DE1214658B - Process for separating cerium from other rare earths - Google Patents

Description

FEDERAL REPUBLIC OF GERMANY

GERMAN

PATENT OFFICE

EDITORIAL

Int. CL:

COIf

German KL: 12 m -17/00

Number: 1214 658

File number: U11614IV a / 12 m

Filing date: April 9, 1965

Opening day: April 21, 1966

The invention relates to a method for separating cerium from other rare earths in aqueous radioactive solution by co-extraction of cerium and rare earths into an organic dialkyl phosphoric acid solution. Then cerium is oxidized to the tetravalent state, and the rare earths other than cerium are mixed with an aqueous acidic persulfate-Ag + solution back-extracted into an aqueous acidic phase.

Solutions containing radioactive cerium and other lanthanide rare earths occur in waste products such as: B. aqueous or organic solutions or precipitates that arise in the treatment of uranium irradiated with neutrons. Before such waste products are further treated to separate the various radioactive fission products, they are usually stored for some time to remove short-lived radioactive and therefore dangerous isotopes, e.g. B. Pm ¹⁴⁸ to be removed; the waste products can then be more easily handled and treated with ion exchange resins with little or no damage to the latter from radioactivity. The removal of some radioactive isotopes through treatment or degradation also enables the storage of a larger amount of promethium and a reduction in protective measures. Pure cerium, as it is obtained by the method according to the invention, is used in power sources for satellites. Ce ¹⁴⁴ , one of the radioactive isotopes commonly found in the waste products described above, however, has too long a half-life to allow its degradation through storage within a reasonable period of time. It must therefore be removed by a chemical process.

So far, attempts have been made to separate the cerium by oxidation to tetravalent and by solution extraction the trivalent rare earths from the tetravalent cerium. These procedures lead to satisfactory results in non-radioactive solutions; but they are for radioactive solutions totally inadequate. It is very likely that radioactivity, in particular gamma rays, destroys the oxidizing agent, and it may also degrade the organic solvent that serves as an extractant. If z. B. Potassium permanganate is the oxidizing agent, it must be in the radioactive Solution to be replenished very soon. One has the same experience with lead dioxide and also with Sodium bismuth as an oxidizing agent for extraction and also with peroxide acetates, which are used as excretory agents find use for cerium.

Process for separating cerium from others
Rare Earth

Applicant:

United States Atomic Energy Commission,

Germantown, Md, (V. St. A.)

Representative:

Dipl.-Ing. J. Strasse, patent attorney,

Hanau / M., Frankfurter Landstr. 1

Named as inventor:

Lane Allan Bray,

Francis Paul Roberts, Richland, Wash. (V. St. A.)

Claimed priority:

V. St. v. America April 20, 1964 (361 629)

It is known that persulfates counteract the adverse effects of radioactivity at room temperature resist; however, they then oxidize the cerium at such a slow rate that their application becomes impractical. Higher temperatures are undesirable because then the degradation effect of the Radioactivity becomes too great. When a silver ion catalyst is added to the persulfate, the rate of oxidation is greater. However, the tetravalent cerium is not very stable and easily becomes in the trivalent state reduced, so that its extraction in and / or its retention in the organic Solvent quantitatively brings no yield and a moderate separation from the trivalent resulting in rare earths.

The invention aims to provide a method for separating cerium from other rare lanthanides Soils that are present in radioactive aqueous solutions at the same time, created by solution extraction in which the cerium is in tetravalent form almost in its entirety and over a relatively long period of time State can be maintained so that the trivalent rare earths easily and selectively through a long and thorough contact with the extractant can be extracted from him, and in which the organic solvent and the oxidizing agent reduce the effect of the present Resist radioactivity.

