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Critical Reviews in Environmental Science and
Technology
ISSN: 1064-3389 (Print) 1547-6537 (Online) Journal homepage: http://www.tandfonline.com/loi/best20
Recent advances on hydrometallurgical recovery
of critical and precious elements from end of life
electronic wastes - a review
Manivannan Sethurajan, Eric D. van Hullebusch, Danilo Fontana, Ata Akcil,
Haci Deveci, Bojan Batinic, João P. Leal, Teresa A. Gasche, Mehmet Ali
Kucuker, Kerstin Kuchta, Isabel F. F. Neto, Helena M. V. M. Soares & Andrzej
Chmielarz
To cite this article: Manivannan Sethurajan, Eric D. van Hullebusch, Danilo Fontana, Ata
Akcil, Haci Deveci, Bojan Batinic, João P. Leal, Teresa A. Gasche, Mehmet Ali Kucuker,
Kerstin Kuchta, Isabel F. F. Neto, Helena M. V. M. Soares & Andrzej Chmielarz (2019): Recent
advances on hydrometallurgical recovery of critical and precious elements from end of life
electronic wastes - a review, Critical Reviews in Environmental Science and Technology, DOI:
10.1080/10643389.2018.1540760
To link to this article: https://doi.org/10.1080/10643389.2018.1540760
© 2019 The Author(s). Published with
license by Taylor & Francis Group, LLC.
Published online: 17 Jan 2019.
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Recent advances on hydrometallurgical recovery of
critical and precious elements from end of life
electronic wastes - a review
Manivannan Sethurajan
a
, Eric D. van Hullebusch
a,b
, Danilo Fontana
c
,
Ata Akcil
d
, Haci Deveci
e
, Bojan Batinic
f
,Jo
~
ao P. Leal
g,h
, Teresa A. Gasche
g
,
Mehmet Ali Kucuker
i
, Kerstin Kuchta
i
, Isabel F. F. Neto
j
,
Helena M. V. M. Soares
j
, and Andrzej Chmielarz
k
a
Department of Environmental Engineering and Water Technology, IHE Delft Institute for Water
Education, Delft, The Netherlands;
b
Institut de Physique du Globe de Paris, Sorbonne Paris Cit
e,
University Paris Diderot, Paris, France;
c
ENEA, Italian National Agency for New Technologies,
Energy and Sustainable Economic Development, Rome, Italy;
d
Mineral-Metal Recovery and
Recycling (MMR&R) Research Group, Mineral Processing Division, Department of Mining
Engineering, Suleyman Demirel University, Isparta, Turkey;
e
Hydromet B&PM Group, Mineral&Coal
Processing Division, Department of Mining Engineering, Karadeniz Technical University, Trabzon,
Turkey;
f
Department of Environmental Engineering and Safety at work, University of Novi Sad,
Novi Sad, Serbia;
g
Centro de Ci^
encias e Tecnologias Nucleares (C
2
TN), Instituto Superior T
ecnico,
Universidade de Lisboa, Bobadela, LRS, Portugal;
h
Centro de Qu
ımica Estrutural (CQE), Instituto
Superior T
ecnico, Universidade de Lisboa, Bobadela, LRS, Portugal;
i
Institute of Environmental
Technology and Energy Economics, Hamburg University of Technology, Hamburg, Germany;
j
LAQV/REQUIMTE, Departamento de Engenharia Qu
ımica, Faculdade de Engenharia, Universidade
do Porto, Porto, Portugal;
k
Instytut Metali Nie_
zelaznych, Gliwice, Poland
ABSTRACT
Waste electrical and electronic equipment (WEEE) contains
economically significant levels of precious, critical metals and
rare earth elements, apart from base metals and other toxic
compounds. Recycling and recovery of critical elements from
WEEEs using a cost-effective technology are now one of the
top priorities in metallurgy due to the rapid depletion of their
natural resources. More than 150 publications on WEEE man-
agement, leaching and recovery of metals from WEEE were
reviewed in this work, with special emphasize on the recent
research (2015–2018). This paper summarizes the recent pro-
gress regarding various hydrometallurgical processes for the
leaching of critical elements from WEEEs. Various methodolo-
gies and techniques for critical elements selective recovery
(using ionic liquids, solvent extraction, electrowinning, adsorp-
tion, and precipitation) from the WEEEs leachates are dis-
cussed. Future prospects regarding the use of WEEEs as
secondary resources for critical raw materials and its techno-
economical and commercial beneficiaries are discussed.
KEYWORDS
Critical raw materials;
Critical and precious metals;
E-Wastes; Hydrometallurgy;
Metal selective recovery;
Rare earth elements; WEEEs
CONTACT Manivannan Sethurajan biotek_mani@yahoo.co.in Department of Environmental Engineering
and Water Technology, IHE Delft Institute for Water Education, Westvest 7, 2611 AX Delft, The Netherlands.
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/best.
ß2019 The Author(s). Published with license by Taylor & Francis Group, LLC.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://
creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided
the original work is properly cited.
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY
https://doi.org/10.1080/10643389.2018.1540760
Abbreviations: E-Waste: Electronic waste; WEEE: Waste elec-
trical and electronic equipment; CRM: Critical raw materials;
PCB: Printed circuit board; LCD: Liquid crystal display; CRT:
Cathode ray tube; Fl. Lamp: Fluorescent lamp; HDD: Hard disk
drives; LED: Light emitting diode; EU: European Union; UNEP:
United Nations Environmental Program; REE: Rare earth elem-
ent; ITO: Indium-tin oxide; PM: Precious metal; NiMH battery:
Nickel-hydride battery; CPU: Central processing unit; RAM:
Random access memory; LiBs: Li-ion batteries; SFL: Spent fluor-
escent lamps
GRAPHICAL ABSTRACT
Pictorial representation of the hydrometallurgical recovery of
critical and precious elements from WEEE
Highlights
Review focusing on the utilization of electronic waste as second-
ary resources
Different types, origin and composition of WEEE are summarized
Various lixiviants for critical elements leaching from WEEE are reviewed
Strategies for critical and precious elements recovery from leachates
are discussed
Techno-economic benefits of hydrometallurgy of electronic waste are
highlighted
1. Introduction
Enormous amounts of waste electrical and electronic equipment (WEEE)
are being generated in recent years. According to recent statistics from
2 M. SETHURAJAN ET AL.
European Union statistics institute, approximately 8.3 to 9.1 million tons of
WEEE wastes were generated in the EU by 2005 and is expected to increase
to 12.3 million tons by 2020 (European commission, 2008)(Figure 1). On
the other hand, worldwide WEEE generation will reach 50 million tons by
2018 (Balde, Wang, Kuehr, & Huisman, 2015).TheseWEEEnotonlycon-
tains organic pollutants (like polybrominated diphenyl ethers, chlorofluoro-
carbons etc.) but also contains significant concentrations of base, critical and
precious metals as well as rare earth elements (Robinson, 2009;Is¸ıldar, van
de Vossenberg, Rene, van Hullebusch, & Lens, 2017). Therefore, discarded
electric and electronic devices have the potential to be a very promising sec-
ondary source of critical and precious elements (Ghosh, Ghosh, Parhi,
Mukherjee, & Mishra, 2015). The major economic driver for recycling elec-
tronic waste is the recovery of critical raw materials (CRM). United Nations
Figure 2. List of critical raw materials (European Commission report, 2014).
Figure 1. Amount of WEEE generated in EU countries in 2014 (Cyprus - 2013 data) (European
Union statistics, 2017).
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 3
Environment Program (UNEP) and European Union (EU) have classified
several elements such as Ga, In, W, Nd, Pd, Ta, REE etc., as critical raw
materials which are essential to EU economy (Figure 2) (Buchert, Sch€
uler,
Bleher, & Programme des Nations Unies pour l’environnement, 2009;
European Commission, 2014). However, the list has been recently updated
and 9 new raw materials (such as baryte, bismuth, hafnium, helium, natural
rubber, phosphorus, scandium, tantalum and vanadium) have also been
addressed as critical raw materials (European Commission, 2017).
While there is a gradual depletion of primary ores of these critical and
precious elements, they are found in relatively high concentrations in elec-
tronic wastes. Waste printed circuit boards (PCBs), waste liquid crystal dis-
plays (LCDs), spent cathode ray tubes (CRTs), spent fluorescent lamps,
waste hard disk drives (HDDs), spent light emitting diodes (LEDs) and
spent batteries are the fastest growing WEEE and contain many critical and
precious elements (Willner & Fornalczyk, 2013; Askari, Ghadimzadeh,
Gomes, & Ishak, 2014; Natarajan, Tay, Yew, & Ting, 2015). For instance,
Indium-tin oxide (ITO) forms the basis of LCDs and rise crucial demand
for indium (In). European Commission (2014) reported that the total world
consumption of yttrium is estimated 7,650 Mg and the main uses of
yttrium are 79% and 21% for phosphors and ceramics, respectively. The
yttrium demand has been increasing by around 8% per year and its supply
is also expected to increase at a similar rate (European Commission, 2014).
On the other hand, europium is a fundamental element for phosphors pro-
duction, almost 96% of the global Eu consumption (425 Mg) is used for
phosphors production (European Commission, 2014). Similarly, precious
metals such as Au are essential to fabricate PCBs and chip-on-board LEDs.
