- Email: [email protected]
Recovery of uranium(VI) from carbonate solutions by foaming
265
Journal of the Less-Common Metals, 71 (1980) 265 - 276 @I Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
RECOVERY FOAMING
K. SHAKIR
OF URANIUM(V1)
CARBONATE
SOLUTIONS
BY
and S. SAMY
Nuclear Chemistry Cairo (Egypt)
(Received
FROM
Department,
Nuclear
Research
Centre,
Atomic
Energy
Establishment,
July 17, 1979)
Summary
The removal of uranium(V1) from ammonium and sodium carbonate solutions was investigated using the foam separation technique. The extent of removal was found to depend on many factors and successful recoveries were attained only at low carbonate concentrations. Better recoveries were achieved by adding S-hydroxyquinoline to the uranyl carbonate solutions whereby many1 triquinolinate was formed. The factors that can affect the foam separation processes were investigated. At the optimum conditions recoveries approaching 100% were achieved.
1. Introduction
In any chemical method for the treatment of uranium ores, the first step is digestion by an acid or an alkali. Alkaline leaching is usually carried out with aqueous solutions of carbonate. The usefulness of carbonate solutions arises from the high solubility and stability of the uranyl tricarbonate complex UOs(COa)a 4- . Uranium(V1) is currently recovered from the pregnant carbonate leach liquors by raising the pH of the solution to 11 or to higher values whereby uranium is precipitated as a mixture of uranates [l, 21. Successful results are obtained only when the uranium concentration is high (above 2.5 (g UaOs) 1-l) [2, 31 and, therefore, other procedures have to be used for the recovery of uranium from solutions of low uranium concentrations. For such purposes foam separation techniques seem to be very promising. These techniques are simple and operate very well in very dilute aqueous solutions. The foam separation of metal ions is performed by allowing the metal ion to react with a surfactant to form a surface-active product (called a sublate) which can be concentrated at the surface of the solution by passing a gentle stream of gas bubbles [4, 51. If the sublate is soluble, then it can be separated by a partition mechanism and the process is termed foam fractionation. However, if the sublate is insoluble removal takes place by flotation.
The foam separation of ur~ium from carbonate media depends on its ability to form highly charged anionic carbonate complexes which can be removed by cationic surfactants. The process has been studied by only a few investigators. In a series of papers Jacobelli-Turi and coworkers [6 - 81 have reported on the possible foam separation of uranium from carbonate solutions and have succeeded in separating uranium from vanadium. Using various collectors, Jude and Fratila [9] have obtained recoveries exceeding 95% for the foam separation of uranium from mine water containing low carbonate condensations. More extensive studies have been performed by Shakir f IO, II] who has ~vestiga~d the effects of many factors on the foam separation of uranium from sodium carbonate solutions. Excellent recoveries were attained but only at low carbonate concentrations. It is known that oxine (Shydroxyquinoline) can combine with uranium in carbonate solutions to form a singly charged anion [12, 131 UO~(C~HsNO)s-. From elec~os~tic considerations, the uranyl triqu~olinat~ complex should be removed at a collector/uranium(VI) molar ratio of a quarter of that which would be required for the removal of UOz(COs)s4-. The present work describes studies of the effect of the addition of oxine on the foam separation of uranium(VI) from ammonium or sodium carbonate solutions using the cationic surfactant cetyl tr~ethyl ~monium bromide (CTAB) as a collector. The results are compared with those obtained in the absence of oxine. It is worthy of mention that solutions of ammonium and sodium carbonate differ from each other in that, for solutions of the same pH and ionic strength, the 0H-/COs2- ratio in an ~monium carbonate solution is much higher than that in a sodium carbonate solution.
2. Experimental A stock solution of 0.1 &I uranyl tric~bo~ate was prepared 1141 from AR uranyl nitrate and sodium carbonate. The exact concen~ation of uranyl ion in this stock solution was determined gravimetrically by the S-hydroxyquinoline method [ 151 a The CTAB collector was a BDH product and collector solutions were freshly prepared daily. The collector was evaporated with absolute ethyl alcohol before dissolution to convert any micelle into the ionic form. Absolute ethanol was used as solvent for the collector as well as a frother at a dose of 1 ml of alcohol per 450 ml of uranyl solution. The flotation system has been described elsewhere [ 161 and the flotation cell was made of a no. 4 sintered glass disc 5.8 cm in diameter fused to a Pyrex glass column about 33 cm in height drawn at the bottom into the form of a funnel. The flotation pracedure was similar to that previously described [ 101 and consisted in transferring 450 ml of the uranyl solution to the flotation cell and passing pure nitrogen gas at a rate of about 41 cm3 mm-‘. The solution of the collector in ethyl alcohol was injected from a syringe in one
267
injection while vigorously stirring the solution in the cell. That instant was recorded as the zero time. Samples of the bulk solution were withdrawn at predetermined time intervals for uranium analysis. Unless otherwise specified the initial uranium concentration was adjusted to 1 X 10m4 M and the bubbling period was 60 min. In most experiments this time was sufficient to achieve the ultimate uranium removal. Oxine was a BDH product and, when used, it was added to the uranyl carbonate solution 30 mm before tr~sfe~~g to the flotation cell. For uranium analysis, an aliquot of the solution was evaporated with nitric acid and then heated on a bare flame to destroy the surfactant. Depending on the uranyl ion concentration, uranium was determined spectrophotometrically using either the thoron [ 171 or the arsenazo III [ 181 method. High sodium ion concentrations were found to affect the analysis deleteriously and where necessary uranium was separated from sodium by extraction with a solution of di(2-ethylhexyl)phosphoric acid in toluene [ 19, 201.
3. Results 3.1. Effect of pH The results obtained for the removal of uranium from ammonium or sodium carbonate solutions of constant ionic strength and varying pH values (adjusted with either the corresponding bicarbonate or hydroxide) are plotted in Fig. 1; it can be seen that the percentage removal increases with increasing alkalinity to reach an optimum value at about pH 11.7 and then decreases with further increase of the PH.
9
10
11
12
PH Fig. 1. Effect of pi-I on the removal of U(V1) from ammonium carbonate (0, *, A, A) and sodium carbonate (V,v) solutions: 0, !J = 0.1, C&Z, = 3, HOx = 3 x 10e4 M;n, g = 0.2, C~/~~=~,HOX=~X~O-~M;.,~=O.~,C~/C =3,HOx=3~10-~M;~,~=O.l, cc/C,= 3;V, @ = 1.2, Cc/C, = 7, HOx = 7 x 10E4 M;r, /A = 1..2, C&T, = 7.
