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Isolation of plutonium in chloride media—II
J. inorg, nucl. Chem., 1969, Vol. 31, pp. 3247 to 3254.
Pergamon Press.
Printed in Great Britain
ISOLATION OF P L U T O N I U M IN CHLORIDE MEDIA-II SOLVENT
EXTRACTION
WITH DI(2-ETHYLHEXYL)PHOSPHORIC ACID*
J. J. FARDYt Visiting Scientist, Australian Atomic Energy Commission, Lucas Heights, Sydney, Australia and J. M. CH1LTON Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830 (Received 17 February 1969)
Abstract-Plutonium extraction by di(2-ethylhexyl)phosphoric acid from hydrochloric acid solution varies with the different valence states in the order Pu(IV) > Pu(VI) > Pu(lll). Behavior of transplutonium actinides, corrosion products, and fission products was examined in this system, and it was found that by extracting Pu(IV) from 6 M HCI it can be separated from most of these ions. Separation from zirconium, which is quantitatively extracted, is achieved by removal of the plutonium from the organic phase as Pu(III) by the addition of an organic-soluble reductant. Stripping of plutonium by aqueous-soluble reductants at elevated temperature is also feasible. Results of laboratory studies to determine optimum conditions for plutonium recovery and decontamination in the production of transplutonium actinides are presented. INTRODUCTION AS POINTED o u t in t h e p r e v i o u s p a p e r in this s e r i e s [ l ] , it is d e s i r a b l e to r e m o v e p l u t o n i u m f r o m h y d r o c h l o r i c a c i d s o l u t i o n s o f i r r a d i a t e d t a r g e t s in t h e i s o l a t i o n of transplutonium elements. The mono-acidic ester, di(2-ethylhexyl)phosphoric a c i d , is k n o w n to h a v e a s t r o n g affinity f o r c e r t a i n h i g h l y - c h a r g e d m e t a l l i c ions in s t r o n g a c i d s o l u t i o n s [2, 3]. P l u t o n i u m e x t r a c t i o n has b e e n e x a m i n e d in n i t r a t e s y s t e m s [4, 5], b u t f e w d a t a e x i s t f o r c h l o r i d e m e d i a . W e e x a m i n e d the e x t r a c t i o n o f p l u t o n i u m in t h e v a r i o u s o x i d a t i o n s t a t e s a n d p r e s e n t a s e p a r a t i o n p r o c e d u r e f o r isolating plutonium from trans-plutonium elements and from most corrosion and fission p r o d u c t s . EXPERIMENTAL Materials
All chemicals used in these studies were of reagent grade or the highest purity obtainable from commercial sources, with the exception of those purified as described below. *Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. 1. J. M. Chilton andJ. J. Fardy, J. inorg, nucl. Chem. 31, 1171 (1969). 2. C. A. Blake, Jr., C. F. Baes, K. B. Brown, C. F. Coleman and J. C. White, Proc. 2rid Int. Conf. peaceful Uses atom. Energy, Geneva 1958, Vol. 28, p. 289. United Nations, New York (1958). 3. K. Kimura, J. chem. Soc. Japan 33 [8], 1038 (1960). 4. Y. N. Kosyakov, E. S. Gurayev and G. N. Yakolev, Separation of Transuranium Elements by Solution Extraction in Alkyl Phosphoric Acid. Paper given at Transplutonium Symposium, Argonne National Laboratory, Illinois, May 1963. (Gmelin Ref. No. AED-CONF-63-062-1). 5. D. E. Horner and C. F. Coleman, ORNL-3051 (1961). 3247
3248
J.J.
F A R D Y and J. M. C H I L T O N
The di(2-ethylhexyl)phosphoric acid ( H D E H P ) was obtained from a commercial source and contained less than 5% mono(2-ethylhexyl)phosphoric acid (H2MEHP). This was purified to remove the di-acidic ester and other impurities. A 0.5 M solution of the commercial H D E H P in diethylbenzene (DEB) was washed with an equal volume of 1 M NaOH, and the middle phase of the three that formed was separated. This was treated with excess 1 M HCI, separated, and washed three times with one-half volume of ethylene glycol. Commercial grade diethylbenzene was purified by passing it through columns of silica gel and alumina to remove water and organic surfactants. Practical grade 2,5-di-tert-butyl-hydroquinone (DBHQ), and 2-ethyl-1-hexanol (2-EHOH) were used without further treatment. The radioactive tracer isotopes, 1~4Ce, asZr-Nb, asSr, 13~Cs, 241Am, 242Cm, and 252Cf were obtained from the Oak Ridge National Laboratory Isotopes Sales Department.
