Simultaneous Solvent Extraction of Co and Ni from Copper Raffinate Waste Solution

Abstract

The extraction and stripping of Co, Ni, Mn, and Mg ions from raffinate solution of the Sarcheshmeh copper complex containing Cu (0.14 g/L), Ni (0.15 g/L), Co (0.06 g/L), Fe (10.72 g/L), Zn (2.4 g/L), Mn (4.83 g/L), and Mg (8 g/L) was comprehensively studied using D2EHPA and LIX 984 extractants. To design the solvent extraction experiments, the response surface method (RSM) was employed. The optimal and most efficient conditions and extraction rates of nickel and cobalt were considered for the application of a central composite design (CCD). The design of experiments (DOE) was carried out using three operating variables: the equilibrium pH of the solution (4–6), extractant concentration (10–20%), and aqueous-to-organic phase ratio (1–3). The results indicated that the highest extraction of Co and Ni occurred within 5 min at a mixing speed of 500 r/min and 40 °C. The results showed that the equilibrium pH of the aqueous solution had a greater influence on nickel and cobalt extraction than the other parameters. According to the research results, 99% of cobalt and 94% of nickel were extracted simultaneously under optimum conditions of pH = 6, [LIX984N] = 10%, and A/O = 3. In the stripping stage, 95% of nickel ions were recovered in one step using 1 M sulfuric acid, and 80% of cobalt ions were recovered in three steps using 5 M hydrochloric acid. Finally, 98% of Zn, 99% of Co, and 94% of Ni were extracted in two stages with D2EHPA and LIX984N extractants.

1. Introduction

Cobalt, nickel, and their compounds are in growing demand, have a wide range of applications based on their different properties, and are used in many advanced materials. Nickel–cobalt alloys are used in electroforming and electrotyping due to their high strength, corrosion resistance, and magnetic properties. The electrodeposition process can be optimized by adjusting parameters such as current density, pulse frequency, and bath temperature [1,2,3].

They are widely used in different sectors, including in batteries [4,5,6], superalloys [7], catalysts [8,9,10], electroplating [1,2,3], etc. [11]. The offering of exclusive physical properties by cobalt (Co) makes it a high-potential candidate for cutting-edge, high-tech applications, such as in high-strength materials; in the aerospace industry as an alloying element for high-temperature application magnets; and, most importantly, in rechargeable batteries [12]. Cobalt is primarily used in glass painting [13,14], tile manufacturing [15,16], ceramics [14,17], and pottery [18]. Nickel is the primary raw material for stainless steel, owing to its high corrosion resistance; it is also used in kitchenware [19], mobile phones, power generation [20], etc. [11].

Solvent extraction (SX) is the preferred method for the separation and recovery of metals from aqueous solutions under certain conditions. This method has several advantages over other techniques used to recover different metals. A short extraction time, low operating cost, high selectivity, and the production of a product with high purity are the characteristics of this method. Solvent extraction has become an effective alternative for the recovery and separation of heavy metals such as Co and Ni from process streams. It has been widely used in industries around the world [11,21,22].

Polymer membranes, including PIMs (Polymer Inclusion Membranes) [23,24], PPMs (Plasticized Polymer Membranes) [25], and other innovative polymer systems, are also highly effective for the recovery of nickel and cobalt, providing efficient, selective, and sustainable solutions for metal recovery from various sources. However, these membranes have significant kinetic limitations. Their operating speed is limited by membrane thickness and the exchange surface area.

It was observed that the selective extraction of nickel could be achieved using the Cyanex 302 extractant. At first, after removing iron, zinc, and copper, the conditions of the nickel in the solution ([Cyanex 302] = 30% and pH = 4, 97%) were extracted [26]. In a research study, after removing zinc from the solution, using 15% D2EHPA extractant (A/O = 3 and pH = 7), 90% of nickel ions were extracted from a solution containing copper, aluminum, manganese, iron, and calcium [27].

In 2017, the separation and extraction of nickel and cobalt were investigated using Versatic10 and Cyanex 272 extractants, and the study succeeded in separating nickel and cobalt from magnesium and manganese using 20% Versatic10, along with 5% TBP. After that, using the Cyanex 272 extractant and TBP, cobalt and nickel were separated [28]. In another research study, the extraction of cobalt and nickel from a solution containing the nickel, cobalt, manganese, aluminum, iron, zinc, and chromium was discussed. After the precipitation of iron and the removal of zinc and manganese, cobalt was first extracted by Cyanex 272 and separated from nickel. After that, nickel was extracted by the NaTOPS 99 extractant [29].

