US2785951A

US2785951A - Bismuth phosphate process for the separation of plutonium from aqueous solutions

Info

Publication number: US2785951A
Authority: US
Inventors: Stanley G Thompson; Glenn T Seaborg
Original assignee: Individual
Current assignee: Individual
Priority date: 1944-01-26
Filing date: 1944-01-26
Publication date: 1957-03-19
Anticipated expiration: 1974-03-19

Description

Mrch 19, 1957 s. G. THOMPSON Erm. 2,785,951 BISMUTH PHOSPHATE PROCESS FOR THE SEPARATION OF PLUTONIUM FROM AQUEOUS SOLUTIONS Filed Jan. 26, 1944 V f/IoJp/hzfe )Vrecg'azzazz I pfefzfafe jeff/m? i l rzzczzozz 4 Prec-Dz'a/z/z j United States Patent() BlSMUTI-I PHOSPHATE PROCESS FOR 'I HE SEP- ARATION F PLUTONIUM FROM AQUEOUS SOLUTIONS Stanley G. Thompson and Glenn T. Seaborg,y Chicago,

lll., assignors to the United States of America as represented `by the United States Atomic Energy Commission Application `lanuary 26, 1944, Serial No. 519,714

Claims. (Cl. 23-14.5)

The invention relates to the separation of element 94 from other substances and more particularly to the separation of element 94 from substances of the kind present in neutron irradiated uranium, such as uranium, fission products, and the like, which are considered to be foreign porducts.

It is an object of the invention to obtain element 94 in a relatively pure state by a convenient process which is suitable for use on both a commercial and laboratory scale, and which isparticularly adapted for concentrations of element 94 such as are obtained from neutron irradiated uranium Where fission products and other substances resulting from a neutron bombardment of uranium are present. Other objects and advantages will be apparent from the following detailed description.

In the drawing, a diagrammatic representation of one embodiment of the invention is given in the form of a flow sheet.

As described herein, the isotope of element 93 having a mass of 239 is referred to las 93239 and the isotope of element 94 having a mass o'f 239 is referred to as 94239. Element 94 may also be spoken of as plutonium, symbol Pu. Reference herein to any of the elements is to be understood as denoting the element generically, Whether in its free state or in the form of a compound, unless indicated otherwise by the context.

Neutron irradiated uranium may be prepared by reacting uranium with neutrons from any suitable neutron source, but preferably the neutrons used are obtained Y from a chain reaction of neutrons with uranium.

Neutron irradiation of uranium produces 92U233 which has a half-life of 23 minutes and by beta decay becomes 93233. This element has a half-life of 2.3 days and by beta decay becomes 94239. Neutron irradiated uranium contains 93239, 94239 and a large number of radioactive fission products produced by reaction of neutrons on fissionable atoms, such as U235 which is present in uranium from natural sources. It also contains minor amounts of other products such as UX1 and UXz. The amount of 93239 and 94239 combined is generally minute, such as, for example, approximately 0.02% by weight. By storing the neutron irradiated uranium for fa suitable period of time, the 93233 is converted almost entirely to 94329. The fission products are present in the neutron irradiated uranium generally to an extent of about 0.02% by weight. Because the fission products in general are highly radioactive, it is preferred that these materials be removed.

The ssion products consist of a large number of elements which may be classified into two groups; a light group with atomic numbers from 35 to 45; and a heavy group with atomic numbers from 51 to 60. The fission products with which we are particularly concerned are those having a half-life of more than three days since they remain in the neutron irradiated reaction mass in substantial quantities at least one month after reaction. These products are chiefly Sr89, Y (57 day half-life), Zr, Cb, and Ru of the group of atomic numbers from 35 to 45; and Te12", Tem, 1131, Xe133, Cs (many years e ICC half-life), Ba (12 days half-life), Lam, and Ce of 20 day and 200 day half-lives from the group of 'atomic numbers from 51 to 60, inclusive.

With respect to the present invention, the foreign products, that is, the fission products, uranium, and other substances present in neutron irradiated uranium, may be classified into those foreign products which are insoluble in the presence of phosphate ions and those which are soluble in the presence of phosphate ions. While the concentrations of certain of the phosphate insoluble foreign products may be so low in certain instances to prevent them from being thrown out of solution as insoluble phosphates, such foreign products may, nevertheless, be carried out of solution by other substances which are precipitated as phosphates.

In its broadest aspect, the invention lprovides a method for separating plutonium from fission products which comprises separating such of the fission products as are in a phoshpate solubility state opposite to that of plutonium and then converting plutonium to an opposite phosphate solubility state and separating plutonium from those fission products that now have a phosphate solubility state opposite to that of the converted plutonium.

The solubility of plutonium in the presence of phosphate ions is changed by changing the oxidation state of the plutonium, as it has been discovered that plutonium has more than one oxidation state; a lower oxidation state in which the element forms an insoluble phosphate in the presence of phosphate ions; and also a higher oxidation state or states in which the element does not form a phosphate insoluble in the presence of phosphate ions. As the foreign products form substances which are in part insoluble and in part soluble in the presence of phosphate ions, the process contemplates separating plutonium in its phosphate insoluble (lower oxidation) state from the phosphate soluble foreign products, then converting the plutonium contained in the phosphate insoluble portion to its higherV oxidation (phosphate soluble) state and separating the latter from the remaining foreign products which are in a phosphate insoluble state. The soluble plutonium phosphate in which plutonium is present in its higher oxidation state is referred to as plutonyl phosphate, (PuOz)3(PO4)2, in the claims appended to this specification. The above process, being based on oxidizing and reducing the plutonium to change its solubility in the presence of phosphate ions, mayV for convenience be referred to as an oxidation-reduction separation. Such oxidation-reduction separation may be used as a complete process for separating plutonium from fission products, uranium, or other foreign products. The steps of oxidation and reduction may be employed in either sequence desired.

