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Solvent extraction of vanadium from chloride solutions using di-(2-ethylhexyl)-phosphoric acid
J. inorg, nucl. Chem., 1967, Vol.29, pp. 2019 to 2025. PergamonPress Ltd. Printed in Northern Ireland
SOLVENT EXTRACTION OF VANADIUM FROM CHLORIDE SOLUTIONS USING DI-(2-ETHYLHEXYL)PHOSPHORIC ACID T. RIGG a n d J. O. GARNER Peace River Mining and Smelting Ltd., Edmonton, Alberta, Canada
(Received 17 January 1967) Abstract--Extraction of vanadium (IV) from chloride solutions by di-(2-ethylhexyl)phosphoric acid (HDPA) in kerosene has been found to take place principally through the formation of neutral mononuclear vanadyl complexes with mono-ionized dimers of HDPA. N o evidence has been obtained of chloro- or hydroxy-group participation in the extracted complex. At very high solvent loadings it appears that the extracted complex involves H D P A monomer rather than the dimer. Most effective extraction is obtained when the diluent has little or no propensity for forming hydrogen bonds with the HDPA.
THE SOLVENTextraction of vanadium from chloride solutions is of considerable interest in the application of the Research Council of Alberta chloride leach process ~1) to sedimentary iron ores such as those occurring in the Peace River district of Alberta, Canada.(9..s) The Peace River ore contains an average of about 0.2 ~ V~O5 which is fairly evenly distributed throughout the deposit. In the RCA process the ore is leached with hydrochloric acid, whereupon virtually all of the vanadium is leached out and eventually appears in the mother liquor remaining on separation of the iron as hydrated ferrous chloride. This mother liquor is normally subjected to hydrolysis for the purpose of recovering hydrochloric acid. However, it is obvious that if a suitable extractant could be found, it would be relatively easy to recover the vanadium from the feed to the hydrolyser. Di-(2-ethylhexyl)phospboric acid (HDPA) has been successfully used in the wellknown Dapex process3 4) It was therefore decided to investigate the behaviour of this reagent in chloride solutions. Preliminary experiments showed that fully-reduced vanadium was not extracted to any significant extent and fully-oxidized solutions gave poor and erratic results. It was found that extraction could be carried out efficiently only when the vanadium was present in the tetravalent state, presumably as vanadyl ion, VO ~+ or V(OH)~ 2+. EXPERIMENTAL Vanadium(IV) solutions were prepared by mixing ammonium metavanadate (NH4VO,) with its own weight of chromatography-grade cellulose powder and heating in a crucible to ,~600°C for 20 min. The residue was leached with 1.0 N hydrochloric acid and filtered. The solution was evaporated to a small volume and the acid content adjusted prior to dilution to the required vanadium tx~ C. P. GRAVENOR,G. J. GOVETTand T. RICG, Can. L Min. Bull. 57, 421 (1964). c2) D. J. KIDD, Research Council of Alberta Geol. Div. Preliminary Report 59-3 (1959). c3~ G. A. GRoss, Geology of Iron Deposits in Canada Vol. 1. Geol. Surv. Canada Economic Geology Report No. 22 (1965). c4~K. B. BROWN and C. F. COLEMAN,Progress in Nuclear Energy, Series Ill--Process Chemistry, Vol. 2, p. 3. Pergamon Press, London (1958). 2019
2020
T.R.1OG and J. O. GARNER
concentration. On some occasions the leach solution had a green tinge instead of the pure blue colour of the vanadyl group. This was attributed to incomplete reduction of vanadium(V) giving rise to a colloidal dispersion of vanadium pentoxide. Such dispersions are inextractable and give rise to yellow colorations in what would otherwise be colorless raffinates. This problem could be overcome by adding 1-2 g of hydroxylamine hydrochloride to the leach solution before evaporating down. The state of oxidation of the final solutions was checked using a platinum--calomel electrode system. Only those solutions registering an e.m.f, between 325 and 400 mV were used. pH was controlled by adding hydrochloric acid or ammonia. The vanadium content of the various aqueous phases was determined by the colorimetric peroxide method. Solvent phases were analysed by first stripping the vanadium with 6.0 N HC1. The concentration of the solutions to be analysed was adjusted by evaporation and/or dilution