Accepted Manuscript Title: Waste Printed Circuit Board Recycling Techniques and Product Utilization Author: Pejman Hadi Meng Xu Carol S.K. Lin Chi-Wai Hui Gordon McKay PII: DOI: Reference:

S0304-3894(14)00770-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2014.09.032 HAZMAT 16280

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

24-4-2014 2-9-2014 8-9-2014

Please cite this article as: P. Hadi, M. Xu, C.S.K. Lin, C.-W. Hui, G. McKay, Waste Printed Circuit Board Recycling Techniques and Product Utilization, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.09.032 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Highlights

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There is a major environmental issue about the printed circuit boards throughout the world. Different physical and chemical recycling techniques have been reviewed. Nonmetallic fraction of PCBs is the unwanted face of this waste stream. Several applications of the nonmetallic fraction of waste PCBs have been introduced.

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Waste Printed Circuit Board Recycling Techniques and Product

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Utilization

Chemical and Biomolecular Engineering Department, Hong Kong University of Science

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Pejman Hadia, Meng Xua, Carol S.K. Linb, Chi-Wai Huia and Gordon McKaya,c*

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and Technology, Clear Water Bay Road, Hong Kong SAR

School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue,

Division of Sustainable Development, College of Science, Engineering and Technology,

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Kowloon, Hong Kong SAR

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Hamad Bin Khalifa University, Qatar Foundation, Doha, Qatar Corresponding Author: Chemical and Biomolecular Engineering Department, Hong

Kong University of Science and Technology, Clear Water Bay Road, Hong Kong SAR, Tel: +852 23588412, Fax: +852 23580054, E-mail: [email protected]

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Abstract E-waste, in particular waste PCBs, represents a rapidly growing disposal problem

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worldwide. The vast diversity of highly toxic materials for landfill disposal and the

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potential of heavy metal vapors and brominated dioxin emissions in the case of

incineration render these two waste management technologies inappropriate. Also, the

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shipment of these toxic wastes to certain areas of the world for eco-unfriendly

“recycling” has recently generated a major public outcry. Consequently, waste PCB

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recycling should be adopted by the environmental communities as an ultimate goal.

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This article reviews the recent trends and developments in PCB waste recycling techniques, including both physical and chemical recycling. It is concluded that the

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physical recycling techniques, which efficiently separate the metallic and nonmetallic

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fractions of waste PCBs, offer the most promising gateways for the environmentallybenign recycling of this waste. Moreover, although the reclaimed metallic fraction has

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gained more attention due to its high value, the application of the nonmetallic fraction has been neglected in most cases. Hence, several proposed applications of this fraction have been comprehensively examined.

Keywords: PCB waste; Recycling; Metal-nonmetal separation; Nonmetallic fraction

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Contents 1. Overview of E-Waste Statistics .................................................................................................................. 5 2. Composition of E-Waste-Printed Circuit Boards ....................................................................................... 7

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2.1 Printed Circuit Board Assemblies ........................................................................................................ 7 3. Environmental Implications of E-Waste Disposal.................................................................................... 11 4. Waste PCB Recycling .............................................................................................................................. 12

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4.1 Chemical Recycling Techniques ........................................................................................................ 12 4.1.1 Vacuum Pyrolysis........................................................................................................................ 12 4.1.2 Centrifugal Separation and Vacuum Pyrolysis ............................................................................ 13

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4.1.3 Vacuum Pyrolysis and Mechanical Processing ........................................................................... 14 4.1.4 Supercritical Fluid ....................................................................................................................... 14 4.1.5 Bioleaching Processes ................................................................................................................. 16

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4.2 Physical Recycling Techniques .......................................................................................................... 18 4.2.1 Corona Discharge and Electrostatic Force................................................................................... 19 4.2.2 Magnetic Separation .................................................................................................................... 20

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4.2.3 Gravity Separation....................................................................................................................... 21 5. Applications of the Non-Metallic Fraction of PCB Waste (NMF) ........................................................... 22 5.1 Phenolic Molding ............................................................................................................................... 22

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5.2 Construction Industry Filler................................................................................................................ 22

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5.3 Polymer Composite Fillers ................................................................................................................. 23 5.4 Porous Material Production ................................................................................................................ 24 5.5 Adsorbent Production ......................................................................................................................... 25

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6. Conclusions .............................................................................................................................................. 27 Acknowledgement.................................................................................................................................... 28 References ................................................................................................................................................ 29

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1. Overview of E-Waste Statistics With advancements in the electronic world almost occurring on a day-to-day basis and

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increased availability of products to the public, the production of electrical and electronic

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devices has been one of the fastest-growing sectors and consequently, it is not surprising to see a staggering increase of electronic wastes over the past several decades. Thus the

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future handling and treatment of waste electric and electronic equipment (WEEE) or ewaste, is a topic of worldwide concern [1]. Undoubtedly, the global amount of WEEE

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being produced and in turn disposed of is sharply increasing [2]. Although it is hard to

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give an accurate estimate of the global e-waste production due to faulty and sometimes non-existent data, the UN estimate of the global WEEE production was 20-50 million

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tons per year [3]. According to Kiddee et al., 500 million computers were discarded

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between 1997 and 2007 in the United States and 610 million computers became obsolete in Japan by the end of 2010. Also the statistics indicate the catastrophic annual generation

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of more than 1.1million tons of e-waste in China, particularly from manufacturing industry, end-of-life appliances and imports from developed countries [4]. A more recent study by Dwivedy showed that the total WEEE amount in India in 2007-2011 is 2.5 million tons with an annual growth rate of e-waste being within 7-10%. Despite India being a signatory of the Basel Convention for Transboundary Movement of Hazardous Substances, there has been a spurt in such imports in the absence of proper regulations [5]. The exportation of e-waste from the United States to developing countries in contravention of the Basel Convention Agreement is currently receiving more attention

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[6]. Bilateral agreements have been signed between the US and the importing countries which allow the transfer of hazardous materials. It has been reported that around 80% of the US e-waste collected initially for recycling objectives is being exported to developing

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nations for backyard recycling practices. China being the center of the exports as well as

the informal recycling operations faces tragic health, safety and environmental issues due

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to the unregulated use of chemicals and wastes [7]. High blood levels of heavy metals in

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the residents of the recycling regions, elevated heavy metal content of the freshwaters and high dioxin quantity in the air validate the adverse effects of informal recycling in these

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zones [8]. Although the receiving countries endeavor to avoid this unfair trade, a growing amount of e-waste is imported to the developing countries each year. This could be due to

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the two-sided economic advantages offered by this unethical business.

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Besides all the hazards originating from e-waste, manufacturing mobile phones and

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personal computers consumes considerable fractions of the gold, silver and palladium mined annually world-wide [9]. Notably, these precious metals occur at concentrations

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more than ten-fold higher in PCBs than in commercially mined minerals. Hence, if PCBs were treated as mines to recover the precious metal within them, a smaller amount of energy would be required when compared to mining virgin materials [10]. Although only very limited amounts of most of these elements exist in each portable unit, the leverage of the soaring price of these metals and the total number of phones produced (more than 1.2 billion annually worldwide) is considerable [11].

In January 2003, The European Union Council addressed the serious issue of electronic waste streams by the Waste Electrical and Electronic Equipment (WEEE) Directive,

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which alongside the Restriction of Hazardous Substances (RoHS) Directive have become EU law as of February 2003. Today, these directives are in fact cornerstones regarding pre-production, production, and post-production of e-waste streams [12].

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Several countries around the world such as China, Brazil, Canada, the United States, and the UK, have in recent years followed the same legislative trends and have developed (or

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partially developed) e-waste laws and directives. However, currently the majority of the

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countries in the world such as India, Russia, New Zealand, and Iran, have no directives specifically concerning WEEE and at times have sufficed with amendments and clauses

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in other general bills and acts.

2. Composition of E-Waste-Printed Circuit Boards

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The definition of e-waste covers a vast array of consumer and business items of

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equipment the centre of which is the printed circuit board or PCB. The electronic and consumer devices include both large and small items ranging from fridges, washing

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machines, televisions, personal computers and laptops to the smaller items such as mobile phones, CD/DVD players, radios, shavers, modems and cameras. Although overall, the PCB fraction by weight represents approximately only 3 wt% of e-waste, the complex array of toxins present in these PCBs makes them very specific and hazardous wastes that must be treated most prudently [13].