Where the plutonium is associated with substances of the kind present in neutron irradiated uranium, it is generally preferred to separate as much of the uranium as possible from the plutonium before treating the plutonium in accordance with the oxidation-reduction separation, as the presence of uranium interferes with the reactions by which the plutonium is separated, particularly if the uranium is present in concentrations which are relatively high as compared to that of plutonium. Various methods for partially separating the plutonium from uranium and fission products may be used in conjunction with the oxidation-reduction separation. A process which has been found to be particularly suitable for use as a preliminary Astep in the oxidation-reduction separation of plutonium is a phosphate precipitation step in which the plutonium is separated as yan insoluble phosphate from substantially all, of the titanium and major amounts of fission products. The partially purified plutonium is then further purified by being treated in accordance with the oxidation-reduction separation which, when used with the phosphate precipitation u step or other preliminary separation method, may be considered as an oxidation-reduction phase of the overall separatory process.

lncarrying out the preliminary phosphate precipitation phase where theplutonium is present in concentrations above approximately 800 mg. per -liter of solution, the plutonium may be separated from substantially all of the uranium and many of the fission products present in the solution by precipitating the plutonium and minor amounts of fission products as insoluble phosphates through the addition of an excess of phosphate ions to the solution, which is adjusted to approximately one normal acid content. If the solution contains an appreciable `amount of uranium, it may also be desirable to have present ions such as sulphate ions, phosphate ions at high'acid concentrations, or the like, which will form a complex coinpound of uranium and thereby prevent the uranium from precipitating as an insoluble phosphate. In this concentration, the plutonium is present above its limit of solubility and will be thrown down as an insoluble phosphate precipitate which is easily removed from the other'substanees in solution. Y

Such high plutonium concentrations may be present in neutron irradiated uranium, especially where the uranium is bombarded with neutrons obtained from a chain reaction for long periods of time. In extracting the plutonium, the neutron irradiated uranium is dissolved in nitric acid and diluted with water to form a solution containing 20% uranyl nitrate hexahydrate, UO2(NO3)2.6H2O, which Solution will also contain the plutonium, fission products, and other substances as nitrates. The solution is Vadjusted to an acid content of approximately one normal. Suiicient sulphate ions or other complexing ions are added to prevent precipitation of the uranium and other substances present, this being done by adding sulphuric acid, sodium sulphate, or other soluble sulphate. The solution is preferably aciditied with H2804 which will also introduce the necessary sulphate ions. An excess of phosphate ions, preferably in the form of phosphoric acid, is added to the solution to precipitate the plutonium. The insoluble plutonium phosphate and minor amounts of fission products in the form of phosphates are thrown down and may be separated from the solution by any convenient means such asriiltration, centrifugation, or the like.

Where the plutonium is present in relatively low concentrations as, for example, mg. per liter of solution, as is usually the case in a solution of neutron irradiated uranium, it is below its limit of solubility in the solution so that the addition of phosphate ions will not cause the plutonium to be thrown down as a precipitate. To carry out the preliminary phosphate precipitation phase with such low concentrations of plutonium, it has been found desirable to use suitable substances which will be converted into insoluble phosphate carriers by phosphate ions and which will carry the plutonium phosphate out of solution. Such carriers must necessarily be phosphates which are relatively insoluble in acid solutions in which uranium or uranyl phosphate is soluble, such as, for example, bismuth phosphate and zirconium phosphate.

Bismuth phosphate, in particular, has been found to be very effective in carrying7 plutonium lout of solution as it has been discovered that plutonium may be removed from the uranyl nitrate hexahydrate solution to the extent of 95% or more where bismuth phosphate is present in bismuth: plutonium ratios varying from :1 to 200,000zl. The carrying elect that bismuth phosphate exhibits on plutonium may be due in whole or part to a surface association with the plutonium such as adsorption, complexes formed Which are in the nature of compounds, or other types of associations. it is also possible that the reason for this behavior resides in the fact that the bismuth phosphate precipitate is isomorphous with plutonium phosphate. Separation of the insoluble phosphates from the solution by conventional means'such as filtration is facilitated by the-crystalline structure of the precipitated bismuth phosphate. Thus, while many carriers may be suitable for carrying the plutonium phosphate from solution, preferably the carrier is one which, like bismuth phosphate, is isomorphic with plutonium phosphate. However, while We have reason to believe that the use o-f a carrier which is isomorphic with plutonium phosphate leads to particularly desirable results, we do not wish to be bound by any theory as to the action of the carrier.

The following experiment and accompanying table is given to sho-w that bismuth phosphate has a very high carrying power for element 94 in concentrations ranging from slightly above tracer concentrations to concentrations Where a carrier is not needed; that is, where the concentration of plutonium exceeds the solubility of plutonium phosphate in the solution.

