Single-Stage Extraction and Separation of Co²⁺ from Ni²⁺ Using Ionic Liquid of [C₄H₉NH₃][Cyanex 272]

Abstract

The purpose of this study was to optimize the extraction conditions for separating Co²⁺ from Ni²⁺ using N-butylamine phosphinate ionic liquid of [C₄H₉NH₃][Cyanex 272]. A Box–Behnken design of response surface methodology was used to analyze the effects of the initial pH, extraction time, and extraction temperature on the separation factor of Co²⁺ from sulfuric acid solution containing Ni²⁺. The concentrations of Co²⁺ and Ni²⁺ in an aqueous solution were determined using inductively coupled plasma-optical emission spectrometry. The optimized extraction conditions were as follows: an initial pH of 3.7, an extraction time of 55.8 min, and an extraction temperature of 330.4 K. The separation factor of Co²⁺ from Ni²⁺ under optimized extraction conditions was 66.1, which was very close to the predicted value of 67.2, and the error was 1.7%. The equation for single-stage extraction with high reliability can be used for optimizing the multi-stage extraction process of Co²⁺ from Ni²⁺. The stoichiometry of chemical reaction for ion-exchange extraction was also investigated using the slope method.

1. Introduction

Cobalt is a strategic metal that has many industrial applications, e.g., rechargeable batteries, superalloys, cemented carbide, and colorants [1,2]. This metal can be extracted or recycled from natural and secondary resources, e.g., Ni-Co laterites [3], stainless steel scarps [4], and spent lithium–ion batteries [5]. Therefore, to obtain the cobalt products, a leaching process is needed [6]. The multicomponent mixtures of metal ions, e.g., Co²⁺, Ni²⁺, Mn²⁺, Al³⁺, Zn²⁺, and others, are coexisting in the leaching solutions [4]. Efficient separation of Co²⁺ from leaching solution containing Ni²⁺ is the key technology for the final preparation of high-purity cobalt or nickel products [7,8].

As a matter of fact, two metals, cobalt and nickel, are difficult to completely separate due to similar physicochemical properties [9,10]. Based on a novel scale micro-reactor, the efficient separation of Co²⁺ from Ni²⁺ in sulfuric acid solution was achieved using the organic extractant of Cyanex 272, and this separation method was a two-stage counter-current extracted process [11]. Using the five-stage extracted process, the separation factor of Co²⁺ from Ni²⁺ (β_Co/Ni) could be reached 9.5 × 10³ using the new organic extractant of di-decylphosphinic acid [12]. About >99% of Co²⁺ and 11% of Ni²⁺ were extracted using the extractant of primene JMT-Cyanex 272 with the process of four-stage counter extraction [13], and the efficient separation of the two metal ions was achieved based on this process. In addition, extractants of MEXTRAL 507P [14], INET-3 [14], and PC88A [15] were also used for the separation of Co²⁺ from Ni²⁺. According to the above published articles, the efficient separation of Co²⁺ from Ni²⁺ in sulfate solution was generally realized using multi-stage extraction, and the experimental conditions of single-stage extraction were one of the essential factors for designing multi-stage extraction.

However, solvent extraction using organic extractants has several drawbacks, such as the irreversible loss of the organic phase [16] and easy volatilization of extractants [17]. For example, the oxidation reaction of primary amine N1923 may occur in the separation of V(V) from the leaching solution, and this process results in irreversible loss of organic extractant [18]. In addition, some organic extractants are easily volatile during extraction, and the organic vapors that fill the workshop may harm workers and cause environmental pollution problems [19]. Therefore, the demerits of solvent extraction using common organic extractants may lead to increased production costs and environmental problems [20,21]. How to reduce or avoid these problems in scientific research is one of the current research directions [22].

To our best knowledge, ionic liquids (ILs) have been used in solvent extraction for separating metal ions [4,23,24] due to their unique properties, i.e., low vapor pressure, high dissolving capability, chemical/thermal stability [25], and designable structure [26,27]. For example, the efficient and sustainable separation of Ho, Er, and Lu from Y was achieved using the functionalized ionic liquid of [N₁₈₈₈][NDA] [28]; the new ionic liquid of [N₁₄₁₁₁][DBP] exhibited high efficiency in uranium separation [29]; and the extraction efficiencies of Cd(II), Cu(II), and Zn(II) were about 85, 67, and 69% using the hydrophobic trioctylammonium-based ionic liquid of [HTOA][adipate] [30]. If more ionic liquids are used as extracted agents for the separation of Co²⁺ from Ni²⁺, the environmentally friendly separation process may be further investigated based on the single-stage extraction [31], and different kinds of ionic liquids may be applied in the recovery of metal ions from secondary waste resources.

