US2906597A - Reduction in pu recovery processes - Google Patents

Description

United States Patent 2,906,597 REDUCTlON IN Pu nncov'nnr PROCESSES David M. Ritter, Seattle, and Robert 11's. maul, Richland, Wash assignors to the United States of America as represented by the United States Atoniic Energy Commission No Drawing. Application December 21, 1945 Serial N0. 636,494

2 Claims. (Cl. 2 314.5)

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 substances of the kind present in neutron irradiated uranium as exemplified by uranium and especially fission products, and the like radioactive contaminants. More particularly, invention concerns a separatory and concentration procedure employing a combination of carriers and involving the use of an oxalate reagent wherein improved reduction and carrying are obtained.

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 half life of about 23 minutes and by beta decay becomes 93 The 93 has a half life of about 2.3 days and by beta decay becomes 94 Thus, neutron irradiated uranium contains 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 number of radioactive fission products. As it is undesirable to produce a large concentration of these fission products which must, in View of their high radio-activity, 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 is equal to approximately 0.02 percent by Weight of the uranium mass. At this concentration of these substances, the concentration of fission elements which must be removed is approximately the same percentage.

, A number of processes have already been proposed for accomplishing the separation and concentration of Pu. Certain of these processes are generically known as the bismuth phosphate type process and the wet fluoride" type of process. These processes are the invention of others and the details of the processes are. described in copending applications, as for example, application Ser. No. 519,714 of Thompson and Seabo'rg, now.U.S.

Patent 2,785,951, to be referred to hereinafterfwhich types of treatment are referred to herein for. background although the present invention is concerned particularly with intermediate phases of theprocedureswhich in some instances may be considered as associated with the former and other instances associated with the latter. type process. That is, for example, the present invention concerns-the treatment of a solution containing Pu in a higher valence state as exemplified by a nitric acid solution containing Pu from a lanthanum fluoride or bismuth phosphate precipitation, or other similar solutions containing.Pu In processes of the aforementioned type, either combination processes where several different types of carriers are used or processes in which the same carrier is used at various points in the process it is necessary to change the Pu to PM for reasons to be set forth in greater detail hereinafter. It has already been proposed in existing processes to use hydrogen peroxide, various ferrous compounds or the like for this purpose. While such reagents function to convert the Pu to Pu it has been found that by the reagents of the present invention, as exemplified by oxalic acid, the conversion of Pu to Pu is better accomplished and other steps of the process are improved in that at least as higher better recovery of Pu may be obtained while in certain instances employing smaller concentrations of some of the reagents for forming the carrier precipitates in the subsequent steps.

That is, by the present invention carried out, in conjunction With steps such as the MP0,, and LaF carrying which have been previouslyworked out, but altered in accordance with the present invention in the reduction phase the advantage of improved reaction may be obtained as will be described.

The meaning of the terms bismuth phosphate type of process, Wet fluoride type of process, reduction, Pu Pu product precipitate, and similar terms will be 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 a method of separating plutonium characterized by certain steps in which different reagents and procedure than have heretofore been used are employed.

Still another object is to provide an improved procedure for converting Pu to Pu Still another object is to provide a procedure for treating solutions containing Pu A still further object is 'to provide a process for the reduction of solutions carrying Pu resulting from LaF or BiPO4 by-pr'oduct precipitation, which may be used in place of or in conjunction with usual reduction.

Still another object is to provide a new and improved reduction operation which lends itself to the use of and to coupling with steps already known or practiced for the separation and recovery of Pu.

Another object is to provide a process for converting Pu to Pu which may employ someof the materials used in existing processesand which may be carried out in existing equipment without change, or with little equipment change.

' reduction treatment of solutions containing. Pu in relatively small or largeamounts.

Still another ebjea' is to provide'a process for the either Other objects will appear hereinafter. :VAS' already indicated, it hasbeen found that Pu in admixture with various extraneous material may be separated and'concentrated by the use of a seriesof steps involving oxidation and reduction and'the utilization of tion of these features, not only are the advantages pre-' viously obtained in the processes stillobtainable, but

further advantages are obtained suchas better carrying of Pu and lesscorrosion.

