Selective Removal of Copper from Nickel–Copper Leach Solution by Electrolysis Cells with High Convection

Abstract

In this study, the selective separation of copper ions (Cu²⁺) from leaching solutions containing both Cu²⁺ and nickel ions (Ni²⁺) was investigated using electrolysis cells with high convection (Chemelec). Firstly, the electrochemical behavior of solutions containing only single Cu²⁺, single Ni²⁺, and both Cu²⁺ and Ni²⁺ was investigated using cyclic voltammetry (CV). Cu²⁺ reduction was observed at −0.15 V in solutions that contained either simply Cu²⁺ or a Cu²⁺-Ni²⁺ combination, whereas oxidation happened at 0.17 V in a single step. The CV analysis of a Ni²⁺-containing solution at pH 1 revealed that Ni²⁺ was not reduced. Potentiostatic selective Cu²⁺ reduction experiments were conducted at cathode potentials of −0.3 V, −0.4 V, and −0.5 V. Increasing the potential from −0.3 V to −0.5 V enhanced copper recovery from 84% to 94%. The current efficiency remained above 90% across all three potentials during 3 h experiments. At −0.5 V, extending the experiment time from 3 h to 5 h resulted in copper recovery exceeding 99%, while current efficiency declined from 90% to 80%. The cathode products were analyzed using X-ray diffraction (XRD), revealing that the main phase consisted of metallic copper.

1. Introduction

Copper and nickel are essential metals in modern technology with a wide range of industrial applications [1,2]. Copper is the third most widely used metal after iron and aluminum, and it plays a key role in the green energy transition [2,3]. Nickel is used in the production of many important industrial products, especially stainless steel, batteries, catalysts, and electroplating [4,5]. A significant portion of copper and nickel production is derived from sulfide ores. Approximately 70% of copper production involves the enrichment of chalcopyrite (CuFeS₂) ores through crushing, grinding, and flotation, followed by pyrometallurgical and electrolytic processes [6,7]. Nickel ores are mainly classified into copper–nickel sulfide ores and lateritic (oxidic) nickel ores. Copper–nickel sulfide ores constitute 30–40% of the total nickel ores, while lateritic nickel ores make up the remaining 60–70%. Although copper–nickel sulfide ores represent a smaller proportion compared to lateritic nickel ores, they account for more than 60% of total nickel production due to the simpler production process [8,9]. Nickel sulfide ores are generally processed by crushing, grinding, flotation, pyrometallurgical nickel or nickel matte production, and refining [6,10,11]. Lateritic nickel ores are typically processed by hydrometallurgical methods such as high-pressure acid leaching (HPAL) or heap leaching [11,12].

Nickel sulfide ores are usually copper-rich, with copper and nickel closely associated [13]. Copper and nickel have many similar physicochemical properties, such as complete mutual solubility and density. These two metals are closely related and completely miscible in both the liquid and solid states. Therefore, they are often found intergrown in many minerals, making it difficult to separate them using direct beneficiation methods [14,15]. During nickel production and refining, copper tends to accumulate continuously in the nickel solution [11,16]. Copper is an impurity in nickel production solutions and needs to be strictly controlled [17]. Since the redox potential of Cu²⁺ (+0.340 V vs. SHE) is more positive than that of Ni²⁺ (−0.230 V vs. SHE), the concentration of copper ions in nickel electrolysis solutions should be minimized [18].

Various methods, such as chemical precipitation [14,19,20], solvent extraction [21,22], ion exchange resins [16,17], and electro-winning [8,23], are widely used for copper removal from Cu²⁺-Ni²⁺ solutions, either individually or in combination. These methods often face challenges in economically removing trace metals or are unsuitable when metals are present in low concentrations. For example, chemical precipitation presents difficulties in residue management and imposes high production costs for precipitants [24,25]. Solvent extraction raises concerns about toxicity due to contamination caused by phase entrainment and solvent evaporation in the resulting solutions. Electrochemical tank refining systems are insufficient for removing low concentrations of copper [8,16,26,27].

