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Extraction of chromium(III) from sodium chloride solutions by means of carboxylic acids
Hydrometallurgy, 15 (1985) 191--202 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
191
EXTRACTION OF CHROMIUM(III) FROM SODIUM CHLORIDE SOLUTIONS BY MEANS OF CARBOXYLIC ACIDS
WtEb-~W APOSTOLUK and ADAM BARTECKI Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University of Wrocfaw, Wybrze2e Wyspia6skiego 27, 50-370 WrocJaw (Poland) (Received December 7, 1984; accepted in revised form July 7, 1985)
ABSTRACT Apostoluk, W. and Bartecki, A., 1985. Extraction of chromium(III) from sodium chloride solutions by means of carboxylic acid. Hydrometallurgy, 15: 191--202, Studies of Cr(III) extraction with carboxylic acids showed that the extraction process takes place at a pH of the aqueous phase ranging from 4 to 5. It was shown that sodium chloride is active with respect to chromium(III) as a salting-out agent. For extraction of Cr(III) with hexanoic acid it was shown that in the organic phase trinuclear complexes of the [Cr(OH)R2.HR ] 3 formula are formed.
INTRODUCTION T h e r e are o n l y a f e w p a p e r s o n e x t r a c t i o n o f C r ( I I I ) b y m e a n s o f c a r b o x y l i c acids. T h e t o p i c was m e n t i o n e d f o r t h e first t i m e in t h e 1 9 6 0 s a n d t h e n t h e l o c a t i o n o f c h r o m i u m ( I I I ) in t h e so-called selectivity series was discussed. As an e x a m p l e , A l e k p e r o v et al. [1] p l a c e d c h r o m i u m in t h e selectivity series o f m e t a l ion e x t r a c t i o n w i t h n a p h t h e n i c acids b e t w e e n B e ( I I ) a n d M n ( I I ) . G i n d i n et al. [2] e x t r a c t e d C r ( I I I ) w i t h a m i x t u r e o f C~--C9 c a r b o x y l i c acids f r o m 2 M NaC104 s o l u t i o n o v e r a p H range o f 2.5 to 3, suggesting t h a t in t h e organic p h a s e a m o n o m e r i c c o m p l e x o f an i n d e f i n i t e f o r m u l a is f o r m e d . In o u r p r e v i o u s p a p e r [3] we have f o u n d t h a t t h e extract i o n p r o c e s s o f C r ( I I I ) f r o m NH4C1 s o l u t i o n s b y m e a n s o f h e x a n o i c and d e c a n o i c acid s o l u t i o n s in c a r b o n t e t r a c h l o r i d e o c c u r s o v e r a p H r a n g e o f t h e a q u e o u s p h a s e o f 4 t o 5, highly p o l y m e r i z e d c o m p l e x e s being f o r m e d in t h e organic phase. We have n o t s u c c e e d e d in establishing e x p l i c i t l y e i t h e r t h e c o m p o s i t i o n o f t h e s e c o m p l e x e s or t h e i r degree o f p o l y m e r i z a t i o n . T h e basic r e a s o n f o r t h e lack o f a n y q u a n t i t a t i v e d a t a o n C r ( I I I ) e x t r a c t i o n w i t h c a r b o x y l i c acids is e x p e r i m e n t a l d i f f i c u l t y , n a m e l y t h e e q u i l i b r i u m s t a t e in t h e e x t r a c t i o n s y s t e m is r e a c h e d v e r y slowly - in m o s t cases a f t e r several t e n s o f h o u r s o f p h a s e c o n t a c t i n g .
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192 This paper is an a t t e m p t to investigate in more detail than previously the extraction equilibria of Cr(III) with carboxylic acids and to interpret them more thoroughly. EXPERIMENTAL R eagen ts
Cr(III) solutions used as the initial aqueous phase were prepared from CrC13.6H20, NaC1 and NaOH. All these reagents made by POCh (Poland) were analytically pure. Organic phases were prepared by dissolving a suitable carboxylic acid in CC14 (POCh), analytical grade. The following carboxylic acids were used: 2-methylpropanoic acid, (pure 99%), BDH Chemicals; hexanoic acid, {pure 99%), Fluka AG; heptanoic acid, {pure), Loba-Chemie; octanoic acid, {pure), International Enzymes Ltd.; nonanoic acid, (pure), BDH Chemicals; dodecanoic acid, (pure 99%), International Enzymes Ltd.; hexadecanoic acid, {pure), Reachim; 2,2-dimethylpropanoic acid, (pure 98%), Fluka AG. Procedure
All Cr(III) solutions were prepared 6 months before examination. The electronic spectra of these solutions are characterized by the presence of two bands located at a b o u t 410 and 570 nm respectively, which correspond to the violet Cr(H20)] ÷ complex as the main c o m p o n e n t of the aqueous phase. Extraction was carried out in 250 ml flasks at the volume ratio V o / V a = 1 at 293 -+1 K. The flasks were shaken in a thermostatted Elpan 357 shaker. The time required to reach the equilibrium state was 48 h. The pH of the aqueous phase was corrected by adding the necessary amount of a dilute NaOH solution while the ionic strength was adjusted by means of NaC1. After shaking the flasks were left for several hours. The contents of flasks were then transferred into separatory funnels and the phases separated. The aqueous phase was filtered and pH and chromium concentration were determined. Analytical methods
Carboxylic acid concentration in the organic phases was determined alkalimetrically using phenolphthalein as an indicator. Chromium concentrations in the aqueous phases were determined spectrophotometrically by means of diphenylcarbazide [4]. Chromium concentrations in the organic phases were calculated on the basis of the metal balance in the system, pH measurements in the aqueous phases were performed in a M e r a - E l m a t N512 pHmeter provided with a combined SAgP electrode and calibrated by means of verified pH standards. All results presented below are mean values obtained from measurements and analyses performed for the two samples prepared in the same way.
