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Solvent extraction of niobium and tantalum
Journal ofthe Less-Common Metals Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
SOLVENT
VI.
EXTRACTION
EXTRACTION
FLUORIC
OF
WITH
TANTALUM
OF THE
ALCOHOLS
FROM
HYDRO-
METALS
AND D. SEVDIC
Laboratory of Analytical Chemistry, BoSkoviC”, Zagreb (Yugoslazlia) (Received
AND
DI-N-OCTYLAMINO
ACID SOLUTIONS
C. DJORDJEVIC
KIOBIUM
233
June 24th.
Faculty of Science,
The University.
and Institute
“Rudjer
1968)
SUMMARY
Studies solutions, (DOAP),
of the extraction
of niobium
and tantalum
from hydrofluoric
with di-n-octylamino ethanol (DOAE) and di-n-octylamino in chloroform, have been carried out. The metal/amine ratio
mined in the organic phase by the slope analysis. and tantalum
with DOAE
were separated
The extracting
acid
propanol was deter-
species of niobium
from the 0.04, 0.4 and
I M
HF systems,
respectively. Different extracting species for niobium and tantalum were obtained, and characterized by analysis and infrared spectra. Species containing NbOFs2- and TaOFdp anions were separated
from 0.04 M HF systems.
INTRODUCTION
Simple and N-substituted long chain amines have proved to be suitable agents for the separation of niobium and tantalum in hydrofluoric acid systemsiy2. The chemical mechanism, and the reason for the observed difference Nb(V) and Ta(V) have not been studied previously. This observation
in behaviour of has prompted us
to investigate the extraction behaviour of these two metals in more detail, in particular with regard to the formation of the extracting species. Bearing in mind the existence of several possible fluorometallates in the aqueous solutions3, which, in general, are dissimilar for niobium and tantalum, differences in the extraction species formation and the extraction process may be expected. The extractants used in this investigation, di-n-octylamino ethanol (DOAE) and di-n-octylamino propanol (DOAP), contain, in addition to the basic nitrogen, the alcoholic OH group, which may or may not participate in the extraction mechanism. The results of our experiments, in particular the characterisation of the separated extracting species, have permitted the deduction of the type of extraction mechanism for the extraction of Nb(V) Ta(V) from hydrofluoric solutions with N-substituted amino alcohols. J. Less-Common
and
Metals, 16 (1968) 233-239
234
C. DJORDJEVI6,
D. SEVDIi:
EXPERIMENTAL
Reagents 2-di-~-octylamino-eth~oI~1~ (DOAE) and 3-di-~-octylamino-propanol/l~ (DOAP) were prepared as previously describedz. Chloroform (Merck p.a.) was used as the organic solvent. In some experiments, DOAE and DOAP were treated with I M HF before use. No difference in the extraction behaviour was observed between the untreated and the HF-treated extractants. Radioactive tracers and stadard metal solzttions Standard hydrofluoric acid solutions of the metals were obtained from oxalic acid solutions, of known metal concentration. The metal hydroxides were precipitated with ~monia, washed with 2% ammonium chloride solution, and dissolved in hydrofluoric acid of known concentration. In the same way, radioactive hydrofluoric acid solutions of Q5Nband 182Ta were prepared. Q5Nbwas obtained from the Radiochemical Center Amersham, in the form of a 0.5 o/o oxalate solution. lQ2Ta was obtained by neutron irradiation of the metal oxalate solutions in the nuclear reactor (Institute “Boris KidriE”, VinCa).
The experiments were carried out in a constant temperature room at 24°C. Equal volumes (2 ml) of the aqueous and organic phase were used. Extraction was carried out in polythene vessels, the stirring time being xg min. It has been found that in all the systems described, equilibrium was reached after 7-10 minutes of stirring The phases were separated by centrifuging and the y-activity of the organic and aqueous phase aliquots (I ml) was determined on a scintillation counter. Reproducible results were obtained. Se~aqat~o~ of the extq~ct~on species The organic phase obtained after the extraction procedure was transferred into a platinum crucible, which was then placed in a vacuum vessel and heated on a water bath. The chloroform was slowly evaporated under a water pump vacuum, The viscous residue thus obtained was then dried in a vacuum of 10-3 mm Hg. Pale yellow, viscid substances, giving reproducible analyses for samples prepared in several independent experiments, were obtained. Analysis Niobium and tantalum were determined by igniting the samples in a platinum crucible, heating the residue at g5o”C, and weighing as MzO~. Fluorine was determined by two separate methods after destroying the samples by fusion with a sodium-potassium carbonate mixture. In the first method, the alkaline melt was dissolved in water, fluorine steam-distilled* and determined by titration with thorium nitrate and alizarin S as indicatorj. In the second method, fluorine was precipitated as CaFz by the addition of an excess of CaClQ.The excess of calcium was determined by EDTA and Eriochrom Black T as indicators. Both methods gave identical results. Carbon, hydrogen and nitrogen were determined micro-analytically. j.
