Gold From Scrap

Association of of Metallurgical Metallurgical Engineers of of Serbia AMES

Scientific paper UDC: 661.061.34:628.4.043

HYDROMETALLURGICAL PROCESS FOR EXTRACTION OF METALS FROM ELECTRONIC WASTE-PART I: MATERIAL CHARACTERIZATION AND PROCESS OPTION SELECTION

Željko Kamberovi ć *, Marija Kora ć, Dragana Ivšić, Vesna Nikoli ć, Milisav Ranitović Department of Metallurgical Engineering, Faculty of Technology and Metallurgy, Belgrade, Serbia Received 11.11.2009 Accepted 28.12.2009.

Abstract

Used electronic equipment became one of the fastest growing waste streams in the world. In the past two decades recycling of printed circuit boards (PCBs) has been based on pyrometallurgy, higly polluting recycling technology whic causes a variety of environmental problems. The most of the contemporary research activities on recovery of base and precious metals from waste PCBs are focused on hydrometallurgical techniques as more exact, predictable and easily controlled. In this paper mechanically pretrated PCBs are leached with nitric acid. Pouring density, percentage of magnetic fraction, particle size distribution, metal content and leachability are determined using optical microscopy, atomic absorption spectrometry (AAS), X-ray fluorescent spectrometry (XRF) and volumetric analysis. Three hydrometallurgical process options for recycling of copper and precious metals from waste PCBs are proposed and optimized: the use of selective leachants for recovery of high purity metals (fluoroboric acid, ammonia-ammonium salt solution), conventional leachants (sulphuric acid, chloride, cyanide) and eco-friendly leachants (formic acid, potassium persulphate). Results presented in this paper showed that size reduction process should include cutting instead of hammer shredding for obtaining suitable shape & granulation and that for further testing usage of particle size -3 +0.1mm is recommended. Also, Fe magnetic phase content could be reduced before hydro treatment treatment. Key words: electronic waste, printed circuit boards, recycling, hydrometallurgy, copper, precious metals

*

Corresponding author: Željko Kamberović [email protected]

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Introduction

Fast electronic industry development brought the great benefits in everyday life, but its consequences are usually ignored or even unknown. Used electronic equipment became one of the fastest growing waste streams in the world. From 20 to 50 million tonnes of waste electical and electronic equipment (WEEE, e-waste) are generated each year, bringing significant risks to human health and the environment [1]. EU legislative restricts the use of hazardous substances in electrical and electronic equipment (EEE) (Directive 2002/95/EC) such as: lead, mercury, cadmium, chromium and flame retardants: polybrominated biphenyls (PBB) or polybrominated diphenyl ethers (PBDE) and also promotes the collection and recycling of such equipment (Directive 2002/96/EC). They have been in implementation since February 2003. Despite rules on collection and recycling only one third of electrical and electronic waste in the European Union is reported as appropriately treated and the other two thirds are sent to landfills and potentially to sub-standard treatment sites in or outside the European Union. In December 2008 the European Commission proposed to revise the directives on EEE in order to tackle the fast increasing waste stream of these products [2]. Recycling of printed circuit boards (PCBs), as a key component in the WEEE, in past two decades have been based on recovery via material smelting. This is highly polluting, primitive recycling technology that can cause a variety of environmental problems. It is mostly processed, sometimes illegally, in developing countries, for instance China, India, Pakistan and some African countries [3,4]. Goosey and Kellner in their detailed study [5] have defined the existing and potential technologies that might be used for the recycling of PCBs. They pointed out that metals could be recycled by by mechanical processing, pyrometallurgy, hydrometallurgy, biohydrometallurgy or a combination of these techniques. Pyrometallurgy, as traditional method to recover precious pr ecious and non-ferrous metals from e-waste, includes different treatments on high temperatures: incineration, melting etc. Pyrometallurgical processes could not be considered as best available recycling techniques anymore because some of the PCB componenets, especially plastics and flame retardants, produce toxic and carcinogenic compounds. The most of the research activities on recovery of base and precious metals from waste PCBs are focused on hydrometallurgical techniques for they are more exact, predictable and easily controlled [6,7]. In recent years the great number of investigations have been conducted in order to solve the problem of WEEE and develop appropriate recycling techniques. According to Cui and Zhang [7] recycling of e-waste can be broadly divided into three major steps: a) disassembly-mechanical pretreatment: selectively removing hazardous and valuble components for special treatment and it is necessary step for further operations, b) concentrating: increasing the concentration of desirable materials using mechanical and/or metallurgical processing and c) refining: metallurgical treatment and purification of desirable materials. Hydrometallurgu, i.e. leaching and cementation process in Serbian mine Bor was first mentioned in 1907 when 200 tons of copper were produced. Since those days till today copper hydrometallurgy has not mount at Serbia and nearby region [8]. Hydrometallurgical processing consist of: leaching – transfering desirable components into solution using acides or halides as leaching agents, purification of the

