Keywords

Introduction

The production of electrical and electronic equipment (EEE) is one of the fastest growing domains of the manufacturing industry in the world. In response to the fast advancement in technology, the demand for EEE has increased dramatically. The high diversity of EEE coupled with a rapid obsolescence of products, due to advancement in technology, and an increasing level of household equipment have led to a significant increase in electrical and electronic equipment waste (e-waste). For example, the e-waste flow is projected to grow from 3 to 5% per year in European Union (EU) [1]. Currently, from 20 to 25 million tons per year of e-waste are being generated globally with major share of EU, USA and Australasia. In EU, e-waste has been projected to increase by 45% between 1995 and 2020 [2]. Countries included in the Emerging Market Economies such as China and India are expected to become significant e-waste producers in the next decade [3].

The EU WEEE directive classifies e-waste as electrical or electronic equipment, which is discarded including all components which are part of the product at the time of discarding [1, 2]. The EU WEEE directive categorizes e-waste into ten different classes from large household appliances to automatic dispensers. E-waste has a complex composition of ferrous, non-ferrous, plastic and ceramic materials. E-waste is classified as hazardous material; therefore, it should be handled properly. However, the presence of valuable metals in e-waste such as Au, Ag, Pt, Ga, Pd, Ta, Te, Ge, and Se makes it attractive for recycling. Recovering materials from e-waste is more profitable than processing primary raw materials largely due to the savings in energy associated with e-waste recycling. According to Boliden Rönnskär (Skelleftehamn, Sweden), extracting metals from e-waste requires only from 10 to 15% of the total energy required in smelting and refining of metal ore concentrates. The e-waste recycling rate for the EU is significantly higher, 35%, than the e-waste recycling rate of the USA, 27% [4].

Currently, increasing demand for valuable metals, complexities of the available raw materials, and earth’s intrinsic limitations pose a challenge to the entire valuable metals production system. The United Nations Environment Program (UNEP) is also calling for an urgent re-think of metals recycling practices as global demand for metals continues to soar. Consequently, urban mining such as the recovery of precious metals from electronic waste (e-waste) streams through sustainable recycling processes have been developing. The main purpose of this paper is to critically review energy efficient valuable metals recovery from e-waste streams by pyrometallurgical and hydrometallurgical processes . Main features of these processes including their advantages and disadvantages are described. And, innovative ideas for different steps of the thermochemical processes in the valuable metals and energy recovery from the e-waste streams are discussed.

Classification and Composition of E-waste

According to the Association of Plastics Manufacturers in EU (APME), average materials consumption in electric and electronic equipment are 38 wt% ferrous, 28 wt% non-ferrous, 19 wt% plastics, 4 wt% glass, 1 wt% wood, and 10 wt% others [5]. Thus, despite its common classification as a waste, e-waste constitutes considerable amounts of secondary resource.

Metals in e-waste can be grouped into precious metals—PMs (Au and Ag), platinum group metals—PGMs (Pd, Pt, Rh, Ir and Ru), base metals—BMs (Cu, Al, Ni, Sn, Zn and Fe), metals of concern—MCs (Hg, Be, In, Pb, Cd, As and Sb), and scarce elements—SEs (Te, Ga, Se, Ta and Ge) [6]. The initiation for the recovery of valuable metals is important for e-waste management, recycling, sustainability and resource conservation. The extraction of PMs, PGMs, and BMs from e-waste is a major economic drive due to their associated value. As shown in Table 1, printed circuit boards (PCBs) are the most precious part in e-waste streams. PCBs are found in electrical and electronics appliances such as televisions, computers, mobile phones, etc. For example, flat screens contain one or more PCBs equipped with electronic components and connectors. Considerable amounts of precious metals are contained both in the components and connectors as well as in the solders [7]. For example, PCB from an LCD television constitutes 575 mg of Ag, 138 mg of Au, and 44 mg of Pd [7]. In general, precious metals in PCB account for more than 80% of the total intrinsic value even though their composition is less than 1 wt% [8].

Table 1 Weight distribution of PMs, PGMs, and BMs in e-waste [6]
Full size table

Generally, PCBs are composed of 40 wt% metals, 30 wt% plastics and 30% ceramics [9]. PCBs are coated with base metals (BMs) such as Sn, Ag or Cu to make them conductive. Polymers and industrial plastics are the other major constituents of PCBs that contain polyethylene, polypropylene, epoxies and polyesters. Large numbers and various kinds of small components are attached to PCBs. During the recycling process, generally, PCBs are crushed into smaller sizes (less than 1–2 mm) and various techniques including magnetic, electrostatic, electrowinning, and selective dissolution are implemented to separate the components [9,10,11,12,13,14,15,16,17]. The sustainable resource management demands the isolation of hazardous metals from e-waste and also optimization of the recovery of PMs. The loss of PMs during the recycling chain will adversely affect the process economy. The value distribution of PMs in PCBs and calculators is more than 80%. After PMs, Cu is the next highest valuable metal to be extracted from e-waste [6].

