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Extraction of some heavier lanthanides in acidic chloride solutions by di(2-ethylhexyl) phosphoric acid
J. inorg,nucl.Chem.,1968,Vol.30, pp. 1617to 1633. PergamonPress. Printedin Great Britain
EXTRACTION OF SOME HEAVIER LANTHANIDES ACIDIC CHLORIDE SOLUTIONS BY Di(2-ETHYLHEXYL) PHOSPHORIC ACID*
IN
THOMAS C. OWENS and MORTON SMUTZ Institute for Atomic Research and Department of Chemical Engineering, Iowa State University, Ames, Iowa
(Received 17 October 1967) Abstract--Equilibrium data have been obtained for the systems MCI3-HCI-H20-1 M D2EHPA (in Amsco Odorless Mineral Spirits), where M is used to designate the trivalent lanthanides Gd, Dy, Er, and Yb, over a wide concentration range of the lanthanide chloride in aqueous solution. For a particular acidity and a given lanthanide concentration in the aqueous phase, the extractability increased as the atomic number of the lanthanide increased. Equilibrium acid concentration was used as a parameter for the equilibrium curves. The general shape of these curves changed as the aqueous acid concentration increased from 1.4 M HCI to 5.0 M HCI. The separation factor of Dy with respect to Gd,/3Dy,~, was also obtained in acidic chloride solutions. The total lanthanide chloride concentration, the total acidity, and the molar ratio of GdCla to DyCl3 in the equilibrium aqueous phase had only slight effect on the separation factor. The separation factor was in the range of 9-13 for the conditions studied. It was found that Yb and Lu were not removed from the organic phase by stripping four times with double volumes of 6 M HCI and other methods of back-extracting which have been previously used. A 20 per cent hydrofluoric acid solution, however, completely removed the solute from the organic phase as the insoluble fluoride.
INTRODUCTION
THE EXTRACTION o f lanthanides into di(2 ethylhexyl) phosphoric acid (hereafter designated D 2 E H P A ) has been studied by a number of investigators, but equilibrium data at greater than tracer concentrations have become available[I-3] only recently. The purpose of this research was to extend the studies to the heavier lanthanides. Peppard e t a/.[4], showed that the extractability of D 2 E H P A for the lanthanides increased with atomic number, was inversely third power dependent on the H + activity in the aqueous phase, and was limited by the formation of a gelatinous third phase when the solute concentration of the organic phase was too high. In extracting the heavier lanthanides with this solvent at high aqueous concentrations it is, therefore, necessary to use relatively high aqueous phase acidities to avoid gel formation in the organic phase. *This work was performed under contract with the Atomic Energy Commission. Contribution No. 2206. 1. 2. 3. 4.
T. Goto and M. Smutz, J. inorg, nuel. Chem. 27, 1369 (1965). T. Lenz and M. Smutz, J. inorg, nucl. Chem. 28, 1110 (I 966). S. G. K. Nair and M. Smutz,J. inorg, nucl. Chem. 29, 1787 (1967). D . F . Peppard, G. W. Mason, J. L. Maier and W. J. Driscoll,J. inorg, nucl. Chem. 4,334 (1957). 1617
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T . C . OWENS and M. SMUTZ
Sato[5, 6] suggested that the anion would be present in the organic phase if the extractions were carried out at high acidities. Another study [7] was also concerned with the extent of chloride extraction into D2EH PA at high acidities of the equilibrium aqueous phase. EXPERIMENTAL
Materials. The lanthanide oxides used in this study, obtained from the Rare-Earth Separations Group of the Ames Laboratory, had a purity greater than 99-9 per cent with respect to the presence of other lanthanide oxides as determined by emission spectroscopy. The acids and all other reagents were of reagent grade. The D2EHPA, obtained from Union Carbide, was mixed with Amsco Odorless Mineral Spirits (hydrocarbon diluent) as obtained from American Mineral Spirits Company to form a 1 M solution of D2EHPA on a monomeric basis (hereafter designated 1 M D2EHPA). The undiluted D2EHPA was 98.8 per cent monoacidic. Preparation of stock solutions. The lanthanide oxides were dissolved in excess HCI, and the excess HCI was evaporated by heating. Then HC1 was added until a slight excess of HCl was present. Equilibration of aqueous and organic phases. A known volume of aqueous feed solution was equilibrated with an equal volume of the organic solvent (1 M D2EHPA) in a separatory funnel by shaking the two phases vigorously for 3 min, allowing the phases to separate for at least 1 hr, and then shaking vigorously for an additional 2 rain. The separatory funnel was then allowed to sit undisturbed for at least 12 hr before the phases were separated and the analysis begun. ,4nalysis of the aqueous and organic phases. After separation of the phases, the organic phase was often somewhat cloudy, probably due to entrained aqueous material. To remove the cloudiness, the organic phase was centrifuged for at least 15 min in a clinical centrifuge. After centrifuging, the organic phase was quite clear, though not colourless. A portion of the organic phase was then measured volumetrically into a separatory funnel where it was contacted four times with equal volumes of approximately 6M HCI. This removed the solute quantitatively from the organic phase for all systems except when either YbC13 or LuCI3 was the solute. The aqueous strip solution was then analyzed for lanthanide content, and the concentration of lanthanide in the organic phase was calculated. When either YbCIz or LuCl3 was the solute, the lanthanide concentration in the organic phase was obtained by difference between the initial aqueous feed and final aqueous concentrations of the solute. The total lanthanide concentration in the initial and final aqueous solutions and in the organic backextract was determined by EDTA titration using pyridine as the buffer and arsenazo as the indicator according to the procedure given by Fritz et al.[8]. The acidity of the initial and final aqueous solutions was measured using the cation exchange resin Dowex 50 × 8 as outlined by Adams and Campbell[9]. The cation exchange resin exchanged three hydrogen ions for one lanthanide ion; then the total hydrogen ion present was titrated using standardized sodium hydroxide with phenolphthalein as the indicator. The EDTA was standardized by titrating a solution of known lanthanide chloride concentration. This known lanthanide concentration was obtained by precipitating the lanthanide in an aliquot of the solution as the oxalate, and then burning the oxalate to the oxide. The oxide was then weighed on an analytical balance. Separation factor studies. The methods of equilibrating the phases, stripping the organic phase and determining the total concentration of the solutes in both the organic and aqueous phases have been discussed previously The composition of the phases containing both Gd and Dy was determined using a spectrophotometric method given by Banks and Klingman[10]. Aliquots of the equilibrated aqueous samples and the organic strip solution were precipitated with oxalic acid and ignited to the 5. 6. 7. 8. 9. l 0.
Taichi Sato,J. inorg, nucl. Chem. 25,109 (1963). Taichi Sato,J. inorg, nucl. Chem. 27, 1853 (1965). T. C. Owens, Ph.D. Thesis, Iowa State University (1967). J. S. Fritz, R. T. Oliver and D.J. Pietrzyk,,4nalyt. Chem. 30, 1111 (1958). J. F. Adams and M. H. Campbell, U. S. ,4 tomic Energy Commission Report HW-76363 (1963). C. V. Banks and D. W. Klingman,,4 nalytica chim. ,4 cta 15,356 (1956).
Extraction of some heavier lanthanides in acidic chloride solutions
1619
oxides. Known weights of these oxide mixtures were dissolved to form approximately 1 M perchlorate solutions, and the relative amounts of Gd and Dy were determined using a Beckman DU-2 spectrophotometer. The Gd absorbance was read at 272.8 m/~ with a slit width of 0.05 mm using a hydrogen lamp. The Dy absorbance was read at 908.0 m/~ with a slit width of 0.03 mm using a tungsten lamp. Presence of chloride in the equilibrium organic phase. To test for the presence of chloride in the extracted species an aqueous solution of silver nitrate was mixed with a portion of previously centrifuged equilibrium organic phase. The presence of the characteristic white precipitate indicated the presence of chloride in the organic phase. Removal of Yb and Lu from the organic phase using 20 per cent HF. The solutes Yb and Lu were removed from the organic phase by contacting the solvent four times with double volumes of 20 per cent hydrofluoric acid. The lanthanide was precipitated as the lanthanide fluoride. The lanthanide fluoride was filtered and dried, and then heated to 700°C in an HF atmosphere to insure the formation of the fluoride. The weight of the fluoride salt indicated that all of the lanthanide had been removed from the organic phase. Qualitative flame emission techniques were used directly on the organic phase to determine the extent of the removal of the solute. This method indicated that all of the Lu and Yb had been removed from the solvent when a 20 per cent hydrofluoric acid strip solution was used. DISCUSSION
Symbols M, N -lanthanide cations with a+3 valence. [ ] a - activity of the species in the aqueous phase. [ ] o - activity of the species in the,organic phase. ( )a--concentration of the species in the aqueous phase, moles/1. ( ) o - concentration of the species in the organic phase, moles/1. KM-- equilibrium constant of the reaction in which M is extracted. ~(Ea°)M-thermodynamic distribution coefficient, defined as the total activity of all M containing species in the organic phase divided by the total activity of all M containing species in the aqueous phase at equilibrium. (Ea°)M-distribution coefficient, defined as the total concentration of all M containing species in the organic phase divided by the total concentration of all M containing species in the aqueous phase at equilibrium. fl~M- stability constant of the species M A~+(~-o. [MAi+(a-i)]a [M+3la [m-1]a i
T/3M,~-- thermodynamic separation factor. -- T(Ea°)M when both M and N are present together. T(Ea°)N'
/3 M,N-- separation factor.
