I
T. H. SIDDALL Ill Savannah River Laboratory,
E. 1.
du Pont de Nemours & Co., Aiken, S. C.
Trialkyl Phosphates and Dialkyl Alkylphosphonates in Uranium and Thorium Extraction Phosphonates are strikingly supeiior to phosphates in extracting uranium and thorium and separating them from zirconium
SEVERAL
sources (7-3) indicate the ability of organic compounds that contain the phosphoryl group to extract lanthanides, actinides, and zirconium. However, most of the work reported has dealt with tri-n-butyl phosphate (TBP) and not with efforts to improve selectivity by altering the substituents on the phosphorus atom. The objective of the work covered in this article was to examine the potential of several trialkyl phosphates and dialkyl alkylphosphonates for extracting uranium and thorium from nitric acid solutions and for separating them from zirconium.
Survey of Extractants The extraction of tracer uranium, thorium, zirconium, and nitric acid by five dialkyl alkylphosphonates and six
trialkyl phosphates is shown in Figures 1 through 7 . The extractants were all diluted to 1.09M with n-dodecane and all measurements were made at 30' C. The data for uranium, thorium, and zirconium are summarized in Table I. The phosphonates are strikingly superior to the phosphates-they extract uranium about 2.5 times better and thorium about 7.1 times better-and they are about 4.6 times more efficient in separating uranium from zirconium, and about 12.7 times more efficient in separating thorium from zirconium. The effects of changing alkyl substituents seem to be parallel in the two classes of extractants. Five appears to be the optimum number of carbons in a n-alkyl chain (using isoamyl as a standin for n-amyl in the phosphate class). Uranium and thorium extraction and
.-CONCENTRATION OF NITRIC ACID IN THE A P f O U S PHASE AT EOUIUBRIUM. MOLS/LITER-
-
I
Figure' 1. Distribution of tracer uranium between nitric acid and trialkyl phosphates Distribution varies when alkyl substituent in phosphate i s changed
separation from zirconium reach a maximum with the amyl compounds. I n both classes the 2-ethylhexyl group is more favorable than the n-alkyl groups for uranium but not for thorium. Attachment of the alkyl group through a secondary carbon atom favors the extraction of uranium. Tris( 1,1,7 trihydroperfluoroheptyl) phosphate extracts uranium very little, if at all. A small amount of yellow scum was formed in the organic phase when this compound was brought in contact with a solution of uranyl nitrate. Otherwise there was no evidence of extraction. The ineffectiveness of tris(1,1,7-trihydroperfluoroheptyl) phosphate as a n extractant for uranium is probably due to the electroinductive effect of the fluorine atoms. Trineopentyl phosphate is a poorer
Figure 2. Distribution of tracer uranium between nitric acid and dialkyl alkylphosphonates Extraction increased about a factor of 10 over that with trialkyl phosphates
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Figure 3. Distribution of tracer thorium between nitric acid and trialkyl phosphates
Figure 4. Distribution of tracer thorium between nitric acid and dialkyl alkylphosphonates
Distribution i s not affected as much b y changes in alkyl substituent as i s the case with uranium
Extraction increased b y as much as a factor of 30 over that with phosphates
extractant for uranium, thorium, and zirconium than are the other phosphates. This inferiority is probably due to steric hindrance by the neopentyl group. Work with this compound was limited by its small solubility in n-dodecane (about 0.3M a t room temperature). The extraction of nitric acid is relatively unaffected by the class of extractant or by differences in the substituents within a class. T h e distribution coefficients of nitric acid a t low concentrations of nitric acid are only about twice as
Table I.
