left. The translational force will act afterwards, but the Magnus force will modify its direction having as an effect the removal of the particles from the magnetic drum. The effect of the Magnus force will rise up the particles. In this way, under the influence of the translational force, the light particles will be moved to the left, falling finally in the C1 collector. 4. New industrial application of ECS ECS is efficient skilled in separating NMPs whose size ranged from 5 to 10 mm. The materials contained in large-scale e-waste, such as waste computers, refrigerator cabinets, toner cartridges, can be completely liberated when crushed into 5–10 mm. Thus, ECS may be the most suitable technology for separating NMPs from the crushed large-scale e-waste.
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Fig. 4. (a) Horizontal drum ECS; (b) the sketch of vertical drum eddy current separator; (c) the sketch of the wet eddy current separator.
The lines of recovering waste toner cartridges and refrigerator cabinets had been constructed (Ruan et al., 2011; Ruan and Xu, 2011a,b) (Fig. 5). Waste toner cartridges recovering line was comprised of coarse crushing, magnetic separation, ECS, and bag-type dust collector (Fig. 5(a)). Waste toner cartridge was comprised of steels, toners, aluminum, and plastics (Table 1). Coarse crushing was employed to liberate the materials of waste toner cartridges. Magnetic separation was used to separate steels from the crushed materials. Bag-type dust collector was adopted to collect toners during the crushing process of toner cartridges. ECS was employed as crucial process for separating aluminum from crushed materials. In experiment, about 500 kg various waste toner cartridges with different sizes (5 cm × 5 cm × 18 cm to 10 cm × 12 cm × 40 cm) and types were fed into the recovery line. Aluminum were crushed into flake granule with sizes concentrating on 10 mm × 10 mm × 1 mm to 15 mm × 15 mm × 2 mm, and then separated from other particles by ECS. The recovery rates of steel (magnet), toner, aluminum, and plastic were 98.4%, 95%, 97.5%, and 98.8%, respectively. Waste refrigerator cabinet was comprised of steels, plastics, aluminum, copper, polyurethane foam, and CFC-11 (see Table 1). Thus, the recovering line of waste refrigerator cabinets was comprised of Table 1 Comprised materials of waste toner cartridge and refrigerator cabinet. Comprised materials Steels/iron Plastics Aluminum Copper Toner Magnet CFC-11 Polyurethane foam Total
Waste toner cartridge (wt.%) 39.3 34.6 11.7 – 7.5 6.9 – – 100
Waste refrigerator cabinet (wt.%) 53.5 22.4 6.3 2.5 – – 2.7 12.6 100
closed crushing, activated carbon adsorption tower, magnetic separation, air current separation, and ECS (Fig. 5(b)). Crushing was performed to liberate the materials. Activated carbon adsorption tower was used to collect CFC-11 emitted from polyurethane foam in the crushing process. Air current separation was designed to collect the crushed polyurethane foam powder. Magnetic separation was employed to separate steels from the crushed materials. ECS was adopted to separate aluminum and copper particles from plastic. In experiment, 50 waste refrigerators cabinets (1547.7 kg) with different sizes and types were fed into the automated production line. The comprised materials were liberated from the cabinets in shearing process, and the released CFC-11 gas was adsorbed by the activated carbon adsorption tower. Air current separation collected 196.5 kg polyurethane foam. 828.5 kg ferrous, 347.7 kg plastic, 97.7 kg aluminum, and 39.6 kg copper were obtained by the processes of magnetic/ECS. Recovery rate of cabinet reached 97.6% by the recovery line. Traditional eddy current separators were employed in the two lines. Traditional eddy current separator always has high separation rate (above 95%) in its applications for mineral processing and municipal solid waste treatment (Cui and Forssberg, 2003). Unfortunately, separation rates of ECS in the two recovering lines were less than 85%, far below the standard separation rate of 95%. The reasons of this gap may be: (1) crushing process of e-waste produces complex particles’ shapes (Fig. 5(a)); besides flake, large proportion of cuboid, triangular, and spherical particles, are presented in the liberated materials. The complex shape decrease the effects of ECF so that the separation rate is decreased; (2) nonferrous metals in e-waste are firmly tight combined to inert materials, greater degree of crushing is therefore needed to liberate the nonferrous materials completely; The size of NMPs in crushed e-waste are smaller than in mineral and municipal solid waste; Small size of particle decreases the separation rate; (3) kinds of nonferrous metals that are present in e-waste (Fig. 5(b)), ECS is not an efficient tool to separated different kinds of nonferrous metals as outlined
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Fig. 5. (a) Crushed materials of waste toner cartridges after magnetic separation and toner removed, (b) the flowcharts and the production line of recovering waste toner cartridges, (c) the flowcharts and the production line of recovering of waste refrigerator cabinets.
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(2012038-15) and Development Program of China (863 program 2012AA063206). The authors are grateful to the reviewers who help us improve the paper by many pertinent comments and suggestions. References
Fig. 6. Approaches to improve separation rate of ECS.
by the low separation rate between copper and aluminum in the recovering line of waste refrigerator cabinets. 5. Measures for improving separation rate of ECS in its new industrial application Approaches to improve separation rate of ECS were concluded in Fig. 6. Separation rate of ECS will increase with the increasing volume, cross area, and conductivity of particle. Flake shape particle always gets better separation rate than irregular shape particle. The increasing of intensity of magnetic field, number of magnetic poles, and radius of magnetic drum will improve separation rate. High rotation speed of magnetic field contributes to high separation rate. However, high feeding speed has negative effect on separation rate. In order to improve the separation rate of ECS in its industrial application for e-waste recovery, crushing should be controlled to keep NMP at great size (volume) and two-dimension area as long as the materials can be completely liberated. The shape of sieve mesh should have a circle-like design shape. On the premise of saving cost, eddy current separator should employ high intensity magnets, more pairs of magnetic poles, and small radius of magnetic drum. Rotation speed of magnetic drum should be increased. Meanwhile, feeding speed should keep be as low as possible. New eddy current separator, such as the innovative two-magnetic-drum separator, is the pressing demand for better application of ECS in e-waste recovering growing industry. 6. Conclusion The models of eddy current force and movement behavior of NMP in ECS were summarized in this paper. Meanwhile, the developing process of eddy current separator and its industrial applications in e-waste (waste toner cartridges and refrigerator cabinets) recovering were also presented. For improving separation rate of ECS in industrial application of e-waste recovering, some suggestions were made related to crushing process, separator design, and separator operation. The aim of this paper is to demonstrate the effectiveness of ECS technology as practical and available tool for recovering non-ferrous metals from e-waste which is now being ignored. Acknowledgments This work was supported by the National Natural Science Foundation of China (51308488), the Natural Science Foundation of Jiangsu province (BK20130449), the Science and Technology Cooperation Fund Program of Yangzhou City-Yangzhou University
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