Symbosium on Production
Production Technology of Neptunium-237 and Plutonium-238
1
Technology of .Yefitunium-237
and
Plzrtonzum-23S, Dzvzsion of .Yurlear Chemictry and Technology, 746th Meeting nf
the Amerzcan Chemical Soczetj, Denver, Colo.,
.Januarjl 7964. Chairman, Morton Smutz, Department of Chemical Engineering, I o w a State University
PRODUCTION OF NEPTUNIUM DIOXIDE J . A.
PORTER
Saaannah Rioer Laboratory, E. I. du I'ont de .Ternours €3 Co., Aiken,
S.C .
A process for the production of neptunium dioxide was developed and demonstrated successfully in pilotand plant-scale applications. Two routes to the oxide were investigated-precipitation of neptunium( IV) oxalate and precipitation of neptunium( IV) peroxide, with subsequent calcination of either compound to neptunium dioxide. The oxalate process was selected for plant operation because it i s less sensitive to operating variables and has greater potential for purifying the neptunium from fission products. N e p l tunium(lV) oxalate precipitates quantitatively from solutions containing 1 to 4th nitric acid and 5 to 50 grams of neptunium per liter. Reprocessing of the wastes i s usually not required, because losses to the filtrate are normally less than 10 mg. per liter of neptunium. The process yields a neptunium(1V) oxalate precipitate that i s dense, granular, and easily filtered and dried. Calcination of the oxalate a t 500" to 550" C. produces neptunium dioxide satisfactory for fabrication of targets for irradiation.
LYas required for the production of neptunium-237 dioxide suitable for fabrication into target elements for irradiation to produce plutonium-238. B) -product neptunium was to be isolated from various process solutions and unconverted neptunium was to be recovered from irradiated target elements T h e final step in the separation processes was concentration of neptunium by anion exchange procedures to yield a product solution containing 1 to 2hf " 0 3 and 5 to 50 grams per liter of neptunium. T h e most logical method for preparing S p O Zfrom such a solution was the precipitation of either neptunium(1V) oxalate or neptunium(1V) peroxide, followed by calcination. Although both neptunium( IV) oxalate and neptunium(1V) peroxide had been prepared previoudy ( 4 ) , the data required for a plant-scale process were not available ; studies were therefore undertaken to evaluate both precipitations as plant processes. T h e studies indicated that both the neptunium peroxide and the neptunium oxalate precipitations could be successfully applied on a plant scale. T h e oxalate precipitation was selected for plant operation because of its superior performance.
A
PROCESS
Process Description
T h e oxalate precipitation process which was selected for the plant production of neptunium dioxide utilizes nitric acid feed solutions containing neptunium in the IV-, V-, or VIvalence states. Feed solutions prepared by the anion exchange procedure ( 7) contain predominantly N p ( I V ) , but oxidation to higher valence states can occur upon standing. T h e rate of oxidation is slower if hydrazine or another suitable inhibitor is added to the Np(1V) solution a t the time of preparation.
In the oxalate process, the neptunium is reduced to the IV state with ascorbic acid prior to precipitation. If hydrazine inhibitor was not originally added to the feed solution, it is added prior to valence adjustment to protect the ascorbic acid from chemical degradation. Neptunium(1V) oxalate hexahydrate is precipitated by the controlled addition of oxalic acid solution to the adjusted feed solution. T h e valence adjustment, precipitation, and a subsequent digestion step are performed at approximately 50" C. to ensure rapid adjustment of valence and the formation of a dense, granular precipitate. T h e slurry of neptunium(I\') oxalate is filtered a t room temperature, washed, and air-dried. T h e filtrate and wash waste normally contain less than 10 mg. of Np per liter, and recovery operations are not required. Neptunium dioxide is produced from the air-dried neptunium(1V) oxalate by calcining to a final temperature of 500" to 550" C. in an air or nitrogen atmosphere. Experimental
Preparation of Reagents. Stock solutions of purified N P * ~ 'for the preliminary precipitation studies were prepared by anion exchange. T h e anionic nitrate complex of N p ( I V ) was absorbed from 8M " 0 3 on a column of Dowex 1-X4 resin, and the resin was then washed with 10 bed volumes of 8M "03. Solutions of N p ( I V ) were eluted from the that contained 0.05M hydrazine column with 0.35M " 0 3 as an oxidation inhibitor. Solutions of Np(V) were prepared by eluting the column with 0.35M " 0 3 , and then healing the eluate to 5.5' C. for 30 minutrs. T h e valence state of the neptunium \vas verified by a spectrophotometer.