= (Ea°)M when both M and N are present together. (Ea°)s '
The extraction of the trivalent lanthanide ions by D 2 E H P A , which is dimeric in non-polar solvents[11, 12], from aqueous solutions having low solute concentration and rather low acidities has been widely reported. Some investigators 11. O . F . Peppard, J. R. Ferraro and G, W. Mason, J. inorg, nucl. Chem. 7, 231 (1958). 12. J. R. Ferraro, G. W. Mason and D. F. Peppard, J. inorg, nucL Chem. 22, 285 (1961 ).
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T . C . OWENS and M. SMUTZ
[5, 13, 14] have found that the anion present in the aqueous phase may also be present in the extracted species. The identities of anion-containing species have not been established, but the extraction of metal chloride complexes and undissociated HC1 seems possible. Reactions which allow for (1)the extraction of the trivalent metal by the ion exchange reaction, (2) the extraction of metal chloride complexes, and (3) the extraction of HCI are given by Equations (1) and (2). M+3+ n C I - + b(n) (HG)c¢,o ~ MCl,,Hb~,,~oo+,,-3Gb¢,o~o,) + (3--n) H+,n = 0,1,2,3.
(1)
H + + CI- + aX ~ HCI.aX.
(2)
These equations assume that HC1 is extracted as a neutral species, and that metal chloride complexes, with the exception of the case when n = 3, are extracted by an ion-exchange reaction. When the neutral molecule is extracted, an extraction reaction other than an ion exchange reaction is necessary. In Equation (2) the quantity a is constant, and X is the solvent entity which may extract HCI. The quantities b(n) and c(n) are variables that depend on the discrete variable n. When n --- 0 in Equation (1), the extraction of the trivalent lanthanide ion, the values on b(n) and c(n) are 3 and 2, respectively. The value ofc(n) depends on the type of solvent molecule (i.e. monomer, dimer, or trimer, etc.) which extracts the solute species, and the value of b(n) denotes the number of these solvent entities required to perform the extraction. The equilibrium constants for the extraction reactions given by Equations (1) and (2) are shown in Equations (3) and (4), respectively. KM(n) =
[MCl"Hb°°¢°°+"-3G~'°c°')]°[H+]a¢3-"~ [M+3], [Cl-]a n [(HG)c~,~]o~"~ Kncl =
[HCI'aX]a [H+]a[Cl-]a[X]a a
n = 0 , 1 , 2 , 3.
(3)
' (4)
The brackets refer to the activity of the species, and it is important to note that the values of the equilibrium constants given in Equation (3) depend on the value ofn. The effect of the acidity of the aqueous phase on the extraction can be determined by considering Equations (3) and (4). The extraction of the neutral HCI would increase as the acid activity is increased, as would be expected. According to Equation (3), the increase of acid activity decreases the extraction of the metal chloride complexes, except when the neutral species is extracted and there is no acid dependency. However, the acid activity also affects the relative activities of the extractable species. When the acid activity increases, the proportion of the higher metal chloride complexes in the aqueous phase also increases. Therefore increased acid activity seems to favor the extraction of neutral HCI and the more 13, V. E. Shaw and D. J, Bauer, U. S. Bur. Mines Rep. 7691 (1964). 14. T. (3. Lenz and M. Smutz, J. inorg, nucl. Chem. 30, 621 (1968).
Extractionof someheavierlanthanidesin acidicchloridesolutions
1621
complexed metal species while the lower acid activity favors the extraction of the trivalent metal (i.e. n = 0 in Equation (1))~ Due to the high acid concentration in the aqueous phase, it may be assumed [15] that the only metal chloride complexes are M ÷3, MC1÷2, MCI2+1, and MCI3. The distribution of the metal among these species can be given by the stability constants which are shown in Equation (5). [MCli+tz-°]a rim = [M+3]a[Cl_]al,
i = 1,2, 3.
(5)
Equations analogous to Equations (3) and (5) may be written for the extraction of metal N. Combining all of the equations, as was done by Hesford and McKay [15] will result in Equation (6) for the thermodynamic separation factor where all of the reactions of Equation (1) occur simultaneousl' r.
T(Ea°)M = T~[~M,N= r(EaO)N
~ KN(n) [CI-]~"[(HG)~,,)]o ~'° [H+]a<3-')"
n=0
(6) 3 KM(n) [Cl-]."[(HG)~oo]o ~"~ [H+la~3-,~ 71=0
l1
1
This equation for r/3M,N is the extension referred to by Lenz and Smutz[14], and is also particular to the case when a single anion, chloride in this study, is present. The first factor of Equation (6) is a ratio which depends on (1) the equilibrium constants of the extraction reactions, (2) the concentration of the anion, (3) the aggregation of the solvent, (4) the solvent dependency in the extraction reactions, and (5) the acid concentration of the aqueous phase. The second factor is essentially a ratio of the ability of the two metals to complex with the anion in the aqueous phase. Because of the lanthanide contraction, it is probable that as the atomic number increases throughout the series, the degree of complexing with the anion would also increase. This single consideration would indicate that D2EHPA would preferentially extract the lower lanthanides; however, this is not the case since it is observed that as the atomic number is increased the extractability increases very significantly. Therefore, the degree of aggregation of the solvent and the preference of the solvent for one lanthanide over another, together with effects of the aqueous phase, determine the performance of the solvent in a particular system. The effects of the aqueous phase may be related to complexing in the aqueous phase and changes in the coordination number of the metal ions in the aqueous phase as their atomic number increases. The presence of three-dimensional polymers when the ratio of the concentration of the trivalent lanthanide to the concentration of the active solvent is greater than ~ has been noted[4, 16]. This explanation of the phenomenon appears to be satisfactory only as a first approximation, since previous work[17] in this group 15. E. Hesfordand H. A. C. McKay,Trans. Faraday Soc. 54, 573 (1958). 16. D. F. PeppardandJ. R. Ferraro,J. inorg, nucl. Chem. 10,275 (1959). 17. T. C. Owens,M. S. Thesis, IowaState University(1965).
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T . C . OWENS and M. S M U T Z
while using the undiluted solvent, indicated that the gelatinous third phase may form when the ratio of the metal concentration and the active solvent concentration was much less than ~. These studies also indicated that even though the ratio of metal concentration to solvent concentration was greater than ~ the organic phase was still a liquid, though quite viscous, and not a gelatinous material. The extraction of metal chloride complexes by the ion-exchange reactions given in Equation (1) would allow the ratio of metal concentration to solvent concentration in the organic phase to exceed ~ without gel formation. Similarly, the extraction of the undissociated lanthanide chloride would result in a greater ratio of metal concentration to solvent concentration in the light phase. RESULTS
Equilibrium data for the systems MCI3-HCI-H20-1 M D 2 E H P A Equilibrium data f o r the systems MCI3-HCI-H20-1 M D 2 E H P A were obtained where the metal chlorides investigated were GdCI3, DyCI3, ErCI3, and YbC13. Based on the extraction reactions given by Equations (1) and (2), the following three variables must be considered: (1) the aqueous phase lanthanide chloride activity, (2) the aqueous phase activity of HCI, and (3) the activity of the metal-containing species in the organic phase. As may be seen from Equation (1), the amount of extraction decreases as the acid activity of .the aqueous phase increases for a particular activity of the solute in the aqueous phase, except when n = 3. Since the experimental determination of the pertinent activities was not possible, the respective concentrations were measured. Figures 1-4 present the I
I
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OIB O~ D ~ + ~
016
0.8 M 1.4M 2,8M 5,0M
H CI. H CI. H CI. HCI
014
~012 0~ o
¢'0.1o
/0 ~ ~
"0--
"0" ~ ~ 0 - - ~ ,_0.....
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r l - ~
-Q-- - -
--O--- ~
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rl
1,50
1.75
Q
O.OC 0.25
0.50
0.75
1.00
t25
MOLES
2,00
2.25
~_50
G d C I 3 / L I T E R , AOUEOUS
Fig. 1. Equilibrium data for the system GdCI~-HCI-H~O-! M D2EHPA.
Z75
Extraction of some heavier lanthanides in acidic chloride solutions t
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1623 I
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0.18 A ~ 1.4M HCI. n ~ 27 M H CL x ~ 5 0 M H CL
0.16
0.14 &
-~
/
~0.12
~_~_.&~
_ -i,~ ' ~ a ~ -'~
I k
~o.lo ...i ~0.08 _J ~0.06
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0.02
/X t
0.00 0
0.25
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0.50
0.75
1.00
1.25
I
MOLES
I
1.50
I
1.75
I
2.00
I
~25
I
2.50
2.75
~ C ~ / L K E R , AQUEOUS
Fig. 2. Equilibrium data for the system DyC13-HCI-H20-1 M D2EH PA. I
I
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t
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0.1 8
0 - 1.4MHCI. & ~ 2.SMHCl. n ~ 5.0M H CI.
i 0,1 6
0.14
0.12 0
m" ill
I b
..~
o.lo T
0.08
_
~ 0.06
-
i i
~ j
~
/ / / 0.04
0.02
0.00
-d / ?
I 0.25
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I
0.50
0.75
LO0
L25
MOLES
I 1.50
I 1.75
I ZOO
I 2.25
I 2.50
Er CIs/LITER, AOUEOUS
Fig. 3. Equilibrium data for the system ErCI3-HCI-H20-1 M D2EH PA.