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leach solution to remove impurities by solvent extraction, adsorption or ion-exchange, then recovery of base and precious metals from the solution by electrorefining process, chemical reduction, or crystallization. The most efficient leaching agents are acids, due to their ability to leach both base and precious metals. Generally, base metals are leached in nitric acid [9, 10]. The most efficient agent used for solder leach is fluoroboric acid [11]. Copper is leached by sulphuric acid or aqua regia [12]. Aqua regia is also used for gold and silver [11], but these metals are usually leached by thiourea or cyanide [13]. Palladium is leached by hydrochloric acid and sodium chlorate [7]. Biohydrometallurgy is a new, cleaner and one of the most promising ecofriendly technologies. Biosorption is a process employing a suitable biomass to sorb heavy metals from aqueous solutions [14]. This is physico-chemical mechanism based on ion-exchange [15], metal ion surface complexation adsorption or both [16]. Oishi et al. [17] conducted research on recovery of copper from PCBs by hydrometallurgical techniques. Proposed process consists of leaching, solvent extraction and electrowinning. In the first stage of research conducted by Veit et al. [12] mechanical processing was used as comminution followed by size, magnetic and electrostatic separation. After pretreatment, the fraction with concentrated Cu, Pb and Sn was dissolved with acids and treated in an electrochemical process in order to recover the metals separately, especially copper, with two different solutions: aqua regia and sulphuric acid. Frey and Park [11] performed research for recovery of high purity precious metals from PCBs using aqua regia as a leachant. The most significant achievement of this research was synthesis of pure gold nanoparticles. Sheng and Etsell [9] investigated leaching of gold from computer chips. The first stage was leaching of base metals with nitric acid and the second, leaching of gold with aqua regia due to its flexibility, ease and low capital requirement. Non-metallic materials are also recovered this way, mainly plastic and ceramics. Quinet et al. [18] carried out bench-scale extraction study on the applicability of economically feasible hydrometallurgical processing routes to recover silver, gold and palladium from waste mobile phones. Selective extraction of dissolved metals from solution is very difficult and demanding process [19]. Experimental

Electronic waste is defined as a mixture of various metals, particularly copper, aluminum and steel, attached to different types of plastics and ceramics. The samples for experimental research presented in this paper were milled PCBs with obtained by mechanical pretreatment of waste computers. The mechanical pretreatment of end-of-life computers was performed at SETrade, Belgrade. The first stage was manual disassembling of computers, liberation of PCBs and removal of the batteries and capacitors. Liberated PCBs were milled in QZ-decomposer, separating magnetic materials from non magnetic fractions, while aluminum was manually removed from conveyor belt. Material was then milled in shredder Meccano Plastica, after which material was not exposed to another magnetic separation. Characterization of granulated waste PCBs included determination of following parameters: pouring density, percentage of magnetic fraction, particle size distribution,

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rate of metal components leachability using optical microscopy, atomic absorption spectrometry (AAS), X-ray fluorescent spectrometry (XRF) and volumetric analysis. Also, appropriate hydrometallurgical system for evaluation of metal components and optimization of process parameters: temperature, time, solid:liquid ratio and mixing velocity are selected. Sieve analysis was performed using Taylor type sieves and mass of fractions obtained after 30 minutes sieving was measured. Percentage of each fraction was calculateded. The pouring density of total sample and of each fraction was measured using Hall flowmeter funnel (ASTM B13). Pouring density of total sample was 889 kg/m 3. The sample was subjected to the magnetic separation process using two permanent magnets each weighing 100g. Percentage of magnetic material content was 5.39%. Content of metallic and non-metallic components of entire sample as well as for each fraction was determined by leaching with 50 vol.% HNO 3 near to boiling temperature with agitation followed by filtration after cooling. Metallic fraction is transferred to liquid. Solid non-metallic residue mass is measured after filtration. Experimental results are shown in Table 1. Table 1. Characteristics of PCBs granulated samples mm Fraction, Metallic part, ρ, kg/m3

wt.% 5.000 2.500 2.000 1.800 1.250 1.000 0.800 0.630 0.500 0.400 0.315 0.250 0.100 -0.100

7.07 37.24 6.99 8.50 11.68 5.61 5.42 4.81 1.64 2.75 2.47 1.16 2.82 1.83

wt.% 800 880 830.15 986.33 1235.63 1474.61 1136.59 1022.48 958.63 908.81 794.15 656.74 632.61 622.05

43.84 50.44 55.47 38.86 31.57 48.48 61.25 43.95 50.20 44.26 41.31 37.87 35.86 38.50

Results of the sieve analysis showed that the greatest percent of sample was in fraction +2.5mm. Metallic part was mostly contained in fraction +0.8 mm. Figures 1a-f are presenting some of the PCB fractions before and after dissolving in nitric acid and removal of metallic components.