hSS-COWWBO’B
&ht‘dS,
I6
(X968)
233-239
SOLVENT TABLE ANALYSIS
EXTRACTION
OF NIOBIUM
AND
TANTALUM
WITH
DOAE
23.5
I OF EXTRACTING
SPECIES
(%)
-
A queows $hase compositiora :
0.04 M HF
0.4 M HF
Composition (9/,)
Extracting species .-. Nb Ta
Nb
TlZ
Nb
Ta
Metal c fl
1r.y
10.4 56.0
25.0 44.4 8.5 2.9 14.8
9.4 58.3
20.4 48.8
11.4 3.7 10.3
9.5 3.? 15.0
31.1 39.3 7.6 3.0 13.2
r::: 3.1 12.4
N F
IMHF
.._.-...
II.0
3.8 11.9
The analytical results obtained are summarized in Table I. Analytical data for niobium compounds agree with the formula [DOAEH]z [NbOFsj for all the three HF concentration ranges. C~~H*~~~O~F~Nbrequires: C, 55.71;; H, 10.3’+4; N, 3.694; F IZ.Z’/ . Nb IZ.O~/,; For tantalum, however, the samples separated from 0.04 M I& systkrs are only in good agreement with the formula [DOAEH] [TaOFd]. CisH40N02F4Ta requires: C, 38.6qb; H, 7.2yJ,; N, 2.5”(,; F, 13.6;/,; Ta, 32.8q.G. The analysis of the compounds separated from 0.4 M HF systems agree with an equimolar mixture of [DOAE] [TaOF*] and [DOAEH~~Ta~~ which requires : C, +@ :i, ; H, 8.3 T-,; N, 2.99/,; F, 14.5:&; Ta, 25.1O:,. The samples separated from I M HF systems, according to the analysis, agree with the formula [DOAEH]zTaFT, as &,H80N202F7Ta requires: C, 48.80/,; H, 9.0”;~; N, 3.2’j’,; F, 15.0~;; Ta, 20.4:/o. Infrared spectra were measured on the 127 Infracord spectrophotometer in the region 4000-700 cm-l. A mulling agent was not necessary for the separated viscid substances. RESULTS
Edraction behaviour Table II shows the dependence of the niobium and tantalum extraction,
with
TABLE II EXTRACTION
OF
CONCENTRATIONS
0.02
0.05 0.1
0.2
0.4 0.6 0.8 I.0 I.2
7.4 1.6 1.8 2.0
NIOBIUM OF IO-5
268 72 37 13 4.0 2 .2 1.4 1.3 0.75 0.56 0.42
0.31 0.24
AND
TANTALIUM
FROM
HYDROFLUORIC
ACIII
SOLUTIONS
WITH
METAL
h!f
333
200
620
220
130 62 22
430 270 140 55 33 22 15 ‘3
II.5
75 34 21 ‘4 IO
5.6 2.7 1.6 1.1
8.2
0.82
6.5 5.1 4.1 3.3
0.58 0.42 0.30 0.22
10 8.6 7.2 6.0
J. ~ess-Co~~o~
Metals, 16 (1968)
233-239
C. DJORDJEVIC,
236
D. SEVDIt
DOAE and DOAP, upon the hydrofluoric acid concentration. The metal (10-5 M) and reagent (10-2 M) concentrations were kept constant. No significant difference was observed between the two extractants and the increase of the hydrofluoric acid concentration decreased the degree of extraction of bothmetals. The extraction coefficients for tantalum were in general higher by about an order of magnitude. The dependence of niobium and tantalum extraction upon the concentration of the reactant in the organic phase was studied in 0.04 M, 0.4 M and I M HF, respectively, with a metal concentration of 10-5 M. The results obtained for niobium are represented in Fig. I. By the slope analysis, a DOAEjNb ratio of z is obtained in all the three hydrofluoric acid concentration ranges examined. A different extraction behaviour is observed for tantalum, as shown in Fig. 2. The slope analysis gives DOAE/ Ta ratios of I, 1.5 and 1.4, respectively, for the HF concentrations given above. In account of the large excess of DOAE concentration, as compared with the metal concentration, the total amine concentration is plotted in the figures.
