Liquid-Liquid Extraction of V(IV) from Sulphate Medium by Cyanex 301 Dissolved in Kerosene

The equilibrium of extraction of V(IV) in the V(IV)- SO₄^2- (H⁺, Na⁺)-Cyanex 301 (HA)-kerosene system has been studied. Significant extraction occurs above pH 1 within 10 min. ^CD (extraction ratio at constant pH_(eq) and [HA]_(o,eq)) value is slightly decreased with increasing [V(IV)]_(ini). ^CD is found to be directly proportional to [H⁺]^-n (n ≤ 2), [HA] ² and (1+1.58 [SO₄^2-]). The process is endothermic (DH = 16 kJ/mol). Apparent K_exvalues at 303 K are 10^_-1.419 and 10^_-0.94 in 0.10 and 1.50 mol/L SO₄^2- medium, respectively. The loading capacity is calculated to be7.87 gV(IV) per100 g Cyanex 301. Kerosene appears as the best diluent. Stripping to the extents of 100%, 94% and 97.7% are possible in single stage by 1 mol/L H₂SO₄, HCl and HNO₃, respectively. Separations of V(IV) from Cu(II) (at pH 0), Zn(II) (at pH 0.5) and Fe(III) (at pH 1.0) by Cyanex 301 are possible.

• Extraction Equilibrium; Vanadium(IV); Cyanex 301; Kerosene; Sulphate

1. Introduction

Vanadium is widely used to prepare ferro-vanadium and the oxidative catalyst, V₂O₅. Almost 90% vanadium is used to prepare ferro-vanadium (0.1% - 0.3% V) for automobile parts and high speed tools. Besides its use as oxidative catalyst, vanadium compounds are used in ink, dye, paint and varnish, insecticide, photographic chemicals, medicine and glass industries [1]. From carnotite (K₂U₂V₂O₁₁∙3H₂O), it is extracted with uranium by the DAPEX process. It is also extracted from vanadinite (3Pb₃(VO₄)₂∙PbCl₂). But in the present world, these ores together with patronite (V₂S₃) are rare on the earth’s crust. So to meet up the demand, the processing of low grade ores and waste materials (tar sand, waste desulphurization catalyst, waste SO₂-oxidation catalyst of contact process, slag, ash, rock etc.) are desirable. Prior roasting and leaching [2,3] followed by solvent extraction is convenient for such purpose. It can build up concentration by using low organic to aqueous phase ratio (O/A) in extraction and high O/A in stripping.

Works published before 1976 on solvent extraction of V(IV) by various extractants have been summarized by Sekine and Hasegawa [4]. D2EHPA is a promising extractant for V(IV) and V(V) [5-11]. Saji and Reddy [12] have reported the extractions of V(IV) and V(V) by EHEHPA. In the field of solvent extraction, a delayed development is the use of organo-phosphinic compounds (Cyanex reagents) introduced by American Cyanamid and Cytec Canada Inc. Cyanex 302 and Cyanex 301 are mono-and di-sulphide analogues of Cyanex 272 (bis- 2,4,4-trimethylpentylphosphinic acid). The sulphur substitution decreases the pK_a values (6.4, 5.6 and 2.6 for Cyanex 272 [13], Cyanex 302 [14] and Cyanex 301 [15], respectively) permitting to work at lower pH [16]. Cyanex reagents differ from other commercial organophosphorous reagents (e.g. D2EHPA, DDPA, TBP, EHEHPA etc.) in that the former reagents contain P-C bonding, whereas the latter reagents contain P-O-C bonding. The presence of P-C bonding in Cyanex reagents renders them to be less susceptible to hydrolysis and less soluble in water [17].

