Waste Management 33 (2013) 585–597

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Waste Management

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Review

Triboelectrostatic separation for granular plastic waste recycling: A review

Guiqing Wu, Jia Li ⇑, Zhenming XuSchool of Environmental Science & Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 21 June 2012Accepted 18 October 2012Available online 28 November 2012

Keywords:Plastic wasteWaste recyclingTriboelectrostatic separation

0956-053X/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.wasman.2012.10.014

⇑ Corresponding author.E-mail address: weee@sjtu.edu.cn (J. Li).

The world’s plastic consumption has increased incredibly in recent decades, generating more and moreplastic waste, which makes it a great public concern. Recycling is the best treatment for plastic wastesince it cannot only reduce the waste but also reduce the consumption of oil for producing new virginplastic. Mechanical recycling is recommended for plastic waste to avoid the loss of its virgin value. Asa mechanical separation technology, triboelectrostatic separation utilizes the difference between surfaceproperties of different materials to get them oppositely charged, deflected in the electric field and sepa-rately collected. It has advantages such as high efficiency, low cost, no concern of water disposal or sec-ondary pollution and a relatively wide processing range of particle size especially suitable for thegranular plastic waste. The process of triboelectrostatic separation for plastic waste is reviewed in thispaper. Different devices have been developed and proven to be effective for separation of plastic waste.The influence factors are also discussed. It can be concluded that the triboelectrostatic separation of plas-tic waste is a promising technology. However, more research is required before it can be widely applied inindustry.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The past decades have witnessed an incredible and consistentgrowth in the consumption of plastics due to their good safety,low cost, durability, lighter weight than competing materials, andextreme versatility and ability to be tailored to meet specific tech-nical needs (Siddique et al., 2008). It has been reported that in 2010the global production of plastics increased to 265 million tonnes,confirming the long term trend of plastic production growth of al-most 5% per year over the past 20 years (as shown in Fig. 1), whilethere is still room for further growth (PlasticsEurope, 2011). Eachyear, around 4% of global oil production represents the cost forthe creation of plastic raw materials. An additional equivalent 4%of global oil production is required as energy to convert the plasticmaterials into prototype or finished products at the same time(EuPC, 2009).

Plastics are widely applied in packaging, building and construc-tion, automotive and electrical and electronic equipment, withpackaging being the largest segment. Although plastic productsusually have excellent durability, more than half of them are dis-carded as waste each year. The increased demand for plastic hasgenerated rapid growth in production as well as disposal of plasticwaste. It can be concluded that plastic waste has become one of thelarger categories in municipal solid waste (MSW), especially inindustrial countries. For example, in the US, plastic waste found

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in MSW has increased from 9.5% in 1994 (USEPA, 1995) to 12.4%in 2010 (USEPA, 2011). Fig. 2 illustrates the composition of MSWin the US. Since the total amount of MSW is increasing rapidly withurban development and population growth, a constant growth ofplastic waste can be expected in both developing and developedcountries (Chen et al., 2011). As a consequence, the question ofthe disposal of plastic waste generated by industry and household-ers has gained a growing public concern.

Plenty of toxic materials including dioxins and hydrochloricacid can be easily produced and cause huge damage to the environ-ment if plastic waste is not managed properly (Ali and Siddiqui,2005; Mølgaard, 1995; Simoneit et al., 2005; Wey et al., 1998).Landfill is becoming more and more expensive due to the increas-ing volume of waste and the decreasing landfill capacity for dis-posal. More significantly, landfill of plastic waste is a waste ofvaluable resources. It also causes a series of problems, such as addi-tives leaching and land occupation (Lea, 1996; Oehlmann, 2009).Incineration is widely applied in energy recycling for plastic waste.Plenty of energy can be recycled during the process and be used forelectricity generation, combined heat and power, or some otherprocesses (Astrup et al., 2009b). However, incineration can alsobe rather risky since many toxic components are found in the flyash and the residues in concentration that exceed the admissiblelimits, such as polycyclic aromatic hydrocarbons (PAHs),polychlorinated biphenyls (PCBs), polychlorinated dioxins (PCDs)and polychlorinated dibenzofurans (PCDFs), which may causecarcinogenesis, teratogenesis and mutagenesis (Chung, 2010;Gilpin et al., 2005; Li et al., 2001).

Fig. 1. World plastics production 1950–2010 (PlasticsEurope, 2011).

Fig. 2. The composition of MSW in the US in 2010 (USEPA, 2011).

586 G. Wu et al. / Waste Management 33 (2013) 585–597

Compared to landfill and incineration, recycling of plastic wasteis much more acceptable and environment friendly. Recycling isnot only an approach for disposing plastic waste, but also an effec-tive way to reduce the requirement for virgin plastics production,which can contribute to saving with respect to global warming(Astrup et al., 2009a). The terminology for plastics recycling isquite complex. Commonly it can be divided roughly into two maincategories: mechanical recycling and chemical recycling. Mostthermoplastics such as poly ethylene terephthalate (PET), polypro-pylene (PP) and polyethylene (PE) have high potential to be re-melted and mechanically recycled, while for thermoset plastics,chemical recycling are more adaptable. Compared to chemicalrecycling, mechanical recycling is more convenient and has a lowdegree of pollution generation and cost. Mechanical recycling isalso a better way to maintain the intrinsic value of plastic andavoid the loss of non-renewable resources. Sadat-Shojai summa-rized the methods of polyvinyl chloride (PVC) recycling and com-pared their strong and weak points as shown in Table 1.Recycling of PVC is not a representative example of polymer recy-cling, since PVC includes Cl in its macromolecular chains in con-trast to other polyolefins (low density polyethylene (LDPE), highdensity polyethylene (HDPE), PP, etc.) or polyesters which includeonly C, H and O, but it is also useful in evaluating the processes ofrecycling other similar plastic waste, especially for mixed plasticwaste which contain Cl or other toxic elements or compounds(Sadat-Shojai and Bakhshandeh, 2011). It shows that mechanical

recycling is a promising method with low pollution, low cost andthe most government support.

