Materials Transactions, Vol. 56, No. 9 (2015) pp. 1579 to 1584
© 2015 The Japan Institute of Metals and Materials
Fundamental Studies on a Recycling System for Precious and Rare Metals
Using a Propylene Carbonate Solvent Containing CuBr2 and KBr
Kana Umehara+1 and Yasunari Matsuno+2
Department of Materials Engineering, Graduate School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
Previously we studied a novel process for recycling gold from secondary sources: the leaching of gold using dimethyl sulfoxide solutions
containing copper bromide and precipitation with water, which could offer a number of advantages, including eco-friendliness, ease of operation
and low cost. In this study, we have further investigated a more environmentally benign solvent, Propylene Carbonate (PC), with CuBr2 and KBr
for the leaching and recovery of precious and rare metals. The mechanism of dissolution was investigated using electrochemical measurements.
Metal wires were dissolved in a PC solution with 0.2 M of CuBr2 and 0.2 M of KBr at 343 K. Next, 10 ml of dilute sulfuric acid aqueous solution
at pH 1 was added to the solution at ambient temperature and shaken to biphasically separate the dissolved metals. The contents of each element
in the sulfuric acid and PC phases were measured by ICP-OES. The results of the electrochemical measurements indicated that the anodic
dissolution of sample metals in the PC containing CuBr2 occurred at relatively negative potentials and was paired with the cathodic reduction of
Cu2+ to Cu+. It was found that Au, Pd, Cu, Sn, Co, Ni and Zn could be dissolved at relatively fast rate, while Ag, Ta, Ti and W could not be
dissolved. In addition, 98% of Au and 94% of Pd remained in the PC phase, while most other dissolved metals migrated to the sulfuric acid
phase. This indicated that the dissolved Au and Pd could be effectively separated from other metals via biphasic separation with sulfuric acid.
Next, the gold in the PC phase was recovered by the reduction of ascorbic acid or calcination. The cost analysis for recovering gold by this
system resulted in 0.34 USD/g-Au. [doi:10.2320/matertrans.M2015202]
(Received May 19, 2015; Accepted June 19, 2015; Published July 31, 2015)
Keywords: biphasic separation, copper dibromide, precious metal, propylene carbonate, recycling
1.
Introduction
The recovery of precious and rare metals from secondary
sources, such as waste electric and electronic equipment
(WEEE), has received considerable attention because of its
many industrial applications and high market prices.1,2) To
date, various techniques have been developed to recover
these metals from secondary sources. Among these techniques, hydrometallurgical processes have been recognized
as promising options for the treatment of secondary sources
because of their relatively low capital cost, potential for
high metal recoveries and suitability for small-scale applications.3,4) However, restrictions on waste disposal, safety
concerns and stringent environmental regulations have
motivated the further development of other economically
viable technologies that pose fewer health risks and are more
eco-friendly.5)
Therefore, we had presented a novel method for recovering
gold: the leaching of gold by dimethyl sulfoxide (DMSO)
solutions containing CuBr2 followed by the precipitation of
gold with water. The critical components of this method
include the following: 1) copper (II) in DMSO acts as an
oxidizing agent and is reduced to copper (I) by oxidizing gold
in the presence of bromide ions; and 2) the addition of water
in the DMSO solutions will drastically change the solution
characteristics, in particular resulting in high redox potentials
of gold in mixed solutions. Therefore, water can be used for
the precipitation of the dissolved gold. This system offers a
number of advantages, including eco-friendliness, ease of
operation, low cost and the minimization of chemical sludge
production.6,7)
Here, we present the use of a more environmentally benign
solvent called propylene carbonate (PC) for this system. PC
+1
Graduate Student, The University of Tokyo
Corresponding author, E-mail: [email protected]
+2
is an outstanding aprotic solvent with a high relative
permittivity (¾r = 64.9), can dissolve many inorganic salts,
and has a wide liquid range (freezing point = ¹54.5
centigrade; boiling point = 241.7 centigrade).8) In addition,
PC is non-toxic and has a low vapor pressure at ambient
temperature, which is currently listed in the solvent category
of the US EPA Safer Chemicals Ingredients List for use in
DfE-labeled products.9) Compared with DMSO, PC contains
no sulfur in its molecular structure, which emits no sulfur
dioxide when combusted.
