Electrochemical sulfur removal from chalcopyrite in molten NaCl-KCl

Article abstract of DOI:10.1016/j.electacta.2016.07.088

Electrolysis of solid chalcopyrite (CuFeS₂) against a graphite inert anode has been studied in equimolar NaCl-KCl melt at 700?°C. During electrolysis, S^2? ions are released from the solid CuFeS₂ cathode, transfer to the graphite anode and discharge to S₂ gas. The reduction mechanism of CuFeS₂ was investigated by cyclic voltammetry, potentiostatic and constant voltage electrolysis together with spectroscopic and scanning electron microscopic analyses. The reduction contains mainly three stages: the insertion of Na⁺ or K⁺ into CuFeS₂, forming L_xCuFeS₂ (L?=?Na or K, x?≤?1); the partial reduction of L_xCuFeS₂ to L_x-wCuFe_1-yS_2-z and Fe; the complete reduction to a mixture of Cu and Fe, which can be magnetically separated. After the separation, pure Cu can be obtained by leaching out the residual Fe with acid. Electrolysis at a cell voltage of 2.4?V has led to a rapid reduction of CuFeS₂. The current efficiency and energy consumption were 85% and 1.68 kWh/kg-CuFeS₂, respectively.

Full text of DOI:10.1016/j.electacta.2016.07.088

Accepted Manuscript
Title: Electrochemical sulfur removal from chalcopyrite in
molten NaCl-KCl
Author: Mingsheng Tan Rui He Yating Yuan Zhiyong Wang
Xianbo Jin
PII:
S0013-4686(16)31593-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2016.07.088
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21-3-2016
13-7-2016
15-7-2016
Please cite this article as: Mingsheng Tan, Rui He, Yating Yuan, Zhiyong Wang,
Xianbo Jin, Electrochemical sulfur removal from chalcopyrite in molten NaCl-KCl,
Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.07.088
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Electrochemical sulfur removal from chalcopyrite in molten NaCl-KCl
Mingsheng Tan, Rui He, Yating Yuan, Zhiyong Wang, Xianbo Jin*
College of Chemistry and Molecular Sciences, Wuhan University, Wuhan, 430072, P.
R. China
*
Corresponding author: Tel & Fax +86 27 68756319
E-mail address: xbjin@whu.edu.cn (X. B. Jin)
1

ABSTRACT
Electrolysis of solid chalcopyrite (CuFeS ) against a graphite inert anode has been
2
2
-
studied in equimolar NaCl-KCl melt at 700 °C. During electrolysis, S ions are
released from the solid CuFeS cathode, transfer to the graphite anode and discharge
2
to S gas. The reduction mechanism of CuFeS was investigated by cyclic
2
2
voltammetry, potentiostatic and constant voltage electrolysis together with
spectroscopic and scanning electron microscopic analyses. The reduction contains
+
+
mainly three stages: the insertion of Na or K into CuFeS , forming L CuFeS (L=Na
2
x
2
or K, x≤1); the partial reduction of L CuFeS to L CuFe S and Fe; the complete
x
2
x-w
1-y 2-z
reduction to a mixture of Cu and Fe, which can be magnetically separated. After the
separation, pure Cu can be obtained by leaching out the residual Fe with acid.
Electrolysis at a cell voltage of 2.4 V has led to a rapid reduction of CuFeS . The
2
current efficiency and energy consumption were 85 % and 1.68 kWh/kg-CuFeS2,
respectively.
Keyword: chalcopyrite metallurgy, metal sulfides, molten salts, solid state
electrolysis, copper.
2

