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ISSN: 2090-4568
Cui and Anderson, J Adv Chem Eng 2016, 6:1
DOI: 10.4172/2090-4568.1000142
Journal of Advanced
Chemical Engineering
Review Article
Open Access
Literature Review of Hydrometallurgical Recycling of Printed Circuit
Boards (PCBs)
Hao Cui* and Corby G Anderson
Kroll Institute for Extractive Metallurgy, Colorado School of Mines, Golden, CO 80401, USA
Abstract
This study provides an up-to-date review of recycling of printed circuit boards (PCBs), specifically in
hydrometallurgical treatment. Waste printed circuit boards, which are rich in base and precious metals, are the
essential component of end-of-life electrical and electronic equipment. From the economic and environmental
perspectives, the efficient recycling of PCBs is of importance. For the extraction of metals from PCBs, a large amount
of work has been done to establish an environmentally friendly and economic way to recover metals from PCBs
based on physical, pyrometallurgical and hydrometallurgical processes. Among those processes, hydrometallurgy
is a promising treatment due to its low capital cost, high selectivity and lower environmental impact. This review
emphasizes the recycling of PCBs by physical and hydrometallurgical treatments.
Keywords: PCBs; Recycling; Physical separation; Hydrometallurgical
treatment
Introduction
Due to the rapid development of technology and incredible market
growth, electronic equipment has an ever shortening lifespan, which
contributes to the fastest increase of e-waste. The definition of e-waste
is the end-of-life electric and electronic equipment without the intent
of reuse [1]. E-waste generally consists of six categories, including
lamps, small IT, screens, temperature exchange equipment and small
equipment, as well as large equipment, such as LED lamps, cell phones,
refrigerators, printers and dryers, as shown in reference [1]. According
to data in 2014, there are approximately 41.8 million tonnes of e-waste
generated through the world, and it is expected that the amount of
e-waste will reach 49.8 million tonnes in 2018, with an annual growth
rate of four to five percentages [1]. Figure 1 shows the trend of e-waste
generation in the world from 2010 to 2018. The management options of
e-waste are involved in reuse, refurbishment and repairs of electronics,
and the end-process for recovering metals, as well as disposal [1].
Since e-waste has high content of base and precious metals and
toxic substances, such as mercury, lead and plastic additives, how to
efficiently recover those metals is of importance, not only because of its
high economic potential, but also due to potential environmental and
human health risks. Table 1 [2] shows that the content of the metals that
contain iron and steel, copper, aluminum and printed circuit boards,
as well as other non-ferrous metals are almost 60%, which could
contribute to the fact that e-waste could be regarded as the secondary
sources for those metals [3-6].
Printed circuit boards, the essential part of electronics, contain
more abundant base and precious metals than their ores, respectively.
Table 2 shows the content of PCBs. Therefore, the main driving forces
of developing metal recovery methodologies are high contents of
base and precious metals and stringent environment regulations.
Other than metals, a large number of polymers and ceramics can be
found in PCBs as well. It is reported that plastics in PCBs commonly
contain isocyanates and phosgene from polyurethanes, acrylic and
phenolic resins, epoxides and phenols such as chip glass [7]. Presently,
incineration and landfill are the only ways on a large scale to practically
treat the non-metallic fraction physically separated from PCBs, which
results in a severe consequence where some organic matters can either
produce hazardous substance as the composition of off-gas or convert
into some poisonous compounds [8,9].
J Adv Chem Eng
ISSN: 2090-4568 ACE an open access journal
A major challenge for metal recovery is the heterogeneity
and complexity of PCBs, which includes metal diversity and their
liberation. Generally, a schematic process of recycling PCBs, shown in
Figure 2 [10], is involved in disassembly, upgrading and refining [11].
Disassembly is used for separation of a target component, particularly
metals, from their organic substance. Upgrading includes mechanical,
pyrometallurgical and hydrometallurgical treatments. Refining is a final
step to get high purity metals. The industrial treatment of E-scrap is
carried out via mainly pre-treatment and pyrometallurgy. Aurubis, one
of the largest copper producers in the world, also efficiently processes a
variety of recycling raw materials including: electronic scrap, precious
metal-bearing copper scrap, copper-iron material, as well as tin/
lead-bearing recycling raw materials, etc. [12]. The Kayser Recycling
System (KRS) is well suited at the Aurubis unit in Lünen, Germany, to
recycle materials with low levels of copper and precious metals [13].
The main pyrometallurgical operations are two reduction processes
in a submerged lance furnace and top blown rotary converter,
respectively, and an oxidation process in an anode furnace. The final
product that contains 99% copper is cast into copper anodes. Due
to the fact that precious metals can be resolved in copper, lead and
matte [14], electrowinning is always utilized as the refining process
to further purify copper and remove and recover precious metals
in the anode slimes [13]. Likewise, as shown in Figure 3, Umicore
[15] built up an integrated metals smelter and refinery at Hoboken
Plant, Antwerp, Belgium, and used copper as a solvent to separate
precious metals from other metals that are collected in a lead slag.
After leaching and electrowinning, precious metals are obtained
as residues that require being refined. The lead slag is sent to the
base metals operation in order to be transformed to the impure lead
bullion that is sent to refinery.
*Corresponding author: Hao Cui, Kroll Institute for Extractive Metallurgy,
Colorado School of Mines, Golden, CO 80401, USA, Tel: +13036535825; E-mail:
hcui@mines.edu
Received December 16, 2015; Accepted January 05, 2016; Published January
18, 2016
Citation: Cui H, Anderson CG (2016) Literature Review of Hydrometallurgical
Recycling of Printed Circuit Boards (PCBs). J Adv Chem Eng 6: 142.
doi:10.4172/2090-4568.1000142
Copyright: © 2016 Cui H, et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits unrestricted
use, distribution, and reproduction in any medium, provided the original author and
source are credited.