so that suitable aliquots containing up to 5"0 mg of vanadium could be conveniently taken. Standard vanadium solutions were prepared by completely digesting accurately-weighed amounts of pure ammonium metavanadate in 6.0 N HCI. The specific colour development procedure used was as follows: an aliquot of the solution was pipetted into a 50-ml volumetric flask, 5 ml of dilute sulphuric acid (1 + 1) was added, followed by 3 ml of 85Yo phosphoric acid and finally 1 ml of 30 ~o hydrogen peroxide. The contents of the flask were diluted to the mark and thoroughly mixed. The optical density of the colorimetric solutions was determined at a wavelength of 450 m/* with a Coleman "Universal Model 14" spectrophotometer equipped with 16-mm cylindrical cells. Extractions were carried out in 250-ml separating funnels, generally with equal volumes (50 ml) of aqueous and solvent phases. The phases were mixed by shaking at 100 c/min for 5 min, and were then allowed to stand until perfectly clear. No trouble was experienced with emulsions. All extractions and colorimetric measurements were made at the ambient temperature of 23 :f: I°C. HDPA was supplied by K and K Labs. Inc., Plain View, New York, and was diluted as required with Esso JP-1 kerosene. RESULTS P r e l i m i n a r y experiments showed t h a t e x t r a c t i o n o f v a n a d i u m ( I V ) increased with p H a n d H D P A c o n c e n t r a t i o n a n d r e q u i r e d only a s h o r t time to a p p r o a c h e q u i l i b r i u m distribution. H y d r o c h l o r i c acid ( > 1.0 N ) was f o u n d to be very effective in stripping the v a n a d i u m f r o m the solvent. T h e d e p e n d e n c e o f e x t r a c t i o n o n m i x i n g time is shown in T a b l e 1. T a b l e 2 shows t h a t the d i s t r i b u t i o n r a t i o is virtually i n d e p e n d e n t o f TABLE 1.--DEPENDENCE OF VANADIUM EXTRACTION ON MIXING TIME. VANADIUM SOLUTION INITIALLY 2"04
g/l, pH
1"9. SOLVErCr/AQOEOUS RATIO 1/1. SOLVENT10 ~ HDPA IN KEROSENE Mixing time (min)
V in extract (g/l)
0.5
1-79
1
1.81
2 5 15
1.81 1.81 1.83
v a n a d i u m c o n c e n t r a t i o n over a wide range. F o r these experiments the initial ionic strength o f the solutions was a d j u s t e d t o c o r r e s p o n d with t h a t o f the m o s t c o n c e n t r a t e d s o l u t i o n in the series b y a d d i n g s o d i u m chloride. H o w e v e r , ionic strength v a r i a t i o n s a p p e a r e d t o have n o significant effect o n the e x t r a c t i o n a n d all o t h e r results r e p o r t e d in this w o r k were o b t a i n e d w i t h o u t such adjustment. T h e effect o f v a r i a t i o n s in H D P A c o n c e n t r a t i o n a n d in acidity was d e t e r m i n e d by c a r r y i n g o u t extractions at a c o n s t a n t initial v a n a d i u m c o n c e n t r a t i o n o f 2-03 g/1 a n d
Extraction of vanadium from chloride solutions using di-(2-ethylhexyl)phosphoricacid 2021 TABLE 2.--DEPENDENCE OF DISTRIBUTION RATIO ( D e ) ON VANADIUM CONCENTRATION. SOLVENT/AQUEOUS RATIO
111; FINALpH 0.98 :k 0"05; SOLVENT 1070 HDPA IN KEROSENE Initial V concentration (g/l)
D~
0.20 0.40 0-71 1.02 2.03 4.06
4.67 4.34 4.57 4.03 4.10 4.28
final aqueous phase p H values in the range 0"00 to 2"35. H D P A concentrations of 3-0, 10.0 and 30"0 70 (volume) were used. The results are shown in Fig. 1, where it is also shown that substantial amounts of inorganic phosphate (5 g H3POJ1) in the aqueous phase have no effect on the extraction of vanadium. 3.0
2.0
1.0
d 0
-I.0
-2.0 -LO
I
0
1.0
2.0
log [HA][H'] -'
Fro. 1.--Vanadium(IV) distribution D, as a function of log [HA][H+]-t for different concentrations of HDPA in kerosene. Initial vanadium concentration in aqueous phase 2 g/1. /k = 370, O = 107o and × = 3070 HDPA. [] denotes results obtained with 10~o HDPA when the aqueous phase contained 5 g H3POdl. Very high solvent loading leads to decreased distribution ratios, as illustrated by the extraction isotherm for 1 70 H D P A shown in Fig. 2. The nature of the diluent was found to have a profound effect on the extraction. The results shown in Table 3 were obtained with 10 ~o (volume) H D P A in a variety ofdiluents. Equal volumes ofaqueous and organie phases were used. Phase separation was good in all cases except with 1-octanol.
