Electrochemical Oxidation of Sulfide Copper-Nickel Alloys:
Thermodynamics, Passivating Films and Chemism
Selivanov E.N., Nechvoglod O.V., Pankratov A. A.
Establishment of the Russian Academy of Sciences Institute of Metallurgy of Ural
Division RAS
101 Amundsen St., Ekaterinburg, 620016 Russia
e-mail: pcmlab@mail.ru
ABSTRACT
Thermodynamically stable forms and compounds of copper, nickel, and sulfur
ions in water solutions have been determined at preset values of pH and the sulfide
electrode potential. Also, probable potentials at the electrodes have been found for the
passage of the metals into solution and sulfur to the elemental form. The phase
composition of the passivating films formed upon the electrochemical oxidation of
copper and nickel sulfides and their alloys in a sulfate solution has been evaluated in
experiments. The chemism of the electrochemical oxidation of complex sulfide-metal
copper-nickel alloys has been explained.
Introduction
The electrochemical oxidation of copper and nickel sulfide alloys is
accompanied by the formation of intermediate and final sparingly soluble products [14], which affect the dissolution kinetics. Depending on the parameters, the oxidation
of copper and nickel sulfides and their alloys has different chemisms. To evaluate the
character of passivation and propose a method for intensification of the oxidation of
the sulfides, it is necessary to have information about the mechanism of
electrochemical processes and data on the phase composition of intermediate and final
products of reactions.
With the aim of developing a technological scheme for processing of sulfide
raw materials by an electrochemical method, it is interesting to study the behavior of
the phase components of a sulfide alloy during oxidation under polarization
conditions and establish regular features of the electrochemical oxidation of sulfide
alloys at the electrode/electrolyte interface. Investigations on specific features of the
electrochemical processes, which take place during the electrolysis of copper-nickel
sulfide alloys, are significant for advancing the theoretical basis of metallurgical
processes in production of nonferrous metals and developing technologies for
processing of sulfide materials, which will exclude emission of sulfur dioxide.
The aim of this study was to establish specific features of the formation of
passivating films and determine the sequence of transformations during the
electrochemical oxidation of sulfide-metal alloys.
Experimental
The regions of stability of the forms of nickel, copper, and sulfur in aqueous
solutions were determined by the method of thermodynamic simulation using an HSC
5.1 program package. The thermodynamic simulation was performed on the following
assumptions:
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- the calculations were made taking into account the properties of pure
stoichiometric substances;
- the thermochemical calculations were made disregarding the reaction rate;
- although a negative value of pH is devoid of the physical meaning, it still
characterized a region with a high acidity of the solutions.
The surface of sulfides was oxidized to experimentally evaluate the composition of
the products after the electrochemical oxidation of the copper-nickel sulfide alloys.
Synthesized nickel and copper sulfides, as well as copper-nickel and nickel converter
mattes were taken as the initial samples. The composition of sulfide alloys is shown in
Table 1.
Table 1 - The chemical composition of samples
Sample
Содержание, %
(Ni+Cu)/S Cu/Ni
Ni
Cu
Fe
Co
S
Nickel sulfide
72,1
-
-
-
27,5
2,62
-
Copper sulfide
71,3
-
-
-
20,8
3,76
-
Copper-nickel matte
22,5
46,7
2,5
0,6
21,6
3,20
2,1
Nickel matte
39,9
31,2
2,6
0,6
20,8
3,42
0,8
A layer of sparingly soluble products was formed in an electrolytic cell for 15
minutes at a current density of 1000 A/m2 in a 100 g/dm3 solution of a sulfuric acid.
The surface area of the anode was 1 cm2. During the experiment, 0.025 Ah electricity
passed through the test sample. Oxidation products of the anode material appeared on
the electrode surface during the experiment. When a passivating layer was formed, the
cell voltage changed during electrooxidation: 1.8 to 2.2 V for nickel sulfide Ni3S; 1.04
to 6.00 for copper sulfide; 1.8 to 2.4 V for the copper-nickel converter matte with
Cu/Ni = 2/1; and 1.8 to 2.4 V for the nickel converter matte.
