The extraction of mineral acids by the phosphine oxide Cyanex 923

hydrometallurgy ELSEVIER

Hydrometallurgy 42 (1996) 245-255

The extraction of mineral acids by the phosphine oxide Cyanex 923 F.J. Alguacil, F.A. Ldpez Centro Nacional de lnvestigaciones Metal~rgicas (CSIC), Avda. Gregorio del Amo 8, Ciudad Universitaria, 28040 Madrid, Spain

Received 24 February 1995; accepted 22 October 1995

Abstract The distribution equilibria of mineral acids: H2SO 4, H3PO 4, HC1, HCIO4 and HNO 3 between aqueous solutions and organic solutions of the phosphine oxide Cyanex 923 in toluene or decane are described. Partition studies have shown that the organic diluent only slightly influences the acid extraction. The extraction mechanism can be related to the solvation of the acid and formation of the L - HmX÷m- (m = 1, 2 or 3) species in the organic phase, where L is the extractant; only in the case of initial high HNO 3 concentrations is the formation of the L • (HNO3) 2 species apparent in this phase. The effect of temperature on the acid extraction is also evaluated.

I. I n t r o d u c t i o n It is known that high molecular weight amines are capable o f extracting acids from aqueous solutions, although in the case of neutral or solvation extractants there has not been the same amount o f information about their role in this particular field of interest and there have only been extended studies on the use of TBP (tributylphosphate) in the extraction o f acids [1-4]. It is also known that, generally speaking, solvation extractants can extract acids from solutions in the same manner that amines do; that is by sharing a pair of electrons through a donor atom. The study of these reagents is o f importance because at high acidities the extraction of the acid can compete favourably with the extraction of metals and normally tends to decrease the latter, and because these reagents can also be used to eliminate or recover acids from effluents, allowing recycling to the circuit. The introduction o f new, commercially available extractants to the solvent extraction market, such as phosphine oxides, which have the capacity to extract metals from acidic 0304-386X/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved SSDI 0 3 0 4 - 3 8 6 X S S D ! 0304-386X(95)00101-8

246

F.J. Alguacil, F.A. Lbpez / Hydrometallurgy 42 (1996) 245-255

aqueous solutions, makes it of interest to study acid extraction, to understand metal extraction further. Cyanex 923 extractant seems to be one of the most promising reagents, especially because, as a liquid, this extractant can be used without organic diluents and therefore allows high loading. The present work studied the extraction of the mineral acids: H2SO 4, HC1, HCIO 4, HNO 3 and H3PO 4, commonly found in hydrometallurgical processes, by the new extractant Cyanex 923 to obtain information about the behaviour of this reagent and to help in understanding its probable future applications in the recovery of metals a n d / o r acids from acidic aqueous solutions or effluents.

2. Experimental Cyanex 923 extractant was kindly supplied by the Spanish branch of American Cyanamid Co., its composition and main properties have been described elsewhere [5,6]. The extractant was used without purification. All other reagents were of A.R. grade. Equilibrium experiments were conducted by contacting the corresponding aqueous and organic phases in separatory funnels thermostatically maintained at the required temperature and provided with mechanical shaking. Contact time was in all cases 10 min, enough to achieve equilibrium, and an organic/aqueous volume ratio of 1 was employed. Aqueous acid concentration was determined by titration of the phase with bromothymol blue as indicator, whereas organic acid concentration was determined by direct titration in ethanol media of the corresponding phase, also using bromothymol blue as indicator. In all cases standard sodium hydroxide solutions were used as titrator reagent. IR measurements were carried out in a Nicolet Magna 550 spectrometer using CsI windows.

3. Results and discussion 3.1. Extraction o f mineral acids

The extraction of mineral acids was studied under different acid or extractant concentrations. The results obtained are shown in Figs. 1-5 and in Tables 1-4. In the case of sulphuric acid, extraction of acid concentrations higher than 3 M is not possible using decane as diluent of the organic phase because a third phase is formed. Extraction of perchloric acid was only studied using an aromatic diluent, because the use of an aliphatic one, such as decane, leads to third phase formation even at low acid a n d / o r extractant concentrations. From the results obtained, it is shown that, in general terms, the extraction of mineral acids by Cyanex 923 diluted in decane or toluene can be expressed in a simplified form by the general formula: H+~Xa'~- + Lorg ~ HXLor,

(1)