Significant concentration of gold (2 gkg
1
) is present in the spent chip-on-
board LEDs (Murakami, Nishihama, & Yoshizuka, 2015). WEEEs are
highly heterogeneous and practically it is not possible to have a generic
recycling technologies.
Currently WEEE is processed at complex smelters by pyrometallurgy,
which has demerits such as high energy requirements, non-selectivity, losses
of rare earth elements (REE), and high capital costs (Tunsu & Retegan, 2017).
However, for low-volume streams (shredder dusts, LEDs, PCBs) which escape
from collection chains of pyrometallurgical recyclers, an entirely different
technology is needed. On the other hand, high grade resources of critical raw
materials are depleting considerably (Hennebel, Boon, Maes, & Lenz, 2015).
In order to meet the market demand, it is important to explore other ways of
metals recycling and recovery strategies from these WEEE. Pyrometallurgy is
not suitable for these kind of wastes because of higher capital costs and
energy requirements compared to hydrometallurgical process.
4 M. SETHURAJAN ET AL.
Several studies on the hydrometallurgy of spent WEEE were reported,
but drawbacks such as acid toxicity, heavy metal pollution, sludge gener-
ation, etc. were highlighted. Cui and Zhang (2008) reviewed pyro and
hydrometallurgical processes proposed for the recovery of metals from
WEEEs. However, this review is almost a decade old and more novel meth-
odologies were proposed in the recent years. Tuncuk, Stazi, Akcil, Yazici,
and Deveci (2012) reviewed the possible applications of hydrometallurgy to
recover metals from of PCBs but had the drawback of focusing in only one
type of WEEE i.e. PCB. Zeng, Li, and Singh (2014) overviewed the recy-
cling of lithium ion batteries, but the emphasis was only on Li and Co.
Recycling and recovery of yttrium (Y), europium (Eu), cerium (Ce), lan-
thanum (La), and terbium (Tb) in phosphors from waste fluorescent lamps
were reported (Tan, Li, & Zeng, 2015). In this review, emphasis was given
only to REE from fluorescent lamps. Extensive review on hydrometallurgi-
cal extraction of gold from different WEEEs were reported by Akcil, Erust,
Gahan, Ozgun, Sahin, and Tuncuk (2015) and G€
okelma, Birich, Stopic, and
Friedrich (2016). In these reviews, gold was given sole attention and did
not target many other metals present in WEEE. Zhang, Wu, Wang, Li,
Zhang, and Zuo (2015) overviewed different processes reported for the
leaching and recovery of In from particular WEEE i.e. LCDs. Similarly,
Chagnes and Pospiech (2013) reviewed various hydrometallurgical proc-
esses for the recycling of spent lithium-ion batteries. There are also a lot of
studies on the biohydrometallurgical approaches for the solubilization of
metals from different WEEEs (Er€
ust, Akcil, Gahan, Tuncuk, & Deveci,
2013; Ilyas & Lee, 2014;Is¸ıldar, van de Vossenberg, Rene, van Hullebusch,
& Lens, 2017). In the present review, up-to date reported strategies for the
leaching and recovery of critical and precious elements from different end
of life electronic wastes are reviewed. Recovery of REE, critical and pre-
cious elements bearing WEEEs such as waste printed circuit boards, waste
liquid crystal displays, spent cathode ray tubes, spent fluorescent lamps,
waste hard disk drives, spent light emitting diodes and spent batteries are
given a special attention. Their elemental composition and various hydro-
metallurgical (leaching and recovery phases) operations proposed for the
recovery of critical elements are reviewed.
2. WEEE as a secondary source for critical raw materials
WEEE such as spent PCBs, spent LCDs, spent fluorescent lamps, spent
LEDs and spent batteries contain significant concentration of different crit-
ical and/or precious metals (Figure 3). Various characteristics (such as the
physico-chemical properties and elemental composition) of each of the dif-
ferent WEEE were discussed below in detail.
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 5
2.1. Spent LCDs
Indium is a scarcely available metal, which finds application in electronics
industry especially in the production of LCD panels. In the recent past dec-
ades, LCDs have replaced conventional CRTs because of their lower power
consumption capacity (Dodbiba, Nagai, Wang, Okaya, & Fujita, 2012).
Indium is used for the fabrication of Indium-Tin-Oxide (ITO) in LCD pan-
els. ITO has some unique characteristics, such as electricity conduction, can
bind strongly to glass and is also transparent, which attracts maximum In
usage in LCD design (Kri
stofov
a, Rudnik, & Mi
skufov
a, 2017). About 70% of
total In consumption is accounted to ITO production (Tolcin, 2012). On the
other hand, natural In resources in Earth crust is as low as 0.25ppm
(Schwarz-Schampera & Herzig, 2002). Dzhalindite, indium hydroxide mineral
is the most common and predominant primary resource of In. In supply will
remain for fewer than 14 years based on the current rate of extraction, which
urges the necessity of recycling of In bearing wastes (Kri
stofov
a, Rudnik, &
Mi
skufov
a, 2017). Spent LCDs are one such wastes which contain significant
concentration of In. A cross section of a LCD panel is shown in Figure 4.As
mentioned earlier, ITO is one of the major parts of spent LCDs (Figure 4). In
is present as indium (III) oxide (In
2
O
3
), which contributes to 90% (by weight)
of in ITO (remaining 10% is made of tin (IV) oxide). Typical spent LCD
panels on an average contain 530 mgkg
1
of In, 346 mgkg
1
of As and
24 mgkg
1
of Sb (Savvilotidou, Hahladakis, & Gidarakos, 2015).
Figure 3. Pictorial representation of different types of WEEE and their major critical
raw materials.
6 M. SETHURAJAN ET AL.
2.2. Spent batteries
2.2.1. Li-ion batteries
Electrical batteries can be classified into two types such as (1) primary bat-
teries (non-rechargeable) and (2) secondary batteries (rechargeable). The
primary batteries are mainly made of Zn-C-MnO
2
. These primary batteries
can again be divided into two types (1) acidic batteries which uses ammo-
nium chloride and (2) alkaline batteries that use potassium hydroxide.
Rechargeable batteries are made of Co and Li and are called as Lithium-ion
batteries (LiBs). LiBs are the most used type batteries and are widely used
in portable electronic devices such as mobile phones, laptops, recorders,
cameras and MP3 players (Shuva & Kurny, 2013). Consumption of Li in
the electronics industry continues to grow in the recent past years
(Sakultung, Pruksathorn, & Hunsom, 2007). For instance in 2007, 2.04 bil-
lion LiB units were produced and in 2010 it went up to about 4.6 billion
units (Scrosati, Krebs, Beck, & Bartels, 2007; Zeng, Deng, Luo, Luo, & Zou,
2012). Due to huge increase in production, excessive spent LiB wastes have
been generated in the recent decades. Spent LiBs can be considered as haz-
ardous waste, because it can cause adverse effects to the environment, ani-
mals and human health (Shin, Kim, Sohn, Yang, & Kim, 2005). On the
other hand, these spent LiBs contain significant concentration of valuable
Figure 4. Cross section of a thin-film-transistor-LCD panel.
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 7
metals like Co and Li. X-ray diffraction studies shows that LiCoO
2
and
Co
3
O
4
are the major mineral phases present in the spent LiBs. For
example, Lee and Rhee (2003) found out that 27.5% (wt%) of LiCoO
2
is
present in spent LiBs (Zheng et al., 2016). There are many different LiB
units produced and the concentration of Co and Li present in the spent
LiB can vary accordingly. As Co is one of the critical elements, spent LiB
wastes can be considered as a potential secondary resource of Co.
However, precautions and safety measurements should be taken prior dis-
mantling the spent Li-batteries. Even the spent LiBs contain residual volt-
age and can produce strong heat and flames due to self-ignition (because
of residual charge) and internal short circuit (Nan, Han, & Zuo, 2005;
Vieceli et al., 2018). Therefore, precautionary steps like (i) refrigeration
using NaCl or water, (ii) cryogenic activities like immersion in liquid nitro-
gen for 4–6 min and (iii) promoting short circuit and discharging the bat-
teries using electric iron powder were proposed in the literature (Li, Wang,
& Xu, 2016; Vieceli et al., 2018).
2.2.2. Ni-MH batteries
Spent NiMH (nickel-hydride) battery scrap is a valuable material for recov-
ery of cobalt and REEs. It is worth to mention that the battery used in
hybrid cars contains 3 kg of REEs and 1.5 kg of Co for 11 kg nickel content.
Mechanical or thermo-mechanical processing (grinding, separation, roast-
ing) of this waste leads usually to separation of the most valuable fraction
often called “black mass.”The black mass is susceptible for hydrometal-
lurgy. Typical elemental composition of the black mass contain from 5 to
15% of REEs, 2–6% of Co, and also other metals like Ni (29–51%), Zn
(1–8%) and Mn (2–8%) (Innocenzi & Vegli
o, 2012; Becker et al., 2016;
Petranikova, Herdzik-Koniecko, Steenari, & Ekberg, 2017).