In PC, copper (I) is rather stable, while copper (II) is
poorly solvated.10) Thus, copper (II) in the PC is expected to
act as an oxidizing agent and be reduced to copper (I) by
oxidizing other substances. In addition, PC solvates water to
some extent but does not do so completely. After leaching
the metals into the PC solutions, a biphasic separation with
aqueous solutions can be conducted to separate the dissolved
metal ions. Therefore, there are many advantages to use PC
for our system.
In this paper, we have conducted fundamental studies to
investigate the possibilities of using PC solutions containing
copper (II) and bromide to leach and recover precious (e.g.,
Ag, Au, Pd) and rare metals (e.g., Co, Ni, Ti, W, Ta). Because
we intend to apply this method to recover these metals from
secondary sources, we also investigated other base metals
(e.g., Cu, Sn, Zn) that are contained in waste electronics.4,11)
First, electrochemical measurements of gold oxidation and
copper ion reduction in the PC solutions containing copper
(II) and bromide were conducted. Second, the dissolution
rates for the metals were investigated in the solutions. Third,
the dissolved metal ions were separated by a biphasic
separation using dilute sulfuric acid aqueous solution. Then,
the gold separated in the PC phase by the biphasic separation
was recovered by reduction using ascorbic acid or calcination. Finally, a preliminary optimized cost analysis for gold
recovery was conducted.
1580
2.
K. Umehara and Y. Matsuno
Materials and Methods
2.1 Materials
KBr (99%), tetra-n-butylammonium perchlorate, TBAP
(96%), PC (99%), ascorbic acid (99.6%) and sulfuric acid
(95%) were purchased from Kanto Chemical Co. Inc., Tokyo,
Japan. CuBr2 (99%) was purchased from Wako Pure
Chemical Industries, Ltd., Osaka, Japan. Commercially
available distilled water was purchased from Daiwa Ltd.,
Tokyo, Japan.
2.2 Methods
2.2.1 Electrochemical measurement
Because gold is the most noble metal with high redox
potentials, we focused on gold to investigate the electrochemical behavior in this study. Steady-state polarization
and cyclic voltammetry were used to measure the anodic
dissolution of gold and the redox potential of copper (I) and
copper (II) in PC solutions, respectively. The original setup
described elsewhere6) has been adapted as follows.
A double-junction (i.e., liquid-junction) reference electrode consisting of an Ag/Ag+ reference electrode (ALS Co.,
Ltd., RE-7 for organic solvent) covered with a salt bridge of
acetonitrile (AN) containing 0.01 M of TBAP was used. All
measured potentials in this study are shown in comparison to
this reference electrode. A platinum disk (º = 3 mm, ALS
Co., Ltd.) was used as the working electrode for cyclic
voltammetry. All electrochemical experiments were performed at 343 K. For the steady-state polarization measurements, PC solutions containing 0.01 M CuBr2 were used.
For the cyclic voltammetry measurements, PC solutions
containing 0.01 M CuBr2 and 0.4 M TBAP as a supporting
electrolyte were used. The sweep rate was set to 50, 100 and
200 mV/s, and all scans were started at the corresponding
rest potential.