Apparatus and procedures All determinations of equilibrium distribution coefficients, Ea°, defined as the ratio of the concentrations in the organic and aqueous phases, were made by analyses of both phaSes for the element of interest. At room temperature (25 ° _ 1°), equilibrium between the phases was obtained by mixing 5 ml of the aqueous phase containing the element under test with the same volume of extractant in a 1 oz glass bottle for 5 rain on a vortex mixer. Equilibrium is obtained in this time, as shown by the consistency in coefficients measured after further agitation for 10 and 15 rain. Special glass bottles were fabricated for measurements at elevated temperatures. A flat-bottom glass container, approximately 20 ml capacity, was surrounded with an outer glass jacket with inlet and outlet ports. Water at the desired temperature was circulated around the bottle from a constant-temperature bath ( - 0-1°). Agitation of the two phases was accomplished by a magnetic stirrer and a Teflon-covered stirring bar. Analyses for alpha-active elements were carded out by planchet counting on a 2~r gas-flow proportional counter and by alpha pulse height analysis. Measured volumes of gamma-active elements were counted in a well-type scintillation counter, and pulse height analysis was used when necessary. Analyses for nonradioactive elements were carded out by standard spectrophotometric procedures. RESULTS
AND
DISCUSSION
Extraction of plutonium and other metal ions The extraction of plutonium from HCI solutions by 1.0 M H D E H P in DEB is summarized in Fig. 1. The extraction order is Pu(IV) > Pu(VI) > Pu(IIl) at all acid concentrations. Therefore, in any separations involving this reagent, it must be remembered that either oxidation or reduction of a Pu(IV) solution will decrease the amount of plutonium extracted. Extraction coefficients of the transplutonium elements, curium and californium, are shown in Fig. 2. Americium extraction is about the same as curium and, hence, about a factor of 30 less than californium. This ratio is lower than the factor of 100 obtained in the 2-ethylhexyl phenylphosphonate-HCl system studied by Baybarz [6]; however, the shapes of the extraction isotherms of these elements are similar for both systems. Even though plutonium extraction is greater at lower acidity (Ea ° = 240 at 2 M HC1), the maximum separation from the transplutonium elements is obtained by extraction from 6 M HCI. Separation from certain corrosion and fission products is also obtained in this extraction system. Since zirconium, which is present as a corrosion product from Zircaloy plant equipment and also as a fission product, is very strongly extracted at all HCI concentrations (Ea ° > 2000), it is removed from all of the trivalent transplutonium elements, which remain in the aqueous phase. Subsequent separation from the extracted Pu(IV) is achieved by reducing plutonium to the 6. R. D. Baybarz, Nucl. Sci. Engng 17,457 (1963).
Isolation of plutonium in chloride media- I 1
~000
-
r
lJill!
3249
I i lll---
e~Pu (IV)
100 -
.,, le
I q
1o w
8 z 0 F-
q z
3(~ o;
0.t
Pu(Ill)
0.01
f III11 z
0.005
~o
to0
HCI coNCENTRATION IN AQUEOUS (N)
Fig. I. Plutonium extraction by 1.0 M H D E H P in D E B as a function of HCI concen-
tration.
trivalent state so that it can be selectively stripped. Other corrosion products, Al, Cr, and Ni, and the fission products, '*iCe, 9°Sr, and lSTCs, are poorly extracted from 2 to 12 M HCI solutions (E, ° < 0.005) and thus are easily separated from plutonium but remain with the trivalent actinides. The data displayed in Fig. 3 show that Fe(III) exhibits the same minimum in its extraction isotherm at 6 M HCl as curium and californium; significant Fe(II) extraction occurs only in acid concentrations greater than 6 M. Optimum plutonium separation from these contaminants also occurs in 6 M HCI solutions.
3250
J . J . F A R D Y and J. M. C H I L T O N
m
0.1
~&
252Cf
--e
0 o Z
0.0t
w
%o
o,ool o.ooo5 o
2
6 8 40 HCl CONCENTRATIONIN AQUEOUS ( N )
42
44
Fig. 2. Curium and californium extraction by 1.0 M H D E H P in DEB as a function of HCI concentration.