Introduction
Over 200 copper minerals were found in the earth’s crust, while only about 10 % have
industrial value. Chalcopyrite (CuFeS ) is the most important mineral for Cu
2
extraction. Presently, chalcopyrite is treated by the smelting process: CuFeS is firstly
2
calcinated to FeO, CuO, Cu and SO ; after slagging the FeO with SiO , the melting
2
2
CuO is reduced to Cu by anthracite or natural gas. Obviously, this traditional process
is environmentally unfriendly, which suffers from double emission of SO and CO
2
2
[
1].
To circumvent the handling of hazard SO , various hydrometallurgical techniques
2
have been tried, aiming to decompose the metal sulfide into metal and sulfur [2]. One
approach is direct leaching, which involves the use of large amount of acid, and at the
same time, oxidant is needed to simultaneously convert the sulfur ions to S to avoid
H S emission [2]. Another approach is direct electrochemical leaching. Similar to the
2
industrial electrolysis of Ni S , the chalcopyrite concentrate is used as solid anode,
3
2
during electrolysis, the metal ions are liberated into the solution, transfer to and
deposit at the cathode. Unfortunately, the anode will be passivated promptly by the
deposition of non-conducting sulfur [2, 3].
Molten salt electrolysis of Cu S has been reported for decades. Obviously, pure Cu S
2
2
melt is not a suitable electrolyte due to its high electronic conductivity [4]. It was
reported that addition of CuCl, an ionic melt, to a mole fraction of 70% or higher can
significantly decrease the electronic conductivity of Cu S [4, 5]. The Cu S + CuCl
2
2
3

electrolyte with the mole fraction of Cu S not exceeding 30% was thus suggested as
2
electrolyte for the electrowinning of Cu [5]. The electrolysis temperature was
suggested to be higher than the boiling point of sulfur (444.6 °C), and therefore, the
anodic product could be always sulfur vapor to be simply swept out by argon. But
relatively high current efficiencies were only obtained at very low apparent current
2
densities (up to 15 mA/cm ) [5]. In the case of Cu S + FeS + CuCl electrolyte, it is
2
interesting that Cu can be selectively deposited. However, the low productivity and
the formation of dendrite copper deposition at the cathode inhibits the practical
application of these electrolytic processes [5]. More recently, molten salt electrolysis
of Cu S has been investigated in Cu S + BaS melt at 1105 °C [6]. The advantages of
2
2
2
this high temperature process includes high electrolysis current (2.5 A/cm ), and
deposition of liquid copper to avoid formation of dendrite at the cathode, but the low
current efficiency (9 ~ 28%) has to be improved.
Alternatively, electrochemical split of various metal sulfides to respective metals and
elemental sulfur have been realized by solid cathode electrolysis in molten chlorides
[
7-10]. During electrolysis, the metal sulfides and the resulting metals stay in solid so
2
-
no occurrence of dendrite formation at the cathode. It is the S ions that leave the
cathode, diffuse through the electrolyte, and discharge to S gas at the graphite anode.
2
Unlike the molten salt electrolysis discussed above that require fusion or dissolution
of metal sulfide, the solid metal sulfide cathode electrolysis requires an insignificant
dissolution of the metal sulfide in the electrolyte. As reported, the electrolysis can
2
-
proceed quickly as long as the electrolyte can promptly transfer S ions.
4

Solid cathode electrolysis of metal sulfides were initially investigated in CaCl based
2
molten chlorides that are often used for solid oxide electrolysis [11-13]. However,
2
-
although the CaCl based electrolytes are good for O transfer, they have limited
2
2
-
dissolution capacity for S ions, and CaS byproduct remains in the cathode even after
prolonged electrolysis at temperatures above 800 °C. In addition, the residual CaO in
the electrolyte due to the easy hydrolysis of CaCl results in anodic consumption of
2
graphite consequently carbon contamination to the molten electrolyte and the metal
2
-
product [9, 10]. On the contrary, NaCl and KCl almost cannot dissolve O ions but
2
-
have great S solubility (for example, 39 mol/L in NaCl at 712 °C [14]). They are
also much less hygroscopic than CaCl . In molten NaCl-KCl, it has been reported that
2
electrolysis of WS (MoS ) to W (Mo) and sulfur can proceed rapidly with high
2
2
current efficiency, and no corrosion to the graphite anode takes place after long-term
electrolysis [7, 8, 15].
This work aims to removal of sulfur from chalcopyrite by solid cathode electrolysis in
the equimolar NaCl-KCl melt at 700 °C. The reduction mechanism of CuFeS has
2
been elucidated according to experimental results from cyclic voltammetry (CV),
potentiostatic and constant voltage electrolysis. We will also demonstrate the novel
magnetism-assistant separation of Cu from the final cathodic product, a mixture
consisting of mainly metallic Cu and Fe.
Experiment
The as-received analytical grade NaCl (352g) and KCl (448g) (99.5%, Sinopharm
Chemical Reagent Co., Ltd, China) were mixed together and loaded into a graphite
5

