US2990243A - Adsorption of plutonium and/or fission products from aqueous solution - Google Patents

Description

United States Patent O ABSORPTION OF PLUTONIUM AND/OR FISSION PRODUCTS FROM AQUEOUS SOLUTION Roy H. Beaton, Richland, Wash, assignor to the Unlted States of America. as represented by the United States Atomic Energy Commission Nb Drawing. F iled Aug. 10, 1945, Ser. No. 610,184

5 Claims. (Cl. 23-145) This invention relates to a method of separating different elements from each other and particularly to the separation of element 94 from other elements.

It is known that when uranium is subjected to neutron bombardment there is formed in small quantities a new element having an atomic weight of 239 and atomic number of 93, known as neptunium (symbol Np). This new element by radioactive decay is transformed through a half-life of 2.3 days to a further new element having an atomic weight of 239 and atomic number 94.

Natural uranium comprises largely isotope U together with about /139 as much U and a very much smaller amount of U When this mixture of isotopes, either as metallic uranium or as a uranium compound, is subjected to bombardment by neutrons or undergoes a self-sustaining neutronic chain reaction, a number of nuclear reactions take place. Isotope U captures neutrons to form U which undergoes beta decay to form transuranic elements:

where x is a small number greater than unity.

Substantially all of the fission fragments have mass numbers within the range 77-158, although small quantities of isotopes of lower and higher mass numbers may result from unbalanced binary fissions, ternary fissions, or other reactions of infrequent occurrence. A very large majority of the fission fragments comprise a light group of mass numbers 84-706 and a heavy group of mass numbers 128-150.

The various fission fragments and the decay products of the initial fission fragments are referred to herein as fission products. These fission products fall within a range of atomic numbers from about 32 to about 64, including the light group of fragments referred to above and their decay products which have atomic numbers ranging from about 35 to about 46; and the heavy group of frag ments and their decay products which have atomic numbers ranging from about 51 to about 60.

The various radioactive fission products have half-lives ranging from a fraction of a second to thousands of years. Those having very short half-lives may be eliminated by aging the material for a reasonable period before handling. Those with very long half-lives do not have sufficiently intense radiation to endanger personnel protected by moderate shielding. On the other hand, the fission products having half-lives ranging from.- a few days to a few years have dangerously intense radiations which cannot be eliminated by aging for practical storage periods. These products are chiefly radioactive isotopes of Sr, Y, Zr, Cb, and Ru of the light group and Te, I, Cs, Ba, La, Ce and Pr of the heavy group.

The total amount of 93 and 94 produced from ice U in natural uranium by neutron bombardment is a function of neutron density and of the time of bombardment. Since 94 is fissionable under the conditions for fission of U the net yield of 94 per unit of time will decrease as the ratio of U content to 94 content of the mass decreases. For this reason, a neutronic reaction for 94* production is suitably terminated when only a fraction of the U has been converted to fission products. The reaction mass at this point contains a large amount of U a much smaller amount of U still smaller amounts of 93 94 and fission products, and traces of other products such as UX and UXg. By aging such a mass for a suitable period of time, the 93 may be substantially completely converted to 94 with simultaneous conversion of the short-lived radioactive fission products to longer-lived or stable isotopes.

The plutonium content of the aged reaction mass will usually be considerably less than 1 percent of the weight of the unreacted uranium, and may even be less than one part by weight per million parts by weight of uranium, The concentration of fission products will be of the same order of magnitude. The separation of plutonium from such as mass involves extraction from the unreacted uranium, decontamination by separating the radioactive fission products, and concentration of the decontaminated material to obtain a product from which a relatively pure plutonium compound can be directly recovered.

The term element 94 is used throughout this specification to designate the element having atomic number 94. The designation 94 refers to the isotope of element 94 having a mass number of 239. Element 94 is also referred to in this specification as plutonium, symbol Pu. Likewise, element 93 means the element having atomic number 93, referred to as neptunium, symbol Np.

