US2877090A - Process using bismuth phosphate as a carrier precipitate for fission products and plutonium values - Google Patents

Description

State PROCESS USING BISMUTH PHOSPHATE AS A CAR- RIER PRECIPITATE FOR FISSION PRODUCTS AND PLUTGNIUM VALUES No Drawing. Application July 16, 1945 Serial No. 605,447

8 Claims. (Cl. 23-14.5)

This invention relates to a procedure for processing materials containing the element of atomic number 94, known as plutonium, for separating the plutonium from extraneous matter such as substance of the kind present in neutron irradiated uranium exemplified by uranium and especially fission products, and the like radioactive contaminants. More particularly, this invention concerns improvements in certain phases of a separatory and concentration procedure involving the use of a by-product bismuth phosphate carrier wherein certain supplemental additions are made.

As described herein, the isotope of element 94 having a mass of 239 is referred to as 94 and is also called plutonium, symbol Pu. In addition, the isotope of element 93 having a mass of 239 is referred to as 93. 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.

Elements 93 and 94 may be obtained from uranium by various processes which do not form a part of the present invention including irradiation of uranium with neutrons from any suitable neutron source, but preferably the neutrons used are obtained from a chain reaction of neutrons with uranium.

Naturally occurring uranium contains a major portion of U a minor portion of U and small amounts of other substances such as UX and UX When a mass of such uranium is subjected to neutron irradiation, particularly with neutrons of resonance or thermal energies, U' by capture of a neutron becomes U which has a certain half life and by beta decay becomes 93 This 93 has a certain half life and by beta decay becomes 94 Thus, neutron irradiated uranium may contain both 93 and 94 but by storing such irradiated uranium for a suitable period of time, the 93 is converted almost entirely to 94 In'addition to the above-mentioned reaction, the reaction of neutrons with fissionable nuclei such as the nucleus of U results in the production of a large num ber of radioactive fission products. As it may be undesirable to produce a large concentration of these fission products which must, in view of their high radioactivity, be. separated from the 94 and further as the weight of radioactive fission products present in neutron irradiated uranium is proportional to the amounts of 93 and 94 formed therein, it is preferable to discontinue the irradiation of the uranium by neutrons when the combined amount of 93 and 94 reaches a predetermined percentage by weight of the uranium mass, at which concentration of these substances, the, concentration of fission,

elements which must be removed is approximately the same percentage. r

. A number of processes have already been. proposed for accomplishing the separation and: concentration of Pu. One of these. processes is generically known as the bismuth phosphate type process. These processes are the 2,877,090 Patented Mar. 10, 1959 a CC invention of others, and details of the processes are described in copending applications as for example appli cation Ser. No. 519,714, now U. S. Patent No. 2,785,951, issued March 19, 1957, to be referred to hereinafter which gives details relative to the aforementioned bismuth phosphate process. Consequently, all of the details of operation of the aforementioned processes are not described herein.

As set forth in said application Ser. No. 519,714, it has been discovered that plutonium has more than one oxidation state, including a lower oxidation state or states referred to herein as Pu" in which the element is characterized by forming insoluble phosphates and other in soluble compounds and a higher oxidation state or states referred to as Pu in which the element forms soluble phosphates and other soluble compounds. As further set forth in Ser. No. 519,714, said lower oxidation state includes Pu, and said higher oxidation state includes plutonium in the hexavalent state. It should therefore be understood that, as used herein, Pu includes plutonium in the tetravalent state, and Pu includes plutonium in the hexavalent state.

That is, by having the metal as Pu a product precipitation or extraction may be accomplished in which a carrier precipitate brings down the Pu" leaving behind in solution a substantial portion of fission product and other sources of contamination. I However, some of the activity may be carried along with the precipitate or otherwise may be occluded with the Pu Certain contaminants such as barium, columbium, Zirconium, and lanthanum activities may be quite difficult to separate from Pu.

The carrier precipitate containing Pu after suitable dissolution, is oxidized so that Pu is obtained which remains in solution. A by-product carrier precipitate, such as a bismuth phosphate by-product precipitate, under the oxidizing conditions carries down fission products leaving the Pu in solution. However, in this step some contaminants may not separate out or otherwise contamination remains so that the solution remaining which contains the Pu may still be contaminated although in some smaller amounts. These various cycles may be repeated several times until sufficient decontamination is accomplished. It should be kept in mind that since the Pu is in an environment of radioactivity which may give many millions of counts a minute per milligram of fission product present, that the rapid, cleancut separation of the Pu presents a considerable problem.

