FR2661667A1 - PROCESS FOR OBTAINING MIXED OXIDES OF TYPE (A / B) O2. - Google Patents

Abstract

a) Process for obtaining mixed oxides of the (A / B) O2 type, where A is uranium b) Process characterized in that f) the metals together using an organic cation are precipitated in the form of anionic complex compound, g) the decomposition of the separated precipitate is carried out in air at 400 to 800 degrees C and h) as a reducing gas atmosphere a carbon monoxide atmosphere is used. c) The invention relates to a process for obtaining mixed oxides of the (A / B) O2 type

Description

"Process for obtaining mixed oxides of type (A / B) O 21 'The invention

  relates to a process for obtaining mixed oxides of the type (A / B) 02 based on two metals A and B in which A represents Uranium, a) which comes from an aqueous solution of the metals in which the Uranium is in hexavalent form as U 02 and the metal B is tetravalent as B 4 +, b) Carbonate is added to the solution, c) a p H value is adjusted in the alkaline zone, d) the metals are precipitated, e) the deposit precipitate is separated and treated again, for which the treatment again comprises the process steps: of composition and

  heating under a reduced gas atmosphere

  A process of this type is known from German patent 2,811,959. According to this process, the crystals

  mixed (U / Pu) 02 soluble in nitric acid 10-

  molars as a preliminary stage for the manufacture of sintered nuclear fuel compounds obtained by mixing a solution of uranyl nitrate and a solution of plutonium nitrate, in that the plutonium is also oxidized in the mixture to the valence level of uranium, which is then put back into a 20% solution of ammonium carbonate, oxalate or carbamate, while NH 3 is added little by little at the same time until the p H value is adjusted to 8.1 to 8.5 as well as C 02 in excess, that the temperature is maintained at 45 to 600 C for the precipitation of a U / Pu complex, that l '' the product obtained is dried, decomposed to about 1000 under vacuum, calcined in a reducing atmosphere at 300 to 7000 C, and then under air supply in (U / Pu) 02 with a desired ratio Oxygen / Metal As a reducing atmosphere a

  Argon / Hydrogen atmosphere is proposed.

  Uranium and Plutonium occur in

  precipitation as a cationic complex.

  The method proposes a step of oxidizing the

  plutonium from the tetravalent state to the hexavalent state.

  Oxidation can be carried out by overheating and concentration of the starting solution at 134 to

  1500 C It is however possible to carry out the oxidation

  electrochemically or using agents

  oxidation like Ago or 03 During overheating-

  fe and by electrochemical oxidation expensive devices are necessary for the execution of the process, which consume a lot of energy The addition of oxidizing agents is unfavorable due to possible impurities of the mixed oxide which has formed (Ago) or in

  reason for security related objections (03).

  The invention provides a simplified method of the type mentioned first for which the oxidation of Plutonium from the tetravalent state to the hexavalent state

is superfluous.

  The objective is resolved in accordance with the

  vention, in that f) the metals together using an organic cation are precipitated in the form of an anionic complex compound, g) the decomposition of the separated precipitate is carried out in air at 400 to 8000 C and h) as reducing gas atmosphere a carbon monoxide atmosphere is used.

  According to another characteristic of the

  vention the metal B is formed from Thorium or

Plutonium or Cerium.

  According to another characteristic of the

  vention as organic cation pentadecyl, Hexadecyl-, heptadecyl and octadecylpyridinim is

put in play.

  According to another characteristic of the

  ention the precipitation of metals takes place at a temperature between 20 e C and the freezing point of

the solution.

It is known to European patents

  0 224 277, O 228 051 and O 228 679 to selectively separate

  uranium and plutonium by each other

use of organic cations.

  These methods cannot solve the problem according to the invention, because at the same time the uranium and the plutonium are precipitated one after the other.

  the other Between the first and the second precipitated-

  tion, the valence of nuclear fuel still

  kept in solution should be changed.

  In addition, a process for the production of Uranium and / or Plutonium is known from the German patent application (OS) published under No. 3,442,642.

  of a sulfuric or nitric solution by precipitate-

  tion using organic cations and separation and subsequent transformation of the deposited precipitate, in which the deposited surfactant precipitate is calcined to ash in Uranium and / or Plutonium oxides. In this process Uranium and Plutonium do not

  are not, however, also precipitated jointly.

  The joint precipitation of Uranium and Plutonium is not possible, according to this process in principle, since one works in an acid medium This also comes from the example of execution in which the Plutonium is precipitated using Cetyl- (hexadecyl-) pyridinium in acid solution, is separated, then

  kneaded with UO 2 powder and subjected to a treatment

  This is how, also according to this process, no mixed oxide can be obtained by

  joint precipitation of Uranium and Plutonium.