It is an object of the invention to separate plutonium from such radioactive fission products.

Another object of the present invention is to provide a simple and eflicient means for separating plutonium from impurities and for concentrating plutonium solutions into substantially smaller volumes of solution than the original solution.

Further objects will be apparent from the following description.

In accordance with the present invention, it has been found that plutonium (particularly plutonium in a trivalent or tetravalent state) and/or fission products may be removed from solutions containing very small concentrations thereof by contacting the solution with titanium dioxide. selective removal of the plutonium is effected so that at least a partial separation of the plutonium from the fission products is secured. This treatment may be conducted by passing the solution through a porous mass of titanium oxide granules or by agitating the solution with such granules or by precipitating the titanium oxide or its hydrate in situ.

Preferably, the titanium oxide is formed into porous aggregates which may be bonded or briquetted together with or without a suitable binder. Especially suitable aggregates may 'be secured by briquetting titania (hydrated titanium dioxide) with a sodium aluminum silicate such as mica and calcining at a high temperature, for example, 800 to 1000 C. Carbon also may be used as a bonding agent as well as numerous carbonizable binders conventionally used to bond briquettes. Moreover, titanium dioxide-silica compositions such as titanium dioxide-silica gel may be used for the removal of plutonium and/ or fission products.

Usually the process is performed after treatment of neutron irradiated uranium to separate some portion of the uranium from the plutonium. In such a case, plutonium more or less contaminated with fission products If fission products are present in the solution, a

Me of replacing some phenolic groups.

from a solution of neutron irradiated uranium which has aged until a major portion of the 92 has decayed to 94 is precipitated or removed from an aqueous solution containing the uranium in the uranyl state of valence. This may be done by precipitation with a carrier such as bismuth phosphate as described in an application for "United States Letters Patent, Serial No. 478,570, filed March 9, 1943, by Stanley G Thompson and Glenn T.

Seaborg, now Patent No. 2,799,553, issued July 16, 1957. Alternatively, the process may be performed by passing the aqueous solution containing the uranium, plutonium and fission products through an adsorbent such as Amberlite IR-1 which adsorbs the plutonium and some of the uranium and fission products. The adsorbent is then eluted with a strong acid or with acid sulphate such as 1.25 M aqueous sodium or potassium acid sulphate solution, or percent oxalic acid to remove plutonium and fission products. Where a large amount of uraniumhas been adsorbed, a portion of the uranium may be eluted by a dilute acid, for example, 0.25 M H 80 or 1 N nitric acid before leaching out the plutonium.

Following the initial separation of plutonium from uranium, the plutonium solution obtained by dissolving the carrier or eluting the adsorbent is subjected to .the action of the titanium dioxide as herein contemplated.

After adsorption and removal of the treated solution the titanium dioxide containing the adsorbed plutonium may be extracted or eluted with dilute aqueous nitric acid or .sodium acid sulphate solution to recover the plutonium.

The Amberlite IR-l adsorbent referred to in this specification is an ion exchange resin manufactured by the Resinous Products and Chemical Company of Philadelphia, Pennsylvania. It is a phenol formaldehyde condensation product containing sufiicient sulfonic groups to insure efficient operation at moderately low pH. It is .usually obtained as the sodium salt and is readily converted to the acid form by treatment with dilute acid treatment solution. The structure may be represented by a network of groups having the following probable formula in which the sulfonic acid groups are not shown since it is not definitely known how many are present or in what position they are located.

CH, CH CH; CH:

Since the hydroxyl groups do not take part in the coudensation action, they are free to react with cations capa- The ion exchange resin is a very porous material and rapid diffusion .of ions takes place. This gives a high adsorption capacity an agent capable of forming a complex chelate compound or complex salt of plutonium. For example, a solution containing plutonium may be treated with citric or tartaric acid or other compound capable of reacting with plutonium to form a slightly ionized or unionized plutonium compound. Such solutions may be subjected to the action of the titanium compound herein contemplated and fission products selectively removed.

