Study on the synergistic extraction of vanadium by mixtures of acidic organophosphorus extractants and primary amine N1923

Separation and Purification Technology 156 (2015) 835–840

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Study on the synergistic extraction of vanadium by mixtures of acidic organophosphorus extractants and primary amine N1923 Dandan Jiang a, Naizhong Song a, Sufen Liao a, Yu Lian b, Jiutong Ma a,⇑, Qiong Jia a,⇑ a b

College of Chemistry, Jilin University, Changchun 130012, PR China College of Science, Changchun Institute of Technology, Changchun 130012, PR China

a r t i c l e

i n f o

Article history: Received 12 July 2015 Received in revised form 2 November 2015 Accepted 3 November 2015 Available online 5 November 2015 Keywords: Extraction Vanadium Acidic organophosphorus extractant Primary amine N1923

a b s t r a c t In the present study, the solvent extraction of vanadium from chloride medium with acidic organophosphorus extractants, primary amine N1923, and their mixtures were investigated. The organophosphorus acids included di(2-ethylhexyl) phosphoric acid (D2EHPA), 2-ethylhexylphosphonic acid mono-(2ethylhexyl) ester (HEHEHP), and bis(2,4,4-trimethylpentyl) phosphinic acid (Cyanex272). The extraction effects from chloride medium were studied in detail, illustrating that all the three mixtures of organophosphorus acids and N1923 have synergistic effects on vanadium. As a representative, extraction stoichiometries of vanadium with D2EHPA + N1923 were studied with the methods of slope and constant mole. The extraction capacity of temperature dependency was also investigated. Moreover, the separation of vanadium, nickel, and chromium with mixtures of D2EHPA + N1923 was studied. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Vanadium is extensively applied due to its physical properties of tensile strength, hardness, and fatigue resistance. However, there is no mineral deposit containing vanadium in nature. It is generally found in vanadium-titanomagnetite, phosphate rock, bitumen, black shale, and fly ash produced from oil industry and petroleum coke [1]. Several separation processes have been proposed to recover vanadium, among which liquid–liquid extraction is the most widely applied technology for metal ion separations due to its advantages including operation in a continuous mode, employment of simple equipment, achievement of high sample throughput and easy scale-up. During the past decades, various reagents have been reported for the extraction of vanadium, including tri-butyl-phosphate [2], di(2-ethylhexyl) phosphoric acid (D2EHPA) [3], 2-ethylhexylphosphonic acid mono-(2-ethylhexyl) ester (HEHEHP) [4], bis(2,4,4trimethylpentyl) phosphinic acid (Cyanex272) [3], primary amine [5,6], primene 81R and alamine 336 [7], trioctylamine [8], trioctylamine [9], Aliquat336 [10], and hydroxy-oxime derivative [11]. To explore new kinds of extractants or extracting systems has also attached more and more attention for vanadium extraction. The extraction of metal ions with the mixture of two kinds of extractants instead of using a single extractant is called synergistic

⇑ Corresponding authors. E-mail addresses: [email protected] (J. Ma), [email protected] (Q. Jia). http://dx.doi.org/10.1016/j.seppur.2015.11.008 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.

extraction. Synergistic extraction can not only enhance the extraction efficiency, but also in some cases significantly improve the selectivity. To date, various mixtures of two extractants have been investigated and applied to the extraction of vanadium, such as D2EHPA/Cyanex272 [3], 3,5-dichlorophenol/trioctylphosphine oxide [12], and tricaprylmethylammonium nitrate/organic acidified primary amine N1923 [13]. For instance, Noori et al. [3] investigated the selective recovery and separation of nickel and vanadium in sulfate media using D2EHPA, Cyanex272, and their mixtures. Adding Cyanex272 to D2EHPA in the organic phase leads to a right shifting of extraction isotherm of nickel and a slight left shifting of the extraction isotherm of vanadium and improves the separation of nickel over vanadium. To optimize the recovery and separation process of nickel and vanadium from the sulfate leach liquor, the influence of different D2EHPA to Cyanex272 ratios and various temperatures were studied. Zhao et al. [13] employed pure tricaprylmethylammonium nitrate ([A336][NO3]) and primary amine N1923 for the separation of vanadium(V) from chromium(VI). The optimal proportion of [A336][NO3] and [RNH3] [NO3] was studied, showing that the mixed [A336][NO3] and [RNH3][NO3] exhibited an obvious synergistic effect for V(V). The work demonstrated that quaternary ammonium IL containing a commercial organic extractant is an efficient and sustainable IL-based extraction strategy for the separation of vanadium from chromium. A survey of literatures showed that acidic organophosphorus reagents have been widely used for the extraction of vanadium.