Illustrations of the types of carrier, precipitates which maybe formed are described in'U.S. Pate'nt2,785,951 of Thompson and Seaborg, aforementioned, and reference'is made to that patent for further disclosure, de-

tails thereof being omitted from the present disclosure except where necessary to anunde'rstanding of the present invention; As set forth' infsaid applica'tion, it has been discovered that plutonium has more than one oxidation state, including-a lower oxidation state or states herein referred to as Pu" in which the element is characterized, for example, by forming insoluble phosphates and fluorides and a higher oxidation state or states'referred to as Pu in which the element forms soluble phosphates and fluorides. j When a' carrier precipitate is formed .in a solution containing Pu ,such precipitate is referred to as a byproduct precipitate since, it carries down fissionprodnets and other extraneous material. .When the .carrier precipitate is formed, in a solution containing Pu this precipitate is referred to as a product precipitate "since theplutonium Pu" is a component primarily carried. The precipitation and other steps in the processiof the present invention will be carried out' much the same as, existing procedures. That is, irradiated material from which Pu is to be recovered is treated by an existing or standard process for eliminating ,at least a part of the extraneous matter. At some point in'the proc essia solution is obtained containing Pu from which a by-product precipitate has been separated. This solution containing Pu is then treated in accordance r .4 I r ent invention. From the resulting solution containing P11 a lanthanum fluoride product precipitate formed with the usual or smaller HF additions carries the Pu substantially completely. The preceding general description may be enlarged upon in various respects.

thecasings may be removed in any, suitable manner. That is, for example, dissolve the aluminum casings from the metalwhich has undergone ,a .chain reaction involving fission. 'Then dissolve the metal slugs from which the casings have been removed in the preceding step. In other words any steps for placing the materials in a better conditionfor handling and processing maybe applied. I V

The solution resulting from the preceding steps may be reduced by any'of the several reducing procedures 7 described such as by incorporating a peroxide, or ferrous compounds or by the present inventions A bismuth phosphate precipitation procedure. is then carried out on the solution in the (r) state.. 1 I

The product precipitate obtained is dissolved and the resultant solution oxidized and otherwise treated in accordance with the usual bismuth phosphate byr product precipitation procedure under (0) conditions to gen peroxide or ferrous compoundshave heretofore been with the present invention .witha source of oxalate ion 7 for converting the Pu to Pum. Then further product precipitation treatments are applied'mu ch. the same as in existing procedure excepting that smaller quantities of reagents may be used in forming the precipitates. That is, for example, if, a lanthanum fluorideproduct precipitate is formedafter the treatment of the present invention, it maybe satisfactorily formed with 1a lower concentration of' HF or other source of fluoride ion than hithertoconsiderednecessary.

employed. I

In accordance with our improvement such solutions containing Pu are reduced with oxalic acid. For example, a 0.05 to .125 Msolution of oxalic acid added to the solution to be reduced and the reaction maintained for approximately one hour at 75 C. gives satisfactory reduction. The strength of the oxalic acid may vary, for

example, from about .022 to .15 M oxalic acid, a particularly useful range being between about .04 and .05 M'.

After the reduction treatment, a product precipitation by means of lanthanum fluoride or other desired treat ment applicable to' a solution containing Pu may be applied. It has been found that by the use of the oxalic acid recoveries of Pu are better. Also, in general, lower I concentrations of HF or other reagent are required in the lanthanum fluoride formation which reduces corrosion.

