Accepted Manuscript Waste Printed Circuit Boards Recycling: An Extensive Assessment of Current Status B. Ghosh, M.K. Ghosh, P. Parhi, P.S. Mukherjee, B.K. Mishra PII:
S0959-6526(15)00137-7
DOI:
10.1016/j.jclepro.2015.02.024
Reference:
JCLP 5193
To appear in:
Journal of Cleaner Production
Received Date: 6 December 2013 Revised Date:
28 January 2015
Accepted Date: 8 February 2015
Please cite this article as: Ghosh B, Ghosh MK, Parhi P, Mukherjee PS, Mishra BK, Waste Printed Circuit Boards Recycling: An Extensive Assessment of Current Status, Journal of Cleaner Production (2015), doi: 10.1016/j.jclepro.2015.02.024. 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.
Words: 15470
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Waste Printed Circuit Boards Recycling: An Extensive Assessment of Current Status B. Ghosh1,2,* , M .K. Ghosh1,2, P. Parhi3, P.S. Mukherjee1,2, B.K. Mishra1,2 Academy of Scientific and Innovative Research;
2
CSIR-Institute of Minerals and Materials Technology, Bhubaneswar-751013, INDIA
3
School of Applied Science & Central of Industrial Technology, KIIT University,
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1
Bhubaneswar-751024, INDIA
* Corresponding author: Email:
[email protected]: Fax: +91 674 2581637/2581160
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Abstract
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The rapid proliferation of electronic devices in the last two decades has compelled the researchers to find a remedy for one of the most toxic and hazardous waste materials - the waste Printed Circuit Boards. Numerous articles have been published demonstrating the process routes for recycling of this toxic but otherwise useful waste due to nearly 30% metal content. In this paper, more than 150 related articles mostly published in the last 15 years and covering the broad areas like characterization of waste printed circuit boards, health hazards
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associated with the processing and the different routes of recycling have been analyzed to provide a comprehensive overview on this topic. Physical separation processes employing electrostatic separator, magnetic separator, froth floatation, etc., has been reviewed for separation of metals and non-metals, along with useful utilizations of the non-metallic
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materials. The recovery of metals from this waste material through pyrometallurgical,
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hydrometallurgical or bio-hydrometallurgical routes is also critically discussed.
Keywords: Printed Circuit Boards (PCBs) recycling; Resource utilization; Metal recovery; Metallurgical process; Leaching.
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Introduction
With the phenomenal advancement and growth in electronic industries, the number of consumer and business electronic products per capita has been raised manifold in the last two decades in tandem with the downward price of newer products. At the same time, the average lifetime of electronic products has also been reduced drastically, resulting in massive
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generation of End-of-life (EoL) electronic goods, popularly known as waste electrical and electronic equipment (WEEE) or Electronic waste (E-waste). Current estimate shows that nearly 45 million tons of E-waste are generated globally per annum, and the number is growing at an exponential rate (Ogunseitan, 2013).
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Printed Circuit Board (PCB) is the integral component of any electronic equipment as it electrically connects and mechanically supports the other electronic components. The basic
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structure of the PCBs is the copper-clad laminate consisting of glass-reinforced epoxy resin and a number of metallic materials including precious metals. The concentration of precious metals especially Au, Ag, Pd and Pt is much higher than their respective primary resources, making waste PCBs an economically attractive urban ore for recycling. Additionally, PCBs also contain different hazardous elements including heavy metals, flame-retardants that pose grave danger to the eco-system during conventional waste treatment of landfilling and
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incineration. Consequently E-wastes including waste PCBs are either kept in stores or shipped to the developing countries where the poorest stratum of the population finds economic benefits by recovering precious metals in rudimentary methods (Chi et al., 2011b).
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The backyard operations in Asia and Africa especially in China, India and Ghana, are worrisome because of the adoption of primitive recycling techniques that lead to most of the hazardous elements leach into nearby water streams or soil. Moreover, in some processes,
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hazardous compounds such as dioxins and furans may form during thermal degradation of waste PCBs. Consequently, environment as well population surrounding the recycling sites are highly affected. For example, mean blood lead level (BLL) (15.3±5.79 µg/dL, n=165) of children living in Guiyu, one of the desired destinations of E-waste in China, is alarmingly higher than mean BLL (9.94±4.05 µg/dL, n=61) of children living in a neighboring town without E-waste processing (Huo et al., 2007). Besides, soil, water and air around E-waste processing sites are 100 times more contaminated by polybrominated diphenyl ethers (PBDEs), heavy metals and polycyclic aromatic hydrocarbons (PAHs) than other places (Guo et al., 2009b; Leung et al., 2006; Tang et al., 2010).
ACCEPTED MANUSCRIPT Manufacturers, environmental agencies and governments around the world are now looking for a systematic and environment friendly recycling technology for these waste PCBs, by which one can recover all the metal values as well as non-metal ones while imparting minimum impact on the environment. While some articles (Huang et al., 2009; Li et al., 2004; Luda, 2011; Sohaili et al., 2012) have reviewed the potential aspects of different routes,
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however, all the issues regarding total recycling of waste PCBs were not addressed. Hence, the purpose of this article is to provide a comprehensive review of various reported processes for PCBs recycling, their advantages and shortfalls towards achieving a cleaner process of
2.
Quantity of Waste PCBs generation
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waste utilization, with especial attention towards extraction of metallic values.
In order to cater the demand, resulted from the colossal development and innovations in
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electronic industries, there is a tremendous growth of PCB manufacturing industries across the globe. Fig. 1 shows the trends of the investments in PCB industries and futuristic growth worldwide. From this trend, it can be assumed that in the coming years, there will be a steep increase in PCB productions and eventually it will lead to enormous waste PCBs generation. Fig. 2 shows the production of PCBs in different countries and regions. China, which shares
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nearly half of the total PCB industries’ market, is the fastest growing nation in the PCBs production. Currently, waste PCBs account for ~3% of the total E-waste generated globally (Dalrymple et al., 2007). In China alone, considering the total E-waste that is generated and imported, more than 500,000 tons of waste PCBs needs to be treated in a single year (Zhang
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electronic goods.
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et al., 2012) and the amount is growing each year due to reducing average lifetime of
Characterization of Waste PCBs
Due to the diverse and complex nature of waste PCBs, characterization in terms of types, structure, components and composition is important to establish the route and process for eco-recycling. 3.1
Structure
All PCBs essentially consist of three basic parts: (1) A non-conducting substrate or laminate, (2) Conducting substrate printed on or inside the laminate and,
ACCEPTED MANUSCRIPT (3) The components attached to the substrate. Depending on the structure and alignment, PCBs can be classified as single-sided, doublesided or multilayered. Single- and double-sided PCBs have the conducting layer on one or both sides of the laminates and with or without plated through-holes to interconnect the sides. These types of PCBs can be found in consumer and automotive electronics. The substrate can
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be cellulose paper reinforced phenolic resin for consumer electronics or low-loss Teflon for radio-frequency applications (Ritchey and Coombs, 2008). Out of the several types of substrate, the most commonly used is the glass-fiber reinforced epoxy resins.
The conducting circuits on the substrate are either printed or etched with thin copper foil.
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Various etch resistant materials such as gold, nickel, silver, tin, tin-lead are used to protect the copper during etching. Various components depending upon the specific functions and
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applications, are attached to the substrate by techniques like socket pedestal device (SPD), through-hole device (THD), surface mounted device (SMD), screw joint device (SJD) and rivet joint device (RJD) (Huang et al., 2007). 3.2
Chemical composition
The elemental composition of PCBs varies depending on the type of PCBs and its
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applications. In general, PCBs contain ~28% metals, ~23% plastics and the remaining percentage as ceramics and glass materials (Zhou and Qiu, 2010). The substrate is mainly made of epoxies or cyanate resins (for multilayered) or phenolic resins (for single layered PCBs). Along with the resin, different types of hardener are required for cross-linking to form
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thermoset plastics. The most commonly used hardeners are dicyanodiamide, 4,4’diaminodiphenyl sulfone, 4,4’-diaminodiphenyl methane (Hall and Williams, 2007b).