2.3. Spent LEDs
Light emitting diodes (LEDs) are widely used in electronics industry, espe-
cially in television displays. LEDs consume less energy than the traditional
illuminants. Zhan, Xia, Ye, Xiang, and Xie (2015) state that the waste LEDs
are poly-metallic and contain Ga (2.1 mgkg
1
), In (1.1 mgkg
1
) and Au
(16.7 mgkg
1
). Apart from the spent LEDs wastes, there is another Ga
containing WEEE gallium arsenide scraps. Gallium arsenide (GaAs) sludges
are co-products generated during the production of LEDs and they contain
significant concentrations of Ga and As. XRD analysis of these scraps
reveal that they contain gallium arsenide (GaAs) and gallium phosphide
(GaP) compounds (Hu, Xie, Hsieh, Liou, & Chen, 2015). Approximately
42–50% of Gallium and 25–50% of As were found in the waste GaAs
8 M. SETHURAJAN ET AL.
scraps (Lee & Nam, 1998; Chen, Tsai, Tsai, & Shu, 2012; Hu, Xie, Hsieh,
Liou, & Chen, 2015).
2.4. Spent fluorescent lamps (SFLs)
The lamp tube contains low-pressure mercury vapor, an inert gas such as
argon or helium and tri-chromatic phosphor coating the inner lamp tube
surface (Wu, Yin, Zhang, Wang, & Mu, 2014). An electronic discharge by
the cathode inside the glass tube stimulates the mercury vapor causing it to
emit radiation in the UV range (k
Hg
¼253.7 nm) (Wu, Yin, Zhang, Wang,
&Mu,2014). The inner tube wall is coated with powder containing yttrium
and europium doped oxide lattices to absorb the invisible UV radiation
emitted from the interaction of mercury (Hg) and electrons for visible
wavelength (The Hong Kong Observatory, 2012; Wu, Yin, Zhang, Wang, &
Mu, 2014; NEC Lighting, Ltd, 2015)(Figure 5).
Wu, Yin, Zhang, Wang, and Mu (2014) reported that the share of red,
green and blue phosphor in standard chemical composition is 55, 30, and
15%, respectively. The elemental composition of standard tri-chromatic
fluorescent lamp is reported in Table 1. Expectation of the total global
lighting market and lamp type shares are given in Figure 6.
Figure 5. Structure of double-capped linear fluorescent lamp (CREE Inc., 2015).
Table 1. The chemical composition of pure tri-chromatic phosphate (adapted from Wu
et al., 2014).
Elements Rare earth oxide Contents (%) Elements Rare earth oxide Contents (%)
YY
2
O
3
46.9–51.2 Ce CeO
3
4.1–5.3
Eu Eu
2
O
3
3.9–4.4 Tb Tb
4
O
7
2.2–2.6
Ce CeO
3
4.1–5.3 Mg MgO 2.7–4.0
Ba BaO 2.1–3.2 –– –
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 9
2.5. Spent PCBs
Waste printed circuit boards (PCBs) account for about 3% of nearly 50 Mt/
year global WEEE generation (Kaya, 2016;Is¸ıldar, van de Vossenberg,
Rene, van Hullebusch, & Lens, 2017). They are heterogeneous in nature,
which constituted of metals (40%), ceramic (30%) and plastics (30%).
Among these fractions, the driving force for recycling waste PCBs is the
recovery of precious metals (in particular Au) (Tuncuk, Stazi, Akcil, Yazici,
& Deveci, 2012). Because PCBs contain many base (such as Cu, Ni and Fe)
and precious metals (PMs), mainly Ag, Au and Pd (Priya & Hait, 2018).
PMs concentrations in PCBs are much higher than those found in the nat-
ural deposits (Ebin & Isik, 2017). PCBs could be seen as a respectable poly-
metallic secondary source for urban mining.
3. Pre-treatment of WEEE
Pre-treatment is the first and obligatory step to recover critical and pre-
cious elements from WEEE. WEEE is inherently complex and heteroge-
neous in type and composition with considerable variations in metal and
materials content, which makes it extremely difficult to develop and imple-
ment recycling processes for selective recovery of the contained elements.
In addition to the heterogeneity of the different WEEE, there is also a sig-
nificant change in the composition of the WEEE, because of the technology
developed over the years (Cui & Forssberg, 2003; Stevens & Goosey, 2008;
Yazıcı, Deveci, Yazici, & Akcil, 2015; Yamane, de Moraes, Espinosa, &
Ten
orio, 2011; Yazıcı& Deveci, 2009; Kucuker, 2018). Pre-treatment of
WEEE is often required prior to metallurgical extraction processes for
Figure 6. Expectation of the total lighting market in terms of lamp type shares (Adopted from
McKinsey & Company, 2012). LED: Light emitting diodes; CFL: Compact fluorescent lamps; LFL:
Linear fluorescent lamps.
10 M. SETHURAJAN ET AL.
selective recovery/separation of the desired components with economic or
pollution potential, increasing the technical effectiveness and reducing the
cost of these processes (Yazıcı& Deveci, 2009). Pre-treatment is the first
stage of a recycling operation for WEEE feedstock (Figure 7), which
involves mainly dismantling/disassembly and physical processes such as
size reduction and physical separation. Sufficient liberation and efficient
separation of metals/materials are essential in WEEE pre-treatment opera-
tions. These processes have been proven as indispensable for the conven-
tional WEEE recycling, i.e., for the recovery of “mass relevant”fractions
presented in WEEE, such as ferrous and non-ferrous metals, plastic, glass
and other. However, part of precious and critical elements are lost during
the pre-treatment processes (Figures 8 and 9). Loss of precious and critical
elements in overall recycling chain is mainly caused by the fact that these
substances in pre-treatment phase end up in output streams (e.g., shred-
ding dusts (Marra, Cesaro, & Belgiorno, 2018)), which implies that further
optimization of WEEE pre-treatment stage is needed (Chancerel, Meskers,
Hagel€
uken, & Rotter, 2009). Chemical pre-treatment for beneficiation of
Figure 7. Unit operations/processes in the treatment of WEEE for recovery of the con-
tained values.
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 11
the contained values, removal of hazardous components, energy recovery
etc. can be also exploited before the extraction of metals. In general, pre-
treatment phase consists of:
Manually disassembly/dismantling
Mechanical treatment processes
Combination of manual and mechanical pre-processing
3.1. Manually disassembly/dismantling
Dismantling/disassembly of WEEE is essentially the first step of recycling
for selective recovery of components and devices (capacitors, batteries,
screens, PCB etc.) for re-use and/or further treatment. The main focus of
Figure 8. Mass flow diagram showing the distribution of (a) PMs and (b) REEs in different frac-
tions (redrawn from Marra et al., 2018).
12 M. SETHURAJAN ET AL.
manually dismantling phase is to ensure removal of hazardous or otherwise
environmentally relevant components defined in Annex VII of the WEEE
Directive 2012/19 (EC Directive, 2012). In addition, more specific disman-
tling targeting the most valuable parts/units such as central processing unit
(CPU), random access memory (RAM) etc. and/or hazardous components
can be performed to obtain high value products (Lee, Chang, Fan, &
Chang, 2004). From processing point of view, dismantling allows pre-separ-
ation and enrichment of valuable metal-bearing parts from non-metallic
parts such as plastics and ceramics, increasing their potential for recycling
and improving economics of the overall recycling process (Cui &
Forssberg, 2003). It is always difficult and costly to process, separate and
recover values downstream from such heterogeneous and complex mix-
tures. Dismantling/disassembly is often carried out manually, making this
step labour intensive, while automated systems are also developed for spe-
cific applications (Elo & Sundin, 2014; Park, Kim, Han, & Park, 2015;
Kopacek, 2016).
Figure 9. Mass balance of the pre-processing of 1,000kg of input WEEE (redrawn from
Chancerel et al., 2009).
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 13
3.2. Mechanical treatment processes
Manual sorting and dismantling is usually followed by a traditional recy-
cling processes, where metals and materials contained in WEEE are
liberated and separated based on their specific physical characteristics
such as weight, size, shape, density, and electrical and magnetic charac-
teristics (de Oliveira, Bernardes, & Gerbase, 2012). After selective sort-
ing/recovery through dismantling, metal bearing components such as
printed circuit boards (PCBs) are often subjected to physical pre-treat-
ment essentially for improving the technical and economic aspects of
the following treatment processes. The first stage of physical pre-treat-
ment is the size reduction of WEEE down to a suitable size for the
selected treatment process. Shredders and hammer mills are extensively
used for size reduction of WEEE (Dalrymple et al., 2007; Kaya, 2016).
In a recent study, Electro Dynamic Fragmentation of PCB was demon-
strated as an unconventional method for size reduction and liberation of
components (Martino, Iseli, Gaydardzhiev, Streicher-Porte, &
Weh, 2017).
The degree of size reduction required is determined by the following
process/operation. In this regard, fine grinding (typically <200 mm) is pre-
requisite for stirred tank leaching as the main stage of a hydrometallurgical
process while relatively coarse material can be fed to pyrometallurgical
processes such as smelting. In a similar way, physical separation processes
as pre-treatment is more effective at relatively coarse sizes; albeit, their sep-
aration efficiency relies essentially on the degree of liberation of target
phases (Yazıcı& Deveci, 2009). Degree of liberation is often a function of
particle size also depending on the type of WEEE. In this regard, reducing
the size of waste computer PCB down to 5 mm was reported to be
required for a high degree of liberation (97%) of metals e.g., copper, alu-
minium and ferromagnetics (Zhang & Forssberg, 1997;1998).
Notwithstanding this, even very fine grinding (e.g., <75 mm) could not be
sufficient for complete liberation of metals from PCB (Ogunniyi, Vermaak,
& Groot, 2009; Yazıcı, Deveci, Alp, Akcil, & Yazıcı,2010).