E-waste Recycling Technologies

There are three major reasons for e-waste processing: environmental concerns, energy savings and resource efficiency. E-waste recycling consists of three main steps: collection, preprocessing, and recovery of valuable materials and disposal of non-recyclable ones [18]. The recycling methodology broadly comprises of shredding, sorting, grading, compacting, baling, or processing segregated plastics and metal components, followed by separation, identification, and testing as relevant [19]. Each step is critical for the recovery of metals and the recycling economy. E-waste collection is facilitated by appropriate government policies, effective advertisement for public awareness, and by installing separate collection facilities at public places. The goal of the EU WEEE directive is to increase the collection rate of discarded electronic and electrical products from 65% by 2012 to 85% by 2016 [1, 2]. End of life electronic components are sorted at the collection facility where useable components are returned to the consumer supply chain.

Pre-processing of e-waste is one of the most important steps in the recycling chain. The discarded equipment are manually dismantled at collection facilities and individual components are isolated. During the early stage, housing, wiring boards and drives, and other components are disassembled. Mechanical processing is an integrated part of this stage where e-waste scrap is shredded into pieces using hammer mills [18]. Metals and non-metals are separated during this stage using techniques similar to that used in the mineral dressing, e.g., screening, magnetic, eddy current and density separation techniques. The recoveries of SEs, which exist as traces in the e-waste stream, require special recovery technology [19].

The final stage in the recycling chain of e-waste is the end processing, where the non-metal and metal fractions of e-waste are further processed. The non-metallic fractions of PCBs are mainly composed of thermoset resins and glass fibers. Thermoset resins cannot be re-melted due to their chain structure. However, chemical processes that include gasification, pyrolysis, supercritical fluid de-polymerization and hydrogenolytic degradation can be applied for producing chemical substances and fuels [20]. For example, the non-metallic fractions can be used in pyrometallurgical processes as fuels and reducing agents.

Recycling Through Pyrometallurgical and Hydrometallurgical Processes

Pyrometallurgical processing techniques, including conflagrating, smelting in a plasma arc furnace, drossing, sintering, melting, and varied reactions in a gas phase at high temperatures for recovering BMs and PMs from e-waste is the conventional method used in the past two decades. In the process, the crushed scraps are liquefied in a furnace or in a molten bath to remove plastics, in which the refractory oxides form a slag phase together with other metal oxides. Recently, Cui and Zhang [21] revised the process of metals recovery from e-waste streams, including hydrometallurgy and pyrometallurgical processes. They reported that hydrometallurgical processes are more predictable and controllable compared to pyrometallurgical processes. However, hydrometallurgical process routes have certain limitations in recovering PMs, at industrial scale. Optimal recovery of PMs can be achieved through pyrometallurgical process routes, this make them more economical and eco-efficient compared to the hydrometallurgical process routes [21].

For example, the Noranda process at Quebec, Canada, recycles about 1 × 105 tons of e-waste per year [22]. Materials entering the reactor are immersed in a molten metal bath at about 1250 °C, which is churned by a mixture of O2-rich air (up to 39% oxygen), this effectively reduces energy consumption due to combustion of plastics and other inflammable materials in feeding. In the process, impurities including Fe, Pb, and Zn are converted to oxides, forming silica-based slag aided by the agitated oxidation zone, followed by cooling and milling of the slag for further recovery of metals prior to its disposal. PMs content of the Cu matte is removed before being transferred to the converters, which yield liquid blister copper. The PMs, including Au, Ag, Pt, and Pd together with SEs and Ni constitute about 0.9%, which are recovered through electrorefining process at the anodes.

Pyrometallurgical processing for the recovery of metals from e-waste is widely applied by Boliden Rönnskär (Skelleftehamn, Sweden) [23]. The scraps with high Cu content are processed in the Kaldo Furnace and around 1 × 105 tons of scraps including e-waste was reportedly being processed in the Kaldo Furnace per year, as per an APME report during the year 2000. E-waste blended with lead concentrates is processed in a Kaldo rector with skip-hoist assisted feeding [24] and the required oxygen for combustion in oil-oxygen burner is provided through an oxygen lance in the system, while off-gases are subjected to additional combustion air at around 1200 °C. A standard gas handling system recovers thermal energy through the integrated steam network. The mixed Cu-alloy produced by the Kaldo furnace is processed in a copper converter for recovery of valuable metals (such as Cu, Ag, Au, Pd, Ni, Se, and Zn), while the dust content (containing Pb, Sb, In, and Cd) is subjected to other processing operations to recover the metals.