268
3.2. Effect of oxine addition on U(VI) removal at variouspH values Figure 1 also shows that the addition of oxine significantly improves the percentage U(V1) removal. At all the ammonium carbonate concentrations tested, removals approaching 100% could be achieved at pH values above 9. At the lower pH values the percentage removal decreases slightly with decrease of the pH. The pattern of removal of uranium from NaaCOs solutions (Fig. 1) is not affected by oxine addition, although the percentage removal is significantly increased. 3.3. Effect of the period of bubbling As shown in Fig. 2, the percentage removal increases with the period of bubbling until a maximum is reached. This maximum, or ultimate percentage removed, depends markedly on the pH and on the oxine concentration. As indicated by the initial slopes of the removal curves, the pH and excess oxine also markedly affect the rate of removal. 3.4. Determination of the removal mechanism To throw some light on the nature of the uranyl complexes that prevail at the high and low pH values, both in the presence and in the absence of oxine, the rate data obtained for U(V1) removal were analysed using the following rate equations: log(Mm - Mt ) = log M, - K, t
(1)
-M,)=logb-clogt
(2)
h#fm
f -
60
z b E d
40
0 0
20
40
Time
60
( min )
Fig. 2. Rate of U(V1) removal from ammonium carbonate solutions: A, /J = 0.1, pH = 9.4, 0.3,pH= 9.4,&/C,= 3,HOx = 2x104M;.,p= CJC, = 3, HOx = 2 x 104’ M;o,/J= O.l,pH=9.4,C,/C,= 3;a,1_1=0.1, pH= ll.O,C,/C,= 3;v,,=O.l,pH=9.4,C,/C,= 3, HOx = 3 x 1O-4 M.
269
where Mm is the maximum fraction removed, M; is the fraction removed after time t, IT, is the rate constant and b and c areconstants. As shown by Rubbin and coworkers [21, ZZ] eqns. (1) and (2) describe removals by petition and flotation respectively. The observed rate data do not conform to the logarithmic expression (eqn. (2)) but can b e reasonably fitted to the semilog~ithmic equation (eqn. (1)). The semilogarithmic plot obtained at pH 9.4 in the absence of oxine is a straight line which passes through the origin (Fig. 3) whereas the semilogarithmic plots obtained at pH 11 in the absence of oxine or at pH 9.4 in the presence of oxine show two distinct removal rates. This suggests that only one uranyl species exists at pEI 9.4 in the absence of oxine whereas two (or probably more) species exist at the other tested conditions.
I 45
I 20 Time (
\
mki )
Fig. 3. Semilogarithmic plots of the rate data obtained for the removal of U(W) from (NH*)#03 solutions: A, p = 0.1, pH = 9.4, C&T, = 3, HOx = 2 x lo-* M; CJ,p = 0.3, pH = 9.4,C,~C,=3,HOx=ZX~O4M;~,~=0.1,pH=9.4,C~C,=3;A,~=0.1,pfI=11.0, M. C&2,= 3;v,~=0.1,pH=9.4,CcIC,= 3, HOx= 3x10
3.5. Effect of collector ccmcentratiwz As shown in Fig. 4, the recovery of uranium from oxine-free systems increases with increase of the ~o~e~~or~ur~iurn ratio until a critical concentration is reached at which further increase in the collector results in a decrease in the precentage removal. The critical collector concentration seems to depend on the ionic strength of the medium and increases as the ionic strength is decreased. The percentage removal obtained at a given C, /C, value considerably increases on adding oxine to the system. It also increases with the collector concentration until an optimum value is reached at which the removal curve tends to level off,
270
100
60
$ 3
60
f
40
2Of-I Coltector
Concentration
f
Mx lo&f
Fig. 4. Effect of the collector concentration on the removal of U(W) from sodium carbonate (0, 0) and ammonium carbonate (0, A, +, A,v, O,V) solutions: ar /A = 1.2, pH = 11.2, HOx= 7x10-4M;~,~= 1.2,~H=l1.2;~,~~0.1,pH=9.4,HO~~3xl.O-~M;*,~= 1.2,pH=lO.?,HOx==3X10M;*,~=0,1,pH=9.3;4~=1.2,pW=10.8;~,~= 1.2, pW=9.1,HOx=3x10-4M;~,~=1_2,pH~9.1;~,~=0.6,pH=9.1.
3.6, Effect of o&e concentration The results obtained for the effect of oxine concentration on the foam separation of uranium are plotted in Fig. 5 which shows that the percentage removal increases with increase of the oxine concentration to reach an optimum value. The oxine concentration which is required to achieve the optimum removal appears to depend on the pH and on the ionic strength of the solutiun. A large excess of oxine dele~~ou~y affects the percentage E?MUVd
3.7. Effect of ionic strength As shown in Fig. 6, the percentage removal is greatly affected by the ionic strength of the medium and is considerably reduced as this strength is increased. Generally the removals obtained for the carbonate-or&e systems are much higher and seem to be less affected by the ionic strength, especially at the low carbonate concentrations, than the removals obtained from oxinefree carbonate systems.
271
Oxine
concentration
( M x10m4)
Fig. 5. Effect of oxine concentration on the removal of U(V1) from ammonium carbonate (0, 0) and sodium carbonate (a) solutions: 0, /..l= 0.1, pH = 11, CJC, = 3; 0, /J = 0.1, pH = 9.4, C,/C, = 3;4 /-r= 1.2, pH = 11.2, C,/C, = 4.
3.8. Effect
of uranium(VI)
concentration
The results obtained for the removal of uranium from (NH4)#Z03 solutions of initially different uranium concentrations and with constant values of pH, ionic strength, and collector and oxine concentrations are shown in Fig. 7. It can be seen that removals approaching 100% are obtained for uranyl ion concentrations of 1 X 1O-5 M and probably less. For uranium(V1) concentrations above 1 X lop5 M, the percentage removal decreases
12
06
Ionic
Strength
700
I 1
Urarwm
I 2
concentration
( MxlCh
Fig. 6. Removal of U(V1) from Na2C03 solutions of varying ionic strength in the presence (0) and in the absence (0) of oxine: pH = 11.3;CJC, = 7; HOx = 7 X 10m4 M. Fig. 7. Effect of U(V1) systems using constant 1.2; pH = 10.8.
concentration on its removal from ammonium carbonate-oxine collector (7 X 10e4 M) and oxine (7 X 10v4 M) concentrations:
/J =
272
with increase of the metal ion concentration and this is obviously caused by the inadequate collector:uranium and/or oxine:uranium ratio.
