Microwave assisted pyrolysis of halogenated plastics recovered from waste computers

Waste Management xxx (2017) xxx–xxx

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Microwave assisted pyrolysis of halogenated plastics recovered from waste computers Luca Rosi a,b,⇑, Mattia Bartoli a, Marco Frediani a,b a b

Department of Chemistry ‘‘Ugo Schiff ”, University of Florence, Via della Lastruccia, 3-13, 50019 Sesto Fiorentino, Florence, Italy Consorzio Interuniversitario Reattività Chimica e Catalisi (CIRCC), Via Celso Ulpiani 27, Bari, Italy.

a r t i c l e

i n f o

Article history: Received 19 January 2017 Revised 12 April 2017 Accepted 13 April 2017 Available online xxxx Keywords: WEEE Pyrolysis Microwave assisted pyrolysis Fuels

a b s t r a c t Microwave Assisted Pyrolysis (MAP) of the plastic fraction of Waste from Electric and Electronic Equipment (WEEE) from end-life computers was run with different absorbers and set-ups in a multimode batch reactor. A large amount of various different liquid fractions (up to 76.6 wt%) were formed together with a remarkable reduction of the solid residue (up to 14.2 wt%). The liquid fractions were characterized using the following different techniques: FT-IR ATR, 1H NMR and a quantitative GC–MS analysis. The liquid fractions showed low density and viscosity, together with a high concentration of useful chemicals such as styrene (up to 117.7 mg/mL), xylenes (up to 25.6 mg/mL for p-xylene) whereas halogenated compounds were absent or present in a very low amounts. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction The EU-28, plus Norway and Switzerland (EU28+2), currently generates around 10 million tons of WEEE each year (UNEP, 2005; FISE UNIRE, 2015), and this is rapidly increasing (Huisman et al., 2015). WEEE consists of a complex mix, including a wide range of different materials such as polymers and metals, and organic molecules which are included as flame retardants (Dalrymple et al., 2007; De Marco et al., 2008; Alston et al., 2011; Muhammad et al., 2015). It has been estimated that roughly 20–30 wt% of WEEE consists of plastics (FISE UNIRE, 2015). The composition of WEEE depends also on the age of the item produced, as the manufacturing process has changed over the years (Martinho et al., 2012). The 17% of Italian WEEE derives from small appliances and significant proportion of this comprises of the plastic casing from computer bodies and typewriters (FISE UNIRE, 2015). The large and ever increasing production of electrical materials and electronic devices in the EU has created an imperative towards the proper management and disposal of WEEE. Over the last decade, remarkable efforts have been devoted to the development of a recycling chain for WEEE. This starts from WEEE collection, through the sorting of various components, to their separation ⇑ Corresponding author at: Department of Chemistry ‘‘Ugo Schiff ”, University of Florence, Via della Lastruccia, 3-13, 50019 Sesto Fiorentino, Florence, Italy. E-mail address: [email protected] (L. Rosi).

and extraction by traditional methods. WEEE sorting methods have improved significantly recently, but the complexity and the diversity of the WEEE mix hampers a complete and useful recycling of the individual components. (Kiddee et al., 2013). The recycling of WEEE also presents several environmental problems connected with the high quantity of metals contained within it, and also because of the presence of various halogenated and nonhalogenated flame retardants (Schlummer et al., 2005). The most widespread halogenated are brominated organic compounds, primarily polybrominated diphenyl ethers (PBDEs), of which the most common are penta-BDE, octa-BDE and deca-BDE, that have been classified as Persistent Organic Pollutants (POPs). Furthermore, yet more polymers have been found and identified in WEEE, such as poly(acrylonitrile-butadienestyrene) (ABS), high impact polystyrene (HIPS), polypropylene (PP), polycarbonate (PC), polyvinyl chloride (PVC), and polystyrene (PS) (Alston et al., 2011). The pyrolysis process can potentially be exploited to convert plastics into fuel or other more valuable chemicals (Sharuddin et al., 2016), and can be considered a potentially worthwhile process in the recovery and re-use of the polymeric material content of WEEE (Al-Salem et al., 2009; Alston et al., 2011; Martinho et al., 2012; De Marco et al., 2008; Muhammad et al., 2015). In the realm of pyrolysis, MAP is an established process which offers some advantages over conventional pyrolysis methods- for example, it involves the rapid heating of the material involved and increased production speed- and so provides a volumetric heating that improves heating efficiencies as compared with conventional techniques (Appleton et al., 2005). Most recently, MAP

http://dx.doi.org/10.1016/j.wasman.2017.04.037 0956-053X/Ó 2017 Elsevier Ltd. All rights reserved.

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L. Rosi et al. / Waste Management xxx (2017) xxx–xxx

has been used to convert different polymeric materials such as tires (Undri et al., 2013a), polyolefin (Undri et al., 2014b), polyesters and biomasses (Bartoli et al., 2016a, 2016b), into fuels and liquid fractions with some efficiencies, and some studies have already been devoted to the MAP of WEEE (Andersson et al., 2012). Andersson and co-workers (Andersson et al., 2012) were focused on the influence of some reaction parameters on oil, gas and a solid residue from the treatment MAP of WEEE, and on the mass reduction during the process but no analysis of the fractions were provided. In this study MAP of WEEE was investigated as a function of two different absorbers and experimental set-ups as possible source of liquid fuels and chemicals. WEEE was in essence processed in a multimode microwave batch reactor. Different set-ups and MW absorbers were tested to enhance the quantity and quality of the liquid fraction, and to reduce the solid residue. Liquid fractions were analyzed through a quantitative GC–MS method, 1H NMR, and FT-IR. Furthermore, the presence and nature of the resulting halogenated compounds was checked in the liquid fractions after pyrolysis.