crucible (inner diameter: 9.0 cm; height: 23.5 cm, AR grade, Jing Shi Ceramics Co.,
Ltd. Shanghai, China), which was then placed in a stainless steel reactor under argon
(99.999%, Wuhan Iron and Steel (Group) Corp., China) protection. The temperature
of the reactor was controlled by a programmable furnace. The equimolar NaCl-KCl
mixture was heated to 300 °C and kept at 300 °C for 10 h to remove moisture, then at
a heating rate of 3 °C/min the temperature was raised to 700 °C for the
electrochemical experiments. Pre-electrolysis was carried out by applying 2.3 V
between a nickel foil cathode and a graphite anode (15 mm in diameter, 99.99%,
JingShi Ceramics Co., Ltd. Shanghai, China) in order to remove possible impurities in
the melt.
Cyclic voltammograms (CVs) of solid chalcopyrite (ore concentrate, containing about
8
7% CuFeS in mass with SiO and Al O as the main impurities, Jinchuan Group Co.,
2 2 2 3
Ltd., Gansu, China) in the equimolar NaCl-KCl melt were recorded using a
computer-assisted IviumStat (IVIUM Technologies B.V., Eindhoven, Netherlands)
electrochemical workstation. The working electrode was a Mo metallic cavity
electrode [16] (MCE, including two caves with 0.5 mm in both diameter and
thickness) filled with the CuFeS powders (the total load of chalcopyrite was about
2
0
.45 mg, corresponding to about 0.39 mg CuFeS ). The potential was controlled
2
against a quartz sealed Ag/AgCl reference electrode [17]. For the constant potential or
cell voltage electrolysis, the chalcopyrite powders were pressed (8 MPa) into
cylindrical pellets (2.0 cm in diameter, ~1.90 g chalcopyrite or ~1.65 g CuFeS ), then
2
sandwiched between two molybdenum meshes to form assemble cathodes. The
6

electrolysis was controlled by a multichannel four-electrode potentiostat (Neware,
Shenzhen, China). After each electrolysis, the cathode was pulled out, cooled in argon,
washed with distilled water and dried under vacuum at 80 °C. The product was
characterized by X-ray diffraction spectroscopy (XRD; Shimadzu 6000, Kyoto, Japan,
Cu-Kα, λ=1.54178 Å), and scanning electron microscope (FE-SEM; FEI Sirion Field
Emission Gun SEM system, Eindhoven, Netherlands) together with energy dispersive
X-ray (EDX; EDAX ,GENESIS 7000, AMETEK, Inc., Mahwah, NJ, USA) analysis.
Results and discussion
Fig. 1a and b show the CVs of the blank Mo cavity electrode (MCE) [16, 18] and the
MCE loaded with CuFeS powders at 700 °C. At the blank MCE, the large cathodic
2
current at about -2.4 V followed by a re-dissolution peak at about -2.3 V on the anodic
branch (Fig. 1a), labeled as redox couple c/a, can be attributed to the
reduction/re-oxidation of the cations from the melt. The small anodic peak at 0.05 V
(labeled as a0) possibly corresponds to the oxidation of molybdenum. Two
consecutive CVs of the CuFeS are shown in Fig. 1b. There are five reduction peaks
2
c1- c5 and an oxidation peak a1 on the first CV. Considering that the SiO and Al O
3
2
2
impurities in the chalcopyrite concentrate are almost inert during the study, the
multiple reduction peaks between 0 and -2.0 V suggest the complexity of
electrochemical reduction of CuFeS in the NaCl-KCl melt. However, these reduction
2
peaks could be hardly seen in the following cycles, indicating that all the CuFeS2
powders in the cavity have been reduced at the first negative scan. The total integral
charge between -0.3 V and -2.0 V (before the formation of Na or K) was calculated to
7