  Uranium and the second metal of the mixed oxide which can represent for example Plutonium, Thorium or Cerium, are present in aqueous solution at different degrees of Valence The addition of either precipitating reagents or redox agents alone leads to precipitates containing essentially only one of the two metals In reality for a joint precipitation of the two metals, it is essential that the metals jointly be precipitated from an alkaline solution containing carbonates or bicarbonates, using an organic cation in the form of complex compounds

anionic.

  According to the invention the metals are co-

  precipitated by the organic cation, from the solution containing carbonates and / or bicarbonates in the form of Carbonato or Hydroxocomplexes or in the form of mixed Carbonato-Hydroxocomplexes As organic cations, cations are particularly suitable

  hydrophobic Pentadecyl-, hexadecyl- (Cetyl), heptade-

  cyl and octadecylpyridinium which are used

in the form of their salts.

  To obtain the solution containing carbonates and / or bicarbonates, the corresponding alkali metal and / or ammonium salts can be used. Preferably sodium carbonate

  or sodium bicarbonate are used.

  The sum of the carbonate and bicarbonate concentrations must rise five times or 20 times preferably ten times the sum of the metal cations to be precipitated Under these conditions more than

99% of the metals are precipitated.

  The p H value of the solution must be between 8 and 10 Particularly preferred is

a p H value of 9.

  The temperature for the precipitation of the metal must be in the case of the use of octadecylpyridinium below 20 C, in the case of the use of pentadecyl-, hexadecyl and heptadecylpyridinium, below 40 C Particularly preferred are temperatures below OOC up to, just above, the freezing point of

the solution.

  The molar ratio of the metals in the precipitate at constant p H values at constant temperature, and at a molar ratio fixed in advance of the metal ions UO 22 + and B 4 + can be adjusted by variation of the sum of the concentrations of carbonates

  and bicarbonates in a continuously variable manner.

  The precipitate formed is calcined after separation, in air at a temperature ranging from 400 to 8000 ° C. and then reduced from 500 to 1100 ° C. in a stream of carbon monoxide For the preparation of the mixed oxides U / Th the temperature is preferably between 500 and 700 OC For the preparation of mixed oxides U / Ce temperatures between 900 and 11000 C are preferred. The duration of the reduction typically amounts to approximately 30 minutes, it however depends

of the composition of the precipitate.

  An essential advantage of the process according to the invention, in particular during the preparation of nuclear fuels, consists in that the oxides

  mixtures are easily soluble in nitric acid.

  Particular mixed oxides are obtained-

  easily soluble when the molar ratio of Uranium to metal B is at least 1: 1 For higher contents of Uranium the solution improves Mixed oxides obtained according to the process according to the invention, of Uranium and Plutonium or Thorium and Cerium in the 1: 1 molar ratio can still be easily dissolved in 40% nitric acid (5 to 6 molars) at temperature

  ordinary, at least at high temperature.

  An additional essential advantage resides in the fact that very dilute solutions of metals can also be used. These solutions can also be solutions of residue of the processes, in which Uranium and for example Plutonium have

been implemented.

  The invention will be explained more precisely on the basis of exemplary embodiments in this

following.

  For these tests, the Plutonium will be simulated by Thorium which behaves chemically

in a very similar way.

Example 1

  Precipitation of Th 4 + aqueous solution at

  a metal concentration of 10-1 mol / l.

  100 ml of a solution of 0.1 was prepared

  mol Th 4 + in distilled water (solution 1).

  In addition, 100 ml of a 0.1 molar NCetylpyridinium chloride solution was prepared in a 0.2 molar ammonium carbonate solution at p H 9.

(solution 2).

  Solution 1 was added to solution 2 for as long, until the formation of precipitate (pure white, fine, well filterable) was

completed.

  Thorium was in a rush

more than 99%.

Example 2

  Precipitation of Th 4 + of a solution

  with 10-2 mol / i carbonates at high temperature. 100 ml of a solution of

  Thorium Th 4 + 0.1 molar, N- chloride

  0.1 molar cetylpyridinium, and Ammonium Carbonate

  0.1 molar nium at p H 9 in distilled water and the mixture is heated to 60 ° C. This solution was then cooled to 4 ° C. and filtered at ordinary temperature.

  precipitate was white, fine and well filterable.

  Thorium was in the precipitate at

more than 99%.

Example 3

  Joint precipitation of UO 22 + and Th 4 + (ratio 2: 1) from a 1 molar nitric solution using a Cation / Carbonate solution for a total concentration of metals of 1 molar also. Two solutions were prepared 1 Solution of 1.66 g of uranyl nitrate and 0.94 g of thorium nitrate in 5 ml of acid

1 molar nitric.

  This corresponds to a U / Th molar ratio of 2: 1. 2 Solution of 9.60 g (0.1 molar) of Ammonium Carbonate and 17.90 g (0.05 molar) of N-cetylpyridinium chloride in 1000 ml of distilled water. Solution 1 has been added to solution 2 and there is immediately produced a yellow and well filterable precipitate, which contains uranium and Thorium with a yield of more than 99% respectively.

