US2918349A - Extraction of plutonium values from organic solutions - Google Patents

Description

2,918,349 Patented Dec. 22, 19 59 EXTRACTION OF PLUTONIUM VALUES FROM ORGANIC SOLUTIONS Glenn T. Seaborg, Chicago, Ill., assignor to the United States of America as represented by the United States Atomic Energy Commission N Drawing, Application October 12, 1945 Serial No. 622,098

7 Claims. (Cl. 23-145) The present invention relates to the solvent extraction of heavy metal compounds, and more particularly to .the extraction of a compound of a transuranic element from an organic solvent solution thereof by means of subjecting the latter to extraction with an aqueous solution in the presence of an acid medium.

.An object 'of this invention is to provide a method for the separation and purification of a transuranic element contained in an organic solvent solution together with other lighter element impurities.

Another object of this invention is to provide a process for the extraction of plutonium from the aforesaid organic solvent solution by means of an aqueous solution in the presence of an acid medium whereby a substantial separation of said light element impurities from the plutonium is effected.

Other objects of this invention will be apparent from the description which follows:

It is known that plutonium can be produced in small quantities by the bombardment of natural uranium with neutrons. The designation plutonium or element 94 as used throughout the present description refers to the transuranic element having an atomic number of 94. The expression 94 means the isotope of element 94 has an atomic weight or mass of 239. Similarly, the terms element 93 or Np refer to the new element known as neptunium having an atomic number of 93.

Uranium is composed of three isotopes, namely, U U 5, and U the latter being present in excess of 99 percent of the whole. When U is subjected to the action of slow or thermal neutrons, a fourth isotope, U is produced which has a half-life of 23 minutes and undergoes beta decay to Np which decays further by beta radiation with a half-life of 2.3 days to yield plutonium. In addition to the formation of the transuranic elements, neptunium and plutonium, there are simultaneously pro- .duced other elements of lower atomic weight known as fission fragments. These fission fragments are composed of two distinct element groups, i.e., a light and heavy element group. The light group contains elements having atomic numbers of between about 35 and 46 while the heavier group is composed of elements of atomic numbers varying between about 51 and 60. The elements of these groups as originally produced are considerably overmassed and undercharged, and hence are highly unstable. By beta radiation, however, they quickly transform themselves into isotopes of these various elements having longer half-lives. The fission fragments and their resulting decay products are commonly known as fission products.

The various radioactive fission products have half-lives ranging from a fraction ofa second to thousands of years. Those having very short half-lives may be substantially eliminated by ageing 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 2 cannot be eliminated by ageing for practical storage periods. These products are chiefly radioactive isotopes'of Sr, Y, Zr, Cb, and R11 of the light group and Te, l, Cs,

Ba, La, Ce, and Pr of the heavy group.

It may be readily seen that plutonium produced as generally set forth above is contaminated with considerable quantities of uranium and fission products. In fact, the plutonium constitutes only a very minor portion of the irradiated mass, i.e., generally less than one percent thereof. In view of such a low concentration of plutonium in the irradiated metal, it becomes apparent that the procedure employed to recover that element must be highly efficient in order to be at all practicable.

There have been devised a number of procedures for the removal and concentration of plutonium from'extremely dilute solutions thereof. In general, such methods involve the formation of various insoluble compounds in said dilute solutions capable of carrying plutonium in the reduced state. The carrier precipitate and plutonium thus obtained are then dissolved and the plutonium is oxidized to PuO in which state of oxidation it is soluble in the presence of said carrier. Under these conditions, the plutonium remains in solution and the fission products are removed when the carrier is added. Thereafter, the dissolved plutonium is reduced to a valence state in which it is carriable by the aforesaid carrier and removed from solution in the form of a carrier precipitate which may again be dissolved and the plutonium purified further if considered necessary or desirable by repeating the above cycle. This procedure, however, is obviously cumbersome and time-consuming, since it requires a number of such steps in order to effect a substantial removal of fission products and other impurities.