609 559/366

3 4

Preferably the oxidation and the profit-containing is then exploited by the aqueous

running at room temperature. Batch solution deposited. After that, the orga-

Contrary to all expectations, it turned out that the niche phase was combined with an aqueous solution, if certain critical concentrations of the bring the oxidizing agent in the concentrated silver catalyst, contains the persulfate anion and the acid 5 trationen according to the invention. As already . content are adhered to, if a quantitative order is carried out above, these concentrations are critical Conversion and retention of the cerium in the tetravalent state and must be turned on to achieve optimal results be maintained over a considerable period of time. The persulfate can be in acid form or can be obtained and particularly used sufficiently as an alkali salt; Ammonium, nalang, around the extraction of the trivalent lanthanidi- io trium or potassium persulphate are seen alike rare earths from tetravalent cerium.

and to be carried out carefully. The invention is therefore The silver ion can be in the form of any water through it characterized in that the critical concentrations of soluble salt are added, but in the case of one Trations 1 to 2 molar for the acid content, 0.02 to nitric acid solution, the nitrate is preferred. the 0.01 molar for the silver cation and about 0.2 molar for. 15 oxidizing solution for the back-extraction of the seldas Are persulfate anion. Preferably, the ten earths except cerium must free mineral acid dehydrated Oxidation solution from a 1 m saltpetre hold, preferably in a concentration of 1 m acid, 0.01 m silver nitrate and 0.2 m alkali persulfate in the case of nitric acid. Since this concentration is the solution. gives the highest degree of separation, it is not so

This invention becomes critical in Example 1 below, as are the concentrations of persulfate and

explained in more detail. Silver ions; satisfactory results can also

The process comprises the following steps: Ready- still with an acid content between 0.5 ni and not present an aqueous batch solution of radio approximately 10 m can be obtained. With these conditions active cerium and other isotopes of the lanthanide and concentrations becomes an almost quantitative exception Earth; Adjusting the pH of the batch traction of the trivalent rare earths and one almost solution between 1 and 4; Bringing together the quantitative retention of cerium in the organic Batch solution is accomplished with an organic one in phase solution. For the essentials just described Water-immiscible dialkylation stages were different temperatures under phosphoric acid, whereby the cerium and other lanthanum seeks. Halfway through temperatures up to 50 ° C Indian rare earths are extracted together; Ab- 30 produced satisfactory results and the fourth race an organic phase from the exhausted valuable cerium in this oxidation state over a aqueous batch solution; Bring together the organically appropriate period of time in the oxidation stage Let phase with an aqueous oxidation solution, the lower temperatures of with 1 to 2 m-mineral acid, 0.02 to 0.01 m-silver ion 35 ° C and 25 ° C as advantageous because they are a and about 0.2 m-persulfate anion, which gives cerium in the 35 longer extraction times without self-reduction tetravalent state is oxidized and allowed in the organi- of cerium. Room temperature was by far that See phase is held while the other three are best and is therefore preferred at this disposal Return rare earths to the aqueous solution.

to be extracted; Separating the aqueous solution from After separating the cerium-containing organic

the organic phase; Reconverting the 40 phase from the aqueous solution, which is the trivalent

contains tetravalent cerium in the organic phase in tri-rare earths, the organic phase becomes

valuable cerium; Bringing together the organic self-reduction of tetravalent cerium in the three-

Leave the phase with a weak state that is degraded by the mineral acid. It turned out that

pulling solution, which removes the cerium from the organic for this purpose approximately 15 hours at room temperature

see phase is separated, and deposition of the separated 45 are required. If it is desired, this reduction

To accelerate the drawn organic phase from the aqueous cer- tion, reducing agents,

solution. such as B. hydrogen peroxide are added.

Various known, water immiscible After the reduction is completed, the cerium dialkyl phosphoric acids are suitable for the process according to the organic phase with a weak bodice invention. For example, di- (2-ethyl- 50 ralic acid, preferably with nitric acid, with a hexyl) -phosphoric acid, bis- (2-ethylhexyl) -phosphorus concentration of about 2 m,
acid and octylphenylphosphoric acid are used. All extracts given in this description. The solvents can be applied in undiluted form tion stages can be carried out by known means, or they can be with an orga, either in a batch operation or in niche solvents, such as. B. paraffin oil, benzene, 55 a continuous countercurrent process.
Toluene or xylene. The best results should be If the trivalent rare earths were further wasted with a paraffin oil solution with 0.4 m di- should be turned, z. B. for medical purposes, (2-ethylhexyl) phosphoric acid and 0.2 m-tri-n-butyl- it might be desirable to get them from the sulfate and phosphate. to separate the silver associated with them. this