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As received

After dissolution

a)

b)

c)

d)

e)

f)

Figure1. PCB fractions before and after dissolving in HNO3 a&b) 1.8mm; c&d) 1.0mm; e&f) 0.1mm

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Analysis of chemical composition of granulated PCBs was performed using volumetric analysis, AAS and XRF spectrometry. Materials used for presented analysis were both granulated samples and samples after sieve analysis dissolved in 50 vol.% HNO3. Volumetric analysis was performed using standard sodium tiosulphate solution for treatment of samples dissolved in 50 vol.% HNO 3. Results showed that copper content in granulated PCBs was 21.61 wt%. Also, distribution of copper in fractions was determined by volumetric analysis as presented in Table 2. Table 2. Distribution of copper in fractions

fraction, mm 5.000 2.500 2.000 1.800 1.250 1.000 0.800 0.630 0.500 0.400 0.315 0.250 0.100 -0.100

Cu, wt.% 21.96 18.37 21.81 13.90 17.82 22.75 26.34 17.53 24.26 20.22 15.08 11.16 11.45 11.47

Presented results show that copper is mostly concentrated in fraction +0.8 mm. AAS was used for analyzing solutions of each fraction, obtained by dissolving in 50 vol.% HNO3, in order to determine content of Cu, Zn, Fe, Ni, Pb. It was performed by Perkin Elmer 4000 spectometer calibrated with standard solutions for each measured metal. Results of experimental analysis are shown in Table 3.

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Table 3. Chemical composition of WPCBs each fraction in wt.% mm Cu Zn Ni Fe 5.000 11.06 1.89 1.79 5.99 2.500 30.50 2.25 1.93 0.18 2.000 30.24 2.28 1.21 2.28 1.800 24.14 2.04 0.29 0.13 1.250 35.31 1.73 1.07 1.20 1.000 33.38 2.23 0.36 1.22 0.800 27.62 2.21 0.61 0.51 0.630 28.99 1.68 0.59 0.88 0.500 40.42 1.81 0.61 1.45 0.400 40.16 1.24 0.97 1.30 0.315 23.17 1.27 0.61 1.68 0.250 14.44 1.18 0.31 1.77 0.100 7.87 1.31 0.20 2.36 -0.100 6.32 2.89 0.58 5.22

Pb 0.89 0.71 1.53 0.78 3.85 7.15 5.91 3.56 3.44 3.50 3.76 2.09 1.50 2.12

Fraction +5mm contained ~6% of Fe, which means that magnetic separation was not efficient enough for this size of particles. XRF spectrometry was used for direct analysis of granulated PCBs samples. Characteristic parts like contacts, solders and composites were analysed. XRF analysis was performed on Skyray EDX 3000. Measurement spots labeled lom-1 to 4 are presented at Figure 2 and results in Table 4.

Figure 2. Measurement spots for XRF analysis

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Table 4. Results of XRF analysis

Lom 1 Lom 2 Lom 3 Lom 3 Lom 4

Cu 96.254 95.072 73.121 30.749 95.03

Ag 3.746 4.928 6.45 14.762

Rh

Pd

2.521 8.806

Pt

Au

2.353

17.943 43.33

4.97

XRF analysis showed that metal content varies from sample to sample and it highly depends on measuring spot. Based on detailed literature review and presented experimental results, several process option were selected as an appropriate hydrometallurgical process for extraction of metals from electronic waste was. Process option 1-The use of selective leachants and recovery of high purity metals from PCBs

This process option involves four main stages: 1. mechanical pre treatment that includes shredding, magnetic separation, eddy current separation and classification [11], 2. solder leach with fluoroboric acid and Ti(IV) ion as oxidizing agent [20], 3. recovery of copper that includes leaching with ammonia-ammonium salt solution, purification by solvent extraction with organic LIX 26 and electrowinning [17] 4. recovery of high purity precious metals (Au, Ag ang Pd) using aqua regia [11]. Schematic preview of process option 1 is presented in Figure 3.