400
tO
~ Fe ( I I
-
~
~
-
~,
~
-
= -
Ill)
_
P/
o.ot m
m
0,004
O
2
4 6 8 40 42 HCl CONCENTRATIONIN AQUEOUS(N)
44
Fig. 3. Fe(lI) and Fe(lll) extraction by 1.0 M H D E H P in DEB as a function of HCI concentration.
Isolation of plutonium in chloride media- 11
3251
Solvent and acid dependences of plutonium extraction The extraction coefficient for Pu(IV) in 6 M HCI is first power dependent on the H D E H P concentration as seen in Fig. 4, while Fig. 1 shows that the extraction coefficient for tetravalent plutonium into 1 M H D E H P is inversely proportional to the first power of the hydrogen ion concentration in the aqueous phase. Like tetravalent plutonium, trivalent plutonium extraction is first power dependent on the H D E H P concentration. However, the extraction of Pu(VI) is more complex, displaying an increase in power dependence with increasing H D E H P concentration (Fig. 4), whereas Fig. 1 shows that the negative slope of the extraction curve increases with increasing acid concentration. In non-polar solvents, such as DEB, H D E H P is dimeric[7]. It is possible to formulate the following extraction mechanism for many divalent and trivalent metal ions: M b++ b(HA)~(org) --~ M(HA~)b(or~)+ bH +
(1)
in which HA represents the monoacidic ester of phosphoric acid. However, the extractions of the tetravalent ions, Th(IV), Np(IV), and Zr(IV) show a more 40O
-II, l l t0 Z
I IIllZ
~,/SLOPE =1.0 ~
W
f IIi2
-'-
e/
= - -
_ _
hi 0 U Z
t
_ _ -'--
g
---
~
--
__
/
I--X
- -
J
-
__ - "
-
•
X L OPE
--
a.
--
- -
=I.0
--
PuI ~ , I
oo
O,Ot __
a~,
t
/
Il~l:ltlll" I~t
-
I I lllillll'
I IIIIIII
13.t
t0
MOLARITY OF HDEHP IN DEB Fig. 4. Plutonium extraction from 6.0 M H C I as a function of H D E H P
concentration.
7. D . F . Peppard, J. R. Ferraro and G. W. Mason, J. inorg, nucl. Chem. 4, 371 (1957).
3252
J.J.
F A R D Y and J. M. C H I L T O N
complex behavior [8]. Peppard and Mason [8] represented the extraction of thorium in chloride solutions as M 4++ 3(HA)2(org) ~ MA2(HAz)z(org)+ 4H +
(2)
from which the distribution ratio Ea° (their K) is expressed as Ea ° = kl [(HA)2]~ors) [H+] 4
(3)
in which kl is the equilibrium constant for Equation (2). Our results indicate that Pu(IV) extraction is also complex in 6 M HCI solutions. The anion and solvent dependences for Pu(IV) differ markedly from the third power solvent dependence and inverse fourth power hydrogen ion dependence exhibited by thorium. Consequently, a reaction analogous to Equation (2) does not represent the Pu(IV) behavior. In other studies[9] it has been shown that thorium extracted from nitrate systems contains nitrate in the extracted species, and this extraction can be expressed as M 4++ NO3- + 3(HA)2(org) ~- M(NOa)(HAsh(org)+ 3H +.
(4)
It has been proposed by analogy that in high HCI concentrations the extracted species should contain chloride[9]. If it can be assumed that the corresponding species in the P u ( I V ) - H D E H P system contains three chlorides, then the extraction can be depicted by the expression Pu 4+ + 3C1- + (HA)2(org) ~ Pu(CIa)(HA2)torg)+ H +.
(5)
On the other hand, possible impurities in the H D E H P reagent, changing ionic strength, and the possibility of competitive cation and neutral species extraction mechanisms at this high acid strength[5] can account partially for the observed relationships.