After size reduction for achieving the required degree of liberation,
physical separation methods can be readily exploited for the separation
of metals from WEEE prior to the pyrometallurgical and hydrometallur-
gical extraction processes. The main advantages of physical separation
include their simplicity and low-costs. A variety of separation methods
are available based on the differences in physical properties including
specific gravity, conductivity, magnetic susceptibility, brittleness and
hydrophobicity of the phases (Wills & Finch, 2015). Table 2 summarises
physical separation methods available/studied for beneficiation of WEEE.
Magnetic separation, which is usually performed with low intensity
14 M. SETHURAJAN ET AL.
magnetic drum separators, is often used as the first stage of physical
separation for removal of ferrous metals as magnetic fraction (Yazıcı&
Deveci, 2009; Tuncuk, Stazi, Akcil, Yazici, & Deveci, 2012). Air classifi-
cation can be used to separate essentially fluffy material or fine light
plastics (Lee, Chang, Fan, & Chang, 2004; Zhao, Wen, Li, & Tao, 2004;
Eswaraiah, Kavitha, Vidyasagar, & Narayanan, 2008). Light metals such
as Al with a high ratio of conductivity/density (>10) are separated by
eddy-current separation from non-conductive (non-metallic) fraction and
heavy non-ferrous metals i.e. base and precious metals, are recovered
through electrostatic separation based on their conductivity (Table 2),
and using permanent magnets, such as iron-boron-neodymium magnets
(Zhang & Forssberg, 1998). Finally, due to differences in specific density
of different materials contained in WEEE, gravity separation techniques
such as shaking tables, heavy media separation, jigging, etc., can be used
to separate materials of different specific gravity by their relative move-
ment in response to gravity (de Oliveira, Bernardes, & Gerbase, 2012).
Despite the improved liberation of metals, the effectiveness of physical
separation methods tends to deteriorate with decreasing particle size, this
also depends on the difference in magnitude of the physical property of
Table 2. Physical separation methods available for recovery of metals from WEEE (adapted
from Cui & Forssberg, 2003; Yazıcı& Deveci, 2009).
Method Exploited property Separation of materials Particle size References
Magnetic
Separation
Magnetic
susceptibility
Ferrous (ferromag-
netics) metals from
non-ferrous metals
and non-metals
(para-/
dia-magnetics)
<5 mm Zhang and Forssberg (1997);
Zhang and Forssberg (1998);
Veit et al. (2005); Yazici and
Deveci (2015); Zhang
et al. (2017)
Electrostatic
Separation
Electrical
conductivity
Metals (conductive)
from non-metals
0.1–5 mm Zhao, Wen, Li, and Tao (2004);
Li, Lu, Guo, Xu, and Zhou
(2007); Wen et al. (2005);
Zhang et al. (2017)
Eddy-
current
Separation
Electrical
conductivity/
specific gravity
Light metals i.e. Al
from conductive but
heavy (base and
precious) metals
and non-conductive
materials (plastics
and ceramics)
>5 mm Zhang and Saito (1998); Yazıcı,
Deveci, Alp, Akcil, and
Yazıcı(2010)
Gravity Separation Specific gravity Metals from
non-metals
0.05–10 mm Galbraith and Devereux (2002);
Zhao, Wen, Li, and Tao
(2004); Eswaraiah, Kavitha,
Vidyasagar, and Narayanan
(2008); Duan et al. (2009);
Veit, Juchneski, and
Scherer (2014)
Flotation Surface properties Non-metals (hydropho-
bic) from metals
0.075–1 mm Ogunniyi and Vermaak (2009);
Vidyadhar and Das (2013);
Gallegos-Acevedo, Espinoza-
Cuadra, and Olivera-
Ponce (2014)
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 15
fractions (Veit et al., 2005; Yazici & Deveci, 2015; Zhang et al., 2017). In
effect, each separation method can be most effective in a certain size range
(Table 2) (Wills & Finch, 2015). Size reduction operations inevitably gener-
ate fine/dust fraction from which recovery of metals by most conventional
separation methods such as magnetic, eddy-current or electrostatic separ-
ation is difficult and inefficient (Zhao, Wen, Li, & Tao, 2004; Li, Lu, Guo,
Xu, & Zhou, 2007; Yazıcı, Deveci, Alp, Akcil, & Yazıcı,2010; Yazıcı&
Deveci, 2015). This inefficiency of physical separation processes can lead to
prohibitively high metal losses (10–35%) (Goosey & Kellner, 2002;
Hageluken, 2006; Marra, Cesaro, & Belgiorno, 2018). However, there are
potential beneficiation methods such as flotation and centrifugal gravity
separation for recovery of metals from fine size fractions (Galbraith &
Devereux, 2002; Wen et al., 2005; Duan et al., 2009; Ogunniyi & Vermaak,
2009; Veit, Juchneski, & Scherer, 2014).
3.3. Fate of critical and precious elements in pre-treatment processes
The recovery of a specific material from end-of-life WEEE input stream
increases with decreasing of impurities in the final material after the pre-
treatment stage (Chancerel, Meskers, Hagel€
uken, & Rotter, 2009; Meskers &
Hagel€
uken, 2009). Loss of precious and critical elements through the manual
dismantling, and/or shredding processes result in the overall reduction of the
recycling efficiency (Bigum, Brogaard, & Christensen, 2012). Fully automated
disassembly is not currently technically feasible and not expected to become
economically viable in near future (Duflou et al., 2008). Technical optimiza-
tion with focus to ameliorate manual disassembly during the pre-treatment
stage is crucial. Also, generally standard practice to shredding whole WEEE
devices leads to significant losses of precious metals - which cannot be com-
pensated by the downstream sorting and refining processes (Buchert,
Manhart, Bleher, & Pingel, 2012).
Losses of precious metals in PCBs caused by shredding was examined
through an industrial test (Chancerel, Meskers, Hagel€
uken, & Rotter, 2009),
where the difference in concentration between unshredded and pre-
shredded PCBs was determined. The results showed 7% less precious met-
als after the pre-shredding phase, while the difference between pre-
shredded PCBs and shredded PCBs indicated additional loss of 62% pre-
cious metals in the PCBs. Ueberschaar, Otto, and Rotter (2017) showed
that relatively small shares of gallium bearing components on PCBs or in
LEDs lead to a dilution with other materials in conventionally applied recy-
cling processes. Ending in the pyrometallurgical process for copper and
precious metals refining, gallium is transferred as oxidized form to the slag.
Thus, gallium rich components must be separated prior to any mechanical
16 M. SETHURAJAN ET AL.
processing with other material. Mechanochemical technology can be used
as a means of pre-treatment, and then hydrometallurgical technology to
recycle metals from some specific wastes, including WEEE. Through this
way, the recovery rate of metal was significantly higher than ordinary
hydrometallurgy (Zhang & Xu, 2016). This was demonstrated by Lee et al.
(2013) in their study on indium recycling from waste LCD panels, Zhang
and Saito (1998) for recovering yttrium (Y) and europium (Eu) in waste
phosphor, Lee, Zhang, and Saito (2000) on recovering Co and Li from
spent lithium ion batteries. Marra, Cesaro, and Belgiorno (2018), demon-
strated that about 80% of REE will be trapped up in dusts (because of these
conventional pre-treatment techniques) and then the dusts have to be
treated by other specific processes (Figure 8).
Although major losses of precious and critical elements are occurring dur-
ing the pre-treatment phase, in order to enhance their recovery, some of the
crucial improvements should be implemented even in steps that precede pre-
treatment phase (Figure 9). Besides increasing the collection rates for all
product groups that contain precious and critical elements, more reliable and
transparent information about the content of these metals in different equip-
ment groups and their components, should be available. Also, structure and
design of electrical and electronic products in order to facilitate manual disas-
sembly and recycling processes need to be additionally optimized.
Within pre-treatment phase, it is necessary to optimize processes by
improvement of manual disassembly and separation of target components in
WEEEs (which are rich in precious and critical elements). Besides removal of
the WEEE components that are legally stipulated (Annex VII of the WEEE
Directive 2012/19, EC Directive, 2012), parts of equipment such as batteries
containing cobalt, neodymium hard disk magnets, small PCBs, etc., should
also be removed and fed into a suitable recycling process (Buchert, Manhart,
Bleher, & Pingel, 2012). Outputs from pre-treatment phase must be fractions
with characteristics appropriate for end-processing facilities. More invest-
ments and further research should be focused on promising technologies for
automatic recognition, sorting, and dismantling of WEEE, in order to recover
precious and critical elements from heterogeneous WEEE flows, more effi-
ciently (Chancerel, Meskers, Hagel€
uken, & Rotter, 2009; Buchert, Manhart,
Bleher, & Pingel, 2012). Finally, quantitative targets for the recycling of
WEEE (Annex V of the WEEE Directive 2012/19, EC Directive, 2012), are
not formulated specifically in terms of material or components, but relate to
the weight percent of the complete devices, which leads to negative incentives
for the recovery of precious and critical elements. Therefore, revision of the
WEEE Directive in terms of setting targets for the recycling rates for specific
critical metals and/or product groups, is recommended (Buchert, Manhart,
Bleher, & Pingel, 2012; Chancerel, Meskers, Hagel€
uken, & Rotter, 2009). In
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 17
general, a compromise between the quality and quantity of grade-recovery
fraction has to be carried out to minimize the losses of valuable metals and
the distribution of precious metals over the outputs of the operating condi-
tions for WEEE pre-processing.