F.J. Alguacil, F.A. l~pez / Hydrometallurgy 42 (1996)245-255

~t 0 tO f~ I £3

-

247

n

03 C)

decane /

/

/ / ,~ /

r20 9917

/

& H2SO4 3M-decane

Z

/FJ

-2

r20 9994

0

-i

log [Cyanex 923] Fig. 1. Extraction of salphuric acid at different Cyane× 923 concentrations. Temperature 20°C. Initial Cyane× 923 concentrations: 0.025-0.50 mol/l.

where L represents the extractant. From this equation is obtained: log OHx = log gex t -t- log [L]org

(2)

From Figs. 1-5 the log gex t values were obtained for each extraction system and are presented in Table 5. In order to refine the extraction constant values obtained graphically, a numerical treatment was carried out using the LETAGROP-DISTR program [7]. Table 5 summa-

-0.~

a

el 0 Z "1"

o~

-i.(

J _/

~o999~

J~

-2.4

-2.0

ecane

~ HNO3 1M-toluene

-1.6

-1.2

log [Cyanex

-0.8

-0.4

923]

Fig. 2. Extraction of nitric acid at different Cyanex 923 concentrations. Temperature 20°C. Initial Cyanex 923 concentrations: 0.025-0.50 mol/l.

248

F.J. Alguacil, F.A. L6pez / Hydrometallurgy 42 (1996) 245-255 -1.0

-1.5 &

"1" c3 - 2 . 0 o e r ~ 0 , 9 9 6 4

-2.5

/

A HCl IM-toluene

A/" -3.0

,

-2.0

r2.0.9864

t , t , J -1.6 -1.2 -0.8 tog [Cyanex 9 2 3 ]

,

,

-0.4

Fig. 3. Extraction of hydrochloric acid at different Cyanex 923 concentrations. Temperature 20°C. Initial Cyanex 923 concentrations: 0.025-0.50 mol/l. rizes the results of the numerical calculations; results fitted well with those obtained graphically and also with the proposed stoichiometries. In the case of nitric acid, at high initial acid concentrations and when toluene is used as diluent (Table 2), the molar ratio [HNO3]/[Cyanex 923] generally had a value very close to 2; this behaviour may be explained by the extraction of additional acid and formation of the double species according to the reaction: (3)

H ~ + NO~- + HNO3Lorg ~ HNO 3 • HNO3Lorg

-0.4 -0.6 -0.8 -I.0 0 I C13

-1.2

~ o

-1.4 -1.6 -1.8 -20

5

i

.

,

.

*

.

i

.

e

.

*

-1.8 -1.6 -1.4 -1.2 -•.Ù - 0 8

.

*

-0.6

log [Cyanex 923]

Fig. 4. Extraction of perchloric acid at different Cyanex 923 concentrations. Temperature 20°C. Dotted line shows 95% confidence interval. Initial Cyanex 923 concentrations:0.025-0.50 mol/1.

F.J. Alguacil, F.A. L6pez/ HydrometaUurgy 42 (1996) 245-255

249

-10

-1.5 0 &

•"r

-2.0

D o

M

-2.5 •

aecmne

Q

~olumne

Hmm04 6M ~JocRna

toluene

-3.0

i

i

-20

i

,

I

-I 6 -1.2 - 0 8 -0.4 log [ C y a n e x 9 2 3 ]

i

O0

Fig. 5. Extraction of phosphoric acid at different Cyanex 923 concentrations. Temperature 20°C. Initial Cyanex 923 concentrations: 0.025-0.50 m o l / l . 0.1 M H3PO 4 average r2: 0.999; 1 M H3PO 4 average r2: 0.999; 6 M H3PO 4 average r2: 0.998.

Table 1 Sulphuric acid extraction by Cyanex 923 at high initial acid concentrations [Cyanex 923]i,maI (mol/I)

[H2SO4]aq (g/l)

[H2 SO4]org (g/l)

DH2SO4

[H2SO4]/[C923] org. phase

0.025 0.06 0.13 0.25 0.50

977.26 972.06 963.63 949.91 917.38

2.77 7.99 16.37 30.09 62.67

2.83.10 -3 8.22.10 -3 1.70.10 -3 3.17.10 -2 6.83.10 -2

1.13 1.36 1.28 1.23 1.28

Organic phase diluent: toluene. Initial aqueous phase: H 2 S O

4

980 g / l . Temperature 20°C.