Volume 6 • Issue 1 • 1000142
Citation: Cui H, Anderson CG (2016) Literature Review of Hydrometallurgical Recycling of Printed Circuit Boards (PCBs). J Adv Chem Eng 6: 142.
doi:10.4172/2090-4568.1000142
Page 2 of 11
Figure 1: Global e-waste generation from 2010 to 2018 (data source [1]).
Material
Composition (Wt%)
Iron and steel
47.9
Non-flame retarded plastic
15.3
Copper
7
Glass
5.4
Flame retarded plastic
5.3
Aluminum
4.7
Printed circuit boards
3.1
Other
4.6
Wood and plywood
2.6
Concrete and ceramics
2
Other metals (non-ferrous)
1
Rubber
0.9
Table 1: E-waste material composition [2].
Metallic
Element
Birloaga et
al. [3]
Yang et al. [4] Oishi et al. [5]
Behnamfard
et al. [6]
Cu (wt%)
30.57
25.06
26
19.19
Al (wt%)
11.69
4.65
3.2
4.01
Fe (wt%)
15.21
0.66
3.4
1.13
Sn (wt%)
7.36
1.86
4.9
0.69
Ni (wt%)
1.58
0.0024
1.5
0.17
Zn (wt%)
1.86
0.04
2.6
0.84
Pb (wt%)
6.70
0.80
3.0
0.39
Mn (wt%)
-
-
0.11
0.04
Sb (wt%)
-
-
0.16
0.37
Au (ppm)
238
-
-
130.25
Ag (ppm)
688
-
-
704.31
separation. The results showed that the metallic elements, such as Fe,
Cu, Ni and Al, could be concentrated, even though further optimization
of the parameters was required. Li et al. [17] also investigated a process
that contained a two-step crushing, corona electrostatic separation and
recovery. It appeared that an effective separation of metals from base
plates could be accomplished. In the study done by Veit et al. [18], 43%
of iron on average in whole PCBs was in the fraction of the magnetic
concentrate. After electrostatic separation, the concentrate of
electrostatic components contained 50% copper, 25% tin and 7%
lead, which could be further processed by chemical processes. A
novel mechanical process [19] looked into the effect of wet impact
crushing and falcon centrifugal machine on upgrading the metal
grade. The study indicated that the grade of the metal concentrate
was up to 92.36% with the recovery of 97.12%. Furthermore, the
water medium in the wet impact crushing cannot only cool the
machine, but help the discharge of the crushed materials in order to
control the over-crushing. Although physical separation is a concern
due to low cost and high environmental friendliness, the high metal
loss (10%-35%) [20] can cause a negative effect in economics. For
instance, Yazici et al. [20] indicated that the magnetic separation
could recover up to 96% Fe and 93% Ni, while more than 60% losses
of copper, gold and palladium occurred because of their association
with iron alloy.
A gravity separation was studied by Huge Marcelo Veit et al.
[21] to investigate the utilization of Mozley concentrator for preextraction of metals from PCBs. The Mozley concentrator consists of
a flat tray and separation V-shaped tray for fine and coarse particles,
respectively. The water flow rate and tilt-tray angle were considered
as parameters to be optimized. It is reported that the material size
fraction of -1+0.25 mm is used in the gravity process, after taking
the loss of materials and interference of fine particles in the gravity
process into consideration. It appeared that it was possible to preconcentrate 85% copper, 95% tin, 96% nickel, and 98% silver, while
aluminum and gold could not be recovered due to its density and
lamellar form, respectively [21].
Table 2: Representative material composition of PCBs.
Pre-Treatment
Both disassembly and mechanical processes are used for the
liberation and separation of the metallic components from waste PCBs
in order to expose the metals for subsequent chemical processes. The
mechanical process is considered the most environmentally friendly
methodology to recover metals. Generally, a mechanical process could
contain shredding, grinding, magnetic separation and electrostatic
separation. However, the major challenge for the physical process is
poor recovery of base and precious metals.
Many studies have been conducted on the physical process. A study
was performed to investigate the feasibility of the mechanical process
flowsheet proposed by Yoo et al. [16], which consists of milling by a
stamp mill, size classification, gravity separation and two-step magnetic
J Adv Chem Eng
ISSN: 2090-4568 ACE an open access journal
Figure 2: A typical schematic flowsheet of PCB recycling [10].
Volume 6 • Issue 1 • 1000142
Citation: Cui H, Anderson CG (2016) Literature Review of Hydrometallurgical Recycling of Printed Circuit Boards (PCBs). J Adv Chem Eng 6: 142.
doi:10.4172/2090-4568.1000142
Page 3 of 11
to recycling of PCBs contain supercritical water oxidation (SCWO),
which is in the presence of oxygen gas, and supercritical water
depolymerization (SCWD) under reducing atmosphere [27].
Figure 3: New flowsheet for Umicore’s integrated metals smelter and
refinery [15].
Flotation also has been investigated to reversely separate metals from
organic matters with regard to fine particles, since gravity separation,
magnetic separation and electrostatic separation could not be effective
because of the similar physical qualities when the particles are so fine.
The plastic is naturally hydrophobic; thus, an e-waste flotation study
without reagents was carried out to mainly investigate following kinetic
parameters with the 65% passing 35 μm materials: airflow rate, pulp
density and impeller speed. It is found that natural hydrophobicity of
the plastic was confirmed by the experiments and the mechanism of
the flotation kinetics was the first order kinetics. Gold and palladium
could be recovered with 64% recovery at enrichment ratio of 3:1
[22]. Mäkinen et al. [23] showed that even though flotation, without
reagents, could produce the concentrated metal products, a relatively
large amount of copper, nickel, lead and antimony were found in the
froth, which contributed to severe consequences of disposal and loss
of metals. Moreover, Vidyadhar et al. [24] reported that under the
conditions of a stirrer speed of 1198 rpm, frother dosage of 0.61 kg/
ton, and pulp density of 9.02%, as well as air flow of 5.00 lph, 37% metal
content with 76% mass yield was obtained, which meant that nearly
95% metal value was recovered [24].