4. Hydrometallurgical treatment of WEEE for recovery of
critical elements
Hydrometallurgy of WEEE consists of at least two main unit operations,
namely (1) leaching (solubilization of metals from WEEE into leachates
using aqueous chemicals) and (2) recovery (selectively recovering the dis-
solved metals from the leachates) (Figure 10). Various leaching and recov-
ery processes for the extraction of critical raw materials from WEEE is
described below in detail.
4.1. Leaching of critical elements from WEEE
Various hydrometallurgical processes have been described in the literature
for the leaching of REE, precious and critical metals from WEEE. There
Figure 10. General flowsheet of hydrometallurgy of WEEE.
18 M. SETHURAJAN ET AL.
are various process parameters such as (1) particle size, (2) lixiviant type,
(3) concentration of the lixiviant, (4) temperature, (5) pH, (6) solid to
liquid ratio, (7) agitation and (8) redox potential that control the leaching
kinetics and extent. In some cases, addition of oxidants and reductants will
also be provided to improve the leaching efficiency. In the below sections,
leaching of REE, critical and precious metals from different WEEE will be
discussed in detail.
4.1.1. Leaching of critical elements from WEEE
4.1.1.1. Indium leaching from LCD. Several studies on the effect of mineral
acids on the leaching of Indium from LCDs have been published (Li, Liu, Li,
Liu, and Zeng, 2011; Dodbiba, Nagai, Wang, Okaya, & Fujita, 2012; Silveira,
Fuchs, Pinheiro, Tanabe, & Bertuol, 2015; Savvilotidou, Hahladakis, &
Gidarakos, 2015)(Table 3). Yang, Kubota, Baba, Kamiya, and Goto (2013)
studied the effect of individual mineral acids (such as HCl, HNO
3
and
H
2
SO
4
) on the leaching of Indium from LCD panel glass. The results showed
that the hydrochloric acid leached more In than sulfuric and nitric acids at
lower solid to liquid ratio (Yang, Retegan, & Ekberg, 2013). On the other
hand, Li, Liu, Li, Liu, and Zeng (2011) reported that both sulfuric and hydro-
chloric acid leach more than 99% of In from ITO targets (at a given time).
However, information regarding the pre-treatment (i.e., removal of the plastic
films) was not clearly mentioned. Mixture of different acids (like
HCl þHNO
3
and HCl þH
2
SO
4
) was also investigated to solubilize In from
LCDs. Hydrochloric acid and nitric acid mixture found to have higher leach-
ing capacity than the sulfuric acid mixture (Savvilotidou, Hahladakis, &
Gidarakos, 2015). The main reactions of acidolysis of ITO are illustrated as
follows (Li, Liu, Li, Liu, and Zeng, 2011):
In2O3þ6Hþ!2In
3þþ3H2O (1)
Leaching efficiency of In from LCDs can be influenced by particle size,
temperature, solid to liquid phase ratio and lixiviant (acid) concentration.
Increase in acid concentration leads to increase in the leaching efficiency of
In from LCDs (Silveira, Fuchs, Pinheiro, Tanabe, & Bertuol, 2015). Indium
leaching from LCDs is temperature and pulp density dependent
(Savvilotidou, Hahladakis, & Gidarakos, 2015). Lower the pulp density,
higher is the leaching efficiency from LCDs. Usually, maximum leaching of
In was achieved in higher temperature range (80 C–90 C) (Kri
stofov
a,
Rudnik, & Mi
skufov
a, 2017).
4.1.1.2. Cobalt leaching from spent Batteries. Various chemical leaching proc-
esses were reported to leach out Co from spent batteries (Table 3). Since
Co is present as Co(III) in LiCoO
2
, reductive leaching by reductive
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 19
Table 3. Different hydrometallurgical approaches proposed for the leaching of critical metals from WEEE.
WEEE type
Metal content
(w/w %) Leachant Optimum conditions Leaching yield Reference
ITO powders In - 71.21% H
2
SO
4
0.75 M H
2
SO
4
, 1:100 solid/
liquid ratio, 90 C,
150 min, and stirring
at 500 rpm.
Maximum 99 %
of In
Li, Liu, Li, Liu, and Zeng (2011)
HCl 1.5 M HCl, 1:100 solid/liquid
ratio, 90 C, 150 min, and
stirring at 500 rpm.
Maximum 98 %
of In
LCD In - 0.02% H
2
SO
4
1.0 M H
2
SO
4
, 1:2 solid/liquid
ratio, 20 C, 24 h, and stir-
ring at 350 rpm.
More than 85% of
In was leached
Yang et al. (2013)
HCl 1.0 M HCl, 1:2 solid/liquid
ratio, 20 C, 24 h, and stir-
ring at 350 rpm.
Maximum 80% of
In was leached
LCD from
mobile phones
In - 0.61% H
2
SO
4
1.0 M H
2
SO
4
, 1:50 solid/
liquid ratio, 90 C, 1 h,
and stirring at 500 rpm.
A maximum of
96.4 wt.% In
Silveira, Fuchs, Pinheiro, Tanabe,
and Bertuol (2015)
LCD
from computers
In - 0.05% HCl HCl:H
2
O (3:2), 1:5 solid/liquid
ratio, 80 C, 1 h, and
under a mild agitation.
Approximately of
60% of
In leached
Savvilotidou, Hahladakis, and
Gidarakos (2015)
Mobile
phone batteries
Co - 30.5% HCl 5 M HCl, 80 C, 1.5:100 solid-
liquid ratio and 60 min
Maximum 84% of
Co was leached
Sakultung, Pruksathorn, and
Hunsom (2007)
Lithium
ion batteries
–HCl þH
2
O
2
3 M HCl þ3.5% H
2
O
2
,80
C,
1:20 solid-liquid ratio,
400 rpm and 40 min.
Maximum 79% of
Co was leached
Shuva and Kurny (2013)
Lithium
ion batteries
Co 33.20% H
2
SO
4
þH
2
O
2
1
st
step - Decomposition
with NH
4
OH to leach Al
and Cu 2
nd
step - 2 M
H
2
SO
4
þ4% H
2
O
2
,70
C,
1:10 solid-liquid ratio,
400 rpm and 60 min.
Maximum 99% of
Co was leached
Nayl, Elkhashab, Badawy, and El-
Khateeb (2014)
Lithium
ion batteries
LiCoO
2
- 27.5% HNO
3
þH
2
O
2
1 M HNO
3
þ1.7 % H
2
O
2
,
1:50 solid to liquid ratio,
400 rpm, 30 min
and 75 C.
More than 95% of
Co was leached
Lee and Rhee (2003)
Lithium
ion batteries
–Citric acid þH
2
O
2
4MC
6
H
8
O
7
þ1% H
2
O
2
,
1.5:100 solid to liquid
ratio, 5 h and 90 C.
More than 99% of
Co was leached
Zheng et al. (2016)
(continued)
20 M. SETHURAJAN ET AL.
Table 3. Continued.
WEEE type
Metal content
(w/w %) Leachant Optimum conditions Leaching yield Reference
Lithium
ion batteries
Co - 53.8% Malic acid þH
2
O
2
1.5 M C
4
H
5
O
6
þ2% H
2
O
2
,
1:50 solid to liquid ratio,
40 min and 90 C.
More than 90% of
Co was leached
Li et al. (2010)
Lithium
ion batteries
Co - 35.52% Tartaric
acid þH
2
O
2
2MC
4
H
6
O
6
þ4% H
2
O
2
,
1.7:100 solid to liquid
ratio, 30 min and 70 C.
More than 98% of
Co was leached
He, Sun, Mu, Song, and Yu (2016)
Lithium
ion batteries
Co - 57.94% Succinic
acid þH
2
O
2
1.5 M C
4
H
6
O
4
þ4% H
2
O
2
,
1.5:100 solid to liquid
ratio, 40 min and 90 C.
More than 99% of
Co was leached
Li et al. (2015)
Lithium
ion batteries
–Glycine þascorbic
acid
0.5 M C
2
H
5
NO
2
þ0.02
C
6
H
8
O
6
, 1:500 solid to
liquid ratio, 6 h and 80 C.
More than 95% of
Co was leached
Nayaka, Pai, Santhosh, and
Manjanna (2016)
Ga-As
waste scraps
Ga - 48.6% HNO
3
2 M HNO
3
, 500 rpm, 2 h
and 60 C.
Maximum 99% of
Ga was leached
Lee and Nam (1998)
Ga-As
waste scraps
–HNO
3
4 M HNO
3
, 0.3:100 solid to
liquid ratio, 500 rpm, 1 h
and room temperature.
Maximum 98% of
Ga was leached
Chen, Tsai, Tsai, and Shu (2012)
Ga-As
waste scraps
–H
2
SO
4
5MH
2
SO
4
, 0.3:100 solid to
liquid ratio, 500 rpm, 1 h
and room temperature.
Maximum 30% of
Ga was leached
Chen, Tsai, Tsai, and Shu (2012)
Ga-As
waste scraps
Ga - 50.8% HNO
3
1.5 M HNO
3
, 2.5:100 solid to
liquid ratio, 200 rpm, 1.5 h
and 40 C.