In 2017, the Cyanex 301 extractant was investigated for the extraction of nickel and cobalt from a solution containing iron, calcium, manganese, magnesium, aluminum, cobalt, and nickel ions. After iron removal, 96.2% cobalt and 95.7% nickel were extracted [22].

Zho and co-authors proved that the Cyphps IL101 extractant is effective in extracting cobalt and nickel from solution containing ions of nickel, cobalt, manganese, zinc, magnesium, iron, aluminum, copper, and chromium. After precipitation of iron, chromium, and aluminum and removal of copper and zinc, in the upper layers, this extractant succeeded in extracting more than 95% of the nickel and cobalt elements [30].

According to the results of research conducted in 2013, the Alamine 336 extractant also performed well in cobalt extraction, and under the conditions of pH = 4.3 and Alamine 336 = 10%, 93.6% of cobalt was extracted [31]. The mixture of Versatic 10 and LIX 84-I extractant was also reported in a study that extracted 99% of nickel and separated nickel ions from cobalt after removing iron, aluminum, and copper [32].

In 2012, Cheng and co-authors extracted 99.6% nickel and 96.9% cobalt from solution containing Zn, Mn, Mg, Ca, Si, Na, and Fe, using a mixture of Versatic10, LIX63, and TBP extractants to separate aluminum and chrome [33].

In another study, using a Ni-Cyanex 272 extractant, over 95% of cobalt was extracted in one step [34]. An artificial leaching solution containing the elements of nickel, cobalt, manganese, zinc, copper, magnesium, and calcium was used for recovery of nickel and cobalt using a mixture of Versatic10 and Acorga CLX 50 extractants, which, at pH = 6.3, extracted more than 99% of nickel and cobalt with 10% (v/v) Versatic10 and 20% (v/v) CLX 50 [35]. For nickel extraction, the utilization of a combination of dialkyl phosphinic acid extractants (Cyanex 272 and Versatic 10) has also been reported. Initially, it removed 98, 80, 70, 46, and 10% of zinc, copper, manganese, cobalt, and calcium, respectively. Furthermore, magnesium was removed in one step using 20% Cyanex 272 by volume at pH 3.9. However, nickel was not separated from magnesium and calcium. For this purpose, at pH = 5.1, nickel was successfully removed by 20% (v/v) Cyanex272 and 10% (v/v) Versatic 10 [36].

In 2004, research was conducted on cobalt and nickel extraction using Cyanex 272 and Versatic10 extractants. In the first step, nickel and cobalt were separated from other impurities by the Versatic10 extractant; then, cobalt ions were extracted from nickel by the Cyanex 272 extractant [37].

In 2013, research was conducted on nickel extraction using the LIX984N extractant. This research showed that 95% of nickel ions were extracted after the removal of chromium and cadmium under conditions of 10% [LIX984N] and pH = 5.3 [38].

Solvent extraction, despite its effectiveness, has certain drawbacks, such as solvent loss through evaporation or degradation, the complexity of controlling operational parameters to ensure efficient separation, and the relatively high costs associated with solvent procurement, recycling, and disposal.

The novelty of the present study can be summarized as follows. First, a combined system of D2EHPA and LIX984N extractants was applied for the simultaneous extraction and separation of Co, Ni, and other metallic ions from copper raffinate solution, which has been rarely reported in previous studies. Second, the solvent extraction experiments were designed and optimized using Response Surface Methodology (RSM) and Central Composite Design (CCD) to evaluate the effects of key operating parameters such as equilibrium pH, extractant concentration, and phase ratio. Third, the influence of equilibrium pH on the extraction behavior of nickel and cobalt was comprehensively investigated and identified as the most dominant factor. In addition, a multi-step stripping approach was developed to achieve efficient and selective recovery of Ni and Co using sulfuric and hydrochloric acids. Finally, experiments were performed using the raffinate solution from the Sarcheshmeh copper complex, which provides practical relevance and demonstrates high extraction efficiencies (99% Co and 94% Ni) under optimized conditions. For this purpose, iron and aluminum were initially removed from the solution using hydrogen peroxide, with the pH adjusted via precipitation process. A solvent extraction step with D2EHPA was used to extract 98% of zinc ions. After that, nickel and cobalt ions were extracted using LIX984N. In the stripping stage, the two ions—nickel and cobalt—were completely separated from each other by sulfuric acid and hydrochloric acid.