In recent past, the extraction behavior of V(IV) from medium by Cyanex 272 [18,19] and by Cyanex 302 [20] and of V(IV) and V(V) in presence of Mo(VI), W(VI), U(VI), Ti(IV), Al(III), Cr(III), Fe(III) from Cl^- medium by Cyanex 272 and HA [17] had been reported. There is no report on the equilibrium of the extraction of V(IV) from medium by HA. The present paper reports the extraction characteristics of V(IV) from medium by HA dissolved in kerosene in order to determine the effects of various parameters on extraction ratio and the value of K_ex; and also to propose mechanism. The loading of HA by V(IV) and the stripping ability of various mineral acids are also investigated. Finally, the possibilities of separation of V(IV) from some 3d-block cations in binary mixtures have been examined.

2. Materials and Methods

2.1. Materials

Cyanex 301 is a product of Cytec Canada Inc. and received as a gift. It contains 75% - 83% R₂PS₂H, 5% - 8% R₃PS, 3% - 6% R₂PSOH and ~2% unknown compound [21] and used without further purification since R₃PS and R₂PSOH have the extracting powers too. Kerosene purchased from the local market is distilled (200˚C - 260˚C) to collect the colorless aliphatic fraction. As-received NH₄VO₃ (99%, Riedel-deHaen) and VOSO₄∙xH₂O (99.9%, Alfa Aesar-Johnson-Mathey), H₂O₂ (30% (w/v), MerckGermany) are used. Analytically pure diluents (besides kerosene) are the products of Riedel-deHaen and E. Merck-India.

2.2. Analytical

The aqueous [V(IV)] has been measured by the HNO₃ oxidative-H₂O₂ method [22] at 450 nm using a UV-visible Spectrophotometer (UV-1650 PC, Shimadzu, Japan). For standard and test solution preparations, NH₄VO₃ and VOSO₄∙xH₂O, respectively, are used. A Mettler Toledo pH meter (model MP 220) is used for pH measurement.

2.3. Procedure for Extraction

The procedure for extraction is given elsewhere [20,23]. Two phases are agitated at O/A = 1 (O = 20 mL) and 303 K (otherwise stated) for a predetermined time (15 min). The phase separation is quick; and the aqueous phase after disengagement is analyzed for its pH_(eq) and V(IV)- content. Then “D” is calculated as usual [20,23].

2.4. Procedure for Loading

The loading procedure is given elsewhere [24]. An aliquot of 100 mL 0.20 mol/L HA-kerosene solution has been used for V(IV)-loading from an aqueous solution containing 1.0 g/L V(IV) and 0.10 mol/L at pH_(ini) = 2.60. After each stage of contact, cumulative [V(IV)]_(o,eq) is calculated to monitor the progress of stripping.

2.5. Procedure for Stripping

The stripping procedure is similar to the extraction procedure. The fully loaded organic solution obtained in the loading study is diluted to contain 200 mg/L V(IV) and 0.10 mol/L HA in kerosene. Vanadium(IV) in this solution has been stripped by (0.1, 0.3 or 1.0) mol/L (H₂SO₄, HCl or HNO₃) solution. A shaking time of 1 h is allowed arbitrarily. After equilibration and phase separation, the aqueous phase is analyzed for its [V(IV)] in order to calculate % V(IV) stripped by:

2.6. Treatment of Extraction Equilibrium Data

R₂PS₂H is the principal constituent of Cyanex 301. This species is monomeric in non-polar diluents (owing to low electronegativity of sulphur) [21,25]. Consequently, Cyanex 301 has been abbreviated as HA. In aqueous solution, VO²⁺ can form complexes with co-existing OH^- and or. On considering the existence of [VO(OH)_jL_k] (L is or and the charge of the complex is neglected for simplicity) in the aqueous phase and the monomeric charge-less extracted complex does not contain any OH^- and or, the equilibrium for its extraction by HA can be represented as (“x”, “2 − j” and “k” are experimental extractant, pH and co-existing ligand dependences, respectively):

On defining “D” as

K_ex of Equation (1) can be expressed as:

