The permanent expansion of the market of electrical and electronic equipment (EEE) and the shorter innovation cycles, lead to a faster replacement of these appliances, making EEE a fast-growing source of waste (WEEE). As stated in Directive 2012/19/EU1 on waste electrical and electronic equipment, the content of hazardous components in EEE is a major concern during the waste management phase, and recycling of WEEE is not currently undertaken to a sufficient extent, resulting in a loss of valuable resources.Support for this work was provided by: Ministry of Education and Science (Spain) (CTQ2008-05520 project); Valencian Community Government (Spain) (PROMETEO/2009/043/FEDER project)

Conesa, Juan A.

Font, Rafael

Moltó Berenguer, Julia

Ortuño García, Nuria

Effect of Temperature, Atmosphere and Metals on the Thermal Degradation of Printed Circuit Boards

Repositorio Institucional de la Universidad de Alicante

EFFECT OF TEMPERATURE, ATMOSPHERE AND METALS ON THE THERMAL DEGRADATION OF PRINTED CIRCUIT BOARDS  Ortuño N*, Conesa JA, Moltó J, Font R  Chemical Engineering Department, University of Alicante, P.O. Box 99, E-03080 Alicante, Spain. *e-mail: nuria.ortuno@ua.es   Introduction  The permanent expansion of the market of electrical and electronic equipment (EEE) and the shorter innovation cycles, lead to a faster replacement of these appliances, making EEE a fast-growing source of waste (WEEE). As stated in Directive 2012/19/EU1 on waste electrical and electronic equipment, the content of hazardous components in EEE is a major concern during the waste management phase, and recycling of WEEE is not currently undertaken to a sufficient extent, resulting in a loss of valuable resources. It has been estimated that printed circuit boards (PCB) comprise approximately 6 wt % of all WEEE, representing over 500 000 tonnes of PCBs generated in the EU per year2. Due to the heterogeneous mix of organic material, metals, and glass fiber, printed circuit boards are specially problematic to recycle3 and low recycling rates are reported of about 15 %4. Thermal treatments have been widely investigated as recycling techniques for e-waste, but little attention has been given to the pollutants evolved during these processes. Polymers used in printed circuit boards are either physically blended with or chemically bonded to brominated flame retardants, which may result in emission of brominated organic pollutants during the thermal decomposition of these residues. Moreover, the presence of Fe and Cu can catalyze debromination/hydrogenation reactions, accelerating the formation of chlorinated and brominated dioxins and furans, so further research is needed on the influence of metals in the emissions of halogenated pollutants from PCB waste incineration5. The present work aims to characterize the emissions from pyrolysis and combustion of waste PCB from mobile phones, before and after the removal of the metallic fraction, and at two different temperatures. The study comprises the analysis of gases, halogens and hydrogen halides, carbon oxides, light hydrocarbons, polycyclic aromatic hydrocarbons (PAHs), and brominated phenols (BrPhs), among other semivolatile compounds. Furthermore, polybrominated dibenzo-p-dioxins and dibenzofurans (PBDD/Fs) have been analyzed. Materials and methods Waste printed circuit boards were separated and crushed to fine dust (sample named “PCB”, corresponding to the whole printed circuit boards). To remove the metallic fraction, part of the sample was treated with a H2O:HCl:H2O2 (2:1:1 vol.) solution, followed by washing with deionized water and drying at 110 ºC (sample named “nmf-PCB”, corresponding to the non metallic fraction). Elemental and metal analyses (see Table 1) were performed to check the process and characterize the samples. Using a tubular quartz reactor located inside a horizontal laboratory furnace (see Figure 1), pyrolysis and combustion runs were carried out at 600 and 850 ºC, in order to study the decomposition products under different operating conditions. In this reactor, the sample is placed in a quartz boat (holder) which is introduced inside the furnace at 1 mm/s, performing the sampling of the gases at the reactor outlet. Sample amounts used correspond to slightly substoichiometric oxygen conditions (oxygen defect), favoring the formation of compounds of incomplete combustion, in order to simulate the system operation under adverse conditions and poor combustion (75 mg of “PCB” and 40 mg of “nmf-PCB”).  