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The principal reinforcing material for PCB substrate is cloth, made of glass fibers or silica. Other inorganic materials such as alumina, alkaline and alkaline earth-oxides and small amounts of other mixed oxides such as barium titanate, are also present (Sum, 1991). Ceramic materials such as BeO, glasses can also be found in the bridges and slots of PCBs. Around 10-20% of the PCB is made up with copper, which forms the conducting layer for electrical connection between different components. Precious metals especially Au and Pd are used as contact materials in joints. Typical Pb/Sn solders, which are used for joining different components in PCBs, account for 4-6% of the total PCB weight. Components that are mounted on PCBs also contain different metallic values such as Ga, In, Ti, Si, Ge, As, Sb,
ACCEPTED MANUSCRIPT Se, Te, Ta, etc. The platinum group metals are present in relays, switches or in sensors (Sum, 1991). 3.3
Toxicity
In the last few years, a number of articles have been published on the adverse health effects such as abnormalities in thyroid function, decreased lung function, premature birth, reduced
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birth weights and birth lengths, genotoxicity, adverse neonatal outcomes, etc., caused due to exposure to E-waste processing (Deng et al., 2006; Fu et al., 2008; Li et al., 2008b; Liu et al., 2011; Liu et al., 2009a; Schoeters et al., 2006; Wong et al., 2007).
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Toxic heavy elements especially Pb, Cd, Hg, As and Cr are significantly present in PCBs. Elevated levels of Cr in umbilical cord blood in infants are correlated with DNA damage and are correlated as well with the mother's exposure to E-waste recycling (Li et al., 2008b). Liu
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et al. (2009a) reported that E-waste recycling workers had 20-fold higher chromosomal aberrations than those not exposed to E-waste. The authors conclude that E-waste exposure induces cytogenetic damage within the general population near the E-waste processing site. Once Cd enters the body, it binds with hydrosulfo, carboxy and imidazolyl groups in enzymes or proteins and interferes with the physiological balance of other divalent metals,
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affects the biological activity of macromolecules and metal enzymes; causes multi-system and multi-organ injury (Schoeters et al., 2006). Around 2-5% Pb is present in the waste PCBs in the form of Pb/Sn solder and is not
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considered to be hazardous in general (Ogilvie, 2004). Nevertheless, ingestion of this heavy metal over a long period may cause malfunction of organs and chronic syndromes. Primitive recycling of E-waste may threaten the health of children by increasing blood lead level and
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altering children’s temperament (Liu et al., 2011). Although low in concentration, Sb can be present as antimonite [Sb(OH)-6] which is far more toxic than Pb and is readily absorbed by plants (Robinson, 2009). In addition, antimony trioxide, which is suspected to be carcinogenic, can also be present with polymers as flame retardant “synergist” (Veit et al., 2006). The plastic materials present in the PCBs mostly consist of C-H-O polymers such as polyethylene, polypropylene, polyesters, polycarbonates, phenol, formaldehyde, etc., whereas the rest of the plastics are mainly halogenated and nitrogen-containing polymers. The
ACCEPTED MANUSCRIPT brominated flame-retardants (BFRs) commonly used in PCBs are organobromide compounds that have an inhibitory effect on the ignition of combustible plastics. BFRs react with atmospheric oxygen in the presence of copper to form brominated analogous dioxins and furans, PBDD/Fs (polybrominated dibenzo-p-dioxins/furans) and ozone depleting brominated organic compounds such as CH3Br (Chi et al., 2011b; Lehto et al., 2003; Wong et al., 2007).
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Legler (2008) had reviewed the endocrine disrupting (ED) effects of BFRs. Author provided new insights into the in vivo effects of BFRs on thyroid hormone, estrogen and androgen pathways and the novel (in vitro) findings on the mechanisms underlying ED effects.
Dioxins introduce a wide spectrum of toxic effects in the human body. Short-term exposure
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to high levels of dioxins is known to damage liver function and cause chloracne, an acne-like eruption of blackheads, cysts and pustules (Marinković et al., 2010). Long-term exposure is
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associated with the disturbances in the nervous, immune, reproductive and endocrine systems. Tetrachlorodibenzo-p-dioxin’s (TCDD) persistence in the body can cause atherosclerosis, hypertension, diabetes and nervous system damage (Pelclová et al., 2006). From the epidemiologic studies, an association between polybrominated diphenyl ether (PBDE) exposure and decreased thyroid function, impaired spermatogenesis (Abdelouahab et al., 2011) and endocrine disruption (Darnerud, 2008; Legler, 2008) is also well established.
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Moreover, the workers in informal recycling sites suffer from irritation in the respiratory track as well as intoxication due to continuous exposure to concentrated acids used for precious metals recovery.
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Nevertheless, most of the hazardous elements that are present in PCBs do not pose a great threat to the environment as such due to their low concentrations, but their concentration
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raised multiple times during open burning or informal recycling. Recently Grant et al. (2013) extensively reviewed the health consequences associated with E-waste in which waste PCBs are one of the key elements. 4.
Issues and Scopes
According to Goosey and Kellner (2002), only 15% of the total discarded PCBs in UK are subjected to any form of recycling and remaining un-processed portions are either dumped as landfill or incinerated or lost in “hidden flow”. In fact, the common and primary techniques of E-waste disposal in developed countries are landfilling and incineration. Apart from that, around 50-80% of E-waste collected for recycling in developed countries, ends up in the
ACCEPTED MANUSCRIPT developing countries like China and India where these are treated in informal way (Wong et al., 2007). For long-term sustainability, an integrated waste management system primarily consisting of three key factors - reduce, reuse and recycle or better known as 3R policy (Terazono et al., 2006), is the preferable approach. Considering the 3R policy, the highest emphasis should
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always be given on the recycle and reuse of the waste electronics to reduce the quantity of waste PCBs ending up in the disposal. Although there are some scopes in reusing the individual parts of complex electronic products, the reusability of waste PCBs is rather limited due to various technical reasons. From the structure of PCBs, it is evident that
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individual components are closely integrated with the boards as well as with each other. Therefore, disassembly and separation of each component for reuse is not only a very
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complex job but also, the products made from these components may have very little or no reselling value. Hence, recycling in terms of recovery of metallic and non-metallic materials seems to be a realistic solution for the waste PCBs utilization. Recycling in an environmentally sound way, reduces the consumption of natural resources, lowers the carbon footprint and lessens the environmental hazards. Besides economic evaluation of the recycling option, which requires both energy and resources, product Life Cycle Assessment
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(LCA) is an important tool to envisage the environmental and resource consequences for the sustainable development. Since the end of 1990s, a number of models for integrated waste management including E-waste management have been proposed based on the LCA (Ahluwalia and Nema, 2007; Andrae and Andersen, 2010; Duan et al., 2009; Finnveden et
demonstrated
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al., 1995; Harrison et al., 2001; Winkler and Bilitewski, 2007). Hula et al. (2003) multi-criteria analysis (MCA) method to determine the economics and
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environmental benefits from material recovery based on the product structure, materials, location of recycling facilities, applicable regulations, geography as well as cultural context. There are several factors to consider in developing a new recycling technology for waste PCBs driven by innovations, social and environmental impact, an integrated waste management policy and economy of the process. Some of the key factors are: (1) The waste PCBs are diverse and complex in terms of type, size, shape, components and composition. With time, the composition of PCBs is continuously changing, making it more difficult to obtain a stable material composition.
ACCEPTED MANUSCRIPT (2) The presence of plastics, ceramics and metals in PCBs in a complex manner leads to great difficulty in liberation and separation of each fraction. (3) Presence of numerous metallic elements leads to very complex recovery process. The recovery process becomes more complicated when the elements are available in minute concentration.
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(4) The driving force for recycling is the recovery of metal values, which is nearly 30% of the total weight of waste PCBs. The non-metallic materials (~70%) have rather less economic value.
(5) The objective of the most recycling processes is to recover maximum metallic values
Recycling of waste PCBs
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5.
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from waste PCBs but sometimes these processes are not very environment-friendly.
