Combined Oxidative Leaching and Electrowinning Process for Mercury Recovery from Spent Fluorescent Lamps

Full text

Combined oxidative leaching and electrowinning process for mercury
recovery from spent fluorescent lamps
Cihan Ozgur
a
, Sezen Coskun
b,
⇑
, Ata Akcil
c
, Mehmet Beyhan
a
, Ismail Serkan Üncü
d
, Gokhan Civelekoglu
a
a
Department of Environmental Engineering, Suleyman Demirel University, TR32260 Isparta, Turkey
b
Egirdir Vocational School, Suleyman Demirel University, TR32500 Egirdir, Isparta, Turkey
c
Mineral-Metal Recovery and Recycling Research Group, Mineral Processing Division, Department of Mining Engineering, Suleyman Demirel University, TR32260 Isparta, Turkey
d
Department of Electrical and Electronic Engineering, Suleyman Demirel University, TR32260 Isparta, Turkey
article info
Article history:
Received 30 September 2015
Revised 30 January 2016
Accepted 21 March 2016
Available online 31 March 2016
Keywords:
Mercury recovery
Electrowinning
Leaching
E-waste
Spent fluorescent lamps
abstract
In this paper, oxidative leaching and electrowinnig processes were performed to recovery of mercury
from spent tubular fluorescent lamps. Hypochlorite was found to be effectively used for the leaching
of mercury to the solution. Mercury could be leached with an efficiency of 96% using 0.5 M/0.2 M
NaOCl/NaCl reagents at 50 °C and pH 7.5 for 2-h. Electrowinning process was conducted on the filtered
leaching solutions and over the 81% of mercury was recovered at the graphite electrode using citric acid
as a reducing agent. The optimal process conditions were observed as a 6 A current intensity, 30 g/L of
reducing agent concentration, 120 min. electrolysis time and pH of 7 at the room temperature. It was
found that current intensity and citric acid amount had positive effect for mercury reduction. Recovery
of mercury in its elemental form was confirmed by SEM/EDX. Oxidative leaching with NaOCl/NaCl
reagent was followed by electrowinning process can be effectively used for the recovery of mercury from
spent fluorescent lamps.
Ó2016 Elsevier Ltd. All rights reserved.
1. Introduction
Waste of electrical and electronic equipment commonly known
as e-waste includes various forms of all electric and electronic
apparatus which are at the end of its life. Some e-wastes such as
televisions, computer monitors and fluorescent lamps contain Hg
(mercury) (Bhutta et al., 2011; Garlapati, 2016). Due to the toxic
effect of mercury, the disposal of e-wastes together with municipal
wastes causes environmental problems in the landfill areas. Mer-
cury is essential to the operation of fluorescent lamps. Distribution
of electrical and electronic waste was determined in EU Directive
2002/96. Spent lamps were calculated 1.7% of total electrical and
electronic wastes as the 5B lighting equipment e-waste group by
European Union in 2005 (Erust et al., 2013; EU Directive
2002/96). E-waste stream is very fast growing in the modern world
(Huang et al., 2009; Behnamfard et al., 2013) and these wastes
should to be designed considering their recycle and reuse potential
(Petter et al., 2014; Sahin et al., 2015). Though the lighting industry
has achieved significant reductions in mercury content, the mer-
cury is still an important component for the working of fluorescent
lamps (NEMA, 2002; Tunsu et al., 2015). When the spent
fluorescent lamps are improperly discarded, mercury may contam-
inate soil and water resources and it can be harmful to the humans
and other organisms. Therefore, the recovery of mercury from
spent fluorescent lamps would reduce the amount of waste, thus
reducing the potential environmental risks (Durão et al., 2008;
Coskun and Civelekoglu, 2014, 2015).
Hydrometallurgy process called as leaching had been widely
applied for metal recovery from electronic wastes because of its’
flexible and energy-saving characteristics (Kinoshita et al., 2003;
Lai et al., 2008). However, acidic-leaching was usually applied to
obtain high mercury yields in the studies (Rey-Raap and
Gallardo, 2013; Tunsu et al., 2014), potential health risks and envi-
ronmental impacts of using acidic reagents should be considered.
In addition, acidic reagents used in the process caused to extraction
of other metals (e.g., Al, Mn, Cu, Zn and Cd) solution and it may lead
to reduce the efficiency of mercury leaching (Kalb et al., 1999;
Coskun and Civelekoglu, 2014, 2015). Therefore, oxidative sodium
hypochlorite (NaOCl)/sodium chloride (NaCl) was selected and
conducted as oxidative leaching solution to extract mercury as
Hg(II) complex as mercury tetra chloride, HgCl
4
2
according to Eq.
(1) (Twidwell and Thompson, 2001).
Hg
ðsolidÞ
þHOClþ3Cl

