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Dehydrochlorination of plastic mixtures
Journal of Analytical and Applied Pyrolysis 49 (1999) 97 – 106
Dehydrochlorination of plastic mixtures H. Bockhorn *, A. Hornung, U. Hornung, P. Jakobstro¨er, M. Kraus Institut fu¨r Chemische Technik, Uni6ersita¨t Karlsruhe (TH) 4 -Kaiserstr. 12, 76128 Karlsruhe, Germany Accepted 22 October 1998
Abstract Dehydrochlorination of plastic mixtures from domestic waste as well as from other chlorine containing mixtures such as electronic scrap is an essential reaction step in waste incineration, pyrolysis and chemical recycling of polymers. For designing pyrolysis procedures, controlled combustion processes and to control the emissions from incinerators, the behaviour of polymers in thermal decomposition with regard to decomposition products and the kinetics of decomposition must be known. The kinetic data, for thermal decomposition of commodity plastics, confirms that in mixtures of different plastics the dehydrochlorination of, e.g. poly (vinyl chloride) (PVC) can be conducted at moderate temperatures and prior to the thermal degradation of the polymer skeleton. In stepwise low temperature pyrolysis mixtures of, e.g. PVC, polystyrene and polyethylene have been separated into hydrogen chloride, the monomer of polystyrene and aliphatic compounds from polyethylene decomposition. The degree of conversion of chlorine from PVC into hydrogen chloride in the low temperature (330°C) first step is about 99.6%. A similar behaviour for dehydrochlorination is obtained during the thermal degradation of electronic scrap. The hydrogen chloride evolution from PVC occurs in the same way as in mixtures of commodity plastics with a maximum rate of HCl loss at 280°C, when heated at 2 K min − 1. Brominated flame retardants are decomposed or evolved at higher temperatures (\300°C). A possibility to fix bromine in the residue is to add calcium carbonate to the electronic scrap before pyrolysis. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Stepwise pyrolysis; Thermal decomposition; Polymers; Kinetic of decomposition; PVC; HCl; Electronic scrap
* Corresponding author. Tel.: +49-721-6082120; fax: +49-7216-084820. 0165-2370/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 9 8 ) 0 0 1 2 4 - 7
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H. Bockhorn et al. / J. Anal. Appl. Pyrolysis 49 (1999) 97–106
1. Introduction The incineration of plastic refuse is a widely used method in waste management. The incineration of plastic waste is connected with environmental problems, e.g. the formation of dioxines and chlorinated compounds from poly (vinyl chloride) (PVC) is reported in [1]. These problems enlarge if electronic scrap is incinerated. On one hand, in addition to chlorine from PVC, electronic scrap also contains brominated flame retardants and polychlorinated biphenyls; on the other hand, nitrile containing polymers from, e.g. acrylonitrile-butadiene styrene (ABS) or styrene acrylonitrile copolymer (SAN) may be precursors for hydrogen cyanide. The high amount of organohalogen and other harmful substances is prohibitive for commercial processes for re-utilization of the polymer fraction of electronic scrap. Their elimination, reduction or conversion into harmless compounds will help to overcome this problem. One method for the elimination of organohalogens is decomposition by thermal pretreatment of electronic scrap. For designing thermal pretreatment procedures, the behaviour of polymers during thermal decomposition with regard to the decomposition products and the kinetics of decomposition must be known. The kinetics of decomposition of various plastics and plastic mixtures have been investigated by isothermal and dynamic measurements [2 – 7]. The kinetic data confirms that different molecular structures of commodity plastics lead to different reaction mechanisms of decomposition, different reaction rates and, particularly, a different temperature dependency of the decomposition rates. In particular, dehydrochlorination of PVC by a free radical chain mechanism occurs at comparably low temperatures. This behaviour suggests a combination of separation and decomposition of polymer mixtures by means of stepwise pyrolysis with different temperatures in the single steps, including a thermal pretreatment for dehydrochlorination of PVC. For the elimination of chlorine by thermal degradation of PVC a rechlorination of other components in the mixture must be prevented. In this work, the feasibility of thermal pretreatment for the elimination of halogens from electronic scrap is investigated. The results from kinetic investigations are presented, e.g. the formal kinetic parameters of thermal degradation of various plastics and the formation of the main pyrolysis products from electronic scrap are compiled. The decomposition of organohalogen components in electronic scrap has been investigated under dynamic conditions with simultaneous thermogravimetry-mass spectrometry and with an isothermal reactor. The experimental results suggest a thermal pretreatment for eliminating harmful compounds from plastic mixtures from domestic waste and from mixtures of thermosetting plastics like electronic scrap. One question is, whether chlorine from PVC can be removed in the same manner as from mixtures of PVC with other plastics. Other problems are bromine containing flame retardants and polychlorinated biphenyls which are present in electric and electronic devices. The results of laboratory scale dehydrochlorination of mixtures of PVC polyethylene and polystyrene have been presented previously [8,9].