4, Discussion As shown in Fig, 1, percentage removals of about 46 I 63 were obtained in the pH range 9.4 - 10.7 when a collector/uranium molar ratio of 3 was used for the foam separation of uranium(V1) from ammonium carbonate solutions of ionic strength 12= 0.1. These results can be interpreted by the following considerations. In the foam separation systems studied the ionic strength was kept constant, except at the fuwest pH values where NHBHCOs was used and the ionic strength was adjusted with (NH4)&0s and NH*OH. Thus, as the pH was increased the CQs2- concentration decreased. In the pH range 9.4 - 10.7 the carbonate concentration (and consequently the COs2-/U(VI) ratio) is fairly high and the uranium should therefore be totally present as the uranyl tricarbonate complex UOs (COB), 4- . From electrostatic considerations it would be expected that this tetravalent ion would be removed with the strong cat-ionic collector CTAB at a ratio C, ,E, of 4. In accord with this, a maximum percentage remuval of 75 should be achieved with the CTABf uranium molar ratio of 3 used in this investigation. Experimentally, the obtained percentage removal is lower (about 46 at pH 9.4) because of the need for excess collector to stabilize the phases and to overcame the competitive effects of the carbonate and the other anions present in the medium. The high percentage removal obtained at pH 11 cannot be attributed to only the decrease of the COs2-/OH- ratio and the consequent decrease of the com~titive effect of the carbonate anion (assuming that CUa2- has a greater flotation tendency than OH-). This high percentage removal can be attributed to the formation of uranates and their subsequent flotation at a low collector/metal ion ratio due to their low charge densities. The formation, in the present case, of a compound other than the uranyl tricarbonate complex, presumably a uranate, is suggested by the rate data analysis which indicates that, in the case of the oxine-free systems, only one uranyl species (obviously U0,(CQs)s4-) exists at pH 9.4, whereas two (or more) species are formed at pH II (Fig. 3). These species are probably uranyl tricarbonate and ammonium uranates. It can also be seen in Fig. 1 that the percentage removal of U(V1) from sodium carbonate increases with increase of the pH to reach an optimum value at approximately pH 11.7 and then decreases with further increase of the alkalinity. According to the literature [ 1,2], uranates begin forming in carbonate media at pH values above 11, Therefore the decrease of the percentage removal at pH values above 11.7 can be attributed to the formation of uranates and/or the decreased ionization of the collector at high hydroxyl ion concentrations. However, it should be mentioned that the structures of uranates are complex and vary with the pH and other precipitation conditions
213
[23 - 311. Therefore the decrease of the percentage removal from NasCOs media of pH 11.7 and the increase of the percentage removal from (NH4)aC03 solutions of pH 11 may be due to differences in the nature and consequently of the flotation tendency of the uranate species formed in the two cases. For the U(VI)-COs2- systems investigated, no insoluble sublates were formed with CTAB. This observation, coupled with the fact that the rate data are best described by the semilogarithmic rate equation (eqn. l), indicates that the foam separation systems were homogeneous and that the flotation was controlled by a partition mechanism. In systems where the sublate is soluble, ion-pair attraction between the collector cations and the metal complex anions is probable so that electroneutrality in the foam is maintained [ 211. Accordingly, the presence of a large amount of cations other than uranium (i.e. high ionic strength) should decrease uranium removal. Since uranyl tricarbonate is tetravalent and has a higher charge than the carbonate, bicarbonate or the hydroxyl ions, should be removed in preference to the other three anions. At uo2w3)34high ionic strength, i.e. at high total concentration of HCOs-, COa2- and OH-, attraction between these anions and CTAB becomes more probable and U(V1) removal decreases as a direct function of the increase of the free anions/U02(C0s)s4- ratio. This explains the decrease in percentage removal at a given collector concentration when the ionic strength is increased (Fig. 6). Trials to obtain complete removal by increasing the collector concentration were unsuccessful. At low collector concentrations the ultimate uranium removal directly increases with increase of the collector concentration but beyond certain limits uranium removal decreases (Fig. 4). This seems to be due to the critical micelle concentration being exceeded with consequent formation of micelles which are known to affect flotation deleteriously [4]. The fact that the critical micelle concentration is affected by the concentration of ions of charges opposite to that of the collector [32] explains the dependence of the critical collector concentration on the ionic strength of the solution [lo] . As the ionic strength is increased the total carbonate concentration increases and consequently the critical micelle concentration is lowered with a corresponding lowering of the critical collector concentration. With the aims of improving the foam separation of uranium and of extending the process to solutions of relatively high ionic strength, attention was directed towards the quinolinate complex of the metal ion. It is known that the addition of the Squinolinol to a solution of the very stable uranium complex U02(COs)s4- causes the uranium to precipitate at an elevated pH (11-12) as an orange solid [ 12,131. In dilute aqueous solutions precipitation does not occur, but instead a red soluble complex U02(C9HGN0)s- is formed. Since the charge of the uranyl-S-quinolinate complex is a quarter of that of many1 tricarbonate it is to be expected that the amount of collector that would be required to float a given quantity of U(V1) would be four times lower when uranium exists as U02(C9HGN0)athan when it exists in the form of U02(C03)34-. On this basis, the effect of
274
oxine on the foam separation of U(V1) from carbonate solutions was investigated. The results obtained (Fig. 1) indicate that the addition of oxine to the uranium-carbonate systems markedly improves the percentage removal though it does not significantly affect the general shape of the curve of pH removal. Both in the presence and in the absence of oxine the percentage removal increases with the alkalinity to reach a maximum at pH 11.7 or slightly higher. At higher alkalinities the percentage removal decreases owing to the decomposition of the uranyl carbonate and oxinate [ 331 complexes and the precipitation of uranates. At pH values below about 9, the percentage removal of U(V1) from the carbonate-oxinate systems decreases with decrease of the alkalinity (Fig. 1) implying that HCOs- deleteriously affects the removal of U(V1) from carbonate-oxinate solutions. Also as indicated by the results obtained for the effect of the ionic strength on the removal of uranium from NasCOsoxinate systems of pH 11.2 (Fig. 6), the carbonate ion impairs the foam separation results. These deleterious effects of COs2- and HCOs- on the percentage removal of uranium from carbonate-oxinate systems are not attributable only to their competition with the uranyl tricarbonate and oxinate complexes for the collector cations, but can be also attributed to the effects of HCOs- and COs2- on the degree of formation of the uranyl triquinolinate complex itself. It is known that B-hydroxyquinoline (HOx) is only slightly soluble in the pH range 5.5 - 8.5 [34] and that its solubility increases with increase of the pH because of its dissociation according to the formula HOx=Ox-+H+
(3)
The formation of a uranyl triquinolinate can be described by the equations 3HOx + U02(COs)s
4- = U02(Ox)s-
complex
+ 3HCOs-
from HOx or Ox-
(4)
and 30x-
+
uo2(co3)34-
e
UO2(Ox)3_
+
3cos2-
(5)
These equations imply that the formation of U02(Ox)sshould decrease with increase of either or both of COs2- and HCOs- in the medium. In agreement with eqns. (4) and (5), the observed percentage removal increases with increase of the HOx concentration to an optimum value (Fig. 5). Obviously the oxine concentration that is required to achieve the optimum depends on the uranium concentration, the collector concentration and the total carbonate ions concentration. With the other factors held constant, a lower amount of uranium in the system results in a higher percentage removal because there will be a greater excess of collector and oxinate ions (Fig. 7). However, a very high excess of HOx impairs the percentage removal (Fig. 5) owing to the competition of the excess quinolinate ions with the uranyl complexes for the collector cations.