With the aim of reducing the irreversible loss of the organic phase and decreasing the environmental pollution in the extraction process, the task-specific ionic liquid (TSIL) of [C₄H₉NH₃][Cyanex 272] was synthesized using a one-step reaction. The FT-IR, ¹H NMR, and ¹³C NMR of TSIL were made with the aim of analysis of purity and impurities. The optimization of extraction conditions for the separation of the two metal ions was determined by the Box–Behnken design (BBD) [32], which is a typical method of response surface methodology (RSM) [32]. Optimization of the extraction conditions for metal ions, e.g., Cu²⁺ [33], Cr³⁺ [34], and V⁴⁺ [35], were successfully performed using the BBD. The BBD was used to optimize the initial pH value (pH_ini), extraction time (t_e), and extraction temperature (T_e) with the aim of achieving the maximum separation factor of Co²⁺ from Ni²⁺ (β_Co/Ni). In addition, the extraction mechanism was also investigated using the slope method according to the published papers.

2. Materials and Methods

2.1. Chemicals and Reagents

The bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex 272, C₁₆H₃₅O₂P, purity: ≥92 wt%) was purchased from Zhengzhou Hecheng New Materials Tech Co., Ltd. (Zhengzhou, China). Butylamine (C₄H₉NH₂, purity: ≥99 wt%), Sodium hydroxide (NaOH, purity: ≥92 wt%), n-Hexane (C₆H₁₄, purity: ≥99 wt%), Anhydrous ethanol (≥99 wt%), NiSO₄·6H₂O (purity: ≥98.5 wt%,) CoSO₄·7H₂O (purity: ≥98 wt%), and H₂SO₄(purity: ≥98 wt%) were purchased from Shanghai Titan Scientific Co., Ltd. (Shanghai, China). Standard solutions of cobalt and nickel (1000 μg/mL) were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. (Shanghai, China). The synthesized [C₄H₉NH₃][Cyanex 272] and n-Hexane were used as extractant and diluent (Figure 1), respectively. All the chemicals and reagents were of reagent grade and used without further purification unless otherwise stated.

The simulated sulfuric acid solution containing 14 g/L of Co²⁺ and 14g/L of Ni²⁺ [4,23] was prepared by dissolving the NiSO₄·6H₂O and CoSO₄·7H₂O in deionized water. With the aim of obtaining a purity solution, the simulated solutions should pass through a filtration membrane with an aperture of 0.45 microns. The pH_ini of the simulated solutions was adjusted by adding H₂SO₄ or NaOH solutions.

2.2. Analytical Methods

The spectra of FT-IR were obtained using a SEPCTRUM GX1 instrument (PerkinElmer, Waltham, MA, USA) at room temperature. The ¹H and ¹³C NMR was obtained using an NMR spectrometer (Avance-III 400 MHz, Bruker, Zurich, Switzerland). A pH meter (Delta 320, Zurich, Switzerland) with an uncertainty of 0.01 was used to measure the pH_ini of the solutions. The concentrations of the two metal ions were analyzed using inductively coupled plasma-optical emission spectrometry (ICP-OES, Optima 8000, PerkinElmer, Waltham, MA, USA). The simulated solutions containing Co²⁺ and Ni²⁺ were diluted by deionized water to the concentrations of 0.1–40 ppm for the next tests.

2.3. Synthesis of the TSIL

The [C₄H₉NH₃][Cyanex 272] was synthesized in one step reaction (Figure 2), and the reaction process was observed by thin layer chromatography (TLC silica gel 60 F-254 on aluminum plate, Merck, NJ, USA). The spectra of the product are provided in the Supplementary Materials.

2.4. Single-Factor Experiment

The experiments were carried out by mixing an equal volume of the organic phase and simulated solution (each 15 mL) in a separating funnel using a water-bath thermostatic oscillator (SHA-C, Jie Rui Er, Changzhou, China). When the extraction was completed, the two immiscible phases were separated for analysis. The pH values in the sulfuric acid solution before and after extraction were determined using a pH meter. The concentrations of two metals before and after extraction were determined by ICP-OES. The effects of pH_ini, t_e, and T_e were first studied using single-factor experiments as follows: one extraction condition was changed when the other conditions were kept constant in every experiment, and the influences of each condition were studied by analyzing the concentration of metal ions in the solutions. All experiments were carried out three times, and the average values were obtained for the diagraph analysis under normal pressure unless otherwise stated.