EXAMPLE I A uranyl nitrate solution containing 20% uranyl nitrate hexahydrate, 25 mgm. Bit3 per l0 ml., and l N in HNOS was prepared. To small volumes (100, l0 or l 10-3 ce.) of this was added appropriate quantities of Fut1 (as a solution of the nitrate) to obtain the desired BizPu ratio. This was followed'by the addition of suiilcient 3.6 M HsPOe to makeV the solution 0.36 M H3PO4. The solutions were heated at C. for 45 minutes and then centrifuged.V Aliquots of the supernatant liquid were taken for alpha-counting. The precipitates were washed 3 times with .04 cc. of 3.6 M HaPOi, dissolved in concentrated HC1 and transferred to platnium dishes for alpha-counting. The 94 in the aliquots of the supernatant liquids was separated from uranium by LaFa precipitation; that is, by insuring the presence of the 94 in the reduced state, adding a solution containing lanthanum ion followed by the addition of hydrofluoric acid solution and digesting for a relatively short interval, whereupon lanthanum fluoride precipitates and carries the 94 out of solution. Results of the experiments are summarized in the table below.

Table I Activity rc- Activity rccovered in the covered in the Percent Ratio BizPu supernatant bismuth phos- 94 presolution 1 phate precipicipitatcd (Counts per tate (Counts minute) per minute) 11, 920 98. Q 92 6. 300 98. 5 92 13, 800 Q9. 4 184 12, 600 98. 5 l2 744 98. 4 6 134 95. 5

1 These counts are corrected for a uranium blank by subtracting 18 counts per minute obtained by precipitating 4 104 grams oi La as thc fluoride in the presence of uranyl nitrate containing no Pu.

2 The concentration of Pu in this experiment was such as to probably exceed the solubility of plutonium phosphate in uranyl nitrate.

In accordance with one embodiment of the invention, neutron irradiated uranium containing element 94 inconcentrations above tracer amounts as, for example, concentrations of approximately V0.01 percent by weight of element 94 is dissolved in nitric acid `to form uranyl nitrate hexahydrate, as Well as nitrates of plutonium, neptunium (element 93), and the various fission products. In any case, an excess of nitric acid is used to insure the dissolving ofl substantially all substancesY present.

To prevent precipitation of uranium and other Vforeign products during the phosphate precipitation phase, it. is desirable, first, that the acidity of`the solution be adjusted to between approximately 0.50 normal and 2.0 normal, and, secondly, that sulphate ions or other complexing ions be present in the solution.V With respect to the acidity of the solution, it has been found that an approximately onenormal acid solution is 'most suitable for precipitation of the plutonium and bismuth as phosphates while leaving the uranium 'and other foreign products in solution. Belowv approximately 0.50 normal acid concentration, uranyl phosphate tends to precipitate and befthrwn down with the plutonium and bismuth phos# nitrate hexahydrate) in order to keep the loss of element- Precipitation of uranium and other foreign products as phosphates is further prevented by the presence of complexing ions such assulphate ions i'n the solution, which may be introduced by the sulphuric acid used to acidify the solution. Usually, however, the solution willV bef adjusted tothe proper acid content without sufficient sulphate ions having been introduced to prevent precipitation ot` the uranium phosphate. In such instances, sources of sulphate ions other than sulphuric acid, such as NazSOr, NaHSOl, or other sulphate salts may be used for this purpose. It is believed that the sulphate ions form a complex with the uranyl ion which substantially prevents the latter from precipitating as an insoluble phosphate. Other complexing ions that may be used for this purpose are phosphate ions, P0473, in' high acid concentrations, carbonate ions, C03-2, in low acid concentrations, or other elements or radicals that will complex the uranyll ion, UO2+2, and thereby substantially preventV the formation of insoluble uranyl phosphate.

As plutonium is generally present in relatively loul concentrations, a carrier such as bismuth phosphate is used to carry the plutonium phosphate out of solution. Bit?v ions may be introduced into the solution by the addition of bismuth nitrate orother bismuth compound preferably in acid solution. The concentration of Bi+3 ions may be' varied quite widely, depending upon the particular concentration of plutonium in the solution. Usually, 25 mg. of Bi+3 per l0 cc. of solution is sufcient. When plutonium is present in tracer amounts it has been found that a concentration of mg. of Bi+3 per l0 cc. of solution will precipitate with phosphate ions and carry substantially all of the plutonium out of solution.

An excess of phosphoric acid is added to the solution topr'eci'pitate the bismuth, plutonium, and minor amounts of fission products as insoluble phosphates. While the concentration of the phosphate ion may be varied greatly, it has been found that generally the higher the acidity of the solution, the greater should be the concentration of the phosphate ion. A phosphoric acid concentration `in the solution of approximately 0.4M to 0.8M is usually suitable. However, Where phosphate ions are also used to complex the UO2++ ions, higher concentrations of phosphoric acid in the solution Such as concentrations of between 1M to 1.5M are necessary to precipitate the bismuth and plutonium. It is necessary that the solution be of the proper acidity and/ or that it have a suitable amount of sulphate ions or other complexing ions present when the phosphoric acid is added, otherwise some of -the uranium will be precipitated as an insoluble uranyl 6 tion may be heated duringor'a'fter the addition of the phosphate and bismuth ions. Preferably,rthe solution is heated for approximatelyan hour atl about 75 C., whereupon a substantially complete precipitation of the bisinuth phosphate takes place, carrying with it substantially all of the plutonium phosphate as well as minor amounts of fissi-on products.