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Table 1 Rate coefficients and apparent activation energies for the thermal decomposition of different thermoplastics Polymer
Ea (kJ mol−1)
log (k0/min−1)
PVC dehydrochlorination: PVC, iso.a PVC, dyn.
136 95 143.59 2.9
11.48 9 0.09 12.5 90.18
1.54 1.54
7 3
Polystyrene, isothermal Polystyrene, dynamic
1729 4 322.8 92.4
12.47 90.02 24.61 9 0.19
1.04 1.09
7 3
Polyethylene, isothermala Polyethylene, dynamic PVC 2nd degradation step: Poly(vinyl chloride), iso.* Poly(vinyl chloride), dyn.
26893 262.1 91.9
17.78 9 90.01 18.00 9 0.14
0.8–1.4 0.83
7 3
217 9 5 234.1 92.1
14.9 9 0.02 16.35 90.15
1–1.8 1.64
7 3
Polyamide 6, isothermal e-caprolactam, isothermal Polyamide 6, dynamic
211.0 93 205.0 91 210.8
14.9 90.05 14.5 9 0.03 15.01
1–1.29 0.98 0.82
7 7 7
Polypropylene, isothermal Polypropylene, dynamic Polyethylene therephthalate, isothermal Polyethylene thereph-thalate, dynamic
220 95 223.79 1.6 214 92 238.7
a
15.06 9 0.06 15.9 90.2 15.2 90.04 18.00
Reference
n
1.1 0.77 1.15 1.15
3
The data is obtained from isothermal and dynamic measurements.
2. Experimental Investigations with thermoplastics are performed with pure polymers without stabilizers, fillers and colors. The polymers are trademarks of BASF (Ludwigshafen, Germany); polyamide 6 (Ultramid B3™), polystyrene (Polystyrol 143 B™), polyethylene (Lupolen 5261™) and PVC (Vinoflex S 5715™). The experiments with electronic scrap are performed with samples also used in large scale rotary kiln pyrolysis experiments at Berlin-Consult (Germany) and were provided by AEG-Sachsenwerk (Regensburg, Germany) and homogenized by BASF (Ludwigshafen, Germany). The used electronic scrap fraction comes mainly from high voltage components. The plastic fraction of this particular scrap consists
Table 2 Rate coefficients and apparent activation energies for several components -evolved during thermal decomposition of electronic scrap Polymer
Ea (kJ·mol−1)
log (k0/min−1)
n
Bisphenol A Styrene 4-phenylbutyronitrile
141.2 97 142.094.2 100.3 94.5
11.24 9 0.02 9.28 90.02 7.19 90.02
0.86 1.22 1.44
100
H. Bockhorn et al. / J. Anal. Appl. Pyrolysis 49 (1999) 97–106
Fig. 1. Decrease of the halogen content during pyrolysis of electronic scrap.
of thermosetting plastics like polyester resins and phenolic plastics and thermoplastics like acrylonitrilebutadiene styrene (ABS), styrene acrylonitrile copolymer (SAN) and others. It has been ground to a powder (particle diameter 0.25 mm) after removing of metal parts.
Fig. 2. Comparison of HCl formation of PVC degradation and electronic scrap.
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Fig. 3. Hydrogen chloride evolution from electronic scrap in comparison with the rate of conversion (heating rate of 2 k min − 1; sample size, 20 mg).
2.1. Determination of formal kinetic parameters Dynamic as well as isothermal experiments were applied to determine formal kinetic parameters. The dynamic experiments were carried with a thermobalance coupled with a quadrupole mass spectrometer (DuPont 951 thermogravimetric analyzer/BalzersQMG 420). The thermobalance was driven by a programmable thermal controller (Eurotherm 818P). It was operated with heating rates from 2 to 20 K min − 1
Fig. 4. Some common brominated flame retardants [12].
