Trans. Nonferrous Met. Soc. China 28(2018) 2143-2150

Electrolytic production of Cu-Ni alloys in CaCl₂-Cu₂S-NiS molten salt

Levent KARTAL^1,2, Servet TIMUR¹

1. Department of Metallurgical & Materials Engineering, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey;

2. Department of Metallurgical & Materials Engineering, Corum, Hitit University, Turkey

Received 15 November 2017; accepted 14 August 2018

Abstract: An alternative metal/alloy production method, known as direct electrochemical reduction (DER), was introduced for the fabrication of CuNi alloys from mixed sulfides (Cu₂S, NiS) under both galvanostatic and potentiostatic conditions. The influences of the process parameters (e.g., cell voltage and current) on the compositions of the reduced compounds were investigated to yield industrially desirable alloys, namely, CuNi10, CuNi20, and CuNi30. The electrochemical behaviors of Cu₂S and NiS in CaCl₂ melt were examined at a temperature of 1200 °C via cyclic voltammetry (CV). Based on the CV results, the cathodic reduction of Cu₂S occurred in one step and cathodic reductions of NiS occurred in two steps, i.e., Cu₂SCu for copper reduction and NiSNi₃S₂ Ni for nickel reduction. Galvanostatic studies revealed that it was possible to fabricate high-purity CuNi10 alloys containing a maximum sulfur content of 320×10^-6 via electrolysis at 10 A for 15 min. Scanning electron microscopy along with energy-dispersive X-ray spectrometry and optical emission spectroscopy (OES) examinations showed that it was possible to fabricate CuNi alloys of preferred compositions and with low levels of impurities, i.e., less than 60×10^-6 sulfur, via DER at 2.5 V for 15 min.

Key words: molten salt electrolysis; electro-reduction; copper extraction; copper sulfide; nickel sulfide; Cu-Ni alloys

1 Introduction

Copper–nickel (Cu-Ni) alloys have been commonly used in many different industries, namely, shipping, offshore oil, gas production, and desalination and for the fabrication of silver-colored coins, electrical resistors, and heater elements due to their outstanding properties such as high corrosion resistance against seawater, tarnish resistance, color, and resistivity [1-3].

Copper and nickel are industrially produced according to multi-step pyrometallurgical processes in which sulfide ores are processed via smelting and conversion operations. In these procedures, Cu-Ni-Fe-S concentrates are turned into mattes with high Cu, Ni, and Cu-Ni contents. The conversion of mattes into their metal forms is performed in two steps for nickel and copper at ~1600 and ~1200 °C, respectively. Such high-energy and high-temperature methods require considerable capital investment, time, and labor as well as extra precautions to avoid potential environmental issues, such as SO₂ emission [4-6]. Formation of SO₂ as a by-product is always considered as a significant problem even though it is possible to be captured in the form of sulfuric acid using modern, expensive SO₂ capture systems [6,7]. As an alternative to conventional extraction methods, a new metal production approach for titanium was introduced in the late 1990s, in which electro-reduction reactions occur on cathodically polarized titanium dioxide pellets to produce metallic titanium. This direct electrochemical reduction (DER) process has attracted considerable attention due to its fewer number of processing stages, less labor requirement, and low energy consumption as well as its simplicity and feasibility for small-scale applications [8-13]. Therefore, many attempts have been made to adopt this method for the production of other metals (Fe, Nb, Cr, etc.) [14-16] and alloys (Fe-Ti, ZrMn₂, CeCo₅, Mg-Sr, Mg-Li-La, TiNbTaZr, etc.) [17-22]. During DER, the reduced metal remains as a solid phase on the cathode holder, whereas O^2- on the cathodically polarized charged material transfers to the anode(s) to oxidize as CO/CO₂ in the case of graphite anode(s) [23-28].

According to a principle similar to the DER of oxides, it is possible to reduce metal sulfides to their metallic forms, where S^2- transfers to the anode(s) to discharge as elemental sulfur gas. To date, very few electrolytic reduction studies have been conducted for metal sulfides [29]. For instance, the productions of molybdenum [30,31], aluminum [32], tungsten [33], antimony [34], titanium [35], vanadium [36], copper– iron [37], and copper [29,38,39] from their sulfides were investigated.