There is a tendency for the nitric acid used for dissolving the neutron irradiated uranium to oxidize a small portion of the plutonium to the phosphate-soluble state. To insure the presence of plutonium in the resulting solution in a reduced state, it may be desirable to add small amounts of a reducing agent to said solution thereby making certain that substantially a-ll of the Pu present will be in the phosphate insoluble state. Reducing agents such as salts of ferrous iron, hydrogen peroxide, hydrazine, or the like, may be used for this purpose. Although we generally prefer to have some reducing agent present at this stage as a precautionary measure, for the purpose indicated, this preliminary step for the separation of uranium is not to be confused with the subsequent procedure for the separation and concentration of plutonium, which we call the oxidation-reduction separation.

The fission products that are carried out of solution with the plutonium and bismuth phosphates 'are' principally radioactive zirconium and columbium phosphates. Minor amounts of other fission products, such as cerium and lanthanum, which form phosphates soluble in `acid solution, may be carried down to some degree with the bismuth phosphate. As it is desirable to lower the amount of radioactive fission products (which are emitters of beta and/ or gamma radiation) and particularly the phosphate-soluble radioactive fission products, present in or carried by the insoluble phosphates, the original solution may lhave added to it before the introduction of the phos# phate ions, i. e., before the bismuth phosphate precipitation step, small amounts of holdback carriers, which are preferably non-radioactive isotopes of the fission products and particularly non-radioactive cerium and lanthanum. The holdback carriers dilute the phosphate soluble, radioactive fission products with the inctive form of such products so that the phosphate soluble fission products which are carried by the bismuth phosphate precipitate will include both the radioactive and inactive forms of the fission products, thereby to that extent decreasing the amount of phosphate soluble radioactive' fission products carried by the bismuth phosphate.

The following tables set forth the percent of 94 carried by the bismuth phosphate precipitate, when different variables such as acid concentration, sulphate ion concentration, temperature, time of heating, etc., are varied one at a time, in order to determine the conditions that will be most favorable for the separation of 94. The tables are given by way of illustration and not to limit the invention. Unless otherwise specified the conditions of the experiments below were as follows: 20%

UO2(NO3) 2.6H2O

1 N HNo or nzsor, @36M HgPor, 25 mg. Bi+3 per 1o cc., temperature of precipitation 95 C., .time of heating during precipitation l hour, and tracer amounts of 94.

Table II EFFECT OF VARIATION IN HaPOt CONCENTRATION HzP 04 concentration (Mol ar .54 .36 .18 .12 .09 .06 .03 1 N HNOS-Percent 94 1n 7 Table 1n TIME F HEATING AT 95 C.

Timeat95o 5 1o 15 2o so so 2' 5 s min. min. mln. min. mln. mln. hrs. hrs. hrs.

1 N HNO-Percent 94 in BiPO4 97 99 97 99 100 1 N H -Percent 94 in B :S04 iPOr 98 99 99 99 99 Table IV VARIATION IN ACID CONCENTRATION Acid normality (HQSO orHNOg 1.00 1.25 1.50 HNOS-Percent 94 in Table V EFFECT OF VARIOUS PRECIPITATION TEMPERATURE FOR ONE HOUR Temperature 25 O. 40 C. 60 C. 80 C. 95 C.

1 N HNOa-Percent 94 in BiPO slight pe- 53 97 100 99 1 N HaSOi-Pereent 94 in BiPO( 55 85 98 99 99 Table Vl VARIATION IN AMOUNT OF BISMUTH CARRIER-IN l N HiSO4 ONLY MgBw/iocc 25 2n 15 10 Percent 94mB1Po4 99 9s 9s 9G Table VII VARIATION IN UO2(NO3)2.6H2O CONCENTRATION IN 1 N HzSO4 SOLUTION ONLY Percent U02(NO5)2.6H2O 0. 0 10. 0 12 17. 6 20. 0 22. 4 Percent 94 in BiPO4 99 100 100 99 98 95 Table VIII COBINATIONS 0F HNO, H2504, AND NmSOi Hi S 0 4 normality HNO normality NazS04 normality Percent 94 in BiPOi The following example is given to aid in the explanation of the invention and is not intended to limit the invention to the details described therein.

EXAMPLE Il 9.4 ce. of solution is prepared containing 2.5 gm. of UO2(NO3)2.6H2O containing 94, l cc. of l0 N HzSO4 and 25 mg. of Bi+3 (58 mg. Bi(NO3)a.5H2O). 0.6 cc. of 6M HaPOi is then added. The solution is heated to 95 C. and maintained at this temperature for 1 hour. The BiPO4 carrying the 94 is separated from the solution by filtration or centrifugation, and, for purposes of analysis, dissolved in 'about l cc. concentrated HC1. The HCl solution is diluted to about 10 cc. and the 94 precipitated using LaFa as carrier, in the manner described above. By Y this method, consistent recovery of 94 to the extent of about 98% is readily obtained.

Generally speaking, the BiPOi precipitate will also remove about 17% of the gamma activity due to fission products( originally present in A the neutron irradiated uranium mass. This activity corresponds to aboutn one- CFI y 90C. for Ifrom '-ve minutes to 'two'hours` or longer.

third of the total Zr, Cb fission activity present in the original material. The final LaFa precipitate contains about 5% of the gamma and about 8% of the betafission activity present in this same material.