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However, as compared with sulfuric acid, few studies about the extraction of vanadium from chloride media using organophosphorus acids have been performed. Moreover, synergistic extraction of vanadium based on such extractants have been seldom investigated [3]. Hence in the present study, the extraction of vanadium from acidic chloride solutions with mixtures of several acidic organophosphorus reagents (D2EHPA, HEHEHP, Cyanex272) and primary amine N1923 was studied in detail. The distribution data were analyzed graphically and numerically to investigate the extraction stoichiometries. The thermodynamic functions were also determined.

phorus acids, N1923, and their mixtures was studied. Results were shown in Figs. 1–3, indicating the extraction efficiency of V(IV) with D2EHPA + N1923, HEHEHP + N1923, and Cyanex272 + N1923, respectively. XN1923 represents the mole fraction of N1923 in the organic phase, whereas D describes the distribution ratio, i.e., the ratio between the concentration of V(IV) in the organic phase and that in the aqueous phase. It can be observed that all the three mixtures have obvious synergistic extraction effects on V(IV). The synergistic enhancement factor, R, can be obtained according to synergistic extraction theory [14].

R¼

2.1. Reagents and apparatus The stock solutions were prepared by dissolving NaVO3 (Aladdin, Shanghai) in hydrochloric acid (HCl) of required concentration. pH of the aqueous phase was adjusted by the addition of HCl or sodium hydroxide (NaOH) solutions. The initial V(IV) concentrations were maintained at 2.0
103 mol L1. A constant ionic strength (l = 1.0 mol L1) adjusted by NaCl was employed for all extraction experiments. All the other reagents and chemicals used were of analytical reagent grade. N1923, D2EHPA, and HEHEHP were obtained from Shanghai Rare-Earth Chemical Co., Ltd., China. Cyanx272 was supplied by Cytec Canada, Inc. All the extractants were used without further purification and dissolved in n-heptane to the required concentrations. The concentration of organophosphorus acids were determined by titration with standard NaOH solution while that of primary amine N1923 by titration with standard HCl solution. N1923 was acidified by an equivalent amount of HCl solution to form the ammonium salt. pH measurements were performed on a pHS-3C digital pH meter (Shanghai Rex Instruments Factory, China). 2.2. Extraction procedures The aqueous and organic phases, of which the volumes were both 5 mL, were mechanically shaken for 30 min at 20 °C unless otherwise stated. Preliminary experiments confirmed that 30 min was enough for reaching extraction equilibrium. After the phases were separated by gravity, the concentration of V(IV) in the aqueous phase was determined by ferrous ammonium sulfate titration with N-phenylanthranilie acid as the indicator. A microburette with a maximum volume of 2 mL was employed for the titration operations. Distribution ratios (D) were calculated from the concentrations of V(IV) in the aqueous phase and that in the organic phase which was obtained by mass balance. After loading of V(IV) by the extractants, the loaded organic phase was equilibrated with HCl solutions of different concentrations for 30 min (1/1 A/O ratio). The aqueous solution was separated from the organic phase and the metal concentrations were determined. Then the stripping ratio was calculated as ([V(IV)]a,e/ [V(IV)]o,i)
100%, where [V(IV)]a,e and [V(IV)]o,i represented the equilibrium concentration of V(IV) in stripping acid and the initial concentration of V(IV) in the organic phase, respectively. 3. Results and discussion 3.1. Extraction effects of V(IV) with mixtures of organophosphorus acids and N1923 When the concentration, pH, and ionic strength of V(IV) solutions were kept constant, the extraction of V(IV) with organophos-