Other than the conventional physical processes mentioned above,
attention to supercritical water, whose temperature and pressure are
over its critical point (374°C, 218 atm), shown as Figure 4 [25], has
been drawn as a technique to pre-treat PCBs before chemical processes.
Compared with ambient liquid water, several unique properties, such
as lower dielectric constant, lower energy of hydrogen bond, and high
solubility of organic compound, were possessed by water near its critical
point. Reactions could occur in a homogeneous supercritical phase
because of the high miscibility of organic compounds in supercritical
water, which contributes to a fact that there are no interphase mass
transport limitations to lower reaction kinetics [26]. Moreover, high
pressure and temperature of supercritical water provide high enough
velocity and temperature rise to destroy particle structure and
make particles more porous. These further result in the diffusion of
supercritical water through particle structures and the creation of bighole structures [27,28]. Typically, the supercritical techniques applied
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ISSN: 2090-4568 ACE an open access journal
As for SCWO, it has been studied in decomposing toxic organic
substances of PCBs. In the study of Chien et al. [9], the resin conversion
in the supercritical water oxidation process of PCBs was 0.90 at 793 K
for 10 min. It was determined that NaOH enhanced the supercritical
water oxidation of PCBs and entrainment of Br in the liquid phase. The
structure of copper in the solid residue was also identified and the results
showed that Cu2O, CuO and Cu(OH)2 are the main copper phases, after
a supercritical water oxidation. Xiu et al. [27] investigated both SCWO
and SCWD with regards to the separation of organic substances from
metals and ceramics. The effects of temperature, retention time and
initial pressure were studied. It appeared that as temperature increased,
the weight loss of solid phase increased for both methods. However,
the degradation rate of organic matter on behalf of SCWO was higher
than that under the SCWD process, which could be attributed to the
combination of hydrolysis and oxidation in the case of SCWO process,
compared with SCWD where only hydrolysis occurred. In addition,
polymers with high molecular weight were eventually decomposed into
low molecular compounds along with SCWD, however, in the process
of SCWO, most organic matter was eventually oxidized into CO2 and
H2O, resulting in high pressure generated in the reactor. The higher
final pressure in the process of SCWO also provided an explanation that
SCWO was a superior method. Xiu et al. [27] also found HCl could be
applied to acid leaching of base metals after the pretreatment of SCWO.
That could be explained by the fact that SCWO converted copper metal
to copper oxides that could be soluble in HCl, even though HCl is a
non-oxidizing acid.
However, since water not only has high critical temperature and
pressure, but also unique properties, such as ion product and dielectric
point, which could contribute to high requirement of reactors, several
alternatives are undertaken. Supercritical methanol was studied by Xiu
et al. [28] to separate polymers and metals from printed circuit boards.
The reasons for utilizing methanol as a supercritical fluid are that the
critical point of methanol is 240°C, 8.09 MPa, which is lower than that
of water. Additionally, the boiling point of methanol is lower than that
of water. The results indicated that temperature, pressure, reaction
time and the solid/liquid ratio affected the performance of supercritical
methanol. The highest conversion happened either at 380°C with 120
Figure 4: (a) The P-T phase diagram of a one-component fluid, with
critical point; (b) The same diagram in P-V space with coexistence curve
and several isotherms, including critical point [25].
Volume 6 • Issue 1 • 1000142
Citation: Cui H, Anderson CG (2016) Literature Review of Hydrometallurgical Recycling of Printed Circuit Boards (PCBs). J Adv Chem Eng 6: 142.
doi:10.4172/2090-4568.1000142
Page 4 of 11
min or 420°C with 60 min. The difference with the two conditions was
that the oil recovery at higher temperature with shorter reaction time
was less than that at low temperature with longer reaction time. Based on
the elemental analysis of ICP-OES, It turned out that most metals were
concentrated after the supercritical methanol process. Particularly, the
content of copper after the treatment was approximately three times of
that in the original material, and silver were significantly concentrated
up to 7902 ppm.
Recycling of Non-Metallic Fraction
As mentioned, non-metallic fraction in PCBs can be separated
from metallic fraction by physical separation, such as magnetic
separation, flotation, electrostatic separation and gravity separation.
However, some challenges, such as how to get a clean separation
between non-metallic and metallic fraction, are still remaining with
regards to physical recycling of non-metallic fraction. In addition, there
are mainly four methods to chemically recycle non-metallic fraction,
including pyrolysis, gasification, supercritical fluid depolymerization
and hydrogenolytic degradation [29]. Other than landfill or
combustion, the non-metallic fraction can be reused in different fields,
such as building materials, additives, etc. Li et al. [17] reported that
the non-metallic fraction attained from a crushing-corona electrostatic
separation could be hot-pressed to the nonmetallic plate with a few
additives. The nonmetallic plate could be used as building materials.
Guo et al. [30] further reported that the non-metallic plates that had
flexural strength of 68.8 MPa and Charpy impact strength of 6.4 kJ/
m2 were obtained, when the nonmetallic plate contained 20 wt% nonmetallic materials from PCBs and the particle size of nonmetallic
fraction was less than 0.07 mm. Guo et al. [31] also investigated the
replacement of non-metallic fraction from PCBs in the production of
wood plastic composite. The results indicated that the flexural strength
and tensile strength could be enhanced by using the non-metallic
fraction, compared with using wood floor.
