Removal of Bromine from Waste Plastics by Pyrolysis with

Hydrotalcite

N. Morita1, M. Nakayasu

2, Y. Kawabata

3, and H. Nakagome

4

1 Manuscript received January 20, 2017; revised January 20, 2017.

2N. Morita is with the Department of Urban Environment Systems, Graduate School of Engineering, Chiba

University, Japan. (E-mail: Naoyuki_Morita@chiba-u.ne.jp). 3M. Nakayasu is with the Department of Urban Environment Systems, Graduate School of Engineering,

Chiba University, Japan. (E-mail: Masami_1_Nakayasu@education.metro.tokyo.jp). 4Y. Kawabata is with the Department of Urban Environment Systems, Graduate School of Engineering,

Chiba University, Japan. (E-mail: tanoshimokita@gmail.com). 5H. Nakagome is a Professor at Chiba University, Japan. (E-mail: nakagome@tu.chiba-u.ne.jp).

Abstract. In recent years, there has been increasing interest in the pyrolysis of waste plastics to fuels as a

form of recycling. One associated challenge is the frequent presence of halogen-based flame retardants in

various consumer plastics. In this study, waste polyethylene mixed with tetrabromobisphenol A was

subjected to thermal decomposition, using hydrotalcite (HT) to reduce the halogen concentration in the

product oil. Samples with and without HT or with various inert additives were pyrolyzed at 450 °C under

nitrogen in a glass reactor and the product oils, solid residues, and gaseous products were investigated. The

addition of HT was found to increase the yield of the product oil. As well, no brominated compounds were

detected in the oil when using HT. Scanning electron microscopy provided evidence that brominated

compounds are captured by HT in the solid residue during thermal decomposition, thus eliminating bromine

from the product oil and gases.

Keywords: pyrolysis, chemical products, hydrotalcite, brominated flame retardants.

1. Introduction

Plastics have numerous uses in various products, especially in the electrical and electronic equipment

(EEE) that is so widely distributed in our society. As these devices are replaced with newer models, their

lifetimes become increasingly short and significant amounts of waste are generated. This is particularly true

because EEE include not only computers and mobile phones, but also hair dryers, refrigerators, and cathode

ray tubes [1]. After use, these EEE become waste (WEEE) that requires significant processing [2]. In the

European Union alone, 17 kg of WEEE is produced per person annually, with an estimated total of 8.3 to 9.1

million tons generated in 2005 [3]. Recently, the EU has introduced laws that aim to promote the reuse,

recycling, and reduction of EEE while limiting the landfill and incineration of WEEE. These products are

made of materials that can already be reused, such as glass and metals, but also contain more than 15

different types of engineering plastics, making recycling difficult [4], [5]. Waste plastics can undergo

thermal, chemical or physical recycling, although the removal of flame retardant materials added to these

products is an important issue in these recycling processes. The recycling of plastics via the thermal

decomposition of WEEE has attracted attention as a promising technology. During pyrolysis, the polymers

are converted into gaseous compounds that can be used as fuels, along with residual carbon [6]. There has

been much research regarding the treatment of waste plastics using thermal decomposition with catalysts [7]-

Corresponding author. Tel.: +81-43-290-3466

E-mail address: Naoyuki_Morita@chiba-u.jp

International Proceedings of Chemical, Biological and Environmental Engineering, V0l. 101 (2017)

DOI: 10.7763/IPCBEE. 2017. V101. 8

54

[18]. However, WEEE plastics are often treated with halogenated flame retardants, such as potentially

harmful polybromines, and so require special treatment. Examples include polybrominated

dibenzoparadioxins (PBDDs) and polybrominated dibenzofurans (PBDFs) [19]. Many of the plastics used

for EEE also contain antimony trioxide as a synergistic component in conjunction with bromine compounds.