Extraction dependency of Nb(V) on DOAE concentration in presence of: ( 0) 0.04 M HF; (m) 0.4 M HF; (A) I M HF. (Nb = IO-~ &I)
Fig. I.
Fig. z. Extraction dependency of Ta(V) on DOAE concentration in the presence of: ( o) 0.04 M HF; ([3) 0.4 M HI?; (a) I M HF. (Ta = 10-5 M)
DOAP shows an analogous behaviour in all the systems, and for this reason the results obtained are not given. Slight differences were found only in the extraction dependence upon the DOAP concentration. By slope analysis DOAP/Nb ratios of 1.9, 1.S and 1.8 were observed for 0.04, 0.4 and I M HF systems, respectively. For tantalum, values of 1.5 were obtained in all the three hydrofluoric acid concentration ranges. Extracting species and ~xtya~t~onmechanism The extraction ability and behaviour of DOAE and DOAP proved to be very similar for bath niobium and tantalum. For this reason the tedious experiments of the separation of the extracting species were carried out only with DOAE. Niobium and J. Less-Common
Metals,
16 (1968) 233-239
SOLVENT EXTRACTION
OF NIOBIUM AND TANTALUM
237
tantalum extracting species were separated from systems containing 0.04, 0.4 and I M HF. The composition of the niobium extracting species, according to the analyses (see Table I), corresponds to the formula [DOAEH]s [NbOF]s for all the three examined systems. Less satisfactory analytical results which were obtained in the I M HF extraction system can be explained by the presence of impurities in the residue of the organic phase. In this system, extraction coefficients for niobium were much lower than in 0.04 M HF, and therefore some amount of the free extractant was probably present in the organic phase. The structure of the niobium extracting species, derived from organic phase residue analyses, agrees well with the extraction studies in solution, where the DOAE/Nb ratio of z was obtained by slope analyses in all the three hydrofluoric acid concentration ranges. The infrared spectrum of the separated niobium extracting species, is compared with the spectrum of DOAE, in Fig. 3. The OH stretching of the alcoholic group is clearly resolved at about 3400 cm-l, similar to the band in the free DOAE. In addition to the group vibrational frequencies of the substituted ammonium cation, a strong band is present at 920 cm-i, which may be assigned to the Nb = 0 stretching.
L
LOJI
Im,
zcol
cm
Fig. 3. Infrared spectra: (a) DOAE,
m
-1
(b) niobium, and (c) tantalum extracting species. J. Less-Common
Metals, 16 (1968) 233-239
C. DJORDJEVIk,
238
D. SEVDIC
The extracting species separated for tantalum in 0.04, 0.4 and I M HF, respectively, were not identical. At the low 0.04 M HF concentration, where efficient extraction occurs, (see Table II), the analysisof the organic phase residue corresponded to the formula [DOAEH] [TaOFaj. The DOAE/Ta ratio obtained by slope analysis in the corresponding solutions agreed well with this structure. The infrared spectrum of this substance is given in Fig. 3. The alcoholic group OH stretching is present in the expected position. The Ta=O stretching occurs at about goo cm-r in addition to the group vibrational frequencies of the substituted ammonium cation. The chemical analysis of the organic phase residue, obtained from tantalum systems with 0.4 M HI?, does not agree with the formula of a single ion-pair species. The infrared spectrum, however, does not show the presence of a Ta-0-Ta bond, and therefore does not indicate a polymer formation. For this reason we believe that in 0.4 M HF systems a mixture of two or more extracting species is transferred into the organic phase. Under identical experimental conditions, the composition of the organic phase residue should be the same, because the same equilibrium constants govern the concentration of the metal species. We have in fact, observed a constant composition of the residues separated in several independent experiments. The analytical results (see Table I) correspond well with an equimolar mixture of [DOAEH] [TaOFd] and [DOAEH]a [TaF,]. However, the same analytical data may possibly agree with some other combination of three components, for example. Substances corresponding, according to the analysis, to the formula [DOAEH]z [TaFTj, were separated from systems containing I M HF. However, these analytical data may be due to some mixture of two or more extracting species, containing excess of the free extractant. Under these conditions, the tantalum extraction coefficient is much lower than in 0.04 M HF. DISCUSSION
