US2831750A - Separating protoactinium with manganese dioxide - Google Patents

Description

Unite Patented Apr. 22, 1958 SEPARATING PROTOACTINIUM WITH MANGANESE DIOXIDE Glenn T. Seaborg, Chicago, 111., John W. Gofman, Berkeley, Calif., and Raymond W. Stoughton, Oak Ridge,

Tenn, assignors to the United States of America as represented by the United States Atomic Energy Commission No Drawing. Application November 3, 1944 Serial No. 561,837

8 Claims. (Cl. 23-145) The invention relates to the preparation of masses and compositions of the isotope of uranium having a mass number of 233, said isotope being designated as U or 23s An object of the invention is to provide an improved method for preparing and isolating P21 and U Other objects and advantages of the invention will become apparent as the following detailed description progresses.

In this specification and the claimsthe name of the element is used to designate the element generically,

ill

of Pa and further that U so produced undergoes fission with neutrons of such low energies as below 1 million electron volts (1 m. e. v.) and even with thermal the preponderant component of the composition and pref erably as a composition containing not in excess of about 10 percent by weight of impurities. Further we have discovered that U may be effectively prepared as a concentrate by separating Pa from neutron irradiated thorium preferably by means of a carrier such as manganese dioxide and thereafter permitting the separated Pa to decay to U In addition we have found that efiective concentrates of these materials may be secured by treatment of neutron irradiated thorium which has been subjected to high intensity irradiation forfa time suflicient to produce Pa -i-U in concentrations within a predetermined range.

The reaction of thorium with slow and moderately fast neutrons may be summarized as follows:

23.5 min 27.4 days 'p zaa 1 ,2513 p- Um 5- half life half life The fission products which are. produced as a result of the fission of U with slow and-moderately fast neutrons are, -so far as we have been able to determine, the same as those produced by the fission of U They consist of a large number of elements which generally fall into a light group with atomic numbers from to 46 incl.

and a heavy group with atomic numbers from 51 to iucl., and which undergo beta decay. The fission products which have a half life of more than three days will remain in the reaction mass in substantial quantities at least one month after the termination of the reaction, and the removal or elimination of these products by our process is particularly advantageous. Among these prodnets are: Sr, 1, Zr, Cb, Ru, Te, 1, Xe, Cs, Ba, La, and Ce of a 20 day half life, and Ce of a 200 day half life.

In accordance with one embodiment of this invention,

the mass of thorium is subjected to the-action of neutrons, the majority of which have energies below 1 million electron volts, and the reaction of the neutrons with the thorium is terminated prior to the time when the neutrons are absorbed by the U at the same rate that they are absorbed by the Th This limit is approximately when the weight ratio of U to unreacted Th is 1 to 100. In other Words, the reaction of Th with neutrons should preferably be terminated prior to when the amount of U is approximately 1 percent of the amount of thorium present in the mass. When the reaction is terminated at or prior to this point there is also no danger during the reaction of a substantial decomposition of the U taking place by a nuclear self-sustaining chain a reaction.

Th in order to reduce the amount of fission products and at the same time have a practical amount of U and Pa for isolation by batch process, the reaction is erminated at a weight ratio of U +Pa to 'l'h of not less than about 1 to 1 million and frequently between about 1 to 10,000, and 1 to 1,000.

The reaction of thorium either in metallic state or a compound such as an oxide or carbonate of thorium with neutrons to produce Pa and U may be carried out with neutrons from any suitable neutron source. Where the neutron source provides fast neutrons, the fast neutrons are slowed to neutrons having energies of below 1 millionelectron volts by interposing neutron slowing material between the fast neutrons and the thorium. Such neutron slowing materials include carbon-containing, deuterium-containing, or hydrogen-containing material such as graphite, paratfin, or deuterium oxide. Suflicient neutron slowing material is used so that at least a majority of the neutronsare slowed to energies of below about 1 million electron volts, since at higher energies there is very little production of U and considerable fission of the thorium. We may interpose the neutron slowing material between the fast neutrons and the thorium-containing mass, or we may admix neutron slowing material with the'thorium. An intimate mixture of thorium with neutron slowing material maybe readily obtained by using hydrated thorium compounds such as the compound Th(OH) .xH O. Since the slow neutron absorption cross sectionofthorium is some ten to forty times larger than that of hydrogen, we may suitably use a ratio as high as about two to four hydrogen atoms per thorium atom without losing any more than 10 percent of the neutrons as a resultof absorption by hydrogen.

