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Thiosulfate leaching of gold from waste mobile phones
Journal of Hazardous Materials 178 (2010) 1115–1119
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Thiosulfate leaching of gold from waste mobile phones Vinh Hung Ha a , Jae-chun Lee b,∗ , Jinki Jeong b , Huynh Trung Hai a , Manis K. Jha c a
Institute for Environmental Science and Technology, Hanoi University of Technology (HUT), 1 – Dai Co Viet, Hanoi, Viet Nam Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), 92, Gwahang-no, Yuseong-gu, Daejeon 305-350, South Korea c Metal Extraction & Forming Division, National Metallurgical Laboratory (CSIR), Jamshedpur, India b
a r t i c l e
i n f o
Article history: Received 7 December 2009 Received in revised form 19 January 2010 Accepted 19 January 2010 Available online 25 January 2010 Keywords: Gold Thiosulfate leaching Mobile phone PCBs Recycling
a b s t r a c t The present communication deals with the leaching of gold from the printed circuit boards (PCBs) of waste mobile phones using an effective and less hazardous system, i.e., a copper–ammonia–thiosulfate solution, as an alternative to the conventional and toxic cyanide leaching of gold. The influence of thiosulfate, ammonia and copper sulfate concentrations on the leaching of gold from PCBs of waste mobile phones was investigated. Gold extraction was found to be enhanced with solutions containing 15–20 mM cupric, 0.1–0.14 M thiosulfate, and 0.2–0.3 M ammonia. Similar trends were obtained for the leaching of gold from two different types of scraps and PCBs of waste mobile phones. From the scrap samples, 98% of the gold was leached out using a solution containing 20 mM copper, 0.12 M thiosulfate and 0.2 M ammonia. Similarly, the leaching of gold from the PCBs samples was also found to be good, but it was lower than that of scrap samples in similar experimental conditions. In this case, only 90% of the gold was leached, even with a contact time of 10 h. The obtained data will be useful for the development of processes for the recycling of gold from waste mobile phones. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Worldwide, billions of peoples are using mobile phones as fast communication devices. Nowadays, mobile phones serve not just as a personal luxury or an addition to traditional land line telephones but also as a primary means of communication in some areas of the world where communication infrastructure is not in place [1]. Due to rapid economical growth, technological advances and the obsolescence of electronic equipment in the market, the amount of waste mobile phones has been growing. The lifetime of these devices is reducing day by day. In fact, most users upgrade their phones due to technological advances and fashion obsolescence; mobile phones are usually taken out of use well before they cease to operate, and, consequently the potential lifespan of a mobile phone is under 3 years and all of them eventually have to be discarded. This consumer behaviour has resulted in hundreds of millions of mobile phones that are taken out of use each year [1]. Worldwide estimates are that, by 2005, there will be over 500 million mobile phones weighing 250,000 t stockpiled in drawers, closets and elsewhere, waiting for disposal [2]. Mobile phones contain toxic elements, such as lead, mercury, chromium, nickel, beryllium, antimony and arsenic as well as valuable metals, such as gold, silver, palladium and platinum
∗ Corresponding author. Tel.: +82 42 868 3613; fax: +82 42 868 3415. E-mail address: [email protected] (J.-c. Lee). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.01.099
[3]. Therefore, recycling of waste mobile phones is required for both environmental protection and resource conservation. Cyanide leaching has been widely used to recycle gold from electronic scraps. In recent years, thiosulfate leaching of gold has received much attention due to stringent environmental regulations and public concerns over the use of cyanide. Acceptable gold leaching rates using thiosulfate were achieved in the presence of ammonia with cupric ions acting as the oxidant [4–10]. The major problem with the thiosulfate system is that, for solutions containing copper(II), ammonia and thiosulfate, the solution chemistry is complex and continually changes due to the homogeneous reaction between copper(II) and thiosulfate [11,12]. Several studies highlighted the effect of solution chemistry on gold leaching and showed that the gold leaching rate decreases as the reaction between copper(II) and thiosulfate proceeds [13–15]. Also, it has been shown that the oxidation of thiosulfate is catalyzed by the presence of copper and ammonia. This communication presents a study of the copper–ammonia– thiosulfate system for the leaching of gold from scraps and PCBs of waste mobile phones. The new approach for applying this complex leaching system for gold recovery from waste mobile phones is important from both environmental and economical points of view. Various process parameters, such as concentrations of thiosulfate, ammonia, and copper and leaching time, were investigated for gold leaching from scraps and PCBs of waste mobile phones. The results obtained from laboratory scale studies will be useful to develop non-toxic gold recovery processes from wastes for commercial scale.
