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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 &
Etsell, 2007; Birloaga, De Michelis, Ferella, Buzatu, & Veglio, 2013; Gurung
et al., 2013).
4.1.3.1. Silver, gold and palladium leaching from PCBs by mineral acids.
Despite most common leaching agents are not selective for a specific PM,
there are some alternative leaching systems, mostly inorganic acids, that can
leach Ag selectively (relatively to other PMs) from PCBs residues (Table 5).
This selective leaching can be carried out taking into account the redox
potentials of the aqueous phase. At lower redox potentials, Au and other PMs
are not solubilized while Ag is, and it can form stable complexes with halides
(Lister, Wang, & Anderko, 2014). By this way, it is possible to perform a pre-
liminary leaching of Ag together with other base metals presents in the PCBs.
Nitric (HNO
3
) and sulfuric (H
2
SO
4
) acids as well as cuprous chloride or
ammonium sulfate systems, at suitable oxidant conditions, are potential
leaching agents described in the literature to perform a selective Ag leach-
ing (Table 5). If H
2
SO
4
is used as the lixiviant, an additional oxidant is
necessary to increase the redox potential of the medium. Several oxidants
were suggested in the literature like hydrogen peroxide (Birloaga, Coman,
Kopacek, & Veglio, 2014; Quinet, Proost, & Van Lierde, 2005), metallic cat-
ions (Fe
3þ
and Cu
2þ
), oxygen (Quinet, Proost, & Van Lierde, 2005; Yazici
& Deveci, 2014) and aqueous ozone (Vinals, Juan, Roca, Cruells, &
Casado, 2005).
Joda and Rashchi (2012) and Neto, Sousa, Brito, Futuro, and Soares
(2016) studied the ability of HNO
3
to leach Ag. Joda and Rashchi (2012)
achieved a leaching efficiency of 82.7% for Ag (along with 94% for Cu)
using 4 M HNO
3
at 65 C within 72 min. On the other hand, Neto, Sousa,
Brito, Futuro, and Soares (2016) reached a leaching of 97% of Ag (and 78%
of Cu) along with less than 3% of Au using 2 M HNO
3
at 50 C
within 210 min.
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 31
Serpe et al. (2015) studied the use of NH
3
in combination with an IO
3
/
I
mixture, which allows oxidizing Ag and Cu and separating them by
selective AgI precipitation. Base metals were previously dissolved using cit-
ric acid. Serpe et al. (2015) approach is of particular interest because it
allows the selective leaching of Ag and still insures the posterior leaching
of Au using an iodine mediated leaching system.
Yazici and Deveci (2013) tested the extraction of Ag and Pd from
PCBs with H
2
SO
4
-CuSO
4
-NaCl solutions. Over a leaching period of
120 min, at a Cl
/Cu
2þ
ratio of 21 and 80 C, the complete extraction
of Cu as well as >90% Ag together with 58% of Pd was reached.
However, the extraction of Au was very limited when a cupric chloride
leaching system was used (Yazici & Deveci, 2015). Lister, Wang, and
Anderko (2014) reported that the use of Fe
3þ
in acidic sulfate allowed
recovering more than 95% of Ag and Cu while Au and Pd remained
intact in the residue.
Zhang and Zhang (2013) developed a process for cuprous chloride syn-
thesis and simultaneous extraction of Ag and Pd from waste PCBs without
using aggressive acids or strong oxidants.
The traditional medium for dissolving Au, as well as the platinum group
metals, is aqua-regia, a mixture of three parts of concentrated HCl to one
part of concentrated HNO
3
(Cui & Zhang, 2008). Aqua-regia is known to be
the best reagent to dissolve Au and it is used for refining Au with highest
quality. However, due to the reaction between HNO
3
and HCl that results in
its decomposition, aqua-regia quickly loses its effectiveness and cannot be
reused. Moreover, aqua-regia is highly corrosive, which is a major disadvan-
tage that implies the necessity for the construction of suitable reactor for
these extreme conditions and thus limiting its industrial feasibility. Precious
metals (e.g. Au) leaching by aqua-regia is shown in Eq. (12).
Au s
ðÞþ11 HCl aq
ðÞ
þ3 HNO3aq
ðÞ
!2HAuCl4aq
ðÞ
þ3 NOCl g
ðÞþ6H2Oaq
ðÞ
(12)
Sheng and Etsell (2007) found a quicker leaching of Au from PCBs using
aqua-regia through the implementation of a sequential three-stage leaching
process, which combined two first steps with HNO
3
and the last one using
aqua-regia. The third step increases Au and Ag extraction. Park and Fray
(2009) also tested the ability of aqua-regia for leaching PMs from PCBs.
The authors reported that aqua-regia was very effective because it was pos-
sible to leach out Ag, Pd and Au simultaneously. Ag was relatively stable in
aqua-regia and remained unreacted. On the other hand, a quantitative
recovery of Pd (93%) and Au (97%) was achieved after leaching and subse-
quent separation processes as a precipitate of Pd(NH
4
)
2
Cl
6
and as
32 M. SETHURAJAN ET AL.
nanoparticles of gold (after liquid-liquid extraction followed by reduction),
respectively.
Other than mineral acids, there are also various other lixiviants like cya-
nides, thio-compounds and halides were proposed for the leaching of PMs,
which are discussed below in detail.
4.1.3.2. Silver, gold and palladium leaching from PCBs by cyanides. Cyanide is
the most used reagent for Au extraction from ores and secondary sources.
More than 90% of Au and Ag extracted from the natural ores are extracted
using cyanide lixiviant systems. The mechanism of PMs dissolution in
cyanide solution is essentially an electrochemical process. The order of
activity for PMs is: Au >Ag >Pd >Pt. A generalized equation of Au leach-
ing by cyanides is illustrated in Eq. (13).
4Au s
ðÞþ8CN
aq
ðÞ
þO2g
ðÞþ2H2Oaq
ðÞ
!4Au CN
ðÞ
2aq
ðÞ
þ4OH
aq
ðÞ
(13)
Maximum dissolution of these metals in cyanide solution can be
obtained at pH 10–10.5. At this pH, the cyanidation process is safe and
economically and environmentally more friendly because most of the free
cyanide present in solution is in the form of the cyanide anion, which
avoids its volatilization as HCN that is highly toxic. Aqueous solutions of
cyanide degrade rapidly in sunlight, but, the less-toxic products, such as
cyanates and thiocyanates, may persist for some years (Yarar, 2002).
However, a series of environmental accidents at various Au mines that
caused severe contamination of rivers and groundwater occurred in the last
years. These facts hinder the implementation of future applications
of cyanide.
In last three decades, numerous potential non-cyanide leaching systems
have been studied for extracting PMs (in particular Au) from PCBs (Cui &
Zhang, 2008; Syed, 2012, Zhang, Li, Xie, Zeng, & Li, 2012)(Table 5).
Among them, thiosulfate, thiourea and halides have been intensively inves-
tigated and will be reviewed below in detail.
4.1.3.3. Silver, gold and palladium leaching from PCBs by thiosulfates.
Thiosulfate can be considered a good candidate for replacement of cyanide
for the PMs extraction due to its lower environmental impact and low cost.
However, leaching efficiencies from thiosulfate leaching was comparatively
lesser than cyanides (Zhang, Chen, & Fang, 2009). Au leaching by thiosul-
fates is shown in Eq. (14).
4Au s
ðÞ
þ8S2O32þO2g
ðÞþ2H2Oaq
ðÞ
!4AuS
2O3
ðÞ2½
3aq
ðÞ
þ4OH
aq
ðÞ
(14)
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 33
The principal problems with thiosulfate leaching are related with the
high consumption of reagent and the slower reaction kinetics than cya-
nides. However, the leaching rates can be improved in the presence of
ammonia and using Cu
2þ
as an oxidant (Cui & Zhang, 2008). The dissol-
ution of PMs in thiosulfate solution is an electrochemical reaction catalyzed
by the presence of Cu
2þ
. This cation acts as an oxidant and at suitable con-
centrations can significantly improve the leaching rate. However, the pres-
ence of ammonia is essential to stabilize Cu
2þ
, since Cu
2þ
as well as other
metals can increase the decomposition of thiosulfate (Arslan & Sayiner,
2008). Moreover, it is important to emphasize that, in the case of direct
thiosulfate leaching of PCBs, the Cu dissolved from PCBs may adversely
affect the leaching process through decomposition of thiosulfate.
The biggest disadvantage of thiosulfate leaching is the low chemical sta-
bility of this compound. The stability of thiosulfate decreases at high tem-
perature and low pH values. Alkaline conditions are necessary to prevent
thiosulfate decomposition, which is quickly degraded under acidic condi-
tions. The pH range is dictated by the ammonia/ammonium buffer point
(pKa ¼9.25 at 25 C). A pH range of 9–10 is generally preferred at ambient
temperature because thiosulfate appears to be less prone to degradation in
this region and Cu
2þ
-ammonia complex is also stable.
Ficeriova, Balaz, and Gock (2011) reported a successful leaching of Au
(98%) and Ag (93%) from waste PCBs using ammonium thiosulfate during
48h after a pre-treatment step to reduce the particle size (<0.80 mm).
Petter, Veit, and Bernardes (2014) found that the leaching process using
sodium thiosulfate was more efficient when an elevated concentration of
CuSO
4
(between 0.015 and 0.030 M) was added. Oh, Lee, Yang, Ha, and
Kim (2003) and more recently Camelino, Rao, Padilla, and Lucci (2015)
developed a two-step leaching process for recovering metals from PCBs. In
a first step, base metals were leached from PCBs using H
2
SO
4
and H
2
O
2
.
In the second step, a thiosulfate leaching solution was used. Higher Au
leaching yields were obtained by Oh, Lee, Yang, Ha, and Kim (2003) when
a higher thiosulfate concentration was used (0.2 M) and a longer leaching
time (48 hours). Is¸ıldar, van de Vossenberg, Rene, van Hullebusch, and
Lens (2017) demonstrated Au leaching of more than 90% within 6.73 hours
using 0.038 M copper sulfate, 0.3 M 0.38 M ammonium hydroxide at
10.76% pulp density (w/v).
4.1.3.4. Silver, gold and palladium leaching from PCBs by thiourea. Thiourea is
also a potential non-cyanide lixiviant that reacts selectively with PMs to
produce stable cationic complexes (Birloaga, De Michelis, Ferella, Buzatu,
& Veglio, 2013). Leaching studies using thiourea suggested that the lone
pairs of electrons on nitrogen and sulfur atoms of thiourea have a better
34 M. SETHURAJAN ET AL.
potential for a coordination bond between Au and Ag compared to cyanide
(Gurung et al., 2013; Akcil et al., 2015). A generalized equation of Au
leaching thiourea is depicted in Eq. (15).
Au s
ðÞþ2CS NH2
ðÞ
2aq
ðÞ
þ2Fe
3þaq
ðÞ
!Au CS NH2
ðÞ
2
2
þaq
ðÞ
þ2Fe
2þaq
ðÞ
(15)
Thiourea leaching process has a fast kinetic reaction with Au and Ag as
well as a low impact on the environment compared to cyanide. Leaching of
PMs with thiourea can result in a leaching efficiency more than 90% in a
short leaching time but the reagent consumption is higher when compared
to cyanide; as consequence, the process is more expensive (Tanriverdi,
Mordogan, & Ipekoglu, 2005). However, when compared with thiosulfate
leaching, it involves lower operating costs because it consumes a smaller
amount of leaching reagent.
Thiourea should be used under relatively restricted conditions as it is
fairly stable in acidic and neutral mediums but it decomposes rapidly in
basic solution. The leaching is usually carried out in the pH range between
1.0 and 2.0. Thiourea leaching requires the presence of an external oxidant
in order to accelerate the leaching rate. Gurung et al. (2013) revealed that
addition of Fe
3þ
maximizes Au and Ag leaching.
Lee, Tang, and Popuri (2011) obtained complete extraction of Au and Ag
from WEEE with a two steps leaching with thiourea, H
2
SO
4
, and ferric sul-
fate at ambient temperature. Jing-Ying, Xiu-Li, and Wen-Quan (2012)
observed that the leaching rate of Au was highly affected by the thiourea
concentration. The leaching rate of Au increased with the thiourea mass con-
centration but decreased for values higher than 24g L
1
. When the content
of thiourea is too high, thiourea is easily oxidized by ferric ion in acidic solu-
tion and formamidine disulfide is produced. With a lixiviant containing
24 gL
1
thiourea and Fe
3þ
concentration of 0.6% at room temperature, a
leaching of 90% of Au and 50% of Ag from PCBs of waste mobile phones
was achieved. In many instances, a two-step leaching was proposed, in which
Cu was leached completely in the first step and more than 80% of Au and
70% of Ag was leached in the second step with acidic thiourea (Lee, Tang, &
Popuri, 2011; Birloaga, De Michelis, Ferella, Buzatu, & Veglio, 2013;
Behnamfard, Salarirad, & Veglio, 2013; Birloaga & Veglio, 2016).
High capital costs and easy oxidation of thiourea are major drawbacks
for the commercial application of PMs leaching by thiourea.
4.1.3.5. Silver, gold and palladium leaching from PCBs by chlorides. Chlorine
was extensively used as a leaching reagent for Au extraction from ores and
concentrates, even at industrial scale (Cui & Zhang, 2008; Syed, 2012).
Chlorination is a practicable alternative for Au leaching due to its high
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 35
dissolution rate that is achieved by controlling the redox potential. The use
of chloride to leach PMs requires the presence of a strong oxidant, such as
H
2
O
2
or NaClO
3
. Under these conditions, Cl
2
is produced, which is
extremely toxic and corrosive and should be manipulated only at specific
conditions and with resistant equipment. Due to these facts, chloride leach-
ing is more difficult to apply than cyanide leaching because special stainless
steel and rubber-lined equipment are required to resist to the highly corro-
sive acidic and oxidizing conditions. Moreover, the chlorine gas is highly
poisonous and must be controlled to avoid any health risk (Syed, 2012).