The most crucial challenge for mechanical recycling is that plas-tic waste needs to be separated effectively. Mechanical recycling ishigh-sensitive to the impurities. Different types of plastic are usu-ally not compatible with each other. All of them have differentphysical characteristics such as melting point, density and hard-ness, so mixed plastics cannot present their original characteristicsand the practical value descends. More significantly, the chemicalimmiscibility makes it desperately sensitive to the purity. Forexample, even small quantity of PVC in another main plastic woulddecrease the recycling ratio of plastics by forming compounds ordeteriorating the nature of other materials (Wey et al., 1998); itis also reported that PET in a PVC recycle stream will significantlyreduces the value of the recycled material by forming solid lumpsof crystalline PET (Hopewell et al., 2009). Therefore, mixed plasticwaste is valueless. However, it can get greatly increased value afterit was separated into pure components. Nowadays households arethe main source of plastic waste stream and they are still mainlycollected by curbside collections, which are usually mixed of differ-ent kinds of plastic waste (Al-Salem et al., 2009; Hopewell et al.,2009), separation of high efficiency seems to be an urgent needin practical recycling. Hence, in order to improve the value andthe recycling rate of plastic waste, it is really necessary to build asound and effective separation process for plastic waste separation.

Optical sorting is used as a pre-sorting process for material(size: +40 mm �60 mm) before size reduction rather than a sepa-ration method for plastic waste scraps. Automatic devices basedon optical, X-ray, and near infrared (NIR) technologies are widelyused in plastic recycling facilities all over the world. However,due to the poor spectral signature obtained, black fragments inthe plastic stream can hardly be processed through this way. It isalso difficult to separate the mixed plastics which have similarproperties such as same color and peak (Arvanitoyannis andBosnea, 2001; Huth-Fehre et al., 1995; Scott, 1995). The diversityof different density of various plastics makes it possible to findan appropriate medium to separate the heavier plastics from light-er ones. It can be conducted either with dry particles using air ta-bles or zigzag air classifiers or by water-based solutions orsuspensions as separating medias (Dodbiba et al., 2003a; Gentet al., 2009; Hu and Calo, 2006). However, since many plastics haveoverlapping density ranges and similar typical densities as shownin Table 2 (Malcolm Richard et al., 2011) (for example, PVC is1.39 g cm�3 and PET is 1.37 g cm�3), it is not easy to separate themby density separation. Selective flotation has high recovery andpurity, especially is suitable for some plastics such as PVC andpolyoxymethylene (POM) which are difficult to be separated bydensity media separation (Shent et al., 1999). However, themaximum size of particles for flotation is no more than 500 lm(Malcolm Richard et al., 2011), and the selective flotation cannotbe achieved without changing surface properties for the plasticswith similar surface properties, such as PVC and PET (Burat et al.,2009). For wet density separation or flotation, water is needed inthe whole process, which causes a concern about secondary pollu-tion. Dewatering or drying the mixture after separation cannot beavoided.

As a dry technique, electrostatic separation utilizing coronacharging has been successfully applied to separate metal/non-me-tal mixtures (Gente et al., 2003; Hou et al., 2010; Li et al., 2007).However, such technologies are only suitable for separating con-ductors from dielectrics, but not able to separate a mixture of dif-ferent dielectrics such as mixed plastics. As a consequence,triboelectrostatic separation has been studied for materials separa-tion especially for separation of insulators.

Triboelectrostatic separation is definitely one of the mostimportant and promising materials-processing techniques. In the

Table 2Ranges in densities and typical densities of several common plastics (MalcolmRichard et al., 2011).

Plastic type Abbreviation Range indensities(g cm�3)

Typicaldensity(g cm�3)

. . .

Low density polyethylene LDPE 0.918–0.93 0.932Medium density

polyethyleneMDPE 0.926–0.940 0.935

High density polyethylene HDPE 0.941–0.965 0.94Polystyrene PS 1.03–1.07 1.06Cross-linked polyethylene XLPE 1.15–1.28 1.15. . .

Poly(vinyl chloride) PVC 1.37–1.42 1.39Poly(ethylene terephthalate) PET 1.35–1.40 1.37. . .

Polytetrafluoroethylene PTFE 2.07–2.20 2.17

Table 3Comparison of different methods for separation of plastic waste.

Method ofseparation

Separationbasis

Wetordry

Particlesize forprocessing

Features

Optical sorting Differencebetween colorsand peaks

Dry >40–60 mm

Low pollution,especiallysuitable fordrinking bottles.

Densityseparation

Differencebetweendensities

Wet/dry

It dependson thedevice

Simple, costlycompetitive.

Flotation Differencebetweensurfaceproperties

Wet <500 lm High efficiency,flexible.

TriboelectrostaticSeparation

Differencebetweeneffectivesurface workfunction

Dry 0.1–13 mm

Suitable formost plastics,efficient, lowpollution.

Table 1The comparison of different approaches for disposing of PVC waste (Sadat-Shojai and Bakhshandeh, 2011).

Method ofdisposing

Sensitivity toimpurities

Degree ofpollutiongeneration

Costs Recycledproduct(s)

Properties of therecycled material

Number of plantsin operationaround the world

Accepting bycountries

Landfilling Non-sensitive Very high Low-cost No materialsrecycled

– Large Non-acceptable

Incineration Usuallynon-sensitive

Very high Usuallylow-cost

Energy Usually energetically not efficient. Large Non-acceptable

Mechanicalrecycling

High-sensitive Low Middle-cost PVC It is dependent on feed materialand processing variables of recycling.

Fair Highly-acceptable

Chemicalrecycling

Relatively sensitive Usually low Usuallyhigh-cost

Diverse rawmaterials

It is dependent on feed materialand processing variables of recycling.

Small Low acceptable

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last century, triboelectrostatic separation methods have undergonecontinuous development and are well established in the mineralprocessing industry for separation of minerals, beneficiation of coalor raw ore (Ban et al., 1997; Kwetkus, 1998; Li et al., 1999). Since1970s in the last century, this technique has been researched forseparating mixed plastics (Pearse and Hickey, 1978). In 1990s, itwas found that this progress could be suited for separating mixedplastic materials especially the tailings and plastics from car orcable scrap (Stahl and Beier, 1996). Identification and sorting ofwaste packing materials including different plastics can also bedone by using of triboelectrostatic techniques (Hearn and Ballard,2005). Since triboelectrostatic separation is a dry process, it is freeof the problem of the disposal of waste water and can be easilyoperated. The recoveries and purities in the researches indicatethat it is an effective method for separating plastic waste of reason-able size. Since it has many advantages over other separationmethods, more and more relevant researches of triboelectrostaticseparation for plastic recycling have been carried out in recentyears (Dascalescu, 2011; Inculet et al., 1994; Iuga et al., 2005;Lungu, 2004; Park et al., 2008a).