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2. Experimental 2.1. Materials and reagents Two types of mobile phone waste, i.e., scrap and PCB samples, were used as received for the leaching experiment. Scrap samples were obtained from keyboards of waste mobile phones, and PCBs were collected from damaged or obsolete mobile phones (see Fig. 1). The average weight of the PCBs of waste mobile phones collected for the experiment was 20 g each. Waste PCBs and scrap consist of reinforced epoxy resin, thin glass fibre, interconnecting metal wires and plates. The PCBs contained an average metallic content of 0.12% Au, 35.1% Cu, 4% Sn and 2.7% Pb. All experiments were carried out using solutions prepared by dissolving analytical grade reagents in deionized water. Solutions of different concentrations used for the leaching experiments were prepared by dissolving required amounts of the chemicals in deionized water. To prepare the solution to be used in the leaching experiments for each set, ammonia water and copper(II) sulfate were mixed and added to the ammonium thiosulfate solution, and the pH of the solution was adjusted to 10–10.5 with ammonia water. 2.2. Leaching tests The leaching experiments were performed in a 500 mL opentop reactor using a shaker. A leaching solution of 300 mL with the desired quantity of reagent was used for each experiment. The scrap and PCB samples were suspended in the upper portion of the leaching reactor using nylon thread, ensuring no contact with the reactor wall during leaching. The shaking speed was maintained at 200 RPM. All experiments were performed at room temperature,
25 ◦ C. Samples of the solution were taken at a fixed interval of time during the leaching experiments and analyzed instantly. The percentage of gold leaching was calculated based on a leach liquor analysis. The material balance for gold leaching was checked by analyzing the leached residue sample. Satisfactory mass balance was obtained. 2.3. Analytical methods The gold concentration in the leach solutions was determined with an atomic absorption spectrometer (AAnalyst 400, PerkinElmer Inc.). After the leaching of gold, the obtained leached residue was dried and again leached by aqua regia to determine the quantity of unleached gold present in the residue. The thiosulfate concentration was determined by the iodimetric method. To eliminate the effect of the cupric ammonia complex on iodine titration, the required amount of acetic acid (10% solution) was added prior to titration with the indicator Vitex (modified starch). The concentration of the cupric ammonia complex was determined at a wavelength of 608 nm using a UV–vis spectrophotometer (UV1601PC, Shimadzu Co.). Solutions containing the cupric ammonia complex present a dark blue coloration, which is key to the colorimetric determination. The solution absorbance was recorded at regular time intervals by computer. 3. Results and discussion 3.1. Leaching of gold by thiosulfate Experiments for the leaching of gold from waste mobile phones were carried out using solutions containing 0.06–0.2 M ammonium thiosulfate, 5–30 mM copper sulphate and 0.1–0.4 M ammonia with a contact time of 2 h. Gold dissolution in an ammoniacal thiosulfate solution is known to be an electrochemical reaction catalyzed by the presence of cupric ions [16]. The role of copper(II) ions in the oxidation of metallic gold to aurous Au+ ions is shown in the following reaction: Au + Cu(NH3 )4 2+ + 5S2 O3 2− = Au(S2 O3 )2 3− + Cu(S2 O3 )3 5− + 4NH3 (1) In addition to the above chemical reaction, some other reactions occur to degrade thiosulfate to tetrathionate. Eq. (2) presents an oxidation reaction, which is promoted by the copper(II) ion [17]: 2Cu(NH3 )4 2+ + 8S2 O3 2− = 2Cu(S2 O3 )3 5− + 8NH3 + S4 O6 2−
(2)
By maintaining the appropriate concentration of ammonia and thiosulfate, the conversion from the Cu(II) to the Cu(I) state can be controlled to obtain efficient leaching of gold. In this case, copper plays the role of catalyst due to the redox reaction between the copper(II) and the copper(I) state. 3.2. Effect of thiosulfate concentration
Fig. 1. (a) Scraps and (b) PCBs of waste mobile phones used for the experiment.
The effect of thiosulfate concentration on the leaching of gold from waste mobile phones was examined by varying its concentration over a range of 0.06–0.2 M. The results presented in Fig. 2(a)–(d) indicate that the leaching of gold increased with an increase in thiosulfate concentration and then after reaching maximum leaching, decreased with a further increase in thiosulfate concentration. The leaching behaviour of gold was complicatedly affected by variation of the concentrations of thiosulfate, copper, and ammonia in the solution. Variation of the concentration of thiosulfate changes the stability region of the gold species with a significant change in the pH value. If the potentials of the copper–ammoniacal thiosulfate system are too low, gold remains
V.H. Ha et al. / Journal of Hazardous Materials 178 (2010) 1115–1119
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Fig. 2. The influence of solution chemistry on the leaching of gold in different concentrations of ammonia (a) 0.1 M; (b) 0.2 M; (c) 0.3 M; (d) 0.4 M (contact time: 2 h, pH 10–10.5, room temperature).