In this study, copper removal from Cu²⁺-Ni²⁺ leaching solutions was investigated using a Chemelec electrolysis cell, which enables high metal recovery with high current efficiency from solutions with low metal content. Chemelec cells are characterized by a high electrode surface area and enhanced electrolyte convection. They stand out due to their simple structure, scalability according to needs, and low installation and operating costs. Developed for metal recovery, the Chemelec cell features a grid-shaped electrode. Inert glass particles are used to disperse the barrier formed on the cathode surface. The grid-shaped electrodes allow the fluid inert glass particles to move freely within the cell. This movement enables the particles to approach the electrode surfaces from multiple directions, improving electrolyte mixing. In Chemelec cells, titanium or stainless steel grids are used as cathodes, while platinized or ruthenium dioxide-coated titanium anodes (DSAs) are used as anodes [28,29,30]. Figure 1 shows a schematic drawing of the Chemelec cell. It has been shown that this kind of cell can be used for the recovery of metals from industrial wastes with low metal concentrations, particularly by integrating them into the rinsing tanks of electroplating plants [28,30,31,32]. In addition to recovering metallic copper from the solution, electrolysis systems avoid the formation of copper slag, which occurs with other systems, as well as the additional steps required for its treatment.

In Chemelec cells, high electrolyte convection and inert glass beads reduce the diffusion layer on the electrode surface, increasing the limit current value and making it economical to recover metals at high current yields from low-concentration solutions [33,34]. Studies on metal removal using electrolysis cells with high convection have primarily focused on single-metal removal from different kind of solutions. Chemelec cells have been applied to the recovery of copper [27,35,36], nickel [37], cobalt [38], tin [39], cadmium [36,40], and lead [40]. Gorgievski et al. investigated the recovery of copper from acid mine drainage waters using a Chemelec cell and recovered 92% of copper from tailings containing 1.3 g/L Cu²⁺ at a current efficiency of over 60% [35]. Bettley et al. succeeded in reducing the Ni content to below 100 ppm in their studies using watt baths containing 1.5 g/L Ni [37]. Chaudhary et al. investigated Sn removal using Chemelec cells with solutions containing Pb as a single metal and impurity, reporting that the Sn concentration was reduced to 1 ppm under optimized conditions [39]. Segundo et al. investigated the removal of Pb and Cd from waste solutions and reported that Pb removal exceeded 99%, while Cd removal exceeded 94% at the end of experiments with single-metal solutions [40]. Boyanov et al. recovered 95% of Cu and Cd from waste solutions containing Cu and Cd using a Chemelec cell after 7 h of experiment time [36]. Chaudhary et al. recovered 99% of cobalt from waste solutions using a Chemelec cell under conditions of 8 h of experiment time and a current density of 15 A/m² [38].

In this study, copper removal from mixed Cu²⁺-Ni²⁺ solutions with different Cu²⁺ ratios and constant Ni²⁺ content was investigated for the first time using an electrolysis cell with high convection, which has low installation and operating costs, produces no waste, and does not involve the use of chemicals. The electrochemical behavior of solutions containing Cu²⁺, Ni²⁺, and mixed Cu²⁺-Ni²⁺ was investigated using CV, and Cu²⁺ removal was carried out under potentiostatic conditions.

2. Materials and Methods

2.1. Materials

The metal salts NiSO₄·6H₂O (99.99% purity) and CuSO₄·5H₂O (99.99% purity), used to prepare Cu²⁺ and Ni²⁺ solutions, were purchased from Merck (Darmstadt, Germany) and used without further purification. All solutions were prepared using deionized water. Sulfuric acid (H₂SO₄, 70%) was supplied by Fisher Scientific (Hampton, NH, USA) and used for pH adjustments.