Recycling of PCBs consists of several stages and in the following sections, each stage has been discussed in details. 5.1
Pretreatment of PCBs
5.1.1 Disassembly
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Dismantling of different electronic components mounted on PCBs is one of the most important steps in the recycling process. Selective disassembly of hazardous components from the PCBs ensures that toxic elements do not enter the mainstream of the recycling process. Components such as batteries, capacitors, etc., are treated differently and in more
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dedicated facility as these components contain toxic heavy metals (Duan et al., 2011). Some of the components that are attached as SPD can be separated from the board by applying a
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small force. Yang et al. (2009a) calculated the minimal force and/or acceleration required for disassembling and concluded that although small removal force is required for dismantling SMDs, minimal acceleration is dependent on the wetting angle. On the other hand, THDs require insignificant removal force, but it increases with the pin bend angle. The electronic components attached to PCBs through soldering can be dismantled by melting the Sn-Pb solder (Li et al., 2004). In recent years, many articles have been published on the feasibility of using air and certain liquids (such as methylphenyl silicone oil, water soluble ionic liquid) as heating medium and ultrasonication for effective removal of solder joints (Huang et al., 2007; Wa et al., 2005; Zeng et al., 2013). Nevertheless, care needs to be taken to avoid thermal degradation of plastics as generations of toxic gases from resins and adhesives is a
ACCEPTED MANUSCRIPT common occurrence during pyrolysis (Barontini et al., 2005). The disassembly process may be manual, automatic or semiautomatic. The most practical need for automated disassembly process is the real-time evaluation of product conditions and a flexible disassembly algorithm that modifies itself depending on the product conditions. The image processing and database are adopted to identify the different components and later, different PLC modules can be used
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in designing further route for dismantling. Many approaches have been reported for automatic (Knoth et al., 2002; Stobbe et al., 2002; Yokoyama et al., 1999) or semiautomatic disassembly (Knoth et al., 2000; Zebedin et al., 2001) but till now, manual disassembly is the most common technique adopted in most of the recycling plants. The application of different
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disassembly techniques for PCBs has been extensively reviewed by Li et al. (2004), where it is reported that automatic disassembly process can be more economical than manual one.
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5.1.2 Liberation of metallic and non-metallic fraction
After removal of the hazardous components, different mineral processing unit operations such as shredding, crushing and grinding can be used to liberate metals from cladding materials such as resin, fiberglass and plastics. Various types of hammer crushers, rotary crushers, disc crushers, shredders, cutters equipped with a bottom sieve, are used for liberation. Ball milling and disc milling are also reported for pulverizing the PCBs after
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cutting into small sizes (Hanafi et al., 2012). As the PCBs are made of reinforced resin, copper wires and glass fibers (multilayer), the conventional crushers may not achieve good liberation. In contrast, shredding or cutting, which works on the principle of shearing, is found to be more useful. Unlike mineral ores, PCBs do not have a particular size fraction for
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liberation; instead, different types of elements are liberated at different size fractions. Zhang and Forssberg (1999) studied the liberation characteristics of PCBs and the effect of shape
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and size on liberation. It was concluded that below 6 mm size, ferromagnetic and copper are completely liberated and at the same time, aluminium is found to be liberated in much coarser fraction. Koyanaka et al. (1999) reported that glass fiber-reinforced epoxy resin undergoes brittle fracturing more readily than metallic materials and concentrates in the finer fraction during impact milling of PCB scraps. Earlier, Yoo et al. (2009) demonstrated the use of stamp mill for the liberation of various metallic components. Vidyadhar and Das (2012) also reported that after milling to below 150 µm size, no interlocking of metallic and non-metallic particles is observed. Nevertheless, one of the major challenges in crushing and grinding of waste PCBs is the generation of fine dust that may be very difficult to handle during subsequent processes. During continuous crushing, the local temperature of PCBs may also
ACCEPTED MANUSCRIPT be increased (Janáčová et al., 2009; Li et al., 2010) and this may result in localized pyrolysis as well as agglomeration of plastics, thus complicating the process further. Some of the works (Yuan et al., 2007) have also been devoted relating to the use of low temperature crushing but it also involves high energy cost which again has an adverse effect on the environment. 5.2
Recovery/Separation of non-metallic fractions
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The organic materials in PCBs such as polymers can be converted into fuel gases by applications of thermal energy and currently is one of the major sources of energy in some of the recycling plants (Brusselaers et al., 2005). In recent years, however, focus has been shifted towards recovery of the plastic materials and other non-metallic components for
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alternative uses. 5.2.1 Physical processes
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The objective of physical recycling process is to recover the non-metallic materials as it is, without any loss of valuable metals. Degree of liberation plays a crucial role in physical separation process as effectiveness of separation depends on shape, size and size distribution of the particles.
Particles shape based separation is one of the basic methods that powder industries use to
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control the powder properties (Furuuchi and Gotoh, 1992; Furuuchi et al., 1993; Ohya et al., 1993). While grinding, the metal particles turn into spherical shape due to their high malleability and ductility and non-metallic particles such as plastics and glass fibers remain non-spherical in shape due to brittle fracture. Koyanaka et al. (1997) examined the shape and
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size properties of copper from printed wiring boards, milled by a swing-hammer type impact mill, followed by recovery of copper through shape-based separation technique using an
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inclined vertical plate. The density-based separators are widely used to separate lighter fraction from heavier ones based on the density difference. The metallic fraction (Sp. Gr. 2.6 - 19.3) can be separated from the plastics (Sp. Gr. 1-1.8) using heavy density liquids such as tetrabromoethane (Hanafi et al., 2012). However, efficiency of the separation is poor as the particle shape and size play a crucial role in the separation. Peng et al. (2004) performed the density-based separation of waste PCB fines (50-300 µm) in an inclined separation trough and achieved more than 95% recovery of metallic materials. Air classification method, in which separation is based on the settling velocity of the particles in the air, has also been reported for the separation of plastics from the metals (Eswaraiah et al., 2008).
ACCEPTED MANUSCRIPT Electrostatic separation is an another promising technology for separating non-conducting materials from conducting ones, owing to its advantages of less environmental hazards, low energy consumption and easy operation (Wei and Realff, 2003). Xue et al. (2012) reported the electrostatic separation of a mixture of copper, silicon and woven glass reinforced resin and proposed that multiple-stage separation is necessary for effective separation of
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conductors, semiconductors and non-conductors. Both fundamental and practical aspects concerning design of suitable electrostatic separator for industrial applications have been reported by a number of researchers (Iuga et al., 1989; Li et al., 2008a; Zhang and Forssberg, 1998). The differences in density and electrical conductivity between plastics, metals and
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ceramics provide an excellent condition for application of corona electrostatic separator. Li et al. (2007) employed corona discharging electrostatic separator for PCBs recycling and found that corona electrostatic separator is suitable for particles with size ranges between 0.6 to 1.2
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mm, but productivity decreases with finer size fraction. One of the major drawbacks in corona electrostatic separator is the pinning effect of large insulating particles and that can be enhanced by increasing the image force and/or decreasing the centrifugal force (Zhang and Forssberg, 1998). Eddy current based electrostatic separator is also successfully employed to separate plastic particles from metal/plastic mixture as well as non-ferrous metals from
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ferrous metals (He et al., 2010; Schlett et al., 2002; Zhang et al., 1998, 1999). The separation efficiency depends on the different trajectories of particle movements due to eddy current (induced in the non-ferrous particles) and external magnetic field, which deflects the ferrous particles to a higher degree.
