Application of Electrostatic Separation to the Recycling of Plastic

Publication Date (Web): November 30, 2007. Copyright ... The aim of this work is to develop a reliable and relatively inexpensive technology for the e...
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Environ. Sci. Technol. 2008, 42, 249–255

Application of Electrostatic Separation to the Recycling of Plastic Wastes: Separation of PVC, PET, and ABS C H U L - H Y U N P A R K , †,‡ H O - S E O K J E O N , * ,‡ HYO-SHIN YU,‡ OH-HYUNG HAN,§ AND JAI-KOO PARK† Hanyang University, Department of Geoenvironmental System Engineering, Seoul, Korea, Korea Institute of Geoscience and Mineral Resources, Daejeon, Korea, Chosun University, Department of Resource Engineering, Gwangju, Korea

Received March 21, 2007. Revised manuscript received October 17, 2007. Accepted October 17, 2007.

Plastics are widely used in everyday life as a useful material, and thus their consumption is growing at a rate of about 5% per year in Korea. However, the constant generation of plastic wastes and their disposal generates environmental problems along with economic loss. In particular, mixed waste plastics are difficult to recycle because of their inferior characteristics. A laboratory-scale triboelectrostatic separator unit has been designed and assembled for this study. On the basis of the control of electrostatic charge, the separation of three kinds of mixed plastics, polyvinyl chloride (PVC), poly(ethylene terephthalate) (PET), and acrylonitrile-butadienestyrene (ABS), in a range of similar gravities has been performed through a two-stage separation process. Polypropylene (PP) and high-impact polystyrene (HIPS) were found to be the most effective materials for a tribo-charger in the separation of PVC, PET, and ABS. The charge-to-mass ratio (nC/g) of plastics increased with increasing air velocity in the tribo charger. In the first stage, using the PP cyclone charger, the separation efficiency of particles considerably depended on the air velocity (10 m/s), the relative humidity (20 kV), and the splitter position (+2 cm from the center) in the triboelelctrostatic separator unit. At this time, a PVC grade of 99.40% and a recovery of 98.10% have successfully been achieved. In the second stage, using the HIPS cyclone charger, a PET grade of 97.80% and a recovery of 95.12% could be obtained under conditions of 10 m/s, over 25 kV, a central splitter position, and less than 40% relative humidity. In order to obtain 99.9% PVC grade and 99.3% PET grade, their recoveries should be sacrificed by 20.9% and 27%, respectively, with moving the splitter from the center to a (+)6 cm position.

1. Introduction The worldwide production of plastics has reached a level of 150 million tons per year, and plastic consumption keeps growing at a rate of approximately 5% annually (1). About 4 million tons of plastic wastes are generated every year in * Corresponding author tel.: +82-42- 868-3582; e-mail: hsjeon@ kigam.re.kr. † Hanyang University. ‡ Korea Institute of Geoscience and Mineral Resources. § Chosun University. 10.1021/es070698h CCC: $40.75