Example I For the purpose of control and comparison, this first I example is in accordance with existing procedure. That is, this example shows the use of peroxide, one of the reagents heretofore used for reduction, at a step in a i 7 process where it is proposed that the oxalate reducing me- ""A further general understanding of the" application of our process in the field of separation and recoverybf Pu may be had from the following; Asuitablesource of materials containing Pu is obtained. This source may be the same as described in US. Patent 2,785,951

aforementioned and is exemplified by a nitric acid .so-

V by extraction and decontamination by the bismuthphos phate method. The solution may be oxidized and sub-' jected to a standard fluoride by-product precipitation step foneliminating fission products' and other undesired components. The oxidized solution containing Pu may then be treated in accordance with the pres dium of the present invention be utilized. Two liters of oxidized product solution from which a BiPO by-product precipitate had' been precipitated were precipitate was washed into a singlelusteroid and studied.-

The waste liquor was analyzed and the yield figured ,on this basis. a

The exact mannerof studying or analyzing the solu tionsisnot a limitation on the present invention and any If the irradiated material has been enclosed in casings, 1

procedure applicable to radioactive substances may be employed. For example, by taking the alpha count on.

Approximately 2,000 m1. of oxidized productsolution from which a MP; by-produc't precipitate had been precipitated were used. This solution contained a total of 14,750 c./m. of Pu. V

The solution was reduced by the addition of 31.5 grams of solid H C O .2H O. This made the solution 0.125 M therein. The solution was heated to 75 C. and held for 1 hour followed by cooling to room temperature. 225 mg./l. of Lat++ was then added followed by the dropwise addition of HF over a 30 minute period to make the solution 0.2 N in HF. 'This was then digested for 60 minutes, centrifuged in 100 ml. lusteroids and the precipitate gathered, washed and studied as above. The mother liquor was analyzed and the yield figure based on this analysis. In this example, even though the conditions of .2 N in HF were used, the yield was 99.24%.

Example IH Another run, in accordance with the present invention, was carried out on 180 ml. of a 10 N, 2 cycle BiPO solution as a starting material. This solution was made 5 N in HNO and oxidized for one hour at 50 C. with 0.01 M NaBiO The BiPO was precipitated by diluting to 1 N HNO and the resultant precipitate was digested for /2 hour, filtered, and washed with 100 m1. of 1 N I-INO 0.1 M H P0 The volume was now 1900 ml.

Reduction was accomplished by the use of H C O of 0.125 M at 75 C. for one hour. The solution was then cooled and 225 mg./l. of La+++ added. The precipitation was accomplished by the addition of HF slowly over a period of A2 hour to make the solution .2 N therein. The precipitate was then digested for one hour, centrifuged in 100 m1. lusteroids, the precipitate aggregated and studied. The yield was based on the loss in the final mother liquor as in the preceding examples, and was found to be greater than 98%.

Since from the several examples there was evidence that H C O reduced advantageously further runs were carried out to establish conditions for its use and to evaluate subsequent product carrying. These further runs showed that Pu is satisfactorily reduced by 0.05-0.125 M oxalic acid (1 hour at 75 C.) and that no diflieulty is experienced in carrying with LaF in 0.2-0.5 N HF after its use.

In two runs 0.125 M H C O was used and the reduced product was carried from 0.2 N HF by co-formed LaF Since the precipitate was intended for metathesis studies, the efficiency of reduction and carrying was based on alpha analysis of the reduced waste liquor. In two subsequent runs, reduction was carried out with 0.05 M H C O and efiiciency was determined by analysis of the reduced product solution as well as analysis of the product precipitate and reduced waste liquor. Lanthanurn fluoride was co-formed in 0.2 N HF and in 0.5 N HF.

The results of these various runs indicated that substantially complete reduction in an HF-free system can be obtained by means of H C O over the preferred range of 0.05 M to 0.125 M and that subsequent co-formed precipitation of LaF affords substantially complete recovery of Pu at the lower HF concentrations of 0.2 N to 0.5 N.

