Efficient separation and recovery of V(V) from Cr(VI) in aqueous solution using new task-specific ionic liquid of [C₁₂H₂₅NH₃][Cyanex 272]

2.1 Chemicals

The simulated aqueous solution containing V(V) and Cr(VI) was made by dissolving NaVO₃(0.47 mol, 59.31 ± 0.01 g) and Na₂CrO₄·4H₂O(0.038 mol, 8.89 ± 0.01 g) in 1000 mL ultrapure water. The concentrations of V(V) and Cr(VI) in the aqueous solution were 23.70 g/L and 1.95 g/L, respectively. The concentrations of two metal ions were in accordance with values of the leaching solution from V-Cr slag (Jing et al., 2017a, 2017b). The source and mass fraction of chemicals used for the experiments were listed in Table 2. The water was ultrapure water, and its specific resistance is more than 18.2 MΩ·cm (Millipore Milli-Q). The synthesized [C₁₂H₂₅NH₃][Cyanex 272] and sulfonated kerosene were used for the extractant and diluting agent.

2.2 Synthesis of [C₁₂H₂₅NH₃][Cyanex 272]

The organic acid-base reflux reaction of Cyanex 272 (20.00 ± 0.01 g, 68.9 mmol) and C₁₂N₂₅NH₂ (12.80 ± 0.01 g, 68.9 mmol) was carried out in anhydrous ethanol (Fig. 1). The reaction temperature was 365.2 ± 0.1 K, the reaction time was 210 min, and the reaction pressure was ordinary pressure. When the reaction was finished, the organic phase was dehydrated using anhydrous sodium sulfate, and the TSIL of [C₁₂H₂₅NH₃][Cyanex 272] was obtained by treating the organic phase with rotary evaporator. The yield of [C₁₂H₂₅NH₃][Cyanex 272] was approximately 93.5 wt% (taking the Cyanex 272 being a reference).

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Fig. 1

Synthetic reaction of [C₁₂H₂₅NH₃][Cyanex 272].

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2.3 Instrument analysis

A pH meter (Mettler Toledo, Switzerland, Delta320) with a combined glass electrode was used to measure the acidity of the aqueous phase. The concentrations of metal ions in the aqueous phase were determined using an inductive couple plasma-optical emission spectrometry (PerkinElmer, United States, ICP-OES, Optima 8000). The ¹H and ¹³C NMR of synthetic TSIL were analyzed using a nuclear magnetic resonance spectrometer (Bruker, Switzerland, Avance III 400 MHz). The IR spectra of TSIL and extracted complex were tested by a fourier transform infrared spectroscope (Spectrum Two N, PerkinElmer, United States). Phase identification of recovered NH₄VO₃ was made using an X-ray diffractometer (XRD) with Cu Ka radiation (Rigaku Corporation, Japan, Smartlab). Morphology of powder was analyzed using a high-resolution cold-field emission-based scanning microscopy (SEM, SU8010, Hitachi, Japan).

2.4 Liquid–liquid extraction and stripping process

Single-factor extracted experiments were performed by mixing 10 mL aqueous phase and 20 mL organic phase in a separating funnel. The organic phase was consisted of extractant (i.e. [C₁₂H₂₅NH₃][Cyanex 272]) and diluent (i.e. sulfonated kerosene). The initial pH of the aqueous phase (pH_ini) was adjusted using diluted sulfuric acid or NaOH solution. A mixture of two insoluble phases was reacted in an oscillating water bath (SHA-C, Jie Ruier, China). When the extracted reaction was completed, two mixed phases were still and stratified under the action of gravity. The loaded organic phase containing V(V) and a trace of Cr(VI) was obtained from the upper layer of the separating funnel, and the raffinate (i.e. aqueous solution) was got from the lower layer of the funnel. A pH meter was used to measure the pH_ini and the equilibrium pH of raffinate. The ICP-OES was used to analyse the concentrations of metal ions in the aqueous phase.