A number of techniques have been studied for the recycling of plastics in WEEE, keeping in mind the health

hazards of these compounds, including thermal decomposition [20]-[24], catalytic processes [25]-[30] and

reduction with ammonia-containing polymers. The use of various other decomposition agents has also been

investigated [31], [32]. Our own group has previously reported that bromine compounds in plastics can be

eliminated by adding hydrotalcite (HT) during the pyrolysis process [33]-[36]. HT is a layered double metal

complex hydroxide, consisting of divalent Mg2+ and trivalent Al3+ host layers with anion species contained

in the interlayer guest layers [37]. HT has received increasing attention in recent years as an ion exchange

material [37]-[40] with applications in catalytic gas removal [41]-[44]. The high anion exchange capacity of

HT has been of significant interest and this material has been used as an adsorbent for the removal of various

contaminants from aqueous solutions. The adsorption of inorganic anions such as borate, nitrate, fluoride,

phosphate, sulfate, chromate, arsenate, and selenate has been studied [45]. The adsorption of anions by HT is

thought to result from the formation of an interlayer outer spherical complex following an anion exchange

reaction [46]. HT has also been considered as a catalyst, and there have been reports that the use of HT can

improve biodiesel yields from 62% to 77% [47], [48]. In addition, the catalytic activity of HT during the

aerobic oxidation of benzyl alcohol has been demonstrated [49]. The present study investigated the effects of

adding HT on the elimination of bromine compounds during the thermal decomposition of polyethylene (PE).

Specifically, a model polymer was pyrolyzed at 450 °C in the presence of HT, and the level of bromine in

the resulting oil was assessed.

2. Experimental

2.1. Materials

Samples simulating a bromine-containing plastic resin (BR-PE) were prepared by mixing 20 g of waste

PE obtained from crushed computer mice (Fujitsu, Ltd.) with tetrabromobisphenol A (TBBA, Tokyo Kasei

Co., Ltd.). Synthetic HT (DHT-4A, Kyowa Chemical Industry Co., Ltd.) was employed, adding either 10 or

20 g of the HT to 20 g of the BR-PE. In addition, for comparison purposes, either 4Å molecular sieves (1/16,

Wako Pure Chemical Industries, Ltd.), 13X molecular sieves (1/16, Wako Pure Chemical Industries, Ltd.), or

sea sand (prewashed with methanol, 425 to 850 μm, Wako Pure Chemical Industries Co., Ltd.) was added to

the BR-PE. A summary of the experimental samples is provided in Table I.

Table I: Experimental sample compositions.

Condition Sample

I Disposable Polyethylene

II Disposable Polyethylene + TBBA（BR-

PE）

III BR-PE + Molecular sieves 4A

IV BR-PE + Molecular sieves 13X

V BR-PE + Sea sand

VI BR-PE + HT（PE:HT=1:1）

VII BR-PE + HT（PE:HT=2:1）

2.2. Experimental Apparatus and Procedures

The experimental apparatus used in this study is shown in Fig. 1. A sample was placed in a glass reactor

and a flow of nitrogen gas at 50 mL/min was applied for 60 min to remove oxygen. After purging, the

nitrogen flow was stopped and the thermal decomposition experiment was conducted. The sample

temperature, as determined using a thermocouple, was raised to 450 °C at 5° C/min. After the temperature

reached 450 °C, heated was ceased and the reactor was allowed to naturally cool to room temperature. The

gas generated during thermal decomposition passed through a cooling pipe where it condensed into an oil

55

that was collected in a container. The gaseous products that did not condense were collected in a gas pack

after bubbling through an alkaline aqueous solution.

Fig. 1: The experimental apparatus

2.3. Analysis

The product oil was diluted by a factor of 100 with hexane (Wako Pure Chemical Industries, special

grade reagent) and subsequently analyzed by gas chromatography/mass spectrometry (GC/MS, Shimadzu

GC-MS-QP2010ultr). The primary product compounds were identified using the spectral library associated

with the instrument software. Pure helium was used as the carrier gas in conjunction with an Rtx-1 column

(i.d. 0.25 mm, 0.25 μm film thickness). The initial column temperature was 40 °C for 30 min, after which it

was raised to 230 °C at 5°C/min.

The residue remaining after the thermal decomposition was analyzed by scanning electron microscopy

(SEM, Hitachi High-Tech TM 3030) in conjunction with energy dispersive X-ray spectroscopy (EDS,

Bruker QUANTAX). The concentration of bromine was quantitatively determined by selecting five points in

the EDS image of the sample and reporting the average value. The gaseous products were analyzed by gas

chromatography (Shimadzu GC-2014) both qualitatively and quantitatively.

3. Results and Discussion

3.1. Product Oil The yields of product oil obtained from pyrolysis are plotted in Fig. 2. These data confirm that the

addition HT increased the yield of oil.