Table 2 Nitric acid extraction by Cyanex 923 at high initial acid concentrations [C923]init (mol/l)

[HNO3]init (g/l)

[HNO3]aq (g/l)

[HNO3]org (g/l)

DHNO3

[HNO3]//[C923] org. phase

0.025 a 0.06 a 0.13 a 0.25 a 0.50 a 0.025 t, 0.06 b 0.13 b 0.25 b 0.50 b

378 378 378 378 378 630 630 630 630 630

376.24 373.78 369.62 361.18 344.30 626.28 619.61 612.17 600.77 572.17

1.74 4.22 8.38 16.82 33.71 3.71 10.40 17.83 29.23 57.83

4.63' 10 -3 1.13' 10 -2 2.27- 10 -2 4.66" 10 -2 9.79" 10 -2 5.92" 10 -3 1.68- 10 -2 2.91 " 10 -2 4.87- 10 -2 1.01 • 1 0 - t

1.10 1.12 1.02 1.07 1.07 2.36 2.75 2.18 1.86 1.84

Organic phase diluent: a decane, b toluene. Temperature 20°C.

F.J. Alguacil, F.A. L6pez/ Hydrometallurgy 42 (1996) 245-255

250

Table 3 Hydrochloric acid extraction by Cyanex 923 at high initial acid concentrations [C923]ini t (tool/l)

[HCl]init (g/l)

[HCI]aq (g/l)

[HC1]org (g/l)

DHCI

[HC1]/[C923] org. phase

0.06 a 0.13 a 0.25 a 0.50 a 0.025 b 0.06 b 0.13 b 0.25 b 0.50 b

219 219 219 219 365 365 365 365 365

217.10 215.24 211.41 203.93 363.65 361.31 358.21 352.41 340.0

1.90 3.76 7.59 15.07 1.35 3.69 6.79 12.59 25.0

8.75" 10-3 1.75" 10 -2 3.59" 10 -2 7.39' 10 -2 3.71" 10- 3 1.02" 10-2 1.90" 10 -2 3.57" 10 -2 7.35" 10 -2

0.87 0.87 0.79 0.83 1.48 1.69 1.43 1.38 1.37

Organic phase diluent: a decane, b toluene. Temperature 20°C.

Table 4 Perchloric acid extraction by Cyanex 923 at high initial acid concentrations [Cyanex 923]init (mol/l)

[HClO4]aq (g/I)

[HClO4]org (g/I)

DH¢IO4

[HCIO4]/[C923] org. phase

0.025 0.06 0.13 0.25 0.50

902.19 899.07 892.74 881.99 859.68

2.31 5.43 11.76 22.51 44.82

0.0026 0.006 0.0132 0.0255 0.0521

0.92 0.90 0.90 0.90 0.89

Initial aqueous phase: HCIO4 904.5 g / L . Temperature 20°C

Table 5 Results obtained from the graphical and numerical treatment Acid

Diluent

log Kext graphical

log Kext numerical

U

tr(log Kext)

H2SO4 1 M H2SO4 3 M H2SO 4 1 M HNO 3 1 M HNO 3 1 M HCI i M HCI 1 M HCIO4 1 M H3PO 4 0.1 M H3PO 4 1 M HaPO 4 6 M H3PO 4 0.1 M H3PO 4 1 M H3PO 4 6 M

decane decane toluene decane toluene decane toluene toluene decane decane decane toluene toluene toluene

-0.8198 -0.5413 - 1.2321 0.4133 0.6712 -0.8755 - 0.9369 0.1915 -0.9574 -0.8986 -0.5287 -0.9556 -0.9159 -0.5394

-1.58-t-0.18 -2.314-0.17 - 1.664-0.21 0.62 4- 0.0001 0.93 + 0.07 -0.95+0.09 - 1.00 4- 0.11 0.124-0.15 -0.79MAX-0.41 -0.884-0.02 -0.564-0.04 -0.864-0.02 -0.914-0.03 -0.594-0.05

0.053 0.029 0.085 0.0002 0.0005 0.008 0.023 0.012 0.81 0.00096 0.00059 0.010 0.0012 0.00089

0.0781 0.0566 0.0707 2.07.10- 5 0.0245 0.0292 0.0374 0.0485 0.2024 0.0077 0.0137 0.0073 0.0086 0.0161

log Kext values at 20°C. U = sum of squares; cr = standard deviation.