!HgCl
2
4ðaqueousÞ
þOH

K
f
¼5:010
15
ð1Þ
http://dx.doi.org/10.1016/j.wasman.2016.03.039
0956-053X/Ó2016 Elsevier Ltd. All rights reserved.
⇑
Corresponding author.
E-mail address: sezencoskun@sdu.edu.tr (S. Coskun).
Waste Management 57 (2016) 215–219
Contents lists available at ScienceDirect
Waste Management
journal homepage: www.elsevier.com/locate/wasman

In order to recovery of mercury from the leaching solution, it
needs to be reduced to its elemental state (Hg
0
). After oxidative
leaching, mercury can be recovered as a Hg
0
from the leachate
by suitable separation processes such as cementation, ion
exchange, solvent extraction, biodegradation/bioreduction, hetero-
geneous photocatalysis, electrowinning and hybrid use of these
processes (Cui and Zhang, 2008; Bussi et al., 2010; Chaturabul
et al., 2015). The electrowinning technology was successfully
applied electroplating process to recover heavy metals from con-
centrated leaching solutions (Vegliò et al., 2003). This process is
attractive due to its versatility, energy efficiency, simple equip-
ment, easy operation, and low operation cost (Jüttner et al.,
2000; Meunier et al., 2006; Lai et al., 2008). However, little infor-
mation is available about recovery of mercury using electrowin-
ning from spent fluorescent lamps. The dissolved mercury in
leachate can be recovered by cathodic reduction according to Eq.
(2). On the other hand, mercury agglomerating may occur at the
electrode surfaces. This may lead to Hg
2
Cl
2
precipitation according
to Eq. (3). When the solution pH goes down below 4, Hg
2
Cl
2
is
formed (Hummer et al., 2006). Therefore, neutral pH (pH of 7) con-
dition was provided for electrowinning experiments in the current
study
HgCl
2
4ðaqueousÞ
þ2e

