I
J. A. LEARY, A. N. MORGAN, and W. J. MARAMAN University of California, Los Alamos Scientific Laboratory, Los Alamos, N. M.
Plutonium Peroxide Precipitation Precipitation of the hexagonal structure of plutonium peroxide is an excellent concentration and purification step in plutonium metal preparation. Formation of an undesirable cubic structure is avoided by proper control of process variables
EARLY
development work by A. V. Henrickson in these laboratories indicated that plutonium could be efficiently concentrated and purified by peroxide precipitation. Because occasionally yield or filtration characteristics of the precipitate made precipitations unsatisfactory, a more detailed study was undertaken, which resulted in methods for selectively preparing either a facecentered cubic or a hexagonal structure. Large scale precipitation of the hexagonal form consistently provides good yield and good purification in the plant for the remotely controlled production of plutonium metal ( I I). The chemistry of acidic solutions of plutonium and hydrogen peroxide is rather complicated. However, if the oxidation and reduction of plutonium by hydrogen peroxide are disregarded, the reactions of plutonium(1V) with hydrogen peroxide proceed in the following manner. When a small quantity of hydrogen peroxide is added to a plutonium(1V) solution, a brown com-. plex is formed which contains two plutonium atoms, two peroxy oxygen atoms, and one hydroxyl group ( 2 ) . [Peroxide oxygen can be expressed either on a molecular basis, as ( 0 2 7 , or on a n equivalent basis, as (0-). The latter designation is used throughout this discussion. By this convention H202 has two peroxy oxygens.] As more hydrogen peroxide is added, the brown complex is converted to a red complex having two plutonium atoms and four peroxy oxygens. Thus, the brown complex has one peroxy oxygen per pluto-
literature Background Subject Plutonium metal is produced by a process that includes peroxide precipitation in a remotely operated plant Plutonium peroxide has variable composition and structure
Ref.
(11) (89 6, 7,
10)
nium atom; the red complex has two. O n further addition of hydrogen peroxide, green plutonium peroxide having approximately three peroxy oxygens (5, 7) per plutonium atom is precipitated. Hamaker and Koch ( 3 ) have reported that dried plutonium peroxide contains indefinite amounts of anionic constituents such as nitrate, chloride, oxide, and especially sulfate, depending on the precipitation medium. X-ray analysis of the precipitates of Hamaker and Koch (3),as well as those of Koshland (7),normally showed a twodimensional hexagonal lattice with al = 4.00 A. The hexagonal sheets, each containing two plutonium atoms, were held together by the anionic constituents such as nitrate, sulfate, bisulfate, and chloride. Frequently the preparations were mixtures of this hexagonal phase and a second phase, described as possibly cubic with a lattice parameter of a = 16.5 A. I n this early work, it was not possible to correlate the structure with method of preparation.
Experimental
Determination of Structure and Chemical Composition. Generally, pure plutonium(II1) stock solution was prepared by dissolving 99.8% plutonium metal ,in the desired acid. However, nitric acid solutions were prepared by dissolving the metal in a small quantity of perchloric acid, then diluting with nitric acid. An aliquot of each stock solution was then diluted with the appropriate sodium salt-acid solution and a n equivalent amount of hydrogen peroxide was added to oxidize the plutonium to the quadrivalent state. Ten molar hydrogen peroxide (Baker's 30% reagent) solution was added dropwise a t room temperature to precipitate the plutonium. All solutions were stirred vigorously during precipitation and the resulting slurry was normally digested for 16 hours and separated by centrifugation. The precipitate was washed three times with 50y0and once with absolute ethyl alcohol, then dried by passing filtered
air a t room temperature through the powder for 1 hour prior to x-ray analysis. Several preparations were digested for periods varying from 1 to 72 hours. X-ray analyses, also made on several slurries prior to separation and drying, showed no evidence of structural changes due to variations in either digestion time or separation procedure. All the preparations for structural analysis were conducted on 100 mg. of plutonium scale, while preparations that also were analyzed for chemical composition were conducted on a 5-gram scale.
Purification and Yield Measurements. These precipitations were conducted both in the laboratory glove-box enclosures, and in the remotely operated plutonium metal production plant. Approximately 200 ml. of 30% hydrogen peroxide (Becco, Food Machinery and Chemical Corp.) was added to a plutonium nitrate feed solution and digested for approximately 1 hour at room temperature (Figure' 1). This converted the plutonium to the quadrivalent state. The solution was then cooled to 15' C. and cold 30% hydrogen peroxide was added a t a rate of approximately 100 ml. per minute with stirring until a preciFitate formed. Thereafter the hydrogen peroxide was added a t a rate of approximately 400 ml. per minute until the total volume of reagent added was equal to one half the volume of starting feed solution. The resulting supernatant liquor was then approximately 9% in hydrogen peroxide. The peroxide slurry was digested at 15' to 20' C. for 30 minutes, then filtered through a medium grade platinum sintered disk mounted in a platinum-lined boat. Vigorous stirring was required while hydrogen peroxide was being added, and during digestion. The plutonium peroxide was washed six times with 500-ml. portions of 6y0 hydrogen peroxide, followed by two washings with 250-ml. portions of absolute ethyl alcohol, then dried by pulling air through the cake. Analytical. To minimize the errors resulting from decomposition of plutoVOL. 51, NO. 1
JANUARY 1959
27
4.0 liters of Feed Solution (Pu) 0.37 M ("09) 2.7 M (HJSOI) 0.1 5 M
= = = t = 25OC.