Before the batch solution with the organic 60 can be done by first checking the pH

Solution to extract together the cerium and the discontinued, and then the rare earths from the

other rare earths is brought together, it is aqueous solution by means of an organic solution: -

desirable, as already briefly mentioned, to be extracted by means of, e.g. B. Di- (2-ethylhexyl) -phos-

pH value between 1 and 4 to achieve the best result- phosphoric acid, from which it is then with a weak

nits to adjust. For this purpose, all mineral acids can be back extracted. This stage

ral acids are used, but nitric acid's process is not part of this invention,

acid preferred. The organic phase, all of which are The following are five examples to illustrate

, rare earth components including the cerium of the method according to the invention.

example 1

An aqueous waste solution was used which contained 800 Curies Ce ¹⁴⁴ per liter, 100 Curies Pm ¹⁴ ₇ per liter, as well as small amounts of other fission products of lanthanide rare earths and Sr ⁹⁰ , Ru ¹⁰⁶ and Zr-Nb ⁹⁵ . The nitric acid concentration was adjusted to a pH of 1.5 to 1.8.

A series of co-extractions was carried out. Each time, 11 of the waste aqueous solution became with 2.21 of a paraffin oil diluent with 0.2 m-tributyl phosphate and 0.4 m-di- (2-ethylhexyl) -phosphoric acid brought together at 35 ° C. The organic phases that result from each of these co-extraction attempts were found for studies of the determination of working conditions, which are described below are applied.

These experiments involved the extraction of trivalent rare earths other than cerium with an aqueous oxidizing solution containing silver ion, persulfate ion, and nitric acid in various concentrations. The contact temperature was 3 5 ⁰ C, and the volume ratio of organic solution to aqueous separating solution was equal to 1. After extraction and phase separation, a sample was taken from both phases and analyzed to determine the distribution coefficients of the obtained cerium. A persulfate concentration of 0.2 m was used in all tests. The results are summarized in Table I.

Table I (continued)

Tabel I. Ce in the organic Contact time ENT ₃ Ag + phase ° / o Minutes molar molar 99.3 60 1.0 0.1 84.1 120 1.0 0.1 16.6 180 1.0 0.1 98.8 60 1.0 0.05 92.0 120 1.0 0.05 42.5 180 1.0 0.05 99.1 60 1.0 0.02 98.9 120 1.0 0.02 98.2 180 1.0 0.02 94.9 240 1.0 0.02 99.2 60 1.0 0.01 99.4 120 1.0 0.01 99.5 180 1.0 0.01 • 99.2 240 1.0 0.01 99.0 300 1.0 0.01 96.9 30th 2.0 0.10 51.9 60 2.0 0.10 1.3 120 2.0 0.10 99.1 60 2.0 0.05 98.6 120 2.0 0.05 92.4 180 2.0 0.05 61.6 240 2.0 0.05 99.2 70 2.0 0.02 98.8 160 2.0 0.02 97.4 220 2.0 0.02 88.9 280 2.0 0.02 98.9 60 2.0 0.01 98.8 120 2.0 0.01 98.6 180 2.0 0.01 97.2 240 2.0 0.01

Contact time ENT ₃ Ag + Ce in the organic 5 Minutes molar molar phase 300 2.0 0.01 7o 60 4.0 0.01 94.0 IO 120 4.0 0.01 99.2 240 4.0 0.01 97.8 300 4.0 0.01 96.7 360 4.0 0.01 96.0 420 4.0 0.01 94.1 15th 70 4.0 0.02 91.7 160 4.0 0.02 96.1 220 4.0 0.02 93.5 60 2.0 - 86.4 120 2.0 - 2.97 20th 180 2.0 - 2.33 2.11