2. Materials and methods Samples of WEEE were recovered from a large selection of plastic waste computer bodies, monitor cases, and keyboards, stored temporarily in our Chemistry Department on their way to being sent to recycling plants. Some of these bodies and monitor cases were subsequently disassembled and reduced into pieces. A representative 5 kg sample of various different plastic components were ground into smaller ‘granules’ of about 1
1 cm and then mixed together to obtain as homogeneous and representative samples as possible (Fig. 1). MAP experiments in general do not strictly require a powdered sample due to the heating process, which is volumetric. However, the only limitation of this approach is that the thickness of the sample should be more or less comparable to the MW penetrating power in order to avoid the dissipation of MW power, and to prevent the conductive heating transfer process, from the wall of the reactor to the center of the sample, taking place (Farag et al., 2012). The carbon powder employed as a MW absorber was the solid obtained from the MAP of tires, minus the metal wire component. (C: 89.01%, H: 0.83%, N: 0.48%, S: 2.0%). A detailed characterization of this carbon powder was previously published in 2013 by Undri et al. (2013a, 2014a). Iron (purity 99.9%) was also employed as a MW absorber, and dimethylsulfoxide-d6 (DMSO-d6) (isotopic purity 99.8%) both of which were supplied by Sigma Aldrich and used without any further purification. Analytical standards for the GC–MS and acetonitrile used in the study (99.99%, GC grade) were purchased from Sigma Aldrich and were used as received.

Fig. 1. WEEE samples from waste monitors cases, keyboards, and computer bodies.

Molecular sieves (4 Ǻ) supplied by Carlo Erba were activated by heating under vacuum (453 K for 30 min) before their use. Kinematic viscosity was detected according to the ASTM method D 2854-00, using an Ostwald viscometer thermostated at 298.14 K, with a Julabo model ME-18 V. Cyclohexane, chlorobenzene and 1,4-dimethylbenzene were used as standards (Haynes, 2011). Density was determined with a pycnometer at 298.14 K. Fourier transform infrared spectroscopy (FT-IR) analyses were performed with a Shimadzu model IRAffinity-1, equipped with a Golden Gate single reflection diamond ATR (Attenuated Total Reflectance) accessory supplied by Specac for the analysis of liquids and solids in reflectance mode, and a sapphire cell (10 cm) was used for gas analysis. 1 H NMR spectra were recorded with an NMR Varian Mercury 400, using dimethylsulfoxide-d6 (DMSO-d6) as a solvent. Residual hydrogens of the solvent were employed as internal standards, and spectra were referenced to tetramethylsilane (TMS). Gas chromatographic analyses were performed using a Shimadzu GC–MS QP5050A equipped with a capillary column PetrocolTM DH 24160-U, (100 m length, 0.25 mm diameter, 0.5 mm stationary phase), using a 1:30 split ratio and a quadrupole mass (MS) detector, with a 70 eV electron impact ions generator, operating in the mass range 40–450 m/z. The oven operated at 298 K for 15 min, then was heated at 2.5 K/min up to 523 K, and was then kept at this temperature for a further 15 min. The total ion chromatography (TIC) was obtained with a signal/ noise ratio of five, and the composition was reported as percent peak areas, reproducibility 0.1%. Compounds were tentatively identified using the NIST mass spectral library. Relative response factors were evaluated according to the equation recently reported for GC-FID (Undri et al., 2015) and extended to GC–MS (Bartoli et al., 2016b). ICP-MS analyses were performed by the Microanalysis Laboratory of the University of Florence on a representative sample of WEEE after acid digestion. The bromine and chlorine contents of the liquid fractions from pyrolysis were determined by Idro-Consult Laboratori Riuniti s.r.l. in Florence. The samples were prepared according to the EPA method 5050, using the Gallenkamp autobomb bomb calorimeter. The bomb combustate solution was analyzed by ion chromatography according to the EPA method 9056 (coverage factor k = 2 with a level of confidence of 95%).

2.1. Pyrolysis details Pyrolysis were carried out in a MW multimode reactor working at 2.45 GHz, equipped with four external magnetrons, each absorbing an electric power of 2 KW for a total of 8 KW, and as a whole capable of delivering a microwave power of up to 6 KW inside the oven. The reactor was designed and supplied by Bi.Elle s.r.l. (Italy), and equipped with a wide-angle measuring infrared thermometer, which provided information on the overall temperature inside the oven but not the temperature on the sample surface. In fact, temperature measurement during MW heating is quite challenging (Menéndez et al., 2010), because microwaves interact with common probes such as thermocouples or with chemical thermometers. An IR thermometer can be used to evaluate the temperature of the vapor phase during MAP, whereas an optic fiber probe can be employed to measure the temperature of the solid, point by point. In order to monitor the MAP process globally, the temperature was measured with an infrared thermometer and calibration of this thermometer was run with an optic fiber, according to the method reported by Undri et al. (2013b). The calibration curve is provided in the supplementary materials.