For the preliminary runs the starting solutionwas a 10 N HNO solution of BiPO -product of a planttype. A BiPO by-product'precipitation was made and the oxidized product solution was reduced with B20 04, using a l hour digestion at 75 C. Concentrations during reduction were 0.1 M H PO l M HNO 0.125 M 11 C 0 At room temperature, 225 mg. .per liter of Lef was added to the reduced solution, concentrated HF was added slowly over 30 minutes to obtain a con.- centration of 0.2 N HF, and the precipitate was separated after 60 minutes of agitation. The results, based on alpha analyses of the Waste liquors, are summarized in the following table:

Total starting count (c./rn.) 14, 5 40 11, 330 Reduced count in waste (c./m.) 10 ,l 1

Total count in waste (c./m.) (aite r Percent reduction Percent carrying A second product solution in 10 N HNO (approximate alpha count 790 c./ml.) was used'for the subser quent runs. After a standard BiPO by-product precipitation, the oxidized product solution was reduced with H C O in 2512 stainless steel, using a l-hour digestion at 75 C. Concentrations during reduction were 0.1 M H PO 1 M HNO and 0.05 M H C O The efficiency of the reduction is indicated by the data given in the following table:

Total starting 'count (c./m./ml.) 76.8 Reduced count in product solution (c./m./rnl.) 75.0 Percent reduction 97.7

N H 0-6 N .HF

Total starting count (elm. ml.) 76. 8 76. 8 Precipitate analysis (c./m. ml.) 7.5. 8 7 4.3 Waste liquor analysis (c./m./ml 0.1 0.1 Percent recovery 98. 7 96. 7 Percent material balance 98. 8 96. 8

By a co-formed precipitate is meant the formation of :the precipitate in accordance with the procedure given in certain of the examples. That is, a co-formed precipitate is produced by an operation described as a hydrogen fluoride strike. A source of lanthanum ions is first incorporated in the solution in which the carrier is to'be formed. Thereafter, the source of fluoride ions, such as the addition of hydrogen fluoride, is made to cause the formation of the lanthanum fluoride precipitate. While certain procedure has been described in the preceding examples, the exact manner of forming the precipitates is not a limitation on our invention.

The present invention relative to the use of oxalate reduction may be used when the carrier precipitates are formed by other procedures for example in conjunction with the formation of a preformed precipitate. A preformed precipitate is produced by incorporating a source of, for example, fluoride ions such as excess hydrogen fluoride into the solution containing Pu in which the carrier precipitate is to be formed. Thereafter, a source of lanthanum ions is added to the solution. words, a preformed precipitate, a conventional type of precipitate formation in this art is produced by procedure described as a lanthanum strike.

In other such reagent is useful from thestandpoint that the oxalate ion is'incor porated and at the same time only hydrogen ions 'incidentally'introduced into the solution when the oxalic acid ionizes, .various other sources of oxalate, ons

may be used. Such other sources are exemplified by sodium and potassium" oxalates as the incidental intro- .duction "of sodium ori potassium io'ns is not detrimental to the process. The mechanism ofnthereaction has been described herein as reduction since as indicated the valence of the plutonium is changed from a higher to a lower valence. It may be that the oxalate ion aids the process by some other action such as exerting an influence' on the formation of complexes or otherwise functions to improve the carrying of the plutonium. Inasmuch as the reactions on the present materials may be complicated and'becauseof the'small'quantities involved as well as the presence of radioactive materials, the complete mechanis'm'of carryingjmay'in some respects be obscure, and it is not desired to be limited to any theory of operation. For the purpose of description, the treatment herein described as introducing oxalate ions into the process at certain phases thereof will generally be referred to as oxalate reduction. Such terminology is intended to embrace the addition of oxalic acid, sodium oxalate, or various other sources of the oxalate ion or mixtures of such reagents, V n

It has beenshown from the foregoing that in the processes for the recovery and separation of plutonium based on alternate oxidation and reduction stages using a combination of carriers such as bismuth phosphate and lanthanumv fluoride, the processes may be'improved byin terposing the treatment of the present invention comprising oxalate reduction. Not only doesthe oxalate reduction of the present invention accomplish change of valence state of the plutonium from a higher valence state to a lower valence state, but as shown in the examples, it enables the production of satisfactory carrying' by the use of lower concentrations 'of reagents as exemplified by the use of HF between ,2 N and .5 N. Heretofore, concentrations of HF have usually been higher than .5 N and in the present invention such higher contents may also be used if desired but are not required for good results. As indicated in the present examples,

the oxalate reduction is preferably carried out at around 75 C., however, various other'temperatures maytbe'used within the range of slightly above room temperature. to 100 C. Ithas been found that the utilization of the herein-described invention does; not hamper the subsequent treatment of the carrier precipitates by existing tates which a're formed 'in accordance with the. present nvention using only .2 normalHF may be subjected to methods heretofore used. It is therefore'appar'ent that practice. For example, the lanthanum fluoridev precipias described.