Studies of the extraction of niobium and tantalum from hydrofluoric acid solution proved to be interesting from several points of view. The difference in extraction behaviour of niobium and tantalum which enables, under certain conditions, a successful separation of these two metalsz, has been explained by the formation of different extracting species. The increase of hydrofluoric acid concentration decreases the degree of metal extraction, indicating that extractable metal species are present only in dilute hydrofluoric acid solutions. The fluorometallates of niobium and tantalum in different hydrofluoric acidity ranges are not identical in composition.3 In dilute 0.04 M HF systems we have found the previously reported7 NbOF$anion, which is extracted from the aqueous phase in up to I M HF. There are probably some other fluorometal complexes also present in the aqueous phase as part of the complex fluoroniobates equilibria. With these long chain amino-alcohols, however, the NbOFs- proved to have suitable properties for extraction in the organic phase. In the case of tantalum, the TaOFd- anion was found to be one of the equihbrium fluorotantalate species responsible for the efficient tantalum extraction in the dilute hydrofluoric acid media. The oxotetrafluorotantalatejti) has not been reported previously, probably because the dilute 0.04 M HF solutions have not been investigated in this respect. The presence of this anion has nevertheless been assumed previously in another extraction system*. An agreement between the solution studies (slope analysis) and the separated J. Less-Common Met&,
r6 (1968) zz_p-23g
SOLVENT
EXTKACTION
OF NIOBIUM
AND TANTALUM
239
extracting species structure has been found for both metals in systems with higher extraction coefficients. With a decrease of the tantalum extraction efficacy in higher hydrofluoric acid concentrations, the residue of the organic phase consists of a mixture containing probably more than one component; some free extractant may also be present. In the medium hydrofluoric acid concentration range studied, the extracting species formed at the lower and higher acid concentrations, respectively, are extracted simultaneously in a ratio dependent upon the solution equilibria. From these results it can be deduced that the method used for the extracting species study, i.e., the careful separation of the organic phase residue, can be applied successfully only for the extraction systems where high metal extraction coefficients are observed. The slope analysis is then in a good agreement with the separated extracting species composition. If, however, the mechanism of the extraction involves more than one extracting species formation, the slope analysis is of less assistance. The mechanism of niobium and tantalum extraction with DOAE and DOAP in hydrofluoric acid solutions has proved to be of the ion association type. The experiments have shown that the extraction depends to a great extent upon the acid and fluoride concentration. From the data obtained, however, it is not possible to deduce the whole course of the reaction. In the hydrofluoric acid concentration range in which these studies were carried out the presence of more than one fluoro metal species is to be expected. Their concentration is governed by complex equilibrium constants, and by hydrogen ion, metal and fluoride ion concentrations; little is known about the nature of such solutions. The presence of MOFe-, (M==Nb, Ta) species* was derived from solution studies carried out in low HF concentration. It has been reported also that fluoride ion concentration has great influence on the complex metal ion formation”. In addition, in hydrofluoric acid systems, the extractants used probably10 form salts of the type ~R~R’~H~~/HF~~~. Further polymerization of HFs- anions may take place, and at higher HF concentrations, chemical reaction with the alcohol group of the extractant may be expected. The separated extracting species represent, therefore, the final product of a complicated extraction process which cannot be expressed by a simple reaction course. However, these studies show that niobium and tantalum in llydrofluoric acid systems do not form extracting species of identical compositions, and that the extraction mechanism with long chain amino alcohols is of the ion association type. REFERENCES I H. STARCHART AND F. HECHT, Mikrochim. Acta, (rgGz) 1152. 2 C. DJORDJEVIC AND D. SEVDIC, Croat. Chem. Acta, 39 (1967) 155. 3 F. FAIRBROTHER, The Chemistvy of Niobium and Tantalum, Elsevier, Amsterdam, 1967, p. 80. .+ N. H. FURMAN, Scott’s Standard Method ofChemical Atialysis, 1’01. r, Van Nostrand, London, 1962, p. 432. 5 G. PIETZKA AND P. ERLICH, Angew. Chem.. 65 (1953) 131. 6 I<. BELCHER AND S. J. CLARK, Anal. Chim. Acta, 8 (1953) 222. 7 0. 1~. KELLER, JR., Inorg. Chem., 2 (1963) 783. 8 C. DJORDJEVIC AND H. GORIEAN, J, Iraorg. Nucl. Chem., 28 (1966) 1451. 9 0. L. KELLER, JR. AND A. CHETHAM-STRODE, JR., Inorg. Ckem., 5 (1966) 367. IO A. S. WILSON AND N. A. WOGMAN, J. Phys. Chem., 66 (1962) 1552. .f. Less-Common
Metals, 16 (1968)
233-239
SOLVENT
VI.