2.1 Printed Circuit Board Assemblies

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Modern electronic devices can contain up to 60 different elements including valuable and hazardous materials. The most complex and valuable materials are found on printed circuit boards which will be further addressed [14].

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Printed circuit boards (PCBs) are in fact the platform upon which microelectronic components such as semiconductor chips and capacitors are mounted. They are used to

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support the electronic components as well as to connect them using conductive pathways,

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tracks or signal traces etched from copper sheets laminated onto them [15]. In literature, a PCB is also referred to as printed wiring board (PWB) or etched wiring board. A PCB

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populated with electronic components is a printed circuit assembly (PCA), also known as a printed circuit board assembly (PCBA). In this study the term PCB is used in place of

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PCBA unless otherwise stated. For the manufacturing of a PCB, conducting layers of thin

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copper foil and insulating dielectric composite fibers are used which are classified

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according to their grade material. For computers and communication equipment, FR-4 is the most common grade, while for home electronics and television, FR-2 is

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predominantly used. Notably FR stands for flame retardant and denotes the flammability safety of the woven fiberglass-reinforced epoxy laminates. Today, highvalue equipment increasingly contains FR-4 boards due to the high thermal resistance and infinitesimal water absorption. For temperature performance higher than that of FR-4, other resins such as polyimides, cyanates, PTFE, other fluoropolymers, epoxy-PPE blends, and even ceramics are often used [16]. Due to the risk that circuit boards might ignite due to high temperatures in the processing of the components and connections (i.e. during the soldering process), and also due to flammability risks as the result of electric energy impacts, materials

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with low flammability are required for the production of PCBs. Traditionally brominated fire retardants have been the most important and popular fire retardants in this sector [17]. Although phosphorous- or nitrogen-based fire

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retardants are available as an alternative and post-industrialized countries are

fire retardants are still overwhelmingly predominant [18].

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gradually moving towards ending halogenated fire retardants in PCBs, brominated

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A major issue derived from the advancements in the speed and functionality of components used on PCBs, is the availability of materials for the PCB substrate that

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are compatible with these products and their process needs. This includes the

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stresses created by higher temperatures during the assembly process. In addition, the expansion of the components and the substrate due to heat must match [19]. All

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these constraints further adds to the issue of the material complexity included in

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PCBs.

The majority of PCBs are made by bonding a layer of copper over the entire

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substrate. This copper can be applied to either one, or both sides of the substrate. Then, the unwanted copper is removed, leaving only the desired copper traces. A minority of boards are produced by directly adding traces of copper to the bare surface. The amount of copper used is in relation with the current that the conductor must carry [19].

Up to this point, no components are put into place. In order to populate the board, components must be attached by electrically and mechanically fixing them on with a molten metal solder. In the early years of microelectronics, the solder was most often a tin-lead alloy. However, with environmental legislation in the EU, Japan and 9

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the USA restricting the use of lead, many new solder compounds have been developed [20]. Since PCBs are manufactured in various types and sizes, ranging from single-layered

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to multi-layered, and single sided to double sided, and that the components placed on each PCB also can be variant both in function and in material, any “average”

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given of the constituent materials or of the size and weight of PCBs must be

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approached with caution. Average values are often very much dependent on the boards under study, but Parsons [21] has provided general approximate average

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compositions for PCBs as indicated in Table 1.

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(Table 1)

Again, it should be noted that these values should be considered with caution due to

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composition values.

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the variety of PCBs and the difficulty of providing an accurate estimate for

As previously mentioned, a large fraction of the WEEE precious metals is found on

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the printed circuit boards (PCBs). One metric ton of circuit boards can contain between 80 and 1,500 g of gold and between 160 and 210 kg of copper. To put this number into context, it should be stated that these concentrations are 40 to 800 times the amount of gold in gold ore, and 30 to 40 times the concentration of copper in copper ore mined in the United States [22]. Almost all electric and electronic equipment have printed circuit boards. It has been reported that printed circuit boards constitute around 3 wt% of all WEEE produced [15]. This number is only an average - the weight percent of the printed circuit board in a mobile phone, for example, being much higher than that of a dishwasher. 10

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PCBs represent the most economically attractive portion of WEEE. Yet, the fact that such a highly complex concoction of various valuable and sometimes hazardous materials are intermingled in such a small volume, poses serious engineering

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challenges for the recovery and recycling of the constituent materials. The heterogeneous mix of organics, metals, fiber glass, toxic materials including heavy

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metals, and plastics makes the PCB processing a challenging task.

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3. Environmental Implications of E-Waste Disposal

According to the US Environmental Protection Agency, 80-85% by weight of e-waste

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was traditionally destined for landfills whose leachate undoubtedly contaminates the soil and groundwater in adjacent regions [23–30]. Spalvins et al. [31] verified the existence of

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lead at higher concentrations when electronic waste was mixed with municipal solid

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waste due to the more aggressive environmental of the acidic leachate for lead leaching.

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Also, the transfer of brominated flame retardants (BFRs) from e-waste containing simulated landfill to water and soil has been explored by Danon-Schaffer et al. [32] where their simulation results indicate the existence and persistence of polybrominated diphenyl ethers (PBDEs) in landfills. The next major disposal route is via incineration and a number of problems have been identified including estrogenic compounds [33,34] and the presence of chlorinated dioxins PCDD/Fs and PBDD/Fs [35–38] in the emissions. Vehlow et al. [39] demonstrated that co-combustion of e-waste with municipal solid waste resulted in the formation brominated and chlorinated dioxins and furans with the furans exceeding the dioxins by a factor of 3 to 4. In addition, the fate of heavy metals in a pilot scale e-waste-

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containing incinerator has been simulated by Long et al. [40]. It was shown that the heavy metals were enriched in fly and bottom ashes after incineration. Also, the vaporization of several heavy metals led to their emission in the exhaust gas.

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Despite the high value of precious metals in PCB e-waste, the high level of toxic

materials and separation technologies has limited the exploitation of materials recovery

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from PCB waste.

4. Waste PCB Recycling

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4.1 Chemical Recycling Techniques

In this type of recycling, the printed circuit boards are depolymerized into smaller useful

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molecules by several techniques, such as pyrolysis, gasification or application of

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supercritical fluids. The obtained products (fuels and gases) are refined by conventional

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approaches and the metallurgical approaches are employed for the treatment of the metallic fraction.

4.1.1 Vacuum Pyrolysis

Vacuum pyrolysis has been investigated by a large number of researchers because of the advantage of the low pressures and temperatures applied. Under these conditions, the organics are distilled off as gases and liquids, but do not undergo cracking decomposition. They can be condensed and collected as fuel to sustain the heat energy required for vacuum pyrolysis or have the potential to be used as chemical feedstock [41– 45]. However, the solid residue still contains metals with the non-metallic glass fibers 12

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and requires further processing. Furthermore, there is extensive evidence that the pyrolysis processes destroy the brominated flame retardants yielding hydrogen bromide and organobrominated compounds [46–49]. Zhou et al. [50] demonstrated the pyrolysis

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products of two types of waste printed circuit boards. They showed that, in both cases, 20-30 wt% oil and 4-6 wt% gas were achieved which could be used as fuel, while the

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pyrolysis residue contained various metals, glass fibers and other inorganic materials

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which could be recycled for further processing and use. The analysis of the fuel by Long et al. [51] revealed that the obtained oil was mainly composed of phenolic and furanic

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compounds and the recycled gas consisted of carbon monoxide, carbon dioxide, methyl bromide, hydrogen bromide and several alkanes and alkenes. Yang et al. [52] suggested

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the use of the heavy fraction of the pyrolysis oil derived from waste printed circuit boards

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as asphalt modifier. They showed that the physical and water resistance properties of

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asphalt can be enhanced by incorporating this oil into asphalt. They attributed this phenomenon to the formation of a chemical network structure between the phenol

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substitutes with asphalt.

4.1.2 Centrifugal Separation and Vacuum Pyrolysis A novel two step separation process has been proposed involving centrifugal separation and vacuum pyrolysis [53,54]. In the first stage, waste PCBs were heated to a temperature around 240˚C by immersing in diesel oil in a rotating drum. Non-condensing pyrolysis gas was driven off and collected and the solder was the only component melting at this temperature and was separated by the rotating centrifugal force of the drum leaving a residue without solder. In the second stage, the residue was placed in the

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pyrolysis reactor and a vacuum pressure lower than 1.5 kPa was applied. The reactor was heated to 600˚C using the pyrolysis gas from the first stage and the furnace was held at this temperature for a specific time. Liquids and gases were condensed and collected. The

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role of the vacuum centrifugal separation was the separation of solder from the base plate using the low melting point of the solder compared with rest of the pyrolysis residues.