Hydrometallurgical Treatment
There are a number of monographs on hydrometallurgical
treatment with regards to recycling e-waste in recent years because of
its low capital cost, low environment impact and easy management,
compared with pyrometallurgy [32]. A hydrometallurgical process
mainly consists of leaching, purification and recovery of metals.
Extraction of base metals
A large amount of base metals, such as copper, lead, zinc and
cadmium, etc. can be found in waste printed boards. Generally,
leaching, as the first step of hydrometallurgical treatment, is to dissolve
the constituents of the e-scrap to form a pregnant solution by a suitable
lixiviant. Acid leaching with an oxidant reagent is widely used for the
first stage leaching of base metals from PCBs. The base metal leaching,
particularly, copper, was generally conducted by using acid such as
H2SO4, HNO3, aqua regia and HClO with various oxidants including
H2O2, O2, Fe3+ and Cl2.
Hydrometallurgical extraction of copper has been well-established
with regards to copper ore, however, there is no flowsheet that could
be applicable to recycle copper from PCBs on the industrial scale.
Typically, copper is dissolved in sulfuric acid to produce impure
copper bearing pregnant solution, and then the pregnant solution goes
through solvent extraction to upgrade copper purity. Eventually, pure
copper is obtained by further electrowinning. A sulfuric acid leaching
with H2O2 is shown in Equations (1) and (2).
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Cu0+H2O2+H2SO4=Cu2++SO42−+2H2O
(1)
Zn0+H2O2+H2SO4=Zn2++SO42−+2H2O
(2)
In the study of selective leaching of valuable metals from PCBs, it
was found that 100% of the copper and zinc were leached out within 8 h
by employing 2M H2SO4 and 0.2M H2O2 at 85°C, meanwhile, 95% of the
iron, nickel and aluminum were dissolved within 12 h. Whereas lead
had low dissolution rate in the H2SO4-H2O2 and (NH4)2S2O3-CuSO4NH4OH systems, thus, the solid residue, mainly lead sulfate, undertook
NaCl leaching solution to form PbCl2, followed by a solid/liquid separation
[33]. In the investigation by Deveci et al. it was clear that concentration
of H2O2 and temperature were the most significant factors through a 23
full factorial design [34]. Yang et al. studied the effects of particle size,
temperature and initial copper concentration with regards to leaching
of copper. The results indicated that the fine particle which was smaller
than 1 mm was efficiently treated, and temperature and initial copper
concentration had insignificant effects on copper leaching [35]. Moreover,
a negative effect was mentioned that the high stirring rate decreased the
copper extraction, which could be attributed to the fact that the increasing
stirring speed caused H2O2 degradation [3].
In acidic sulfate solution, even though H2O2 was extensively used
as the oxidant as mentioned above, it suffers from its remarkably
high consumption due to its decomposition and high temperature.
Thus, ferric ion is regarded as an oxidant alternative because of its
low cost and regeneration [36]. The downstream can be purified by
goethite and jarosite precipitation before electrowinning [37,38].
An environmental assessment was investigated by Fogarasi et al. to
evaluate two copper processes which were the direct electrochemical
oxidation and mediated electrochemical oxidation using the Fe3+/Fe2+
redox couples [39]. Unfortunately, there was no leaching data reported.
Furthermore, Yazici and Deveci investigated the effect of ferric ions
on sulfate leaching of metals from PCBs. The study indicated that
high extractions of copper and nickel were obtained, and increasing
temperature, ferric concentration and acid concentration positively
affected the metal extraction, while the increase of solid radio was an
adverse effect on leaching of metals. The addition of air or hydrogen
peroxide could maintain the high radio of Fe3+/Fe2+ [36].
The H2SO4-CuSO4-NaCl system has attracted attention not only
because of the fact that chloride solution has higher solubility and
activity of metals compound compared with in sulfate phase, but
also due to the advantage of copper ions, which means the usage of
copper can avoid the contamination of Fe3+ and decrease the high cost
when H2O2 is used [40,41]. The mechanism in H2SO4 system is shown
in Equations (3)-(7). Ping et al. showed that in the electro-oxidation
conditions, the recovery of Cu was 100% within 3.5 h [40].
Cu+Cu2+=2Cu+
+
−
(3)
−
−
2
Cu +Cl =CuCl CuCl+Cl =CuCl
−
2
(4)
−
4CuCl +O2+2H2O=2[Cu(OH)2·CuCl2]+4Cl
(5)
2[Cu(OH)2·CuCl2]+H2SO4=CuSO4+CuCl2+2H2O
(6)
CuSO4+5H2O=CuSO4·5H2O
(7)
Further investigation was performed with regard to other metals
such as Fe, Ni, Ag, and Au. The experimental results indicated that the
Cl−/Cu2+ ratio is the influential factor for the extraction process. At 1%
solid ratio, more than 90% Fe, Ni and Ag could be recovered with 58%,
under the condition of a Cl−/Cu2+ radio of 21 and 80°C. Furthermore,
the addition of oxygen could increase the recovery of the metals even at
a higher solid ratio [41].
Volume 6 • Issue 1 • 1000142
Citation: Cui H, Anderson CG (2016) Literature Review of Hydrometallurgical Recycling of Printed Circuit Boards (PCBs). J Adv Chem Eng 6: 142.
doi:10.4172/2090-4568.1000142
Page 5 of 11
In addition to the sulfuric acid treatment of copper, other acids are
also employed. Kasper et al. studied copper extraction by using aqua
regia, followed by electrowinning. The material fed to the acid leaching
was a metal concentrate by the mechanical process containing 60%
copper. 95% of the copper purity was obtained on the cathodes [42].