The bismuth phosphate precipitate obtained from the phosphate precipitation phase includes plutonium and minor amounts of fission products as phosphates. Substantially all of the uranium and major amounts of fission products have been removed and the plutonium is ready for further purification by the oxidation-reduction phase to be described.

The oxidation-reduction separation alone may be used to separate plutonium from substances of the kind pres'- ent in neutron irradiated uranium, including uranium. Where the plutonium is present with such substances, as where neutron irradiated uranium is treated directly, the oxidation-reduction separation is somewhat less eicient than where the plutonium has been subjected to a preliminary separation treatment which removes most of the uranium and fission products, in that the uranium interferes with the reactions by which the plutonium is separated. The use of a preliminary phosphate precipitation separation in conjunction with the oxidationreduction separation provides a complete process which is particularly effective in obtaining plutonium in a pure state, especially where the original source of the plutonium is neutron irradiated uranium.

In carrying out the oxidation-reduction phase, the particular form in which the plutonium is to be treated is dependent somewhat upon the concentrations of plutonium in the original solution. Where the plutonium is present in relatively high concentrations, that is, above approximately 800 mg. per liter of solution, the plutonium will be in the form of an'insoluble plutonium phosphate precipitate. Where the concentrations of plutonium are low, `as for example, 10 mg. per liter of solution, the preliminary phosphate precipitation phase will yield the plutonium as an insoluble phosphate carried by bismuth phosphate.

However, as the concentrations of plutonium in neutron irradiated uranium are normally so low as to require the use of a carrier such as bismuth phosphate, the` material to be treated by the oxidation-reduction phase is usually bismuth phosphate carrying plutonium phosphate and minor amounts of fission products in the form of phosphates.

The bismuth phosphate precipitate is dissolved in an excess of concentrated acid, such as ten normal nitric acid. The solution is diluted with water to a suitable -acidity for oxidation of the plutonium, such dilution being dependent somewhat upon Athe oxidizing agents used; for most oxidizing agents, the acidity should be between about two normal and ten normal. If desired, the oxidizing agent or agents can be added before or after dilution.

The oxidizing agent or agents in amounts sufficient to oxidize the plutonium and convert it to its phosphate soluble state are added to the solution. While any oxidizing agent having a potenti-al greater than 1.0 volt will tend to oxidize the plutonium, oxidizing agents that have been found to be most suitable for this purpose are dichromates such as K2Cr207. Na2Cr2O7 or the like,

peroxydisulfate in the presence of silver nitrate.

Where dicromates, such as KgCrzO7 or NaCr2O7, are used, the yacidity of the solution should Ibe between 2N and 6N. The dichromate is added in the amount of .OOlM to .1M and is permitted to react for one-half hour to four hours at between 50 C. to 95 C. Where the solution has an acidity of 2N and a kdichromate in the amount of .()GlM is used, the solution is preferably heated for approximately one hour at 75 C. Where sodium bismuthate, NaBiO3, in the amount of .005M is used, the solution should be between 3N and 7N acidity and the solution vmaintained at a temperature of from A50" C. to

nitric. acid therefore converts all these substances to soluble nitrates. The hydroxide treatment may be used at'any point in either the phosphate precipitation phase or oxidation-reduction phase in which the plutonium is present with bismuth phosphate.

After the plutonium has been treated by the oxidationreduction phase in which bismuth phosphate is used as a carrier, the plutonium and bismuth are obtained as phosphates. The plutonium may be separated from the bismuth by any one of several methods, For example, the bismuth phosphate carrying plutonium phosphate may be dissolved in an acid such as hydrochloric acid and a carrier, such as lanthanum fluoride or the like, may be precipitated from the solution to carry down the plutonium away from the bismuth in the manner already described. The carrier should be of such a nature that it is either required in lesser amounts than bismuth phosphate to 4carry a given amount of plutonium or such that it is more soluble in certain solvents than is bismuth phosphate so that it may be dissolved in such a small amount of solvent that the plutonium will be present in relatively high concentrations and consequently, may be precipitated therefrom as a substantially pure plutonium compound. Where bismuth phosphate is converted to bismuth hydroxide at one or more points throughout the process, the volume of the solution may be decreased to a point Where plutonium will be present in suchlliigh concentrations that it may be separated from the bismuth by being precipitated as a plutonium compound without the use of an additional carrier.

EXAMPLE III As a further specific embodiment of the invention, and referring particularly to the drawing, the following is given by Way of illustration. A bismuth phosphate precipitate 2, carrying plutonium and minor amounts of lission products, was obtained from a preliminary phosphate precipitation phase as applied in the manner described above to a solution 1 comprising 1000 gms. of uranyl nitrate hexahydrate which had been subjected to an extended neutron bombardment. ln carrying out the 'preliminary phosphate precipitation step l, l gms. of Bit3 was precipitated as bismuth phosphate from 4 liters of 20% U02 (NO3)2.6H2O 1N H2804 in the presence of 0.36M phosphoric acid and the bismuth phosphate precipitate 2 was then washed with an acid mixture of 1N in HNOs and 0.4M in H3PO4. The waste filtrate 3 was discarded.