Plutonium stripping (back-extraction) Several methods can be used to strip plutonium from the organic solvent: (1) reduction of the plutonium to Pu(III), which has a low affinity for the acid ester, (2) formation of an aqueous-soluble plutonium complex which is stronger than the organophosphate complex, and (3) adjustment of the hydrogen ion concentration in the aqueous phase to reverse the metal ion-hydrogen ion exchange. This third method was not considered to be practicable because of the large extraction coefficients over the range of acid concentrations from 2 to 12 M HCI. Reduction of the plutonium to Pu(III) will result in efficient stripping and leave the zirconium in the organic phase. Additions of the aqueous-soluble reductants, ascorbic acid, hydroquinone, or hydroxylamine hydrochloride, to HCI strip 8. D . F . Peppard and G. W. Mason, Nucl. Sci. Engng 16,382 (1963). 9. D . F . Peppard, G. W. Mason and S. McCarty, J. inorg, nucl. Chem. 13, 138 (1960).
Isolation of plutonium in chloride m e d i a - II
3253
solutions at room temperature failed to quantitatively remove the plutonium. However, the stripping coefficients did increase with increasing acidity of the strip solution. Ascorbic acid and hydroquinone are equally ineffective over the complete acid range (So" = 0.3 in 6 M HCI), and hydroxylamine hydrochloride is even less effective (S0" = 0.008 in 6 M HCI). At 50 ° all of these reagents are more effective; in general, the stripping increases with increasing acidity. For 0.2 M hydroquinone at room temperature the strip coefficients in 6 M and 10 M HC1 solutions are 0.3 and 4.8, while at 50 ° these values increase to 6.6 and 36.0. However. an increase of acid concentration from 6 M to 10 M at 50 ° did not result in an increase in stripping with ascorbic acid; a visible darkening of the 10 M acid solution indicated decomposition of the reagent. Laboratory studies show that Pu(IV) extraction with H D E H P does not decrease significantly with increasing temperature; therefore, the more efficient stripping cannot be explained by this mechanism. Reductant stability and increased solubility of the reductant in the organic phase at elevated temperatures appear to be the major parameters in this mode of stripping. Since the aqueous-soluble reductants are relatively unsatisfactory for plutonium stripping at room temperature, an organic-soluble reductant, 2,5-di-tertbutylhydroquinone was examined[10]. The solubility of D B H Q in DEB is low; qo'
~Z
~0o
w 0 Z
9
~
~o-~
I.x w
Z
~ to-~
0
2
4 6 8 tO 12 HGI CONCENTRATION IN AQUFOU5 (N)
14
Fig. 5. Plutonium stripping from 1-0 M H D E H P , modified with 25 volume % of 0.2 M D B H Q in 2-EHOH, as a function of HCI concentration. 10. C. F. Coleman, Final Cycle Plutonium Separation by Amine Extraction, ORNL-CF-61-5-74, May (1961).
3254
J. J. FARDY and J. M. CHILTON
therefore, a 0.2 M solution of DBHQ in 2-EHOH was used. The addition of 25 v/o of this solution to the 1 M HDEHP-DEB extractant gave efficient stripping into 5-9 M HC1 solutions (see Fig. 5). Dilution of the organic phase with 25 v/o of either DEB or 2-EHOH without any DBHQ lowered the extraction coefficient of the Pu(IV) only slightly. Thus, this concentration of the polar diluent, 2-EHOH, does not participate significantly in stripping through inactivation of the HDEHP by suppression of its dimer formation[11]. An equivalent quantity of DBHQ, in the absence of 2-EHOH, can be dissolved in the extractant at higher temperatures, but lower Pu stripping reflects poor heat stability. Hydroquinone dissolves readily in 2-EHOH and can be added to the organic phase, but it is less efficient than DBHQ. Several aqueous-soluble complexing reagents for plutonium were investigated. Carbonate solutions were unsatisfactory because of emulsification and difficulty in phase separations. Citric acid does not form a complex strong enough to shift the equilibrium to the aqueous phase. Oxalate solutions were very effective in stripping the plutonium; for example, a 0.5 M solution of oxalic acid in 6 M HCI gave a strip coefficient of 115. However, this reagent also strips zirconium from the organic phase with a coefficient of 800; thus no separation of this impurity from plutonium is achieved.
Plant process application A process based on the above data has been used for separating tetravalent plutonium from transplutonium elements, corrosion products, and fission products. The success of this plutonium isolation scheme has led to its routine use in the Transuranium Processing Facility at Oak Ridge National Laboratory [ 12]. 11. G. W. Mason, S. McCarty and D. F. Peppard, J. inorg, nucl. Chem. 2,4,967 (1962). 12. D.E. Ferguson, ORNL-4145, 132 (1967).
