Hydrometallurgical Process for Extraction of Metals from Electronic waste-part I: Material Characterization and Process Option Selection

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.

Full text

Association of Metallurgical Engineers 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 Kamberović *, Marija Korać, Dragana Ivšić, Vesna Nikolić,
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.
Key words: electronic waste, printed circuit boards, recycling, hydrometallurgy,
copper, precious metals
* Corresponding author: Željko Kamberović kamber@tmf.bg.ac.rs

232 MJoM Vol 15 (4) 2009 p.231-243
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
mechanical processing, pyrometallurgy, hydrometallurgy, biohydrometallurgy or a
combination of these techniques.
Pyrometallurgy, as traditional method to recover precious 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

Kamberović et al- Hydrometallurgical process for extraction of metals... 233
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 eco-
friendly 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,

234 MJoM Vol 15 (4) 2009 p.231-243
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/m3.
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.% HNO3 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,
wt.%
ρ, kg/m3 Metallic part,
wt.%
5.000 7.07 800 43.84
2.500 37.24 880 50.44
2.000 6.99 830.15 55.47
1.800 8.50 986.33 38.86
1.250 11.68 1235.63 31.57
1.000 5.61 1474.61 48.48
0.800 5.42 1136.59 61.25
0.630 4.81 1022.48 43.95
0.500 1.64 958.63 50.20
0.400 2.75 908.81 44.26
0.315 2.47 794.15 41.31
0.250 1.16 656.74 37.87
0.100 2.82 632.61 35.86
-0.100 1.83 622.05 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.

Kamberović et al- Hydrometallurgical process for extraction of metals... 235
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

236 MJoM Vol 15 (4) 2009 p.231-243
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.% HNO3. 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 Cu, wt.%
5.000 21.96
2.500 18.37
2.000 21.81
1.800 13.90
1.250 17.82
1.000 22.75
0.800 26.34
0.630 17.53
0.500 24.26
0.400 20.22
0.315 15.08
0.250 11.16
0.100 11.45
-0.100 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.

Kamberović et al- Hydrometallurgical process for extraction of metals... 237
Table 3. Chemical composition of WPCBs each fraction in wt.%
mm Cu Zn Ni Fe Pb
5.000 11.06 1.89 1.79 5.99 0.89
2.500 30.50 2.25 1.93 0.18 0.71
2.000 30.24 2.28 1.21 2.28 1.53
1.800 24.14 2.04 0.29 0.13 0.78
1.250 35.31 1.73 1.07 1.20 3.85
1.000 33.38 2.23 0.36 1.22 7.15
0.800 27.62 2.21 0.61 0.51 5.91
0.630 28.99 1.68 0.59 0.88 3.56
0.500 40.42 1.81 0.61 1.45 3.44
0.400 40.16 1.24 0.97 1.30 3.50
0.315 23.17 1.27 0.61 1.68 3.76
0.250 14.44 1.18 0.31 1.77 2.09
0.100 7.87 1.31 0.20 2.36 1.50
-0.100 6.32 2.89 0.58 5.22 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

238 MJoM Vol 15 (4) 2009 p.231-243
Table 4. Results of XRF analysis
Cu Ag Rh Pd Pt Au
Lom 1 96.254 3.746
Lom 2 95.072 4.928
Lom 3 73.121 6.45 2.521 17.943
Lom 3 30.749 14.762 8.806 2.353 43.33
Lom 4 95.03 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.

Kamberović et al- Hydrometallurgical process for extraction of metals... 239
Electronic Scrap
(General composition:
Cu, Al, Fe, Sn, Pb, Zn, Ni, Ag, Au, Pd + non metallic
Shredding
Magnetic & Eddy
Current separation
Iron/steel fraction
aluminum fraction
Residue: about 90 wt.% of the total
(Cu, Sn, Pb, Zn, Ni, Ag, Au, Pd + non metallic
Solder leach
(HBF4)Solder recovery:
electrowinning
solution
solder: 7 wt.% of the total
Sn: 4.2 wt.%
Pb: 2.8 wt.%
Non metalic
Residue: about18 % of the total
(Cu, Zn, Ni, Ag, Au, Pd)
Copper leach
[Cu(II)/NH3/NH4+]
24h, 25 Csolution Copper recovery:
electrowinning
16% of the total
Residue: about 2 wt.% of the total
(Zn, Ni, Ag, Au, Pd)
Recovery of precious
metals
Separation of nickel and
zinc
Leaching
Electrowinning
Solvent
Extraction
PCB: ground
Solution:Cu(II)/NH3/NH4+
Duration: 24h, 25°C
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.

240 MJoM Vol 15 (4) 2009 p.231-243
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.

Kamberović et al- Hydrometallurgical process for extraction of metals... 241
Fig. 5. Flow sheet of gold recovery from gold-plated PCBs (GPCB), gold-coated glass
bangles (GCGB) and gold-coated mirrors (GCM)

242 MJoM Vol 15 (4) 2009 p.231-243
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/carroll-
text.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, Vth 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

Kamberović et al- Hydrometallurgical process for extraction of metals... 243
processing and electrometallurgy, Journal of Hazardous Materials, 137 (3), 2006,
1704–1709
[13] Ž. Kamberović, D. Sinadinović, M. Korać, Metallurgy of gold and silver (in
Serbian), SIMS, 2007
[14] K. Chojnacka, Biosorption and bioaccumulation – the prospects for practical
applications, Environment International, 2009, Article in Press, Corrected Proof
[15] 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
[16] B.C. Qi, C. Aldrich, Biosorption of heavy metals from aqueous solutions with
tobacco dust, Bioresource Technology, 99 (13), 2008, 5595–5601
[17] 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
[18] 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
[19] 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), 518-
522
[20] 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