Waste Management xxx (2015) xxx–xxx

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Review

Precious metal recovery from waste printed circuit boards using cyanide and non-cyanide lixiviants – A review Ata Akcil a,⇑, Ceren Erust a, Chandra Sekhar Gahan a,b, Mehmet Ozgun a, Merve Sahin a, Aysenur Tuncuk a a b

Mineral-Metal Recovery and Recycling (MMR&R) Research Group, Mineral Processing Div., Dept. of Mining Eng., Suleyman Demirel University, TR32260 Isparta, Turkey Department of Microbiology, School of Life Sciences, Central University of Rajasthan, Bandar Sindri-305817, NH-8, Kishangarh Tehsil, Ajmer district, Rajasthan, India

a r t i c l e

i n f o

Article history: Received 17 November 2014 Accepted 15 January 2015 Available online xxxx Keywords: E-waste Cyanide Metal Thiosulphate Thiourea Precious

a b s t r a c t Waste generated by the electrical and electronic devices is huge concern worldwide. With decreasing life cycle of most electronic devices and unavailability of the suitable recycling technologies it is expected to have huge electronic and electrical wastes to be generated in the coming years. The environmental threats caused by the disposal and incineration of electronic waste starting from the atmosphere to the aquatic and terrestrial living system have raised high alerts and concerns on the gases produced (dioxins, furans, polybrominated organic pollutants, and polycyclic aromatic hydrocarbons) by thermal treatments and can cause serious health problems if the flue gas cleaning systems are not developed and implemented. Apart from that there can be also dissolution of heavy metals released to the ground water from the landfill sites. As all these electronic and electrical waste do posses richness in the metal values it would be worth recovering the metal content and protect the environmental from the pollution. Cyanide leaching has been a successful technology worldwide for the recovery of precious metals (especially Au and Ag) from ores/concentrates/waste materials. Nevertheless, cyanide is always preferred over others because of its potential to deliver high recovery with a cheaper cost. Cyanidation process also increases the additional work of effluent treatment prior to disposal. Several non-cyanide leaching processes have been developed considering toxic nature and handling problems of cyanide with non-toxic lixiviants such as thiourea, thiosulphate, aqua regia and iodine. Therefore, several recycling technologies have been developed using cyanide or non-cyanide leaching methods to recover precious and valuable metals. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction In the fast developing world today, technological advancements in electrical and electronic equipments (television, computers, printer, telephone, modem, fax machines, copy machines, LED/ LCD monitors, laptops, printed circuit boards, medical equipments, etc.) have a lesser life span compared to olden days due to rapid increase in demand of advanced products. Short life span of the electrical and electronic equipment (EEE) has generated huge tonnage of waste Electrical and Electronic Equipments (WEEE) called ‘‘E-waste’’. The reasons for decreasing life span of electrical and electronic devices are as follows:  Incoming of highly advanced and technically skilled devices/ equipments with cheaper price and more features.

⇑ Corresponding author. Tel.: +90 246 2111321; fax: +90 246 2370859. E-mail address: [email protected] (A. Akcil).

 Rapid growth in life style of human beings with modern facilities having user friendly electrical and electronic equipments.  Stiff competition amongst individuals to use and small enterprises and industries to produce and sell best products made on advanced technologies. The average annual growth of the E-waste market was predicted to be 8.8% (2004–2009) resulting with scrapped computers outnumbering the production rate (BCC, 2005; Kang and Schoenung, 2005). Fifteen countries amongst all the EU countries categorised dumped/unused refrigerator, personal computers (PC), television (TV), copy machine and small home appliances Ewaste with a resultant increase in waste generation from 3.3 to 4.3 kg/person (Wildmer et al., 2005). Studies conducted on the generation of E-waste predicted 19.1 kg/year/person in 27 EU countries by 2012 with total of 10.5 million tons of E-waste by 2014 (Huisman, 2010). Increasing concern over environmental issues in the global scenario in the recent years has mobilised researchers, scientists and industries together with government

http://dx.doi.org/10.1016/j.wasman.2015.01.017 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Akcil, A., et al. Precious metal recovery from waste printed circuit boards using cyanide and non-cyanide lixiviants – A review. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.01.017

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A. Akcil et al. / Waste Management xxx (2015) xxx–xxx

authorities to execute several recycling strategies for metal recovery from E-waste. The primary reason for development of recycling strategies on E-waste is due to the stricter policies and regulations imposed on the industries and landfill zones. Moreover the 2002/ 96/EC regulation proposes obligatory recovery of metal values from E-wastes. In accordance to EU regulations, Turkish Ministry of Environment and Forestry has also prepared the ‘‘Regulation of Controlling Waste of Electric and Electronic Equipments (WEEE)’’ (No. 2830) with stringent policies and regulations on recycling/ reusing of E-wastes in 2012 as follows:  Municipalities are obliged to collect and store E-wastes separately from conventional wastes.  The manufacturers must take the responsibility of recycling their discarded products in their production centre/separate recycling centre/outsource to licensed recycling centres.  The manufacturers must carry out their own research or outsource research projects to develop better recycling technologies for E-wastes (Regulation of Controlling Waste Electric Electronic Equipments, 2012). Kiddee et al. (2013a,b) presented an overview of toxic substances present in E-waste, their potential environmental and human health impacts together with management strategies currently being used in certain countries. Several tools including Life Cycle Assessment (LCA), Material Flow Analysis (MFA), Multi Criteria Analysis (MCA) and Extended Producer Responsibility (EPR) have been developed to manage E-wastes especially in developed countries. Recycling E-wastes is predicted to be the appropriate method of E-waste management, developing environment friendly, technically feasible and economically appropriate methods of recycling. However this could be an important source of metal recovery from secondary resources at the time when primary metal resources are depleting. Furthermore, steps should also be taken to educate the consumers/users of the electrical and electronic products regarding E-waste collection centres to increase the collection rate to recover higher content of metal values prior to its disposal in landfills. E-waste is well known to contain several environmentally hazardous organics such as fire retardants (Cl/Br, etc.) and several inorganic elements (He et al., 2006). Advancement in the technological throughput with innovative approaches accounting to the rapid growth in science and technology and market demand for better qualitative products with cheaper costs has increased the consumption of electrical-electronic equipments. This has resulted with generation of 20–50 million tons E-wastes annually worldwide with 35% increase per year (Schwarzer et al., 2008). Table 1 shows the equipments/devices included in the Ewaste category together with the amount of waste generated in UK. The table also states that the home/household appliances contribute highest amongst all the E-wastes.