In copending application-United States Serial No. 481,- 660, filed April 3, 1943, and patented as U.S. Patent No. 2,811,415 on October 29, 1957, a process for separating plutonium compounds from aqueous nitric acid solutions, aqueous hydrochloric acid solutions, or other suitable aqueous media is described and claimed and essentially involves the extraction of said compounds from such aqueous solutions by suitable organic solvents. Such a process can be utilized for the separation of element '94 from aqueous solutions in any phase of the separation procedure previously described for the recovery of said element from neutron-irradiated uranium. While this procedure serves to effectively recover the plutonium from the original solution and at the same time to separate that element from various other impurities, no substantial decrease is etfected in the percentage of certain types of fission products associated therewith and hence separation of element 94 from the organic solvent by distillation would-result in the procurement of an impure product.

By the present invention it has been discovered that plutonium compounds may be readily and effectively separated fro-m organic solvent solutions of the type obtained in the aforesaid copending application containing said compounds together with fission products. By sub jecting such solutions to extraction in this manner, it has been found, plutonium of relatively high purity is obtainable inasmuch as the fission products which have been extracted with the plutonium as well as other undesirable impurities tend to remain behind in the organic solvent phase, particularly when a limited amount of acid aqueous medium is used for the extraction. In such a process, the plutonium may reenter the aqueous solution in the trivalent, tetravalent or hexavalent state.

Recovery of element 94 from its original aqueous solution may likewise be effected by first subjecting the solut on containing ionic plutonium in its tetravalent or hexavalent state to extraction with a suitable organic solvent and thereafter re-extracting the resulting organic solvent solution with an aqueous solution containing a suitable reducing agent such as, for example, sulphur dioxide, hydrazine, hydroxylamine, or the like. In em ploying this specific embodiment of the present invention, it has been observed, on contact of the aqueous solution with the. organic solvent, element 94 is reduced to its trivalent state thereby becoming insoluble in said organic solvent and passing into the aqueous phase. Also during this operation, particularly where hexavalent plutonium is originally present, a portion of the latter at least may be reduced to the tetravalent state and pass into the aqueous phase where it is reduced further to the trivalent state. By this general procedure a substantially quantitative recovery of the plutonium present in the organic solvent is effected, while at the same time the quantity of impurities associated with the plutonium is materially reduced.

Likewise, it will be apparent that organic solvent solutions of element 94, which have been obtained from the original aqueous plutonium solution containing a suitable salting-out agent, may be subjected to extraction with an aqueous solution of a reducing agent of the general type specified above to secure an aqueous solution of substantially pure plutonium.

The aqueous solutions from which plutonium may be extracted in accordance with the present invention suitably contain plutonium of at least +4 valence state, said ions being substantially free from strong complexing agents. Preferably, no ion should be present in the aqueous solution which forms ionic complexes with plutonlum more strongly than does the nitrate ion. The

.hydroxyl ion, and the anions of acids which ionizein aqueous solutions to a substantially smaller degree than nitric acid, are particularly disadvantageous from the standpoint of complexing tetravalent plutonium. Thus, hydroxyl, sulphate, phosphate, fluoride, and oxalate ions tend to complex tetravalent plutonium sufficiently to decrease its extractability into organic solvents. It is therefore desirable to minimize the concentrations of free ions of this class in the aqueous or organic solvent solutions to be extracted. Most of the complexing ions can be excluded in the preparation of the aqueous solutions. Alternatively, an interfering ion may itself be complexed by another ion, or its concentration as a free ion may otherwise be minimized. by control of the concentration of another ion which can combine with it. Thus, the fluoride ion can be complexed by zirconyl ion, and the hydroxyl ion can be suppressed by hydrogen ion, to

reduce their interference with the extraction of tetravalent plutonium.

The extraction. of hexavalent plutonium is relatively free from interference by hydroxyl ion. Hexavalent plutonium can be extracted from aqueous solutions of any hydrogen ion concentration suflicient to prevent the precipitation of a basic plutonium compound. An aqueous solution of plutonyl nitrate may thus be extracted without the necessity of free nitric acid in the solution. In the case of tetravalent plutonium, on the other hand, it is desirable to maintain free acid in the aqueous solution. Solutions of plutonous nitrate for extraction with organic solvents should have a pH not substantially above 2.5, and preferably should have a concentration f free nitric acid of at least 1 N.