undissolved and copper will precipitate as a sulfide [16]. In addition, higher thiosulfate concentrations increase thiosulfate consumption, which causes an increase in the concentrations of degradation products, such as sulfate, trithionate and tetrathionate [11,14]. Since the stability of gold complexes with thiosulfate depends upon the concentration of thiosulfate in the solution, it is expected that the leaching of gold will increase with an increasing concentration of thiosulfate until the optimum solution composition is reached. Therefore, the concentration of thiosulfate has to be controlled by maintaining the appropriate concentration ratio of ammonia to thiosulfate in the solution so that copper can play the catalytic role involving the facile transfer between the cupric and cuprous states. 3.3. Effect of copper(II) concentration It has been reported by several researchers that the presence of copper(II) ions in the solution promotes the dissolution of gold in thiosulfate media [4,10]. Thus, experiments were carried out to study this influence on the concentrate studied in the present work. The copper ion is an effective catalytic agent for the dissolution of gold with thiosulfate [16]. As shown in Fig. 2(a)–(d), the results of leaching experiments show that the leaching efficiency of gold increased with copper concentration when it was less than 15 mM, while at higher copper concentrations the leaching of gold became almost independent of copper concentration. The reported increase in the dissolution of gold in copper thiosulfate solutions containing ammonia has been attributed to the formation of copper(II)–amine complexes. Such results are consistent with that reported by Langhans et al. [7]. Jeffrey [18] suggested that the leaching rate of gold is limited by the diffusion of cop-
per(II) to the surface of gold at low concentrations of copper, but at higher copper concentrations the reaction is chemically controlled. Under the experimental conditions used in this study, copper concentration had a much more pronounced effect on the stability regions of the copper species, where Cu(S2 O3 )3 5− becomes more stable than the Cu(NH3 )4 2+ complex for NH3 –S2 O3 2− –Cu2+ concentrations in solution [16]. In addition, the precipitation of tenorite occurs with an increase in copper concentration in the solution. It would appear that a higher ammonia to thiosulfate ratio would be required to achieve a higher Cu(NH3 )4 2+ concentration in the solution compared to Cu(S2 O3 )3 5− [16]. However, a high Cu(NH3 )4 2+ concentration in the solution will also result in higher losses of thiosulfate through its conversion to tetrathionate, and, for this reason, the leaching efficiency of gold may be decreased. 3.4. Effect of ammonia concentration The effect of ammonia concentration on gold leaching was studied. The results presented in Fig. 2(a)–(d) revealed that ammonia plays an important role in the leaching of gold in thiosulfate solutions. It stabilizes copper in the cupric state for the oxidation of gold. An increase in ammonia concentration increased the leaching efficiency of gold, while lower leaching of gold was obtained when the ammonia concentration was higher than 0.3 M, except at copper concentrations less than 15 mM. In the presence of ammonia, the reduction of copper(II) by thiosulfate occurs by the reaction shown in Eq. (2). The leaching reaction of gold in the presence of ammonia is considerably slower than that in the absence of ammonia [19]. Consequently, increasing the total ammonia concentration but keeping the other reagent concentrations, potential and pH
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copper and ammonia and leaching time, were studied. Solution chemistry had more influence on the leaching of gold from such scraps and PCBs. The solution containing 15–20 mM Cu(II), 0.1–0.14 M thiosulfate and 0.2–0.3 M ammonia were found to be suitable for gold leaching. Within 2 h 98% of the gold was leached out from mobile phone scraps with a solution containing 20 mM Cu(II), 0.12 M thiosulfate and 0.2 M ammonia. In similar conditions, only 90% of the gold was leached from waste PCBs, even with a contact time of 10 h. When the Cu(II) concentration was decreased to 15 mM, the leaching reaction for gold was found to be very slow. Therefore, the dissolution of gold is extremely dependent on the thiosulfate, Cu(II) and ammonia concentrations used to leach the gold. Acknowledgements Fig. 3. Effect of time on leaching of gold (a) from mobile phone scraps in 0.12 M thiosulfate, 0.2 M ammonia and 20 mM copper; (b) from PCBs in 0.12 M thiosulfate, 0.2 M ammonia and 20 mM copper; (c) from PCBs in 0.12 M thiosulfate, 0.2 M ammonia and 15 mM copper (pH 10–10.5, room temperature).