While neutrons obtained from any suitable source of high neutron outputmay be used, it is desirable ;to subject the thorium to neutrons from a high intensity source in order that suitable concentrations of Pa and U may be obtained in a reasonable length of time.

Preferably the thorium is subjected to slow neutrons from a source of neutrons capable of supplying at least 5 neutrons per second to the thorium mass and where a relatively high concentration of U +Pa is desired shouldweigh no more than about 20 tons. Preferably this mass should be of such a thickness that at least percent and preferably 75 percent or more of the neutrons supplied are absorbed. Such high neutron intensity may be obtained by subjecting thorium to the action of neutrons obtained by slowing down secondary neutrons obtained from a self-sustaining chain reaction of U U or 94 with neutrons.

By placing thorium adjacent to a neutron chain reacting mass comprising uranium and/or 94 in amount suflicient to establish a self-sustaining neutron chain reactiondispersed in a neutron slowing medium such as carbon or D 0, between.5 10 and 10 neutrons per second are supplied to the thorium and at least 50 to 75 percent are absorbed so that a ratio of U +l a to Th of more than 1 to 1 million may be attained in a reasonable length of time, such as one to three months. In such a case the degree of bombardment desired may be completed before the preponderant amount of Pa formed has decayed to U The method of chemical separation here involved is based upon the initial separation and recovery of Pa folowed by conversion of the Pa to U and the subsequent separation of these two elements. 7

A suitable method which is particularly effective for isolatingla and U described below is given by way of example and it is to be understood that the invention is not limited to the details. The details of this method for isolating Pa and U were developed by Working with a number of small samples of these radioactivities formed in small bombardments, and with part of the isotopes which were obtained from 5 kg. of thorium nitrate .(Th(NO .4H O) which was bombarded in the form of an aqueous saturated solution with neutrons obtained from about 14,000 micro-ampere-hours of deuterons on beryllium. The separations were carried out before any appreciable quantity of Pa had decayed to U Example 1 TheSkg. of neutron bombarded thorium nitrate containing Pa +U was diluted up to 26 liters with water and'then made-approximately 0.5 N in nitric acid. To this total solution was added about 400 grams of manganous chloride. The solution was heated and the manganese was precipitated as manganese dioxide from the hot solution by the addition of a soluble manganate or permanganate, especially an alkali manganate or permanganate such as potassium permanganate. Each precipitate of manganese dioxide was centrifuged out separately. The protoactinium is carried down with the manganese dioxide in a substantially quantitative manner under these conditions.

The combined manganese dioxide precipitates were dissolved in a mixture of hydrogen peroxide and hydrochloric acid. After decomposing the hydrogen peroxide by boiling, about 200 mg. of zirconium oxychloride were added to this solution, the zirconium was precipitated as zirconium phosphate by the addition of phosphoric acid, and the precipitate was centrifugedout separately. Zirconium phosphate carries down the protoactinium essentially quantitatively, thereby separating the latter from the relatively large amount of manganese dioxide with which it was originally associated.

It was now necessary to remove the Pa from the rather large amount of zirconium with which it was present, and to purify it from uraniumand thorium. The zirconium phosphate containing protoactinium phosphate and. other phosphates was brought into solution by treatment. with. dilute hydrofluoric acid. This step also gave a separationfrom thorium, which was converted to the insoluble fluoride at this point and sep- I point.

arated. The solution was cooled with ice-water and zirconium and protoactinium hydroxides were precipitated by adding dilute sodium hydroxide solution; the solution must be kept cold in this precipitation since otherwise difilculty will be experienced in redissolving the precipitated hydroxides. The hydroxide precipitate was then dissolved in nitric acid and the protoactinium was carried away from most of the zirconium by another series of manganese dioxide precipitations. Some of the zirconium comes along with the manganese dioxide precipitate in this procedure so that a-further removal of the zirconium, as well as the manganese, from the protoactinium was necessary. This involved going through the above described cycle two more times, finally ending up with a small zirconium phosphate precipitate containing substantially all the protoactinium that was originally present.