2. Experimental Procedures

2.1. Materials and Analysis

Figure 1 shows the structural formulas of the main chemical composition for (A) D2EHPA and (B) LIX984N. The raffinate solution used in this research was obtained from the Sarcheshmeh copper mine, whose elemental analysis is given in

Table 1. Composition of the copper raffinate solution.

Elements	Zn	Mg	Al	Co	Cu	Fe	Mn	Ni
Concentration, g/L	2.4	8	11.37	0.06	0.14	10.72	4.83	0.15

, and the raffinate is the actual leach solution from which copper was separated through plant-scale processes in the complex.

Given the presence of elements such as iron, zinc, and magnesium, 97% of the iron ions were initially removed from the solution by a precipitation process at pH 3 and 80 °C. Subsequently, a solvent extraction stage using the D2EHPA extractant at pH 2.5 with an A/O ratio of 1:1 achieved the selective extraction of 98% of the zinc ions and 90% of the magnesium ions. In the next step, cobalt and nickel ions were extracted using the extractant LIX984N. In the solvent extraction step, time, mixing intensity, and temperature were maintained as fixed parameters to ensure repeatable extraction performance and minimize variability in metal transfer. The experiments in this stage were conducted according to a Design of Experiments (DOE) framework. After completion of the extraction stage, the organic phase containing substantial ions of nickel and cobalt was subjected to stripping to separate the two metals, achieved using sulfuric acid and hydrochloric acid, respectively. During the phase separation step, the aqueous and organic phases were brought into contact, then allowed to separate for a defined period. During this contacting step, nickel and cobalt ions were transferred from the organic phase to the aqueous phase, facilitating their separation. The entire extraction and stripping sequence was conducted under controlled temperature and pH conditions to minimize co-extraction and ensure high selectivity for the target metals.

2.2. (DOE) Design of Experiments

Optimal extraction conditions were achieved by using a central composite design (CCD). The analysis was performed using Design Expert 11 software. In this method, three variables with ten tests, along with two repetitions of the central point, were selected to determine the optimal conditions of equilibrium pH (4–6), extractant concentration (10–20%), and the ratio of aqueous phase to organic phase (1–3).

After designing the experiment using DOE, the data were analyzed using Analysis of Variance (ANOVA). The ANOVA method helps determine if there are significant differences between the means of multiple groups. The F value is a ratio that compares the variance among group means to the variance within the groups, indicating how much the group means differ relative to the variability within each group. The p-value represents the probability of observing the data—or something more extreme—under the null hypothesis stating that there are no differences between the group means. A low p-value (typically less than 0.05) suggests that the observed differences are statistically significant.

2.3. Batch Experiments

Initially, the extraction process was performed using a glass beaker and a hot-plate magnetic stirrer. By adding diluted sodium hydroxide and sulfuric acid to the raffinate solution, the solution pH was adjusted to a determined value. A Metrohm digital pH meter was used to measure the preliminary pH of the aqueous solution. In each experiment, a determined amount of the organic phase (approximately 20 mL) was poured to a specified amount of the aqueous phase in proportion to the prepared ratio. All experiments were performed under strictly controlled and documented operating conditions to ensure reproducibility, including pre-equilibration of both phases to within ±0.5 °C of the set temperature. The initial mixing time prior to phase contact was standardized to 5 min to achieve consistent interfacial conditions.

pH adjustment of the aqueous phase was performed using dilute sodium hydroxide (NaOH) and mineral acids (H₂SO₄ or HCl) as required, and the initial pH of the aqueous phase was measured with a calibrated digital pH meter (Metrohm) prior to contact. For each run, the pH was measured again after a short equilibration (2–3 min) to verify stability prior to phase contact and to capture any drift due to temperature or solution equilibration. The temperature was set at 40 °C. The two phases were thoroughly stirred until equilibrium. The mixed solution was stirred for an additional 30 min at 500 rpm. For separation of the achieved phases, the mixture was transferred into a separation decanter funnel. An inductively coupled plasma optical emission spectrometer (ICP-OES, Agilent 5110, Santa Clara, CA, USA) was used to determine the concentrations of nickel and cobalt ions in the aqueous solution before and after extraction treatment. Equations (1) and (2) were used to calculate the elements’ distribution coefficients and extraction percentages, respectively [39].