The Equation (2) represents the basic equation for solvated chelate formation by reaction of a metal ion with an acidic extractant. All concentration terms and pH in Equation (2) refer to the equilibrium values. Although it is difficult to collect D-values experimentally at a constant set of pH_(eq), [HA]_(o,eq) and []_(eq), it is possible to modify the experimental D-values to ^CD values at a chosen set of constant pH_(eq), [HA]_(o,eq) and []_(eq). Since in almost all experiments, 0.10 mol/L (~25 times greater than [V(IV)]) has been used; it can be assumed that []_(eq) will not be significantly changed from []_(ini). Consequently, after determining the approximate pH and extractant dependences and rounding up these values, log ^CD can be calculated by:

Moreover, [H₂A₂]_(o,eq) is equal to

Consequently, according to Equation (2), log ^CD should be independent of [V(IV)] if the solutions behave ideally; while it should depend on pH_(eq), [HA]_(eq) or [L] at a constant set of other parameters. Moreover, as K_ex is related to temperature by Van’t Hoff equation, log ^CD will also depend on temperature.

3. Results and Discussion

3.1. Extraction Equilibrium

Some preliminary experiments indicate that V(IV) is extractable by HA at pH ~ 0.50. When 0.20 g/L V(IV) containing 0.10 mol/L at pH_(ini) of 2.30 is extracted by 0.10 mol/L HA in kerosene at 303 K and O/A = 1, then it is found that [V(IV)]_(o) is increased up to phase contact of 10 min. It is therefore concluded that the equilibration time is 10 min (15 min is allowed in subsequent experiments).

Variation of “D” with [V(IV)]_(ini) is found out at four different set of experimental parameters. It is found in all cases that [V(IV)]_(o) is increased, but ‘D’ is decreased continuously with increasing [V(IV)]_(ini) in the aqueous phase. This is contrary to Equation (2) which is valid at constant [HA]_(o,eq) and pH_(eq). The observed decreasing behavior might be due to the non-constancy of [HA]_(o,eq) and pH_(eq) for various extents of V(IV) extraction. On calculating log ^CD (by Equation (3) on considering “2 − j” = 1.50 at constant pH_(eq) of 1.40 or 1.67 at constant pH_(eq) of 1.80 and x = 2.00 for all systems), log ^CD vs. log ([V(IV)]_(ini), mol/L) plots are drawn in Figure 1. Plots indicate the independency of ^CD on [V(IV)] at least up to 300 mg/L. Systems with higher [V(IV] behave nonideally.

At a constant [HA]_{(o, eq)}, the plot of log D vs pH_(eq) should be a straight line with slope equaling to “2 − j” (cf. Equation (2)). For low [] of 0.10 mol/L, Figure 2 represents log ^CD vs pH_(eq) plots at constant [HA]_(o,eq) of 0.20 and 0.30 mol/L. In both cases, straight lines are not obtained. Three or four points at lpHr produce straight lines of slope 2. At hpHr, the slope is decreased gradually and at pH 2.15, it becomes 1.6. It is concluded that single type of extractable species is formed from two different types of aqueous V(IV) species, viz. VO²⁺ and VO(OH)⁺. In the first case, two H⁺ are liberated; whereas, in the second case, one H⁺ is liberated, per V(IV) being extracted into the organic phase. As an evidence to this statement, the formation of same extractable species from VO²⁺, VO(OH)⁺ and VO(OH)₂ by D2EHPA may be cited [5]. The effect of pH on extraction at hcr of [] (1.50 mol/L) has also been investigated (cf. Figure 2). A

straight line having slope of 1.72 is obtained. It is therefore concluded that the pH dependency in the present system depends on []. As [] is increased, pH dependency is decreased.