constant increases the stability region of the Cu(NH3 )4 2+ complex [16]. On the other hand, in the absence of ammonia, gold dissolution by thiosulfate is passivated by the build-up of sulphur coatings as a result of the decomposition of thiosulfate on the gold surface [9,20]. It is suggested that ammonia prevents gold passivation by being preferentially adsorbed on gold surfaces over thiosulfate and leached as an amine complex [9]. But, at high concentrations of ammonia and high pH, some solid copper species, such as CuO, Cu2 O and (NH4 )5 Cu(S2 O3 )3 , may form, possibly hindering gold dissolution by coating the gold surface [16]. However, at pH > 9 the leaching of gold depends on the thiosulfate concentration rather than with ammonia concentration [21]. This suggest that the predominant gold species is Au(S2 O3 )2 3− rather than Au(NH3 )2 + . 3.5. Effect of leaching time Based on the above results, experiments were carried out to study the effect of leaching time on the thiosulfate leaching of gold using the optimum solution composition: 0.12 M thiosulfate, 0.2 M ammonia and 20 mM copper(II). It can be seen from Fig. 3 that the leaching behaviour of scraps and PCBs are significantly different in thiosulfate solutions containing copper and ammonia. From the scrap samples the leaching of gold was rapid. Within 2 h 98% of the gold was leached out from the mobile phone scraps with a solution containing 20 mM copper, 0.12 M thiosulfate and 0.2 M ammonia. The leaching of gold from the PCB sample was also found to be good, but it was lower than that of scrap samples under similar experimental conditions. In this case, the leaching of gold was only 90% with a contact time of 10 h. When the copper concentration was decreased to 15 mM (0.12 M thiosulfate and 0.2 M ammonia), the leaching kinetics for gold were found to be very slow. This behaviour may be attributed to the presence of copper in the PCBs. Copper in the PCBs is dissolved to Cu(NH3 )2 + by Cu(NH3 )4 2+ as proposed by Oishi et al. [22], resulting in a decrease of the oxidizing agent (Cu(NH3 )4 2+ ) for gold. The purification and recovery of gold from thiosulfate leach solution is carried out using solvent extraction [23], ion exchange [24], activation carbon adsorption [10], etc. From such solution, the pure gold metal could be obtained using electrowinning [10,25] or reductive precipitation [26]. 4. Conclusions Laboratory scale experiments were carried out to study the leaching of gold from scraps and PCBs collected from waste mobile phones. Various parameters, such as concentration of thiosulfate,
This paper is based on work supported by the Resources Recycling R&D Centre (Korea) under the project entitled “Development of total recycling technology for WEEE”. The first author expresses his warmest thanks to the National Research Foundation of Korea for supporting a research fellowship during the course of investigation. References [1] BASEL/MPPI, Mobile phone partnership initiative MPPI, Guidance: environmentally sound management of end-of-life mobile phone, The Basel Convention on the control of transboundary movements of hazardous waste and their disposal, Draft 4.1A Guidance, Basel Convention/UNEP, 2004, April, p. 21. [2] E. Most, Calling all Cell Phones, Collection, Reuse, and Recycling Programs in the US, Inform Inc., New York, 2003, http://www.informic.org. [3] O. Osibanjo, I.C. Nnorom, Material flows of mobile phones and accessories in Nigeria: environmental implications and sound end-of-life management options, Environ. Impact Assest. Rev. 28 (2008) 198–213. [4] M.G. Aylmore, Treatment of a refractory gold–copper sulfide concentrate by copper ammoniacal thiosulfate leaching, Miner. Eng. 14 (6) (2001) 615–637. [5] A.C. Grosse, G.W. Dicinoski, M.J. Shaw, P.R. Haddad, Leaching and recovery of gold using ammoniacal thiosulphate leach liquors (a review), Hydrometallurgy 69 (2003) 1–21. [6] P.L. Breuer, M.I. Jeffrey, An electrochemical study of gold leaching in thiosulfate solutions containing copper and ammonia, Hydrometallurgy 65 (2002) 145–157. [7] J.W. Langhans, K.P.V. Lei, T.G. Carnahan, Copper-catalyzed thiosulphate leaching of low-grade gold ores, Hydrometallurgy 29 (1992) 191–203. [8] M.I. Jeffrey, P.L. Breuer, W.L. Cho, A kinetic study that compares the leaching of gold in the cyanide, thiosulfate, and chloride systems, Metall. Mater. Trans. B: Process. Metall. Mater. Process. Sci. 32B (2001) 979–986. [9] T. Jiang, J. Chen, S. Xu, A kinetic study of gold leaching with thiosulphate, in: J.B. Hiskey, G.W. Warren (Eds.), Hydrometallurgy, Fundamentals, Technology and Innovations, AIME, Littleton, Colorado, USA, 1993, pp. 119–126. [10] C. Abbruzzese, P. Fornari, R. Massidda, F. Veglio, S. Ubaldini, Thiosulphate leaching for gold hydrometallurgy, Hydrometallurgy 39 (1995) 265–276. [11] J.J. Byerley, S.A. Fouda, G.L. Rempel, Kinetics and mechanism of the oxidation of thiosulphate ions by copper(II) ions in aqueous ammonia solution, J. Chem. Soc., Dalton Trans. (1973) 889–893. [12] P.L. Breuer, M.I. Jeffrey, The reduction of copper(II) and the oxidation of thiosulfate and oxysulfur anions in gold