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In all of the experiments, samples were placed in a 1000 mL borosilicate Erlenmeyer flask inside the oven and connected through a vertical flat pipe to two condensing systems cooled at 273 K and 263 K respectively. Liquids were collected in a flask, and gas in a gas holder. As reported in previous papers (Undri et al., 2013a, 2014a), two experimental set-ups were used, namely set up A and set up B. The two set ups differed in the inclusion of a fractionating column placed between the oven and the condensing system in set up B. In this set up, the flat pipe was replaced by the fractionating column (200 mm long. internal diameter 30 mm) which was filled with either glass spheres or molecular sieves. Figures of the two set-ups are included in the supplementary materials (Figs. S1 and S2). The glass spheres had each a diameter of 4 mm and were pre-heated at 393 K with a band heater, while the molecular sieves

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were pre-heated to 463 K to promote their activation. This fractionating system avoided high boiling fractions from leaving the oven (Frediani et al., 2012). WEEE samples were mechanically mixed with the MW absorber prior to the pyrolysis, and the experiments were carried out in a N2 atmosphere without any carrier gas, thus avoiding the dilution of the gas and vapor products, and simplifying the overall set up. The gas flow was achieved by the gas formed during pyrolysis, and the pressure drop was provided by the condensing system and the gas-holder. Pyrolysis was run and stopped when gas evolution was not further detected. Either carbon or iron were used as MW absorbers, and a microwave power of 3 KW was set. One of the key points to note is that achieving a reproducibility in the MAP of WEEE is an important issue, due to the great variability of the starting feedstock

Fig. 2. Some representative FT-IR spectra from WEEE analysis, showing the presence of some polymers.

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available. Therefore each test was repeated three times and the mean value was used to evaluate the influence of the reaction conditions, apparatus set-ups, and microwave absorber on yields, and on the composition of the liquid fraction collected.

3. Results and discussion 3.1. Characterization of WEEE samples 3.1.1. FT-IR analysis Polymer type analysis of several individual granules of WEEE samples was carried out via FT-IR ATR, in order to obtain information about functional groups. Three representative FT-IR spectra, as an example, are reported in Fig. 2. The spectra collected are typical of styrenic polymers such as ABS and HIPS (mCAH 3180–3160 cm1, mCAH 2950–2850 cm1, mCAN bond at 2200 cm1, mC@C 1660 cm1), both of which are major components of WEEE plastics. Furthermore polyester-based polymer composites such as PC (mC@O1750 cm1, dCAO1250–1100 cm1) and PVC (CACl gauche 650 cm1) were also identified.

3.1.2. ICP-MS analysis The ICP-MS analysis of WEEE samples carried out after acid digestion, showed Sb as the main metal present, together with other metals component such as Zn, Co, Ni, Cr, Pb, Cd, and As (see Table 1). The mixed WEEE plastics had no visible metal pieces, so it can be assumed that all the metals were present within the polymer as fillers. The significant amount of Sb and Zn in WEEE is likely due at the presence of antimony trioxide and zinc borate. Antimony trioxide is predominantly added as a synergist to aid the flame retardant propriety of brominated flame retardants which latter are used in styrenic polymers such as HIPS and ABS. Zinc, primarily as borate, is added as a flame retardant in PVC, usually in conjunction with antimony trioxide (Pritchard, 1998). These experimental data are in line with the prevailing presence of styrenic polymers and PVC in samplings of WEEE sent for pyrolysis.

Table 1 Metals present in the WEEE samples. Metal

Concentration [mg/g]

Standard deviation [mg/g]