Hydrometallurgical Processes

Various investigators studied the extraction of PMs and BMs from e-waste using hydrometallurgical routes [15, 25,26,27,28]. These routes are based on traditional hydrometallurgical technology of metals extractions from their primary ores. Similar steps of acid or caustic leaching are employed for selective dissolution of PMs from e-waste. The pregnant solution is separated and purified for the enrichment of metal content thereby impurities are removed as gangue materials. The isolation of metal of interest is conducted through solvent extraction, adsorption and ion exchange enrichment processes. Finally, metals are recovered from solution through electrorefining (electrometallurgy) or chemical reduction processes [29,30,31,32,33]. Cui and Zhang [21] reviewed the processes for recovering metals from e-waste. Hydrometallurgy and pyrometallurgical processes were evaluated and discussed. It has been reported that hydrometallurgical processes have some benefits compared to pyrometallurgical processes because they are more exact, predictable and controllable [34].

Solvents especially halides, cyanides, thiourea and thiosulfates are used for the leaching of PMs from their primary ores. Process factors including pH, temperature and stirring control the dissolution of metals from their primary ores. The recovery of PMs from the leached solution is carried out by cementation, solvent extraction, adsorption on activated carbon, and ion exchange methods. Similar techniques could be employed for extracting metals from e-waste, however, its complex nature makes the process complicated compared to natural ores.

Park and Fray [8] proposed a hydrometallurgical method for recovering PMs from e-waste. Aqua regia was used as leachant and a fixed ratio of 1/20 between metals and leachant was exercised. Ag and Pd were extracted during the first stage with 98 and 93% recovery, respectively. For Au, a liquid-liquid extraction method was adopted with toluene and a recovery of 97% was reported [8]. It is noted that HNO3, H2SO4, and HCl-based solutions are commonly employed for dissolving PMs from e-waste . From the leachants, PMs are recovered employing methods similar to those used in the mineral industry.

Limitations of Hydrometallurgy Route

Hydrometallurgical processes have been successfully used to recover PMs from e-waste . However, these processes have limitations that restrict their industrial scale application. These limitations include [21, 35, 36]:

  • hydrometallurgical processes are slow and less profitable compared to the pyrometallurgical processes,

  • mechanical processing of e-waste for efficient dissolution is time consuming, and causes 20% loss of PMs,

  • there are risks of PM loss during dissolution and subsequent steps,

  • leachant such as cyanide are hazardous and should therefore be used with high safety standards

  • halide leaching needs special equipment made of stainless steel and rubbers due to strong corrosive acids and oxidizing conditions,

  • the use of thiourea leachants is limited in gold extraction due to its high cost and consumption. Moreover, further developments are required to improve the current technology of thiourea-based gold leaching,

  • the consumption of thiosulfate is comparatively higher and the overall process is slower, which limits its application for gold extraction from ores as well as from e-waste .

Pyrometallurgical Processes

State-of-the-art smelters and refineries can extract valuable metals and isolate hazardous substances efficiently. Such recycling facilities can close the loop of valuable metals and reduce environmental impact arising from large quantities of e-waste. Currently, e-waste recycling is dominated by pyrometallurgical routes [37], whereas the steel industry embraces the ferrous fractions for the recovery of Fe, and the secondary aluminum industry takes the aluminum fractions. Pyrometallurgical processes works with the steps of disassemble, separation/upgrading and purification that are fundamentally similar to those of mechanical or hydrometallurgical processes . Smelting in furnaces, incineration, combustion and pyrolysis are typical e-waste recycling processes by the pyrometallurgical process routes. The recovery of precious metals is not achieved by leaching, crushing or grinding, but by smelting in furnaces at high temperatures. In these processes, metals are sorted by exploiting their chemical and metallurgical properties, e.g., PMs are segregated into a solvent metal phase (Cu or Pb). Plastic components of e-waste cannot easily be recycled due to the mix of flame retardants, pigments and mixed types of plastics. However, smelting processes utilize the energy content of the plastics. Energy usage is reduced due to the combustion of plastics and other flammable materials in the e-waste feed, which partially substitute the role of coke as a reducing agent and energy source.