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Fig. 5. Decrease of the bromine content during pyrolysis of electronic scrap.
between ambient temperature and 800°C. Pure helium was used as a purge gas (flow rate 100 cm3 min − 1). The quadrupole mass spectrometer (Balzers-QMG 420) was connected to the thermobalance via an open coupling with a differential pressure reduction via a platinum orifice (10 mm diameter). The coupling could be heated up to 500°C to prevent condensation of the evolved products. The high pressure end of the coupling was located directly above the heated polymer sample. The low pressure end of the coupling was inserted directly into the ion source of the mass spectrometer. The ionizing voltage of the cross-beam electron impact ionization source amounted to 30 eV. More details are given in [2]. The isothermal experiments were carried with a gradient free reactor. The gradient free reactor with on-line mass spectrometry enables the determination of formal kinetic parameters of thermal degradation reactions of solids or liquids under isothermal conditions up to 1 MPa. The reactor consists of a heated sliding device carrying the sample within a platinum crucible. After preheating the sliding device the sample is moved quickly into the reaction chamber. Heat transfer to the sample is accomplished through mixing of the reactor contents by means of a small gas turbine. Perfect mixing also provides gradient free conditions within the reaction chamber. The reaction products are analysed on-line with a quadrupole mass spectrometer (Balzers, QMG 421, analyzer 400). More details are given in [2,3,7]. All data are given for thermal degradation under inert atmosphere (helium).
2.2. Analytic methods for the determination of chlorine and bromine contents in electronic scrap samples The pyrolysis of electronic scrap has been performed in a separate oven (Du Pont) equal to the one of the thermobalance. Samples up to 500 mg have been
H. Bockhorn et al. / J. Anal. Appl. Pyrolysis 49 (1999) 97–106
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investigated under isothermal and dynamic conditions and a helium flow of 175 ml/min. The evolved pyrolysis products were captured in a cold trap with liquid nitrogen. Products, which could not be condensed in the cold trap, were directly send to a Wickbold apparatus. With the Wickbold procedure [10,11], chlorine and bromine contents of the gaseous pyrolysis products and liquid products of the cold traps as well as of the solid residues have been determined. Supplementary EDRFA (energy dispersive X-ray fluorescence analysis) experiments have been performed to investigate the bromine content. With this method the bromine content is determined via a quantitative calibration. Furthermore, the metal content is obtained using EDRFA. Additional investigations of the liquid pyrolysis products are performed with GC-MS and GC-FID.
3. Results
3.1. Formal kinetic parameters of the thermal degradation of 6arious plastics and the formation of main pyrolysis products from electronic scrap Data for the overall rate of thermal decomposition for different plastics is given in Table 1. The data in Table 1 refers to a rate expression for the thermal decomposition as given by − Ea da =k0e RT (1 − a)n dt
where a is the conversion (m0 −m/m0 −m ), with m0 being the initial mass, m the actual mass and m the final mass. Ea is the apparent activation energy and n is the apparent reaction order. Analogous kinetic investigations have been performed with electronic scrap under isothermal conditions with the gradient free reactor. Due to the polyesters SAN and ABS some of the main decomposition products from electronic scrap degradation are styrene, bisphenol A and 4-phenylbutyronitrile. The formal kinetic parameters of the formation of these components are presented in Table 2. Comparing Table 1 and Table 2, the differences in the energies of activation and the pre-exponential factors are higher for the commodity thermoplastics (Table 1) than for the decomposition products from electronic scrap (Table 2). The higher the differences, the better stepwise pyrolysis can be applied. Examples for a separation of different plastics by stepwise pyrolysis including dehydrochlorination and the realisation of this process in laboratory scale are given in [7–9]. In case of electronic scrap, the pre-exponential factors of the formation of the main pyrolysis products seem to be different enough to separate these three products. However, such a procedure is only applicable if harmful chlorine or bromine containing components can be separated.