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Kamberović et al- Hydrometallurgical process for extraction of metals... Electronic Scrap (General composition: Cu, Al, Fe, Sn, Pb, Zn, Ni, Ag, Au, Pd + non metallic

Shredding

Iron/steel fraction

Magnetic & Eddy Current separation

aluminum fraction Residue: about 90 wt.% of the total (Cu, Sn, Pb, Zn, Ni, Ag, Au, Pd + non metallic

Solder leach (HBF4 )

Non metalic

solution

solder: 7 wt.% of the total Sn: 4.2 wt.% Pb: 2.8 wt.%

Residue: about18 % of the total (Cu, Zn, Ni, Ag, Au, Pd)

24h, 25 C

Copper leach + [Cu(II)/NH 3/NH4 ] Residue: about 2 wt.% of the total (Zn, Ni, Ag, Au, Pd)

Recovery of precious metals

Solder recovery: electrowinning

solution

Copper recovery: electrowinning 16% of the total

Leaching

PCB: ground Solution:Cu(II)/NH3/NH4+ Duration: 24h, 25°C

Solvent Extraction Separation of nickel and zinc

Electrowinning

Figure 3. The recycling process of metals contained in PCB waste

Process option 2- The use of conventional leachants for recovery of metals from waste PCBs

This process option represents bench-scale method for extraction and recovery of copper and precious metals from waste PCBs. After comminution, material was subjected to serial of hydrometallurgical processing routes: sulphuric acid leaching and precipitation for Cu recovery; chloride leaching followed by cementation for Pd, Ag, Au and Cu recovery and cyanidation and activated carbon adsorption for recovery of Au and Ag. The proposed flowsheet is presented in Figure 4.

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Figure 4. Proposed flowsheet for the recovery of precious metals from WPCBs [18]

Process option 3- The use of green leachants for recovery of metals from waste PCBs

This process option is particulary based on recovery of gold from electronic waste using an “eco-friendly” or “green” reagents. After communition, non-toxic reagents formic acid and potassium persulphate are used for Au leaching at boiling temperature. Base metals, obtained as by-products, in a further steps could be recoverd by electrowining. Gold is recoverd by melting. This process option is presented in Figure 5.


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Fig. 5. Flow sheet of gold recovery from gold-plated PCBs (GPCB), gold-coated glass bangles (GCGB) and gold-coated mirrors (GCM)

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Conclusion

On the basis of experimental results it can be concluded that properties of investigated material is in accordance with literature and it could be a representative for selection of proper hydrometallurgical recycling technique. AAS chemical analysis has shown that fraction above 5 mm contained high amount of Fe and should be avoided by more efficient magnetic separation. Also, -0.1 mm fracton can cause various difficulties in process, great loses due to large content of metals in this fraction and decreased leachability. Final selection of the process which could be applied for further analysis depends on input materials characteristics. There is no completely green option. Selection of suitable hydrometallurgical process highly depend on leaching tests and techno-economical analysis and possible solution for electronic waste lies in combination of proposed process options. Literature

[1]

S. Herat, International regulations and treaties on electronic waste (e-waste), International Journal of Environmental Engineering, 1 (4), 2009, 335 - 351 [2] Recast of the WEEE and RoHS Directives proposed in 2008, http://ec.europa.eu/environment/waste/weee/index_en.htm [3] Basel Action Network and Silicon Valley Toxics Coalition, Exporting Harm: The High-Tech Trashing of Asia, Seattle and San Jose, 2002 [4] Carroll, High-Tech Trash, National Geographic Magazine Online, 2002, http://ngm.nationalgeographic.com/ngm/2008-01/high-tech-trash/carrolltext.html [5] M. Goosey, R. Kellner, A Scoping study end-of-life printed circuit boards, Intellect and the Department of Trade and Industry, Makati City, 2002 [6] J. C. Lee, H. T. Song, J. M. Yoo, Present status of the recycling of waste electrical and electronic equipment in Korea, Resources, Conservation and Recycling 50 (4), 2007, 380–397 [7] J. Cui, L. Zhang, Metallurgical recovery of metals from electronic waste: A review, Journal of Hazardous Materials, 158 (2-3), 2008, 228–256 [8] Ž. Kamberović, D. Sinadinović, M. Sokić, M. Korać, Hydrometallurgical treatment of refractory and polymetallic copper ores, V th Congress of the Metallurgists of Macedonia, 17-20 septembar, Ohrid, Makedonija, 2008, IL-02-E [9] P.P. Sheng, T.H. Etsell, Recovery of gold from computer circuit board scrap using aqua regia, Waste Management and Research, 25 (4), 2007, 380–383 [10] R. Vračar, Vučković N., Kamberović Ž., Leaching of copper(I) sulphide by sulphuric acid solution with addition of sodium nitrate, Hydrometallurgy, Elsevier, vol 70/1-3 (2003) 143 - 151 [11] Y. J. Park, D. Fray, Recovery of high purity precious metals from printed circuit boards, Journal of Hazardous Materials 164 (2-3), 2009, 1152 –1158 [12] H. M. Veit, A. M. Bernardes, J. Z. Ferreira, J. A. Soares Tenório, C. de Fraga Malfatti, Recovery of copper from printed circuit boards scraps by mechanical


