steam distillation of mixtures of methyl benzoate-linalool and ethyl benzoate-1-menthol from a n aqueous solution of sodium benzoate, for the Structures of methyl benzoate and ethyl benzoate are obviously more closely similar to that of sodium benzoate than are the structures of linalool and 1-menthol. T h e mechanism of the biased steam distillation, however, has not been completely established. Further studies in this field are in progress.
Acknowledgment
T h e author thanks M. D. Sutherland and the late Sinichiro Fujise for giving much advice in the latter part of this work. literature Cited K.-H., Ind. Research) Tatzcan ' (l)
9 35 (1958). RECEIVED for review May 22. 1962 RESUBMITTED August 13, 1964 ACCEPTED September 17, 1964
PRECIPITATION OF PLUTONIUM TRIFLUORIDE G L E N N A.
B U R N E Y A N D
F R A N K W . TOBER
Savannah River Laboratorv, E. I . du Pont de S r m o u r s t 3 Co.. Aiken, S. C.
A process was developed to precipitate plutonium trifluoride from plutonium nitrate solution.
Large crystals are precipitated b y controlling the ratio of nitric acid to hydrofluoric acid to maintain the required plutonium trifluoride solubility during addition of the plutonium nitrate solution to the precipitant. The solubility of the trifluoride i s decreased prior to filtration b y lowering the ratio of nitric acid to hydrofluoric acid. The soluLosses of plutonium to the filtrate are minimized b y adding bility product of PuF3 i s 2.4 + 0.4 X ascorbic acid to the feed immediately before precipitation to reduce Pu(IV) and adding sulfamic acid to decrease the rate of oxidation of Pu(ll1). The plutonium trifluoride is easily filtered and dried to the anhydrous salt and is pure enough for subsequent reduction to plutonium metal.
THE o,bjFcti,ve of this work was to develop a plant process for
trifluoride florved continuously from the first vessel into the second-stage vessel, where more fluoride was added. .4lthough the one-stage process was found to perform adequately for the laboratory studies. the two-stage process was selecteti for plant operation because it was quicker and offered more favorable nuclear safety.
precipitation of plutonium trifluoride. an intermediate in the production of plutonium metal ( 2 ) . Although plutonium trifluoride had been prepared in the laboratory, no study had defined conditions for effectively maintaining plutonium in the trivalent state in a nitrate system or for precipitating the trifluoride under conditions which would yield the large, easily filtered crystals required for a production process. In the development of the process, the effects of the important variables were studied, including reductants for Pu(IV), composition of the precipitant. the ratio of nitric acid to hydrofluoric acid during formation and digestion of the precipitate. the degree of agitation, and the method of combining the plutonium solution with the precipitant. Also, the separation of plutonium from cationic impurities present in the feed solution was measured. This precipitation process has been used successfully in a production plant for several years. T h e plant process and the conversion of plutonium trifluoride to plutonium metal have been described by Orth (2).
Most of the laboratory tests were made with 5-gram batches of plutonium. T h e precipitator was a 500-ml. polyethylene vessel 5.5 cm. in diameter. T h e contents were stirred by a paddle having two sets of four blades that measured 4.8 cm. from tip to tip. T t e paddle was rotated at approximately 600 r.p.m. The solutions were added to the precipitator through capillary polyethylene tubing. The precipitate \cas filtered through a disk of fritted Teflon (Du Pont) with a diameter of 1.8 cm. and a mean pore size of less than 10 microns. The fritted material was not uniform; to compensate for the difference in porosity of different filters, the filtration time ivas measured relative to the time required to transfer a n equal volume of \+ater through the filter under identical conditions. T h e plutonium(II1) nitrate feed solution was prepared by cation exchange. T h e hydrofluoric acid and nitric acid were reagent grade.