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3.2. In6estigation of the dehalogenation of electronic scrap A sample of electronic scrap containing 1.72 wt.% bromine and 0.94 wt.% chlorine has been investigated with regard to the decomposition of the organohalogen components. The most important halogenated compounds in electronic scrap are brominated flame retardants, polychlorinated biphenyls and PVC. To investigate the decrease of organohalogens dependence on pyrolysis temperature, the sample was heated at a constant heating rate (2 K min − 1) to different temperatures or treated isothermally and the halogen content of the residue was determined using the Wickbold procedure and/or EDRFA. Fig. 1 presents a comparison between the decrease of the halogen content in the residue and the degradation of the entire sample measured with thermogravimetry (heating rate 2 K min − 1). The chlorine content decreases up to 400°C from 0.94 to 0.16 wt.%; higher temperatures do not decrease the chlorine content in the residue. Therefore, the chlorine residue seems to be inorganic and it remains in the carbon free residue of the Wickbold decomposition. The observed temperature range of hydrogen chloride evolution based on mass spectrometic analysis is between 220 and 400°C for a heating rate of 2 K min − 1 (Fig. 2). This is identical to the temperature range of the dehydrochlorination of PVC. The ion currents at m/e =36 and 38 (H35Cl, H37Cl) in TG-MS-measurements of PVC and electronic scrap show identical shapes. The maximum HCl evolution rate at 280°C as well as the occurring shoulder of the ion currents from PVC and electronic scrap degradation are similar. The comparison of the ion currents of hydrogen chloride with the first derivative of the normalized weight of the electronic scrap (da/dt) from TG-MS shows that dehydrochlorination takes place at lower temperatures than the decomposition of remaining skeleton, compare Fig. 3. Therefore, the removal of HCl from PVC in electronic scrap occurs similarly as in mixtures of commodity plastics. The decomposition of the organobromides in electronic scrap occurs at higher temperatures compared with the dehydrochlorination of PVC close to the decomposition of the remaining skeleton (Fig. 1). A remarkable amount of bromine remains in the residue, possibly due to the reaction with copper or calcium carbonate (filler) forming copper and calcium bromide. An indication for copper bromide is that the remaining residue burns in the Wickbold-apparatus with a strong green flame, which points to the presence of volatile copper halides. The formation of copper(I)bromide during pyrolysis might proceed via elemental bromine. Bromine or hydrogen bromide are obtained during degradation of flame retardants, examples of which are given in Fig. 4, interacting with the present polymers [12]. Dynamic and isothermal measurements show that the bromine content in the residue decreases with increasing temperature up to 400°C (Fig. 5). Therefore, brominated products are evolved during the total pyrolysis procedure. The amount of bromine in the product oil increases and the amount of bromine in the gas phase is approximately constant. In contrast to chlorine, a separation of bromine during
H. Bockhorn et al. / J. Anal. Appl. Pyrolysis 49 (1999) 97–106
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thermal decomposition fails and scavengers have to be used to demobilize bromine to yield ‘halogen free’ decomposition products. One possibility to fix bromine in the residue is to add calcium carbonate to the electronic scrap before pyrolysis. For an addition of 20 wt.% calcium carbonate and a pyrolysis temperature of 400°C the content of bromine in the residue is doubled. The normalized bromine content of 80% is comparable with the results obtained at 300–330°C. A pyrolysis temperature of 300°C seems to be suitable to evolve the chlorine and to demobilise bromine in the residue. Although the evolution of halogen and organohalogen is spread over a wide temperature range, a separation from the other decomposition products might be possible due to mineralization with metals (like copper) or calcium carbonate. Further experiments are in progress.
4. Conclusion The formal kinetic parameters of the dehydrochlorination of PVC confirm that the HCl evolution can be separated from the degradation products of thermoplastics in plastic mixtures at low temperatures. For processing mixtures containing PVC no further pretreatment of the mixture is necessary. The dehydrochlorination takes place quantitatively at low temperatures. Investigation of the decomposition of electronic scrap shows, that the chlorine content from PVC can be removed in the same manner as from thermoplastic mixtures. The thermal degradation of organohalogen to volatile products occurs close to the main decomposition of the entire sample. Therefore, in addition to thermal pretreatment a demobilisation of organic halogens by means of copper or calcium carbonate is necessary. A separation of the pyrolysis products of electronic scrap into halogen free and halogen containing fractions seems to be possible if the bromine content is demobilised in the residue.
Acknowledgements We gratefully acknowledge the support of this work by AIF.
References [1] W. Kaminsky, Ullmann’s Encyclopedia of Industrial Chemistry, vol. A 21, Verlag Chemie, Weinheim 1992, p. 57. [2] H. Bockhorn, A. Hornung, U. Hornung, S. Teepe, J. Weichmann, Combust. Sci. Technol. 116/117 (1996) 129. [3] R. Knu¨mann, H. Bockhorn, Combust. Sci. Technol. 101 (1994) 285. [4] M. Paabo, B.C. Levin, Fire Mater. 11 (1987) 55. [5] A. Guyot, Polym. Degrad. Stab. 15 (1986) 219 – 235. [6] I. Lu¨derwald, F. Merz, M. Rothe, Angew. Makromol. Chem. 67 (1978) 193.