In order to improve the transfer of plutonium from the original aqueous solution to the organic phase, it is generally desirable to incorporate a salting-out agent in the aqueous solution. A salting-out agent, for this purpose, has the same characteristics as a salting-out agent for previously known solvent extraction processes, i.e., high solubility in the solution to be extracted and low solubility in. the extract phase. The preferred saltingout agents for use in the present invention are those having a common ion with the compound being extracted. Thus, if a nitrate of plutonium is being extracted, the salttng-out agent is preferably an inorganic nitrate, Ex-

NaNO Ca( a)2 KNO 3)z LiNO Mg( 3)2 NH NO a)s Mn(NO t m The concentration of the salting-out agent which is desirable in any particular case will depend on the valence of the cation and the concentration of the common anion due to any free acid in the solution. In the case of 1 N nitric acid solutions, it is desirable to employ a concentration of'a univalent nitrate of at least 3 M, and preferably 5-10 M. Equivalent concentrations of polyvalent nitrates may be employed at the same acld concentration, and the salt concentration may suitably be increased or decreased with decrease or increase in acid concentration.

The resulting organic solvent solution of element 94 and fission products is then extracted with an acidifiedaqueous solution, as a result of which element 94 passes into the aqueous phase leaving a substantial portion of the associated impurities in the organic solvent. Suttable acids for use in such solutions arenitric acid, hydrochloric acid, perchloric acid and the like.

. If desired, extraction of tetravalent plutonium from its solution in an organic solvent may be facilitated by use of a complexing agent capable of complexing tetravalent plutonium in the aqueous solution. For example, the organic solution containing tetravalent plutonium may be extracted with an aqueous medium contalnmg sulphate or oxalate ions or other anion of an acid which is more weakly ionized than nitric acid. Where the plutonium is in the hexavalent state reducing agents capable of reducing plutonium to tetravalent or trivalent state such as sodium nitrite, ferrous ions, bromite ion may be incorporated.

In carrying out the process of this invention, previously known extraction procedures and apparatus may be em- I ployed. The extraction may be elfected by batch, contom of the column and a suitable means for withdrawing the aqueous acid solution employed in re-extraction. With certain obvious changes, solvents heavier than water can be employed with substantially equally good results.

The plutonium may be recovered from the aqueous extract phase by any suitable procedure such as evaporation of the aqueous acid solution, crystallization with an isomorphous crystalline carrier, or adsorption on a solid adsorbent,

It is apparent that the procedures described above may be applied to the extraction of plutonium from solutions containing various impurities of lower atomic weight which have partition coefficients between organic solvent and aqueous phases lower than that of plutonium. The purification effected by this procedure is especially advantageous in separating plutonium from the fission products contained in solutions derived from neutron-irradiated uranium. Repeated extractions with one or more types of acidified aqueous solutions may be used to effect purification and concentration of plutonium to an extent sufficient to enable final recovery of a pure compound of plutonium.

The process may be applied to extraction of plutonium from water or aqueous medium by various solvents which at? Substantially immiscible with the. aqueous solution to be extracted and which contain at least 1 atom capable of donating an electron pair to a coordination bond. Such solvents suitably comprise water-immiscible organic compounds containing an oxygen, sulphur, or nitrogen electron-donor atom. It will be evident, however, that most nitrogen-containing organic compounds of this type are basic in nature and will be unsuitable for the extraction of plutonium from acidic aqueous solutions. Such compounds may be used, if desired, to extract substantially neutral solutions containing small amounts of hexavalent plutonium. Most organic solvents containing oxygen or sulphur donor atoms may be used for the extraction of acidic aqueous solutions as well as neutral solutions, and the oxygenated organic solvents are the preferred extractants for use in the present process.