Havlik et al. proposed a method that hydrochloric acid was used as
the leaching solution to leach the sample thermally treated by either
pyrolysis or burning. An improvement of the copper extraction was
accomplished, when the burning temperature increased, which could
be explained by the fact that copper is released from the PCBs and is
oxidized. At the pyrolysis temperature of 900°C, the highest extraction
of copper was achieved because copper oxide was more efficiently
leached out in a non-oxidizing environment than pure copper metal
[43]. Kim et al. [44] were highly interested in electro-generated
chlorine in a hydrochloric acid solution. The effect of cuprous ions and
leaching kinetics of copper from PCBs were investigated. It appears
that depression of cuprous ions is helpful for increasing the leaching
rate of copper, which is due to the fact that the chloride generation
preferably occurs in the anode [44]. Moreover, a comparison was made
to differentiate the performances in a combined reactor, which could
be used as both the chlorine generator and metal leaching, and in two
separated reactors. The results showed that current and temperature
were of importance in terms of the kinetics of copper dissolution and
metals leaching. The generation of cuprous ions has a negative effect on
the leaching efficiency of the metals. Furthermore, it was observed that
the surface layer diffusion was the kinetics law of copper dissolution,
which meant that from the viewpoint of kinetics, the rate control step
of copper leaching was the diffusion of the lixiviant through the porous
product layer [45]. In a recent study, Yazici and Deveci further studied
cupric chloride leaching of copper as well as other metals (Fe, Ni,
Ag, Pd and Au) from PCBs. The study showed that almost complete
extraction of copper, nickel and iron were achieved over a leaching
period of 120 minutes at 79mM initial Cu2+. Increasing the initial
copper concentration remarkably enhanced the metal extraction except
for gold. Increasing temperature and oxygen supply could also increase
the extraction of palladium and silver to 90% and 98%, respectively,
which could be attributed to maintaining the high radio of Cu2+/Cu+
and thermodynamically favorable reaction between palladium/ silver
and dissolved oxygen [46].
Pressure leaching of copper has been extensively investigated
in recent years due to two benefits: high concentration of oxygen in
solution and fast kinetics [47]. However, a limited amount of literature
mentioned the application of pressure oxidation leaching in recycling
PCBs. In the study of Jha et al. [48], it was found that at 150°C with
2M H2SO4 and 15% H2O2 under the oxygen pressure of 20 bar, 97.01%
copper could be recovered from the liberated metal sheets, which was
pretreated by organic swelling.
Some efforts were made in recycling copper by employing the
Cu(I)-ammine complex [5,49]. On the basis of the thermodynamic
prediction, the oxidation-reduction potential of Cu(NH3)42+/Cu(NH3)2+
was greater than that of Cu(NH3)2+/Cu, and the oxidation-reduction
potential of Cu(I)/Cu was greater than hydrogen potential, which
meant, in this case, that Cu(NH3)42+ could be regarded as an oxidant
and Cu(I) could be reduced to metallic copper. The theoretical power
consumption was lower than the conventional electrowinning process.
The results indicated that the Cu(II)-ammine complex had the positive
effect on the leaching rate of copper [49]. Oishi et al. found that the
Cu(I) ammine contained the impurities, such as Zn, P, Mn and some
Ni, in the both ammonium sulfate and chloride systems. However, Zn,
Pb, Ni and Mn could be removed in the sequent purification process
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by using LIX 26 (alkyl substituted 8-hydroxy-quinoline). The main
impurity for copper concentrate was lead, which was due to similar
potential between copper and lead [5].
In addition, the application of ionic liquid was studied to recycle
copper from PCBs because of its outstanding properties, such as
negligible volatility and high conductivity. Zhu et al. [50] reported
that [EMIM+][BF4-] could completely separate electronic components
and solders from printed circuit boards at 240°C due to the hydrogen
bond of [EMIM+][BF4-]. Zeng et al. [51] investigated the utilization
of water soluble ionic liquid [BMIm]BF4 in dismantling electronic
components and tin solder from PCBs as functions of temperature,
retention time and turbulence. The results indicated that approximate
90% of the electronic components could be separated at 250°C, 12
minutes and 45 rpm. And this study also reported that due to high
efficiency and reusability of ionic liquids, [BMIm]BF4 dismantling is
more favorable than mechanical dismantling, when the capacity of
the plant is over 3,000 tons/day. In 2007, Dong et al. [52] investigated
the behaviors of [bmim]HSO4 in leaching of chalcopyrite. The results
suggested that [bmim]HSO4 acted as an acid and catalyst to facilitate
the dissolution of chalcopyrite. In the study by Huang et al. [bmim]
HSO4 was further studied to recycle copper from PCBs. Particle size,
ionic liquid concentration, H2O2 dosage and solid-liquid radio, as well
as temperature were considered as the parameters. It was observed that
99.92% copper could be reached under the conditions of 25 mL 80%
(v/v) ionic liquid, 10 mL 30% H2O2, solid/liquid ratio of 1/25 at 70°C
within 2 retention hours. Moreover, diffusion through a product layer
was the controlling step, using the shrink core model [53].
There is a scenario that leaching is not necessary if metals obtained
from PCBs have been concentrated to high purity (60%-80%) [54]
from physical separation. Under this circumstance, electrometallurgy
could be superior to produce copper. Xiu et al. [55] reported that after
the strong SCWO process, recoveries of copper and lead approached
to 100%. In the electrokinetic process, as shown in Figure 5, the solid
residue was suspended in 1M HCl solution. The anode and cathode,
two platinum-coated plates, were isolated by two porous glass frits. The
results indicated that the increase of current density promoted the rise
of copper recovery, however, the excessively high current density led
to high potential gradient that resulted in more side reactions. Thus,
97.6% of copper concentration with recovery of 84.2% was obtained
at 20 mA/cm2 current density and 11 h reaction time. More than 74%
copper recovery was attained by cathode deposition as two phases: Cu
and Cu2O [55]. The same electrokinetic set-up was also applied in the
extraction of heavy metals (Cd, Cr, As, Ni, Zn and Mn) with different
acids [56]. It appeared that HCl could recover 70% Cd, which could
be attributed to the conjugation of low pH and Cd-Cl complexes. The
high extraction of Cr, Zn, and Mn was found in the presence of citric
acid [56]. Chu et al. [54] also investigated the electrolysis process as
Figure 5: Schematic drawing of the electrokinetic set-up [55].