Quinet, Proost, and Van Lierde (2005) applied chloride leaching (HCl
and NaCl) to recover Pd from WEEE using two oxidants: HNO
3
and
H
2
O
2
. Both oxidants (HNO
3
and H
2
O
2
) achieved a similar (93–95%) recov-
eries of Pd when HCl was used as the lixiviant. Zhou, Zheng, and Tie
(2005) patented a process for recovering PMs from WEEE containing plas-
tics. A leaching of 92% of Au and Pd content was obtained using HCl and
NaClO
3
. Behnamfard, Salarirad, and Veglio (2013) obtained an almost total
(98%) leaching of Pd using the chlorination (HCl 5 M and NaClO 10% (w/
v)) after three previous subsequent extraction steps, already described
above. Yazici and Deveci (2013) tested Cu
2þ
and Cl
as oxidant and lig-
and, respectively, to recover Ag from PCBs. The extent of Ag extraction
was dependent of the amount of Cl
concentration. At low Cl
concentra-
tions, Ag precipitated as AgCl. The increase of Cl
concentration enhanced
the Ag extraction due to the formation of silver chloride complexes, being
the highest (92%) extraction of Ag achieved when 4 gL
1
Cu
2þ
and 46.6 g
L
1
Cl
were used at 80 C. He and Xu (2015) studied a chlorination pro-
cess to recycle Au from PCBs. Results showed that the process was efficient
and less-pollutant. Moreover, more than 90% of Au could be recovered by
controlling the redox potential (above 1100 mV) of the leaching solution.
Xing and Lee (2017) studied the mixtures of HCl and several oxidizing
agents, such as H
2
O
2
, NaClO and HNO
3
, to dissolve Au and Ag from
anode slime resultant from the treatment of copper sludge. The authors
reported a complete dissolution of Au from the anode slime after addition
of the oxidizing agent to the HCl solution. While a complete dissolution of
Cu, Zn, Ni, and Sn was achieved with a mixture of HCl with either H
2
O
2
or NaClO, a low leaching of Ag (less than 10%) was verified. Imre-Lucaci,
Nagy, Imre-Lucaci, and Fogarasi (2017) studied the Au extraction from
PCBs using a H
2
O
2
-HCl leaching system followed by electrowinning. A
high efficient Au dissolution took place when a mixture of 1 M H
2
O
2
and
5 M HCl was used. Lu, Song, and Xu (2017) described a two-stage chlorin-
ation leaching process for extracting selectively Cu and Au from waste
memory modules from PCBs by controlling the redox potential of the solu-
tion using NaClO
3
. Under optimal experimental conditions, an almost total
36 M. SETHURAJAN ET AL.
extraction of Cu (96.5%) and Au (93%) was obtained at redox potentials of
0.4 and 1.1V, respectively.
4.1.3.6. Silver, gold and palladium leaching from PCBs by iodides. Besides
being less reactive than chloride, iodide allows achieving a faster dissolution
of PMs. The use of iodine-iodide system to leach Au and other PMs is
extremely advantageous because iodide leaching is considered to be non-
toxic, noncorrosive and very selective to Au (Konyratbekova, Baikonurova,
& Akcil, 2015). Moreover, both iodine and iodide can be recovered and
reused. Under general conditions, iodine dissolves in the presence of iodide
to form triiodide ion, which acts as oxidant for elemental Au originating
the Au-iodide complex (Konyratbekova, Baikonurova, & Akcil, 2015). The
Au-iodide complex is the most stable compound formed by Au and a
halogen (Zhang, Chen, & Fang, 2009). However, high rate of reagent con-
sumption during the leaching and high reagent cost limits its industrial
application (Syed, 2012; Ghosh, Ghosh, Parhi, Mukherjee, & Mishra, 2015).
Addition of oxidants to iodine leaching systems enhances the Au extrac-
tion and decreases the iodine consumption resulting in a more economical
and cheaper process (Xu, Chen, Chen, & Huang, 2009; Xu, Chen, Chen, &
Huang, 2010). Xu, Chen, Chen, and Huang (2009 and 2010) studied the
utilization of H
2
O
2
, as oxidant, in iodine leaching of Au from fine particle
size fractions of PCBs. It was observed that 0.2% iodine resulted in low Au
leaching rate but increasing the iodine concentration to 1–2% with 1% of
H
2
O
2
, 95% of Au was leached. Sahin et al. (2015) investigated a two-step
leaching of Au from waste PCBs. Firstly, Cu and other base metals were
removed (2 M H
2
SO
4
, 0.2 M H
2
O
2
for 120 min, at 80 C). In a second step,
93% of Au was obtained when iodine leaching tests were performed in the
presence (2% of H
2
O
2
) of oxidant.
Serpe et al. (2015) described an iodine-iodide leaching process where the
vast majority (>99%) of Au was dissolved from the waste PCBs under opti-
mized leaching. Xiu, Qi, and Zhang (2015) studied the possibility of using
iodine (oxidant)-iodide (complexing agents) system for leaching Au, Ag
and Pd from a pre-treated (with supercritical water þhydrochloric acid)
waste PCBs. Previously, the organic matter of the PCBs was removed using
super critical water oxidation and the base metals were leached using HCl
1 M. The iodine/iodide ratio was crucial for leaching Au, Ag, and Pd with
high yield; a maximum leaching of Au and Pd was obtained when the iod-
ine/iodide molar ratio was 1:5 (1:6 for Ag).
4.2. Recovery of critical elements from leachates
Researchers recently focused on the recovery of critical elements from sec-
ondary sources because of increasing demand for high-purity critical
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 37
elements. Since it has been observed that the WEEE leachates contain sub-
stantial quantities of multi metals with potentially high economic values, it
is important to develop proper selective recovery strategies. Leachates from
WEEE, and other domestic and industrial metallic waste materials, often
contain a large number of different soluble metals, and a major challenge is
how to recover these metals as separate entities, for example using selective
technologies (Johnson & Du Plessis, 2015). There are a lot of methods for
the recovery of metal ions from aqueous solution, such as precipitation,
liquid-liquid extraction, electrowinning, cementation and adsorption
(Fujiwara, Ramesh, Maki, Hasegawa, & Ueda, 2007; Zhu, Zheng, & Wang,
2015; Anastopoulos, Bhatnagar, & Lima, 2016, Kucuker, 2018).
4.2.1. Critical and precious metals and REE recovery by chemical precipitation
Precipitation is a well-established metal recovery techniques to recover met-
als from multi metallic leachates. Certain chemicals like sulfides, hydroxides
and carbonates when added to a polymetallic solution, could change the
ionic equilibrium of the system and precipitate the metals as respective salts
(e.g. metal-sulfides or metal-carbonate or metal-hydroxide) (Sethurajan,
Lens, Horn, Figueiredo & van Hullebusch, 2017). Precipitation is widely
used to remove metals from contaminated wastewater or acid
mine drainage.
Metal precipitation reaction follows three important phases like (1) nucle-
ation, (2) growth of nucleus, and (3) aggregation and crystallization (Lewis,
2010). pH and concentration of the metals are two important factors that
affect the metal precipitation. Sludge generation and high requirements of
chemicals to adjust the pH are the major draw backs of the metal precipita-
tion technique.
4.2.1.1. Hydroxide precipitation. Strong bases like sodium hydroxide (NaOH),
and lime or hydrated lime (Ca(OH)
2
) are the commonly applied chemicals
the precipitation of metal hydroxides. Weak bases (e.g. ammonia solution)
can also be used to precipitate metal hydroxides, however at higher pH it
can form stable complexes because of the dissolution of metal hydroxides
(Contestabile, Panero, & Scrosati, 2001). A generalized equation for the
metal hydroxide precipitation can be written as follows,
M2þþ2OH
MOH
ðÞ
2(16)
where M is a divalent metal ion.
Metal removal by hydroxide are widely used because of its relatively simple
operation and low capital cost (Huisman, Boks, & Stevels, 2003). A major dis-
advantage of this process is the high solubilities of the metal hydroxide com-
plexes precipitated, when the pH is not optimum. A soluble metal complex
38 M. SETHURAJAN ET AL.
M(OH)
þ
will be formed with respect to the change in the pH. Contestabile,
Panero, and Scrosati (2001) demonstrated a selective recovery of Co from
spent batteries as Co(OH)
2
using NaOH. They observed that the pH was
increased from initial pH 6.0 to 8.0 during the precipitation reaction. Silveira,
Fuchs, Pinheiro, Tanabe, and Bertuol (2015) demonstrated a selective recov-
ery of In (as In(OH)
3
) using NH4OH. They investigated the indium hydrox-
ide precipitation in the pH range 5.0 - 9.0 and found out that more than 90%
of In can be precipitated in the pH range 6.0–9.0. However, at pH 7.4, a max-
imum of 99.8% of In was precipitated (Silveira, Fuchs, Pinheiro, Tanabe, &
Bertuol, 2015). Lee et al. (2013) proposed selective recovery of REE (Nd in
particular) from NdFeB magnet scrap leachate by altering the system pH. The
pH of the leachate was highly acidic (0.13 <pH <0.02) and it was adjusted
using sodium hydroxide. It was found that at pH 0.6, more than 95% of Nd
can be selectively recovered as Nd-hydroxide precipitates, from the sulfuric
acid leached NdFeB magnet wastes. However it is worth to note that the
selective recovery of Nd from HCl leached NdFeB magnet wastes was not
successful as H
2
SO
4
leachate.
4.2.1.2. Sulfide precipitation. Ferrous sulfide (FeS), calcium sulfide (CaS),
sodium sulfide (Na
2
S), sodium hydrosulfide (NaHS), ammonium sulfide
((NH
4
)
2
S), hydrogen sulfide (H
2
S) are the major chemicals used for the met-
als sulfides precipitation (MSP). MSP has various advantages over other
methods including the metal sulfide precipitates are less soluble, selective
metal precipitation can be done, reaction rates are fast, settling properties are
far better and sulfide precipitates can be reused (Lewis, 2010). In addition to
that, MSP also offers selective metal precipitation and can successfully be
applied to extremely low concentration (ppb) of metals (Kim, Kim, Lee, &
Pandey, 2011). Operating pH plays an important role in the precipitation of
metal sulfide and also various metal sulfides has the tendency of solubility
with respect to pH (Lewis, 2010; Sethurajan, Lens, Horn, Figueiredo & van
Hullebusch, 2017). Concentration of the sulfide is the key factor in the MSP,
if it exceeds or depletes either sulfides or metals will remain in the solution
(Veeken, Akoto, Pol, & Weijma, 2003). Also, metal removal by MSP has other
various barriers (Lewis & Van Hille, 2006) to overcome such as (1) formation
of poly sulfides due to the localization of supplied sulfides, which results in
excessive consumption of sulfide and low metal removal and (2) low solubil-
ity of metal sulfides and higher supersaturation which resulted in the forma-
tion of fine particles with unfavorable solid-liquid separation difficulties.
Optimization of sulfide dosage is the limiting parameter for its less known
commercial applications, as the metal sulfides have very less solubility which
will have an impact in the process sensitivity (Veeken, Akoto, Pol, & Weijma,
2003; Lewis, 2010). Lewis and Van Hille (2006) proposed that the gaseous
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 39
hydrogen sulfide source could decrease the level of supersaturation and in
turn the formation of fine particles was controlled. Li, Liu, Li, Liu, and Zeng
(2011) demonstrated a selective recovery of In from In/Sn leachate. Sn was
firstly removed by H
2
S (gas) as tin sulfide (SnS). Ga-sulfide precipitation of
WEEE leachate was investigated and reported (Chen, Tsai, Tsai, & Shu, 2012;
Hu, Xie, Hsieh, Liou, & Chen, 2015). Chen, Tsai, Tsai, and Shu (2012)
observed that 49% of Ga could be precipitated from nitric acid medium,
using Na
2
S at initial pH 3.0. However, 49% of precipitation is relatively a low
recovery and to overcome this Hu, Xie, Hsieh, Liou, and Chen (2015) pro-
posed drop-wise addition of sulfide to the leachate. Drop wise addition
(5 Lmin
1
)ofNa
2
S to Ga and As containing leachate, resulted in 98% select-
ive recovery of Ga-precipitates (Hu, Xie, Hsieh, Liou, & Chen, 2015).
4.2.1.3. Other precipitation techniques. Carbonates precipitation is also used
to precipitate the metals as metal carbonates, in which straight precipitation
by chemicals such as calcium carbonate is used or conversion of hydroxides
to carbonates is applied for the precipitation of metals (Wang, yung-tse, &
Shammas, 2005). Carbonates precipitation can also be applied in combin-
ation with hydroxide precipitations. Operation at low pH, faster settling
and good metal removal were some of the merits of carbonates precipita-
tion. Sometimes oxalate precipitation was also used to precipitate REE
from leachates. De Michelis, Ferella, Varelli, and Vegli
o(2011) illustrated
oxalate precipitation of Y from spent fl. lamp waste leachate. Rabatho,
Tongamp, Takasaki, Haga, and Shibayama (2013) investigated the selective
recovery of REE from spent magnetic sludge. Nitric acid mediated mag-
netic sludge leachate contain 28 gL
1
of Nd, 0.7–0.8 gL
1
of Dy,
4.0–6.6 gL
1
of Fe and 0.4–0.5 gL
1
of B. Firstly, Fe was removed as
Fe(OH)
3
using sodium hydroxide at pH 2.0–3.0. However 20–25% of REE
(Nd and Dy) was co-precipitated or trapped with Fe-precipitate. Finally,
oxalic acid was used to precipitate REE as their oxalate salts from the Fe-
depleted leachate. More than 70% of Nd could be recovered as
Nd
2
(C
2
O
4
)
3
10H
2
O (neodymium oxalate hydrate) using 1.1 M oxalic acid.