In this study, for the first time, we examine the electrolytic production of copper–nickel alloys from their sulfides (Cu₂S and NiS) in CaCl₂ electrolyte. Accordingly, the possibility of controlled CuNi alloy production with different compositions and the reduction mechanism of Cu₂S–NiS mixtures have been explored using cyclic voltammetry (CV) and galvanostatic and potentiostatic methods. The technique suggested in this study can also be used for the recovery of heavy metals from their precipitated sulfides, which are frequently produced in the mining and metallurgical industries during the treatment of process solutions, waste waters, and mining effluents [40].

2 Experimental

The materials used in this study are listed in Table 1 with their specifications. The schematic depiction of the experimental setup utilized for the molten salt electrolysis is shown in Fig. 1.

Table 1 Materials used in cyclic voltammetry measurements and molten salt electrolysis

During electrolysis, a graphite rod was used as the anode and a graphite crucible containing a mixture of Cu₂S, NiS, and CaCl₂ was polarized as the cathode. CaCl₂, utilized as the electrolyte, was kept in an oven at 150 °C to prevent re-moistening. CaCl₂ granules were filled into the graphite crucible, which was then heated in a medium-frequency induction furnace (50 kHz, 30 kW, 40 A). Before starting heating, the electrolysis cell was purged with highly pure argon (99.9% purity) at a flow rate of 50 mL/min. Then, the temperature of the system was increased to 1200 °C and maintained for 30 min to ensure the homogenization of the electrolyte before initiating electrolysis.

Fig. 1 Schematic illustration of experimental setup

The mechanism of the DER of Cu₂S-NiS was further investigated via CV. The CaCl₂-Cu₂S-NiS melt was left for 30 min to achieve equilibration of the constituents before performing CV analysis. The CV measurements were conducted on a Gamry reference 3000 electrochemical workstation. A tungsten wire was used as the WE, and graphite rods were used as the CE and the quasi-reference electrode. The cyclic voltammograms of the system were obtained at 1200 °C with a scanning rate of 200 mV/s.

In addition to the CV measurement, electrolysis at constant current and voltage was performed according to the experimental conditions listed in Table 2. After the electrolysis, the extracted cathode products accumulated at the bottom of the graphite crucible were poured into a graphite mold. The mold was cooled down to room temperature, and the solidified electrolyte on the cathode product was crushed and washed with hot water.

The obtained products were analyzed using optical emission spectroscopy (OES, GNR-S7 MLP), X-ray diffraction (XRD, Siemens D5000), and scanning electron microscopy (SEM, JSM-840, JEOL) equipped with energy-dispersive X-ray spectrometry (EDS, EDS link, Oxford).

Table 2 Experimental conditions for molten salt electrolysis conducted at 1200 °C

3 Result and discussion

3.1 Cyclic voltammetry

Table 3 gives the Gibbs free energy change in the copper and nickel sulfides as well as in calcium chloride at 1200 °C calculated using HSC chemistry 5.1 version. The decomposition potentials were additionally derived from the given standard Gibbs free energy at 1200 °C. Since the decomposition potentials of the Cu and Ni sulfides were much lower than that of CaCl₂, the reduction of Cu and Ni was expected to occur without any Cl₂ emission.

Table 3 Gibbs free energy change (△G^Θ) along with decomposition voltages of Cu and Ni sulfides and CaCl₂ at 1200 °C

The electrochemical behaviors of Cu₂S and NiS in molten CaCl₂ were studied using CV. The cyclic voltammograms of the system obtained at 1200 °C and a scanning rate of 200 mV/s are shown in Figs. 2 and 3. As shown by the theoretical decomposition voltage values (Table 3) and CV diagrams (Figs. 2 and 3), the reduction voltages of the Cu and Ni sulfides significantly changed. Noteworthy differences between these theoretical and practical values could be attributed to the kinetics and dynamics of the cathode reactions. Generally, cell reactions for molten salt electrolysis containing MeS are given as follows [29,33]:

Anode: S^2--2e=1/2S₂(g)                      (1)