2.5. Optimizing the Extraction Conditions by BBD

Design-Expert Ver. 8.0.6 (Trial version, Stat-Ease, Minneapolis, MN, USA) was used to optimize the extraction conditions [36]. According to the single-factor experimental results, three independent factors, i.e., pH_ini (A), t_e (B), and T_e (C), were found to be responsible for the β_Co/Ni. With the aim of analyzing the A, B, and C, three-factor and three-level Box–Behnken experimental design was used to conduct response surface analysis [37], and the three factors were optimized at three levels (−1, 0, −1) (

Table 1. Independent variables and levels used for Box–Behnken design.

No.	Variables	Level
		−1	0	1
A	pHini	1.50	3.75	6.00
B	te (min)	15.00	52.50	90.00
C	Te(K)	298.15	315.65	333.15

).

Based on the experimental data, regression analysis was made using the following second-order polynomial equation:

$$\begin{array}{l}{\beta }_{\mathrm{Co}/\mathrm{Ni}}=D_0+D_{\mathrm{A}}A+D_{\mathrm{B}}B+D_{\mathrm{C}}C +\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{D}_{\mathrm{A}}{}_{\mathrm{A}}{A}^2+D_{\mathrm{B}}{}_{\mathrm{B}}{B}^2+D_{\mathrm{C}}{}_{\mathrm{C}}{C}^2+\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}{D}_{\mathrm{A}}{}_{\mathrm{B}}AB+D_{\mathrm{B}}{}_{\mathrm{C}}BC+D_{\mathrm{A}}{}_{\mathrm{C}}AC\end{array}$$

(1)

where D₀ is a constant coefficient; D_A, D_B, and D_C represent coefficients of the linear equation for three independent factors, i.e., pH_ini (A), t_e (B), and T_e (C). D_AA, D_BB, and D_CC represent coefficients of the quadratic equation for three factors, i.e., A², B², and C². D_AB, D_BC, and D_AC represent coefficients of interaction effects of variables, i.e., AB, BC, and AC.

The significance of the equations was studied according to analysis of variance (ANOVA). The significance of each coefficient and the interaction between the independent variables were also investigated using the obtained p-value. In addition, real experiments were made to evaluate the accuracy of the optimized extraction conditions.

2.6. Calculations

The extraction percentages of Co²⁺ (E_Co), Ni²⁺ (E_Ni), and the β_Co/Ni were calculated as Equations (2)–(4).

$$E_{\mathrm{Co}}=\frac{V_1{C}_{1\left(\mathrm{Co}\right)}-V_2{C}_{2\left(\mathrm{Co}\right)}}{V_1{C}_{1\left(\mathrm{Co}\right)}}\times 100$$

(2)

$$E_{\mathrm{Ni}}=\frac{V_1{C}_{1\left(\mathrm{N}i\right)}-V_2{C}_{2\left(\mathrm{Ni}\right)}}{V_1{C}_{1\left(\mathrm{Ni}\right)}}\times 100$$

(3)

$${\beta }_{\mathrm{Co}/\mathrm{Ni}}=\frac{E_{\mathrm{Co}}\times (100-E_{\mathrm{Ni}})}{(100-E_{\mathrm{Co}})\times E_{\mathrm{Ni}}}$$

(4)

where V₁ (L) and V₂(L) represent the volumes of the stock and raffinate solution, respectively. C_1(Co) (g/L), C_2(Co) (g/L), C_1(Ni) (g/L), and C_2(Ni) (g/L) represent the concentrations of Co²⁺ and Ni²⁺ in the stock and raffinate solution.