2.2. Methods

The experimental studies in this research consist of two parts: electrochemical analyses and potentiostatic Cu²⁺ removal experiments. To determine the potentiostatic reduction conditions, the ion behavior was first investigated electrochemically, followed by Cu²⁺ removal through potentiostatic experiments. For electrochemical measurements, a 6 mm-diameter glassy carbon (GC) electrode was used as the working electrode (WE), a coiled platinum wire as the counter electrode (CE), and an Ag/AgCl electrode as the reference electrode (RE). All reported potentials are plotted against Ag/AgCl. All electrochemical and potentiostatic experiments were conducted using an Autolab PGSTAT101 with a 10A booster. The amount of metal remaining in the solution was determined using inductively coupled plasma (ICP-OES) (PerkinElmer Avio 500, Waltham, MA, USA). The X-ray diffraction (XRD) patterns of the cathode products were obtained using Bruker D2 Phaser diffractometer (Billerica, MA, USA) with CuKα radiation (1.54 Å wavelength).

2.3. Electrolysis Cells with High Convection

The schematic depiction of the experimental setup utilized for copper removal from Cu-Ni mixed solution is shown in Figure 2.

Potentiostatic reduction experiments were conducted in a high-convection electrolysis cell, which was fabricated using a 3D printer (BambuLab, Shenzhen, China) from polypropylene (BASF Ultrafuse). The cell consists of three electrodes: two anodes and one cathode. The electrochemical reaction takes place in a compartment with dimensions of 0.05 × 0.03 × 0.04 m³ (length × width × height). The effective reactor volume of the cell was 60 cm³. To enhance ion transport, inert glass spheres (1.0 mm diameter, 2.5 g/cm³ density) were used. The amount of glass spheres was adjusted so that their height was 2 cm without flow and 3 cm under fluidization. The distance between the electrodes was 8 mm. A 316L perforated steel plate (orifice diameter = 1.5 mm, 60 × 50 × 1 mm³; length × width × thickness) was used as the cathode. Expanded porous platinized titanium was used as the anode. An Ag/AgCl electrode served as the reference electrode in the potentiostatic experiments, as in CV experiments.

3. Results

3.1. Electrochemical Behavior of Cu²⁺ Sulfate Solution

First, the electrochemical behavior of solutions containing different concentrations of Cu²⁺ was investigated using the CV technique. Cu²⁺ reduction can occur in either one step or two steps, depending on the solvent type and ambient conditions [23,26,41]. In the two-step reduction process, Cu²⁺ is first reduced to Cu⁺ according to Equation (1), and then, Cu⁺ is further reduced to Cu according to Equation (2):

Cu²⁺ + e⁻ → Cu⁺      E° = +0.158 V

(1)

Cu⁺ + e⁻ → Cu      E° = +0.522 V

(2)

In the one-step reduction process, Cu²⁺ is directly reduced to Cu⁰ in a single step, as shown in Equation (3):

Cu²⁺ + 2e⁻       Cu⁰ E° = +0.337 V

(3)

During the reduction of Cu²⁺ from sulfate solutions, cathodic reactions proceed according to Equations (1)–(3), while at the anode, water undergoes decomposition, producing oxygen as described in Equation (4):

2H₂O → O₂ + 4e⁻ + 4H⁺ E° = +1.23 V

(4)

Sulphuric acid is widely used in the leaching of ores and concentrates due to its low cost, ease of availability, and high metal solubility [42]. For this reason, the production of many metals via electrolytic methods is typically conducted in sulfate solutions [43]. Since the copper content in ores, waste materials, and other metal sources varies more significantly than that of nickel, a wide range of Cu²⁺ concentrations was selected for investigation. The Cu²⁺ concentrations analyzed for their electrochemical behavior using CV are listed in

Table 1. Cu²⁺ concentrations used in CV analyses.

Samples	1	2	3	4
Cu2+/g/L	4.5	1.5	0.75	0.5

.

