Pyrolysis Dehalogination

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Review

Pyrolysis and dehalogenation of plastics from waste electrical and electronic

equipment (WEEE): A review

Xiaoning Yang, Lushi Sun ⇑, Jun Xiang, Song Hu, Sheng Su

State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, 430074 Wuhan, PR China

a r t i c l e i n f o

Article history:

Received 12 April 2012Accepted 24 July 2012Available online 27 August 2012

Keywords:

WEEE plasticsPyrolysisDehalogenationSb2O3

a b s t r a c t

Plastics from waste electrical and electronic equipment (WEEE) have been an important environmentalproblem because these plastics commonly contain toxic halogenated flame retardants which may causeserious environmental pollution, especially the formation of carcinogenic substances polybrominateddibenzo dioxins/furans (PBDD/Fs), during treat process of these plastics. Pyrolysis has been proposedas a viable processing route for recycling the organic compounds in WEEE plastics into fuels and chemicalfeedstock. However, dehalogenation procedures are also necessary during treat process, because the oilscollected in single pyrolysis process may contain numerous halogenated organic compounds, whichwould detrimentally impact the reuse of these pyrolysis oils. Currently, dehalogenation has become a sig-nificant topic in recycling of WEEE plastics by pyrolysis. In order to fulfill the better resource utilization of the WEEE plastics, the compositions, characteristics and dehalogenation methods during the pyrolysisrecycling process of WEEE plastics were reviewed in this paper. Dehalogenation and the decompositionor pyrolysis of WEEE plastics can be carried out simultaneously or successively. It could be ‘dehalogen-ating prior to pyrolysing plastics’, ‘performing dehalogenation and pyrolysis at the same time’ or ‘pyrol-ysing plastics first then upgrading pyrolysis oils’. The first strategy essentially is the two-stage pyrolysiswith the release of halogen hydrides at low pyrolysis temperature region which is separate from thedecomposition of polymer matrixes, thus obtaining halogenated free oil products. The second strategy

is the most common method. Zeolite or other type of catalyst can be used in the pyrolysis process forremoving organohalogens. The third strategy separate pyrolysis and dehalogenation of WEEE plastics,which can, to some degree, avoid the problem of oil value decline due to the use of catalyst, but obviously,this strategy may increase the cost of whole recycling process.

2012 Elsevier Ltd. All rights reserved.

1. Introduction

With the development of social economy, the demand forelectronic products increases rapidly and simultaneously, the re-fresh speed of these products is becoming progressively faster.

Therefore, much electronic equipment that is used only for a shorttime has not been able to satisfy people’s needs and becomes ob-solete, and so the production of WEEE shoot up (seen in Fig. 1).According to a report of UNEP (2009), the global production of WEEE is around 40 million tonnes per year. And it can be expected

0956-053X/$ - see front matter 2012 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.wasman.2012.07.025

Abbreviations: ABS, acrylonitrile–butadiene–styrene; BFRs, brominated flame retardants; Br-ABS, acrylonitrile–styrene–butadiene flame retarded with tetrabromobi-sphenol-A; Br-HIPS, high impact polystyrene flame retarded with decabromodiphenyl oxide; DDE, decabromodiphenyl ethane; DDO, decabromodiphenyl oxide; DSC,differential scanning calorimetry; DTG, differential thermogravimetric; EEE, electrical and electronic equipment; ER, epoxy resin; FR, flame retardant; FT-IR, fouriertransform-infrared spectroscopy; GC-AED, gas chromatograph equipped with atomic emission detector; GC-ECD, gas chromatograph equipped with electron capturedetector; GC-FID, gas chromatograph equipped with flame ionization detector; GC–MS, gas chromatography–mass spectrometry; GC-TCD, gas chromatograph equipped withthermal conductivity detector; HBCD, 1,2,5,6,9,10-hexabromocyclodecane; HIPS, high impact polystyrene; IQ, intelligence quotient; LDPE, low-density polyethylene; MFs,metallic fractions; NMFs, non-metallic fractions; OHCs, organohalogen compounds; PA, polyamide; PBDD/Fs, polybrominated dibenzo dioxins/furans; PBDEs, polybrominateddecabromodiphenyl ether; PBT, poly(butylene terephthalate); PC, polycarbonate; PCBs, printed circuit boards; PE, polyethylene; PE/ABS–Br, mixtures of high densitypolyethylene and acrylonitrile–butadiene–styrene copolymer containing a polybrominated epoxy type flame retardant; PET, poly(ethylene terephthalate); PP, polypropylene;PPE, polypheylene ether; PR, phenolic resin; PS, polystyrene; PS/ABS–Br, mixtures of polystyrene and acrylonitrile–butadiene–styrene copolymer containing apolybrominated epoxy type flame retardant; PXDD/Fs, polybrominated, chlorinated or mixed brominated–chlorinated dibenzo dioxins/furans; PVC, poly(vinyl chloride);TBBPA, tetrabromobisphenol-A; TG, thermogravimetric; TGA, thermogravimetric analysis; TH, thyroid hormone; TSH, thyroid-stimulating hormone; WEEE, waste electricaland electronic equipment.⇑ Corresponding author. Tel./fax: +86 27 87545526.