Cathode: MeS(solid)+2eMe(solid)+S^2-(in molten salt)  (2)

Fig. 2 Cyclic voltammogram in pure CaCl₂ and CaCl₂–Cu₂S mixture at 1200 °C (WE: A=0.32 cm²; CE: scan rate=200 mV/s)

Fig. 3 Cyclic voltammogram in pure CaCl₂ and CaCl₂-NiS mixture at 1200 °C (WE: A=0.32 cm²; CE: scan rate=200 mV/s)

The CV results of Cu₂S showed a wide reduction peak at 0.05 V, labeled as A. On the anodic side of the CV, the minimum oxidation current was approximately -0.20 V and it peaked at 0.15 V, marked as A′. After A′, another oxidation process was observed at about 0.67 V, labeled as B′. It was apparent that the electro- desulfidation of Cu₂S proceeded in at one-step, as described below:

Cathode: Cu₂S+2e=2Cu+S^2-                     (3)

Anode: S^2--2e=1/2S₂(g)                            (4)

Net reaction: Cu₂S=2Cu+1/2S₂(g)              (5)

In the absence of NiS, no obvious redox peaks were observed in the potential range of -1.5 V to 1.0 V. However, after adding 3 g NiS into the CaCl₂ melt, two reduction peaks at about -0.20 V (peak A) and -0.45 V (peak B) appeared during the negative scan (blue line in Fig. 3). The reason for the appearance of these two sequential peaks could be the stepwise reduction of NiS. First, NiS was partially dissociated to Ni₃S₂ and S^2- in molten chlorides, and newly formed Ni₃S₂ was then reduced to Ni:

Cathode: 3NiS+2e=Ni₃S₂+S^2-                  (6)

Cathode: Ni₃S₂+4e= 3Ni+2S^2-                  (7)

Anode: 3S^2--6e=3/2S₂(g)                           (8)

Net reaction: NiS=Ni+1/2S₂(g)                    (9)

Lastly, the voltammetric behavior of mixed Cu₂S/NiS in molten CaCl₂ at 1200 °C was investigated to understand the electro-reduction mechanism of mixed sulfides (Fig. 4). The reduction of the mixed CaCl₂-Cu₂S-NiS melt initiated at about 0.10 V, as seen in the CaCl₂-NiS mixture (Fig. 3). Peak A was possibly caused by the formation of Cu-Ni₃S₂ compounds from the reduction of Cu₂S and NiS. After peak A, another reduction process started at about -0.30 V (marked as B), which was probably due to the formation of Ni reduced from Ni₃S₂. During the positive scan, oxidation peaks A′, B′, and C′ at about 0.35, 0.5, and 0.90 V were observed, which were likely belonged to the oxidation of Cu, Cu₂S-Ni and Ni₃S₂, respectively.

Fig. 4 Cyclic voltammogram in pure CaCl₂ and CaCl₂- Cu₂S-NiS mixture at 1200 °C (WE: A=0.32 cm², CE: scan rate= 200 mV/s)

3.2 Galvanostatic electrolysis

The electrochemical production of Cu-Ni alloys via the DER technique was initially performed at constant currents of 10 and 5 A.

The cell potential variation as a function of electrolysis time exhibited a sharp increase after considerable reduction of the charged Cu₂S/NiS powders (Fig. 5). As expected, the cell potential was low at relatively low currents. A sharp jump was observed after 20 min of electrolysis at 5 A, whereas relatively less increment was observed for electrolysis at 10 A beyond about 10 min.

Fig. 5 Cell voltage versus time in CaCl₂-Cu₂S-NiS melt at 1200 °C

The OES results of the produced alloys and the starting mixture ratios are given in Table 4. Apparently, the amount of applied current did not significantly influence the chemistry of the Cu-Ni alloys. As shown in the OES results, the overall matrix compositions obtained at 5 A were close to 9:1 for Cu/Ni (Table 4). Hence, the macro investigation confirmed the composition of the alloys. The chemical composition of the obtained Cu-Ni alloy was close to the Cu/Ni mass ratio in the charged sulfide powder mixture.