EXTRACTION
EXTRACTION
FLUORIC
OF
WITH
TANTALUM
OF THE
ALCOHOLS
FROM
HYDRO-
METALS
AND D. SEVDIC
Laboratory of Analytical Chemistry, BoSkoviC”, Zagreb (Yugoslazlia) (Received
AND
DI-N-OCTYLAMINO
ACID SOLUTIONS
C. DJORDJEVIC
KIOBIUM
233
June 24th.
Faculty of Science,
The University.
and Institute
“Rudjer
1968)
SUMMARY
Studies solutions, (DOAP),
of the extraction
of niobium
and tantalum
from hydrofluoric
with di-n-octylamino ethanol (DOAE) and di-n-octylamino in chloroform, have been carried out. The metal/amine ratio
mined in the organic phase by the slope analysis. and tantalum
with DOAE
were separated
The extracting
acid
propanol was deter-
species of niobium
from the 0.04, 0.4 and
I M
HF systems,
respectively. Different extracting species for niobium and tantalum were obtained, and characterized by analysis and infrared spectra. Species containing NbOFs2- and TaOFdp anions were separated
from 0.04 M HF systems.
INTRODUCTION
Simple and N-substituted long chain amines have proved to be suitable agents for the separation of niobium and tantalum in hydrofluoric acid systemsiy2. The chemical mechanism, and the reason for the observed difference Nb(V) and Ta(V) have not been studied previously. This observation
in behaviour of has prompted us
to investigate the extraction behaviour of these two metals in more detail, in particular with regard to the formation of the extracting species. Bearing in mind the existence of several possible fluorometallates in the aqueous solutions3, which, in general, are dissimilar for niobium and tantalum, differences in the extraction species formation and the extraction process may be expected. The extractants used in this investigation, di-n-octylamino ethanol (DOAE) and di-n-octylamino propanol (DOAP), contain, in addition to the basic nitrogen, the alcoholic OH group, which may or may not participate in the extraction mechanism. The results of our experiments, in particular the characterisation of the separated extracting species, have permitted the deduction of the type of extraction mechanism for the extraction of Nb(V) Ta(V) from hydrofluoric solutions with N-substituted amino alcohols. J. Less-Common
and
Metals, 16 (1968) 233-239
234
C. DJORDJEVI6,
D. SEVDIi:
EXPERIMENTAL
Reagents 2-di-~-octylamino-eth~oI~1~ (DOAE) and 3-di-~-octylamino-propanol/l~ (DOAP) were prepared as previously describedz. Chloroform (Merck p.a.) was used as the organic solvent. In some experiments, DOAE and DOAP were treated with I M HF before use. No difference in the extraction behaviour was observed between the untreated and the HF-treated extractants. Radioactive tracers and stadard metal solzttions Standard hydrofluoric acid solutions of the metals were obtained from oxalic acid solutions, of known metal concentration. The metal hydroxides were precipitated with ~monia, washed with 2% ammonium chloride solution, and dissolved in hydrofluoric acid of known concentration. In the same way, radioactive hydrofluoric acid solutions of Q5Nband 182Ta were prepared. Q5Nbwas obtained from the Radiochemical Center Amersham, in the form of a 0.5 o/o oxalate solution. lQ2Ta was obtained by neutron irradiation of the metal oxalate solutions in the nuclear reactor (Institute “Boris KidriE”, VinCa).
The experiments were carried out in a constant temperature room at 24°C. Equal volumes (2 ml) of the aqueous and organic phase were used. Extraction was carried out in polythene vessels, the stirring time being xg min. It has been found that in all the systems described, equilibrium was reached after 7-10 minutes of stirring The phases were separated by centrifuging and the y-activity of the organic and aqueous phase aliquots (I ml) was determined on a scintillation counter. Reproducible results were obtained. Se~aqat~o~ of the extq~ct~on species The organic phase obtained after the extraction procedure was transferred into a platinum crucible, which was then placed in a vacuum vessel and heated on a water bath. The chloroform was slowly evaporated under a water pump vacuum, The viscous residue thus obtained was then dried in a vacuum of 10-3 mm Hg. Pale yellow, viscid substances, giving reproducible analyses for samples prepared in several independent experiments, were obtained. Analysis Niobium and tantalum were determined by igniting the samples in a platinum crucible, heating the residue at g5o”C, and weighing as MzO~. Fluorine was determined by two separate methods after destroying the samples by fusion with a sodium-potassium carbonate mixture. In the first method, the alkaline melt was dissolved in water, fluorine steam-distilled* and determined by titration with thorium nitrate and alizarin S as indicatorj. In the second method, fluorine was precipitated as CaFz by the addition of an excess of CaClQ.The excess of calcium was determined by EDTA and Eriochrom Black T as indicators. Both methods gave identical results. Carbon, hydrogen and nitrogen were determined micro-analytically. j.