Table 3 shows, in brief, a comparison among the main separa-tion methods of plastic waste. Triboelectrostatic separation hashigh efficiency, low cost and no concern of water disposal or sec-ondary pollution, with a wider processing range of particle sizeespecially suitable for the crushed or granular plastic waste. Forthe mixed plastic waste scraps of particle size in millimeters, tribo-electrostatic separation has undoubtedly great advantages, whichindicates a promising future for this technique in plastic recycling.However, more research should be carried out until it can bewidely utilized in industry. This paper aims to introduce the tribo-electrostatic separation technology for the plastic waste and pres-ent the recent progress in separating plastic waste by it.

2. Triboelectrostatic separation

It is well known that when two materials with different surfaceproperties contact each other, they may get charged. This tribo-charging phenomenon is also known as ‘contact electrification’ or‘frictional electrification’ when materials rub against each other(Lowell and Rose-Innes, 1980). As for short contact during collision,it can also be called ‘impact charging’ (Matsusaka and Masuda,2003). In practice, it is usually not easy to distinguish the processfor charging, so the term ‘triboelectric charging’ or ‘tribo-charging’is used in such a broad sense (Matsusaka et al., 2010).

It is generally believed that the mechanism of charge transfer intribocharging can be explained by three mechanisms: electrontransfer, ion transfer, and material transfer (Lee, 1994; Lowelland Rose-Innes, 1980; Matsusaka et al., 2010; Matsusaka andMasuda, 2003; Saurenbach et al., 1992). Among all the mecha-nisms, electron transfer mechanism is the most important andwidely accepted since it has successfully explained the metal–metal contact electrification. When two different materials cometo contact, electrons transfer happens until their Fermi levelequals. Difference in work functions between them is the maindriving force (Harper, 1951). As for insulators, there is no available‘free electron’ in them, so work function theory is not applicable.Some modified models such as surface state model (Anderson,1994; Gutman and Hartmann, 1992), molecular–ion-state model(Duke and Fabish, 1978) have been proposed and quantum chem-ical calculation (Yoshida et al., 2006) is also introduced to explainthe tribocharging phenomenon between insulators. Although thereare still many unknowns, it is widely accepted that the electrontransfers only happen on the surfaces of insulators; electrons move

Table 4Comparison of published triboelectric series.

Diaz and Felix-Navarro (2004) Iuga et al. (2005) Fujita et al. (1995) Park et al. (2008b) Matsushita et al. (1999) Park et al. (2007a)

"Positive chargeNylon 6.6 AlPVAcPVOHPMMA PMMA PMMA PMMA

GPPSABS ABS ABSPP Calibre

PC PCPS PMMA HIPS PS

RubberPET PET PET

PE PE PE HDPE PE CuPI PET LDPE HDPEPP PP HOMOPP PP PPPET COPPPVC PVC PVC Soft-PVC

Hard-PVC PVC PVCPTFE PTFE PTFE;Negative charge

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from the filled surface of insulator 1 to the empty surface of insu-lator 2 until their Fermi level equals; the main driving force for thecharge transfer is the difference in the effective work functions ofthe two surfaces.

Tribocharging was used in electric spray guns for dry powdercoating in order to minimize the disruption of the deposited layer.It was also used in electro-photography where the latent image onthe surface of the photoconducting drum is developed by pouringcharged toner particles over the drum surface (Taylor and Secker,1994).

Triboelectrostatic separation utilizes tribocharging phenome-non to get different materials charged in opposite polarities andthen feed them into electric field to separate them by their differ-ent charge polarities. The particle trajectories are deflected in theelectric field according to the polarities and the amounts of charge.After contact, two solids with different charge polarities are sepa-rated by electric field and collected.

Since plastics usually have different surface properties i.e. effec-tive surface work functions, triboelectrostatic separation can becertainly available for the plastic waste processing. It is difficultto measure the exactly effective work function of every granularplastic, but the triboelectric series of plastics can express a se-quence of the relative work functions of plastics as an importantindicator for the material separation, since it determines the polar-ity of the materials get charged with (Park et al., 2008b). A numberof researchers have drawn up triboelectric series to predict thepolarity of the charge that is transferred from one surface to an-other. Several published triboelectric series of plastics studied bydifferent methods are shown in Table 4.

When two kinds of plastic contact each other, the upper one inthe triboelectric series will get positively charged and the otherone will be negatively charged. The triboelectric series indicatesthe differences between the surface properties of plastics as wellas the possibility of the triboelectrostatic separation for plasticwaste. Based on the principle of tribocharging, many investigationsof devices of triboelectrostatic separation for mixed plastics have

Fig. 3. Flowchart of triboelectrostat

been studied. For example, PVC is a commonly hazardous materialthat should be removed from other plastic waste and it is almostthe most negative except for polytetrafluoroethylene (PTFE) inthe series. Many researchers take the advantage of it, getting themixed plastic waste tribocharged, getting PVC negatively chargedand other plastics positively charged, thus PVC can be removedfrom other plastics by electrostatic deflection and be collected onthe positive electrode (Jeon et al., 2009; Lee and Shin, 2002; Parket al., 2007b).

It can also be concluded from Table 4 that there is no universaltriboelectric series. Some results remain controversial such as thesequence of PP, PE and PET. This is due to the fact that the exper-imental conditions varied from experiment to experiment. It hasalso been reported that the additives in the commercial plasticsalso influence the results of the triboelectric series (Matsushitaet al., 1999). Nevertheless, the triboelectric series can provide aroughly prediction for the recycling of plastic waste in the tribo-electrostatic separation.

3. Triboelectrostatic separation for plastic waste

Triboelectrostatic separation for recycling plastic waste is abso-lutely an economically satisfactory mechanical process for its sim-plicity. Fig. 3 shows a flowchart for the separation of plastics. To beseparated, plastic waste is usually crushed and screened into prop-er size, the mixture granules get charged by a tribocharger, andthen are fed into a electric field for deflection and then collected.

3.1. Pretreatment

Plastic waste is usually crushed to the optimum size range bycutting mills or shredders (Dodbiba et al., 2005; Park et al.,2007a). Sieves are usually needed for the classification.

The suitable particle size for triboelectrostatic separationvaries from device to device. It has been reported that the

ic separation for plastic waste.

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triboelectrostatic separation method is effective for the particles ofsize in the range of 1–13 mm (Outotec, 2008). It is also reportedthat the cyclone separator can be effective for plastic particles ofsize in the range around 150 lm (Yanar and Kwetkus, 1995).