193
Theoretical considerations In our previous work [3] we have shown that effective extraction of Cr(III) from NH4C1 solutions with carboxylic acids takes place over the pH limits of the aqueous phase ranging from 4 to 5 while the values of the partition coefficient D lie between 0.1 and 10. Over this pH range the Cr(III) ions in dilute solutions undergo hydrolysis to a considerable extent. For example, 10 -s molal Cr(III) solution at pH = 4.5 and unit ionic strength (NaC104) contains at 298 K about 30% of the Cr(H2 O)~ ÷ species, 30% of the Cr(OH) 2+ species, 30% of Cr3(OH)~ + and 5% of Cr2(OH)~ + [5]. For this reason one cannot neglect the effect of hydrolysis on the extraction process of Cr(III) and rule out the formation of Cr(III) hydroxy- or oxycomplexes with carboxylic acids in the organic phase. It should be emphasized that the metal hydroxy- or oxycomplexes with carboxylic acids may be formed in the organic phase when metal aquocomplex and water participate in the extraction process [6,7,8] and if the hydrolyzed metal ion species occurring in the aqueous phase participate in the extraction process [9]. Having the above in mind, one can assume that extraction of Cr(III) with carboxylic acids takes place according to one of the following three equations j
Cr~a) + j(3
+ x ) / 2 (HR)~(o) ~- [CrR3- xHR]j(o) + 3j H ÷
{1)
j Cr~a) + j ( 3 - m + x)/2 (HR)2(o) + m H20 ~- [Cr(OH)mR3_rn'XHR]j(o)
+ 3j H +
(2)
j Cr(OH)~(a~ )+ + j ( 3 - m + x)/2 (HR)2(o) ~ [Cr(OH)mR3_m'xHR]j(o ) +/(3-m)
H+
(3)
where: HR - - m o n o m e r i c carboxylic acid molecule, j --degree of complex polymerization in the organic phase, x - number of monomeric acid molecules attached to the metal ion in a complex in the organic phase. The extraction equilibria given by eqns. (1)~ (2) and (3) may be described by the equation logD = l o g K + ( j - 1 )
lOgCM(a) + a p H + b l o g [ ( H R ) ] 2 ( o ) - j l o g a
M
(4)
where: log K = log Kex + l o g j - 1/2b log 2, C M ( a ) - - equilibrium metal concentration in the aqueous phase, a -- s'toichiometric coefficient equal to the number of hydrogen ions involved in reactions (1), (2) and {3), b -- stoichiometric coefficient equal to the number of carboxylic acid involved in reactions (1), (2) and (3), aM -- complexation function (side reaction coefficient) of metal in the aqueous phase, resulting in the formation of ionic species which do not participate in the extraction process. It is evident t h a t the plot of log D vs. pH for the extraction of polymeric metal complex shows considerable curvature, but when hydrolysis of the extracted metal occurs in the aqueous phase then the curve becomes flatter.
194 In many cases, for experimental D values in the range of 0.1 < D < 10 the data may be represented by a straight line. The degree of polymerization of a metal complex in the organic phase may be determined by the graphical m e t h o d using, for instance, a single equilibrium model [11]. This model was successfully used by us for interpretation of the equilibria of metal extraction with carboxylic acids in previous works [ 10,11 ]. In order to investigate the composition of the Cr(III) complex in the organic phase and to find out which chemical equation applies to the extraction process, one can use a m e t h o d developed by Jaycock and Jones [12]. According to that m e t h o d eqn. (4) is expressed in linear form Y = pX + C
(5)
where: Y = l o g D X = (j - 1) log CM(a) + a pH + b log [(HR)2](o) C = log K - j log a M and then the slope of a straight line given by eqn. (5) is determined. The coefficient p for the best matched coefficients of eqn. (4) should be as close to unity as possible. The simplest case occurs if the value of the side reaction coefficient aM = 1 since this enables us to determine directly the extraction constant Kex. In other cases Kex may be determined only if the value of aM is known. DISCUSSION E f f e c t o f the p h a s e c o n t a c t t i m e on e x t r a c t i o n o f Cr(III)
Some earlier experiments [3] showed that the contact time between the aqueous and organic phase significantly affects the extraction process of Cr(III). In the present work it was assumed that the phases should be contacted continuously for a suitable period of time. In all tests performed, after the required pH correction, turbidity of the aqueous phase was observed. However, at the equilibrium state neither phase exhibited turbidity and the organic phase was blue-violet. Figure 1 shows how Cr(III) is extracted from the aqueous phase containing 0.0109 M CrC13 with 0.104 M hexanoic acid solution depending on the phase contact time. As shown, the respective extraction curves presented in the log D--pH coordinate system are straight lines whose slope increases with prolongation of the phase contact time. The relationship logD = f (pH) obtained for t = 48 h is a straight line with slope equal to 2.62 and it seems that it corresponds to the equilibrium state since any prolongation of the phase contact time does not affect the slopes of extraction curves. Therefore, in all other studies the phase contact time was 48 h. The effect observed may be related to a non-equilibrium extraction in which highly h y d r o l y z e d Cr(III) species participate and thus the equilibrium
195
~
log D
1.6
~
log D - -
/
1.6
./ 0.8
0.8
0~
0.4
0.0
0.0 i
-0.4
~ I
~,.o
-0.~ I
L,.s
I
s.o
~H
~.o
z,.s
s.o pH
Fig. 1. D e p e n d e n c e o f c h r o m i u m ( I I I ) e x t r a c t i o n o n t h e p h a s e c o n t a c t t i m e . c o t ( t o t a l ) = 1.09 m M ; CNaC1 = 0 . 1 0 M ; [ H R 2 ] ( o ) = 0 . 1 0 4 M; c o n t a c t t i m e : [] - - 24 h, o - - 36, • - - 48 h. Fig. 2. I n f l u e n c e o f s o d i u m c h l o r i d e c o n c e n t r a t i o n o n c h r o m i u m ( I I I ) e x t r a c t i o n b y m e a n s o f h e x a n o i c acid. c c r ( t o t a l ) = 1 . 0 9 raM; [(HR)~ ](o) = 0 . 1 0 4 M; CNaC1 : O - 0 . 1 0 M, • - - 0 . 5 0 M, <>-- 1 . 0 0 M, • - - 3 . 0 0 M.
state is reached after the long period of time as a consequence of some secondary reactions (e.g. elimination of OH groups by carboxylate ions).
Effect o f NaCl concentration on extraction o f Cr(III) with hexanoic acid The effect of NaC1 concentration in the aqueous phase on extraction of Cr(III) was investigated by using a 0.104 M hexanoic acid solution in carbon tetrachloride. The initial CrC13 concentration in the aqueous phase was 1.09 mM. Four Cr(III) extraction series from aqueous phases of various NaC1 concentrations equal to 0.10, 0.50, 1.0 and 3.0 M, respectively, were performed. The results obtained are shown in Fig. 2 as a plot log D vs. pH. The relationships log D = f (pH) have a linear character and, as shown, with the increasing NaC1 concentration in the aqueous phase the extraction of Cr(III) increases too. An increase in the Cr(III) extraction may be explained as due to the salting-out effect of sodium chloride with respect to the chromium complexes formed. As a rule, the complexation by the chloride ions decreases
196
metal extraction b y means of carboxylic acids. In this case, it is possible that complexation of Cr(III) by the chloride ions positively affects the extraction process (e.g. CrC12÷ complex takes part in the extraction). In all other experiments in the aqueous phase a constant NaC1 concentration of 0.10 M was maintained.