ð1Þ

where DA, DB, and Dmix represents the distribution ratios when V(IV) is extracted by organophosphorus acids, N1923, and their mixtures, respectively. The R values in the mixing systems based on organophosphorus acids and N1923 were listed in Table 1. Synergistic effects can be found at all XN1923 values in the range of 0.1–0.9 for all the three mixing systems. Up to date, the reason why a mixture containing two or more extractants has synergistic or antagonistic effect on metal ions is not very clear. In a binary mixing system, three extraction reactions, i.e., the extraction with the two single extractant and the mixtures, simultaneously exist, all of which may affect the extraction effects. 3.2. Extraction stoichiometry of V(IV) with single extractant It is well known that organophosphorous-based acidic extractants exit as dimmers in nonpolar organic diluents. There have been some reports about the extraction of V(IV) with organophosphorus acidic extractant, by which the following equation was generally obtained [15–17]. KA

þ VO2þ ðaÞ þ 2H2 A2ðoÞ
VOA2  2HAðoÞ þ 2HðaÞ

ð2Þ

where ‘‘a”, ‘‘o”, and KA represent aqueous phase, organic phase, and extraction constant respectively. The distribution ratio, DA, can be described as

log DA  2pH ¼ 2 log ½H2 A2 ðoÞ þ log K A

ð3Þ

As a representative, the analytical data of the extraction of V(IV) with D2EHPA were listed in Table 2, indicating a logarithm value of KA of 0.54. 12

N1923 D2EHPA N1923+D2EHPA

10 8

D

2. Experimental

Dmix DA þ DB

6 4 2 0 0.0

0.2

0.4

0.6

0.8

1.0

X N1923 Fig. 1. Extraction of VO2+ with mixtures of D2EHPA and N1923. [VO2+] = 2.0
103 mol L1, pH = 2.0, l = 1.0 mol L1, CN1923 + CD2EHPA = 0.05 mol L1.

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12

2

N1923 HEHEHP N1923+HEHEHP

10

1

logDAB

D

8 6

-1

4 2 0 0.0

slope: 1.93

0

-2 0.2

0.4

0.6

0.8

0.0

1.0

0.5

1.0

X N1923 Fig. 2. Extraction of VO2+ with mixtures of HEHEHP and N1923. [VO2+] = 2.0
103 mol L1, pH = 2.0, l = 1.0 mol L1, CN1923 + CHEHEHP = 0.05 mol L1.

12

2.0

2.5

Fig. 4. Effect of equilibrium pH on the distribution ratio. [VO2+] = 2.0
103 mol L1, l = 1.0 mol L1, CD2EHPA = 0.03 mol L1, CN1923 = 0.02 mol L1.

-2.0

N1923 Cyanex272 N1923+Cyanex272

10

1.5

pH eq

(A) -2.5

logDAB - 2pH

D

8 6 4

slope: 1.04

-3.0

slope: 1.01 -3.5

2 0 0.0

0.2

0.4

0.6

0.8

-4.0 -2.4

1.0

-2.1

X N1923

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

D2EHPA + N1923 HEHEHP + N1923 Cyanex272 + N1923

1.2 1.1 1.1

1.4 1.1 1.1

1.7 1.3 1.3

2.1 1.7 1.7

2.0 2.3 2.3

1.8 1.8 1.8

1.5 1.4 1.4

1.4 1.2 1.2

1.2 1.1 1.1

-1.2

(B) -2.5

logDAB - 2pH

XN1923

-1.5

-2.0

Fig. 3. Extraction of VO2+ with mixtures of Cyanex272 and N1923. [VO2+] = 2.0
103 mol L1, pH = 2.0, l = 1.0 mol L1, CN1923 + CCyanex272 = 0.05 mol L1.

Table 1 Synergistic enhancement coefficients of V(IV) with mixtures of organophosphorus acids and N1923.a

-1.8

log [H2 A 2 ] (o) , mol L-1

slope: 1.01 -3.0

slope: 1.05 -3.5

a The total concentration of the extractants in every mixing system was 0.05 mol L1.

Table 2 Analytical data of VO2+ concentration, D2EHPA concentration, and equilibrium constant. No.