Table 1. Composition (wt %) of printed circuit boards (PCB) and non metallic fraction (nmf-PCB). Sample PCB nmf-PCB Elemental analysis:  C 20.4 36.4 H 1.9 3.4 N 0.7 1.4 S - - X-Ray Fluorescence analysis: O 24.5 21.7 Cu 24.2 0.50 Si 10.5 15.3 Br 5.7 12.2 Ca 4.5 3.8 Al 3.3 1.5 Pb 0.9 0.03 Sn 1.4 0.5 Ni 0.3 0.1 Ba 0.2 0.7 P 0.4 0.5 (-): not detected   Organohalogen Compounds Vol. 76, 194-197 (2014) 194 For each experimental condition, four different runs were carried out on each sample, as well as a blank run, due to the different sampling and analysis techniques used to quantify the following groups of compounds: - Hydrogen halides and halogen gases were collected by passing the evolved gases through two consecutive impingers containing diluted solutions of H2SO4 and NaOH, according to U.S. EPA method 266. The solutions were analyzed by ion chromatography. - Gases and volatile compounds were collected in Tedlar® bags and analyzed by gas chromatography coupled to thermal conductivity (TCD), flame ionization (FID) and mass spectrometer (MS) detectors. - Semivolatile compounds were adsorbed on Amberlite® XAD-2 resin at the outlet of the reactor and extracted with dichloromethane/acetone (1:1 vol.) by accelerated solvent extraction (Dionex ASE® 100). These compounds, including polycyclic aromatic hydrocarbons (PAHs), were analyzed by HRGC-MS in SCAN mode, according to U.S. EPA method 8270D7, while bromophenols were analyzed in SIR mode. An Agilent HP5-MS (30 m x 0.25 mm i.d. x 0.25 µm) was used as chromatographic column. - PBDD/Fs were collected with the same sorbent resin in a separate run. The resin was extracted with dichloromethane and toluene in two subsequent steps using a Dionex ASE® 100; then the samples were purified using an automated clean-up system (Power Prep®), and finally concentrated. The samples were analyzed by gas chromatography coupled to high resolution mass spectrometry (HRGC-HRMS), according to the standard method for its chlorinated analogues8. A Restek TRB-Meta X5 chromatographic column (15 m x 0.25 mm x 0.25 µm) was used for the analysis. Recoveries of the 13C-labeled PBDD/Fs were between the acceptable limits, except for 13C-OBDD/F, which are not reported. Throughout the experimental process (sampling, extraction, purification, concentration and analysis) the samples were protected from light, in order to prevent photodegradation of the brominated compounds. Results and discussion Table 2 shows the results of the majority compounds obtained in the analyses of gases, volatile and semivolatile compounds of the two samples in pyrolysis (P) and combustion (C) runs at 600 and 850 ºC. Apart from CO and CO2, HBr was the main gas emitted from the decomposition of printed circuit boards. Minor quantities of Br2, HCl and Cl2 were also detected in all runs. Inorganic bromine emissions accounted for 24 - 37 % of initial bromine content for sample “PCB” and 39 - 53 % in the case of “nmf-PCB”, showing an increase with temperature and a less accused dependence on the presence of oxygen. Among light hydrocarbons, the most abundant were methane, ethylene, propylene, benzene and toluene, with similar trends for both samples. These compounds are easily oxidized in combustion runs, hence yields were higher in pyrolysis runs, and increased in the run at 850 ºC. Phenol, benzofuran and styrene were the majority semivolatile compounds, emitted in all experiments, except in combustion at 850 ºC. Both samples followed similar trends, with the highest semivolatile yields observed in the pyrolysis run at 600 ºC. Yields are higher for the decomposition of the non-metallic fraction, and this can be due to the fact that the polymer content in sample “nmf-PCB” is aproximately twice the polymer content from sample “PCB”.  Figure 1. Scheme of the laboratory scale tubular reactor. Organohalogen Compounds Vol. 76, 194-197 (2014) 195 Table 2. Gases, volatile and semivolatiles compounds from pyrolysis (P) and combustion (C) of printed circuit boards (PCB) and non metallic fraction (nmf-PCB). EXPERIMENT P600 C600 P850 C850 SAMPLE PCB nmf-PCB PCB nmf-PCB PCB nmf-PCB PCB nmf-PCB COMPOUND mg compound/kg sample (ppm) Gases and volatile compounds               Inorganic bromine:         HBr 14900 46300 12700 45900 20200 58100 11800 59600 Br2 1200 1600 1000 3700 1100 2800 6700 5800 Carbon oxides:         CO - - 88980 288030 - - 94840 295630 CO2 18510 51230 441580 594950 31940 57470 677440 506180 RCO = CO/(CO+CO2) 0% 0% 17% 33% 0% 0% 12% 37% Main light hydrocarbons:        methane   3570 8540 2910 8310 10980 23310 30 30 ethylene  1370 2340 930 1540 3550 6430 - - propylene  2070 4310 310 340 1140 2030 - - benzene   1060 1910 650 1290 13790 24470 - - toluene  1050 1600 - 1070 3350 7240 - - Semivolatile compounds and PAHs             Main 16 priority PAHs:        naphthalene 39 73 28 57 5790 11320 1 3 acenaphthylene 3 - 2 2 2170 6240 - - fluorene 22 62 - - 750 1450 - - phenanthrene 9 21 6 17 1470 3080 - - Other semivolatiles:         styrene 780 1050 40 90 1670 2910 - - phenol 27740 53660 7380 24200 2800 6620 - - benzofuran 1990 2700 680 1350 2390 3640 - -  (-): not detected or < 1 ppm             Only in the pyrolysis at 850 ºC were all 16 priority PAHs detected, and also with the highest yields. In general, the most abundant were naphthalene, acenaphthylene, fluorene and phenanthrene.  