This zirconium phosphate precipitate, which now contained only about 10 mg. of zirconium, was dissolved in hydrofluoric acid, and the zirconium and protoactinium then precipitated as the hydroxides as described above. This hyrdoxi-de precipitate was then converted to the corresponding nitrates by dissolving in nitric acid. Another precipitation of the zirconium phosphate (carrying the protoactinium). was then made in order to be sure that all the uranium and thorium, to amounts less than a microgram, were removed. The final Zirconium phosphate was converted into zirconium sulphate by dissolving in hydrofluoric acid, precipitating the zirconium as the hydroxide, dissolving the hydroxide in sulphuric acid, and evaporating to dryness. The protoactinium accompanies the zirconium through this series of conversions. The final zirconium sulphate (containing about 10 mg. of zirconium and. the protoactinium) was dissolved in about 30 cc. of 0.33 M solution of ammonium fluoride or other alkali fluoride such as KP and the hydrogen ion concentration of the solution was adjusted until it corresponded to that of the methyl red indicator end The Pa was now isolated by an electrolytic procedure; in this procedure the Pa was deposited in a very thin adherent layer and was separated from the zirconium at the same time. The electrolysis chamber consisted of a clear Bakelite chamber fitted with a brass plug which screwed into the lower end and upon which rested a copper disk upon which the Pa was deposited. This copper disk served as the cathode, while a motordriven platinum stirrer served as the anode. The electroplating was carried out for about 8 hours at about 100 milliamperes of current, applying about 15 volts across the electrolysis chamber. The Pa is plated out nearly quantitatively (about 90 percent) under these conditions. No zirconium plates out under these conditions, so that a good separation of protoactinium from zirconium is obtained by this procedure. The Pa may then be stored until it has decayed substantially to U or it may be allowed to decay partially to U and the U separated from the Pa by any method of separating uranium from protoactinium.

By way of example, a separation of U from Pa was made by us as follows:

The thin plated film containing the Pa and U and which had aged about two months was dissolved in 6 N hydrochloric acid to which a few drops of 6 N nitric acid had beentadded. Since asmall amount of copper was dissolved from the backing plate in this procedure this was removed by an H 8 precipitation from the acid solution. No Fa on U was carried down by the copper sulphide in this precipitation. After the removal of the H S and the reduction of the amount of 6 N hydrochloric acid to about 2 00., by boiling the solution, there was added about 0.1 mg. of zirconium as zirconium oxychloride. There was then added about a hundredth of a cc. of percent phosphoric acid and the precipitated zirconium phosphate was removed by centrifugation.

' protoactinium.

The major portion of the Pa approximately 70 percent or more, was removed in this step. The yield of Pa in this precipitate is lower than that obtained when a large excess of phosphoric acid is used; however, an excess of phosphoric acid must be avoided because the acidity of the solution must be kept very low in the electrolysis procedure which follows: It is essential that less than 0.1 percent of the Pa remain with the U to avoid the simultaneous electrolytic deposition of Pa with the U and hence further separations were necessary. Further separations of the Pa from the U were effected by adding zirconium, about 0.1 mg. at a time in successive portions, to the solution. The Pa was not carried down quantitatively by the zirconium phosphate under these conditions so that it was necessary to make about 15 precipitations of zirconium phosphate, by the addition of zirconium to the phosphoric acid solution in this manner. After performing this number of precipitations, less than. 0.1 percent of the Pa remained in the solution. Practically all of the U remains in solution in this procedure; however, it should be emphasized that in order for this to be the case it is necessary that the Pa? be removed with small successive portions of zirconium phosphate in the manner which has been described. 7