Double sulfate precipitation is another form of metal recovery by chemical
precipitation. For instance, REEs were precipitated as Na(REE)(SO
4
)
2
by
using concentrated NaOH (5 M) (Nan, Han, Yang, Cui, & Hou, 2006;Li
et al., 2010; Innocenzi & Vegli
o, 2012; Becker et al., 2016).
4.2.2. Critical and precious metals and REE recovery by solvent extraction
Solvent extraction (SX) or otherwise referred as liquid-liquid distribution
which requires two liquid phases which are completely immiscible with
each other. Liquid-liquid distribution is an equilibrium process and can be
explained by the following equation (Bertuol, Tanabe, Meili, & Veit, 2015),
40 M. SETHURAJAN ET AL.
MaqnþþnHR $MRnþnH
aqþ(17)
where HR - organic solvent, MR
n
- metal-organic species (extracted), H
þ
-
proton released by the organic solvent in exchange for the cationic metal
species M
nþ
. Various factors such as (1) selection of suitable organic
extractant (2) selection of proper diluent and (3) pH can affect the solvent
extraction efficiency. Also, lixiviant (e.g., Cl) used to leach, could also play
an important role in the SX recovery efficiency of CRMs. For instance, In
recovery from mild acidic HCl leachate was found to be better than mild
acidic H
2
SO
4
or HNO
3
leachate (Kri
stofov
a, Rudnik, & Mi
skufov
a, 2017).
It is known that the extraction efficiency of aliphatic diluents (such as hex-
ane, octane, and Solvent 70), is higher than the polar diluents (such as
cyclohexanone, 1-octanol, and chloroform) (Mohammadi, Forsberg, Kloo,
De La Cruz, & Rasmuson, 2015). In recent times, solvent extraction is
applied to many electronic waste materials like spent LCDs (Yang, Kubota,
Baba, Kamiya, & Goto, 2013) and spent NdFeB magnets (Gergoric, Ekberg,
Steenari, & Retegan, 2017). Bis(2-ethylhexyl) phosphoric acid D2EHPA (or
DEHPA), tributyl phosphate TBP, bis(2,4,4-trimethylpentyl) phosphinic
acid Cyanex 272, or a mixture of different phosphine oxides known as
Cyanex 923 are some of the common extractants used to recover critical
metals (Kri
stofov
a, Rudnik, & Mi
skufov
a, 2017). Yang, Kubota, Baba,
Kamiya, and Goto (2013) investigated the separation of In from the LCD
waste by using di-2-ethylhexyl phosphoric acid (D2EHPA). Indium and
other metals concentration in sample feed were selected based on 10 gram
of LCD waste per 100 mL of lixiviant (HCl or H
2
SO
4
) and contains In
2 mM, Sn 0.2mM, Zn 2.5mM, Cu 2.5 mM, Fe 3 mM and Al 6 mM. It was
observed that, 0.25 M was the optimal D2EHPA concentration that can
selectively recover maximum In from acidic leachate (pH <1.0).
Temperature was also found to influence the extraction of In from the
leachate. When the temperature is lower (<20 C), In extraction efficiency
is higher than In extraction obtained at higher temperature (>20 C)
(Yang, Kubota, Baba, Kamiya, & Goto, 2013). This is due the exothermic
nature of the D2EHPA mediated In extraction from the leachate. Ria~
no
and Binnemans (2015) demonstrated the separation of Nd and Dy by using
ionic liquids in the solvent extraction process. Gergoric, Ekberg, Steenari,
and Retegan (2017) studied the recovery of REE by solvent extraction from
the waste NdFeB magnets leachate by using D2EHPA. The NdFeB magnets
were first sulfated, roasted and leached with water to solubilize the REEs.
The leach liquor contain Nd 9.1 mM, Dy 2.7 mM, Pr 3.2 mM, Gd 0.69 mM,
Co 0.17 mM and B 0.55 mM. Iron concentration was below detectable.
D2EHPA was used as the organic extractant, with a wide range of concen-
trations (0.3, 0.6, 0.9, and 1.2 M). Different diluents such as Solvent 70,
hexane, octane, toluene, 1-octanol, cyclohexanone and chloroform were
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 41
used in order to see the effects of diluents. All the REE (Nd, Dy, Gd and
Pr) were separated as a group and it was also observed that aliphatic
diluents illustrate more extraction efficiency than polar diluents. 0.3 M
D2EHPA in hexane was found to be the best operating condition for max-
imum extraction and separation between heavy REEs (Nd, Dy and Pr) and
light REE (Gd) and 100% stripping was achieved with 2 M or higher HCl.
Petranikova, Herdzik-Koniecko, Steenari, and Ekberg (2017) reported
multi-stage solvent extraction for the recovery of REEs using TBP and
Cyanex 923 dissolved in kerosene, from Ni-MH battery scraps. In the first
step, an extractant consisting of 8% Cyanex 923, 10% TBP, 82% kerosene
was used in order to remove Zn and Fe (with 99.9% efficiency). It was fol-
lowed by three-stages of washing and four stages of stripping. Finally, Al
and REEs were extracted using a mixture of 70% Cyanex 923, 10% TBP,
10% kerosene, 10% 1-Decanol. The final raffinate was characterised by high
purity (>99.9% Ni). Co-extracted Co, Mn and Ni were removed from
organic phase using 0.9 M NaNO
3
and 0.1 M HNO
3
mixture.
Figure 12. Some common cations and anions constituents of ionic liquids.
42 M. SETHURAJAN ET AL.
4.2.3. Critical and precious metals and REE recovery by adsorption
Adsorption is one of the most effective methods among the metal recovery
strategies due to its simplicity and wide-range availability (Fujiwara,
Ramesh, Maki, Hasegawa, & Ueda, 2007; Anastopoulos, Bhatnagar, &
Lima, 2016). Adsorption is an alternative process, if the metal concentra-
tion in the leachate is sufficiently low (Cunha et al., 2015, Zazycki, Tanabe,
Bertuol, & Dotto, 2017, Kucuker, 2018).
Adsorption studies have mainly been focusing on the removal of heavy
metal ions from industrial effluents, being the detoxification of these solu-
tions prior to disposal as the primary goal (Volesky, 2007). On the other
hand, adsorption technique has been using for the recovery of precious
metal from aqueous solution since 1951 (McQuiston & Chapman, 1951;
Syed, 2012). Until now, there has been a growing tendency to introduce
new developments to this process for metal recovery from primary and sec-
ondary solutions (Syed, 2012). However, limited research has been carried
out on the metal recovery from secondary sources using adsorption pro-
cess. Mechanism and kinetic of adsorption of PMs and REEs from leachate
have been investigated by researchers in batch mode using a number of
parameters that can potentially influence the efficiency of the adsorption
process: namely, pH, temperature, initial metal concentration, time and agi-
tation rate (Syed, 2012; Kucuker, Nadal, & Kuchta, 2016). Syed (2012), Das
and Das (2013), Jha et al. (2016) and Anastopoulos, Bhatnagar, and Lima
(2016) summarized the published literature (1995–2016) on the use of sorb-
ents for REEs and PMs adsorption. According to the literature survey,
recovery of precious metals and rare earth elements from leachates through
adsorption is a promising approach. However, further research should also
focus on the development of adsorption aspect to generate operational and
cost data with the ultimate aim of commercialization.
4.2.4. Critical and precious metals and REE recovery by ionic liquids
A fascinating development in the hydrometallurgy of metals is the use of
ionic liquids (IL) to perform or enhance that extraction. Some of the more
common cations and anions used are presented in Figure 12.
Ionic liquids are environmentally friendly solvents with favourable prop-
erties such as extremely low vapour pressure, low combustibility, excellent
thermal stability, and a wide temperature range in its liquid state. The low
volatility and combustibility of ionic liquids (ILs) together with the high
extractability presented in many cases make its use in extraction methods a
promising approach (Sun, Luo, & Dai, 2013; Park et al., 2014).
In many instances IL were used as mere solvents trying to improve the
conditions of which can be called classical extractants. A comprehensive,
but not exhaustive, list of these studies follows. They used N,N-
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 43
dioctyldiglycol amic acid (DODGAA) as extractant and 1-octyl-3-methyli-
midazolium bis(trifluoromethyl sulfonyl) imide, [C
8
mim][NTf
2
] as solvent
(Yang et al., 2012); triphosphine trioxide as extractant and [EBPip][NTf
2
]
or [EOPip][NTf
2
] as solvents (Turgis et al., 2016); octyl(phenyl)-N,N-diiso-
butylcarbamoylmethyl phosphine oxide (CMPO) as an extractant and
[Bmim][PF
6
] or [Bmim][NTf
2
] as solvents (Nakashima, Kubota,
Maruyama, & Goto, 2005); choline hexafluoroacetylacetonate as extractant
and choline bis(trifluoromethylsulfonyl)imide as solvent (Onghena, Jacobs,
Van Meervelt, & Binnemans, 2014); di(2-ethylhexyl)phosphoric acid
(HDEHP) as an extractant and various imidazolium ILs or one pyrrolidi-
nium as solvents for the separation among lanthanides (Sun, Bell, Luo, &
Dai, 2011). The selective recovery of metals by ionic liquids depend on dif-
ferent extraction parameters like the metal loading in the feed phase, per-
centage of water in the feed solution, equilibration time, and type of
hydrated melt (Rout, Kotlarska, Dehaen, & Binnemans, 2013).
Yang, Kubota, Baba, Kamiya, and Goto (2013) studied the use of
DODGAA as an extractant and [C
4
mim][NTf
2
] as a solvent for the separ-
ation of REE from other metals in fluorescent lamps phosphors leachate
(Yang, Kubota, Baba, Kamiya, & Goto, 2013). Another hydrometallurgical
process using the undiluted ionic liquid trihexyl(tetradecyl)phosphonium
chloride for the separation of the transition metals iron, cobalt, copper,
manganese and zinc from the rare earths neodymium and samarium was
studied (van der Hoogerstraete, Wellens, Verachtert, & Binnemans, 2013).
Dialkylphosphate ionic liquids were also proposed to separate Nd from
nitric acid leached magnet leachate (Rout, Kotlarska, Dehaen, &
Binnemans, 2013). Ria~
no and Binnemans (2015) investigated the selective
recovery of Nd and Dy (from waste magnets) by using a combination of
the ionic liquid trihexyl(tetradecyl)phosphonium nitrate and a selective
complexing agent ethylenediaminetetraacetic acid (EDTA).
Also the recovery of REEs from metal hydride batteries was addressed in
few instances (Larsson & Binnemans, 2014; van der Hoogerstraete &
Binnemans, 2014). By using trihexyl(tetradecyl)phosphonium chloride
(Cyphos IL 101) or tricaprylmethylammonium chloride (Aliquat 336) ionic
liquids (Larsson & Binnemans, 2014), it is possible to separate cobalt, man-
ganese, iron and zinc from REE’s. By using trihexyl(tetradecyl)phospho-
nium nitrate, it was demonstrated to achieve good separations between Co/
Sm and Ni/La (van der Hoogerstraete & Binnemans, 2014).
A more recent approach is the use of bifunctional ionic liquid extrac-
tants. Cations and anions of well-known extractants were modified in order
to enhance the ionic liquid properties of those extractants. Yang et al.
(2012) proposed that bifunctional ionic liquid Aliquat336 could extract
more than 95% of REE from a very acidic solution. Modification of other
44 M. SETHURAJAN ET AL.
industrial extractants like di(2-ethylhexyl)phosphoric acid (HDEHP) and 2-
ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) could
produce a typical acid-base coupling bifunctionalized IL’s which provide a
good separation between early and late REEs (Sun & Waters, 2014).
Bifunctional ionic liquid extractant (bif-ILE) [A336][P507] was also pro-
posed for the extraction of mid-heavy rare earths elements (REEs) (Shen
et al., 2016).
4.2.5. Critical and precious metals and REE recovery by electrowinning
Electrowinning (EW) is also one of the efficient technologies that helps
to recover metals from the metal containing solutions or leachates.
Selective recovery of the target metal is one of the main advantages of
electrowinning. EW has some other merits such as (1) less or no sec-
ondary waste generation, (2) no hazardous chemical usage and (3) com-
paratively lesser investment cost. EW technology was successfully applied
to electronic scraps for the selective recovery of base metals such as Cu
and Pb (Mecucci & Scott, 2002; Madeno
glu, 2005). However, EW tech-
nology application on the selective recovery of critical and precious met-
als is still in its early stages. Selective separation of precious metals (Au)
by EW is challenging especially in presence of Cu (Grosse, Dicinoski,
Shaw, & Haddad, 2003). However, Chehade et al. (2012) demonstrated
on the selective separation of pure Cu, Ag, Au and Pd from the PCBs
(containing (wt %) Cu 18.49%, Au 0.04%, Ag 0.16%, Pd 0.01%, Cu
0.06 gL
1
, Cd 0.04 gL
1
). The PCBs were first digested using aqua-regia
and then electrowinning was applied to the leachate. Four sequential
EW chambers were used and in each chambers one metal was electro
deposited on the cathode. Copper was the first to selectively recover by
this technology, followed by gold, palladium and finally silver. A max-
imum of 0.04 kg of Au, 0.18kg of Ag, 0.01kg of Pd and 21.00 kg of Cu
was recovered from 125 kg of PCBs (Chehade et al., 2012).
