Recycling Polymer-Rich Waste Printed Circuit Boards at High Temperatures: Recovery of Value-Added Carbon Resources
High-temperature transformation studies were carried out on polymer-rich waste printed circuit boards (PCBs) in the temperature range of 750–1,350 °C in argon atmosphere. Copper-rich metallic fractions started to separate out as foils/droplets at temperatures above 950 °C producing significant quantities of carbonaceous residue. In-depth characterisation of the residue was carried out using X-ray diffraction, Raman spectroscopy, SEM/EDS, surface area analysis, and LECO measurements. The recovery of carbons from waste PCBs reached up to 25 % of total weight. These carbons generally had a disordered structure with 3–4 layers stacking along the c-axis. The presence of metals in the carbonaceous residue became negligibly small at 1,350 °C, significantly enhancing the quality of the carbonaceous product (carbon content: 52–74 wt%). This study has shown that potentially vast reserves of carbon could be recovered from e-waste through appropriate recycling, while minimising the impact of waste on the environment.
KeywordsRecycling E-waste Disordered carbons PCBs Thermal transformations
Electronic waste (e-waste) is one of the fastest growing solid waste streams around the world today . Rapid uptake of information technology with the advent of new designs and technology at regular intervals and intense marketing in the electronics sector is causing the early obsolescence of many electronic items. With high consumer demand in affluent countries, and even amongst developing economies, the manufacturing and sales of equipment in the electronic industry is now ~$1 trillion annually, yielding 30–50 million tonnes of obsolete equipment worldwide each year [2, 3]. The international association of electronics recyclers projects that ~3 billion computer products units including CPUs, monitors, notebooks, keyboards, printers, copiers, faxes, etc. will be scrapped by 2015 . In Australia, an estimated 37 million computers will be in landfills or on their way; another 4 million computers are expected to be sold every year and less than 1.5 % will be recycled . A similar scenario exists for TVs, mobile phones, fluorescent tubes and other electronics. With an overall recycling rate of only ~10–18 % worldwide, most of the discarded electronic equipment are stored in warehouses, off-shored to developing economies, trashed in landfills or incinerated [6, 7, 8].
E-waste is known to be a complex mixture of plastics, metals, ceramics and other trace impurities; it can contain up to 1,000 different substances . PCBs typically contain 40 wt% metals, 30 wt% organics and 30 wt% ceramics. However, there is a great variance in the composition of PCB (PCB: the central component in electrical and electronic devices), wastes coming from different appliances, from different manufacturers and of different ages. After removing batteries and capacitors, PCBs from computers/TVs can contain up to 70 wt% organics, whereas PCBs from mobile phones contain ~20 wt% organics . There is a potential material resource of over 40 million tonnes a year in e-waste which could be used again and again if appropriate recycling practices are put in place.
A number of approaches such as metallurgical (hydro, bio and pyro) techniques, chemical techniques and low-temperature pyrolysis are being used to recover materials from the e-waste [11, 12]. Due to economic reasons, recycling efforts are generally focused on the recovery of copper and other/precious metals from the waste electronics as the metallic concentrations in waste printed circuit boards (PCBs) can be several times higher than their respective ores . The nonmetallic fraction containing large amounts of carbon is either burnt to provide energy during recycling or is trashed as a waste by-product. Our focus in this article is to investigate the formation and characterisation of residual carbons during high-temperature transformations of waste PCBs.
A number of pyrolysis investigations on waste PCBs/e-waste have been reported in the literature. In the Haloclean process, the e-waste is first heated at 350 °C and then at 450 °C under nitrogen in a horizontal rotary kiln; this process generates a gas/oil fraction that can be concentrated as oil and a solid residue from which metals can be recovered after further treatment . Zhou et al.  conducted a three-step pyrolysis process: in the first stage, a PCB was pyrolysed at 600 °C under vacuum; the second step involved the vacuum centrifugal separation of the solder. In the last step, the solder was re-melted, and the products generated were ~72 wt% of solid residue composed of metals, glass fibres and other inorganic materials; 22 wt% of oil fraction containing tin and lead; and 6.35 wt% gaseous products . Another investigation involved the heating of a mixture of PCBs and molten salts, KOH and NaOH at 300 °C under argon flow. Two solid products were obtained: a metallic fraction composed of wires and foils and a calcium carbonate/calcium silicate powder . de Marco et al. investigated four different materials: polyethylene (PE) wires containing Al and Cu from e-waste, table phones, mobile phones and PCBs; these were heat treated at 500 °C under nitrogen atmosphere. The solid products generated were inorganic materials consisting mainly of metals and a black powder/char. The amount of char recovered was low for PE materials and PCBs, and high for both types of phones. Pyrolysis oil consisting of organic compounds was also generated; gaseous products were mainly hydrocarbons along with large amounts of CO and CO2 .