The volume of the solution was then reduced, by evaporation, to a volume of about 0.1 cc. To this solution, which contained about 0.10 cc. of 6 N hydrochloric acid and of the order of 0.001 cc. of 85 percent phosphoric acid, there was added 0.02 cc. of glacial acetic acid and the solution was diluted by theaddition of cc. of water. This solution was placed in an electrolysis chamber which consisted of a clear Bakelite chamber fitted with a brass plug that screwed into the lower end and upon which rested a platinum foil that the U was to be deposited upon. The electroplating was carried out for about 8 hours at about 90 milliamperes of current applying about 8 volts across the electrolysis chamber (the actual electrode potential'for U 3 is not known). The U is plated out nearly quantitatively (greaterrthan 90 percent) under these conditions and the plate usually appears 'as a well adhering film of. pure U (probably in the form of the trit aoctaoxide,,U O together with a small amount of, organic matter which can be burned off theplate. It should be emphasized that in order for the uranium to be quantitatively deposited the acidity of the solution must be kept very low as described above. (In addition, it may be pointed out that this electrolysis may be performed from an 0.3 M ammonium fluoride solution, with approximately the same yields, and in some experiments we have used this procedure.) The weight of the U sample prepared in the manner outlined above, as determined from its alpha activity and from the halflife of U (1.2 10 years), was found to be 3.8 micrograms.

The preferred method for separating protoactinium from zirconium is the electrolysis from ammonium fluoride solution, described above, wherein microgram (or less) quantities of protoactinium are deposited while as much as 10 mg. of zirconium remain in solution. Another method is the precipitation of the protoactinium with manganese dioxide. In this procedure a certain amount of zirconium is also carried along with the manga-- nese dioxide; this small amount of zirconium can be removed from the manganese by precipitation as zirconium phosphate which carries the protoactinium with it. After dissolving the zirconium-protoactinium phosphate in dilute hydrofluoric acid solution, the protoactinium can be re-.

moved by another manganese dioxide precipitation and this cycle may be repeated as often as is necessary in order to remove practically all the zirconium from the In a third method the protoactinium is dilute hydrochloric acid solution; six or eightprecipitations will carry about percent of the protoactinium, while carrying a negligible amount of the zirconium, which remains in solution as a complex oxalate. (The protoactinium can then be removed from the lanthanum oxalate by dissolving the latter in hydrochloric acid and precipitating a very small amount of zirconium phosphate from this solution.) A fourth method involves the fractional precipitation of zirconium iodate. If there is added to a 1 to 6 N I-ICl or HNO solution, containing a mixture of zirconium and protoactinium, a small amount of sodium iodate, sufficient to precipitate only a small fraction of the zirconium as zirconium iodate, it is found that the protoactinium concentrates to a large extent in this first fraction. This precipitate can be dissolved in concentrated hydrochloric acid and again a partial precipitation of zirconium iodate performed; in eachcycle a large concentration of protoactinium is obtained. By removing several zirconium iodate precipitates in each cycle, the protoactinium can be brought along quantitatively. Finally, a solution containing a salt of zirconium and Pa in solution can be made 6 to 10 N in hydrochloric acid and subjected to fractional crystallization for the separation of zirconium oxychloride in crystalline form, whereupon the protoactinium concentrates in the mother liquor.

Example 2 A. solution containing 300 grams per liter of neutron irradiated thorium nitrate tetrahydrate and an acidity of 1 N nitric acid was prepared. This solution contained about 2 parts by weight of Pa |U per million parts of thorium. Manganous nitrate was added in excess of the amount required to form 1.1 grams per liter of Mn0 and sufficient potassium permanganate was added to precipitate a total of 1.1 grams per liter of MnO in two approximately equal portions and the MnO containing the Pa was recovered.

The MnO- was dissolved in a mixture of nitric acid and hydrogen peroxide and the solution was boiled to decompose excess peroxide and diluted to about 20 grams per liter of manganous nitrate per liter and an acidity of 1 normal nitric acid. MnO was again precipitated in theproportion equivalent to about 10 percent by weight of the manganous nitrate in solution. The process of dissolving and precipitating Mn0 was repeated several times and the MnO finally obtained was dissolved in nitric acid and concentrated to 4 M m-anganous ion. Thereupon the aqueous solution was permitted to age until a major portion of the Pa in solution had decayed to U This U was then extracted from the solution with ether.

In accordance with a further modification of the invention, other methods may be used for recovering a Pa concentrate which may be aged to form U For example an aqueous solution of neutron irradiated thorium such as a nitric acid solution may be treated with hydrogen fluoride substantially in excess of that required to form thorium fluoride. In such a case the thorium precipitates leaving Pa in solution. This Pa may then be treated to recover a Pa by convenient means such as we have heretofore described.