I 2.75
1624
T.C. OWENS and M. SMUTZ I
I
f
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I
I
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I
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I
1
016
J
J
"n.
a
5 0 M HCI. 7 . 7 M HCL
0.1 6
/ / 0.1 4 _o n,. 0.12 0 0.10
/d
0.08 ,J o 0.06
0.04
0.02
ooo
I 0.25
I 0.50
I 0.75
I
I
1.00
1.25
MOLES
I I l~ 0
I I f7 ~
I ZO O
I Z .2 5
I 2 .~ 0
I 2~~
Yb CI3/LITER,AQUEOUS
Fig. 4. Equilibriumdatafor the systemYbCI3-HCI-H20-I M D2EHPA. equilibrium data in graphical form indicating the concentration of the lanthanide in the organic phase as the aqueous lanthanide chloride concentration and acidity were varied. The points indicate experimental results; however, the concentration of HC1 in the equilibrium aqueous solution must necessarily be associated with each point. Equilibrium curves have been drawn on each of the figures to indicate the equilibrium data at constant HCI concentration in the aqueous phase. Table 1 gives the equilibrium data used in plotting Figs. 1-4. The equilibrium concentrations of the lanthanide in both the aqueous and organic phase, and the equilibrium acidity in the aqueous phase are given. Figures 1-4 show that at relatively low acidities (0.8 and 1.4 M) the shape of the equilibrium curve resembles the shape of equilibrium curves obtained by Lenz and Smutz[2] and Nair and Smutz[3] for La, Pr, Nd, and Sm. The organic phase concentration of the solute increased to a maximum and then decreased slowly as the lanthanide concentration of the aqueous phase increased. Then, as the equilibrium acidity increased above 1.4 M HCI, the organic concentration increased rapidly to its maximum value and stayed nearly constant as the lanthanide concentration of the aqueous phase increased. At rather high solute concentrations in the aqueous phase, when Er was present in the system, there was a slight increase in the organic phase concentration of solute for acidities of 1.4 and 2-5 M HC1. At still higher acidities, regardless of the lanthanide present in the system, the solute concentration in the organic phase increased steadily as the lanthanide chloride concentration of the aqueous phase concentration increased.
Extraction of some heavier lanthanides in acidic chloride solutions Table 1, Equilibrium data for the system MCla-HC1-H20-1 M D2EHPA where M designates Gd, Dy, Er, and Yb Aqueous conc. (moles/L)
Final acidity (moles HCI/1.)
Organic conc. (moles/l.)
0.049 0.049 0-255 0"255 0.501 0.501 0'746 0'744 0.992 0.991 1"25 1.24 1.48 1-49 1-73 1.73 1.99 1"98 2'23 2.22 2"47 2'47 2-72 2"71
0"812 0.811 0"776 0"779 0-808 0.808 0'846 0.849 0-821 0"824 0"845 0-845 0.787 0-765 0"773 0"803 0.735 0'776 0"769 0-786 0'781 0"832 0"701 0"768
0.069 0-069 0.097 0.097 0-100 0.100 0"099 0.099 0'098 0"098 0.094 0"094 0.091 0-091 0.086 0.086 0.083 0"083 0"078 0"078 0-075 0'076 0'072 0-072
0-076 0"076 0'267 0-267 0.514 0"514 0"766 0.766 1"02 1.02 1'28 1.27 1"52 1"52 1"76 1-76 2.02 2.02 2'26 2'26 2.50 2.49
1-43 1.43 1.43 1-42 1.41 1.39 1.40 1"40 1'37 1'41 1'40 1"37 1-40 1'40 1-40 1"40 1.38 1.40 1.38 1-32 1.46 1.47
0-027 0.027 0.048 0.048 0.055 0"055 0"055 0"055 0'053 0"053 0-049 0-049 0"045 0"045 0"041 0.041 0.039 0-039 0.036 0-036 0.035 0.035
GdCls
1625
1626
T . C . OWENS and M. SMUTZ Table 1. (Contd.) Aqueous cone. (moles/L)
Final acidity (moles HC1/I.)
Organic cone. (moles/k)
0.074 0.074 0.281 0-281 0.535 0-537 0.788 0.789 1.04 1'04 1.28 1.28 1.54 1-53 1.78 1.78 1.99 1.98
2.89 2.88 2.81 2.81 2.83 2.86 2-80 2.79 2.83 2.82 2.85 2.84 2.87 2-87 2-86 2.84 2.87 2.84
0.0026 0.0026 0-0078 0.0078 0-0103 0.0101 0.0112 0.0112 0.0113 0-0112 0.0107 0.0109 0.0107 0.0108 0.0110 0.0110 0.0117 0.0118
0.056 0.056 0.251 0.251 0.504 0"504 0.763 0.764 1.02 1.02 1.28 1.27 1.27 1.27
4'97 4.98 4-99 4.98 5"02 5"01 4-95 4.93 4"96 4"95 5.00 4'91 5"08 5"09
0.0002 0.0002 0.0011 0.0011 0.0021 0.0021 0.0031 0.0031 0.0043 0.0043 0.0066 0.0066 0.0071 0.0071
0.065 0.065 0.265 0-270 0.510 0.509 0.768 0.766 1.01 1.01 1.25 1.25 1.48 1.49
1.38 1-37 1 "38 1 "37 1.37 1.35 1.37 1 "34 1-35 1-35 1 "37 1 "37 1.37 1 "36
0'108 0'108 0.128 0" 128 0"132 0.132 0"130 0"132 0-130 0.130 0-127 0"126 0.126 0"125
GdCIn (Contd.)
DyCI3
Extraction of s o m e heavier lanthanides in acidic chloride solutions Table 1. (Contd.)
Aqueous conc. (moles/L)
Final acidity moles HCI/I.)
Organic conc. (moles/l.)
1.73 1-73 1"98 1.98 2'23 2.47 2.47 2.45 2.44
1-41 1.42 1.47 1"43 1.44 1.43 1.47 1.55 1-47
0.123 0.121 0-118 0.116 0.116 0.113 0-116 0-116 0.116
0" 116 0"316 0.317 0.564 0"565 0.817 0.817 1-08 1 "08 1.34 1.34 1"60 1-59 1.85 1-85 2.09 2.10 0.065 0.065 0.276 0'276 0-518 0.518 0.761 0.762 1-01 1.00 1.25 1.26 1.42 1.43
2'81 2"78 2-78 2.75 2.74 2"72 2"74 2.76 2'76 2"73 2.73 2.61 2.62 2-65 2-60 2.74 2.68 4.98 5.00 4.98 4"96 5.00 4-99 4.94 4.94 4.92 4.92 5.04 5.01 5.10 5"03
0.040 0.059 0.059 0"066 0.066 0.069 0.069 0-068 0-069 0"069 0.069 0.070 0.070 0-070 0.069 0.070 0.071 0.006 0-005 0.017 0.017 0.023 0.024 0'028 0.028 0.032 0.032 0.036 0-036 0.042 0.042
0.037 0-037 0.040 0.040 0-225 0.225
1.46 1.48 1.54 1-53 1-48 1.48
0.122 0.122 0-114 0.115 0-142 0.143
DyC13 (Contd.)
ErC13
1627
1628
T . C . OWENS and M. S M U T Z Table 1. (Contd.)
ErCIa
Aqueous conc. (moles/L)
Final acidity (moles HCI/I.)
Organic cone. (moles/l.)
0.252 0.253 0.486 0"487 0-753 0.753 0.999 1.00 1.24 1.24 1.49 1.49 1-74 1-75 1 "96 1 "96 2.20 2-21 0.047 0.047 0.236 0"237 0.528 0.531 0.778 0-780 ! '03 1'04 1 '29 I "29 1 "54 1 "54 1 "79 1 "78 2"03 2"04 2"14
1 '52 1 '51 1.43 I "43 1.40 1.38 1"38 i "39 1 '35 1 "38 1 "35 1.34 1.32 1-30 1.72 1.77 1 "75 I "74 2'53 2.53 2.53 2"53 2.55 2"56 2'52 2.51 2.40 2.40 2.35 2.38 2"27 2-27 2"74 2.57 2"50 2'54 2"59
0"136 0"136 0.145 0-146 0" 145 0" 145 0'142 0.142 0"142 0-142 0-142 0-141 0" 142 0' 142 0.139 0-139 0'141 0"141 0"086 0-085 0.117 0"117 0"113 0"113 0"115 0.114 0"115 0-114 0"116 0'115 0'116 0"115 0.117 0.118 0"122 0"122 0"124
0.085 0-085 0-260 0.259 0.500 0.501 0.749 0.747 0.997 0.995
4.94 4.97 5.05 5.06 5.06 5.05 5"05 5"09 5.11 5"11
0-034 0.034 0.060 0.060 0.074 0'074 0.081 0-081 0"086 0-086
(Contd.)
Extraction of some heavier lanthanides in acidic chloride solutions Table 1. (Contd.) Aqueous conc. (moles/L)
Final acidity (moles HC 1/1.)
Organic conc. (moles/l.)