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Zhou et al. [50,55] showed that temperature and rotational speed were two critical factors

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that affected the solder removal efficiency. The experimental results indicated that almost all solder was separated from the pyrolysis residue when the temperature was 400 ˚C, and

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the rotating drum was rotated at 1200 rpm for 10 min.

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4.1.3 Vacuum Pyrolysis and Mechanical Processing

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Long et al. [51] showed that when vacuum pyrolysis is coupled with various mechanical

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separation methods, the separation of different components were conducted efficiently. The four stage separation process begins with an initial cutting of the waste PCB material

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followed by a vacuum pyrolysis process producing an oil and a gas with the majority being a solid residue which undergoes crushing and size classification [51]. These fractions of classified pyrolysis residues were separated into a light fraction of mostly non-metallic components and a heavy fraction of copper by gravity separation using a vertical zig-zag air flow separator. This method yielded reasonably high quality metallic and non-metallic fractions.

4.1.4 Supercritical Fluid

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The novel method of using supercritical fluids for the metal – nonmetal separation is attracting more attention. Supercritical water exhibit diminished hydrogen bonding characteristics at its supercritical conditions allowing organic species, oxygen and water

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to form a homogeneous phase, resulting in a more efficient oxidation due to the removal of the mass transport limitations. Chien et al. [56] indicated that the oxidation of waste

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printed circuit boards was highly enhanced in the presence of sodium hydroxide, where

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the major fraction of bromine was remained in the liquid phase, whereas copper remained in the solid residue as copper oxide and copper hydroxide. Xing and Zhang [57]

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succeeded in complete decomposition of brominated epoxy resins to HBr enriched in water by controlling the temperature, water content and holding time. They demonstrated

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that sub- and supercritical water treatment methods led to the efficient separation of glass

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fibers and copper, respectively. Xiu and Zhang [58] combined the supercritical

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degradation with an electrokinetic process and recovered copper and lead under optimum supercritical and electrokinetic conditions. Both the recovery rate and purity of the

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reclaimed metal ions were shown to be sufficiently high. In this process, copper migrated to the cathode compartment and deposited on the cathode, whereas lead moved towards either anode or cathode and little was deposited on the cathode. This allowed the efficient separation of the metals from waste printed circuit boards together with the degradation of brominated compounds. A similar study was conducted by Xiu et al. [58], where they used acid leaching for metals separation. Besides supercritical water, other supercritical fluids were also employed for the recycling of PCBs. Sanyal et al. [59] used supercritical carbon dioxide as solvent with an additional small amount of water to separate the PCB components into copper foil, glass

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fiber and polymer. The lack of formation of hydrocarbons and noxious substances, commonly observed in PCB pyrolysis, strengthens the environmentally-friendly nature of supercritical carbon dioxide recycling process. Wang et al. [60] investigated the

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extraction of flame retardants from waste PCBs using supercritical carbon dioxide and

demonstrated the high efficiency of the process. Supercritical methanol used by Xiu and

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Zhang [61] yielded phenol-containing oil and bromine-containing gas. Under low

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treatment temperatures, the oil contained considerable amount of flame retardants, whereas high temperatures resulted in the complete decomposition of flame retardants.

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Also, HBr could be recovered from the gas for further reuse. Moreover, a high content of

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metals was retained in the solid residue.

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4.1.5 Bioleaching Processes

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Although predominantly targeting the valuable metal fraction recovery from waste PCB, bioleaching could benefit the materials recovery from e-waste in two ways. The use of

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microorganisms to extract metals by generating weaker organic acids will save on the manufacture of the currently used strong inorganic acids for metal leaching and also save the environment significantly in terms of treating and disposing of strong inorganic acid waste compared to the weaker and more readily treatable organic acids generated by the microorganism cultures. Furthermore, as the field of bioleaching develops it should be possible to develop strains to target specific metals and therefore perform selective metal extraction from the wastes thus minimizing further treatment technologies and further reducing pollution. The removal of the metallic components via bioleaching will then leave the nonmetallic fraction for processing with relatively low contamination from the

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weak organic leaching acids. There is very extensive research currently being undertaken in bioleaching as it has implications far beyond treating waste PCBs, including, the mining industry and the treatment of other wastes containing metals [62–68].

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Choi et al. [69] studied the bioleaching of copper present in waste PCBs using

Acidithiobacillus ferrooxidans and showed that the copper content leached from this

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waste increased as the amount of the ferrous ion increased in the solution up to 7 g.L-1.

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They attributed this phenomenon to the oxidation potential of ferrous ion according to the

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following reaction:

(1)

The direct chemical leaching out of a part of copper was also presumed to partially assist

(2)

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the process:

reaction (2).

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The increase in the pH of the leachate confirmed the formation of hydroxide ions through

can be oxidized again to

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It has been pointed out that the produced

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presence of A. ferrooxidans as follows [66,70]: (3)

In addition, a large portion of the leached copper ions was found to be fixed in the precipitate. Therefore, it was observed that the addition of citric acid, as a complexing agent, raised the solubility of the leached metal ions and increased the amount of copper in the solution rather than the precipitate. Yang et al. [70] investigated the factors influencing the copper bioleaching and found that process variables such as ferrous ion concentration, pH level of the medium and stock solution quantity affects the copper

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bioleaching significantly. Similar findings were reported by other researchers [71,72]. Liang et al. [66] investigated the effect of the mixed culture of two acidophiles, namely Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans, on the bioleaching of

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copper, nickel, zinc and lead and found that the extraction efficiency of all the metals

were enhanced when mixed culture were applied compared to the individual cultures.

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They attributed this bioleaching enhancement to the increased redox potential and

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lowered pH value in the case of mixed culture.

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4.2 Physical Recycling Techniques

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The drive to recover the valuable metals in particular gold, silver, palladium and copper has received tremendous attention in recent years using extraction processes such as

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leaching, mechanical and hydrometallurgical processing techniques [73–79]. In addition

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several review articles focusing on metal recovery for re-use are also available [80–83]. Because of the large amount of literature and reviews on metal recovery, the remainder of

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this review will concentrate on methods of separating the non-metallic fraction of PCB waste from the metallic fraction and the potential applications of this non-metallic fraction, NMF.

Averaging the compositions of a range of waste PCBs, the main components and their percentages are shown in Table 2 [84]. (Table 2) Despite the great desire to recover the metals, frequently carried out at the disregard and untreated disposal of most of the non-metallic fraction, NMF, it can be seen that the NMF is of the order of 70% by weight of the waste PCB. Although still in its infancy, the more

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recent separation technologies are now considering this major NMF fraction in their designs and operation, realizing that value-added applications for NMF will appear in the future. We will now review these more recent approaches to separating the NMF from

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the metallic fraction before further recovery and purification processes take place.

Several methods involving mechanical-physical separation have been reported. The

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techniques include magnetic separation, size separation, density based separation,

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electrical conductivity and combinations of these. These processes usually rely on the physical differences between the metallic and nonmetallic fractions of the waste PCBs

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[81,85–88].

4.2.1 Corona Discharge and Electrostatic Force

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The corona-electrostatic method is perhaps the most effective separation technology for

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the metallic and non-metallic fractions at present [89–93]. The method has the advantage that it is environmentally friendly, producing no wastewater and no gaseous emissions.

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The PCBs with the metallic components removed must be reduced to very small particles which can be achieved by accelerating them at high speed to impact on a hardened plate. Then the small particles, typically less than 0.6 mm are passed along a vibratory feeder to a rotating roll to which is applied a high voltage electrostatic field using a corona and an electrostatic electrode [94]. The non-metallic particles become charged and remain attached to the drum eventually falling off into storage bins; whereas the metallic particles discharge rapidly in the direction of an earthed electrode. It has been found that particle sizes of 0.6 – 1.2 mm is the most suitable size for separation in industrial applications. Therefore, a two-step crushing process has been

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proposed to achieve this particle size [91]. Li et al. [92] found that as the angle of the static electrode reduced and the corona electrode angle was increased, the separation efficiency was enhanced. It was reported that applied voltage of 20-30 kV, center

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distance of 21 cm, static electrode radius of 1.9 cm, corona wire radius of 11.4 cm, static electrode angle of 20˚ and corona electrode angle of 60˚ were the optimum operating

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parameters influencing the separation efficacy. Considerable work is continuing in this

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area with particular focus on the electrostatic behavior of the system and the field

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intensity [95–97].