(Prereduction Step)
Digest for -1 hr. Cool to 1 5 O c.
c 2. IO liters of 30% HZOY
Plutonium Peroxide Precipitation
1
( a d d slowly)
Digest for 0.5 hr. Cool to l o c c.
*
.
Filter
Filtrate to recovery ~.
I
J.
-
Plutonium Peroxide Cake
Wash solutions to recovery
Wash twice, 0.25 liter of absolute ethanol Air-dry peroxide cake
(To Hydrofluorination)
Figure 1.
Precipitations were carried out in a remotely controlled plant
nium peroxide, many analyses were conducted on the washed wet precipitate. Consequently, it was frequently necessary to determine the ratio of each constituent to plutonium in the precipitate, rather than to use gravimetric factors. Generally, plutonium was determined by radioassay, using the appropriate corrections for americium content and isotopic composition. However, the gravimetric factor for plutonium from dried plutonium peroxide was determined by electrometric titration against ceric sulfate. Peroxide oxygen was determined iodometrically. Sulfate and chloride were determined gravimetrically as barium sulfate and silver chloride,
Table I.
respectively. Perchlorate was determined as nitron perchlorate, while nitrite was determined by a modified Winkler method. These procedures have been reported in detail (70).
Effect of Acidity. Two distinct crystalline phases of plutonium peroxide could be prepared, as well as mixtures of the two phases, by adjusting acidity of the precipitation medium (Table I). I n nitric acid solution, precipitations from high acidity-i.e., 21M or greaterresulted in a hexagonal structure, while precipitation from 0.45M acid resulted
Two Crystalline Phases of Structure of Plutonium Peroxide Were Made b y Adjusting Acidity of Precipitation Medicine
Prep. NO.
A.
Concentration," Moles 'Liter Hf NO, C1-
Pu
H202
0.003 0.033 0.033 0.033
3.00 3.00 3.00 3.00
0.45 1.0 2.0 3.2
3.0 3.0 3.0 3.0
0.053 0.053 0.053
1.00 1.00 1.00
0.51 2.8 3.6
0.176 0.229 0.264 0.176
3.30 1.35 0.25 3.30
1.10 1.44 4.10 4.10
0.033 0.033 0.176
1.00 3.00 3.00
0.55 4.0 4.2
... ... ... ... ... ...
"03
19 20 21 22
B.
HCI 15 16 17
C.
HClOd 4 5 6 7
D. &SO4 25 26 27
ClO;
SO;-
Structure
... ... ... ...
0.25 0.25 0.25 0.25
... ...
...
FCC
...
Hex. Hex.
... ...
3.0 3.0 3.8
... ... ...
...
FCC FCC
...
... ... ...
... ,.. ... ... ... ... 0.37 2.0 2.0
FCC
...
...
...
... ...
1.60 2.12 4.63 4.63
... ...
0.53
Mixt.
Hex.
FCC ECC Hex. Hex.
Hex. Hex.
Uncorrected for changes resulting from reactions such as precipitation or complexing.