It is well-known that for insulators tribocharging only happenson the surfaces of the particles, as deep as 30 nm as for polymers(Lee, 1994; Watson and Yu, 1997). Hence, too large size decreasesthe specific surface area relatively, leading to the decrease of theelectric field force, which is the main separation power of the pro-cess. Besides, with the increase of the particle size, the electric fieldstrength needed for electrostatic deflection of the particles in-creases exponentially (Dodbiba et al., 2005). Dodbiba et al. studiedthe effect of particle size influencing the effectiveness of triboelec-tric separator. Considering the forces acting on a charged particleof spherical shape, it is proposed that the relationship of particlesize (D), electric field strength (E), the magnitude of the surface po-tential (Vs), and the density (qs) can be described as the followinginequality (Dodbiba et al., 2002):

D � 2:79� 10�4 � ½ErVs=ðkqsÞ�1=3 ð1Þ

where coefficient k, found experimentally, is dimensionless and laidbetween 1 and 1.8. So it can be concluded that the particle size fortriboelectrostatic separation has a maximum limit.

On the other hand, small size is also not favorable, since it willcost more energy in the milling step, cause the increase of the col-lision between the same kind of plastic particles and lead to the de-crease of the separation efficiency. Small particle size may alsobring out some problems such as adhesion on the wall of the char-ger, blocking and the decrease of recovery. Particle size is undoubt-edly an important factor that influences the final efficiency of theseparation. As plastic waste is usually in various sizes when it iscollected, in order to control the particle size for the next opera-tion, crushing and screening are definitely essential.

The materials may get charged in the crushing and screeningstep, which would probably impede the charging efficiency andinfluence the purity and recovery of the whole process. For the re-search purpose as well as high reproducibility, neutralization of theparticles is needed. The charge accumulated in this step can beneutralized by means of ionized air produced by a discharger (Parket al., 2008a).

Washing and drying is also useful for discharging the plasticparticles. Since surface work function would increase with surfacecontamination and oxidation, it is really important to keep the sur-face of particles clean in triboelectrostatic separation (Trigwellet al., 2003). Washing and drying is also applicable for minimiza-tion of contamination on the particle surface, which benefits thereproducibility of tribocharging efficiency (Lee and Shin, 2002).

Fig. 4. Rotating tube for plastic particle tribocharging.

3.2. Charging

After the crushing and screening step, plastic waste is supposedto be in a suitable range of particle size for the charging purpose.

Rub can be used to increase the charge transfer between thegranular plastics to several orders of magnitude greater than in asimple touching contact (Taylor and Secker, 1994). Some research-ers provide the supporting evidence that charge transfer increasedwith rubbing pressure (Haenen, 1976). Because the plastics areinsulators, the electric charge will be accumulated with repeatedrubbing. For aggregating the rubbing pressure and frequency,numerous devices have been investigated in recent years for plas-tic particle charging in the triboelectrostatic separation. Accordingto charging mechanism, these devices are classified to be ‘‘solidsingle phase’’ or ‘‘gas–solid two-phase’’. The ‘‘solid single phase’’charging mechanism means the interactions only exist amongsolid particles during the charging process. While the ‘‘gas–solid

two-phase’’ charging mechanism use the interactions betweengas and solid to charge the mixed solid particles. The devices of so-lid single phase contain rotating tube (Inculet et al., 1998), rotaryblades (Matsushita et al., 1999) and vibrating devices (Blajanet al., 2010; Higashiyama et al., 1997; Lungu, 2004). The gas–solidtwo-phase devices contain cyclone (Dodbiba et al., 2002; Yanarand Kwetkus, 1995), fluidized bed (Calin et al., 2008; Calin et al.,2007; Dascalescu, 2011; Iuga et al., 2005), and propeller-typetribocharger (Miloudi et al., 2011a).

3.2.1. Solid single phase tribochargingRotating tube has been used as a tribocharger for separating

plastic waste since 1990s (Inculet et al., 1994, 1998). In this char-ger, plastic particles can be fed continuously through the tubewhile it is rotating. The cylindrical tube rotates along an axis whichis inclined slightly to the horizontal. Thus the particles can passthough the tube by the force of gravity. In order to create more stir-ring for the mixture, in the tube there are ribs made of same mate-rial as the tube itself. The ribs extend radially inwardly from thewall of the tube (as shown in Fig. 4). Results show that degree ofmixing is much improved by the ribs (Inculet, 1994). Rotating tubehas shown the feasibility of getting PP/polystyrene (PS) and PP/high density polyethylene (HDPE) particles effectively charged atvarious relative humidities and normal ambient temperatures. Un-like fluidized bed or cyclone tribocharger whose process involvessubstantial power to compress the fluidization air, it has advanta-ges for its mechanical simplicity and modest power requirements.Continuous operation is also favorable for industrial populariza-tion. However, its disadvantages should not be ignored. The parti-cle–particle collision frequency is relatively low compared tofluidized bed or cyclone charger. As a consequence, the chargingefficiency may be not enough for the plastics which are hard tobe charged. By adjusting the inclining angle or increasing the tubelength to increase the residence time, the charge magnitude can beimproved.

Similar to the rotating tube, rotary blades is also a kind of devicethat can stir the plastic mixture and get them charged throughrotation. A friction mixer with rotary blades was used for separa-tion of plastics in 1999 (Matsushita et al., 1999). The rotor is pro-vided with rotary blades made of plastic material. As shown inFig. 5, transverse blades mounted at helical positions on a rotationaxis and an oblique blade. After the mixture of plastics enters themixer through a feed port it is continuously stirred until the lidof the outlet port opens. The plastic pieces are charged by rubbingagainst each other or against the outer cylinder and the rotor dur-ing this process. Rotary blades can be operated in an intensivespeed, since it is an enclosing chamber. As it is not continuous,

Fig. 5. Friction mixer with rotary blades (Matsushita et al., 1999).

Fig. 6. Contour plots of the charge/mass ratio of (a) ABS and (b) HIPS granularsamples as a function of the oscillation frequency n and the crank length R (Blajanet al., 2010).

Fig. 7. Contacting model of granular waste plastics in fluidized bed.

590 G. Wu et al. / Waste Management 33 (2013) 585–597

the residence time can be optimized and the particles can stay tur-bulent until they get enough charge accumulated. The exact charg-ing efficiency is unknown, but purity and recovery of the separationbased on this charging result can be above 90%. However, in recentseveral years, there is hardly report about this equipment.