The loaded organic phase containing V(V) and a trace of Cr(VI) was scrubbed twice using fresh water with scrubbing time of 10 min, a volume ratio of loaded organic phase to the aqueous phase of 1: 1, and scrubbing temperature of 298.2 ± 0.1 K. The loaded organic phase was then stripped with diluting NH₃·H₂O solution. Unless otherwise stated, all single-factor experiments including extraction and stripping were repeated three times under ordinary pressure, and their average value was used for plotting or analyzing the reaction mechanism.

2.5 Formulas

Extraction percentages of V(V) (E_V), Cr(VI) (E_Cr), and the separation factor (β_V/Cr) of V(V) from Cr(VI), distribution coefficient of V(V) (D_V), the stripping percentage of V(V) (S_V), were calculated as Eqs. (1)–(5).

3.1 NMR and IR spectrum analysis of [C₁₂H₂₅NH₃][Cyanex 272]

The real figures of Cyanex 272, C₁₂H₂₅NH₂, and [C₁₂H₂₅NH₃][Cyanex 272] are shown in Fig. 2a. The Cyanex 272 and [C₁₂H₂₅NH₃][Cyanex 272] are liquid, but the C₁₂H₂₅NH₂ is a solid body. The ¹H NMR (400 MHz, CDCl₃, ppm) of [C₁₂H₂₅NH₃][Cyanex 272] is shown in Fig. 2b, and the specific spectrum analysis is as follows: 2.72(t, 2H, CH₂, J = 16 Hz), 1.87(d, 2H, CH₂), 1.67(s, 2H, CH₂), 1.59(d, 2H, CH₂), 1.40(m, 2H, 2 × CH), 1.25(m, 22H, 11 × CH₂), 1.11(m, 9H, 2 × CH₃ + 1·NH₃), 0.88(t, 21H, 7 × CH₃, J = 20 Hz). The ¹C NMR (400 MHz, CDCl₃, ppm) of [C₁₂H₂₅NH₃][Cyanex 272] is shown in Fig. 2c, and the displacements of carbon are resolved as follows: 14.13, 22.70, 23.95, 23.99, 24.26, 24.31, 25.71, 27.04, 29.37, 29.64, 29.66, 30.31, 30.33, 31.18, 31.92, 40.14, 40.89, 41.02, 41.76, 53.39, 53.50, 53.77, 53.89. The IR spectrum of [C₁₂H₂₅NH₃][Cyanex 272] is shown in Fig. 2d, and characteristic peaks are analyzed as follows: asymmetric stretching vibration of –CH₂ (2923 cm⁻¹), symmetric stretching vibration of –CH₃ (2854 cm⁻¹) (Suresh et al., 2016), in-plane bending vibration of –CH₂ (1467 cm⁻¹), in-plane bending vibration of –CH₃ (1363 cm⁻¹) (Tayyari et al., 2016), bending vibration of P⚌O (1132 cm⁻¹) (Frost et al., 2013), stretching vibration of C—C(1022 cm⁻¹) (Zhao et al., 2015a, 2015b).

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Fig. 2

Images of Cyanex 272, C₁₂H₂₅NH₂, and [C₁₂H₂₅NH₃][Cyanex 272](a); ¹H NMR (b) and ¹³C NMR (c) of [C₁₂H₂₅NH₃][Cyanex 272]; IR spectrum of [C₁₂H₂₅NH₃][Cyanex 272](d).

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3.2 Effect of pH_ini on E_v and β_V/Cr

The equilibrium pH of the aqueous phase is one of the crucial factors affecting the extraction efficiency of metal ions (Hu et al., 2014a, 2014b). For example, the E_V increase gradually using extractant of Mextral 973H when the equilibrium pH ranges from −0.8 to 1.5 (Zhang et al., 2018). Compared with the single extractant (Zhang et al., 2016), synergistic extractant of Aliquat 336 and TBP have a wider range of equilibrium pH for the separation of V(V). In the extraction of V(V) using the LIX63 and PC88A, the pH_ini was ranged from 1.2 to 6.0 when the equilibrium pH was varied from 1.6 to 5.4 (Nguyen and Lee, 2015). Therefore, the equilibrium pH is determined by its pH_ini in the same extraction system. In addition, the pH_ini is easy to adjust in real experiments or industrial application.