Table 4 Molten salt electrolysis under galvanostatic conditions and product compositions obtained in CaCl₂-Cu₂S-NiS melt at 1200 °C

The microstructures of the products are given in Fig. 6 along with the EDS results. The background light-gray phase (denoted by B) was composed of 85.71% Cu and 13.86% Ni, whereas the dark gray regions (denoted by A) were composed of 34.92% Cu, 62.09% Ni, and 2.8% Fe. It was shown that a small amount of iron was also collected at the cathode. Interestingly, there were no dark gray areas detected in the sample synthesized at 10 A. The general chemical composition was calculated to be 86.59% Cu, 12.88% Ni, and 0.53% Fe. The possible reason for this diverse microstructure was the high reduction rate at a higher current.

3.3 Potentiostatic electrolysis

The cell potentials increased significantly with time in the constant current experiments. In these cases, the potentials could reach the decomposition potentials of the melt. To avoid the decomposition of CaCl₂, a second series of experiments were performed at constant voltages. Based on the theoretical calculations (see Table 3, ~3.05 V for CaCl₂ at 1200 °C) and the CV results, cell voltages of 1.0, 1.5, 2.0, and 2.5 V were chosen. Figure 7 shows the current variations as a function of electrolysis time at different voltages. Apparently, all current curves recorded at different voltages exhibited similar characteristics in that initial high current decreased with time and reached stable values eventually. The electrolysis reached the steady-state background current level after 10 min for all applied voltages.

Fig. 6 SEM images of samples synthesized at 5 A (a) and 10 A (b) in CaCl₂-Cu₂S-NiS melt at 1200 °C

Fig. 7 Current versus time at different voltages in CaCl₂- Cu₂S-NiS melt at 1200 °C for Cu/Ni mass ratio of 9:1

The chemical analysis of the obtained products revealed that the Cu, Ni and Fe contents were 88%, 11%-12%, and 0.35%, respectively (Table 5). The sulfur content of the alloys decreased to less than 0.3%.

Table 5 Chemical analysis of obtained products via OES

SEM with EDS analysis (Fig. 8(a)) showed the uniform structure of the synthesized copper–nickel alloys (denoted by B) with small Ni-Cu-Fe-S-rich islands (denoted by A). An optical image of the same sample revealed that the enrichment of the Ni-Fe-S phases occurred at the grain boundaries (Fig. 8(b)). Minor iron contamination of less than 0.5% was detected in the synthesized products due to the iron content in the initial raw material and the wide range of working voltages, which covered the iron reduction potential. Based on the metallographic analysis and EDS results, it could be deduced that the Cu₂S-NiS mixture in the CaCl₂ melt could be converted into high-purity CuNi alloys (99%) in a short period of time (i.e., 15 min).

Fig. 8 SEM image of reduced sample at cell potential of 1.5 V along with EDS analyses (a) and optical micrograph after electrolysis in CaCl₂-Cu₂S-NiS melt at 1200 °C for 15 min (b)

The entrapped Ni-Cu-S phases could be fully separated from the converted metallic parts by a slow cooling method after pouring the molten mixture into the graphite mold. This problem could be easily overcome for further industrial implementation by integrating a controlled cooling system. With this attachment, the light density of the Ni-Cu-S matte phase accumulated on the top surface of the solidified alloys.

3.4 Influence of Cu/Ni ratio on Cu-Ni alloy production

The influence of the Cu/Ni ratio on the composition of the Cu-Ni alloys was studied at constant voltage (1.5 V). As observed in the current variations with respect to electrolysis time given in Fig. 9, the initial current values exhibited declining trends, which was probably due to the same reason as that in the galvanostatic experiments, namely, the almost complete reduction of charged sulfide mixtures. There were three main areas in the current–time curve: sharp decline region I, stable constant current region II, and decreasing trend III. These current tendencies with time indicated the reduction rates of the charged particles. The reductions initiated with relatively high rates (I) and reached steady conditions (II) and were finally completed with decreasing rates (III). The reduction of CuNi10, CuNi20, and CuNi30 showed behaviors similar to that observed in their current–time curves: the higher the nickel content, the larger the region II was.