Experimental
Discussion
Two methods were studied for precipitating plutonium trifluoride. T h e first utilized a single-stage precipitator, the second utilized a two-stage unit. I n the one-stage precipitation, all of the plutonium nitrate feed solution and some of the fluoride precipitant were added simultaneously and at controlled rates to a nitric acid-hydrofluoric acid diluent solution in the precipitator. After feed addition was complete, more hydrofluoric acid was added to depress the solubility of plutonium trifluoride. In the two-stage precipitation. the plutonium feed and a fraction of the fluoride precipitant lvere added simultaneously a n d a t controlled rates to a nitric acid-hydrofluoric acid diluent solution in the first-stage vessel. A slurry of plutonium
A number of variables influence the physical properties and solubility of plutonium trifluoride : the plutonium valence, nitric acid and hydrofluoric acid concentrations in the precipitation media. the degree of agitation of the precipitate. and the manner in which the reactants are combined. The effects of these variables were studied in the laboratory. and conditions were defined for a plant precipitation process. Feed Composition and Valence Adjustment. T h e precipitation of essentially pure plutonium trifluoride is required for a practical plant process. The presence of plutonium tetrafluoride leads to high losses to the filtrate and to a precipitate with undesirable physical properties. In contrast to the tetrafluoride, the trifluoride is crystalline and contains no
28
l&EC PROCESS DESIGN A N D DEVELOPMENT
14
I
Table I. Effect of RLeducing Agents on Precipitation of PuF~ Concn: of Bulk Concn. of Relative Pu zn Density .4ge of Reductanf. Filtration Filtrate, of PuF3, Feed, Hr. .M Time Mg.,lL. G. Pu/CG.
I
Aniinoguanidine Sulfate 24
0 0 0 0 3 0 3
71 142 71 141
10 14
2.9
4.8
1 0 1 0
Ascorbic Acide 144 0.05 144 0.10 500h 0.10 1.4 7 1.4 Sulfamir Ascor6ic acid added to f8?ed30 minutes 6eforeprecipitation. acid in f e e d zeus replenishfd because most of original was converted to sulfate aftrr 500-hours storage.
water of crystallization. so that it is easily dried to the anhydrous salt. The solubility of the trifluoride is lower than that of the tetrafluoride: approximately 10 mg. of plutonium per liter in the precipitation medium us. more than 100 mg. per liter. The feed solution is produced from a dilute solution of plutonium( 111) nitrate by a cation exchange process. Typical composition of the feed is 30 to 70 grams of plutonium per liter. 4 to 5.M nitric acid. 0.2.M sulfamic acid, and 0.3,W hydroxylamine nitrate. Sulfaniic acid reduces the rate of oxidation of Pu(1II) in this solution to 4 to 6 7 , per day by reacting rapidly \vith nitrous acid. .4ddition of 0 . 3 M aminoguanidine sulfate to the cation exchange eluate immediately after elution decreases the rate of oxidation of Pu(II1) to l to 27, per day, presumably by the same mechanism. but aminoguanidine \vas not chosen for use because its presence decreases the bulk density of the trifluoride. increases the filtration time, and does not completely eliminate losses caused by the more soluble tetrafluoride. Optimum results for the precipitation are attained by adding ascorbic acid to the feed a few minutes before precipitation; ascorbic acid reduces any Pu(1V) rapidly and completely to Pu(II1). Addition of ascorbic acid to the diluent solution !vas not effective. The results of the tests of aminoguanidine and ascorbic acid for maintaining Pu(II1) in the feed are given in Table I . Conditions for Preciipitation. Laboratory studies demonstrated that the precipitation of plutonium trifluoride responds to clasric conditions of icrystal growth. The general chemical equation illustrates the factors to be controlled better than the ionic equarion ; hydrofluoric acid is only slightly ionized, so that ionic fluoride is a function of total acidity. PU(rO3)3
K q D= 2.4 Ka
=
-+ 3HF eP u F + ~ 3”03
=t= 0.4
X
(solubility of PuF3)
7.2 X 1 O F (ionization of HF)
Optimum conditions for preparation of large crystals are attained b!- adding the plutonium(II1) nitrate and the hydrofluoric acid simultaneously and a t controlled rates to an initial volume of nitric acid-h;stal growth is enhanced under conditions of moderate solubility-Le., high HNO3/HF ratios-the concentrations of the two acids are carefully controlled throughout the addition of the plutonium nitrate feed. Studies at a
HN03/HF Ratio in Supernate, rnol/rnol Figure 1.
s
Solubility of PuFBin supernate
20 -I60
-I20
-
s g b
E
2
Time after Initial Addition, minutes Figure 2.
Effect of time on solubility of PuFI
single feed rate and with a single design of equipment indicated that a HNO,,/HF ratio of a t least 4 was necessary to produce plutonium trifluoride with a high bulk density (approximately 2 grams of Pu per cc.) and which filtered rapidly. With a HNOSIHF ratio less than 3, the bulk density decreased (approximately 1.3 grams of Pu per cc.) and the filtration time often increased. The instantaneous and “equilibrium” solubilities as a function of the “03,HF ratio were determined (Figure 1). Within the experimental limits, there is a relatively sharp increase in the instantaneous solubility at a ratio of approximately 6. (The experimental limits were the several seconds required for separation of the supernatant solution during which time the precipitation continued.) The “equilibrium” values are the solubility values measured 18 hours after precipitation. In Figure 2. the solubility of plutonium trifluoride is shown as a function of time at H N O J H F ratios of approximately 5 and 30. Although the solubilities of plutonium trifluoride in the VOL. 4
NO. 1
JANUARY
1965
29
Table II.