Pergamon Press.
Printed in Great Britain
ISOLATION OF P L U T O N I U M IN CHLORIDE MEDIA-II SOLVENT
EXTRACTION
WITH DI(2-ETHYLHEXYL)PHOSPHORIC ACID*
J. J. FARDYt Visiting Scientist, Australian Atomic Energy Commission, Lucas Heights, Sydney, Australia and J. M. CH1LTON Chemical Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. 37830 (Received 17 February 1969)
Abstract-Plutonium extraction by di(2-ethylhexyl)phosphoric acid from hydrochloric acid solution varies with the different valence states in the order Pu(IV) > Pu(VI) > Pu(lll). Behavior of transplutonium actinides, corrosion products, and fission products was examined in this system, and it was found that by extracting Pu(IV) from 6 M HCI it can be separated from most of these ions. Separation from zirconium, which is quantitatively extracted, is achieved by removal of the plutonium from the organic phase as Pu(III) by the addition of an organic-soluble reductant. Stripping of plutonium by aqueous-soluble reductants at elevated temperature is also feasible. Results of laboratory studies to determine optimum conditions for plutonium recovery and decontamination in the production of transplutonium actinides are presented. INTRODUCTION AS POINTED o u t in t h e p r e v i o u s p a p e r in this s e r i e s [ l ] , it is d e s i r a b l e to r e m o v e p l u t o n i u m f r o m h y d r o c h l o r i c a c i d s o l u t i o n s o f i r r a d i a t e d t a r g e t s in t h e i s o l a t i o n of transplutonium elements. The mono-acidic ester, di(2-ethylhexyl)phosphoric a c i d , is k n o w n to h a v e a s t r o n g affinity f o r c e r t a i n h i g h l y - c h a r g e d m e t a l l i c ions in s t r o n g a c i d s o l u t i o n s [2, 3]. P l u t o n i u m e x t r a c t i o n has b e e n e x a m i n e d in n i t r a t e s y s t e m s [4, 5], b u t f e w d a t a e x i s t f o r c h l o r i d e m e d i a . W e e x a m i n e d the e x t r a c t i o n o f p l u t o n i u m in t h e v a r i o u s o x i d a t i o n s t a t e s a n d p r e s e n t a s e p a r a t i o n p r o c e d u r e f o r isolating plutonium from trans-plutonium elements and from most corrosion and fission p r o d u c t s . EXPERIMENTAL Materials
All chemicals used in these studies were of reagent grade or the highest purity obtainable from commercial sources, with the exception of those purified as described below. *Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. 1. J. M. Chilton andJ. J. Fardy, J. inorg, nucl. Chem. 31, 1171 (1969). 2. C. A. Blake, Jr., C. F. Baes, K. B. Brown, C. F. Coleman and J. C. White, Proc. 2rid Int. Conf. peaceful Uses atom. Energy, Geneva 1958, Vol. 28, p. 289. United Nations, New York (1958). 3. K. Kimura, J. chem. Soc. Japan 33 [8], 1038 (1960). 4. Y. N. Kosyakov, E. S. Gurayev and G. N. Yakolev, Separation of Transuranium Elements by Solution Extraction in Alkyl Phosphoric Acid. Paper given at Transplutonium Symposium, Argonne National Laboratory, Illinois, May 1963. (Gmelin Ref. No. AED-CONF-63-062-1). 5. D. E. Horner and C. F. Coleman, ORNL-3051 (1961). 3247
3248
J.J.
F A R D Y and J. M. C H I L T O N
The di(2-ethylhexyl)phosphoric acid ( H D E H P ) was obtained from a commercial source and contained less than 5% mono(2-ethylhexyl)phosphoric acid (H2MEHP). This was purified to remove the di-acidic ester and other impurities. A 0.5 M solution of the commercial H D E H P in diethylbenzene (DEB) was washed with an equal volume of 1 M NaOH, and the middle phase of the three that formed was separated. This was treated with excess 1 M HCI, separated, and washed three times with one-half volume of ethylene glycol. Commercial grade diethylbenzene was purified by passing it through columns of silica gel and alumina to remove water and organic surfactants. Practical grade 2,5-di-tert-butyl-hydroquinone (DBHQ), and 2-ethyl-1-hexanol (2-EHOH) were used without further treatment. The radioactive tracer isotopes, 1~4Ce, asZr-Nb, asSr, 13~Cs, 241Am, 242Cm, and 252Cf were obtained from the Oak Ridge National Laboratory Isotopes Sales Department.