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T h e more detailed study of the neptuniurn(1V) oxalate precipitation was performed bvith process solutions having variable neptunium and nitric acid concentrations and containing -108 d (min.) (ml.) of Pu?38 and 10: to 106 y c,'(min.) (ml.) of fission products. Hydrazine >vas added to inhibit the oxidation of the Np(I1.). and usually no more than a few per cent of oxidized neptunium were detected in freshly prepared solutions. .4fter the solution had stood for several weeks, however, significantly larger concentrations of oxidized neptunium were present in the process qolutions. .4 307, solution of hydrogen peroxide was used in the peroxide precipitation studies. -411 other solutions required in these studies tvere prepared from reagent grade chemicals. Solubility Studies, The solubility of neptunium(I\-) peroxide in hydrogen peroxide--nitric acid solutions and the ) in oxalic acid-nitric acid solubility of n e p t ~ n i u m ( I \ ~oxalate solutions were determined by similar techniques. The pure compounds were equilibrated at room temperature with the various reagent mixtures. and the solubilities were determined by analyses of the saturated solutions. T h e solubility values were verified by an alternative method in ivhich the neptunium compounds were precipitated in situ. Septunium analyses were perfarmed by alpha counting. T h e oxalic acid or hydrogen peroxide was titrated \vith standard potassium permanganate solution, and the nitric acid was titrated with standard sodium hydroxide solution. Precipitation Studies. Most of the studies of the process variables in the neptunium(1V) oxalate and neptunium( IV) peroxide precipitations were performed in ordinary laboratory glassware scaled to accommodate about 1 gram of neptunium. T h e precipitates were filtered by vacuum on medium porosity fritted glass disks. Further development of the oxaIate precipitation was performed in a pilot plant facility that could process as much as 100 grams of neptunium per batch. T h e pilot plant equipment was constructed of stainless steel. T h e filtration characteristics of the precipitates were compared by measuring "filtration factors." T h e filtration factor is defined as the ratio of the time required to filter a given slurry through a given filter to the time required to pass an equal volume of water through the same filter under the same conditions.
T h e following procedure is recommended for the precipitation of neptunium(1V) peroxide.
Feed Preparation. Adjust the nitric acid concentration to 3 to 4 M . No valence adjustment is required. Precipitation. Maintain adequate mixing. Add a n equal volume of 30Y0 hydrogen peroxide at a controlled rate over a period of 90 minutes. Maintain the temperature of the slurry at 18" to 25' C. Digestion. Cool the slurry to 8' C. and hold at this temperature for 30 minutes. Filtration. Filter as rapidly as possible through a medium porosity- frit. Washing. Wash the precipitate with about three cake volumes of a 1.5M nitric acid-157, hydrogen peroxide solution. Drying. Dry as required for subsequent processing. T h e precipitation of neptunium(1V) peroxide by this procedure yields a slurry that exhibits a filtration factor of about 6. This denotes moderately good filtration characteristics, as plant processes are operable with filtration factors as high as 10. T h e addition of 0.05M sulfate to the feed solution will reduce the filtration time by about 40% without affecting product losses, but the presence of sulfate in the product may be undesirable for some applications. T h e normal loss of neptunium to the filtrate and wash solutions is approximately 30 mg. per liter, only slightly higher than would be expected from the solubility data. This loss is usually considered to be negligible; however, recovery of the product losses can be accomplished, if required, by anion exchange processing after thermal decomposition of the peroxide. T h e process variables should be controlled near the recommended conditions for the most satisfactory operation. Increased hydrogen peroxide concentrations in the slurry will
Figure 1. Solubility of neptunium(1V) peroxide as a function of hydrogen peroxide concentration
Precipitation of Neptunium(1V) Peroxide