leaching solutions, Hydrometallurgy 70 (1–3) (2003) 163–173. [13] J.J. Byerley, S.A. Fouda, G.L. Rempel, The oxidation of thiosulfate in aqueous ammonia by copper(II) oxygen complexes, J. Inorg. Nucl. Chem. Lett. 9 (1973) 879–883. [14] J.J. Byerley, S.A. Fouda, G.L. Rempel, Activation of copper(II) amine complexes by molecular oxygen for the oxidation of thiosulfate ions, J. Chem. Soc., Dalton Trans. (1975) 1329–1338. [15] P.L. Breuer, M.I. Jeffrey, Copper catalysed oxidation of thiosulfate by oxygen in gold leach solutions, Miner. Eng. 16 (1) (2003) 21–30. [16] M.G. Aylmore, D.M. Muir, Thiosulfate leaching of gold—a review, Miner. Eng. 14 (2) (2001) 135–174. [17] P.L. Breuer, M.I. Jeffrey, Thiosulfate leaching kinetics of gold in the presence of copper and ammonia, Miner. Eng. 13 (10) (2000) 1071–1081. [18] M.I. Jeffrey, Kinetic aspects of gold and silver leaching in ammonia-thiosulfate solutions, Hydrometallurgy 60 (1) (2001) 7–16. [19] M.I. Jeffrey, P.L. Breuer, C.K. Chu, The importance of controlling oxygen addition during the thiosulfate leaching of gold ores, Miner. Process. 72 (2003) 323–330. [20] A.M. Pedraza, I. Villegas, P.L. Freund, B. Chornik, Electrooxidation of thiosulphate ion on gold: study by means of cyclic voltammetry and Auger electron spectroscopy, Electroanal. Chem. Int. Electrochem. 250 (2) (1988) 443–449.
V.H. Ha et al. / Journal of Hazardous Materials 178 (2010) 1115–1119 [21] R.Y. Wan, Importance of solution chemistry for thiosulfate leaching of gold, in: Proceedings of the World Gold 97, Singapore Society for Mining, Metallurgy and Exploration, Inc., Littleton, 1997, pp. 159–162. [22] T. Oishi, K. Koyama, H. Konishi, M. Tanaka, J.-C. Lee, Influence of ammonium salt on electrowinning of copper from ammoniacal alkaline solutions, Electrochim. Acta 53 (2007) 127–132. [23] J. Zhao, Z. Wu, J. Chen, Extraction of gold from thiosulfate solutions with alkyl phosphorus esters, Hydrometallurgy 46 (1997) 363–372.
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[24] M.I. Jeffrey, D.M. Hewitt, X. Dai, S.D. Brunt, Ion exchange adsorption and elution for recovering gold thiosulfate from leach solutions, Hydrometallurgy 100 (2010) 136–143. [25] M.P. Gallagher, Affinity of activated carbon towards some gold(I) complexes: dissolution of finely disseminated gold using a flow electrochemical cell, Hydrometallurgy 25 (1989) 305–316. [26] A.K. Perez, H.D. Garaviz, Method for recovery of precious metals from difficult ores with copper ammonium thiosulfate, U.S. Patent 4,654,078 (1987).
Contents lists available at ScienceDirect
Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
Short communication
Thiosulfate leaching of gold from waste mobile phones Vinh Hung Ha a , Jae-chun Lee b,∗ , Jinki Jeong b , Huynh Trung Hai a , Manis K. Jha c a
Institute for Environmental Science and Technology, Hanoi University of Technology (HUT), 1 – Dai Co Viet, Hanoi, Viet Nam Mineral Resources Research Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), 92, Gwahang-no, Yuseong-gu, Daejeon 305-350, South Korea c Metal Extraction & Forming Division, National Metallurgical Laboratory (CSIR), Jamshedpur, India b
a r t i c l e
i n f o
Article history: Received 7 December 2009 Received in revised form 19 January 2010 Accepted 19 January 2010 Available online 25 January 2010 Keywords: Gold Thiosulfate leaching Mobile phone PCBs Recycling
a b s t r a c t The present communication deals with the leaching of gold from the printed circuit boards (PCBs) of waste mobile phones using an effective and less hazardous system, i.e., a copper–ammonia–thiosulfate solution, as an alternative to the conventional and toxic cyanide leaching of gold. The influence of thiosulfate, ammonia and copper sulfate concentrations on the leaching of gold from PCBs of waste mobile phones was investigated. Gold extraction was found to be enhanced with solutions containing 15–20 mM cupric, 0.1–0.14 M thiosulfate, and 0.2–0.3 M ammonia. Similar trends were obtained for the leaching of gold from two different types of scraps and PCBs of waste mobile phones. From the scrap samples, 98% of the gold was leached out using a solution containing 20 mM copper, 0.12 M thiosulfate and 0.2 M ammonia. Similarly, the leaching of gold from the PCBs samples was also found to be good, but it was lower than that of scrap samples in similar experimental conditions. In this case, only 90% of the gold was leached, even with a contact time of 10 h. The obtained data will be useful for the development of processes for the recycling of gold from waste mobile phones. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Worldwide, billions of peoples are using mobile phones as fast communication devices. Nowadays, mobile phones serve not just as a personal luxury or an addition to traditional land line telephones but also as a primary means of communication in some areas of the world where communication infrastructure is not in place [1]. Due to rapid economical growth, technological advances and the obsolescence of electronic equipment in the market, the amount of waste mobile phones has been growing. The lifetime of these devices is reducing day by day. In fact, most users upgrade their phones due to technological advances and fashion obsolescence; mobile phones are usually taken out of use well before they cease to operate, and, consequently the potential lifespan of a mobile phone is under 3 years and all of them eventually have to be discarded. This consumer behaviour has resulted in hundreds of millions of mobile phones that are taken out of use each year [1]. Worldwide estimates are that, by 2005, there will be over 500 million mobile phones weighing 250,000 t stockpiled in drawers, closets and elsewhere, waiting for disposal [2]. Mobile phones contain toxic elements, such as lead, mercury, chromium, nickel, beryllium, antimony and arsenic as well as valuable metals, such as gold, silver, palladium and platinum
∗ Corresponding author. Tel.: +82 42 868 3613; fax: +82 42 868 3415. E-mail address: [email protected] (J.-c. Lee). 0304-3894/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jhazmat.2010.01.099
[3]. Therefore, recycling of waste mobile phones is required for both environmental protection and resource conservation. Cyanide leaching has been widely used to recycle gold from electronic scraps. In recent years, thiosulfate leaching of gold has received much attention due to stringent environmental regulations and public concerns over the use of cyanide. Acceptable gold leaching rates using thiosulfate were achieved in the presence of ammonia with cupric ions acting as the oxidant [4–10]. The major problem with the thiosulfate system is that, for solutions containing copper(II), ammonia and thiosulfate, the solution chemistry is complex and continually changes due to the homogeneous reaction between copper(II) and thiosulfate [11,12]. Several studies highlighted the effect of solution chemistry on gold leaching and showed that the gold leaching rate decreases as the reaction between copper(II) and thiosulfate proceeds [13–15]. Also, it has been shown that the oxidation of thiosulfate is catalyzed by the presence of copper and ammonia. This communication presents a study of the copper–ammonia– thiosulfate system for the leaching of gold from scraps and PCBs of waste mobile phones. The new approach for applying this complex leaching system for gold recovery from waste mobile phones is important from both environmental and economical points of view. Various process parameters, such as concentrations of thiosulfate, ammonia, and copper and leaching time, were investigated for gold leaching from scraps and PCBs of waste mobile phones. The results obtained from laboratory scale studies will be useful to develop non-toxic gold recovery processes from wastes for commercial scale.
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V.H. Ha et al. / Journal of Hazardous Materials 178 (2010) 1115–1119
2. Experimental 2.1. Materials and reagents Two types of mobile phone waste, i.e., scrap and PCB samples, were used as received for the leaching experiment. Scrap samples were obtained from keyboards of waste mobile phones, and PCBs were collected from damaged or obsolete mobile phones (see Fig. 1). The average weight of the PCBs of waste mobile phones collected for the experiment was 20 g each. Waste PCBs and scrap consist of reinforced epoxy resin, thin glass fibre, interconnecting metal wires and plates. The PCBs contained an average metallic content of 0.12% Au, 35.1% Cu, 4% Sn and 2.7% Pb. All experiments were carried out using solutions prepared by dissolving analytical grade reagents in deionized water. Solutions of different concentrations used for the leaching experiments were prepared by dissolving required amounts of the chemicals in deionized water. To prepare the solution to be used in the leaching experiments for each set, ammonia water and copper(II) sulfate were mixed and added to the ammonium thiosulfate solution, and the pH of the solution was adjusted to 10–10.5 with ammonia water. 2.2. Leaching tests The leaching experiments were performed in a 500 mL opentop reactor using a shaker. A leaching solution of 300 mL with the desired quantity of reagent was used for each experiment. The scrap and PCB samples were suspended in the upper portion of the leaching reactor using nylon thread, ensuring no contact with the reactor wall during leaching. The shaking speed was maintained at 200 RPM. All experiments were performed at room temperature,