In the rst cycle Il of the oxidation-reduction phase, the bismuth phosphate precipitate 2 was dissolved in 10N HNOS, the acidity reduced to 6N HNOS by dilution, and the solution made 0.1M` in KaCrzO'z. The plutonium was oxidized by heating this solution at 95 C. for 2.25 hours. The solution was then diluted to 1N acidity by adding distilled water `and sufficient H3PO4 was added to bring the concentration of HsPO4 to .05M. The solution was heated to approximately 90 C. to throw down the bismuth phosphate precipitate 4 which was then removed by filtration and discarded. The plutonium in the filtrate 5 was reduced by passing in a rapid stream of SO2 gas for 5 minutes and allowing the solution to stand for approximately 1 hour. Bit'` was then added as a N HNOs solution to make the Bit3 concentration 0.8 mg. per cc. (This is one-thirdV the amount of Bi+3 used in the phosphate precipitation phase and serves to reduce the volume in the subsequent cycle by a factor of 3.) It was found that the Bi+3 did not precipitate completely as BiPOfi, probably because of complex formation between the Cr+3 and HaPO4. Therefore, the HaPOi concentration was adjusted to 70.10M and the solution heated to 90 C. The bismuth phosphate carrying plutonium precipitated substantially completely was washed and the precipitate V6 was separated by filtration.

The waste filtrate 7 was discarded.

substances present in the original uranyl nitrate hexah'y drate solution showed approximately 95% recovery of the plutonium and a re'ductionfin fission product activity associated with"the plutonium Vby a factor of `103. As this amount of fission product activity was still in excess of that desired, the above described oxidation-reduction phase was carried through a second and third cycles to obtain a reduction in the amount of fission product activity associated with the plutonium by a factor of l0".

In the second cycle III, the bismuth phosphate precipitate 6 was dissolved in nitric acid and treated in the same manner as in the first cycle, except for a reduction by a factor of 3 in solution volumes and amounts of reagents used. Thus, the bismuth phosphate precipitate 6 was dissolved in nitric acid, the plutonium oxidized to its phosphate soluble oxidation state, and a precipitate obtained by the addition of phosphoric acid. The waste precipitate 8, containing bismuth phosphate, was separated from the solution by filtration and discarded. The filtrate 9, Icontaining the plutonium, was treated with a sulphur dioxide to reduce the plutonium. The plutonium was then precipitated, the precipitate 1l) being removed from thefsolution by ltration, The waste filtrate 11V was discarded. This completed the second cycle. Y

The third cycle IV was carried out in the same manner as the second cycle except for a further reduction by a factor of 3 in the volumes of solutions and amountsof reagentsV used. The precipitate 10, containing the plutoniurn, was dissolved and the plutonium oxidized to its phosphate soluble oxidation state. The solution was diluted to obtain'the Waste bismuth phosphate precipitate 12`which Wasremoved from the solution by filtration and discarded. The filtrate 13 was treated with sulphur dioxide to reduce the plutonium. Upon precipitation, the

- plutonium was c ontained in the bismuth phosphate precipitate t4 as a rfinal purified product, the plutonium being precipitatedwith llO mg. of Bit3 as bismuth phosphate, which was the smallest amount of Bit3 that could be used to separate the plutonium from this solution,

Vwhich was 0.36M in H3PO4. The waste filtrate 15V was discarded. Y Y

An analysis of the fractions containing plutonium in the phosphate precipitation phase and in the three oxidation-reduction cycles is given in the followingtable:

l Total absorber thickness including sample and counter window, etc. were Vgenerally maintained at w10 ing/cm2 This thickness applies particularly to precipitate fractions containing the plutonium. However-,such sample 'thicknesses could not be maintained with many of the tlltrates.

2 Counted through 2.75 grams ci lead per square centimeter.

Counted through lOgrams of lead per square centimeter.

Table X is a material balance chart showing plutonium recovery and fission product activities in various fractions as Shown in the drawing for a phosphate precipitation phase and two oxidation-reduction cycles.

In this table, the columns denoted as A refer to the fractions in terms of counts per minute `per gram of UO2(NO3)2.6H2O. The columns denoted as B refer to the radioactivity in terms o f percent of the total fraction. The totalrecovery of plutonium is indicated in the last line ofthe table designated'as P.

(le+4 may be used in an amount of .02M with a solution having an acidity of from 2N to 7N and maintained at 75 C. for from one to two hours. Where Ce+4 is lused with dichromate such as K2Cr20 the amounts used are preferably .002M to .02M of Ce+4 with .01M to .1Mjof K2Cr207. The solution is adjusted further to 2N to 10N acidity and maintained for fifteen minutes to two hours at 50 C. to 90 C. The foregoing ranges given as to amounts, acidity, time, and temperature are the preferredV limits where the particular oxidizing agents are used.

The plutonium is thus oxidized to a higher oxidation state in which it remains soluble in the presence of phosphate ions. Also present in the solution, which is between 2N and 10N acidity, are dissolved bismuth phosphate and fission products. The solution is diluted or neutralized to approximately one normal acidity to precipitate bismuth phosphate. If desired, phosphoric acid in suicient amount to bring the solution to between 0.1M to 0.8M in Ha-PO, may be added to aid in precipitating the bismuth phosphate. The bismuth phosphate will be precipitated and will carry out of solution certain radioactive fission products such as zirconium, columbium, and, possibly, cerium and some others. The plutonium, of course, remains in solution as it is in a phosphate soluble state.