Apparatus and procedures All determinations of equilibrium distribution coefficients, Ea°, defined as the ratio of the concentrations in the organic and aqueous phases, were made by analyses of both phaSes for the element of interest. At room temperature (25 ° _ 1°), equilibrium between the phases was obtained by mixing 5 ml of the aqueous phase containing the element under test with the same volume of extractant in a 1 oz glass bottle for 5 rain on a vortex mixer. Equilibrium is obtained in this time, as shown by the consistency in coefficients measured after further agitation for 10 and 15 rain. Special glass bottles were fabricated for measurements at elevated temperatures. A flat-bottom glass container, approximately 20 ml capacity, was surrounded with an outer glass jacket with inlet and outlet ports. Water at the desired temperature was circulated around the bottle from a constant-temperature bath ( - 0-1°). Agitation of the two phases was accomplished by a magnetic stirrer and a Teflon-covered stirring bar. Analyses for alpha-active elements were carded out by planchet counting on a 2~r gas-flow proportional counter and by alpha pulse height analysis. Measured volumes of gamma-active elements were counted in a well-type scintillation counter, and pulse height analysis was used when necessary. Analyses for nonradioactive elements were carded out by standard spectrophotometric procedures. RESULTS
AND
DISCUSSION
Extraction of plutonium and other metal ions The extraction of plutonium from HCI solutions by 1.0 M H D E H P in DEB is summarized in Fig. 1. The extraction order is Pu(IV) > Pu(VI) > Pu(IIl) at all acid concentrations. Therefore, in any separations involving this reagent, it must be remembered that either oxidation or reduction of a Pu(IV) solution will decrease the amount of plutonium extracted. Extraction coefficients of the transplutonium elements, curium and californium, are shown in Fig. 2. Americium extraction is about the same as curium and, hence, about a factor of 30 less than californium. This ratio is lower than the factor of 100 obtained in the 2-ethylhexyl phenylphosphonate-HCl system studied by Baybarz [6]; however, the shapes of the extraction isotherms of these elements are similar for both systems. Even though plutonium extraction is greater at lower acidity (Ea ° = 240 at 2 M HC1), the maximum separation from the transplutonium elements is obtained by extraction from 6 M HCI. Separation from certain corrosion and fission products is also obtained in this extraction system. Since zirconium, which is present as a corrosion product from Zircaloy plant equipment and also as a fission product, is very strongly extracted at all HCI concentrations (Ea ° > 2000), it is removed from all of the trivalent transplutonium elements, which remain in the aqueous phase. Subsequent separation from the extracted Pu(IV) is achieved by reducing plutonium to the 6. R. D. Baybarz, Nucl. Sci. Engng 17,457 (1963).
Isolation of plutonium in chloride media- I 1
~000
-
r
lJill!
3249
I i lll---
e~Pu (IV)
100 -
.,, le
I q
1o w
8 z 0 F-
q z
3(~ o;
0.t
Pu(Ill)
0.01
f III11 z
0.005
~o
to0
HCI coNCENTRATION IN AQUEOUS (N)
Fig. I. Plutonium extraction by 1.0 M H D E H P in D E B as a function of HCI concen-
tration.
trivalent state so that it can be selectively stripped. Other corrosion products, Al, Cr, and Ni, and the fission products, '*iCe, 9°Sr, and lSTCs, are poorly extracted from 2 to 12 M HCI solutions (E, ° < 0.005) and thus are easily separated from plutonium but remain with the trivalent actinides. The data displayed in Fig. 3 show that Fe(III) exhibits the same minimum in its extraction isotherm at 6 M HCl as curium and californium; significant Fe(II) extraction occurs only in acid concentrations greater than 6 M. Optimum plutonium separation from these contaminants also occurs in 6 M HCI solutions.
3250
J . J . F A R D Y and J. M. C H I L T O N
m
0.1
~&
252Cf
--e
0 o Z
0.0t
w
%o
o,ool o.ooo5 o
2
6 8 40 HCl CONCENTRATIONIN AQUEOUS ( N )
42
44
Fig. 2. Curium and californium extraction by 1.0 M H D E H P in DEB as a function of HCI concentration.