According to Equation (2), the plot of log ^CD vs. log [HA]_{(o, eq)} at a constant pH_(eq) should be a straight line with slope giving moles of HA(x) associated with 1 mol of V(IV) in extracted species. The log D vs log [HA]_{(o, ini)} plots (as there will be a very little variation between pH_(ini) and pH_(eq) and between [HA]_{(o, ini)} and [HA]_{(o, eq)}) are shown in Figure 3. Straight lines having slope of 2 are indeed obtained at both lcr and hcr of. It is

therefore concluded that the HA-dependency is always 2 irrespective of [], though the pH-dependency is dependent of [].

The log ^CD vs log ([], mol/L) plot is displayed in Figure 4" target="_self"> Figure 4. Experimental points fall on a curve. In lcr of, ^CD is seldom changed; whilst in the hcr, it is considerably increased with increasing []. The curve in the figure is theoretical and represented by:

which is obtained to fit experimental points as described in the caption of Figure 4. It is seen that [] has little effect on extraction when its concentration is kept ~0.10 mol/L; but at hcr of [], log ^CD is almost directly proportional to log [].

The Van’t Hoff plots are shown in Figure 5. It is found that ^CD is increased with increasing temperature and the straight line relationship holds. Slopes of the lines are −870 (DH = 16.70 kJ/mol) and −830 (DH = 15.95 kJ/mol) for pH = 1.35 and 1.25 systems, respecttively. Therefore, the process is endothermic with low DH value of ~16 kJ/mol.

It is evident from these studies that the value of “x” is 2 irrespective of the experimental parameter but the value of “k” is 0 at low [] and −1 at high []. The value of “2 − j” is 2 in low pH_(eq) and <2 in high pH_(eq). At lcr of and at lpH_(eq)r, “2 − j” = 2 implies that “j” = 0; but at hpH_(eq)r, “2 − j” < 2 (but >1) implies that 1 < j < 2. On the other hand, at hcr of SO₄^2- and at both lpH_(eq)r and hpH_(eq)r, “2 − j” < 2 implies j > 0.

3.2. Evaluation of Extraction Equilibrium Constant

The foregoing results give the equation for log^CD at 303

K in lcr of sulphate as:

Based on Equation (5), value of log K_ex has been evaluated from intercepts of the straight lines or asymptotic lines in Figures (2)-(4) and tabulated (Table 1). The average log K_ex is −1.419 at lcr of with stand. dev. of 0.105. Besides, log ^CD at 303 K in hcr of can be put as:

On using Equation (6), log K_ex is evaluated as −0.94 with stand. dev. of 0.026.

3.3. Extraction Mechanism

The empirical equation for K_ex at 303 K is:

The 1st and 2nd ionization constants of H₂SO₄ are 10³ [26] and 10⁻² [27], respectively. These values suggest that will be more available than in the working pH region. So, L in Equation (1) represents. As the values of “x”, “k”, “l” and “(2 − j)” are known at different experimental conditions, Equation (1) will provide extraction mechanisms. Although in Equation (1), “L” is presented as a product (liberated during complex formation); experimental results indicate that it is associated with V(IV) during complex formation. As “x” is always 2, non-solvated chelate (VOA₂) is formed at lcr of, whereas, solvated complex (VOSO₄·2HA) is formed at hcr of. Typical equilibria are suggested as:

1) in lcr of and lpHr:

2) in lcr of and hpHr (limiting):

3) in hcr of (limiting):

An alternative option of the formation of [VO∙SO₄∙2HA]_(o) may be the formation of [VO(HSO₄)(A)∙HA]_(o) with the simultaneous liberation of a proton. These are only presumptions from the experimental results and not proven by other means.