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4.2.2 Magnetic Separation

This process is widely used to separate the ferromagnetic metals from non-magnetic

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wastes. Although a magnet can simply be used for this purpose, there are some problems

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associated with this method. One of the major issues is the agglomeration of the particles which results in the attraction of some nonferrous fraction (such as NMF) attached to the

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ferrous fraction. This will lead to the low efficiency of this method. However, several authors have attempted in optimizing the efficiency of magnetic separation methods [89]. Veit et al. [89] employed a magnetic field of 6000-6500 G to separate the ferromagnetic elements, such as iron and nickel. The chemical concentration of the magnetic fraction was 43% Fe and 15.2% Ni on average. However, there was a considerable amount of copper impurity in the magnetic fraction as well. Yoo et al. [98] used a two-stage magnetic separation. In the first stage, a low magnetic field of 700 G was applied which led to the separation of 83% of nickel and iron in the magnetic fraction and 92% of copper in the non-magnetic fraction. The second magnetic separation stage was

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conducted at 3000 G which resulted in a reduction in the grade of the nickel-iron concentrate and an increase in the copper concentrate grade.

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4.2.3 Gravity Separation

Gravity separation is based on the separation of the materials according to their different

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specific gravities. The relative movement of the materials relative to gravity and external

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forces such as fluid flow causes the separation of the components. Nonetheless, this separation is not only dependent on the density of the components, but also on their size.

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Hence, in order to have a proper separation, the size factor should be excluded by controlling the particle sizes [85].

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The principle of the air classification technique is based on the suspension of the particles

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in a flowing air stream and the separation of the particles based on their density

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difference. The particles experience two forces in this approach acting in opposite directions; gravity forces and drag forces. When the density of the particle is low, the

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gravity force dominates the drag force and thus the particle moves downwards, whereas high particle density results in the dominance of the latter and upper movement of the particle. Accordingly, the particles with different densities can be separated [99]. Zheng et al. used an air classification technique for the separation of metals and nonmetals and found that the maximum copper content in the nonmetallic fraction was only 1.6% [100]. One of the major disadvantages of this method is the simultaneous difference of particle size and density. Long et al. confirmed the great dependence of air classification method on particle size. It was demonstrated that the separation of copper into the low-density fraction was enhanced as the particle size increased. When the particle size of the crushed

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material was smaller than 0.45 mm, the grade of the copper was drastically decreased. Several applications of the non-metallic materials fractions recovered by this technique

ip t

will be discussed later.

cr

5. Applications of the Non-Metallic Fraction of PCB Waste (NMF) 5.1 Phenolic Molding

us

Phenolic molding compounds (PMC) are produced from phenolic resins for various

an

applications under high temperatures and pressures. Wood flour is the most common organic filler used in the production of PMCs. With the depletion of wood resources and

M

its increasing cost, it is an urgent assignment to find alternatives to this filler. The nonmetallic glass fiber fraction reclaimed from printed circuit board scraps has been

d

recently applied in phenolic molding compounds [101,102]. Guo et al. [103] have

te

partially replaced the wood flour with reclaimed nonmetallic fraction of PCBs. The

Ac ce p

theory was based on the applying a shear force for a mixture of NMF and a crosslinking substance. Nevertheless, due to the production of volatile gases, primarily phenols, during the process and consequent creation of voids, the flexural strength and the dielectric strength reached their minimum values at a certain NMF content. Also, the ability of the composite for the flow was drastically decreased by increasing the NMF content in the composite. They also showed that in order to obtain the most desirable mechanical properties, the particle size of NMF particles should not exceed 70 µm [104].

5.2 Construction Industry Filler

22

Page 22 of 42

This non-metallic glass fiber/resin fraction has been used in a range of applications as fillers [105–107]. Another growing application is the use of the nonmetallic fraction in the construction industry in the production of wood plastic composite [108,109]. It has

ip t

been incorporated as a filler in asphalt and in concrete [2,110–112]. Researchers have sought improved construction materials with better mechanical strength, less

cr

environmental impact, and less cost. The compressive and flexural strengths are the two

us

most critical properties that must be considered for construction materials. The small particle size of the nonmetallic fraction of PCBs coupled with the coarse glass fibers

an

makes both the microstructure and mechanical strength superior. Although the introduction of NMF as filler in asphalt improved the elasticity and stiffness of the

M

prepared composite, asphalt ductility was considerably decreased due to the existence of

d

the stress concentration between the asphalt and the NMF. Also, the increase in the

te

viscosity of the composite resulted in a great challenge in its flow [111]. Wang et al. [113] also pointed out some of the disadvantages of the incorporation of

Ac ce p

NMF in cement mortar. They demonstrated that the compressive strength, flexural strength and tensile bond strength of the composites were significantly decreased by the addition of NMF.

5.3 Polymer Composite Fillers The re-use of non-metals recycled from the waste PCBs is growing steadily as more applications are developed. The use of the glass fiber non-metallic fraction can be incorporated into polyester composite as a reinforcing filler [114] and similar success has been achieved by their inclusion on polypropylene composites [100,115]. Zheng et al.

23

Page 23 of 42

have utilized the silane coupling agent- modified NMF as reinforcing filler in thermoplastic polypropylene (PP). They have observed that the tensile and flexural properties of the composite were greatly enhanced by the addition of modified NMF.

ip t

They assigned this improvement to the inherent properties of fiberglass in NMF, such as high length-to-diameter ratio, high elastic modulus and low elongation. They also

cr

reported the considerable influence of the NMF particle size on the mechanical properties

us

of the composite. The enhancement in the mechanical properties by using smaller filler particle sizes was attributed to the transfer of the stress from the matrix to the filler

an

particles, thereby resulting in higher tensile and flexural strengths [100,116,117]. The environmental hazards of using the composite have been evaluated by copper and

M

bromine leaching test. It was shown that the leaching of copper complies with the

te

bromine [100].

d

identification standards for hazardous wastes, whereas there is no relative measure for

Similar experiments were conducted by Xu et al. where pimelic acid-modified NMF was

Ac ce p

incorporated into polypropylene [118]. They revealed that the amount of pimelic acid used for the modification of NMF has a significant effect on the properties of the ultimate composite due to the α to β transformation of crystals and induction of β-crystal formation due to the surface effects of PA on the crystals nucleation.

5.4 Porous Material Production Ke et al. [119] have produced porous carbons from nonmetallic fraction of FR-3 type waste PCBs via physical and chemical activation techniques. It has been shown that chemical activation technique has resulted in the production of enormously high surface

24

Page 24 of 42

area activated carbons, while the surface areas of the materials obtained by physical activation were also acceptable. Also, the employment of chemical activation methods at temperatures as high as 900˚C induced the creation of a mesoporous structure in the

ip t

activated carbon. It was reported that the decomposition of the resin and the production

of volatile compounds, such as carbon monoxide and dioxide and hydrogen bromide, was

cr

responsible for the low yield of the carbons.

us

Also, recovered fiberglass from NMF with a porosity of 94-95% and a thickness of 15-60 mm was used as sound absorber [120]. It was demonstrated that all the best-performing

an

samples could absorb the incident sound energy at the corresponding frequencies. Outstanding sound attenuation property of this material was attributed to the

M

interconnecting micro-voids in the internal structure of the material. The transmission of

d

the incident sound waves into the voids and the visco-thermal effect via the friction

Ac ce p

conversion to heat.

te

between the air and fibers brought about the dissipation of the sound energy and its

5.5 Adsorbent Production

A recent development in the utilization of NMF is the production of an adsorbent/exchange resin type material by chemical activation of the non-metallic fraction of the waste PCB [13]. The NMF was impregnated in a caustic solution at a certain ratio and was subsequently activated at relatively high temperatures. The produced material generated a surface area of over 200 m2/g compared with a surface area of less than 1 m2/g for the original material. Also, the surface properties of the original and the activated materials showed that the original material had no active