Therefore, in this study, the effect of pH_ini on E_V, E_Cr, and β_V/Cr is shown in Fig. 3 with the operating conditions of the volume ratio of organic phase to aqueous phase (O/A) is 2:1, the concentration of [C₁₂H₂₅NH₃][Cyanex 272] (C₁) is 0.24 mol/L, extraction time (t_e) is 35 min, and extraction temperature (T_e) is 298.2 ± 0.1 K is shown in Fig. 3. The pH_ini of the aqueous phase is difficult to accurately adjust to a fixed value, therefore, the range of pH_ini error is set to ± 0.02 (Fig. 3a). From images of partial extraction experiments, the phase interface is distinct after oscillation reaction and static layering. The E_V, E_Cr, and β_V/Cr decrease when pH_ini ranges from 1.5 to 5.0 (Fig. 3b). When the pH_ini are 1.5, 2.0, 2.5, 3.0, 3.5, 4.0, 4.5 and 5.0, the average of E_V from the three parallel trials are 98.7, 85.4, 77.5, 74.1, 69.4, 66.4, 54.8, 45.3 %, and the E_Cr are 7.2, 6.1, 5.3, 4.9, 4.6, 4.1, 3.8, 3.6, and the β_V/Cr are 1077.9, 89.9, 61.6, 55.6, 47.0, 46.3, 30.7, 22.5. When pH_ini = 1.5, the E_V and β_V/Cr are larger than the values under the other pH_ini. Therefore, the pH_ini = 1.5 is a reasonable extractive condition for the efficient separation of V(V) from Cr(VI) in the subsequent studies.

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Fig. 3

Images of partial extraction experiments (a); Effect of pH_ini on E_v and β_V/Cr (b).

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3.3 Effect of C₁ on E_V and β_V/Cr

The C₁ were set as 0.17, 0.19, 0.22, 0.24, 0.26, 0.29, and 0.31 mol/L, and the concentration of V(V) (C_V) in the aqueous solution was 0.47 mol/L. The proportions of C₁ to C_V were 0.36, 0.40, 0.46, 0.51, 0.55, 0.62, and 0.66. The effect of C₁ on E_V, E_Cr, and β_V/Cr using [C₁₂H₂₅NH₃][Cyanex 272] dissolved in sulfonated kerosene is shown in Fig. 4 under the conditions of pH_ini = 1.5, O/A = 2:1, t_e = 35 min, and T_e = 298.2 ± 0.1 K. The E_V and E_Cr increase with the growth of C₁, but the β_V/Cr first increase and then decrease with the increasing of the C₁. The E_V and β_V/Cr are 99.5 and 1.07 × 10³ when C₁ = 0.26 mol/L. The extractive stoichiometry of V(V) using [C₁₂H₂₅NH₃][Cyanex 272] can be expressed as Eq. (6) (Ning et al., 2014), AB represents the [C₁₂H₂₅NH₃][Cyanex 272], the main species of V(V) in the range of equilibrium pH is V₄ ${\mathrm{O}}_{12}^{4-}$ , and y represents the coefficient of the chemical reaction equation.

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Fig. 4

Effect of C₁ on E_v and β_V/Cr.

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According to Eq. (6), increasing the C₁ can promote the degree of extractive reaction. Therefore, E_V increases as C₁ increases, and this conclusion is consistent with the experiment results. However, the β_V/Cr is determined by the E_V and E_Cr, and its maximal value is 1.07 × 10³ when C₁ = 0.26 mol/L. Therefore, the C₁ of 0.26 mol/L is the suitable condition for the efficient separation for V(V) from Cr(VI).