Effect of initial CrCl3 concentration in the aqueous phase on extraction of chromium (III) with hexanoic acid Studies on the effect of initial CrC13 concentration on the extraction process of Cr(III) with hexanoic acid were performed in order to determine the degree of polymerization of complexes formed in the organic phase. Hexanoic acid concentration in the organic phase was 0.104 M. Initial CrC13 concentration in the aqueous phase was varied from 0.218 mM to 10.9 mM.
tog D
I
I
I
2.0
1.6
1.2
0.8
~
0.~
0.0
-O.t,
]
I
G.O
~.5
510
pH
Fig. 3. E f f e c t o f initial CrC13 c o n c e n t r a t i o n in t h e a q u e o u s p h a s e o n c h r o m i u m ( I I I ) ext r a c t i o n w i t h h e x a n o i c acid. CNaC1 = 0 . 1 0 M; [ ( H R ) 2 ] (o) = 0 . 1 0 4 M; ccr (totaD: * - - 0 . 2 1 8 raM, ~ - - 0 . 5 4 5 raM, • - - 1.09 raM, o - - 5 . 4 5 raM, • - - i 0 . 9 raM.
197 p H o s - -
'
'
,
--
,
]+.6
~..2 ~_
I_ 3.5
I
1
3.0
2.5
I
2.0 1.0~ C Cr (totat }
Fig. 4. pH0. s vs. log cot(total ).
The results obtained are presented in Fig. 3 as a plot of log D vs. pH. These relationships over the entire pH range under investigation and for all initial CrC13 concentrations are linear with slopes slightly lower than 3 (2.50-2.80). Evident dependence of Cr(III) extraction on the initial CrC13 concentration in the aqueous phase indicates that in the organic phase polynuclear chromium(III) complexes are formed. The pH0.s values were determined in order to plot the function pH0.s = f [log CCr(total)] presented in Fig. 4. This plot is a straight line with a slope of -0.222. The single equilibrium model was used to find the value o f j = 2.99 for n = 3 which indicates that in the organic phase trinuclear chromium(III) complexes with hexanoic acid Occur.
Effect o f hexanoic acid concentration on extraction o f Cr(III) Studies on the effect of extraction agent concentration on metal extraction enables us to determine the composition of the complex formed in the organic phase. This effect was investigated by making a series of chromium(III) extractions with hexanoic acid solutions in CC14 for four different extractant concentrations: 0.1; 0.4; 0.8 and 1.0 M NaC1 concentration and initial CrC13 concentration in the aqueous phase were constant and were 0.10 M and 5.45 mM, respectively. The results obtained are presented in Fig. 5. As shown, in this case these relationships are also linear with the slope slightly lower than 3. The values of pH0.s f o u n d were used to plot the function of pH0.s vs. log [(HR)2](o) shown in Fig. 6. This plot is a straight line whose slope is -0.689. Usirig" the single equilibrium model for n = 3 the value of x is 1.14 which indicates that for further considerations one should assume the value x = 1. This means that in the trinuclear chromium(III) complex each Cr(III) ion has one molecule of the extractant, i.e. this complex corresponds to the formula [Cr.R3- HR]3.
198 i
{og D
,
I/ •
, //
2.0
1.6
1.2
0.8
o.~
II 0.0
/" ./
-o.~ I
I
I
I
3.5
L,.O
1,.5
5.0
pH
Fig. 5. E f f e c t o f h e x a n o i c acid c o n c e n t r a t i o n o n c h r o m i u m ( I I I ) , e x t r a c t i o n . CNaC1 = 0 . ] 0 M; c o t ( t o t a l ) = 5 . 4 5 raM; [ ( H R ) 2 ] ( o ) : [] - - 0 . 1 0 M, • - - 0 . 4 0 M, © - - 0 . 8 0 M, • - - 1.00 M.
pHo.s
I
I
T--
/+.5
/*.1
3.7 -1.o
Fig. 6. pH0. s vs. log [ ( H R ) 2 ] ( o ) .
I -o.5
I 0,0 Log [ ( H R ) z ]
199
The extraction of Cr(III) with hexanoic acid was assumed to proceed according to reaction (1) and in the organic phase the [CrR3" HR]3 complexes are formed. This assumption was verified by using the Jaycock and Jones method described earlier [12]. Coefficients a and b of eqn. (4) were expressed by means of the values j = 3 and x = 1 and the value o f coefficient p in eqn. (5) was calculated by the least squares method. Since this value, p = 0.47, differs considerably from 1, the assumption concerning the extraction process according to reaction (1) and composition of the complex in the organic phase was considered not to be satisfied. Further calculations were based upon eqns. (2) and (3). Calculations performed for various combinations of coefficients a and b in eqn. (4), under the assumption that extraction of Cr(III) takes place according to reaction (2), gave the values of coefficient p in eqn. (5) ranging from 0.50 to 0.60, that is, also very different from unity. Satisfactory results were obtained for calculations performed assuming that extraction of Cr(III) takes place according to reaction (3) and the values of coefficients a and b from eqn. (4) are 6 and 4.5, respectively.
Yi¸
0
2.0
0
1.6 ![
0 0
1.2[-
0
! 0.8 ~ I r
~
'•O
OJ+~-
0 0.0I"
0 O
-0.hl / "~
I 16
~L 17
]
is
_ _
X
Fig. 7. Y = f(X). CNaCl = 0.10 M; [(HR)2](o) = 0.104 M; ccr(total) = 5.45 m M , c)-- exper-
imental points.The fulllinehas been calculatedby the leastsquares method.