2.2.2 Metal dissolution experiments
As was already mentioned, copper (II) in the PC is
expected to act as an oxidizing agent and be reduced to
copper (I) by oxidizing other substances. Then, the following
equilibria have been assumed to hold in the presence of
bromide:
Cuþ þ Br $ CuBr ðsolidÞ
ð1Þ
Cuþ þ 2Br $ CuBr2 ð2Þ
It is known that up to a bromide/copper (I) ratio of less
than 2, reaction (1) is predominant, and CuBr (solid) is
precipitated. Conversely, at higher bromide/copper (I) ratios,
the entire amount of copper (I) is complexed by reaction (2)
and is solvated in the PC solution.10) Therefore, in this study,
KBr was added into the PC solutions containing CuBr2 to
obtain a bromide/copper (I) ratio equal to 3, leading to the
stable solvation of copper (I).
All the metal dissolution experiments were conducted
as follows. Two millimoles each of CuBr2 and KBr were
dissolved in 10 ml of PC in a conical flask and maintained at
343 K. Then, each 1.2­3.0 mmol of Ag, Au, Co, Cu, Ni, Pd,
Sn, Ta, Ti, W, and Zn wires with a diameter of 0.2­0.5 mm
was placed in the solvent for dissolution on a shaking
apparatus. The remaining metal wire was removed from the
apparatus at various time intervals (0.5­8 h) so that its weight
could be measured to calculate the amount of dissolved
metals.
2.2.3 Biphasic separation of the dissolved metals
After the dissolution of each metal in 10 ml of PC with
2.0 mmol of CuBr2 and KBr, 10 ml of dilute sulfuric acid
aqueous solution (0.1 M) was added to the solution at
ambient temperature and shaken to produce a biphasic
separation of the dissolved metals. The contents of each
element in the PC and sulfuric acid phases were measured by
ICP-OES using an SII SPS5520.
2.2.4 Recovery of gold by reduction with ascorbic acid
or calcination
After biphasic separation, the sulfuric acid aqueous
solution phase was removed, and the dissolved Au in the
PC phase was recovered by reduction. 0.28 M ascorbic acid
was added to the PC phase as the reducing agent, shaken and
stood for 7 days at ambient temperature. The precipitated
substances were filtered and dried at 353 K for 24 h, after
which its weight was measured. The content of the
precipitated substances was analyzed using a FE-SEM
(Hitachi S4200) and EDS, the details of which were
described elsewhere.6) The recovery rate was calculated from
the amounts of the dissolved and recovered Au. In addition,
the dissolved Au in the PC phase was recovered via
calcination. After the calcination of the PC solution, the
impurities in the remaining substance were removed by
rinsing with pure water; the resulting material was then
analyzed by FE-SEM and EDS.
2.2.5 Cost analysis
A preliminary cost analysis was conducted to estimate the
optimum cost for the recovery of gold using the proposed
system. The optimum amounts of the solvent, solutes,
sulfuric acid, ascorbic acid, and the treatment of the waste
solution required for this system to recover 1 gram of gold
was calculated based on the results obtained above. The
prices for the solvents (PC), solutes (CuBr2 and KBr),
sulfuric acid, and reducing agent (ascorbic acid) were
obtained from an Internet survey. The cost for the treatment
of the waste solution was obtained from a facility located in
Tokyo, Japan.
3.
Results and Discussions
3.1 Electrochemical measurement
Figure 1 shows the steady-state polarization curves of gold
in the PC solution containing 0.01 M CuBr2. The rest (i.e.,
corrosion) potential was +377 mV. A limiting current of
0.36 µA was observed for cathodic polarization at approximately +0.25 mV. Conversely, no limiting current was
clearly observed for anodic polarization.
Figure 2 shows the cyclic voltammograms for 0.01 M
CuBr2 and 0.4 M TBAP in the PC for the platinum electrode
and those for 0.4 M TBAP in the PC as background
measurements. In the presence of 0.01 M CuBr2, cathodic
peaks were observed at +480, +474 and +468 mV at scan
rates of 50, 100 and 200 mV/s, respectively (labeled C1).
After changing the direction of the scan to ¹218 mV, the
current returned to zero and then showed anodic peaks at
+604, +607 and +616 mV at scan rates of 50, 100 and
200 mV/s (labeled A1).