The voltammetry graphs of the solutions containing Cu²⁺ concentrations, as listed in Table 1, are shown in Figure 3. First, the electrochemical behavior of the solution containing 0.5 g/L Cu²⁺ was investigated at different scan rates. In Figure 3a, the scan begins at 0 V, proceeding in the cathodic direction until the first reduction peak appears, corresponding to the reduction of Cu²⁺ to Cu. The graph clearly indicates that electron transfer is quasi-reversible. The reduction of Cu²⁺ occurs in a single step at −0.15 V, while the oxidation of Cu occurs in a single step at 0.17 V. The peak current values increase with the scan rate, and the peak potentials shift, becoming more negative at the cathodic peak and more positive at the anodic peak. The voltammetry graphs for the 0.75 and 1.5 g/L Cu²⁺ solution are presented in Figure 3b and Figure 3c, respectively. In these graphs, the reduction of Cu²⁺ was also observed at −0.15 V and oxidation at 0.17 V, similar to the 0.75 g/L solution. Finally, the CV graph of the solution with the highest Cu²⁺ concentration (4.5 g/L Cu²⁺) is shown in Figure 3d. Unlike the results at lower concentrations, the anodic peak height remains unchanged at scan rates of 20, 50, and 100 mV/s, whereas the cathodic peak height increases with scan rate. Although the onset values of the reduction and oxidation peaks remain constant, the peak potentials shift, becoming more negative in the cathodic region and more positive in the anodic region. It is observed that the peak current increases simultaneously with the increase in scan rate at all concentrations.

To better understand the electrochemical behavior as a function of Cu²⁺ concentration, the voltammetry graphs obtained at a 100 mV/s scan rate for different concentrations are presented together in Figure 4a. As expected, the anodic and cathodic peak current values increase with increasing concentration. From the graph in Figure 4a, it is observed that while the peak onset values remain unchanged, the peak potentials shift, becoming more negative for reduction and more positive for oxidation. To better visualize the concentration-dependent peak shift, the voltammetry graphs for the lowest (0.5 g/L Cu²⁺) and highest (4.5 g/L Cu²⁺) copper concentrations are shown together in Figure 4b. It is evident that as the concentration increases, the reduction peak shifts to a more negative potential, while the anodic peak shifts to a more positive potential.

3.2. Electrochemical Behavior of Ni²⁺ Sulfate Solution

After analyzing Cu²⁺-containing solutions, the electrochemical behavior of the Ni²⁺-containing solution was investigated at pH 1 using solutions with 1.5 g/L Ni²⁺. It is well known that high acidity inhibits the deposition of nickel on the cathode [44]. Studies report that Ni²⁺ reduction does not occur at low pH values or occurs in combination with H⁺ reduction in a single step [23], as described by Equation (5):

Ni²⁺ + 2e⁻ = Ni     E° = −0.23 V

(5)

2H ⁺ + 2e⁻ = H₂     E° = 0.00 V

(6)

The pH of the 1.5 g/L Ni²⁺ solution was adjusted to 1, and the solution was analyzed within the potential range of −0.8 V to 1.0 V. The voltammetry graph obtained is shown in Figure 5. It can be seen from Figure 5 that Ni²⁺ reduction did not occur, and H⁺ reduction began at −0.7 V [25]. The absence of an oxidation peak in the CV graph indicates that Ni²⁺ reduction does not take place and that all observed reduction peaks correspond to H⁺ reduction.

The absence of Ni²⁺ reduction at pH 1 indicates that mixed solutions containing Ni²⁺ and Cu²⁺ can be separated without Ni²⁺ reduction.

3.3. Electrochemical Behavior of Cu²⁺-Ni²⁺ Mixed Solution

After determining the electrochemical behavior of synthetic solutions containing individual Cu²⁺ and Ni²⁺, the electrochemical behavior of solutions with varying Cu²⁺ concentrations and a constant 1.5 g/L Ni²⁺ was investigated using CV at different scan rates. In all experiments, the solution pH was adjusted to 1 using sulphuric acid. CV analyses were performed at different scan rates (10–100 mV/s) within a potential range of −0.85 V to 0.8 V to minimize the influence of H⁺ reduction on the cathodic peak. The Cu²⁺ and Ni²⁺ concentrations of the solutions analyzed in this section are provided in

Table 2. Cu²⁺ and Ni²⁺ concentrations of solutions used in CV analyses.