25

Page 25 of 42

functional groups, while a considerable amount of hydroxyl moieties were observed on the surface of the activated material. Also, surface analysis results confirmed the doping of potassium onto the porous adsorbent which was hypothesized to function as ion

ip t

exchanger. A comprehensive study of the properties of this material has been described elsewhere [121]. The activated adsorbent has been used to remove single component

cr

heavy metal ions from water [122,123] and also it can separate binary mixtures of metals

us

by selective or simultaneous adsorption [124,125]. Figure 1 shows the adsorption capacities of the e-waste derived resin for several single component metal ions [126]. It

an

was demonstrated that the single-component heavy metal adsorption capacities of the activated material was significantly high and not only higher than the original material,

M

but also much beyond those of several widely-used commercial adsorbents/ion

d

exchangers. Adsorption capacities of the activated material were in the range of 2-4

te

mmol metal ion/g activated material depending on the type of the metal to be adsorbed [122,123,125], while most commercial resins have capacities in the region of 2.0 to 2.2

Ac ce p

mmol metal ion/g resin for almost all the metals. Also, it was confirmed that the adsorption efficiency of this material does not decrease in multi-component systems and, in some cases, its capacity is even enhanced due to the synergistic effect of the two metals [124,125]. Depending on the difference between the properties of the heavy metals in multi-component systems, simultaneous or selective adsorption might occur [124,125]. This is one of the few applications of the non-metallic waste PCB fraction that could attract a high value if it succeeds through to commercialization. (Figure 1)

26

Page 26 of 42

6. Conclusions For several years, waste PCBs have been poorly managed. The valuable metal components have been recovered using environmentally-unfriendly strong inorganic acid

ip t

leaching processes or pyrolysis and hydrometallurgical processes which emit eco-

cr

unfriendly gaseous pollutants. The 70% by weight nonmetallic fraction has been

traditionally discarded to landfill or used as very low cost fillers in the construction

us

industry. The present review indicates that while substantial research needs to be done to pave the way forward for successful, environmentally friendly and economic waste PCB

an

recycling, significant progress has been made both in the methods for separating PCB

M

waste into its metallic and nonmetallic fractions but also in identifying more economically attractive uses for the 70% by weight nonmetallic component of e-waste.

Corona electrostatic methods are now capable of producing two streams from PCB

te

-

d

In terms of separation technologies:

waste comprising a metallic and a nonmetallic portion with little cross-contamination;

Ac ce p

the method is dry at room temperature and as such is almost zero polluting depending on the quality of the dust extraction system; -

Supercritical fluids may eventually offer an attractive separation process by selectively dissolving out fractions of PCB waste using different solvents at room temperatures and effective solvent recovery systems would reduce pollution to almost zero levels depending on the vapor recovery process;

-

Bioleaching has considerable potential to offer by selectively extracting the metals at low temperatures by microorganism generated organic acids, thus significantly

27

Page 27 of 42

reducing the pollution from strong acid leaching and also leaving an unpolluted nonmetallic residue for further processing.

ip t

In terms of the research into the utilization of the 70% w/w nonmetallic fraction of PCB waste, the potential opportunities to generate and recycle value added products has not

Incorporating the nonmetallic fraction into plastic moulds has a higher value than

us

-

cr

made the same progress and is limited to:

when it is used as a filler in cement and asphalt industries;

The application in the production of adsorbent/resins for water treatment applications

an

-

would represent a major breakthrough if scale-up succeeds; Research into the production of silicon or the recovery of silica would also be

M

-

d

attractive opportunities for new research on nonmetallic e-waste and provide

Ac ce p

te

sustainable recycling into the microelectronics industry if successful.

Acknowledgement

The authors would like to thank the Hong Kong Research Grant Council for their support of this research.

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References J. Cui, L. Zhang, Metallurgical recovery of metals from electronic waste: a review., J. Hazard. Mater. 158 (2008) 228–56.

[2]

X. Niu, Y. Li, Treatment of waste printed wire boards in electronic waste for safe disposal., J. Hazard. Mater. 145 (2007) 410–6.

[3]

United Nations Environment Programme, E-waste: The Hidden Side of IT Equipment’s Manufacturing and Use, 2005.

[4]

P. Kiddee, R. Naidu, M.H. Wong, Electronic waste management approaches: an overview., Waste Manag. 33 (2013) 1237–50.

[5]

M. Dwivedy, R.K. Mittal, An investigation into e-waste flows in India, J. Clean. Prod. 37 (2012) 229–242.

[6]

O. Ogunseitan, The Basel Convention and e-waste: translation of scientific uncertainty to protective policy, Lancet Glob. Heal. 1 (2013) 313–314.

[7]

R. Kahhat, J. Kim, M. Xu, B. Allenby, E. Williams, P. Zhang, Exploring e-waste management systems in the United States, Resour. Conserv. Recycl. 52 (2008) 955–964.

[8]

T. Fujimori, H. Takigami, T. Agusa, A. Eguchi, K. Bekki, A. Yoshida, et al., Impact of metals in surface matrices from formal and informal electronic-waste recycling around Metro Manila, the Philippines, and intra-Asian comparison., J. Hazard. Mater. 221-222 (2012) 139–46.

cr

us

an

M

d

te

Ac ce p

[9]

ip t

[1]

L. Jing-ying, X. Xiu-li, L. Wen-quan, Thiourea leaching gold and silver from the printed circuit boards of waste mobile phones., Waste Manag. 32 (2012) 1209–12.

[10]

P. Wäger, R. Hischier, M. Eugster, Environmental impacts of the Swiss collection and recovery systems for Waste Electrical and Electronic Equipment (WEEE): a follow-up., Sci. Total Environ. 409 (2011) 1746–56.

[11]

A. Behnamfard, M.M. Salarirad, F. Veglio, Process development for recovery of copper and precious metals from waste printed circuit boards with emphasize on palladium and gold leaching and precipitation., Waste Manag. 33 (2013) 2354–63.

[12]

A. Gottberg, J. Morris, S. Pollard, C. Mark-Herbert, M. Cook, Producer responsibility, waste minimisation and the WEEE Directive: case studies in ecodesign from the European lighting sector., Sci. Total Environ. 359 (2006) 38–56. 29

Page 29 of 42

P. Hadi, P. Gao, J.P. Barford, G. McKay, Novel application of the nonmetallic fraction of the recycled printed circuit boards as a toxic heavy metal adsorbent., J. Hazard. Mater. 252-253 (2013) 166–70.

[14]

W. Rankin, Minerals, Metals and Sustainability: Meeting Future Material Needs, 2011.

[15]

A. Canal Marques, J.-M. Cabrera, C.D.F. Malfatti, Printed circuit boards: a review on the perspective of sustainability., J. Environ. Manage. 131 (2013) 298–306.

[16]

R. Sanapala, Characterization of FR-4 printed circuit board laminates before and after exposure to lead-free soldering conditions, 2008.

[17]

G. Grause, M. Furusawa, A. Okuwaki, T. Yoshioka, Pyrolysis of tetrabromobisphenol-A containing paper laminated printed circuit boards., Chemosphere. 71 (2008) 872–8.

[18]

M. Goosey, R. Kellner, Recycling technologies for the treatment of end of life printed circuit boards (PCBs), Circuit World. 29 (2003) 33.

[19]

K. Mitzner, PCB Design for Signal Integrity, in: Complet. PCB Des. Using OrCad Capture Layout, 2007: pp. 109–166.

[20]

Y. Xia, X. Xie, Reliability of lead-free solder joints with different PCB surface finishes under thermal cycling, J. Alloys Compd. 454 (2008) 174–179.

[21]

D. Parsons, Printed circuit board recycling in Australia, in: 5th Aust. Conf. Life Cycle Assess., 2006: pp. 22–24.

Ac ce p

te

d

M

an

us

cr

ip t

[13]

[22]

E. Grossman, High Tech Trash: Digital Devices, Hidden Toxics, and Human Health, 2007.

[23]

Z. Jun-hui, M. Hang, Eco-toxicity and metal contamination of paddy soil in an ewastes recycling area., J. Hazard. Mater. 165 (2009) 744–50.

[24]

A. Arukwe, T. Eggen, M. Möder, Solid waste deposits as a significant source of contaminants of emerging concern to the aquatic and terrestrial environments - A developing country case study from Owerri, Nigeria., Sci. Total Environ. 438C (2012) 94–102.

[25]

S.E. Musson, K.N. Vann, Y.-C. Jang, S. Mutha, A. Jordan, B. Pearson, et al., RCRA toxicity characterization of discarded electronic devices., Environ. Sci. Technol. 40 (2006) 2721–6.