Maximum 99% of
Ga was leached
Hu, Xie, Hsieh, Liou, and
Chen (2015)
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 21
lixiviants were proposed. Hydrogen peroxide (H
2
O
2
) is the commonly used
reducing agent, that can reduce Co(III) to Co(II) which is more susceptible
to leaching than Co(III). Inorganic acids (such as sulfuric (H
2
SO
4
), nitric
(HNO
3
) and hydrochloric (HCl) acids) along with H
2
O
2
were commonly
used lixiviants to solubilize Co from spent batteries. HCl was found to be a
better leachant than sulfuric and nitric acid (Sakultung, Pruksathorn, &
Hunsom, 2007). The higher Co leaching efficiency by HCl could be attrib-
uted by the dissociation constants (K
a
) of the acids. For instance, the dis-
sociation constant of HCl is 10
6
, which is higher than that of H
2
SO
4
(10
3
)
and HNO
3
(28) (Sakultung, Pruksathorn, & Hunsom, 2007). The leaching
of Co from lithium-cobalt oxide (present in the spent batteries) by HCl
and H
2
SO
4
is illustrated by equations 2–5:
2 LiCoO2s
ðÞþ8HCl
aq
ðÞ
!2 CoCl2þCl2g
ðÞþ2 LiCl aq
ðÞ
þ4H2O (2)
LiCoO2s
ðÞþ6HCl
aq
ðÞ
þH2O2aq
ðÞ
!CoCl2aq
ðÞ
þCl2þLiCl aq
ðÞ
þ4H2O
(3)
4 LiCoO2s
ðÞþ6H2SO4aq
ðÞ
!4 CoSO4aq
ðÞ
þ2Li
2SO4aq
ðÞ
þ6H2Og
ðÞþO2g
ðÞ
(4)
2 LiCoO2s
ðÞþ3H2SO4aq
ðÞ
þH2O2aq
ðÞ
!2 CoSO4aq
ðÞ
þLi2SO4aq
ðÞ
þ4H2Og
ðÞþO2g
ðÞ
(5)
Apart from the type of acid, other factors such as acid concentration,
temperature and solid to liquid phase ratio could also affect the leaching of
Co from LiBs. Leaching kinetics studies reveal that increase in acid concen-
tration and temperature increase the leaching of Co, while the increase in
solid to liquid ratio decrease the leaching efficiency. An increase in acid
concentration leads to an increase of protons in the system which in turn
leach more Co. However, if the acid concentration exceeds 6 M, then the
increase in leaching efficiency is negligible (Sakultung, Pruksathorn, &
Hunsom, 2007). The cobalt leaching by HCl (þH
2
O
2
), H
2
SO
4
(þH
2
O
2
),
and HNO
3
(þH
2
O
2
) follows shrinking core kinetics model while the reac-
tion rate was controlled by surface chemical reaction (Lee & Rhee, 2003;
Shuva & Kurny, 2013; Nayl, Elkhashab, Badawy, & El-Khateeb, 2014). The
activation energy required to leach Co from spent batteries were estimated
to be 28.33 kJmol
1
(by HCl), 30.1–41.4 kJmol
1
(by H
2
SO
4
) and
52.3 kJmol
1
(by HNO
3
) (Lee & Rhee, 2003; Shuva & Kurny, 2013; Nayl,
Elkhashab, Badawy, & El-Khateeb, 2014). The high activation energy
required confirms that the Co leaching (from spent batteries) by inorganic
acids is temperature dependent.
Even though the Co leaching by inorganic acids proved efficient, toxic
and hazardous Cl
2
, Sox, and NOx will also be co-generated as by-products.
In order to overcome this problem, several other organic lixiviants such as
22 M. SETHURAJAN ET AL.
glycine (Nayaka, Pai, Santhosh, & Manjanna, 2016), malic acid (Li et al.,
2010), oxalic acid (Sun & Qiu, 2012), citric acid (Zheng et al., 2016), acetic
acid (Golmohammadzadeh, Rashchi, & Vahidi, 2017), succinic acid (Li
et al., 2015) and tartaric acid (He, Sun, Mu, Song, & Yu, 2016) were also
proposed to leach out Co from spent LiBs. Similar to inorganic acids, the
leaching efficiency of Co from LiBs is increasing with increase in organic
acids concentration and temperature. The apparent activation energy
required to leach cobalt by citric acid was found to be 45.72 kJmol
1
. The
leaching efficiency decreases with increase in solid to liquid ratio as like the
case of inorganic leachants (Li et al., 2010; Zheng et al., 2016). Co leaching
by organic leachants follow shrinking core model, while the reaction rate is
controlled by chemical reaction.
4.1.1.3. Gallium leaching from spent LEDs and Ga/As scraps. Lee and Nam
(1998) studied the Ga leachability from the Ga-As waste scraps, which con-
tain 47% of Ga and 51% of As. Nitric acid (2 M) was observed to leach out
99% of Ga within 2 hours at a relatively high temperature (60 C). Increase
in the solid to liquid ratio (in nitric acid medium) increases the leachability
of Ga, which could be influenced by the generation of NO
2
gas which was
due to an exothermic self-catalytic reaction. Similarly, increase in tempera-
ture and acid concentration also increases the Ga solubilisation. Chen, Tsai,
Tsai, and Shu (2012) investigated the leaching of Ga from Ga-As scraps
using sulfuric and nitric acids and reported that leaching efficiency of nitric
acid (>95%) was higher than that of sulfuric acid (<30%). In contrast to
Lee and Nam (1998), Chen, Tsai, Tsai, and Shu (2012) observed that
increase in Ga-As/nitric acid ratio decrease the leaching efficiency of Ga.
Figure 11. Recycling routes for SFLs (Binnemans et al., 2013).
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 23
Table 4. Different hydrometallurgical approaches proposed for the leaching of rare earth elements from WEEE.
WEEE type
Metal content
(w/w %) Leachant Optimum conditions Leaching yield Reference
Fluorescent lamps Eu - 1.39%
Y - 1.29%
Pressure acid
leaching with
HNO
3
þH
2
SO
4
4 M acid (HNO
3
þH
2
SO
4
) mixture, 4 h,
125 C and 5 MPa
96.4% of Y and
92.8% of Eu
Rabah (2008)
Fluorescent lamps Y - 7.2% H
2
SO
4
4MH
2
SO
4
, 20% pulp density, 90 C,
24 h and 200 rpm
85% of Y De Michelis, Ferella, Varelli,
and Vegli
o(2011)
Fluorescent lamps Y - 4.57% H
2
SO
4
2MH
2
SO
4
, 20% pulp density, 70 C,
24 h and 100 rpm
99% of Y Innocenzi & Vegli
o,2012
Fluorescent lamps YOX (Y
2
O
3
:Eu
3þ
)
- 20%
Ionic liquids 2 g of [Hbet][Tf
2
N], 5% H
2
O (wt%),
10 mg g
1
(SFL solid per gram of
ionic liquid), 90 C, 24 h and 600 rpm
99% of YOX Dupont and
Binnemans (2015a)
NdFeB magnets Nd - 35.10%
Dy - 1.10%
HNO
3
1 M HNO
3
þ0.3MH
2
O
2
,83.3 g L
-1
pulp
density, 20 min at 80 C.
98 % Nd and
81 % Dy
Rabatho, Tongamp, Takasaki,
Haga, and Shibayama (2013)
NdFeB magnets Nd - 31.27% H
2
SO
4
3MH
2
SO
4
,1:50 solid to liquid ratio,
27 C and 15 min.
95% of Nd Lee et al. (2013)
NdFeB magnets Nd - 19.10% H
2
SO
4
Roasting at 600 C, 5 h 3 M H
2
SO
4
,
110.8 g L
-1
pulp density, 70 C
and 4 h.
99% of Nd Yoon et al. (2014)
NdFeB magnets Nd - 23.00%
Dy - 6.26%
Pr - 6.52%
Gd - 2.25%
Water 1) Mixed with 14.5 M H
2
SO
4
and drying
at 110 C, 24 h 2) Roasting at 750 C,
1 h 3) H
2
O, 1:50 solid to liquid ratio,
25 C, 1 h at 225 rpm
98% of REE mixture
€
Onal, Borra, Guo, Blanpain, and
Van Gerven (2015)
NdFeB magnets Nd - 25.95%
Dy - 4.22%
Ionic liquids Roasting at 950 C, 3-15 h 1:1 wt/wt
[Hbet][Tf
2
N]-H
2
O mixture, 10 mg g
1
(magnets solid per gram of ionic
liquid), 80 C, 48 h and 600 rpm
99% of REE mixture Dupont and
Binnemans (2015b)
NiMH batteries REE total (La þCe)
-5%
H
2
SO
4
2MH
2
SO
4
,80
C, 3 h 35% La and 35% Ce
were leached
Innocenzi & Vegli
o(2012)
NiMH batteries La - 2.7%
Nd - 4.5%
Sm - 6.2%
Pr - 2.29%
Ce - 2.58%
Baking followed
by H
2
O and
H
2
SO
4
leaching
Pre-treatment - Baking with 2 mL H
2
SO
4
at 300 C Leaching - H
2
O, at 75 Cin
1 h, 500 rpm, 1:50 solid to liquid ratio
80.4% La, 98.8% Ce,
98.2% Nd, 98.5%
Pr, 99.2% Sm
were leached
Marra, Cesaro, and
Belgiorno (2017)
24 M. SETHURAJAN ET AL.
But this could be explained by the usage of low temperature (room tem-
perature) in case of Chen, Tsai, Tsai, and Shu (2012). An apparently low
activation energy of 39.9 kJmol
1
was required to leach 99% of Ga by
nitric acid (2 M), also suggest that Ga leaching from Ga-As scraps is a tem-
perature dependent reaction. Hu, Xie, Hsieh, Liou, and Chen (2015) also
reported similar findings on the nitric acid leaching of Ga from Ga-
As scraps.