28
INDUSTRIAL AND ENGINEERING CHEMISTRY
in a pure face-centered cubic phase having a cell constant a = 16.46 A. In hydrochloric acid solutions, a similar acidity effect was obtained. but the phase alteration occurred a t an acidity of approximately 3 M . The cubic stmcture also was obtained from perchloric acid solutions at acidities up to 1.44M. At a concentration of 4.10M perchloric acid, the hexagonal structure was obtained. However, in preparations 6 and 7 the perchlcrate ion concentration also was increased to 4.63AM, so that the effect of acidity is ambiguous Acidity clearly affects the structure of plutonium peroxide (Table I). In general. precipitations made from less than 2,M total acidity have the cubic structure, except in the nitric acid system where an acidity of less than 1 M is required for the pure cubic phase. At high acidityi.e.. -3M and above-pure hexagonal plutonium peroxide normally is obtained. Effect of Perchlorate Ion Concentration. Attempts to prepare plutonium peroxide from 0.4M perchloric acid solution were unsuccessful because the precipitate formed as a colloid that could be detected with a Tyndall cone. This colloid could be coagulated partially by the addition of sulfate. but was easily peptized when washed with 50% ethanol. When the plutonium perchlorate solution was made 0.16.44 in sulfate before adding hydrogen peroxide, the precipitate could be filtered and washed without difficulty. An interesting effect of perchlorate ion concentration on structure was observed by precipitating plutonium peroxide from solutions containing small amounts of sulfate. As shown in Table 11, increasing the perchlorate concentration from 0.25 to 4.OM at a constant acidity of 0 . 4 8 M caused a change from cubic to hexagonal structure, whether the perchlorate was added as the sodium or the magnesium salt. Larger precipitarions made for chemical analysis under the same conditions as preparations 9 and 10 of Table I1 exhibited the identical change in structure. However. perchlorate ion was not detectable in either precipitate. (Based on the lower limit of detectability, the mole ratio of perchlorate-plutonium was less than 0.02.) Consequently the change in structure appears to be due to the high ionic strength (-4.7) of the precipitation medium in preparation 10. Effect of Hydrogen Peroxide Concentration. Increasing the hydrogen peroxide concentration from 1 to 3M did not alter the structure of cubic plutonium peroxide precipitated from 0.45M nitric acid solution (Table 111). Similarly in the case of hexagonal peroxide precipitated from 4.10M perchloric acid, varying the hydrogen peroxide concentration from 0.25 to 3.30M did not alter the structure. Effect of Sulfate Concentration. The
NUCLEAR TECHNOLOGY structure appears to be independent of sulfate concentration (Table IV). Preparations 23 and 24 indicate that a change of from 0.10 to 0.90M in total sulfate concentration did not alter the structure of hexagonal plutonium peroxide, while the cubic form was obtained from a 0.37M sulfate solution at lower acidity and also was obtained at low acidity in the absence of sulfate (preparation 18).
omp position of P~u+oniumPeroxide Oxidation State of Plutonium in Precipitate. In the early work it was reported that plutonium peroxide was a plutonium(1V) compound (7). How-
Table II. Prep. No. 14 9 10 11
Increasing Perchlorate Ion Concentration Changed Structure of Plutonium Peroxide from Cubic to Hexagonal Concentration, Moles/Liter HzOz Hf (210; 0.25 0.40 1.00 ' '33.48 (1.25 1.00 0.48 4.0" 1.00 0.48 4.0b
Pu 0.052 0.033 0.033 0.033
Structure FCC FCC Hex. Hex.
Perchlorate added as Mg(C104)z.
Table IV. Structure of Plutonium Peroxide fs Independent of Sulfate Concentration [(Pu) = 0.033Ml prep. Concentration, Moles/Liter StrutNo. HZOZ Hf SO;NO, ture 3 . 0 FCC !8 1 . 0 0.45 0 FCC 25 1.0 0.55 0.37 0 Hex. 23 3.0 2 . 0 0.10 3.0 Hex. 2 . 0 0.90 3 . 0 24 3.0
Increasing Hydrogen Peroxide Concentration Did Not.Alter Structure of Plutonium Peroxide
Prep.
No. 18 19 6 7
SO;0.50 0.16 0.16 0.16
...
Perchlorate added as NaClOd.
Table 111.
ture. Because the plutonium is in the tetrapositive state (7, 70), additional anions such as sulfate, nitrate, hydroxide, or oxide must be present to satisfy the plutonium valency completely. Analyses of the wet hexagonal precipitates indicated that the hexagonal structure contains more peroxy oxygenplutonium than does the face-centered cubic. Preparations 6 and 7 of Table V were washed by reslurrying with 6M perchloric acid. The maximum peroxy oxygen-plutonium ratio that can be attributed to incomplete washing is 0.01 in preparation 7. Moreover, preparation 6 was made by adding so little hydrogen peroxide that the ratio was only 1.92 in the precipitation medium. However, the peroxy oxygen-plutonium ratio in the precipitate was increased to 3.34. Neither preparation 6 nor 7 could be separated and dried without destruction, so the dry preparation was not analyzed. Preparation 26 contained a mole ratio greater than 3 after copious washing. The average ratio of peroxy oxygen-plutonium was 3.37 f 0.11
ever, these preparations were of mixed structure. More recently it has been shown that both the pure cubic and pure hexagonal forms are compounds of quadrivalent plutonium (70). Mole Ratio of Peroxy Oxygen to' Plutonium Mole Ratio. By the iodometric method, the peroxy oxygen-plutonium ratios were determined for both the cubic and the hexagonal structure precipitates. As shown in Table V, the cubic structure contains three atoms of peroxy oxygen per plutonium atom. The average ratio was 3.03 f 0.04 (average deviation) regardless of whether the sample was analyzed wet or immediately after drying for 1 hour a t room tempera-
Concentration, Moles/Liter HzOz H+ NO, 3.0 1.0 0.45 . 3.0 0.45 3.0 0.25 4.10 3.30 4.10
Pu
0.033 0.033 0.264 0.176
... ...