30

Page 30 of 42

M.S. Noon, S.-J. Lee, J.S. Cooper, A life cycle assessment of end-of-life computer monitor management in the Seattle metropolitan region, Resour. Conserv. Recycl. 57 (2011) 22–29.

[27]

C. Luo, C. Liu, Y. Wang, X. Liu, F. Li, G. Zhang, et al., Heavy metal contamination in soils and vegetables near an e-waste processing site, South China., J. Hazard. Mater. 186 (2011) 481–90.

[28]

P.-Y. Liu, Y.-X. Zhao, Y.-Y. Zhu, Z.-F. Qin, X.-L. Ruan, Y.-C. Zhang, et al., Determination of polybrominated diphenyl ethers in human semen., Environ. Int. 42 (2012) 132–7.

[29]

C.S.C. Wong, S.C. Wu, N.S. Duzgoren-Aydin, A. Aydin, M.H. Wong, Trace metal contamination of sediments in an e-waste processing village in China., Environ. Pollut. 145 (2007) 434–42.

[30]

O. Tsydenova, M. Bengtsson, Chemical hazards associated with treatment of waste electrical and electronic equipment., Waste Manag. 31 (2011) 45–58.

[31]

E. Spalvins, B. Dubey, T. Townsend, Impact of electronic waste disposal on lead concentrations in landfill leachate., Environ. Sci. Technol. 42 (2008) 7452–8.

[32]

M.N. Danon-Schaffer, A. Mahecha-Botero, J.R. Grace, M. Ikonomou, Mass balance evaluation of polybrominated diphenyl ethers in landfill leachate and potential for transfer from e-waste., Sci. Total Environ. 461-462 (2013) 290–301.

[33]

X. Bi, B.R.T. Simoneit, Z. Wang, X. Wang, G. Sheng, J. Fu, The major components of particles emitted during recycling of waste printed circuit boards in a typical e-waste workshop of South China, Atmos. Environ. 44 (2010) 4440–4445.

Ac ce p

te

d

M

an

us

cr

ip t

[26]

[34]

C. Owens, C. Lambright, K. Bobseine, B. Ryan, L. Gray, B. Gullett, et al., Identification of estrogenic compounds emitted from the combustion of computer printed circuit boards in electronic waste, Environ. Sci. Technol. 41 (2007) 8506– 11.

[35]

S. Wen, F. Yang, J.G. Li, Y. Gong, X.L. Zhang, Y. Hui, et al., Polychlorinated dibenzo-p-dioxin and dibenzofurans (PCDD/Fs), polybrominated diphenyl ethers (PBDEs), and polychlorinated biphenyls (PCBs) monitored by tree bark in an Ewaste recycling area., Chemosphere. 74 (2009) 981–7.

[36]

H. Duan, J. Li, Y. Liu, N. Yamazaki, W. Jiang, Characterization and inventory of PCDD/Fs and PBDD/Fs emissions from the incineration of waste printed circuit board., Environ. Sci. Technol. 45 (2011) 6322–8.

[37]

W.J. Hall, P.T. Williams, Pyrolysis of brominated feedstock plastic in a fluidised bed reactor, J. Anal. Appl. Pyrolysis. 77 (2006) 75–82. 31

Page 31 of 42

H. Duan, J. Li, Y. Liu, N. Yamazaki, W. Jiang, Characterizing the emission of chlorinated/brominated dibenzo-p-dioxins and furans from low-temperature thermal processing of waste printed circuit board., Environ. Pollut. 161 (2012) 185–91.

[39]

J. Vehlow, B. Bergfeldt, K. Jay, H. Seifert, T. Wanke, F.E. Mark, Thermal treatment of electrical and electronic waste plastics, Waste Manag. Res. 18 (2000) 131–140.

[40]

Y.-Y. Long, Y.-J. Feng, S.-S. Cai, W.-X. Ding, D.-S. Shen, Flow analysis of heavy metals in a pilot-scale incinerator for residues from waste electrical and electronic equipment dismantling., J. Hazard. Mater. 261 (2013) 427–34.

[41]

A.M. Cunliffe, N. Jones, P.T. Williams, Recycling of fibre-reinforced polymeric waste by pyrolysis : thermo-gravimetric and bench-scale investigations, J. Anal. Appl. Pyrolysis. 70 (2003) 315–338.

[42]

J. Moltó, R. Font, A. Gálvez, J. Conesa, Pyrolysis and combustion of electronic wastes, J. Anal. Appl. Pyrolysis. 84 (2009) 68–78.

[43]

W.J. Hall, P.T. Williams, Separation and recovery of materials from scrap printed circuit boards, Resour. Conserv. Recycl. 51 (2007) 691–709.

[44]

G. Jie, L. Ying-Shun, L. Mai-Xi, Product characterization of waste printed circuit board by pyrolysis, J. Anal. Appl. Pyrolysis. 83 (2008) 185–189.

[45]

H.-L. Chiang, K.-H. Lin, M.-H. Lai, T.-C. Chen, S.-Y. Ma, Pyrolysis characteristics of integrated circuit boards at various particle sizes and temperatures., J. Hazard. Mater. 149 (2007) 151–9.

Ac ce p

te

d

M

an

us

cr

ip t

[38]

[46]

F. Barontini, V. Cozzani, Formation of hydrogen bromide and organobrominated compounds in the thermal degradation of electronic boards, J. Anal. Appl. Pyrolysis. 77 (2006) 41–55.

[47]

M. Blazso, Z. Cze, C. Csoma, Pyrolysis and debromination of flame retarded polymers of electronic scrap studied by analytical pyrolysis, J. Anal. Appl. Pyrolysis. 64 (2002) 249–261.

[48]

M.P. Luda, N. Euringer, U. Moratti, M. Zanetti, WEEE recycling: Pyrolysis of fire retardant model polymers., Waste Manag. 25 (2005) 203–8.

[49]

Y.C. Chien, H.P. Wang, K.S. Lin, Y.J. Huang, Y.W. Yang, Fate of bromine in pyrolysis of printed circuit board wastes., Chemosphere. 40 (2000) 383–7.

32

Page 32 of 42

Y. Zhou, W. Wu, K. Qiu, Recovery of materials from waste printed circuit boards by vacuum pyrolysis and vacuum centrifugal separation., Waste Manag. 30 (2010) 2299–304.

[51]

L. Long, S. Sun, S. Zhong, W. Dai, J. Liu, W. Song, Using vacuum pyrolysis and mechanical processing for recycling waste printed circuit boards., J. Hazard. Mater. 177 (2010) 626–32.

[52]

F. Yang, S. Sun, S. Zhong, S. Li, Y. Wang, J. Wu, Performance of the heavy fraction of pyrolysis oil derived from waste printed circuit boards in modifying asphalt., J. Environ. Manage. 126 (2013) 1–6.

[53]

Y. Zhou, K. Qiu, A new technology for recycling materials from waste printed circuit boards., J. Hazard. Mater. 175 (2010) 823–8.

[54]

Y. Zhou, W. Wu, K. Qiu, Recovery of materials from waste printed circuit boards by vacuum pyrolysis and vacuum centrifugal separation., Waste Manag. 30 (2010) 2299–304.

[55]

Y. Zhou, W. Wu, K. Qiu, Recycling of organic materials and solder from waste printed circuit boards by vacuum pyrolysis-centrifugation coupling technology., Waste Manag. 31 (2011) 2569–76.

[56]

Y.-C. Chien, H.P. Wang, K.-S. Lin, Y.. Yang, Oxidation of printed circuit board wastes in supercritical water, Water Res. 34 (2000) 4279–4283.

[57]

M. Xing, F.-S. Zhang, Degradation of brominated epoxy resin and metal recovery from waste printed circuit boards through batch sub/supercritical water treatments, Chem. Eng. J. 219 (2013) 131–136.

Ac ce p

te

d

M

an

us

cr

ip t

[50]

[58]

F.-R. Xiu, Y. Qi, F.-S. Zhang, Recovery of metals from waste printed circuit boards by supercritical water pre-treatment combined with acid leaching process., Waste Manag. 33 (2013) 1251–7.

[59]

S. Sanyal, Q. Ke, Y. Zhang, T. Ngo, J. Carrell, H. Zhang, et al., Understanding and optimizing delamination/recycling of printed circuit boards using a supercritical carbon dioxide process, J. Clean. Prod. 41 (2013) 174–178.

[60]

H. Wang, M. Hirahara, M. Goto, T. Hirose, Extraction of flame retardants from electronic printed circuit board by supercritical carbon dioxide, J. Supercrit. Fluids. 29 (2004) 251–256.