Samples	1	2	3	4	5
Cu2+/g/L	4.5	3	1.5	0.75	0.5
Ni2+/g/L	1.5	1.5	1.5	1.5	1.5

.

CV graphs of the solutions prepared at the concentrations listed in Table 2 are shown in Figure 6. From these graphs, it is evident that Ni²⁺ reduction does not occur, while H⁺ reduction takes place at −0.70 V [25]. The Cu²⁺ reduction and oxidation peak values are approximately the same as those observed in the solution containing only Cu²⁺. The reduction of Cu²⁺ to Cu occurs at −0.15 V, while the oxidation of Cu to Cu²⁺ occurs at 0.17 V.

To better visualize the effect of Cu²⁺ concentration differences, the voltammetry graphs for various Cu²⁺ concentrations with a constant 1.5 g/L Ni²⁺ are combined and presented in Figure 7a. The peak current values increased with rising Cu²⁺ concentration, and the peak positions shifted in both negative and positive directions. Figure 7b displays the voltammetry graphs for the solutions with the lowest and highest Cu²⁺ concentrations. It is more clearly observed that the initial oxidation and reduction values remained unchanged, while the peak positions shifted in both positive and negative directions.

To determine whether nickel is reduced, the CV graphs of solutions containing only 0.5 g/L copper ions and those containing 0.5 g/L copper ions along with 1.5 g/L nickel ions are presented together in Figure 8. The anodic and cathodic reactions occurring at the same potentials in both graphs indicate that nickel reduction does not take place, and the observed peaks correspond to copper reduction and oxidation. This shows that at pH 1, the reduction potentials of Ni²⁺ and Cu²⁺ are clearly different, and as long as the correct electrode potential is applied, the separation of the two ions will occur [45].

3.4. Potentiostatic Separation of Cu²⁺ from Cu²⁺-Ni²⁺ Mixed Sulfate Solution

After determining the oxidation and reduction conditions at different concentrations and pH values using CV, potentiostatic separation experiments were conducted at −0.3 V, −0.4 V, and −0.5 V cathode potentials. Initially, experiments were performed at different potentials for a fixed duration of 3 h. Figure 9 shows the time-dependent current variation for these experiments. Similarly, the current initially increased from 0 to a peak value for each potential and then gradually decreased over time to the residual current level. This is likely due to the continuous consumption of Cu²⁺ ions in the solution over time, leading to a decrease in their concentration. As clearly shown in Figure 9, the highest current value was observed at the highest potential, as expected, while the current decreased with decreasing potential.

The variation in Cu²⁺ concentrations in the solution over 3 h at different potentials is shown in Figure 10. The figure indicates that the Cu²⁺ content decreased proportionally with the applied potential each hour, with the lowest Cu²⁺ concentration recorded at 140 ppm after 3 h at −0.5 V. These experiments confirmed that Ni²⁺ reduction did not occur, as indicated by the CV analyses, and the Ni²⁺ concentration remained constant at 1.5 g/L across all potentials.

Figure 11 shows the copper recovery and charge values at different potentials. The lowest copper recovery was 80% at −0.3 V, while the highest recovery reached 94% at −0.5 V. As the potential increased from −0.3 V to −0.5 V, the charge increased from 5834 to 7001.

Table 3. Current efficiencies, charges and copper yields at different experimental times and potentials.