hSS-COWWBO’B
&ht‘dS,
I6
(X968)
233-239
SOLVENT TABLE ANALYSIS
EXTRACTION
OF NIOBIUM
AND
TANTALUM
WITH
DOAE
23.5
I OF EXTRACTING
SPECIES
(%)
-
A queows $hase compositiora :
0.04 M HF
0.4 M HF
Composition (9/,)
Extracting species .-. Nb Ta
Nb
TlZ
Nb
Ta
Metal c fl
1r.y
10.4 56.0
25.0 44.4 8.5 2.9 14.8
9.4 58.3
20.4 48.8
11.4 3.7 10.3
9.5 3.? 15.0
31.1 39.3 7.6 3.0 13.2
r::: 3.1 12.4
N F
IMHF
.._.-...
II.0
3.8 11.9
The analytical results obtained are summarized in Table I. Analytical data for niobium compounds agree with the formula [DOAEH]z [NbOFsj for all the three HF concentration ranges. C~~H*~~~O~F~Nbrequires: C, 55.71;; H, 10.3’+4; N, 3.694; F IZ.Z’/ . Nb IZ.O~/,; For tantalum, however, the samples separated from 0.04 M I& systkrs are only in good agreement with the formula [DOAEH] [TaOFd]. CisH40N02F4Ta requires: C, 38.6qb; H, 7.2yJ,; N, 2.5”(,; F, 13.6;/,; Ta, 32.8q.G. The analysis of the compounds separated from 0.4 M HF systems agree with an equimolar mixture of [DOAE] [TaOF*] and [DOAEH~~Ta~~ which requires : C, +@ :i, ; H, 8.3 T-,; N, 2.99/,; F, 14.5:&; Ta, 25.1O:,. The samples separated from I M HF systems, according to the analysis, agree with the formula [DOAEH]zTaFT, as &,H80N202F7Ta requires: C, 48.80/,; H, 9.0”;~; N, 3.2’j’,; F, 15.0~;; Ta, 20.4:/o. Infrared spectra were measured on the 127 Infracord spectrophotometer in the region 4000-700 cm-l. A mulling agent was not necessary for the separated viscid substances. RESULTS
Edraction behaviour Table II shows the dependence of the niobium and tantalum extraction,
with
TABLE II EXTRACTION
OF
CONCENTRATIONS
0.02
0.05 0.1
0.2
0.4 0.6 0.8 I.0 I.2
7.4 1.6 1.8 2.0
NIOBIUM OF IO-5
268 72 37 13 4.0 2 .2 1.4 1.3 0.75 0.56 0.42
0.31 0.24
AND
TANTALIUM
FROM
HYDROFLUORIC
ACIII
SOLUTIONS
WITH
METAL
h!f
333
200
620
220
130 62 22
430 270 140 55 33 22 15 ‘3
II.5
75 34 21 ‘4 IO
5.6 2.7 1.6 1.1
8.2
0.82
6.5 5.1 4.1 3.3
0.58 0.42 0.30 0.22
10 8.6 7.2 6.0
J. ~ess-Co~~o~
Metals, 16 (1968)
233-239
C. DJORDJEVIC,
236
D. SEVDIt
DOAE and DOAP, upon the hydrofluoric acid concentration. The metal (10-5 M) and reagent (10-2 M) concentrations were kept constant. No significant difference was observed between the two extractants and the increase of the hydrofluoric acid concentration decreased the degree of extraction of bothmetals. The extraction coefficients for tantalum were in general higher by about an order of magnitude. The dependence of niobium and tantalum extraction upon the concentration of the reactant in the organic phase was studied in 0.04 M, 0.4 M and I M HF, respectively, with a metal concentration of 10-5 M. The results obtained for niobium are represented in Fig. I. By the slope analysis, a DOAEjNb ratio of z is obtained in all the three hydrofluoric acid concentration ranges examined. A different extraction behaviour is observed for tantalum, as shown in Fig. 2. The slope analysis gives DOAE/ Ta ratios of I, 1.5 and 1.4, respectively, for the HF concentrations given above. In account of the large excess of DOAE concentration, as compared with the metal concentration, the total amine concentration is plotted in the figures.