400
tO
~ Fe ( I I
-
~
~
-
~,
~
-
= -
Ill)
_
P/
o.ot m
m
0,004
O
2
4 6 8 40 42 HCl CONCENTRATIONIN AQUEOUS(N)
44
Fig. 3. Fe(lI) and Fe(lll) extraction by 1.0 M H D E H P in DEB as a function of HCI concentration.
Isolation of plutonium in chloride media- 11
3251
Solvent and acid dependences of plutonium extraction The extraction coefficient for Pu(IV) in 6 M HCI is first power dependent on the H D E H P concentration as seen in Fig. 4, while Fig. 1 shows that the extraction coefficient for tetravalent plutonium into 1 M H D E H P is inversely proportional to the first power of the hydrogen ion concentration in the aqueous phase. Like tetravalent plutonium, trivalent plutonium extraction is first power dependent on the H D E H P concentration. However, the extraction of Pu(VI) is more complex, displaying an increase in power dependence with increasing H D E H P concentration (Fig. 4), whereas Fig. 1 shows that the negative slope of the extraction curve increases with increasing acid concentration. In non-polar solvents, such as DEB, H D E H P is dimeric[7]. It is possible to formulate the following extraction mechanism for many divalent and trivalent metal ions: M b++ b(HA)~(org) --~ M(HA~)b(or~)+ bH +
(1)
in which HA represents the monoacidic ester of phosphoric acid. However, the extractions of the tetravalent ions, Th(IV), Np(IV), and Zr(IV) show a more 40O
-II, l l t0 Z
I IIllZ
~,/SLOPE =1.0 ~
W
f IIi2
-'-
e/
= - -
_ _
hi 0 U Z
t
_ _ -'--
g
---
~
--
__
/
I--X
- -
J
-
__ - "
-
•
X L OPE
--
a.
--
- -
=I.0
--
PuI ~ , I
oo
O,Ot __
a~,
t
/
Il~l:ltlll" I~t
-
I I lllillll'
I IIIIIII
13.t
t0
MOLARITY OF HDEHP IN DEB Fig. 4. Plutonium extraction from 6.0 M H C I as a function of H D E H P
concentration.
7. D . F . Peppard, J. R. Ferraro and G. W. Mason, J. inorg, nucl. Chem. 4, 371 (1957).
3252
J.J.
F A R D Y and J. M. C H I L T O N
complex behavior [8]. Peppard and Mason [8] represented the extraction of thorium in chloride solutions as M 4++ 3(HA)2(org) ~ MA2(HAz)z(org)+ 4H +
(2)
from which the distribution ratio Ea° (their K) is expressed as Ea ° = kl [(HA)2]~ors) [H+] 4
(3)
in which kl is the equilibrium constant for Equation (2). Our results indicate that Pu(IV) extraction is also complex in 6 M HCI solutions. The anion and solvent dependences for Pu(IV) differ markedly from the third power solvent dependence and inverse fourth power hydrogen ion dependence exhibited by thorium. Consequently, a reaction analogous to Equation (2) does not represent the Pu(IV) behavior. In other studies[9] it has been shown that thorium extracted from nitrate systems contains nitrate in the extracted species, and this extraction can be expressed as M 4++ NO3- + 3(HA)2(org) ~- M(NOa)(HAsh(org)+ 3H +.
(4)
It has been proposed by analogy that in high HCI concentrations the extracted species should contain chloride[9]. If it can be assumed that the corresponding species in the P u ( I V ) - H D E H P system contains three chlorides, then the extraction can be depicted by the expression Pu 4+ + 3C1- + (HA)2(org) ~ Pu(CIa)(HA2)torg)+ H +.
(5)
On the other hand, possible impurities in the H D E H P reagent, changing ionic strength, and the possibility of competitive cation and neutral species extraction mechanisms at this high acid strength[5] can account partially for the observed relationships.