2022
T. RIGGand J. O. GARNER 1.0 o
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0.01
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[vo~qoq Fro. Z--Extraction isotherm for vanadium(IV) in 1 per cent HDPA]kerosene. Final aqueous pH 1"09 4- 0-03. TABLE3.--DILUENTEFFECTONEXTRACTIONOFVANADIUMBYHDPA. INITIAL VANADIUM CONCENTRATION2"03g/l; lnNALpH 0"95 ± 0"05 Diluent
Do
Kerosene (JP-1) n-Hexane Carbon tetrachloride Benzene Methyl isobutyl ketone 1-Octanol
3"19 4.24 1"63 0'99 0.88 0.47
Some of the organic extracts were examined for the presence of chloride ions. The extracts were washed with an equal volume of 0.001N HNOs, then back-extracted into 3"0 N HNOs, and silver nitrate was added to precipitate the chloride. Only very small amounts of chloride were found, as indicated in Table 4. TABLE 4.-----CHLORIDE CONTENT OF V A N A D I U M - H ] ~ P A
EXTRACTS
Diluent
[C1]/[V] ratio
V present in sample (mg)
n-Hexane Carbon tetrachloride Benzene
0"011 0.018 0.022
65"7 50-3 40.4
The absorption spectra of vanadyl solutions in the range 340 to 820 m # showed no change over the p H interval used in the present work, suggesting the absence of significant hydrolysis. Solutions containing up to 2.0 M sodium chloride displayed no variations attributable to chloro-complex formation. All spectra showed only one
Extraction of vanadiumfrom chloridesolutionsusing di-(2-ethylhexyl)phosphoricacid 2023 peak (760 m#). Kerosene solutions of the vanadium complex gave the same type of spectrum, peaking at the same wavelength but of lower intensity. DISCUSSION Extraction at low levels o f solvent loading
The extraction of complexes formed between various metal ions and solvent-soluble ion-exchanging agents such as acetylacetone, 8-hydroxyquinoline and HDPA has been discussed previously in considerable detail by several authors, t5-7~ In particular, RYDBERGCs~has published a detailed theoretical discussion of the formation of composite mononuclear complexes involving agents of this type and gives criteria for the identification of the type of complex formed. The results in Table 2 show that the extraction of vanadium by HDPA is essentially independent of the vanadium concentration at low levels of solvent loading. It is therefore concluded that only mononuclear complexes are involved in extraction under these conditions. Although the distribution ratio for vanadium was strongly dependent on both the final aqueous phase hydrogen ion concentration ([H+]) and the HDPA concentration in the organic phase ([HA]org) the results in Fig. 1 indicate that it is constant for constant values of the product [HA]ordH+]-1. The maximum slope of the line in Fig. 1 is 2.0. On the basis of Rydberg's considerations the most likely explanation of these results is that vanadium is extracted as a neutral complex of the type VO(A)~. Over the rectilinear region in Fig. 1 the extraction can thus be represented by the expression: Dv o c \
[n +] / .
(1)
This is analogous to the findings of PEPPARD et al. (a) in HDPA extraction of the Group III elements from scandium to actinium, which apparently all form neutral complexes of the type MA a and have distribution ratios that can be expressed in terms of the general relationship: ([HA]org~ s
D oc k ~ / "
(2)
The square-law dependence for vanadium extraction by HDPA can be deduced in a fairly straightforward manner as shown, for example, in Ref. (7). The lack of any effects attributable to hydrolysis of the vanadyl ion is readily understandable in the light of the work of ROSSOTTIand RO$SOTTI,(10) who investigated the hydrolysis: VO 2+ + H~O ~ VOOH + + H+;
Kh =
[VOOH+][H +] [VOW+]
(3)
~5~R. E. CoNbrlCKand W. H. McVEY,& Am. chem. Soc. 71, 3182 (1949). ~e~j. RVDBERO, Acta chem. scand. 4, 1503 (1950). ~7~G. H. MomusoNand H. FI~mER,Solvent Extraction in Analytical Chemistry. John Wiley,New York (1957). ts~j. RYDBF.RO,Arkiv Kemi. 8, 101 (1955). ~0~D. F. I~PPhgD,G. W. M~oN, W. J. DrJSCOLLand R. J. SmONEN,J. inorg, nucl. Chem. 7, 276 (1958). {x0~F. J. C. ROSSOXTIand H. S. RossoxTI,Acts chem. stand. 9, 1177 (1955).
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T. Rloo and J. O. GARNER
and obtained the value Kh -----10-6"°. From this value it can be calculated that under the most strongly hydrolysing conditions employed in the present work (pH = 2"35, vanadium concentration ---- 2.0 g/l) less than 0"1 per cent of the vanadyl ion should be hydrolysed. It is therefore considered extremely unlikely that the extracted complex contains hydroxyl groups. The absorption spectra and analytical results likewise indicate that no chloro-complexes are formed. The situation is thus fortuitously much simpler than in other cases such as, for example, HDPA extraction of iron(III) from chloride solutions, where hydrolysis as well as the formation of ehloro-complexes must be taken into account.