ErCI3 (Contd.) 1-24 1-24 1.48 1-48
5" 10 5"06 5-05 5.06
0'093 0.093 0.102 0-101
0.034 0.036 0"187 0.187 0.419 0.420 0"668 0'666 0'901 0"901 1-14 1-14 1"38 1"38 1.58 1.58 0.037 0.037 0.239 0"239 0"487 0'488 0"737 0"739 0"995 0-996 1"25 1'25 1-49 1.49
5-39 5.40 5'29 5'28 5.06 5"08 5"33 5'37 5-31 5"32 5"20 5-19 5-08 4.93 5.05 5.08 5"05 5'05 5.06 5"08 4-99 4-98 5.00 5.00 4-99 4"95 5.01 4.97 4-87 4.89
0'066 0.064 0"115 0"115 0'133 0' 132 0" 134 0-136 0-151 0-151 0"157 0.156 0.175 0"176 0.176 0-178 0.083 0"083 0-123 0"123 0'141 0" 140 0'152 0' 149 0.155 0' 154 0-154 0.153 0-164 0.164
0.044 0.043 0' 198 0.198 0.425 0"425 0"655 0'655
7'64
0"058 0"058 0" 106 0-106 0"!31 0' 131 0' 152 0"152
VbC~
7"72 7-69 7"55 7"57 7"64 7"70
1629
1630
T . C . OWENS and M. SMUTZ
The lowest acidity shown on each of the figures indicates approximately the lowest acidity at which the gelatinous third phase would not form. This is the reason that acidities lower than approximately 5.0 M HC1 were not reported when Yb was present in the extraction system. It is apparent that the shape of the equilibrium curves, at constant acid concentration, depends on the acid concentration in the aqueous phase and not on the identity of the solute present in the system. That is, at acidities of 1.4 M HCI, regardless of the lanthanide present, the equilibrium curve increases to a relative maximum and then decreases, while at 5-0 M HC1, the solute concentration in the organic phase increases as the solute concentration in the aqueous phase increases. The presence of chloride-containing species in the organic phase also depends largely on the acid concentration in the aqueous phase. At lower acidities (0.8 and 1.4 M HCI), the characteristic precipitate was not observed when AgNO3 solution was added to the organic phase. This indicated very little, if any, chloride in the equilibrium organic phase. At higher acidities (-2-5 M HCI), the amount of precipitate increased as the concentration of the lanthanide in the aqueous phase increased. At the highest acidities (5.0 and 7.7 M HC1), the presence of chloride was readily apparent, and the amount of precipitate did not noticeably change with increasing solute concentration in the organic phase. Figures 1-4 show that for a given aqueous lanthanide chloride concentration and HC1 concentration, the organic phase concentration of solute increased as the atomic number of the lanthanide increased. This is shown quite clearly in Fig. 5 where equilibrium data for Gd and Yb at aqueous acidities of 5.0 M HCI are plotted. Separation factor studies. Figure 6 shows results that have been obtained in determining the separation factor of Dy with respect to Gd using 1 M D2EHPA as the solvent and an acidic chloride solution of the lanthanide mixture as the aqueous phase. The molar ratio of DyC13 to GdCI3 varied from 50 : 50 to 30: 70 in the equilibrium aqueous phase and the two levels of equilibrium aqueous acidity investigated were approximately 1.4 M HCI and 2-1 M HCI. Due to the spread in the experimental data, it was difficult to draw conclusions about the effects of total aqueous lanthanide concentration, equilibrium acidity of the aqueous phase, and the molar ratio of the solutes in the aqueous phase, but some trends may be noted. It appeared that at constant acidity and constant molar ratio of the solutes the separation factor of Dy with respect to Gd increased to a maximum at about 1.5 M total lanthanide chloride concentration of the aqueous phase and then decreased as the total aqueous concentration increased further. However, there was not the marked change in the separation factor with total solute concentration that was observed when the separation factor,/3Sm,Nd, was determined[14, 18] for the perchlorate, nitrate, and chloride systems. It also appeared that changing the ratio of GdCI3 to DyClz in the equilibrium aqueous phase from 50:50 to 30:70 caused the separation factor to increase slightly; however, increasing the aqueous acidity appeared to decrease the separation factor slightly. It should be noted that, regardless of the conditions, the separation 18. C. Battista, C. Mize and M. Smutz. J. inorg, nucl. Chem. 28, 1121 (1966).
1631
Extraction of some heavier lanthanides in acidic chloride solutions I
I
1
I
I
I
I
I
I
I
Oot8 --
5M
HCl
0.16
0.14
0.12 -J
~
0.I0
~0.08
0.04
0.02 GdCl3 - 5 M
i
0.00 0
o.z~
i ~-l"'--'-'~
o~
0.:~
,.00
L.z5
HCl
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l
I
i.~
,.75
zoo
2.25
2.~
2.75
M O L E S OF RARE-EARTH CHLORIDE/LITER,
AQUEOUS
Fig. 5. Equilibrium curves for the systems YbCIz-HCI-HzO-I M D2EHPA and GdCIa-HCI-H20-I M D2EHPA at5.0 M HCI. 14.0 - . 130
--
12.0
--
I
I &
b
&
I
& x
4t ,,.o -
X
8 O
I0.0 --
O
x
O
"~
O 9.0--
O
re
8.0--
7.0 6.0-5.0 - -
o-
o
DvClm' GdCI= = 50, 50 AClDITY~ 1.4 M HCl m
ACIDITY ~ 1.4 M HCI
X OyC~, GdCI===30,70
m
~ 2,1 M HCl
o.o ~' o.oo
I o.s
I ~.o
I J,s
I 20
z.5
TOTAL RARE-EARTH CHLORIDE CONCENTRATION IN AQUEOUS PHASE, MOLES/UTER Fig. 6. Separation factors for the system GdCI:~-DyCI~-HCI-H20--1 M D2EHPA.
1632
T . C . OWENS and M. S M U T Z
factor of Dy with respect to Gd range from approximately 9 to 13. This agrees with the value of 10.4 for/3Dy,Gdobtained by Pierce and Peck [ 19]. Extraction of Lu in the system LuCI3-HC1-H20-I M D 2E H PA . In order to determine if the extraction of Lu from a LuCI3-HCI-H20-1 M D 2 E H P A was similar to the extraction of Yb from a Y b C I 3 - H C I - H 2 0 - I M D 2 E H P A system, a single equilibrium point of the former system was determined. The replicated data are shown in Table 2. It was observed that Lu was only slightly more extractable than was Yb under the same conditions of the equilibrium aqueous phase. This was as expected since Pierce and Peck [19] found the separation factor of Lu with respect to Yb to be only 1.86 for tracer levels in an aqueous perchlorate system. Table 2. Equilibrium data for the system L u C h - H C I H 2 0 - 1 M D 2 E H P A at approximately 5.0 M HCI. (Lu)a (moles/L) 1.00 1.00
(HCI)a (moles/L) 5.03 5.04
(Lu)o (moles/l.) 0.160 0.160
As in the case when Yb was the solute, it was not possible to completely strip all of the solute from the organic phase with three portions of 6 M HC1 using volumes double that of the organic phase. Consequently, the organic phase concentration of the solute was calculated by the difference of the concentration of LuCI3 in the initial and final aqueous phases. Stripping of Yb and Lu from equilibrated 1 M D2EHP,4. In initial experiments when using YbC13 as the solute in the highly acidic aqueous phase, it was found that stripping with four equal volumes of 6 M HCI would not remove all of the solute from the equilibrated organic phase. Higher concentrations of HC1 were used with no additional solute stripped, and, when stripped with 12 M HCI, there was a noticeable decrease in the amount of solute removed from the organic phase. Other acids of varying concentrations such as nitric acid, phosphoric acid, perchloric acid, and sulfuric acid were also used, but none of these reagents completely removed the solute from the solvent. The use of a reagent which would monomerize the solvent, and thereby reduce the extraction (2), was also tried. Alcohols, such as n-butanol and ndecanol, were added to the solvent, and then the mixture was contacted with HCI. After contacting the organic solutions with the acid, a third phase formed which seemed to stay with the liquid organic phase. There seemed to be no advantage to using this type of additive to the organic phase. The use of a 20 per cent hydrofluoric acid solution was successful in removing the solute from the equilibrated 1 M D 2 E H P A . The solvent was contacted four to five times with double volumes of hydrofluoric acid. The lanthanide was removed as the insoluble lanthanide fluoride, and there seemed to be no effect on the solvent after the contact. A 5 per cent solution of hydrofluoric acid was also tried, but it did not remove the solute completely. 19. T. B. Pierce and P. F. Peck, Analyst 88, 217 (1963).
Extraction of some heavier lanthanides in acidic chloride solutions
1633
With LuCI~ as the solute, five portions of double volumes of 6 M HCI would remove only about 70 per cent of the solute from the organic phase. This indicated that this lanthanide behaved like Yb. A 20 per cent hydrofluoric acid strip solution was successful in removing all of the solute from the solvent. CONCLUSIONS
(1) The equilibrium curves of the higher lanthanide chlorides at lower acidities are similar in shape to the equilibrium curves observed for the lower lanthanides but differ in shape as the acid concentration of the equilibrium aqueous phase is increased. (2) As the atomic number of the metal increased, the extraction of the metal into the organic phase also increased. (3) The separation factor of Dy with respect to Gd was found to be approximately that published by Pierce and Peck[19] even though an acidic chloride solution of the solutes was used in this study. Under all experimental conditions studied flOy,Gdranged from approximately 9 to 13. (4) It was found that Lu was extracted only slightly better than was Yb under the same conditions of the equilibrium aqueous phase. Stripping the Lu-containing organic phase four times with double volumes of 6 M HCI would not remove all of the solute from the organic phase as was also found when Yb was the solute. (5) A 20 per cent hydrofluoric acid solution was the reagent which would remove all of the Yb or Lu, which had been extracted, from the solvent.
