LOS ALAMOS SCIENTIFIC LABORATORY of the UNIVERSITY of Californiao LOS ALAMOS NEWMEXICO

ClC-l 4 REPORT C(XLECTICN REFWCX)LKX’KN v- LA-3188 COPY ,,i J:

LOS ALAMOS SCIENTIFIC LABORATORY OF THE UNIVERSITY OF CALIFORNIAo LOS ALAMOS NEWMEXICO

[ _—— — PLUTONIUM BY ION EXCHANGE - V. ANION-EXCHANGE SORPTION FROM ACID-ALU~NUM NITRATE LEGAL NOTICE

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LOS ALAMOS SCIENTIFIC LABORATORY OF THE UNIVERSITYOF CALIFORNIA LOS ALAMOS NEW MEXICO

REPORT WRITTEN: September 1964

REPORT DISTRIBUTED: March 9, 1965

THE PROCESSING OF PLUTONIUM BY ION EXCHANGE - V. EQUILIBRIUM ANION-EXCHANGE SORPTION FROM MIXED NTTRIC ACID-ALUMINUM NITRATE SOLVENTS

D. B. James

‘1’Msreport expresses the opinions of the author or r authors and does not necessarily reflect the opinions or views of the Los Alarnos Scientific Laboratory.

Contract W-740 5-E NG. 36 with the U. S. Atomic Energy Commission

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ABSTRACT

A method is described for calculating the trace-loading

equilibrium distribution coefficient, 1$, for the sorption of plutonium

(IV) on Dowex IJd+ anion-exchange resin from mixed aluminum nitrate-

nitric acid solutions. Published data for sorption from salt solutions

and from acid solutions are used to obtain the dependence of $ upon

the molar concentration of nitrate ion and upon the mole ratio of

hydronium ion to water. These parameters are calculated according to

a published technique for twen~-two mixed solutions. values of % calculated from theoretical equations, were compared to experimental

measurements of 1$ for these solutions.

ACKNOWLEDGMENTS

The author wishes to thank the personnel in Group CMB-1 of this

Laboratory, who performed the chemical analyses, andR. S. Cooper who

assisted with the calculations.

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TABLE OF CONTENTS Page

ABSTRACT 2

ACKNOWLEIX3MENTS 2

INTRODUCTION 11

EXPERIMENTAL 6 Reagents 6 Batch Equilibration Experiments 7

RESULTS AND CONCLUSIONS 8

REFERENCES 15

TABLES

Tsble I Distribution Coefficients from Mixed Al(N03)3-HN03 Solvents

FIGURES

Fig. 1. Equilibrium Sorption of Plutonium from Nitrate Solutions 11

Fig. 2. Distribution Coefficients from Mixed Nitric Acid- AluminumNitrate Solvents 13

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INTRODUCTION

(l-3) Previous papers have dealt with the sorption of plutonium

on Dowex lx4 anion-exchange resin from ~ nitric acid. While this is

probably the best universal solvent for plutonium processing by emion

exchange, previous processing operations frequently introduce the

sluminum ion into nitric acid feed solutions. The aluminum ion is not

sorbed by snion-exchange resin from nitric acid,(1$4) but studies by (4) Ryan snd Wheelwright indicate that considerable quantities of

nitrate salts in the aqueous phase may drastically affect both the

kinetics and equilibria of the system. It was the purpose of this

work to investigate the equilibrium sorption of plutonium on

Dowex 1.x4from mixed nitric acid-aluminum nitrate solvents.

Plutonium(IV) is sorbed as the hexanitratoplutonate(IV) ion.(5)

(1)

where the bar indicates”a species associated with the resin phase. An

equilibrium quotient for this reaction could be written,

4 . .

. .

= !5u!?z. (2) ‘“’w FC2

The brackets indicate molar concentrations in the aqueous phase and

millimoles per “oven-dry” grsm of resin in the resin phase. In these high ionic strength solvents there is considerable resin invasion by

both acid and salt. However, with the shove definition the non- — exchange nitrate within the resin phase ties not contribute to I!70-. [13 The trace-level loading distribution coefficient > % is the amount of plutonium per grsm of oven-dry resin dividedby the amount of plutonium

per liter of solution; C is the equivalent capacity of the resin for

anions; and F is the fraction of plutonium present as Pu(l’?03)~in the aqueous phase.

, [H2PU(N03)6]+[~u(N03) ;]+[Pu(N03~]+[mu(No3) 5]+[pu(No3)j]+...+[pJ`] -= F . PU(N03): (3)

By defining,

Kn = m’n=l’2’ (4) ,-

kn = &ai31’‘=”2’““”’“ (5)

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. . and assudng that the contribution of HPu(IVO) ] is insignificant in [ 35 comparison with the other terms in the numerator of equation (3), we

obtain,

-1- HO+2 [1 + +.f-[1 + ‘1K2 fT-H20 2 [JH20 (6)

‘1’&+ *+””” +k6k,**:%[N0iJ’ “

= $H+ , (7) NO- 2 ah-3

where $H aid $N are functions ‘nw ‘f [H30’I /[’2”1 ‘d [“<1 respectively. ~is line= dependence of 1$ on the aqueous concentra-

tions requties not only the assumption shove, but also that Kn and lcn

are not functions O* these variables and that &.. is a constant. ru Certainly these s.kevery rough approximations, but any more detailed

treatment would intioduce alabyrinth of complications.