3. Results and Discussion

3.1. ¹H, ¹³C NMR, and FT-IR of [C₄H₉NH₃][Cyanex 272]

The bis(2,4,4-trimethylpentyl) phosphinic acid, i.e., Cyanex 272 (2.93 g, 10 mmol) and butylamine (0.74 g, 10 mmol) in absolute ethanol were stirred for 3.5 h at 88 ℃ under the reflux condition. A light-yellow oily liquid was obtained with a 93.5% yield after evaporation of solvent using the rotary evaporation system. ¹H NMR (400 MHz, CDCl₃, ppm, C₄H₉NH_2, Supplementary Materials Figure S1): 0.92(t, 3H, 1 × CH₃); 1.13(s, 2H, 1 × CH₂); 1.35(m, 2H, 1 × NH₂); 1.43(m. 2H, 1 × CH₂); and 2.69(t, 3H, 1 × CH₂). ¹H NMR ([C₄H₉NH₃][Cyanex 272], Supplementary Materials Figure S2): 0.93(t, 21H, 7 × CH₃), 1.10 (t, 9H, 2 × CH₃ + 1 × NH₃), 1.36(m, 8H, 4 × CH₂), 1.57(d, 2H, 2 × CH), 1.69(m, 2H, 1 × CH₂), 1.89(s, 2H, 1 × CH₂), 2.78(t, 2H, 1 × CH₂). In the synthetic feedstock of n-butylamine (C₄H₉NH₂), the shift of hydrogen on -NH₂ is about 1.35 ppm, with a small widening on the right side of the peak. For the synthesized ionic liquid of [C₄H₉NH₃][Cyanex 272], compared with the C₄H₉NH₂, the -NH₂ becomes -NH₃, and the probability of hydrogen shift on the -NH₃ is within the range of 1.11pm (Figure 3b). The changing shift of hydrogen on the -NH₃ for synthesized ionic liquid resulted from the formation of new bonds in the ionic liquid. ¹³C NMR (400 MHz, CDCl₃, ppm, Supplementary Materials Figure S3): 13.06(s, 1C, 1 × CH₃), 20.07(s, 1C, 1 × CH₂), 24.04(m, 2C, 2 × CH₃), 25.50(d, 2C), 30.24(t, 7C, 6 × CH₃ + 1 × CH₂), 30.38(d, 2C, 2 × CH), 39.37(s, 1C, 1 × CH₂), 40.88(d, 1C, 1 × CH₂), 41.76(d, 1C, 1 × CH₂), 53.53(m, 2C, 2 × CH₂). FT-IR (cm⁻¹, Supplementary Materials Figure S4): asymmetric stretching vibration peak of methyl group (2950.60 cm⁻¹), asymmetric stretching vibration peak of ethyl group (2900.46 cm⁻¹), symmetric stretching vibration peak of methyl group (2868.16 cm⁻¹), in-plane vibration peak of amino group (1635.51 cm⁻¹), Symmetric deformation peak of ethyl group (1466.30 cm⁻¹), Symmetric deformation peak of ethyl group of methyl group (1363.45 cm⁻¹), stretching vibration peak of carbon–carbon bond (1126.73, 1022.10 cm⁻¹).

3.2. Single-Factor Experiment

The concentration of [C₄H₉NH₃][Cyanex 272] was set as 0.24 mol/L considering the concentration of Co²⁺ in the solution is 0.24 mol/L [4]. With the volume ratio of organic phase to the aqueous phase (O/A) = 1:1, the effect of pH_ini on β_Co/Ni is shown in Figure 3a when t_e = 60 min and T_e = 303.15 K. The β_Co/Ni first increases and then decreases with the growth of pH_ini. According to Equation (4), the β_Co/Ni is proportional to E_Co and inversely proportional E_Ni, and the final value of β_Co/Ni is determined by the ratio of E_Co and E_Ni.

The influence of t_e on β_Co/Ni is shown in Figure 3b when T_e = 303.15 K, pH_ini = 3.5, and O/A = 1:1. The β_Co/Ni first increases and then decreases with the increase in t_e due to the different extraction kinetics of Co²⁺ and Ni²⁺ [38]. The effect of T_e on β_Co/Ni is shown in Figure 3c when t_e = 45 min, pH_ini = 3.5, and O/A = 1:1. The β_Co/Ni increases with the growth of T_e. The reason is that higher temperature reduces the viscosity and surface tension of the extraction system, which is beneficial to mass transfer [37]. In addition to this, the extraction reaction of Co²⁺ using extractant of [C₄H₉NH₃][Cyanex 272] is an endothermic reaction, and the higher T_e is conductive to the reaction [10]. However, considering the cost of the extraction process, the highest temperature in the next experimental design was set at 333.15 K.