Published on Web 11/30/2007

 2008 American Chemical Society

Korea, but only less than 30% of them are recycled (2, 3). Plastic wastes generated from the automotive (4), electronic, and IT fields (5, 6) as well as from packaging materials, fluid containers, clothing, and household products are constantly increasing due to the industrial development and short life span of this equipment (7–9). The incineration and landfill deposition of municipal solid wastes may cause environmental problems and become more expensive. In this regard, the disposal of plastic wastes has become an important issue all over the world due to their increasing volume and the decreasing landfill capacity for disposal (10–14). Plastic wastes can release hazardous substances during disposal procedures or under normal geoenvironmental conditions, and their additives are also released by leaching and contact transference (1). Polyvinyl chloride (PVC) in plastics pollutes the environment by generating hazardous hydrogen chloride gas and dioxins containing chlorine. Also, it decreases the recycling ratio of plastics by forming compounds or deteriorating the nature of other materials even if a small quantity of PVC is present in the main plastics (1, 15). Thus, the development of a material separation technique that can recycle plastic wastes as well as solve disposal problems is a growing necessity. Physical separation techniques that can recycle mixed plastics are classified as electrostatic separation, dry and wet gravity separation, froth flotation, near-infrared ray (NIR), and color sorting (16–18). Poly(ethylene terephthalate) (PET) in the bottles has been separated from polypropylene (PP) or polyethylene (PE) by a combination of wet gravity separation and froth flotation (19). The heavy group plastics such as PVC, PET, polystyrene (PS), and polycarbonate (PC) have also been separated from the light groups, such as highdensity polyethylene (HDPE), low-density polyethylene (LDPE), and PP, using water as a fluidization medium (20). It was reported that PVC of 99.3% purity was separated from PET using reagents such as calcium lignin sulfonate and methyl isobutyl carbinol (21). Wet gravity separation and froth flotation of plastic wastes are considered costly compared to dry separation. In a wet separation process, mixed PVC, PET, acrylonitrile-butadiene-styrene (ABS), rubber, and PC may raise more difficulties because similar specific gravities and flotation agents may cause water disposal problems (16, 17, 22). In NIR, the potentiality of a reliable distinction between five major plastics, that is, PE, PET, PP, PS, and PVC, in household garbage was reported by Huth-Fehre and Feldhop (23). Wienke obtained a median sorting purity of higher than 98% for nonblack plastics. At this time, more than 75 samples per second can be identified with the combination of a InGaAs diode array and a neural network (24). However, this is a difficult task due to the close similarities between the materials, and it needs a further reduction of shadow contributions and a stabilization of the sensor or light source to obtain reproducible measurements, and color sorting is not effective for the separation of particles having similar properties (25–28). Tribocharging occurs when particles are charged with opposite polarities by particle–particle and particle-surface charging mechanisms due to their work function (29–32). The charge transfer mechanism in tribocharging can be explained by three models: electron transfer, ion transfer, and material transfer (32, 33). It has been reported that an electron transfer among them mainly affects the contact charging between materials (32–36). In the process of tribocharging, two materials that are brought into contact or collision can undergo charge transfer according to their work VOL. 42, NO. 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Schematic diagram for a triboelectrostatic separator unit [body, 1300-mm-high and 350-mm-wide; electrode, 450mm-high and 250-mm-wide; pipe, 450-mm-long and 16 mm in diameter; cyclone, 350-mm-high and 150-mm-wide (top); bin, 150-mm-high and 110-mm-wide (top)]. function difference until the point at which their Fermi levels equalize. The final charge will actually be the outcome of two processes: the charge transfer occurs during the contact between materials, and the charge backflow also occurs as they separate (30, 32). Hence, the selective charging of materials is an important parameter, and the triboelectric series is widely used as an indicator of selectively charging plastics in triboelectrostatic separation. The triboelectric series which represents the order of charge polarity of a plastic material may be arranged in the following sequence: (negative) PVC, a copolymer of polypropylene (COPP), homopolypropylene (HOMOPP), LDPE, HDPE, PET, rubber, high-impact polystyrene (HIPS), Calibre, ABS, and polymethyl methacrylate (PMMA) (positive) (13, 37). The types of commonly used tribochargers include cyclone (38, 39), vibrating feeder (43, 44), fluidized bed (45, 46), inclined rotating drum (41, 47), honeycomb, and spiral tube charger. Inculet et al. (41) separated PVC, nylon, PE, and acrylic using a fluidized bed and a rotating tube. Pearse and Hickey (38) and Yanar and Kwetkus (39) measured the charge-to-mass ratio of plastics with a nickel and copper cyclone, using a Faraday cage in consideration of air velocity and relative humidity. Fujita et al. (40) and Higashiyama et al. (43) measured the charge-to-mass ratio of plastics related to the material of a vibrating feeder. Matsushita separated PVC, PE, PP, and PS using rotary blades (42). They reported that separation efficiency increased with decreasing relative humidity and with increasing air velocity and electric field. This work aims at developing a reliable and relatively inexpensive technology for the effective triboelectrostatic separation of PVC, PET, and ABS by a two-stage separation process, based on the control of the charge polarity and charge-to-mass ratio of those particles. Particularly, our study is focused on estimating process variables such as air velocity, electrode potential, splitter position, and relative humidity that affect the separation efficiency of the plastics. Therefore, we designed and assembled a laboratory-scale triboelectrostatic separator unit, including a fluidized bed cyclone tribocharger, and evaluated a charger material affecting the selective charging polarity and charge-to-mass ratio of plastics.

2. Experimental 2.1. Materials and Separation Method. PVC pipe scrap and PET bottles were collected from household wastes and ABS pellets obtained from a local petrochemical plant. PVC, PET, 250

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FIGURE 2. Flowchart for the separation of PVC, PET, and ABS in a two-stage process.