[61]

F.-R. Xiu, F.-S. Zhang, Materials recovery from waste printed circuit boards by supercritical methanol., J. Hazard. Mater. 178 (2010) 628–34.

33

Page 33 of 42

H. Brandl, R. Bosshard, M. Wegmann, Computer-munching microbes: metal leaching from electronic scrap by bacteria and fungi, Hydrometallurgy. 59 (2001) 319–326.

[63]

N.R. Ballor, C.C. Nesbitt, D.R. Lueking, Recovery of scrap iron metal value using biogenerated Ferric iron, Biotechnol. Bioeng. 93 (2006) 1089–1094.

[64]

J. Wang, J. Bai, J. Xu, B. Liang, Bioleaching of metals from printed wire boards by Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans and their mixture., J. Hazard. Mater. 172 (2009) 1100–5.

[65]

Y. Xiang, P. Wu, N. Zhu, T. Zhang, W. Liu, J. Wu, et al., Bioleaching of copper from waste printed circuit boards by bacterial consortium enriched from acid mine drainage., J. Hazard. Mater. 184 (2010) 812–8.

[66]

G. Liang, Y. Mo, Q. Zhou, Novel strategies of bioleaching metals from printed circuit boards (PCBs) in mixed cultivation of two acidophiles, Enzyme Microb. Technol. 47 (2010) 322–326.

[67]

N.P. Marhual, N. Pradhan, R.N. Kar, L.B. Sukla, B.K. Mishra, Differential bioleaching of copper by mesophilic and moderately thermophilic acidophilic consortium enriched from same copper mine water sample., Bioresour. Technol. 99 (2008) 8331–6.

[68]

M. Nemati, J. Lowenadler, S.T. Harrison, Particle size effects in bioleaching of pyrite by acidophilic thermophile Sulfolobus metallicus (BC)., Appl. Microbiol. Biotechnol. 53 (2000) 173–9.

[69]

M.-S. Choi, K.-S. Cho, D.-S. Kim, D.-J. Kim, Microbial Recovery of Copper from Printed Circuit Boards of Waste Computer by Acidithiobacillus ferrooxidans, J. Environ. Sci. Heal. Part A- Toxic/Hazardous Subst. Environ. Eng. 39 (2004) 2973–2982.

[70]

T. Yang, Z. Xu, J. Wen, L. Yang, Factors influencing bioleaching copper from waste printed circuit boards by Acidithiobacillus ferrooxidans, Hydrometallurgy. 97 (2009) 29–32.

[71]

Y. Xiang, P. Wu, N. Zhu, T. Zhang, W. Liu, J. Wu, et al., Bioleaching of copper from waste printed circuit boards by bacterial consortium enriched from acid mine drainage., J. Hazard. Mater. 184 (2010) 812–8.

[72]

N. Zhu, Y. Xiang, T. Zhang, P. Wu, Z. Dang, P. Li, et al., Bioleaching of metal concentrates of waste printed circuit boards by mixed culture of acidophilic bacteria., J. Hazard. Mater. 192 (2011) 614–9.

Ac ce p

te

d

M

an

us

cr

ip t

[62]

34

Page 34 of 42

C. Hagelüken, C.W. Corti, Recycling of gold from electronics: Cost-effective use through “Design for Recycling,” Gold Bull. 43 (2010) 209–220.

[74]

C. Oh, S. Lee, H. Yang, T. Ha, M. Kim, Selective leaching of valuable metals from waste printed circuit boards, J. Air Waste Manage. Assoc. 53 (2003) 897–902.

[75]

R.J. Dawson, G.H. Kelsall, Recovery of platinum group metals from secondary materials. I. Palladium dissolution in iodide solutions, J. Appl. Electrochem. 37 (2006) 3–14.

[76]

N. Gönen, E. Körpe, M.E. Yıldırım, U. Selengil, Leaching and CIL processes in gold recovery from refractory ore with thiourea solutions, Miner. Eng. 20 (2007) 559–565.

[77]

Z. Ping, F. ZeYun, L. Jie, L. Qiang, Q. Guangren, Z. Ming, Enhancement of leaching copper by electro-oxidation from metal powders of waste printed circuit board., J. Hazard. Mater. 166 (2009) 746–50.

[78]

J. Yang, Y. Wu, J. Li, Recovery of ultrafine copper particles from metal components of waste printed circuit boards, Hydrometallurgy. 121-124 (2012) 1–6.

[79]

T. Oishi, K. Koyama, S. Alam, M. Tanaka, J.C. Lee, Recovery of high purity copper cathode from printed circuit boards using ammoniacal sulfate or chloride solutions, Hydrometallurgy. 89 (2007) 82–8.

[80]

M.-S. Lee, J.-G. Ahn, J.-W. Ahn, Recovery of copper, tin and lead from the spent nitric etching solutions of printed circuit board and regeneration of the etching solution, Hydrometallurgy. 70 (2003) 23–29.

Ac ce p

te

d

M

an

us

cr

ip t

[73]

[81]

K. Huang, J. Guo, Z. Xu, Recycling of waste printed circuit boards: a review of current technologies and treatment status in China., J. Hazard. Mater. 164 (2009) 399–408.

[82]

W. He, G. Li, X. Ma, H. Wang, J. Huang, M. Xu, et al., WEEE recovery strategies and the WEEE treatment status in China., J. Hazard. Mater. 136 (2006) 502–12.

[83]

J. Lee, H.T. Song, J.-M. Yoo, Present status of the recycling of waste electrical and electronic equipment in Korea, Resour. Conserv. Recycl. 50 (2007) 380–397.

[84]

P. Hadi, Recovery of Sorption Products from Processing Non-Metallic Waste PCBs, 2013.

[85]

J. Cui, E. Forssberg, Mechanical recycling of waste electric and electronic equipment: a review, J. Hazard. Mater. 99 (2003) 243–63.

35

Page 35 of 42

O. Ogunseitan, J.M. Schoenung, J.D.M. Saphores, A.A. Shapiro, The Electronics Revolution: From E-Wonderland to E-Wasteland, Science (80-. ). 326 (2009) 670– 671.

[87]

K. Zhang, J.L. Schnoor, E.Y. Zeng, E-waste recycling: where does it go from here?, Environ. Sci. Technol. 46 (2012) 10861–7.

[88]

B.H. Robinson, E-waste: an assessment of global production and environmental impacts., Sci. Total Environ. 408 (2009) 183–91.

[89]

H.M. Veit, T.R. Diehl, A.P. Salami, J.S. Rodrigues, A.M. Bernardes, J.A.S. Tenório, Utilization of magnetic and electrostatic separation in the recycling of printed circuit boards scrap., Waste Manag. 25 (2005) 67–74.

[90]

H.M. Veit, A.M. Bernardes, J.Z. Ferreira, J.A.S. Tenório, C. de Fraga Malfatti, Recovery of copper from printed circuit boards scraps by mechanical processing and electrometallurgy., J. Hazard. Mater. 137 (2006) 1704–9.

[91]

J. Li, Z. Xu, Y. Zhou, Application of corona discharge and electrostatic force to separate metals and nonmetals from crushed particles of waste printed circuit boards, J. Electrostat. 65 (2007) 233–238.

[92]

J. Li, H. Lu, S. Liu, Z. Xu, Optimizing the operating parameters of corona electrostatic separation for recycling waste scraped printed circuit boards by computer simulation of electric field., J. Hazard. Mater. 153 (2008) 269–75.

[93]

J. Guo, J. Guo, Z. Xu, Recycling of non-metallic fractions from waste printed circuit boards: a review., J. Hazard. Mater. 168 (2009) 567–90.

Ac ce p

te

d

M

an

us

cr

ip t

[86]

[94]

H. Lu, J. Li, J. Guo, Z. Xu, Movement behavior in electrostatic separation: Recycling of metal materials from waste printed circuit board, J. Mater. Process. Technol. 197 (2008) 101–108.

[95]

J. Wu, J. Li, Z. Xu, Electrostatic separation for recovering metals and nonmetals from waste printed circuit board: problems and improvements., Environ. Sci. Technol. 42 (2008) 5272–6.

[96]

W. Jiang, L. Jia, X. Zhen-Ming, A new two-roll electrostatic separator for recycling of metals and nonmetals from waste printed circuit board., J. Hazard. Mater. 161 (2009) 257–62.