State of aggregation of HDPA HDPA, in common with other dialkyl phosphates, has a strong tendency to form dimers, especially in non-polar solvents, (u'lz) in a manner similar to the carboxylic acids. It is believed that dimerization arises by formation of a hydrogen-bonded 8membered ring structure. Mono-ionization of the dimer thus offers the possibility of forming a relatively stable chelate structure with suitable metallic ions. c9) In the case of the vanadyl ion the complex would have the structure shown below: H RO
O
\/
O
%/
P
P
/~ RO
OR
/\ 0
0
OR
0
OR
vo RO
0 P
RO
P
O
O
OR
H
The dimerization constants of carboxylic acids are known to be very large in nonpolar solvents such as n-hexane and carbon tetrachloride. Dimerization becomes less marked as the ability to form hydrogen bonds increases and, for example, is almost negligible in water. The same type of behaviour seems to characterize the dialkyl phosphates (mxa) and is manifest by the strong dependence of the distribution ratios of various extracted ions on the nature of the solvent diluent, as shown in Table 3. It is particularly interesting to observe that DYRssmq and LmMc14)have found that the distribution ratios for the extraction of calcium by dibutyl phosphoric acid decrease in the same marked fashion in the sequence: n-hexane, kerosene, carbon tetrachloride, benzene, methyl isobutyl ketone. These authors also obtained very similar results for the extraction of uranyl ion by dioctyl phosphoric acid, in which the distribution ratios (xx) D. Dvl~s'~, Acta chem. scand. 11, 1771 (1957). cx~)D. F. I~PPARO, J. R. FER~mO and G. W. MASON, J. inorg, nucl. Chem. 4, 371 (1957); d. tnorg. nuel. Chem. 7, 231 (1958). (is) C. J. HARDY and D. SeArtOILL, U.K. Atomic Energy Authority Report A E R E C/R 2830 (1959); J. inorg, nucL Chem. 11, 128 (1959). (x4) D. D~SSEN and D. H. Lm~l, Acta chem. scand. 14, 1100 (1960).
Extraction of vanadium from chloride solutions using di-(2-ethylhexyl)phosphoricacid 2025 decreased in the sequence: kerosene, n-hexane, carbon tetrachloride, benzene, 2-octanol. It is therefore concluded that the distribution ratio obtained in the extraction of metal ions by dialkyl phosphoric acids is a function of extractant-diluent interaction rather than interaction between the diluent and the specific metal species. In the rather special case of uranyl ion extraction by kerosene solutions of dialkyl phosphoric acids containing some tributyl phosphate the latter can itself contribute to the extraction. However, in the case of vanadyl extraction it was observed that the addition of l0 per cent tributyl phosphate to the kerosene diluent impaired extraction, evidently by interfering with the dimerization process. Inspection of Fig. 1 shows generally good agreement between the experimental points and the line drawn with the theoretical slope of 2.0. On the other hand, the points representing the results of extractions carried out with 30 per cent HDPA are all on the low side of the line. It seems possible that this is due to the likelihood of further polymerization of HDPA to form trimers: 3(HA) 2 ~ 2(HA)3.
(4)
According to BAESand BAKERtm the equilibrium constant for this reaction in n-octane is 10 mole -1. Assuming that this value applies reasonably well to kerosene solutions of HDPA we calculate that about two-thirds of the HDPA will be present as trimer in 30 per cent HDPA-kerosene solutions; this could well account for the smaller vanadium distribution ratios.
Extraction under conditions of very high solvent loading If the mechanism discussed above, whereby vanadium is extracted by mono-ionized dimers of HDPA, were the only extraction mechanism operating in this system it would not be possible to achieve vanadium concentrations in the organic phase greater than 0.25 of the formal HDPA concentration. However, the extraction isotherm depicted in Fig. 2 shows that increasing the aqueous phase vanadium concentration brings about an increase in the vanadium/HDPA ratio up to a limiting value of 0.5. It would thus appear that over the rectilinear portion of the extraction isotherm (where the distribution ratio is constant) vanadium forms complexes essentially exclusively with the mono-ionized HDPA dimer, but at vanadium/HDPA ratios approaching 0"25 the extraction continues to increase (but D~ now decreases) with increasing vanadium concentration, presumably as a result of the formation ofmonomeric HDPA complexes: VO ~- + VO(HA.A)~ = 2VO(A)~ + 2H +.
(5)
The alternative explanation based on the formation of polynuclear vanadium species is discounted on the grounds that no increase in viscosity is noticeable in the highly-loaded solvent extracts. 115~C. F. BAESand H. T. BAKER,J.phys. Chem. 64, 89 (1960).