While prior processes have been carried out satisfactorily and give Pu recovery, in certain phases some difiiculties have been, or may from time to time be experiencedr For example, it has been noted that when the content of fission product runs high, the losses of P11 in some of the steps, such as in the by-product precipitation with a bismuth phosphate carrier, may be greater. Without wishing to be bound by any explanation of mechanism or theory of the reason for this loss, it is possible that the various fission components adsorb or occlude Pu. Consequently, when the lay-product precipitate is formed which carries down the fission products, this Pu may also be carried down either due to the aforementioned attachment or due to the fact that it is inhibited from oxidation which would place it in a soluble state.

I have found that the aforementioned separatory and recovery processes illustrated by the bismuth phosphate type of process, and which are referred to as existing, standard, or conventional processes may have included therein changes and additions for improving such processes. That is, in the instance of by product precipitation, certain operations in accordance with the. present,

invention maybe carried out which permit better oxidae tion and aid in reducing Pu loss in the lay-product precipitation.

The meaning of the termsproduct and by-product precipitate, bismuth phosphate type of process, oxidation, redissolving, direct and reverse strikes, product, and similar terms are apparent to some extent from the preceding description and will be further apparent as the description proceeds.

This invention has for one object to provide improvements in methods for the separation and recovery of plutonium.

Another object is to provide improvements in methods of forming the bismuth phosphate type of precipitate.

Still another and particular object is to improve byproduct precipitation steps.

Another object is to provide procedure for minimizing losses of plutonium in recovery processes where either or both the plutonium and fission concentrations may be relatively high as compared with the contents heretofore encountered.

A still further object is to provide steps which lend themselves to combination with steps already known or practiced.

Another object is to provide a type of process which may employ materials used in existing processes, but with more eflicient action and which may be carried out in existing equipment and are generally similar to existing practice, as respects some of the operations.

Other objects will appear hereinafter.

In order to further illustrate my invention, and some of the points where the invention has particular value, an illustrative process is described below. This illustrative process is also described for defining in further detail features such as bismuth phosphate product precipitation, bismuth phosphate by-product precipitation and the like thereby making it unnecessary in the examples which later follow of repeating'such description in detail.

Neutron irradiated uranium is dissolved in a suitable quantity of solvent, such as 60-70% nitric acid, giving a uranyl nitrate solution containing plutonium. The solution is subjected to treatment with an agent such as sodium nitrite, formic acid, H oxalic acid or the like in excess for a period of about one hour at a temperature from 50 C. to 75 C. whereby any of the Pu which may have been oxidized to the Pu state in the solution step is reduced to the Pu state, hydrazine is eliminated or other action takes place. The concentration of the solution determined on the basis of uranyl nitrate hexahydrate is adjusted to 20% and reagents such as H 80 and fluosilicic acid added.

To the solution is now added a source of bismuth ion to provide a concentration of bismuth ion equivalent to grams of Bi ion in four liters of the uranyl nitrate solution having a concentration of 20% on the basis of uranyl nitrate hexahydrate;' phosphoric acid is also incorporated to make the solution between .3.8 M therein, and a precipitate comprising BiPO which carries the Pu" comes down and is separated from the solution by filtration or centrifugation. This illustrates reduction and an extraction or product precipitation.

The BlPO4 precipitate carrying the Pu" is dissolved in 10 N HN0 The acidity of the solution is reduced to 6 N HNO by dilution and the solution made .1 M in K Cr O Sodium bismuthate and various other oxidizing agents may also be used. On heating the solution at 95 C. for 2.25 hours, the plutonium is oxidized to the Pu state. The solution is then diluted to 1 N acidity by addition of water and H P0 added to provide a suitable phosphate ion concentration for causing the formation of a BiPO, lay-product precipitate. The solution is heated to about 90 C. whereupon BiPO precipitates carrying fission products and preferably none of the Pu The precipitate may be removed by filtration or eentrifugation and discarded. This illustrates oxidation and a by-product precipitation. At this point, in forming a bismuth phosphate by-product precipitate, the procedures such as the additio'fi of a fluoride reagent of the present invention, the incorporation of the Bi reagent in an acid medium of high normality, and the incorporation of the Bi in several additions may be utilized.

In conjunction with by-product precipitation, a procedure termed scavenging may sometimes also be carried out. However, it is unnecessary to go into detail on this feature as the present invention is applicable in all such existing processes whether they are exactly as described above or vary somewhat therefrom.

If repetition of the cycle is contemplated for further decontamination, the Pu in the filtrate is reduced by passing in a rapid stream of S0 gas for five minutes and allowing the solution to stand for approximately one hour, or by providing a source of oxalic acid, Fe+ or similar reducing agents, and the cycle is suitably repeated.