E-mail addresses: [email protected], [email protected] (L. Sun).

Waste Management 33 (2013) 462–473

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that this figure will become bigger in the future. Actually, WEEEhas been one of the waste streams of increasing fastest in munici-pal solid waste. In European Union, this waste is increasing threetimes as fast as the growth of average municipal waste (Church-man-Davies, 2002; Luda et al., 2010; Moltó et al., 2009).

WEEE plastics, which account for about 30% (Sodhi and Reimer,2001; UNEP, 2005) of the total amount of WEEE (seen in Fig. 2), areimportant parts of WEEE. On the one hand, because of the noxiousBFRs included in WEEE plastics, treating them simply by landfill(main method of treating WEEE currently (Miskolczi et al., 2006;Panda et al., 2010; Robinson, 2009; Watson et al., 2010)) not onlywould occupy the decreasing landfilling area, but also pollute ourenvironment and threaten human health. Researches of Eriksson

and Jakobsson (2001) indicated that BFRs had some bad effect tomammals’ nervous system and possibly interfered the reproduc-tive system of mammals. Besides, the decomposition of certaintypes of BFRs in the environment will lead to the formation of moretoxic and accumulative products such as lower brominated biphe-nyl ether congeners and PBDD/Fs (Morris et al., 1992; Stegemanet al., 1993). On the other hand, from the resource point of view,

petroleum-derived WEEE plastics contain numerous organic mat-ters which can be used to produce fuels or chemical feedstock aftertreating them properly (de Marco et al., 2008; Hall and Williams,2008, 2007, 2006b).

So, researches on recycling WEEE plastics have far-reaching sig-nificance to the environment and energy crisis faced by our currentsociety. At present, there are four options available in the manage-ment of WEEE plastics: landfilling, mechanical recycling, energyrecycling and feedstock recycling (seen in Table 1). Various studies,based on these four aspects, have been carried out for the disposalof WEEE plastics. Gamse et al. (2000) had used high-pressure CO2

apparatus and GC-ECD to investigate the solubility behavior of HBCD and TBBA in supercritical CO2 for separating halogenated

flame retardants from polymer matrixes. Taurino et al. (2010) re-ported a faster characterization of WEEE plastics using FT-IR andDSC techniques in order to make mechanical recycling. The trialrun using WEEE in the smelter feed stream in the Umicore smelterat Hoboken, Belgium (Brusselaers et al., 2006) shows that approx-imately 2/3 of the plastics’ energy content was used to replace con-sumption of coke and fuel oil (reducing agent and energy source)and approximately 1/3 was recovered as increased steam produc-tion with no environmental problem when the WEEE plastics con-tained relatively low levels of flame retardants. Yamawaki (2003)studied the gasification recycling technology of WEEE plasticsand found that gasification of WEEE plastics containing BFRs canbe achieved without the generation and regeneration of bromi-nated dioxins. Finally, Onwudili and Williams (2009) reported

the decomposition behavior of commonly used brominated flameretarded plastics, Br-ABS and Br-HIPS, in supercritical water at450 C and 31 MPa with both the absence and presence of alkalineadditives, NaOH and Ca(OH)2, for the production of fuel oils orchemical feedstock.

Apart from the methods mentioned above, pyrolysis, whichaims to provide valuable gasoline-range hydrocarbons or otherchemical compounds, is also an important method. And it was re-garded as the most promising processing route for WEEE plastics(Bagri and Williams, 2002; Brebu et al., 2005; Brebu and Sakata,2006; Luda et al., 2005). Paul T. Williams, in Leeds University Brit-ain, and Yusaku Sakata, in Okayama University Japan, made manyinvestigations with respect to the recycling of WEEE plastics by themethod of pyrolysis. We previously also did lots of experimental

studies on the pyrolysis of waste PCBs in a tube reactor and cameto several useful conclusions (Sun et al., 2003a, 2003b, 2002,

Fig. 1. The WEEE stack in Guiyu China (TIME, 2010).

Fig. 2. The compositions of WEEE (Sodhi and Reimer, 2001; UNEP, 2005).

X. Yang et al. / Waste Management 33 (2013) 462–473 463



2001a, 2001b). But, pyrolysis oils of WEEE plastics generally con-tain a large number of organic brominated compounds whichapparently will hinder the reuse of them. Therefore it is necessaryto upgrade the oil products or take dehalogenating measures dur-ing the pyrolysis process and directly obtain oils that are not con-taminated by halogen. In fact, it is a crucial step, for the recycling of WEEE plastics, to remove organic halogen effectively and cheaply(Blazsó et al., 2002). The dehalogenation measures could be carriedout with the decomposition of WEEE plastics simultaneously orsuccessively. Upon the sequence of pyrolysis and dehalogenation,the recycling process of WEEE plastics by pyrolysis can be classi-fied into three categories (Fig. 3): dehalogenating prior to thedecomposition of plastics, performing dehalogenation and pyroly-sis simultaneously and pyrolysing plastics first, then upgrading the

pyrolysis oils collected in pyrolysis step. In this paper, we reviewand summarize WEEE plastics pyrolysis behavior, various dehalo-genation techniques during the recycling process of them by pyro-lysis and their compositions and characteristics.