Cathode Potentials vs. Ag/AgCl/V	Deposition Time (h)	Quantity of Charge (C)	Current Efficiency (%)	Cu Recovery, %
−0.3	3	5834	94.2	80
−0.4	3	6400	94.3	88
−0.5	3	7001	91.7	94
−0.5	5	7860	86.4	99

shows the charge, current efficiency, and copper recovery data from experiments conducted at different potentials and durations. At the highest applied potential (−0.5 V), the experiment time was extended to 5 h, and the Cu²⁺ content of the solution was analyzed. While current efficiency remained nearly constant at different potential values for 3 h, it decreased to 86.4% when the duration was extended to 5 h. The highest copper recovery of 99.13% was achieved at −0.5 V with a 5 h experimental duration.

Figure 12 shows the graph of current variation over time. Similarly to the previous experiments, the current initially increased and then remained constant at the residual current level after the 4th hour.

The Cu²⁺ concentration change at −0.5 V over 5 h is shown in Figure 13. In parallel with the current change graph in Figure 12, the Cu²⁺ content initially decreased rapidly, followed by a more gradual decline after 3 h. By the end of 5 h, the Cu content of the solution had dropped to 39 ppm, the lowest copper concentration observed in all experiments. Conventional tank electrolysis systems are known to be inadequate for removing metals from dilute solutions. In a two-electrode system operating at similar concentrations, the metal concentration in the solution can be reduced to 280 ppm in 20 h. However, the high-convection Chemelec cell we used reduced it to 39 ppm in just 5 h [23]. Hannula et al. also studied copper removal from industrial wastewater with the classical two-electrode system and stated that 60% of the copper in the solution was removed in more than 20 h [25].

The phases present in the cathode-deposited metal were analyzed using XRD techniques. Figure 14 shows the XRD pattern of the cathode products. The diffractogram reveals two distinct phases: Cu and Cu₂O. The Cu₂O phase is believed to have formed during the drying process of the cathode products. In the XRD analysis, no Ni-containing phases were detected, which aligns with the ICP results. Djouani et al. also studied potentiostatic Cu-Ni separation using the classical two-electrode system and observed a small amount of the Cu₂O phase, similar to our study [23].

The above analysis shows that the potentiostatic method can effectively reduce the Cu²⁺ ion concentration in a mixed solution of Ni²⁺ and Cu²⁺, enabling the separation of the two ions.

4. Conclusions

In this study, the potentiostatic selective separation of Cu²⁺ from Cu²⁺-Ni²⁺ leaching solutions was investigated using an electrolysis cell with high convection. First, the electrochemical behavior of synthetically prepared Cu²⁺, Ni²⁺, and mixed Cu²⁺-Ni²⁺ solutions was analyzed by CV, and the reduction potentials were determined. Then, selective Cu²⁺ reduction was carried out at −0.3 V, −0.4 V, and −0.5 V, and the effect of cathode potential on selective Cu²⁺ reduction, current efficiency, and copper recovery was examined. The obtained results are summarized below:

• CV analyses showed that Cu²⁺/Cu reaction was a quasi-reversible process controlled by diffusion. Cu²⁺ was reduced at −0.15 V and oxidized at 0.17 V in a single step.
• The electrochemical behavior of Ni²⁺ solutions was investigated at pH 1. It was seen that at pH 1, Ni²⁺ reduction did not occur, and only H⁺ reduction took place, with no anodic peak observed.
• The electrochemical behavior of a mixed Cu²⁺ and Ni²⁺ solution, simulating a leaching solution, was studied. Due to the significant difference in the deposition potentials of the two ions, their separation was easily achieved using the potentiostatic method.
• In potentiostatic selective reduction experiments, copper recovery increased from 84% to 94% as the applied potential increased from −0.3 V to −0.5 V.
• Current efficiency remained above 90% for all three potentials in 3 h experiments.
• Extending the experiment duration from 3 h to 5 h at −0.5 V cathodic potential resulted in copper recovery exceeding 99%, while current efficiency decreased from 90% to 80%, leaving a residual Cu²⁺ concentration of 39 ppm in the solution.
• XRD analysis of the cathode products showed that the main structure was composed of copper, with a small amount of the Cu₂O phase due to oxidation during drying.