EXTRACTION OF SOME HEAVIER LANTHANIDES ACIDIC CHLORIDE SOLUTIONS BY Di(2-ETHYLHEXYL) PHOSPHORIC ACID*
IN
THOMAS C. OWENS and MORTON SMUTZ Institute for Atomic Research and Department of Chemical Engineering, Iowa State University, Ames, Iowa
(Received 17 October 1967) Abstract--Equilibrium data have been obtained for the systems MCI3-HCI-H20-1 M D2EHPA (in Amsco Odorless Mineral Spirits), where M is used to designate the trivalent lanthanides Gd, Dy, Er, and Yb, over a wide concentration range of the lanthanide chloride in aqueous solution. For a particular acidity and a given lanthanide concentration in the aqueous phase, the extractability increased as the atomic number of the lanthanide increased. Equilibrium acid concentration was used as a parameter for the equilibrium curves. The general shape of these curves changed as the aqueous acid concentration increased from 1.4 M HCI to 5.0 M HCI. The separation factor of Dy with respect to Gd,/3Dy,~, was also obtained in acidic chloride solutions. The total lanthanide chloride concentration, the total acidity, and the molar ratio of GdCla to DyCl3 in the equilibrium aqueous phase had only slight effect on the separation factor. The separation factor was in the range of 9-13 for the conditions studied. It was found that Yb and Lu were not removed from the organic phase by stripping four times with double volumes of 6 M HCI and other methods of back-extracting which have been previously used. A 20 per cent hydrofluoric acid solution, however, completely removed the solute from the organic phase as the insoluble fluoride.
INTRODUCTION
THE EXTRACTION o f lanthanides into di(2 ethylhexyl) phosphoric acid (hereafter designated D 2 E H P A ) has been studied by a number of investigators, but equilibrium data at greater than tracer concentrations have become available[I-3] only recently. The purpose of this research was to extend the studies to the heavier lanthanides. Peppard e t a/.[4], showed that the extractability of D 2 E H P A for the lanthanides increased with atomic number, was inversely third power dependent on the H + activity in the aqueous phase, and was limited by the formation of a gelatinous third phase when the solute concentration of the organic phase was too high. In extracting the heavier lanthanides with this solvent at high aqueous concentrations it is, therefore, necessary to use relatively high aqueous phase acidities to avoid gel formation in the organic phase. *This work was performed under contract with the Atomic Energy Commission. Contribution No. 2206. 1. 2. 3. 4.
T. Goto and M. Smutz, J. inorg, nuel. Chem. 27, 1369 (1965). T. Lenz and M. Smutz, J. inorg, nucl. Chem. 28, 1110 (I 966). S. G. K. Nair and M. Smutz,J. inorg, nucl. Chem. 29, 1787 (1967). D . F . Peppard, G. W. Mason, J. L. Maier and W. J. Driscoll,J. inorg, nucl. Chem. 4,334 (1957). 1617
1618
T . C . OWENS and M. SMUTZ
Sato[5, 6] suggested that the anion would be present in the organic phase if the extractions were carried out at high acidities. Another study [7] was also concerned with the extent of chloride extraction into D2EH PA at high acidities of the equilibrium aqueous phase. EXPERIMENTAL
Materials. The lanthanide oxides used in this study, obtained from the Rare-Earth Separations Group of the Ames Laboratory, had a purity greater than 99-9 per cent with respect to the presence of other lanthanide oxides as determined by emission spectroscopy. The acids and all other reagents were of reagent grade. The D2EHPA, obtained from Union Carbide, was mixed with Amsco Odorless Mineral Spirits (hydrocarbon diluent) as obtained from American Mineral Spirits Company to form a 1 M solution of D2EHPA on a monomeric basis (hereafter designated 1 M D2EHPA). The undiluted D2EHPA was 98.8 per cent monoacidic. Preparation of stock solutions. The lanthanide oxides were dissolved in excess HCI, and the excess HCI was evaporated by heating. Then HC1 was added until a slight excess of HCl was present. Equilibration of aqueous and organic phases. A known volume of aqueous feed solution was equilibrated with an equal volume of the organic solvent (1 M D2EHPA) in a separatory funnel by shaking the two phases vigorously for 3 min, allowing the phases to separate for at least 1 hr, and then shaking vigorously for an additional 2 rain. The separatory funnel was then allowed to sit undisturbed for at least 12 hr before the phases were separated and the analysis begun. ,4nalysis of the aqueous and organic phases. After separation of the phases, the organic phase was often somewhat cloudy, probably due to entrained aqueous material. To remove the cloudiness, the organic phase was centrifuged for at least 15 min in a clinical centrifuge. After centrifuging, the organic phase was quite clear, though not colourless. A portion of the organic phase was then measured volumetrically into a separatory funnel where it was contacted four times with equal volumes of approximately 6M HCI. This removed the solute quantitatively from the organic phase for all systems except when either YbC13 or LuCI3 was the solute. The aqueous strip solution was then analyzed for lanthanide content, and the concentration of lanthanide in the organic phase was calculated. When either YbCIz or LuCl3 was the solute, the lanthanide concentration in the organic phase was obtained by difference between the initial aqueous feed and final aqueous concentrations of the solute. The total lanthanide concentration in the initial and final aqueous solutions and in the organic backextract was determined by EDTA titration using pyridine as the buffer and arsenazo as the indicator according to the procedure given by Fritz et al.[8]. The acidity of the initial and final aqueous solutions was measured using the cation exchange resin Dowex 50 × 8 as outlined by Adams and Campbell[9]. The cation exchange resin exchanged three hydrogen ions for one lanthanide ion; then the total hydrogen ion present was titrated using standardized sodium hydroxide with phenolphthalein as the indicator. The EDTA was standardized by titrating a solution of known lanthanide chloride concentration. This known lanthanide concentration was obtained by precipitating the lanthanide in an aliquot of the solution as the oxalate, and then burning the oxalate to the oxide. The oxide was then weighed on an analytical balance. Separation factor studies. The methods of equilibrating the phases, stripping the organic phase and determining the total concentration of the solutes in both the organic and aqueous phases have been discussed previously The composition of the phases containing both Gd and Dy was determined using a spectrophotometric method given by Banks and Klingman[10]. Aliquots of the equilibrated aqueous samples and the organic strip solution were precipitated with oxalic acid and ignited to the 5. 6. 7. 8. 9. l 0.
Taichi Sato,J. inorg, nucl. Chem. 25,109 (1963). Taichi Sato,J. inorg, nucl. Chem. 27, 1853 (1965). T. C. Owens, Ph.D. Thesis, Iowa State University (1967). J. S. Fritz, R. T. Oliver and D.J. Pietrzyk,,4nalyt. Chem. 30, 1111 (1958). J. F. Adams and M. H. Campbell, U. S. ,4 tomic Energy Commission Report HW-76363 (1963). C. V. Banks and D. W. Klingman,,4 nalytica chim. ,4 cta 15,356 (1956).
Extraction of some heavier lanthanides in acidic chloride solutions
1619
oxides. Known weights of these oxide mixtures were dissolved to form approximately 1 M perchlorate solutions, and the relative amounts of Gd and Dy were determined using a Beckman DU-2 spectrophotometer. The Gd absorbance was read at 272.8 m/~ with a slit width of 0.05 mm using a hydrogen lamp. The Dy absorbance was read at 908.0 m/~ with a slit width of 0.03 mm using a tungsten lamp. Presence of chloride in the equilibrium organic phase. To test for the presence of chloride in the extracted species an aqueous solution of silver nitrate was mixed with a portion of previously centrifuged equilibrium organic phase. The presence of the characteristic white precipitate indicated the presence of chloride in the organic phase. Removal of Yb and Lu from the organic phase using 20 per cent HF. The solutes Yb and Lu were removed from the organic phase by contacting the solvent four times with double volumes of 20 per cent hydrofluoric acid. The lanthanide was precipitated as the lanthanide fluoride. The lanthanide fluoride was filtered and dried, and then heated to 700°C in an HF atmosphere to insure the formation of the fluoride. The weight of the fluoride salt indicated that all of the lanthanide had been removed from the organic phase. Qualitative flame emission techniques were used directly on the organic phase to determine the extent of the removal of the solute. This method indicated that all of the Lu and Yb had been removed from the solvent when a 20 per cent hydrofluoric acid strip solution was used. DISCUSSION
Symbols M, N -lanthanide cations with a+3 valence. [ ] a - activity of the species in the aqueous phase. [ ] o - activity of the species in the,organic phase. ( )a--concentration of the species in the aqueous phase, moles/1. ( ) o - concentration of the species in the organic phase, moles/1. KM-- equilibrium constant of the reaction in which M is extracted. ~(Ea°)M-thermodynamic distribution coefficient, defined as the total activity of all M containing species in the organic phase divided by the total activity of all M containing species in the aqueous phase at equilibrium. (Ea°)M-distribution coefficient, defined as the total concentration of all M containing species in the organic phase divided by the total concentration of all M containing species in the aqueous phase at equilibrium. fl~M- stability constant of the species M A~+(~-o. [MAi+(a-i)]a [M+3la [m-1]a i
T/3M,~-- thermodynamic separation factor. -- T(Ea°)M when both M and N are present together. T(Ea°)N'
/3 M,N-- separation factor.