200 Calculations performed by the least squares m e t h o d for various extraction series gave the values of coefficient p from eqn. (5) ranging from 0.86 to 0.98. Figure 7 shows the plot of Y = f (X) obtained on the grounds of calculations made for a series of Cr(III) extractions in which the initial CrC13 concentration in the aqueous phase was 5.45 mM and the hexanoic acid concentration in the organic phase was 0.104 M. The slope of the straight line Y = p X + C determined for that series is 0.98 and the correlation coefficient r x y = 0.70. Deviations of the experimental points m a y be explained as due to the fact that in calculations the parameter C was assumed to be constant which is inaccurate since the complexation function a Cr is not constant over the pH range under investigation and thus the value of C varies slightly over this pH range. Naturally, this makes impossible any estimation of the extraction constant Kex. Having in mind that calculations performed for reaction (3) gave the values of a = 6 and b = 4.5 and that these parameters correspond to stoichiometric coefficients in reaction (3), we obtain ](3-m
j(3-m)
+ x ) / 2 = 4.5
= 6
(6) (7)
Since graphical analysis performed using the simple equilibrium model j = 3, we have the values m = 1 and x = 1. This means that in the organic phase during extraction of Cr(III) with hexanoic acid the complexes of general formula [Cr (OH)R2" HR ] 3 are formed and the extraction equaution is 3 Cr/~u~2+ ~v,,j(a) + 9/2 (HR)2(o) ~- [Cr(OH)R2- HR]3(o) + 6 H(a +)
(8)
E x t r a c t i o n o f Cr(III) with various c a r b o x y l i c acids
In order to compare the extraction properties of various carboxylic acids a series of Cr(III) extractions was performed using 0.1 M solutions of the following acids: 2-methylopropanoic, hexanoic, heptanoic, octanoic, nonanoic, dodecanoic, hexadecanoic and 2,2-dimethylpropanoic. The aqueous phase was a solution in which CrC13 and NaC1 concentrations were 1.09 mM and 0.1 M, respectively. The results obtained are presented in Fig. 8 in the log D--pH coordinate system. The extraction curves have a linear nature and their slope is about 3. All the acids examined extract chromium efficiently over the pH range 4 to 5. The longer the carbon chain of the acid molecule, the better its extraction properties for Cr(III), 2,2dimethylpropanoic acid being an exception. In spite of the fact that this acid has a short and significantly branched carbon chain in its molecule, it is the best extracting agent for Cr(III). The t e n d e n c y observed may be theoretically justified. According to Warshawsky [13] who analyzed the extraction processes of metals with various types of extracting agents, the following relationship is valid
201
tog D - - -
T
T
i
1.6
1.2
O.B
--
0./~-
0.0-
-0.~
t I
~.o
o
1
t..s
L _ _
s.o
}H
Fig. 8. C o m p a r i s o n o f t h e e x t r a c t i o n o f c h r o m i u m ( I I I ) w i t h v a r i o u s c a r b o x y l i c a c i d s . CNaC1 = 0 . 1 0 M ; o C t ( t o t a l ) -- 1 . 0 9 r a M ; [ ( H R ) ~ ](o) = 0 . 1 0 M. + - - 2 - m e t h y l p r o p a n o i c a c i d , o -- hexanoic acid, • -- heptanoie acid, 0 -- octanoic acid, • -- nonanoic acid, X dodecanoic acid, ~ -- hexadecanoic acid, A -- 2,2-dimethylpropanoic acid.
pH0.s = pKHR + log DHR -
1/n log/3n
(9)
where: pH0.s -- pH of half-extraction of a metal, KHR--dissociation constant of the extractant, DHR -- distribution coefficient of the extractant between aqueous and organic phase, ~n - overall stability constant of the MRn complexes. Since in the homologous series of n-carboxylic acids PKHR = constant while/3n varies only slightly, eqn. (9) indicates that pH0.s for a given metal depends, above all, on extractant distribution between the phases. As shown in Ref. [14], the distribution constant KD, HR of an n-carboxylic acid between the aqueous and organic phase depends on the carbon chain length in the molecule and, in addition, the increment of the distribution constant per one added methylene group is practically constant and is lOgKD,HR = 0.56 ÷ 0.60
(10)
202
Exceptions to the rule are the carboxylic acids with highly branched chains which are more effective than the corresponding acids with straight chains. This effect was observed, e.g. by Cattrall and Walsh [15] who studied the extraction process of Fe (III) with various carboxylic acids. CONCLUSIONS
1. The extraction process of Cr(III) with carboxylic acids from NaC1 solutions was found to take place over a pH range of the aqueous phase of 4 to 5. An increase in NaC1 concentration in the aqueous phase has a favourable effect on the extraction process of chromium (III). 2. The extraction process of Cr(III) with hexanoic acid takes place with the formation of a trinuclear complex [Cr(OH)R2-HR]3 in the organic phase. 3. A comparison of the results of Cr(III) extraction with various carboxylic acids confirms the general rule that the acids with long carbon chains are better extractants than those with short chains.
REFERENCES
6 7 8 9 10 11 12 13 14 15
Flett, D.S. and Jaycock, M.J., Extraction of metals by carboxylic acids, in: Marinsky, J.A. and Marcus, Y. (Eds.), Ion Exchange and Solvent Extraction, Vol. 3, Marcel Dekker, New York, 1973, Chap. 1, pp. 1--50. Gindin, L.M. et al., Izv. Sib. Otdel. AN SSSR, Ser. Khim. Nauk, 3 (1967) 195. Apostoluk, W. and Bartecki, A., Symposium Pracovniku Banskego Prumyslu, HORNICKA PRIBRAM VE VEDE A TECHNICE, Sekce N, 1981, pp. 313--330. Marczenko, Z., Spektrofotometryczne oznaczanie pierwiastk6w PWN, Warszawa, 1979, pp. 232--234. Baes, Jr, C.F. and Mesmer, R.E., The Hydrolysis of Cations, John Wiley and Sons, New York, London, Sydney, Toronto, 1976, p. 219. Bartecki, A. and Apostoluk, W., Hydrometallurgy, 5 (1980) 367. Apostoluk, W. and Bartecki, A., Mat. Sci., 3 (1980) 115. Kodama, N., Yamada, H. and Tanaka, M., J. Inorg. Nucl. Chem., 38 (1976) 2063. Berger, S.A. and Graff, S.M., J. Inorg. Nucl. Chem., 37 (1975) 1031. Bartecki, A. and Apostoluk, W., J. Inorg. Nucl. Chem., 40 (1978) 109. Bartecki, A., Apostoluk, W. and Mager, J., J. Inorg. Nucl. Chem., 41 1979) 1461. Jaycock, M.J. and Jones, A.D., Extraction of metal ions from aqueous solutions by means of carboxylic acids, Dyrssen, D., Liljenzin, J.O. and Rydberg, J. (Eds.), Solvent Extraction Chemistry, North Holland, (1967) p. 160. Warshawsky, A., Minerals Sci. Eng., 5 (1973) 36. Kojima, I., Masashi, Y. and Tanaka, M., J. Inorg. Nucl. Chem., 32 (1970) 987. Cattrall, R.W. and Walsh, M.J., J. Inorg. nucl. Chem., 36 (1974) 1643.