F.J. Alguacil, F.A. LOpez/ Hydrometallurgy 42 (1996)245-255

251

Table 6 IR characteristics spectral data for Cyanex 923 and acid-loaded organic solutions of Cyanex 923 in toluene R 3PO

H 2SO4

2960-2860

3434 2960-2860

HNO3 2960-2860 2410 1637

1605

HCI 3477 2960-2860

HCIO4

H 3PO4

2960-2860

2960-2860

1157

1156

1605

1463 1420-1393 1365 1310 1164

1163 1119

1164

1100-1000 1098 1092 1050-900 1050-900 959 958 907 900 835 809 641 612

Probable assignment v O-H asym. and sym. C-H def. P-O-H v a-NO 2 O-H asym. CH 3 sym. CH 3 VI NO2 sym. P-CH 3 stretching P=O 1:3 SO2deg. P-O (PO43- ) v 3 C104 stretching deg. SO,2stretching deg. SO42stretching sym. SO42stretching P-O v 3 PO3stretching P-O v4 CIO~rocking N-O stretching P-C 8 NO2 v4 SO2-

asym = asymmetrical; sym = symmetrical; def = deformation; deg = degenerate. Wave numbers in cm- 1.

Possibly, this reaction m a y be due to the fact that the first species further extracts another acid m o l e c u l e through H b o n d i n g , b e h a v i o u r s o m e t i m e s found in a m i n e - a c i d extraction systems a n d T B P [8,9].

3.2. Infrared spectra

The infrared spectra were m e a s u r e d from the organic phases obtained in the extraction of 1 M acid solutions by C y a n e x 923, 20% v / v in toluene at 20°C. Table 6 gives the wave n u m b e r s and probable a s s i g n m e n t s for the different C y a n e x 9 2 3 - a c i d extraction systems; the bands c o r r e s p o n d i n g to an u n l o a d e d organic phase o f C y a n e x 923 are also given [ 1 0 - 1 2 ] . The spectrum o f an organic phase of C y a n e x 923 20% v / v in toluene showed the following bands: o n e peak at 1164 c m -1, that can be attributed to the stretching vibration of the P = O b a n d o f the p h o s p h i n e oxide; one weak b a n d near 1310 c m - ~ , that can be assigned to the 8 s y m m e t r i c m o d e of the P - C H 3 group; and a b a n d at 809 c m -~ , due to the stretching vibration o f the P - C b o n d in the p h o s p h i n e oxide. Other bands in the spectrum appeared at 2956 a n d 2857 c m - i and are assigned to the C - H stretching

252

F.J. Alguacil, F.A. Lrpez / Hydrometallurgy 42 (1996) 245-255

vibration of the alkyl chains associated with the P = O group; one peak at 1463 cm - t is assigned to the ~ asymmetrical CH 3 vibration and two weak bands at 1420 and 1393 cm-1 correspond to the ~r symmetrical mode of the CH 3 group. In the case of the organic phase loaded with sulphuric acid, the band which appears at 1163 cm -1 can be assigned to the stretching vibration of the P = O bond of the phosphine oxide. There is no appreciable displacement with the wave number obtained with the free Cyanex 923, which indicates that there is no strong interaction between this phosphine oxide and the extracted sulphuric acid. In this spectrum the bands assigned to the vibrations of the sulphate ion are: 1119 cm -l (weak), 1092 cm -1 (strong) and 1050-900 cm -1 (two weak bands); the probable assignment of these bands are respectively: the band at 1119 cm-1 to the v 3 vibration mode of the sulphate group, the band at 1092 cm -1 and one of the bands in the 1050-900 cm -l range to the S - O degenerate stretching and the other band to the S - O symmetrical stretching. Another band at 612 cm - l could be assigned to the v4 vibration mode of the sulphate group. These results seem to confirm that, in the extracted species, the sulphate group maintained the point group T d symmetry. The spectrum also showed two low intensity bands at 3434 cm-1 (weak and broad) and 1605 cm -1 (weak) that are also indicative that there is a certain degree of solvation of the H2SO4L species by water molecules. The spectrum of the organic phase loaded with nitric acid showed these characteristics: the band at 1164 cm - l almost disappears, indicative of the substitution of the P = O bond by another type of phosphorous-oxygen bond. This conclusion is supported by the appearance in the spectrum of peaks at 2410 c m - l (broad and weak), 959 cm -1 and 907 c m - l ; the probable assignment of these peaks is that the first is due to the deformation vibration of the P - O - H bond and the other two are due to the stretching mode of the same group and to the stretching vibration of the P - O bond, respectively. In the case of the NO 3 group, the spectrum showed very clearly the peaks attributable to the O - N O 2 (nitrate) group. Thus, the bands at 1637 (weak) and 1365 cm -1 (strong) are due to the asymmetric and symmetric (v 5 and v l) stretching vibrations of the - N O 2 group, whereas the band at 835 cm_ l (medium) can be assigned to the non-planar rocking mode of the N - O group and the band at 641 cm - l (medium) to the ~ NO 2 vibration. The pattern of wave numbers obtained showed that the nitrate group is present in the organic phase as a covalent group (lower symmetry, point group C2v symmetry) rather than the nitrate ion (point group D3h symmetry). The spectrum does not clearly show bands that can be attributed to the presence of water in the organic phase, which indicates that solvation by water is less than in the case of the extraction of sulphuric acid. From the results obtained, it seems that although, the stoichiometry of the species extracted by Cyanex 923-nitric acid at low aqueous acid concentrations can be generally represented by HNO3L, the structure R3POHONO 2 seems to be more adequate, according to the IR results. For organic phases of Cyanex 923 in toluene loaded with hydrochloric acid the corresponding spectrum showed the band attributable to the P = O bond of the phosphine oxide unaltered at 1164 cm -1, indicating that there is no strong interaction between this