275
To recover the collector and oxine from the foam phase for recycling purposes, it is suggested first to heat the scum with dilute acid to destroy the carbonate and then to treat it with an alkali solution whereby uranium is precipitated as the alkali uranate [ 4, lo] whilst the collector and oxine remain in solution as ammonium hydroxide and oxinate. The collector can then be converted into its bromide and separated from the oxine by shaking with a solvent, as for example benzene or Ccl,, which can extract oxine and leaves the collector.
Nomenclature
cc Cnl Cc/C, HOX Mm
Mt t
p
initial collector concentration initial uranium concentration molar ratio of collector concentration to uranium oxine (8hydroxyquinoline) maximum fraction of uranium removed uranium removed at time t time of bubbling in minutes ionic strength
concentration
References 1 Swedish Patent 148,719 (1957) to P. 0. Stelling and J. E. K. Svenko. C. F. W. D. Wilkinson, Uranium Metallurgy, Vol. 1, Interscience, New York, 1962, p. 95. 2 F. A. Forward and J. Halpern, J. Met., 6 (1954) 1408. 3 P. Mouret, P. Pottier, P. Soudan and J. Le Bris, 2nd Int. Conf. on Peaceful Uses of Atomic Energy, Geneva, Vol. 3, United Nations, Geneva, 1958, p. 356. 4 F. Sebba, Zon Flotation, Elsevier, New York, 1st edn., 1962. 5 R. Lemlich, Adsorptive Bubble Separation Techniques, Academic Press, New York, 1st edn., 1972. 6 C. Jacobelli-Turi, A. Barocas and F. Salvetti, Gazz. Chim. Ital., 93 (1963) 1493. 7 A. Barocas, C. Jacobelli-Turi and F. Salvetti, J. Chromatogr., 14 (1964) 291. 8 C. Jacobelli-Turi, A. Barocas and S. Terenzi, Ind. Eng. Chem., Process Des. Dev., 6 (1967) 161. 9 E. Jude and N. Fratila, Proc. 10th Mineral Processing Con@., London, 1973, Institute of Mining and Metallurgy, London, 1974, p. 707. 10 K. Shakir, J. Appl. Chem. Biotechnol., 23 (1973) 339. 11 K. Shakir, Chem. Ser., 6 (1974) 70. 12 W. E. Clifford, P. Noble, Jr., and E. P. Bullwinkel, U.S. Atomic Energy Commission
Rep. RMO-2623. 13 E. P. Bullwinkel and P. Noble, Jr., J. Am. Chem. Sot., 80 (1958) 14
I. I. Chernyaev,
2nd Znt. Conf. on Peaceful 15 16
17
2955. G. V. Ellert, R. N. Shcholokov and V. P. Marakou, Uses of Atomic Energy, Geneva, Vol. 28, United Nations,
V. A. Golovnyo,
Geneva, 1958, p. 235. A. I. Vogel, Quantitative Inorganic Analysis, Longmans, p. 471. K. Shakir, Sep. Sci., 8 (1973) 345. S. P. Sangal, Micro&em. J., 7 (1963) 331.
London,
2nd edn.,
1953,
276 18 A. A. Nemodruk and L. P. Glukhova, J. Anal. Chem. USSR, 18 (1963) 85. 19 D. F. Peppard, G. W. Mason, I. Hucher and F. A. J. A. Brandas, J. Inorg. Nucl. Chem., 24 (1962) 1387. 20 K. Kimura, Bull. Chem. Sot. Jpn, 33 (1960) 1038. 21 A. J. Rubin, J. D. Johnson and J. C. Lamb III, Ind. Eng. Chem., Process Des. Deu., 5 (1966) 368. 22 A. J. Rubin, J. Am. Water Works Assoc., 60 (1968) 832. 23 J. Sutton, J. Inorg. Nucl. Chem., 1 (1955) 68. 24 J. K. Dawson, E. Wait, K. Alcock and D. R. Chilton, J. Chem. Sot., (1956) 3531. 25 J. Maly and V. Vesely, J. Inorg. Nucl. Chem., 7 (1958) 119. 26 E. V. Garner, J. Inorg. Nucl. Chem., 21 (1961) 380. 27 E. H. P. Cordfunke, J. Inorg. Nucl. Chem., 24 (1962) 303. 28 M. Saxena, J. Indian Chem. Sot., 39 (1962) 831. 29 M. E. A. Hermons and T. Markestein, J. Inorg. Nucl. Chem., 25 (1963) 461. 30 P. C. Depets and B. 0. Loopstra, J. Inorg. Nucl. Chem., 25 (1963) 945. 31 V. Baran and M. Tympl, J. Inorg. Nucl. Chem., 28 (1966) 89. 32 M. L. Corrin and W. D. Harkins, J. Am. Chem. Sot., 69 (1947) 683. 33 J. E. Ricci and F. J. Loprest, J. Am. Chem. Sot., 77 (1955) 2119. 34 H. Irving, J. A. D. Ewart and J. T. Wilson, J. Chem. Sot., (1949) 2672.