The above data show that at nitric acid concentrations of 1 m and 2 m, the cerium was kept practically completely in its tetravalent state and so for the first hour in the organic phase, while thereafter a reduction to the trivalent state within a more or less short period of time occurred which naturally affected the deposition. Trivalent cerium was deposited from the organic phase together with the other trivalent lanthanides. However, where the silver ion concentration was 0.01M, quantitative cerium deposition (99 ° / ₀ ) and almost quantitative deposition (94 ° / ₀ ) with 2M-nitric acid could be maintained for 5 hours with 1M-nitric acid will. With a nitric acid concentration of 4M, the cerium was reduced to a remarkable degree after 2 hours when the silver concentration was 0.02M; however, 4 m-nitric acid with an optimal silver concentration of 0.01 m gave a satisfactory cerium content over 5 hours. Without the silver catalyst and a nitric acid concentration of 2 m there was hardly any cerium oxidation, as is evident from the low cerium deposition in the organic phase.

In some experiments the organic phases for the back extraction of the cerium were in a aqueous cerium solution treated again. For this purpose the organic phases were washed with nitric acid with a concentration of about. 2 m brought together at room temperature. It needed usually between 15 and 16 hours to convert about 98% of the cerium to nitric acid, when no reducing agent has been used.

Example 2

Another series of experiments was carried out similar to that of Example 1, but using equal volumes of organic extractant and aqueous batch solutions at 35 ° C. The organic extractant had the same composition as in Example 1, the aqueous separating solution was always 0.2 Hi-K ₂ S ₃ O ₈ , 1 m-nitric acid,

but had different concentrations of silver ions. The results are summarized in Table II.

Table II

Contact time Ag + Ce in the
organic phase Minutes molar ° / o 20th 0.1 99.3 40 0.1 99.5 60 0.1 99.3 90 0.1 98.4 120 O ₅ I. 84.1 150 0.1 41.5 180 0.1 16.6 20th 0.05 98.9 40 0.05 99.0 60 0.05 98.8 90 0.05 97.3 120 0.05 92.0 150 0.05 75.1 180 0.05 42.5 210 0.05 21.6 20th 0.02 96.4 40 0.02 99.2 60 0.02 99.1 90 0.02 99.0 120 0.02 98.9 150 0.02 98.6 180 0.02 98.2 210 0.02 96.7 240 0.02 94.9 270 0.02 90.0 300 0.02 82.7 30th 0.01 95.4 30th 0.01 90.1 60 0.01 99.2 60 0.01 99.4 120 0.01 99.4 120 0.01 99.4 180. 0.01 99.5 180 0.01 99.4 240 0.01 99.2 240 0.01 98.4 300 0.01 99.0 300 0.01 96.2 360 0.01 97.4 360 0.01 90.2

10

Table III

Contact time K2S2O3 Ce in the
organic phase Minutes molar % 30th 0.1 - 76.2 60 0.1 95.7 120 0.1 95.1 180 0.1 84.0 240 0.1 57.8 300 0.1 31.0 360 0.1 16.0 20th 0.2 96.4 40 0.2 99.2 60 0.2 99.1 90 0.2 99.0 120 0.2 98.9 150 0.2 98.6 180 0.2 98.2 210 0.2 96.7 240 0.2 94.9 270 0.2 90.0 300 0.2 82.7

50

These experiments show, like those of Example 1, that with a silver concentration of 0.01 m at allow far the best results. The same results were also obtained with analogous experiments obtained in which the nitric acid concentration of the aqueous separating solutions was 2 m.

Example 3

In this example, the results of various tests with a separating solution of 0.1 m-potassium persulfate, 0.02 m-Ag ⁺ and 1 m-nitric acid were compared with the analogous values from Table II; the conditions including the composition of the organic extractant were the same in both cases, with the exception that in table Π the persulfate concentration was 0.2 m.

20th

25 From the above it can be seen that a persulfate concentration of 0.1 M is insufficient for a period longer than 2 hours and that the concentration of 0.2 M for the persulfate is then superior.