'rier repeatedly or preferably a combination" of carriers Accordingto the best evidence available, the oxidation state oflplutonium secured insolution by the action of the oxidizing agents referred to hereinand-in eitedcopending application Ser. No. 519,714, nevus. Patent No. 2,785,951, 'is greater than four, and the oxidation state secured ,in solution by' the, action of the reducing agents referred to herein and therein is no greater'than four. It should thereforebe understood that, as'u'se'd herein, .Pu "'and Pu in the (0) condition (or state) ar e'b'oth synonymouslwith tplutoniu'm in an oxidation state greaterfthan'four? and PuW and Pu in the (r) condition (or'st ate) are both synonymous with P111- tonium in an oxidation state no greater than four? It is vto'be understood that all; the matter contained in the above description and examples shall be interpreted as illustrative and not limitative of the'scope of this invention;

- 1. In processes :for the separation and recovery of plutonium values from an aqueous solution containing the same, comprising carrier precipitation of said plutonium values'from said solution by means of precipitation therein of bismuth phosphate while maintaining the plutonium f solution to a valence state no greater than four by means.

of incorporatingoxalate ions therein to a concentration 'within the approximate range of 0.022 to 0.15 molar, prior to said precipitation of lanthanum fluoride therein.

2; 'I he process of claim 1 in which the oxalate ions Iare incorporated in the form of oxalic acid.

Referencfes Cited in the file of this patent 7 UNITED STATES PATENTS 1,526,943 f T Thews Feb/l7, 1925 50 r 2,776,185 1 1 We'rner et a1. Jan. 1-, 7

, OTHER, REFERENCES,

. Seaborg: The Chemical andRadibactive Properties of the Heavy 'ElementsfChemical & Engineering News,

vol. 23, No. 23, pp. 2190-2193 (Dem 10, 1945).

Seaborget aL: National Nuclear Energy Series, vol.

IV-14A,' page 575, McGraw-Hill Book Co., Inc., N.Y.

Claims (1)

1. IN PROCESS FOR THE SEPARATION AND RECOVERY OF PLUTONIUM VALUES FROM AN AQUEOUS SOLUTION CONTAINING THE SAME, COMPRISING CARRIER PRECIPATION OF SAID PLUTONIUM VALUES FROM SAID SOLUTION BY MEANS OF PRECIPATION THEREIN OF BISMUTH POSPHATE WHILE MAINTAINING THE PLUTONIUM IN A VALENCE STATE NOT ABOVE FOUR, DERIVATION FROM THE RESULTING CARRIER PRECIPATE OF AN AQUEOUS ACIDIC SOLUTION CONTAINING SAID PLUTONIUM VALUES IN A VALENCE STATE GREATER THAN FOUR, AND THEREAFER CARRIER PRECIPATION OF SAID PLUTONIUM VALUES FROM SAID ACIDIC SOLUTION BY MEANS OF PRECIPATION THEREIN OF LANTHANUM FLUORIDE WHILE MAINTAINING THE PLUTONIUM IN A VALENCE STATE NOT ABOVE FOUR THE IMPROVEMENT METHOD WHICH COMPRISES THE COMBINATION WITH THE AFORESAID PROCEDURE, OF REDUCTION OF PLUTONIUM IN A VALENCE STATE GREATER THAN FOUR IN SAID ACIDIC SOLUTION TO A VALENCE STATE GREATER THAN FOUR BY MEANS OF INCORPORATING OXALATE IONS THEREIN TO A CONCENTRATION WITHIN THE APPROXIMATE RANGE OF 0.022 TO 0.15 MOLAR, PRIOR TO SAID PRECIPATION OF LANTHANUM FLUORIDE THEREIN.