Fundamental Studies on a Recycling System for Precious and Rare Metals Using a Propylene Carbonate Solvent
1581
0.2
3
0.18
Total dissolu on, 䠿 / mol/L
Current, log|I| / μA䞉cm-2
2.5
2
1.5
1
0.16
0.14
0.12
0.1
0.08
0.2 M CuBr2, 0.3 M KBr
0.06
0.2 M CuBr2, 0.2 M KBr
0.04
0.02
0.5
0
㻜
㻡
㻝㻜
0
㻜
㻜㻚㻝
㻜㻚㻞
㻜㻚㻟
㻜㻚㻠
㻜㻚㻡
㻜㻚㻢
㻜㻚㻣
me, t / hr.
㻝㻡
㻞㻜
㻞㻡
㻜㻚㻤
Potential, E / V
Fig. 1 Steady-state polarization curves of gold in PC solution containing
0.01 M CuBr2.
Fig. 3 Changes in the amount of dissolved Au with time in the PC
containing CuBr2 and KBr at 348 K.
0.25
40
30
Current, I / μA䡡mm-2
Total dissolu on, 䠿 / mol/L
A2
50 mV/sec (BG)
100 mV/sec (BG)
200 mV/sec (BG)
20
A1
50 mV/sec
100 mV/sec
10
200 mV/sec
0.2
Cu
0.15
Ni
Sn
0.1
Pd
Zn
0.05
Co
0
0
0
-10
-0.8
-0.6
-0.4
-0.2
0
0.2
Poten al, E / V (vs.
0.4
0.6
0.8
1
Fig. 2 Cyclic voltammograms for 0.01 M of CuBr2 and 0.4 M of TBAP in
the PC for a platinum electrode.
The current densities for the cathodic peaks (C1) and
anodic peaks (A1) are of nearly the same magnitude.
Therefore, these peaks can be considered to correspond to
the following reaction:
ð3Þ
From the results shown in Fig. 2, the redox potential of
reaction (3) for the PC solution containing 0.01 M CuBr2 was
determined to be +541 mV, which is more positive compared
with the rest (corrosion) potentials of gold, which are shown
in Fig. 1. All of these results suggest that the anodic
dissolution of gold in the PC containing CuBr2 occurs at
relatively negative potentials and is paired with cathodic
reaction (3).
It should be noted that other cathodic peaks at +673, +657
and +648 mV (labeled C2) and anodic peaks at +844, +865
and +884 mV (labeled A2) were also observed at scan rates
of 50, 100 and 200 mV/s, respectively, which were not
observed in the absence of CuBr2 (i.e., the background
measurements). In addition, the anodic peaks at A2 are
significantly larger compared with the cathodic peaks at C2.
These peaks can probably be attributed to the irreversible
oxidation of bromide as shown in the following equation,
which should be investigated in the further work:
2Br $ Br2 þ 2e
3
4
5
6
7
8
9
me, t / hr.
Ag|Ag+)
Cu2þ þ 2Br þ e $ CuBr2 2
C2
C1
-20
1
ð4Þ
Fig. 4 Changes in the amount of dissolved Cu, Ni, Sn, Pd, Zn, Co with
time in the PC containing CuBr2 and KBr at 348 K.
3.2 Leaching metals using the PC with CuBr2 and KBr
In this section, the results of the leaching experiments
performed in this study are summarized. Figure 3 shows the
changes in the amount of dissolved Au with respect to time,
and Fig. 4 shows those for Cu, Ni, Sn, Pd, Zn and Co.
3.2.1 Ag, Ta, Ti and W
Ag, Ta, Ti and W did not dissolve in the PC solutions. It is
assumed that Ta, Ti and W form surface oxide films in the air
and are passivated,12) which prevented their dissolution in the
solutions. In the case of DMSO solutions containing CuBr2
and KBr, Ag dissolved at a relatively fast rate.7) The reason
that Ag did not dissolve in the PC solutions should be
investigated in future work.
