Elsevier

Waste Management

Volume 57, November 2016, Pages 64-90
Waste Management

Review
Recovery of metals and nonmetals from electronic waste by physical and chemical recycling processes

https://doi.org/10.1016/j.wasman.2016.08.004Get rights and content

Highlights

  • The primary objective of PCB recycling is to minimize the harmful environmental impact and ensure the maximum recovery of both metallic (30%) and nonmetallic materials to conserve scarce sources.

  • PCBs represent the most valuable portion of WEEE and account for about 3–5% of nearly 50 million tpy global e-waste.

  • Desoldering PCB assemblies by thermal and/or chemical treatment in order to remove reusable valuable electronic components and eliminate hazardous materials before recycling processes is the first and most important step.

  • Physical separation processes are simple, convenient and environmentally sound. They benefit from low capital and operating costs and suffer from a high metal loss (10–35%) and BFR dust problem.

  • Today, more than 70% of waste PCBs is treated in smelters. Pyrometallurgical route suffers from the limitation of selective refining, heavy metals and BFR fumes. Hydrometallurgical route is more selective towards metal recovery from waste pretreated PCBs and creates less environmental hazards than pyrometallurgical approach.

  • For Au/Ag recovery, thiourea or cyanide; for Cu, H2SO4 + H2O2 or ammonia; for Pb, HNO3 and for Sn, HCl and for solder, HBF4 + H2O2 are the best leaching reagents.

Abstract

This paper reviews the existing and state of art knowledge for electronic waste (e-waste) recycling. Electrical and/or electronic devices which are unwanted, broken or discarded by their original users are known as e-waste. The main purpose of this article is to provide a comprehensive review of e-waste problem, strategies of e-waste management and various physical, chemical and metallurgical e-waste recycling processes, their advantages and disadvantages towards achieving a cleaner process of waste utilization, with special attention towards extraction of both metallic values and nonmetallic substances. The hazards arise from the presence of heavy metals Hg, Cd, Pb, etc., brominated flame retardants (BFRs) and other potentially harmful substances in e-waste. Due to the presence of these substances, e-waste is generally considered as hazardous waste and, if improperly managed, may pose significant human and environmental health risks.

This review describes the potential hazards and economic opportunities of e-waste. Firstly, an overview of e-waste/printed circuit board (PCB) components is given. Current status and future perspectives of e-waste/PCB recycling are described. E-waste characterization, dismantling methods, liberation and classification processes are also covered. Manual selective dismantling after desoldering and metal-nonmetal liberation at −150 μm with two step crushing are seen to be the best techniques. After size reduction, mainly physical separation processes employing gravity, electrostatic, magnetic separators, froth floatation, etc. have been critically reviewed here for separation of metals and nonmetals, along with useful utilizations of the nonmetallic materials. The recovery of metals from e-waste material after physical separation through pyrometallurgical, hydrometallurgical or biohydrometallurgical routes is also discussed along with purification and refining. Suitable PCB recycling flowsheets for industrial applications are also given. It seems that hydrometallurgical route will be a key player in the base and precious metals recoveries from e-waste.

E-waste recycling will be a very important sector in the near future from economic and environmental perspectives. Recycling technology aims to take today’s waste and turn it into conflict-free, sustainable polymetallic secondary resources (i.e. Urban Mining) for tomorrow. Recycling technology must ensure that e-waste is processed in an environmentally friendly manner, with high efficiency and lowered carbon footprint, at a fraction of the costs involved with setting multibillion dollar smelting facilities. Taking into consideration our depleting natural resources, this Urban Mining approach offers quite a few benefits. This results in increased energy efficiency and lowers demand for mining of new raw materials.