2. WEEE plastics

2.1. Definition of WEEE plastics

WEEE embraces diverse forms of electric and electronic equip-ment that are at the end of their lives or have been discarded by

their original owners. Consequently, WEEE plastics can be definedas the end of life plastic materials from waste stream of WEEE,encompassing plastic outer casing, components or consumablesof discarded EEE such as refrigerators, air conditioners, televisionsand computers. In this article, we use the term ‘‘WEEE plastics’’ inaccordance to the above definition.

2.2. Compositions of WEEE plastics

Plastics are low-cost materials and are widely used becausethey can be easily processed into light but durable materials withlow thermal and electrical conductivity (Brebu et al., 2004a). Theycan be used in EEE for insulation, noise reduction, sealing, housing,

interior structural parts, functional parts and interior electroniccomponents (Nnorom and Osibanjo, 2008). Different EEE may usedifferent polymer types for their specific purpose such as conform-ing to environmental requirements or stress intensity criterions.This fact leads to WEEE plastics a complex mix of materials of dif-ferent, often incompatible, polymer types (Freegard et al., 2005;Schlummer et al., 2007; Watson et al., 2010). Fig. 4 shows typicalcompositions of WEEE plastics investigated by Freegard et al.(2005). Thermoplastics, such as HIPS, ABS, PVC and PC are the mostimportant parts of WEEE plastics. The combination of HIPS and ABSonly account for approximate 55% of all WEEE plastics (Freegardet al., 2005). There are only a small number of thermosets among

Table 1

Four options in the management of WEEE plastics.

Options Definitions Comments

Landfilling Landfilling, which is the least preferred option, refers to roughly buryWEEE plastics under layers of earth

Landfilling is not a harmless method. It is well known that all landfills leak(UNEP, 2005), which would cause the leakage of harmful materials into soil,polluting precious underground water

Mechanicalrecycling

Mechanical recycling is reprocessing of WEEE plastics to form newsimilar plastic product with nearly same or less performance level than

the original products

It is necessary to separate and sort heterogeneous WEEE plastics first beforethe process of mechanical recycling. In addition, due to the possible

formation of toxic PBDD/Fs when exposed to thermal stress (Nnorom andOsibanjo, 2008; Schlummer et al., 2006), special precaution is requiredduring recycling process

Energyrecycling

Energy recycling aims at acquiring thermal energy by incinerating WEEEplastics in a boiler or other industrious equipment

The main bottleneck of this type of recycling is the possible formation of PBDD/Fs within the incineration courses. To control emissions of PBDD/Fs,high temperature (>850 C) and long residence time (>2 s) are required(Mark et al., 2006)

Feedstockrecycling

Feedstock recycling aims to convert the organic materials in WEEEplastics into fuels, original monomers or other valuable chemicals bymethods such as pyrolysis, gasification

Feedstock recycling is seen as one of the most valuable options in thetreatment of WEEE plastics. Pyrolysis, as an important feedstock recyclingmethod, only consumes 10% of the energy content of WEEE plastics (Bhaskaret al., 2002; Brebu et al., 2004a; Nnorom and Osibanjo, 2008). But, likewise,pyrolysis process may cause the formation of PBDD/Fs (Ebert and Bahadir,2003; Hornung et al., 2003; Schlummer et al., 2007; Thoma and Hutzinger,1987; Weber and Kuch, 2003)

Fig. 3. Three strategies to recycling WEEE plastics by pyrolysis.

464 X. Yang et al./ Waste Management 33 (2013) 462–473



WEEE plastics. These kinds of plastics include mainly ER and PR contained at PCBs (Sun et al., 2003a). Waste PCBs are special WEEEwhich represent about 2% of the total amount of WEEE but containmore harmful materials (Ongondo et al., 2011). In general, recy-cling waste PCBs is more complex. It needs to divide waste PCBsinto metallic fractions (MFs) and non-metallic fractions (NMFs)first. Then, these two fractions can be recycled by different meth-ods respectively (Guo et al., 2009). NMFs, though consist of moreimpurities than other WEEE plastics, still can be recycled by waysthat are similar with the recycling of waste plastics.

2.3. Flame retardants in WEEE plastics

The use of flame retardants in certain EEE’s plastic componentsis critical to ensure safety. There are more than 175 different typesof flame retardants (Birnbaum and Staskal, 2003) available whichinclude about 27% of halogenated organic compounds (Harjuet al., 2008), as well as other types of flame retardants such asphosphorus-containing compounds. About 30% of the plastics usedin EEE contain either halogenated or non-halogenated flame retar-dants (Vehlow et al., 2000). Plastics with halogenated flame retar-

dants are about 41% (186,000 tonnes) of all flame retarded EEEplastics (seen in Fig. 5).