25 ◦ C. Samples of the solution were taken at a fixed interval of time during the leaching experiments and analyzed instantly. The percentage of gold leaching was calculated based on a leach liquor analysis. The material balance for gold leaching was checked by analyzing the leached residue sample. Satisfactory mass balance was obtained. 2.3. Analytical methods The gold concentration in the leach solutions was determined with an atomic absorption spectrometer (AAnalyst 400, PerkinElmer Inc.). After the leaching of gold, the obtained leached residue was dried and again leached by aqua regia to determine the quantity of unleached gold present in the residue. The thiosulfate concentration was determined by the iodimetric method. To eliminate the effect of the cupric ammonia complex on iodine titration, the required amount of acetic acid (10% solution) was added prior to titration with the indicator Vitex (modified starch). The concentration of the cupric ammonia complex was determined at a wavelength of 608 nm using a UV–vis spectrophotometer (UV1601PC, Shimadzu Co.). Solutions containing the cupric ammonia complex present a dark blue coloration, which is key to the colorimetric determination. The solution absorbance was recorded at regular time intervals by computer. 3. Results and discussion 3.1. Leaching of gold by thiosulfate Experiments for the leaching of gold from waste mobile phones were carried out using solutions containing 0.06–0.2 M ammonium thiosulfate, 5–30 mM copper sulphate and 0.1–0.4 M ammonia with a contact time of 2 h. Gold dissolution in an ammoniacal thiosulfate solution is known to be an electrochemical reaction catalyzed by the presence of cupric ions [16]. The role of copper(II) ions in the oxidation of metallic gold to aurous Au+ ions is shown in the following reaction: Au + Cu(NH3 )4 2+ + 5S2 O3 2− = Au(S2 O3 )2 3− + Cu(S2 O3 )3 5− + 4NH3 (1) In addition to the above chemical reaction, some other reactions occur to degrade thiosulfate to tetrathionate. Eq. (2) presents an oxidation reaction, which is promoted by the copper(II) ion [17]: 2Cu(NH3 )4 2+ + 8S2 O3 2− = 2Cu(S2 O3 )3 5− + 8NH3 + S4 O6 2−
(2)
By maintaining the appropriate concentration of ammonia and thiosulfate, the conversion from the Cu(II) to the Cu(I) state can be controlled to obtain efficient leaching of gold. In this case, copper plays the role of catalyst due to the redox reaction between the copper(II) and the copper(I) state. 3.2. Effect of thiosulfate concentration
Fig. 1. (a) Scraps and (b) PCBs of waste mobile phones used for the experiment.
The effect of thiosulfate concentration on the leaching of gold from waste mobile phones was examined by varying its concentration over a range of 0.06–0.2 M. The results presented in Fig. 2(a)–(d) indicate that the leaching of gold increased with an increase in thiosulfate concentration and then after reaching maximum leaching, decreased with a further increase in thiosulfate concentration. The leaching behaviour of gold was complicatedly affected by variation of the concentrations of thiosulfate, copper, and ammonia in the solution. Variation of the concentration of thiosulfate changes the stability region of the gold species with a significant change in the pH value. If the potentials of the copper–ammoniacal thiosulfate system are too low, gold remains
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Fig. 2. The influence of solution chemistry on the leaching of gold in different concentrations of ammonia (a) 0.1 M; (b) 0.2 M; (c) 0.3 M; (d) 0.4 M (contact time: 2 h, pH 10–10.5, room temperature).
undissolved and copper will precipitate as a sulfide [16]. In addition, higher thiosulfate concentrations increase thiosulfate consumption, which causes an increase in the concentrations of degradation products, such as sulfate, trithionate and tetrathionate [11,14]. Since the stability of gold complexes with thiosulfate depends upon the concentration of thiosulfate in the solution, it is expected that the leaching of gold will increase with an increasing concentration of thiosulfate until the optimum solution composition is reached. Therefore, the concentration of thiosulfate has to be controlled by maintaining the appropriate concentration ratio of ammonia to thiosulfate in the solution so that copper can play the catalytic role involving the facile transfer between the cupric and cuprous states. 3.3. Effect of copper(II) concentration It has been reported by several researchers that the presence of copper(II) ions in the solution promotes the dissolution of gold in thiosulfate media [4,10]. Thus, experiments were carried out to study this influence on the concentrate studied in the present work. The copper ion is an effective catalytic agent for the dissolution of gold with thiosulfate [16]. As shown in Fig. 2(a)–(d), the results of leaching experiments show that the leaching efficiency of gold increased with copper concentration when it was less than 15 mM, while at higher copper concentrations the leaching of gold became almost independent of copper concentration. The reported increase in the dissolution of gold in copper thiosulfate solutions containing ammonia has been attributed to the formation of copper(II)–amine complexes. Such results are consistent with that reported by Langhans et al. [7]. Jeffrey [18] suggested that the leaching rate of gold is limited by the diffusion of cop-