4.1.2. Leaching of rare earth elements (REE) from WEEE
4.1.2.1. Yttrium and europium leaching from spent fluorescent lamps.
Phosphors from spent fluorescent lamps (SFLs) is a potential source of
REEs, which can be recovered (Porob, Srivastava, Nammalwar,
Ramachandran, & Comanzo, 2012). Binnemans et al. (2013) illustrated the
recycling routes for REEs from SFLs in Figure 11.
Over the recent years, several researchers have used all common mineral
acids to recover REEs from SFLs powders (Table 4) and they could not fig-
ure out what is the most effective mineral acid for reaching the maximal
leaching yields of the target elements (Virolainen, 2013). There are some
reasons for this issue: first selectivity over matrix component and second
suitability of the leachate to downstream processing (Virolainen, 2013).
Rabah (2008) proposed that more than 90% of Y and Eu could be leached
from SFls by pressure acid (HNO
3
þH
2
SO
4
) leaching. De Michelis,
Ferella, Varelli, and Vegli
o(2011) reported that temperature plays a signifi-
cant role in the leaching of from SFLs. Hot acidic leaching (4 M H
2
SO
4
at
90 C) could leach more than 95% of Y could from SFLs (De Michelis,
Ferella, Varelli, & Vegli
o, 2011). The leaching of Eu and Y from their
respective oxide (present in the spent fluorescent lamps) by H
2
SO
4
is illus-
trated by the equations 6 and 7:
Eu2O3s
ðÞþ3H2SO4aq
ðÞ
!Eu2SO4
ðÞ
3aq
ðÞ
þ3H2Og
ðÞ (6)
Y2O3s
ðÞþ3H2SO4aq
ðÞ
!Y2SO4
ðÞ
3aq
ðÞ
þ3H2Og
ðÞ (7)
Dupont and Binnemans (2015a) studied and proposed that thermomor-
phic properties of carboxyl-functionalized ionic liquid: betainium bis(tri-
fluoromethylsulfonyl)imide, [Hbet][Tf2N] can be exploited for the leaching
and extraction of Y and Eu from waste fluorescent lamp phosphors.
Thermomorphic properties of the [Hbet][NTf2]) (betainium bis(trifluoro-
methylsulfo-nyl)imide) (thermomorphism means that the solubility of the
IL with water can be thermally changed even becoming immiscible induc-
ing a separation that can be tuned while varying the weight fraction in
water) benefit the selective leaching of T and Eu from the waste lamp
phosphors. The ionic liquid used in this study has protonated carboxyl-
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 25
functionalized and it showed the ability to selectively dissolve REE oxides
(Y
2
O
3
and Eu
2
O
3
) as shown in equation 8 (Dupont & Binnemans, 2015b),
REE2O3þ6 Hbet
½
Tf2N
½
!2 REE bet
ðÞ
3
Tf2N
½
3þ3H2O (8)
High water content in ionic liquid (5 wt %) and high temperature
(90 C) was found to positively influence the leaching of REE by ionic
liquids. This is because, high water content and temperature decreases vis-
cosity and improves the diffusion of the lixiviant and consequently
increases the leaching efficiency.
4.1.2.2. REE leaching from spent magnet scraps. Various hydrometallurgical
solubilization of REE (especially neodymium and dysprosium) from mag-
nets were reported (Lee et al., 2013; Rabatho, Tongamp, Takasaki, Haga, &
Shibayama, 2013; Yoon et al., 2014;€
Onal, Borra, Guo, Blanpain, & Van
Gerven, 2015)(Table 4). Lee et al. (2013) studied the leaching of Nd from
magnetic scrap using various lixiviants (such as HCl, H
2
SO
4
, HNO
3
and
NaOH). NdFeB compounds in the magnets dissolve as shown in the equa-
tions 9–11, in the acidic medium. Neodymium readily dissolves in the
acidic pH and forms hydrogen gas. Similarly, iron and boron also dissolve
in presence of acids and generate hydrogen gas.
Nd s
ðÞþHþXaq
ðÞ
!Nd3þaq
ðÞ
þH2g
ðÞþXaq
ðÞ (9)
Fe s
ðÞþHþXaq
ðÞ
!Fe2þaq
ðÞ
þH2g
ðÞþXaq
ðÞ (10)
Bs
ðÞþHþXaq
ðÞ
!B3þaq
ðÞ
þH2g
ðÞþXaq
ðÞ (11)
Various factors (leachant concentration, solid to liquid ratio, temperature
and leaching time) affecting the leaching of REE from magnet wastes were
also investigated. The leaching efficiency of NaOH was lower when com-
pared to that of the mineral acids. Temperature affects the leaching of REE
significantly. Increase in temperature leads to the decrease in the REE
leaching efficiency. Linear relationship between temperature and leaching
efficiency suggests that the REE leaching rate by acids is controlled by
shrinking core kinetics (Yoon et al., 2014). Similarly, increase in solid to
liquid ratio decrease the leaching yield. This is because the increase in pulp
density leads to lower availability of reagent per unit weight of WEEE than
that of lower pulp density. On the other hand, increase in the acid concen-
tration increases the solubilisation of REE from magnet scraps. Increasing
acid concentration leads to increase in protons that attack and dissolve
more REE than in the case lower acid concentration (Lee et al., 2013;
Rabatho, Tongamp, Takasaki, Haga, & Shibayama, 2013; Yoon et al., 2014).
Acid concentration (3 N), 2% (w/v) pulp density, temperature (27 C) and
26 M. SETHURAJAN ET AL.
15 min of leaching time were found to be the optimum conditions to leach
out more than 95% of Nd from the waste NdFeB magnet.
Rabatho, Tongamp, Takasaki, Haga, and Shibayama (2013) studied the
effects of (NH
4
)
2
SO
4
,H
3
PO
4
, HNO
3
and HCl acid solutions on the leach-
ing Nd and Dy from waste magnetic sludge. The leaching efficiency of Nd
and Dy by using (NH
4
)
2
SO
4
and H
3
PO
4
was lesser (less than 5% Nd and
40% Dy) when compared to that of HCl and HNO
3
(more than 80% of Nd
and Dy). Rabatho, Tongamp, Takasaki, Haga, and Shibayama (2013) also
revealed that addition of oxidizing agent (H
2
O
2
) could enhance the leach-
ing efficiency of REE from the magnetic sludge. Both HCl and HNO
3
(in
presence of H
2
O
2
) could leach out more than 95% of REE, however HNO
3
seems to be a better leachant because of its poor leaching capacity on Fe.
Higher Fe leaching and Fe concentration in the leachate impede the further
selective recovery of REE from the leachates.
Yoon et al. (2014) investigated the kinetics of sulfuric acid mediated Nd
leaching from waste NdFeB magnets. The waste magnets were first roasted
for 5 h at 600 C prior leaching with sulfuric acid. The results obtained in
this study is comparable with data reported by Lee at al. (2013), except for
the effect of temperature. Yoon et al. (2014) states that increase in tempera-
ture in turn increases the Nd leaching efficiency from waste magnets in
contrast to the results of Lee at al. (2013). However, Yoon et al. (2014)
claims that the effect of temperature could be influenced by the inclusion
of ash layer formation due to the roasting. The leaching kinetics of Nd
from the magnets follow shrinking core model, while the reaction rate is
controlled by ash layer diffusion. The activation energy required to leach
Nd from the magnets was found to be 2.26 kJmol
1
(for 2.5 M H
2
SO
4
)
and 2.77 kJmol
1
(for 3.5 M H
2
SO
4
).
€
Onal, Borra, Guo, Blanpain, and Van Gerven (2015) proposed a combin-
ation of roasting followed by water leaching for the solubilization of REE
from scrap magnets. Firstly, crushed magnet waste was treated with sulfuric
acid and roasted to convert all the impurities to their oxides while the REE
remain as sulfates. Later, the roasted sludge was water leached to solubilize
more than 98% of REE (Nd, Dy, Pr and Gd).
Dupont and Binnemans (2015b) proposed a functionalized ionic liquid
betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf2N] for the effect-
ive leaching and separation of Nd and Dy from the waste
FeNdB magnets.