[VO2+](a), (mol L1)

log[H2A2](o), (mol L1)

log KA

Average log KA

1 2 3 4 5

1.7
103 1.3
103 0.9
103 0.6
103 0.4
103

2.02 1.72 1.54 1.41 1.31

0.53 0.55 0.53 0.57 0.55

0.54 ± 0.03

The extraction of V(IV) with primary amine N1923 has also been reported, however, there are no studies of the extraction stoichiometry, especially from other media except sulfuric acid

-4.0 -2.4

-2.1

-1.8

-1.5

-1.2

log [(RNH3Cl)3 ] (o) , mol L-1 Fig. 5. Effect of extractant concentration on the distribution ratio. [VO2+] = 2.0
103 mol L1, pH = 2.0, l = 1.0 mol L1. (A) j – [(RNH3Cl)3](o) = 0.02 mol L1, d – [(RNH3Cl)3](o) = 0.01 mol L1; (B) h – [H2A2](o) = 0.02 mol L1, s – [H2A2](o) = 0.01 mol L1.

solutions [6,18]. In the present work, N1923 exists predominantly as trimeric species since it was acidified by an equivalent amount of hydrochloric acid to form the ammonium salt [19]. When other experimental conditions were fixed, the relationship between distribution ratio and N1923 concentration in the organic phase was

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2.0

½H2 A2 ðoÞ ¼ C H2 A2  C VO2þ


N1923 D2EHPA N1923+D2EHPA

log D

1.5

2DA þ xDAB 1 þ DA þ DB þ DAB

½ðRNH3 ClÞ3 ðoÞ ¼ C ðRNH3 ClÞ3  C VO2þ


1.0

slope: -1.27 slope: -1.50

0.5

slope: -1.08

0.0 2.8

3.0

3.2

3.4

3.6

3.8

-1

1000/T (K ) Fig. 6. Effect of temperature on the extraction of VO2+ with mixtures of D2EHPA, N1923, and D2EHPA + N1923. [VO2+] = 2.0
103 mol L1, pH = 2.0, l = 1.0 mol L1. j – CN1923 = 0.05 mol L1, d – CD2EHPA = 0.05 mol L1, N CD2EHPA = 0.03 mol L1, – CN1923 = 0.02 mol L1.

ð7Þ

DB þ yDAB 1 þ DA þ DB þ DAB

ð8Þ

To determine the x, y values and obtain the final extraction reaction, the dependencies of pH and extractant concentrations on the distribution ratio were investigated in detail. For pH dependency studies, the distribution ratio values were determined at varying pH of V(IV) solutions when other experimental conditions were kept constant. Results were shown in Fig. 4, illustrating an m value of about 2.0 could be obtained. Similarly, the distribution ratio values were studied at varying extractant concentrations to investigate the effect of them on the extraction behaviors. Results were indicated in Fig. 5, indicating that the slopes of log DAB  2pH versus log[H2A2](o) and log[(RNH3Cl)3](o) were both about 1.0. Therefore, the following equation could be concluded for the extraction of V(IV) with D2EHPA + N1923. K AB

þ VO2þ ðaÞ þ H2 A2ðoÞ þ ðRNH3 ClÞ3ðoÞ
VO  A2  ðRNH3 ClÞ3ðoÞ þ 2HðaÞ

ð9Þ investigated. The following equation was obtained to express the extraction of V(IV) with primary amine N1923. KB



VO2þ ðaÞ þ 2ClðaÞ þ ðRNH3 ClÞ3ðoÞ
VO  Cl2  ðRNH3 ClÞ3ðoÞ

ð4Þ

where KB describes extraction constant with primary amine N1923. The distribution ratio, DB, can be described as 

The logarithm of equilibrium constant, log KAB, could be calculated as 0.60 ± 0.02. According to Eqs. (2), (4), and (9), the following forming reactions could be reduced. b1

VOA2  2HAðoÞ þ ðRNH3 ClÞ3ðoÞ
VO  A2  ðRNH3 ClÞ3ðoÞ þ H2 A2ðoÞ

ð5Þ

ð10Þ

By similar method to obtain KA, extraction constant with D2EHPA, the logarithm of KB was determined to be 1.95 ± 0.02.