Introduction

E-waste comprises of waste electric and electronic equipments (WEEE/EEE) or goods which are not fit for their originally intended use. Such EEEs may be TVs, telephones, radios, computers, printers, fax machines, DVDs, CDs, washing machines, refrigerators, dryers, vacuum cleaners, etc. Fig. 1 shows the composition distribution of e-waste. Half of the e-waste is coming from electrical appliances and the rest from electronic goods. Fig. 2 shows the four sources of e-waste. Small/large home appliances, hospital medical equipments, government office machines (information technology (IT) and telecom equipments) and private sector offices and industrial equipments and machines are main source of e-waste. Consumer and lighting equipments; electrical and electronic tools; entertainment devices; toys and sports equipments and monitoring and controlling equipments are also important source of e-waste. EEEs can become e-waste due to rapid advancement in technology; development in society; change in style, fashion and status; greater demands on EEE; nearing the end of their useful life and not taking precaution while handling them. The replacement of EEE becomes more frequent, which results in large quantities of e-waste need to be disposed (Zhou and Qiu, 2010).

E-waste is one of the fastest growing waste streams and it has been estimated that these items already constitute about 8% of municipal waste (Widmer et al., 2005). E-waste contains lots of valuable resources together with plenty of heavy metals and hazardous materials, which are considered both an attractive polymetallic secondary source and an environmental contaminant. E-waste represents a rapidly growing disposal problem worldwide. Therefore, recycle of valuable metallic and/or nonmetallic materials from them are necessary and compulsory in many developed/developing countries. With the phenomenal technological advancement and growth in electronic industries, the number of consumer and business electronic products per capita has been raised manifold in the last three 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 generation of End-of-Life (EoL) electronic goods. The United Nations (UNs) estimate of the global WEEE production was 14 million tons in 1992, 24 million tons in 2002, 49 million tons in 2012 and more than 50 million tons today; and the number is growing at an exponential rate (Fig. 3). The rate of e-waste generation is increasing by 10% every year (Sakunda, 2013). The USA, China, Japan, Germany and Russia were the biggest e-waste generating countries in 2012. The USA, Australia and the UK had the biggest e-waste production per capita in 2012. E-waste amount is 5–30 kg per person per year and grows at 3 times faster than the municipal waste (Zhou and Qiu, 2010, Wang et al., 2015, Kaya, 2016a).

Table 1 shows the hazardous substance occurrences in WEEE and possible adverse affects to the human health. Hg is used in relays, switches, batteries, liquid crystal displays (LCDs) and gas discharge lamps (i.e. fluorescent tubes in scanners and photocopiers). Yearly, about 22% of the Hg produced in the world is used in electronics industry. Rechargeable batteries contain Pb, Cd, Li and Ni. Old TVs, Personal Computers (PCs) and Cathode Ray Tubes (CRTs) contain Pb in cone glass, Ba in electron gun getter and Cd in phosphors. PCBs have Pb, Sn and Sb in solder; and Cd and Be are found in switches. Polyvinylchloride (PVC) and BFRs are main components of plastics. Cr6+ are found in data types and floppy discs. Condensers and transformers contain polychlorinated biphenyls (PBBs). Chlorofluorocarbon (CFC) can be found in cooling units and insulation foams. Americium (Am) can be found in smoke detectors. LCDs include liquid crystals embedded between thin layers of glass and electrical control elements. Liquid crystals are mixture of 10–20 substances which belong to the groups of substituted phenylcyclohexanes, alkylbenzenes and cyclohexylbenzenes. These substances contain O, F, H and C and are suspected to be hazardous.

Pb causes damages on human central and peripheral nervous systems, blood systems and kidney. It also affects brain development of children. Cr causes asthmatic bronchitis and damages on DNA. Cd has a toxic irreversible effects on human health, accumulates in kidney and liver. Cd also causes neural damage. Hg causes a chronic damage to the brain and respiratory system. Plastics and PVC produce dioxins after burning. They cause reproductive and development problems, damage immune system and interfere with regulatory hormones.