ClO; 0.25 0.25 4.63 4.63
Structure
FCC FCC Hex. Hex.
Table V. Ratio of Peroxy Oxygen to Plutonium in Precipitates (Digestion temperature, 25" C. The hexagonal stsucture contains more peroxy oxygenplutonium than the face-centered cubic) Treatment Prep.
No. 4 9
Digestion time, hr.
Analysis
o-/pu Mole Ratio
Wet Dry
3.10 3.10
Wet
3.03
1 2
3% HzOs 0.1M H ~ S O C CaHsOH
2 2
0.01M HCl CiHsOH
Wet Dry
2.99 2.98
Wet Wet
3,34 3.53
Wet
3.50
Wet Dry Wet Dry
3.34 3.11 3.15 2.96
Times washed Washed with Cubic Structure
120 16
4 6 4
1
2
9 5 16
6 7
1
0.5 0.5
'
CzHaOH 0 . 1 M HCl CzHsOH Duplicate
Hexagonal Structure 4 6M HClOi 3 6M HClOi
for the wet and 3.02 f 0.07 for the dry hexagonal precipitates.
Mole Ratio of Sulfate to Plutonium. The mole ratio of sulfate-plutonium was determined for preparations 9 (cubic) and 10 (hexagonal), precipitated at an identical total sulfate concentration of 0.16M. The cubic structure contained 0.24 f 0.02 mole of sulfate per mole of plutonium, whereas the hexagonal structure had a mole ratio of 0.37 f 0.01. Hexagonal preparation 26, made from a total sulfate concentration of 2.OM, also contained 0.38 f 0.01 mole of sulfate per mole of plutonium. When cubic peroxide was precipitated from l.GM nitric acid and 0.10M total sulfate, the ratio was 0.25; hexagonal precipitations a t the same sulfate concentration contain 0.36 mole of sulfate per mole of plutonium. Thus over a wide range of total sulfate concentrations, the hexagonal form normally takes up more sulfate-i.e., 0.38 f 0.01 mole per mole of plutonium-than the cubic form.
2.99
Duplicate 7 26
10
0.5 16
3 3 2 3
1% H1102 CeH6OH 0.1M HCl CaHsOB ,
I
Mole Ratios of Chloride to Plutonium and Nitrate to Plutonium. The cubic VOL. 51, NO. 1
JANUARY 1959
''
29
structure of preparation 16 had a chloride-plutonium mole ratio of 0.029, while hexagonal preparation 17 had a ratio of 0.160. By varying acidity, both structures also were precipitated from 2.5M total nitrate and 0.15M total sulfate solutions. The cubic structure has a mole ratio of nitrate-plutonium of 0.05; this ratio was 0.02 for the hexagonal form. Density of Plutonium Peroxide. Although the bulk densities of plutonium peroxides are highly variable, the cubic form normally has the higher bulk density. For example, hexagonal preparation 23 (Table IV) had a dry tamped bulk density of 0.17 gram per ml., whereas a cubic precipitation from a similar solution had a dry tamped density of 0.70 gram per ml. Particle densities also were determined by measuring the amount of 0.01M sulfuric acid solution displaced by a known weight of powder in a pycnometer. The hexagonal form had a density of 3.43> compared to 3.71 gram per cc. for the cubic form. Discussion
I n the case of cubic plutonium peroxide, only three fourths of the plutonium(1V) valence is taken up by peroxide oxygen, Consequently, the remaining fourth must be satisfied by one equivalent of oxide, hydroxide, or the anion of the acid system from which the peroxide was precipitated. However, the analyses indicate that much less than an equivalent of sulfate, chloride, or perchlorate is associated with one mole of plutonium in the precipitate. In view of the colloidal nature of the cubic structure, these anions probably are present in an adsorbed state. Consequently the cubic structure may have the empirical formula Pu2(0-),j(O--).xHzO, where x is approximately 6. Such a compound could be formed by linking plutonium(1V) dimers such as (PuOPu) with peroxide groups to give a structure as
l
0
oI , I 0 I 0 I 1
0
-0-0-Pu-0-Pu-0-00 ' ' 0
Table VII. Good Purification of Plutonium Is Given by Peroxide Precipitation
Element Group
I
INDUSTRIAL AND ENGINEERING CHEMISTRY
Element Na
cu
Ag
I1
Be
Mg
Grams of Element per lo6 Grams of Pu Nitrate Plutonium feed solutior peroxide > 100 15 > 100 20 > 100 < 20 15 < 1 90 3 90 < 1 5 2 4 < 0.5 2 2 75 3 1,000 30