3.4 Effect of t_e on E_V and β_V/Cr

When pH_ini = 1.5, O/A = 2:1, C₁ = 0.26 mol/L, and T_e = 298.2 ± 0.1 K, the effect of t_e on E_v, E_Cr and β_V/Cr using the extractant of [C₁₂H₂₅NH₃][Cyanex 272] is shown in Fig. 5. The E_v, E_Cr and β_V/Cr are increased with the growth of t_e. When t_e are 25, 30, and 35 min, the E_V are 99.5, 99.5, and 99.5 %, and the E_Cr are 15.6, 15.8, and 15.7 %, and the β_V/Cr are 1.08 × 10³, 1.05 × 10³, and 1.07 × 10³. Extracted reaction of vanadium using extractants of N235 reaches an equilibrium state within 5 min because that the little change of E_v is made when the t_e exceeds 5 min (Ye et al., 2018). For example, extraction of V(IV) using the synergistic extractant of D2EHPA and PC88A reaches an equilibrium in 12 min due to the slight change as the t_e is prolonged from 12 to 16 min (Shi et al., 2017). In this study, The E_v, and β_V/Cr are not changed or changed less when t_e are 25, 30, and 35 min. and the extractive reaction can be reached equilibrium state when t_e = 25 min. Therefore, t_e = 25 min is an appropriate extractive time for separation of V(V) from Cr(VI). The best β_V/Cr is 1.08 × 10⁵ using the new TSIL with the optimum operating conditions, which is better than the values of β_V/Cr for [C₈mim][PF₆] (Hu et al., 2014a, 2014b), [A336][NO₃] (Zhao et al., 2015a, 2015b), and [RNH₃][NO₃] (Zhao et al., 2015a, 2015b). In other words, the TSIL of [C₁₂H₂₅NH₃][Cyanex 272] is a good extractant for recovery and separation of V(V) from Cr(VI).

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Fig. 5

Effect of t_e on E_v and β_V/Cr.

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3.5 Extraction mechanism for the separation of V(V)

The stoichiometry of extracted reaction can be determined using the slope method (Jiang et al., 2015, Li et al., 2020), and extracted reaction using [C₁₂H₂₅NH₃][Cyanex 272] for V(V) can be expressed as Eq. (7) (Ning et al., 2014).

For Eq. (7), the equilibrium constant K_ex is expressed as:

The form of Eq. (9) is changed by taking logarithm:

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Fig. 6

Slope method for the investigation of extractive mechanism.

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In order to further study the extractive mechanism, the powder of the [C₁₂H₂₅NH₃][Cyanex 272]₄·H₄V₄O₁₂ (Fig. 7) was obtained using the process of solvent-out crystallization (Zhao et al., 2019). The solvent-out crystallization was listed as follows: (1) the loaded organic phase was first obtained from the finished extraction process; (2) the powder of extracted complex was obtained using the solvent-out crystallization with the anhydrous ethanol; (3) the wet powder was dried for 4 h at the temperature of 353.2 ± 0.1 K in the blast drier; (4) the dry powder of extracted complex was used for the IR spectrum analysis (Fig. 7). Peaks in the extracted complex are analyzed as follows: asymmetric stretching vibration of –CH₂ (2917 cm⁻¹), symmetric stretching vibration of –CH₃ (2849 cm⁻¹) (Suresh et al., 2016), in-plane bending vibration of –CH₃ (1458 cm⁻¹) (Tayyari et al., 2016), the asymmetric and symmetric stretching vibrations of V = O (969 and 940 cm⁻¹) (Nandi and Sarkar 2012), the stretching vibrations of V-O-V (721, 617 and 525 cm⁻¹) (Zhang et al., 2016). Compared with the IR spectrum of fresh TSIL of [C₁₂H₂₅NH₃][Cyanex 272], there peaks of ν_sym(–CH₃), ν_asym(–CH₂), and δ(–CH₃) in the extracted complex has been happened red shift, indicating the formation of new extracted complex.

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Fig. 7

IR spectra of extractant and extracted complex.

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3.6 Stripping for TSIL-based organic phase and its recycling use

When pH_ini = 1.5, O/A = 2:1, C₁ = 0.26 mol/L, T_e = 298.2 ± 0.1 K, and t_e = 25 min, the E_v, E_Cr and β_V/Cr were about 99.5 %, 15.6 % and 1.08 × 10³, and the concentrations of V(V) and Cr(VI) in the loaded organic phase were about 23.58 and 0.30 g/L. The loaded organic phase containing V(V) and a small amount of Cr(VI) was first scrubbed twice using the fresh water (the volume ration of deionized water to TSIL-based organic phase was 0.5:1 for every time), and the loaded Cr(VI) in organic phase was almost completely removed using this process. After scrubbing, a dilute solution of NH₃·H₂O was used as a stripping agent to recover V(V) from the organic phase (Joo et al., 2016, Wei-Sheng and Hsing-Jung 2018), and the stripping reaction can be expressed as Eq. (13).