high with the phosphonates as with the phosphates. At higher concentrations of acid the differences become small. Similar but even smaller differences in behavior are to be noted within a class. In so far as conclusions can be drawn from the behavior of a limited number of compounds, there are empirical rules for choosing a n extractant for uranium. T h e branched alkyl radicals are to be preferred to the normal radicals. The branching should preferably occur at the 1 position but may be effective in the
Comparison of Extractants a t 30' C. Shows That Other Chemically Similar Extractants Are More Selective Than Tri-n-butyl Phosphate Distribution Coefficient
Z U/Zr Th/Zr Phosphate" Tri-n-butyl 25 2.9 0.22 105 13 Triisoamyl 34 4.2 0.12 280 35 Tri-n-hexyl 38 3.0 0.14 270 21 Tricyclohexyl 106 3.5 0.64 166 5.5 Tri-n-octyl 33 2.4 0.14 240 17 Tris(2-ethylhexyl) 58 2.5 0.14 410 18 Average 49 3.1 0.23 245 17 Phosphonatea Di-n-butyl n-butyl 92 24 0.17 530 140 Di-n-butyl cyclohexyl 125 17 0.14 890 120 Di-n-amyl n-amyl 133 33 0.092 1450 360 Di-n-hexyl n-hexyl 89 26 O.07Ob 1270 370 Bis( 2-ethylhexyl) 176 10.6 0.12 1470 90 Average 123 22 0.12 1122 216 a All data for phosphates are for 3.011.1 HNOa in aqueous phase at equilibrium. All data for phosphonates are for 0.80-VHNOa in aqueous phase at equilibrium. These are typical concentrations of nitric acid that might be used in separations processes with the two classes of extractants. Extrapolated value. Compound
42
U
Th
INDUSTRIAL AND ENGINEERING CHEMISTRY
2 position. However, the choice of compounds is limited because of compounds which form solid addition compounds of small solubility. The compounds formed from 2-hexanol or possibly 2-pentanol should be superior extractants, without a t the same time forming the insoluble addition compound. If only the normal compounds are available, amyl and hexyl groups are to be preferred to butyl. Occurrence of a Third Phase Certain extractants formed addition compounds with uranyl nitrate which separated out as a solid phase. Solutions of tri-n-butyl phosphate and di-nbutyl n-butyl phosphonate formed a third liquid phase under a variety of conditions when in contact with aqueous solutions containing thorium nitrate. The solubility in hydrocarbons of the addition compounds with uranium appears to be governed by a t least three rules : 1. The solubility of the addition compounds increases when the chain length of normal paraffin diluents is decreased. 2. When the alkyl substituents in the extractant molecule are normal isomers, solubility of the addition compound increases as the chain length of the alkyl groups increases. Addition compounds of the n-butyl phosphate and phosphonate are easily crystallized from hexane in a dry ice-acetone bath. The
NUCLEAR TECHNOLOGY
CONCENTRATION OF NITRIC ACID IN THE AQUEOUS PHASE AT EQUILIBRIUM MOLSILITER
Figure 5. Distribution of tracer zirconium between nitric acid and trialkyl phosphates After tricyclohexyl phosphate TBP shows poorest selectivity against zirconium extraction
amyl addition compounds are crystallized only with difficulty from n-hexane and the hexvl comDounds are not crvstallized a t all.' 3. The solubilitv of the addition compounds increases with increasing temperature. There is some evidence that solubility increases as the diluent molecule is made more symmetric. Certain characteristics of three of these addition compounds are described below. Trineopentyl Phosphate. The addition compound of uranyl nitrate with trineopentyl phosphate was very insoluble in n-dodecane. A good x-ray pattern of the solid addition compound was obtained. The unit cell for uranium atoms was orthorhombic with dimensions of 17.1, 11.9, and 13.3 A. Tricyclohexyl Phosphate. The addition compound of tricyclohexyl phosphate with uranyl nitrate was only slightly soluble in n-dodecane. Tricyclohexyl phosphate is itself a solid which melts at 65" C. and is soluble in n-dodecane at room temperature, at least to the extent of 0.5M. Apparently the addition compounds of uranyl nitrate with trialkyl phosphates are thermally unstable even when highly purified. A sample of the addition compound of tricyclohexyl phosphate was prepared from phosphate which had been three times crystallized from
CONCENTRATION OF NITRIC ACID
Figure 6. Distribution of zirconium between nitric acid and d ia Ikyl a lkylphosphonates Butyl group again shows poor selectivity against extraction of zirconium
menthanol. At about 120' C. the compound decomposed without melting. During decomposition nitrogen dioxide was evolved and other symptoms similar to the violent decomposition of tri-nbutyl phosphate-uranyl nitrate were observed.