BFRs, which are the most common flame retardants in EEE plas-tics, have about 75 categories among which 30–40 are extensivelyused (Freegard et al., 2005; Tohka and Zevenhoven, 2002). TBBPA,DDO, HBCD and DDE (properties seen in Table 2) are the mostwidely used BFRs. TBBPA and its congeners singly occupy 50% mar-ket share worldwide (Tohka and Zevenhoven, 2001). All these BFRsappear to be persistent and bioaccumulative, resulting in foodchain contamination, including human milk, producing adversehuman health effects (DiGangi et al., 2010). Study conducted byChevrier et al. (2010) found a significant association amongstexposure to PBDEs and lower TSH during pregnancy, which mayhave implications for maternal health and fetal development. Table

3 lists some other researches about the bad effect of BFRs in peopleor animals.

3. Recycling WEEE plastics by pyrolysis

3.1. General pyrolysis behavior of plastic polymers

Pyrolysis is a course whereby organic materials are heatedwithout the presence of oxygen until the materials decompose intogases, oils and chars which can be used as chemical feedstock orfuels. In pyrolysis of plastic polymers, macromolecular structuresof polymeric materials are broken down into smaller moleculesand a wide range of hydrocarbons are formed. The quality of pyro-lysis product and product distribution depends on temperature,residence time and some other factors (seen in Table 4). Tempera-ture is the most important operating variable. High temperature(>600 C) favor the production of simple small gaseous molecules;low temperature (<400 C) lead to more viscous liquid products(Buekens, 2006). Reactor type, which determines heat and massexchange, residence time and the escape of primary products, is

also an important, and may be decisive (Sasse and Emig, 1998), fac-tor for product selectivity. With regard to the recycling of wasteplastics for as much, light liquid products as possible and littleamount of carbon residue. Fluidized bed reactor may be a betterchoice. It has advantages of yielding more oil products, displayinga better temperature control performance and preserving morevaluable normal hydrocarbons (Scheirs, 2004). Many researchers(Guo et al., 2010; Hall and Williams, 2006a, 2006b) had studiedthe pyrolysis of WEEE plastics in a fluidized bed reactor. Fig. 6shows the schematic diagram of the fluidized bed reactor used inan experiment of Hall and Williams (2006a) which measures 1 min height by 120 mm in diameter. In their work, brominated HIPSwas pyrolysed at temperatures of 450, 500 and 550 C with aaim of investigating the flash pyrolysis behavior of HIPS containing

flame retardants of DDO, characterizing the pyrolysis products anddetermining the distribution of bromine and antimony among gas,liquid and solid products. The sample pyrolysed and formed anaverage of 94 wt%, 89.9 wt% and 93 wt% oil with a small amountof gas and solid products at three temperatures respectively.GC–MS analysis showed that oil products mainly comprised of benzene, toluene, ethylbenzene, styrene and cumene. And it wasrevealed that there was a significant concentration of moleculeswith a molecular mass of 125–140 Da (Fig. 7) in all three oils.But more than 98% of the noxious bromine existed in the oil prod-ucts, and only less than 2% bromine released in the form of HBr orBr2 that were absorbed by the alkali solution of Na2CO3 andNaHCO3.

Obviously, this research implied that pyrolysis alone might not

be enough, for WEEE plastics, to yield pure products that are halo-gen-free and can be used directly in industries. Therefore, assisting

Fig. 4. Various polymers in WEEE plastics (Freegard et al., 2005) (ABS, acrylonitrile–butadiene–styrene; HIPS, high impact polystyrene; PA, polyamide; PBT, poly(butyl-ene terephthalate); PC, polycarbonate; PP, polypropylene; PPE, polypheylene ether;PS, polystyrene; PVC, poly(vinyl chloride)).

Fig. 5. Use of flame retarded plastics in EEE ( Vehlow et al., 2000) (FR, flameretarded; E&E, electric and electronic).




measures of destructing organohalogens are needed for the recy-

cling of WEEE plastics by the means of pyrolysis.

3.2. Dehalogenation measures during the recycling routes of WEEE

plastics by pyrolysis

3.2.1. Dehalogenating prior to the decomposition of plastics

The method of dehalogenating first and then decomposing plas-tics essentially is two-stage pyrolysis method which is character-ized with the release of halogen hydrides at low pyrolysistemperature region and the decomposition of polymer matrixesat relative high temperature region, which, therefore, is applicableto those WEEE plastics that display two-stage pyrolysis feature.

3.2.1.1. For chlorine contained at WEEE plastics. PVC is common

chlorine contained at WEEE plastic. Numerous studies (Bockhornet al., 1998; Kim, 2001; Ma et al., 2002; Slapak et al., 2000; Zhu

et al., 2008) indicated that its pyrolysis process has two stages.

Zhu et al. (2008), who analyzed qualitatively the pyrolysis behaviorof PVC using TG-FT-IR (Mettler-Toledo TGA/SDTA851e thermo-gravimetry analyzer coupled with Nicolet Netxus 670 Fouriertransform spectrometer, carrier gas: N2, heating rate: 30 C/min),pointed out that the pyrolysis of PVC could be divided into twomain stages (Fig. 8). The first stage, in the temperature range of 280–400 C, mainly involved dehydrochlorination and resulted inthe formation of some conjugated double bonds that break duringthe second stage. The mass transition ratio of 65% and the temper-ature of maximum rate of 320 C were observed in the first stage.The second decomposition stage of PVC took place in the temper-ature range of 400–560 C, corresponding to the mass transition ra-tio of 35%, when PVC main backbone began to decomposegenerating some hydrocarbons. The temperature of the maximum

rate of the mass changes as determined from DTG curve was482 C. In addition, Bockhorn et al. (1999) had used pyrolysis-

Table 2

Properties of four BFRs TBBPA, DDO, HBCD and DDE (DSBG, 2002; Li et al., 2005; Ou et al., 2001).