Volume 6 • Issue 1 • 1000142
Citation: Cui H, Anderson CG (2016) Literature Review of Hydrometallurgical Recycling of Printed Circuit Boards (PCBs). J Adv Chem Eng 6: 142.
doi:10.4172/2090-4568.1000142
Page 6 of 11
functions of the concentrations of CuSO4·5H2O, H2SO4 and NaCl,
current density and time with regards to the concentrated metal
scraps containing 83.42% copper. It was found that the increase of the
concentration of CuSO4·5H2O simultaneously increased the current
efficiency and particle size of copper powder in the cathode. Increasing
the concentration of H2SO4 and current density were effective at the
increase of current efficiency. The optimization indicated the copper
purity of 98.06% was obtained under the optimal conditions of 50 g/L
CuSO4·5H2O, 40 g/L NaCl, 118 g/L H2SO4, and 80 mA/cm2 with 3 h.
Additionally, recycling of other valuable base metals, including
lead, tin, nickel and zinc, is also of significance in terms of economic and
environmental perspectives. Studies on recycling of lead and tin from
solder are reported using alkaline [57,58], nitric [58-60], hydrochloric
[58] or fluoroboric acids [61] as leaching reagents. Ranitović et al. [58]
reported that NaOH was not a reliable leaching reagent to extract lead
and tin due to the dissolution and precipitation of metal hydroxides.
HNO3 is capable of extracting lead, yet, tin could not be extracted
resulting from the fact that tin is oxidized to insoluble form. On the
contrary, it is also shown that 90% of tin could be extracted using HCl
at high temperature; however, the precipitation of AgCl resulted in an
unacceptable loss of Ag [58]. In the investigation of lead leaching from
solder, Jha et al. [59] reported that 99.99% lead could be leached out
from the swelling liberated solder under the conditions of 90°C, 0.2M
HNO3 and S/L radio of 1:20 (g/mL) in 120 min. Meanwhile, 98.74% tin
could also be recovered at 90°C with 3.5M HCl for 120 min. Mecucci
et al. [60] proposed the flowsheet, shown as Figure 6, to selectively
separate copper, lead and tin. The flowsheet contains shredding, nitric
acid leaching and electrodeposition, as well as electrolyte regeneration.
It was found that copper and lead could be completely dissolved in
the nitric acid of 6M, whereas at the high concentration of HNO3, the
formation of metastannic acid resulted in the fact that tin was able to be
removed as a precipitate (metastannic acid). The reaction was shown
in Equation (8).
Sn+4HNO3=H2SnO3↓+4NO2+H2O
HCl. Other than solvent extraction, cementation is employed to extract
copper as well. Behnamfard added metallic iron to copper solution,
then metallic copper precipitated [6]. Kumari et al. [67] also reported
that in a process of pyrolysis-beneficiation-leaching-solvent extraction,
after recovering sulfuric acid and iron from the leach liquor using 70%
TEHA in kerosene and air sparging, respectively, 99.99% copper could
be extracted using 10% LIX 841C in two stages at pH 2.5 and O/A ratio
1/1. Similarly, nickel could be completely recovered from the raffinate in
two stages when 1% LIX 841C was used at pH 4.58 and O/A ratio 2/1.
Cu2++mRH+n(LH)2=Cu(RmL2Hm+2n−2)+2H+
(9)
Extraction of precious metals
Precious metals have been used in electric and electronic industries
due to their excellent electrical conductivity, low contact electrical
resistance and corrosion resistance [68], even though rare earths
have started partially replacing precious metals in electronic industry.
Therefore, a large number of e-waste contains significant amount
of precious metals, particularly gold, silver, and palladium. It is of
importance to recycle precious metals from e-waste. For instance, the
gold content in PCBs is 35-50 times higher than gold ore [68], even
though it has been noticed that the gold content in printed circuit
boards is decreasing. Extraction of precious metals from PCBs,
including leaching, purification and recovery, is the second stage after
the recovery of base metals. The most common leaching reagents for
precious metal leaching include cyanide, thiourea and thiosulfate
because of the stable metal complex formed [32]. Table 4 [69] and
Table 5 summarize the alternatives to cyanide and make a comparison
of several common reagents for gold extraction. Senanayake [70]
summarized a series of equations to illustrate the mechanism of gold
complex formation regarding different lixiviants, and also showed the
linear correlations of stability constants of Au(I)-complexes, following
the order: CN−>HS−>S2O32−>SC(NH2)2>OH−>I−>SCN−>SO32−>Br−>
(8)
The results indicated the feasibility of simultaneous recovery of
lead and copper through electrodeposition, and tin could be recovered
by electrodeposition after the dissolution of metastannic acid in the
presence of hydrochloric acid. Park et al. studied the separation of
zinc and nickel ions in a diluted aqua regia using TBP, Cyanex 272 and
Cyanex 301. It appeared that over 99% zinc could be recovered using
Cyanex 301 at low pH (pH<6), while 20% nickel could be extracted.
The separation factor was approximately 21,700 at pH 6 [62]. As
mentioned above, there are some potential leaching reagents that are
capable of extracting base metals. Table 3 summarizes the advantages
and disadvantages of four reagents for base metal extraction.