Table 1 The tonnage and amount of basic e-wastes by the year 2000 (ICER, 2000). Equipment

Units (Millions)

Weight (tonnes)

% of Total

Large household appliances Small household appliances IT equipment Telecoms Radio, TV and audio Lamps Medical Monitoring and control Toys Electronic electrical tools Automatic dispensers

10 15 22 7 12 77 No data 8 8 6 No data

392 30 357 8 72 12

43 3 39 1 8 1

8 8 28

1 1 3

Total

165

915

100

WEEE contains a variety of hazardous inorganic substances such as Hg and Pb, which may cause some environmental problems when it is not properly managed (Xu et al., 2009, 2010; Li and Lu, 2010). Secondary metal resources such as WEEE contain base metal such as Cu, in particular and precious metals Au, Ag, Pt and Pd comparable to the metal content in ores and concentrates (Havlik et al., 2011; Tuncuk et al., 2012; Birloaga et al., 2013). Therefore, recycling of WEEE is of prime importance from both environmental and economic benefit (Wildmer et al., 2005; Robinson, 2009; Yazici and Deveci, 2013). The period between 1994 and 2003 observed generation of huge E-waste due to discarding of 500 million personal computers, which contained 2.8 million tons of plastic, 0.7 million tons of lead (Pb), 1339 tons of cadmium (Cd), 848 tons of chromium (Cr) and 282 tons of mercury (Hg) (Puckett et al., 2002). High content of base metal (Fe, Cu, Al, Pb and Ni) and precious metal (Ag, Au, Pt and Pd) present in E-waste results makes it a potential source of secondary resources for metal recovery (Table 2). E-waste such as DVD players, printed circuit boards, computers and electronic scrap has maximum content of base metals whilst the electronic devices such as personal computers and mobile phone cards contained highest level of precious metal. The metal content of Cu and Au in primary metal resources like ores/concentrate was 0.5–1% and 1–10 g/ton respectively, whilst in case of secondary metal resources such as E-waste it was 20% and 250 g/ton respectively. This justifies that the metal content in secondary metal resources would be worth recovering if feasible technologies can developed with environmental friendly methods (USGS, 2001; Goosey and Kellner, 2003; Cui and Zhang, 2008). The weight percentage of metals obtained from PCB’s is shown in Table 3 with highest copper content amongst all other metals. The metal content in E-wastes varies with the source and type of the E-waste. A typical computer circuit board contains 20% Cu and 250 g/ton Au, whilst mobile phone contains 13% Cu and 350 g/ton Au (Hagelüken, 2006). It is very important to note that the E-wastes contain 13–26 times higher Cu content and 35–50 times more Au content compared to ores/concentrates (Zhang and Forssberg, 1998; Cui and Zhang, 2008). As the precious metal content in Waste PCB’s is higher than the ores/concentrates it would be worth to recycle waste for economical and environmental advantage. The metal contents in circuit board (Table 4) describe its economic value, for which the recovery of metal values from E-waste could be preferred in future (Yu et al., 2009). The content of precious metals in telephone, calculators and printed circuit boards is about 70% whilst about 40% in TV boards and DVD players (Cui and Zhang, 2008). Various economical and environmental advantages of the metal recovery from E-wastes are as follows (Zhang and Forssberg, 1998): Conservation of primary metal resources. Decrease in the amount of solid waste generated. Recovery of non-metal material (plastic, etc.). Recovery of Ferrous metals, Non-ferrous metals and precious metals.  Energy savings at a greater than the primary metal resources.  Prevention of environmental pollution caused by heavy metals, solvent based flame retardant, plastics and toxic gas released the from E-waste.    

The metal recovery from E-wastes and the amount of energy savings is shown in Table 5, where highest energy savings ratio is obtained from aluminium recovery process. Precious metal content in the E-wastes plays a crucial role in choosing and developing metal recovery methods. Hagelüken classified E-wastes as high, medium and low grade based on its gold content (Hagelüken, 2006). In general high grade primary metal

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A. Akcil et al. / Waste Management xxx (2015) xxx–xxx Table 2 Various types of E-wastes and their metal content (Cui and Zhang, 2008). E-Waste

Fe (wt%)

Cu (wt%)

Al (wt%)

Pb (wt%)

Ni (wt%)

Ag (ppm)

Au (ppm)

Pd (ppm)

TV board scrap PC board scrap Mobile phone scrap Portable audio scrap DVD player scrap Calculator scrap PC mainboard scrap Printed circuit boards scrap TV scrap (CRT’s removed) Electronic scrap PC scrap Typical electronic scrap E-scrap sample 1 E-scrap sample 2 Printed circuit boards E-scrap (1972 sample) E-waste mixture

28 7 5 23 62 4 4.5 12 – 8.3 20 8 37.4 27.3 5.3 26.2 36

10 20 13 21 5 3 14.3 10 3.4 8.5 7 20 18.2 16.4 26.8 18.6 4.1

10 5 1 1 2 5 2.8 7 1.2 0.71 14 2 19 11.0 1.9 – 4.9

1.0 1.5 0.3 0.14 0.3 0.1 2.2 1.2 0.2 3.15 6 2 1.6 1.4 – – 0.29

0.3 1 0.1 0.03 0.05 0.5 1.1 0.85 0.038 2.0 0.85 2 – – 0.47 – 1.0

280 1000 1380 150 115 260 639 280 20 29 189 2000 6 210 3300 1800 –

20 250 350 10 15 50 566 110 S2O2 3 > SC(NH2)2 > OH > I > SCN >   SO2 3 > NH3 > Br > Cl > CH3CN. In Au(III) the stability of the complexes were lined up as CN > OH > SCN > Br > Cl. Leaching gold and silver from ores/concentrates/wastes via cyanidation has been applied for centuries as it results with highly efficient recovery with a simple process. However, in this process cyanide is well known as a toxic chemical posing serious threats to the natural habitat. Increasing environmental pressure on the use of cyanide has motivated researchers and scientists together with industries to test other possible lixiviants such as halogens (Hilson and Monhemius, 2006), thiocyanate (Kononova et al., 2007), thiosulphate (Abbruzzese et al., 1995; Jeffrey et al., 2001) and thiourea (Chen et al., 1980; Deschênes and Ghali, 1988; Murthy and Prasad, 1996; Lacoste-Bouchet et al., 1998; Murthy et al., 2003; Li and Miller, 2007) for precious metal leaching (Au, Ag and Pt). All the non-cyanide lixiviants provide a safer and environmentally friendly process with reasonably good and faster leaching kinetics compared to cyanide (Pant et al., 2012). 2.1. Cyanide leaching The commercial use of cyanide for gold and silver recovery has been carried out in New Zealand since 100 years. Use of non-cyanide lixiviant has also been well studied and researched by several researchers, scientists and industries, but the advantages of cyanide over non-cyanide lixiviants have made cyanide preferable. However the cyanide has been efficient, user-friendly if and only handled carefully obeying all the safety regulations, and economical with the acceptable risk to humans and is preferred over all other non-cyanide lixiviants. It has been observed that 875 gold and silver mines are being operational by the year 2000, where > 90% of gold and silver recovery processes uses cyanide lixiviant (Mudder and Botz, 2004; Akcil, 2010). Out of the total global demand of 360,000 tons of sodium cyanide annually, approximately one-third of the total i.e., 120,000 tons is used potentially for gold and silver recovery. However, it has been observed that 6% of the total hydrogen cyanide produced is converted to sodium cyanide (Chemical Market Reporter, 1998, 1999). There are plenty of beneficial uses for cyanide, and cyanide products used safely around the world, out of which a small portion is used in mining. Restricting use of cyanide in gold and silver mining cannot obliterate the risks arising from cyanide (The Gold Institute, 2000). In current use of cyanide leaching technology, the ore goes through size reduction and processed at pH value of 11 in an oxygen environment and then it is processed with cyanide solution for gold extraction (Gurdal, 2008). The cyanide ion is an anion that can be found as complexes (e.g., weak, moderately strong and strong), as free cyanide or as simple compounds in cyanidation solutions. Depending on the pH of the