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Low-intensity drum magnetic separators are generally used to recover ferrous materials from the non-magnetic fraction. Researchers at Daimler-Benz in Ulm, Germany, have developed a
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mechanical treatment approach in which magnetic separators are used to remove ferrous elements before fine grinding (Goosey and Kellner, 2003). Veit et al. (2005) used the magnetic separators for enrichment of valuable metallic fraction before electrostatic separation of PCB fines. Magnetic separators are not very much useful for crushed PCBs because the particles are agglomerated during the process and the non-magnetic materials escape with ferrous one. Nevertheless, it can be used as groundwork before electrostatic separation for easy handling of non-conducting materials. A major issue in physical processing of waste PCBs is the poor recovery of metallic fraction from fines generated during comminution and liberation. Moreover, due to the brittle nature of glass fibers and epoxy resins, a significant fraction of non-metallic materials often reports
ACCEPTED MANUSCRIPT in different base and precious metals, leading to a very complex materials composition. Ogunniyi and Vermaak (2009) tried froth flotation of the comminution fines for enrichment of metallic fraction and found that Au and Pd are among the elements best enriched in the sink. Similar studies were also carried out by Vidyadhar and Das (2012) with PCB fines (-1 mm) and it was reported that metallic fraction can be effectively recovered with less than 4%
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loss of metal values. 5.2.2 Chemical process
The enrichment of metallic materials is often carried out by various temperature-assisted chemical processes. Pyrolysis is one of the common techniques used for degradation of
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plastics to oils, gases, tar, etc., and easy separation of metallic materials and glass fibers. It is carried out in the absence of oxygen or in the presence of some inert gases at a temperature
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range of 400 to 700⁰C. A significant amount of work on pyrolysis of waste PCBs has been reported (Chien et al., 2000a; Guo et al., 2010; Hall and Williams, 2007a; Quan et al., 2009) including the micro-wave induced pyrolysis (Sun et al., 2011; Zhang et al., 2010) and vacuum pyrolysis (Long et al., 2010; Zhou and Qiu, 2010). Experimentally, pyrolysis of waste computer PCBs generated approximately 22.7% oil, 4.7% gases and 70% copper-rich residue (Hall and Williams, 2007b). The composition of the pyro-gases and oils varies
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depending upon the pyrolysis temperature, residence time in the reactor, type of reactors and even the PCBs size fractions (Chiang et al., 2007). The pyro-gases have high calorific value, and the oils can be recycled further as raw materials for chemical industries. Nevertheless, a major concern in recycling of waste PCBs by pyrolysis is the presence of dioxin precursors
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such as Dibenzofurans, 4-methyl-Benzofluroethane and other brominated compounds in pyrolysis oils (Sun et al., 2011; Zhou et al., 2007). Some of the recent studies (Luyima et al.,
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2012; Sun et al., 2011) suggest that the addition of CaCO3, Fe2O3 during pyrolysis process can control the brominated compounds and other organic compounds like benzene. Gasification is carried out in the presence of a controlled amount of oxygen or air or steam at high temperature (1000 ℃). The gaseous product compositions in gasification and pyrolysis are nearly the same, but the solid residue differs mainly in terms of chemical reactivity (Havlik et al., 2010). Co-combustion is a similar technique used to decompose the plastic fraction of PCBs. In gasification and co-combustion, certain amount of bromine is found to be present in the char or ash product, and most of the bromine goes into the syngas, where it can be collected by wet scrubbing. Zheng et al. (2009b) reported the recovery of glass fibers from the nonmetallic fraction of the PCBs in a fluidized bed reactor. They have achieved
ACCEPTED MANUSCRIPT 94.8% recovery of glass fibers with purity more than 95.4% in the temperature range from 400 ℃ to 600 ℃, followed by air-cyclone separator. In recent years, supercritical fluids have been used as an effective medium for the destruction of epoxy adhesive layer from PCBs and to produce small organic molecule (Ozaki et al., 2000; Tagaya et al., 2004; Wang et al., 2004; Xiu and Zhang, 2009). Organic materials are
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rapidly oxidized to CO2 and H2O in a very short period in the presence of supercritical fluids such as water (Tc > 647 K, Pc > 218 atm) or methanol (Tc > 513 K, Pc > 79.84 atm.) (Chien et al., 2000b; Xiu and Zhang, 2009, 2010). During the process, alkali cations are used to capture bromine to make it environmental friendly. The average extraction efficiency was found to be
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over 90% using supercritical ethanol as the extraction solvent (Wang and Zhang, 2012). The adhesive layer of epoxy resin in PCBs can be destroyed by supercritical CO2 to liberate
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different materials layers and to recover phosphate-based flame retardants Triphenyl phosphate (TPPO4) efficiently at 343 K and 25 MPa (Wang and Zhang, 2012). A similar study (Sanyal et al., 2013) indicates that supercritical CO2 and an additional amount of water can be very useful in de-lamination of PCB substrate and further separation of copper foils, glass fibers and polymers.
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Use of organic solvents to dissolve the bromine epoxy resin and separate out metals, glass fibers and other polymers is also another attractive option. Zhu et al. (2013) used dimethyl sulphoxide (DMSO) as an organic solvent to dissolve bromine epoxy from waste PCBs and found that complete separation is possible when waste PCBs of 16 mm2 size fraction are
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treated under a solid-liquid ratio of 1:7 at 185 ℃ for 60 minutes. 5.2.3 Alternative uses of non-metallic fraction
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Currently most of the non-metallic fractions are subjected to landfilling, incineration and open dumping causing potential threat to the environment as well as loss of resources (Guo et al., 2009a; Hadi et al., 2013; Li et al., 2012). The nonmetallic powder can be directly used as filler material or as composites, possessing similar properties of traditional filler materials (Zheng et al., 2009a). Hong and Su (1996) used nonmetals as reinforcing fillers in the polyester composite. Non-metals recovered from paper-based waste PCBs can also replace wood-flour in the production of wood plastic (polyethylene) composites (Guo et al., 2007). In an analogy, PCB non-metallic fraction as reinforcing fillers in polypropylene (PP) modified by a silane coupling agent could be successfully added as a substitute for traditional fillers (Yokoyama and Iji, 1995). Similar
ACCEPTED MANUSCRIPT study was carried out by Zheng et al. (2009a) with computer PCBs as reinforcing filler in polypropylene. According to them, both tensile and flexural properties coupled with heat resistance properties could be improved significantly with the addition of the non-metallic fraction. Muniyandi et al. (2013) also observed similar results by using the non-metallic fraction as filler material in rHDPE (High Density Polyethylene) for production of
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rHDPE/PCB composite. Moreover, addition of MAPE (Maleic anhydride modified polyethylene) to improve the interfacial adhesion between PCB non-metallic fraction and rHDPE, further improves the flexural strength, tensile strength and impact strength by 71%, 98% and 44% respectively compared to the rHDPE/PCB composites.
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The non-metallic fraction of PCBs can also be used with some effectiveness as a partial replacement for inorganic aggregates in concrete applications to decrease the dead weight of
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structures (Mou et al., 2007). The glass fibers and resins powder contained in the nonmetallic fraction can also be used to strengthen the asphalt (Guo et al., 2009a). Sun et al. (2013) studied the feasibility of using glass fibers reclaimed from waste PCBs, as sound and thermal insulation materials. The results indicate that Recovered Glass Fibers (RGF) perform better than commercial sound absorbing materials. Recently, Hadi et al. (2013) published a research article based on the potential application of non-metallic from waste PCBs as toxic
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heavy metal adsorbent mainly for Cu and Pb ions.
In summary, physical recycling process for recovery of non-metallic is much more advantageous than chemical recycling as the former is relatively convenient and environment
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friendly, equipment and environment control measures are less and the potential applications of products are diverse.
Separation & Recovery of Metals
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5.3
5.3.1 Selection of target metals In spite of having diverse compositions, the main driving force for recycling of PCBs is the recovery of metals. Nevertheless, the recovery of each metal may not be feasible due to economic reason and technological limitations. The recyclability of a metal can be determined by the “contribution score” of the individual metal that is related to weight content, environmental hazards associated with the metal, energy consumption, natural resources depletion, etc. The most widely used assessment index is the resource recovery efficiency (RRE) (Legarth et al., 1995) which compares different metals based on their weight content, recycling efficiency and world reserves and is expressed as:
ACCEPTED MANUSCRIPT RRE = ∑
×
~ ∑
(1)
Where, E is the recovery percentage, F is the amount of resource/ton of scrap, P and C are the annual production and consumption of the primary resource respectively, R is the world reserve of the resource, and i counts the type of the resources in the scrap.
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Another approach by Huisman et al. (2003) calculates Quotes for environmentally Weighted RecyclabiliTY (QWERTY) score to determine the environmental performance by the following equation: , ,
(2)
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= ∑
Where, EVWactual,i is actual environmental impact for the weight of material i. EVWmax,i
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maximum environmental impact for the weight of material i. EVWmin and EVWmax are total defined minimum and maximum environmental impact for the complete product, respectively.