3.4. Effect of Diluent

In order to determine the effect of diluent on V(IV)-distribution, D-values have been measured when the same aqueous phase has been extracted separately by 0.10 mol/L HA in different diluents keeping all other parametric conditions ([V(IV)] = 200 mg/L, pH_(ini) = 2.00 and [] = 0.01 mol/L) identical. It is observed that the extraction ratio increases in the following order with the variation of diluent: CHCl₃ (e = 4.807; D = 0.42) < 1,2-C₂H₄Cl₂ (e = 10.42; D = 0.54) < C₆H₄-(CH₃)₂ (e = 2.26; D = 0.68) < cyclo-C₆H₁₂ (e = 2.02; D = 1.31) = C₆H₅Cl (e = 5.69; D = 1.31) < C₆H₅-CH₃ (e = 2.385; D = 1.64) < n-C₇H₁₆ (e = 1.921; D = 2.08) = C₆H₆ (e = 2.274; D = 2.08) < CCl₄ (e = 2.228; D = 2.34) < petroleum benzin (D = 3.62) < kerosene (e = 2.00; D = 3.93). The study helps draw the conclusion that kerosene is a very good diluent followed by petroleum benzin and CCl₄ for the extraction of V(IV) by Cyanex 301. CHCl₃, 1,2-C₂H₄Cl₂ and C₆H₄ (CH₃)₂ are not recommended. 79.72% V(IV) extraction in kerosene phase is decreased to only 29.70% V(IV) extraction in CHCl₃ phase.

3.5. Loading of Cyanex 301 with V(IV)

The cumulative [V(IV)]_(o) (g/L) has been plotted against the number of phase contact in Figure 6. It is observed that the loading of V(IV) in the organic phase is ended up at the 13^th contact. An aliquot of 1 L 0.20 mol/L HA is saturated with 5.07 g V(IV) and so the loading capacity is calculated as 7.87 g V(IV) per 100 g HA. The loading capacity is considerably high, and so it can be recommended for a large scale separation of V(IV) from an aqueous solution. The extraction of 5.07 g V(IV)/L by 1 L 0.20 molar HA at saturated loading implies the HA/V(IV) mole ratio of 2.01 which is identical to that obtained from the extractant dependence study. The loading results indicate that the mechanism of extraction at high loading is not changed from that suggested at low loading.

3.6. Stripping of V(IV)-Loaded Organic Phase by Mineral Acids

The maximum V(IV) loaded organic phase containing 5.07 g/L V(IV) with theoretically no free extractant, after proper dilution and adjustment of free [HA], has been subjected for stripping by 0.1, 0.3 and 1.0 mol/L H₂SO₄, HNO₃ and HCl solutions 303 K and O/A = 1. The stripping results are given in Table 2. It is found that stripping percentage is more or less acceptable in all three acids used alone. In all cases, stripping percentage is increased with increasing concentration of acid. It is seen that 71.50% stripping by 0.10 mol/L H₂SO₄ is increased to 100% stripping by 1 mol/L H₂SO₄. Similarly, 45% stripping by 0.10 mol/L HCl is increased to 94% stripping with 1 mol/L HCl; whereas, 78% stripping by 0.10 mol/L HNO₃ is increased to ~98% stripping by 1 mol/L HNO₃. Sulphuric acid (1 mol/L) is sufficient to strip off V(IV) quantitatively. Nitric acid and hydrochloric acid

Table 1. Evaluation of the values of K_ex at 303 K.

can also be used in stripping if two stage stripping is practiced.

It is reported that Cyanex 302 and Cyanex 301 undergo oxidation in oxidizing environment [28-31] (oxidation products being R₂(P=S)-S-S-(S=P)R₂, R₂(P=S)OH and R₂(P=O)OH). It can be demonstrated, however, that when V(IV) is extracted repeatedly from fresh aqueous solutions ([V(IV)] = 0.20 g/L, [] = 0.10 mol/L, pH_(ini) = 1.50) by 0.30 mol/L HA in kerosene (fresh in the first step and regenerated afterwards) and stripped subsequently with 1 mol/L H₂SO₄, then (79 ± 2)% extraction and 100% stripping are observed from the 1st - 25th extraction-stripping steps. It is therefore concluded that HA does not undergo any sort of oxidation as also reported by Sole et al. [32].