FIGURE 3. Charge polarity and charge-to-mass ratio of PVC, PET, and ABS in relation to charger material (10 m/s air velocity and 30% relative humidity). and ABS at an equal weight ratio were mixed. The triboelectrostatic method was employed for separation tests. Figure 1 shows a schematic diagram of a triboelectrostatic separator and peripheral equipment used in this work. It consists of a feeding zone (6, 7, 8, and 9), charging zone (1), separation zone (2, 3, and 4), and collecting zone (5). Figure 2 shows a flow sheet for the separation of PVC, PET, and ABS in a twostage process. They were shredded using a cutting mill (pulverizette 19, Fritsch GmbH, Germany), and then a representative fraction of the plastics was sieved as -3.35 +

FIGURE 4. Effect of air velocity on the separation efficiency of PVC, PET, and ABS in a triboelectrostatic separation unit (first stage tests: 20 kV electrode potential, +2 cm splitter position, and 30% relative humidity; second stage tests: 25 kV electrode potential, central splitter position, and 40% relative humidity). 1.18 mm. The charge of the plastic particles induced by the cutting mill was neutralized with a discharger (Kasuga Denki Inc., Japan). In the first stage, the mixture was fed into a cyclone charger made of PP with flowing air. Then, PVC was negatively charged and PET and ABS positively charged by the charger. The negatively charged PVC particles were deflected to the positive electrode under the influence of the electric field between the electrodes and the positively charged PET and ABS to the negative electrode. In the second stage, the charged PET and ABS particles were neutralized with the discharger and fed into the cyclone charger with flowing air. Then, the PET was negatively charged and ABS positively charged by the HIPS charger. The charged PET was deflected to the positive electrode and ABS to the negative electrode. The charge of the particles was measured with a Faraday cage. For example, the positive (+) charge on particles induced the negative (-) charge on the surrounding cage, and the charge induced on the inner wall of the cage was equal in magnitude but opposite in sign to the introduced charge. Then, the charge of the same magnitude and sign appeared on the outside of the cage, and thus the final charge was measured with an electrometer after being collected with

FIGURE 5. Effect of electrode potential on the separation efficiency of PVC, PET, and ABS in a triboelectrostatic separation unit (first stage: 10 m/s air velocity, +2 cm splitter position, and 30% relative humidity; second stage: 12 m/s air velocity, central splitter position, and 40% relative humidity). a Faraday cup. The Faraday cage used in this study was a model KQ-1400 (Kasuga Denki Inc., Japan), and its measurement ranged from (1 to (9999 nC. The weight of individual plastics was measured with an electronic balance (BP 2100s, Sartorius). Hence, the charge density or specific charge of plastic particles was determined as a charge-to-mass ratio (nC/g). In addition, the data shown in figures are the average values from the tests repeated at least three times. 2.2. Charge Polarity and Charge-to-Mass Ratio of PVC, PET, and ABS. The tribocharger material significantly affects the charge polarity and charge-to-mass ratio of plastics. Experiments were performed in order to compare the charge polarity and charge-to-mass ratio of plastics in the charger. PP laid between PVC and PET in the triboelectric series was selected as the tribocharger material in the first stage and HIPS between PET and ABS selected in the second stage. A total of 30 g each of PVC, PET, and ABS were fed into the cyclone charger at 30% relative humidity and 10 m/s air velocity. 2.3. Triboelectrostatic Separation. Air Velocity Tests. In the first stage, plastic separation as a function of air velocity VOL. 42, NO. 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Effect of the splitter position on the separation efficiency of PVC, PET, and ABS in triboelectrostatic separation unit (first stage tests: 10 m/s air velocity, 20 kV electrode potential, and 30% relative humidity; second stage tests: 12 m/s air velocity, 25 kV electrode potential, and 40% relative humidity).

FIGURE 7. Effect of the relative humidity on the separation efficiency of PVC, PET, and ABS in triboelectrostatic separation unit (first stage tests: 10 m/s air velocity, 20 kV electrode potential, and +2 cm splitter position; second stage tests: 12 m/ s air velocity, 25 kV electrode potential, and central splitter position).

was tested with a PP charger under the conditions of an electrode potential of 20 kV, a splitter position of +2 cm from the center, and a relative humidity of 30%. In the second stage, it was also tested with a HIPS charger under conditions of a 25 kV electrode potential, central splitter position, and 40% relative humidity. At this time, air velocity was controlled in the range 2–12 m/s with an air conditioner, dryer, and flow meter as shown in Figure 1. The net charge of mixed plastics was measured with a Faraday cage. Electrode Potential Tests. In the first stage, plastic separation as a function of electrode potential was tested with a PP charger, at 10 m/s air velocity, a +2 cm splitter position, and 30% relative humidity. In the second stage, it was also tested with a HIPS charger at 12 m/s air velocity, a central splitter position, and 40% relative humidity. At this time, electrode potential was controlled, ranging from 10 to 30 kV with a power supply. Splitter Position Tests. The determination of the splitter position was tested under constant conditions of a 10 m/s air velocity, 20 kV electrode potential, and 30% relative humidity in the first stage and then 12 m/s air velocity, 25