Based on the above two approaches, Le et al. (2013) developed the Model for Evaluating Metal Recycling Efficiency from Complex Scraps (MEMRECS) for prioritizing the selection
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of target metals. The main advantage of this approach is that it includes the environmental impact as well as natural resource conservation aspects for calculating recyclability. According to this model, the metal recovery priority should be on the precious metals such as Au, Ag and Pd along with some base metals such as Cu, Sn and Ni. Nevertheless, the actual
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score may vary depending upon the weight content of metals in waste PCBs. 5.3.2 Metal recovery through pyrometallurgical route
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Pyrometallurgy is the traditional approach for metal recovery from the waste PCBs but selective recovery of individual metals can hardly be done by this route. Pyrometallurgical techniques include incineration, smelting in plasma arc furnace, blast furnace or copper smelter, high temperature roasting in presence of selective gases to recover mainly nonferrous metals. Currently, more than 70% of waste PCBs are treated in smelters rather than through mechanical processing (Cui and Zhang, 2008). The main advantage of pyrometallurgical treatment is its ability to accept any forms of scrap. Hence, electronic scrap can be used as a part of raw materials in the smelters for recovery of copper along with gold and silver (Sum, 1991). A recycling method, developed by Technical University Berlin in 1997, turned waste PCBs into a Cu-Ni-Si alloy, a mixed oxide (mainly Pb and Zn) and
ACCEPTED MANUSCRIPT environmentally agreeable slag by a top blown reactor (Bernardes et al., 1997; Li et al., 2004). At Boliden Ltd. Ronnskar smelter, Sweden, waste PCBs are directly fed into copper converter to recover Cu, Ag, Au, Pd, Ni, Se and Zn while the dust containing Pb, Sb, In and Cd is processed separately for metal recovery (Theo, 1998). At Umicore’s integrated metal smelter and refinery, electronic scraps are first treated in IsaSmelt furnace to recover precious
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metals along with Cu the form of Cu-bullion. Cu is first recovered from this bullion through copper leaching and electrowinning followed by precious metals recovery from copperleached residue in precious metal refinery (Hagelüken, 2006). Fig. 3 shows a flow-sheet for copper recovery as copper cathode from waste electronic equipment by a pyrometallurgical
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route combined with electrolysis (Antrekowitsch et al., 2006; Gramatyka et al., 2007).
The downside of the pyrometallurgical approach is the pollution and energy consumption
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during the process. As a result, E-waste recycling plants often entail extensive emission control system for environmental protection. In vacuum pyrometallurgy, metals with different vapor pressures can be separated by distillation or sublimation and thereafter, it can be recovered through condensation at specific conditions Vacuum Metallurgy Separation (VMS) has been reported for the recovery of Bi, Sb, Pb and other heavy metals with high vapor pressure (Zhan and Xu, 2009, 2011). Zhou et al. (2007) in an innovative pyrometallurgical
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process, claimed more than 99% recovery of Cu at 1200 ℃. Using NaOH as slag forming material, waste PCBs are first pyrolysed under reducing conditions forming slag and metal mixture. Molten metal and part of the slag are then oxidized in the next step to purify and recover copper and precious metal, followed by reduction of the molten Cu2O in slag to Cu in
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another step using the recycled pyrolysis gas. Pyrolysis gas thus loses its toxicity during reduction of copper and reaction with slag. Recently, Flandinet et al. (2012) demonstrated a
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process where molten KOH-NaOH eutectic not only dissolved glass and oxides, but also destructed organics to recover copper-rich metallic values. Most of the halides were captured in the molten bath during the process and the exhaust gas contained around 30% hydrogen, is suitable as fuel gas.
5.3.3 Metal recovery through hydrometallurgical route Hydrometallurgical route is more selective towards metal recovery from waste PCBs or pretreated PCBs, easier to control over reaction and creates less environmental hazards than pyrometallurgical approach. The base metals recovery has a substantial impact on the economics of the process due to larger available amount in waste PCBs. Moreover, recovery
ACCEPTED MANUSCRIPT of base metals also ensures the enrichment of precious metals in the solid residue, making it easier to leach out subsequently.
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5.3.3.1 Cu recovery
Copper is mainly present in the elemental form (in mechanically treated PCBs) or as an alloy (in thermally treated PCBs). A number of articles have been published in recent years pertaining to the leaching of copper from waste PCBs (Castro and Martins, 2009; Koyama et
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al., 2006; Masavetas et al., 2009; Md Fazlul et al.). Copper can effectively be leached using acidic or ammoniacal media (Behnamfard et al., 2013; Havlik et al., 2010; Kamberović et al.,
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2009; Koyama et al., 2006; Masavetas et al., 2009; Oh et al., 2003). On industrial scale, sulfuric acid is the most preferred reagent for Cu leaching from scrap materials due to lower reagent price and easier regeneration. However, in this case further separation process becomes complicated due to poor selectivity of inorganic acids as a leaching agent. Oh et al. (2003) reported a sulfuric acid leaching study with a non-magnetic portion (58%) obtained after magnetic separation of conducting metallic mass from crushed computer PCBs. By
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leaching under the conditions of 2M H2SO4, 0.2M H2O2, 85 ℃ for 12 h, more than 95% extraction of Cu along with some other base metals (Fe, Zn, Ni, and Al) was achieved. Maguyon et al. (2012) used nitric acid media to simultaneously extract and deposit copper from waste PCBs containing ~95% Cu, generated in PCB manufacturing facility. The
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optimized conditions for the acid treatment step were: 120 mg PCB waste/ mL Conc. HNO3 loading ratio and 4 h contact time. It was concluded that nitric acid treatment of crushed
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PCBs shows better Cu recovery than in aqua regia. However, the acid extract could not be used directly for Cu electro-deposition due to high acid content and required to be further diluted accordingly. Electrochemical deposition achieved 98% recovery efficiency at 3 A current and 14.25 kWh-kg-1 Cu energy consumption. Ammoniacal leaching, on the other hand, is superior in terms of selectivity towards copper. Cu forms the amine complexes Cu[(NH3)2]+ and Cu[(NH3)4]2+ with NH3 and amine stability can be controlled by optimizing pH of the solution, oxidation potential, NH3 concentration, etc. Koyama et al. (2006) demonstrated the thermodynamic feasibility of Cu leaching in the nitrogen atmosphere using Cu(NH3)42+ as an oxidizing agent instead of conventionally used
ACCEPTED MANUSCRIPT oxygen. Cupric ammine oxidizes elemental Cu to form Cu(I)-amine complex according to the following reaction. Cu + Cu(NH3)42+ = 2Cu(NH3)2+
(3)
It is observed that effect of temperature on leaching rate is insignificant, but the concentration
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of Cu(II)-amine complex enhances the leaching rate and presence of Cu(I)-amine complex slightly depresses the same. Liu et al. (2009b) also tried to recover copper from crushed and ground PCBs in NH3/NH5CO3 solution with aeration. During the distillation of NH3 from leach liquor, copper carbonate is precipitated which is later calcined to recover pure copper
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oxide with a particle size of 0.2 µm. In a recent study, waste PCBs were first smelted to produce copper bearing alloy, which was later treated through NH3/NH4Cl using CuCl2 as oxidants (Lim et al., 2013). It is observed that 98% Cu can be selectively leached under the
200 rpm, 30 ℃ and 1% pulp density. 5.3.3.2 Precious metal recovery
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following conditions: 2 kmol.m-3 NH4Cl and 5 kmol.m-3 NH3 solution, 0.1 kmol.m-3 CuCl2,
Currently, Around 300 tons of gold are used in electronic industries along with other precious and strategic metals like Ag, Pd, Pt, Nb, Ta, etc. (Montero et al., 2012). Most of the precious
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metals are present in elemental form and in proximity of other metals, which makes very difficult to separate the individual ones. In spite of dynamic research on this field, the progress on the recovery of precious metals from a complex system is negligible. These metals are highly unreactive in the normal environment and require high oxidation potential
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during leaching. To improve the selectivity of the precious metals and minimize the impurities, leaching is preferably carried out after the removal or recovery of base metals.
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Sheng and Etsell (2007) in sequential steps first dissolved the base metals in nitric acid, followed by leaching of the first step leach residue in aqua regia to extract gold and finally precipitation of gold with ferrous sulfate. Nevertheless, the construction of the leaching reactor suitable for highly corrosive nitric acid and aqua regia, limits its industrial feasibility. Currently, the active research has been shifted towards the development of less corrosive reagents such as cyanide, halide, thiourea and thiosulfate for precious metals leaching from waste PCBs. Cui and Zhang (2008) had extensively reviewed precious metals extraction methods from electronic waste through pyrometallurgical, hydrometallurgical and biohydrometallurgical processing routes. A critical comparison of the main leaching methods was drawn from the point of view of both economic feasibility and environmental impact.