Brominated flame r etardants Chemical structur es Bromine percentage (%) Melting point (C) TGA 5% losing temperature (C)

Tetrabromobisphenol- A (TBBPA) 58.5 181 305

Decabromodiphenyl oxide (DDO) 83 303–307 353.10–362

1,2,5,6,9,10-Hexabromocyclodecane (HBCD) 74.71 178–188 280

Decabromodiphenyl ethane (DDE) 82.3 345 332

Table 3

Health risks of some BFRs.

BFRs Health risks References

PBDEs Cryptorchidism Main et al. (2007)PBDEs OHCs present in the serum of pregnant women would transfer over the placenta to the infants Meijer et al. (2008)PBDEs Diabetes Lim et al. (2008)PBDEs Highest exposure individual scored five less IQ points Herbstman et al. (2010)TBBPA Current use of TBBPA may be a matter of concern for human health Van der Ven et al. (2008)TBBPA Affected three different proteins involved in important processes of neonatal brain development Viberg and Eriksson (2011)HBCD HBCD may disrupt TH-mediated brain development Ibhazehiebo et al. (2011)HBCD HBCD can potentially impact the thyroid system of fish Palace et al. (2010)

Table 4

Factors affecting product distribution (Buekens, 2006).

Factors of influence Effect

Chemical composition of theresins

The primary pyrolysis products relate directly to the chemical structure and composition of the resin, and also to the mechanism of itsdecomposition (purely thermal or catalytic)

Pyrolysis temperature andheating rate

Higher operating temperatures and high heating rates both enhance bond breaking and favor the production of small molecules

Pyrolysis time Longer residence times favor a secondary conversion of primary products, yielding more coke, tar, as well as thermally stable products,thus gradually obscuring the effect of original polymer structure

Reactor type Determines mainly the quality of heat transfer, mixing, gas and liquid phase residence times, and the escape of primary productsOperating pressure Low pressure reduces the condensation of reactive fragments forming coke and heavies




GC–MS to investigate the dehydrochlorination of plastic mixturescontaining PVC and reported that the HCl evolution from the ther-moplastic mixtures occurred at low temperatures in the same wayas PVC and could be separated from the degradation of plastics inthe mixtures without being influenced by the effect caused by theirblending (Fig. 9). In short, on the base of pyrolysis behavior of chlo-rinated plastics, it is feasible to remove HCl at low temperaturesand then decompose the polymer matrix further at high tempera-ture region with a goal of obtaining relative pure pyrolysis oil thatis not contaminated by chlorine through two-stage pyrolysis meth-od. Studies of López et al. (2011) about the two-stage pyrolysis of plastic mixtures (PE, PP, PS, PET and PVC) in an autoclave (Fig. 10)proved the dehalogenating effectiveness of two-stage pyrolysis forchlorinated plastics and revealed that maximum dechlorination

rate could be achieved at the pyrolysis temperature of 300 C witha lasting time of 60 min at the first step.

3.2.1.2. For bromine contained at WEEE plastics. The pyrolysisbehavior of WEEE plastics containing bromine is different fromthat containing chlorine. When there are not any flame retardantsynergists Sb2O3 existed in this type of plastics, the release of HBr would not occur at different temperature range from thedecomposition of polymer matrix as do chlorinated plastics. Con-versely, these two events coincide (Barontin et al., 2005; Grauseet al., 2010). When the plastics contain synergists Sb2O3, however,the decomposition will undergo in two distinct regions. Sb2O3 ini-tiates the decomposition through reacting with the HBr evolvedfrom the flame retardants at the low temperature region, whichis just the reason why Sb2O3 can be used as flame retardant syner-gist. Fig. 11 shows the mass loss patterns of two brominated highimpact polystyrene samples with the presence of Sb2O3 and with-

out ( Jakab et al., 2003). Obviously, the decomposition of the sampleI, containing Sb2O3 additive, starts at lower temperature than the

Fig. 6. Schematic diagram of the fluidized bed reactor as described by Hall and Williams (2006a).

Fig. 7. Molecular mass range of the pyrolysis oils analyzed by size exclusionchromatography in relation to pyrolysis temperature (Hall and Williams, 2006a).

Fig. 8. PVC thermogravimetric (TG) and differential thermogravimetric (DTG)curves (Zhu et al., 2008).




sample II that does not contain Sb2O3, which leads to a separatedDTG peak. In fact, the effect of Sb2O3 can remove the brominatedadditives and Sb2O3 from HIPS by heating the plastics to 330 Cand then, once the brominated compounds and Sb2O3 had been re-moved, HIPS could be pyrolysed to form bromine-free products(Bhaskar et al., 2007; Jakab et al., 2003). In other words, the two-

stage pyrolysis method may be also applicable for recycling bromi-nated plastics containing Sb2O3.