kV electrode potential, and 40% relative humidity in the second stage. The “-” and “+” signs in the splitter position signify moving the direction from the center to the negative and the positive electrodes, respectively. The splitter position was moved toward either the positive or negative electrode by 2 cm at a time. Relative Humidity Tests. Relative humidity was controlled, ranging from 20 to 70% in the first stage and from 30 to 70% in the second stage, with a dehumidifier (model AD0502XA of Whirlpool Corp., U.S.A.). Relative humidity was tested under constant conditions of 10 m/s air velocity, 20 kV electrode potential, and a +2 cm splitter position in the first stage and then 12 m/s air velocity, 25 kV electrode potential, and a central splitter position in the second stages. Separated plastic particles were hand-picked under naked eyes and weighed. The separation efficiency was expressed as both grade and recovery in the figures.

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3. Results and Discussion 3.1. Charge Polarity and Charge-to-Mass Ratio of PVC, PET, and ABS. Figure 3 shows the charge polarity and charge-

FIGURE 8. Products collected in bins and particles attached on electrodes in a triboelectrostatic separation unit (PVC, black; PET, green; ABS, white). (a) First-stage separation of PVC from ABS and PET. (b) First-stage products: ABS and PET in the left bin and PVC in the right bin. (c) Second-stage separation of ABS from PET. (d) Two-stage products: ABS in the left bin and PET in the right bin. to-mass ratio of PVC, PET, and ABS determined with a cyclone charger made of PP and HIPS materials. The charge-to-mass ratios of PVC, PET, and ABS in the PP charger were -13.3, +8.1, and +11.6 nC/g, and those in the HIPS charger were -16.7, -9.0, and +6.0 nC/g. PVC in the PP charger was charged negatively, and PET and ABS were charged positively. Also, PVC and PET in the HIPS charger were charged negatively, and ABS was charged positively. The results could be explained by the difference between the work function of the various plastics (13, 37, 46). In the case of using a PP charger, PVC is charged negatively because its work function is higher than that of PP, whereas PET and ABS are charged positively, with their work functions being lower than that of PP. This enables PVC to separate from PET and ABS in the first stage. In the case of using a HIPS charger, the charge polarity of PVC and PET is charged negatively because their work functions are higher than that of HIPS, whereas ABS is charged positively, with its work function being lower than that of HIPS. Thus, the PET can be separated from the ABS in the second stage. 3.2. Effect of Air Velocity on Separation Efficiency. When mixed plastics are fed into a cyclone charger, tribocharging occurs. The surface charge-to-mass ratio and separation efficiency of plastics can be improved by adjusting the air velocity. Figure 4 shows the effect of air velocity on the net-chargeto-mass ratio and separation efficiency of plastics in a triboelectrostatic separation unit. In the first stage using a PP cyclone charger, the charge-to-mass ratio of PVC increased as the air velocity increased, having a saturation charge value of 24 nC/g at a 10 m/s air velocity. An increase of air velocity in a fluidized bed may cause an increase in the impact force and frequency between particles and their contacting surfaces. The increase in the charge-to-mass ratio is due to