191
EXTRACTION OF CHROMIUM(III) FROM SODIUM CHLORIDE SOLUTIONS BY MEANS OF CARBOXYLIC ACIDS
WtEb-~W APOSTOLUK and ADAM BARTECKI Institute of Inorganic Chemistry and Metallurgy of Rare Elements, Technical University of Wrocfaw, Wybrze2e Wyspia6skiego 27, 50-370 WrocJaw (Poland) (Received December 7, 1984; accepted in revised form July 7, 1985)
ABSTRACT Apostoluk, W. and Bartecki, A., 1985. Extraction of chromium(III) from sodium chloride solutions by means of carboxylic acid. Hydrometallurgy, 15: 191--202, Studies of Cr(III) extraction with carboxylic acids showed that the extraction process takes place at a pH of the aqueous phase ranging from 4 to 5. It was shown that sodium chloride is active with respect to chromium(III) as a salting-out agent. For extraction of Cr(III) with hexanoic acid it was shown that in the organic phase trinuclear complexes of the [Cr(OH)R2.HR ] 3 formula are formed.
INTRODUCTION T h e r e are o n l y a f e w p a p e r s o n e x t r a c t i o n o f C r ( I I I ) b y m e a n s o f c a r b o x y l i c acids. T h e t o p i c was m e n t i o n e d f o r t h e first t i m e in t h e 1 9 6 0 s a n d t h e n t h e l o c a t i o n o f c h r o m i u m ( I I I ) in t h e so-called selectivity series was discussed. As an e x a m p l e , A l e k p e r o v et al. [1] p l a c e d c h r o m i u m in t h e selectivity series o f m e t a l ion e x t r a c t i o n w i t h n a p h t h e n i c acids b e t w e e n B e ( I I ) a n d M n ( I I ) . G i n d i n et al. [2] e x t r a c t e d C r ( I I I ) w i t h a m i x t u r e o f C~--C9 c a r b o x y l i c acids f r o m 2 M NaC104 s o l u t i o n o v e r a p H range o f 2.5 to 3, suggesting t h a t in t h e organic p h a s e a m o n o m e r i c c o m p l e x o f an i n d e f i n i t e f o r m u l a is f o r m e d . In o u r p r e v i o u s p a p e r [3] we have f o u n d t h a t t h e extract i o n p r o c e s s o f C r ( I I I ) f r o m NH4C1 s o l u t i o n s b y m e a n s o f h e x a n o i c and d e c a n o i c acid s o l u t i o n s in c a r b o n t e t r a c h l o r i d e o c c u r s o v e r a p H r a n g e o f t h e a q u e o u s p h a s e o f 4 t o 5, highly p o l y m e r i z e d c o m p l e x e s being f o r m e d in t h e organic phase. We have n o t s u c c e e d e d in establishing e x p l i c i t l y e i t h e r t h e c o m p o s i t i o n o f t h e s e c o m p l e x e s or t h e i r degree o f p o l y m e r i z a t i o n . T h e basic r e a s o n f o r t h e lack o f a n y q u a n t i t a t i v e d a t a o n C r ( I I I ) e x t r a c t i o n w i t h c a r b o x y l i c acids is e x p e r i m e n t a l d i f f i c u l t y , n a m e l y t h e e q u i l i b r i u m s t a t e in t h e e x t r a c t i o n s y s t e m is r e a c h e d v e r y slowly - in m o s t cases a f t e r several t e n s o f h o u r s o f p h a s e c o n t a c t i n g .
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192 This paper is an a t t e m p t to investigate in more detail than previously the extraction equilibria of Cr(III) with carboxylic acids and to interpret them more thoroughly. EXPERIMENTAL R eagen ts
Cr(III) solutions used as the initial aqueous phase were prepared from CrC13.6H20, NaC1 and NaOH. All these reagents made by POCh (Poland) were analytically pure. Organic phases were prepared by dissolving a suitable carboxylic acid in CC14 (POCh), analytical grade. The following carboxylic acids were used: 2-methylpropanoic acid, (pure 99%), BDH Chemicals; hexanoic acid, {pure 99%), Fluka AG; heptanoic acid, {pure), Loba-Chemie; octanoic acid, {pure), International Enzymes Ltd.; nonanoic acid, (pure), BDH Chemicals; dodecanoic acid, (pure 99%), International Enzymes Ltd.; hexadecanoic acid, {pure), Reachim; 2,2-dimethylpropanoic acid, (pure 98%), Fluka AG. Procedure
All Cr(III) solutions were prepared 6 months before examination. The electronic spectra of these solutions are characterized by the presence of two bands located at a b o u t 410 and 570 nm respectively, which correspond to the violet Cr(H20)] ÷ complex as the main c o m p o n e n t of the aqueous phase. Extraction was carried out in 250 ml flasks at the volume ratio V o / V a = 1 at 293 -+1 K. The flasks were shaken in a thermostatted Elpan 357 shaker. The time required to reach the equilibrium state was 48 h. The pH of the aqueous phase was corrected by adding the necessary amount of a dilute NaOH solution while the ionic strength was adjusted by means of NaC1. After shaking the flasks were left for several hours. The contents of flasks were then transferred into separatory funnels and the phases separated. The aqueous phase was filtered and pH and chromium concentration were determined. Analytical methods
Carboxylic acid concentration in the organic phases was determined alkalimetrically using phenolphthalein as an indicator. Chromium concentrations in the aqueous phases were determined spectrophotometrically by means of diphenylcarbazide [4]. Chromium concentrations in the organic phases were calculated on the basis of the metal balance in the system, pH measurements in the aqueous phases were performed in a M e r a - E l m a t N512 pHmeter provided with a combined SAgP electrode and calibrated by means of verified pH standards. All results presented below are mean values obtained from measurements and analyses performed for the two samples prepared in the same way.