Proper e-waste management for all countries is necessary; because, e-waste pollutes the ground water, acidifies the soil, generates toxic fume and gas after burning, accumulates fastest in municipal disposal areas and releases carcinogenic substances into the air. For a proper e-waste management, the most favored to least favored option hierarchy pyramid is given in Fig. 4. For a proper e-waste management; waste prevention conserves scarce resources; minimization reduces material usage and reuse uses materials again. They are the most favored options and are on top of the e-waste hierarchy pyramid. Burning (i.e. incineration or pyrolysis for energy recovery prior to disposal) and disposal by landfilling are the least favored options in the e-waste management pyramid. Disposal does not conserve any resources. Recycling e-waste is an intermediate polymetallic secondary resource recovery option. Open dumping is the most common form of e-waste disposal in the most developing countries. Burial or landfill disposal allows heavy metals to be leached into the ground water or methane off gassing. Combustion of organic substances in waste by incineration makes hazardous material airborne, generates ashes and heat. Leaching of the ashes may cause water and soil contamination. E-waste constitutes 40% of Pb and 70% of heavy metals in landfills (Sepülveda et al., 2010; Kaya, 2016b).

BFRs are used in both PVC and in other types of plastics to reduce the flammability of PCBs, cables and plastic covers of WEEEs. Incineration has a risk of generating and dispersing contaminants and toxic substances. The gases released during the burning and residue ash is often toxic and requires expensive flue gas purification systems. Studies have shown that Cu in PCBs and cables acts as catalyst for dioxin formation when BFRs are incinerated. These BFRs when exposed to low temperature (600–800 °C) uncontrolled burning can lead to the generation of extremely toxic polybrominated/polychlorinated dioxins (PBDDs/PCDDs and furans (Fs). PVC which can be found in e-waste in significant amount is highly corrosive when burned and also induces the formation of dioxins. Concerns have also been raised about the use of stabilizer Cd metals and phthalate plasticizers in PVC. Phenolic BFRs and glass fiber are generally used in PCBs (Tsydenova and Bengtsson, 2011).

Grabda et al. (2009) investigated the bromination of ZnO by thermal decomposition of tetrabromobisphenol (TBBPA). They found that the bromination of ZnO occurred at 272 °C (DSC) and above 290 °C (furnace) effectiveness of 41, 64 and 81% dependent on experimental conditions. Volatilization of the formed ZnBr2 began at 340 °C and had a yield at 650 °C. Oleeszek et al. (2013) investigated the distribution of Cu, Ag and Au during thermal treatment with BFRs. They found that 50% of Cu and Ag can evolve from sample residues in the form of volatile CuBr and AgBr above 600 and 1000 °C, respectively. Au was resistant to HBr and remained unchanged in the residue. Incineration also leads to the loss of valuable trace elements which could have been recovered, if they had been sorted and processed separately.

In landfill, e-waste is placed in a hole, compacted and covered with soil, i.e. buried under soil. It reduces the amount of rats, lessens the danger of fire and decreases the bad odor. A double liner system (i.e. compacted plastic clay and plastic geomembrane liner) at the bottom prevents liquid waste from seeping into the ground water and collects leachate to seep through the solid waste. Improper treatment of e-waste generates serious soil, air and water pollution problems. Current improper e-waste handling includes:

  • open burning of circuit boards and cables for metals,

  • acid/cyanide stripping of valuable metals and

  • CRT cracking and dumping.

For a proper e-waste management, EEE producers’ extended responsibilities (EPR) include proper labeling the materials to assist recycling, limit the toxic constituents in the products, use green/recyclable raw materials in the production, minimize the waste amount in the product, offer take-back program options, etc. Responsibilities of governments are implementation of legislations and laws; strict regulations against illegal dumping of e-waste; heavy fines on industries and encourage non-governmental organizations (NGOs) for awareness. E-waste includes at least 57 valuable elements found in periodic table (Fig. 5). E-waste contains valuable resources which offer opportunities for Urban Mining and job creation. The USA Environmental Protection Agency (EPA) and UNs estimate that only 15–20% of e-waste is recycled, the rest of these consumer electronics go directly to landfills and incineration (United Nations University, 2009). According to Assocham report in India, only 1.5% of total e-waste is recycled by formal recycling sector in an environment friendly way. The rest is recycled by informal recycling sector in the world (http://www.assocham.org/newsdetail.php?id=5725).