be about 0.8 C, in good agreement with the theoretical charge needed (~0.82 C) for
the full reduction of ~0.39 mg CuFeS . The peak a1 in Fig. 1b should then be the
2
oxidation of metallic Cu and Fe, which was confirmed by comparing to the CVs of
MCEs loading with Fe powders (Fig. 1c) and Cu powders (Fig 1d).
In order to further confirm that all the sulfur of CuFeS in the MCE can be completely
2
removed by the single cathodic scan, the reduction product after the scan was analyze.
Fig. 2a shows the raw chalcopyrite powders, which consists of much nonuniform
particles with some larger than 50 micrometers. After the first cathodic scan, the
powders loaded in the Mo cave (Fig. 2b) became more porous (Fig. 2c), in line with
2
-
the expected volume change after removal of S ions. The morphology also changed
significantly (Fig. 2d). EDX analysis shows that the original strong peak of S in the
precursor disappears in the product. These findings demonstrate that the reduction of
CuFeS can proceed very quickly, which can be explained from five aspects: (1) the
2
load of CuFeS in each Mo cave is relatively low (~ 0.2 mg); (2) the relatively large
2
overpotential when scan to -2.2 V; (3) the electronic conductivity of the metal sulfide
allows the reduction to take place at all the metal sulfide/melt interfaces; (4) the high
2
-
porosity metal generated by removal of S allows fast ionic diffusion and (5) the
2
-
NaCl + KCl melt has a very high S solubility (~39 mol/L in NaCl at 712 °C) [7, 8,
4,15].
In our previous study, we found that MoS in the NaCl-KCl melt could undergo
1
2
+
+
+
electrochemical insertion of Na and/or K at the initial stage of reduction [7]. Na
+
and K can also be inserted into CuFeS most likely combining with the valence
2
8

change from Fe (III) to Fe (II), forming compounds like NaCuFeS and KCuFeS
2
2
+
+
[
19-21]. CuFeS may undergo insertion of Na and/or K during reduction as well.
2
CVs in different potential ranges were recorded to clarify the situation. Fig. 3a shows
three consecutive CVs of CuFeS in the potential range of c1 and c2 (from 0.05 to
2
-1.0 V) at a scan rate of 50 mV/s. It is clear that there are mainly two redox couples,
i.e. c1/a1 and c2/a2. These peaks are fairly stable in the three cycles, and particularly,
c1/a1 can be subdivided, featuring the continuous insertion/de-insertion behaviors.
However, when the negative potential limitation was extended to -1.3 V, all the
reduction and re-oxidation currents decreased significantly with the cycle number,
suggesting the gradual consumption of CuFeS . Considering that the c3/a3 redox
2
couple is as reversible as the c2/a2 couple, c3 could be another insertion reaction. On
the other hand, it is clear that the biggest reduction peak c4, which might correspond
to the formation of metallic product, begins at about -1.2 V.
In order to make further understanding of the electrochemical reduction process, a
series of potentiostatic electrolysis of CuFeS pellets were carried out at different
2
potentials between -0.9 V and -1.6 V, and the cell voltages were recorded
simultaneously. The CuFeS in each pellet was about 1.65 g in mass, and the
2
theoretical charge Q needed for complete reduction was about 3471 C. As shown in
Fig. 4, when the imposed potential was -0.9 V, the current-time plot exhibited a
typical initial large spike followed by a quick decay to the background level. The
corresponding cell voltage change should include the variations of anodic polarization
and Ohm drop, both would decrease with the current. We thus observed a high initial
9