193
Theoretical considerations In our previous work [3] we have shown that effective extraction of Cr(III) from NH4C1 solutions with carboxylic acids takes place over the pH limits of the aqueous phase ranging from 4 to 5 while the values of the partition coefficient D lie between 0.1 and 10. Over this pH range the Cr(III) ions in dilute solutions undergo hydrolysis to a considerable extent. For example, 10 -s molal Cr(III) solution at pH = 4.5 and unit ionic strength (NaC104) contains at 298 K about 30% of the Cr(H2 O)~ ÷ species, 30% of the Cr(OH) 2+ species, 30% of Cr3(OH)~ + and 5% of Cr2(OH)~ + [5]. For this reason one cannot neglect the effect of hydrolysis on the extraction process of Cr(III) and rule out the formation of Cr(III) hydroxy- or oxycomplexes with carboxylic acids in the organic phase. It should be emphasized that the metal hydroxy- or oxycomplexes with carboxylic acids may be formed in the organic phase when metal aquocomplex and water participate in the extraction process [6,7,8] and if the hydrolyzed metal ion species occurring in the aqueous phase participate in the extraction process [9]. Having the above in mind, one can assume that extraction of Cr(III) with carboxylic acids takes place according to one of the following three equations j
Cr~a) + j(3
+ x ) / 2 (HR)~(o) ~- [CrR3- xHR]j(o) + 3j H ÷
{1)
j Cr~a) + j ( 3 - m + x)/2 (HR)2(o) + m H20 ~- [Cr(OH)mR3_rn'XHR]j(o)
+ 3j H +
(2)
j Cr(OH)~(a~ )+ + j ( 3 - m + x)/2 (HR)2(o) ~ [Cr(OH)mR3_m'xHR]j(o ) +/(3-m)
H+
(3)
where: HR - - m o n o m e r i c carboxylic acid molecule, j --degree of complex polymerization in the organic phase, x - number of monomeric acid molecules attached to the metal ion in a complex in the organic phase. The extraction equilibria given by eqns. (1)~ (2) and (3) may be described by the equation logD = l o g K + ( j - 1 )
lOgCM(a) + a p H + b l o g [ ( H R ) ] 2 ( o ) - j l o g a
M
(4)
where: log K = log Kex + l o g j - 1/2b log 2, C M ( a ) - - equilibrium metal concentration in the aqueous phase, a -- s'toichiometric coefficient equal to the number of hydrogen ions involved in reactions (1), (2) and {3), b -- stoichiometric coefficient equal to the number of carboxylic acid involved in reactions (1), (2) and (3), aM -- complexation function (side reaction coefficient) of metal in the aqueous phase, resulting in the formation of ionic species which do not participate in the extraction process. It is evident t h a t the plot of log D vs. pH for the extraction of polymeric metal complex shows considerable curvature, but when hydrolysis of the extracted metal occurs in the aqueous phase then the curve becomes flatter.
194 In many cases, for experimental D values in the range of 0.1 < D < 10 the data may be represented by a straight line. The degree of polymerization of a metal complex in the organic phase may be determined by the graphical m e t h o d using, for instance, a single equilibrium model [11]. This model was successfully used by us for interpretation of the equilibria of metal extraction with carboxylic acids in previous works [ 10,11 ]. In order to investigate the composition of the Cr(III) complex in the organic phase and to find out which chemical equation applies to the extraction process, one can use a m e t h o d developed by Jaycock and Jones [12]. According to that m e t h o d eqn. (4) is expressed in linear form Y = pX + C
(5)
where: Y = l o g D X = (j - 1) log CM(a) + a pH + b log [(HR)2](o) C = log K - j log a M and then the slope of a straight line given by eqn. (5) is determined. The coefficient p for the best matched coefficients of eqn. (4) should be as close to unity as possible. The simplest case occurs if the value of the side reaction coefficient aM = 1 since this enables us to determine directly the extraction constant Kex. In other cases Kex may be determined only if the value of aM is known. DISCUSSION E f f e c t o f the p h a s e c o n t a c t t i m e on e x t r a c t i o n o f Cr(III)
Some earlier experiments [3] showed that the contact time between the aqueous and organic phase significantly affects the extraction process of Cr(III). In the present work it was assumed that the phases should be contacted continuously for a suitable period of time. In all tests performed, after the required pH correction, turbidity of the aqueous phase was observed. However, at the equilibrium state neither phase exhibited turbidity and the organic phase was blue-violet. Figure 1 shows how Cr(III) is extracted from the aqueous phase containing 0.0109 M CrC13 with 0.104 M hexanoic acid solution depending on the phase contact time. As shown, the respective extraction curves presented in the log D--pH coordinate system are straight lines whose slope increases with prolongation of the phase contact time. The relationship logD = f (pH) obtained for t = 48 h is a straight line with slope equal to 2.62 and it seems that it corresponds to the equilibrium state since any prolongation of the phase contact time does not affect the slopes of extraction curves. Therefore, in all other studies the phase contact time was 48 h. The effect observed may be related to a non-equilibrium extraction in which highly h y d r o l y z e d Cr(III) species participate and thus the equilibrium
195
~
log D
1.6
~
log D - -
/
1.6
./ 0.8
0.8
0~
0.4
0.0
0.0 i
-0.4
~ I
~,.o
-0.~ I
L,.s
I
s.o
~H
~.o
z,.s
s.o pH
Fig. 1. D e p e n d e n c e o f c h r o m i u m ( I I I ) e x t r a c t i o n o n t h e p h a s e c o n t a c t t i m e . c o t ( t o t a l ) = 1.09 m M ; CNaC1 = 0 . 1 0 M ; [ H R 2 ] ( o ) = 0 . 1 0 4 M; c o n t a c t t i m e : [] - - 24 h, o - - 36, • - - 48 h. Fig. 2. I n f l u e n c e o f s o d i u m c h l o r i d e c o n c e n t r a t i o n o n c h r o m i u m ( I I I ) e x t r a c t i o n b y m e a n s o f h e x a n o i c acid. c c r ( t o t a l ) = 1 . 0 9 raM; [(HR)~ ](o) = 0 . 1 0 4 M; CNaC1 : O - 0 . 1 0 M, • - - 0 . 5 0 M, <>-- 1 . 0 0 M, • - - 3 . 0 0 M.
state is reached after the long period of time as a consequence of some secondary reactions (e.g. elimination of OH groups by carboxylate ions).
Effect o f NaCl concentration on extraction o f Cr(III) with hexanoic acid The effect of NaC1 concentration in the aqueous phase on extraction of Cr(III) was investigated by using a 0.104 M hexanoic acid solution in carbon tetrachloride. The initial CrC13 concentration in the aqueous phase was 1.09 mM. Four Cr(III) extraction series from aqueous phases of various NaC1 concentrations equal to 0.10, 0.50, 1.0 and 3.0 M, respectively, were performed. The results obtained are shown in Fig. 2 as a plot log D vs. pH. The relationships log D = f (pH) have a linear character and, as shown, with the increasing NaC1 concentration in the aqueous phase the extraction of Cr(III) increases too. An increase in the Cr(III) extraction may be explained as due to the salting-out effect of sodium chloride with respect to the chromium complexes formed. As a rule, the complexation by the chloride ions decreases
196
metal extraction b y means of carboxylic acids. In this case, it is possible that complexation of Cr(III) by the chloride ions positively affects the extraction process (e.g. CrC12÷ complex takes part in the extraction). In all other experiments in the aqueous phase a constant NaC1 concentration of 0.10 M was maintained.