As indicated, certain of the operations for obtaining the bismuth phosphate product and by-product precipitates or other carrier precipitates are not a limitation on the present invention. The product precipitation under (r) conditions, as indicated, may be accomplished by using any of a number of reducing agents of which sulfur dioxide, hydrogen peroxide, oxalic acid, sodium nitrite, ferrous iron, and the like are mentioned for illustration. Likewise, the by-product precipitation under (0) conditions, as indicated, may be accomplished by a number ofoxidizing agents for obtaining the Pu state; Dichromates, bismuthates, permanganates and the like are mentioned for illustration.

' It will be further noted in forming the bismuth phosphate carriers that this may be accomplished by either adding the source of phosphate ions to a solution already containing the bismuth ions or by adding the source of bismuth ions to the liquid containing phosphate ions. In the art, it has been customary to define such procedures by the terms, direct strike and reverse strike. In the direct strike a bismuth phosphate is precipitated by adding the source of phosphate ions to the plutonium containing liquid which also contains bismuth ions as well as sulfuric acid and the other components referred to above. In the reverse strike the bismuth reagent is added to the plutonium containing liquid which also contains the phosphate ions and other components.

Also it will be noted that the solutions containing Pu which may be treated by my invention may be the same type of solutions as heretofore treated, or may be solutions in which the concentrations of components are much higher. As apparent from the foregoing, one common type of solution containing Pu subject to separation' and recovery procedures comprises a nitric acid containing liquid having a content of Pu therein. The nitric acid solution also contains other materials such as radioactive substances.

The description is now directed, in particular, to some of the features of the present invention. As has been indicated, this invention relates to improving the emciency of bismuth phosphate processes. As described above, bismuth phosphate carrier precipitates may be used both for decontamination, when the materials to be treated are under oxidation conditions, and for the carrying of Pu when the materials are under reducing conditions. However, in carrying out the aforementioned steps, some losses of product and other difiiculties have been encountered.

It has been found, in accordance with the present invention, that small additions of fluorides such as sodium fluoride in conjunction with the formation of a bismuth phosphate by-product precipitate may reduce any Pu loss. This, primarily, may be due to the solvent action of the fluoride addition dissolving off occluded Pu permitting its oxidation and/or preventing it being carried on the bismuth phosphate by-product precipitate. I

"The featuresof the present invention generallydescribed above will be further apparent from a consideration of the following specific examples.

EXAMPLE I In accordance with this: example the Pu containing solution was treated by a bismuth phosphate by-product carrier precipitate formation excepting that in certain of the runs a small content of sodium fluoride was incorporated. In some of the runs the content of fission products was usual, and in others there was at least three times the usual quantity of fission activity. Results of these several runs under this example are shown in the following table:

The extraction precipitate referred to in the table comprised a bismuth phosphate carrier precipitate formed in a solution containing Pu This precipitate containing Pu was dissolved in nitric acid, diluted and oxidized with oxidizing agents in accordance with existing practice as described earlier herein. A bismuth phosphate by-product precipitate was accomplished by incorporating bismuth ions and phosphate ions. The differences exemplified in the runs of the table were that in some of the runs a small content of sodium fluoride was present during the oxidation step and during the by-product precipitate formation. As may be observed from the table, the loss of Pu in the by-product precipitate when sodium fluoride was present, was substantially smaller than in the parallel runs where no fluoride was present.

EXAMPLE H In accordance with this example a master extraction (1800 milliliters) was carried out, and the solution of the extraction precipitate, namely the solution of a bismuth phosphate precipitate carrying Pu (formed under reducing conditions), was divided into two equal portions. The first portion was processed through an oxidation and a by-product precipitation without the addition of any NaF while NaF was added to the second portion.

That is, NaF was added to the extent that the oxidation was carried out in the presence of .03 N NaF. In the runs of this example the bismuth reagent was added as a 2 N solution. The concentration of phosphoric acid was .8 M. The manner of addition and oxidation and all of the other details of operation were in keeping with usual practice and the same as described earlier herein.

By-product loss of Pu in the instance where no sodium fluoride was added was twice that in the instance where the fluoride was added. In this particular example the loss of Pu was .69 percent without the fluoride addition and approximately .35 percent with the fluoride addition. Since the loss was only .69, the efficiency of the process at the onset was relatively good. However, under such conditions the addition of the fluoride in accordance with this invention reduced this loss.

Therefore, from the preceding description it may be seen that I have provided when by-product losses are large, and in some cases optionally when the results indicate that low Pu losses are already obtained, a process for improving the by-product bismuth phosphate carrier formation which comprises the incorporation of a small amount of sodiumrfluoride, hydrogen fluoride or other simple, soluble fluoride.