5

solution, ‘‘free cyanide’’ can be in the form of either cyanide anion (CN) or hydrocyanic acid (or hydrogen cyanide, HCN). HCN is a relatively weak acid and is predominantly found in waters with a pH below approximately 8.5, where volatilisation of cyanide takes place (Randol International Ltd., 1985; Flynn and McGill, 1995). At an optimal gold extraction pH of 10.5 or greater, most of the free cyanide in the solution is in the form of the cyanide anion (CN), where cyanide loss by volatilisation is limited. Otherwise, the cyanidation process would neither be practical and safe nor economically and environmentally feasible (Kuyucak and Akcil, 2013). Native copper readily dissolves in cyanide solutions i.e. complete dissolution at 45 °C (Marsden and House, 2006). Considering that fact and the noticeable copper content of waste printed circuit boards, direct cyanide leaching of waste printed circuit boards could be disadvantageous since direct cyanidation would increase the consumption of cyanide and decrease precious metal recoveries. Montero et al. (2012) studied column leaching of waste printed circuit boards using cyanide solutions and reported low leaching recoveries for gold (48%) and silver (52%) even at a high cyanide concentration of 4 g/L. They also noted simultaneous dissolution of copper (77%) during leaching. Therefore, an acidic sulphate leaching in the presence of a suitable oxidant can be implemented to dissolve/remove copper and other base metals prior to extraction of precious metals in cyanide as well as thiosulfate, thiourea and chloride media. Quinet et al. (2005), Oh et al. (2003) and Kamberovic´ et al. (2011) suggested sulphuric acid leaching of Ewaste in the presence of H2O2 or O2 prior to precious metal extraction. The dissolution mechanism has been debated under both acidic and alkaline conditions. Dissolution involves an electrochemical process in which the anodic reaction is gold oxidation whilst the cathodic reaction is oxygen reduction. Senanayake (2008) has illustrated the gold ion diffusion through the interfaces into the solution as shown in Fig. 2. Gold cyanidation is an electrochemical process which relies on the fact that gold dissolves in the alkaline cyanide solution and forms gold cyanide complex (Au(CN) 2 ) in the so called anodic reaction. Another half reaction, in which oxygen is reduced, is called cathodic reaction (Eqs. (1) and (2)). The overall reaction is clearly shown in Eq. (3).

Cathode reaction : 4Au þ 8CN ! 4AuðCNÞ2 þ 4e

ð1Þ

Anode reaction : O2 þ 2H2 O þ 4e ! 4OH

ð2Þ

Sum : 4Au þ 8CN þ O2 þ 2H2 O ! 4AuðCNÞ2 þ 4OH

ð3Þ

The chemical reaction for the gold recovery by cyanide leaching can otherwise be written as shown in Eqs. (4)–(6) (Habashi, 1970).

2Au þ 4CN þ O2 þ 2H2 O ! 2AuðCNÞ2 þ H2 O2 þ 2OH

ð4Þ

2Au þ 4CN þ H2 O2 ! 2AuðCNÞ2 þ 2OH

ð5Þ

Fig. 2. Anodic cyanidation model for gold; boundary i: gold-film interface, boundary o: film-solution interface (Senanayake, 2008).

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A. Akcil et al. / Waste Management xxx (2015) xxx–xxx

Overall reaction formula: ‘‘Elsner’s equation’’

4Au þ 8CN þ O2 þ 2H2 O ! 4AuðCNÞ2 þ 4OH

ð6Þ

In principle, is limited either by mass transfer of oxygen or cyanide from liquid bulk to the particle surface by rate of intrinsic leaching reactions on the surface of the particles or by intra-particle diffusion is omitted in gold cyanidation studies due to the generally non-porous characteristics of the particles. Many researchers have concluded that increasing the dissolved oxygen concentration increases the rate of dissolution. Few other researchers also stated that dissolved oxygen concentration has no significant effect on cyanide consumption and further discussed that faster leaching kinetics were observed by using higher dissolved oxygen concentrations (Ogunniyi and Vermaak, 2007, 2009; Gurdal, 2008; Arslan and Sayiner, 2009; Guo et al., 2009; Ogunniyi and Vermaak, 2009). The solubility of gold in cyanide solution depends on the concentration of oxygen, cyanide together with temperature, pH, surface area of gold ore/concentrate/waste, stirring rate and other anion/cations. The cyanide ion is hydrolysed to molecular hydrogen cyanide and hydroxyl ions, based on the pH of the leaching environment. Approximately at pH 9.3, half of the total cyanide is present as free cyanide and the other half is as toxic hydrogen cyanide. As > 90% of the cyanide is present in the leaching solution forms toxic hydrogen cyanide at pH 8.4, care should be taken to avoid lowering of pH below 9.0 for the safety of the workers (Gurdal, 2008). Although cyanide is toxic for human being and harmful to the environment, the information available on cyanide is good enough for safety handling. Its unfortunate that even after having proper information regarding the toxicity of cyanide, there has been accidents in past possibly because of human error. However, the accidents from cyanide have now reduced by number and are expected to decrease much more in future. Still, the safety of cyanide in general and consequences on the environment is serious issue but the benefits of cyanide are much more. Therefore, the use of cyanide is advantageous if handled carefully without any harmful effects (Mudder, 1999). The metal content in printed circuit boards (PCB’s) is high (Table 6), therefore, hydro-metallurgical processing of PCB’s started in 1970s aiming Ag and Au recovery in particular (Randol International Ltd., 1985). Cyanide is largely used lixiviant for gold mining since years together because of low cost and several other advantages over non-cyanide lixiviants. This process is based on dissolution of gold and silver from the circuit boards in cyanide solution (Flynn and