ACCEPTED MANUSCRIPT For gold leaching, cyanide is being used for more than a century since it was patented in 1888 by Mac Arthur and Forest brothers (Senanayake, 2004). The basic reactions involved in cyanide leaching are: (4)
Complex formation: 4Au + 8CN− → 4Au(CN)2− + 4e
(5)
Reduction: O2 + 2H2O + 4e → 4OH−
(6) (7)
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Overall: 4Au + 8CN− + O2 + 2H2O → 4Au(CN)2− + 4OH−
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Oxidation: 4Au → 4Au+ + 4e
Using sodium cyanide solution in column leaching technique Montero et al. (2012) recovered 46.6% Au, 51.3% Ag, 47.2% Nb along with 62.3% Cu from the crushed PCBs. Quinet et al.
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(2005) also examined the feasibility of a hydrometallurgical route on bench scale to recover precious metals from the waste PCBs, containing 27.37% Cu, 0.52% Ag, 0.06% Au and 0.04% Pd. The first step involves oxidative sulfuric acid leaching to dissolve Cu and part of the Ag, followed by an oxidative chloride leaching to dissolve Pd and Cu, and finally recovery of gold, silver and palladium by cyanidation. The optimized flow-sheet claims 95%, 93% and 99% recovery of Au, Ag and Pd respectively. Nevertheless, the rising
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environmental issues regarding the use of cyanide has eventually prompted the needs to find new substitutes for gold leaching (Hilson and Monhemius, 2006). In recent years, the recovery of gold by thiourea has gained worldwide attention due to its
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less environment impact. Unlike cyanide, thiourea forms a cationic complex with gold in acidic medium and can dissolve up to 99% gold as per the following reaction:
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Au + 2CS(NH2)2 → Au(CS(NH2)2)2+ + e
(8)
Ficeriová et al. (2008, 2011) studied the amenability of thiourea leaching for precious metals recovery from electronic waste. The standard conditions for leaching are: ambient temperature, 10 g/L each of H2SO4 and CS(NH2)2 and 5 g/L ferric sulfate. While as such PCBs under the above conditions could dissolve 76% Au in 2 h, it is possible to achieve 90% gold dissolution with the crushed sample under the same leaching conditions. However, such single step leaching of the PCBs is not very selective because along with 90% gold dissolution, 68% Cu, 45% Fe, 43% Pb and 28% Zn dissolve simultaneously. Later in similar study Behnamfard et al. (2013) achieved 85.76% and 71.36% extractions of Au and Ag
ACCEPTED MANUSCRIPT respectively in 3 h by leaching the residue after two-stage Cu- leaching with 20 g/L thiourea, 6 g/L Fe3+, 10 g/L H2SO4 at S:L ratio 1:10 (g/mL) under ambient temperature. Once the leach liquor is treated with 8 g/L sodium borohydrate as a reducing agent more than 99.5% gold and silver are precipitated. However, the cost and consumption of thiourea are important factors for process development.
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Alike thiourea, another non-toxic and non-corrosive reagent is thiosulphate. It forms a stable complex with gold in aqueous solution as follows: Au + 2S2O32- → Au(S2O3)23- + e E0 = +0.153 V
(9)
to form Au(I) complex as shown in Eq.(10)
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4Au + 8S2O32- + O2 + 2H2O → 4Au(S2O3)23- + 4OH-
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Gold dissolves in alkaline thiosulfate solution using dissolved oxygen as the oxidizing agent
(10)
However, the above reaction proceeds very slowly without the presence of any catalyst ion as an oxygen carrier. Due to high pH requirement of thiosulfate leaching, Cu(II) ions in ammonia i.e. Cu(NH3)42+ can act as an effective catalyst for gold dissolution.
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Au + 5S2O32- + Cu(NH3)42+ → Au(S2O3)23-+ 4NH3+ Cu(S2O3)35-
(11)
The cupric tetra-amine complex ions are regenerated by the reaction between dissolved oxygen and Cu(S2O3)35- as per Eq. (12)
(12)
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Cu(S2O3)35- + 4NH3 + ¼ O2 + ½ H2O → Cu(NH3)42+ + 3S2O32- + OH-
Oh et al. (2003) could extract 95% Au within 48 h and 100% Ag within 24 h, when residue
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after physical processing and base metal leaching was treated in 0.2M (NH4)2S2O3, 0.02M CuSO4, and 0.4M NH4OH at 40 ℃. Similar study carried out by Ha et al. (2010) indicated rapid extraction of gold (98% in 2 h) from PCB scrap in 20mM Cu(II), 0.12M thiosulfate and 0.2M ammonia solution, but under similar conditions, 90% gold recovery was feasible after 10 h from waste mobile phones PCBs. Ficeriová et al. (2011) demonstrated that when leaching is carried out with “as-received” PCB samples (S:L=1:11.25) using 0.5M (NH4)2S2O3, 0.2M Cu(II) and 1M NH3, only 16% Au and 12% Ag is recovered after 48 h. However, with the pre-treated waste (-800 µm), recovery is increased to 98% and 93% for Au and Ag respectively. Tripathi et al. (2012) also conducted similar studies with waste mobile phone PCBs. In the case of PCB granules (0.5 to 3 mm) 56.7% Au could be leached in 8 h at
ACCEPTED MANUSCRIPT ambient temperature under the optimized conditions of (NH4)2S2O3: 0.1M, CuSO4: 40mM, S:L 10 g/L and pH: 10-10.5. Surprisingly when as such PCBs are leached at 40 g/L S:L ratio under the above conditions, 78.8% gold recovery is achieved as compared to 30.35% achieved with PCB granules. According to the authors, higher surface area of PCB granules results in high Cu extraction that in turn leads to high losses of thiosulfate ions by its
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conversion to tetrathionate and other polythionates. An eco-friendly process by preferentially dissolving other base metals and silver from a goldplated circuit board in boiling 20% potassium persulfate solution is reported by Syed (2006). Potassium persulfate is a strong oxidizing agent and nearly completely dissolves the base
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metals retaining gold in the solid residue. Nearly 99.5% pure gold is obtained by melting the leached residue with borax and KNO3 mixture. Both potassium persulfate and its ultimate
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product potassium sulfate are non-toxic.
Although halide has been reported as a promising alternative for precious metals recovery from ores/concentrates, but not much attention has been paid towards its use in gold extraction from PCBs. Halide leaching esp., chlorine/chloride leaching has proved to be wellestablished route for dissolution of gold in acidic medium. Nevertheless, the handling of
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chlorine gas and requirement of special reactor hinders its application in the full-scale process. Sodium hypochlorite leaching is another effective lixiviant for gold but not well reported with regard to gold extraction from waste PCBs. Quinet et al. (2005) achieved 9395% Pd extraction efficiency during oxidative leaching using HNO3 or H2O2 in chloride
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medium (HCl and NaCl). Xu et al. (2010) attempted the iodide leaching of gold from waste PCBs and achieved 95% gold recovery at ambient temperature in 4 h under the conditions: 1.0%~1.2% iodide concentration, n(I2):n(I-)=1:8~1:10, H2O2 concentration 1% ~2%, S:L 1:10
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and pH 7. Iodide leaching is considered to be non-toxic, non-corrosive and very selective towards gold as gold-iodide complex is the most stable compound formed by gold and halogen (Zhang et al., 2009). However, very high rate of reagent consumption during the leaching and reagent cost are the main obstacles in this process. 5.3.3.3 Leaching of other metals Besides Cu and precious metals, other metals, which may add to the economics, are Sn, Ni, Pb, etc. For many years, Pb-Sn solder alloys were used during the manufacturing of PCBs. The European Union passed a major legislation in 2003 stating that all PCBs and electronic
ACCEPTED MANUSCRIPT assemblies must be lead-free. Hence, the use of lead-free solder alloys such as Sn-Cu-Ag, SnCu-Co, etc., containing comparatively higher amount of tin are now used. Sn and Pb can be leached out from the PCBs along with Cu by inorganic acid. Tin leaching using different inorganic acids was studied in detail with the objective of recovering Cu and Sn (Castro and Martins, 2009). They could achieve maximum 89.1% and 98.1% Sn
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recoveries in 2h at 60⁰C using 3N HCl and (3N HCl + 1N HNO3) leach system respectively. Tin and copper were recovered as precipitates after adjusting the leach liquor pH by NaOH. Non-thermal treatment of E-waste prior to leaching operation helps in more efficient Sn leaching as Sn is located on the surface of PCBs and remains in a good contact with the
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reagent. Peres et al. (2012) focused on the production of tin metal through electrolysis of Sncontaining solution. Leaching of PCB powders in 2.18N H2SO4 at 60 ℃ generated a leach
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liquor containing 2 g/L Sn and 2.8 g/L Cu. However, authors used synthetic solution (2 g/L Sn) in electrolysis experiments to optimize parameters such as time, electrolyte temperature and current density through full factorial design. It is concluded that 97.5% Sn can be recovered from the solution within 35 minutes at 300 A/m2 cathodic current density. Jha et al. (2012) recently reported the leaching behavior of lead from the solder materials present in the outer layer of waste PCBs, liberated by organic swelling. More than 99% Pb dissolution was
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possible with 0.2M HNO3 at S:L ratio 1:100 (g/mL) and 90 ℃ in 45 minutes. Tin remaining in the residue was further leached with 3.5M HCl at 90 ℃ for 120 minutes at S:L ratio 1:20 (g/mL) to achieve 98.74% leaching efficiency.