Plutonium stripping (back-extraction) Several methods can be used to strip plutonium from the organic solvent: (1) reduction of the plutonium to Pu(III), which has a low affinity for the acid ester, (2) formation of an aqueous-soluble plutonium complex which is stronger than the organophosphate complex, and (3) adjustment of the hydrogen ion concentration in the aqueous phase to reverse the metal ion-hydrogen ion exchange. This third method was not considered to be practicable because of the large extraction coefficients over the range of acid concentrations from 2 to 12 M HCI. Reduction of the plutonium to Pu(III) will result in efficient stripping and leave the zirconium in the organic phase. Additions of the aqueous-soluble reductants, ascorbic acid, hydroquinone, or hydroxylamine hydrochloride, to HCI strip 8. D . F . Peppard and G. W. Mason, Nucl. Sci. Engng 16,382 (1963). 9. D . F . Peppard, G. W. Mason and S. McCarty, J. inorg, nucl. Chem. 13, 138 (1960).
Isolation of plutonium in chloride m e d i a - II
3253
solutions at room temperature failed to quantitatively remove the plutonium. However, the stripping coefficients did increase with increasing acidity of the strip solution. Ascorbic acid and hydroquinone are equally ineffective over the complete acid range (So" = 0.3 in 6 M HCI), and hydroxylamine hydrochloride is even less effective (S0" = 0.008 in 6 M HCI). At 50 ° all of these reagents are more effective; in general, the stripping increases with increasing acidity. For 0.2 M hydroquinone at room temperature the strip coefficients in 6 M and 10 M HC1 solutions are 0.3 and 4.8, while at 50 ° these values increase to 6.6 and 36.0. However. an increase of acid concentration from 6 M to 10 M at 50 ° did not result in an increase in stripping with ascorbic acid; a visible darkening of the 10 M acid solution indicated decomposition of the reagent. Laboratory studies show that Pu(IV) extraction with H D E H P does not decrease significantly with increasing temperature; therefore, the more efficient stripping cannot be explained by this mechanism. Reductant stability and increased solubility of the reductant in the organic phase at elevated temperatures appear to be the major parameters in this mode of stripping. Since the aqueous-soluble reductants are relatively unsatisfactory for plutonium stripping at room temperature, an organic-soluble reductant, 2,5-di-tertbutylhydroquinone was examined[10]. The solubility of D B H Q in DEB is low; qo'
~Z
~0o
w 0 Z
9
~
~o-~
I.x w
Z
~ to-~
0
2
4 6 8 tO 12 HGI CONCENTRATION IN AQUFOU5 (N)
14
Fig. 5. Plutonium stripping from 1-0 M H D E H P , modified with 25 volume % of 0.2 M D B H Q in 2-EHOH, as a function of HCI concentration. 10. C. F. Coleman, Final Cycle Plutonium Separation by Amine Extraction, ORNL-CF-61-5-74, May (1961).
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J. J. FARDY and J. M. CHILTON
therefore, a 0.2 M solution of DBHQ in 2-EHOH was used. The addition of 25 v/o of this solution to the 1 M HDEHP-DEB extractant gave efficient stripping into 5-9 M HC1 solutions (see Fig. 5). Dilution of the organic phase with 25 v/o of either DEB or 2-EHOH without any DBHQ lowered the extraction coefficient of the Pu(IV) only slightly. Thus, this concentration of the polar diluent, 2-EHOH, does not participate significantly in stripping through inactivation of the HDEHP by suppression of its dimer formation[11]. An equivalent quantity of DBHQ, in the absence of 2-EHOH, can be dissolved in the extractant at higher temperatures, but lower Pu stripping reflects poor heat stability. Hydroquinone dissolves readily in 2-EHOH and can be added to the organic phase, but it is less efficient than DBHQ. Several aqueous-soluble complexing reagents for plutonium were investigated. Carbonate solutions were unsatisfactory because of emulsification and difficulty in phase separations. Citric acid does not form a complex strong enough to shift the equilibrium to the aqueous phase. Oxalate solutions were very effective in stripping the plutonium; for example, a 0.5 M solution of oxalic acid in 6 M HCI gave a strip coefficient of 115. However, this reagent also strips zirconium from the organic phase with a coefficient of 800; thus no separation of this impurity from plutonium is achieved.
Plant process application A process based on the above data has been used for separating tetravalent plutonium from transplutonium elements, corrosion products, and fission products. The success of this plutonium isolation scheme has led to its routine use in the Transuranium Processing Facility at Oak Ridge National Laboratory [ 12]. 11. G. W. Mason, S. McCarty and D. F. Peppard, J. inorg, nucl. Chem. 2,4,967 (1962). 12. D.E. Ferguson, ORNL-4145, 132 (1967).