5. Techno-economic feasibility of hydrometallurgy of WEEE
Pyro-metallurgical recycling of WEEE were demonstrated and integrated
at commercial level (Ebin & Isik, 2017). Mobile plant for WEEE treat-
ment by a full hydro-metallurgical process exists as well, but it has not
been implemented on an industrial scale yet (Zeng, Li, & Singh, 2014;
Innocenzi, De Michelis, & Vegli
o, 2017). Innovative development of a
sustainable hydrometallurgical process requires the application of know-
ledge and experience gained in a variety of chemical processing steps
and economic and environmental evaluation of many parameters.
Current recycling technologies adopted by mobile recycling plants do
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 45
not permit to reach an economic advantage for all the valuable materials
coming from WEEEs, especially if recycling plants are focused on a par-
ticular waste stream or product (Innocenzi, De Michelis, & Vegli
o, 2017;
De Michelis & Kopacek, 2018). Nevertheless as already stated, the main
driving force for WEEE recycling is the recovery of metals.
Unfortunately this may not be feasible due to economic reasons and
technological limitations.
The recyclability of a metal can be determined by the “contribution
score”of the individual metal which is related to weight content, environ-
mental hazards associated with the metal, energy consumption, natural
resources depletion, etc. The most widely used assessment index is the
resource recovery efficiency (RRE) (Legarth et al., 1995) which compares
different metals based on their weight content, recycling efficiency and
world reserves and is expressed as:
RRE ¼XiEiFi=Pi
ðÞ
Ci=Ri
ðÞ
XiEiFi=Ri(18)
Where E is the recovery percentage, F is the amount of resource/ton of
scrap, P and C are the annual production and consumption of the primary
resource respectively, R is the world reserve of the resource, and i counts
the type of the resources in the scrap.
In order to determine the environmental performance, Huisman devel-
oped the QWERTY index (Quotes for environmentally Weighted
RecyclabiliTY) (Huisman, Boks, & Stevels, 2003) for calculating product
recyclability defined in equation (14):
QWERTYi¼XiEVWactual;iEVWmax;i
ðÞ
=EVWmin EVWmax
ðÞ
(19)
Where, EVW
actual
,
i
is the actual environmental impact for the weight of
material i, EVW
max
,
i
is the maximum environmental impact for the weight
of material i, EVW
min
and EVW
max
are the total defined minimum and
maximum environmental impact for the complete product, respectively.
Based on the above mentioned two approaches, Le, Yamasue, Okumura,
and Ishihara (2013) developed the Model for Evaluating Metal Recycling
Efficiency from Complex Scraps (MEMRECS) for prioritizing the selection
of target metals. This approach not only includes the weight of each metal
fraction but also comprises two critical aspects associated with sustainable
issue: natural resources conservation and environmental impact reduction.
According to these models, the recovery priority should be on precious
metals such as Au, Ag and Pd along with some base metals such as Cu, Sn,
and Ni.
In the light of these considerations, an economic evaluation of hydrome-
tallurgical processing routes and the techno-economical assessment has to
be reached through the development of gold amount variation models,
46 M. SETHURAJAN ET AL.
present in waste material, as a crucial economical component of PCBs.
Other parameters to be evaluated are the total capital cost and operating
cost, involving all economic factors in final executive summary, regarding
total plant direct costs, total plant indirect costs, labor, utilities and raw
material costs. In this way, using proper algorithms it is possible to assess
the operational time, needed to achieve economical sustainability of the
hypothetical hydrometallurgical plant.
This set of considerations are fundamental in setting the benchmark for
metal recycling strategy, and it is also helpful in technological selection or
technological improvement for metal recycling from waste PCBs in particu-
lar and scraps containing various metal fractions in general.
The future of the WEEE treatment industry will require manufacturing
firms to be highly agile enterprises, capable of exploiting rapid market
changes by increasing flexibility in their physical infrastructures and pro-
duction processes.
In this field, noteworthy is the PCRec project (www.pcrec-network.eu),
funded by the European Institute of Technologies and currently in progress
in Europe: it has been built a network of infrastructures conceived as a
response to the present necessity to overcome the limited capacity of any
single research groups to face small to medium enterprises (SME’s) com-
plex innovation needs and to maximize synergetic collaboration between
research infrastructures and enterprises. The goal is to strengthen the over-
all capacity and to respond to the present and upcoming innovation needs
and improve the exploitation of European secondary resources from
Hi-tech EoL products. Also, in the European FP7 framework HydroWEEE-
demo project, a mobile plant was designed and demonstrated for the recov-
ery of metals from WEEE (Innocenzi, De Michelis, & Vegli
o, 2017). The
results showed the process could be commercially feasible for PCBs. And
for REEs, net gain could be positive only when the market price of RE con-
centrate increase from 14 e/kg (price in 2017) to 20 e/kg or more and also
the plant works at its highest capacity (Innocenzi, De Michelis, &
Vegli
o, 2017).
6. Conclusions and research needs
Huge loads of electronic wastes are generated and discarded in the environ-
ment. There are different types of WEEE (spent PCBs, spent LCDs, spent
LEDs, spent batteries, spent magnets and spent fluorescent lamps) that con-
tain different REE, critical and precious metals in significant concentra-
tions. Apart from metal values, WEEE also contain toxic elements and
harmful pollutants. Leaching and selective recovery of the heavy metals is
the best solution to meet the growing critical raw materials demands and
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 47
also to reduce the environmental impacts caused by the WEEEs in the
environment. There are a number of leaching processes suggested by vari-
ous researchers for distinctly different WEEE and also different metal
recovery techniques that have been demonstrated to be successful for the
recovery of REE, critical and precious metals from the WEEE leachates.
Elemental composition of the WEEE and the metals targeted plays an
important role in the selection of appropriate hydrometallurgical processes.
Good understanding on the bulk chemical composition of the WEEE,
knowledge on the leaching strategies and the understanding of the metal
recovery process will help to use the end of life WEEEs as alternative for
critical raw materials. WEEE is not only heterogeneous in nature, but also
significantly changed in the elemental composition (due to update and cost
cutting in manufacturing techniques). Pre-treatment is the foremost step to
recycle the WEEE and care must be taken not to lose much of the valuable
CRMs and to remove as much as possible hazardous and non-economic
parts of WEEE. For instance, Hg removal in case of waste LCDs and plastic
and filter dusts removal in case of waste PCBs. This impedes the recovery
of metals by conventional metallurgical operations and compel for an
updating of the existing technologies.
The most common leaching agents for dissolution of critical metals REE
from WEEE including mineral acids and organic acids and cyanide, aqua-
regia, thiosulfate and thiourea were proposed for precious metals. Sulfuric
acid was found to be the best leachant for REE and critical metals.
Selection of lixiviant also plays a role in the recovery efficiency of REE
from the leachates. For instance, solvent extraction of critical metals from
HCl system is better than H
2
SO
4
/HNO
3
systems, while selective precipita-
tion of REE is better in H
2
SO
4
than in HCl system. Optimization of leach-
ing parameters such as lixiviant concentration, temperature, pulp density,
agitation and particle size are very important to achieve maximum leaching
efficiency. In some cases, (1) mixture of lixiviants, (2) addition of oxidis-
ing/reducing agents and (3) multi-step leaching could increase the leach-
ing efficiency.
In the case of precious metals, aqua regia allows the highest gold dissol-
ution rate among different leaching agents, but it is applied usually at
laboratory scale because of the strong oxidation and high-corrosion power
which limits its industrial applications. Furthermore, management of the
highly acidic wastewaters is very difficult. Over the last century, cyanide
leaching has been widely used to recover gold from gold minerals and sec-
ondary sources its high efficiency and relatively low cost. The main draw-
back of this method is the production of a huge amount of cyanide
contaminated wastewaters, which can lead to a serious damage to people
and the environment: for this reason, this method is being gradually
replaced by other methods.
48 M. SETHURAJAN ET AL.
Another important issue to address is the upscaling of the new leaching
systems at industrial scale. For instance, stability of the leaching system and
PM-complexes are determinant for its implementation at large scale. The
presence of Fe(III) and Cu(II) catalysts during PM leaching using thiourea
and thiosulfate, respectively, provokes a rapid oxidation of the ligands and,
thus, an increase of the reagent consumption. Therefore, despite the intense
research, which occurred in the last three decades on the study of non-
cyanide lixiviants (including mainly thiourea, thiosulfate and halides) for
extracting Au and other PMs, unless halides, all other alternatives are
clearly more complex to operate at industrial scale than cyanide. Thus, fur-
ther development is needed before they can be considered as real alterna-
tives when thinking in a future commercial implementation.
In the last step, various methodologies are available for the recovery of
the metals from the leachate such as precipitation, adsorption, solvent
extraction, electrowinning and ionic liquids. Selective recovery is one of the
important aspects to consider and techniques such as precipitation, ionic
liquids can offer such advantage. While precipitation could generate sec-
ondary sludge production, adsorption and electro-winning can be used to
overcome this issue. Single stage operation still has some limitations and
might not solve all problems because WEEE is a complex matrix. Some
combined and integrated recovery technologies have begun to be
put forward.
In conclusion, the reason for preferring hydrometallurgy over pyrometal-
lurgy is because of reduced gas emission compared to pyro process which
releases toxic gases (dioxins/furans) and volatile metals, dust, Cl
2
,Br
2
,SO
2
and CO
2
together with Pb, Hg, Cr
6þ
, Cd and flame retardants. Moreover
lower dust emissions are generated and energy consumption is reduced.
Other advantages are the high recovery rate, no slag generation and easy
working condition. In hydrometallurgical processes, a large amount of
liquid wastes and sludge are produced and must be disposed carefully.
Another drawback is represented by the slow leaching kinetics.
Earliest cost benefit analysis show encouraging results on the prices and
the net gain (for instance, REE from spent fluorescent lamps), however
rigorous techno-economic feasibility studies and demonstration are needed
prior the commercial implementation.
Acknowledgments
The authors would like to thank the financial support provided by the European network
for innovative recovery strategies of rare earth and other Critical metals from electrical and
electronic waste (RECREEW), COST action program. M. Sethurajan and E.D. van
Hullebusch thank the Experienced Water Postdoc Fellowship COFUND Programme (FP7-
PEOPLE-2013-COFUND). J.P. Leal and T.G. Almeida thank Fundac¸~
ao para a Ci^
encia e a
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 49
Tecnologia for financial support under projects ENVIREE (ERA-MIN/0002/2014), REEuse
(PTDC/QEQ-EPR/1249/2014) and C2TN (UID/Multi/04349/2013). A. Akcil and H. Deveci
thank TUBITAK and SDU BAPYB for financial support under projects INTENC/113Y011,
116M012 and 4957-D2-17. Isabel F.F. Neto and Helena M.V.M. Soares thank the financial
support with reference LAQV (UID/QUI/50006/2013 - POCI/01/0145/FEDER/007265)
from FCT/MEC through national funds and co-financed by FEDER, under the Partnership
Agreement PT2020. Isabel F.F. Neto acknowledges a grant scholarship (SFRH/BD/87299/
2012) financed by FCT.
ORCID
Ata Akcil http://orcid.org/0000-0002-9991-0543
References
Akcil, A., Erust, C., Gahan, C. S., Ozgun, M., Sahin, M., & Tuncuk, A. (2015). Precious
metal recovery from waste printed circuit boards using cyanide and non-cyanide lixi-
viants-a review. Waste Management,45, 258–271. https://doi.org/10.1016/j.wasman.2015.
01.017
Anastopoulos, I., Bhatnagar, A., & Lima, E. C. (2016). Adsorption of rare earth metals: A
review of recent literature. Journal of Molecular Liquids,221, 954–962. https://doi.org/
10.1016/j.molliq.2016.06.076
Arslan, F., & Sayiner, B. (2008). Extraction of gold and silver from Turkish gold ore by
ammoniacal thiosulfate leaching. Mineral Processing and Extractive Metallurgy Review,
29(1), 68–82. https://doi.org/10.1080/08827500601141784
Askari, A., Ghadimzadeh, A., Gomes, C., & Ishak, M. B. (2014). E-waste management:
towards an appropriate policy. European Journal of Business and Management 6 (1),
37–46.
Balde, C. P., Wang, F., Kuehr, R., & Huisman, J. (2015). The global e-waste monitor 2014:
Quantities, flows and resources. Tokyo & Bonn: United Nations University, 1–79.
Becker, K., Chmielarz, A., Szołomicki, Z., Gotfryd, L., Piwowo
nska, J., Pietek, G., & Pokora,
M. (2016). Hydrometalurgiczny recykling akumulator
ow Ni-MH i Li-ion. Rudy i Metale
Nie_
zelazne Recykling,61 (6), 235–243. doi:10.15199/67.2016.6.1
Behnamfard, A., Salarirad, M. M., & Veglio, F. (2013). Process development for recovery of
copper and precious metals from waste printed circuit boards with emphasize on palla-
dium and gold leaching and precipitation. Waste Management,33(11), 2354–2363.
https://doi.org/10.1016/j.wasman.2013.07.017
Bertuol, D. A., Tanabe, E. H., Meili, L., & Veit, H. M. (2015). Hydrometallurgical process-
ing. In Electronic waste (pp. 61–71). Cham, Germany: Springer International Publishing.
https://doi.org/10.1007/978-3-319-15714-6_7
Bigum, M., Brogaard, L., & Christensen, T. H. (2012). Metal recovery from high-grade
WEEE: a life cycle assessment. Journal of Hazardous Materials,207,8–14. https://doi.
org/10.1016/j.jhazmat.2011.10.001
Binnemans, K., Jones, P. T., Blanpain, B., VanGerven, T., Yang, Y., Walton, A., & Buchert,
M. (2013). Recycling of rare earths: a critical review. Journal of Cleaner Production,51,
1–22. https://doi.org/10.1016/j.jclepro.2012.12.037
50 M. SETHURAJAN ET AL.
Birloaga, I., De Michelis, I., Ferella, F., Buzatu, M., & Veglio, F. (2013). Study on the influ-
ence of various factors in the hydrometallurgical processing of waste printed circuit
boards for copper and gold recovery. Waste Management,33(4), 935–941. https://doi.
org/10.1016/j.wasman.2013.01.003
Birloaga, I., Coman, V., Kopacek, B., & Veglio, F. (2014). An advanced study on the hydro-
metallurgical processing of waste computer printed circuit boards to extract their valu-
able content of metals. Waste Management,34(12), 2581–2586. https://doi.org/10.1016/j.
wasman.2014.08.028
Birloaga, I., & Veglio, F. (2016). Study of multi-step hydrometallurgical methods to extract
the valuable content of gold, silver and copper from waste printed circuit boards. Journal
of Environmental Chemical Engineering,4(1), 20–29. https://doi.org/10.1016/j.jece.2015.