The metal fractions separated during the preprocessing of e-waste are composed of Fe, Al, Cu, Pb and PMs. After Fe and Al, Cu and Pb are the main constituents of a typical e-waste. Therefore, it is logical to send e-waste to smelters that accept Cu/Pb scrap. Currently, Cu and Pb smelters work as e-waste recyclers for the recovery of Pb, Cu and PMs. In these pyrometallurgical processes, e-waste/Cu/Pb scrap is feed into a furnace, whereby metals are collected in a molten bath and oxides form a slag phase layer. Details of the Cu smelting process are explained in Section “Copper Smelting Route”.

Copper Smelting Route

Primary and secondary copper smelters are adopted to recycle and extract PMs from e-waste. It is reported that due to their excellent gas cleaning units, copper smelting processes are more environmentally friendly compared to lead smelters that generate toxic fumes [38]. Copper smelting facilities near urban areas will minimize the cost of e-waste transportation, and therefore the recycling process costs can be lowered. In these processes, PMs are recovered via conventional electrorefining processes where they are segregated in slimes [38]. Usually, copper smelting routes including matte and black copper are used for e-waste recycling. In the primary copper smelting, copper matte (~40 to 60%) and blister copper (~98.5%) are produced. Furthermore, blister copper is refined to produce anode copper (~99%). In the black copper route (secondary copper smelting) crude copper is produced during a reduction process and is refined by oxidation in a converter. The black copper smelting process consists of reduction and oxidation cycles. Impurities are mostly segregated into the vapor phase and are discharged in the off gas [38].

Limitations of Pyrometallurgical Processes

Pyrometallurgical processes are generally more economical, eco-efficient and maximize the recovery of PMs, however, they have certain limitations [6, 21]:

  • recovery of plastics is not possible because plastics replace coke as a source of energy ,

  • smelting cannot recover Al and Fe since they are oxidized and dissolved into the slag,

  • smelting flame retardants and polyvinyl chloride (PVC) present in e-waste leads to the formation of dioxins, requiring special emission controls,

  • a large investment is required for installing integrated e-waste recycling plants that maximize the recovery of valuable metals and also protect the environment by controlling hazardous gas emissions,

  • instant burning of fine dust of organic materials can occur before reaching the metal bath. In such cases, agglomeration may be required to effectively harness the energy content and also to minimize the health risk posed by fine dust particles,

  • ceramic components in feed material can increase the volume of slag generated in the blast furnaces, which increases the risk of losing PMs from BMs,

  • partial recovery and purity of PMs are achieved by pyrometallurgical routes. Therefore, subsequent hydrometallurgical and electrochemical techniques are necessary to extract pure metals from BMs,

  • process control and optimization of the smelting and refining processes are challenging due to complex feed materials,

  • smelting cannot recover certain product components, such as chips or bare fiberglass boards.

Selected Industrial Processes for Metals Recovery from E-waste

Currently, industrial processes for recovering metals from e-waste are based on combined pyrometallurgical, hydrometallurgical and electrometallurgical processes. In pyrometallurgical processes, e-waste is blended with other materials and incorporated into primary/secondary smelting processes (e.g., into copper or lead smelters). Copper smelting is the main process route for e-waste recycling where PMs are collected in copper matte or black copper. In the final stage of Cu production, i.e., the electrorefining process, pure Cu metal is produced and the PMs are separated into slimes where they are recovered using hydrometallurgical processes . Currently, various industrial processes are used globally for extracting metals from e-waste, including the Umicore integrate smelting and refining facility, the Noranda process in Quebec, Canada, Boliden Rönnskär smelters in Sweden, Kosaka’s recycling plant in Japan, the Kayser recycling system in Austria and the Metallo-Chimique N.V plants operated in Belgium and Spain.

Metals Recovery from E-waste at Rönnskär Smelters and Noranda Process

Boliden Rönnskär (Skelleftehamn, Sweden) is world’s largest recycler of Cu and PMs from e-waste. The majority of the e-waste sent to Rönnskär comes from EU and North America [8]. Figure 1 shows the process flow sheet diagram of the Boliden Rönnskär smelter. A variety of scrap from the non-ferrous and electronic industry is introduced into the process at different stages depending on the purity and requirement of the final product. For instance, high Cu containing scrap is fed into the converting process directly, but the low-grade e-waste is fed into the Kaldo furnace (see Fig. 1). At Boliden Rönnskär (Skelleftehamn, Sweden) smelter, the annual recycling capacity of e-scraps is 1.2 × 105 tons. The feed material of the Kaldo converter consists of blended lead concentrates and e-waste that are combusted with the supply of oxygen and oil. The main steps in the Boliden Rönnskär smelters are drying, roasting, smelting, converting and refining, as schematically shown in Fig. 1.