While effective results may be secured using titanium 'dioxide alone, it is found to be advantageous to form a titanium dioxide-sodium aluminum silicate mixture and to briquette and calcine this mixture at a temperature of 4 600 to 1000 C. for a period of about 15 minutes to three hours. About 5 to 30 parts by weight of mica or similar silicate per 95 to 70 parts by weight of titanium dioxide is used, and in general hydrated titanium dioxide is used in preparation in the composition.

. The mica orother silicate serves as a binder to holdthe TiO together. Whether it takes partin the adsorption or forms a.titanium dioxide reaction product whencalcined is not known.

In accordance with a further modification, it 'hasbeen found that the selectivity of the titanium dioxide for plutonium rather than for fission products generally may be controlled at least to a substantial degree by the temperature of calcination. For example, hydrated titanium dioxide and 5 percent by weight of mica may be calcined at not less than 900 to 1000" C. for about 30-90 minutes to form a product which will selectively adsorb plutonium. On the other hand, if the temperature of calcination is 600-800 C., the product adsorbs substantial quantities of fission products. Prolonged calcination reduces the adsorption characteristics and, therefore, calcination for more than about 2 to 3 hours generally is not allowable.

Other titanium'dioxide-silica compositions are elfective for removal of fission products and/or plutonium. For example, hydrated titanium dioxide may be precipitated together with silica gel to form a titanated silica gel containing from about 5 to 60 parts by weight of TiO per 95 to 40 percent parts by weight of SiO These products may be used without calcination.

The following examples are illustrative.

Example 1 Neutron irradiated uranium aged for 50 days after irradiation and containing about 1 to 2 grams plutonium, in ionic tetravalent state, and about 1.5 to 2.5 grams of fission products per metric ton of uranium was dissolved in nitric acid and excess nitric acid to form an aqueous solution containing 5 percent UO (NO .6H O and a pH of 2.6.

15 liters of this solution was passed through a column containing Amberlite IR resin (4060 mesh) at a rate of 60 gaL/ftP/hr. to adsorb plutonium, fission products and some uranium. This column was 10 millimeters in diameter and 41 cm. long and contained 50 grams of the resin, and had been conditioned by passing a quantity of 5 percent uranyl nitrate solution therethrough until replaceable cations had been largely replaced by uranyl groups. After the adsorption treatment, the resin was then extracted with 500 cc. of 0.25 M sulphuric acid to remove uranium and with 1000 cc. of 1 M H PO to remove most of the fission products which emit gamma rays. The resin was then extracted with 800 cc. of a solution 0.5 M in H and 1 M in Na SO at a downward flow of 20 gal./ft. /hr. to recover the plutonium. This solution contained tetravalent plutonium ions, fission products and some uranyl uranium dissolved therein.

The sodium acid sulphate solution was diluted to 25 liters, the pH adjusted to 3.5, and passed through a column 19 millimeters in diameter and 33 inches high filled with briquetted titanium dioxide. This titanium dioxide was prepared by briquetting a mixture comprising percent by weight of hydrated titanium dioxide and 5 percent by weight of mica at a pressure of 15,000 pounds per square inch to form aggregates of 40-60 mesh and calcining the aggregates at 1000 C. for 45 minutes. 98 percent of the plutonium and less than one percent of the fission products were removed from the solution by the titanium dioxide.

Example 2 Experiments were carried out by calcining the titanium dioxide mica composition of Example 1 at various temperatures and then in each case pre-soaking a one gram sample of the calcined composition in water for one hour followed by shaking for 10 minutes with 10 milliliters of solution having a pH of 3.5 and being 1.5 molar in sodium sulphate containing small quantities of tetravalent plutonium and fission products. The amount of plutonium and fission products adsorbed was then determined. The fission product removal was determined by observing the magnitude of beta radiation of the solution before and after treatment. The results are shown in the following table:

Example 3 The process of Example 2 was repeated using commercial acid cake titanium dioxide which had not been calcined. All of the plutonium and 97 percent of the fission products emitting beta rays were absorbed in 10 minutes by the SiO When this type of titanium dioxide was calcined at 1000 C. for 60 minutes, it adsorbed 40 percent of the plutonium and substantially none of the fission products in 10 minutes.