In a recent study by our group, the pyrolysis of waste PCBs at 1,150 °C in argon atmosphere led to a clear separation of various metals from a carbonaceous and ceramic residue; this was attributed to the poor affinity of carbon with copper/copper-based alloys . In the present investigation, studies were carried out on thermal transformations of waste PCBs in the temperature range from 750 to 1,350 °C. In this article, we report the generation of significant quantities of carbonaceous residues following the separation of most of the metallic phase as droplets or foils. Along with an in-depth investigation on polymer degradation and gaseous release, a range of analytical tools were used to characterise these carbonaceous residues. This investigation was carried out on polymer-rich single-sided PCBs that dominate the electronics market in sheer volumes and are a dominant fraction of waste PCBs from obsolete, old-generation electronic devices.
Materials and Methods
This assembly was pushed into the cold zone (200–300 °C) of a high-temperature horizontal tube furnace with the help of a graphite rod. The reaction assembly was held there for 10 min to avoid thermal shock, and then pushed into the hot zone of the furnace, maintained in the temperature range 750–1,350 °C. A few studies were also carried out at 1,550 °C as well. High-purity argon gas was passed through the furnace tube at a rate of 1 L/min during the heat treatment. The gas outlet was connected to an Infrared (IR) gas analyser for a continuous monitoring of CO, CO2 and CH4 gases produced during the heat treatment. Previous studies from our group had carried out heat treatment of PCBs as a function of time (1, 2, 5, 10, 15, 20, and 30 min); the thermal degradation of PCBs was found to be nearly complete within 15 min at these temperatures . In this study, the exposure of waste PCBs to high temperature was therefore carried out for 20 min; specimens were pulled back into the cold zone and kept there for 30 min to avoid thermal cracking and the re-oxidation of residual products.
Experiments were repeated several times at each temperature to enhance the reproducibility of results. Figure 1b shows samples before and after the heat treatment. The reaction products after the heat treatment could be separated into metallic droplets/foils, and a dark carbonaceous phase/slag. Relative proportions of various phases in the residue showed sample-to-sample variation and could not be ascertained accurately. Even though a significant proportion of metals were separated, a small quantity of residual metals still remained within the carbonaceous residue as indicated by ICP analysis. It was also difficult to separate out slag phases from carbons; however, a rough estimate of relative proportion of carbon in the residue was obtained from LECO analysis. These results have been provided in later sections. The weights of the samples were measured before and after the heat treatment; the carbon content of the carbonaceous residue was measured using LECO CN TruSpec Analyser.
The low-temperature thermal degradation of PCB polymers was also investigated using TGA/FTIR (Thermogravimetric-analyser model Perkin Elmer Pyris 1) for temperatures up to 1,350 °C to study volatile release during continuous heating at the rate of 20 °C/min. The carbonaceous phase was analysed using x-ray diffraction, Raman spectroscopy, Scanning Electron Microscopy (SEM/EDS) model Hitachi S3400X, and Inductively Coupled Plasma Optical Emission Spectrometers model, Perkin Elmer Optima 7300DV (ICP-OES) techniques. Our previous studies on carbonaceous residues from high-temperature pyrolysis of waste PCBs had shown the generation of carbon microfibres and foams . BET surface area analysis was carried out by using Micromeritics Tristar 3000 to determine the area of pores generated by escaping gases during polymer degradation. Samples were dried for 3 h at 150 °C under vacuum. Surface area and pore distribution were studied through the nitrogen physio-sorption technique. Twenty-point adsorption and 20-point desorption isotherms were generated and were used to determine specific surface area using the BET model. The pore distribution was determined through BJH model .