Fig. 1
figure 1

Process flow sheet diagram of the Boliden Rönnskär (Skelleftehamn, Sweden) smelter, modified from [39]. The feed materials and useful end products are encircled with red (left) and green (right) lines, respectively

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The Kaldo furnace at Boliden Rönnskär, to which the e-waste is fed, produces a mixed Cu-alloy that is treated in a copper converter for the recovery of metals including Cu, Ag, Au, Pt, Pd, Ni, Se, and Zn. The volatile metals such as Pb, Sb, In, and Cd are segregated into the vapor phase that is recovered by a separate process. The off gases emission is treated for producing sulfuric acid and SO2 gas.

The Noranda process is another commercial pyrometallurgical process for the recovery of metals from e-waste . The feed material for this process is composed of e-waste and copper ore concentrates. At this recycling facility, about 1 × 105 tons of e-waste is processed for metal recovery. A blend of e-waste and copper concentrate is fed into the molten bath at 1250 °C and the process temperature is maintained by injecting supercharged oxygen. Energy usage in the Noranda reactor is reduced due to the combustion of plastics in the e-waste feedstock. During the oxidation process, impurities including Fe, Pb and Zn are converted into oxides, and segregated into a silica-based slag. The slag is cooled and processed for the recovery of metals before disposal. PMs are segregated in liquid copper that is processed into a copper converter for higher purity. The blister copper is refined in the anode furnace and casted into anodes with 99.1% purity. The remaining residue (0.9%) contains PMs including Au, Ag, Pt, and Pd, and some other recoverable metals such as Ni, Se and Te. Finally, PMs are recovered through electrorefining process.

In general, PMs are recovered with a combination of pyrometallurgical and hydrometallurgical processes . It is worth noting that during mechanical separation of Fe, Al and plastics from e-waste, there is a risk of inevitably PMs losses. The PMs are closely tied up with non-ferrous metals and plastics in the PCBs. The acceptance of Fe, Al and plastics in the Cu smelting can enhance the overall recovery of PMs. Huisman and Stevels [40] reported that the direct smelting route for mobile phones is a more eco-efficient solution compared to the indirect smelting of disintegrated mobile phone components. However, batteries should be separated before smelting in the furnace [41]. Most of the integrated e-waste recycling plants directly use PCBs in the smelting furnaces, which have many advantages such as:

  • maximization the overall segregation of PMs in the Cu fraction, causing the final recovery to be higher,

  • partial replacement of coke with plastics as a source of energy during the smelting process.

Pyrometallurgical routes are beneficial to segregate and upgrade valuable metals in the BMs from e-waste that are further treated by hydrometallurgical routes to recover metals. Industrially, IsaSmelt, Kaldo, rotary and plasma arc furnaces are used for collecting valuable metals in the BMs. A summary of selected industrial process is given in Table 2.

Table 2 Summary of selected pyrometallurgical processes for recovering valuable metals from e-waste . TSL: top submerged lanced [42]
Full size table

Summary and Conclusions

Traditional methods of managing e-waste which include disposing in landfills, burning in incinerators or exporting abroad for disposal are no longer options due to the strict environmental regulations. Fortunately, the presence of valuable metals in e-waste and increasing demand for the metals as well as complexities of the currently available primary raw materials make recycling an attractive and viable option both in terms of environment and economics. Moreover, it is efficient in terms of resource management by closing the loop of metals. Consequently, urban mining such as the recovery of PMs from e-waste streams through sustainable recycling processes have emerged. The sustainable recycling practices address the scarcity and complexity of primary resources and reduce consumption of energy while managing environmental issues related to hazardous materials from the e-waste streams. Therefore, recycling of e-waste is important for both resource and waste management. However, e-waste recycling is limited due to challenges such as insufficient collection facilities, higher transportation cost or lack of integrated and automatic smelting and refining facilities near urban areas.

Currently, pyrometallurgical, hydrometallurgical or a combination of both routes are used for recovering valuable metals from e-waste streams. Fundamentally, hydrometallurgical routes are similar to those used in the minerals processing industry, which include leaching and metal extractions from leachates. Pyrometallurgical process routes are more economical and eco-efficient particularly for the recovery of PMs, which result in considerable loss when applying hydrometallurgical processes . In the pyrometallurgical process routes the non-metallic fractions can be used as fuels and reducing agents. The thermal energy can be effectively recovered through integrated steam networks. Limitations in applying the pyrometallurgical processes include difficulties to recover Fe and Al, as they oxidize in the process, and the required strict control of emissions to avoid pollution. Both process routes have advantages and disadvantages, which therefore should be chosen for specific feed materials and desired products.