Septunium(1V) peroxide was precipitated readily from solutions containing 5 to 50 grams per liter of Np(IV), (V), or (VI) in 3 to 4 M " 0 3 by the addition of 3oY0 hydrogen peroxide. T h e hydrogen peroxide rapidly reduced the higher valence states to the I V state, which was precipitated. T h e precipitate was grey-green in color. and the precipitation did not progress through the highly colored, soluble peroxy complexes that are characteristic in the formation of plutonium(1V) peroxide ( 2 ) . However, two crystalline forms of neptunium (IV) peroxide were prepared that were quite similar to the two forms of plutonium( I\') peroxide formed under similar conditions (,5). T h e crystalline form? derived from solutions of low acidity (-1 M ) . was face-centered cubic, and the precipitate was difficult to filter; the structure formed in solutions of higher acidity was hexagonal, and the precipitate was more easily filtered. T h e solubility of neptunium(1V) peroxide a t 23' C. varies with the concentration of nitric acid and hydrogen peroxide, as shown in Figures 1 and 2. A discussion and interpretation of these solubility data have been published ( 3 ) . 290
l & E C PROCESS DESIGN A N D DEVELOPMENT
1.0
10.0
HN03. M
Figure 2. Solubility of neptunium(lV) peroxide as a function of nitric acid concentration
o1ubilit.y losses. but the recommended value provides an acceptable compromise betlvcen losses and waste volumes. Nitric acid concentrations in the feed in the range of 3 to 4M arc rrcommended because the slurries produced have acid conceiitrations in the 1 . 5 to 2-11 range Lvhere minimum solubility l o w s result. Also. and of particular importance. reduction of the oxidized neptunium to the I\' state by hydrogcn peroxide is do\\- or incomplete at lo\ver acidities. 'The lo\\. temperature digestion step is performed to lo\ver the solubility of the nrptuniuni(1V) peroxide; filtrate losses of neptunium average about 40 mg. per liter \\.hen the digestion is performed at room temperature. 'The purification of neptunium by peroxide precipitation \vas not studied extensively. Separation from a number of cationic impurities. Fuch as iron. \vould probably be excellent. Separation of neptunium from plutonium. zirconium. niobium, and protactinium \vas poor. Plutonium \vas carried completely Lvith the precipitate, .%bout 907, of the Pa233daughter product of XpZ3:and about 307, of the Zrg5-NbgS\vere carried. Precipitation of Neptuniurn(1V) Oxalate
Septunium(I\.) oxalate \vas precipitated readily from solutions containing 5 to 50 grams per liter of Np(1V). (V), or (\-I) in 1 to 4.1.1 HSO3 by the addition of oxalic acid after valence ad-justment ivith ascorbic acid. T h e precipitate was bright green. dense. and granular. and showed excellent filtration characteristics. 'The solubility of neptunium(I\') oxalate at 23' C . varies as a function of the concentration of nitric acid and oxalic acid. as shown in Figure 3. At oxalic acid concentrations near 0.1.2.1, the solubility varies only betxveen 6 and 10 mg. per liter of neptunium as the nitric acid concentration is varied between 1 and 4 M . indicating that satisfactory precipitation performance can be obtained over a rather \vide range of conditions. T h e folloning procedure is recommended for the precipitation of neptunium(1V) oxalate:
Feed Preparation. Adjust the nitric acid concentration within the range of 1 to 4 M . Adjust to 0.05M in hydrazine. if not already present. Valence Adjustment and Precipitation. Heat the feed solution to SO =k 5' C. and maintain adequate mixing. Add the amount of 1 M ascorbic acid stoichiometrically required to reduce the oxidized neptunium species to the I V state and provide a 0 . 0 3 M excess. Immediately afterward, add the stoichiometric amount of 1M oxalic acid plus a 0.1M excess at a controlled rate over a 30- to 45-minute period. Digestion. Agitate the slurry for 30 minutes at 50 i 5 C. Filtration. Cool the slurry to room temperature, and filter i t through a medium porosity frit. Washing. Wash the precipitate with about three cake volumes of a 1 M nitric acid-0.1 .M oxalic acid solution. Drying. Dry as required for subsequent processing. T h e precipitation of neptunium( I\.) oxalate by this procedure yields a slurry that has a filtration factor of about 2.0 to 2.5, denoting excellent filtration characteristics. T h e normal loss of neptunium to the filtrate and wash solutions is approximately 6 mg. per liter. in good agreement Lvith the losses expected from the solubility data. This small quantity of neptunium normally does not warrant recovery. When recovery is required, however. it is easily accomplished by anion exchange processing. T h e oxalic acid that is present does not interfere with the recovery by anion exchange. 'The oxalate precipitation process is rather insensitive to most process variables. 'The process step that requires most rareful control is the valence ad,justment. Of a number of potential reductants tested for neptunium valence adjustment.