Vibration has been validated to be an effective way for tribo-charging of plastic granules (Laurentie et al., 2010). Thus vibratingdevices are also chosen as tribocharger in many researches. Hig-ashiyama et al. have demonstrated a tribocharger utilizing avibrating feeder equipped with a charging plate. In order to in-crease the contact area, on the plate there are several triangulargrooves, in which the particles move from one end toward theother while contacting with both sides of the groove to be charged(Higashiyama et al., 1997). Similar charging equipments which uti-lized vibration effect have also been reported in some papers. Lun-gu investigated a vibrating pot as a tribocharger for the charging ofPE and PS, which is vibrating on the vertical direction for a certaintime span (Lungu, 2004). Dascalescu et al. studied a vibratory de-vice as chargers in the separation of disk-like polyamide particles.An electromagnetic device commonly employed for sieving appli-cations was used as the vibratory device, and a PP box wasamounted on the plate as the charger (Dascalescu et al., 2005).The typical charging contour plots for acrylonitrile butadiene sty-rene (ABS) and high impact polystyrene (HIPS) charging in Al vi-brated zigzag-shaped square pipes can be observed in Fig. 6. Itcan be concluded that vibrating equipments are also quite effectivefor plastic particles charging.

3.2.2. Gas–solid two-phase tribochargingFluidized bed is one of the most popular kinds of tribochargers

and was extensively studied in industrial applications, such as finecoal preparation (Dwari and Hanumantha Rao, 2009) and polymer-ization reactors (Rokkam et al., 2010). The air tumbling in the deviceprovides a great environment for the particle–particle or particle–wall collisions, which is the main mechanism of the tribochargingin this kind of devices (Calin et al., 2008). As shown in Fig. 7, duringthe process of fluidized bed, air flow was introduced from the bot-tom filter. The air turbulent is generated in the chamber and parti-cles are ‘‘conveyed’’ upwards by the gas flow. During the process,

the trajectories of particles are affected by the forces of Fg, Fr andFa (Fig. 7A), where Fg is gravity, Fr is air resistance and Fa is air dy-namic pressure. Because the value of Fa changes randomly with airturbulent, there are mainly three kinds of contacting (Fig. 7B): con-tacting between different kinds particles, contacting between samekind particles and contacting between particles and wall material.

Fluidized bed can be designed in different shapes. Fig. 8 demon-strates a typical cylindrical fluidized bed, which has an air input atthe bottom and a central pipe allowing the particles to flow

Fig. 8. Typical cross-section of cylindrical fluidized bed used for tribochargingparticles.

Fig. 9. Fluidized bed device for tribocharging of plastic granules (Iuga et al., 2005).

Fig. 10. Propeller-type device for the tribochargin

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through the pipe while retaining its acquired charge. This structurehas been utilized by several researches to separate mixed plasticwaste (Inculet et al., 1998; Lee and Shin, 2002). Fig. 9 shows an-other form of fluidized bed. It contains air distributors with differ-ent mesh apertures correlated with particle size and a changeabletribocharging chamber that be replaced according to the triboelec-tric series of the feedstock. No matter what kind of layout the flu-idized bed is, the main purpose is to provide an environment thatparticles can rub against each other and get enough charge for theneed of deflection in the electric field.

Propeller-type tribocharger for triboelectrostatic separation ofplastic waste has been developed in recent years. As shown inFig. 10a, at the lower end of a cylindrical chamber made of PVC,there is a coaxial propeller. It can produce a helical air motion thatis supposed to facilitate the tribocharging by granule-to-propeller,granule-to-cylinder wall, and granule-to-granule collisions. After acertain residence time when the charge is considered to become en-ough for the deflection, the cylindrical chamber can pivot with re-spect to a horizontal axis and the granules can be evacuated bygravity into an appropriate collector, as shown in Fig. 10b. This oper-ation can be performed with the propeller still in motion, so that tofacilitate the recovery of all the charged granules, which mightotherwise remain stuck to the walls of the cylinder. Propeller-typecharger has been used in the separating of polyamide (PA)/PC andHIPS/ABS (Miloudi et al., 2011a; Miloudi et al., 2011b). The chargingefficiency of propeller charger can be also shown in Fig. 11, it clearlyindicates that propeller is a good device for particle charging.

Cyclone is another common charger widely used in mineral sep-aration processing. Similar to fluidized bed, cyclone charger alsoneeds pressurized air flow to transport the particles into the char-ger as well as create a turbulent ambience where the collisionshappen and the particles can get oppositely charged. As shown inFig. 12, the airflow pushes the particles in and makes them spirallymove and rub against each other. This kind of device has been suc-cessfully utilized for charging in the triboelectrostatic separation ofPE/PVC (Yanar and Kwetkus, 1995), PE/PET (Dodbiba et al., 2002),ABS/PS/PP (Dodbiba et al., 2003b). The mechanisms of particlecharging in the cyclone are quite similar to those of a fluidizedbed. Park et al. designed a device named fluidized bed cyclonetribocharger. It has been reported that through this kind of charger,particles of cross-linked polyethylene (XLPE)/PVC (Park et al.,2007a), PVC/PET (Park et al., 2007b) and PVC/PET/ABS (Parket al., 2008a) can be tribocharged effectively, and separated suc-cessfully in the subsequent electrostatic deflection. The frictionforce here is much higher than any other equipment, so the

g of plastic granules (Miloudi et al., 2011a).

Fig. 11. Electric field (2 cm above) at the surface of ABS and HIPS granular layers(Miloudi et al., 2011a).

Fig. 12. Cyclone tribocharger (Dodbiba et al., 2002).

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charging efficiency can be really high. The power requirements arealso higher than others. Since the particles tend to move close tothe wall, the abrasion of device is also a serious problem (Dodbibaet al., 2002).

Table 5Comparison between different tribochargers.