Although most normally liquid organic compounds containing an atom with excess electrons, such as oxygen, sulphur, or nitrogen, are capable of forming a coordination bo-nd, it will be evident to those skilled in the art that certain molecular structures can interfere with this electron-donating property. Electron-attracting constituents such as halogen atoms can off-set the electron donating property of an atom such as oxygen, if present in s ufiicient number and proper relationship to the donor atom. For this reason, it is preferable to employ compounds containing only carbon, hydrogen and electrondonor atoms. It will also be apparent that certain molecular configurations can give rise to steric hindrance which may interfere sufiiciently to prevent the formation of coordination bonds. Tertiary carbon atoms adjacent an electron-donor atom and long chains of nondonor atoms linked to a donor atom are especially undesirable in this respect. The preferred solvents are those in which the donor atom is linked to a hydrogen atom or to non-tertiary carbon atoms and in which at least one component linked to the donor atom contains less than four consecutive non-donor atoms.

The following are examples of suitable solvents for use in the present process:

Ethyl ether Z-phenoxyethanol Z-benzyloxyethanol 1,2-diethoxyethane l-ethoxy-Z-butoxyethane Methylisobutylcarbinol Methyl ethyl ketone Methyl amyl ketone Methyl isobutyl ketone Mesityl oxide 2-ethylbutanol The examples which follow are illustrative of various modifications of the process of the present invention and demonstrate the adaptability of such a procedure to the recovery of plutonium from organic solvent solutions which contain the impurities normally associated with that element.

EXAMPLE I A solution of 5 N in ammonium nitrate and 3 N in nitric acid, which contained 1.09 milligrams/ml. of plutonium, 6.2 milligrams/ml. of zirconium, and 6.2 milligrams/ml. of lanthanum, was agitated with an equal volume of methyl isobutyl ketone for a period of approximately ten minutes after which the ketone layer was separated, washed with an equal volume of a solution 5 N in ammonium nitrate and 3 N in nitric acid in order to assist in the removal of fission products and other impurities, and then subjected to extraction with a volume of water which was approximately 0.75

6 percent of the volume of ketoneemployed. "The'orig'i-nal feed solution was subjected to three such extraction cycles and the resulting aqueous extract was analyzed-for :plutonium content on the basis of itsgalpha radiation and that of the original feed solution. The plutonium content of the extract was found to be 83.8 percent of that in h initial u ion s s ro a i is an l s s indicate the product thus obtained to be 99.75 percent plutonium and 0.25 percent zirconium. Lanthanum and other impurities commonly associated with element 94 were not present in detectable amounts.

EXAMPLE II A continuous batch extraction was carried out with plutonium-containing solutions in which the plutonium content in each instance was 150;]. g. The plutonium was first extracted from each aqueous solution which was 10 N in ammonium nitrate and 1 N in nitric'acid, by means of the various organic solvents indicated below. The solvent was introduced at the bottom of an extraction column, having a capacity of 15 ml., through a fritted glass disc and rose through the column of product solution to the top where it was withdrawn. The, resulting sol vent solution was then subjected to re-extraction by introducing the same at the bottom of a second column of the same capacity and containing 1 N nitric acid, in a similar manner. In both instances the organic solvent was forced through the aqueous solution in the extraction and re-extraction columns continuously by displacement from a storage column using water under a constant pressure head.

It will be apparent, of course, that on recycling the organic solvent a substantially quantitative separation of the plutonium from the original solutio'n can be effected.

EXAMPLE III A SO-gram sample of uranyl nitrate hexahydrate prepared from neutron-bombarded uranium was melted and sufiicient quantities of sodium dichromate and nitric acid to render the mixture 0.2 M and 0.5 N in these two respective reagents were added to oxidize the plutonium to its hexavalent state. The resulting solution which contained uranyl nitrate, pluto'nyl nitrate, and fission products was agitated with diethyl ether, and more than percent of the plutonium, substantially all of the uranium and about one percent of the fission products dissolved in the ether layer of the resulting two-phase systern. The ether layer was then shaken with small portions of water saturated with sulphur dioxide in order to reduce and extract the plutonium. The combined water po'rtions contained only a'small fraction of the original uranium and about 90 percent of the plutonium.