The titanium products containing the adsorbed fission products and/ or plutonium may be treated for recovery of the adsorbed products by extraction, elution, etc., or may be used as such as a source of radiation such as alpha, beta or gamma radiation from the absorbed agents.

It is understood that the present invention is not limited by any particular theory of operation but only by the following claims.

I claim:

1. A process of separating tetravalent plutonium and fission product values contained in an aqueous solution comprising passing said aqueous solution over an adsorbent composition consisting of 70 to 100% by weight titanium dioxide and 30 to by weight of a binder and recovering the major portion of one of the constituents from the adsorbent and the major portion of the other constituent from the solution.

2. A process of recovering plutonium values from a solution containing said values in the tetravalent state in association with fission product values comprising passing said solution over an adsorbent composition consisting of 95% titanium dioxide and 5% mica which has been calcined at 800 to 1000 C. for not over three hours, whereby the plutonium values are adsorbed by the adsorbent composition, and recovering the plutonium.

3. A process for the selective separation of fission product values from a substantially uranium-free aqueous solution containing plutonium values and fission product values comprising adding to said aqueous solution a hydroxypolycarboxylic acid selected from the group consisting of citric and tartaric acids whereby a complex with the plutonium values present is formed, contacting the resultant solution with a solid adsorbent composition consisting essentially of titanium dioxide, adsorbing said fission product values on said adsorbent and separating said adsorbent containing fission product values adsorbed thereon from the solution containing unadsorbed plutonium values.

4. A method of preparing an adsorbent composition adapted to adsorb fission product metal values which comprises calcining a composition consisting of from to parts hydrated titanium dioxide and from 5 to 30 parts by Weight sodium aluminum silicate at about 800 to 1000 C. for a period of about 15 minutes to 3 hours.

5. An adsorbent composition consisting of a calcined mixture containing from 70 to 95 parts by weight TiO and from 5 to 30 parts by weight sodium aluminum silicate.

References Cited in the file of this patent UNITED STATES PATENTS 1,059,531 Ebler Apr. 22, 1913 2,399,395 Shriver Apr. 30, 1946 2,811,416 Russell et al. Oct. 29, 1957 OTHER REFERENCES Searle: Chemistry and Physics of Clays, 2nd ed., pages 239, 413, 414. Ernest Benn Ltd., London (1933).

Freundlich: Colloid and Capillary Chemistry, pages 220, 221, 222. Third edition translated from German, 1922.

Myers et al.: Ind. and Eng. Chem., 55 vol. 33, page 697 1941 (TPl A58).

ABC Document CN-1071, Production and Extraction Report from period ending November 11, 1943, 18 pages.

Claims (2)

1. A PROCESS OF SEPARATING TETRAVALENT PLUTONIUM AND FISSION PRODUCT VALUES CONTAINED IN AN AQUEOUS SOLUTION COMPRISING PASSING SAID AQUEOUS SOLUTION OVER AN ADSORBENT COMPOSITION CONSISTING OF 70 TO 100% BY WEIGHT TITANIUM DIOXIDE AND 30 TO 3% BY WEIGHT OF A BINDER AND RECOVERING THE MAJOR PORTION OF ONE OF THE CONSTITUENTS FROM THE ADSORBENT AND THE MAJOR PORTION OF THE OTHER CONSTITUENT FROM THE SOLUTION.

5. AN ADSORBENT COMPOSITION CONSISTING OF A CALCINED MIXTURE CONTAINING FROM 70 TO 95 PARTS BY WEIGHT TIO2 AND FROM 5 TO 30 PARTS BY WEIGHT SODIUM ALUMINUM SILICATE.