McGill, 1995). The basic assessments of cyanide and aqua regia leaching methods are clearly shown in Table 7. Studies on the recovery of precious metals from E-wastes using cyanide as a lixiviant are shown in Table 8. 2.2. Thiosulphate leaching Non-cyanide leaching using ammonium thiosulphate solution as a lixiviant has been thoroughly researched and developed since last 20 years. Several researchers around the globe have put lot of efforts in understanding the effects of ammonium thiosulphate leaching. Leaching tests carried out using ammonium thiosulphate solution as a lixiviant resulted with relatively low recovery of gold compared to cyanide leaching with relatively higher reactive species consumption. However, the results obtained also showed relatively lower consumption of thiosulphate lixiviant compared to cyanide. Therefore it was concluded that the thiosulphate as a non-cyanide lixiviant for gold recovery must be applied more economically in ores containing carbonate and copper compared to cyanide leaching. However, there exists little information about pilot/demonstration/full scale operation with thiosulphate leaching of gold (Wan et al., 1995; Wan and Le Vier, 2003; Molleman and Dreisinger, 2002; Muir and Aylmore, 2002; Fleming et al., 2003; Wan and LeVier, 2003; Senanayake, 2005; Tanriverdi et al., 2005; Arslan and Sayiner, 2009). The most commonly used lixiviant for gold leaching is cyanide but the residual cyanide content in the effluents and waste water could be harmful for the environment if left untreated (Zhang et al., 2009; Sepúlveda et al., 2010). On the contrary, thiosulphate leaching is nontoxic and economical compared to cyanide. Therefore, even with low gold recovery by thiosulphate leaching, it can be an environment friendly technology. The main problem with thiosulphate leaching is the low recovery of gold and is not costeffective for commercial application (Zhang et al., 2009). Cupric (Cu2+) is added in thiosulphate leaching as catalyst in low concentrations i.e. 30–120 mg/L. Thiosulphate leaching is performed in the presence of ammonia to stabilise copper since copper as well as other metals can increase the decomposition of thiosulphate (Marsden and House, 2006). In the case of direct thiosulphate leaching of waste printed circuit boards, dissolved copper may adversely effect the leaching process through decomposition of thiosulphate. Therefore, it can be suggested that thiosulphate leaching can be applied after a pre-leaching stage in which copper was removed. Studies conducted on thiosulphate lixiviant for gold recovery as a non-cyanide lixiviant suggests that thiosulphate is a potential substitute to cyanide in future. Presence of Cu2+ and NH3 in

Table 6 Metal content of different printed circuit boards (Zhang et al., 2012). Source/metals

(1)

(2–4)

(5)

(6–7)

(8)

(9–10)

(11)

(12)

(13)

Cu Al Pb Zn Ni Fe Sn Sb Au/ppm Pt/ppm Ag/ppm Pd/ppm

23.73 4.7 4.48 0.75 3.32 7.47 3.65 1.82 800 – 800 210

23.47 1.33 0.99 1.51 2.35 1.22 1.54 – 570 30 3301 294

20 5 1.5 – 1 7 – – 250 – 1000 110

20 2 2 1 2 8 4 0.4 1000 – 200 50

26.8 1.9 – 1.5 0.47 5.3 1.0 0.06 80 – 3300 –

10 7 1.2 1.6 0.85 12 – – 280 – 110 –

15.6 – 1.35 0.16 0.28 1.4 3.24 – 420 – 1240 10

22 – 1.55 – 0.32 3.6 2.6 – 350 – – –

1.85 4.78 4.19 2.17 1.63 2.0 5.28 – 350 4.6 1300 250

Total

50

33

35

40

40

21

22

30

38

(1): Hao et al. (2008), (2–4): Yu et al. (2011), (5): Ogunniyi and Vermaak (2007, 2009), (6): (Hagelüken (2006) (7): Shuey et al. (2006), (8): Sum (2005), (9): Zhao et al. (2004), (10,11): Zhang and Forssberg (1997a,b), (12): Kim et al. (2004) (13): Iji and Yokoyama (1997).

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A. Akcil et al. / Waste Management xxx (2015) xxx–xxx Table 7 Basic assessment indices of cyanide, aqua regia and other reagents leaching methods (Zhang et al., 2012). Leaching method

Cyanide Aqua regia Thiourea Thiosulfate Chloride Bromide Iodide a

Economica feasibility

Environmentala impact

Research levela

Leaching rate

Reagent cost

Corrosive

Toxicity

Reliability

3 4 4 2 5 5 5

5 4 4 2 4 2 3

5 0 4 5 0 2 5

0 3 4 4 3 3 5

5 5 4 2 4 2 3

Scale is between 0 and 5.

Table 8 The samples study on recovering precious metals from E-wastes with cyanide. Source of waste

Medium conditions

Recovery (%)

References

E-waste

Cyanide leaching (pH > 10 and temp 25 °C) to the chloride leaching tailings

Ag – 93% Au – 95% Pd – 99%

Quinet et al. (2005)

PCB of cell phones

In a commercial cyanide process (GalvastripperÒ of Galva (2013)) (in potassium cyanide concentration of 6–8%, at 25 °C, in 2–4 h, in pH 12.5, at S/L ratio 1/20)

Au – 60–70%

Petter et al. (2014)

thiosulphate solution acts as a catalyst enhancing the gold recovery (Langhans et al., 1992; Abbruzzese et al., 1995; Breuer and Jeffrey, 2000; Aylmore and Muir, 2001; Jeffrey et al., 2001). The chemical reaction with Cu2+ and S2O2 3 is given in Eq. (7) stating reduction of Cu2+ to Cu+ (Byerley et al., 1973a,b) and furthermore Eq. (8) states the oxidation of Cu+ to Cu2+ in the presence of oxygen. Thiosulphate was assumed to oxidise in the presence of oxygen and copper but the chemical reaction of thiosulphate and copper was much quicker (Byerley et al., 1973a,b, 1975; Chu et al., 2003). 2 2 2CuðNH3 Þ2þ ! 2CuðS2 O3 Þ5 4 þ 8S2 O3 3 þ S4 O6 þ 8NH3

ð7Þ

2CuðS2 O3 Þ5 3 þ 8NH3 þ 0:5O2 þ H2 O 2  ! 2CuðNH3 Þ2þ 4 þ 6S2 O3 þ 2OH

ð8Þ

Gold leaching in a thiosulphate solution containing copper and ammonia is an electrochemical reaction as shown in Eqs. (9) and (10).  Au þ 2S2 O2 ! AuðS2 O3 Þ3 3 2 þe 2  CuðNH3 Þ2þ ! 2CuðS2 O3 Þ5 4 þ 3S2 O3 þ e 3 þ 4NH3

ð9Þ ð10Þ

Ficeriová et al. (2011) stated that the ammonium thiosulphate leaching of waste material to recover Au and Ag was economical and eco-friendly too. However, a pre-treatment step is required for size reduction of PCB’s prior to leaching. The thiosulphate leaching was carried out for 48 h and resulted with 98% gold and 93% silver recovery. The recovery of other metals were 84% Cu, 82% Fe, 77% Al, 76% Zn, 70% Ni, 90% Pd, 88% Pb and 83% Sn using hydrogen peroxide, sodium chloride, sulphuric acid and aqua regia solutions. This leaching method provided an insight to obtain higher recovery of metals with faster kinetics. Several studies on the metal recovery from E-wastes with thiosulphate solution can be seen in Table 9. 2.3. Thiourea leaching Another non-cyanide lixiviant, thiourea is also as potential lixiviant for precious metal recovery and has been widely accepted by several researchers because of low toxicity and faster kinetics. The leaching yields obtained for gold and silver from ores/concen-

trates and wastes using thiourea lixiviant is promising. Studies on thiourea leaching suggest that the electron pairs between nitrogen and sulphur atoms have a better potential for a coordination bond between gold and silver, compared to cyanide (Chen et al., 1980; Deschênes and Ghali, 1988; Murthy and Prasad, 1996; LacosteBouchet et al., 1998). Therefore, it is assumed that thiourea can be a potential non-cyanide lixiviant for commercial application and is highly selective to precious metals (Gurung et al., 2013). Separation of metals in alkali solution is difficult and unstable; therefore the leaching is carried out in acidic medium. Addition of ferric ion into thiourea leaching system for gold and silver recovery is advantageous as shown in Eqs. (11) and (12) (Kai et al., 1997; Murthy et al., 2003; Gonen et al., 2007).