[97]

W. Jiang, L. Jia, X. Zhen-Ming, Optimization of key factors of the electrostatic separation for crushed PCB wastes using roll-type separator., J. Hazard. Mater. 154 (2008) 161–7.

36

Page 36 of 42

J.-M. Yoo, J. Jeong, K. Yoo, J.-C. Lee, W. Kim, Enrichment of the metallic components from waste printed circuit boards by a mechanical separation process using a stamp mill., Waste Manag. 29 (2009) 1132–7.

[99]

C. Eswaraiah, T. Kavitha, S. Vidyasagar, S.S. Narayanan, Classification of metals and plastics from printed circuit boards (PCB) using air classifier, Chem. Eng. Process. Process Intensif. 47 (2008) 565–576.

ip t

[98]

cr

[100] Y. Zheng, Z. Shen, C. Cai, S. Ma, Y. Xing, The reuse of nonmetals recycled from waste printed circuit boards as reinforcing fillers in the polypropylene composites., J. Hazard. Mater. 163 (2009) 600–6.

us

[101] J. Guo, Q. Rao, Z. Xu, Application of glass-nonmetals of waste printed circuit boards to produce phenolic moulding compound., J. Hazard. Mater. 153 (2008) 728–34.

an

[102] J. Ozaki, S.K.I. Djaja, A. Oya, Chemical recycling of phenol resin by supercritical methanol, Ind. Eng. Chem. Res. 39 (2000) 245–249.

M

[103] J. Guo, J. Li, Q. Rao, Z. Xu, Phenolic molding compound filled with nonmetals of waste PCBs, Environ. Sci. Technol. 42 (2008) 624–628.

te

d

[104] J. Guo, Q. Rao, Z. Xu, Effects of particle size of fiberglass-resin powder from PCBs on the properties and volatile behavior of phenolic molding compound., J. Hazard. Mater. 175 (2010) 165–71.

Ac ce p

[105] S. Yokoyama, M. Iji, Recycling of thermosetting plastic waste from electronic component production processes, in: IEEE Int. Symp. Electron. Environ., Ieee, 1995: pp. 132–137. [106] M. Iji, Recycling of epoxy resin compounds for moulding electronic components, J. Mater. Sci. 33 (1998) 45–53. [107] P. Mou, D. Xiang, X. Pan, L. Wa, J. Gao, G. Duan, New solutions for reusing nonmetals reclaimed from waste printed circuit boards, in: IEEE Int. Symp. Electron. Environ., 2005: pp. 205–209. [108] C. Clemons, Wood-plastic composites in the United States. The interfacing of two industries, For. Prod. 52 (2002) 10. [109] J. Guo, Y. Tang, Z. Xu, Performance and thermal behavior of wood plastic composite produced by nonmetals of pulverized waste printed circuit boards., J. Hazard. Mater. 179 (2010) 203–7. [110] R. Siddique, J. Khatib, I. Kaur, Use of recycled plastic in concrete: a review., Waste Manag. 28 (2008) 1835–52. 37

Page 37 of 42

[111] J. Guo, J. Guo, S. Wang, Z. Xu, Asphalt modified with nonmetals separated from pulverized waste printed circuit boards., Environ. Sci. Technol. 43 (2009) 503–8. [112] P. Panyakapo, M. Panyakapo, Reuse of thermosetting plastic waste for lightweight concrete., Waste Manag. 28 (2008) 1581–8.

ip t

[113] R. Wang, T. Zhang, P. Wang, Waste printed circuit boards nonmetallic powder as admixture in cement mortar, Mater. Struct. 45 (2012) 1439–1445.

cr

[114] S.G. Hong, S.H. Su, The use of recycled printed circuit boards as reinforcing fillers in the polyester composites, J. Environ. Sci. Heal. 31 (1996) 1345–1359.

us

[115] Y. Zheng, Z. Shen, C. Cai, S. Ma, Y. Xing, Influence of nonmetals recycled from waste printed circuit boards on flexural properties and fracture behavior of polypropylene composites, Mater. Des. 30 (2009) 958–963.

an

[116] Y. Zheng, Z. Shen, C. Cai, S. Ma, Y. Xing, The reuse of nonmetals recycled from waste printed circuit boards as reinforcing fillers in the polypropylene composites., J. Hazard. Mater. 163 (2009) 600–6.

d

M

[117] Y. Zheng, Z. Shen, C. Cai, S. Ma, Y. Xing, In situ observation of polypropylene composites reinforced by nonmetals recycled from waste printed circuit boards during tensile testing, J. Appl. Polym. Sci. 114 (2009) 1856–1863.

te

[118] B. Xu, Z. Lin, J. Xian, Z. Huo, L. Cao, Y. Wang, et al., Preparation and characterization of polypropylene composites with nonmetallic materials recycled from printed circuit boards, J. Thermoplast. Compos. Mater. In Press (2014).

Ac ce p

[119] Y. Ke, E. Yang, X. Liu, C. Liu, W. Dong, Preparation of porous carbons from nonmetallic fractions of waste printed circuit boards by chemical and physical activation, New Carbon Mater. 28 (2013) 108–113. [120] Z. Sun, Z. Shen, S. Ma, X. Zhang, Sound absorption application of fiberglass recycled from waste printed circuit boards, Mater. Struct. (2013). [121] P. Hadi, J. Barford, G. Mckay, Electronic waste as a new precursor for adsorbent production, SIJ Trans. Ind. Financ. Bus. Manag. 1 (2013) 128–135. [122] P. Hadi, J. Barford, G. McKay, Toxic heavy metal capture using a novel electronic waste-based material-mechanism, modeling and comparison., Environ. Sci. Technol. 47 (2013) 8248–55. [123] M. Xu, P. Hadi, G. Chen, G. McKay, Removal of cadmium ions from wastewater using innovative electronic waste-derived material, J. Hazard. Mater. 273 (2014) 118–123.

38

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[124] P. Hadi, J. Barford, G. McKay, Selective toxic metal uptake using an e-wastebased novel sorbent–Single, binary and ternary systems, J. Environ. Chem. Eng. 2 (2014) 332–339.

ip t

[125] P. Hadi, J. Barford, G. McKay, Synergistic effect in the simultaneous removal of binary cobalt–nickel heavy metals from effluents by a novel e-waste-derived material, Chem. Eng. J. 228 (2013) 140–146.

Ac ce p

te

d

M

an

us

cr

[126] P. Hadi, C. Ning, W. Ouyang, C.S.K. Lin, C.-W. Hui, G. McKay, Conversion of an aluminosilicate-based waste material to high-value efficient adsorbent, Chem. Eng. J. 256 (2014) 415–420.

39

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Table 1. Materials composition of selected printed circuit boards by weight (%), excluding materials in

Board 2 FR4 (more copper & ICs)

Board 3 Phenolic board (TV, monitor)

Copper

7

27

36

Iron

12

2

10.7

Glass fibre & SiO2 filler

24

15

Plastics

23

5

Ferrite

5

0

Epoxy

7

8

Phenolic

0

0

Gold

0.03

Bismuth

0.005

Chromium

0.002

Board 4 FR4 (more copper & ICs, lead free solder)

us

cr

Material

Board 1 FR4 (less copper and ICs)

ip t

very small quantity [21]

27 2 15

7

5

an

13

0

0

8

6

0

0.1

0

0.1

0.05

–

3.45

0.1

–

0.1

0.3

3

0.2

0

2.3

0.2

0.1

0.2

Silver

0.3

0.04

0

0.1

Tin

0.3

3

0.2

2.5

Zinc

3

0.5

–

0.5

Aluminum

7

1

22

1

ICs complex equiv

9

35

1

35

d

Ac ce p

Nickel

te

Lead

M

3

Table 2. Typical material composition of populated PCBs.

Component

Mass % 40

Page 40 of 42

16

Solder

4

Iron

3

Nickel

2

Silver

0.05

Gold

0.03

Palladium

0.01

ip t

Copper

cr

>70

Ac ce p

te

d

M

an

us

Glass-reinforced plastic

41

Page 41 of 42

ip t cr

4

us

2

an

qe (mmol/g)

3

1

2

Ce (mmol/L)

3

4

Ac ce p

0

te

0

d

M

1

Cu Pb Zn Co Ni

Figure 1. Adsorption capacity of the waste PCB-derived adsorbent for different metals (qe and Ce represent the adsorption capacity of the adsorbent and equilibrium concentration of the metals in the solution, respectively) [126].

42

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