Co Zn Ni Cr Sb Pb Cd As

0.66 29.50 2.62 0.70 73.58 2.87 1.50 0.59

0.01 0.45 0.11 0.03 2.31 0.26 0.05 0.04

3.2. Pyrolysis yields and composition MAP set-ups, experimental condition and yields of products are reported in Table 2. The temperature pattern in pyrolysis experiments, showed two different trends depending on the MW absorber, as shown in Fig. 3. A rapid increase of temperature was observed during MAP, followed by its stabilization before the end of the experiments, using iron (ID2 and ID4), while a smoother increment of temperature was observed in the presence of carbon (ID1 and ID3). These behaviors may be ascribed to the intense increase in the loss factor of carbon with the rising temperature, as reported by Menéndez et al. in 2010 The loss factor of the iron is also influenced by temperature, although it is more affected by the shape and the average dimension of its particles (Menéndez et al., 2010). The amount of the liquid fraction formed in the pyrolysis of plastic WEEE was 63.5% (ID1) using set-up A with carbon as the MW absorber, and it decreased when the microwave absorber was iron (ID2, 55.3%) in the same set up. Accordingly, the amount of solid formed was higher using iron (42.4%) than carbon (26.2%), and the gas formed was 10.4% with carbon and 2.3% with iron. These results were in line with what was found in previous research (Undri et al., 2014b). However, very different behavior was observed with set-up B; in fact, it allowed for a higher yield in the liquid fraction when iron was the MW absorber (76.6%, ID4) with respect to carbon (52.8%, ID3). As a consequence, the gas and solid fractions were formed in low amounts using iron; the gas fraction was 2.2% in ID4, with respect to 19.7% for ID3; and the solid fraction was 21.2% in ID4 with respect to 27.5% for ID3. The higher yield of liquid and gas fractions obtained when setup A was replaced by set-up B using iron as the MW absorber may be attributed to the fractionating column that increased the residence time of the products in the oven, and consequently increased the overall cracking of the WEEE- thus providing a large amount of liquid and simultaneously low volatile compounds. The different yield trend between ID3 and ID4 may be attributable to the presence of carbon particles in ID3 dragged away from the bulk. They apparently can promote cracking reactions in the gas phase with a consequent increase of the gas fraction. Instead, in ID4, the rapid increase of temperature involved in the melting of the feedstock at the beginning and then afterwards in the cracking process, was accompanied by a comparatively much reduced dragging of particles away from the high viscosity mass of the melting feedstock. The use of SiO2-Al2O3-based materials, as well as of different kind of zeolites, can potentially allow for the enhancement of the quality of the pyrolysis products, as reported previously by several authors (Kunwar et al., 2016; Muhammad et al., 2015). Ma et al. also reported an improvement of the liquids from the pyrolysis of WEEE, with a decrease of up to 50% of brominated compounds. Their use of a zeolite-type acid catalyst like HY involved a decrease of up to 20% of the liquid fraction, with a

Table 2 MAP set-ups and yields of products. Test

WEEE

Absorber

Set-up

WEEE/Absorber

[g] ID1 ID2 ID3 ID4 ID5

200.9 201.0 201.2 200.4 199.9

C Fe C Fe C

A A Ba Ba Bb

2.00 2.01 2.00 2.02 2.00

Time

Tmax

[min]

[K]

Yields [wt%] Char

Liquid

Gas

40 30 40 60 50

702 702 702 659 723

26.2 42.4 27.5 21.2 14.2

63.5 55.3 52.8 76.6 75.6

10.4 2.3 19.7 2.2 10.2

Tmax: Maximum value of reached temperature. a Glass spheres in the fractionating column. b Molecular sieves in the fractionating column.

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Fig. 3. Temperature of the WEEE during the pyrolysis.

Fig. 4. FT-IR spectra of gas fraction from MAP of WEEE.

concomitant rise in gas volume due to an increase in cracking processes (Ma et al., 2016). To avoid the decrease in yield of the liquid fraction, we employed molecular sieves, located in a column placed between the oven and the condensing system, just outside the oven. The purpose of this was to exploit the

lower acidity/basicity of the molecular sieve, and also to achieve a reduced cracking and a magnification of the surface adsorption processes. In test ID5, glass spheres were instead substituted for the molecular sieves in the fractionating column inserted between the oven and the condensing system, in order to eval-

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Table 3 Viscosity and density of liquid fractions from MAP of WEEE. Test

Density [g/mL]

Viscosity [cP]

HHV [MJ/kg]

ID1 ID2 ID3 ID4 ID5

0.90 0.93 0.88 0.90 0.90

0.76 n.a. 0.69 0.73 0.84

40.6 34.3 38.6 41.2 37.0

uate their eventual catalytic influence on the composition of the liquid fraction (Muhammad et al., 2015). In fact, there was a higher yield of the liquid fraction with the molecular sieves than in the corresponding test with the glass spheres (ID3). This behavior may be ascribed to the catalytic activity of the molecular sieves in promoting a more efficient radical degradation on their surfaces, instead of the catalytical cracking reactions achieved in an acid/based site when zeolite-type catalysts are used.

Table 4 Main compounds identified in liquid fractions of MAP of WEEE through GC–MS quantitative analysis. Compounds

a.ca

r.t. min

RRF

ID1 mg/ml

ID2 mg/ml

ID3 mg/ml

ID4 mg/ml

ID5 mg/ml

1-Butene 1-Bromopropane 2-Butene Bromomethane 2-Propenenitrile 1,3-Butadiene Propanenitrile 2-Methyl-2-propenenitrile 2,2-Dimethylpropanenitrile 2-Methylpropanenitrile Benzene Toluene 4-Ethenylcyclohexene o-Xylene m-Xylene Styrene p-Xylene Ethylbenzene 1,2,3-Trimethylbenzene 2-Propenylbenzene Propylbenzene 1-Ethyl-3-methylbenzene 2-Propylbenzene Phenol a -Methylstyrene 1-Ethenyl-4-methylbenzene 2-Methyl-3-phenylpropene 2-butylbenzene 1-Ethenyl-2-methylbenzene 1-Propenylbenzene 2-Ethylphenol 3-Butenylbenzene Indene Acetophenone Benzonitrile 1-Phenyl-1-butene 4-Methylphenol Butylbenzene 3-Bromophenol 4-Bromophenol 1-Phenyl-2-methylpropene 2-Butenylbenzene 2,6-Dimethylphenol Phenyl tert-butyl ketone 3-Methylbenzonitrile 2-Methylbenzonitrile 2-Ethenyl-1,4-dimethylbenzene 2,3-Dimethylphenol 3-Phenyl-1-pentene 2-Ethylbenzonitrile 4-Ethylphenol 1,2,3,4-Tetrahydro-1-hydroxy-naphthalene 3,4-Dimethylphenol 1,1a,6,6a-Tetrahydro-cycloprop[a]indene 1-Methyl-1H-indene 1-Ethenyl-4-ethylbenzene Pentylbenzene 1,1-Dimethyl-1H-indene Naphthalene 2,4,6-Trimethylphenol 3-Methyl-1-phenyl-2-butene