[13] [14] [15]

[16] [17] [18] [19]

[20]

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processing and electrometallurgy, Journal of Hazardous Materials, 137 (3), 2006, 1704–1709 Ž. Kamberović, D. Sinadinović, M. Korać, Metallurgy of gold and silver (in Serbian), SIMS, 2007 K. Chojnacka, Biosorption and bioaccumulation – the prospects for practical applications, Environment International, 2009, Article in Press, Corrected Proof G. Naja, V. Diniz, B. Volesky, Predicting metal biosorption performance, In: Proceedings of the16th International Biohydrometallurgy Symposium, S. T. L. Harrison, D. E. Rawlings, J. Peterson, Eds., Compress Co.: Cape Town, South Africa, 2005, 553−562 B.C. Qi, C. Aldrich, Biosorption of heavy metals from aqueous solutions with tobacco dust, Bioresource Technology, 99 (13), 2008, 5595–5601 T. Oishi, K. Koyama, S. Alam, M. Tanaka, J. C. Lee, Recovery of high purity copper cathode from printed circuit boards using ammoniacal sulphate or chloride solutions, Hydrometallurgy 89 (1-2), 2007, 82–88 P. Quinet, J. Proost, A. Van Lierde, Recovery of precious metals from electronic scrap by hydrometallurgical processing routes, Minerals and Metallurgical Processing, 22 (1), 2005, 17–22 J. Pavlović, S.Stopić, B.Friedrich, Ž. Kamberović, Selective Removal of Heavy Metals from Metal-bearing Wastewaters in Cascade Line Reactor, Environmental Science and Pollution Research-ESPR, 7, Vol.14, (2007), 518522 Gibson at al. Patent US 6.641.712 B1, 2003, Process for the recovery of tin, tin alloys or lead alloys from printed circuit boards

Association of Metallurgical Engineers of Serbia AMES

Scientific paper UDC: 628.477.6

HYDROMETALLURGICAL PROCESS FOR EXTRACTION OF METALS FROM ELECTRONIC WASTE-PART II: DEVELOPMENT OF THE PROCESSES FOR THE RECOVERY OF COPPER FROM PRINTED CIRCUIT BOARDS (PCB)

Željko Kamberović1*, Marija Korać2, Milisav Ranitović2 1

Faculty of Technology and Metallurgy, University of Belgrade, Serbia Innovation center of the Faculty of Technology and Metallurgy, Belgrade, Serbia

2

Received 13.06.2011 Accepted 15.08.2011

Abstract

Rapid technological development induces increase of generation of used electric and electronic equipment waste, causing a serious threat to the environment. Waste printed circuit boards (WPCBs), as the main component of the waste, are significant source of base and precious metals, especially copper and gold. In recent years, most of the activities on the recovery of base and precious metals from waste PCBs are focused on hydrometallurgical techniques as more exact, predictable and easily controlled compared to conventional pyrometallurgical processes. In this research essential aspects of the hydrometallurgical processing of waste of electronic and electrical equipment (WEEE) using sulfuric acid and thiourea leaching are presented. Based on the developed flow-sheet, both economic feasibility and return on investment for obtained processing conditions were analyzed. Furthermore, according to this analysis, SuperPro Designer software was used to develop a preliminary techno-economical assessment of presented hydrometallurgical process, suggested for application in small mobile plant addressed to small and medium sized enterprises (SMEs). Following of this paper, the described process is techno-economically feasible for amount of gold exceeding the limit value of 500ppm. Payback time is expected in time period from up to 7 years, depending on two deferent amounts of input waste material, 50kg and 100kg of WEEE per batch. Key words: waste printed circuit boards, recycling, hydrometallurgy, copper leaching, gold leaching, techno-economical assessment

*

Corresponding author: Željko Kamberović, [email protected]