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[7] U. Hornung, Bestimmung der Vergasungskinetik von Kunststoffen mit Hilfe eines isotherm betriebenen Kreislaufreaktors, Fortschritt-Berichte VDI, Reihe 3, Band 485, VDI-Verlag, Du¨sseldorf 1997. [8] H. Bockhorn, A. Hornung, U. Hornung, J. Anal Appl. Pyrol. 46 (1998) 1 – 13. [9] A. Hornung, H. Bockhorn, U. Hornung, Chem. Eng. Technol. 09 (1998) 21. [10] DIN 51408: Pru¨fung flu¨ssiger Mineralo¨l-Kohlenwasserstoffe, Bestimmung des Chlorgehaltes, Verbrennung nach Wickbold. [11] B. Stachel, G. Behringer, U. Schacht, Fresenius J. Anal. Chem. 350 (1994) 375. [12] M. Lewin, S.M. Atlas, E.M. Pearce, Flame Retardant Polymeric Materials, Plenum, New York, 1975.
.
Dehydrochlorination of plastic mixtures H. Bockhorn *, A. Hornung, U. Hornung, P. Jakobstro¨er, M. Kraus Institut fu¨r Chemische Technik, Uni6ersita¨t Karlsruhe (TH) 4 -Kaiserstr. 12, 76128 Karlsruhe, Germany Accepted 22 October 1998
Abstract Dehydrochlorination of plastic mixtures from domestic waste as well as from other chlorine containing mixtures such as electronic scrap is an essential reaction step in waste incineration, pyrolysis and chemical recycling of polymers. For designing pyrolysis procedures, controlled combustion processes and to control the emissions from incinerators, the behaviour of polymers in thermal decomposition with regard to decomposition products and the kinetics of decomposition must be known. The kinetic data, for thermal decomposition of commodity plastics, confirms that in mixtures of different plastics the dehydrochlorination of, e.g. poly (vinyl chloride) (PVC) can be conducted at moderate temperatures and prior to the thermal degradation of the polymer skeleton. In stepwise low temperature pyrolysis mixtures of, e.g. PVC, polystyrene and polyethylene have been separated into hydrogen chloride, the monomer of polystyrene and aliphatic compounds from polyethylene decomposition. The degree of conversion of chlorine from PVC into hydrogen chloride in the low temperature (330°C) first step is about 99.6%. A similar behaviour for dehydrochlorination is obtained during the thermal degradation of electronic scrap. The hydrogen chloride evolution from PVC occurs in the same way as in mixtures of commodity plastics with a maximum rate of HCl loss at 280°C, when heated at 2 K min − 1. Brominated flame retardants are decomposed or evolved at higher temperatures (\300°C). A possibility to fix bromine in the residue is to add calcium carbonate to the electronic scrap before pyrolysis. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Stepwise pyrolysis; Thermal decomposition; Polymers; Kinetic of decomposition; PVC; HCl; Electronic scrap
* Corresponding author. Tel.: +49-721-6082120; fax: +49-7216-084820. 0165-2370/99/$ - see front matter © 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 9 8 ) 0 0 1 2 4 - 7
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H. Bockhorn et al. / J. Anal. Appl. Pyrolysis 49 (1999) 97–106
1. Introduction The incineration of plastic refuse is a widely used method in waste management. The incineration of plastic waste is connected with environmental problems, e.g. the formation of dioxines and chlorinated compounds from poly (vinyl chloride) (PVC) is reported in [1]. These problems enlarge if electronic scrap is incinerated. On one hand, in addition to chlorine from PVC, electronic scrap also contains brominated flame retardants and polychlorinated biphenyls; on the other hand, nitrile containing polymers from, e.g. acrylonitrile-butadiene styrene (ABS) or styrene acrylonitrile copolymer (SAN) may be precursors for hydrogen cyanide. The high amount of organohalogen and other harmful substances is prohibitive for commercial processes for re-utilization of the polymer fraction of electronic scrap. Their elimination, reduction or conversion into harmless compounds will help to overcome this problem. One method for the elimination of organohalogens is decomposition by thermal pretreatment of electronic scrap. For designing thermal pretreatment procedures, the behaviour of polymers during thermal decomposition with regard to the decomposition products and the kinetics of decomposition must be known. The kinetics of decomposition of various plastics and plastic mixtures have been investigated by isothermal and dynamic measurements [2 – 7]. The