Chronologically serious international programs combating e-waste problem are the UNs Development Programme (UNDP)’s Basel Convention (No transboundary movement of hazardous waste) in 1989. The Swiss Economic Association for the Suppliers of Information, Communication and Organizational Technology (SWICO) was proposed e-waste project with an Advance Recycling Fee (ARF). The Swico recycling system has been developed to ensure that WEEE could be taken back free of charge in Switzerland since 1994. Japan’s Home Appliances Recycling Law (i.e. take-back, recycle end products) was implemented in 2001.

Restriction of the use of certain Hazardous Substances in electrical and electronic equipment (RoHS) directive (2002/95/EC) brings restriction for complement of EEE that are shipped to the European market after the 1st July 2006 and limits the use of six hazardous substances (i.e. four heavy metals: Pb, Hg, Cd, Cr6+, and two BFRs: polybrominated biphenyls (PBB), and polybrominated diphenyl ethers (PBDE), with some limited exemptions). The commission decision 2005/618/EC of 18 August 2005 established maximum concentration values of 0.1% by weight in homogenous materials (i.e. plastics, ceramics, glass, metals, alloys, paper, board, resins, coatings) for Pb, Hg, Cr6+, PBB, and PDBE and 0.01% by weight in homogenous materials for Cd. Manufacturers are fully accountable for ensuring that their products are in compliance. Failure to comply with the RoHS requirements will result in the removal of manufacturers’ products from the market. The purpose of EU directive 2002/96/EC of 27 January 2003 on WEEE (reuse, recycle, tack-back, recycling cost) is, as a first priority, the prevention of WEEE, and in addition, the reuse, recycling and other forms of recovery of such wastes so as to reduce the disposal of waste. It also seeks to improve the environmental performance of all operators involved in the life cycle of EEE, e.g. producers, distributors and consumers and in particular those who directly involved in the treatment of WEEE. OECD’s Environmentally Sound Management of Waste (Reclaim e-waste) was implemented in 2004. Finally, EUs’ HYDROWEEE (FP7-SME) project deals with the recovery of base and precious metals from WEEE including lamps and spent batteries by hydrometallurgical processes. The idea was to develop a mobile plant using hydrometallurgical processes to extract metals like Cu, Mn, Zn, In and Y in a high purity (>95%) (http://www.4980.timewarp.at/sat/hydroWEEE/).

According to the UNs 2014 Global e-Waste Surveillance Report, 503,000 tons of e-wastes were produced in Turkey (http://unu.edu/news/news/ewaste-2014-unu-report.html). E-waste amount was 6.5 kg/person in 2014. Turkey is the 17th biggest e-waste producer in the world. Between 2006 and 2012, about 30,500 tons of WEEEs were collected and separated in Turkey by 30 licensed recyclers. Waste PCBs with electronic parts are exported to Belgium, Germany and France for further recycling from Turkey. In the EU, 9.3 million tons of e-waste was collected and only 35% of them are recycled in 2012. 50–80% e-waste collected in the USA and other developed countries exported to third world countries. E-waste export map is shown in Fig. 6 (http://ewasteguide.info/europe-breaking).

PCBs represent the most economically attractive portion of WEEE and account for the weight for about 3–5% (Jiang et al., 2012, Kaya, 2016b). Waste PCBs constitute a heterogeneous mixture of metals, nonmetals and some toxic substances. By containing many electronic components (ECs), such as resistors, relays, capacitors, and integrated circuits (ICs), waste PCBs have a metal content of nearly 30% Cu;10–20%, solder Pb; 1–5%, Ni; 1–3%, Fe; 1–3%, Ag; 0.05%, Au 0.03%; and Pd 0.01%, especially the purity of precious metals in PCBs is more than 10 times that of rich minerals (Zhou and Qiu, 2010). It can be seen clearly that except the hazardous substances, a lot of valuable materials contained in PCBs make them worth being recycled. Therefore, developing a non-polluting, efficient and low cost processing technology for recycling of PCBs can not only avoid environmental pollution, but also help to recycle valuable resources, which have a great significance for continuous improvement of the human living environment, standards and resources recycling.