The local composition of the passivating layer was evaluated on a JSM-5900LV
X-ray spectral microanalyzer.
Results
Purbe diagrams were constructed for the Ni-S-H2O Cu-S-H2O, and S-H2O
systems at pH = 2 to 14 and the range of potentials from 2.0 to 2.0 V. In the Ni-SH2O system (Fig. 1) the region of existence of metallic nickel covered the whole
interval of the varying pH values. Therefore, metallic nickel could be obtained in both
an acid and an alkaline medium. The precipitation potential of metallic nickel
decreased to 1.0 V in an alkaline medium (pH = 14). An increase in the potential
caused a consecutive alternation of the regions of existence of the sulfides
Ni3S2→NiS→NiS2. At potentials of 0.0 to 0.5 and pH from 2 to 7 NiSO4 was
formed.
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Fig. 1. Phase diagram of the Ni-S-H2O system at 25°C.
In an alkaline medium the NiS2→NiS transition was possible as the electrode
potential increased. The further rise of the potential at the sulfide caused the formation
of nickel oxide and hydrates (pH larger than 7).
Thus, the Ni3S2→NiS→NiS2→NiSO4+S transitions depend on the electrode
potential and pH of the solution. The formation of NiS2 and S in an alkaline medium
is thermodynamically impossible.
In the Cu-S-H2O system (Fig. 2) metallic copper was stable over the whole
interval of pH at potentials lower than 0.2 V. As the potential increased, the
formation of the copper sulfide Cu2S was probable in the whole interval of the acidity
studied. At a higher potential an adjacent region was the region of existence of the
sulfide CuS. In a medium with a considerable acidity the formation of elemental
sulfur was possible over a narrow region of the potentials.
Fig. 2. Phase diagram of the Cu-S-H2O system at 25°C.
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The Cu2S→CuS→Cu2S→Cu→Cu2O transitions were probable in alkaline
solutions, and their occurrence depended on the electrode potential and the pH value.
The anodic dissolution of sulfides in an alkaline medium leads to the formation of
oxides and hydrates. Considering that the potentials of the anodic dissolution of nickel
and copper sulfides in an acid medium are much higher than in an alkaline medium,
the sulfides can be more favorably dissolved in energy terms at small pH values.
According to the diagram for S-H2O (Fig. 3), elemental sulfur was formed after
saturation of the solution with H2S in an acid medium. For sulfur to pass to solution as
SO42- or S2O82- requires rising the potential. In the region of positive potentials up to
2.0 V a stable dissolved form was sulfate-ions. At a potential higher than 2.0 V sulfur
with the valence +6 oxidized to the state +7 and passed from a sulfate-ion to a
thiosulfate-ion. This transformation is purely electrochemical and does not involve
hydrogen protons or hydroxyl groups.
---- region of stability of water and its decomposition to hydrogen and oxygen
 region of stability of SO 24 and S2O 82 
Fig. 3. Phase diagram of the S-H2O system at a temperature of 25°C.
Thus, from the Purbe diagrams it was possible to determine thermodynamically
stable forms and compounds of copper, nickel, and sulfur ions in aqueous solutions at
preset values of pH and the electrode potential. They also allowed determining the
probable potentials of sulfide electrodes for the passage of the metals to solution and
sulfur to its elemental state.
The regions occupied by elemental sulfur, sulfide (NiS), and nickel oxysulfate
(NiONiSO4) were found on the electrode surface (Fig. 4 and Table 1).
According to the literature data [1-8], Ni3S2 can be oxidized either in one stage
with the liberation of elemental sulfur and nickel cations or in two stages with the
formation of an intermediate compound (NiS):
Ni3S2 - 6e¯ = 3Ni2+ + 2S0,
(1)
Ni3S2 - 2e¯ = 2NiS + Ni2+,
(2)
2+
0
2NiS - 4e¯ = 2Ni +2 S .