!Hg
0
þ4Cl

ð2Þ
Hg
0
þHg
2þ
þ2Cl

!Hg
2
Cl
2ðsÞ
ð3Þ
In the present study, electrowinning process was applied for
convert mercury to its metallic state after oxidative (hypochlorite)
leaching of mercury from spent tubular fluorescent lamps. The
optimal process conditions for high efficiency of recovery were
identified.
2. Material and methods
2.1. Sample preparation and oxidative leaching tests
Spent fluorescent lamps were collected from hospitals, schools
and factories in the city center of Isparta, Turkey. T8 and T12 linear
(tubular) types of spent fluorescent lamps were selected for study
owing to their high rate of consumption around the world (Coskun
and Civelekoglu, 2015). These lamps also have higher mercury con-
tent than the other mercury-containing lamps such as T2, T5 and
compact fluorescent lamps (CFLs) (Hu and Cheng, 2012). Each
spent fluorescent lamp was manually dismantled under vacuum
in laboratory. The oxidative leaching experiments were conducted
on pulverized mixture samples of lamps (50–50% mixture of the T8
and T12 lamps) to simulate a realistic situation. Sample prepara-
tion method was described in detail by Coskun and Civelekoglu
(2015).
To determine the initial mercury concentration of lamps, 20 g
pulverized samples were extracted with 25 mL water and 25 mL
aqua regia (HCl/HNO
3
, v/v - 3/1) in polypropylene flasks using a
magnetic stirrer at 200 rpm (Heidolph MR Hei-Tec 3001) at room
temperature for 18 ± 2 h. Each sample was filtered through funnels
using following the mixing stage. The initial mercury concentra-
tions were determined using atomic absorption spectrophotome-
ter (AAS) (PerkinElmer-FIMS 400, attached with flow injection
automated system). NaOCl (6–14% active chlorine, Merck
105614) and NaCl (extra pure, Merck 106400) were used as chem-
ical (oxidative) leaching reagents to extract mercury from pulver-
ized lamp samples. The leaching tests were performed in 250 mL
polypropylene flasks placed in temperature-controlled water baths
(GFL 1086) with mechanical stirrers. During the leaching tests, the
pH of the solutions was monitored using a digital pH meter (WTW
multi, 340i). The solutions were filtered (20
l
m, pure cellulose
filter papers), and analyzed for their mercury content to quantify
leaching efficiency. Each sample was diluted by a factor of 1:10
using nitric acid solution (pH = 2) to avoid the precipitation of met-
als and then stored at 4 °C for further analysis using AAS. Quantifi-
cation of leaching efficiency was determined by comparing initial
mercury concentration in extracted lamp sample solutions and
final mercury concentration in filtered leaching solutions. All mer-
cury analysis were based on Method 7471B (Mercury in solid or
semisolid waste-manual cold vapor technique) from USEPA’s ‘‘Test
methods for evaluating solid waste-physical/chemical method”
(SW-846) (USEPA, 1998).
2.2. Electrowinning tests
Leaching process was followed by electrowinning and oxidative
leaching solution was used as electrolyte in this process. The reac-
tion system was comprised of a 150 mL glass reactor with one gra-
phite electrode (0.5 cm diameter, 6 cm length), acting as the
cathode, one dimensionally stable anode (DSA
Ò
) (2 cm width,
5 cm length), which is made of titanium substrate with a thin layer
of iridium oxide (IrO
2
) acting as the anode of the electrolytic cell. A
power supply (GW Instek GPR-1820 HD) that provides electric cur-
rent (0–10 A and 0–18 V) was used to provide electric current.
While the applied current was constant (e.g., 2, 4 or 6 A), the volt-
age value was varied from 7 to 12 V between the two electrodes
during the experiments. All electrowinning tests were carried at
room temperature and pH of 7. The pH values were adjusted using
reagent grade NaOH and/or HCl solutions with different molar con-
centrations (0.2 M–0.5 M–1 M). The electrodes were connected
parallel to the power supply and the mercury was analyzed with
AAS.
Citric acid (C
6
H
8
O
6
, Merck-818707) was added into filtered
leaching solution as a reducing agent. The effects of the current
intensity (A), citric acid amount (g/L), electrolysis time (min.) on
process yield were evaluated on basis of 2
3
full factorial designs
(Table 1)at25°C of temperature. The central point tests were used
to evaluate the experimental error of electrowinnig process and
therefore the SE (standard error) for the effects. The experimental
matrix was designed according to Yates Algorithm (Montgomery,
1991).
The graphite electrodes were analyzed before and after of elec-
trowinnig process by scanning electron microscope coupled with
an energy-dispersive X-ray spectrophotometer (SEM/EDX) (FEI
Quanta250 FEG) to investigate the elemental compositions.
3. Results and discussion
3.1. Leaching tests
Oxidative leaching tests were carried out to determine the opti-
mal leaching conditions in terms of simultaneous mercury extrac-
tions. Mercury could be leached by NaOCl/NaCl reagent with an
efficiency of 96% from real spent fluorescent lamps at 2-h contact
time, 50 °C of temperature, pH 7.5 and 120 rpm agitation speed
(Coskun and Civelekoglu, 2015). The addition of chloride ions
Table 1
Factors and levels investigated in electrowinning tests.
Code Factor (variable) Level
10 +1
A Current intensity (A
a
)246
B Citric acid amount (g/L) 5 15 30
C Electrolysis time (min) 30 60 120
a
Ampère (A).
216 C. Ozgur et al. / Waste Management 57 (2016) 215–219

provided to increase the solubility of mercury through forming the
soluble and stable complex of HgCl
4
2
at neutral conditions. This
process was found to be more environmental friendly than the
acidic leaching of mercury. All leaching tests were carried out using
this combination, prior to conducting the electrowinning tests.
3.2. Electrowinning tests
Complex of HgCl
4
2
which was present in leaching solution
transferred to the elemental mercury during electrowinning pro-
cess. Electrowinning efficiencies were displayed in Fig. 1a. As seen
from this figure, recovery efficiency of factor 1 (minimum current
intensity, citric acid amount and electrolysis time) was found
lower than the central point tests. When all of these factors
increased to the average value (factor 0: A, current intensity 4 A;
B, citric acid amount 15 g/L; and C, time 60 min.), the recovery effi-
ciency also increased from a rate of 27–62% (Fig 1a). Citric acid was
used as the reductant agent for the conversion of Hg (II) to Hg (0).
Efficiency of factor A (maximum current intensity 6 A; minimum
citric acid amount 5 g/L; and minimum electrolysis time 30 min.)
and factor C (minimum current intensity 2 A; minimum citric acid
amount 5 g/L; and maximum electrolysis time 120 min.) was
measured low (Fig. 1a). However, factor B was found higher
recovery yield than the factor A and C. According to these results,
citric acid amount was more effective than current intensity and
electrolysis time in the process. In addition, AB and ABC factors
had high recovery efficiencies (about 77% and 81%, respectively).
Therefore, the recovery efficiency was increased by the increasing
current intensity and citric acid amount. The maximum
electrowinning efficiency was calculated as 81% (Fig. 1a, ABC).
Efficiency of mercury recovery was slightly increased with
maximum of all factors (current intensity: A, citric acid amount:
B and electrolysis time: C).
ANOVA method was used to evaluate the effect of the main fac-
tors on the process of electrowinning and the results were summa-
rized in Fig. 1b. F test statistics was used to evaluate factor
coefficients for significance at 95% (p = 0.05) confidence level. The
calculated SE value using central point experiment of electrowin-
ning was 6.76. The factors ‘‘C, ‘‘AC” and ‘‘ABC” were determined
to be statistically insignificant (p > 0.05) and these factors were
excluded from the figure (Fig. 1b).
It was found that current intensity (A) and citric acid amount
(B) had positive effect for mercury reduction (Fig 1b, A; +19% and
B; +28%). Furthermore, interaction effects of these variables (AB)
Factors
AB C 0
Hg(0) recovery (%)
0
20
40
60
80
100
a
Main and interaction effects
AC
1AB ACBCABC
B AB AC BC ABC
Effects (%)
-20
-10
0
10
20
30
40
b
Fig. 1. Electrowinning process mercury extraction yield (a) and extraction effects (b).
C. Ozgur et al. /Waste Management 57 (2016) 215–219 217