In the USA, e-waste recycling ratio for computers in 2010 was 40% (423,000 t), monitors 33% (595,000 t), mobile phones 11% (19,500 t), keyboards and mice 10% (67,800 t) and TVs 17% (1,045,000 t). In the USA, about 40,000 mobile phones discarded every day (www.powershow.com). A personal computer material composition includes about 26% silica/glass, 23% plastics, 20% ferrous metal, 14% Al and 17% other metals (such as Pb, Cu, Zn, Hg, and Cd). A CRT panel contains 0–2% Pb, frit 65–75% Pb, funnel glass 22–25% Pb and neck 28–30% Pb. A 43 cm CRT monitor contains about 950 gr Pb. Manufacturing of a computer and monitor requires 240 kg fossil fuels, 2.2 kg chemicals, 1.5 tons of water. Recycling 1 million laptops saves the energy equivalent to the electricity used by 3657 homes a year. 41 smart phones contain about 1 gr of Au. Every year 1 million smart phones recycled and 16 tons Cu, 350 kg Ag, 34 kg Au and 1.5 kg Pd can be recovered.

Besides all the hazards originating from e-waste, manufacturing mobile phones and PCs consumes considerable fractions of the Au, Ag and Pd mined annually worldwide. 43% of total production of gold in the world is used in electronics. A large fraction of the WEEE precious metals is found on the PCBs. Since PCBs as becoming more complex and smaller, the amount of materials is constantly changing. 1 tons of circuit boards can contain between 80 and 1500 g of Au and between 160 and 210 kg of Cu. These concentrations are 40–800 times the amount of gold in Au ore, and 30–40 times the concentration of copper in Cu ore mined in the USA. Globally, 267.3 tons of Au and 7275 tons of Ag are consumed annually by electronic industry (Vats and Singh, 2015). According to 2014 UNs’ e-waste report, yearly about 300 tons of Au were recovered and the e-waste market size was 52 billion dollars. It has been proved that it is worthwhile to recycle electronic scrap in spite of the fact that the content of precious metals (Au, Ag and Pd) steadily decreases (http://unu.edu/news/news/ewaste-2014-unu-report.html).

E-waste, in particular waste PCBs, represents a rapidly growing disposal problem worldwide. Considering that the lifetime of a mobile phone is approximately 1 year and of a computer 2–5 years, it is estimated that about 100 million mobile phones and 17 million of computers are discarded annually in the world due to malfunctioning equipment or because technologies become obsolete. 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:

  • 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.

  • The presence of plastics, ceramics and metals in PCBs in a complex manner leads to great difficulty in liberation and separation of each fraction.

  • Presence of numerous metallic elements leads to a very complex recovery process. The recovery process becomes more complicated when the elements are available in ppm concentration.

  • The driving force for recycling is the recovery of metal values, which is nearly 30% of the total weight of waste PCBs. The nonmetallic materials (∼70%) have rather less economic values (i.e. filler material).

  • The objective of most recycling processes is to recover maximum metallic values from waste PCBs but sometimes these processes are not very environment-friendly.