VOCl2 ðRNH3 ClÞ3ðoÞ þH2 A2ðoÞ
VO A2 ðRNH3 ClÞ3ðoÞ þ2HþðaÞ þ2ClðaÞ

log DB ¼ log K B þ log ½BðoÞ þ log ½Cl ðaÞ

3.3. Extraction stoichiometry of V(IV) with mixtures of D2EHPA and N1923 The synergistic extraction stoichiometry of V(IV) with mixtures of D2EHPA and N1923 was studied as a representative. The synergistic extraction was assumed as followings

VO2þ ðaÞ

þ ð2 

 mÞClðaÞ

þ xH2 A2ðoÞ K AB

þ yðRNH3 ClÞ3ðoÞ
VO  Cl2m  A2x  ðRNH3 ClÞ3yðoÞ þ mHþðaÞ ð6Þ where KAB denotes the equilibrium constant of the synergistic extraction. Concentrations of the two extractants in the organic phase were obtained as follows.

b2



ð11Þ VOA2  2HAðoÞ þ VO  Cl2  ðRNH3 ClÞ3ðoÞ b3



þ ðRNH3 ClÞ3ðoÞ
2VO  A2  ðRNH3 ClÞ3ðoÞ þ 2HþðaÞ þ 2ClðaÞ

ð12Þ

The formation constants, could be obtained as, log b1 = log KAB log KA = 1.14, log b2 = log KABlog KB = 1.35, and log b3 = 2log KAB log KAlog KB = 0.21. Obviously, Eq. (10) contributes most to the synergistic extraction equations among the formation reactions, which means that the complex obtained from the single D2EHPA system is more prone to react with the other extractant, N1923, and to form the final synergistic extraction compound, VOA2(RNH3Cl)3.

Table 3 Thermodynamic parameters of VO2+ extraction processes. Extractrant

T (K)

1000/T (K1)

log D

DH (kJ mol1)

DG (kJ mol1)

DS (J mol1 K1)

D2EHPA

293.15 303.15 313.15 323.15 333.15

3.41 3.30 3.19 3.09 3.00

0.60 0.70 0.83 0.93 1.03

20.63

3.03

60.05

N1923

293.15 303.15 313.15 323.15 333.15

3.41 3.30 3.19 3.09 3.00

0.68 0.85 1.02 1.17 1.29

28.75

10.94

135.41

D2EHPA + N1923

293.15 303.15 313.15 323.15 333.15

3.41 3.30 3.19 3.09 3.00

0.96 1.10 1.24 1.36 1.48

24.28

3.37

94.30

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0.5

60

N1923 D2EHPA N1923+D2EHPA

Stripping %

0.4

D

0.3

0.2

V Ni Cr

40

20

0.1

0.0 0.0

0.2

0.4

0.6

0.8

0 0.0

1.0

0.2

X N1923 Fig. 7. Extraction of Ni2+ with mixtures of D2EHPA and N1923. [Ni2+] = 2.0
103 mol L1, pH = 2.0, l = 1.0 mol L1, CN1923 + CD2EHPA = 0.05 mol L1.

1.0

D

0.6

0.4

0.2

0.0

0.6

CHCl , mol L -1

0.8

1.0

Fig. 9. Stripping behavior of VO2+, N2+, and Cr3+ from the loaded D2EHPA + N1923 phase. CD2EHPA = 0.03 mol L1, CN1923 = 0.02 mol L1.

Result showed that the plots of log D vs. 1000/T for the extraction of V(IV) gave straight lines, by which DH could be obtained. It can be observed that DH values are positive (Table 3), indicating that the extraction procedures of V(IV) with D2EHPA, N1923, and D2EHPA + N1923 are all endothermic in nature. The obtained DG and DS were also presented in Table 3. The negative value of DG in the mixing system confirm the feasibility of the extraction process and spontaneous nature of the extraction reaction, while the positive value of DS implies that the extraction process is more random in nature.

N1923 D2EHPA N1923+D2EHPA

0.8

0.4

3.5. Separation studies of VO2+, Ni2+, and Cr3+ with mixtures of D2EHPA and N1923 0.0

0.2

0.4

0.6

0.8

1.0

X N1923 Fig. 8. Extraction of Cr3+ with mixtures of D2EHPA and N1923. [Cr3+] = 2.0
103 mol L1, pH = 2.0, l = 1.0 mol L1, CN1923 + CD2EHPA = 0.05 mol L1.

3.4. Thermodynamic study Fig. 6 illustrated the relationship between experimental temperature (T) and extraction efficiency (D). The relevant thermodynamics parameters, enthalpy change (DH), Gibbs free energy change (DG), and entropy change (DS), can be calculated according to the following equations,

D log D DH ¼ 2:303R D 1T

ð13Þ

DG ¼ RT ln K

ð14Þ

DG ¼ DH  T DS ) DS ¼

DH  DG T

ð15Þ

where R is the universal gas constant (8.314 J mol1 K1).