Charger type Successful examples Experimentalparticle size

Advan

Solid single phaseRotating tube HDPE & PP; PP & PS Inculet et al. (1994) 2–5 mm Mech

requiRotary blades PS, PE, PP, PET Matsushita et al. (1999) 4 mm Inten

Vibratingdevices

ABS & HIPS Blajan et al. (2010) 0.25–5 mm Mechrequi

Gas–solid two phasesFluidized-bed PMMA, PS, PE Calin et al. (2007) 2–5 mm High

PET & PVC Iuga et al. (2005) 2–4 mmCyclone PE & PVC Yanar and Kwetkus (1995); PET

& PE Dodbiba et al. (2002)150 lm;1–3.5 mm

High

Propeller-typecharger

ABS & HIPS Miloudi et al. (2011a) 1–2 mm Relati

The comparison of different tribochargers is shown in Table 5,demonstrating the main advantages and disadvantages of each de-vice. There is no perfect device for tribocharging of plastic granules.Solid single phase tribocharging devices are recommended for theirmechanical simplicity and relatively low power requirements. Theyhave been proved to be effective in some cases. However, theirdegree of mixing or rubbing frequency cannot compete that ofgas–solid two-phase tribocharging. Therefore, solid single phasechargers are recommended when plastics do not need too muchpower to get enough charge. All the gas–solid two-phase tribo-charging need extra power for air flow to maintain the extremelyturbulent environment for particle collision, but the rubbing fre-quency in these devices can be very high. For the plastics difficultto be charged, gas–solid two-phase tribocharging is more efficient.It is also suitable for magnifying the charge difference betweenplastics with slight difference in triboelectric charging property.

3.3. Electrostatic deflection

Tribocharged mixed plastic particles are commonly fed into anelectric field separator and deflect to high voltage static electrodesaccording the Coulomb forces acting on them. The configuration ofelectric field plays an important role in the separation process. De-signs utilized in the triboelectrostatic separation of plastic wastecan be roughly divided into three types: free-fall electric field,roll-type electric field and vertical electric field for up-flow fluid-ized particles.

3.3.1. Free-fall electric fieldFree-fall electric field is the most widely used in triboelectrostat-

ic separation process. When charged plastic particles enter the elec-tric field, affected by electric field force, gravity and air resistance,they will deflect to the electrodes according to their polarities.The particle position at the bottom is influenced by the particle size,mass, surface charge, electric field strength, and some aerodynam-ics parameters. For specific feedstock, the particle size and mass isdetermined, and the separation efficiency is more likely to be influ-enced by the surface charge and electric field strength. High electricfield strength guarantees enough electric field force for deflection.However, high field intensity strongly attracts the particles to theelectrodes and causes impacts which would get the particles oppo-sitely charged and deflect to the opposite electrode thus impede thepurities of separation products (as shown in Fig. 13). Inclined elec-trodes and screen mesh type electrodes are often utilized in somecases to mitigate the impact effect. Free-fall separation system alsosuffers from restrictions. It is lack of controllability of gravitationalforce, which is stronger than the Coulomb forces for particles of big

tages Disadvantages

anical simplicity and low powerrements. It can be operated continuously.

Low rubbing frequency.

sive collision. High rotating speed will make theparticles adhere to the wall.

anical simplicity and low powerrements.

Low rubbing frequency betweenparticles.

collision frequency. Continuous operation. High power requirements.

frictional speed. High power requirements andsevere abrasion.

vely low abrasion and low adhesion. Not continuous, slightly high powerrequirements.

Fig. 13. Free-fall separation system.

Fig. 14. Roll-type triboelectrostatic separator: (1) Vibratory feeder, (2) Groundedrotating roll electrode, (3) HV static electrode, (4) Brush, and (5) Collectingcompartments (Tilmatine et al., 2010).

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size and impedes the separation efficiency. Although free-fall sepa-ration system has advantages such as mechanical simplicity andmodest power requirements, it cannot compete other kinds of elec-tric field when separation efficiency taking to consideration.

3.3.2. Roll-type electric fieldTypical schematic representation of roll-type electric field is

shown in Fig. 14. Roll-type triboelectrostatic separation methodhas been proven to be more appropriate in sorting of coarse plasticgranules (size >2 mm) than free-fall separation system (Tilmatineet al., 2010), especially for particles of poor tribocharging. The cen-trifugal force positively associates with the electric field force todetach the positively charged particles from the roll, while the neg-atively charged particles adhere to the roll electrode until they areswept down by the brush (as shown in Fig. 14). To improve thepurity of the product, the particles are required to be monolayeron the roll, so a belt conveyor and a vibrating feeder are neededin some cases (Miloudi et al., 2011b).

The two separation systems discussed above usually have collect-ing bins or trays (collectors) at the bottom of them. When particlesget through the electric field, they deflect in different trajectoriesand fall into their different target collectors (as shown in Figs. 13and 14). Since the absolute values of horizontal shifts of both kindsof particles are usually not the same, the collectors are not symmet-rical in some designs. Sometimes there is an extra collector in themiddle for collecting the middling, which should get a secondaryseparation. The configuration of collectors is also a parameter thatwould influence the recovery as well as the purity of product.

Both free-fall and roll-type electric field suffer from the poorpredictability of the outcome of tribocharging. An optimal voltageof electrodes should be optimized according to the charging effi-ciency of the particles time after time, as a low voltage may benot enough to attract the charged particles while a high voltagemay make the particles impact the electrodes and rebound intothe wrong collectors.

3.3.3. Fluidized bed triboelectrostatic separatorAn orthogonal physical field with electric field and fluidized bed

has been designed as a type of triboelectrostatic separator to solvethe rebounding problem (Dascalescu, 2011; Dragan et al., 2010).This is a device where charging and deflection happen at the sametime. As shown in Fig. 15, air flow gets into the device from the bot-tom, and the particles maintain in fluidized state until they get ex-actly enough charge to be attracted to the electrodes. The particleswhich do not accumulate enough charge will fall back to the fluidbed chamber to get more charge thus the electrode rebounding ef-fect would not be a problem. The particles adhering to the elec-trodes can be either aspirated away or transported away by abelt conveyor. Experiments have shown its superiority in separat-ing plastic mixtures. However, the restrictions it suffers from canneither be ignored. As one sort of particles in the mixture may becharged and collected faster than the others, the composition ofthe mixture in the fluidized bed may vary in time. The compositionof the mixture has been proved to have a great influence on theoutcome of the tribocharging process, thus it may impose con-straints to the integration of this separation system in a continuousindustrial process (Dragan et al., 2010).

4. Impact factors for triboelectrostatic separation

The effectiveness of particle charging has a great influence onthe output of any electrostatic separation process (Calin et al.,2008). It is important to optimize the factors that influence the effi-ciency of tribochargers.