In studies led by Sakata (Bhaskar et al., 2007; Mitan et al., 2008),polyethylene/polypropylene/polystyrene mixed plastics with highimpact polystyrene containing DDE or DDO as flame retardantswith or without Sb2O3 as a synergist were pyrolysed in a quartzglass reactor (Fig. 12) using two-stage method. The first step tookplace at 330 C for 2 h and the second step took place at 430 C tillthe finish of the pyrolysis reaction (Fig. 12). The experimental re-sults indicated that under the controlled temperature programmedpyrolysis conditions, step 1 oils had higher brominated compoundsthan step 2 in all samples and the presence of antimony could fur-ther increase the rate of bromine release by forming SbBr3 at lowpyrolysis temperatures. Thus oil products that contain relative lessbromine can be obtained in the second step. Nevertheless process-ing brominated plastics containing antimony by the two-stagepyrolysis method does not convert the toxic organic bromines intoinorganic bromine but just concentrate them in the first step oils.Thus, it is necessary to investigate appropriate way to process oilscollected in the first pyrolysis step that contain higher amount of bromine.

3.2.2. Performing dehalogenation and pyrolysis at the same time

The method of dehalogenating and pyrolysing simultaneously isto remove halogen during the pyrolysis process of halogenatedplastics through devising specific experimental setup, controllingreaction parameter and reaction procedure or using catalysts, addi-tives and so on. In fact, a plethora of scholars’ researches (Brebuet al., 2005; Blazsó and Czégény, 2006; Hall and Williams, 2008;Miskolczi et al., 2008) in this field were based on this strategy,namely removing halogen directly in the process of pyrolysis.

3.2.2.1. Slow pyrolysis with long residence time. Considering the factthat the residence time of the volatile products in the hot zone of abatch or fluidized bed reactor is extremely short and there is notsufficient time for the organobromines to degrade completely, Mis-kolczi et al. (2008) designed a tube reactor with a length of 1.55 m(Fig. 13) and investigated the pyrolysis of two flame retardant plas-

tics HIPS (8.5 wt% DDO and 5.5 wt% Sb2O3) and ABS (18.2 wt%TBBPA and 3.8 wt% Sb2O3) at the temperatures of 360, 380, 400,420 and 440 C respectively in this reactor with a aim of acquiringinformation about their pyrolysis reaction characteristics, bromineor other organic compound distribution in solid, liquid and gasproducts. Because of the unusually lengthened reactor, the firstdegradation products can stay in the hot zone of the reactor for alonger time and thus there is sufficient time for them to decom-pose totally. For those first brominated degradation products, theycan also decompose secondly and convert organic bromine furtherinto non-toxic inorganic bromine such as HBr which can be ab-sorbed by alkali solution. The experimental evidences indicatedthat without any catalysts or other additives existing, the pyrolysisof flame retarded plastics over long residence time in a tube reactor

has advantages of converting more organic bromine into inorganicbromine than fluidized bed and other fixed bed pyrolysis.

3.2.2.2. Pyrolysing plastics and dehalogenating with catalysts simul-

taneously. Pyrolysis in conjunction with a catalyst is also an effec-tive method for processing plastics which contain BFRs. Hall andWilliams (2008), studied the destruction of brominated HIPS andABS over two zeolite catalysts Y-Zeolite and ZSM-5 in a verticalfixed bed reactor. In their study, the two samples were pyrolysedat 440 C and pyrolysis products were analyzed using GC–MS,GC-FID/ECD and FT-IR. The fixed bed reactor is shown in Fig. 14,which measured 260 mm in length by an internal diameter of 45 mm and was externally heated by a 1.5 kW tube furnace. Halland Williams’ work showed that both the two zeolite catalysts

can be used to remove brominated compounds from the volatilepyrolysis products of Br-HIPS and Br-ABS. And the Y-Zeolite’s effect

Fig. 9. HCl evolution from electronic scrap (plastic mixtures) in comparison withthe rate of conversion (heating rate of 2 K/min; sample size, 20 mg) (Bockhorn et al.,1999).

Fig. 10. Experimental setup as described by López et al. (2011).

Fig. 11. The comparison of TG/DTG curves of two samples HIPS + DDO + Sb 2O3 (I)and HIPS + DDO(II) (ShimadzuTGA-51 thermobalance, heating rate, 5 C/min)( Jakab et al., 20 03).

468 X. Yang et al. / Waste Management 33 (2013) 462–473



of removing organobromines was more obvious than the ZSM-5’s(Fig. 15). However, Hall and Williams also pointed out that theuse of both zeolites can bring about a reduction in the mass of oil produced during pyrolysis due to coke formation on the surfaceof the catalysts and also would harm the value of pyrolysis prod-ucts, for example, reducing the amount of valuable styrene. In fact,the debrominating effect of Y-Zeolite and ZSM-5 is only theadsorption to the brominated compounds without converting or-

ganic bromine into inorganic bromine. Thus, the use of the zeolitecatalysts during the pyrolysis of flame retarded plastics wouldraise questions about the proper way to process spent catalysts.In addition, an investigation into the effectiveness of the zeolitecatalysts after they have been reactivated will also be required, if the technique is to be used on a commercial scale. Apart fromthe two zeolites, Blazsó and Czégény (2006) reported the debromi-nating effect of the other four zeolites 4A, 13X, Al-MCM-41 andNaY for the flame retardant TBBPA. According to their research,over molecular sieves of larger pore size (13X and NaY zeolite),an important decrease of TBBPA amount is observed indicatingeffective trapping ability of these catalysts for brominatedaromatic compounds. But, molecular sieve 4A of small pore sizewas found to have only little debrominating activity and no debro-

minating effect was observed over Al-MCM-41 mesoporouscatalyst.