As is well known, vanadium often coexists with other metal ions since it is present in over 50 different minerals. For example, vanadium and nickel are used as alloying elements in steel industries, therefore, the extraction of mineral resources of these two metals has attracted much attention especially in industrialized countries [3]. The separation of vanadium from chromium is regarded to be an uneasy task due to their similar physicochemical properties [6]. In the present study, the extraction behaviors of Ni2+ and Cr3+ with D2EHPA + N1923 were investigated where the concentrations of Ni2+ and Cr3+ in the aqueous phase were determined by cupric sulfate back titration and ferrous ammonium sulfate titration, respectively (Figs. 7 and 8). It could be observed that D2EHPA + N1923 systems also showed synergistic effects for Ni2+ and Cr3+ at some ratio of extractants. The separation factors between VO2+ and Ni2+ or Cr3+ with D2EHPA + N1923 were calculated, i.e., bV/Ni = DV/DNi and bV/Cr = DV/DCr, respectively. Results were shown in Table 4. It is obvious that bV/Ni values obtained from D2EHPA + N1923 system are higher than those obtained from single D2EHPA extractant. As reported in various studies, D2EHPA has been extensively employed to the extraction of vanadium [1,3,16,17,20,21]. The addition of N1923 into D2EHPA extractant benefitted the separation of VO2+ and Ni2+.

Table 4 Separation factors of VO2+, Ni2+, and Cr3+ with mixtures of D2EHPA and N1923. Extractant

D2EHPA

Concentration (mol L1) bV/Ni bV/Cr

0.01 4.3 1.1

D2EHPA + N1923 0.02 8.1 1.9

0.03 13.6 3.3

0.04 19.3 4.7

0.05 24.1 5.5

0.04 + 0.01 37.2 7.4

0.03 + 0.02 52.0 12.7

0.02 + 0.03 24.8 11.6

0.01 + 0.04 41.1 9.8

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D. Jiang et al. / Separation and Purification Technology 156 (2015) 835–840

In order to study the selective separation method for the recovery of VO2+ from the loaded organic phase, the stripping behavior of VO2+, Ni2+, and Cr3+ was investigated. HCl solutions were employed as the stripping agent with concentrations ranged from 0.01 to 1.0 mol L1. The organic phase composed of 0.02 mol L1 N1923 and 0.03 mol L1 D2EHPA loaded with 0.002 mol L1 VO2+, Ni2+, and Cr3+ were back-extracted with HCl. Results were shown in Fig. 9, indicating that the stripping ratio firstly increased with increasing HCl concentrations and then almost kept constant when HCl concentration was high enough. The obvious differences in the stripping ratio of VO2+, Ni2+, and Cr3+ illustrated that it is of potential to selectively separate VO2+ from the loaded organic phase containing Ni2+ and Cr3+. 4. Conclusions The current work focused on the synergistic extraction of vanadium from hydrochloric solutions with three organophosphorus acids, D2EHPA, HEHEHP, Cyanex272, and their mixtures with primary amine N1923. All the three mixing systems have synergistic effects on vanadium extraction. Stoichiometry studies about the extraction of vanadium with D2EHPA + N1923 illustrated that VOA2(RNH3Cl)3 is the final extracted compound together with 2 hydrogen ions released. Thermodynamic experiments confirm that the synergistic extraction process is endothermic in nature. The synergistic extraction system showed higher separation abilities of vanadium, nickel, and chromium compared with single D2EHPA extractant. Further studies are being continued to give a deep insight for the applications of the synergistic effects to the extraction and separation of vanadium in industry. Acknowledgement The project was supported by Jilin Provincial Science & Technology Department (No. 20140101112JC). References [1] W. Li, Y. Zhang, T. Liu, J. Huang, Y. Wang, Comparison of ion exchange and solvent extraction in recovering vanadium from sulfuric acid leach solutions of stone coal, Hydrometallurgy 131–132 (2013) 1–7. [2] Z.G. Deng, C. Wei, G. Fan, M.T. Li, C.X. Li, X.B. Li, Extracting vanadium from stone-coal by oxygen pressure acid leaching and solvent extraction, Trans. Nonferr. Met. Soc. China 20 (2010) S118–S122.

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