Di-n-butyl Cyclohexyl Phosphonate. The addition compound of di-n-butyl cyclohexyl phosphonate with uranyl nitrate was very soluble in hydrocarbons at room temperature. T h e solubility increased as the chain length of the hydrocarbon was decreased. When 2,2, 4,6,G-pentamethylheptane was substituted for n-dodecane, the solubility of the addition compound was slightly increased. Samples of the addition
Table 11. Solubility of Addition Compound of Di-n-butyl Cyclohexyl Phosphonate with Uranyl Nitrate Depeqds on Choice of Hydrocarbon Diluent Temp. of Complete Miscibility, Hydrocarbon %-Hexane Decalin
2,2,4,6,6-Pentamethylheptane n-Dodecane n-Heradecane
IN THE A Q ~ E O U SPHASE AT EQUILIBRIUM, MOLWLITER
O
c.
7 About 15 32 38 46
compound were added to various hydrocarbons in the ratio of 1 gram of addition compound to 2 ml. of hydrocarbon. Solubility data are given in Table 11. T h e melting point of the addition compound was found to be 65' C. Formation of a Third Liquid Phase in Presence of Thorium Nitrate. T h e marked tendency of extractant dissolved in aliphatic diluent to form a second liquid organic phase when mixed with aqueous thorium nitrate appears to be restricted to tri-n-butyl phosphate and di-n-butyl n-butyl phospKonate. The tendency of tri-n-butyl phosphate solutions in aliphatic diluents to form a second organic phase when in contact with thorium nitrate has been described ( 4 ) . Qualitative experiments with 1.1M solutions of di-n-butyl n-butyl phosphonate showed that di-n-butyl n-butyl phosphonate tended to form a third phase to about the same extent as trin-butyl phosphate. . The tendency to form a third phase decreases markedly with amyl phosphates and phosphonates, and was not observed with hexyl compounds and higher homologs. A 1.09M solution of triisoamyl phosphate in n-dodecane was equilibrated with an equal volume of 1.4M solution of thorium nitrate. No second organic phase was observed a t room temperature, though a t about 0' C. there was a faint cloudiness in the organic VOL. 51,
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Figure 7. Distribution of nitric acid for dialkyl alkylphosphonates and trialkyl phosphates Extraction of nitric arid is insensitive to change in alkyl substituents and is increased only moderately b y substituting an alkyl for an alkoxy group
01-N-BUTYL N-BUTILPHOSPHONATE
$ - 01-N-BUTYL CICLOHEXYLPHOSPHONATE DI-N-AMYL H-AMYLPHOSPHONATE
- DI-N-HEXYL N-HEXYLPHOSPHONATE V - 815 (I-ETHYLHEXYL) I-ETHYLHEXYLPHOSPHONATE 0 - TRI-ti-BUTYL PHOSPHATE + TRllSOAMYL PHOSPHATE X - T R I - N - H E X Y L PHOSPHATE A
~
Y
V
-
TRlS (5-ETHYLHEXYLI PHOSPHATE
CONCENTRATION OF NITMC ACID IN THE AQUEOUS PHASE AT EQUILIBRIUM, MOLS/LITER
phase which usually precedes the separation of a separate phase. The limiting concentrations of nitric acid and thorium nitrate for maintaining a twophase system were determined for 1.09,M di-n-amyl n-amyl phosphonate in n-dodecane at 30" C. (Table 111). A third phase was observed for 1.09.44 di-n-butyl cyclohexyl phosphonate only a t very high concentrations of thorium. This extractant appeared to have about
Tab'e 'I1* No Second Organic Phase Appears Until High Thorium 'rations Are Reached in System Thorium Nitrate-Nitric Acid-Water-Di-namyl n-Amylphosphonate in n-Dodecane Th(N03)d Aqueous Phase Concn., HSOi ThjNox)~ Oreanic -I
coiicn.,
niole/l. 0.0 0.27 0.54 0.77 ~
conen., moles/l.