3.2.2 Au
As shown in Fig. 3, the PC solutions with 0.2 M of CuBr2
and 0.2 M of KBr, which contain 0.2 M of copper (II) and
0.6 M of bromide, dissolved approximately 0.12 M of Au and
reached equilibrium after 22 h, while those with 0.2 M of
CuBr2 and 0.3 M of KBr, which contain 0.2 M of copper (II)
and 0.7 M of bromide, dissolved approximately 0.17 M of
Au.
It was anticipated that the dissolution of gold occurs via
the following anodic electrochemical half-reactions:6)
AuðsÞ þ 4Br $ AuBr4 þ 3e
AuðsÞ þ 2Br $ AuBr2 þ e
ð5Þ
ð6Þ
Thus, the following overall reactions can be obtained:
AuðsÞ þ 3Cu2þ þ 10Br $ AuBr4 þ 3CuBr2 ð7Þ
1582
K. Umehara and Y. Matsuno
Table 1 Comparison of initial gold dissolution rates.
Dissolution rate,
mol·m¹2·h¹1
Leaching agents
Ref.
< 0.004
13)
SOCl2 and pyridine
0.3
13)
0.2 M CuBr2, 0.2 M KBr in DMSO
1.61
6)
0.2 M CuBr2, 0.2 M KBr in PC
1.34
This work
Cyanide
Sn
AuðsÞ þ Cu2þ þ 4Br $ AuBr2 þ CuBr2 ð9Þ
As the dissolution of Cu proceeded, the bromide/copper (I)
ratio in the solutions decreased. The precipitation of
substances was observed and could be attributable to the
following reaction:
Cu þ Cu2þ þ 2Br $ 2CuBr ðsolidÞ
ð10Þ
3.2.4 Ni, Sn, Pd, Zn, Co
It was anticipated that these metals dissolve via the
following anodic electrochemical half-reaction, where M
stands for Ni, Sn, Pd, Zn and Co:
M þ xBr $ M Brx þnx þ ne
ð11Þ
Thus, the following overall reaction can be obtained:
M þ nCu2þ þ ð2y þ xÞBr
$ M Brx þnx þ yCuBr2 Ni
Co
Cu
ð8Þ
The stoichiometric ratios of gold to copper (II) and
bromide in (7) and (8) suggest that the concentration of
bromide is the limiting factor for the maximum concentration
of dissolved gold, which is supported by the result that
the increase in the bromide concentration in the solutions
increases the maximum concentration of dissolved gold.
The initial gold dissolution rates (i.e., the average rate in
the first 30 min) were compared with those obtained in
previous investigations,6,13) which are shown in Table 1. The
initial gold dissolution rates in the PC solutions with CuBr2
and KBr were larger than those in conventional cyanide
leaching agents and the SOCl2/pyridine mixture, and of the
same order as that in the DMSO solutions with CuBr2 and
KBr.
3.2.3 Cu
As shown in Fig. 4, the dissolution of copper increased
with time and did not reach equilibrium within 8 hours. It
was anticipated that in the initial stage of dissolution, Cu
dissolved via the following reaction (9):
Cu þ Cu2þ þ 4Br $ 2CuBr2 Zn
ð12Þ
The dissolution of Ni and Sn increased with time and did not
reach equilibrium after 8 h. Conversely, the dissolution of Pd,
Zn and Co reached equilibrium at 0.08 M in 4.0 h, 0.05 M in
4.0 h and 0.04 M in 2.0 h, respectively. The electrochemical
oxidation and complex formation for these metals are
complicated; thus, future work should be conducted to
investigate the dissolution mechanism of these metals.