All samples were collected in triplicate where feasible, and appropriate blanks and quality controls were included to ensure analytical reliability. Concentration determinations were corrected for dilution factors and instrument blank responses, and quality control charts were maintained to track any drift during the batch series.

$$\mathrm{D}={\displaystyle \frac{{\left[\mathrm{A}\right]}_{\mathrm{o}\mathrm{r}\mathrm{g}}}{{\left[\mathrm{A}\right]}_{\mathrm{a}\mathrm{q}}}}$$

(1)

$$\mathrm{E}\mathrm{x}\mathrm{t}\left(\%\right)=(1-{\displaystyle \frac{1}{1+\mathrm{D}\left(\frac{{\mathrm{V}}_{\mathrm{o}}}{{\mathrm{V}}_{\mathrm{a}\mathrm{q}}}\right)}})\times 100$$

(2)

where V_aq indicates the volume of the aqueous solution and Vo represents the volumes of the organic phases. D implies the distribution coefficient, which can be defined as the ratio of the A metal concentration in the organic phase ([A]_org) and in the aqueous phase to obtain the extraction equilibrium state ([A]_aq). The experimental setup is shown in Figure 2.

3. Results and Discussion

3.1. Data Analysis and Contour Plots

The results of nickel and cobalt extraction from raffinate solution under different laboratory conditions are shown in

Table 2. Co, Ni, Mn, and Mg extraction under various operating conditions.

Row	Parameter 1: pH	Parameter 2: [LIX984N] (%)	Parameter 3: A/O	Co Ex (%)	Ni Ex (%)	Mn Ex (%)	Mg Ex (%)
1	6	20	3	99.16	99.60	8.12	18.60
2	5	15	2	85.00	73.75	1.15	6.74
3	6	20	1	99.50	98.50	8.30	22.69
4	6	10	3	99.00	94.00	5.81	9.70
5	4	20	1	27.00	31.00	7.50	5.11
6	4	20	3	15.00	16.12	4.50	4.24
7	5	15	2	87.00	72.00	1.65	4.65
8	4	10	3	16.00	14.00	3.40	3.44
9	4	10	1	25.00	20.00	6.25	3.58
10	6	10	1	99.00	96.00	6.18	11.81

. The achieved range of cobalt extraction is approximately 15.0–99.5%, and that of nickel ranges from 16.12 to 99.6%. The ANOVA method for cobalt and nickel extraction is presented in

Table 3. Achieved model results for cobalt extraction.

	Sum of Squares	Degrees of Freedom	Mean Square	F Value	p-Value Prob > F
Model	13,494.94	7	1927.85	1137.72	0.0009
pH	12,298.35	1	12,298.35	7258.04	0.0001
[LIX984N]	0.3472	1	0.3472	0.2049	0.6951
A/O	53.39	1	53.39	31.51	0.0303
pH × [LIX984N]	0.0139	1	0.0139	0.0082	0.9361
pH × A/O	56.89	1	56.89	33.57	0.0285
A/O × [LIX984N]	0.8889	1	0.8889	0.5246	0.5442
pH × pH	1085.07	1	1085.07	640.37	0.0016

and

Table 4. Analysis of Variance for the model of nickel extraction.

	Sum of Squares	Degrees of Freedom	Mean Square	F Value	p-Value Prob > F
Model	12,063.39	7	1723.34	296.84	0.0034
pH	11,550.48	1	11,550.48	1989.52	0.0005
[LIX984N]	41.50	1	41.50	7.15	0.1161
A/O	76.76	1	76.76	13.22	0.0680
pH × [LIX984N]	8.04	1	8.04	1.38	0.3604
pH × A/O	36.04	1	36.04	6.21	0.1303
A/O × [LIX984N]	9.64	1	9.64	1.66	0.3266
pH × pH	340.94	1	340.94	58.73	0.0016

, respectively. For nickel and cobalt, the F value equals 296.84 and 1137.72, respectively, which shows the model’s validity. For nickel and cobalt, there is only a 0.34% and 0.09% probability, respectively, that such a large amount of F occurs due to noise. If the obtained p-value is higher than the F value and also less than 0.05, the model is acceptable. In the case of cobalt, the pH, A/O, (pH × A/O), and (pH × pH) parameters are significant, and in nickel, the pH and (pH × pH) parameters are significant.