kinetic data confirms that different molecular structures of commodity plastics lead to different reaction mechanisms of decomposition, different reaction rates and, particularly, a different temperature dependency of the decomposition rates. In particular, dehydrochlorination of PVC by a free radical chain mechanism occurs at comparably low temperatures. This behaviour suggests a combination of separation and decomposition of polymer mixtures by means of stepwise pyrolysis with different temperatures in the single steps, including a thermal pretreatment for dehydrochlorination of PVC. For the elimination of chlorine by thermal degradation of PVC a rechlorination of other components in the mixture must be prevented. In this work, the feasibility of thermal pretreatment for the elimination of halogens from electronic scrap is investigated. The results from kinetic investigations are presented, e.g. the formal kinetic parameters of thermal degradation of various plastics and the formation of the main pyrolysis products from electronic scrap are compiled. The decomposition of organohalogen components in electronic scrap has been investigated under dynamic conditions with simultaneous thermogravimetry-mass spectrometry and with an isothermal reactor. The experimental results suggest a thermal pretreatment for eliminating harmful compounds from plastic mixtures from domestic waste and from mixtures of thermosetting plastics like electronic scrap. One question is, whether chlorine from PVC can be removed in the same manner as from mixtures of PVC with other plastics. Other problems are bromine containing flame retardants and polychlorinated biphenyls which are present in electric and electronic devices. The results of laboratory scale dehydrochlorination of mixtures of PVC polyethylene and polystyrene have been presented previously [8,9].
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Table 1 Rate coefficients and apparent activation energies for the thermal decomposition of different thermoplastics Polymer
Ea (kJ mol−1)
log (k0/min−1)
PVC dehydrochlorination: PVC, iso.a PVC, dyn.
136 95 143.59 2.9
11.48 9 0.09 12.5 90.18
1.54 1.54
7 3
Polystyrene, isothermal Polystyrene, dynamic
1729 4 322.8 92.4
12.47 90.02 24.61 9 0.19
1.04 1.09
7 3
Polyethylene, isothermala Polyethylene, dynamic PVC 2nd degradation step: Poly(vinyl chloride), iso.* Poly(vinyl chloride), dyn.
26893 262.1 91.9
17.78 9 90.01 18.00 9 0.14
0.8–1.4 0.83
7 3
217 9 5 234.1 92.1
14.9 9 0.02 16.35 90.15
1–1.8 1.64
7 3
Polyamide 6, isothermal e-caprolactam, isothermal Polyamide 6, dynamic
211.0 93 205.0 91 210.8
14.9 90.05 14.5 9 0.03 15.01
1–1.29 0.98 0.82
7 7 7
Polypropylene, isothermal Polypropylene, dynamic Polyethylene therephthalate, isothermal Polyethylene thereph-thalate, dynamic
220 95 223.79 1.6 214 92 238.7
a
15.06 9 0.06 15.9 90.2 15.2 90.04 18.00
Reference
n
1.1 0.77 1.15 1.15
3
The data is obtained from isothermal and dynamic measurements.
2. Experimental Investigations with thermoplastics are performed with pure polymers without stabilizers, fillers and colors. The polymers are trademarks of BASF (Ludwigshafen, Germany); polyamide 6 (Ultramid B3™), polystyrene (Polystyrol 143 B™), polyethylene (Lupolen 5261™) and PVC (Vinoflex S 5715™). The experiments with electronic scrap are performed with samples also used in large scale rotary kiln pyrolysis experiments at Berlin-Consult (Germany) and were provided by AEG-Sachsenwerk (Regensburg, Germany) and homogenized by BASF (Ludwigshafen, Germany). The used electronic scrap fraction comes mainly from high voltage components. The plastic fraction of this particular scrap consists
Table 2 Rate coefficients and apparent activation energies for several components -evolved during thermal decomposition of electronic scrap Polymer
Ea (kJ·mol−1)
log (k0/min−1)
n
Bisphenol A Styrene 4-phenylbutyronitrile
141.2 97 142.094.2 100.3 94.5
11.24 9 0.02 9.28 90.02 7.19 90.02
0.86 1.22 1.44
100
H. Bockhorn et al. / J. Anal. Appl. Pyrolysis 49 (1999) 97–106
Fig. 1. Decrease of the halogen content during pyrolysis of electronic scrap.
of thermosetting plastics like polyester resins and phenolic plastics and thermoplastics like acrylonitrilebutadiene styrene (ABS), styrene acrylonitrile copolymer (SAN) and others. It has been ground to a powder (particle diameter 0.25 mm) after removing of metal parts.