4.1. Materials of the tribocharger

The material from which the tribocharger is made can be animportant factor influences the efficiency of the tribocharging pro-cess. As discussed above, the tribocharging of plastics is an extre-mely complex phenomenon. For most chargers, there are mainlythree mechanisms mainly influence the net charge accumulatedby particles of a binary plastics mixture: particle–wall collisions,collisions between particles of different material, and collisions be-tween particles of the same material (Calin et al., 2008; Inculetet al., 1994; Iuga et al., 2005). The polarity and magnitude of thecharge accumulated on the surface of the particles are influencedby the combined action of all these three mechanisms, and mostof the charge comes from the collisions between particles of differ-ent material. The most expected result is that one kind of the par-ticles gets all positively charged and the other gets all negativelycharged. Therefore, the material of the charger chamber is morepreferred to have a work function laid in the middle of the

Fig. 15. Fluidized bed-type triboelectrostatic separator.

594 G. Wu et al. / Waste Management 33 (2013) 585–597

triboelectric series between the two plastics to be separated, thusthe particle–wall collisions will not change the expected polarityof any charging particle (Iuga et al., 2005). Considering the tribo-electric series, Park et al. utilized PP cyclone charger to removePVC from PET and ABS, and then used HIPS cyclone to get ABSand PET charged with opposite polarities, separating PVC, PETand ABS successfully (Park et al., 2008a). On the other hand, thedifference in charge density between the particles varies fromcharger to charger. Choosing a material of charger that can makethe highest difference in charge density of the plastics should alsobe taken into consideration (Park et al., 2007a). As such, plastics inthe middle of triboelectric series are usually used as the material ofcharger in the most investigations.

However, the charging process will increase roughness of theinner wall of the charger (Dodbiba et al., 2002). Considering thewear resistance to particle–wall collisions and friction, metalmaterials for the charging chamber are more preferred. On theother hand, the metal materials can help to carry the charge onthe wall to ground, avoiding an accumulation of the charge onthe charger surface, so that the amount of the charge acquired bythe plastic particles can be increased (Higashiyama et al., 1997).

The charger can be made from the same material as some of theparticles in the mixture being processed through the charger.Therefore, making the charger from the minority material in themixture can serve to bring up the level of the charge at whichthe majority material emerges from the charger to the magnitudeof the charge induced in the minority material (Inculet, 1994).

4.2. Residence time

Residence time in the charger is also a parameter that deter-mines the final charge of the particles as well as the recovery ofthe whole process. In most tribocharger, the magnitude of the ac-quired charge is a non-linear function of the residence time. Thecharge increases with the increase of residence time and becomessaturated at longer values of it (Dascalescu et al., 2005). However,the residence time is not necessary to be long enough for the chargesaturation since there are also problems for longer residence time.

When the residence time gets long enough and charge becomes sat-urated through the particle–particle collisions, the particle–wallcollisions will continue happening and some particles may getcharged opposite to the original polarity, thus the recovery and pur-ity of the product will decrease (Matsushita et al., 1999). Besides,the longer the residence time is, the larger the mass of material ad-here to the inner wall of charger is, which may decrease the recov-ery (Dodbiba et al., 2005). After all, the residence time is aim atgetting enough charge for the deflection. Over charged or inade-quately charged particles are not favorable. Therefore, a proper res-idence time should be optimized according to different conditions.

4.3. Mechanical factors

Parameters in different chargers such as air velocity, rotatingspeed and vibrating frequency also influence the frequency of thecollisions and the degree of mixing. Usually, the degree of mixingcan be improved with the increase of these factors, which benefitsthe particle–particle collisions and improve the magnitude ofcharge accumulated. However, the frequency of particle-chargercollisions is also improved at the setting of higher air velocity orrotating speed. The total charge depends on the balance betweenthese two physical mechanisms. Therefore, depending on the con-figuration of the tribocharging device, the increase of air velocitydoes not necessarily leads to higher levels of charge (Draganet al., 2010). On the other hand, it needs more power to maintainhigher air velocity or rotating speed. So these operational parame-ters should be optimized according to different plastic mixturesand different chargers.

4.4. Variable ambient conditions

The relative humidity of the ambient in the charger has beenproven to be an important factor that can significantly affect themagnitude of charge that the plastic particles can get. Most pub-lished works indicate that the polarity of charge does not changebut the magnitude of charge decreases with the increase of the rel-ative humidity. Németh et al. believed that the charging and

G. Wu et al. / Waste Management 33 (2013) 585–597 595

discharging behavior of plastics could be explained by the forma-tion of water films onto plastic surface (Németh et al., 2003). Someplastics such as PA, polymethyl methacrylate (PMMA) and POMhave been proven to have a high ability to take up water. A highvalue of atmospheric humidity offers a high number of water mol-ecules that can be introduced into these polymers and form water-containing swollen layers, which can introduce more ions in thesystems and decrease the surface resistivity. Therefore, more leak-age causes and charge cannot be accumulated with the increasingrelative humidity (Greason, 2000). Some literatures also indicatethat higher humidity would impede the effectiveness of charging,as well as the output of the separation process.

In summary, charger material with a medium work function,proper residence time, relatively high degree of mixing and lowhumidity would increase the effectiveness of plastic particlecharging.

5. Numerical study on the triboelectrostatic separation ofplastic waste

Numerical simulation and modeling can be quite effective indesign and optimization of new processes or devices. Some differ-ent modeling approaches have been proposed to simulate thetriboelectrostatic separation process or optimize it.

By proposing reasonable assumptions, particle trajectory in theelectric field can be simulated and the recovery can be estimated.Ha et al. studied the separation of PVC from mixed plastics, andcomputed the trajectories of PVC particles under different condi-tions. The particle trajectories are obtained using a Lagrangianmethod as a function of different important variables such as Rey-nolds number. Stokes number, electrostatic force, electric chargeand electric field distribution, inclined angle of plane electrodes,particle rebounding, particle charge decay rate after impact onthe electrode surface, etc., in order to determine the optimal designconditions. The present predicted results for the cumulative yieldrepresent well the experimental ones (Ha et al., 2003). Unlike thecomplicated computation of the former work, a unified modelingapproach has been presented by Wei and Realff. It contains threesteps: (1) modeling the trajectory, (2) modeling the recovery, and(3) empirical partition curve fitting (Wei and Realff, 2005b).According to this approach, they studied the design and optimiza-tion of free-fall separators (Wei and Realff, 2003) as well as roll-type (drum-type) separators (Wei and Realff, 2005a). Simplifyingthe simulation by neglecting the particle–particle interactions inthe electric field, air drag force, interparticle collisions and therebounding effect, trajectories of plastic particles of different de-signs can be computed easily. To optimize the free-fall separators,the objective function is set as the maximization of the total profit,which is the revenue from selling recycled products minus the an-nual unit capital cost. Since the revenue is proportional to therecoveries and the capital cost is a function of plate area and thevoltage, the profit of different designs can be calculated straightforwardly (Wei and Realff, 2003). The simulation of particle trajec-tory in the electric field cannot only provide a prediction of theseparation efficiency of existing equipment but also compare theseparation efficiency devices of different designs and differentoperating conditions. For example, a second stage was found tobe preferable only at high feed flow rate and product prices be-cause the additional revenue is enough to cover the additional cost(Wei and Realff, 2003, 2005a).