C C C P C C C C P C C S C C C C S C P C C C P S C C P C C P C C C P P P P C P C C C C P C P C C C C C P C C C P C P C C P

11.93 12.10 12.29 12.51 15.49 18.04 18.05 20.12 21.75 21.91 26.77 37.56 44.25 47.02 47.10 48.97 49.17 49.29 51.73 53.15 54.00 54.54 54.65 54.71 56.05 56.68 58.26 58.54 59.18 60.12 60.22 60.49 60.58 61.10 61.18 61.48 61.54 61.60 61.98 62.07 62.14 62.22 64.20 64.31 64.96 65.05 65.39 66.63 67.36 67.36 67.63 67.75 67.82 67.89 67.96 68.10 68.37 69.84 70.12 70.57 70.73

0.07 0.02 0.07 0.01 0.09 0.09 0.08 0.01 0.13 0.12 0.16 0.16 0.38 0.39 0.39 0.37 0.49 0.36 0.52 0.46 0.49 0.50 0.50 0.34 0.47 0.55 0.57 0.60 0.51 0.52 0.37 0.59 0.61 0.43 0.44 0.60 0.38 0.63 0.17 0.63 0.61 0.61 0.63 0.64 0.47 0.47 0.64 0.48 0.70 0.59 0.49 0.70 0.49 1.12 0.69 0.72 0.77 0.83 0.75 0.58 0.70

0.0 0.0 0.0 0.0 1.8 1.0 0.0 18.7 0.4 0.0 2.3 33.4 0.1 0.1 0.1 44.7 25.6 0.0 4.6 0.3 0.4 0.1 0.0 1.4 16.0 0.3 0.1 0.0 1.0 0.1 0.0 0.1 0.2 0.1 0.0 0.1 0.0 0.6 0.2 0.0 0.1 0.0 0.0 0.0 0.2 0.0 0.1 0.0 0.0 0.2 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.4 0.0 0.0

0.2 0.0 0.0 0.3 7.1 0.0 1.9 92.9 1.3 0.0 2.1 31.6 0.2 0.3 0.3 53.6 23.9 0.0 7.4 1.7 0.7 0.0 0.0 17.9 20.2 0.0 0.0 0.0 0.0 0.0 1.9 0.3 0.2 0.0 0.0 0.0 0.6 0.7 0.0 0.5 0.3 0.0 1.2 0.0 0.8 0.0 0.2 0.6 0.0 0.6 0.3 0.0 0.3 0.0 0.3 0.1 1.2 0.2 0.2 0.4 0.2

0.1 0.1 0.1 0.0 14.2 0.0 6.4 11.5 0.0 2.6 6.5 50.4 0.1 0.2 0.3 47.3 26.6 0.6 5.7 2.3 0.8 0.1 0.4 7.6 19.3 0.0 0.1 0.1 0.0 0.3 0.0 0.2 0.6 0.0 0.2 0.0 0.5 0.2 0.0 0.1 0.0 0.1 0.0 0.0 0.0 0.8 0.3 0.0 0.0 0.6 0.3 0.1 0.0 0.2 0.1 0.0 0.3 0.1 0.9 0.0 0.0

0.1 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.8 38.5 0.5 0.0 0.0 117.7 13.2 0.0 0.0 3.3 0.4 0.0 2.3 0.0 26.2 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.2 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.1 0.0 0.0

0.0 0.0 0.0 0.0 1.8 0.0 1.2 21.6 0.4 0.0 0.7 11.3 0.0 0.0 0.1 14.1 6.3 0.0 1.5 0.6 0.2 0.0 0.1 0.0 6.2 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.3 0.0 0.1 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.1 0.0 0.2 0.0 0.1 0.0 0.0

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L. Rosi et al. / Waste Management xxx (2017) xxx–xxx Table 4 (continued) Compounds