F.J. Alguacil, F.A. Lrpez / Hydrometallurgy 42 (1996) 245-255

253

monoprotic acid and the phosphine oxide. Bands at about 3477 cm-1 (weak and broad) and 1605 cm - l (weak) are due to the probable presence of water in the extracted species. These results indicate a certain degree of solvation by water of the HC1L species. In the case of the extraction of perchloric acid, the spectrum of loaded organic phases showed a peak at 1157 cm - l (shoulder) and a strong band at 1098 cm - I . The small displacement of the band attributable to the P = O group of the phosphine oxide to lower values indicates a small interaction between the phosphine oxide and the perchloric acid. The strong intensity of the band at 1098 cm - l should be due to the coincidence of this band with that due to the v 3 vibration mode of the CIO4 group that appeared in the 1200-950 cm -1 range. A band near 900 cm -1 should also be attributable to this group (v 4 vibration mode). The spectrum does not show any band that should be assigned to the presence of water in the organic phase, thus the extracted species HCIO4L is not solvated by water molecules. Finally, the spectrum of an organic phase loaded with phosphoric acid showed a medium intensity band at 1156 c m - I ; this peak should be assigned to the P = O bond of the phosphine oxide and the displacement indicates a certain degree of interaction between the P = O of the phosphine group and the hydrogen of the acid. The bands that can be attributed to the phosphate group appeared at 1100-1000 cm-~ ( P - O degenerate stretching) and 958 cm -1 (v I vibration mode of the PO43- group). These bands represent the presence of the phosphate ion (point group Td symmetry) in the organic phase. In this case the presence of water in the organic phase is not probable, due to the lack in the spectrum of the bands attributable to this molecule; this also indicates that the species extracted by phosphoric acid-Cyanex 923 are not solvated by water molecules.

-0.4 -"

A

A o H~S04

-08

HNO 3 ¢ H3PO 4 o HCl

,~ -1.2 0

-1.6

-2.0 1

29

3.0

I

I

I

31 3.2 33 I O 0 0 / T , K ~1

I

34

35

Fig. 6. Arrheniusplot for the extraction of mineral acids by Cyanex923 in decane. Organicphase: Cyanex923

10% v/v in decane. Aqueous phase: 1 M acid, except 3 M H2SO4.

254

F.J. Alguacil, F.A. L6pez/ Hydrometallurgy42 (1996) 245-255 O0 --~

"--

&

A

-0.5,

J.

• . • •

Hzso ~ HNO~ H~PO~ HCl

-10 0

-15

-2.03 0

3al

3 '2 3 3' 3 4~ 1000/T, K-1

3 ~.5

3.6

Fig. 7. Arrhenius plot for the extraction of mineral acids by Cyanex 923 in toluene. Organic phase: Cyanex 923 20% v / v in toluene. Aqueous phase: 1 M acid.

3.3. The influence of temperature T h e extraction o f different acid solutions with C y a n e x 923 10% or 20% v / v in d e c a n e or t o l u e n e was carried out at temperatures b e t w e e n 10°C and 70°C d e p e n d i n g on the organic diluent. Results are g i v e n in Figs. 6 and 7; in the case o f H 2 S O 4, HC1, H C 1 0 4 and H a P O 4 a q u e o u s solutions, increasing t e m p e r a t u r e d e c r e a s e d the extraction o f the acid by C y a n e x 923 and only in the case o f nitric acid solutions is the acid extraction slightly i n c r e a s e d by an increase in temperature. In all the systems the b e h a v i o u r is i n d e p e n d e n t o f the organic diluent. Table 7 s h o w s the c h a n g e in enthalpy (the heat o f reaction) obtained for each system. Only the extraction o f nitric acid s h o w e d an e n d o t h e r m i c character. The values o f A G ° and A S ° at 20°C w e r e also calculated (Table 7).