Au þ 2CSðNH2 Þ2 þ Fe3þ ! AuðCSðNH2 ÞÞþ þ Fe2þ

ð11Þ

Ag þ 3CSðNH2 Þ2 þ Fe3þ ! AgðCSðNH2 Þ2 Þ3þ þ Fe2þ

ð12Þ

Ferric iron oxidises thiourea easily in acidic solutions forming formamidine disulphide (Eq. (13))

2CSðNH2 Þ2 þ 2Fe3þ ! ðSCN2 H3 Þ2 þ 2Fe2þ þ 2Hþ

ð13Þ

Formamidine disulphide is an unstable product in acidic solution and quickly breaks down into elemental sulphur and cyanamide (Eq. (14)).

ðSCN2 H3 Þ2 ! CSðNH2 Þ2 þ NH2 CN þ S

ð14Þ

Finally thiourea disappears with a stable ferric sulphate production as shown in Eq. (15). þ Fe3þ þ SO2 4 þ CSðNH2 Þ2 ! ðFeSO4  CSðNH2 Þ2 Þ

ð15Þ

Li et al. (2012) conducted thiourea leaching changing several parameters such as particle size, thiourea concentration, Fe3+ ion concentrations and temperature. The results obtained showed recovery of 90% Au and 50% Ag on a particle size of 100 mesh in 24 g/L thiourea, 0.6% Fe3+ concentration and 25 °C for 2 h of leaching period (Li et al., 2012). Lee et al. (2011) obtained complete gold and silver extraction from thiourea leaching of E-wastes, after roasting, size reduction and magnetic separation. The studies regarding metal recovery rates using thiourea solutions from Ewastes are provided with more details in Table 10.

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Table 9 Studies regarding recovering metals from E-wastes with thiosulphate. Source of waste

Used amount of chemicals

Medium conditions

Recovery (%)

References

PCB

0.12 M (NH4)2S2O3 15 mM Cu 0.2 M NH3

In pH 10–10.5, at 25 °C, in 10 h (200 rpm)

90% Au

Ha et al. (2010)

PCB

0.5 M (NH4)2S2O3 0.2 M CuSO45H2O 1 M NH3

In 80 g/L solid–liquid ratio, in pH 9, at 40 °C, in 48 h

98% Au 93% Ag

Ficeriová et al. (2011)

PCB

0.1 M (NH4)2S2O3 40 mM CuSO4

In 10 g/L solid–liquid ratio, in pH 10–10.5, at 25 °C, in 8 h (250 rpm)

78.8% Au

Tripathi et al. (2012)

PCB

72.71 mM thiosulphate, 10 mM copper (II) ion and 0.266 M ammonia concentrations

At 20 °C and in pH 10 (400 rpm)

>90% Au

Ha et al. (2014)

PCB of cell phones

0.1 M Na2S2O3 0.2 M NH4OH 0.015–0.03–0.05 M CuSO4 0.01–0.05–0.1 M H2O2

At 25–26 °C, in pH 9.9–10.9, 1/20 S/L ratio, in 4 h

15% Au 3% Ag

Petter et al. (2014)

PCB of cell phones

0.1 M (NH4)2S2O3 0.2 M NH4OH 0.015–0.03–0.05 M CuSO4 0.01–0.05–0.1 M H2O2

At 25–26 °C, in pH 9.0–10.1, 1/20 S/L ratio, in 4 h

15% Au 3% Ag

Petter et al. (2014)

Mobile phone cards

0.12 M (NH4)2S2O3 20 mM Cu, 0.2 M NH3

In pH 10–10.5, at 25 °C, in 2 h (200 rpm)

98% Au

Ha et al. (2010)

Amongst all the cyanide and non-cyanide lixiviants discussed above for the gold recovery, it has been found that cyanide leaching is the most efficient, economical and easy to control. However, the thiourea leaching can result with a recovery percentage above 90% in a short duration of leaching time, but the thiourea consumption would be too high and an expensive process compared to cyanide and thiosulphate lixiviant (Tanriverdi et al., 2005). It is necessary to investigate both benefits and consequences during the tests conducted for gold recovery using both cyanide and noncyanide lixiviants. Cyanide lixiviant has been very effective for gold leaching with economically feasible Au amounts from low grade ores but cyanide can cause environmental problems if the management is improper disobeying all safety regulations. On the contrary, non-cyanide lixiviant such as thiosulphate is a low-cost, environmentally friendly method, having low chemical stability

with low metal recovery and thiourea has high leaching rate with perceived carcinogenic effects because of low chemical stability. The other possible non-cyanide lixiviants could be halogen for leaching precious metals but are easy lixiviants. However, use of halogens has been well studied by several researchers with very little success in last few years but is relatively safe, reliable for leaching with high chemical stability (Tuncuk et al., 2012). 2.4. Others non-cyanide lixiviants Other than the aforesaid cyanide and non-cyanide lixiviants several other lixiviants like inorganic acid/oxidising reactive systems has been well investigated for leaching of metals from E-wastes (Gloe et al., 1990; Mecucci and Scott, 2002; Kinoshita et al., 2003; Oh et al., 2003; Madenoglu, 2005; Quinet et al.,

Table 10 Studies regarding recovering metals from E-wastes with thiourea. Source of waste

Used amount of chemicals

Medium conditions

Recovery (%)

References

E-waste

24 g/L CS(HN2)2 0.5 M H2SO4 %0.6 Fe3+

154 lm particle size, in 40 mL solution, at 25 °C, in 2 h

90% Au 50% Ag

Li et al. (2012)

PCB

20 g/L CS(HN2)2 6 g/L Fe3+ 10 g/L H2SO4

1–3 lm particle size, 10% S/L rate, in pH 1.4, at 25 °C (600 rpm)

82% Au 84% Ag

Li and Miller (2007)

PCB

10 g/L CS(HN2)2 5 g/L Fe3+ 10 g/L H2SO4

In 400 ml leaching solution, 50 gr PCB, at pH 1, in 120 min, at 20 °C

97% Au 94% Ag

Ficeriová et al. (2008)

PCB

14 g CS(HN2)2 2.6 Fe2(SO4)3 3.6 N H2SO4 0.5 M CS(HN2)2 0.05 M H2SO4

0.84 mm particle size, in 200 ml leaching solution, in 24 h, at 20 °C (150 rpm)

100% Au 100% Ag

Lee et al. (2011)

53–75 lm particle size, 5% S/L rate, in 2 h, at 45 °C (500 rpm)

32% Ag

Gurung et al. (2013)

PCB

0.5 M CS(HN2)2 0.01 M Fe3+ 0.01 M H2SO4

53–75 lm particle size, 5% S/L rate, in 2 h, at 50 °C (500 rpm)

68% Au

Gurung et al. (2013)

PCB

20 g/L CS(HN2)2 6 g/L Fe3+ 10 g/L H2SO4

95%) using nitric acid (HNO3) (Eq. (19)). The same researchers used electrolytic method to recover these metals from leaching solutions.