11.021
Buchert, M., Manhart, A., Bleher, D., & Pingel, D. (2012). Recycling critical raw materials
from waste electronic equipment. Freiburg:
€
Oko-Institut eV,49,30–40.
Buchert, M., Sch€
uler, D., Bleher, D., & Programme des Nations Unies pour
l’environnement. (2009). Critical metals for future sustainable technologies and their recy-
cling potential. Darmstadt, Germany: UNEP DTIE; €
Oko-Institut.
Camelino, S., Rao, J., Padilla, R. L., & Lucci, R. (2015). Initial studies about gold leaching
from printed circuit boards (PCB’s) of waste cell phones. Procedia Materials Science,9,
105–112.
Chagnes, A., & Pospiech, B. (2013). A brief review on hydrometallurgical technologies for
recycling spent lithium-ion batteries. Journal of Chemical Technology and Biotechnology,
88(7), 1191–1199. https://doi.org/10.1002/jctb.4053
Chancerel, P., Meskers, C. E., Hagel€
uken, C., & Rotter, V. S. (2009). Assessment of precious
metal flows during preprocessing of waste electrical and electronic equipment. Journal of
Industrial Ecology,13(5), 791–810. doi:10.1111/j.1530-9290.2009.00171.x
Chehade, Y., Siddique, A., Alayan, H., Sadasivam, N., Nusri, S., & Ibrahim, T. (2012,
March). Recovery of gold, silver, palladium, and copper from waste printed circuit
boards. In Proceedings of the International Conference on Chemical, Civil and
Environment Engineering (ICCEE) (pp. 24–25). Dubai, United Arab Emirates.
Chen, W. T., Tsai, L. C., Tsai, F. C., & Shu, C. M. (2012). Recovery of gallium and arsenic
from gallium arsenide waste in the electronics industry. CLEAN-Soil, Air, Water,40(5),
531–537. doi:10.1002/clen.201100216
Contestabile, M., Panero, S., & Scrosati, B. (2001). A laboratory-scale lithium-ion battery
recycling process. Journal of Power Sources,92(1), 65–69. https://doi.org/10.1016/S0378-
7753(00)00523-1
CREE Inc. (2015). Fluorescent Lamps Construction. url: http://image.slidesharecdn. com/
creegenltgtraining-12619621707885-phpapp02/95/cree-gen-ltg-training-4028.
jpg?cb¼1261940621 (visited on 12/08/2015).
Cui, J., & Forssberg, E. (2003). Mechanical recycling of waste electric and electronic equip-
ment: a review. Journal of Hazardous Materials,99(3), 243–263. https://doi.org/10.1016/
S0304-3894(03)00061-X
Cui, J. R., & Zhang, L. F. (2008). Metallurgical recovery of metals from electronic waste: A
review. Journal of Hazardous Materials,158(2–3), 228–256. https://doi.org/10.1016/j.
jhazmat.2008.02.001
Cunha, J. M., Klein, L., Bassaco, M. M., Tanabe, E. H., Bertuol, D. A., & Dotto, G. L.
(2015). Cobalt recovery from leached solutions of lithiumeion batteries using waste mate-
rials as adsorbents. The Canadian Journal of Chemical Engineering,93, 2198–2204. doi:
10.1002/cjce.22331
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 51
Das, N., & Das, D. (2013). Recovery of rare earth metals through biosorption: an overview.
Journal of Rare Earths,31(10), 933–943. https://doi.org/10.1016/S1002-0721(13)60009-5
Dalrymple, I., Wright, N., Kellner, R., Bains, N., Geraghty, K., Goosey, M., & Lightfoot, L.
(2007). An integrated approach to electronic waste (WEEE) recycling. Circuit World
33(2), 52–58. http://dx.doi.org/10.1108/03056120710750256.
De Michelis, I., Ferella, F., Varelli, E. F., & Vegli
o, F. (2011). Treatment of exhaust fluores-
cent lamps to recover yttrium: Experimental and process analyses. Waste Management,
31(12), 2559–2568. https://doi.org/10.1016/j.wasman.2011.07.004
De Michelis, I., & Kopacek, B. (2018). Hydro WEEE project: Design and construction of
a mobile demonstration plant. In: Waste Electrical and Electronic Equipment Recycling
(pp. 357–383). Woodhead Publishing. https://doi.org/10.1016/B978-0-08-102057-9.00013-5
de Oliveira, C. R., Bernardes, A. M., & Gerbase, A. E. (2012). Collection and recycling of
electronic scrap: A worldwide overview and comparison with the Brazilian situation.
Waste Management,32(8), 1592–1610. https://doi.org/10.1016/j.wasman.2012.04.003
Directive, E. C. (2012). Directive 2012/19/EU of the European Parliament and of the
Council of 4 July 2012 on waste electrical and electronic equipment, WEEE. Official
Journal of the European Union L,197,38–71.
Dodbiba, G., Nagai, H., Wang, L. P., Okaya, K., & Fujita, T. (2012). Leaching of indium
from obsolete liquid crystal displays: Comparing grinding with electrical disintegration
in context of LCA. Waste Management,32(10), 1937–1944. https://doi.org/10.1016/j.
wasman.2012.05.016
Duan, C., Wen, X., Shi, C., Zhao, Y., Wen, B., & He, Y. (2009). Recovery of metals from
waste printed circuit boards by a mechanical method using a water medium. Journal of
Hazardous Materials,166(1), 478–482. https://doi.org/10.1016/j.jhazmat.2008.11.060
Duflou, J. R., Seliger, G., Kara, S., Umeda, Y., Ometto, A., & Willems, B. (2008). Efficiency
and feasibility of product disassembly: A case-based study. CIRP Annals,57(2), 583–600.
https://doi.org/10.1016/j.cirp.2008.09.009
Dupont, D., & Binnemans, K. (2015a). Rare-earth recycling using a functionalized ionic
liquid for the selective dissolution and revalorization of Y
2
O
3
:Eu
3þ
from lamp phosphor
waste. Green Chemistry,17(2), 856–868. doi:10.1039/C4GC02107J
Dupont, D., & Binnemans, K. (2015b). Recycling of rare earths from NdFeB magnets using
a combined leaching/extraction system based on the acidity and thermomorphism of the
ionic liquid [Hbet][Tf 2 N]. Green Chemistry,17(4), 2150–2163. doi:10.1039/
C5GC00155B
Ebin, B., & Isik, M. I. (2017). Pyrometallurgical processes for the recovery of metals from
WEEE. In WEEE Recycling (pp. 107–137). Elsevier.
Elo, K., & Sundin, E. (2014). Process concepts for semi-automatic dismantling of LCD tele-
visions. Procedia CIRP,23, 270–275. https://doi.org/10.1016/j.procir.2014.10.104
Er€
ust, C., Akcil, A., Gahan, C. S., Tuncuk, A., & Deveci, H. (2013). Biohydrometallurgy of
secondary metal resources: a potential alternative approach for metal recovery. Journal of
Chemical Technology and Biotechnology,88(12), 2115–2132. https://doi.org/10.1002/jctb.
4164
Eswaraiah, C., Kavitha, T., Vidyasagar, S., & Narayanan, S. S. (2008). Classification of met-
als and plastics from Printed Circuit Boards (PCB) using air classifier. Chemical
Engineering and Processing,47, 565–576. https://doi.org/10.1016/j.cep.2006.11.010
European Commission. (2008). Commission staff working paper accompanying the pro-
posal for a directive of the European Parliament and of the Council on waste electrical
and electronic equipment (WEEE) (recast) - Impact Assessment.
52 M. SETHURAJAN ET AL.
European Commission. (2014). Report on critical raw materials for the EU, Report of the
Adhoc Working Group on defining critical raw materials.
European Commission. (2017). Communication from the Commission to the European
Parliament, the Council, the European Economic and Social Committee and the
Committee of the Regions-A European Agenda for the collaborative economy. Brussel.
European Union statistics. (2017). Waste statistics - electrical and electronic equipment.
Retrieved from http://ec.europa.eu/eurostat/statistics-explained/index.php/Waste_statis-
tics_-_electrical_and_electronic_equipment#Further_Eurostat_information. March 23,
2018.
Ficeriova, J., Balaz, P., & Gock, E. (2011). Leaching of gold, silver and accompanying metals
from circuit boards (PCBs) waste. Acta Montanistica Slovaca,16(2), 128–131.
Fujiwara, K., Ramesh, A., Maki, T., Hasegawa, H., & Ueda, K. (2007). Adsorption of plat-
inum (IV), palladium (II) and gold (III) from aqueous solutions on l-lysine modified
cross linked chitosan resin. Journal of Hazardous Materials,46,39–50. https://doi.org/
10.1016/j.jhazmat.2006.11.049
Galbraith, P., & Devereux, J. L. (2002). Beneficiation of printed wiring boards with gravity
concentration. In IEEE International Symposium on Electronics & the Environment, 6–9
May, 242–248.
Gallegos-Acevedo, P. M., Espinoza-Cuadra, J., & Olivera-Ponce, J. M. (2014). Conventional
flotation techniques to separate metallic and nonmetallic fractions from waste printed
circuit boards with particles nonconventional size. Journal of Mining Science,50(5),
974–981. https://doi.org/10.1134/S1062739114050172
Gergoric, M., Ekberg, C., Steenari, B. M., & Retegan, T. (2017). Separation of heavy rare-
earth elements from light rare-earth elements via solvent extraction from a neodymium
magnet leachate and the effects of diluents. Journal of Sustainable Metallurgy,3(3),
601–610. https://doi.org/10.1007/s40831-017-0117-5
Ghosh, B., Ghosh, M. K., Parhi, P., Mukherjee, P. S., & Mishra, B. K. (2015). Waste printed
circuit boards recycling: An extensive assessment of current status. Journal of Cleaner
Production,94,5–19. https://doi.org/10.1016/j.jclepro.2015.02.024
G€
okelma, M., Birich, A., Stopic, S., & Friedrich, B. (2016). A review on alternative gold
recovery re-agents to cyanide. Journal of Materials Science and Chemical Engineering,
4(8), 8–17. doi:10.4236/msce.2016.48002
Golmohammadzadeh, R., Rashchi, F., & Vahidi, E. (2017). Recovery of lithium and cobalt
from spent lithium-ion batteries using organic acids: Process optimization and kinetic
aspects. Waste Management,64, 244–254. https://doi.org/10.1016/j.wasman.2017.03.037
Goosey, M., & Kellner, R. (2002). A Scoping Study: End-of-Life Printed Circuit Boards,
Intellect and the Department of Trade and Industry, Makati City, 44 s.
Grosse, A. C., Dicinoski, G. W., Shaw, M. J., & Haddad, P. R. (2003). Leaching and recov-
ery of gold using ammoniacal thiosulfate leach liquors (a review). Hydrometallurgy,
69(1), 1–21. https://doi.org/10.1016/S0304-386X(02)00169-X
Gurung, M., Adhikari, B. B., Kawakita, H., Ohto, K., Inoue, K., & Alam, S. (2013).
Recovery of gold and silver from spent mobile phones by means of acidothiourea leach-
ing followed by adsorption using biosorbent prepared from persimmon tannin.
Hydrometallurgy,133,84–93. https://doi.org/10.1016/j.hydromet.2012.12.003
Hageluken, C. (2006). Improving metal returns and eco-efficiency in electronics recycling-a
holistic approach for interface optimisation between pre-processing and integrated metals
smelting and refining, IEEE International Symposium on Electronics & the
Environment, May 8, 2011, San Francisco, pp. 218–233.
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 53
He, Y. C., & Xu, Z. M. (2015). Recycling gold and copper from waste printed circuit boards
using chlorination process. RSC Advances,5(12), 8957–8964. doi:10.1039/C4RA16231E
He, L. P., Sun, S. Y., Mu, Y. Y., Song, X. F., & Yu, J. G. (2016). Recovery of lithium, nickel,
cobalt, and manganese from spent lithium-ion batteries using L-tartaric acid as a leach-
ant. ACS Sustainable Chemistry & Engineering,5(1), 714–721. doi:10.1021/
acssuschemeng.6b02056
Hennebel, T., Boon, N., Maes, S., & Lenz, M. (2015). Biotechnologies for crucial raw mater-
ial recovery from primary and secondary sources: RD priorities and future perspectives.