The final equation according to the parameters is obtained as follows:

$$\begin{gathered} \mathrm{C}\mathrm{o} \mathrm{E}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n} (\%)=\left(86\right)+\left(39.21\right)\times \mathrm{p}\mathrm{H} +\left(0/2083\right)\times \left[\mathrm{L}\mathrm{I}\mathrm{X}984\mathrm{N}\right] -\left(2/58\right) \times {\displaystyle \frac{\mathrm{A}}{\mathrm{O}}} - \left(0/0417\right) \times \mathrm{p}\mathrm{H} \times \left[\mathrm{L}\mathrm{I}\mathrm{X}984\mathrm{N}\right] \\ +\left(2/58\right)\times \mathrm{p}\mathrm{H} \times {\displaystyle \frac{\mathrm{A}}{\mathrm{O}}} - \left(0/3333\right)\times \left[\mathrm{L}\mathrm{I}\mathrm{X}984\mathrm{N}\right] \times {\displaystyle \frac{\mathrm{A}}{\mathrm{O}}} - \left(26/04\right){\mathrm{p}\mathrm{H}}^2 \end{gathered}$$

(3)

$$\begin{gathered} \mathrm{N}\mathrm{i} \mathrm{E}\mathrm{x}\mathrm{t}\mathrm{r}\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}\left(\%\right)=\left(72/87\right)+\left(38\right)\times \mathrm{p}\mathrm{H}+\left(2/28\right)\times \left[\mathrm{L}\mathrm{I}\mathrm{X}984\mathrm{N}\right]-\left(3/10\right)\times {\displaystyle \frac{\mathrm{A}}{\mathrm{O}}}-\left(1\right)\times \mathrm{p}\mathrm{H}\times \left[\mathrm{L}\mathrm{I}\mathrm{X}984\mathrm{N}\right]+\left(2/12\right)\times \mathrm{p}\mathrm{H}\times {\displaystyle \frac{\mathrm{A}}{\mathrm{O}}} \\ -\left(1/10\right)\times \left[\mathrm{L}\mathrm{I}\mathrm{X}984\mathrm{N}\right]\times {\displaystyle \frac{\mathrm{A}}{\mathrm{O}}}-\left(14/6\right){\mathrm{p}\mathrm{H}}^2 \end{gathered}$$

(4)

Figure 3A and Figure 3B, show the model’s validity based on nickel and cobalt extraction values, respectively. As can be seen, the results of the experiments are consistent with the model predictions presented in the software. The colored dots represent the software predictions based on Equations (3) and (4), while the drawn line reflects the experimental results corresponding to the composition.

3.2. Diagrams of the Main Effects of Parameters on Nickel and Cobalt Extraction by LIX984N

3.2.1. pH Effect

LIX984N is considered a chelated acid extractant, and its performance is depicted in Equation (5). According to this relationship and Le Chatelier’s principle, with increasing pH (decreasing ${\mathrm{H}}^{+}$), the reaction progresses toward the right, that is, metal extraction. The extraction percentage of nickel and cobalt changes simultaneously with pH changes under constant conditions of an extractant concentration equal to 10% and an (A/O) ratio = 1:1, as shown in Figure 4. According to this graph, the amount of nickel and cobalt extraction increases with increasing pH. Keeping the concentration of the extractant and the ratio of aqueous-to-organic phases constant, with an increasing pH from 4 to 6, the extraction percentage of nickel and cobalt increases from 20% and 25% to 96% and 99%, respectively. The blue sections indicate low ion extraction, while the red sections indicate high ion extraction.

$${\mathrm{M}}^{2+}+{\mathrm{H}}_2\mathrm{X} \leftrightarrow \mathrm{M}\mathrm{X}+2{\mathrm{H}}^{+}$$

(5)

3.2.2. The Effect of LIX984N Extractant Concentration

Figure 5 shows the effect of LIX984N extractant concentration on the extraction percentage of nickel and cobalt ions (pH = 6). According to this figure, with an increasing concentration of LIX984N, the nickel and cobalt extraction percentage increases with a slight slope. The reason for this small change is the high capacity of this extractant. The effect of extractant concentration on nickel ions is greater than that on cobalt, so by increasing the extractant concentration to 20%, the extraction of cobalt and nickel reaches 31% and 27%, increasing from 20% and 25%, respectively. At pH = 6, with an increase in the extractant concentration from 10% to 20%, the extraction of nickel and cobalt reaches increases from 96% and 99% to 98.5% and 99.5%, respectively. In general, low metal concentrations do not require high extractant concentrations. In this case, a large amount of extractant is not used, which also reduces the co-extraction of associated metals. For example, at pH = 6, the ratio of the aqueous phase to the organic phase is 1:1 and [LIX984N] = 10%, and the extraction of manganese and magnesium is 11.88% and 6.8%, respectively; with an increase in the extractant concentration to 20% and keeping other conditions constant, the amount of extraction of manganese and magnesium ions reaches 22.69% and 8.30%, respectively. As a result, with low concentrations, conditions can be created such that the extractant absorbs only the desired metal, reducing the absorption of impurities.