Journal of the Less-Common Metals, 71 (1980) 265 - 276 @I Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
RECOVERY FOAMING
K. SHAKIR
OF URANIUM(V1)
CARBONATE
SOLUTIONS
BY
and S. SAMY
Nuclear Chemistry Cairo (Egypt)
(Received
FROM
Department,
Nuclear
Research
Centre,
Atomic
Energy
Establishment,
July 17, 1979)
Summary
The removal of uranium(V1) from ammonium and sodium carbonate solutions was investigated using the foam separation technique. The extent of removal was found to depend on many factors and successful recoveries were attained only at low carbonate concentrations. Better recoveries were achieved by adding S-hydroxyquinoline to the uranyl carbonate solutions whereby many1 triquinolinate was formed. The factors that can affect the foam separation processes were investigated. At the optimum conditions recoveries approaching 100% were achieved.
1. Introduction
In any chemical method for the treatment of uranium ores, the first step is digestion by an acid or an alkali. Alkaline leaching is usually carried out with aqueous solutions of carbonate. The usefulness of carbonate solutions arises from the high solubility and stability of the uranyl tricarbonate complex UOs(COa)a 4- . Uranium(V1) is currently recovered from the pregnant carbonate leach liquors by raising the pH of the solution to 11 or to higher values whereby uranium is precipitated as a mixture of uranates [l, 21. Successful results are obtained only when the uranium concentration is high (above 2.5 (g UaOs) 1-l) [2, 31 and, therefore, other procedures have to be used for the recovery of uranium from solutions of low uranium concentrations. For such purposes foam separation techniques seem to be very promising. These techniques are simple and operate very well in very dilute aqueous solutions. The foam separation of metal ions is performed by allowing the metal ion to react with a surfactant to form a surface-active product (called a sublate) which can be concentrated at the surface of the solution by passing a gentle stream of gas bubbles [4, 51. If the sublate is soluble, then it can be separated by a partition mechanism and the process is termed foam fractionation. However, if the sublate is insoluble removal takes place by flotation.
The foam separation of ur~ium from carbonate media depends on its ability to form highly charged anionic carbonate complexes which can be removed by cationic surfactants. The process has been studied by only a few investigators. In a series of papers Jacobelli-Turi and coworkers [6 - 81 have reported on the possible foam separation of uranium from carbonate solutions and have succeeded in separating uranium from vanadium. Using various collectors, Jude and Fratila [9] have obtained recoveries exceeding 95% for the foam separation of uranium from mine water containing low carbonate condensations. More extensive studies have been performed by Shakir f IO, II] who has ~vestiga~d the effects of many factors on the foam separation of uranium from sodium carbonate solutions. Excellent recoveries were attained but only at low carbonate concentrations. It is known that oxine (Shydroxyquinoline) can combine with uranium in carbonate solutions to form a singly charged anion [12, 131 UO~(C~HsNO)s-. From elec~os~tic considerations, the uranyl triqu~olinat~ complex should be removed at a collector/uranium(VI) molar ratio of a quarter of that which would be required for the removal of UOz(COs)s4-. The present work describes studies of the effect of the addition of oxine on the foam separation of uranium(VI) from ammonium or sodium carbonate solutions using the cationic surfactant cetyl tr~ethyl ~monium bromide (CTAB) as a collector. The results are compared with those obtained in the absence of oxine. It is worthy of mention that solutions of ammonium and sodium carbonate differ from each other in that, for solutions of the same pH and ionic strength, the 0H-/COs2- ratio in an ~monium carbonate solution is much higher than that in a sodium carbonate solution.
2. Experimental A stock solution of 0.1 &I uranyl tric~bo~ate was prepared 1141 from AR uranyl nitrate and sodium carbonate. The exact concen~ation of uranyl ion in this stock solution was determined gravimetrically by the S-hydroxyquinoline method [ 151 a The CTAB collector was a BDH product and collector solutions were freshly prepared daily. The collector was evaporated with absolute ethyl alcohol before dissolution to convert any micelle into the ionic form. Absolute ethanol was used as solvent for the collector as well as a frother at a dose of 1 ml of alcohol per 450 ml of uranyl solution. The flotation system has been described elsewhere [ 161 and the flotation cell was made of a no. 4 sintered glass disc 5.8 cm in diameter fused to a Pyrex glass column about 33 cm in height drawn at the bottom into the form of a funnel. The flotation pracedure was similar to that previously described [ 101 and consisted in transferring 450 ml of the uranyl solution to the flotation cell and passing pure nitrogen gas at a rate of about 41 cm3 mm-‘. The solution of the collector in ethyl alcohol was injected from a syringe in one
267
injection while vigorously stirring the solution in the cell. That instant was recorded as the zero time. Samples of the bulk solution were withdrawn at predetermined time intervals for uranium analysis. Unless otherwise specified the initial uranium concentration was adjusted to 1 X 10m4 M and the bubbling period was 60 min. In most experiments this time was sufficient to achieve the ultimate uranium removal. Oxine was a BDH product and, when used, it was added to the uranyl carbonate solution 30 mm before tr~sfe~~g to the flotation cell. For uranium analysis, an aliquot of the solution was evaporated with nitric acid and then heated on a bare flame to destroy the surfactant. Depending on the uranyl ion concentration, uranium was determined spectrophotometrically using either the thoron [ 171 or the arsenazo III [ 181 method. High sodium ion concentrations were found to affect the analysis deleteriously and where necessary uranium was separated from sodium by extraction with a solution of di(2-ethylhexyl)phosphoric acid in toluene [ 19, 201.
3. Results 3.1. Effect of pH The results obtained for the removal of uranium from ammonium or sodium carbonate solutions of constant ionic strength and varying pH values (adjusted with either the corresponding bicarbonate or hydroxide) are plotted in Fig. 1; it can be seen that the percentage removal increases with increasing alkalinity to reach an optimum value at about pH 11.7 and then decreases with further increase of the PH.
9
10
11
12
PH Fig. 1. Effect of pi-I on the removal of U(V1) from ammonium carbonate (0, *, A, A) and sodium carbonate (V,v) solutions: 0, !J = 0.1, C&Z, = 3, HOx = 3 x 10e4 M;n, g = 0.2, C~/~~=~,HOX=~X~O-~M;.,~=O.~,C~/C =3,HOx=3~10-~M;~,~=O.l, cc/C,= 3;V, @ = 1.2, Cc/C, = 7, HOx = 7 x 10E4 M;r, /A = 1..2, C&T, = 7.