SOLVENT EXTRACTION OF VANADIUM FROM CHLORIDE SOLUTIONS USING DI-(2-ETHYLHEXYL)PHOSPHORIC ACID T. RIGG a n d J. O. GARNER Peace River Mining and Smelting Ltd., Edmonton, Alberta, Canada
(Received 17 January 1967) Abstract--Extraction of vanadium (IV) from chloride solutions by di-(2-ethylhexyl)phosphoric acid (HDPA) in kerosene has been found to take place principally through the formation of neutral mononuclear vanadyl complexes with mono-ionized dimers of HDPA. N o evidence has been obtained of chloro- or hydroxy-group participation in the extracted complex. At very high solvent loadings it appears that the extracted complex involves H D P A monomer rather than the dimer. Most effective extraction is obtained when the diluent has little or no propensity for forming hydrogen bonds with the HDPA.
THE SOLVENTextraction of vanadium from chloride solutions is of considerable interest in the application of the Research Council of Alberta chloride leach process ~1) to sedimentary iron ores such as those occurring in the Peace River district of Alberta, Canada.(9..s) The Peace River ore contains an average of about 0.2 ~ V~O5 which is fairly evenly distributed throughout the deposit. In the RCA process the ore is leached with hydrochloric acid, whereupon virtually all of the vanadium is leached out and eventually appears in the mother liquor remaining on separation of the iron as hydrated ferrous chloride. This mother liquor is normally subjected to hydrolysis for the purpose of recovering hydrochloric acid. However, it is obvious that if a suitable extractant could be found, it would be relatively easy to recover the vanadium from the feed to the hydrolyser. Di-(2-ethylhexyl)phospboric acid (HDPA) has been successfully used in the wellknown Dapex process3 4) It was therefore decided to investigate the behaviour of this reagent in chloride solutions. Preliminary experiments showed that fully-reduced vanadium was not extracted to any significant extent and fully-oxidized solutions gave poor and erratic results. It was found that extraction could be carried out efficiently only when the vanadium was present in the tetravalent state, presumably as vanadyl ion, VO ~+ or V(OH)~ 2+. EXPERIMENTAL Vanadium(IV) solutions were prepared by mixing ammonium metavanadate (NH4VO,) with its own weight of chromatography-grade cellulose powder and heating in a crucible to ,~600°C for 20 min. The residue was leached with 1.0 N hydrochloric acid and filtered. The solution was evaporated to a small volume and the acid content adjusted prior to dilution to the required vanadium tx~ C. P. GRAVENOR,G. J. GOVETTand T. RICG, Can. L Min. Bull. 57, 421 (1964). c2) D. J. KIDD, Research Council of Alberta Geol. Div. Preliminary Report 59-3 (1959). c3~ G. A. GRoss, Geology of Iron Deposits in Canada Vol. 1. Geol. Surv. Canada Economic Geology Report No. 22 (1965). c4~K. B. BROWN and C. F. COLEMAN,Progress in Nuclear Energy, Series Ill--Process Chemistry, Vol. 2, p. 3. Pergamon Press, London (1958). 2019
2020
T.R.1OG and J. O. GARNER
concentration. On some occasions the leach solution had a green tinge instead of the pure blue colour of the vanadyl group. This was attributed to incomplete reduction of vanadium(V) giving rise to a colloidal dispersion of vanadium pentoxide. Such dispersions are inextractable and give rise to yellow colorations in what would otherwise be colorless raffinates. This problem could be overcome by adding 1-2 g of hydroxylamine hydrochloride to the leach solution before evaporating down. The state of oxidation of the final solutions was checked using a platinum--calomel electrode system. Only those solutions registering an e.m.f, between 325 and 400 mV were used. pH was controlled by adding hydrochloric acid or ammonia. The vanadium content of the various aqueous phases was determined by the colorimetric peroxide method. Solvent phases were analysed by first stripping the vanadium with 6.0 N HC1. The concentration of the solutions to be analysed was adjusted by evaporation and/or dilution so that suitable aliquots containing up to 5"0 mg of vanadium could be conveniently taken. Standard vanadium solutions were