= (Ea°)M when both M and N are present together. (Ea°)s '
The extraction of the trivalent lanthanide ions by D 2 E H P A , which is dimeric in non-polar solvents[11, 12], from aqueous solutions having low solute concentration and rather low acidities has been widely reported. Some investigators 11. O . F . Peppard, J. R. Ferraro and G, W. Mason, J. inorg, nucl. Chem. 7, 231 (1958). 12. J. R. Ferraro, G. W. Mason and D. F. Peppard, J. inorg, nucL Chem. 22, 285 (1961 ).
1620
T . C . OWENS and M. SMUTZ
[5, 13, 14] have found that the anion present in the aqueous phase may also be present in the extracted species. The identities of anion-containing species have not been established, but the extraction of metal chloride complexes and undissociated HC1 seems possible. Reactions which allow for (1)the extraction of the trivalent metal by the ion exchange reaction, (2) the extraction of metal chloride complexes, and (3) the extraction of HCI are given by Equations (1) and (2). M+3+ n C I - + b(n) (HG)c¢,o ~ MCl,,Hb~,,~oo+,,-3Gb¢,o~o,) + (3--n) H+,n = 0,1,2,3.
(1)
H + + CI- + aX ~ HCI.aX.
(2)
These equations assume that HC1 is extracted as a neutral species, and that metal chloride complexes, with the exception of the case when n = 3, are extracted by an ion-exchange reaction. When the neutral molecule is extracted, an extraction reaction other than an ion exchange reaction is necessary. In Equation (2) the quantity a is constant, and X is the solvent entity which may extract HCI. The quantities b(n) and c(n) are variables that depend on the discrete variable n. When n --- 0 in Equation (1), the extraction of the trivalent lanthanide ion, the values on b(n) and c(n) are 3 and 2, respectively. The value ofc(n) depends on the type of solvent molecule (i.e. monomer, dimer, or trimer, etc.) which extracts the solute species, and the value of b(n) denotes the number of these solvent entities required to perform the extraction. The equilibrium constants for the extraction reactions given by Equations (1) and (2) are shown in Equations (3) and (4), respectively. KM(n) =
[MCl"Hb°°¢°°+"-3G~'°c°')]°[H+]a¢3-"~ [M+3], [Cl-]a n [(HG)c~,~]o~"~ Kncl =
[HCI'aX]a [H+]a[Cl-]a[X]a a
n = 0 , 1 , 2 , 3.
(3)
' (4)
The brackets refer to the activity of the species, and it is important to note that the values of the equilibrium constants given in Equation (3) depend on the value ofn. The effect of the acidity of the aqueous phase on the extraction can be determined by considering Equations (3) and (4). The extraction of the neutral HCI would increase as the acid activity is increased, as would be expected. According to Equation (3), the increase of acid activity decreases the extraction of the metal chloride complexes, except when the neutral species is extracted and there is no acid dependency. However, the acid activity also affects the relative activities of the extractable species. When the acid activity increases, the proportion of the higher metal chloride complexes in the aqueous phase also increases. Therefore increased acid activity seems to favor the extraction of neutral HCI and the more 13, V. E. Shaw and D. J, Bauer, U. S. Bur. Mines Rep. 7691 (1964). 14. T. (3. Lenz and M. Smutz, J. inorg, nucl. Chem. 30, 621 (1968).
Extractionof someheavierlanthanidesin acidicchloridesolutions
1621
complexed metal species while the lower acid activity favors the extraction of the trivalent metal (i.e. n = 0 in Equation (1))~ Due to the high acid concentration in the aqueous phase, it may be assumed [15] that the only metal chloride complexes are M ÷3, MC1÷2, MCI2+1, and MCI3. The distribution of the metal among these species can be given by the stability constants which are shown in Equation (5). [MCli+tz-°]a rim = [M+3]a[Cl_]al,
i = 1,2, 3.
(5)
Equations analogous to Equations (3) and (5) may be written for the extraction of metal N. Combining all of the equations, as was done by Hesford and McKay [15] will result in Equation (6) for the thermodynamic separation factor where all of the reactions of Equation (1) occur simultaneousl' r.
T(Ea°)M = T~[~M,N= r(EaO)N
~ KN(n) [CI-]~"[(HG)~,,)]o ~'° [H+]a<3-')"
n=0
(6) 3 KM(n) [Cl-]."[(HG)~oo]o ~"~ [H+la~3-,~ 71=0
l1
1
This equation for r/3M,N is the extension referred to by Lenz and Smutz[14], and is also particular to the case when a single anion, chloride in this study, is present. The first factor of Equation (6) is a ratio which depends on (1) the equilibrium constants of the extraction reactions, (2) the concentration of the anion, (3) the aggregation of the solvent, (4) the solvent dependency in the extraction reactions, and (5) the acid concentration of the aqueous phase. The second factor is essentially a ratio of the ability of the two metals to complex with the anion in the aqueous phase. Because of the lanthanide contraction, it is probable that as the atomic number increases throughout the series, the degree of complexing with the anion would also increase. This single consideration would indicate that D2EHPA would preferentially extract the lower lanthanides; however, this is not the case since it is observed that as the atomic number is increased the extractability increases very significantly. Therefore, the degree of aggregation of the solvent and the preference of the solvent for one lanthanide over another, together with effects of the aqueous phase, determine the performance of the solvent in a particular system. The effects of the aqueous phase may be related to complexing in the aqueous phase and changes in the coordination number of the metal ions in the aqueous phase as their atomic number increases. The presence of three-dimensional polymers when the ratio of the concentration of the trivalent lanthanide to the concentration of the active solvent is greater than ~ has been noted[4, 16]. This explanation of the phenomenon appears to be satisfactory only as a first approximation, since previous work[17] in this group 15. E. Hesfordand H. A. C. McKay,Trans. Faraday Soc. 54, 573 (1958). 16. D. F. PeppardandJ. R. Ferraro,J. inorg, nucl. Chem. 10,275 (1959). 17. T. C. Owens,M. S. Thesis, IowaState University(1965).
1622
T . C . OWENS and M. S M U T Z
while using the undiluted solvent, indicated that the gelatinous third phase may form when the ratio of the metal concentration and the active solvent concentration was much less than ~. These studies also indicated that even though the ratio of metal concentration to solvent concentration was greater than ~ the organic phase was still a liquid, though quite viscous, and not a gelatinous material. The extraction of metal chloride complexes by the ion-exchange reactions given in Equation (1) would allow the ratio of metal concentration to solvent concentration in the organic phase to exceed ~ without gel formation. Similarly, the extraction of the undissociated lanthanide chloride would result in a greater ratio of metal concentration to solvent concentration in the light phase. RESULTS
Equilibrium data for the systems MCI3-HCI-H20-1 M D 2 E H P A Equilibrium data f o r the systems MCI3-HCI-H20-1 M D 2 E H P A were obtained where the metal chlorides investigated were GdCI3, DyCI3, ErCI3, and YbC13. Based on the extraction reactions given by Equations (1) and (2), the following three variables must be considered: (1) the aqueous phase lanthanide chloride activity, (2) the aqueous phase activity of HCI, and (3) the activity of the metal-containing species in the organic phase. As may be seen from Equation (1), the amount of extraction decreases as the acid activity of .the aqueous phase increases for a particular activity of the solute in the aqueous phase, except when n = 3. Since the experimental determination of the pertinent activities was not possible, the respective concentrations were measured. Figures 1-4 present the I
I
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1
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OIB O~ D ~ + ~
016
0.8 M 1.4M 2,8M 5,0M
H CI. H CI. H CI. HCI
014
~012 0~ o
¢'0.1o
/0 ~ ~
"0--
"0" ~ ~ 0 - - ~ ,_0.....
/ 0.08
_/ /
.J o 0.06
.L
0.04
0.02
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r l - ~
-Q-- - -
--O--- ~
"tT
rl
1,50
1.75
Q
O.OC 0.25
0.50
0.75
1.00
t25
MOLES
2,00
2.25
~_50
G d C I 3 / L I T E R , AOUEOUS
Fig. 1. Equilibrium data for the system GdCI~-HCI-H~O-! M D2EHPA.
Z75
Extraction of some heavier lanthanides in acidic chloride solutions t
I
t
I
I
I
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I
1623 I
I
0.18 A ~ 1.4M HCI. n ~ 27 M H CL x ~ 5 0 M H CL
0.16
0.14 &
-~
/
~0.12
~_~_.&~
_ -i,~ ' ~ a ~ -'~
I k
~o.lo ...i ~0.08 _J ~0.06
/o
/
/ n
0.04
0.02
/X t
0.00 0
0.25
I
I
I
I
0.50
0.75
1.00
1.25
I
MOLES
I
1.50
I
1.75
I
2.00
I
~25
I
2.50
2.75
~ C ~ / L K E R , AQUEOUS
Fig. 2. Equilibrium data for the system DyC13-HCI-H20-1 M D2EH PA. I
I
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t
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0 - 1.4MHCI. & ~ 2.SMHCl. n ~ 5.0M H CI.
i 0,1 6
0.14
0.12 0
m" ill
I b
..~
o.lo T
0.08
_
~ 0.06
-
i i
~ j
~
/ / / 0.04
0.02
0.00
-d / ?
I 0.25
I
I
I
I
0.50
0.75
LO0
L25
MOLES
I 1.50
I 1.75
I ZOO
I 2.25
I 2.50
Er CIs/LITER, AOUEOUS
Fig. 3. Equilibrium data for the system ErCI3-HCI-H20-1 M D2EH PA.