801 bnccnnotlon
1.0
0.1 01 0 d c Acrd, Y
Figure 3. The effect of oxalic acid concentration on the solubility of neptuniurn(1V) oxalate
100
3 86L
~
I 101 0
I
10
I
a
I
I I
30
40
50
bo
m
Rmction T i m e (t), minutes
Figure 4. The reduction of N p ( V ) to Np(lV) by 0.03M ascorbic acid in nitric acid-0.05M hydrazine
ascorbic acid (or isoascorbic acid) was the most satisfactory reductant that did not contribute to contamination of the product. T h e rate at which ascorbic acid reduces oxidized neptunium species to the IV state is relatively slow, and is strongly influenced by nitric acid concentration and temperature. T h e effect of nitric acid concentration on the reduction of Np(V) to Np(IV) by ascorbic acid at room temperature is shown in Figure 4. T h e rate of reduction is significantly increased at 50' C. Thus? the process should be operated at 50' C. to ensure complete reduction of neptunium at all nitric acid concentrations in the 1 to 4 M range. A 0.03M excess concentration of ascorbic acid above the stoichiometric requirement is sufficient to accomplish the reduction. Good performance is obtained and processing time is minimized by conducting the reduction and precipitation steps simultaneously. If it is desired to operate the process at room temperature, the feed solution must be adjusted to about 4 M HNO3 to obtain complete valence adjustment. Process performance at room temperature is satisfactory. but the resulting slurries require about twice as long to filter as those prepared a t 50' C. T h e oxalate precipitation is expected to provide significant separation of neptunium from certain cationic impurj ties, but this aspect of the process was studied only briefly. Less than lYc of the Rulo3and only 2 to 37, of the Zr9:-NbgS present in the feed solution were carried by the precipitate. while essenVOL. 3
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Table 1.
~
~
Neptunium(1V) Oxalate Precipitation
Effect of Neptunium Concentration, Nitric Acid Concentration, and Temperature
Reductionb Temp.,
F e d Compositiona .Vf, g . / l . HNOS, M
Run
1 2 3 4 5
51.4 26 7 25 6 46 7 26.7 26.7 25 7 21 8 20.1 27 2 23.3
6 -7
O
1.64 2 85
R u n Conditions Precipitationc’ Digestiond Filtration Temp., Temp., Temp.,
c.
O
80 80 50 50 50 50 23 23 23
1 16
1 62 2.52 2.85 5 05 4 00 4 00 3 10 2.10
c.
c.
50 50 50 50 50 50 23 23 23 23 23
50 50 50 50 50 50 23 23 23 23 23 ~~
c. 23 23 23 23 23 23 ~. 23 23 23 23 23
Ftltratione Factor 2 5 ~
2 0 2 3 2.5 ...
.Vp Loss in Filtrate M g ./l.