3.3. Optimization of the Conditions for Extraction Process

Seventeen experiments were first designed using BBD and then carried out one by one, and the results of these experiments are listed in

Table 2. Box–Behnken design for independent variables and observed responses.

Run No.	pHini	te/min	Te/K	βCo/Ni
1	3.75	52.50	315.65	64.41
2	1.50	52.50	333.15	54.72
3	3.75	52.50	315.65	63.27
4	3.75	90.00	333.15	48.67
5	3.75	15.00	298.15	29.71
6	3.75	90.00	298.15	40.21
7	3.75	52.50	315.65	65.51
8	6.00	52.50	333.15	55.82
9	1.50	52.50	298.15	42.26
10	6.00	15.00	315.65	27.21
11	6.00	90.00	315.65	38.27
12	1.50	90.00	315.65	34.46
13	1.50	15.00	315.65	22.87
14	3.75	52.50	315.65	64.72
15	6.00	52.50	298.15	44.15
16	3.75	52.50	315.65	63.89
17	3.75	15.00	333.15	36.42

When concentration of [C₄H₉NH₃][Cyanex 272] was set as 0.24 mol/L and O/A = 1:1.

, which was used to obtain the predicting equation. At least one variable is different in each group of experiments, and three levels of pH_ini are 1.50, 3.75, and 6.00, and three levels of t_e are 15.00, 52.50, and 90.00 min, and three levels of T_e are 298.15, 315.65, and 333.15 K, and the biggest value of β_Co/Ni is 65.51 when pH_ini = 3.75, t_e = 52.50, and T_e = 315.65. According to a regression analysis from the experimental data, the β_Co/Ni could be explained as Equation (5) according to second-order polynomial Equation (1). The D₀ is 64.36; D_A, D_B, and D_C are 1.39, 5.68, and 4.91; D_AA, D_BB, and D_CC are −11.59, −22.07, and −3.54; D_AB, D_BC, and D_AC are −0.13, 0.44 and −0.20, respectively.

$$\begin{array}{l}{\beta }_{\mathrm{Co}/\mathrm{Ni}}=64.36+1.39A+5.68B+4.91C\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}-0.13AB-0.20AC+0.44BC\\ \hspace{1em}\hspace{1em}\hspace{1em}\hspace{1em}-11.59A^2-22.07B^2-3.54C^2\end{array}$$

(5)

The variance of the quadratic prediction Equation (5) is summarized and listed in

Table 3. ANOVA of response quadratic model analysis for the β_Co/Ni..

Scheme	F-Value	p-Value (Prob > F)
Equation (5)	159.5146	<0.0001 a
A	6.691242	0.0361
B	111.1343	<0.0001
C	83.27631	<0.0001
AB	0.030291	0.8668
AC	0.067301	0.8028
BC	0.33025	0.5835
A2	243.8083	<0.0001
B2	884.7419	<0.0001
C2	22.71165	0.0020

F-value is from the F-test, which is most commonly known as the Joint Hypotheses test, and it is a test under the Null hypothesis; p-value is a hypothesis probability and an important basis for judging the significance of factors in statistics; ^a: significant.

. The determination coefficient (R² = 0.99) of Equation (5) is close to 1, which indicates that the effective correlation between predicated and measured values are obtained using this equation. The adjusted determination coefficients (adj R² = 0.98) are also close to 1, which demonstrates that the equation is significant for accurately predicting the β_Co/Ni. In addition to this, the F-value is 159.51 and the p-value < 0.0001, which implies that the equation is significant too. The p-value of the lack of fit is 0.0548, which indicates that the lack of fit is insignificant compared to the pure error. The lower coefficient of variation (C.V.% = 3.25) indicated that small deviation and high reliability of the experimental value. The regression equation can be used for predicting the experiment results instead of the real extractive experiments, and it can also be used to predict and analyze the β_Co/Ni.

The values of A, B, C, A², B², and C² are significant (p < 0.05), and the other values are not significant (p > 0.05). The influence degree of various factors on the β_Co/Ni is ranked in the following order: C > B > A. The coefficients of two factors, i.e., AB, AC, and BC, are 0.8668, 0.8028, and 0.5835, respectively. The effect degree of interaction based on two factors on the β_Co/Ni is ranked as BC > AC > AB. The BC has the biggest effect on the β_Co/Ni compared with the other two interaction factors of AC and AB, and the influent of BC on β_Co/Ni is shown in Figure 4. As shown in Figure 4b, the red zone represents the higher β_Co/Ni under the different T_e and t_e, and the efficient separation of the two metal ions will be operated according to the equation prediction.