4.1.2.3. REE leaching from spent Ni-MH batteries. Various hydrometallurgical
processes were proposed for the leaching of REE from Ni-MH battery
wastes (Pietrelli, Bellomo, Fontana, & Montereali, 2005; Innocenzi &
Vegli
o, 2012; Meshram, Somani, Pandey, Mankhand, & Deveci, 2017)
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 27
(Table 4). Individual or combination of mineral acids (HCl, HNO
3
and
H
2
SO
4
) were widely used for the extraction of REEs from Ni-MH batteries
(Pietrelli, Bellomo, Fontana, & Montereali, 2005; Larsson, Ekberg, &
Ødegaard-Jensen, 2013; Petranikova, Herdzik-Koniecko, Steenari, & Ekberg,
2017). In some cases, oxidants (like H
2
O
2
) were also added along with
mineral acids to improve the REE leaching efficiency. Larsson, Ekberg, and
Ødegaard-Jensen (2013) proposed that the anodic parts of the batteries
could be completely dissolved with 1 M hydrochloric, sulfuric and nitric
acids (at pH 1.0 and temperature 30 C) within 6 hours. 2 M H
2
SO
4
was
found to be efficient to dissolve the black mass within 2 hours of leaching,
however it is not sufficiently effective reagent for metallic nickel (Pietrelli,
Bellomo, Fontana, & Montereali, 2005). Petranikova, Herdzik-Koniecko,
Steenari, and Ekberg (2017) reported that 8 M HCl was optimum for the
leaching of REEE from cathode and anode mixture.
Innocenzi and Vegli
o(2012) proposed a two-step leaching process for
the efficient leaching of lanthanum and cerium from Ni-MH scraps. In the
first step, the electrode material was leached with 3 M H
2
SO
4
at high tem-
perature (80–85 C) for 3 hours. In the second step, the leaching was con-
ducted with 1 M H
2
SO
4
at room temperature (20 C) for 1 hour. The
objective of the second stage was to increase the yield of REEs leaching.
Prior leaching with H
2
SO
4
, washing with water was suggested to remove
the electrolyte residues (KOH) present (Innocenzi & Vegli
o, 2012). Becker
et al. (2016) reported another two-step leaching process for the leaching Co
and REEs from NiMH batteries. First step was to wash with hot water
(95 C, 1 h) followed by roasting at high temperature for 4 hours. Second
step was leaching the roasted calcine with H
2
SO
4
solution at high tempera-
ture (90 C) for 6 hours which resulted in more than 98% leaching of Co
and REEs.
Meshram, Somani, Pandey, Mankhand, and Deveci (2017) proposed a
different two-step leaching process for the leaching of REEs (La, Ce, Nd, Pr
and Sm) from spent Ni-MH batteries. Baking with 2 mL H
2
SO
4
at 300 C,
for 90 min was as a pre-treatment in-order-to transform nickel, zinc and
REEs into sulfate form. After baking, the first step leaching was carried out
using water (at 75 C) leached out 91% of Ni, 94% of Zn and 91% of REEs.
In the first step, low concentration (20–30%) of Co, Fe and Mn were also
leached out. The second step (reductive leaching) was carried out using
NaHSO
3
in H
2
SO
4
(at 95 C) to leach out the residual Co and Mn.
4.1.3. Leaching of precious metals from waste printed circuit boards
Numerous hydrometallurgical processes have been described in the litera-
ture for recovering PMs from waste PCBs (Table 5). Because PCBs contain
a high content of Cu, which can increase the consumption of reagents and
28 M. SETHURAJAN ET AL.
Table 5. Different hydrometallurgical approaches proposed for the leaching of precious metals from WEEE.
Metal content in PCBs Leachant Conditions Leached metal (%) Reference
Au - 0.02% Ag
- 0.07%
H
2
SO
4
2MH
2
SO
4
,5%H
2
O
2
,25
C, 10%
(w/v) pulp density, 200 rpm
Ag (60.76%); Au (0%) Birloaga, Coman, Kopacek, and
Veglio (2014)
Au - 0.01% Ag -
0.07% Pd - 0.01%
H
2
SO
4
0.81 M H
2
SO
4
, 0.41 M Fe(III), 2 h,
80 C and 1% (w/v)
pulp density
Ag (21.4%); Au (0%);
Pd (69.4%)
Yazici and Deveci (2014)
Au - 0.03% Ag -
0.09% Pd - 0.01%
HNO
3
4 M HNO
3
, 72 min, 65 C, 20%
(w/v) pulp density
Ag (87%) Joda and Rashchi (2012)
Au - 0.05% Ag
- 0.04%
HNO
3
2 M HNO
3
, 3.5 h, 50 C, 10% (w/
v) pulp density, 150 rpm
Ag (97%); Au (0%) Neto, Sousa, Brito, Futuro, and
Soares (2016)
Ag - 0.02% Pd
- 0.03%
NaCl- CuSO
4
V
NaCl
/m
CuSO4
¼6, 0.5 h, 60 C Ag (93.9%);
Pd (95.3%)
Zhang and Zhang (2013)
Au - 0.01% Ag -
0.08% Pd - 0.003%
Thiosulfate 0.5 M (NH
4
)
2
S
2
O
3
þ0.2 M
CuSO
4
.5H
2
Oþ1MNH
3
,
48 h, 40 C
Ag (93%); Au (98%);
Pd (90%)
Ficeriova, Balaz, and Gock (2011)
Au - 0.1% Thiosulfate 0.1 M Na
2
S
2
O
3
þ0.2 M NH
3
OH
þ0.015-0.03 M Cu
2þ
Au (15%) Petter, Veit, and Bernardes (2014)
Au - 0.005% Ag -
0.008% Pd
- 0.002%
Thiosulfate 0.2 M (NH
4
)
2
S
2
O
3
þ0.02 M
CuSO
4
þ0.4MNH
4
OH,
48 h, 40 C
Ag (100%); Au (95%) Oh, Lee, Yang, Ha, and Kim (2003)
Au - 0.02% Thiosulfate 0.13 M (NH
4
)
2
S
2
O
3
þ20 mM
Cu
2þ
, 2 h, 20 C
Au (70%) Camelino, Rao, Padilla, and
Lucci (2015)
Au - 0.004% Ag
- 0.05%
Thiourea 24 g L
1
Thiourea þ0.6% Fe(III),
2h, 25C
Ag (50%); Au (90%) Jing-Ying, Xiu-Li, and Wen-
Quan (2012)
Au - 0.01% Ag -
0.07% Pd - 0.003%
Thiourea 20 g L
1
Thiourea þ6g L
1
Fe(III) þ10 g L
1
H
2
SO
4
, 10%
(w/v), 3 h, 25 C, 200 rpm
Ag (71%); Au (84%) Behnamfard, Salarirad, and
Veglio (2013)
Au - 0.01% Ag
- 0.07%
Thiourea 20 g L
1
Thiourea þ6g L
1
Fe(III) þ0.1 M H
2
SO
4
,1h,
25 C, 200 rpm
Ag (75%); Au (90%) Birloaga & Veglio (2016)
Au - 0.06% Ag -
0.52% Pd - 0.04%
Chloride 2 M HCI þ20.5 kg t
1
H
2
O
2
,
33% (w/v), 3 h, 75 C
Ag (3.9%); Au (12.9%);
Pd (93.1%)
Quinet, Proost, and Van
Lierde (2005)
Au - 0.01% Ag -
0.07% Pd - 0.003%
Chloride 5 M HCl þ1% H
2
O
2
þ10%
NaClO, 10% (w/v) pulp dens-
ity, 3 h, 55 C, 300 rpm
Ag (16%); Au (6%);
Pd (98%)
Behnamfard, Salarirad, and
Veglio (2013)
Au - 0.01% Ag -
0.07% Pd - 0.01%
Chloride 4 g L
1
Cu
2þ
þ46.6 g L
1
Cl
,80
C
Ag (92%); Pd (58%) Yazici and Deveci (2013)
(continued)
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 29
Table 5. Continued.
Metal content in PCBs Leachant Conditions Leached metal (%) Reference
Au - 0.28% Chloride 5 M HCl þH
2
O
2
5 M, 4h Au (30%) Imre-Lucaci, Nagy, Imre-Lucaci,
and Fogarasi (2017)
Au - 0.07% Chloride 17 g L
1
NaClO
3
, 80 min, 50 C,
Eh ¼1.1 V
Au (93%) Lu, Song, and Xu (2017)
Au - 1.18% Ag -
4.68% Pd - 0.46%
Chloride 1/20 gmL
1
aqua regia, 3 h Ag (98%); Au (97%);
Pd (93%)
Park and Fray, (2009)
Au - 0.01% Iodide 2.7 mM KI þ0.51 mM I
2
in 10 mL Au (>99%) Serpe et al. (2015)
Au - 0.03% Ag -
0.54% Pd - 0.02%
Iodide Iodine/iodide mole ratio of 1:5,
120 min, 10% (w/v) pulp
density, pH 9
Ag (98%); Au (98%);
Pd (96%)
Xiu, Qi, and Zhang (2015)
30 M. SETHURAJAN ET AL.
decrease PMs recoveries (Montero, Guevara, & dela Torre, 2012; Camelino,
Rao, Padilla, & Lucci, 2015), leaching of PMs is usually carried out after a
previous mild oxidative acid leaching of Cu and other base metals. This
strategy improves the selectivity of the PMs and minimizes the impurities
(Sheng & Etsell, 2007).
An important point to be considered in the leaching of PMs is related
with the selection of the lixiviants and fraction size of PCBs. Various lixi-
viants are proposed for the dissolution of precious metals from WEEE.
Different lixiviants (such as cyanides, mineral acids, thiourea, thiosulfate
and halides) exhibit different leaching mechanisms and each of them have
their own merits and demerits (Syed, 2012; Lee & Srivastava, 2016). It is
evident that fractions with a smaller particle size results in a better dissol-
ution of PMs. This is due to the high surface area of the smallest particles
and, thus, a higher proportion of metals exposed to the lixiviant (Sheng &