cell voltage followed by a quick decrease to a plateau voltage near 0.7 V. Combined
with the CV study, the small reduction current at -0.9 V is probably due to the
formation of L CuFeS (x≤1, L = Na or K) as a result of electrochemical insertion of
x
2
+
+
Na and/or K . The integral cathodic charge of about 261 C (notice that background
current is neglectable) for the 1.65 g CuFeS pellet corresponds to the x in L CuFeS
2
2
x
of about 0.3. Indeed, several small peaks appeared at the XRD pattern of the -0.9 V
electrolysis product (Fig. 5), likely due to the formation of inserted compound.
The shape of current-time curve at -1.2 V was similar to that at -0.9 V (Fig. 4), with
the reduction charge increased to about 560 C, corresponding to x = 0.55 in L CuFeS
x
2
o
if all the charge are for the insertion reaction. The ~ 15 XRD peak of the -0.9 V
product shifted to ~13.5° (the characteristic peak of LCuFeS ), also indicating an
2
increased x after the -1.2 V electrolysis. However, there was still a noticeable cathodic
current after the 1 h electrolysis, which could be indication of another ongoing
reaction, probably the reduction of L CuFeS to Fe considering the appearance of
x
2
XRD peak at about 45°. To confirm these analysis, elemental mapping of the -1.2 V
and 1h electrolysis product was performed. Fig. 6a shows a typical SEM image of the
product, which include two typical morphologies: aggregation of big and irregular
granules and aggregation of fine dendrites. Distributions of elements Na, K, Cu, Fe
and S are displayed in Fig. 6b~f. It is clear that the fine dendrites in the middle consist
mainly of Fe, suggesting the formation of metallic Fe. On the other hand, Na, K, Cu,
Fe and S distributed homogenously in the big particles, indicating the formation of
+
+
Na and K inserted compound, particularly K CuFeS , considering the much stronger
x
2
10

signal of K than Na.
The electrolysis at -1.5 V for 2 h resulted in a reduction charge of about 1560 C,
which is much larger than the amount (865 C) needed for the formation LCuFeS . The
2
XRD pattern of the -1.5 V product shown in Fig. 5 indicates a mixture of L CuFeS ,
x
2
Fe and Cu. The XRD peaks of L CuFeS after the 2 h electrolysis were still very
x
2
strong, suggesting the slow reduction of L CuFeS to Cu and Fe at -1.5 V, and the
x
2
reduction was still ongoing considering the large cathodic current after the 2 h
electrolysis (larger than the background current at -1.6 V). This could be largely due
to the great kinetic barrier to overcome considering the recorded cell voltage was as
high as 1.7 V, while the theoretical decomposition voltage of CuFeS to Cu, Fe and S
2
2
Θ
r
o
gas is only about 450 mV (CuFeS = Cu + Fe + S (g), ΔG = 171.3 kJ/mol, 700 C).
2
2
Fortunately, when the cathodic polarization was increased to -1.6 V, the
electrochemical reduction of CuFeS to Cu and Fe became very quick. The XRD
2
pattern shown in Fig. 5 indicates that the cathode pellet has been completely reduced
in the 2 h electrolysis. The current-time curve (Fig. 4) shows an initial large current,
corresponding to the insertion reaction, followed by two current plateaus, possibly
corresponding to the formation of Fe and Cu respectively. According to the XRD
pattern of the -1.2 V electrolysis product, Fe was first generated during the reduction.
Theoretically, the reduction of iron sulfide is also slightly easier than copper sulfide at
o
7
5
00 C, considering the thermodynamic decomposition voltages of FeS and Cu S are
2
Θ
r
16 mV (2FeS = 2Fe + S (g), ΔG = 199.4 kJ/mol) and 532 mV (2Cu S = 4Cu + S
2
2
2
Θ
r
(
g), ΔG = 205.3 kJ/mol) respectively. Moreover, K, Cu, Fe ternary sulfides with
11