(3)
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A stepwise mechanism is most probable. The first stage includes the oxidation
of nickel in the nonstoichiometric Ni3S2 sulfide, which is accompanied by
demetallization and the formation of NiS and nickel cations. At the second stage,
sulfide sulfur is oxidized to the elemental form, and the nickel cations pass to
solution.
As follows from the X-ray spectral microanalyses data (Fig. 5 and Table 1), the
sulfide film formed on the surface of the sample predominantly consisted of elemental
sulfur and NiS in agreement with the thermodynamic simulation data.
Fig. 4. Electrode surface formed during electrochemical oxidation of the nickel sulfide
Table 1. The X-ray spectral microanalyz data on the local composition of the phases
on the surface of the nickel sulfide (according to Fig. 4)
No.
Phase
Concentration, at %
S
Ni
1
NiSO4-S
23.8-25.8
10.9-11.0
2
S
69.2-70.4
6.2-7.9
3
NiS
45.6-46.4
44.4-47.6
The anodic oxidation of the copper sulfide is effected conditionally via a
stepwise mechanism [1, 2]:
Cu2S – e = CuS + Cu2+,
(4)
2+
0
CuS - 2 e = Cu + S .
(5)
The investigators [1, 2, 7-11] think that in conditions of the anodic polarization
the copper sulfide (I) decomposes to copper and the copper sulfide (II):
Cu2S=Cu+CuS.
(6)
According to the obtained data (Table 2 and Fig. 5), the copper sulfide (I) is
oxidized as a series of intermediate sulfide compounds.
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Table 2. The X-ray spectral microanalyses data on the local composition of the phases
on the surface of the copper sulfide (according to Fig. 5)
No.
Phase
Concentration, at %
S
Cu
1
Cu1.6S
37.6
61.5
2
Cu1.75S
36.8
61.9
3
Cu-CuS
11.7-30.0
69.9-88.1
4
Cu1.6S
38.9
60.4
5
34.2-49.1
33.5-44.2
nCuOmCuSO4
Fig. 5. Electrode surface formed during the electrochemical oxidation of the copper
sulfide Cu1.96S
The film was formed by an internal sublayer having a dense homogeneous
structure and an external layer with dense homogeneous and loose inhomogeneous
regions. Inclusions of black color were detected on the film surface. The products of
electrooxidation of the synthesized copper sulfide were represented by
nonstoichiometric Cu1.8S, Cu1.75S, and CuS and oxide-sulfate inclusions like
mCuO·nCuSO4 formed through the interaction of copper cations with a sulfate
electrolyte in the presence of oxygen liberated on the anode.
In the case of polarization, the first stage includes oxidation of univalent copper,
which is accompanied by the formation of copper cations and copper-deficient
nonstoichiometric sulfides. Demetallization probably progresses in the following
sequence: Cu2.0-1.96S→ Cu1.8S→ Cu1.75S→ Cu1.6S → Cu1.4S → CuS. Sulfide sulfur is
oxidized at the second stage.
The mechanism of decomposition of sulfide copper (I) by the reaction (6)
explains the formation of the regions of metallic copper on the electrode surface. A
film of the reaction products had a dense homogeneous structure formed by fine
particles of secondary sulfides 4-6 m in size. Elemental sulfur was not detected.
Probably, because of a short time of the film formation, the reaction was incomplete
due to the formation of intermediate sulfides.
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According to the X-ray spectral microanalyses data (Fig. 6 and Table 3), the
passivating layer on the surface of the copper-nickel matte electrode consisted of
nonstoichiometric copper sulfides Cu1.8S, Cu1.75S, Cu1.6S, Cu1.4S and Сu1.2S (oxidation
products of univalent copper) and, also, nickel oxysulfates. Micrometer-sized particles
of the copper sulfide formed dense and loose regions. The loose regions appeared due
to a complete dissolution of the nickel sulfides. Dendrites of the copper sulfide
Cu1.96S were included in the matrix of the nickel sulfide. The nickel sulfide NiS and
elemental sulfur, which are products of the anodic oxidation of Ni3S2, were not
detected. The reaction of the electrochemical dissolution of the nickel sulfide Ni3S2
probably was more complete than that of the copper sulfide Cu1.96S. As distinct from
Cu1.2-1.75, the electrodissolution of NiS was more vigorous.