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Oh et al. (2003), using 2M H2SO4 and 0.2M H2O2 at 85 ℃ obtained more than 95% extraction of Fe, Zn, Ni and Al after 12 h of leaching. However, subsequent leaching of the
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leached residue in 2M NaCl solution at ambient temperature more than 95% of Pb was achieved. Brandon et al. (2001) suggested a single leachate route comprising of electrogenerated chlorine in an acidic aqueous solution of high chloride ion activity to produce a multi-metal leach electrolyte containing all of the possible metal values. 5.3.4 Bio-hydrometallurgical route Bio-hydrometallurgical processing is well established as an alternative route for recovering metals especially copper and gold from very low-grade ores and concentrates. The investigations have also been extended to other metals due to low investment cost, less environmental impact, less energy consumption and better control than the conventional
ACCEPTED MANUSCRIPT pyrometallurgy or hydrometallurgy routes (Brierley and Brierley, 2001; Cui and Zhang, 2008). In recent years, bio-hydrometallurgy has also been applied for metal recovery from waste PCBs. The extraction of metals such as Cu, Ni, Zn, Cr and precious metals from PCB scrap is technically feasible by the use of bacteria-assisted reaction (Brandl et al., 2001; Faramarzi et
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al., 2004; Karwowska et al., 2014). Brandl et al. (2001) indicated that it is possible to mobilize metal from E-waste by the use of the microorganisms such as bacteria (Thiobacilli) and fungi (A. Niger, P. Simplicissimum). Faramarzi et al. (2004) used different cryogenic bacterial strains (Chromobacterium violaceum, Pseudomonas fluorescens, Bacillus
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megaterium) to recover gold from waste PCBs and found maximum 14.9% gold dissolution as dicyanaoaurate [Au(CN)2-]. An investigation on the extraction of Cu from waste PCBs
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using bacterial consortium enriched from natural acid mine drainage establishes that extraction of copper is mainly accomplished indirectly through oxidation by Fe(III) ions generated from Fe(II) oxidation by bacteria (Xiang et al., 2010). The Cu recovery rate primarily depends upon the initial pH, Fe2+ concentration and bio-oxidation rate of Fe2+. The sequential reactions are given below
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4Fe2+ + O2 + 4H+ = 4Fe3+ + 2H2O Cu + 2Fe3+ = Cu2+ + 2Fe2+
(13) (14)
Similar results were also achieved while bioleaching of metal concentrates obtained from
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PCB scrap, by mixed culture of acidophilic bacteria (Zhu et al., 2011). The results obtained after column bioleaching of E-waste (Ilyas et al. (2010) using moderately thermophilic bacteria that included Sulfobacilllus thermosulfidooxidans and Thermoplasma acidophilum,
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are shown in the Fig. 4. Table 1 summarizes some reported results on metal extraction from waste PCBs using different types of microorganisms. Nevertheless, metal precipitation especially Cu at higher scrap concentration as well as toxic influence of ingredients on the growth of microorganisms are some of the major drawbacks projected by various researchers during bioleaching of PCB scrap apart from very slow reaction rate (Brandl et al., 2001; Choi et al., 2004; Cui and Zhang, 2008; Xiang et al., 2010; Zhu et al., 2011). Biosorption, another facet of bio-hydrometallurgy, is a procedure to recover metals from the leach liquor. The procedure is based on the different physico-chemical interactions (such as ion-exchange, complexation, coordination and chelation) between metal ions and the charged
ACCEPTED MANUSCRIPT surface groups of microorganisms. Creamer et al. (2006) demonstrated the bioseparation procedure to recover precious metals (Au and Pd) from E-waste leach liquor using Desulfovibrio desulfuricans biomass. It is concluded that bioseparation method is feasible for Au(III), Pd(II) and Cu(II) through a 3-step process (shown in the Fig. 5). In the first step fresh biomass could selectively precipitate Au(III) from leach liquor, while Pd(II) precipitation was
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inhibited due the presence of high amount of Cu(II). In second step, the pre-treated (palladised) biomass was used to catalyze the precipitation of Pd(II) as elemental Pd. The residual solution when treated in the 3-rd step by biogas generated through K. pneumoniae or E. coli, copper can be removed as a mixture of hydroxide and sulfate. In the steps I and II,
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hydrogen sparging was employed for initiation of metal reduction. 5.3.5 Electrochemical process
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Electrochemical processes have less environmental impact than chemical leaching as the main reagent in this process is electron, which is considered as “clean reagent” (Janssen and Koene, 2002; Walsh, 2001). Some of the earlier studies on E-scrap indicate the possible use of electro-chemical process for recovering valuable metals directly from E-scrap. Gold, palladium and silver from plated or coated metal scrap, can be recovered by iodide electrolysis using KI/KOH aqueous solution (Onwughara et al., 2010; Sum, 1991). Another
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approach (Sum, 1991) is the electro-leaching of copper from the waste PCBs to enrich the residue with precious metals. Electro-generated chlorine has been used to leach out metals from waste PCB scrap (shredded to below 4 mm size), followed by electro-deposition of metals at a graphite felt cathode as counter reaction of anodic generation of chlorine
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according to the following reaction (Brandon et al., 2001; Pilone and Kelsall, 2013).
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Anodic reaction: 2Cl- → Cl2 + 2e-
(15)
Leach reactor: Mscrap + (n-z)Cl- + z/2 Cl2 → MCln(n-z)-
(16)
Cathodic reaction: MCln(n-z)- → Mwon + (n-z)Cl-(catholyte) + z/2 Cl2
(17)
Kim et al. (2011) found electrogenerated Cl2 as a promising oxidant for gold leaching from PCBs followed by ion-exchange to further concentrate the gold solution. In the first stage, Cu was leached selectively with about 5% Au extraction followed by a second stage leaching of the first stage leach residue, resulting in 93% Au extraction. Recently Fogarasi et al. (2013) made a comparative environmental assessment on the Curecovery from waste PCBs by two different electro-chemical routes. Both the electrochemical
ACCEPTED MANUSCRIPT processes involve the dissolution of copper from waste PCBs with its simultaneous cathodic electrodeposition from the resulting leach liquor. The first process uses direct electrochemical oxidation in H2SO4 media while in the second dissolution of copper is through mediated electrochemical oxidation using the Fe3+/Fe2+ redox couple in HCl media. The process based on Fe3+/Fe2+ redox couple mediated electrochemical oxidation is found to be superior in
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terms of environment impact to direct electrochemical oxidation because regenerated Fe3+/Fe2+ solution can be used for further processing without addition of fresh reagent. Nevertheless, it is concluded that further studies are required in pilot plant scale to compare
5.3.6 Purification and recovery of metals
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with the existing hydrometallurgical plants in terms of the environment impact.
Various methods and options are available for the purification and selective recovery of the
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target metals from the pregnant leach liquor. These methods include solvent extraction, cementation, ion exchange, precipitation, adsorption, etc. (Habashi, 1999). The route to recover the metal(s) depends upon the economy of the process and how much efficient it is for a specific metal. However, very limited numbers of articles are available on purification and separation of individual metals from PCBs leach liquor.