a.ca

r.t. min

RRF

ID1 mg/ml

ID2 mg/ml

ID3 mg/ml

ID4 mg/ml

ID5 mg/ml

3-(2-Propyl)-phenol, 2-Methoxy-1,3-dimethylbenzene Quinoline 1-Cyclopenten-1-yl-benzene 2-(2-Propenyl)-phenol 1-Methylnaphthalene 4-Pentenylbenzene 3-Methyl-5-phenyl-1-pentene (2-Chloroethyl)-benzene 3-Phenylpropionitrile 3-Methylisoquinoline 4-Bromo-2,6-dimethylbenzene 2,5-Cyclohexadien-1-yl-benzene 1-Phenylcyclohexene 1-Chloro-2-phenylpropane 1-(Bromomethyl)-2-methyl-benzene 2,3-Diphenylbutane 2,4-Dimethyl-4-phenyl-1-butene 2-Phenyl-1-butene 2,6-Dimethylquinoline Diphenylmethane 1-(2-Propenyl)-naphthalene 1-Naphthalenecarbonitrile Dibenzyl 1-(Bromomethyl)-4-methylbenzene 1,2-Diphenylpropane 3,40 -Dimethyl-1,10 -biphenyl 1-Naphthaleneacetonitrile 1,3-Diphenylpropane 1,10 -Cyclopropylidenebis-benzene 1,3-Diphenyl-2-butene 3-Phenyl-1-butanol (E)-Stilbene 2-Methyl-4-phenyl-1-butene 10,11-Dihydro-5H-dibenzo(a,d)cycloheptene 2,4-Dimethyl-4-phenyl-1-butene 1,4-Diphenylbutane 1-Chloro-3-phenylpropane 1,2,3,4-Tetrahydro-1-phenylnaphthalene 2,3,3-Trimethyl-3H-indole Triphenyl phosphate 9-Ethenyl-anthracene 1-Methyl-2-propylbenzene 1,2-bis(4-Pyridyl)-ethane 2-Phenylnaphthalene

C P P P P P P P P C P P P P P C C C P P C P C C P P C C C C P P C P P P C P C P P P P P C

71.45 71.65 74.94 75.43 75.66 75.71 77.45 77.69 77.91 78.07 78.37 78.95 79.29 79.38 79.87 80.26 80.32 80.37 82.04 83.36 84.00 86.92 86.97 88.83 90.49 90.54 91.94 94.42 95.18 95.69 96.11 96.61 97.68 98.20 98.47 98.73 99.72 99.86 100.64 101.65 102.23 104.49 105.90 107.66 110.23

0.57 0.63 0.68 0.79 0.58 0.88 0.84 0.93 0.47 0.64 0.79 0.35 1.00 1.02 0.56 0.37 1.32 0.96 0.80 0.89 1.12 1.16 0.93 1.28 0.42 1.40 1.32 1.10 1.47 1.81 1.56 0.90 1.38 1.07 1.86 1.18 1.64 1.13 1.98 1.01 1.58 1.65 1.08 1.14 2.13

1.0 0.0 0.0 0.1 0.1 0.1 3.6 0.0 1.2 0.0 0.0 0.0 0.1 0.0 0.2 0.5 0.0 0.0 0.0 0.0 0.2 0.1 0.0 0.1 0.1 0.0 0.0 0.0 1.8 0.3 0.1 0.4 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 7.1 0.0 41.5 2.1 0.0 0.0 8.2 0.0 2.5 0.3 0.5 0.2 0.4 0.0 0.0 0.0 1.4 0.4 0.1 0.2 0.4 0.0 0.1 0.0 0.1 0.1 0.0 3.4 0.8 4.5 0.5 0.1 0.2 0.0 0.0 0.1 0.1 0.3 0.2 0.3 0.1 0.1 0.2 0.1

0.2 0.0 0.0 1.4 0.2 0.5 2.9 0.0 1.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 0.5 0.0 0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.4 0.0 0.0 0.0 0.0 0.0 0.0

0.0 0.0 0.1 0.0 0.0 2.3 0.0 131.4 0.0 0.1 0.2 0.0 0.0 0.0 0.2 0.0 0.0 0.0 0.3 3.7 0.1 0.0 0.4 0.1 0.0 0.0 0.0 0.1 0.6 0.0 0.0 0.0 0.0 0.0 0.1 0.1 0.1 0.0 0.1 0.0 0.0 0.0 0.0 0.0 0.0

93.2

86.3

96.7

86.0

91.5

Total assignment [wt%]b a b

Attribution level of confidence: S = standard; C  90%, P < 90%. Calculated as: 100 * (summa of weight of identified compounds)/(weight of liquid fraction).

3.3. Products characterization 3.3.1. Gas Gas fractions were collected and analyzed through FT-IR ATR, and the spectra collected are shown in Fig. 4. The spectra showed the presence of saturated and unsaturated hydrocarbons (mCH 3200–2750 cm1) such as methane, ethane, ethylene, propane, and propene. A very strong band at 2450 cm1 was attributed to the presence of large amounts of CO2, in agreement with the pyrolysis of the polyester compounds which were originally identified in the starting WEEE. Furthermore, the decomposition of polyesters with the formation of CO2 has already been reported by Montaudo et al., 1992. Other bands are present at 3400–3200 (mCN), 2200–2100, and 1500–1400 (mCN) cm1, showing the presence of organic compounds such as nitriles (i.e. acetronitrile). 3.3.2. Liquids 3.3.2.1. Physical properties. The density of the liquid fractions collected was close to 0.9 g/mL and the viscosity was low and close

to 0.8 cP (with the exception of the results for ID2, see Table 3). In experiment ID2, the liquid showed a higher viscosity, suggesting a higher concentration of compounds with a relatively high molecular weight. These products might derive from partially paralyzed polymers, carried outside the reactor by the gas flow. High Heating Values (HHVs) were in the range of 37–41.2 MJ/kg, and proved that liquids from the MAP of WEEE could be suitable for use as fuels. 3.3.2.2. GC–MS characterization. Liquid fractions were thoroughly investigated via GC–MS. The MS data allowed for a quick identification while giving useful information about the nature of the detected compounds. A quantitative GC–MS method was employed to evaluate the concentration of the different compounds present in each liquid fraction. Relative response factors (RRF) of the compounds present were calculated according to the equation reported and used by Bartoli et al., 2016b to obtain their concentrations. A complete list of the compounds, identified through the NIST library, together with their concentrations (mg/mL) and RRF are reported in Table 4, while the GC–MS chromatograms are reported in Fig. 5.