Fig. 12. Experimental setup and temperature program in the experiments of Bhaskar et al. (2007).

Fig. 13. Schematic diagram of the pyrolysis reactor as described by Miskolczi et al. (2008) (1, extruder; 2, electric motor; 3, reactor; 4, distillation column; 5, cooler; 6,separator; 7, filter).

Fig. 14. Schematic diagram of the fixed bed reactor as described by Hall andWilliams (2008).




Furthermore, several other types of catalysts (Brebu et al., 2005,

2004b; Hall et al., 2008; Uddin et al., 1999) also have debrominat-ing effect. Brebu et al. (2004b) studied the thermal decompositionbehavior of fire retardant brominated ABS mixed with PS and PE inthe presence of the catalysts FeOOH, Fe(Fe3O4)–C and Ca(CaCO3)–C.The pyrolysis of each sample was investigated at 450 C in both thepresence and the absence of the catalysts under the experimentalsetup shown by Fig. 16. GC-TCD, GC-FID, GC-AED and GC–MS wereused to characterize pyrolysis products. And it was found that oilsaccounted for 63–74wt% of all pyrolysis products of PE/ABS–Br atdifferent conditions. These oils contained a broad spectrum of sat-urated and unsaturated hydrocarbons, and about 2000 ppm bro-mine. The pyrolysis of PS/ABS–Br produced about 70wt% oilsamong which benzene derivatives were the main compoundsand bromine was of also about 2000 ppm. FeOOH and Fe(Fe3O4)–

C catalysts were effective at reducing the bromine content of thepyrolysis oils from PE/ABS–Br but the effect were weaker for PS/ABS–Br. But, calcium carbon composite Ca(CaCO3)–C had lower ef-fect in removal of bromine, especially on the pyrolysis of PS/ABS–Br mixture.

3.2.3. Pyrolysing plastics first then upgrading pyrolysis oils

This method is based on the thought of first considering theproblem of producing oils with a desirable products compositionand then upgrading these oils contaminated by toxic organic halo-gen through various debromination methods. And so the recyclingof harmful halogenated plastics can be achieved.

From the research of Hall and Williams (2006a) mentioned inSection 3.1, we learn that pyrolysis oils contain torrents of brominewhen no dehalogenating measure is exerted in pyrolysis process.And some of these bromine exists in organic compounds such aspolybrominated phenols, polybrominated benzenes and polybro-minated diphenyl ethers. Obviously, these bromine contained atpyrolysis oils cannot be used before debromination process. Onaverage, catalytic hydrodehalogenation is an effective way of removing the halogen in pyrolysis oils.

Catalytic hydrodehalogenation usually is used to dispose wastehalogenated compounds, for instance polychlorinated biphenyl,farm chemicals. And much type of catalysts can effectively removethe halogenic substituent in aromatics. Wu et al. (2005) found thatin the presence of NaOH, Ni/AC, Ni/c–Al2O3, Ni/SiO2 and Raney Niall can completely eliminate the chlorine in chlorinated benzene(hydrogen pressure: 1.0 MPa; temperature: 333–343 K). Theexperiments of Shen and Sun (2009) proved that metal salts,FeCl24H2O, FeCl3, NiCl2, CoCl2, CuBr and some iron complexes withbenzimidazole ligand are efficient catalysts for hydrodebromina-tion of bromoarenes under mild reaction conditions of room tem-perature and N2 atmosphere with two equivalents of Grignardreagents. Moreover, Pd–Fe/C (Yang et al., 2003) also displayed highdehalogenating activity under certain conditions.

These facts suggest that dehalogenation of the halogenatedpyrolysis oils maybe also could be achieved by catalytically hydro-treating. Vasile et al. (2007) investigated the dehalogenating effectof two commercially available catalysts DHC-8 and a metal-loadedactivated carbon (Mo 2.89%, Ni 4.63%) to the pyrolysis oils fromwaste computer casings (Vasile et al., 2005). Reaction conditionsof hydrogen pressure of 6.5 MPa and temperature of 350 C wereselected in the experiments. According to this research, the hydro-genation over the two catalysts used allows eliminating almost to-tally the hazardous toxic compounds, from pyrolysis oils resultedfrom flame retarded computer scraps.

3.3. The impact of Sb 2O 3 in WEEE plastics to pyrolysis reaction

Sb2O3, which can produce a synergistic flame retardant effect, isfrequently added to the plastic casings of electric and electronic

Fig. 15. GC-ECD chromatograms of the oil produced by the pyrolysis of Br-ABS (left) and Br-HIPS (right) ( Hall and Williams, 2008).