I 2.75
1624
T.C. OWENS and M. SMUTZ I
I
f
I
I
I
I
I
I
I
1
016
J
J
"n.
a
5 0 M HCI. 7 . 7 M HCL
0.1 6
/ / 0.1 4 _o n,. 0.12 0 0.10
/d
0.08 ,J o 0.06
0.04
0.02
ooo
I 0.25
I 0.50
I 0.75
I
I
1.00
1.25
MOLES
I I l~ 0
I I f7 ~
I ZO O
I Z .2 5
I 2 .~ 0
I 2~~
Yb CI3/LITER,AQUEOUS
Fig. 4. Equilibriumdatafor the systemYbCI3-HCI-H20-I M D2EHPA. equilibrium data in graphical form indicating the concentration of the lanthanide in the organic phase as the aqueous lanthanide chloride concentration and acidity were varied. The points indicate experimental results; however, the concentration of HC1 in the equilibrium aqueous solution must necessarily be associated with each point. Equilibrium curves have been drawn on each of the figures to indicate the equilibrium data at constant HCI concentration in the aqueous phase. Table 1 gives the equilibrium data used in plotting Figs. 1-4. The equilibrium concentrations of the lanthanide in both the aqueous and organic phase, and the equilibrium acidity in the aqueous phase are given. Figures 1-4 show that at relatively low acidities (0.8 and 1.4 M) the shape of the equilibrium curve resembles the shape of equilibrium curves obtained by Lenz and Smutz[2] and Nair and Smutz[3] for La, Pr, Nd, and Sm. The organic phase concentration of the solute increased to a maximum and then decreased slowly as the lanthanide concentration of the aqueous phase increased. Then, as the equilibrium acidity increased above 1.4 M HCI, the organic concentration increased rapidly to its maximum value and stayed nearly constant as the lanthanide concentration of the aqueous phase increased. At rather high solute concentrations in the aqueous phase, when Er was present in the system, there was a slight increase in the organic phase concentration of solute for acidities of 1.4 and 2-5 M HC1. At still higher acidities, regardless of the lanthanide present in the system, the solute concentration in the organic phase increased steadily as the lanthanide chloride concentration of the aqueous phase concentration increased.
Extraction of some heavier lanthanides in acidic chloride solutions Table 1, Equilibrium data for the system MCla-HC1-H20-1 M D2EHPA where M designates Gd, Dy, Er, and Yb Aqueous conc. (moles/L)
Final acidity (moles HCI/1.)
Organic conc. (moles/l.)
0.049 0.049 0-255 0"255 0.501 0.501 0'746 0'744 0.992 0.991 1"25 1.24 1.48 1-49 1-73 1.73 1.99 1"98 2'23 2.22 2"47 2'47 2-72 2"71
0"812 0.811 0"776 0"779 0-808 0.808 0'846 0.849 0-821 0"824 0"845 0-845 0.787 0-765 0"773 0"803 0.735 0'776 0"769 0-786 0'781 0"832 0"701 0"768
0.069 0-069 0.097 0.097 0-100 0.100 0"099 0.099 0'098 0"098 0.094 0"094 0.091 0-091 0.086 0.086 0.083 0"083 0"078 0"078 0-075 0'076 0'072 0-072
0-076 0"076 0'267 0-267 0.514 0"514 0"766 0.766 1"02 1.02 1'28 1.27 1"52 1"52 1"76 1-76 2.02 2.02 2'26 2'26 2.50 2.49
1-43 1.43 1.43 1-42 1.41 1.39 1.40 1"40 1'37 1'41 1'40 1"37 1-40 1'40 1-40 1"40 1.38 1.40 1.38 1-32 1.46 1.47
0-027 0.027 0.048 0.048 0.055 0"055 0"055 0"055 0'053 0"053 0-049 0-049 0"045 0"045 0"041 0.041 0.039 0-039 0.036 0-036 0.035 0.035
GdCls
1625
1626
T . C . OWENS and M. SMUTZ Table 1. (Contd.) Aqueous cone. (moles/L)
Final acidity (moles HC1/I.)
Organic cone. (moles/k)
0.074 0.074 0.281 0-281 0.535 0-537 0.788 0.789 1.04 1'04 1.28 1.28 1.54 1-53 1.78 1.78 1.99 1.98
2.89 2.88 2.81 2.81 2.83 2.86 2-80 2.79 2.83 2.82 2.85 2.84 2.87 2-87 2-86 2.84 2.87 2.84
0.0026 0.0026 0-0078 0.0078 0-0103 0.0101 0.0112 0.0112 0.0113 0-0112 0.0107 0.0109 0.0107 0.0108 0.0110 0.0110 0.0117 0.0118
0.056 0.056 0.251 0.251 0.504 0"504 0.763 0.764 1.02 1.02 1.28 1.27 1.27 1.27
4'97 4.98 4-99 4.98 5"02 5"01 4-95 4.93 4"96 4"95 5.00 4'91 5"08 5"09
0.0002 0.0002 0.0011 0.0011 0.0021 0.0021 0.0031 0.0031 0.0043 0.0043 0.0066 0.0066 0.0071 0.0071
0.065 0.065 0.265 0-270 0.510 0.509 0.768 0.766 1.01 1.01 1.25 1.25 1.48 1.49
1.38 1-37 1 "38 1 "37 1.37 1.35 1.37 1 "34 1-35 1-35 1 "37 1 "37 1.37 1 "36
0'108 0'108 0.128 0" 128 0"132 0.132 0"130 0"132 0-130 0.130 0-127 0"126 0.126 0"125
GdCIn (Contd.)
DyCI3
Extraction of s o m e heavier lanthanides in acidic chloride solutions Table 1. (Contd.)
Aqueous conc. (moles/L)
Final acidity moles HCI/I.)
Organic conc. (moles/l.)
1.73 1-73 1"98 1.98 2'23 2.47 2.47 2.45 2.44
1-41 1.42 1.47 1"43 1.44 1.43 1.47 1.55 1-47
0.123 0.121 0-118 0.116 0.116 0.113 0-116 0-116 0.116
0" 116 0"316 0.317 0.564 0"565 0.817 0.817 1-08 1 "08 1.34 1.34 1"60 1-59 1.85 1-85 2.09 2.10 0.065 0.065 0.276 0'276 0-518 0.518 0.761 0.762 1-01 1.00 1.25 1.26 1.42 1.43
2'81 2"78 2-78 2.75 2.74 2"72 2"74 2.76 2'76 2"73 2.73 2.61 2.62 2-65 2-60 2.74 2.68 4.98 5.00 4.98 4"96 5.00 4-99 4.94 4.94 4.92 4.92 5.04 5.01 5.10 5"03
0.040 0.059 0.059 0"066 0.066 0.069 0.069 0-068 0-069 0"069 0.069 0.070 0.070 0-070 0.069 0.070 0.071 0.006 0-005 0.017 0.017 0.023 0.024 0'028 0.028 0.032 0.032 0.036 0-036 0.042 0.042
0.037 0-037 0.040 0.040 0-225 0.225
1.46 1.48 1.54 1-53 1-48 1.48
0.122 0.122 0-114 0.115 0-142 0.143
DyC13 (Contd.)
ErC13
1627
1628
T . C . OWENS and M. S M U T Z Table 1. (Contd.)
ErCIa
Aqueous conc. (moles/L)
Final acidity (moles HCI/I.)
Organic cone. (moles/l.)
0.252 0.253 0.486 0"487 0-753 0.753 0.999 1.00 1.24 1.24 1.49 1.49 1-74 1-75 1 "96 1 "96 2.20 2-21 0.047 0.047 0.236 0"237 0.528 0.531 0.778 0-780 ! '03 1'04 1 '29 I "29 1 "54 1 "54 1 "79 1 "78 2"03 2"04 2"14
1 '52 1 '51 1.43 I "43 1.40 1.38 1"38 i "39 1 '35 1 "38 1 "35 1.34 1.32 1-30 1.72 1.77 1 "75 I "74 2'53 2.53 2.53 2"53 2.55 2"56 2'52 2.51 2.40 2.40 2.35 2.38 2"27 2-27 2"74 2.57 2"50 2'54 2"59
0"136 0"136 0.145 0-146 0" 145 0" 145 0'142 0.142 0"142 0-142 0-142 0-141 0" 142 0' 142 0.139 0-139 0'141 0"141 0"086 0-085 0.117 0"117 0"113 0"113 0"115 0.114 0"115 0-114 0"116 0'115 0'116 0"115 0.117 0.118 0"122 0"122 0"124
0.085 0-085 0-260 0.259 0.500 0.501 0.749 0.747 0.997 0.995
4.94 4.97 5.05 5.06 5.06 5.05 5"05 5"09 5.11 5"11
0-034 0.034 0.060 0.060 0.074 0'074 0.081 0-081 0"086 0-086
(Contd.)
Extraction of some heavier lanthanides in acidic chloride solutions Table 1. (Contd.) Aqueous conc. (moles/L)
Final acidity (moles HC 1/1.)
Organic conc. (moles/l.)