268
3.2. Effect of oxine addition on U(VI) removal at variouspH values Figure 1 also shows that the addition of oxine significantly improves the percentage U(V1) removal. At all the ammonium carbonate concentrations tested, removals approaching 100% could be achieved at pH values above 9. At the lower pH values the percentage removal decreases slightly with decrease of the pH. The pattern of removal of uranium from NaaCOs solutions (Fig. 1) is not affected by oxine addition, although the percentage removal is significantly increased. 3.3. Effect of the period of bubbling As shown in Fig. 2, the percentage removal increases with the period of bubbling until a maximum is reached. This maximum, or ultimate percentage removed, depends markedly on the pH and on the oxine concentration. As indicated by the initial slopes of the removal curves, the pH and excess oxine also markedly affect the rate of removal. 3.4. Determination of the removal mechanism To throw some light on the nature of the uranyl complexes that prevail at the high and low pH values, both in the presence and in the absence of oxine, the rate data obtained for U(V1) removal were analysed using the following rate equations: log(Mm - Mt ) = log M, - K, t
(1)
-M,)=logb-clogt
(2)
h#fm
f -
60
z b E d
40
0 0
20
40
Time
60
( min )
Fig. 2. Rate of U(V1) removal from ammonium carbonate solutions: A, /J = 0.1, pH = 9.4, 0.3,pH= 9.4,&/C,= 3,HOx = 2x104M;.,p= CJC, = 3, HOx = 2 x 104’ M;o,/J= O.l,pH=9.4,C,/C,= 3;a,1_1=0.1, pH= ll.O,C,/C,= 3;v,,=O.l,pH=9.4,C,/C,= 3, HOx = 3 x 1O-4 M.
269
where Mm is the maximum fraction removed, M; is the fraction removed after time t, IT, is the rate constant and b and c areconstants. As shown by Rubbin and coworkers [21, ZZ] eqns. (1) and (2) describe removals by petition and flotation respectively. The observed rate data do not conform to the logarithmic expression (eqn. (2)) but can b e reasonably fitted to the semilog~ithmic equation (eqn. (1)). The semilogarithmic plot obtained at pH 9.4 in the absence of oxine is a straight line which passes through the origin (Fig. 3) whereas the semilogarithmic plots obtained at pH 11 in the absence of oxine or at pH 9.4 in the presence of oxine show two distinct removal rates. This suggests that only one uranyl species exists at pEI 9.4 in the absence of oxine whereas two (or probably more) species exist at the other tested conditions.
I 45
I 20 Time (
\
mki )
Fig. 3. Semilogarithmic plots of the rate data obtained for the removal of U(W) from (NH*)#03 solutions: A, p = 0.1, pH = 9.4, C&T, = 3, HOx = 2 x lo-* M; CJ,p = 0.3, pH = 9.4,C,~C,=3,HOx=ZX~O4M;~,~=0.1,pH=9.4,C~C,=3;A,~=0.1,pfI=11.0, M. C&2,= 3;v,~=0.1,pH=9.4,CcIC,= 3, HOx= 3x10
3.5. Effect of collector ccmcentratiwz As shown in Fig. 4, the recovery of uranium from oxine-free systems increases with increase of the ~o~e~~or~ur~iurn ratio until a critical concentration is reached at which further increase in the collector results in a decrease in the precentage removal. The critical collector concentration seems to depend on the ionic strength of the medium and increases as the ionic strength is decreased. The percentage removal obtained at a given C, /C, value considerably increases on adding oxine to the system. It also increases with the collector concentration until an optimum value is reached at which the removal curve tends to level off,
270
100
60
$ 3
60
f
40
2Of-I Coltector
Concentration
f
Mx lo&f
Fig. 4. Effect of the collector concentration on the removal of U(W) from sodium carbonate (0, 0) and ammonium carbonate (0, A, +, A,v, O,V) solutions: ar /A = 1.2, pH = 11.2, HOx= 7x10-4M;~,~= 1.2,~H=l1.2;~,~~0.1,pH=9.4,HO~~3xl.O-~M;*,~= 1.2,pH=lO.?,HOx==3X10M;*,~=0,1,pH=9.3;4~=1.2,pW=10.8;~,~= 1.2, pW=9.1,HOx=3x10-4M;~,~=1_2,pH~9.1;~,~=0.6,pH=9.1.
3.6, Effect of o&e concentration The results obtained for the effect of oxine concentration on the foam separation of uranium are plotted in Fig. 5 which shows that the percentage removal increases with increase of the oxine concentration to reach an optimum value. The oxine concentration which is required to achieve the optimum removal appears to depend on the pH and on the ionic strength of the solutiun. A large excess of oxine dele~~ou~y affects the percentage E?MUVd
3.7. Effect of ionic strength As shown in Fig. 6, the percentage removal is greatly affected by the ionic strength of the medium and is considerably reduced as this strength is increased. Generally the removals obtained for the carbonate-or&e systems are much higher and seem to be less affected by the ionic strength, especially at the low carbonate concentrations, than the removals obtained from oxinefree carbonate systems.
271
Oxine
concentration
( M x10m4)
Fig. 5. Effect of oxine concentration on the removal of U(V1) from ammonium carbonate (0, 0) and sodium carbonate (a) solutions: 0, /..l= 0.1, pH = 11, CJC, = 3; 0, /J = 0.1, pH = 9.4, C,/C, = 3;4 /-r= 1.2, pH = 11.2, C,/C, = 4.
3.8. Effect
of uranium(VI)
concentration
The results obtained for the removal of uranium from (NH4)#Z03 solutions of initially different uranium concentrations and with constant values of pH, ionic strength, and collector and oxine concentrations are shown in Fig. 7. It can be seen that removals approaching 100% are obtained for uranyl ion concentrations of 1 X 1O-5 M and probably less. For uranium(V1) concentrations above 1 X lop5 M, the percentage removal decreases
12
06
Ionic
Strength
700
I 1
Urarwm
I 2
concentration
( MxlCh
Fig. 6. Removal of U(V1) from Na2C03 solutions of varying ionic strength in the presence (0) and in the absence (0) of oxine: pH = 11.3;CJC, = 7; HOx = 7 X 10m4 M. Fig. 7. Effect of U(V1) systems using constant 1.2; pH = 10.8.
concentration on its removal from ammonium carbonate-oxine collector (7 X 10e4 M) and oxine (7 X 10v4 M) concentrations:
/J =
272
with increase of the metal ion concentration and this is obviously caused by the inadequate collector:uranium and/or oxine:uranium ratio.