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As reported in Table 4, a very large part of the compounds present in the liquids (among 86.0% and 96.7%) were identified and quantified. Styrene and a-methylstyrene were present in large amounts in all the liquid fractions. They were ascribed to the pyrolysis of

styrenic polymers present in WEEE. In the liquid ID4 (carried out using set-up B, with iron as the MW absorber) only products from polystyrene degradation were present, in agreement with the results for polystyrene pyrolysis reported by Undri et al. (2014c). In this latter case, iron might be a very efficient MW absorber,

Fig. 5. Chromatograms of liquid fractions collected from MAP of WEEE.

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L. Rosi et al. / Waste Management xxx (2017) xxx–xxx

and at the same time it might be the promoter of a very efficient radical degradation of nitriles, and consequently only polystyrene degradation products were observed (styrene, 117.7 mg/mL, a-methylstyrene 26.2 mg/mL). The predominant nitrile compound was 2-methyl-2propenenitrile (ID2, 92.9 mg/mL) while other nitriles, such as 2-propenenitrile, propanenitrile, 2-methylpropanenitrile, and 2,2-dimethylpropane nitrile, were formed in lower quantities. Nitriles were formed through the pyrolysis of ABS polymers. In the presence of carbon as the MW absorber, the yield of the nitriles with the two set-ups depended on the kind of nitriles involved. Using iron as the MW absorber, the concentration of nitriles was

Fig. 6. Quinoline formation N-heteroaromatics formations.

from

aromatic

nitriles

as

an

example

Table 5 Total concentration of Chlorine and Bromine into liquid fractions. Test

Chlorine [mg/mL]

Bromine [mg/mL]

ID1 ID2 ID3 ID4 ID5

0.7 1.9 0.6 0.3 0.3

0.3 0.8 0.4 <0.1 <0.1

of

9

higher using set-up A (ID2 versus ID4), and it may have been due to a reduced residence time of the products inside the oven. The use of the molecular sieves (ID5) gave the same type and quantity of products, with the exception of 2-methyl-2propenenitrile. This compound was present in higher amounts in the liquid of ID5 with respect to that present in ID3 (the analogous experimental condition using glass spheres instead of sieves). Alkyl-substituted aromatic compounds, formed through radical rearrangements, were present in all liquids. The use of the molecular sieves (ID5) caused a decrease of the styrene concentration to 14.4 mg/mL, and this might have been due to the catalytic activity of the molecular sieves that can promote further radical reactions, leading to the formation of aromatic compounds containing relatively long aliphatic chains such as 5-phenyl-3-methyl-1-pentene (131.4 mg/mL). In addition to single ring aromatic compounds, quinolone and aliphatic quinolone derivatives were detected in low concentrations, and were formed through a rearrangement of aromatic nitriles, as shown in Fig. 6 for quinoline, and according to the existing literature (Hall and Williams, 2006). The presence of brominated flame retardants and PVC in the feedstock leads to both halogenated organic compounds and halogen salts. However, the presence of halogenated compounds from the degradation of PVC and flame retardants was observed in some of liquid fractions, but in very low amounts. GC–MS analysis showed the presence of low amounts of halogenated compounds mainly deriving from the cross reactions between PVC and styrenic polymers (chlorine aromatic derivatives) and bromine aromatic derivatives, as a result of the presence of brominated flame retardants. The small amounts of halogenated

Fig. 7. 1H NMR of liquid fractions, with that of styrene as reference, solvent signal depleted.

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compounds detected by GC–MS analysis were confirmed by the analysis of the total content of chlorine and bromine that showed concentrations lower than 1.9 mg/mL, as shown in Table 5. Set-up B, contrary to set-up A, considerably reduced the concentration of halogenated compounds. A free brominated aromatic liquid was obtained in ID4 probably due to the formation of iron salts in the bulk of the reaction. The same results were obtained when using molecular sieves in the fractionating system (ID5). This could be ascribed to the greater surface area of the molecular sieves compared to that of the glass spheres. The surface area of the molecular sieves allowed for increased adsorption and releasing phenomena, thus causing a more efficient condensation process, and an increase of the residence time of high boiling compounds inside the oven, effectively causing more degradation. As a consequence, the liquid fraction from ID5 showed a low

content of halogenated compounds, together with a lower yields of styrene (14.1 mg/mL) than in all the other experiments. Some compounds from the thermal degradation of flame retardants, such as triphenyl phosphate, were detected in low concentrations (less than 0.3 mg/mL). Moreover the relevant presence of Sb, as Sb2O3 (a common flame retardant), detected in the feedstock, can promote the formation of SbBr3 in the solid residue, as according to Brebu et al. (2007).