Effect of HN03,"F
Ratio on Bulk Density of PuF3
(Scale: 5 grams of Pu, 0.05M ascorbic acid a d d e d to f e e d )
H,VOa I H F Ratio Pu addition step Digestion on 5 . 5 to 1 . 9 2.4 2.1 2.1 8 to 3 . 1 3.1 4 2.7 10 to 4 . 5 4.1 1 3 to 4 . 2 4.2
Relative Filtration Time 3.4 3.5 3.2 3.6 3.5 3.6 IIGLuIIvc
Solubility tv Bulk of Pu 'n in Density of PuF3, Filtrate, M g . ! L . G. P u ~ C C . 10 1.3 8 1.3 10 1.9 10 2.0 16 2.2 21 2.3
",
two precipitations differ markedly, the shapes of the curves are similar. In both measurements the solubility dccreased rapidly for approximately 20 minutes and then approached "equilibrium" at a much slower rate. Low losses to the filtrate are attained at H N 0 3 I H F ratios of less than 3 ; therefore after the addition of plutonium nitrate is complete. more hydrofluoric acid is added to decrease the ratio. At a HT03; H F ratio of 3> solubility losses are only 10 to 15 mg. of plutonium per liter. I t Xvould be predicted that a further drcrease in the " 0 3 1 H F ratio would lower the solubility of plutonium trifluoride, but this was not demonstrated experimentally. Failure to attain the expected decrease in solubility is not due to colloidal micelles. as indicated by ultracentrifugation. A possible explanation for the high solubility is the presence of a low concentration of quadrivalent plutonium. .4 high bulk density (>2) of plutonium trifluoride is desirable for good performance of the subsequent reduction step ' \cith calcium metal. A s illustrated in Table 11. higher " 0 3 H F ratios produce precipitates of higher density. Ratios during the addition of precipitant: rather than during the digestion step, control the density of the precipitate. Factors Affecting Solubility of Plutonium Trifluoride. T h e solubility product of plutonium trifluoride: 2.4 ?E 0.4 X 10-16: is calculated from data obtained from a number of measurements at appreciably different nitric and hydrofluoric acid concentrations and is consistent with a value selected by other workers ( 7 ) . The solutions all contained sulfamic acid, hydroxylamine. and ascorbic acid. These solubility data were obtained for equilibration times of 4 to 12 hours; equilibrium values cannot be measured, because Pu(II1) is not stable in the nitric acid system. HoLvever. conditions Ivere adjusted to decrease the rate of oxidation to a minimum. The HN03,'HF ratio that determines the ionic fluoride concentration is the most important factor affecting the solubility of plutonium trifluoride in the precipitation medium. However. it is of considerable importance to maintain plutonium as Pu(II1) because Pu(1V) forms the more soluble tetra-
Age of Feed, Hr. 48 71 142 1 47 70 143 71 120 I44
30
Table 111. Ascorbic Acid Concn., M 0 0 0 0 0
0 0 0.05 0.05 0.05
fluoride. Because Pu(1V) is the most stable state in nitric acid solution, reductants and stabilizing agents must be present in the precipitation medium to maintain Pu(II1). T h e effective stabilizers are compounds such as sulfamic acid or hydrazine, which react rapidly bvith nitrous acid and thus reduce the rate of oxidation. Although ascorbic acid is a very effective reductant, it is oxidized after several days to a n insoluble product that carries some plutonium; therefore, it is added a relatively short time before precipitation. The radiolysis products produced by alpha decay of plutonium increase the rate of oxidation. This effect is not important so long as the slurry is mixed; however. if the plutonium trifluoride accumulates on the bottom of the precipitator, the stabilizing agents are depleted rapidly enough to permit oxidation with a resultant increase in soluble plutonium. Fluoride Precipitants. Although the characteristics of the plutonium trifluoride precipitate are principally dependent on the fluoride ion concentration in the precipitation medium, the concentration of hydrofluoric acid precipitant can be varied considerably. Good precipitates were obtained with 2.7 to 28.21 H F \