Fig. 2. Comparison of HCl formation of PVC degradation and electronic scrap.
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Fig. 3. Hydrogen chloride evolution from electronic scrap in comparison with the rate of conversion (heating rate of 2 k min − 1; sample size, 20 mg).
2.1. Determination of formal kinetic parameters Dynamic as well as isothermal experiments were applied to determine formal kinetic parameters. The dynamic experiments were carried with a thermobalance coupled with a quadrupole mass spectrometer (DuPont 951 thermogravimetric analyzer/BalzersQMG 420). The thermobalance was driven by a programmable thermal controller (Eurotherm 818P). It was operated with heating rates from 2 to 20 K min − 1
Fig. 4. Some common brominated flame retardants [12].
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Fig. 5. Decrease of the bromine content during pyrolysis of electronic scrap.
between ambient temperature and 800°C. Pure helium was used as a purge gas (flow rate 100 cm3 min − 1). The quadrupole mass spectrometer (Balzers-QMG 420) was connected to the thermobalance via an open coupling with a differential pressure reduction via a platinum orifice (10 mm diameter). The coupling could be heated up to 500°C to prevent condensation of the evolved products. The high pressure end of the coupling was located directly above the heated polymer sample. The low pressure end of the coupling was inserted directly into the ion source of the mass spectrometer. The ionizing voltage of the cross-beam electron impact ionization source amounted to 30 eV. More details are given in [2]. The isothermal experiments were carried with a gradient free reactor. The gradient free reactor with on-line mass spectrometry enables the determination of formal kinetic parameters of thermal degradation reactions of solids or liquids under isothermal conditions up to 1 MPa. The reactor consists of a heated sliding device carrying the sample within a platinum crucible. After preheating the sliding device the sample is moved quickly into the reaction chamber. Heat transfer to the sample is accomplished through mixing of the reactor contents by means of a small gas turbine. Perfect mixing also provides gradient free conditions within the reaction chamber. The reaction products are analysed on-line with a quadrupole mass spectrometer (Balzers, QMG 421, analyzer 400). More details are given in [2,3,7]. All data are given for thermal degradation under inert atmosphere (helium).
2.2. Analytic methods for the determination of chlorine and bromine contents in electronic scrap samples The pyrolysis of electronic scrap has been performed in a separate oven (Du Pont) equal to the one of the thermobalance. Samples up to 500 mg have been
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investigated under isothermal and dynamic conditions and a helium flow of 175 ml/min. The evolved pyrolysis products were captured in a cold trap with liquid nitrogen. Products, which could not be condensed in the cold trap, were directly send to a Wickbold apparatus. With the Wickbold procedure [10,11], chlorine and bromine contents of the gaseous pyrolysis products and liquid products of the cold traps as well as of the solid residues have been determined. Supplementary EDRFA (energy dispersive X-ray fluorescence analysis) experiments have been performed to investigate the bromine content. With this method the bromine content is determined via a quantitative calibration. Furthermore, the metal content is obtained using EDRFA. Additional investigations of the liquid pyrolysis products are performed with GC-MS and GC-FID.
3. Results
3.1. Formal kinetic parameters of the thermal degradation of 6arious plastics and the formation of main pyrolysis products from electronic scrap Data for the overall rate of thermal decomposition for different plastics is given in Table 1. The data in Table 1 refers to a rate expression for the thermal decomposition as given by − Ea da =k0e RT (1 − a)n dt
where a is the conversion (m0 −m/m0 −m ), with m0 being the initial mass, m the actual mass and m the final mass. Ea is the apparent activation energy and n is the apparent reaction order. Analogous kinetic investigations have been performed with electronic scrap under isothermal conditions with the gradient free reactor. Due to the polyesters SAN and ABS some of the main decomposition products from electronic scrap degradation are styrene, bisphenol A and 4-phenylbutyronitrile. The formal kinetic parameters of the formation of these components are presented in Table 2. Comparing Table 1 and Table 2, the differences in the energies of activation and the pre-exponential factors are higher for the commodity thermoplastics (Table 1) than for the decomposition products from electronic scrap (Table 2). The higher the differences, the better stepwise pyrolysis can be applied. Examples for a separation of different plastics by stepwise pyrolysis including dehydrochlorination and the realisation of this process in laboratory scale are given in [7–9]. In case of electronic scrap, the pre-exponential factors of the formation of the main pyrolysis products seem to be different enough to separate these three products. However, such a procedure is only applicable if harmful chlorine or bromine containing components can be separated.