Effect of initial CrCl3 concentration in the aqueous phase on extraction of chromium (III) with hexanoic acid Studies on the effect of initial CrC13 concentration on the extraction process of Cr(III) with hexanoic acid were performed in order to determine the degree of polymerization of complexes formed in the organic phase. Hexanoic acid concentration in the organic phase was 0.104 M. Initial CrC13 concentration in the aqueous phase was varied from 0.218 mM to 10.9 mM.
tog D
I
I
I
2.0
1.6
1.2
0.8
~
0.~
0.0
-O.t,
]
I
G.O
~.5
510
pH
Fig. 3. E f f e c t o f initial CrC13 c o n c e n t r a t i o n in t h e a q u e o u s p h a s e o n c h r o m i u m ( I I I ) ext r a c t i o n w i t h h e x a n o i c acid. CNaC1 = 0 . 1 0 M; [ ( H R ) 2 ] (o) = 0 . 1 0 4 M; ccr (totaD: * - - 0 . 2 1 8 raM, ~ - - 0 . 5 4 5 raM, • - - 1.09 raM, o - - 5 . 4 5 raM, • - - i 0 . 9 raM.
197 p H o s - -
'
'
,
--
,
]+.6
~..2 ~_
I_ 3.5
I
1
3.0
2.5
I
2.0 1.0~ C Cr (totat }
Fig. 4. pH0. s vs. log cot(total ).
The results obtained are presented in Fig. 3 as a plot of log D vs. pH. These relationships over the entire pH range under investigation and for all initial CrC13 concentrations are linear with slopes slightly lower than 3 (2.50-2.80). Evident dependence of Cr(III) extraction on the initial CrC13 concentration in the aqueous phase indicates that in the organic phase polynuclear chromium(III) complexes are formed. The pH0.s values were determined in order to plot the function pH0.s = f [log CCr(total)] presented in Fig. 4. This plot is a straight line with a slope of -0.222. The single equilibrium model was used to find the value o f j = 2.99 for n = 3 which indicates that in the organic phase trinuclear chromium(III) complexes with hexanoic acid Occur.
Effect o f hexanoic acid concentration on extraction o f Cr(III) Studies on the effect of extraction agent concentration on metal extraction enables us to determine the composition of the complex formed in the organic phase. This effect was investigated by making a series of chromium(III) extractions with hexanoic acid solutions in CC14 for four different extractant concentrations: 0.1; 0.4; 0.8 and 1.0 M NaC1 concentration and initial CrC13 concentration in the aqueous phase were constant and were 0.10 M and 5.45 mM, respectively. The results obtained are presented in Fig. 5. As shown, in this case these relationships are also linear with the slope slightly lower than 3. The values of pH0.s f o u n d were used to plot the function of pH0.s vs. log [(HR)2](o) shown in Fig. 6. This plot is a straight line whose slope is -0.689. Usirig" the single equilibrium model for n = 3 the value of x is 1.14 which indicates that for further considerations one should assume the value x = 1. This means that in the trinuclear chromium(III) complex each Cr(III) ion has one molecule of the extractant, i.e. this complex corresponds to the formula [Cr.R3- HR]3.
198 i
{og D
,
I/ •
, //
2.0
1.6
1.2
0.8
o.~
II 0.0
/" ./
-o.~ I
I
I
I
3.5
L,.O
1,.5
5.0
pH
Fig. 5. E f f e c t o f h e x a n o i c acid c o n c e n t r a t i o n o n c h r o m i u m ( I I I ) , e x t r a c t i o n . CNaC1 = 0 . ] 0 M; c o t ( t o t a l ) = 5 . 4 5 raM; [ ( H R ) 2 ] ( o ) : [] - - 0 . 1 0 M, • - - 0 . 4 0 M, © - - 0 . 8 0 M, • - - 1.00 M.
pHo.s
I
I
T--
/+.5
/*.1
3.7 -1.o
Fig. 6. pH0. s vs. log [ ( H R ) 2 ] ( o ) .
I -o.5
I 0,0 Log [ ( H R ) z ]
199
The extraction of Cr(III) with hexanoic acid was assumed to proceed according to reaction (1) and in the organic phase the [CrR3" HR]3 complexes are formed. This assumption was verified by using the Jaycock and Jones method described earlier [12]. Coefficients a and b of eqn. (4) were expressed by means of the values j = 3 and x = 1 and the value o f coefficient p in eqn. (5) was calculated by the least squares method. Since this value, p = 0.47, differs considerably from 1, the assumption concerning the extraction process according to reaction (1) and composition of the complex in the organic phase was considered not to be satisfied. Further calculations were based upon eqns. (2) and (3). Calculations performed for various combinations of coefficients a and b in eqn. (4), under the assumption that extraction of Cr(III) takes place according to reaction (2), gave the values of coefficient p in eqn. (5) ranging from 0.50 to 0.60, that is, also very different from unity. Satisfactory results were obtained for calculations performed assuming that extraction of Cr(III) takes place according to reaction (3) and the values of coefficients a and b from eqn. (4) are 6 and 4.5, respectively.
Yi¸
0
2.0
0
1.6 ![
0 0
1.2[-
0
! 0.8 ~ I r
~
'•O
OJ+~-
0 0.0I"
0 O
-0.hl / "~
I 16
~L 17
]
is
_ _
X
Fig. 7. Y = f(X). CNaCl = 0.10 M; [(HR)2](o) = 0.104 M; ccr(total) = 5.45 m M , c)-- exper-
imental points.The fulllinehas been calculatedby the leastsquares method.