3.3 Biphasic separation of the dissolved metals
Figure 5 shows pictures for the biphasic separation of
the dissolved metals with sulfuric acid. The upper phase is
sulfuric acid, the color of which is blue, except for Pd,
Au
Pd
Fig. 5 Photos of the biphasic separation of the dissolved metals with
sulfuric acid.
Table 2
(%).
Distribution ratio of each metal in the PC and sulfuric acid phases
Metal
in the PC phase
in the H2SO4aq phase
Zn
9.39
90.6
Co
0.10
99.9
Ni
0.89
99.1
Sn
0.01
99.9
Pd
94.3
5.70
Au
97.9
2.08
indicating the presence of Cu2+ ion. Conversely, the lower
phase is the PC. The PC phase appears transparent for Sn,
Zn, Ni, Co and Cu, indicating an absence of metal ions.
Conversely, the colors of the PC phase for Au and Pd look
dark red, indicating the presence of dissolved Au and Pd ions.
In the case of Sn, white colored precipitates are observed in
the bottom of sulfuric acid phase.
The contents of each metal in the PC and sulfuric acid
phases measured by ICP-OES are shown in Table 2. It was
found that 98% of the Au and 94% of the Pd remain in the
PC phase, while most other metals were separated in the
sulfuric acid phase. These results indicated that the dissolved
Au and Pd can be effectively separated from other metals
using biphasic separation with sulfuric acid.
In the case of Sn, the precipitated substances were found at
the bottom of the sulfuric acid phase. It is known that tin
chloride hydrolyzes by the reactions (13),14) (14), (15) and
(16):15)
SnCl4 þ 2H2 O $ SnO2 þ 4HCl
ð13Þ
SnCl2 þ H2 O $ SnðOHÞCl þ HCl
ð14Þ
SnCl2 þ 6H2 O $ SnCl2 3SnðOHÞ2 þ 6HCl
ð15Þ
SnCl2 þ 2H2 O $ SnðOHÞ2 þ 2HCl
ð16Þ
Based on the E-pH diagram of the Sn-Br-H2O system,7) it is
likely that Sn was precipitated as SnO2 in the sulfuric acid
phase.
Fundamental Studies on a Recycling System for Precious and Rare Metals Using a Propylene Carbonate Solvent
Table 3 Compositions (mass%) of the recovered substances by reduction
with ascorbic acid and calcination, quantified by EDS point analyses.
Reduction with
ascorbic acid
Element
Calcination
O
15.3
3.05
K
0.034
0.027
Cu
0.061
4.97
Br
0.135
0.795
Au
84.4
90.7
Total
100
100
PC : 1 L
CuBr2 : 45 g
KBr : 24 g
gold
Biphasic
separa on
1583
Au : 21.7 g
Reduc on by
ascorbic
acid : 49.3 g
Sulfuric
acid : 1 L
PC to calcina on
Sulfuric
acid : 1 L
Table 4
Recovery rate of gold by reduction with ascorbic acid.
Total precipitation,
w/mg
Gold
content
(mass%)
Gold
content,
wgold/mg
Gold
dissolution,
wgold/mg
Recovery
rate (%)
225
84.4
190
189
100
Table 5
Fig. 6 Process flow diagram for recovering gold using the PC solutions
containing CuBr2 and KBr.
Table 6 Amounts and prices of the solvents, solutes, and the treatment of
the waste solution.
Recovery rates of gold by calcination.
Solvent (PC)
Total remained
substance,
w/mg
Gold
content
(mass%)
Gold content,
wgold/mg
Gold
dissolution,
wgold/mg
Recovery
rate (%)
178
90.7
161
194
83
3.4
Recovery of gold by reduction with ascorbic acid or
calcination
The dissolved gold obtained by the biphasic separation
was recovered by reduction with ascorbic acid. The
recovered substances were analyzed using an EDS point
analysis, which is shown in Table 3. The results were
obtained by averaging 10 points in the EDS analysis. It is
likely that oxygen can be attributed to the remaining organic
substances, such as PC, ascorbic acid, and its oxidized
substance (i.e., dehydroxyl ascorbic acid). Additionally, the
EDS point analysis cannot detect hydrogen. Therefore, it
should be noted that there are uncertainties in the results of
the compositions shown in Table 3.