prepared by completely digesting accurately-weighed amounts of pure ammonium metavanadate in 6.0 N HCI. The specific colour development procedure used was as follows: an aliquot of the solution was pipetted into a 50-ml volumetric flask, 5 ml of dilute sulphuric acid (1 + 1) was added, followed by 3 ml of 85Yo phosphoric acid and finally 1 ml of 30 ~o hydrogen peroxide. The contents of the flask were diluted to the mark and thoroughly mixed. The optical density of the colorimetric solutions was determined at a wavelength of 450 m/* with a Coleman "Universal Model 14" spectrophotometer equipped with 16-mm cylindrical cells. Extractions were carried out in 250-ml separating funnels, generally with equal volumes (50 ml) of aqueous and solvent phases. The phases were mixed by shaking at 100 c/min for 5 min, and were then allowed to stand until perfectly clear. No trouble was experienced with emulsions. All extractions and colorimetric measurements were made at the ambient temperature of 23 :f: I°C. HDPA was supplied by K and K Labs. Inc., Plain View, New York, and was diluted as required with Esso JP-1 kerosene. RESULTS P r e l i m i n a r y experiments showed t h a t e x t r a c t i o n o f v a n a d i u m ( I V ) increased with p H a n d H D P A c o n c e n t r a t i o n a n d r e q u i r e d only a s h o r t time to a p p r o a c h e q u i l i b r i u m distribution. H y d r o c h l o r i c acid ( > 1.0 N ) was f o u n d to be very effective in stripping the v a n a d i u m f r o m the solvent. T h e d e p e n d e n c e o f e x t r a c t i o n o n m i x i n g time is shown in T a b l e 1. T a b l e 2 shows t h a t the d i s t r i b u t i o n r a t i o is virtually i n d e p e n d e n t o f TABLE 1.--DEPENDENCE OF VANADIUM EXTRACTION ON MIXING TIME. VANADIUM SOLUTION INITIALLY 2"04
g/l, pH
1"9. SOLVErCr/AQOEOUS RATIO 1/1. SOLVENT10 ~ HDPA IN KEROSENE Mixing time (min)
V in extract (g/l)
0.5
1-79
1
1.81
2 5 15
1.81 1.81 1.83
v a n a d i u m c o n c e n t r a t i o n over a wide range. F o r these experiments the initial ionic strength o f the solutions was a d j u s t e d t o c o r r e s p o n d with t h a t o f the m o s t c o n c e n t r a t e d s o l u t i o n in the series b y a d d i n g s o d i u m chloride. H o w e v e r , ionic strength v a r i a t i o n s a p p e a r e d t o have n o significant effect o n the e x t r a c t i o n a n d all o t h e r results r e p o r t e d in this w o r k were o b t a i n e d w i t h o u t such adjustment. T h e effect o f v a r i a t i o n s in H D P A c o n c e n t r a t i o n a n d in acidity was d e t e r m i n e d by c a r r y i n g o u t extractions at a c o n s t a n t initial v a n a d i u m c o n c e n t r a t i o n o f 2-03 g/1 a n d
Extraction of vanadium from chloride solutions using di-(2-ethylhexyl)phosphoricacid 2021 TABLE 2.--DEPENDENCE OF DISTRIBUTION RATIO ( D e ) ON VANADIUM CONCENTRATION. SOLVENT/AQUEOUS RATIO
111; FINALpH 0.98 :k 0"05; SOLVENT 1070 HDPA IN KEROSENE Initial V concentration (g/l)
D~
0.20 0.40 0-71 1.02 2.03 4.06
4.67 4.34 4.57 4.03 4.10 4.28
final aqueous phase p H values in the range 0"00 to 2"35. H D P A concentrations of 3-0, 10.0 and 30"0 70 (volume) were used. The results are shown in Fig. 1, where it is also shown that substantial amounts of inorganic phosphate (5 g H3POJ1) in the aqueous phase have no effect on the extraction of vanadium. 3.0
2.0
1.0
d 0
-I.0
-2.0 -LO
I
0
1.0
2.0
log [HA][H'] -'
Fro. 1.--Vanadium(IV) distribution D, as a function of log [HA][H+]-t for different concentrations of HDPA in kerosene. Initial vanadium concentration in aqueous phase 2 g/1. /k = 370, O = 107o and × = 3070 HDPA. [] denotes results obtained with 10~o HDPA when the aqueous phase contained 5 g H3POdl. Very high solvent loading leads to decreased distribution ratios, as illustrated by the extraction isotherm for 1 70 H D P A shown in Fig. 2. The nature of the diluent was found to have a profound effect on the extraction. The results shown in Table 3 were obtained with 10 ~o (volume) H D P A in a variety ofdiluents. Equal volumes ofaqueous and organie phases were used. Phase separation was good in all cases except with 1-octanol.