New Biotechnology,32(1), 121–127. doi:10.1016/j.nbt.2013.08.004
Hu, S. H., Xie, M. Y., Hsieh, Y. M., Liou, Y. S., & Chen, W. S. (2015). Resource recycling
of gallium arsenide scrap using leaching-selective precipitation. Environmental Progress
& Sustainable Energy,34(2), 471–475. doi:10.1002/ep.12019
Huisman, J., Boks, C. B., & Stevels, A. L. N. (2003). Quotes for environmentally weighted
recyclability (QWERTY): concept of describing product recyclability in terms of environ-
mental value. International Journal of Production Research,41(16), 3649–3665. https://
doi.org/10.1080/0020754031000120069
Ilyas, S., & Lee, J. C. (2014). Biometallurgical recovery of metals from waste electrical and
electronic equipment: a review. Chem Bio Eng Reviews,1(4), 148–169. doi:10.1002/
cben.201400001
Imre-Lucaci, A., Nagy, M., Imre-Lucaci, F., & Fogarasi, S. (2017). Technical and environ-
mental assessment of gold recovery from secondary streams obtained in the processing
of waste printed circuit boards. Chemical Engineering Journal,309, 655–662. https://doi.
org/10.1016/j.cej.2016.10.045
Innocenzi, V., & Vegli
o, F. (2012). Recovery of rare earths and base metals from spent
nickel-metal hydride batteries by sequential sulfuric acid leaching and selective precipita-
tions. Journal of Power Sources,211, 184–191. https://doi.org/10.1016/j.jpowsour.2012.
03.064
Innocenzi, V., De Michelis, I., & Vegli
o, F. (2017). Design and construction of an industrial
mobile plant for WEEE treatment: Investigation on the treatment of fluorescent powders
and economic evaluation compared to other e-wastes. Journal of the Taiwan Institute of
Chemical Engineers,80, 769–778
Is¸ıldar, A., van de Vossenberg, J., Rene, E. R., van Hullebusch, E. D., & Lens, P. N. (2017).
Biorecovery of metals from electronic waste. In Sustainable Heavy Metal Remediation
(pp. 241–278). Cham: Springer.
Isıldar, A., Rene, E. R., van Hullebusch, E. D., & Lens, P. N. L. (2017). Two-step leaching
of valuable metals from discarded printed circuit boards, and process optimization using
response surface methodology. Advances in Recycling and Waste Management,2(2), 1–9.
doi:10.4172/2475-7675.1000132
Jha, M. K., Kumari, A., Panda, R., Kumar, J. R., Yoo, K., & Lee, J. Y. (2016). Review on
hydrometallurgical recovery of rare earth metals. Hydrometallurgy,165,2–26. https://
doi.org/10.1016/j.hydromet.2016.01.035
Jing-Ying, L., Xiu-Li, X., & Wen-Quan, L. (2012). Thiourea leaching gold and silver from
the printed circuit boards of waste mobile phones. Waste Management,32(6),
1209–1212. https://doi.org/10.1016/j.wasman.2012.01.026
Joda, N. N., & Rashchi, F. (2012). Recovery of ultra-fine grained silver and copper from PC
board scraps. Separation and Purification Technology,92,36–42. https://doi.org/10.1016/
j.seppur.2012.03.022
54 M. SETHURAJAN ET AL.
Johnson, D. B., & Du Plessis, C. A. (2015). Biomining in reverse gear: Using bacteria to
extract metals from oxidised ores. Minerals Engineering,75,2–5. doi:10.1016/
j.mineng.2014.09.024
Kaya, M. (2016). Recovery of metals and nonmetals from electronic waste by physical and
chemical recycling processes. Waste Management,57,64–90. https://doi.org/10.1016/j.
wasman.2016.08.004
Kim, E. Y., Kim, M. S., Lee, J. C., & Pandey, B. D. (2011). Selective recovery of gold from
waste mobile phone PCBs by hydrometallurgical process. Journal of Hazardous
Materials,198, 206–215. https://doi.org/10.1016/j.jhazmat.2011.10.034
Konyratbekova, S. S., Baikonurova, A., & Akcil, A. (2015). Non-cyanide leaching processes
in gold hydrometallurgy and iodine-iodide applications: A review. Mineral Processing
and Extractive Metallurgy Review,36(3), 198–212. https://doi.org/10.1080/08827508.
2014.942813
Kopacek, B. (2016). Intelligent disassembly of components from printed circuit boards to
enable re-use and more efficient recovery of critical metals, IFAC-PapersOnLine,49(29),
190–195. https://doi.org/10.1016/j.ifacol.2016.11.100
Kri
stofov
a, P., Rudnik, E., & Mi
skufov
a, A. (2017). Hydrometallurgical methods of indium
recovery from obsolete LCD and LED panels. Metallurgy and Foundry Engineering,
42(3), 157. doi:http://dx.doi.org/10.7494/mafe.2016.42.3.157
Kucuker, M. A., Nadal, J. B., & Kuchta, K. (2016). Comparison between batch and continu-
ous reactor systems for biosorption of neodymium (Nd) using microalgae. International
Journal of Plant, Animal and Environmental Sciences,6, 197–203.
Kucuker, M. A. (2018). Biomining Concept for Recovery of Rare Earth Elements (REEs)
from Secondary Sources. Hamburger Berichte; Bd. 48; Verlag Abfall aktuell der
Ingenieurgruppe RUK GmbH, Stuttgart, ISBN 978-3-9817572-8-6.
Larsson, K., Ekberg, C., & Ødegaard-Jensen, A. (2013). Dissolution and characterization of
HEV NiMH batteries. Waste Management,33(3), 689–698. https://doi.org/10.1016/j.was-
man.2012.06.001
Larsson, K., & Binnemans, K. (2014). Selective extraction of metals using ionic liquids for
nickel metal hydride battery recycling. Green Chemistry,16(10), 4595–4603. doi:10.1039/
c3gc41930d
Le, H. L., Yamasue, E., Okumura, H., & Ishihara, K. N. (2013). MEMRECS—a sustainable
view for metal recycling from waste printed circuit boards. Journal of Environmental
Protection,4(8), 803–810. doi:10.4236/jep.2013.48094
Lee, H. S., & Nam, C. W. (1998). A study on the extraction of gallium from gallium arsenide
scrap. Hydrometallurgy,49(1), 125–133. https://doi.org/10.1016/S0304-386X(98)00016-4
Lee, J., Zhang, Q., & Saito, F. (2000). Room temperature extraction of Co and Li from
ground lithium-ion secondary battery scrap. Shigen to Sozai,116 (11), 919–922. https://
doi.org/10.2473/shigentosozai.116.919
Lee, C. K., & Rhee, K. I. (2003). Reductive leaching of cathodic active materials from lith-
ium ion battery wastes. Hydrometallurgy,68(1), 5–10. https://doi.org/10.1016/S0304-
386X(02)00167-6
Lee, C. H., Chang, C. T., Fan, K. S., & Chang, T. C. (2004). An overview of recycling and
treatment of scrap computers. Journal of Hazardous Materials,114(1), 93–100. https://
doi.org/10.1016/j.jhazmat.2004.07.013
Lee, C. H., Tang, L. W., & Popuri, S. R. (2011). A study on the recycling of scrap inte-
grated circuits by leaching. Waste Management & Research,29(7), 677–685. doi:10.1177/
0734242X10380995.
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 55
Lee, C. H., Jeong, M. K., Kilicaslan, M. F., Lee, J. H., Hong, H. S., & Hong, S. J. (2013).
Recovery of indium from used LCD panel by a time efficient and environmentally sound
method assisted HEBM. Waste Management,33(3), 730–734. https://doi.org/10.1016/j.
wasman.2012.10.002
Lee, C. H., Chen, Y. J., Liao, C. H., Popuri, S. R., Tsai, S. L., & Hung, C. E. (2013).
Selective leaching process for neodymium recovery from scrap Nd-Fe-B magnet.
Metallurgical and Materials Transactions A,44(13), 5825–5833. https://doi.org/10.1007/
s11661-013-1924-3
Lee, J. C., & Srivastava, R. R. (2016). Leaching of gold from the spent/end-of-life mobile
phone-PCBs using “greener reagents”.InThe Recovery of Gold from Secondary Sources.
(pp. 7–56). London: Imperial College Press. https://doi.org/10.1142/9781783269907_0002
Legarth, J. B., Alting, L., Danzer, B., Tartler, D., Brodersen, K., Scheller, H., & Feldmann,
K. (1995). A new strategy in the recycling of printed circuit boards. Circuit World,21(3),
10–15. https://doi.org/10.1108/eb044031
Lewis, A., & van Hille, R. (2006). An exploration into the sulphide precipitation method
and its effect on metal sulphide removal, Hydrometallurgy,81(3), 197–204
Lewis, A. E. (2010). Review of metal sulphide precipitation. Hydrometallurgy,104(2),
222–234. https://doi.org/10.1016/j.hydromet.2010.06.010
Li, J., Lu, H., Guo, J., Xu, Z., & Zhou, Y. (2007). Recycle technology for recovering resour-
ces and products from waste printed circuit boards. Environmental Science &
Technology,41(6), 1995–2000. doi:10.1021/es0618245
Li, J., Xu, Z., & Zhou, Y. (2007). Application of corona discharge and electrostatic force
to separate metals and nonmetals from crushed particles of waste printed circuit
boards. Journal of Electrostatics,65(4), 233–238. https://doi.org/10.1016/j.elstat.2006.08.
004
Li, L., Ge, J., Chen, R., Wu, F., Chen, S., & Zhang, X. (2010). Environmental friendly leach-
ing reagent for cobalt and lithium recovery from spent lithium-ion batteries. Waste
Management,30(12), 2615–2621. https://doi.org/10.1016/j.wasman.2010.08.008
Li, Y., Liu, Z., Li, Q., Liu, Z., &Zeng, L. (2011). Recovery of indium from used indium-tin
oxide (ITO) targets. Hydrometallurgy,105(3), 207–212. https://doi.org/10.1016/j.hydro-
met.2010.09.006
Li, L., Qu, W., Zhang, X., Lu, J., Chen, R., Wu, F., & Amine, K. (2015). Succinic acid-based
leaching system: a sustainable process for recovery of valuable metals from spent Li-ion
batteries. Journal of Power Sources,282, 544–551. https://doi.org/10.1016/j.jpowsour.
2015.02.073
Li, J., Wang, G., & Xu, Z. (2016). Environmentally-friendly oxygen-free roasting/wet mag-
netic separation technology for in situ recycling cobalt, lithium carbonate and graphite
from spent LiCoO
2
/graphite lithium batteries. Journal of Hazardous Materials,302,
97–104. https://doi.org/10.1016/j.jhazmat.2015.09.050
Lister, T. E., Wang, P. M., & Anderko, A. (2014). Recovery of critical and value metals
from mobile electronics enabled by electrochemical processing. Hydrometallurgy,149,
228–237. https://doi.org/10.1016/j.hydromet.2014.08.011
Lu, Y., Song, Q., & Xu, Z. (2017). Integrated technology for recovering Au from waste
memory module by chlorination process: Selective leaching, extraction, and distillation.
Journal of Cleaner Production,161,30–39. https://doi.org/10.1016/j.jclepro.2017.05.033
Madeno
glu, H. (2005). Recovery of some metals from electronic scrap (Doctoral disserta-
tion, Ege €
Universitesi).
56 M. SETHURAJAN ET AL.
Marra, A., Cesaro, A., & Belgiorno, V. (2018). Separation efficiency of valuable and critical
metals in WEEE mechanical treatments. Journal of Cleaner Production,186, 490–498.
https://doi.org/10.1016/j.jclepro.2018.03.112
Martino, R., Iseli, C., Gaydardzhiev, S., Streicher-Porte, M., & Weh, A. (2017). Electro
dynamic fragmentation of printed wiring boards as a preparation tool for their recycling.
Minerals Engineering,107,20–26. https://doi.org/10.1016/j.mineng.2017.01.009
McKinsey & Company. (2012). Lighting the Way: Perspectives on the Global Lighting
Market, (2nd ed.). Retrieved from http://www.mckinsey.com//media/mckinsey/dotcom/
clientservice/Automotive%20and%20Assembly/Lighting the way Perspectives on global
lighting market 2012.ashx, March 7, 2015.
McQuiston, F. W., & Chapman, T. G. (1951). Recovery of gold or silver, US Patent,
US2545239 (C01G 5/00).
Mecucci, A., & Scott, K. (2002). Leaching and electrochemical recovery of copper, lead and
tin from scrap printed circuit boards. Journal of Chemical Technology and Biotechnology,
77(4), 449–457. doi:10.1002/jctb.575
Meskers, C., & Hagel€
uken, C. (2009). The impact of different pre-processing routes on the
metal recovery from PCs. http://www. preciousmetals. umicore.com/PMR/Media/escrap/
impactOfDifferentPreprocessing. Pdf (accessed 2017).
Meshram, P., Somani, H., Pandey, B. D., Mankhand, T. R., & Deveci, H. (2017). Two stage
leaching process for selective metal extraction from spent nickel metal hydride batteries.
Journal of Cleaner Production,157, 322–332. https://doi.org/10.1016/j.jclepro.2017.04.144
Mohammadi, M., Forsberg, K., Kloo, L., De La Cruz, J. M., & Rasmuson, Å. (2015).
Separation of Nd (III), Dy (III) and Y (III) by solvent extraction using D2EHPA and
EHEHPA. Hydrometallurgy,156, 215–224. https://doi.org/10.1016/j.hydromet.2015.05.
004
Montero, R., Guevara, A., & dela Torre, E. (2012). Recovery of gold, silver, copper and nio-
bium from printed circuit boards using leaching column technique. Journal of Earth
Science and Engineering,2(10), 590–595.
Murakami, H., Nishihama, S., & Yoshizuka, K. (2015). Separation and recovery of gold
from waste LED using ion exchange method. Hydrometallurgy,157, 194–198. https://
doi.org/10.1016/j.hydromet.2015.08.014
Nakashima, K., Kubota, F., Maruyama, T., & Goto, M. (2005). Feasibility of ionic liquids as
alternative separation media for industrial solvent extraction processes. Industrial &
Engineering Chemistry Research,44(12), 4368–4372. doi:10.1021/ie049050t
Nan, J., Han, D., & Zuo, X. (2005). Recovery of metal values from spent lithium-ion bat-
teries with chemical deposition and solvent extraction. Journal of Power Sources,152,
278–284.