Table 4 summarizes the total weight of the precipitated
substances, its gold content, and the recovery rate of gold. It
is shown that most of the dissolved gold can be recovered by
reduction with ascorbic acid.
The dissolved gold obtained by biphasic separation was
also recovered by calcination. The compositions of the
recovered substance obtained by the EDS point analysis are
shown in Table 3. Table 5 summarizes the total weights of
the remaining substances after calcination, their gold content,
and their recovery rates of gold.
It was found that 83% of the gold was recovered by
calcination. Some gold in the solution likely went into the air
with the soot produced by the calcination, which resulted in a
17% loss of the dissolved gold. The method of calcination
should thus be investigated to improve the recovery rate of
the dissolved gold.
3.5 Cost analysis
Figure 6 shows the optimized process flows of recovering
Amount
Price
Cost (USD)
1 liter
2.0 USD/liter
(1.2­2.4 USD/liter)
2.0
CuBr2: 45 g
20 USD/kg
(5­200 USD/kg)
0.89
KBr: 24 g
3 USD/kg
(2.5­3.8 USD/kg)
0.071
Solute
Sulfuric acid
for biphasic
separation
1 liter
0.1 USD/kg-H2SO4
(H2SO4: 7.6 g) (0.067­0.153 USD/kg)
0.0008
Ascorbic acid
for reduction
of gold
49.3 g
3 USD/kg
(1­5 USD/kg)
0.15
Treatment of
waste solution
1 liter
4.2 USD/liter
4.2
Total
-
7.3
Note: The numbers in the brackets show the variation of the price.
gold based on the recovered gold (i.e., 21.7 g-Au). Table 6
shows the amount and cost of the solvent, solutes, sulfuric
acid, ascorbic acid, and the treatment of waste solution
required for the proposed system. It was estimated that 21.7 g
of Au can be recovered for 7.3 USD; therefore, the cost for
recovering gold by this system is 0.34 USD/g-Au. The cost
to treat the waste solution has the largest share (58% of the
total); however, recovering solutes from the waste solutions
will decrease the cost. In addition, the prices of the solvent
and reagents vary significantly depending on their grade and
lot. Therefore, it should be noted that there is an uncertainty
in the results of the cost analysis results.
4.
Conclusion
It can be concluded that the PC solution containing CuBr2
and KBr is a promising organic liquor regius for leaching and
recovering precious and rare metals.
From the results of the electrochemical measurements, the
high redox potentials of gold are significantly lowered in the
PC in the presence of bromide ions. Therefore, copper (II)
1584
K. Umehara and Y. Matsuno
ions in the PC can act as an oxidizing agent for the gold in the
presence of bromide. The PC solutions can dissolve various
metals (i.e., Au, Pd, Co, Cu, Ni, Sn, Zn). Particularly for Au,
the initial gold dissolution rate in the PC solution is relatively
large compared to those of previous methods. Additionally,
biphasic separation by the addition of a dilute sulfuric acid
aqueous solution after the dissolution of each metal can
selectively separate Au and Pd from other metals because Au
and Pd are contained in the PC phase, while other metals are
contained in the sulfuric acid phase. Au in the PC phase can
thus be recovered by reduction and calcination.
The proposed organic liquor regius can be used to recycle
precious and rare metals from WEEE (Waste Electrical and
Electronic Equipment). Future work should be conducted to
determine the most suitable recycling system of precious and
rare metals from WEEE.
Acknowledgments
This study was supported by the JSPS KAKENHI Grant
Number 25550067, the Environment Research and Technology Development Fund (3K133006) of the Ministry of the
Environment, Japan and Iketani Science and Technology
Foundation.
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