per(II) to the surface of gold at low concentrations of copper, but at higher copper concentrations the reaction is chemically controlled. Under the experimental conditions used in this study, copper concentration had a much more pronounced effect on the stability regions of the copper species, where Cu(S2 O3 )3 5− becomes more stable than the Cu(NH3 )4 2+ complex for NH3 –S2 O3 2− –Cu2+ concentrations in solution [16]. In addition, the precipitation of tenorite occurs with an increase in copper concentration in the solution. It would appear that a higher ammonia to thiosulfate ratio would be required to achieve a higher Cu(NH3 )4 2+ concentration in the solution compared to Cu(S2 O3 )3 5− [16]. However, a high Cu(NH3 )4 2+ concentration in the solution will also result in higher losses of thiosulfate through its conversion to tetrathionate, and, for this reason, the leaching efficiency of gold may be decreased. 3.4. Effect of ammonia concentration The effect of ammonia concentration on gold leaching was studied. The results presented in Fig. 2(a)–(d) revealed that ammonia plays an important role in the leaching of gold in thiosulfate solutions. It stabilizes copper in the cupric state for the oxidation of gold. An increase in ammonia concentration increased the leaching efficiency of gold, while lower leaching of gold was obtained when the ammonia concentration was higher than 0.3 M, except at copper concentrations less than 15 mM. In the presence of ammonia, the reduction of copper(II) by thiosulfate occurs by the reaction shown in Eq. (2). The leaching reaction of gold in the presence of ammonia is considerably slower than that in the absence of ammonia [19]. Consequently, increasing the total ammonia concentration but keeping the other reagent concentrations, potential and pH
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copper and ammonia and leaching time, were studied. Solution chemistry had more influence on the leaching of gold from such scraps and PCBs. The solution containing 15–20 mM Cu(II), 0.1–0.14 M thiosulfate and 0.2–0.3 M ammonia were found to be suitable for gold leaching. Within 2 h 98% of the gold was leached out from mobile phone scraps with a solution containing 20 mM Cu(II), 0.12 M thiosulfate and 0.2 M ammonia. In similar conditions, only 90% of the gold was leached from waste PCBs, even with a contact time of 10 h. When the Cu(II) concentration was decreased to 15 mM, the leaching reaction for gold was found to be very slow. Therefore, the dissolution of gold is extremely dependent on the thiosulfate, Cu(II) and ammonia concentrations used to leach the gold. Acknowledgements Fig. 3. Effect of time on leaching of gold (a) from mobile phone scraps in 0.12 M thiosulfate, 0.2 M ammonia and 20 mM copper; (b) from PCBs in 0.12 M thiosulfate, 0.2 M ammonia and 20 mM copper; (c) from PCBs in 0.12 M thiosulfate, 0.2 M ammonia and 15 mM copper (pH 10–10.5, room temperature).
constant increases the stability region of the Cu(NH3 )4 2+ complex [16]. On the other hand, in the absence of ammonia, gold dissolution by thiosulfate is passivated by the build-up of sulphur coatings as a result of the decomposition of thiosulfate on the gold surface [9,20]. It is suggested that ammonia prevents gold passivation by being preferentially adsorbed on gold surfaces over thiosulfate and leached as an amine complex [9]. But, at high concentrations of ammonia and high pH, some solid copper species, such as CuO, Cu2 O and (NH4 )5 Cu(S2 O3 )3 , may form, possibly hindering gold dissolution by coating the gold surface [16]. However, at pH > 9 the leaching of gold depends on the thiosulfate concentration rather than with ammonia concentration [21]. This suggest that the predominant gold species is Au(S2 O3 )2 3− rather than Au(NH3 )2 + . 3.5. Effect of leaching time Based on the above results, experiments were carried out to study the effect of leaching time on the thiosulfate leaching of gold using the optimum solution composition: 0.12 M thiosulfate, 0.2 M ammonia and 20 mM copper(II). It can be seen from Fig. 3 that the leaching behaviour of scraps and PCBs are significantly different in thiosulfate solutions containing copper and ammonia. From the scrap samples the leaching of gold was rapid. Within 2 h 98% of the gold was leached out from the mobile phone scraps with a solution containing 20 mM copper, 0.12 M thiosulfate and 0.2 M ammonia. The leaching of gold from the PCB sample was also found to be good, but it was lower than that of scrap samples under similar experimental conditions. In this case, the leaching of gold was only 90% with a contact time of 10 h. When the copper concentration was decreased to 15 mM (0.12 M thiosulfate and 0.2 M ammonia), the leaching kinetics for gold were found to be very slow. This behaviour may be attributed to the presence of copper in the PCBs. Copper in the PCBs is dissolved to Cu(NH3 )2 + by Cu(NH3 )4 2+ as proposed by Oishi et al. [22], resulting in a decrease of the oxidizing agent (Cu(NH3 )4 2+ ) for gold. The purification and recovery of gold from thiosulfate leach solution is carried out using solvent extraction [23], ion exchange [24], activation carbon adsorption [10], etc. From such solution, the pure gold metal could be obtained using electrowinning [10,25] or reductive precipitation [26]. 4. Conclusions Laboratory scale experiments were carried out to study the leaching of gold from scraps and PCBs collected from waste mobile phones. Various parameters, such as concentration of thiosulfate,
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