3Me0 þ 8HNO3 ! 3MeðNO3 Þ2 þ 4H2 O þ 2NO

ð19Þ

ðMe : Cu; PbÞ Zhou et al. (2005) developed and patented (Chinese patent) the process for the recovery of precious metals from E-wastes containing plastics. These E-wastes were burnt at a temperature of ranging between 400 and 500 °C for 8–12 h and later leached by HCl and H2SO4 at 90 °C for metal recovery. Later the leaching pulp was filtered for solid–liquid separation and analysed. The silver content with half solid ratios was dissolved with diluted HNO3 at 60 °C and finally the gold and palladium content were recovered using HCl and NaClO3 with a recovery of 92% precious metals. In a similar study conducted by Kinoshita et al. (2003) dissolved Cu and Ni from waste circuit boards by HNO3 with >90% recovery and purified the solution using solvent extraction method. As the nitric acid is oxidative by nature, it allowed recovery of metals that could be dissolved in nitrate media such as Cu and Ag, but it was concluded to be an expensive process compared to other mineral acids. However, use of sulphuric acid could be most affordable one with additives reactants such as H2O2, O2, Fe3+ and can be a preferable one over other processes. Birloaga et al. (2014) referred to two chemical leaching systems for the base and precious metals extraction from waste printed circuit boards (WPCBs); sulphuric acid with hydrogen peroxide have been used for the first group of metals, meantime thiourea with the ferric ion in sulphuric acid medium were employed for the second one. The effects of hydrogen peroxide volume in rapport with sulphuric acid concentration and temperature were evaluated for oxidative leaching process. 2 M H2SO4 (98% w/v), 5% H2O2, 25 °C, 1/10 S/L ratio and 200 rpm were founded as optimal conditions for Cu extraction. Thiourea acid leaching process, performed on the solid filtrate obtained after three oxidative leaching steps, was carried out with 20 g/L of CS(NH2)2, 6 g/L of Fe3+, 0.5 M H2SO4. The resulted solution contains

40 g/L Zn, which accounts for 95% from all the metal content in the solution. The impurities consist of Al (4%) and small amounts of Fe, Ni and Sn. The cement contains mainly Cu, with a purity of approximately 87%. Oh et al. (2003) developed a two-step leaching process, where successful leaching of Cu, Fe, Zn, Ni and Al from circuit boards was carried out using H2SO4 and H2O2 in the first step with >95% recovery. In the second step they recovered Au/Ag using thiosulphate leaching. Quinet et al. (2005) also studied leaching Cu, Au, Ag and Pd from mobile phone circuit boards. Several researchers have used different oxidants such as H2O2, O2, Fe3+ for leaching Cu in H2SO4. Several studies have also been carried out on chemical precipitation, cementation and active carbon adsorption to purify leaching solution and/or for metal (Pd) recovery. Chloride leaching (HCl, NaCl) were applied to E-waste for the recovery of palladium with different oxidants such as HNO3 and H2O2 (Eq. (20)), where as hydrochloric acid (HCl) leaching with both oxidisers led to a recovery of 93–95% Pd (Eq. (21)). 0



Pd þ 2=3HNO3 þ 4Cl þ 2Hþ 

! PdCl4 þ 2=3NO þ 4=3H2 O ðDG ð75  CÞ ¼ 84 kJ=molÞ 0

ð20Þ 



Pd þ H2 O2 þ 4Cl þ 2Hþ ! PdCl4 þ 2H2 O 



ðDG ð75 CÞ ¼ 244 kJ=molÞ

ð21Þ

These researchers also investigated thiourea and cyanide leaching for gold and silver recovery in the following step, and they stated that they were able to obtain a recovery rate of >95% Au and Ag using cyanide leaching. They also studied chemical precipitation, cementation and active carbon adsorption for the recovery of the metals from aqueous state after completion of the leaching. A number of studies were conducted for recovery of metals from the E-waste by leaching methods. It is equally important to also investigate the solution purification after leaching step as it is very important for scaling up the process. Several solution purification steps such as solvent extraction, active carbon adsorption, ion exchange with resins, chemical sedimentation, cementation and electrolysis can be potentially used for purifying leaching solutions and purified metal recovery (Habashi, 1999). Mecucci and Scott (2002) have shown that Cu and Pb can be recovered from circuit boards in HNO3 from leaching solutions using electrolysis. In a similar research, Kinoshita et al. (2003) applied solvent extraction (LIX984) successfully for purifying leaching solution with Cu and Ni. Quinet et al. (2005) studied the recovery of Au, Ag and Pd using sulphuric acid, chlorine and cyanide lixiviants as leachants or oxidants. They also used several additives to the leaching system such as H2O2, O2, Fe3+. In the sulphuric acid leaching addition of H2O2 resulted with better copper recovery at 80 °C (Eq. (22)).

Cu0 þ H2 O2 þ H2 SO4 ! CuSO4 þ 2H2 O ðDG ð80  CÞ ¼ 329 kJ=molÞ

ð22Þ

Few other leaching studies such as Cu(II)–NH3–(NH4)2SO4 (Koyama et al., 2006), KI/I2 and NaCl/hypochloride (Shibata and Matsumoto, 1999) were carried out by several other researchers. Koyama et al. (2006) obtained copper with Cu(II)–NH3–(NH4)2SO4 leaching and then used electrolytic process for the recovery. Shibata and Matsumoto (1999) used KI/I2 and NaCl/Ca(ClO)2 as leaching lixiviant for Au/Ag recovery by solvent extraction. The small laminates that arise during circuit board production can be recovered by dissolving them in H2SO4/HNO3 and later recovered using electrolytic method. Considering the aforesaid facts it can be concluded that electrolysis is one of the potential method for the recovery of Cu from pure solutions, but the needs to be pure solution free from iron and other impurities. This iron can be

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A. Akcil et al. / Waste Management xxx (2015) xxx–xxx

Fig. 3. A proposed flow chart for recovering precious metals from E-wastes (Quinet et al., 2005).

removed by solvent extraction prior to electrolytic process. The other methods could be precipitation/cementation and sedimentation/cementation for the metal recovery (Goosey and Kellner, 2002). Addition of oxidants to iodine leaching enhances the gold recovery and decreases the iodine consumption resulting with an economical and cheaper process (Xu et al., 2009, 2010). Li and Lu (2010) have found sodium hypochloride to be a very good oxidant for iodine leaching of gold. The optimum conditions for 85% gold recovery found was 7–9% S/L ratio, 0.25 mol/L iodide concentrations, neutral pH with 4 h of leaching. Xu et al. (2009) used hydrogen peroxide as oxidant in iodine leaching of Au from circuit boards (fine particle size fractions). Addition of 1% hydrogen peroxide to the iodine leaching medium increased the Au leaching yield to 95%, but additional H2O2 more than 1% led to precipitation of iodine on the gold surface leading to decline in the gold recovery. The reaction of hydrogen peroxide assisting iodine leaching is shown in Eq. (23).