Extraction dependency of Nb(V) on DOAE concentration in presence of: ( 0) 0.04 M HF; (m) 0.4 M HF; (A) I M HF. (Nb = IO-~ &I)
Fig. I.
Fig. z. Extraction dependency of Ta(V) on DOAE concentration in the presence of: ( o) 0.04 M HF; ([3) 0.4 M HI?; (a) I M HF. (Ta = 10-5 M)
DOAP shows an analogous behaviour in all the systems, and for this reason the results obtained are not given. Slight differences were found only in the extraction dependence upon the DOAP concentration. By slope analysis DOAP/Nb ratios of 1.9, 1.S and 1.8 were observed for 0.04, 0.4 and I M HF systems, respectively. For tantalum, values of 1.5 were obtained in all the three hydrofluoric acid concentration ranges. Extracting species and ~xtya~t~onmechanism The extraction ability and behaviour of DOAE and DOAP proved to be very similar for bath niobium and tantalum. For this reason the tedious experiments of the separation of the extracting species were carried out only with DOAE. Niobium and J. Less-Common
Metals,
16 (1968) 233-239
SOLVENT EXTRACTION
OF NIOBIUM AND TANTALUM
237
tantalum extracting species were separated from systems containing 0.04, 0.4 and I M HF. The composition of the niobium extracting species, according to the analyses (see Table I), corresponds to the formula [DOAEH]s [NbOF]s for all the three examined systems. Less satisfactory analytical results which were obtained in the I M HF extraction system can be explained by the presence of impurities in the residue of the organic phase. In this system, extraction coefficients for niobium were much lower than in 0.04 M HF, and therefore some amount of the free extractant was probably present in the organic phase. The structure of the niobium extracting species, derived from organic phase residue analyses, agrees well with the extraction studies in solution, where the DOAE/Nb ratio of z was obtained by slope analyses in all the three hydrofluoric acid concentration ranges. The infrared spectrum of the separated niobium extracting species, is compared with the spectrum of DOAE, in Fig. 3. The OH stretching of the alcoholic group is clearly resolved at about 3400 cm-l, similar to the band in the free DOAE. In addition to the group vibrational frequencies of the substituted ammonium cation, a strong band is present at 920 cm-i, which may be assigned to the Nb = 0 stretching.
L
LOJI
Im,
zcol
cm
Fig. 3. Infrared spectra: (a) DOAE,
m
-1
(b) niobium, and (c) tantalum extracting species. J. Less-Common
Metals, 16 (1968) 233-239
C. DJORDJEVIk,
238
D. SEVDIC
The extracting species separated for tantalum in 0.04, 0.4 and I M HF, respectively, were not identical. At the low 0.04 M HF concentration, where efficient extraction occurs, (see Table II), the analysisof the organic phase residue corresponded to the formula [DOAEH] [TaOFaj. The DOAE/Ta ratio obtained by slope analysis in the corresponding solutions agreed well with this structure. The infrared spectrum of this substance is given in Fig. 3. The alcoholic group OH stretching is present in the expected position. The Ta=O stretching occurs at about goo cm-r in addition to the group vibrational frequencies of the substituted ammonium cation. The chemical analysis of the organic phase residue, obtained from tantalum systems with 0.4 M HI?, does not agree with the formula of a single ion-pair species. The infrared spectrum, however, does not show the presence of a Ta-0-Ta bond, and therefore does not indicate a polymer formation. For this reason we believe that in 0.4 M HF systems a mixture of two or more extracting species is transferred into the organic phase. Under identical experimental conditions, the composition of the organic phase residue should be the same, because the same equilibrium constants govern the concentration of the metal species. We have in fact, observed a constant composition of the residues separated in several independent experiments. The analytical results (see Table I) correspond well with an equimolar mixture of [DOAEH] [TaOFd] and [DOAEH]a [TaF,]. However, the same analytical data may possibly agree with some other combination of three components, for example. Substances corresponding, according to the analysis, to the formula [DOAEH]z [TaFTj, were separated from systems containing I M HF. However, these analytical data may be due to some mixture of two or more extracting species, containing excess of the free extractant. Under these conditions, the tantalum extraction coefficient is much lower than in 0.04 M HF. DISCUSSION