4, Discussion As shown in Fig, 1, percentage removals of about 46 I 63 were obtained in the pH range 9.4 - 10.7 when a collector/uranium molar ratio of 3 was used for the foam separation of uranium(V1) from ammonium carbonate solutions of ionic strength 12= 0.1. These results can be interpreted by the following considerations. In the foam separation systems studied the ionic strength was kept constant, except at the fuwest pH values where NHBHCOs was used and the ionic strength was adjusted with (NH4)&0s and NH*OH. Thus, as the pH was increased the CQs2- concentration decreased. In the pH range 9.4 - 10.7 the carbonate concentration (and consequently the COs2-/U(VI) ratio) is fairly high and the uranium should therefore be totally present as the uranyl tricarbonate complex UOs (COB), 4- . From electrostatic considerations it would be expected that this tetravalent ion would be removed with the strong cat-ionic collector CTAB at a ratio C, ,E, of 4. In accord with this, a maximum percentage remuval of 75 should be achieved with the CTABf uranium molar ratio of 3 used in this investigation. Experimentally, the obtained percentage removal is lower (about 46 at pH 9.4) because of the need for excess collector to stabilize the phases and to overcame the competitive effects of the carbonate and the other anions present in the medium. The high percentage removal obtained at pH 11 cannot be attributed to only the decrease of the COs2-/OH- ratio and the consequent decrease of the com~titive effect of the carbonate anion (assuming that CUa2- has a greater flotation tendency than OH-). This high percentage removal can be attributed to the formation of uranates and their subsequent flotation at a low collector/metal ion ratio due to their low charge densities. The formation, in the present case, of a compound other than the uranyl tricarbonate complex, presumably a uranate, is suggested by the rate data analysis which indicates that, in the case of the oxine-free systems, only one uranyl species (obviously U0,(CQs)s4-) exists at pH 9.4, whereas two (or more) species are formed at pH II (Fig. 3). These species are probably uranyl tricarbonate and ammonium uranates. It can also be seen in Fig. 1 that the percentage removal of U(V1) from sodium carbonate increases with increase of the pH to reach an optimum value at approximately pH 11.7 and then decreases with further increase of the alkalinity. According to the literature [ 1,2], uranates begin forming in carbonate media at pH values above 11, Therefore the decrease of the percentage removal at pH values above 11.7 can be attributed to the formation of uranates and/or the decreased ionization of the collector at high hydroxyl ion concentrations. However, it should be mentioned that the structures of uranates are complex and vary with the pH and other precipitation conditions
213
[23 - 311. Therefore the decrease of the percentage removal from NasCOs media of pH 11.7 and the increase of the percentage removal from (NH4)aC03 solutions of pH 11 may be due to differences in the nature and consequently of the flotation tendency of the uranate species formed in the two cases. For the U(VI)-COs2- systems investigated, no insoluble sublates were formed with CTAB. This observation, coupled with the fact that the rate data are best described by the semilogarithmic rate equation (eqn. l), indicates that the foam separation systems were homogeneous and that the flotation was controlled by a partition mechanism. In systems where the sublate is soluble, ion-pair attraction between the collector cations and the metal complex anions is probable so that electroneutrality in the foam is maintained [ 211. Accordingly, the presence of a large amount of cations other than uranium (i.e. high ionic strength) should decrease uranium removal. Since uranyl tricarbonate is tetravalent and has a higher charge than the carbonate, bicarbonate or the hydroxyl ions, should be removed in preference to the other three anions. At uo2w3)34high ionic strength, i.e. at high total concentration of HCOs-, COa2- and OH-, attraction between these anions and CTAB becomes more probable and U(V1) removal decreases as a direct function of the increase of the free anions/U02(C0s)s4- ratio. This explains the decrease in percentage removal at a given collector concentration when the ionic strength is increased (Fig. 6). Trials to obtain complete removal by increasing the collector concentration were unsuccessful. At low collector concentrations the ultimate uranium removal directly increases with increase of the collector concentration but beyond certain limits uranium removal decreases (Fig. 4). This seems to be due to the critical micelle concentration being exceeded with consequent formation of micelles which are known to affect flotation deleteriously [4]. The fact that the critical micelle concentration is affected by the concentration of ions of charges opposite to that of the collector [32] explains the dependence of the critical collector concentration on the ionic strength of the solution [lo] . As the ionic strength is increased the total carbonate concentration increases and consequently the critical micelle concentration is lowered with a corresponding lowering of the critical collector concentration. With the aims of improving the foam separation of uranium and of extending the process to solutions of relatively high ionic strength, attention was directed towards the quinolinate complex of the metal ion. It is known that the addition of the Squinolinol to a solution of the very stable uranium complex U02(COs)s4- causes the uranium to precipitate at an elevated pH (11-12) as an orange solid [ 12,131. In dilute aqueous solutions precipitation does not occur, but instead a red soluble complex U02(C9HGN0)s- is formed. Since the charge of the uranyl-S-quinolinate complex is a quarter of that of many1 tricarbonate it is to be expected that the amount of collector that would be required to float a given quantity of U(V1) would be four times lower when uranium exists as U02(C9HGN0)athan when it exists in the form of U02(C03)34-. On this basis, the effect of
274
oxine on the foam separation of U(V1) from carbonate solutions was investigated. The results obtained (Fig. 1) indicate that the addition of oxine to the uranium-carbonate systems markedly improves the percentage removal though it does not significantly affect the general shape of the curve of pH removal. Both in the presence and in the absence of oxine the percentage removal increases with the alkalinity to reach a maximum at pH 11.7 or slightly higher. At higher alkalinities the percentage removal decreases owing to the decomposition of the uranyl carbonate and oxinate [ 331 complexes and the precipitation of uranates. At pH values below about 9, the percentage removal of U(V1) from the carbonate-oxinate systems decreases with decrease of the alkalinity (Fig. 1) implying that HCOs- deleteriously affects the removal of U(V1) from carbonate-oxinate solutions. Also as indicated by the results obtained for the effect of the ionic strength on the removal of uranium from NasCOsoxinate systems of pH 11.2 (Fig. 6), the carbonate ion impairs the foam separation results. These deleterious effects of COs2- and HCOs- on the percentage removal of uranium from carbonate-oxinate systems are not attributable only to their competition with the uranyl tricarbonate and oxinate complexes for the collector cations, but can be also attributed to the effects of HCOs- and COs2- on the degree of formation of the uranyl triquinolinate complex itself. It is known that B-hydroxyquinoline (HOx) is only slightly soluble in the pH range 5.5 - 8.5 [34] and that its solubility increases with increase of the pH because of its dissociation according to the formula HOx=Ox-+H+
(3)
The formation of a uranyl triquinolinate can be described by the equations 3HOx + U02(COs)s
4- = U02(Ox)s-
complex
+ 3HCOs-
from HOx or Ox-
(4)
and 30x-
+
uo2(co3)34-
e
UO2(Ox)3_
+
3cos2-
(5)
These equations imply that the formation of U02(Ox)sshould decrease with increase of either or both of COs2- and HCOs- in the medium. In agreement with eqns. (4) and (5), the observed percentage removal increases with increase of the HOx concentration to an optimum value (Fig. 5). Obviously the oxine concentration that is required to achieve the optimum depends on the uranium concentration, the collector concentration and the total carbonate ions concentration. With the other factors held constant, a lower amount of uranium in the system results in a higher percentage removal because there will be a greater excess of collector and oxinate ions (Fig. 7). However, a very high excess of HOx impairs the percentage removal (Fig. 5) owing to the competition of the excess quinolinate ions with the uranyl complexes for the collector cations.