The copper leaching solution after solid-liquid separation can be
either treated by purification or directly taken to the final process,
where precipitation or electrowinning is involved to recover copper or
its compounds [47,63]. Solvent extraction is considered to concentrate
copper from dilute acidic leaching liquors. The common extractants
are hydroxyl oximes (such as LIX984N [64]) and alkyl phosphinic acids
(such as CYANEX 301 and CYANEX 302 [65]). Fouad [66] investigated
solvent extraction of copper by the mixture of Cyanex 301 (RH) and
LIX® 984N (LH). The extraction could be expressed by Equation (9).
The results indicated that the mixture of RH and LH with the radio
of 1:1 had the higher extraction efficiency, compared with using
RH or LH, individually, which could be attributed to the formation
of CuRL2H complex. Moreover, 90.65% of stripping percentage of
copper(II) from the organic phase was obtained by the addition of 6 M
J Adv Chem Eng
ISSN: 2090-4568 ACE an open access journal
Figure 6: Schematic diagram of the process [60].
Volume 6 • Issue 1 • 1000142
Citation: Cui H, Anderson CG (2016) Literature Review of Hydrometallurgical Recycling of Printed Circuit Boards (PCBs). J Adv Chem Eng 6: 142.
doi:10.4172/2090-4568.1000142
Page 7 of 11
Pros
Cons
Highly selective, low reagent
Sulfuric acid cost, well established process
for copper ore
At elevated temperature,
corrosive
Chloride
Fast kinetics at room
temperature, high solubility
and activity of base metals, low
toxicity
Excessive corrosion, difficult
electrowinning of copper, poor
quality of copper
Aqua regia
Fast kinetics, effective
High reagent cost, highly
corrosive, low selectivity
Ionic liquids
Thermally stable,
environmentally friendly
High cost, excessive dosage
Table 3: Comparison of potential leaching reagents for base metals [32,36,46,50].
Reagent
Concentration pH
Basic
Research Extent of
range
range chemistry level
commercialization
Ammonia
High
8-10
Simple
Low
Ammonia/
cyanide
Low
9-11
Simple
Extensive Cu/Au ores
Ammonium
thiosulfate
High
8.5-9.5 Complex
Pilot
Extensive Semi-commercial
Hypochlorite/
High chloride
chloride
6-6.5
Welldefined
Extensive
Historical and
Modern
Bacteria
7-10
Fairly
complex
Low,
growing
None
Natural
High
organic acids
5-6
Fairly
complex
Low
None
Thiourea
1-2
Welldefined
Fairly
popular
Some
concentrates
Thiocyanide low
1-3
Welldefined
Low
None
Aqua regia
High
<1
Welldefined
Low
Analytical and
refining
Acid ferric
chloride
High
<1
Welldefined
Low
Electrolytic Cu
slimes
High
High
Table 4: Suggested alternatives to cyanide [69].
Pros
Cons
Cyanide
Highly effective, low reagent
dosage and cost
Difficult to process wastewater,
environmental risk, low kinetics
Thiourea
Less toxic, high reaction rate,
less interference ions
Poorer stability, high
consumption, more expensive
than cyanide, downstream metal
recovery
thiosulfate
High selectivity, non-toxic and
non-corrosive, fast leaching rate
High consumption of reagent,
downstream metal recovery
Halide
High leaching rate, high
selectivity, relatively healthy and
safe except for bromine
Highly corrosive for chlorine, high
consumption for iodine
Aqua regia Fast kinetics, low reagent dosage
Strongly oxidative and corrosive,
difficult to deal with downstream
Cl−.
−
−
−
2−
3
2Au+H2O2+4L =2Au(I)L2+2OH (L=Cl , S2O , SC(NH2)2)
−
2Au+L2+2L =2Au(I)L2 (L=Cl, Br, I, SCN, SC(NH2)2)
−
(10)
(11)
(12)
Au+1.5L2+L =Au(III)L4 (L=Cl, Br, I)
(13)
Au+Cu(II) or Fe(III)+2L=Au(I)L2+Cu(I) or Fe(II)
(14)
(L=Cl−, S2O32−, SC(NH2)2, SCN−, NH2CH2COO−, NH2CH(CH3)
COO−).
For over a century, cyanide has been extensively used as the leaching
lixiviant to treat both gold mines and secondary gold source due to
J Adv Chem Eng
ISSN: 2090-4568 ACE an open access journal
Thiourea, NH2CSNH2, is considered a most promising alternative
to cyanide regarding leaching of precious metals due to its fast leaching
rate and non-toxicity. The thiourea leaching is conducted at pH=1.5
following the reaction shown as Equation (14). The demerits of thiourea
leaching are high cost and consumption because of its poor stability. Li
et al. [72] examined the thiourea leaching of gold and silver from PCBs
as functions of particle size, temperature, and thiourea concentration,
as well as Fe3+ concentration. It appeared that the optimum condition
for gold leaching happened when 24 g/L thiourea and 0.6% of Fe3+ were
used within 2 h. It is also proved that thiourea is less toxic and highly
efficient [72]. Birloaga et al. reported that 69% of gold was extracted
under the conditions of 20 g/L thiourea, 6 g/L Fe3+, and 10 g/L H2SO4,
as well as 600 rpm. Furthermore, under the same reagent condition, a
multistage cross current leaching was used to reduce the consumption
of thiourea and improve the efficiency of gold leaching from PCBs
[73]. Yin et al. compared thiourea leaching with iodine leaching of
gold from PCBs. It appeared that 93.5% gold was able to be leached
out directly by iodine without pretreatment, while thiourea leaching of
gold was carried out after copper leaching, which resulted in the high
consumption of thiourea [74].