Studies of the extraction of niobium and tantalum from hydrofluoric acid solution proved to be interesting from several points of view. The difference in extraction behaviour of niobium and tantalum which enables, under certain conditions, a successful separation of these two metalsz, has been explained by the formation of different extracting species. The increase of hydrofluoric acid concentration decreases the degree of metal extraction, indicating that extractable metal species are present only in dilute hydrofluoric acid solutions. The fluorometallates of niobium and tantalum in different hydrofluoric acidity ranges are not identical in composition.3 In dilute 0.04 M HF systems we have found the previously reported7 NbOF$anion, which is extracted from the aqueous phase in up to I M HF. There are probably some other fluorometal complexes also present in the aqueous phase as part of the complex fluoroniobates equilibria. With these long chain amino-alcohols, however, the NbOFs- proved to have suitable properties for extraction in the organic phase. In the case of tantalum, the TaOFd- anion was found to be one of the equihbrium fluorotantalate species responsible for the efficient tantalum extraction in the dilute hydrofluoric acid media. The oxotetrafluorotantalatejti) has not been reported previously, probably because the dilute 0.04 M HF solutions have not been investigated in this respect. The presence of this anion has nevertheless been assumed previously in another extraction system*. An agreement between the solution studies (slope analysis) and the separated J. Less-Common Met&,
r6 (1968) zz_p-23g
SOLVENT
EXTKACTION
OF NIOBIUM
AND TANTALUM
239
extracting species structure has been found for both metals in systems with higher extraction coefficients. With a decrease of the tantalum extraction efficacy in higher hydrofluoric acid concentrations, the residue of the organic phase consists of a mixture containing probably more than one component; some free extractant may also be present. In the medium hydrofluoric acid concentration range studied, the extracting species formed at the lower and higher acid concentrations, respectively, are extracted simultaneously in a ratio dependent upon the solution equilibria. From these results it can be deduced that the method used for the extracting species study, i.e., the careful separation of the organic phase residue, can be applied successfully only for the extraction systems where high metal extraction coefficients are observed. The slope analysis is then in a good agreement with the separated extracting species composition. If, however, the mechanism of the extraction involves more than one extracting species formation, the slope analysis is of less assistance. The mechanism of niobium and tantalum extraction with DOAE and DOAP in hydrofluoric acid solutions has proved to be of the ion association type. The experiments have shown that the extraction depends to a great extent upon the acid and fluoride concentration. From the data obtained, however, it is not possible to deduce the whole course of the reaction. In the hydrofluoric acid concentration range in which these studies were carried out the presence of more than one fluoro metal species is to be expected. Their concentration is governed by complex equilibrium constants, and by hydrogen ion, metal and fluoride ion concentrations; little is known about the nature of such solutions. The presence of MOFe-, (M==Nb, Ta) species* was derived from solution studies carried out in low HF concentration. It has been reported also that fluoride ion concentration has great influence on the complex metal ion formation”. In addition, in hydrofluoric acid systems, the extractants used probably10 form salts of the type ~R~R’~H~~/HF~~~. Further polymerization of HFs- anions may take place, and at higher HF concentrations, chemical reaction with the alcohol group of the extractant may be expected. The separated extracting species represent, therefore, the final product of a complicated extraction process which cannot be expressed by a simple reaction course. However, these studies show that niobium and tantalum in llydrofluoric acid systems do not form extracting species of identical compositions, and that the extraction mechanism with long chain amino alcohols is of the ion association type. REFERENCES I H. STARCHART AND F. HECHT, Mikrochim. Acta, (rgGz) 1152. 2 C. DJORDJEVIC AND D. SEVDIC, Croat. Chem. Acta, 39 (1967) 155. 3 F. FAIRBROTHER, The Chemistvy of Niobium and Tantalum, Elsevier, Amsterdam, 1967, p. 80. .+ N. H. FURMAN, Scott’s Standard Method ofChemical Atialysis, 1’01. r, Van Nostrand, London, 1962, p. 432. 5 G. PIETZKA AND P. ERLICH, Angew. Chem.. 65 (1953) 131. 6 I<. BELCHER AND S. J. CLARK, Anal. Chim. Acta, 8 (1953) 222. 7 0. 1~. KELLER, JR., Inorg. Chem., 2 (1963) 783. 8 C. DJORDJEVIC AND H. GORIEAN, J, Iraorg. Nucl. Chem., 28 (1966) 1451. 9 0. L. KELLER, JR. AND A. CHETHAM-STRODE, JR., Inorg. Ckem., 5 (1966) 367. IO A. S. WILSON AND N. A. WOGMAN, J. Phys. Chem., 66 (1962) 1552. .f. Less-Common
Metals, 16 (1968)
233-239