Modeling based on the experimental results usually can have arelatively high fitness. Quadric polynomial regression of limitedexperimental results is usually used to describe the triboelectro-static separation process. The common model can be expressedas the following quadric equation:

Y ¼ a0 þXn

i¼1

aixi þXn

i¼1

Xn

j¼1

aijuiuj ð2Þ

where y is the response of the process (it can be the charging effi-ciency or the recovery of the separation); a0, ai and aij are the modelparameters (so-called regression coefficients) needed to be deter-mined, which will indicate how the factors influence the response;n is the total number of factors and xi is the normalized centered va-lue for each factor ui:

xi ¼ ðui � uicÞ=Dui ¼ u�i ð3Þuic ¼ ðuimax þ uiminÞ=2 ð4ÞDui ¼ ðuimax � uiminÞ=2 ð5Þ

Eqs. (2)-(5) are so useful that several papers have presentedtheir works in the modeling of triboelectrostatic separation processaccording to them. By choosing relevant factors and finishing lim-ited designed experiments, some experimental data can beachieved and the regression of Eq. (2) can be done. Blajan et al.,2010 studied the triboelectrification of granular plastic waste in vi-brated zigzag-shaped square pipes in view of electrostatic separa-tion and build two models. The response Q/M representing thecharging efficiency is influenced by the crank length R� and theoscillation frequency n� of the triboelectrification pipes. The recov-ery of the separation is set as a function of R�, n� and the appliedvoltage U. For example, the response function y (i.e. ABS recoveryfrom mixed plastics with HIPS using vibrated pipes) can be ex-pressed as:

y ¼ 82:83� 2:758n� � 2:998R� � 3:002U� þ 1:648n�R�

� 1:598n�U� � 2:348R�U� ð6Þ

Similar modeling method has been applied in some other casestoo, such as the fluidized-bed (Dascalescu, 2011) and propellertype tribocharger (Miloudi et al., 2011a; Miloudi et al., 2011b).The regression modeling can predict the outcome in a certain rangeof impact factors and simplify the optimization.

Numerical study provides an effective way to design or opti-mize a device. Trajectory simulation and quadric polynomialregression are usually applied in the study of triboelectrostaticseparation of plastic waste. They can save the times of experimentsand predict the outcome effectively. For existing equipment, it maybe difficult to change the structure parameters of it such as thelength or the inclining angle of the electrodes. By numerical simu-lation, it can be very convenient to change the parameters and pre-dict the outcome to provide reference for the devices optimization.

6. Challenges and opportunities for improving triboelectrostaticseparation for granular plastic waste

Triboelectrostatic separation has been considered as one of themost promising methods for plastic recycling, as it is much cheaperand the separation efficiency is much better than conventional sep-aration methods. The bench scale experiments have presented abright future to utilize the technology for plastic recycling. How-ever, there are also some challenges that it has to face before it be-come continuous industrial process.

The charge efficiency influences the separation efficiency signif-icantly but it is extremely sensitive to the change of ambienthumidity. In most cases, low humidity benefits the charging pro-cess. However, it is not easy to control relative humidity below20% in a plant for the limited capacity of the dehumidifier (Parket al., 2007a). Improving the controllability of relative humidityor increasing the maximum tolerance of relative humidity of theequipment would help the development of triboelectrostaticseparation.

596 G. Wu et al. / Waste Management 33 (2013) 585–597

During the charging step, the study of mechanism of the parti-cle collision still needs to be deeply researched. It will be helpful tocontrol the tribocharging efficiency if some collision models inmicrolevel can be built to give a better understanding of the charg-ing behavior of plastic particles. Some numerical models for therecovery of the separation have been presented and proved to behelpful in optimization of the process. More numerical simulationstudy will help the development of the technology.

Triboelectrostatic separation can be effective for separating bin-ary mixture but it cannot process mixtures of three or more kindsof plastic at one time. When plastic waste is collected, more thanthree kinds of plastic may be mixed together, which make it diffi-cult to separate them by one-step triboelectrostatic separation.Two-step triboelectrostatic separation can be a solution for sepa-rating ternary mixture (Dodbiba et al., 2003b; Park et al., 2008a).Combined with other mechanical separation methods such as den-sity based separation, more complex mixtures can also be pro-cessed (Dodbiba et al., 2005).

Since the materials may easily be contaminated or have oxida-tion layers before the processing, which would influence the sur-face properties and the charging efficiency, the reproducibility oftriboelectrostatic separation has long been a serious concern. Fortribocharging of plastic particles, minimization of contaminationand oxidation is indispensable in order to get a consistent and reli-able charging efficiency as well as separation efficiency. From thisview, standardization of plastic waste collecting and pretreatmentbefore triboelectrostatic separation would be necessary, which hasnot been intensively studied yet.

7. Conclusions

Recycling is most appropriate strategy for management of plas-tic waste for both economical and environmental reasons. Mechan-ical recycling is suitable for plastic recycling because of its lowdegree of pollution generation and cost.

Triboelectrostatic separation is undoubtedly a promising pro-cess for granular plastic waste because of its mechanical simplicity,low cost, high separation efficiency and ability to process a rela-tively wide range of particle size than other separation systems.

In triboelectrostatic separation process, mixed plastic wasteusually need to be crushed into proper size, then get charged in atribocharger, be fed into the electric field and collected in differentways. Several different devices have been designed for triboelec-trostatic separation and shown the feasibility to achieve high sep-aration efficiency through these systems. Impact factors and somenumerical modeling have been studied for the optimization fortriboelectrostatic separation. More research needs to be carriedout before triboelectrostatic separation can be widely and success-fully applied in industry.

Acknowledgements

This project was supported in part by the National Natural Sci-ence Foundation of China (51008192), Doctoral Fund of Ministry ofEducation of China (20090073120041), Shanghai Natural ScienceFoundation (10ZR1415900) and Open Project of Key Laboratoryof Solid Waste Treatment and Resource Recycle (09gk01). Theauthors are grateful to the reviewers who help them improve thepaper by many pertinent comments and suggestions.

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