ErCI3 (Contd.) 1-24 1-24 1.48 1-48
5" 10 5"06 5-05 5.06
0'093 0.093 0.102 0-101
0.034 0.036 0"187 0.187 0.419 0.420 0"668 0'666 0'901 0"901 1-14 1-14 1"38 1"38 1.58 1.58 0.037 0.037 0.239 0"239 0"487 0'488 0"737 0"739 0"995 0-996 1"25 1'25 1-49 1.49
5-39 5.40 5'29 5'28 5.06 5"08 5"33 5'37 5-31 5"32 5"20 5-19 5-08 4.93 5.05 5.08 5"05 5'05 5.06 5"08 4-99 4-98 5.00 5.00 4-99 4"95 5.01 4.97 4-87 4.89
0'066 0.064 0"115 0"115 0'133 0' 132 0" 134 0-136 0-151 0-151 0"157 0.156 0.175 0"176 0.176 0-178 0.083 0"083 0-123 0"123 0'141 0" 140 0'152 0' 149 0.155 0' 154 0-154 0.153 0-164 0.164
0.044 0.043 0' 198 0.198 0.425 0"425 0"655 0'655
7'64
0"058 0"058 0" 106 0-106 0"!31 0' 131 0' 152 0"152
VbC~
7"72 7-69 7"55 7"57 7"64 7"70
1629
1630
T . C . OWENS and M. SMUTZ
The lowest acidity shown on each of the figures indicates approximately the lowest acidity at which the gelatinous third phase would not form. This is the reason that acidities lower than approximately 5.0 M HC1 were not reported when Yb was present in the extraction system. It is apparent that the shape of the equilibrium curves, at constant acid concentration, depends on the acid concentration in the aqueous phase and not on the identity of the solute present in the system. That is, at acidities of 1.4 M HCI, regardless of the lanthanide present, the equilibrium curve increases to a relative maximum and then decreases, while at 5-0 M HC1, the solute concentration in the organic phase increases as the solute concentration in the aqueous phase increases. The presence of chloride-containing species in the organic phase also depends largely on the acid concentration in the aqueous phase. At lower acidities (0.8 and 1.4 M HCI), the characteristic precipitate was not observed when AgNO3 solution was added to the organic phase. This indicated very little, if any, chloride in the equilibrium organic phase. At higher acidities (-2-5 M HCI), the amount of precipitate increased as the concentration of the lanthanide in the aqueous phase increased. At the highest acidities (5.0 and 7.7 M HC1), the presence of chloride was readily apparent, and the amount of precipitate did not noticeably change with increasing solute concentration in the organic phase. Figures 1-4 show that for a given aqueous lanthanide chloride concentration and HC1 concentration, the organic phase concentration of solute increased as the atomic number of the lanthanide increased. This is shown quite clearly in Fig. 5 where equilibrium data for Gd and Yb at aqueous acidities of 5.0 M HCI are plotted. Separation factor studies. Figure 6 shows results that have been obtained in determining the separation factor of Dy with respect to Gd using 1 M D2EHPA as the solvent and an acidic chloride solution of the lanthanide mixture as the aqueous phase. The molar ratio of DyC13 to GdCI3 varied from 50 : 50 to 30: 70 in the equilibrium aqueous phase and the two levels of equilibrium aqueous acidity investigated were approximately 1.4 M HCI and 2-1 M HCI. Due to the spread in the experimental data, it was difficult to draw conclusions about the effects of total aqueous lanthanide concentration, equilibrium acidity of the aqueous phase, and the molar ratio of the solutes in the aqueous phase, but some trends may be noted. It appeared that at constant acidity and constant molar ratio of the solutes the separation factor of Dy with respect to Gd increased to a maximum at about 1.5 M total lanthanide chloride concentration of the aqueous phase and then decreased as the total aqueous concentration increased further. However, there was not the marked change in the separation factor with total solute concentration that was observed when the separation factor,/3Sm,Nd, was determined[14, 18] for the perchlorate, nitrate, and chloride systems. It also appeared that changing the ratio of GdCI3 to DyClz in the equilibrium aqueous phase from 50:50 to 30:70 caused the separation factor to increase slightly; however, increasing the aqueous acidity appeared to decrease the separation factor slightly. It should be noted that, regardless of the conditions, the separation 18. C. Battista, C. Mize and M. Smutz. J. inorg, nucl. Chem. 28, 1121 (1966).
1631
Extraction of some heavier lanthanides in acidic chloride solutions I
I
1
I
I
I
I
I
I
I
Oot8 --
5M
HCl
0.16
0.14
0.12 -J
~
0.I0
~0.08
0.04
0.02 GdCl3 - 5 M
i
0.00 0
o.z~
i ~-l"'--'-'~
o~
0.:~
,.00
L.z5
HCl
I
I
I
I
l
I
i.~
,.75
zoo
2.25
2.~
2.75
M O L E S OF RARE-EARTH CHLORIDE/LITER,
AQUEOUS
Fig. 5. Equilibrium curves for the systems YbCIz-HCI-HzO-I M D2EHPA and GdCIa-HCI-H20-I M D2EHPA at5.0 M HCI. 14.0 - . 130
--
12.0
--
I
I &
b
&
I
& x
4t ,,.o -
X
8 O
I0.0 --
O
x
O
"~
O 9.0--
O
re
8.0--
7.0 6.0-5.0 - -
o-
o
DvClm' GdCI= = 50, 50 AClDITY~ 1.4 M HCl m
ACIDITY ~ 1.4 M HCI
X OyC~, GdCI===30,70
m
~ 2,1 M HCl
o.o ~' o.oo
I o.s
I ~.o
I J,s
I 20
z.5
TOTAL RARE-EARTH CHLORIDE CONCENTRATION IN AQUEOUS PHASE, MOLES/UTER Fig. 6. Separation factors for the system GdCI:~-DyCI~-HCI-H20--1 M D2EHPA.
1632
T . C . OWENS and M. S M U T Z
factor of Dy with respect to Gd range from approximately 9 to 13. This agrees with the value of 10.4 for/3Dy,Gdobtained by Pierce and Peck [ 19]. Extraction of Lu in the system LuCI3-HC1-H20-I M D 2E H PA . In order to determine if the extraction of Lu from a LuCI3-HCI-H20-1 M D 2 E H P A was similar to the extraction of Yb from a Y b C I 3 - H C I - H 2 0 - I M D 2 E H P A system, a single equilibrium point of the former system was determined. The replicated data are shown in Table 2. It was observed that Lu was only slightly more extractable than was Yb under the same conditions of the equilibrium aqueous phase. This was as expected since Pierce and Peck [19] found the separation factor of Lu with respect to Yb to be only 1.86 for tracer levels in an aqueous perchlorate system. Table 2. Equilibrium data for the system L u C h - H C I H 2 0 - 1 M D 2 E H P A at approximately 5.0 M HCI. (Lu)a (moles/L) 1.00 1.00
(HCI)a (moles/L) 5.03 5.04
(Lu)o (moles/l.) 0.160 0.160
As in the case when Yb was the solute, it was not possible to completely strip all of the solute from the organic phase with three portions of 6 M HC1 using volumes double that of the organic phase. Consequently, the organic phase concentration of the solute was calculated by the difference of the concentration of LuCI3 in the initial and final aqueous phases. Stripping of Yb and Lu from equilibrated 1 M D2EHP,4. In initial experiments when using YbC13 as the solute in the highly acidic aqueous phase, it was found that stripping with four equal volumes of 6 M HCI would not remove all of the solute from the equilibrated organic phase. Higher concentrations of HC1 were used with no additional solute stripped, and, when stripped with 12 M HCI, there was a noticeable decrease in the amount of solute removed from the organic phase. Other acids of varying concentrations such as nitric acid, phosphoric acid, perchloric acid, and sulfuric acid were also used, but none of these reagents completely removed the solute from the solvent. The use of a reagent which would monomerize the solvent, and thereby reduce the extraction (2), was also tried. Alcohols, such as n-butanol and ndecanol, were added to the solvent, and then the mixture was contacted with HCI. After contacting the organic solutions with the acid, a third phase formed which seemed to stay with the liquid organic phase. There seemed to be no advantage to using this type of additive to the organic phase. The use of a 20 per cent hydrofluoric acid solution was successful in removing the solute from the equilibrated 1 M D 2 E H P A . The solvent was contacted four to five times with double volumes of hydrofluoric acid. The lanthanide was removed as the insoluble lanthanide fluoride, and there seemed to be no effect on the solvent after the contact. A 5 per cent solution of hydrofluoric acid was also tried, but it did not remove the solute completely. 19. T. B. Pierce and P. F. Peck, Analyst 88, 217 (1963).
Extraction of some heavier lanthanides in acidic chloride solutions
1633
With LuCI~ as the solute, five portions of double volumes of 6 M HCI would remove only about 70 per cent of the solute from the organic phase. This indicated that this lanthanide behaved like Yb. A 20 per cent hydrofluoric acid strip solution was successful in removing all of the solute from the solvent. CONCLUSIONS
(1) The equilibrium curves of the higher lanthanide chlorides at lower acidities are similar in shape to the equilibrium curves observed for the lower lanthanides but differ in shape as the acid concentration of the equilibrium aqueous phase is increased. (2) As the atomic number of the metal increased, the extraction of the metal into the organic phase also increased. (3) The separation factor of Dy with respect to Gd was found to be approximately that published by Pierce and Peck[19] even though an acidic chloride solution of the solutes was used in this study. Under all experimental conditions studied flOy,Gdranged from approximately 9 to 13. (4) It was found that Lu was extracted only slightly better than was Yb under the same conditions of the equilibrium aqueous phase. Stripping the Lu-containing organic phase four times with double volumes of 6 M HCI would not remove all of the solute from the organic phase as was also found when Yb was the solute. (5) A 20 per cent hydrofluoric acid solution was the reagent which would remove all of the Yb or Lu, which had been extracted, from the solvent.