an increase in electron transfer between materials with increasing outside energy by air (36, 45, 48). In the case of the efficiency, that is, grade and recovery of PVC, it reached a maximum at 10 m/s of air velocity and then decreased. It is plausible that the falling velocity of the particles increases with an increase in the air velocity. Thus, some particles are moved toward the opposite collection bin after strongly colliding on the splitter or electrode, and then both the PVC grade and recovery decrease. Therefore, 10 m/s of air velocity was determined to be optimum for achieving a relatively high net-charge-to-mass ratio and separation efficiency of plastics in the triboelectrostatic separation. In the second stage, using a HIPS cyclone charger, the charge-to-mass ratio of PET increased with air velocity, being 20.3 nC/g at air velocities over 10m/s. The separation efficiency of PET increased as the air velocity increased to 10 m/s, and then the PET grade remained constant, but the PET recovery somewhat decreased. The charge-to-mass ratio in the first stage is higher than that in the second stage because the work function difference between PVC and PP is larger than that of PET and HIPS. 3.3. Effect of Electrode Potential on Separation Efficiency. When the charged particles enter the electric field formed between the electrodes, they are subjected to a combination of electrostatic, gravity, and drag forces (30, 40, 41). Then, the charged particles will be deflected primarily by electrostatic force and partly by gravity and drag force toward negative and positive electrodes. Figure 5 shows the effect of electrode potential on the separation efficiency of plastics in a triboelectrostatic separation unit. In the first stage, both the PVC grade and recovery increased as the electrode potential increased. At a 5 kV electrode potential, the PVC grade and recovery were 75.50% VOL. 42, NO. 1, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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and 65.42%, respectively. It appears that an electrode potential of 5 kV is not strong enough to pull the charged particles toward the electrodes. At 20 kV, a 99.40% PVC grade and 98.10% PVC recovery were successfully obtained. The results show that the PVC grade and recovery increased with increasing electrode potential. If the charge-to-mass ratio of a particle is high, it can be deflected, although the electrode potential is relatively low (39, 41, 42). In the second stage, both the PET grade and recovery also increased as the electrode potential increased. At 25 kV, the maximum PVC grade and recovery were 97.80% and 95.12%, respectively. The results indicate that the PET grade and recovery curves shift considerably, depending on the potential of the electrodes. The optimum PVC separation efficiency in the first stage was obtained at 20 kV, whereas that of PET was obtained in the second stage at 25 kV. This is attributed to the charge-to-mass ratio (24.0 nC/g) of PVC being larger than that (20.3 nC/g) of PET, as mentioned in Figure 4. 3.4. Effect of Splitter Position on Separation Efficiency. Figure 6 shows the effect of splitter position on the separation efficiency of plastics in a triboelectrostatic separation unit. As shown in the first stage, the PVC grade increased as the splitter position was moved from the negative electrode to the positive one, and the PVC recovery increased as the splitter position was moved from the positive electrode to the negative one. The falling position of particles can vary, depending on charging factors such as work function, charger material, mixture ratio, air velocity, and relative humidity under a definite gravity force, drag force, and electrostatic force (30, 38–48). PVC particles which have a high negative charge-to-mass ratio are strongly deflected to the positive electrode, but some PVC particles which have a neutral charge-to-mass ratio fall freely. Also, in the case of PET and ABS, some PET and ABS particles were not deflected to the negative electrode and behaved similarly to PVC. Such behavior of the particles deteriorated the separation efficiency. Considering the results, a PVC grade of 99.40% and a recovery of 98.10% were obtained at a splitter position 2 cm from the center toward the positive electrode, which seems to be the optimum position. However, a PVC purity of 99.9% could be obtained at a splitter position between +6 cm and the positive electrode, although the PVC recovery considerably decreased by 21.9%. The directions of PET grade and recovery curves in the second stage are opposite each other, like in the first stage, depending on the splitter position. The highest PET grade and PET recovery, 97.80% and 95.12%, respectively, were each obtained at the central splitter position. Also, a PET grade of 99.5% was obtained at a splitter position between +6 cm and the positive electrode, although the PET recovery was comparatively decreased by 28%. 3.5. Effect of Relative Humidity on Separation Efficiency. Figure 7 shows the effect of relative humidity on the separation efficiency of plastics. In the first stage, the PVC grade increased as the relative humidity decreased. The acceptable separation efficiency of PVC could be obtained as long as the relative humidity was less than 30%. Furthermore, the second stage tests show that the separation efficiency of PET remained constant when the relative humidity was less than 40%. With increasing relative humidity, water films are formed on the plastic surface (29, 42), and it disturbs the surface polarization between particles when contacting or colliding with particles. Also, the surface charge of charged particles decreases probably due to the discharge of the electron through the moisture layer attached on the surface (48). From the results above, relative humidity is proved to greatly affect the separation efficiency of mixed plastics in triboelectrostatic separation. 254

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3.6. Photographs of Separating Processes and Separation Products. Figure 8 shows photographs of particles attached on both electrodes and products collected in bins in a triboelectrostatic separation unit. In Figure 8a and b, negatively charged PVC in the PP charger due to the work function between plastics is deflected toward the positive electrode, whereas PET and ABS are deflected toward the negative electrode. From Figure 8c and d, PET and ABS in the HIPS charger are charged negatively, and ABS is charged positively. Thus, PET is deflected toward the positive electrode, whereas ABS is deflected toward the negative electrode. Particles which have a higher charge-to-mass ratio than that in the collection bins are attached on both electrodes at the same electrode potential (Figure 8a and c). Namely, the particle which has a high charge-to-mass ratio can be easily deflected toward the electrode, although the energy of the electrode potential is low. Therefore, the charge-to-mass ratio of the materials seems to be the most important among all process variables in triboelectrostatic separation. The triboelectrostatic separation of PVC, PET, and ABS has been performed successfully in the two-stage process. In future research, based on the systematic approach presented in this article, we will extend it to a multistage process for various plastics separation.

Acknowledgments The authors would like to express special thanks to the Resource Recycling R&D Center, Ministry of Science & Technology of Korea, for financial support.

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