Table 7 Values of AH°, AG°(20°C) and AS°(20°C) for the extraction of mineral acids by Cyanex 923 System

A H°

AG°

AS°

C923-decane-H2SO 4 3 M C923-decane-HNO 3 1 M C923-decane-HCl 1 M C923-decane-H3PO 4 1 M C923-toluene-H2SO4 1 M C923-toluene-HNO 3 1 M C923-toluene-HCi 1 M C923-toluene-HCIO4 I M C923-toluene-H3PO 4 1 M

-7.05 0.65 - 13.52 -21.90 6.28 0.61 13.71 0 21.33

12.89 - 3.46 5.30 4.91 9.26 -5.19 5.58 - 0.67 5.08

-0.068 0.014 -0.064 -0.092 - 0.053 -0.016 -0.066 0.002 - 0.090

Values are given in kJ/mol.

-

-

-

F.J. AIguacil, F.A. L6pez /Hydrometallurgy 42 (1996) 245-255

255

4. Conclusions Cyanex 923 may be used in the extraction of mineral acids from aqueous solutions. The great advantage of this extractant is that it is a liquid and thus its concentration can easily be adapted to any particular acid extraction system. The organic diluent does not seem to influence the acid extraction, although the use of aliphatic diluents should be avoided with H 2 SO 4 concentrations higher than 3 M and with HC104 due to third phase formation. The extraction o f the mineral acids by Cyanex 923 can be represented by the general reaction: rnH~q + Xaq- + Lorg ~ HXLorg The exception to this rule is nitric acid; IR measurements showed that, in this case, the extracted species are better represented by the general stoichiometry R3POHONO2; also, and only in the case of nitric acid, the extraction of an additional mole o f acid at high initial H N O 3 concentrations was detected. Increasing temperature decreases the extraction o f the mineral acids except in the case of nitric acid, extraction o f which slightly increases with temperature. Values of log Kext, A H °, AG ° and A S ° were also obtained.

Acknowledgements Dr. A. Sastre is thanked for her advice about some of the numerical calculations.

References [1] Ritcey, G.M. and Ashbrook, A.W., Solvent Extraction, Part I. Elsevier, Amsterdam (1984), pp. 87-171. [2] Cox, M., In: J. Rydberg, C. Musikas and G.R. Choppin (Editors), Principles and Practices of Solvent Extraction. Marcel Dekker, New York (1992), pp. 357-412. [3] Eyal, A.M., Arbel-Hadad, M., Hadi, S., Canari, R., Haringman, A. and Hazan, B., ln: D.H. Logsdail and M.J. Slater (Editors), Solvent Extraction in the Process Industries, Vol. 2. Elsevier Applied Science, London (1993), pp. 723-730. [4] Rickelton, W.A., In: D.H. Logsdail and M.J. Slater (Editors), Solvent Extraction in the Process Industries, Vol. 2. Elsevier Applied Science, London (1993), pp. 731-736. [5] American Cyanamid Co., Cyanex 923 extractant technical bulletin. Wayne (1991). [6] Alguacil, F.J., Caravaca, C., Martinez, S. and Cobo, A., Hydrometallurgy, 36 (1994): 369-384. [7] Liem, D.H., Acta Chem. Scand., 25 (1971): 1521-1534. [8] Rydberg, J. and Sekine, T., In: J. Rydberg, C. Musikas and G.R. Choppin (Editors), Principles and Practices of Solvent Extraction. Marcel Dekker, New York (1992), pp. 101-156. [9] Alguacil, F.J. and Caravaca, C., Hydrometallurgy, 34 (1993): 91-98. [10] Simmon, W. and Clerc, T., Strukturaufld~'ung Organischer Verbindungen mit SpektroskopischenMethoden. Akademische Verlagsgesellschaft, Frankfurt (1970). [11] Prcstsch, E., Clerc, T., Seibl, J. and Simon, W., Tabellen zur Strukturaufkl'firungOrganischer Verbindungen mit Spektroskopischen Methoden. Springer, Berlin (1976). [12] Nakamoto, K., Infrared and Raman Spectra of Inorganic and Coordination Compounds. Wiley, New York, 3rd ed. (1978).