2Au þ 4I þ H2 O2 ! 2AuI2 þ 2OH

ð23Þ

Another study conducted by Xu et al. (2010) for gold recovery from PCB’s with iodine–iodide solutions, 0.2% iodine resulted with low leaching rate but increased iodine concentration of 1–1.2% resulted with elevated gold recovery of 95%. Quinet et al. (2005) attempted to develop an economically feasible technology for the recovery of precious metals from mobile phones on a bench-scale study using hydrometallurgical method. The feed material contained 27.37% Cu, 0.52% Ag, 0.06% Au and 0.04% Pd. Another study was conducted by grinding four different materials with different particle sizes (1.168 mm,

1.168 + 0.6 mm, 0.6 + 0.3 mm, and 0.3 mm) followed by hydrometallurgical processing such as sulphuric acid leaching, chlorine leaching, thiourea leaching, cyanide leaching, cementation, precipitation, ion exchange and active carbon adsorption. The flow sheet proposed is shown in Fig. 3. Using the method provided in the flow-sheet 93% of silver, 95% of gold and 99% of palladium were recovered. Several studies about the recovery ratio of some metals in Ewastes using other lixiviants are summarised in Table 11. Considering the challenges and deficit of recycling E-waste for the metal recovery of precious metal mostly depends on the effectiveness of each single step involved the process. The maximum recovery of precious metal is rather insignificant mostly up to 50%. There are mostly high losses of gold-bearing fractions during dismantling and pre-processing steps. In practice, less than 20% of the gold recycling potential from WEEE has been observed, which comprises of collection stage resulting as a weakest stage in the chain. There is still a long way to go for organising efficient collection of WEEE. Government’s agencies in all the countries need to take this seriously and facilitate better collection systems. In addition, often in pre-processing high (and avoidable) losses also occur. Pre-processing breaks up devices into main material fractions and channels these into the appropriate end-processing/material recovery processes. In case of computer circuit boards, they are not removed at an early stage and for an appropriate metallurgical recovery processes. Less liberation of metal and dust formation from the shredder process also lead to unintended co-separation of precious metals into the sorting output fractions, from where they cannot be recovered easily. There are several unnoticed problems during the processing for which hydrometallurgical

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A. Akcil et al. / Waste Management xxx (2015) xxx–xxx Table 11 Studies regarding metal recovery with other lixiviants. Source of waste

Used amount of chemicals

Medium conditions

Recovery (%)

References

PCB

1–6 M HNO3 (1st stage leaching)

At 23 and 80 °C, 100 gr PCB (for 300 ml solution), in 15, 30, 60, 120, 240 and 360 min (leaching parameters)

90% Pb

Mecucci and Scott (2002)

0.5 M NaNO3, 1,5 M HCl (2nd stage electrodeposition) 2 M NaNO3, 0.2 M NaOH (3rd stage electrohydrolysis) PCB

83% Cu 99.8% Cu

In chloride medium (HCl and NaCl) oxidative leaching (HNO3 and H2O2) 0.3 kmol/m3 Cu(II) 5 kmol/m3 NH3 1 kmol/m3 (NH4)2SO4

At 75 °C

93–95% Pd

1,5 mm particle size, 10 g PCB, at 25 °C, in 5 h

82% Cu

PCB

Nitric acid and water leaching of two stage (at 70 °C and in 1 h) (1st stage) in 100 ml aqua regia (2nd stage)

1/4 S/L ratio, 425 lm particle size, at 23 °C, in 3 h (2nd stage)

45.5% Au

Sheng and Etsell (2007)

PCB

1.1% iodine 1.5% H2O2 n(I2):n(I) = 1:10

1/10 S/L ratio, in 4 h, at 25 °C, in pH = 7

97.5% Au

Xu et al. (2009)

PCB PCB

0.25 M KI 1 M HCl

%7 S/L ratio, in pH = 7, in 4 h 8 + 3 particle size, at 900 °C in 30 min (pre-treatment thermal), at 80 °C, in 180 min, in 3 g/400 ml (leaching studies)

85% Au 85% Cu 55% Sn

Xu et al. (2010) Havlik et al. (2011)

PCB

0.5–7.5 g/L Cu2+ 4.7–46.6 g/L Cl

At 20–80 °C, solids ratio (1–15% w/v), air/oxygen (2–4 L/min.) and in 2h

>91% Cu, Ni, Fe, Ag

Yazici and Deveci, 2013

PCB

2 M H2SO4, in 20 ml 30% H2O2 (for 100 ml solution)

At 30 °C, in 2 h

90% Au

Birloaga et al. (2013)

PCB

Two step leaching 1st stage: 2 M H2SO4 (98% w/v), 5% H2O2 2nd stage: 20 g/L of CS(NH2)2, 6 g/L of Fe3+, 0.5 M H2SO4

At 25 °C, 1/10 S/L ratio and 200 rpm

95% Zn, 87% Cu, 4% Al

Birloaga et al. (2014)

PCB

10% Diisoamyl sulphide (S201, >98.5%)

A/O ratio 5 and 2 min extraction

99.5% Pd

PCB of cell phones

1/3 (v/v) HNO3

At 25 and 60 °C, in 2 h and at 1/20 S/L ratio

100% Ag

Zhang and Zhang (2014) Petter et al. (2014)

PCB

Quinet et al. (2005) Koyama et al. (2006)

processing could be advantageous (Wildmer et al., 2005; Hagelüken, 2006; Terazono et al., 2006; Huisman et al., 2007; Robinson, 2009; Yazici et al., 2010, 2011; EU, 2012; Bas et al., 2013; Erust et al., 2013; Yazici and Deveci, 2013).

recovery rates. Therefore, all efforts from manufacturer companies would also serve more cost-effective and sustainable recycling.