Several studies highlighted thiosulfate leaching of gold. Thiosulfate
leaching is operated in alkaline condition to prevent thiosulfate
decomposition [75]. An evaluation [76] was conducted to compare
thiosulfate with cyanide and nitric acid. Nearly 65% of gold was leached
in the cyanide solution, and almost 100% of silver was leached in HNO3
solution. However, in the case of thiosulfate leaching, only around
15% gold could be extracted, which gave a negative indication of
thiosulfate leaching of gold. Further, in this study, it was suggested that
the presence of copper ions promoted gold extraction in the sodium
thiosulfate system. Ficeriová et al. found that 98% of gold and 93% of
silver were recovered from pretreated PCBs under the conditions of
0.5M (NH4)2S2O3, 0.2M CuSO4·H2O, and 1M NH3 at 40°C after 48 h.
Up to 90% of palladium was also extracted by leaching in aqua regia
solution with 2 h [77]. Ha et al. [78] reported that 98% gold could be
recovered using a solution containing 20 mM copper, 0.12M thiosulfate
and 0.2M ammonia. The reaction happening in the thiosulfate leaching
is shown in Equation (15).
Au+Cu(NH3)42++5S2O32−=Au(S2O3)35−+Cu(S2O3)35−+4NH3
Table 5: Comparison of potential leaching reagents for gold [10,32,76].
4Au+O2+2H2O+8NaCN=4NaAu(CN)2+4NaOH
its high efficiency and low cost [68]. Most cyanide leaching processes
occur at pH 10, because of the fact that cyanide ion is stable at pH 10.2.
Below pH 8.2, cyanide exists as hydrogen cyanide that is highly volatile
[71], which results in cyanide loss and harmfulness of operators’ health.
Recently, the slow cyanidation rate and severe environmental impact of
cyanide gold leaching accelerate the development of a substitute that is
more effective and environmental-friendly.
(15)
Park et al. proposed a process of recycling precious metals such
as gold, silver and palladium using aqua regia. The results showed
that 98% silver could be recovered without any additives and 93%
palladium was recovered as Pd(NH4)2Cl6, which was a red precipitate.
A solvent extraction was employed to recover gold as nanoparticles,
where toluene, dodecanethiol and sodium borohydride were used [79].
However, the application of aqua regia in extraction of precious metals
is limited in a lab scale because aqua regia is strongly oxidative and
corrosive, and the waste water from leaching is too acidic to be dealt
with [76].
Other than gold and silver, palladium is also considered as a
valuable metal to be recycled. Zhang et al. [80] proposed the process to
recover palladium, including enrichment and dissolution of palladium,
and extraction and stripping of Pd(II). Copper was leached out in
Volume 6 • Issue 1 • 1000142
Citation: Cui H, Anderson CG (2016) Literature Review of Hydrometallurgical Recycling of Printed Circuit Boards (PCBs). J Adv Chem Eng 6: 142.
doi:10.4172/2090-4568.1000142
Page 8 of 11
Figure 7: The flowsheet proposed by Behnamfard et al. [6].
J Adv Chem Eng
ISSN: 2090-4568 ACE an open access journal
Volume 6 • Issue 1 • 1000142
Citation: Cui H, Anderson CG (2016) Literature Review of Hydrometallurgical Recycling of Printed Circuit Boards (PCBs). J Adv Chem Eng 6: 142.
doi:10.4172/2090-4568.1000142
Page 9 of 11
the solution of CuSO4 and NaCl. The overall reaction was shown as
Equation (16).
Cu+Cu2++2Cl−=2CuCl(s)
(16)
When the ratio of [Cu]/[Cu2+] was more than 1.4, palladium
was leached out along with CuCl. Thereafter, palladium was further
dissolved in the solution of CuSO4 and NaCl, where the ratio of [Cu]/
[Cu2+] was less than 0.9. The reaction of palladium dissociation was
shown as Equation (17). Diisoamyl sulfide (S201) was applied to extract
99.4% palladium from leaching solution, then a two-step stripping
was accomplished using docecane with 0.1M NH3. Therefore, 96.9%
palladium was obtained with negligible effect of copper ions.
0.5Pd+Cu2++4Cl−=CuCl2−+0.5PdCl42−
(17)
In the gold industry, gold is recovered from gold-rich leaching
solution by carbon adsorption, following precipitation with zinc dust or
electrowinning [81,82]. Behnamfard et al. [6] proposed the flowsheet,
shown in Figure 7, involving a two-step sulfuric acid leaching, acidic
thiourea leaching with ferric ions, and HCl-NaClO-H2O2 leaching, as
well as precipitation with sodium borohydride (SBH). Approximate
99% copper could be recovered. It was found that 84.31% of gold and
71.36% of silver could dissolve in acidic thiourea with ferric ions, while
palladium could not be dissolved in thiourea solution. In this study,
SBH was used as a reducing reagent to selectively precipitate gold from
silver. The optimal precipitation of gold and silver happened at 8 g/L
SBH in 15 min.
Conclusion
Printed circuit boards (PCBs) contains various valuable materials,
such as copper, gold and silver. From the economic point of view,
recycling of those valuable materials is extremely attractive. Increasing
generation of PCBs and the severe environmental impacts of landfills
promote the development of recycling methodologies. The mechanical
process is of importance for following chemical processes, because
of the need for metal liberation. The hydrometallurgical process has
been studied in terms of its advantages, such as low capital cost and
less environmental impact. The work done shows the promising future
in the world of PCBs recycling. Both ionic liquid and chlorine-based
media have the potential for the extraction of base and precious
metals. However, the flowsheets proposed are limited in lab-scale.
Thus, larger scale studies should be concentrated on to achieve
commercialization.
Acknowledgements
This work was funded by the NSF CR3 consortium through collaboration
with Kroll Institute for Extractive Metallurgy in the George S Ansell, Department of
Metallurgical and Materials Engineering, Colorado School of Mines, USA.
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Volume 6 • Issue 1 • 1000142
Citation: Cui H, Anderson CG (2016) Literature Review of Hydrometallurgical Recycling of Printed Circuit Boards (PCBs). J Adv Chem Eng 6: 142.
doi:10.4172/2090-4568.1000142
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