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Introduction

As a result of rapid technical and technological development, waste electric and electronic equipment (WEEE) is becoming one of the major environmental risks for its high quantity and toxicity in recent years. In terms of materials and components WEEE is non-homogeneous and very complex, and the major challenge for recycling operations is how to respond to poor recovery of metals by mechanical treatment as well to avoid gas handling problems or hazard gas compounds release using pirometallurgical process. Printed circuit boards (PCBs), as a key component of WEEE, can be considered as a significant secondary raw material due to its complex composition, mainly consisted of plastic, glass, ceramics and metals (copper, aluminum, iron, zinc, nickel, lead and precious metals). In the past two decades much attention has been devoted to development of techniques for recycling WEEE, especially copper and precious metals [1, 2, 3]. The state of the art in recovery of precious metals from electronic waste highlights two major recycling techniques, such as pyrometallurgical and hydrometallurgical, both combined with mechanical pre-treatment. Pyrometallurgical processing including incineration, smelting in a plasma arc furnace or blast furnace, sintering, melting and reactions in a gas phase at high temperatures has been a conventional technology for recovery of metals from WEEE [4, 5, 6]. However, state-of-the-art smelters are highly depended on investments. In the last decade, attention has been moved from pyrometallurgical to hydrometallurgical process for recovery of metals from electronic waste [7, 8]. This paper describes essential aspects of the hydrometallurgical processing of WEEE using sulfuric acid and thiourea leaching. Previous studies, reported by authors [9, 10] were conducted in order to investigate optimal processing conditions concerning hydrometallurgical recovery of base and precious metals from WPCBs. In this paper some results obtained in these studies are also presented. Based on these results, regarding characterization and development of the hydrometallurgical treatment of waste material, both economic feasibility and return on investment for obtained processing conditions were analyzed. Furthermore, this analysis was used in order to develop a preliminary techno-economical assessment of presented hydrometallurgical process adopted for use in small mobile plant taking into account limitation of necessary quantities of waste material as well as investment, addressed to small and medium sized enterprises (SMEs). Materials

Material used in presented research is comminuted and mechanically pre-treated waste PCBs, as described in previous studies by authors [1, 9, 10]. Chemical composition of two samples was analyzed using atomic absorption spectroscopy, X-ray fluorescence spectroscopy and, inductively coupled plasma atomic emission spectroscopy. Obtained results together with literature data are presented in Table 1. In this paper, fractions (F), 0.071mm< F <1 mm were used for further analysis. Apparatus and methodology

Previous to leaching tests materials dynamics within leaching reactor was tested as a function of particle size and stirring rate (rpm). Impact of stirring rate on fractions -

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2mm and -1mm was examined in the range of 100 to 700 rpm. Solution used for testing was 30 wt. % NaCl, whose density matches the density of H 2SO4 solution used for leaching. Table 1. Chemical composition of sample of WPCBs [9]

Materials Cu Al Pb Zn Ni Fe Sn

Sample 1* %(w/w) 27.99 0.47 2.17 2.01 1.23 1.18 3.26

Sample 2** %(w/w) 25.24 0.69 2.22 2.05 0.93 0.98 3.17

Literature data [3] %(w/w) 20 2 2 1 2 8 4

Au/ppm 440 890 1000 Pt/ppm 57 17 Ag/ppm 1490 1907 2000 Pd/ppm 50 47 50 Ceramics 20.41 22.14 max 30% Plastics 32.07 32.41 max 30% * WPCBs collected by S.E.Trade d.o.o. Belgrade, fraction –6mm ** WPCBs collected by Institute Mihajlo Pupin, fraction –1+0.071mm In a first step fraction -2mm was examined. It was shown that the increase of the stirring rate did not produce any effect on the waste material which remained at the bottom of the laboratory glass. In the next step fraction -1mm was examined when the intensification of material mixing was noticed at 200 rpm. At 300 rpm, all the granulate particles from the container are raised and caught mixing while floating particles slowly transit into the solution. Finally, at 600 rpm all particles of the waste materials are fully affected by mixing. In the industrial leaching conditions, stirring rate of 600 rpm is relatively high and could cause great trouble for carrying out of the process of leaching. It is assumed that with proper solid:liquid ratio, results obtained at 300 rpm will be satisfactory, and thus this parameter was fixed in the further analysis. According to experimental set up, reported in a previous study [1, 9,10], leaching tests were performed in a glass vessel with 15.6 cm in diameter, with a condenser, steel impeller, oxygen dispersion tube and hydrogen peroxide dozer. Experimental set up is shown in Figure 1. Electrowinning (EW) was performed in a rectangular electrolytic cell with dimensions 100×88×300mm with effective volume 2000 mL made of high density polypropylene. The cathode material was copper (Cu 99,99%) and anode was lead antimony alloy (PbSb7).