different stoichiometric ratios but the same crystal structure (such as K Cu FeS and
2
3
4
KCuFeS ) have been reported. Thus, the step-by-step reduction of LCuFeS to metals
2
2
is possible.
The above findings allow to presume the main steps of the electro-reduction of
CuFeS in the NaCl-KCl melts, which are represented by reactions (1) to (3) below:
2
+
x L + CuFeS + x e = L CuFeS
(1)
(2)
(3)
2
x
2
+
2-
L CuFeS + (2z-w) e = L(x-w)CuFe(1-y)S(2-z) + yFe + wL + zS
x
2
+
2-
L(x-w)CuFe(1-y)S(2-z) + (4-2z+w-x)e → Cu + (1-y)Fe + (x-w)L + (2-z)S
+
+
(
L= Na or K , 0≤w<x≤1, 0< y≤1, 0≤z<2)
It can be confirmed that CuFeS is first reduced to L CuFeS according to Reaction
2
x
2
(1). Then Fe will be generated before Cu, probably accompanying with some low-Fe
ternary sulfides (L(x-w)CuFe(1-y)S(2-z)). The XRD pattern of the partially reduced
L CuFeS at -1.2 V as shown in Fig. 5 can then be well explained by Reaction (2),
x
2
considering some low-Fe sulfides (such as K Cu FeS ) have XRD patterns similar to
2
3
4
LCuFeS [21, 22]. In the present study, no evidence of formation of Cu S or L CuS
y
2
2
x
as reduction intermediate was observed. However, it is not sure whether the low-Fe
ternary sulfides were reduced to Cu and Fe simultaneously, or to Cu S (or L CuS )
2
x
y
and Fe followed by a fast reduction of Cu S to Cu. Nevertheless, Reaction (2) and (3)
2
can include possible reaction steps for the electrochemical reduction of L CuFeS in
x
2
molten NaCl-KCl.
Although the electrochemical reduction of CuFeS could be more complicated, the
2
above experimental results indicate that fast electrolysis can be implemented at a cell
12

voltage larger than 2.3 V (see the recorded cell voltage during electrolysis at -1.6 V,
Fig. 4). Subsequently, constant cell voltage electrolysis was carried out to mimick the
common industrial practices. The CuFeS in the cathode pellet was about 1.65 g, and
2
the applied cell voltage between the solid cathode and the graphite anode was 2.4 V to
ensure that both the thermodynamic and kinetic difficulties encountered in the
reduction of the intermediates could be overcome. The current response is shown in
Fig. 7, suggesting several reduction stages. These currents can be interpreted by
linking to the XRD analyses (Fig. 8) of the partially and fully reduced products.
The first stage lasted for about 3 ~ 6 min with the largest current, suggesting a very
fast reaction (Fig. 7). The XRD pattern of the product after electrolysis at 2.4 V for 5
min (Fig. 8) shows exactly KCuFeS phase (JCPSD No. 51-0744), indicating that the
2
x (in L CuFeS ) could increase to 1 in a short time at a high voltage, and the
x
2
+
electrochemical insertion of K ion via Reaction (1) should be of priority. This is also
+
in line with our observation in Fig. 6, which indicate that the inserted ion is mainly K .
The 15~30 min product consisted of mainly Fe and a small quantity of Cu, indicating
that the second stage (6~30 min) mainly corresponds to the reduction of L CuFeS to
x
2
Fe, which is still fast. The third stage between 30 and 80 min was relatively slow, and
the 60 min product shows significantly increased Cu phase. These three reduction
stages are also in good agreement with the above observation for electrolysis at a
constant potential of -1.6 V. Both the constant potential and cell voltage electrolysis
indicate that the electrochemical reduction of solid CuFeS in the NaCl-KCl melt
2
undergoes an insertion reaction to L CuFeS , followed by a partial metallization with
x
2
13

Fe generated and finally a full metallization to Cu and Fe.
The SEM image of the metallic product from the 120 min electrolysis at 2.4 V is
shown in Fig. 9. There are basically two kinds of morphologies. One is aggregations
of small particles with sizes less than 5 μm. The other is belt-like, with the width of
about tens of micrometers. The EDX analysis indicate that the aggregations of small
particles consist mainly of Fe while the big belts were mainly Cu. On the other hand,
the anodic gas S has been led out by argon. After condensation, the anodic product
2
was in a pale yellow color, which was proved to be S by XRD (not shown here). No
8
corrosion to the graphite anode was found after electrolysis for one week. The
observation that graphite is an ideal non-consumable anode is not only in line with the
results of previous electrolysis of solid WS and MoS in the same electrolyte [7, 8],
2
2
but also agrees with those observations during electrolysis of dissolved metal sulfides
in molten chlorides [23, 24]. According to Fig. 7, there existed some background
current after the completion of the chalcopyrite reduction, which could be of an
electronic nature as often observed during molten salt electrolysis. The current
efficiency was calculated to be about 85 % for the 120 min electrolysis of about 1.65
g CuFeS at a cell voltage of 2.4 V, and the energy consumption was about 1.68
2
kWh/kg-CuFeS2.
We thus have demonstrated an efficient process to remove the sulfur ions from solid
chalcopyrite and convert them to elemental sulfur. A mixture of Cu and Fe metals is
the reduction product of solid chalcopyrite. It is interesting that Cu and Fe in the
electrolysis product are phase separated and have great difference in particle sizes.
14