3.2.3. A/O Phase Ratio Effect

The graph shown in Figure 6 illustrates the effect of changes in the ratio of the aqueous-to-organic phase on the extraction of nickel and cobalt under the conditions of [LIX984N] = 10% and pH = 6. As seen in this figure, with an increase in the ratio of the aqueous phase to the organic phase, the extraction percentage of elements decreases with a slight slope, in accordance with Equation (2). As the aqueous phase rises, the organic phase is saturated by nickel and cobalt ions, reducing the extraction rate of accompanying impurities. For example, under the conditions of pH = 6 and [LIX984N] = 20%, with an increase in the ratio of the aqueous-to-organic phase from 1 to 3, the extraction of magnesium and manganese ions increases from 8.3% and 22.69% to 1.8% and 18.6%, respectively, which helps to obtain purer nickel and cobalt in the organic phase. Another advantage of a high A/O ratio is cost-effectiveness, as a lower extractant volume is utilized.

3.3. Synergistic Effect of the Studied Parameters on Nickel and Cobalt Extraction Using LIX984N

3.3.1. Effect of pH and Extractant Concentration

Figure 7 shows the combined effect of pH and extractant concentration on the extraction of nickel and cobalt when A/O = 1:1. As shown by this figure, the extraction rate of elements increases with an increase in pH at different extractant concentrations. According to the results and graphs, the effect of the extractant concentration on the nickel and cobalt extraction percentage is minimal. The extraction of nickel and cobalt increases slightly due to the extractant’s high capacity. Suppose that at the same time as the pH increases from 4 to 6, the concentration of the extractant also increases from 10% to 20%. In that case, the amount of nickel and cobalt extraction reaches its highest level, i.e., 98.5% and 99.5%, respectively. Also, with the increase in pH, the extraction of disturbing ions increases slightly, which can be managed by considering the lowest concentration of the extractant and reducing the extraction of impurities (Table 2). The blue sections indicate low ion extraction, while the red sections indicate high ion extraction. Moving from the blue end toward the red end, ion extraction should increase.

3.3.2. Effect of A/O Ratio, Extractant Concentration, and pH

The interaction effect of the A/O ratio and pH according to the results presented in Table 2 is illustrated in Figure 8. As seen in this figure, pH is the most critical parameter influencing the extraction of cobalt and nickel using the LIX984N extractant. This relationship shows that increasing the ratio of the aqueous-to-organic phase results in a decrease in the percentage of extraction. The results obtained in this research also show the correctness of this matter. For example, under the conditions of [LIX984N] = 10% and pH = 4, increasing the ratio of the aqueous-to-organic phase from 1 to 3, the extraction of cobalt and nickel decreases from 25% and 20% to 16% and 14%, respectively. The reason for this is the decrease in the volume of the organic phase and the decrease in the extractant’s capacity. However, increasing this ratio reduces the extraction rate of accompanying impurities and makes nickel and cobalt purer in the organic phase. For instance, at pH = 6 and [LIX984N] = 10%, increasing the ratio of the aqueous-to-organic phase from 1 to 3, the extraction of manganese and magnesium is reduced from 6.18% and 11.81% to 81.5% and 9.7%, respectively. According to the results presented in the table and figure, with a phase ratio of A/O = 1, [LIX984N] = 10%, and pH = 6, the amount of cobalt and nickel extraction remains constant, at 99% and 96%, respectively. Increasing the A/O ratio to 3 and the pH to 6, nickel extraction rate increased from 94% to 96%. For cobalt this value remained unchanged and is 99%. As a result, with an increase in the A/O ratio, the extraction percentage of metal ions decreases The blue sections indicate low ion extraction, while the red sections indicate high ion extraction. Moving from the blue end toward the red end, ion extraction should increase. Figure 9) shows three dimensions of the interaction effect of the A/O ratio and extractant concentration on nickel and cobalt extraction. The extractant concentration increases the extraction of elements, and the A/O ratio decreases the extraction of elements. Both parameters have a minimal and linear effect on the extraction of elements.