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Fig.1. Experimental apparatus for leaching: 1.Electro-resistant heater; 2.Reaction glass vessel; 3.Cooler with condenser; 4.Mixer; 5.Oxygen tube; 6.Peroxide dozer; 7.Digital pH and temperature measurement device

Results and discussion

Optimum processing conditions were obtained by variation of different leaching parameters. Hydrometallurgical extraction of copper from the waste material was performed using sulfuric acid as leaching agent. Solid residue after copper leaching step, was leached by thiourea in the presence of ferric ion (Fe 2(SO4)3) as an oxidant in sulfuric acid solution, in order to extract gold. In the case of copper leaching, analysis was performed for both, laboratory and pilot scale, while gold leaching was tested only for laboratory scale. According to summarized results, optimum copper and gold leaching parameters are presented in Table 2. Table 2. Optimal gold and copper leaching conditions

Parameter Acid conc. H2SO4 conc. thiourea conc. Oxidants conc. H2 O2 O2 Fe3+ Solid-liquid ratio Temperature Stirring rate Time

Copper leaching

Gold leaching

Laboratory scale

Pilot scale

1.5-2M /

1.5-2M /

10 g dm-3 20 g dm-3

40mL/h 16L/h / 50-100g/L 75-80°C >300rpm >5h

50 kg/t 80 L/kg/h / 150-200g/L 70°C ~300rpm <10h

/ / 6 g dm-3 20 % of solid 40 C 600 rpm ≥ 200 min °


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EW was performed in order to extract metallic copper from leaching solution. Optimal EW conditions are shown in Table 3, while obtained copper deposit is illustrated in Figure 2. Table 3. Optimal EW conditions

Voltage Current density Temperature Time Steering rate

2.1V 120-200A/m2 40ºC 15h 100rpm

Fig.2. Copper deposited on the cathode

According to presented optimal processing parameters, block diagram for hydrometallurgical recovery of base and precious metals, using selective leaching agents from WPCBs was developed. Block diagram for hydrometallurgical processing of WPCBs using sulfuric acid and thiourea leaching is shown on Figure 3.

Fig.3. Block diagram for hydrometallurgical recovery of base and precious metals [9]

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Although hydrometallurgy is relatively widely used in base and precious metals processing plants, the objective of this work was to discuss the possibility of providing a purely hydrometallurgical alternative to the pyrometalurgical process as more exact, easily controlled, less expensive, and less subjected to losses for recovery of base and precious metals from WEEE. On the other hand, by adopting an entirely hydrometallurgical route for use in small mobile plant, economic benefits for SMEs may be possible, regarding low capital investment and necessary quantities of waste material. Process option investigated in this paper was based on preliminary results carried out on the pilot scale in the scope of the European project “Innovative Hydrometallurgical Processes to recover Metals from WEEE including lamp and batteries – HydroWEEE” (FP 7 – research activities addressed to SMEs). Core objective of this project was development of suitable multi functional hydrometallurgical process for recovery of base and precious metals from fluorescent powders coming from CRT and spent lamps, printed circuit boards, LCD and lithium spent batteries. The experience and knowledge obtained in the preliminary pilot plant tests gave important technological indications for the development of the small mobile plant HydroWEEE. Furthermore results obtained during the activities of project, particularly related to WPCBs in this paper, have been used for the development of techno-economical assessment for the new mobile hydrometallurgical plant. Schematic outline of HydroWEEE mobile plant is presented in Figure 4.

Fig.4. Outline of the mobile pilot plant placed in a transportable container [11]

The techno-economical assessment for hydrometallurgical processing route presented in Figure 3, was applied on the small mobile plant schematically shown in Figure 4. For this purpose the SuperPro Designer software was used. Conceptual outline of hydrometallurgical treatment of WPCBs is shown in Figure 5.