Various separation routes may be designed according to the different physical and
chemical properties between the two metals. One possible way could be direct gravity
separation, considering their size difference. Electrolytic refining in aqueous solution
could be another way, taking advantage of the great different between the equilibrium
dissolution/deposition potentials of Cu and Fe, which is as large as 0.78 V considering
2
+
2+
the Cu /Cu and Fe /Fe couples for Cu and Fe respectively. It is also possible to
separate them by leaching in aqueous solution, noting that some acids can dissolve Fe
but not Cu. Here a magnetism-assistant separation method has been implemented.
Considering Fe is a magnetic metal but Cu is not, it is very possible to separate the
two metals by magnetic force. As shown in Fig. 10a, after magnetic stirring of the
electrolysis product in water, the iron powders were collected on the magnetic stirrer.
The rest powders were let to settle and collected by filtering. XRD analysis shows that
the collected purple bronze powders are mainly Cu, with some small diffraction peaks
of Fe (Fig. 10b). The small amount of iron was then removed by leaching in HCl
solution, and pure Cu was obtained as confirmed by the XRD analysis (Fig. 10b).
These results indicate that after removal of S from the chalcopyrite, the post
separation of Cu and Fe could be easy.
Conclusions
In summary, facile removal of sulfur ions from chalcopyrite (CuFeS ) through solid
2
cathode electrolysis, with a mixture of Cu and Fe generated at the cathode and
elemental sulfur generated at the graphite anode, can be achieved in NaCl-KCl at
7
00 °C. Cyclic voltammetry, potentiostatic and constant cell voltage electrolysis
15

together with XRD, EDX analysis and SEM observation showed that CuFeS was
2
+
first reduced to L CuFeS through the continuous electrochemical insertion of Na
x
2
+
and/or K . In the following reduction, Fe was firstly generated and L CuFeS was
x
2
converted to L(x-w)CuFe(1-y)S(2-z), and finally to a mixture of Cu and Fe. Rapid
electrolysis of CuFeS has been realized at a cell voltage of 2.4 V with a current
2
efficiency of about 85 %. In the final metallic product, the big belt-like Cu (10 ~ 30
μm in width) and small particles of Fe (less than 5 μm in particle size) were phase
separated. Magnetic separation of Cu and Fe from each other was carried out and after
acid leaching of the residual Fe in Cu, pure Cu was obtained. These findings can form
the base for the development of a new, fast, and environmental friendly electrolytic
process for the metallurgy of chalcopyrite and other sulfur-copper mines.
Acknowledgements
This work is supported by NSFCs (21173161, 20973130), the Fundamental Research
Funds for the Central Universities.
16

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[
[
[
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19

Figure Captions
Fig. 1. Cyclic voltammograms (CVs) of (a) the blank Mo cavity electrode (MCE, with
the SEM image inset), (b) MCE loaded with chalcopyrite (CuFeS ) powders (with the
2
SEM image inset), (c) MCE loaded with Fe powders and (d) MCE loaded with Cu
powders electrode in equimolar NaCl-KCl at 700 °C.
Fig. 2. The typical SEM images of (a) chalcopyrite (CuFeS ) powders; (b) MCE
2
loaded with CuFeS ; (c and d) the reduction product of (b) after the first cathodic scan
2
from -0.2 V to -2.3 V (vs. Ag/AgCl) as shown in Fig. 1b. The insets show the
corresponding EDX analysis.
Fig. 3. Consecutive CVs of the MCE loaded with CuFeS powder in the potential
2
range (a) 0.05 ~ -1.0 V and (b) 0.05 ~ -1.3 V at 50mV/s in the equimolar NaCl-KCl
mixture at 700 °C.
Fig. 4. Typical current-time plots recorded during the potentiostatic electrolysis of
pellets at the indicated potentials in molten NaCl-KCl (700 °C). The inset shows the
corresponding cell voltage-time curves.
Fig. 5. XRD patterns of potentiostatic electrolysis products as indicated potentials at
7
00 °C in molten NaCl-KCl.
Fig. 6. (a) SEM image and the corresponding elemental mapping (b) sodium (Na), (c)
potassium (K), (d) copper (Cu), (e) iron (Fe) and (f) sulfur (S) of product obtained at
20