3. Future recommendations

Electrical and electronic equipment contain various fractions of valuable materials. Most of the valuable substances are found in printed circuit boards, which occur in relevant quantities mainly in the categories Office, Information and Communication Equipment as well as Entertainment and Consumer Electronics. Besides well known precious metals such as gold, silver, platinum and palladium also scarce materials like indium and gallium start to play an important role, due to their application in new technologies (e.g. flat screens, photovoltaics). Increasing demand for highly developed versions of electronic and electrical devices together with short life cycle of the existing new products floating in the market has sharply increased E-waste tonnage. This alarming increase of the E-waste can have detrimental effect on the environment if suitable recycling technologies are not adopted by most developing and developed countries. As these E-wastes are a huge reservoir of metal values suitable for metal recovery (precious and base metals) would result with a huge economical benefit. As the environmental regulations for treatment and the landfill of E-waste is getting stricter, there has been increasing research and development activities carried out for E-waste recycling. The harmful effects of the landfill technology causing land and water pollution have motivated industrialists and government organisation to carry out preventive measures with proper management strategies. Compared with primary metal resources such and ores and concentrates, secondary metal resources such as E-wastes are rich

Responsible recycling of E-waste is costly due to the number of steps involved. Developing new and less costly methods is extremely important to provide recycling and contribute to future work. Reuse and recovery of electronics reduces the environmental impact of these products, as well as the impact from primary production of metals and fractions found in electronics for future. Since precious metal content of E-waste particularly gold is the main economic motivation for recycling, loss of these metals should be reduced. Physical separation methods, an environmentally sound route, have been industrially applied prior to metallurgical processes to produce a metal-rich fraction at the expense of high precious metal losses. Direct hydrometallurgical treatment of E-waste in two stages i.e. acidic sulphate leaching for extraction of copper and other base metals, and subsequent precious metal recovery in cyanide/thiourea/thiosulphate/chloride/iodide, could overcome these metal losses. Further, more research should be also undertaken to develop efficient as well as flexible solution purification and metal recovery processes from pregnant leaching solutions since the composition of pregnant solutions is strongly dependent on the e-waste type being processed. In addition, owing to the complex structure of electrical and electronic equipments, effective disassembly of E-waste prior to recycling activities is also of great importance to obtain high metal

4. Summary

Please cite this article in press as: Akcil, A., et al. Precious metal recovery from waste printed circuit boards using cyanide and non-cyanide lixiviants – A review. Waste Management (2015), http://dx.doi.org/10.1016/j.wasman.2015.01.017

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A. Akcil et al. / Waste Management xxx (2015) xxx–xxx

in metal values. Recycling of E-wastes is an important part of waste management to have a pollution free environment. Traditional methods of managing E-waste including disposing in landfills, burning in incinerators or exporting to underdeveloped countries, all of which are not permitted anymore. The presence of precious metals in E-waste makes recycling an attractive and viable option both in terms of environment and economics. Industrially, pyrometallurgical, hydrometallurgical and biohydrometallurgical or a combination of all routes is used for recovering precious metals from E-waste. Many Researchers have been working using several techniques with an objective to have zero waste generation similar to hydrometallurgical process (Hu et al., 2012; Hadi et al., 2013; Yang et al., 2013; Zhou et al., 2013; Zou et al., 2013). Metal recovery from E-wastes using appropriate hydrometallurgical techniques/methods can lead to an excellent future for gold and silver market generating safety use of E-waste for metal recovery with a decreased environmental pollution. Many European researchers have developed pilot scale application to understand the intricacies and develop roadmap for a suitable technology on E-waste recycling. Moreover Industries such as Hoboken, Outokumpu and Norando together with several SME’s and small scale industries in South Korea, Japan have been widely engaged in metal recovery from E-waste. All of these necessarily requires the development of new and applicable technologies for the recycling of E-wastes. Acknowledgments This review was produced by research grants from the Research Projects Coordination Unit of the Suleyman Demirel University (Project no: BAP 3604-YL2-13 and Project no: BAP 2504-YL2-13). The authors would like to express their gratitude for financial support from The Scientific and Technological Research Council of Turkey (TUBITAK), Co-funded by Marie Curie Actions under FP7 and CAYDAG, TUBITAK (Project no: 113Y011). References Abbruzzese, C., Fornari, P., Mesidda, R., Veglió, F., Ubaldini, S., 1995. Thiosulphate leaching for gold hydrometallurgy. Hydrometallurgy 39, 265–276. Akcil, A., 2010. A new global approach of cyanide management: international cyanide management code for the manufacture, transport, and use of cyanide in the production of gold. Miner. Process. Extr. Metall. Rev. 31, 135–149. Andrews, D., Raychaudhuri, A., Frias, C., 2000. Environmentally sound technologies for recycling secondary lead. J. Power Sources 88 (1), 124–129. Arslan, F., Sayiner, B., 2009. Ammoniacal thiosulphate leaching of Ovacik gold ore. In: Ozbayog˘lu, G. (Ed.), Mineral Processing on the Verge of the 21st Century. Balkema, Rotterdam, pp. 517–522. Aylmore, M.G., Muir, D.M., 2001. Thiosulfate leaching of gold––a review. Miner. Eng. 14 (2), 135–174. Bas, A.D., Deveci, H., Yazici, E.Y., 2013. Bioleaching of copper from low grade scrap TV circuit boards using mesophilic bacteria. Hydrometallurgy 138, 65–70. BCC, 2005. Electronic Waste Recovery Business. Report ID: MST037A, 182. Business Communications Company Inc. Birloaga, I., Michelis, I.D., Ferella, F., Buzatu, M., Vegliò, F., 2013. Study on the influence of various factors in the hydrometallurgical processing of waste printed circuit boards for copper and gold recovery. Waste Manage. 33, 935– 941. Birloaga, I., Coman, V., Kopacek, B., Vegliò, F., 2014. An advanced study on the hydrometallurgical processing of waste computer printed circuit boards to extract their valuable content of metals. Waste Manage. 34, 2581–2586. Brandl, H., Bosshard, R., Weggmann, M., 2001. Computer-munching microbes: metal leaching from electronic scrap by bacteria and fungi. Hydrometallurgy 59 (2–3), 319–326. Breuer, P.L., Jeffrey, M.I., 2000. Thiosulfate leaching kinetics of gold in the presence of copper and ammonia. Miner. Eng. 13 (10–11), 1071–1081. Byerley, J.J., Fouda, S.A., Rempel, G.L., 1973a. Kinetics and mechanism of the oxidation of thiosulfate ions by copper(II) ions in aqueous ammonia solution. J. Chem. Soc., Dalton Trans. 8, 889–893. Byerley, J.J., Fouda, S.A., Rempel, G.L., 1973b. The oxidation of thiosulfate in aqueous ammonia by copper(II) oxygen complexes. Inorg. Nucl. Chem. Lett. 9, 879–883. Byerley, J.J., Fouda, S.A., Rempel, G.L., 1975. Activation of copper(II) ammine complexes by molecular oxygen for the oxidation of thiosulphate ions. J. Chem. Soc., Dalton Trans. 13, 1329–1338.

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