3.4. Verification of Equation

Based on the quadratic equation, the optimum extraction conditions of Co²⁺ from Ni²⁺ were calculated as follows: a pH_ini of 3.7, a t_e of 55.8 min, a T_e of 330.4 K, and the predicted β_Co/Ni of 67.2. The verification experiments (Figure 5) were tested with the optimum extraction conditions, and the actual β_Co/Ni was 66.1. The experimental value was close to the predicted value, and the experimental error was small. The experimental results indicate that Equation (5) can better predict the β_Co/Ni for single-stage extraction of Co²⁺ from Ni²⁺.

With the optimized single-stage extraction conditions, the separation factors of Co²⁺ from Ni²⁺ using Cyphos IL 104 [4], Cyanex 301 [39], Primene JMT-Cyanex 272 [13], and [C₄H₉NH₃][Cyanex 272] are 14, 1.8, 45.3, and 67.2, respectively (

Table 4. Comparison of separation factor of Co²⁺ from Ni²⁺ using single-stage extraction.

Extractants	Diluent	Main Conclusion
Cyphos IL 104 [4]	Exxsol D80 or toluene	βCo/Ni is about 14 with optimized extraction conditions, i.e., 0.2M Cyphos IL 104, O/A = 1:1, te of 15 min, and Te of 296 ± 2 K
Cyanex 301 [39]	Kerosene	βCo/Ni is about 1.8 with optimized extraction conditions, i.e., 1.5M Cyanex 301, O/A = 1:1, pHini of 1, te of 30 min, and Te of 298 ± 1 K
Primene JMT-Cyanex 272 [13]	Exxol D100	βCo/Ni is about 45.3 with optimized extraction conditions, i.e., 1:1 percentage composition of JMT-Cy272, 0.4M chloride solution, te of 20 min, and Te of 296 ± 3 K
[C4H9NH3][Cyanex 272] (Paperwork)	n-Hexane	βCo/Ni is about 67.2 with optimized extraction conditions, i.e., 0.24M [C4H9NH3][Cyanex 272], O/A = 1:1, pHini of 3.7, te of 55.8 min, and Te of 330.4 K,

). It is concluded that the [C₄H₉NH₃][Cyanex 272] has better ability to efficient separate Co²⁺ from Ni²⁺ than Cyphos IL 104 [4], Cyanex 301 [39], and Primene JMT-Cyanex 272 [13]. In other words, the [C₄H₉NH₃][Cyanex 272] is more effective in separating the two metal ions than three extractants according to their separation factors of Co²⁺ from Ni²⁺. The new TSIL of [C₄H₉NH₃][Cyanex 272] is a kind of efficient extractant for the separation of the two metal ions.

3.5. Extraction Mechanism of [C₄H₉NH₃][Cyanex 272]

The slope method can be used to determine the stoichiometry of chemical reactions for metal ion extraction [40,41]. The extraction equilibrium constant (K) and distribution ratio of the two phases (D) can be obtained using balanced chemical reactions. The [C₄H₉NH₃][Cyanex 272] is a protic ionic liquid [42], and the cationic group of C₄H₉NH₃]⁺ is very soluble in the aqueous phase (Supplementary Materials Figure S5), but the solubility of [C₄H₉NH₃][Cyanex 272] in the aqueous phase is very poor (Supplementary Materials Figure S6, and the solubility is about 0.45 ± 0.02 g/100 g H₂O at the temperature of 298.2 K and normal pressure). It is assumed that protons can be completely transferred during the extraction reaction. The extractive mechanism of Co²⁺ in the sulfuric acid medium by [C₄H₉NH₃][Cyanex 272] is the ion-exchange reaction [43], and the chemical reaction can be expressed as Equation (6).

$$m[{\mathrm{C}}_4{\mathrm{H}}_9{\mathrm{NH}}_3{\left]\right[\mathrm{Cyanex} 272]}_{\left(\mathrm{org}\right)}+{\mathrm{Co}}_{\left(\mathrm{aq}\right)}^{2+}\stackrel{K_{\mathrm{eq}}}{\rightleftarrows }{\mathrm{Co}[\mathrm{Cyanex} 272]}_2{}_{\left(\mathrm{org}\right)}+m{[{\mathrm{C}}_4{\mathrm{H}}_9{\mathrm{NH}}_3]}_{\left(\mathrm{aq}\right)}^{+}$$