The bismuth phosphate precipitate containing the phosphate insoluble fission products is removed by convenient means, such as filtration or centrifugation, and discarded. The solution containing the plutonium also contains certain iission products which are soluble in the presence of phosphate ions. To separate the plutonium from such iission products, the plutonium is converted to its phosphate insoluble form, that is, its lower oxidation state, by the use of reducing agents. Any strong reducing agent may be used, although reducing agents which have been found particularly suitable for this purpose are H2O2, ferrous nitrate or ferrous lammonium sulfate, hydrazine, SO2, or H2O2 and ferrous nitrate. The reducing agents are permitted to react for approximately one hour at a temperature of from 50 C. to 75 C. While an excess of the reducing agent may be used, such excess should not be too greatv because in certain instances an excess of the reducing agent may decrease the carrying power of bismuth phosphate with respect to plutonium.

The solution now contains plutonium in its reduced state, that is, one in which it is insoluble in the presence of phosphate ions. Additional phosphoric acid may be added to make the phosphoric acid concentration between 0.1M to 0.8M. Bit3 in the form of bismuth nitrate in acid solution is introduced in several portions to form a bismuth phosphate precipitate which carries with it the insoluble plutonium phosphate. To facilitate the precipitation of the bismuth phosphate the solution may be heated, as for one hour at approximately 75 C. The

.plutonium carried by the bismuth phosphate precipitate is thus substantially completely separated from fission products and other foreign products.

Where the oxidation-reduction separation as -above described is used in combination with the preliminary phosphate precipitation step, at the completion of one oxidation-reduction cycle there is a ydecrease in the radioactive fission products associ-ated with the plutonium by a factor of from 102 to 104. To obtain a reduction in the radioactive ssion products by a factor of more than 107, the oxidation-reduction phase may be repeated' from two to four times.

Where the plutonium is initially present in such high concentrations that no carrier need be used in the phosphate precipitation phase, the material to be treated by the oxidation-reduction phase is an insoluble plutonium phos cept that a carrier, such as bismuth phosphate,.need not'` be used, at lleast in the first oxidation-reduction separation: A

In the preliminary phosphate precipitation phase, particularly where bismuth phosphate is used as a carrier, certain foreign products such as zirconium and columbium, and possibly barium or other substances, are carried down with fthe plutonium phosphate.

To decrease the amountV of fission Vproducts carried down with the plutonium phosphate precipitate, an oxidizing agent such as Cr207= may be added to the uranyl nitrate hexahydrate solution to oxidize the plutonium and convert it to its phosphate soluble state. After the plutonium has been oxidized, the phosphate precipitation separation is carried out as described above, that is, the solution is acidied to approximately one normal acid content, the proper amount of sulphate ions is added, and bismuth precipitated with phosphoric acid. As the insoluble bismuth phosphate is precipitated, it carries with it certain foreign products, particularly zirconium and columbium, and possibly barium sulphate. The plutonium is not precipitated as it has been oxidized to its phosphate soluble state. The bismuth phosphate precipitate containing a portion of fission products carried down with it is removed and discarded. The plutonium in the filtrate (which contains substantially all the uranyl nitrate originally present) is reduced by a suitable reducing agent, such as Fe+2 ions, to convert the plutonium to its phosphate insoluble form. The solution containing the dissolved plutonium may then be treated again in accordance with the described phosphate precipitation phase which comprises introducing bismuth ions to yield a bismuth phosphate precipitate which will carry the insoluble plutonium phosphate out of solution, and thus separate the plutonium from uranium and the major portion of the tission products which form soluble phosphates. Such a'direct oxidation-reduction phase removes a relatively large portion of the phosphate-insoluble foreign products which would otherwisebe associated with the plutonium after the phosphate precipitation phase.

It is desirable when carrying out the oxidation-reduction separation in the complete process in which the preliminary bismuth phosphate precipitation separation is used in conjunction with the oxidation-reduction separationto maintain the volume of solution treated as small as possible. This may be particularly desirable where the process is carried out on a large scale. The principall reason for the large volume of solution ordinarily required after the plutonium is associated with bismuth phosphate is the insolubility of bismuth phosphate which makes necessary the use of either largeamounts or dilute nitric acid or of small amounts of concentrated nitric acid to dissolve the bismuth phosphate and plutonium phosphate. Where strong nitric acid is used, the solution must be subsequently diluted to a relatively large volume to obtain the proper acidity for treatment.

To decrease the volume of solution to be treated, the bismuth phosphate precipitate (carrying plutonium phosphate and minor amounts of foreign products which are obtained from the phosphate precipitation phase or from the oxidationreduction phase) may be converted to a form which is more soluble in nitric acid and thus may be dissolved by a relatively small volume of solvent. This may be done by digesting the bismuth phosphate precipi tate with a caustic alkali solution, such as potassium hydroxide or sodium hydroxide, thereby forming insoluble bismuth hydroxide, Bi(OH)a. The bismuth hydroxideis Washed with Water to remove potassium phosphate and is then dissolved in nitric acid. As bismuth hydroxide is much more soluble in nitric acid than is bismuth phosphate, the volume of the bismuth nitrate solution to be treated will be considerably decreased by this step. The plutonium and foreign products associated with the bismuth phosphate are acted upon by the caustic alkali solution in the same manner as the bismuth phosphate, and the subsequent treatment of the bismuth hydroxide with l Counted through 2.75 grams of load per square centimeter. 2 Counted through 10 grams of lead per square centimeter'.