Fig. 2: Yields of product oil

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The chemical components of the product oils are summarized in Table II. Decane was confirmed to be

the most common product, and 3-bromophenol and 2-bromotetradecane were detected as bromine

compounds. These data also demonstrate that product oil containing no bromine compounds was obtained by

adding HT.

Table II: Results of the qualitative analysis of product oils by GC/MS

Detectable substance Molecular

formula

Experimental condition

I II III IV V VI VII

1-octane C８H１６ ○ nd Nd nd nd nd nd

2,4-Dimethylhexane C8H18 ○ nd ○ ○ ○ ○ nd

1-nonen C9H18 ○ nd ○ ○ nd ○ nd

Undecane C11H24 ○ nd ○ ○ nd ○ ○

Decane C10H22 ○ nd ○ ○ nd ○ nd

3-Bromophenol C6H5BrO nd nd ○ nd nd nd nd

Dodecane C12H26 ○ nd ○ ○ nd nd nd

Tetradecane C14H30 ○ nd ○ nd nd ○ nd

2-bromotetradecane C14H29Br nd ○ Nd ○ nd nd nd

1-heptadecane C17H34 ○ nd Nd nd nd nd nd

○: Detected nd: Not Detected

3.2. Residue

Fig. 3: (a) EDS image of the residue from sample II

Fig. 3: (b) EDS image of the residue from sample VI

57

Fig. 3: (c) EDS image of the residue from sample VII

The amount of bromine contained in each residue was quantified by surface observations in conjunction

with EDS. The level of bromine in sample II, without HT addition, was 1.61%. This concentration increased

to 22.4% and 20.1% in samples VI and VII, to which HT had been added, indicating that bromine was

trapped in the residue when HT was included in the specimen.

The results of SEM observations of the residues are shown in Fig. 3. Both small and large particles are

evident on the surface as the amount of added HT increases. It can be inferred from these images that

bromine was adsorbed on the surfaces of small oxide particles, because the large particles are Mg or Al-

based oxides.

3.3. Gas Production

The volume of gas generated by pyrolysis was less than 1 L in all experiments. From this, it can be

inferred that the samples were primarily transitioned to either product oil or solid residue by thermal

decomposition. Analysis of the gases recovered by the gas pack found the presence of H2, CH4 and CO2. It

is assumed that the H2 and CH4 resulted from the thermal decomposition of the PE, while the CO2 came

from the HT.

4. Conclusion

This study investigated the removal of bromine compounds from the oil obtained by the pyrolysis of

waste plastics containing brominated flame retardants, using HT. The results demonstrate the possibility of

obtaining oil containing no bromine compounds. In addition, a higher yield of oil was obtained in

conjunction with the addition of HT. These effects are especially evident when HT is added at a ratio of 2:1.

Thus, it appears that HT has a catalytic effect and there is an optimum addition amount. Qualitative analyses

of the product oils from samples to which HT had been added demonstrate the absence of bromine

compounds, suggesting that HT adsorbs these brominated species. EDS surface observations of the residues

found bromine concentrations of 1.61%, 22.39% and 20.12% in the case of samples II, VI and VII,

demonstrating that the HT adsorbs the bromine compounds. This concentration of the bromine in the residue

upon the addition of HT is believed to have removed brominated compounds from the product oils. PE

typically contains a wax component and, during thermal decomposition, this material often causes clogging

of the cooler piping. However, in the present study, samples VI and VII did not exhibit clogged and also

showed higher product oil yields. It can be inferred from these yields that the HT also serves to decompose

the wax component. The residue from plastic pyrolysis is typically landfilled, and dissolution testing has

demonstrated that the bromine compounds contained in the residue do not leach out. In addition, our

previous work has demonstrated that the use of thermal decomposition can reduce the amount of waste going

to landfill by approximately 80% [35]. In summary, this work has demonstrated that the addition of HT

during the thermal decomposition of waste plastics containing brominated flame retardants substantially

reduces the level of bromine compounds contained in the product oil.

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5. Acknowledgments

The contributions of Ms. Nonoka Suzuki and Ms. Sonoka Hasegawa of the Tokyo Metropolitan Tama

High School of Science and Technology are gratefully acknowledged. The authors also wish to acknowledge

the Kyowa Chemical Industry Co., Ltd. for donating the hydrotalcite used in this work.

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