Emissions of bromophenols are shown in Figure 2, with maximum formation at 600 ºC in oxygen presence. Mono-, di- and tribrominated isomers predominate, more specifically, formation of isomers with a bromine atom in ortho/para positions is favored (2-, 4-, 2,4-, 2,6- and 2,4,6-BrPh), which concur with the most abundant isomers found during thermal degradation of tetrabromobisphenol A9, commonly used as flame retardant in printed circuit boards.  The emissions of 2,3,7,8-substituted PBDD/Fs are shown in Figure 3. In pyrolytic conditions, yields were relatively low and similar regardless of the temperature. In combustion at 850 ºC the emissions were 3 times higher than in pyrolysis, whereas in combustion at 600 ºC, a significant increase was observed (60-fold for sample “PCB” and 10-fold for “nmf-PCB”). Both samples exhibited similar isomer profiles, with 1,2,3,4,6,7,8-HpBDF as the most abundant isomer among furans and 1,2,3,4,6,7,8-HpBDD among dioxins. However, 2,3,4,7,8-PeBDF and 1,2,3,4,7,8-HxBDF are the       Figure 2. Yields of brominated phenols in pyrolysis (P) and combustion (C) at 600 and 850 ºC of two samples: printed circuit boards (“PCB”) and non-metallic fraction (nmf-PCB). Organohalogen Compounds Vol. 76, 194-197 (2014) 196 furans that contribute most to the toxicity of the emissions, while 2,3,7,8-TBDD and 1,2,3,7,8-PeBDD are the ones that enhance toxicity the most. Emissions and levels of toxic equivalents of PBDD/Fs were higher in combustion than in pyrolysis runs, since oxygen promotes the radical halogenation reactions and leads to an increase in PBDD/F formation, particularly furans. It is known that metals catalyze the surface-mediated reaction of precursors, such as brominated phenols, that lead to PBDD/Fs formation5, as it was observed for sample “PCB”, but the effect of the metal presence is only observed in the run with maximum formation. As a general conclusion from the different results obtained for both samples, it is shown that yields of the different compounds emitted depended more on operating conditions (temperature and oxygen ratio), but also on the nature of the material or the presence of metals. Therefore, a strict control of operating conditions is required in order to minimize pollutant emissions from thermal recycling of this kind of wastes. Acknowledgements Support for this work was provided by:  - Ministry of Education and Science (Spain) (CTQ2008-05520 project) - Valencian Community Government (Spain) (PROMETEO/2009/043/FEDER project).   References:  1.  European Comission. (2012); Official Journal of the European Comission. Directive 2012/19/UE of the European Parliament and of the Council on waste electrical and electronic equipment (WEEE) (recast). 2.  Das A, Vidyadhar A, Mehrotra S P. (2009); Resour. Conserv. Recy. 53(8): 464-469. 3.  Williams P. (2010); Waste and Biomass Valorization 1(1): 107-120. 4.  Goosey M, Kellner R. (2002); A Scoping Study: End-of-Life Printed Circuit Boards; Department of Trade and Industry, Intellect & Shipley Europe Limited: London. 5.  Duan H, Li J, Liu Y, Yamazaki N, Jiang W. (2011); Environ. Sci. Technol. 45(15): 6322-6328. 6.  US EPA. (1994); SW-846. Method 26. Determination of hydrogen halide and halogen emissions from stationary sources. Non-isokinetic method. 7.  US EPA. (2007); SW-846. Method 8270D. Semivolatile organic compounds by GC/MS. 8.  US EPA. (1994); SW-846. Method 1613. Tetra- through Octa-Chlorinated Dioxins and Furans by Isotope Dilution HRGC/HRMS. 9.  Ortuño N, Moltó J, Conesa J A, Font R. (2014); Environ. Pollut. 191(0): 31-37.          Figure 3. Yields of 2,3,7,8-brominated dioxins and furans in pyrolysis (P) and combustion (C) at 600 and 850 ºC of two samples: printed circuit boards (“PCB”) and non-metallic fraction (nmf-PCB). Organohalogen Compounds Vol. 76, 194-197 (2014) 197 

Effect of Temperature, Atmosphere and Metals on the Thermal Degradation of Printed Circuit Boards

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