2022
T. RIGGand J. O. GARNER 1.0 o
a --x. O.lO
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f
, Ir~lll
0.001
,
i
i
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0.01
i
i[
0,1
[vo~qoq Fro. Z--Extraction isotherm for vanadium(IV) in 1 per cent HDPA]kerosene. Final aqueous pH 1"09 4- 0-03. TABLE3.--DILUENTEFFECTONEXTRACTIONOFVANADIUMBYHDPA. INITIAL VANADIUM CONCENTRATION2"03g/l; lnNALpH 0"95 ± 0"05 Diluent
Do
Kerosene (JP-1) n-Hexane Carbon tetrachloride Benzene Methyl isobutyl ketone 1-Octanol
3"19 4.24 1"63 0'99 0.88 0.47
Some of the organic extracts were examined for the presence of chloride ions. The extracts were washed with an equal volume of 0.001N HNOs, then back-extracted into 3"0 N HNOs, and silver nitrate was added to precipitate the chloride. Only very small amounts of chloride were found, as indicated in Table 4. TABLE 4.-----CHLORIDE CONTENT OF V A N A D I U M - H ] ~ P A
EXTRACTS
Diluent
[C1]/[V] ratio
V present in sample (mg)
n-Hexane Carbon tetrachloride Benzene
0"011 0.018 0.022
65"7 50-3 40.4
The absorption spectra of vanadyl solutions in the range 340 to 820 m # showed no change over the p H interval used in the present work, suggesting the absence of significant hydrolysis. Solutions containing up to 2.0 M sodium chloride displayed no variations attributable to chloro-complex formation. All spectra showed only one
Extraction of vanadiumfrom chloridesolutionsusing di-(2-ethylhexyl)phosphoricacid 2023 peak (760 m#). Kerosene solutions of the vanadium complex gave the same type of spectrum, peaking at the same wavelength but of lower intensity. DISCUSSION Extraction at low levels o f solvent loading
The extraction of complexes formed between various metal ions and solvent-soluble ion-exchanging agents such as acetylacetone, 8-hydroxyquinoline and HDPA has been discussed previously in considerable detail by several authors, t5-7~ In particular, RYDBERGCs~has published a detailed theoretical discussion of the formation of composite mononuclear complexes involving agents of this type and gives criteria for the identification of the type of complex formed. The results in Table 2 show that the extraction of vanadium by HDPA is essentially independent of the vanadium concentration at low levels of solvent loading. It is therefore concluded that only mononuclear complexes are involved in extraction under these conditions. Although the distribution ratio for vanadium was strongly dependent on both the final aqueous phase hydrogen ion concentration ([H+]) and the HDPA concentration in the organic phase ([HA]org) the results in Fig. 1 indicate that it is constant for constant values of the product [HA]ordH+]-1. The maximum slope of the line in Fig. 1 is 2.0. On the basis of Rydberg's considerations the most likely explanation of these results is that vanadium is extracted as a neutral complex of the type VO(A)~. Over the rectilinear region in Fig. 1 the extraction can thus be represented by the expression: Dv o c \
[n +] / .
(1)
This is analogous to the findings of PEPPARD et al. (a) in HDPA extraction of the Group III elements from scandium to actinium, which apparently all form neutral complexes of the type MA a and have distribution ratios that can be expressed in terms of the general relationship: ([HA]org~ s
D oc k ~ / "
(2)
The square-law dependence for vanadium extraction by HDPA can be deduced in a fairly straightforward manner as shown, for example, in Ref. (7). The lack of any effects attributable to hydrolysis of the vanadyl ion is readily understandable in the light of the work of ROSSOTTIand RO$SOTTI,(10) who investigated the hydrolysis: VO 2+ + H~O ~ VOOH + + H+;
Kh =
[VOOH+][H +] [VOW+]
(3)
~5~R. E. CoNbrlCKand W. H. McVEY,& Am. chem. Soc. 71, 3182 (1949). ~e~j. RVDBERO, Acta chem. scand. 4, 1503 (1950). ~7~G. H. MomusoNand H. FI~mER,Solvent Extraction in Analytical Chemistry. John Wiley,New York (1957). ts~j. RYDBF.RO,Arkiv Kemi. 8, 101 (1955). ~0~D. F. I~PPhgD,G. W. M~oN, W. J. DrJSCOLLand R. J. SmONEN,J. inorg, nucl. Chem. 7, 276 (1958). {x0~F. J. C. ROSSOXTIand H. S. RossoxTI,Acts chem. stand. 9, 1177 (1955).
2024
T. Rloo and J. O. GARNER
and obtained the value Kh -----10-6"°. From this value it can be calculated that under the most strongly hydrolysing conditions employed in the present work (pH = 2"35, vanadium concentration ---- 2.0 g/l) less than 0"1 per cent of the vanadyl ion should be hydrolysed. It is therefore considered extremely unlikely that the extracted complex contains hydroxyl groups. The absorption spectra and analytical results likewise indicate that no chloro-complexes are formed. The situation is thus fortuitously much simpler than in other cases such as, for example, HDPA extraction of iron(III) from chloride solutions, where hydrolysis as well as the formation of ehloro-complexes must be taken into account.