200 Calculations performed by the least squares m e t h o d for various extraction series gave the values of coefficient p from eqn. (5) ranging from 0.86 to 0.98. Figure 7 shows the plot of Y = f (X) obtained on the grounds of calculations made for a series of Cr(III) extractions in which the initial CrC13 concentration in the aqueous phase was 5.45 mM and the hexanoic acid concentration in the organic phase was 0.104 M. The slope of the straight line Y = p X + C determined for that series is 0.98 and the correlation coefficient r x y = 0.70. Deviations of the experimental points m a y be explained as due to the fact that in calculations the parameter C was assumed to be constant which is inaccurate since the complexation function a Cr is not constant over the pH range under investigation and thus the value of C varies slightly over this pH range. Naturally, this makes impossible any estimation of the extraction constant Kex. Having in mind that calculations performed for reaction (3) gave the values of a = 6 and b = 4.5 and that these parameters correspond to stoichiometric coefficients in reaction (3), we obtain ](3-m
j(3-m)
+ x ) / 2 = 4.5
= 6
(6) (7)
Since graphical analysis performed using the simple equilibrium model j = 3, we have the values m = 1 and x = 1. This means that in the organic phase during extraction of Cr(III) with hexanoic acid the complexes of general formula [Cr (OH)R2" HR ] 3 are formed and the extraction equaution is 3 Cr/~u~2+ ~v,,j(a) + 9/2 (HR)2(o) ~- [Cr(OH)R2- HR]3(o) + 6 H(a +)
(8)
E x t r a c t i o n o f Cr(III) with various c a r b o x y l i c acids
In order to compare the extraction properties of various carboxylic acids a series of Cr(III) extractions was performed using 0.1 M solutions of the following acids: 2-methylopropanoic, hexanoic, heptanoic, octanoic, nonanoic, dodecanoic, hexadecanoic and 2,2-dimethylpropanoic. The aqueous phase was a solution in which CrC13 and NaC1 concentrations were 1.09 mM and 0.1 M, respectively. The results obtained are presented in Fig. 8 in the log D--pH coordinate system. The extraction curves have a linear nature and their slope is about 3. All the acids examined extract chromium efficiently over the pH range 4 to 5. The longer the carbon chain of the acid molecule, the better its extraction properties for Cr(III), 2,2dimethylpropanoic acid being an exception. In spite of the fact that this acid has a short and significantly branched carbon chain in its molecule, it is the best extracting agent for Cr(III). The t e n d e n c y observed may be theoretically justified. According to Warshawsky [13] who analyzed the extraction processes of metals with various types of extracting agents, the following relationship is valid
201
tog D - - -
T
T
i
1.6
1.2
O.B
--
0./~-
0.0-
-0.~
t I
~.o
o
1
t..s
L _ _
s.o
}H
Fig. 8. C o m p a r i s o n o f t h e e x t r a c t i o n o f c h r o m i u m ( I I I ) w i t h v a r i o u s c a r b o x y l i c a c i d s . CNaC1 = 0 . 1 0 M ; o C t ( t o t a l ) -- 1 . 0 9 r a M ; [ ( H R ) ~ ](o) = 0 . 1 0 M. + - - 2 - m e t h y l p r o p a n o i c a c i d , o -- hexanoic acid, • -- heptanoie acid, 0 -- octanoic acid, • -- nonanoic acid, X dodecanoic acid, ~ -- hexadecanoic acid, A -- 2,2-dimethylpropanoic acid.
pH0.s = pKHR + log DHR -
1/n log/3n
(9)
where: pH0.s -- pH of half-extraction of a metal, KHR--dissociation constant of the extractant, DHR -- distribution coefficient of the extractant between aqueous and organic phase, ~n - overall stability constant of the MRn complexes. Since in the homologous series of n-carboxylic acids PKHR = constant while/3n varies only slightly, eqn. (9) indicates that pH0.s for a given metal depends, above all, on extractant distribution between the phases. As shown in Ref. [14], the distribution constant KD, HR of an n-carboxylic acid between the aqueous and organic phase depends on the carbon chain length in the molecule and, in addition, the increment of the distribution constant per one added methylene group is practically constant and is lOgKD,HR = 0.56 ÷ 0.60
(10)
202
Exceptions to the rule are the carboxylic acids with highly branched chains which are more effective than the corresponding acids with straight chains. This effect was observed, e.g. by Cattrall and Walsh [15] who studied the extraction process of Fe (III) with various carboxylic acids. CONCLUSIONS
1. The extraction process of Cr(III) with carboxylic acids from NaC1 solutions was found to take place over a pH range of the aqueous phase of 4 to 5. An increase in NaC1 concentration in the aqueous phase has a favourable effect on the extraction process of chromium (III). 2. The extraction process of Cr(III) with hexanoic acid takes place with the formation of a trinuclear complex [Cr(OH)R2-HR]3 in the organic phase. 3. A comparison of the results of Cr(III) extraction with various carboxylic acids confirms the general rule that the acids with long carbon chains are better extractants than those with short chains.
REFERENCES
6 7 8 9 10 11 12 13 14 15
Flett, D.S. and Jaycock, M.J., Extraction of metals by carboxylic acids, in: Marinsky, J.A. and Marcus, Y. (Eds.), Ion Exchange and Solvent Extraction, Vol. 3, Marcel Dekker, New York, 1973, Chap. 1, pp. 1--50. Gindin, L.M. et al., Izv. Sib. Otdel. AN SSSR, Ser. Khim. Nauk, 3 (1967) 195. Apostoluk, W. and Bartecki, A., Symposium Pracovniku Banskego Prumyslu, HORNICKA PRIBRAM VE VEDE A TECHNICE, Sekce N, 1981, pp. 313--330. Marczenko, Z., Spektrofotometryczne oznaczanie pierwiastk6w PWN, Warszawa, 1979, pp. 232--234. Baes, Jr, C.F. and Mesmer, R.E., The Hydrolysis of Cations, John Wiley and Sons, New York, London, Sydney, Toronto, 1976, p. 219. Bartecki, A. and Apostoluk, W., Hydrometallurgy, 5 (1980) 367. Apostoluk, W. and Bartecki, A., Mat. Sci., 3 (1980) 115. Kodama, N., Yamada, H. and Tanaka, M., J. Inorg. Nucl. Chem., 38 (1976) 2063. Berger, S.A. and Graff, S.M., J. Inorg. Nucl. Chem., 37 (1975) 1031. Bartecki, A. and Apostoluk, W., J. Inorg. Nucl. Chem., 40 (1978) 109. Bartecki, A., Apostoluk, W. and Mager, J., J. Inorg. Nucl. Chem., 41 1979) 1461. Jaycock, M.J. and Jones, A.D., Extraction of metal ions from aqueous solutions by means of carboxylic acids, Dyrssen, D., Liljenzin, J.O. and Rydberg, J. (Eds.), Solvent Extraction Chemistry, North Holland, (1967) p. 160. Warshawsky, A., Minerals Sci. Eng., 5 (1973) 36. Kojima, I., Masashi, Y. and Tanaka, M., J. Inorg. Nucl. Chem., 32 (1970) 987. Cattrall, R.W. and Walsh, M.J., J. Inorg. nucl. Chem., 36 (1974) 1643.