3.4. Optimization of Process Parameters

Based on the results (Table 2) and Equations (3) and (4), the model predicted that 99% cobalt and 98.5 nickel could be extracted simultaneously under the following conditions: pH = 6, A/O = 1.6, and [LIX984N] = 20%.

After performing this test and repeating it, simultaneous extraction of nickel (90% and 95%) and cobalt (91% and 94%) was achieved, showing the model’s correctness.

3.5. Stripping of the Loaded Organic Phase

The figures show the effect of acid type and different acid concentrations on the recovery of nickel and cobalt ions. Stripping is the revere of the extraction operation, as described by Equation (5). According to this relationship and Le Chatelier’s principle, as the amount of ${\mathrm{H}}^{+}$ increases, the opposite of the extraction process takes place, and the desired element is obtained as a solution in the aqueous phase. Increasing the acid concentration increases the amount of ${\mathrm{H}}^{+}$ in the environment and increases the recovery of nickel and cobalt. At a 0.5 M sulfuric acid concentration, 70% of nickel and 3% cobalt were stripped from the organic phase. With an increase in the concentration of sulfuric acid up to one molar, the stripping rate of nickel reaches 94% (Figure 10).

The effect of the behavior of hydrochloric acid on nickel and cobalt stripping is different compared to that of sulfuric acid. This acid’s highest stripping efficiency for nickel and cobalt is 94% and 9%, respectively. At a concentration of 0.5 M, 88% of nickel and 3% of cobalt is stripped. With an increase in acid concentration up to 1 M, the stripping efficiency reaches 94% and 4% for nickel and cobalt, respectively (Figure 11).

After recovering nickel from the solution using hydrochloric acid in three steps, 80% of the cobalt ions could be returned from the organic solution to the aqueous phase. Nickel was first stripped from the organic phase. Following this process, approximately 80% of the remaining cobalt ions were successfully transferred from the organic phase to the aqueous phase. The observed difference in the stripping behavior of nickel and cobalt with hydrochloric acid compared to sulfuric acid is mainly due to the distinct chemical interactions and complexation behavior of the metals. Sulfuric acid enhances the formation of soluble sulfate complexes. In contrast, hydrochloric acid increases the formation of metal–chloride species.

3.6. Flowsheet of Co and Ni Removal from Sarcheshmeh Copper Complex Raffinate

Based on extraction and stripping data of Co and Ni, a flowsheet for selective recovery of each metal using solvent extraction was proposed (Figure 12). The proposed flowsheet shown in Figure 12 can be potentially applied to various leach solutions that contain both cobalt and nickel, especially those derived from laterite, sulfide, and secondary resources. In particular, Co and Ni commonly co-exist in pressure acid leach (PAL) solutions of laterite ores, in leach liquors from spent lithium-ion batteries, and in copper refinery raffinate solutions similar to those of the Sarcheshmeh system investigated in this study. These types of solutions usually contain comparable concentrations of Ni and Co, along with Fe, Zn, and Mn; therefore, the proposed extraction–stripping scheme could be effectively employed for their separation and recovery.

4. Conclusions

The recovery of nickel and cobalt from raffinate solution of the Sarcheshmeh copper complex was effectively investigated using the response surface methodology (RSM) with a central composite design (CCD) as the DOE, considering the three parameters of pH, extractant concentration, and the ratio of the aqueous-to-organic phase (A/O). Experiments were conducted using raffinate solution of the Sarcheshmeh copper complex.

The model predictions were verified with high-reliability numbers of about 97% and 98% for nickel and cobalt mining, respectively. The results illustrate that maximum extraction of nickel and cobalt can be achieved simultaneously. ANOVA showed that the effect of the equilibrium pH of the aqueous solution on nickel and cobalt extraction was greater than that of other parameters. The best extraction conditions for nickel and cobalt are pH = 6, [LIX984N] = 10%, and A/O = 3. According to the obtained model, 99% of cobalt and 94% of nickel can be extracted simultaneously under such conditions. In the stripping step, 95% of the nickel ions were stripped with 1M sulfuric acid, an 80% of the cobalt ions were stripped using 5M hydrochloric acid in three steps.