State of aggregation of HDPA HDPA, in common with other dialkyl phosphates, has a strong tendency to form dimers, especially in non-polar solvents, (u'lz) in a manner similar to the carboxylic acids. It is believed that dimerization arises by formation of a hydrogen-bonded 8membered ring structure. Mono-ionization of the dimer thus offers the possibility of forming a relatively stable chelate structure with suitable metallic ions. c9) In the case of the vanadyl ion the complex would have the structure shown below: H RO
O
\/
O
%/
P
P
/~ RO
OR
/\ 0
0
OR
0
OR
vo RO
0 P
RO
P
O
O
OR
H
The dimerization constants of carboxylic acids are known to be very large in nonpolar solvents such as n-hexane and carbon tetrachloride. Dimerization becomes less marked as the ability to form hydrogen bonds increases and, for example, is almost negligible in water. The same type of behaviour seems to characterize the dialkyl phosphates (mxa) and is manifest by the strong dependence of the distribution ratios of various extracted ions on the nature of the solvent diluent, as shown in Table 3. It is particularly interesting to observe that DYRssmq and LmMc14)have found that the distribution ratios for the extraction of calcium by dibutyl phosphoric acid decrease in the same marked fashion in the sequence: n-hexane, kerosene, carbon tetrachloride, benzene, methyl isobutyl ketone. These authors also obtained very similar results for the extraction of uranyl ion by dioctyl phosphoric acid, in which the distribution ratios (xx) D. Dvl~s'~, Acta chem. scand. 11, 1771 (1957). cx~)D. F. I~PPARO, J. R. FER~mO and G. W. MASON, J. inorg, nucl. Chem. 4, 371 (1957); d. tnorg. nuel. Chem. 7, 231 (1958). (is) C. J. HARDY and D. SeArtOILL, U.K. Atomic Energy Authority Report A E R E C/R 2830 (1959); J. inorg, nucL Chem. 11, 128 (1959). (x4) D. D~SSEN and D. H. Lm~l, Acta chem. scand. 14, 1100 (1960).
Extraction of vanadium from chloride solutions using di-(2-ethylhexyl)phosphoricacid 2025 decreased in the sequence: kerosene, n-hexane, carbon tetrachloride, benzene, 2-octanol. It is therefore concluded that the distribution ratio obtained in the extraction of metal ions by dialkyl phosphoric acids is a function of extractant-diluent interaction rather than interaction between the diluent and the specific metal species. In the rather special case of uranyl ion extraction by kerosene solutions of dialkyl phosphoric acids containing some tributyl phosphate the latter can itself contribute to the extraction. However, in the case of vanadyl extraction it was observed that the addition of l0 per cent tributyl phosphate to the kerosene diluent impaired extraction, evidently by interfering with the dimerization process. Inspection of Fig. 1 shows generally good agreement between the experimental points and the line drawn with the theoretical slope of 2.0. On the other hand, the points representing the results of extractions carried out with 30 per cent HDPA are all on the low side of the line. It seems possible that this is due to the likelihood of further polymerization of HDPA to form trimers: 3(HA) 2 ~ 2(HA)3.
(4)
According to BAESand BAKERtm the equilibrium constant for this reaction in n-octane is 10 mole -1. Assuming that this value applies reasonably well to kerosene solutions of HDPA we calculate that about two-thirds of the HDPA will be present as trimer in 30 per cent HDPA-kerosene solutions; this could well account for the smaller vanadium distribution ratios.
Extraction under conditions of very high solvent loading If the mechanism discussed above, whereby vanadium is extracted by mono-ionized dimers of HDPA, were the only extraction mechanism operating in this system it would not be possible to achieve vanadium concentrations in the organic phase greater than 0.25 of the formal HDPA concentration. However, the extraction isotherm depicted in Fig. 2 shows that increasing the aqueous phase vanadium concentration brings about an increase in the vanadium/HDPA ratio up to a limiting value of 0.5. It would thus appear that over the rectilinear portion of the extraction isotherm (where the distribution ratio is constant) vanadium forms complexes essentially exclusively with the mono-ionized HDPA dimer, but at vanadium/HDPA ratios approaching 0"25 the extraction continues to increase (but D~ now decreases) with increasing vanadium concentration, presumably as a result of the formation ofmonomeric HDPA complexes: VO ~- + VO(HA.A)~ = 2VO(A)~ + 2H +.
(5)
The alternative explanation based on the formation of polynuclear vanadium species is discounted on the grounds that no increase in viscosity is noticeable in the highly-loaded solvent extracts. 115~C. F. BAESand H. T. BAKER,J.phys. Chem. 64, 89 (1960).