Example 4

  Joint precipitation of a saturated solution with respect to Uranium, Uranium and Thorium (2: 1 ratio) in a Carbonate solution

  0.1 molar with a Carbonate / Cation solution.

  Two solutions were prepared: 1 50 ml of a solution of 1.30 g (0.05 molar) of uranyl nitrate and 0.74 g (0.025 molar) of Thorium nitrate in a solution of Ammonium Carbonate 0.1 molar (0.48 g per 50 ml) This

  corresponds to a U / Th molar ratio of 2: 1.

  2 A solution of 9.60 g (0.1 molar) of Ammonium Carbonate and 17.90 g (0.05 molar) of N-cetylpyridinium chloride in 1000 ml of distilled water

lée.

  Solution 1 has been added to solution 2 and after approximately 5 minutes precipitation occurs,

  recognizable by its pearly reflections.

  The well-filterable yellow-green precipitate contains Uranium and Thorium with a yield of,

respectively, more than 99%.

Example 5

  Joint precipitation of U 022 + and Th 4 + from an aqueous solution for a total concentration

in metals of <10-2 mol / l.

  1 1 of a respective solution was prepared.

  5 x 10-3 molar UO 22 + and Th 4 + in distilled water to this solution was then added 0.1 mol of solid ammonium carbonate and the value of p H was adjusted to about 9 with ammonia water.

  Finally, 0.5 mol of N- chloride was added

  cetylpyridinium solid and stirred for half a

  hour A light green, fine precipitate formed which could be conveniently separated by filtration. The two metals were more than 99% in the precipitate.

Example 6

  Joint precipitation of UO 22 + and Th 4 + (molar ratio 1: 1) of a solution of Carbonate 5

X 10-3 molar.

  We prepared two solutions of respective-

  100 ml with 5 x 10 + 4 mol / l of Uranyl nitrate, x 10-4 mol / i of Thorium nitrate in 5 x 10-3 mol / l of Ammonium Carbonate After adjusting the p value H to 9 with ammonia was added in each of the two solutions 10-3 mol of the surfactant octadecylpyridinium bromide under

form of solid substance.

  One of the solutions was stirred for 2 hours at 20 ° C, the second was stirred for 2 hours at 40 C.

  Then the solutions rested at temperatures-

  corresponding tures for an additional 4 hours

  res After filtration of the precipitate, the filtrate was analyzed for Uranium, Thorium and organic Cation In addition, the surface tension of the filtrate was determined The following table shows the results: Temperature Concentration Residual tension in mol / l surface Precipitation of the filtrate in

O -1

C n N m Uranium Thorium Surfactant

-6 -7 -5

4 1.53 10 2.57 10 <10 52.7

-6 -7 -5

1.42 10 3.21 10 <10 54.8

  It emerges from this example that the process according to the invention is also suitable for the preparation of mixed oxides of nuclear fuels from a residue solution having a lower metal concentration. As can be seen from the table, the metal concentrations also although the concentration of organic cation (surfactant) also is very low in the filtrate after precipitation.

Example 7

Preparation of a mixed U / Th oxide.

  The precipitate obtained in a corresponding manner

  according to examples 3 and 4, but however with a U / Th molar ratio of 1: 1 was in air at a temperature ranging from 600 to 8000 C and then reduced to

  About 5000 C in a stream of carbon monoxide.

  A complete mixed oxide is obtained with defined lattice constants which roughly characterize a 1: 1 composition. The mixed oxide dissolves in total in 6 molar nitric acid by light

  heating without residue remaining.

Claims (1)

CLAIMS) Process for obtaining mixed oxides of the type (A // B) 02 based on two metals A and B in which A represents Uranium, a) which comes from an aqueous solution of the metals in which l 'Uranium is in hexavalent form as U 02 and the metal B is tetravalent as B 4 +, b) Carbonate is added to the solution, c) a p H value is adjusted in the alkaline zone, d) the metals are precipitated, e) the precipitate from the deposit is separated and treated again, for which the treatment again comprises the process steps: of composition and heating under an atmosphere of reducing gas, characterized in that f) the metals together using an organic cation are precipitated in the form of an anionic complex compound, g) the decomposition of the separated precipitate is carried out in air at 400 to 800 ° C and h) as the atmosphere of reducing gas an atmosphere of carbon monoxide is used e.   20) Method according to claim 1, charac-   terized in that the metal B is formed of Thorium or Plutonium or Cerium.   3) Method according to claim 1, charac-   terized in that as an organic cation pentadecyl, Hexadecyl-, heptadecyl and octadecylpyridinim is put in play.   ) Method according to claim 1, characterized in that the precipitation of metals takes place at a temperature between 200 C and the freezing point of the solution.