Weight Loss and Carbon Content
Average values of metals/oxides and carbons recovered during heat treatment at various temperatures ranging from 750 to 1,350 °C
Temperature of heat treatment (°C)
Weight loss through polymer degradation (wt%)
Metals/oxides recovered (wt%)
Carbons recovered (wt%)
The structural features of residual carbons were also analysed by Raman spectroscopy, where the scattering process includes contributions from various phonon vibration modes of materials. The spectrum from e-waste residues after excitation with 514-nm laser light is shown in Fig. 6b. The spectrum has two peaks at around 1,585 and 1,350 cm−1. The peak at 1,350 cm−1, known as D (defect) band, is a feature representing disorder in graphitic structures [23, 24]. The peak at 1,585 cm−1, known as G (graphite) band, represents highly ordered graphitic structure. The relative intensity ratio of ID/IG bands was determined by computing areas under the peaks, which can be used to determine the extent of disorder/or the degree of graphitisation in the carbon structure. These were determined to be 2.018 at 1,150 °C and 1.83 at 1,350 °C; this result indicates extensive disorder and poor graphitisation in the carbon structure. Both Raman and X-ray diffraction results are in good agreement and indicate these materials to be disordered carbons.
Electron Microscopy Investigations
Surface Area Measurement
Average values of pore width, cumulative pore surface area and cumulative volume along with BET surface area
Temperature of heat treatment (°C)
Pore width (nm)
Cumulative pore surface area (m2/g)
Cumulative pore volume (cm3/g)
BET surface area (m2/g)
The high-temperature pyrolysis of waste PCBs in the temperature range of 750–1,350 °C led to the segregation of metals, carbons and slag oxides; the amounts of carbon bearing materials produced were quite significant. Results in Table 1 indicate a significant carbon yield ranging between 20 and 32 wt%; these numbers are much higher than typical quantities of chars produced by a range of polymers. The pyrolysis of PVC at 740 °C produced a maximum char of 9 wt%; PS, PE and PP gave solid residues of 0.6, 1.8 and 1.6 wt%, respectively, under these conditions. During pyrolysis at 850 °C under nitrogen atmosphere, PVC produced 5.9 wt% char; LDPE and HDPE produced only 0.2 and 2.3 wt% of chars, respectively [25, 26]. Our results indicate that polymer-rich e-waste, such as single-sided boards based on phenol formaldehyde resin, can produce high amounts of residual carbon. With carbon recovery reaching 32 wt% at 1,150 °C; this is amongst the highest levels achieved in high-temperature transformation studies on polymeric waste.
The FTIR spectrum of gases evolved in the temperature range of 40–600 °C during TGA investigations showed the release of a range of organic volatiles, CO/CO2 gases and several brominated compounds from bromine-based flame retardants in PCBs. One of the key issues associated with the heat treatment of waste PCBs is the generation of hazardous dioxins and furans. Guo et al.  have, however, established that the generation of toxic furans and dioxin becomes negligibly small at temperatures above 900 °C; and the associated toxicity could be significantly reduced by adding Ca(OH)2 to leach out bromine from the flame retardants present in waste PCBs. Main gases generated at high temperatures were CO, CO2 and CH4; their cumulative volume was seen to decrease at higher temperatures.
Detailed structural characterisation of carbons was carried out using X-ray diffraction and Raman scattering techniques. Observed carbons were found to have a predominantly disordered structure; the relative proportion of defect/graphitic structures was found to range from 2.018 (1,150 °C) to 1.83 (1,350 °C). The typical (002) peak for graphite occurs at 26.6° representing the separation of basal planes stacked along the c-axis; covalently bonded (sp2) carbon atoms in the basal planes are arranged on a hexagonal lattice. The (101) peak for graphite occurs at 44.67°. However, the carbonaceous residues from e-waste produced peaks that were located at slightly lower angles: (24.6° and 44.12°) at 1,150 °C and (24.8° and 44.12°) at 1,350 °C. This shift indicates larger distances along the c-axis as well as in the basal plane for disordered carbons. Small values of Lc indicate the packing along the c-axis to be only 3–4 layers thick. However, high surface area was achieved for carbon residues at 1,150 °C, which could lead to the possibility of producing activated carbons from waste PCBs. Future studies will investigate the high-temperature behaviour of PCBs at shorter times during the period of gaseous release and study the evolution of pore structure, number density and surface area prior to pore coalescence and condensation.