(6)

The extractive equilibrium constant of Co²⁺ (K_Co) can be listed as Equation (7) according to Equation (6).

$$K_{\mathrm{Co}}=\frac{C_1\cdot {(C_3)}^m}{{(C_2)}^m\cdot C_4}$$

(7)

where the C₁ and C₂ (mol/L) represent the concentrations of Co[Cyanex 272]₂ and [C₄H₉NH₃][Cyanex 272] in the organic phase. The C₃ and C₄ (mol/L) represent the concentration of ${\left[\mathrm{C}4\mathrm{H}9\mathrm{NH}3\right]}_{}^{+}$ and ${\mathrm{Co}}_{}^{+}$ in the aqueous phase. The distribution ratio of the two phases for Co²⁺(D_Co) can be listed as Equation (8).

$$D_{\mathrm{Co}}=\frac{C_1}{C_4}$$

(8)

Equation (9) can be obtained by taking the D_Co into the K_Co.

$$K_{\mathrm{Co}}=\frac{D_{\mathrm{Co}}\cdot {(C_3)}^m}{{(C_2)}^m}$$

(9)

Equation (10) can be obtained by taking the logarithm of K_Co

$$\lg D_{\mathrm{Co}}=\lg K_{\mathrm{Co}}-m\lg (\frac{C_3}{C_2})$$

(10)

The value of m in Equation (10) can be determined by the slope method (Figure 6). The m is 2.02. The value of m is approximately 2.0. Therefore, the equation of chemical reaction can be listed as Equation (11).

$$2[{\mathrm{C}}_4{\mathrm{H}}_9{\mathrm{NH}}_3{\left]\right[\mathrm{Cyanex} 272]}_{\left(\mathrm{org}\right)}+{\mathrm{Co}}_{\left(\mathrm{aq}\right)}^{2+}\stackrel{K_{\mathrm{eq}}}{\rightleftarrows }{\mathrm{Co}[\mathrm{Cyanex} 272]}_2{}_{\left(\mathrm{org}\right)}+2{[{\mathrm{C}}_4{\mathrm{H}}_9{\mathrm{NH}}_3]}_{\left(\mathrm{aq}\right)}^{+}$$

(11)

3.6. Stripping for TSIL-Based Extraction Phase and Its Recycling Use

After single-stage extraction, the TSIL-based extraction phase containing Co[Cyanex 272]₂ can be stripped with 0.5 mol/L H₂SO₄ (Figure 7). The stripping percentage of Co²⁺ from TSIL-based extraction phase was about 99.2% with the optimized stripping conditions, i.e., stripping phase ratio of 1:1, stripping time of 35 min, and stripping temperature of 308 ± 1 K. The product of CoSO₄·7H₂O was obtained from the stripping phase using evaporation crystallization [44]. The product of Ni(OH)₂ can be obtained from the raffinate of the aqueous phase containing Ni²⁺ using the precipitation method [45]. The TSIL-based extraction phase can be recycled for extraction after activation. The specific steps for activation are as follows: (1) the TSIL-based organic phase from stripping is firstly washed using the deionized water (the volume ratio of deionized water to ILs-based organic phase was 1:4), and the aqueous phase containing metal ions, which remained in the organic phase, is completely washed by new deionized water; (2) the reusable TSIL-based organic phase is obtained by the activating reaction of the washed organic phase and commercial n-butylamine solution (the reaction was similar to the Section 2.3), and the recycled TSIL-based organic phase (about 96.4% yield) can be reused for the single-extraction process.

4. Conclusions

In this study, the optimal extraction conditions of Co²⁺ from Ni²⁺ were determined using the RSM method to be: pH_ini of 3.7, t_e of 55.8 min, and a T_e of 330.4 K. The real yield of β_Co/Ni obtained by the validation test was 66.1, and the error was 1.7%, which was very close to the predicted value of 67.2. The result shows that the equation is efficient and feasible, with practical significance. The obtained Equation (5) of single-stage extraction is useful to design the multi-stage extraction for efficient separation of Co²⁺ from Ni²⁺_, and the multi-stage extraction can be used in industrial applications. The chemical reactive equation was determined using the slope method. Additionally, the recycled TSIL-based organic phase with 96.4% yield can be reused for the single-extraction process.