Therefore, the influence of the stripping time (t_s) on the S_V is shown in Fig. 8a when the concentration of NH₃·H₂O (C₂) is 0.42 mol/L, stripping phase ratio (O/A)_s is 1:1, and stripping temperature (T_s) is 303.2 ± 0.1 K. The S_V increases with the growth of t_s when it is in the range of 10–40 min, and the S_V remains almost unchanged when the t_s is in range of 40–60 min. With the increase of t_s from 10 to 40 min, the S_V increases from 55.7 to 94.7 %. When t_s is changed from 40 to 60 min, the S_V keeps at the same value of 94.6 %. The stripping was basically reached equilibrium state when the t_s is 40 min, and this reaction time is the suitable condition for the process of stripping.

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Fig. 8

Effect of t_s on S_V(a); Effect of C₂ on S_V(a).

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The effect of C₂ on S_V is shown in Fig. 8b when t_s is 40 min, (O/A)_s = 1:1, T_s = 303.2 ± 0.1 K. The S_V increase with the growth of C₂ when it ranges from 0.33 to 0.61 mol/L, and the S_V is 99.2 % when C₂ is 0.52 mol/L. The change of S_V is very small when the C₂ ranges from 0.52 to 0.61 mol/L, and the increasing in basicity for aqueous solution will improve the recovery cost of the V-Cr slag. Therefore, the C₂ of 0.52 mol/L is a reasonable concentration of NH₃·H₂O for the stripping process considering stripping efficiency.

To sum up, after scrubbing of the loaded organic phase, then it was stripped with the diluent NH₃·H₂O of 0.52 mol/L when (O/A)_s = 1:1, T_s = 303.2 ± 0.1 K, and t_s = 40 min. The recovered NH₄VO₃ (Fig. 9) was obtained using the processes of stripping, filtration, washing, and desiccation (378.2 ± 0.1 K and 10 h) from the loaded organic phase. The XRD of recovered NH₄VO₃ is shown in Fig. 9a, and the peaks are good consistent with the standard peaks. The SEM image of recovered NH₄VO₃ is presented in Fig. 9b, and the plate-like crystals of NH₄VO₃ are in an aggregate or homogeneous state. The separation and recovery of V(V) from Cr(VI) in aqueous solutions can be reached using the new extracted system including the novel TSIL of [C₁₂H₂₅NH₃][Cyanex 272].

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Fig. 9

XRD patterns (a) and SEM (b) of recovered NH₄VO_3.

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The TSIL-based organic phase containing V(V) and a small amount of Cr(VI) was first scrubbed twice using the fresh water, and TSIL-based organic phase containing Cr(VI) was almost completely removed using this process, and the organic phase containing V(V) was stripped with the diluent NH₃·H₂O, and the recovered NH₄VO₃ was obtained from aqueous phase containing V(V). After stripping, the reusable TSIL-based organic phase (about 97.2 % yield, Fig. 10) was obtained from the TSIL-based organic phase containing V(V), and this reusable TSIL-based organic phase can be reused for extraction of V(V) from simulated solution containing V(V) and Cr(VI). The recovered Cr₂O₃ can be also obtained using the processes of evaporation, drying, dehydration, and calcination from aqueous phase containing Cr(VI) (Ying et al., 2020). The investigation on the recovery of Cr(VI) has not been further carried out in current experiments. In the next step of processing the actual leach liquor, the systematic research will be carried out in our laboratory. Here, the recycling method for Cr(VI) has been referred to the published article (Ying et al., 2020).

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Fig. 10

Recycling use of TSIL-based organic phase [The process in the dashed line has not been carried out in this study, and this treatment process can be referred to the published article(Ying et al., 2020)].

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