In the above description at some points certain specific concentrations of reagents have been given for the pure. pose of illustration. For example, it has been indicated that .03 N, NaF may be used. The choice of thispar ticular value was based to some extent on the fission product content of the materials being processed and other related factors. Higher amounts of NaF, such for example up to .1 N, may be employed. Similar remarks apply in the instance that HF is used in place of, or as a supplement to NaF. In general, too large a content of these fluoride reagents would not be employed because of possible corrosion. Hence, for the definition of addition of fluorides for the purposes described herein, it may be merely indicated that a small content of fluoride is employed. Similar remarks apply to the specification of some of the other reagents. For example, although it is indicated that dissolution may be accomplished in 60% to 70% nitric acid, other lower concentrations of solvent may be employed and a longer period required for dissolution. In gencral, the solvents used for dissolution and related features are described in the copending applications referred to, and such steps may also be in accordance with other practice.

It is to be understood that all matters contained in the above description and examples are illustrative only and do not limit the scope of this invention.

I claim:

1. In a process for recovering and segregating plutonium values from an aqueous solution containing said values together with fission product values which comprises oxidizing plutonium to the hexavalent oxidation state and then providing a suflicient concentration of bismuth and phosphate ions in said solution to precipitate bismuth phosphate, thereby carrying down fission product values with said precipitate, and recovering plutonium values from the remaining solution, the improvement which comprises providing a low concentration of fluoride ions in said solution during the step of precipitating the bismuth phosphate.

2. The process of claim 1 wherein the concentration of fluoride ions is no greater than approximately 0.1 normal.

3. The process of claim 1 wherein the fluoride ions are provided by adding an alkali fluoride to the solution.

4. The process of claim 1 wherein the fluoride ions are provided by adding sodium fluoride to the solution.

5. The process of claim 1 wherein the fluoride ions are provided by adding a source of fluoride ions to the solution after the step of oxidizing the plutonium to the hexavalent state.

6. The process of claim 1 wherein fluoride ions in a concentration of approximately 0.03 normal is provided by adding sodium fluoride to the solution.

7. In a process for recovering and segregating plutonium values from an aqueous solution containing said values together with fission product values which comprises oxidizing plutonium to the hexavalent oxidation state and then providing a suflicient concentration of bismuth and phosphate ions in said solution to precipitate bismuth phosphate, thereby carrying down fission product values with said precipitate, and recovering plutonium values from the remaining solution, the improvement which comprises providing bismuth ions in said solution, then providing phosphate ions in the bismuth-containing solution, and subsequent to the step of oxidizing the plutonium to the hexavalent oxidation state providing a low concentration of fluoride ions in said solution by adding sodium fluoride to the solution.

8. In a process for recovering and segregating plutonium values from an aqueous solution containing said values together with fission product values which comprises oxidizing plutonium to the hexavalent oxidation state and then providing a suflicient concentration of bismuth and phosphate ions in said solution to precipitate bismuth phosphate, thereby carrying downfission product values with said precipitate, and recovering plutonium values from the remaining. solution, the improve ment which comprises providing phosphate ions in said solution, then providing bismuth ions in the phosphatecontaining solution, 'and subsequent to the step of oxidizing the plutonium to the hexavalent oxidation state i s1. providing a low eoncentrationof, fluoride ions in said solution. by adding sodium fluoride to the solution.

References Citedin the tile of this patent Seaborg: The Chemical and Radioactive Properties of the Heavy Elements, Chemical and Eng. News, vol. 23, pp. 2190-2193 (1945).

Claims (1)

1. IN A PROCESS FOR RECOVERING AND SEGREGATING PLUTONIUM VALUES FROM AN AQUEOUS SOLUTION CONTAINING SAID VALUES TOGETHER WITH FISSION PRODUCT VALUES WHICH COMPRISES OXIDIZING PLUTONIUM TO THE HEXAVALENT OXIDATION STATE AND THEN PROVIDING A SUFFICIENT CONCENTRATION OF BISMUTH AND PHOSPHATE IONS IN SAID SOLUTION TO PRECIPITATE BISMUTH PHOSPHATE, THEREBY CARRYING DOWN FISSION PRODUCT VALUES WITH SAID PRECIPITATE, AND RECOVERING PLUTONIUM VALUES FROM THE REMAINING SOLUTION, THE IMPROVEMENT WHICH COMPRISES PROVIDING A LOW CONCENTRATION OF FLUORIDE IONS IN SAID SOLUTION DURING THE STEP OF PRECIPITATING THE BISMUTH PHOSPHATE.