Example 4

A number of experiments were carried out on a non-radioactive batch solution to examine the effects of temperature changes. Temperatures of 25 ° C, 36 ° C and 50 ⁰ C were applied.

The organic solution was a paraffin oil solution, 0.4 m-di (2-ethylhexyl) phosphoric acid, 0.2 m-tributyl phosphate, 0.01 m-cerium and 0.01 m-neodymium. The aqueous separating solution was 0.1 m potassium persulfate, 0.02 m silver nitrate and 1 m nitric acid. There were brought together equal volumes of the organic and aqueous solutions. The results are in Table IV summarized.

Table IV

Time Ce in the organic Phase (%) Minutes 25 ° C 36 ° C 1 18.7 19.4 5 63.0 98.8 30th 99.4 99.4 60 99.6 99.5 90 99.5 99.5 120 99.5 99.5 50 ⁰ C 52.2 99.4 99.5 99.4 99.0 92.1

Show the above results, that a certain time is necessary to oxidize cerium in the tetravalent organic extendable state, and this time is shorter at elevated temperatures (30 minutes at 25 ° C, for about 5 minutes at 36 ° C and 5O ⁰ C). The results show that at 50 ⁰ C after approximately 2 hours the cerium content in the organic

Solution falls, while at 25 ° C and 36 ⁰ C, the extraction is extremely quantitative (99.5 ° / _0).

Example 5

An aqueous batch solution with 35 kilocuries of cerium and 60 kilocuries of μm ¹ * ⁷ and a pH of 2 was mixed with a solution of paraffin oil, 0.4 m-di (2-ethylhexyl) phosphoric acid and 0.2 m-tributyl phosphate extracted. The organic phase thus obtained was brought together at room temperature with an aqueous solution of 2 m-nitric acid, 0.2 m-potassium persulfate and 0.02 m-silver nitrate. The phases thus obtained were separated and the aqueous phase was analyzed. It contained only 0.6 ° / ₀ cerium and promethium altogether.

In the above examples, a solvent was used which is composed of di- (2-ethylhexyl) phosphoric acid, Tributyl phosphate and paraffin oil consisted of 0.4 m-di- (2-ethylhexyl) phosphoric acid and 0.2 m-triao butyl phosphate.

Di- (2-ethylhexyl) phosphoric acid is the active extractant for rare earths. Since they are in is substantially insoluble in water, its use in paraffin oil improves phase separation. The tributyl phosphate acts as an accompanying solvent for the paraffin oil and the sodium salt of oleic (2-ethylhexyl) phosphoric acid. The latter ingredient is not in the solvent during the extraction steps present, but it is formed when the solvent is ready for reuse by treatment with aqueous sodium carbonate is purified. The tributyl phosphate prevents a third phase formation. The di (2-ethylhexyl) phosphoric acid concentration in the mixed solution can vary from 0.01 m to 1 m. The concentration of tributyl phosphate should be about half that of di- (2-ethylhexyl) phosphoric acid to have.

Claims (3)

Patent claims: 1. Process for separating cerium from other rare earths in aqueous radioactive solution by co-extraction of cerium and rare earths into an organic dialkylphosphoric acid solution, Oxidation of the cerium to the tetravalent level and back extraction of the rare earths except cerium with an aqueous acidic persulfate Ag + solution in an aqueous acidic phase, characterized in that that the concentration of the acid 1 to 2 molar, that of the silver ions 0.01 to 0.02 molar and the persulfate ion is about 0.2 molar. 2. The method according to claim 1, characterized in that the organic solvent for the Dialkylphosphoric acid a paraffin oil solution with 0.4 m - di - (2 - ethylhexyl) - phosphoric acid and 0.2 m-tributyl phosphate. 3. The method according to claim 1, characterized in that the aqueous oxidation solution from a 1 m nitric acid, 0.01 m silver nitrate and 0.2 m alkali persulfate solution. 609 559/366 4.66 © Bundesdruckerei Berlin