Electron microscopy results clearly indicated the phase separation of various e-waste constituents after heat treatment. Following polymer degradation at low temperatures and the formation of chars, both metals and ceramic oxides started to segregate out at temperatures above 950 °C. Line scans on residues after heat treatment at 1,350 °C showed extended regions composed mainly of carbon indicating a clear separation of the metallic phase and the precipitation of oxide phases. ICP analysis results showed that the concentration of major metals especially hazardous lead in the carbonaceous residue had dropped to very low levels at 1,350 °C. Even the concentration of base metals from various oxides had become very low. The BET surface area of these carbonaceous resources showed a small dependence on temperature.
Improper handling of e-waste results in vast amounts of toxic waste being sent into landfills which has the potential to leach into soil and ground water supplies. The carbon-bearing waste in landfills leads to the release of greenhouse gases, including huge amounts of CO2 and methane. Up to 50 % of these emissions could be methane, which is 21 times more potent a greenhouse gas than CO2 . With landfilling and incineration becoming less accepted and more expensive, recycling complex hazardous e-waste is no longer a choice but an essential future requirement. In this article, in-depth investigations were reported on the high-temperature transformations of polymer-rich waste PCBs. This study has shown that significant carbonaceous resources could be recovered from recycling electronic waste. These carbons could be used as a source of energy or be utilised in applications such as reduction reactions, carburisation, activated carbons, etc. These results have the potential to make a positive impact on the environment through waste management as the conventional resources have become increasingly scarce, and the need to conserve and recycle resources becomes even greater. This e-waste recycling approach is expected to lead to sustainable developments in the field resulting in increased environment protection and economic development.
R. Cayumil gratefully acknowledges the financial support for this project from the Commonwealth Scientific and Industrial Research Organisation (CSIRO) and the Science and Industry Endowment Fund (SIEF). The authors would like to express their gratitude to the technical support of the Mark Wainwright Analytical Centre Units Electron Microscope and Solid State & Elemental Analysis, of the University of New South Wales.
- 1.COM (2000) Proposal for a directive of the European Parliament and of the council on waste electrical and electronic equipment. Official Journal of the European Communities, Brussels, BelgiumGoogle Scholar
- 2.Gover JE (1993) Review of the competitive status of the United States Electronics Industry. Technological competitiveness: contemporary and historical perspectives on the electrical, electronics, and computer industries. Institute of Electrical and Electronics Engineers IEEE, Piscataway, New York, pp 57–74Google Scholar
- 3.UNEP (2006) Call for global action on e-waste. United Nations Environment ProgrammeGoogle Scholar
- 4.IAER (2006) International Association of electronics recyclers industry report. http://www.iaer.org/communications/indreporthtm
- 5.TEC (2008) Tipping point: Australia’s e-waste crisis. Total Environment Centre, AustraliaGoogle Scholar
- 6.EPA US (2007) Management of electronic waste in the United States: approach 2. Washington DCGoogle Scholar
- 7.EPA US (2008) Electronics waste management in the United States: approach 1. Washington DCGoogle Scholar
- 8.Puckett J, Byster L, Westervelt S, Gutierrez R, Davis S, Hussain A, Dutta M (2002) Exporting harm: the high-tech trashing of Asia. The Basel Action Network, SeattleGoogle Scholar
- 14.Schöner J, Hornung A, Sagi S, Seifert H (2004) Post-treatment of pyrolysis residues of WEEE. Recovery of precious metals. In: International conference on incineration and thermal treatment technologies, Phoenix, Arizona, May 10–14Google Scholar
- 20.Condon JB (2006) Surface area and porosity determinations by physisorption: measurements and theory. Elsevier Science, OxfordGoogle Scholar
- 21.Sahajwalla V, Zaharia M, Kongkarat MS, Khanna R, Rahman M, Saha-Chaudhury N, O’Kane P, Dicker J, Skidmore C, Knights D (2011) Recycling end of life polymers in electric arc furnace steelmaking process: fundamentals of polymer reactions with slag and metal. Energy Fuels 26:58–66CrossRefGoogle Scholar
- 28.Forster P et al (2007) Changes in atmospheric constituents and in radiative forcing. In: Solomon S et al (eds) Climate change 2007: The Physical Science Basis. Cambridge University Press, CambridgeGoogle Scholar