3.3.2.3. 1H NMR. The liquid fractions were also investigated through NMR spectroscopy- a very versatile tool to detect the main classes of organic compounds. 1H NMR spectra of samples ID1-ID5 are shown in Fig. 7, together with the spectrum of styrene as a reference, while the integrals of the same are reported in Table 6.

Table 6 Normalized integrals of liquid fractions from the MAP of WEEE with that of styrene as reference. Aromatic and C@CH-OCC

Phenolic ‘‘OH” and C@CHR

d (ppm)

9.0–6.5

6.5–5.0

ID1 ID2 ID3 ID4 ID5 Styrene

3.9 2.3 2.3 2.1 2.3 75.0

40.1 44.6 43.9 57.3 44.3 25.0

CH2AOAC; CH2AOOC; ring-join methylene, and ArACH2AAr

Protons of CH3Ar, CH2Ar, CHAr and CHCOO

CH, CH2 of alkyl groups; CH2 and CH in b position in an aromatic ring

CH3 in b position and CH2 and CH in c position in an aromatic ring or ethereal oxygen

4.5–3.3

3.3–2.0

2.0–1.6

1.6–1.0

CH3 of alkyl groups, or in c position, or further, of an alkyl chain linked to an aromatic ring 1.0–0.5

10.1 11.2 12.6 20.1 12.8 0.0

5.4 3.2 3.4 3.0 3.4 0.0

18.1 17.9 19.2 9.9 19.5 0.0

5.6 3.5 3.4 1.8 4.8 0.0

12.9 14.5 13.2 4.4 11.1 0.0

Fig. 8. Comparison of FT-IR ATR spectra of liquids from MAP of WEEE.

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All spectra showed the presence of a large amount of substituted aromatic compounds and, among them, styrene was identified and confirmed by the signals of pure styrene, reported as reference in the same sets of data (Fig. 7 and Table 6). Furthermore, alkyl-substituted aromatic compounds were confirmed by the signals in the range of 3.3–2.0 ppm. Very weak and broad signals, respectively at 9.33 ppm and 3.45 ppm, confirmed the presence of Ar-OH compounds such as phenol or its derivatives which were formed by the decomposition of PC or flame retardants.

11

of halogen-free liquids that may be employed as a valuable source of chemicals or fuel without the need of any preliminary treatments. Acknowledgements The authors wish to thank Fondazione Ente Cassa di Risparmio di Firenze (project: ‘‘Value from Waste” n. 2014.0703). Appendix A. Supplementary material

3.3.2.4. FT-IR ATR. Liquid from the MAP of WEEE were also analyzed through FT-IR ATR, and spectra are shown in Fig. 8. All spectra showed bands of saturated, unsaturated, and aromatic hydrocarbons (stretching in the range of 3300–2850 cm1), in agreement with the corresponding 1H NMR spectra and GC– MS analyses. The presence of aromatics was also confirmed by the stretching bands in the 1660 cm1 zone. Other bands in the range of 900–800 cm1 confirmed the presence of multisubstituted aromatics. Bands of nitrile moiety were not revealed, confirming their presence in very low amounts in the liquids. 3.3.3. Solid Char samples from the pyrolysis of WEEE were not analyzed because they were mixed in with the MW absorber. They might in future research be separated using a magnet where iron is the MW absorber, and the iron might then be reused. In the case of carbon, the mixture of carbon and char may be further re-employed as a MW absorber. 4. Conclusions MAP of a selection of Waste from Electric and Electronic Equipment (WEEE) coming from end life computers were carried out with different reaction conditions, apparatus set-ups, and MW absorbers (carbon and iron), obtaining a high quality liquid fraction. Liquids were formed in large amounts, and they had low density and viscosity with the exception of ID2, where partially degraded polymers were present. HHV values were all up to 37 MJ/kg and confirmed the suitability of the use of the resulting liquids as fuels. A larger amount of the liquid fraction (63.5%) was obtained using carbon instead of iron as the MW absorber in set-up A. However in set-up-B, with iron as the MW absorber, the yield of liquid was further improved to 76.6% (ID4) with a styrene concentration of 117.7 mg/mL by using a fractionating column. On the contrary a large gasification (up to 19.7%) of WEEE was reached using set-up B with carbon as the MW absorber (ID3,) with a corresponding decrease of the liquid fraction (52.8%) Very low amounts of halogenated compounds were always detected (less than 1.9 mg/mL) and a free halogenated liquid was obtained using set up B with iron as the MW absorber as consequence of radical degradation in the bulk of melted feedstock. Thus molecular sieves in the fractionating column (ID5), instead of glass spheres, gave an even lower production of solid residue, and increased the production of liquid (75.6%), even if a low concentration of halogenated compounds (0.3 mg/mL) was present due to gas-solid adsorption degradative processes. MAP experiment planning remains a key issue with the need of balancing the parameters to reduce the processing time and improve yields. Collected data suggest that the use of set-up B and Fe may be a reasonable approach to reduce solid residue and obtaining a high-quality liquid using a low cost and high performing MW absorber. In conclusion the MAP of WEEE may be a sound process to solve some environmental problems caused by the disposal of the plastic fractions of WEEE, not just through the reduction of the waste volume but also with the production

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