Nan, J., Han, D., Yang, M., Cui, M., & Hou, X. (2006). Recovery of metal values from a
mixture of spent lithium-ion batteries and nickel-metal hydride batteries.
Hydrometallurgy,84(1), 75–80. https://doi.org/10.1016/j.hydromet.2006.03.059
Natarajan, G., Tay, S. B., Yew, W. S., & Ting, Y. P. (2015). Engineered strains enhance
gold biorecovery from electronic scrap. Minerals Engineering,75,32–37. https://doi.org/
10.1016/j.mineng.2015.01.002
Nayaka, G. P., Pai, K. V., Santhosh, G., & Manjanna, J. (2016). Recovery of cobalt as cobalt
oxalate from spent lithium ion batteries by using glycine as leaching agent. Journal of
Environmental Chemical Engineering,4(2), 2378–2383. https://doi.org/10.1016/j.jece.
2016.04.016
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 57
Nayl, A. A., Elkhashab, R. A., Badawy, S. M., & El-Khateeb, M. A. (2014). Acid leaching of
mixed spent Li-ion batteries. Arabian Journal of Chemistry,10, S3632–S3639. https://
doi.org/10.1016/j.arabjc.2014.04.001
NEC Lighting, Ltd. (2015). Fluorescent Lamps and Principles of Light Emission. Company
Website. Retrieved from https://www.nelt.co.jp/english/products/useful/06.html. March
23, 2018.
Neto, I. F. F., Sousa, C. A., Brito, M. S. C. A., Futuro, A. M., & Soares, H. M. V. M.
(2016). A simple and nearly-closed cycle process for recycling copper with high purity
from end life printed circuit boards. Separation and Purification Technology,164,19–27.
https://doi.org/10.1016/j.seppur.2016.03.007
Ogunniyi, I. O., Vermaak, M. K. G., & Groot, D. R. (2009). Chemical composition and lib-
eration characterization of printed circuit board comminution fines for beneficiation
investigations. Waste Management,29(7), 2140–2146. https://doi.org/10.1016/j.wasman.
2009.03.004
Ogunniyi, I. O., & Vermaak, M. K. G. (2009). Investigation of froth flotation for benefici-
ation of printed circuit board comminution fines. Minerals Engineering,22(4), 378–385.
https://doi.org/10.1016/j.mineng.2008.10.007
Oh, C. J., Lee, S. O., Yang, H. S., Ha, T. J., & Kim, M. J. (2003). Selective leaching of valu-
able metals from waste printed circuit boards. Journal of the Air & Waste Management
Association,53(7), 897–902. https://doi.org/10.1080/10473289.2003.10466230
€
Onal, M. A. R., Borra, C. R., Guo, M., Blanpain, B., & Van Gerven, T. (2015). Recycling of
NdFeB magnets using sulfation, selective roasting, and water leaching. Journal of
Sustainable Metallurgy,1(3), 199–215. https://doi.org/10.1007/s40831-015-0021-9
Onghena, B., Jacobs, J., Van Meervelt, L., & Binnemans, K. (2014). Homogeneous liquid-
liquid extraction of neodymium (III) by choline hexafluoroacetylacetonate in the ionic
liquid choline bis (trifluoromethylsulfonyl) imide. Dalton Transactions,43(30),
11566–11578. doi:10.1039/c4dt01340a.
Park, Y. J., & Fray, D. J. (2009). Recovery of high purity precious metals from printed cir-
cuit boards. Journal of Hazardous Materials,164(2–3), 1152–1158. https://doi.org/10.
1016/j.jhazmat.2008.09.043
Park, J., Jung, Y., Kusumah, P., Lee, J., Kwon, K., & Lee, C. K. (2014). Application of ionic
liquids in hydrometallurgy. International Journal of Molecular Sciences,15(9),
15320–15343. doi:10.3390/ijms150915320
Park, S., Kim, S., Han, Y., & Park, J. (2015). Apparatus for electronic component disassem-
bly from printed circuit board assembly in e-wastes. International Journal of Mineral
Processing,144,11–15. https://doi.org/10.1016/j.minpro.2015.09.013
Petranikova, M., Herdzik-Koniecko, I., Steenari, B. M., & Ekberg, C. (2017).
Hydrometallurgical processes for recovery of valuable and critical metals from spent car
NiMH batteries optimized in a pilot plant scale. Hydrometallurgy,171, 128–141. https://
doi.org/10.1016/j.hydromet.2017.05.006
Petter, P. M. H., Veit, H. M., & Bernardes, A. M. (2014). Evaluation of gold and silver
leaching from printed circuit board of cellphones. Waste Management,34(2), 475–482.
https://doi.org/10.1016/j.wasman.2013.10.032
Pietrelli, L., Bellomo, B., Fontana, D., & Montereali, M. (2005). Characterization and leach-
ing of NiCd and NiMH spent batteries for the recovery of metals. Waste Management,
25(2), 221–226. https://doi.org/10.1016/j.wasman.2004.12.013
Porob, D. G., Srivastava, A. M., Nammalwar, P. K., Ramachandran, G. C., & Comanzo,
H. A. (2012). U.S. Patent No. 8,137,645. Washington, DC: U.S. Patent and Trademark
Office.
58 M. SETHURAJAN ET AL.
Priya, A., & Hait, S. (2018). Comprehensive characterization of printed circuit boards of
various end-of-life electrical and electronic equipment for beneficiation investigation.
Waste Management,75, 103–123. https://doi.org/10.1016/j.wasman.2018.02.014
Quinet, P., Proost, J., & Van Lierde, A. (2005). Recovery of precious metals from electronic
scrap by hydrometallurgical processing routes. Minerals & Metallurgical Processing,
22(1), 17–22.
Rabah, M. A. (2008). Recyclables recovery of europium and yttrium metals and some salts
from spent fluorescent lamps. Waste Management,28(2), 318–325. https://doi.org/10.
1016/j.wasman.2007.02.006
Rabatho, J. P., Tongamp, W., Takasaki, Y., Haga, K., & Shibayama, A. (2013). Recovery of
Nd and Dy from rare earth magnetic waste sludge by hydrometallurgical process.
Journal of Material Cycles and Waste Management,15(2), 171–178. https://doi.org/10.
1007/s10163-012-0105-6
Ria~
no, S., & Binnemans, K. (2015). Extraction and separation of neodymium and dyspro-
sium from used NdFeB magnets: an application of ionic liquids in solvent extraction
towards the recycling of magnets. Green Chemistry,17(5), 2931–2942. doi:10.1039/
C5GC00230C
Robinson, B. H. (2009). E-waste: an assessment of global production and environmental
impacts. Science of the Total Environment,408(2), 183–191. https://doi.org/10.1016/j.sci-
totenv.2009.09.044
Rout, A., Kotlarska, J., Dehaen, W., & Binnemans, K. (2013). Liquid-liquid extraction of
neodymium (III) by dialkylphosphate ionic liquids from acidic medium: the importance
of the ionic liquid cation. Physical Chemistry Chemical Physics,15(39), 16533–16541; doi:
10.1039/c3cp52218k
Sahin, M., Akcil, A., Erust, C., Altynbek, S., Gahan, C. S., & Tuncuk, A. (2015). A potential
alternative for precious metal recovery from e-waste: iodine leaching. Separation Science
and Technology,50(16), 2587–2595. https://doi.org/10.1080/01496395.2015.1061005
Sakultung, S., Pruksathorn, K., & Hunsom, M. (2007). Simultaneous recovery of valuable
metals from spent mobile phone battery by an acid leaching process. Korean Journal of
Chemical Engineering,24(2), 272–277. https://doi.org/10.1007/s11814-007-5040-1
Savvilotidou, V., Hahladakis, J. N., & Gidarakos, E. (2015). Leaching capacity of metals-
metalloids and recovery of valuable materials from waste LCDs. Waste Management,45,
314–324. https://doi.org/10.1016/j.wasman.2015.05.025
Schwarz-Schampera, U., & Herzig, P. M. (2002). Indium: Geology, Mineralogy and
Economics. Berlin, Heidelberg, New York: Springer-Verlag.
Scrosati, B., Krebs, A., Beck, M., & Bartels, J. (2007). An update of the portable battery
market and the rechargeable battery collection in Japan. Proceedings of 12th inter-
national Congress for Battery Recycling ICBR, Budapest, Hungary, June 20–22, 27–34.
Serpe, A., Rigoldi, A., Marras, C., Artizzu, F., Mercuri, M. L., & Deplano, P. (2015).
Chameleon behaviour of iodine in recovering noble-metals from WEEE: Towards sus-
tainability and “zero”waste. Green Chemistry,17(4), 2208–2216. doi:10.1039/
C4GC02237H
Sethurajan, M., Lens, P. N. L., Horn, H. A., Figueiredo, L. H. A. & van Hullebusch, E. D.
(2017) Leaching and recovery of metals, In E. R. Rene, E. Sahinkaya, A. Lewis, P. N. L.
Lens (Eds.), Sustainable heavy metal remediation. Vol. 2: Case studies (pp. 161–206).
Springer book. Cham, Germany: Elsevier.
Shen, L., Chen, J., Chen, L., Liu, C., Zhang, D., Zhang, Y., Su, W., & Deng, Y. (2016).
Extraction of mid-heavy rare earth metal ions from sulfuric acid media by ionic liquid
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 59
[A336] [P507]. Hydrometallurgy,161, 152–159. http://dx.doi.org/10.1016/j.hydromet.
2016.01.015
Sheng, P. P., & Etsell, T. H. (2007). Recovery of gold from computer circuit board scrap
using aqua-regia. Waste Management & Research,25(4), 380–383. doi:10.1177/
0734242X07076946
Shin, S. M., Kim, N. H., Sohn, J. S., Yang, D. H., & Kim, Y. H. (2005). Development of a
metal recovery process from Li-ion battery wastes. Hydrometallurgy,79(3), 172–181.
https://doi.org/10.1016/j.hydromet.2005.06.004
Shuva, M. A. H., & Kurny, A. S. W. (2013). Dissolution kinetics of cathode of spent lith-
ium ion battery in hydrochloric acid solutions. Journal of the Institution of Engineers
(India): Series D,94(1), 13–16. https://doi.org/10.1007/s40033-013-0018-0
Silveira, A. V. M., Fuchs, M. S., Pinheiro, D. K., Tanabe, E. H., & Bertuol, D. A. (2015).
Recovery of indium from LCD screens of discarded cell phones. Waste Management,45,
334–342. https://doi.org/10.1016/j.wasman.2015.04.007
Stevens, G. C., & Goosey, M. (2008). Materials used in manufacturing electrical and elec-
tronic products. In R.E. Hester and R.M. Harrison (Eds.) Issues in Environmental Science
and Technology: 27 Electronic Waste Management, (pp. 40–73), Cambridge, UK: RSC
Publishing.
Sun, X., Bell, J. R., Luo, H., & Dai, S. (2011). Extraction separation of rare-earth ions via
competitive ligand complexations between aqueous and ionic-liquid phases. Dalton
Transactions,40(31), 8019–8023. doi:10.1039/C1DT10873E
Sun, L., & Qiu, K. (2012). Organic oxalate as leachant and precipitant for the recovery of
valuable metals from spent lithium-ion batteries. Waste Management,32(8), 1575–1582.
Sun, X., Luo, H., & Dai, S. (2013). Mechanistic investigation of solvent extraction based on
anion-functionalized ionic liquids for selective separation of rare-earth ions. Dalton
Transactions,42(23), 8270–8275. doi:10.1039/c3dt50148e
Sun, X., & Waters, K. E. (2014). The adjustable synergistic effects between acid-base cou-
pling bifunctional ionic liquid extractants for rare earth separation. AIChE Journal,
60(11), 3859–3868. doi:10.1002/aic.14563
Syed, S. (2012). Recovery of gold from secondary sources—A review. Hydrometallurgy,115,
30–51. https://doi.org/10.1016/j.hydromet.2011.12.012
Tan, Q., Li, J., & Zeng, X. (2015). Rare earth elements recovery from waste fluorescent
lamps: A review. Critical Reviews in Environmental Science and Technology,45(7),
749–776. https://doi.org/10.1080/10643389.2014.900240
Tanriverdi, M., Mordogan, H., & Ipekoglu, U. (2005). Leaching of Ovacik gold ore with
cyanide, thiourea and thiosulfate. Minerals Engineering,18(3), 363–365. https://doi.org/
10.1016/j.mineng.2004.06.012
The Hong Kong Observatory. (2012). How does a compact fluorescent light-bulb save
energy? http://www.hko.gov.hk/education/edu06nature/ele_fluorescent_e.htm. March 23,
2018.
Tolcin, A. C. (2012). U.S. Geological Survey Indium. Mineral Commodity Summaries, U.S.
Geological Survey.
Tuncuk, A., Stazi, V., Akcil, A., Yazici, E. Y., & Deveci, H. (2012). Aqueous metal recovery
techniques from e-scrap: Hydrometallurgy in recycling. Minerals Engineering,25(1),
28–37. https://doi.org/10.1016/j.mineng.2011.09.019
Tunsu, C., & Retegan, T. (2017). Hydrometallurgical processes for the recovery of metals
from WEEE. In WEEE Recycling (pp. 139–175).
Turgis, R., Arrachart, G., Dubois, V., Dourdain, S., Virieux, D., Michel, S., Legeai, S.,
Lejeune, M., Draye, M., & Pellet-Rostaing, S. (2016). Performances and mechanistic
60 M. SETHURAJAN ET AL.
investigations of a triphosphine trioxide/ionic liquid system for rare earth extraction.