Section snippets

Printed circuit boards (PCBs)

PCBs, the base of electronics, are essential part of almost all of the electronic products. PCBs are used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from Cu sheets laminated onto a non-conductive substrate. PCBs are integral part in majority of electronic systems and are commonly found in consumer electronics. PCBs constitute at least 3% of the total electronic scraps by weight. Most recycling approaches

Physical/mechanical recycling techniques

Physical processes are usually employed during the upgrading stage when various metals and nonmetals contained in e-waste are liberated and separated by some kinds of shredding and crushing processes. The drive to recover the valuable metals in particular Au, Ag, Pd and Cu has received tremendous attention in recent years using extraction processes such as physical, chemical and hydro/pyrometallurgical leaching separation techniques. Methodology generally includes PCB assembly desoldering

Chemical recycling techniques

Chemical recycling is to decompose the waste polymers into their monomers or some useful chemicals by means of chemical reactions. Pyrolysis, gasification, depolymerization using supercritical fluids and hydrogenolytic degradation processes are four chemical recycling techniques (Guo et al., 2009). Chemical recycling separates organic and metallic materials. Many chemical recycling processes of waste PCBs have been tested in a laboratory scale. For instance, the pyrolysis process (heating

Metallurgical processes

Metallurgical processes are used in the upgrading and refining stages of the recycling chain. In metallurgical processes, metals are melted by heat (pyrometallurgical processes) or dissolved by a liquid (hydrometallurgical processes) and further sorted by making use of their chemical/metallurgical properties. Pyrometallurgical processing, notably smelting, has become a traditional method to recover metals from e-waste in the last three decades. In hydrometallurgical treatment, the main steps

Purification

Metals from PCBs are dissolved on leaching process and can be recovered by purification techniques such as: liquid/liquid extraction (solvent-extraction), precipitation/cementation and electrolyte refinement (electrowinning/electro-recovery) (Lister et al., 2014, Silvas et al., 2015). Cu can be extracted using LIX 84-Kerosene. Direct solvent extraction (DSX) or synergistic solvent extraction (SSX) can be carried out using Cyanex 301, 302 or 272 type lixiviates along with kerosene and TBP for Ni >

Alternative uses of nonmetallic fraction

In fact, most of the researchers recycle the nonmetallic fractions as filler for thermosetting resin composites and thermoplastic resin composites when considering physical recycling methods. Physical recycling grinds nonmetals into fine particles for addition into new composites as filler if they are clean. The nonmetallic powder obtained from mechanical process can only be used as low-value product (i.e. paint, paving material, cement, plastic and asphalt filling material) (Zhou and Qiu, 2010

Concluding remarks

E-waste is one of the fastest growing municipal solid waste streams worldwide today. WEEEs are one of the largest known sources of heavy metals without effective collection, reuse and recycling systems, they will be dangerous to environment. Recycling of WEEE and reuse of some electrical/electronic parts are a beneficial alternative than disposal. Waste PCBs account for about 3–5% of nearly 50 million t/year global E-waste generations.

For several years, waste PCBs have been poorly managed by

Muammer Kaya was born in Eskisehir-Turkey in 1960. He obtained his B.Sc. in Mining Engineering from Eskisehir Osmangazi University (ESOGU) in 1981. Prof. Dr. Kaya received his M.Sc. and Ph.D. from the Metallurgical Eng. Dept. of McGill University in Canada in 1985 and 1989, respectively. Prof. Kaya is interested in Mineral Processing, Flotation, Recycling and Environmental Protection. He has been working as a full Prof. at ESOGU since 1999. Prof. Kaya has worked as the director of ESOGU

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    Muammer Kaya was born in Eskisehir-Turkey in 1960. He obtained his B.Sc. in Mining Engineering from Eskisehir Osmangazi University (ESOGU) in 1981. Prof. Dr. Kaya received his M.Sc. and Ph.D. from the Metallurgical Eng. Dept. of McGill University in Canada in 1985 and 1989, respectively. Prof. Kaya is interested in Mineral Processing, Flotation, Recycling and Environmental Protection. He has been working as a full Prof. at ESOGU since 1999. Prof. Kaya has worked as the director of ESOGU Research Center for 15 years and the also founder and first manager of ESOGU Vocational School in Eskisehir-Turkey. Prof. Kaya is a member of TMS, CIM and AIME.

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