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Electrowinning is one of the most common routes that people use to recover metal(s) from acidic solution. Although the recovery of metallic copper from acidic leach liquor (CuSO4 solution) is being practiced industrially for a long time, the efficiency is found to be lower than electrowinning from synthetic CuSO4 solution. In addition, extensive corrosion of Pb
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anode is observed due to the presence of impurity ions such as Co2+, Fe3+ and Cl- (Ehsani et al., 2012). Long Le et al. (2011) purified the leach liquor with 50% LIX 984N at pH 1.5 and A/O phase ratio of 1:1.5 in three stages and suggested that copper can be recovered by
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electrowinning or hydrogen reduction. Copper can also be recovered as copper sulfate crystal either from sulfate leach liquor (Yang et al., 2011) or after selective extraction in sulfate medium by solvent extraction. Another process (Fig. 6) developed by Alam et al. (2007) for the selective recovery of metals from waste PCBs involves cementation of Ag by Cu particles, followed by separation of Cu from other base metals using LIX 26 in ammoniacal solution and finally by electrowinning of copper. The other base metals such as Al, Mn, Fe, Co, Zn and Pb can also be recovered as their respective sulfate. A similar scheme reported by (Kamberović et al., 2011) is shown in Fig. 7 for the recovery of selected base metals and precious metals using sulfuric acid and thiourea. Sheng and Etsell (2007) established that precipitation of gold from chloride solution can be achieved by FeSO4. Park and Fray (2009)
ACCEPTED MANUSCRIPT used toluene for solvent extraction of gold from aqua regia solution and dodecanethiol and sodium borohydride solutions were used for gold nanoparticle formation. 6.
Concluding remarks
Waste PCBs account for ~3% of nearly 50 Mt/year global E-waste generations. A significant
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amount of Cu and Au in it attracts crude recyclers in some parts of Asia and Africa leading to substantial environmental and health problems. Due to the heterogeneous composition and hazardous material contents, proper recycling methodology is a still a challenging task.
Selective disassembly of hazardous components from the PCBs minimizes the toxic elements
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ending up in the mainstream of the recycling process. Following the dismantling, good liberation of non-metals from metals is required for effective separation. In general, milling
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to size below 150 µm ensures no interlocking of metallic and non-metallic particles. For physical separation of metals and non-metals, the density-based separators are quite popular. The latest developments include Corona Electrostatic Separator, which is based on the difference in density and electrical conductivity between plastics, metals and ceramics. Potential applications of recovered non-metallic fractions are diverse e.g. as filler material or as composites having similar properties of traditional filler materials. Recovered glass fibers
insulation materials.
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perform better than commercial sound absorbing materials and have great potential as
In contrast to physical separation, chemical separation generally involves decomposition of
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plastics through pyrolysis producing oils, gases, tar, etc. A major shortcoming is the presence of the significant amount of dioxin precursors in pyrolysis oils, which can possibly be
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reduced by adding CaCO3, Fe2O3 during pyrolysis. In recent years, supercritical fluids have been an effective medium for the destruction of epoxy adhesive layer. For metal extraction, pyrometallurgy route has the main advantage of its ability to accept any forms of scrap but suffers from the limitation of selective refining. In a reported study, waste PCBs produced a Cu-Ni-Si alloy, a mixed oxide (mainly Pb and Zn) and slag by a top blown reactor. Vacuum metallurgy separation (VMS) is suitable for Bi, Sb, Pb and other heavy metals with high vapor pressure. Hydrometallurgical route mostly focused on copper recovery. Although sulfuric acid is the most preferable reagent for Cu leaching but downstream processing becomes complicated
ACCEPTED MANUSCRIPT due to poor leaching selectivity in inorganic acids. Ammonia leaching, on the other hand, has more selectivity towards copper. Recycling of PCBs can only be profitable with substantial gold recovery. Thiosulfate and thiourea seem to be promising one alternative to cyanide for gold leaching. An interesting approach for gold recovery is the persulfate based leaching where all base metals including silver dissolve keeping gold in the solid residue, which can
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be later purified by melting. Bioleaching using microorganisms such as bacteria and fungi is technically feasible for the extraction of base metals and precious metals from PCB scrap. Apart from low reaction rate, metal precipitations as well as bacterial toxicity are some of the major issues projected by
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various researchers.
Electrochemical metal extraction route, on the other hand, is considered as more environment
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friendly than chemical leaching because it involves on the electron transfer process aided by the supplied electrode potential with minimum chemical reagents requirement. Aqueous processing route mostly generates multi-metal containing solutions and recovery of individual metal either in metal form or as salt is a challenging task. Cementation of Au/Ag using Cu/Zn powders or Cu solvent extraction followed by Cu electrowinning processes are
are still unresolved.
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no doubt established, but issues regarding other metals recovery from the PCB leach liquor
The authors hope that the coverage of items in the paper will assist the scientific community,
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policy makers and other stakeholders to find the gap areas in achieving a cleaner and economical recycling process. Evidently, more studies are needed in the area of metal separation and recovery from PCB leach liquor. Authors feel hydrometallurgical route will be
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a key player in the metal recovery step following a successful physical separation step.
ACKNOWLEDGEMENT
The authors are thankful to Prof. B. K. Mishra, Director, IMMT-Bhubaneswar for his kind permission to publish this article. REFERENCES WECC Global PCB Production Report For 2011. http://www.eipc.org/eipcpublications/weccreport-2012/ Accessed on July, 2014.
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Recovered figures
References
Thiobacillus thiooxidans + T. ferrooxidans
Cu, Ni, Al, Zn >90%
(Brandl et al., 2001)
Aspergillus niger,
Cu, Sn 65%
(Brandl et al., 2001)
Penicillium simplicissimum
Al, Ni, Pb, Zn >95%
Acidithiobacillus ferrooxidans
Cu 81.6%
Acidiphilium acidophilum (ATCC 27807)
Cu 3.6%, Ni 86%, Zn (Hudec et al., 2005) 40.8%
Sulfobacillus thermosulfidooxidans + acidophilic heterotroph (A1TSB)
Ni 81%, Cu 89%, Al 79%, (Ilyas et al., 2007) Zn 83%
Chromobacterium violaceum
Au 68.5%
A. ferrooxidans
Cu 99%
(Yang et al., 2009b)
A. ferrooxidans,
Cu 99%
(Wang et al., 2009)
A. thiooxidans
Cu 74.9%
A. ferrooxidans + A. thiooxidans
Cu 99.9%
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Microorganisms
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(Choi et. al., 2004)
(Brandl et al., 2008)
genera Acidithiobacillus and Gallionella
Cu 95%
Thermosulfidooxidans sulfobacilllus
Cu 86%, Zn 80%, Al 64%, (Ilyas et al., 2010) Ni 74%
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+Thermoplasma acidophilum
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Chromobacterium violaceum
Cu 24.6%, Au 11.3%
(Xiang et. al., 2010)
(Chi et. al., 2011)
genera Acidithiobacillus and Gallionella
Cu 96.8%, Al 88.2%, Zn (Zhu et. al., 2011) 91.6%
At. ferrooxians, L. ferrooxidans, At. thiooxidans
Cu 95%
(Bas et al., 2013)
Acidithiobacillus Thiooxidans
Cu 98%
(Hong and Valix, 2014)
Table 1. Bioleaching of E-scrap in different published literature
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Fig. 1 Trends in global printed circuit board production and forecast (2012-2016) in millions of $US [Source: Prismark (2012)]
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Fig. 2 PCB production (2011) by major producing Countries/Region (Source: WECC Global PCB Production Report for the year 2012)
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Fig. 3 Recycling process of E-waste containing copper(Antrekowitsch et al., 2006)
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Fig. 4 Percent metals solubilization during whole leaching process included pre-leaching and bioleaching (Ilyas et al., 2010)
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Fig. 5 Diagrammatic representation of proposed 3-step process for platinum group metal (PGM) recovery from E-waste leachate, and removal of Cu2+. Pre-treatment of cells refers to pre-palladisation (Creamer et al., 2006).
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Fig. 6 Proposed flowsheet of the monovalent copper electrowinning process(Alam et al., 2007)
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Fig. 7 Block diagram for hydrometallurgical recovery of base and precious metals (Kamberovic, 2011)
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More than 150 original peer-reviewed research articles published over the span of 20 years have been reviewed here.
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Most of the processes dealing with PCB recycling have been discussed.
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Special emphasis is given on the hydrometallurgical approaches and alternative uses
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of non-metallic materials.