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3.2. In6estigation of the dehalogenation of electronic scrap A sample of electronic scrap containing 1.72 wt.% bromine and 0.94 wt.% chlorine has been investigated with regard to the decomposition of the organohalogen components. The most important halogenated compounds in electronic scrap are brominated flame retardants, polychlorinated biphenyls and PVC. To investigate the decrease of organohalogens dependence on pyrolysis temperature, the sample was heated at a constant heating rate (2 K min − 1) to different temperatures or treated isothermally and the halogen content of the residue was determined using the Wickbold procedure and/or EDRFA. Fig. 1 presents a comparison between the decrease of the halogen content in the residue and the degradation of the entire sample measured with thermogravimetry (heating rate 2 K min − 1). The chlorine content decreases up to 400°C from 0.94 to 0.16 wt.%; higher temperatures do not decrease the chlorine content in the residue. Therefore, the chlorine residue seems to be inorganic and it remains in the carbon free residue of the Wickbold decomposition. The observed temperature range of hydrogen chloride evolution based on mass spectrometic analysis is between 220 and 400°C for a heating rate of 2 K min − 1 (Fig. 2). This is identical to the temperature range of the dehydrochlorination of PVC. The ion currents at m/e =36 and 38 (H35Cl, H37Cl) in TG-MS-measurements of PVC and electronic scrap show identical shapes. The maximum HCl evolution rate at 280°C as well as the occurring shoulder of the ion currents from PVC and electronic scrap degradation are similar. The comparison of the ion currents of hydrogen chloride with the first derivative of the normalized weight of the electronic scrap (da/dt) from TG-MS shows that dehydrochlorination takes place at lower temperatures than the decomposition of remaining skeleton, compare Fig. 3. Therefore, the removal of HCl from PVC in electronic scrap occurs similarly as in mixtures of commodity plastics. The decomposition of the organobromides in electronic scrap occurs at higher temperatures compared with the dehydrochlorination of PVC close to the decomposition of the remaining skeleton (Fig. 1). A remarkable amount of bromine remains in the residue, possibly due to the reaction with copper or calcium carbonate (filler) forming copper and calcium bromide. An indication for copper bromide is that the remaining residue burns in the Wickbold-apparatus with a strong green flame, which points to the presence of volatile copper halides. The formation of copper(I)bromide during pyrolysis might proceed via elemental bromine. Bromine or hydrogen bromide are obtained during degradation of flame retardants, examples of which are given in Fig. 4, interacting with the present polymers [12]. Dynamic and isothermal measurements show that the bromine content in the residue decreases with increasing temperature up to 400°C (Fig. 5). Therefore, brominated products are evolved during the total pyrolysis procedure. The amount of bromine in the product oil increases and the amount of bromine in the gas phase is approximately constant. In contrast to chlorine, a separation of bromine during
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thermal decomposition fails and scavengers have to be used to demobilize bromine to yield ‘halogen free’ decomposition products. One possibility to fix bromine in the residue is to add calcium carbonate to the electronic scrap before pyrolysis. For an addition of 20 wt.% calcium carbonate and a pyrolysis temperature of 400°C the content of bromine in the residue is doubled. The normalized bromine content of 80% is comparable with the results obtained at 300–330°C. A pyrolysis temperature of 300°C seems to be suitable to evolve the chlorine and to demobilise bromine in the residue. Although the evolution of halogen and organohalogen is spread over a wide temperature range, a separation from the other decomposition products might be possible due to mineralization with metals (like copper) or calcium carbonate. Further experiments are in progress.
4. Conclusion The formal kinetic parameters of the dehydrochlorination of PVC confirm that the HCl evolution can be separated from the degradation products of thermoplastics in plastic mixtures at low temperatures. For processing mixtures containing PVC no further pretreatment of the mixture is necessary. The dehydrochlorination takes place quantitatively at low temperatures. Investigation of the decomposition of electronic scrap shows, that the chlorine content from PVC can be removed in the same manner as from thermoplastic mixtures. The thermal degradation of organohalogen to volatile products occurs close to the main decomposition of the entire sample. Therefore, in addition to thermal pretreatment a demobilisation of organic halogens by means of copper or calcium carbonate is necessary. A separation of the pyrolysis products of electronic scrap into halogen free and halogen containing fractions seems to be possible if the bromine content is demobilised in the residue.
Acknowledgements We gratefully acknowledge the support of this work by AIF.
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