Table 2. Stripping of V(IV) loaded organic phase using different acid solutions. [V(IV)]_(o) = 200 mg/L, [Cyanex 301] = 0.10 mol/L, Equilibration time = 1 h, Temp. = (303 ± 0.5) K, O/A = 1 (O = 20 mL).

3.7. Separation Ability of V(IV) from Some Other Metal Ions

In order to examine the effectiveness of HA towards the mutual separations of some 3d-block metal ions viz. Ti(IV), V(IV), Fe(III), Co(II), Ni(II), Cu(II) and Zn(II), the extraction percentages of these metal ions have been estimated. For this purpose, 0.20 g/L metal ion is extracted from 0.10 mol/L (or, [] = H₂SO₄ when [H₂SO₄] > 0.10 mol/L) medium at different pH_(eq) values by 0.10 mol/L HA in kerosene at 303 K and O/A = 1 (O = 20 mL) on equilibration for 1 h. The extraction results given in Table 3 predict the following:

1) V(IV) can be separated from Cu(II) at pH 0 in single step (0% V(IV) extraction and 100% Cu(II) extraction).

2) On using counter-current extraction stages, V(IV)

Table 3. Solvent extraction data of some 3d-block elements by Cyanex 301 dissolved in kerosene. [Cyanex 301] = 0.10 mol/L (in kerosene); [Metal ion] = 0.20 g/L; [] = [H₂SO₄] or 0.10 mol/L, Temp = 303 K, O/A = 1 (O = 20 mL), Equilibration time = 1 h.

NE: non-extractable, CE: complete extraction; ^*Aqueous solution becomes cloudy before extraction but becomes clear after extraction.

can be separated from:

• Zn(II) at pH ~ 0.5 (1% V(IV) extraction and 98% Zn(II) extraction)• Fe(III) at pH 1.0 (4.3% V(IV)-extraction and 99% Fe(III)-extraction)• Co(II) at pH 1.5 (38.7% V(IV)-extraction and 91% Co(II) extraction), and

• Ni(II) at pH 1.5 (38.7% V(IV)-extraction and 95% Ni(II) extraction).

3) Separation of V(IV) from Ti(IV) is difficult but not impossible. Separation can be achieved at pH 2.0 on using counter-current multistage extraction.

4. Conclusions

The following conclusions are drawn:

1) Vanadium(IV) can be extracted by HA at pH above 1. The equilibration time 10 min. Up to at least 0.30 g/L V(IV), the extraction ratio (D) is independent of V(IV) concentration in the aqueous phase.

2) The corrected extraction ratio (^CD) is proportional to [H⁺]⁻² at its lcr, [HA]² and the factor (1 + 1.58 []). The K_ex values at 303 K are 10^-1.419 and 10^{−0.94 in 0.10 and 1.50 mol/L medium, respectively.}

3) The extraction process is endothermic with a DH value of 16 kJ/mol.

4) The extracted species are VOA₂ at lcr and VO·HSO₄·A·HA or VOSO₄ at hcr of.

5) 100 g HA can extract as much as 7.87 g V(IV). This gives HA/V(IV) mole ratio of 2.01 indicating that the mechanism of extraction is not changed with loading.

6) Among the diluents used, kerosene is the best. The least effective diluent is CHCl₃ followed by 1,2-C₂H₄Cl₂ and xylene.

7) 1 mol/L H₂SO₄, HCl and HNO₃ can strip off 100%, 94% and 97.7% V(IV), respectively, in single step.

8) Using HA as extractant, V(IV) can be separated from Cu(II). It can be separated from Zn(II) at pH 0.5 but for clear-cut separation counter-current 2-3 stage extractions may be required. In a similar way, V(IV) can be separated from Fe(III) at pH 1.

Separations V(IV) from Ti(IV), Co(II) and Ni(II) by HA appear to be difficult.

Notations and Abbreviations

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