2.04 1.99 1.97 1.96 ~
I
Phase,
Mole/L. 0.374 0.372 0.372 0.371
~~
Table IV. Mole Ratio of Bis(2-ethylhexyl) 2-Ethylhexyl Phosphonate to Thorium Is Almost 3 to 1 Weight % of Bis(2-ethylhexyl) 2-Ethylhexylphosphonate in Mole Ratio Organic Phase Extractant before Extraction t o Thorium 100 55 32 15.8
44
2.8 2.8 2.9 2.9
INDUSTRIAL AND ENGINEERING
the same properties as di-n-amyl n-amylphosphonate. When a 1.09M solution of tri-set-butyl phosphate was equilibrated with an equal volume of 1'21 thorium nitrate at room temperature. no third phase formed. Stoichiometry of Extraction Reactions of Dialkyl Alkylphosphonates. Katzin and others ( 3 ) found that there must be two complexes of thorium nitrate with di-n-butyl n-butyl phosphonate. When solutions of di-n-butyl n-butyl phosphonate were saturated with tho-~ rium nitrate, the average mole ratio of di-n-butyl n-butyl phosphonate to thorium M'as 2.34 This is consistent with existence of a mixture of tWO complex species. one containing two and the other three molecules of di-n-butvl n-butyl phosphonates per thorium atom. Di-n-hexyl n-hexyl phosphonate and bis(2-ethylhexyl) 2-ethylhexyl phosphonate were chosen for study of the phosphonates as a class. Confirmarory experiments were performed with di-n-bum1 n-butyl phosphonate. I n addition to the technique, as employed by Katzin, of saturating the organic phase with thorium nitrate. a technique employing very dilute thorium nitrate (tracer UX,) was also used. The tracer experiments were also carried out with uranium-233, to determine the stoichiometric relationships with that element. The tracer technique consisted of equilibrating various dilutions of the phosphonates in n-dodecane at 30' C. with samples of an aqueous phase which were kept constant a t 1 . 0 5 M nitric acid. When the logarithm of the distribution coefficient of thorium or
CHEMISTRY
uranium is plotted as a function of the concentration of the phosphonate, the limiting slope is the same as the number of extractant molecules in each complex molecule. The technique of saturation Lvith thorium nitrate was used \vith bis(2-ethylhexyl) 2-ethylhexyl phosphonate (Table IV). The mole ratio of 2.4 a t 22' C. for di-n-butyl n-butyl phosphonate agrees well with that obtained by Katzin. However, the mole ratio with bis(2ethylhexyl) 2-ethylhexyl phosphonate was very nearly 3. The tracer technique \vas used Jvith di-n-butyl n-butyl phosphonate and din-hexylphosphonate and both uranium and thorium, and with bis(2-erhylhexyl) 2-ethylhexyl phosphonate and thorium only. In all cases the limiting slope with thorium was 3 ; with uranium: 2. O n this basis the stoichiometr!- \vith uranium seems to be two organic molecules for each uranium atom. independent of the concentration of uranium. However, the situation with thorium must be more complicated. .A11 the data for thorium ma!- be explained if the following equilibrium is assumed. where P is the extractant molecule and the thorium nitrate is in the aqueous phase: 2 Th(N03),.3P
+ Th(NO,), + 3 Th(h-O,)JP
.At low concentrations of thorium, as in the tracer experiments. the equilibrium is shifted far to the left and it is possible to detect only the complex with three organic molecules per atom of thorium even \Then di-n-butyl nbutylphosphonate is the extractant. .4t high concentrations of thorium the equilibrium is shifted some\zliat IO the right and a mixture of complexes is obtained. With bis(2-erhylhexyl) 2ethylhexyl phosphonate the equilibrium constant must be much smaller than ivith di-n-butyl n-butyl phosphonate. Even a t high thorium concentrations equilibrium is not shifted significantly to rhe right and the complex with tivo molecules of extractant is scarcely detectable.
Cited (1 Alcock, K., Bedford, F. C., Hardwick, W. H., McKay, H. A. C., J . Znorg. >VuclearChtm. 4, 100 (19571. (2)Hesford, E., McKay, H. A. C., Scargill, D., Zbid., 4, 321 (195'). (31 Katzin, L. I., Ferraro, J. R., Fl-endlandt, W. W., McBeth, R. L.. J . A m . Chem. SOC. 78, 5139 (1956). (4) Siddall, T. H.: 111. AECResrarchand Development Rept. DP-181 (1956).
Literat&
RECEIVED for review 5 f a y 28, 1958 ACCEPTED October 20, 1958 Division of Industrial and Engineering Chemistry, Symposium o n Reprocessing Chemistry for Irradiated Fuel, 133rd Meeting, ACS, San Francisco, Calif., April 1958. Information developed during work under contract AT(07-2)-1 with the U. S . .4tomic Energy Commission.
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