275
To recover the collector and oxine from the foam phase for recycling purposes, it is suggested first to heat the scum with dilute acid to destroy the carbonate and then to treat it with an alkali solution whereby uranium is precipitated as the alkali uranate [ 4, lo] whilst the collector and oxine remain in solution as ammonium hydroxide and oxinate. The collector can then be converted into its bromide and separated from the oxine by shaking with a solvent, as for example benzene or Ccl,, which can extract oxine and leaves the collector.
Nomenclature
cc Cnl Cc/C, HOX Mm
Mt t
p
initial collector concentration initial uranium concentration molar ratio of collector concentration to uranium oxine (8hydroxyquinoline) maximum fraction of uranium removed uranium removed at time t time of bubbling in minutes ionic strength
concentration
References 1 Swedish Patent 148,719 (1957) to P. 0. Stelling and J. E. K. Svenko. C. F. W. D. Wilkinson, Uranium Metallurgy, Vol. 1, Interscience, New York, 1962, p. 95. 2 F. A. Forward and J. Halpern, J. Met., 6 (1954) 1408. 3 P. Mouret, P. Pottier, P. Soudan and J. Le Bris, 2nd Int. Conf. on Peaceful Uses of Atomic Energy, Geneva, Vol. 3, United Nations, Geneva, 1958, p. 356. 4 F. Sebba, Zon Flotation, Elsevier, New York, 1st edn., 1962. 5 R. Lemlich, Adsorptive Bubble Separation Techniques, Academic Press, New York, 1st edn., 1972. 6 C. Jacobelli-Turi, A. Barocas and F. Salvetti, Gazz. Chim. Ital., 93 (1963) 1493. 7 A. Barocas, C. Jacobelli-Turi and F. Salvetti, J. Chromatogr., 14 (1964) 291. 8 C. Jacobelli-Turi, A. Barocas and S. Terenzi, Ind. Eng. Chem., Process Des. Dev., 6 (1967) 161. 9 E. Jude and N. Fratila, Proc. 10th Mineral Processing Con@., London, 1973, Institute of Mining and Metallurgy, London, 1974, p. 707. 10 K. Shakir, J. Appl. Chem. Biotechnol., 23 (1973) 339. 11 K. Shakir, Chem. Ser., 6 (1974) 70. 12 W. E. Clifford, P. Noble, Jr., and E. P. Bullwinkel, U.S. Atomic Energy Commission
Rep. RMO-2623. 13 E. P. Bullwinkel and P. Noble, Jr., J. Am. Chem. Sot., 80 (1958) 14
I. I. Chernyaev,
2nd Znt. Conf. on Peaceful 15 16
17
2955. G. V. Ellert, R. N. Shcholokov and V. P. Marakou, Uses of Atomic Energy, Geneva, Vol. 28, United Nations,
V. A. Golovnyo,
Geneva, 1958, p. 235. A. I. Vogel, Quantitative Inorganic Analysis, Longmans, p. 471. K. Shakir, Sep. Sci., 8 (1973) 345. S. P. Sangal, Micro&em. J., 7 (1963) 331.
London,
2nd edn.,
1953,
276 18 A. A. Nemodruk and L. P. Glukhova, J. Anal. Chem. USSR, 18 (1963) 85. 19 D. F. Peppard, G. W. Mason, I. Hucher and F. A. J. A. Brandas, J. Inorg. Nucl. Chem., 24 (1962) 1387. 20 K. Kimura, Bull. Chem. Sot. Jpn, 33 (1960) 1038. 21 A. J. Rubin, J. D. Johnson and J. C. Lamb III, Ind. Eng. Chem., Process Des. Deu., 5 (1966) 368. 22 A. J. Rubin, J. Am. Water Works Assoc., 60 (1968) 832. 23 J. Sutton, J. Inorg. Nucl. Chem., 1 (1955) 68. 24 J. K. Dawson, E. Wait, K. Alcock and D. R. Chilton, J. Chem. Sot., (1956) 3531. 25 J. Maly and V. Vesely, J. Inorg. Nucl. Chem., 7 (1958) 119. 26 E. V. Garner, J. Inorg. Nucl. Chem., 21 (1961) 380. 27 E. H. P. Cordfunke, J. Inorg. Nucl. Chem., 24 (1962) 303. 28 M. Saxena, J. Indian Chem. Sot., 39 (1962) 831. 29 M. E. A. Hermons and T. Markestein, J. Inorg. Nucl. Chem., 25 (1963) 461. 30 P. C. Depets and B. 0. Loopstra, J. Inorg. Nucl. Chem., 25 (1963) 945. 31 V. Baran and M. Tympl, J. Inorg. Nucl. Chem., 28 (1966) 89. 32 M. L. Corrin and W. D. Harkins, J. Am. Chem. Sot., 69 (1947) 683. 33 J. E. Ricci and F. J. Loprest, J. Am. Chem. Sot., 77 (1955) 2119. 34 H. Irving, J. A. D. Ewart and J. T. Wilson, J. Chem. Sot., (1949) 2672.