EXPERIMENTAL

Reagents: The resin used in these experiments was fl?oma single

batch of Dowex IX4 (loo - 200 mesh)NO-. It was converted from the 3 chloride to the nitrate form and its equivalent capacity for anions,

6 3.82 milliequitients per gram of oven-dry resin, was determined as (1) described before. The impurities in the plutonium were less than

0.01 weight percent. All other reagents were analytical grade or better.

Batch Equilibration Experiments: A measured weight of resin was equilibrated with solvent containing no plutonium end filtered “&y” with vacuum. The resin was added to a measured volume of aluminum nitrate-nitric acid solution containing a known amount of these com- pounds and plutonium nitrate. Amounts of resin end plutonium were controlled so that the resin was loaded to less than 1 percent of its capacity, yet the concentration change in the aqueous phase was at least 90 percent. The mixture was shaken for h8 hours with a wrist- action shaker at room temperature, and then the concentration of plutonium in the aqueous phase was again measured by a-counting with a correction for americium present, determined by a-counting.

If the rates of sorption from these mixed solvent systems are no slower than an order of magnitude of that measured for 71Jnitric acid,(2) then equilibriun”should have been reached in this time with this very low degree of resin loading. Preliminary kinetic experiments with these solvents indicate that this is a valid assumption. Several measurements were made tith one system by removing a measured volume of the queous phase and adding a measured volume of either aluminum nitrate or nitric acid solution between equilibrations. ‘These . . sequential.experiments allowed equilibrium to be approached from above

end below} and the results were always consistent with equilibrium

behavior.

RESULTS AND CONCLUSIONS

in vexious aluminum The results of 22 batch measurements of % nitrate-nitric acid solvents are given in the COl~ labeled (~)eao

of Tshle I. Also shown are values of 1$ calcdated by two Mfferent

methods.

In the absence of anything better, one might estimate $ for a

mixed aluminum nitrate-nitric acid solvent from a weighted average of

the Mstribution coefficients for salt solutions and for acid sol~tions

of appropriate concentrations.

The weighted aver~e values, (KJ av,, ~~erecalculated from

%IN03W-IN0 + 3cAI(No ) (%‘) salt (q)avc = ) (8) CHN02+ 3cAl(NOa)a J 2.J

are total stoichiometric molar concentrations a=eCHNO ‘dcAl(N03)3 of nitric ~cid and aluminum nitrate, and (l$)~o ad (~) salt sre the 3 distribution coefficients of plutonium from nitric acid solutions

and from calcium nitrate solutions with molar concentrations one and

~half that 0fcAI(N03)3. These distribution coefficient data for

nitric acid and calcium nitrate are given in Reference 5. The author

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.4 A

rl low-l - 001-I+OJ mmmstn s ● Inlnlnuiln. . . I-3 - I lIilJilA IALA

9 .

. . kindly supplied the original nunbers so that precise calculations

couldbe made.

The values ofl$in Tsblelu.nder (l$)c~ce were calculated from

equation (7). The values of (I$)Salt mom Reference 5 were US@dto

calculate & according to equation (7), assuming that $ makes an

insignificant contribution since C~O = 0.5 in the =q?erimentd “3 solutions. A plot of $N versus NO; is shown in Fig. 1. Then $H [J

was determinedly calculating, at constant NO- the difference [1 3’ (5) for betieen ON Wd l/(~)~o [NO~~, sgain using Ryanls data

(K.Jmo=” ‘dues ‘or [a ‘d [H,o+l/[%ol ‘n ‘itiic acid ‘“’utions J were calculated from the degree of dissociation as determined by the

proton megnetic resonance method of Axtmann et al.(6}7) ~~the

specific gravity of these solutions.‘8) A plot Of $H V@Z’SUS

Values of [1130’j/[H~~ ‘d [NOJ [H3”’1/[%d ‘s ‘hem ‘n ‘ige ‘0 in the mixed aluminum nitrate-nitric acid solutions given in Tsble I

were s3so calculatedly Axtmenn’s method end the specific gravity of

these mixed soluti.ons~g) men $, and $N were extracted from Fig. 1

evaluated according to equation (7). ‘d ‘%)calc.

The avezsge ehsolute percent detiatiOn of (~)cdc. ~d (K#e=o

in Teble I is 24. However, since Axtmannls calculations were only

experimentally verifiable to plus or minus 10 percent and $N changes . . V=Y rapidly with l_NO-jSsuch a detiation is to be eqectede 3 Nevertheless, these calculations are more reliable than the simple

. weighted average, especially for solutions containing l~ge amounts

10 [H30+]/ [H20] . 0,06 0.08 0.10 0.12 0,14 0.16

. .