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Fig.5. Outline of hydrometallurgical treatment of WPCBs

With the intention to open up the possibility for an economic evaluation of presented hydrometallurgical processing route, the techno-economical assessment is achieved through the development of gold amount variation models, present in waste material, as a crucial economical component of WPCBs. The models, developed by SuperPro Designer software, calculate system efficiencies, total capital costs and production (operating) costs. This methodology was applied to the development of all

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models and obtained results are directly comparable regarding time needed for return of investment. Assessment was performed by modeling two different amounts of input waste material, 50kg and 100kg of WEEE per batch, following the presented solid:liquid ratio, whereas for gold this ratio was in the range from 200ppm to 1000ppm. These data were combined to give cost and performance analysis for the integrated system. Prior to hydrometallurgical treatment, the mechanical pre-treatment of WPCBs was performed. This procedure, in addition to other mechanical operations, includes granulation of input waste material according to previously described results [1]. Therefore, presented models exclude mechanical pre-treatment cost. Fixed parameters, i.e. total capital cost and operating cost, were calculated according to prices in Serbia, October 2010, involving all economic factors in final executive summary, regarding total plant direct cost, total plant indirect cost, labor, utilities and raw material costs. Table 4 shows results obtained by performing simulation focused on calculation of payback time related to different amounts of gold. This calculation was used to assess the operational time, needed to achieve economical sustainability of such hydrometallurgical plant. Calculations were focused on the determination of total revenues of presented hydrometallurgical process regarding gold recovery. Total income for each model was calculated according to LME prices, October 2010 [12] for revenues, and it was crucial to determine economic sustainability of the whole process. Finally, based on these results, Diagrams 1 and 2 show the dependence between the operational time needed for return on investment and gold amount present in the waste material, concerning increase of payback time with decreasing gold amount. Table 4. a) Calculated dependence of gold amount vs. payback time, 50 kg of waste input material per batch

EXECUTIVE SUMMARY – 50 kg Total Capital Investment Capital Investment Charged to this Project Operating Cost

148,000$ 148,000$ 97,000$/yr

Gold amount, ppm

Total revenues, $/yr

Return on investment, %

Payback time, years

200 300 440 500 600 700 800 890 1000

62,000 83,000 92,000 100,000 115,000 132,000 148,000 159,000 161,000

-15.97 -1.96 3.14 8.39 14.65 21.29 27.99 32.36 32.95

not feasible not feasible 31.84 11.92 6.83 4.70 3.57 3.09 3.02

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Table 4. b) Calculated dependence of gold amount vs. payback time, 100 kg of waste input material per batch

EXECUTIVE SUMMARY – 100 kg Total Capital Investment Capital Investment Charged to this Project Operating Cost

149,000$ 149,000$ 144,000$/yr

Gold amount, ppm

Total revenues, $/yr

Return on investment, %

Payback time, years

200 300 440 500 600 700 800 890 1000

99,000 129,000 169,000 188,000 219,000 252,000 281,000 299,000 339,000

-21.68 -2.71 17.32 24.88 37.44 50.75 62.30 69.65 85.55

not feasible not feasible 5.77 4.02 2.67 1.97 1.61 1.44 1.17

1000

900

) 800 m p p ( d l 700 o G 600

500

2

4

6

8

10

12

Return on investment (year)

Diagram 1. Dependence of gold amount vs. payback time, 50 kg of waste input material per batch

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1000

900

800 ) m p 700 p ( d l o G 600

500

400

0

1

2

3

4

5

6

Return on investment (year)

Diagram 2. Dependence of gold amount vs. payback time, 100 kg of waste input material per batch

Conclusion

In the presented work the techno-economical feasibility of the hydrometallurgical treatment of WPCBs has been demonstrated, concerning great environmental and economical potentials that the development of an efficient hydrometallurgical route for recovery of base and precious metals may offer. According to these facts the development of such a technology responding to contemporary strict environmental requirements would be much easier. In addition, presented hydrometallurgical technology will allow the production of material with purity suitable for commercial use. According to models evaluated in this paper it is clear that the most important economic criteria is related to gold amount present in the waste material. Following these results, process is techno-economically feasible for amount of gold exceeding the limit value of 500ppm. Gold and silver obtained as cement powder could be sold to refinery or internally refined up to commercial purity Au and Ag metal powders in small a rafination equipment. Relatively insufficient widespread presence of hydrometallurgy in a field of WEEE recycling still signifies domination of the pyrometallurgy. On the contrary to SMEs requirements, it is quite clear that presented technology imply a future alternative to pyrometalurgical process that can be readily applied in small plants, unlike large multisector companies not so dependent on investment as well for market share.


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Acknowledgments

This work has been carried out with financial support of the EU within the project “Innovative Hydrometallurgical Processes to recover Metals from WEEE including lamp and batteries – HydroWEEE”, FP 7 – Funding Scheme: research activities addressed to SMEs and Ministry of Science and Technical Development, Republic of Serbia, project “Innovative synergy of by-products, waste minimization and clean technology in metallurgy”, No. 34033. References

[1]

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Gold From Scrap

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