Table 3. The X-ray spectral microanalyses data on the local composition of the phases
on the surface of a copper-nickel converter matte (according to Fig. 6)
No.
Phase
Concentration, at %
S
Fe
Ni
Cu
1
Cu1.75S
33.7
0.2
0.4
59.5
2
Cu1.6S
35.7
0.3
0.4
57.2
3
Cu1.4S
33.4
0.3
1.6
47.1
4
Cu1.2S
43.0
0.3
52.1
5
15.2-16.2
1.8
10.8-12.9
1.7-2.6
nNiOmNiSO4
6
Cu1.6S
28.1
0.8
1.2
44.2
7
CuSO4
17.1
0.6
2.6
35.2
Fig. 6. Electrode surface formed during the electrochemical oxidation of a coppernickel converter matte
According to the X-ray spectral microanalyses data (Fig. 7 and Table 4), the
surface of the electrochemically oxidized converter matte had homogeneous dense
regions with phases in the form of grains and dendrites, as well as loose leached
regions. A microanalysis showed that the dense homogeneous layer consisted of
nickel sulfides approaching Ni3S2 and NiS as regards the composition. The
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composition of the dendrites corresponded to nNiO·mNiSO4, while fine inclusions of
the copper sulfides were similar to CuS. A metallic phase of a nickel-based solid
solution including copper, iron, and cobalt was located near the dendrites. In
accordance with the conditions of oxidation of the nickel converter matte, the quantity
of electricity passed through the electrode was insufficient for a complete oxidation of
the metal on the electrode surface (up to 30% of the surface area of the initial sample
was covered by nickel crystals 0.1-0.4 mm in size). For this reason, the sulfides were
not oxidized and elemental sulfur was not formed.
Fig. 7. Electrode surface formed during the electrochemical oxidation of a nickel
converter matte
Table 4. The X-ray spectral microanalyses data on the local composition of the phases
on the surface of a nickel converter matte (according to Fig. 6)
No
Phase
Concentration, at %
S
Fe
Co
Ni
Cu
1 nNiO·mNi(Cu)SO4 22.8-34.6
0.1
0.1
15.7-19.7
2.1-8.9
2
Ni(Cu)S
31.1
0.1
0.1
27.6
4.7
3
Ni3S2
31.3
0.2
0.1
42.3
1.0
4
Ni-Cu
3.5-7.1
0.9-1.4 0.9-1.1 71.7-83.8
2.9-5.0
5
Cu(Ni)S
21.3-41.4 0.1-0.8 0.1-0.2 8.2-18.4
12.9-27.9
6
CuS
49.7
0.1
0.0
2.9
43.3
Conclusions
1. According to the thermodynamic simulation data, the oxidation of copper and
nickel sulfides in an acid medium is followed by the transition of sulfide sulfur to the
elemental state and the passage of the metal cations to solution: Cu2.0-1.96S → Cu1.8S
→ Cu1.75S → Cu1.6S → Cu1.4S → CuS → CuSO4+S and Ni3S2 → NiS → NiS2 →
NiSO4+S.
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2. It was confirmed that the electrochemical oxidation of copper and nickel
sulfides under polarization occurs in stages; secondary sulfides and sulfur appear on
the electrode and form a passivating film.
3. The metallic phase and then the sulfide phase are oxidized during the
oxidation of a metallized sulfide-metal Ni-Ni3S2 alloy.
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The work was supported by the Ministry of Education and Science of Russia, project
no. 02.740.11.0821.
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