had positive effect to electrowinning process (+7.8%). Nanseu-Njiki
et al. (2009) observed that an increase in current intensity
also increases the rate of anode dissolution. The higher current
intensity may play a role more turbid the solution, and
consequently favors recovering. Citrate ions (Cit
3
) formed to a
complex with Hg
2+
was reduced to Hg
0
as shown in Eq. (4)
(Kabra et al., 2004).
½Hg
2þ
nCit
3n
þð3n2ÞH
þ
þð18n2ÞOH

!6nCO
2
þð13n2ÞH
2
OþHg
0
ð4Þ
On the other hand, the main effect of electrolysis time (factor C)
did not significantly influence mercury reduction. Interaction
effects of citric acid amount and electrolysis time (BC) had little
negative influence (8.2%) on mercury recovery (Fig 1b). It means
there was no advantage in working at high electrolysis time
because of increasing the energy consumption and process costs
as reported in Hummer et al., 2006.
Overall the results indicated that higher mercury recovery
efficiencies could be reached with electrowinning experiments.
The optimal process conditions were observed to be 6 A current
intensity, 30 g/L of reducing agent concentration and 120 min.
electrolysis time. Similarly, Hummer et al. (2006) reported high
mercury recovery efficiency with a current intensity of 6 A, elec-
trolysis time of 240 min. at the end of electroleaching process. As
seen from the results, electrolysis time was reduced by half in
our study. While white phosphor powders of fluorescent lamps
were used as samples and NaCl was chosen as leaching solution
in aforementioned study, we used pulverized glass and phosphor
powder mixture samples of T8 and T12 lamps to simulate a more
realistic situation. Furthermore, NaOCl and NaCl mixture reagent
was used as leaching solution as distinct from Hummer et al.
(2006).
3.3. Elemental composition experiments
SEM/EDX analyses of graphite electrode were obtained before
and after the electrowinning process. Fig. 2a shows a SEM/EDX
image including elemental analysis of pure graphite electrode.
According to this analysis, 100% C of total weight was measured
elemental composition of graphite electrode (Fig. 2b). After elec-
trowinning test, approximately 46.65% C, 35.61% O, 8.25% Na, 3.
26% Hg, 2.83% Mg, 2.55% Cl and 0.85% Ca was measured on graphite
electrode (Fig. 2d). Elemental mercury was detected in the SEM/
EDX image in Fig. 2c.
4. Conclusions
The oxidative leaching and electrowinning processes were
performed to recovery of mercury from spent tubular fluorescent
lamps. The experiments were conducted on pulverized mixture
samples of T8 and T12 lamp types. Mercury could be leached
with an efficiency of 96% using NaOCl/NaCl reagent. The maxi-
mum electrowinning efficiency was calculated as 81%. It was
found that current intensity and citric acid amount had positive
effect for mercury reduction. Electrolysis time did not signifi-
cantly influence the efficiency of electrowinning process. Recov-
ery of mercury in its elemental form was confirmed by SEM/
EDX. The oxidative leaching with NaOCl/NaCl reagent was fol-
lowed by electrowinning process approach appears to be techni-
cal feasibility of the mercury from spent fluorescent lamps. In
the future studies, the researchers can focus on using of different
types, materials and numbers of electrodes in the process to
improve the mercury recovery. Furthermore, the economical
evaluations should be conducted prior to each specific
application.
Acknowledgements
The authors would like to thank to Erik Zimmerman from Per-
mascand AB for providing the DSA Anodes. This work was sup-
ported by research grants from Scientific and Technical Research
Council of Turkey (TUBITAK) (project no. 110Y264).
Fig. 2. Pure graphite electrode SEM image (a) EDX analysis (b) and graphite
electrode - Hg SEM image (c) EDX analysis (d).
218 C. Ozgur et al. / Waste Management 57 (2016) 215–219

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