Where the oxidation-reduction separation is carried out Without the preliminary phosphate precipitation separation, the presence of uranium will retard the react1ons to a certain extent. The plutonium, nevertheless, can .be separated from substantially all of the uranium and major amounts of fission products by the oxidation-reduction separation alone, if desired. Where uranyl nitrate hexahydrate is treated directly in accordance with the oxidation-reduction separation described above, it is desirable to repeat such separation procedure one or more times to still further decrease the amounts of radioactive fission products associated with the plutonium. In such case, the process may be considered as comprising an oxidation-reduction cycle in the presence of uranium and one or more further oxidation-reduction cycles in the absence of uranium.

While no particular details have heretofore been given as to the preparation of the neutron irradiated mass which serves as the source of 94 in the separation process of the present invention, among the methods which are suitable for preparing such la mass may be mentioned, by way of illustration, the bombardment of metallic uranium for about 40 days with neutrons produced by the action of 12 m. e. v. deuterons on beryllium, followed by aging for a period of about 60 days to permit substantially complete conversion of 93 to 94.

The above detailed description is given for purposes of illustration and the invention is to be limited only by the scope ofthe appended claims.

What is claimed is:

l. A method of separating plutonium from phosphate soluble foreign products yand phosphate insoluble foreign products which -comprises adding bismuth ions to a solution containing plutonium in a valent state not above 4, phosphate insoluble foreign products, and phosphate soluble foreign products, thereby precipitating as phosphates said bismuth, said plutonium, and at least a portion of said phosphate insoluble foreign products, removing said precipitate from said solution, dissolving said precipitate in an aqueous acid medium, oxidizing said plutonium to an oxidation state greater than 4 and thereafter contacting the resulting solution with phosphate ions.

2. A method of separating plutonium from substances present in neutron irradiated uranium including uranium and tission products which comprises forming a solution of neutron irradiated uranium, adding bismuth ions to said solution, maintaining said plutonium in an oxidation state not greater than 4, contacting said solution with phosphate ions thereby precipitating as phosphates said bismuth, said plutonium, and Ia portion of said fission 14 products from said solution, dissolving said precipitate in an aqueous acid medium, oxidizing said plutonium to an oxidation state greater than 4 and precipitating said fission products as insoluble phosphates from said plutonium by contacting the resulting solution with phosphate ions.

3. A method of separating plutonium from substances present in neutron irradiated uranium including uranium and fission products which comprises dissolving neutron irradiated uranium, in yan acid medium to produce a solution of between 0.50 N and 2 N acid content, adding bismuth ions and phosphate ions to said solution which contains a reducing agent to maintain plutonium in its reduced state, precipitating said bismuth, said plutonium, `and minor amounts of said fission products, as phosphates by contacting the solution with phosphate ions, removing said precipitated substances from said solution, dissolving s-aid precipitated substances in an aqueous acid medium whose acidity is greater than that of the solution from which said substances were precipitated, oxidizing said plutonium to its phosphate soluble state, diluting said solution to approximately one normal acidity, precipitating said bismuth and some of said fission products as phosphates by contacting the resulting solution with phosphate ions, and removing said precipitate from said phosphate soluble plutonium.

4. A method of separating plutonium from substances present in neutron irradiated uranium including uranium 'and fission products which comprises dissolving neutron irradiated uranium in nitric acid to form a uranyl nitrate hexahydrate solution, adjusting the acidity of said solution to between 0.50 N and 2 N acid content, adding bismuth ions and phosphate ions to said solution which contains a reducing agent to maintain plutonium in its reduced state, precipitating said bismuth, said plutonium, and minor amounts of said fission products as phosphates by contacting said solution with phosphate ions, removing said precipitated substances from said solution, dissolving said precipitated substances in an acid solution, oxidizing said plutonium to its phosphate soluble state, diluting said solution to approximately one normal acidity, precipitating said bismuth and some of said ssion products as phosphates, by contacting said solution with phosphate ions, removing said precipitate from said phosphat soluble plutonium, reducing said plutonium to its phosphate insoluble state, 'adding bismuth ions to said solution, and precipitating said bismuth and-said plutonium as phosphates from said ssion products which are in solution.

5. In a process for the separation of plutonium from a solution containing that element together with phosphate insoluble foreign products, the s-tep which comprises contacting 'a solution containing plutonium in a valent state greater than 4 and said foreign products with phosphate ions in the presence of bismuth phosphate.

References Cited in the le of this patent UNITED STATES PATENTS Fermi et al. July 2, 1940 OTHER REFERENCES

Claims

5. IN A PROCESS FOR THE SEPARATION OF PLUTONIUM FROM A SOLUTION CONTAINING THAT ELEMENT TOGETHER WITH PHOSPHATE INSOLUBLE FOREIGN PRODUCTS, THE STEP WHICH COMPRISES CONTACTING A SOLUTION CONTAINING PLUTONIUM IN A VALENT STATE GREATER THAN 4 AND SAID FOREIGH PRODUCTS WITH PHOSPHATE IONS IN THE PRESENCE OF BISMUTH PHOSPHATE.