EXAMPLE IV The table below demonstrates the ability of the process of the present invention to separate plutonium from impurities which are commonly associated therewith. The extraction cycles employed consisted of shaking with an aqueo'us solution 10 N in ammonium nitrate, 1 N in nitric acid, and containing a known amount of the particular light element impurity studied with an equal volume of solvent. The solvent was then washed with an contact withan aqueous nitric acid solution in the absence of a reducing agent and separating the resulting extract and ratfinate phases.

2. The process of claim 1 in which the organic solvent 5 is methylisobutyl ketone.

' equal volume of 1 N nitric acid, and the nitric acid solution was thereafter analyzed for the given element to determine the separation factor, which is that fraction of theimpurity originally present that was extracted and back-extracted together with plutonium.

general, it may be said that the use of any equivalent or vmodification of procedure which would naturally occur to those skilled in the art is included in the scope of this invention.

This is a continuation-in-part of copending United States application, Serial No. 481,660 filed April 13, 1943, and-patented as US. Patent No. 2,811,415 on October 29, 1957, and all subject matter therein, not

inconsistent with the subject matter herein, is incorporated in the present disclosure by reference.

What is claimed is:

1. A process for the separation of plutonium from a substantially water-immiscible organic solvent solution containing ionic plutonium in an oxidation state of at least +4 which comprises bringing the plutonium into Table II Separation factors for light element impurities-Multiplied by 1,000 Solvent Al B Be Cs Fe K Li Na Mg P 131 La. Ru Zr Ethyl ether 2 0. 07 7 0. 5 1. 0 0. 2 1, 5 0, 5 Methylisobutyl ketone- 0 1 0 3 0. 2 1. 0 0. 2 0. 1 0. 4 2 0. 4 ethyl-n-amyl ketone-.- 0 2 0 3 1.0 2 1.0 0.7 0.6 2 2 2-ethylbuty1 Cellosolvc.-- 0 2 1. 5 0.3 0.2 0.2 0.5 0.2 Triglycoldichlorlde--- 2 0. 25 0. 08 4 4 0 3 0. 1 1. 0 1.0 Acetophenone.-. 1 3 0.3 0. 3 3 3 0. 3 0. 3 0. 7 Menthone 2 0.001 5 0. 1 0, 2 Cyclohexanone 0. 05 1. 0 5 0. 2 2 10 6 0. 2 Ethylbutyl Cellosolv 5 0. 2 0. 5 0. 5 0, 2 Dlethyl Cellosolve 1 0. 3 5 0. 5 10 6 0. 5 Dlmethyltetrahydrofurane. 2 1. 0 5 0. 5 0, 3 Nltromethaue 2 0. 2 10 6 2 0, 5 Nltroethane.- 5 0. 7 10 4 10 Nltrobenzene 0. 01 3 0. 5 0. 2 4 0. 2 2 Ethyl sulfide. 2 3 5 2 3 4 5 Trlchloroethylene 0. 1 0. l 5 0- 7 6 3 4 3. The process of claim 1 in which the organic solvent is ethyl ether.

4. The process of claim 1 in which the organic solvent is methyl-n-amyl ketone. a i i 5. The process of claim 1 in which the organic solvent is Z-ethylbutyl Cellosolve. I

6. The process of claim 1' in which the nitric acid solution has a concentration of about 1 N.

7. In a process for the separation of plutonium from light element impurities commonly associated therewith in a solution of a substantially water-immiscible organic solvent, the steps which comprise contacting the solution in the absence of a reducing agent with an aqueous inorganic acid solution containing a common anion saltingout agent with regard to the plutonium salt to be extracted, and separating the organic solvent thus treated.

References Cited in the file of this patent UNITED STATES PATENTS

Claims (1)

1. A PROCESS FOR THE SEPARATION OF PLUTONMIUM FROM A SUBSTANTIALLY WATER-IMMISCIBLE ORGANIC SOLVENT SOLUTION CONTAINING IONIC PLUTONIUM IN AN OXIDATION STATE OF AT LEAST +4 WHICH COMPRISES BRINGING THE PLUTONIUM INTO CONTACT WITH AN AQUEOUS NITRIC ACID SOLUTION IN THE ABSENCE OF A REDUCING AGENT AND SEPARATING THE RESULTING EXTRACT AND RAFFINATE PHASES.