10-’

10-6

1 I I 1 I I 3 4 5 6 7 8 [W;] (M)

1. Equilibrium Sorption of Plutonium from Nitrate Solutions.

11 of both salt and acid. The variation of (~) .... with total nitrate v U-L. for various polarities of aluminum nitrate is shown in Fig. 2.

The curves for salt and acid begin to deviate at NO; = 3.9, [1 which corresponds to CaO = 5. Hence$ we might speculate that below 3 a total nitrate molarity of 5 the difference between Ryan’s acid and

salt distribution coefficient curves in Fig. 2 (curves A and H) is due,

for the most pat to incomplete dissociation of the acid. The acid

solution simply provides less nitrate ion for the formation of the

sorbable hexanitrato complex of plutonium.

In nitric acid solutions greater than 5 molar, the difference

between curves A andH is augmentedby the protonation of the sorbable

complex. The formation of HPu(N03)~ and H2Pu(N03)6 reduces the f!raction

of the plutonium that is in the form of sorbs%le complex. The

sbsence of this association at lower concentrations indicates that

~U(N03)~ is at least as strong, if not a stronger acid than nitric

acid.

The trace-level loading distribution coefficient for the sorption

of plutonium f!roma given solvent is a constant only for very low

degrees of resin loading, say less them 1 percent. If one assumes

mat ~u is a constant for sny given solvent for all levels of sorption> (1) then from equation (2) it canbe shown that the equilibrium

distribution coefficient for sny degree of sorption> ~~ is givenby

(9) . ‘=%(’>[?]2)● .. 12 I —- 1 I 1 I - . 50,000 [AI(NOJ3] H H - /H- “o / ,0 A 0.00 / ./ B 0.38 /- //’ . . / c 0.67 / // D 0.91 / // E 1.11 / /) / /’ F 1.26 / #I G 1.50 /“ g /’ 005 M ACID It 20,000 H Lmnw W3$/ } f’

/ / / / Io,ooc / / K; / / / / 5,00t — / / / / / / / / 2,00

I I,00 5 6 7 8 9 10 TOTAL NITRATE (MOLEWLITER)

Fig. 2. Distribution Coefficients from Mixed Nitric Acid-Altinm Nitrate Solvents.

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. . (1) It has been demonstrated that this relationship holds for the entire

range of resin loading for sorption from ‘@ nitric acid. To Verify

equation (9) for O < 2 Pu(NO )~]/C < 1 for a sufficient nunber of 1 3 aluminum nitrate-nitric acid solvents to establish its validity for

all such mixt&es would be a formidable research program. A few

equilibrations at higher loadings have not contradicted equation (9).

WYitten in terms of the aqueous molar concentration of plutonium(IV),

equation (9) takes the form(1) CPU$

%=%{’+”- -} J (lo)

c u= 6 (11) ‘CPU% “

By using Axtmann tS method for c~cii.sting [NOJ and [H30+]/[H20],

the curves in Fig. 1 for estimating $N and $H, end then equations

(7)j (10), ad (11)$ one can predict the equilibrium sorption behavior

of plutonium (IV) on Dowex Idk from any aluminum nitrate-nitric acid

solution.

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Im?lmENcEs

1. D. B. James, “The Processing of Plutonium by Ion Exchange - 1. The Concentration Dependence of Distribution Coefficients on Dowex EL from 7M Nitric Acid,” J. Inorg. Nucl. Chem. ~, ?ll (1963).

2. D. B. James and R. S. Cooper, “Processing of Plutoniumby Ion Exchange - 111. The Calculation of Fixed-Bed Column Performance,” USAEC Publ. I&2838 (lg64)o

3. R. S. Cooper andD. B. James, “Processing of Plutonium by Ion Exchange - IV. The Separation of Plutonim frcznWeakly Sorbed ~purities, ” USAEC Publ. LA-3040 (lg64).

4. J. L. Ryan and E. J. Wheelwright, “The Recovery, Purification, and Concentration of Plutonium by Anion Exchange in Nitric Acid,” USAEC Publ. HW-55893 (del) (1959).

5. J. L. Ryan, “Species Involved in the Anion-Exchange Absorption of Quadrivalent Actinide Nitrates,” J. Phys. Chem. 6A, 1375 (1960).

6. R. C. Axtmannand B. B. Murray, “Dissociation of Nitric Acid in Aluminum Nitrate Solutions,” USAEC. Publ. DP-297 (1958).

7. R. C. AxtmannY W. E. Shuler, andB. B. Murray, “ProtonResonance Shifts in Nitric Acid Solutions of Aluminum Nitrate,” J. Phys. Chem. ~, 57 (1.960).

& “InternationalCritical Tables,” Vol. III, E. W. Washburn, Ed. McGraw-Hill, New York (1928), p. 58.

9* E. L. Christensen} C. W. Kelley, A. J. Beaumont, and J. R. Humphrey, “Distribution of Plutonium and Selected Impuri@ Elements Between Nitrate Solutions and ‘l?ri-n-butylPhosphate,” from “Extractive and Physical Metallurgy of Plutonium and Its Alloys,” W. D. Wilkinson, Ed., Interscience, New York (1960), p. 75.

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