Dalton Transactions,45(3), 1259–1268. doi:10.1039/C5DT03072B
Ueberschaar, M., Otto, S. J., & Rotter, V. S. (2017). Challenges for critical raw material
recovery from WEEE-The case study of gallium. Waste Management,60, 534–545.
https://doi.org/10.1016/j.wasman.2016.12.035
van der Hoogerstraete, T., Wellens, S., Verachtert, K., & Binnemans, K. (2013). Removal of
transition metals from rare earths by solvent extraction with an undiluted phosphonium
ionic liquid: separations relevant to rare-earth magnet recycling. Green Chemistry,15(4),
919–927. doi:10.1039/C3GC40198G
van der Hoogerstraete, T., & Binnemans, K. (2014). Highly efficient separation of rare
earths from nickel and cobalt by solvent extraction with the ionic liquid trihexyl (tetra-
decyl) phosphonium nitrate: a process relevant to the recycling of rare earths from per-
manent magnets and nickel metal hydride batteries. Green Chemistry,16(3), 1594–1606.
doi:10.1039/c3gc41577e
Veeken, A. H., Akoto, L., Pol, L. W. H., & Weijma, J. (2003). Control of the sulfide (S
2-
)
concentration for optimal zinc removal by sulfide precipitation in a continuously stirred
tank reactor. Water Research,37(15), 3709–3717
Veit, H. M., Diehl, T. R., Salami, A. P., Rodrigues, J. D. S., Bernardes, A. M., & Ten
orio,
J. A. S. (2005). Utilization of magnetic and electrostatic separation in the recycling of
printed circuit boards scrap. Waste Management,25(1), 67–74. https://doi.org/10.1016/j.
wasman.2004.09.009
Veit, H. M., Juchneski, N. C. D. F., & Scherer, J. (2014). Use of gravity separation in metals
concentration from printed circuit board scraps. Rem: Revista Escola de Minas,67(1),
73–79. http://dx.doi.org/10.1590/S0370-44672014000100011
Vieceli, N., Nogueira, C. A., Guimar~
aes, C., Pereira, M. F., Dur~
ao, F. O., & Margarido, F.
(2018). Hydrometallurgical recycling of lithium-ion batteries by reductive leaching with
sodium metabisulphite. Waste Management,71, 350–361.
Vidyadhar, A., & Das, A. (2013). Enrichment implication of froth flotation kinetics in the
separation and recovery of metal values from printed circuit boards. Separation and
Purification Technology,118, 305–312. https://doi.org/10.1016/j.seppur.2013.07.027
Vinals, J., Juan, E., Roca, A., Cruells, M., & Casado, J. (2005). Leaching of metallic silver
with aqueous ozone. Hydrometallurgy,76(3–4), 225–232. https://doi.org/10.1016/j.hydro-
met.2004.11.001
Virolainen, S. (2013). Hydrometallurgical recovery of valuable metals from secondary raw
materials, Thesis for the degree of Doctor of Science (Technology), at Lappeenranta
University of Technology, Lappeenranta, Finland.
Volesky, B. (2007). Biosorption and me. Water Research,41(18), 4017–4029.
Wang, L. K., yung-tse, H., & Shammas, N K. (Eds.) (2005). Physicochemical Treatment
Processes. Vol. 3. Totowa, NJ: Humana Press.
Wen, X., Zhao, Y., Duan, C., Zhou, X., Jiao, H., & Song, S. (2005). Study on Metals
Recovery from Discarded Printed Circuit Boards by Physical Methods, Proceedings of
the 2005 IEEE International Symposium, 16–19 May, 121–128.
Wills, B. A., & Finch, J. (2015). Wills’mineral processing technology (8th ed.). (ISBN No:
9780080970530). Butterworth-Heinemann, p. 512.
Willner, J., & Fornalczyk, A. (2013). Extraction of metals from electronic waste by bacterial
leaching. Environment Protection Engineering,39(1), 197–208. doi:10.5277/EPE130115
Wu, Y., Yin, X., Zhang, Q., Wang, W., & Mu, X. (2014). The recycling of rare earths from
waste tricolor phosphors in fluorescent lamps: A review of processes and technologies.
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 61
Resources, Conservation and Recycling,88,21–31. https://doi.org/10.1016/j.resconrec.
2014.04.007
Xing, W. D., Lee, & M. S. (2017). Leaching of gold and silver from anode slime with a mix-
ture of hydrochloric acid and oxidizing agents. Geosystem Engineering,20(4), 216–223.
https://doi.org/10.1080/12269328.2017.1278728
Xiu, F. R., Qi, Y. Y., & Zhang, F. S. (2015). Leaching of Au, Ag, and Pd from waste printed
circuit boards of mobile phone by iodide lixiviant after supercritical water pre-treatment.
Waste Management,41, 134–141. https://doi.org/10.1016/j.wasman.2015.02.020
Xu, Q., Chen, D. H., Chen, L., & Huang, M. H. (2009). Iodine leaching process for recov-
ery of gold from waste PCB. Chinese Journal of Environmental Engineering,3(5),
911–914.
Xu, Q., Chen, D. H., Chen, L., & Huang, M. H. (2010). Gold leaching from waste printed
circuit board by iodine process. Nonferrous Metals,62(3), 88–90.
Yamane, L. H., de Moraes, V. T., Espinosa, D. C. R., & Ten
orio, J. A. S. (2011). Recycling
of WEEE: characterization of spent printed circuit boards from mobile phones and com-
puters. Waste Management,31(12), 2553–2558. https://doi.org/10.1016/j.wasman.2011.
07.006
Yang, H., Wang, W., Cui, H., Zhang, D., Liu, Y., & Chen, J. (2012). Recovery of rare earth
elements from simulated fluorescent powder using bifunctional ionic liquid extractants
(Bif-ILEs). Journal of Chemical Technology and Biotechnology,87(2), 198–205. doi:
10.1002/jctb.2696
Yang, F., Kubota, F., Baba, Y., Kamiya, N., & Goto, M. (2013). Selective extraction and
recovery of rare earth metals from phosphor powders in waste fluorescent lamps using
an ionic liquid system. Journal of Hazardous Materials,254,79–88. https://doi.org/10.
1016/j.jhazmat.2013.03.026
Yang, J., Retegan, T., & Ekberg, C. (2013). Indium recovery from discarded LCD panel
glass by solvent extraction. Hydrometallurgy,137,68–77. https://doi.org/10.1016/j.hydro-
met.2013.05.008
Yarar, B. (2002). Long term persistence of cyanide species in mine waste environments.
Tailings and Mine Waste,2, 197–203.
Yazıcı, E. Y., & Deveci, H. (2009). Recovery of Metals from E-wastes. Madencilik,48(3),
3–18 (In Turkish).
Yazıcı, E. Y., Deveci, H., Alp, I., Akcil, A., & Yazıcı, R. (2010). Characterisation of
Computer Printed Circuit Boards for Hazardous Properties and Beneficiation Studies.
XXV. Int. Mineral Processing Congress (IMPC), 6–10 Sept., Brisbane, Australia,
4009–4015.
Yazıcı, E. Y., Yazıcı, R., Deveci, H., & Alp, _
I. (2010). Eddy current separation of metals
from e-wastes. In G€
ulsoy, €
O. Y., Erg€
un, L. S¸., Can, N. M. and C¸elik, _
I. B. (Eds.),
Proceedings of the XIIth. International Mineral Processing Symposium,6–10 Oct,
Cappadocia, Nevs¸ehir, Turkey, 1207–1215.
Yazici, E. Y., & Deveci, H. (2013). Extraction of metals from waste printed circuit boards
(WPCBs) in H
2
SO
4
-CuSO
4
-NaCl solutions. Hydrometallurgy,139,30–38. https://doi.
org/10.1016/j.hydromet.2013.06.018
Yazici, E. Y., & Deveci, H. (2014). Ferric sulfate leaching of metals from waste printed cir-
cuit boards. International Journal of Mineral Processing,133,39–45. https://doi.org/10.
1016/j.minpro.2014.09.015
Yazici, E. Y., & Deveci, H. (2015). Cupric chloride leaching (HCl-CuCl
2
-NaCl) of metals
from waste printed circuit boards (WPCBs). International Journal of Mineral Processing,
134,89–96. https://doi.org/10.1016/j.minpro.2014.10.012
62 M. SETHURAJAN ET AL.
Yazıcı, E. Y., Deveci, H., Yazici, R., & Akcil, A. (2015). Base and Precious Metal Losses in
Magnetic Separation of Waste Printed Circuit Boards. Proceedings of European
Metallurgical Conference-EMC 2015, 15–17 June, D€
usseldorf, Germany, Vol. 2, 649–662.
Yoon, H. S., Kim, C. J., Chung, K. W., Lee, S. J., Joe, A. R., Shin, Y. H., Lee, S. I., Yoo,
S. J., & Kim, J. G. (2014). Leaching kinetics of neodymium in sulfuric acid from E-scrap
of NdFeB permanent magnet. Korean Journal of Chemical Engineering,31(4), 706–711.
https://doi.org/10.1007/s11814-013-0259-5
Zazycki, M. A., Tanabe, E. H., Bertuol, D. A., & Dotto, G. L. (2017). Adsorption of valuable
metals from leachates of mobile phone wastes using biopolymers and activated carbon.
Journal of Environmental Management,188,18–25. https://doi.org/10.1016/j.jenvman.
2016.11.078
Zeng, G., Deng, X., Luo, S., Luo, X., & Zou, J. (2012). A copper-catalyzed bioleaching pro-
cess for enhancement of cobalt dissolution from spent lithium-ion batteries. Journal of
Hazardous Materials,199, 164–169. https://doi.org/10.1016/j.jhazmat.2011.10.063
Zeng, X., Li, J., & Singh, N. (2014). Recycling of spent lithium-ion battery: A critical
review. Critical Reviews in Environmental Science and Technology,44(10), 1129–1165.
https://doi.org/10.1080/10643389.2013.763578
Zhan, L., Xia, F., Ye, Q., Xiang, X., & Xie, B. (2015). Novel recycle technology for recover-
ing rare metals (Ga, In) from waste light-emitting diodes. Journal of Hazardous
Materials,299, 388–394. https://doi.org/10.1016/j.jhazmat.2015.06.029
Zhang, X., Chen, L., & Fang, Z. (2009). Review on gold leaching from PCB with noncya-
nide leach reagents. Nonferrous Metals,61,72–76.
Zhang, S., & Forssberg, E. (1997). Mechanical separation-oriented characterization of elec-
tronic scrap. Resources, Conservation and Recycling,21(4), 247–269. https://doi.org/10.
1016/S0921-3449(97)00039-6
Zhang, S., & Forssberg, E. (1998). Mechanical recycling of electronics scrap-the current sta-
tus and prospects. Waste Management & Research,16(2), 119–128. doi:10.1177/
0734242X9801600204
Zhang, Y., Li, A., Xie, H., Zeng, X., & Li, J. (2012). Current status on leaching precious
metals from waste printed circuit boards. The 7th International Conference on Waste
Management and Technology.16, 560–568.
Zhang, Y., Liu, S., Xie, H., Zeng, X., & Li, J. (2012). Current status on leaching precious
metals from waste printed circuit boards. Procedia Environmental Sciences,16, 560–568.
https://doi.org/10.1016/j.proenv.2012.10.077
Zhang, Q., & Saito, F. (1998). Non-thermal extraction of rare earth elements from fluores-
cent powder by means of its mechanochemical treatment. Shigen to Sozai,114 (4),
253–257. https://doi.org/10.2473/shigentosozai.114.253
Zhang, G., Wang, H., He, Y., Yang, X., Peng, Z., Zhang, T., & Wang, S. (2017).
Triboelectric separation technology for removing inorganics from non-metallic fraction
of waste printed circuit boards: Influence of size fraction and process optimization.
Waste Management,60,42–49. https://doi.org/10.1016/j.wasman.2016.08.010
Zhang, K., Wu, Y., Wang, W., Li, B., Zhang, Y., & Zuo, T. (2015). Recycling indium from
waste LCDs: a review. Resources, Conservation and Recycling,104, 276–290. https://doi.
org/10.1016/j.resconrec.2015.07.015
Zhang, L., & Xu, Z. (2016). A review of current progress of recycling technologies for met-
als from waste electrical and electronic equipment. Journal of Cleaner Production,127,
19–36. https://doi.org/10.1016/j.jclepro.2016.04.004
CRITICAL REVIEWS IN ENVIRONMENTAL SCIENCE AND TECHNOLOGY 63
Zhang, Z. Y., & Zhang, F. S. (2013). Synthesis of cuprous chloride and simultaneous recov-
ery of Ag and Pd from waste printed circuit boards. Journal of Hazardous Materials,
261, 398–404. https://doi.org/10.1016/j.jhazmat.2013.07.057
Zhao, Y., Wen, X., Li, B., & Tao, D. (2004). Recovery of copper from waste printed circuit
boards, Minerals & Metallurgical Processing,21(2), 99–102.
Zheng, Y., Long, H. L., Zhou, L., Wu, Z. S., Zhou, X., You, L., Yang, Y., & Liu, J. W.
(2016). Leaching procedure and kinetic studies of cobalt in cathode materials from spent
lithium ion batteries using organic citric acid as leachant. International Journal of
Environmental Research,10 (1), 159–168.
Zhou, P., Zheng, Z., & Tie, J. (2005). Technological process for extracting gold, silver and
palladium from electronic industry waste Chinese Patent. CN1603432A (C22B 11/00).
Zhu, Y., Zheng, Y., & Wang, A. (2015). Preparation of granular hydrogel composite by the
redox couple for efficient and fast adsorption of La (III) and Ce (III). Journal of
Environmental Chemical Engineering,3(2), 1416–1425. https://doi.org/10.1016/j.jece.
2014.11.028
64 M. SETHURAJAN ET AL.