Moreover the Pa may be recovered or removed as a concentrate from a nitric acid acidified solution by precipitation of about 10-20 percent by weight of the thorium as thorium iodate which carries down the Pa in the solution. For example a thorium nitrate solution having a nitric acid normality of about 4 may be treated with suflicient potassium iodate to precipitate 10-20 percent of the thorium as thorium iodate in order to remove the Pa. This precipitate may be redissolved and retreated in order to increase the Pa concentration if desired.

In the specification and claims, wherever reference is made to the presence or the addition of phosphoric acid or a phosphate, unless otherwise indicated by the context, it is to be understood that the reference is to orthophosphoric acid or its salts.

Bythe above methods of separating 1 21 and U from foreign products we are able to obtain compositions composed largely or entirely of U compounds, which are substantially free from fission products. The U metal maybe produced from suitable compounds thereof by calcium reduction or any of the other known methods for producing uranium metal from compounds of uranium.

U metal or compounds of U may be shaped into the form of spheres, cylinders, blocks or the like by known methods of shaping uranium metal and cornpounds. Such shaped articles of manufacture may be used as a source of nuclear power as described in our copending application, Serial No. 565,990, filed November 30, 1944'.

While there has been described certain embodiments of'ou'r invention, it is to be understood that it is capable of many modifications. Changes, therefore, may be made without departing from the spirit and scope of the invention as described in the appended claims, in which it is the intention to claim all novelty in the invention as broadly as possible.

We claim:

1. The method of separating Pa from foreign products present in neutron irradiated thorium which comprises forming a solution of neutron irradiated thorium and a manganous salt, then adding a substance selected from the class consisting of soluble manganates and permanganates to precipitate manganese as manganese dioxide whereby protoactiniumis carried down with the manganese dioxide.

2. The method of separating Pa from foreign products present in neutron irradiated thorium which comprises forming a solution of neutron irradiated thoriumand a manganous salt, then adding potassium permanganate to precipitate the manganese as manganese dioxide whereby protoactinium is carried down with the manganese dioxide; dissolving the precipitate, adding a soluble zirconium salt, and adding phosphate ion to precipitate zirconium phosphate whereby protoactinium is carried down with the zirconium phosphate.

3. The method as in claim 1, wherein potassium permanganate is employed as the substance for precipitating manganese dioxide.

4. A method of recovering Pa which comprises forming a solution of neutron irradiated thorium containing Pa and contacting the solution with manganese dioxide which removes Pa from the solution.

5. The method of separating protactinium from uranium and thorium, which comprises forming a solution containing ions of said elements and a manganous salt, then adding a substance selected from the class consisting of soluble manganates and permanganates to precipitate manganese as manganese dioxide whereby protoactinium values are carried down with the manganese dioxide.

6. The method of separating protoactinium from uranium and thorium, which comprises forming a solution containing ions of said elements and a manganous salt, then adding potassium permanganate to p1ecipitate the manganese as manganese dioxide whereby protoactinium is carried down with the manganese dioxide, dissolving the precipitate, adding a soluble zirconium salt, and adding phosphate ion to precipitate zirconium phosphate whereby protoactinium is carried down with the zirconium phosphate.

7. A method of separating Pa from zirconium which com-prises precipitating Pa with a carrier precipitate of manganese dioxide from an aqueous solution containing zirconium and Pa 8. method of separating pro'toactinium values from zirconium which comprises precipitating protoactinium values with a carrier precipitate of manganese dioxide from an aqueous'solution containing zirconium and protoactinium.

References Cited in the file of this patent UNITED STATES PATENTS Schwerin Dec. 8, 1914 Fermi et a1 July 2, 1940 OTHER REFERENCES

Claims (1)

1. THE METHOD OF SEPARATING PA233 FROM FOREIGN PRODUCTS PRESENT IN NEUTRON IRRADIATED THORIUM WHICH COMPRISES FORMING A SOLUTION OF NEUTRON IRRADIATED THORIUM AND A MANGANOUS SALT, THEN ADDING A SUBSTANCE SELECTED FROM THE CLASS CONSISTING OF SOLUBLE MANGANATES AND PERMANGANATES TO PRECIPITATE MANGANESE AS MANGANESE DIOXIDE WHEREBY PROTOACTINIUM IS CARRIED DOWN WITH THE MANGANESE DIOXIDE.