Fig. 16. Schematic experimental setup for pyrolysis as described by Brebu et al.(2004b) (TC: Thermocouple).




equipment (Camino et al., 1991; Pitts, 1972; Yang et al., 2002 ). Theessential of X-Sb flame retardant system is the generation of vola-tile SbX3 in combustion process which can react with high activefree radicals in combustion zone, change the reaction model inthe gas phase and reduce the reaction heat, and thus quench flame

(Ou et al., 2001). Two reaction mechanisms have been proposedleading to SbX3 formation. Lum (1977) investigated the Sb2O3–PVC system through laser probe pyrolysis and quality analysismethods and found that SbCl3 can be formed by direct reactionshown in Eq. (1) between Sb2O3 and HCl released from the pyroly-sis of PVC.

Sb2O3ðsÞ þ 6HClðgÞ ! 2SbCl3ðgÞ þ 3H2OðgÞ ð1Þ

Pitts et al. (1970) believed that the formation of SbCl3 may occurthrough the intermediate products, as shown in Eqs. (2) and (3):

Sb2O3ðsÞ þ 2HClðgÞ ! 2SbOClðsÞ þ H2OðgÞ ð2Þ

5Sb2OClðsÞ ! Sb4O5Cl2ðsÞ þ SbCl3ðgÞ ð3Þ

No matter through which mechanisms SbX3 is formed, the for-mation of SbX3 can increase halogen release rate when X-Sb flameretardant material is heated. This effect, of course, can also influ-ence the pyrolysis reaction of halogenated WEEE plastics contain-ing Sb2O3. According to a study of Jakab et al. (2003), Sb2O3 canreduce the initial temperature of decomposition about 50 C.Moreover, since the formation of SbBr3 absorbs certain amountof bromine, the amount of toxic organic bromine in pyrolysis prod-ucts would decrease. Rzyman et al. (2010) studied the brominationand evaporation of Sb2O3 during thermal treatment of TBBPA usingDSC analysis and a laboratory-scale furnace (Fig. 17). The temper-ature of the furnace was initially stabilized at 140 C for 1 h andthen increased to 250, 290, 340, 390, 440, 490, 540 and 650 C(in separate experimental runs) at a rate of 10 C/min under argon

(99.9999%) flow of 80 ml/min. When the desired temperature wasreached, the reaction tube was cooled rapidly in an argon flow to

prevent any secondary reactions. This research revealed that thebromination of Sb2O3 closely followed the decomposition of TBBPAand took place at a temperature below 340 C. The maximumamount of bromine converted into SbBr3 (above 80%) was obtainedin experimental run at 650 C with a small fraction of bromine ex-

isted in other organic compound or HBr.Sb2O3 could largely affect the pyrolysis reaction of halogenatedplastics. But the effect does not have to be negative to the recyclingof WEEE plastics by pyrolysis. Practically, the reaction betweenSb2O3 and HX may promote the cleavage of X–C bonds formingthe SbX3. So the presence of Sb2O3 not only could decrease the loadon the subsequent debromination process by other method butalso reclaim the Sb element in the form of SbX3 which can be ex-tracted from pyrolysis oils.

4. Conclusions

In the face of the sharp increasing WEEE plastics, it would notonly mitigate the threat of toxic WEEE to human environment

but also relieve the energy pressure resulted from the depletionof fossil fuel to convert the waste plastic into hydrocarbon fuelcheaply and environment-friendly. But as noted previously, thehalogen element in WEEE severely hinders the recycling pace.Dehalogenation comes to the key step of all methods of recyclingWEEE. Pyrolysis, as an important method, has been the focus of agreat deal of researches all over the world. The two-stage pyrolysismethod, which can be classified as the method of first dehalogen-ating and then pyrolysing, shows strong effectiveness for the chlo-rinated WEEE plastics. But, for the Br–Sb flame retarded WEEEplastics, the toxic bromine only could be concentrated in the firststep pyrolysis oils which must be processed further for completelyharmless recycling. Researches based on the method of dehalogen-ation and pyrolysis occurring simultaneously had paid too much

attention to dehalogenation but concerned little to the value of pyrolysis products. It is necessary to find a balance between the

Fig. 17. Schematic experimental set-up for furnace experiments and overview of the analytical procedure as described by Rzyman et al. (2010).




quality and quantity of pyrolysis products and dehalogenationwhich may result in products value and yield decline. Studiesbased on the third thought of pyrolysing plastics first and thenupgrading the pyrolysis oil considered the two problems of pyroly-sis and dehalogenation separately, which avoid the possible valuedecrease of oil products resulted from too much concerns todehalogenation. But the separation of pyrolysis and dehalogen-ation obviously would increase recycling cost, if this method isused to process halogenated plastics industriously. In addition,the presence of Sb2O3 would decrease the stability of plastics andabsorb some amount of bromine by forming SbBr3. However,simultaneously, it would promote the formation of PXDD/Fs to cer-tain extent. In sum, though pyrolysis is a favorable method forrecycling WEEE plastics, there are still many handicaps that needto be overcome.

Acknowledgments

This study was supported by the National Natural Science Foun-dation of China (Nos. 50976038, 51021065).

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Pyrolysis Dehalogination

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