-
1.2 V for 1 h.
Fig. 7. The current-time plots recorded during the constant cell voltage electrolysis at
.4 V.
2
Fig. 8. XRD patterns of raw materials and products obtained at 2.4 V for different
times (chalcopyrite pellets ca. 1.9 g).
Fig. 9. Typical SEM images of the products from constant cell voltage electrolysis of
chalcopyrite pellets at 2.4 V for 2 h. The inset is the corresponding select area EDX
analysis.
Fig. 10. The photos of powders obtained at 2.4 V for 2 h (a) after magnetic stirring, (b)
XRD patterns of corresponding powders through different treatment process as
indicated.
21

Figure 1
0
.08
.00
0.08
1
st cycle
(
a)
-1
a0
(b)
blank 50mVs
0
.04
.00
2nd cycle
a1
0
-
1
a0
5
0mVs
a
0
-0.08
-0.16
-0.24
c1
c2
-
0.04
0.08
c3
c5
-
c4
c
-0.12
-
.06
.04
.02
.00
2.5 -2.0 -1.5 -1.0 -0.5
0.0
-2.5 -2.0 -1.5 -1.0 -0.5
0.0
Potential/V (vs.Ag/AgCl)
Potential/V (vs.Ag/AgCl)
0
0.02
(d)
(
c) Fe powder, 10 mV/s
Cu powder, 10 mV/s
0
0
0
0
.00
-
0.02
-
0.02
0.04
blnak, 50 mV/s
blank, 50 mV/s
-
-0.04
-
2.5 -2.0 -1.5 -1.0 -0.5
0.0
-2.4 -2.0 -1.6 -1.2 -0.8 -0.4 0.0
Potential/V (vs.Ag/AgCl)
Potential/V (vs.Ag/AgCl)
22

Figure 2
S
A
D
B
C
Cu
Si
Fe
Fe
Al
Cu
O
C
Fe Cu
0
2
4
6
8 10
Energy/keV
Cu
Cu
Fe
Fe
O
Al
Si
Fe Cu
C
0
2
4
6
8
10
Energy/keV
23

Figure 3
0.03
0.02
0.01
0.00
a2
(a)
a1
1
2
3
st
nd
rd
-
-
-
0.01
0.02
0.03
c1
-0.4
5
0 mV/s
c2
-1.2
-0.8
0.0
Potential/V (vs.Ag/AgCl)
0
.03
.02
.01
.00
a2
(b)
a3
1
2
3
st
nd
rd
a1
0
0
0
-0.01
-0.02
-0.03
c4
c1
5
0 mV/s
c3
1.2
c2
-0.8
-
-0.4
0.0
Potential/V (vs.Ag/AgCl)
24

Figure 4
4
3
2
1
0
3
2
1
0
-
-
-
1.6V, 2h
1.5V, 2h
1.2V, 1h
-0.9V, 1h
0
30
60
Time/min
90
120
0
30
60
Time/min
90
120
25

Figure 5
2
2
1
LxCuFeS2
3
-
1.6V,2h
2
3
2
3
Fe
Cu
3
1
1
1
3
11
1
-
1.5V,2h
1
1
1
1
2 1
1
-
1.2V,1h
1

1
1
1
-0.9V, 1h
raw
1
0
20
30
40
50
60
70
80
2
Theta/degree
26

Figure 6
(
a)
(b)
(e)
(c)
(f)
(d)
N
F
K
S
C
27

Figure 7
6
4
2
0
2
.4 V, 2h
0
20 40 60 80 100 120
Time/min
28

Figure 8




LxCuFeS2
Cu
Fe
2
.4V, 60min



2.4V, 30min

2.4V, 15min






2.4V, 5min
raw

1
0 20 30 40 50 60 70 80
2Theta/degree
29