Fig. 9 Current versus electrolysis time at different Cu/Ni ratios in CaCl₂-Cu₂S-NiS melt at 1200 °C and 1.5 V for 15 min

According to the OES results (Table 6), the charged Cu₂S/NiS powder mixtures were mostly converted into their metallic forms. SEM with EDS analyses of the products revealed (Fig. 10) that the darker small areas were mainly composed of unreduced Cu₂S-NiS powder (denoted by A) and the lighter regions (labeled as B) had compositions similar to the relevant Cu-Ni alloys. It should be noted that very small amounts of S remained in the dark areas. Based on the SEM-EDS and OES results, it could be concluded that the white background in the three produced alloys had a composition similar to the CuNi10, CuNi20, and CuNi30 alloys with heterogeneously distributed unreduced Cu₂S-NiS areas.

Figure 11 showed that higher nickel content in the synthesized alloys resulted in Cu peak shifts to the right side with (220) orientational growth.

Table 6 OES results of CuNi10, CuNi20, and CuNi30 alloys synthesized in CaCl₂ melt at 1200 °C and 1.5 V for 15 min

Fig. 10 SEM images of alloys produced at different Cu/Ni mass ratios in CaCl₂-Cu₂S-NiS melt at 1200 °C and 1.5 V for 15 min

Fig. 11 XRD patterns of Cu-Ni alloys synthesized at different Cu/Ni ratios in CaCl₂-Cu₂S-NiS melt at 1200 °C and 1.5 V for 15 min

4 Conclusions

1) According to the CV results, the cathodic reductions of Cu₂S occurred in one step and NiS occurred in two steps: Cu₂SCu for copper reduction and NiSNi₃S₂Ni for nickel reduction.

2) Galvanostatic investigations showed that the applied currents did not significantly influence the obtained Cu-Ni alloy compositions, which had Cu/Ni mass ratios close to those of the charged sulfide mixtures.

3) Potentiostatic studies revealed that the sulfur content in the products depended on the applied voltage. It was possible to synthesize high-purity CuNi10 alloy containing a maximum sulfur content of 60×10^-6 by electrolysis at 2.5 V for 15 min.

4) The desired compositions of the Cu-Ni alloys could be achieved by tuning the Cu/Ni mass ratios.

5) This developed DER technique could be applied for the recovery of heavy metals from precipitated sulfides produced in the mining and metallurgical industries during the treatment of process solutions, waste waters, and mining effluents.

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CaCl₂-Cu₂S-NiS熔盐电解生产Cu-Ni合金

Levent KARTAL^1,2, Servet TIMUR¹

1. Department of Metallurgical & Materials Engineering, Istanbul Technical University, 34469 Maslak, Istanbul, Turkey;

2. Department of Metallurgical & Materials Engineering, Hitit University, Corum, Turkey

摘  要：介绍一种金属/合金的生产方法，用于恒电流和恒电位条件下由混合硫化物(Cu₂S, NiS)生产Cu-Ni合金，称为直接电化学还原(DER)。研究槽电压和槽电流等工艺参数对还原得到的化合物组成的影响，以生产工业所需的CuNi10, CuNi20和CuNi30等合金。在1200 °C下采用循环伏安法(CV)考察Cu₂S和NiS在CaCl₂熔体中的电化学行为。根据CV研究结果，Cu₂S的阴极还原是一步完成的，即Cu₂SCu；NiS的阴极还原则分两步进行，即NiSNi₃S₂Ni。恒电流研究表明，在10 A电流下电解15 min，可制备出最高硫含量为320×10^-6的高纯CuNi10合金。扫描电子显微镜以及能量色散X射线能谱和光学发射光谱(OES)测试结果表明，在2.5 V电压下直接电化学还原15 min，可制备出杂质含量低(即硫含量小于60×10^-6)的所选成分的Cu-Ni合金。

关键词：熔盐电解；电还原；铜提取；硫化铜；硫化镍；Cu-Ni合金

(Edited by Xiang-qun LI)

Corresponding author: Levent KARTAL; E-mail: kartall@itu.edu.tr

DOI: 10.1016/S1003-6326(18)64859-9