Liquid purification or separation – Processes – Ion exchange or selective sorption
Patent
1996-02-05
1997-12-30
Nessler, Cynthia L.
Liquid purification or separation
Processes
Ion exchange or selective sorption
210669, 210670, 210673, 210682, 210912, C02F 142
Patent
active
057026084
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION AND RELATED ART
The present invention relates to a process for the decontamination of radioactive nitric aqueous effluents containing strontium, particularly in trace form, and sodium. Such effluents are more particularly obtained in spent nuclear fuel reprocessing installations.
In such installations, the reprocessing operations start with a dissolving of the fuel in nitric acid followed by a series of extraction-reextraction stages by means of an organic solvent based on tributyl phosphate in order to separate from the dissolving nitric solution the three following fractions: and matrix.
All these operations give rise to liquid effluents which, although only being weakly radioactive, must still undergo decontamination. These effluents have a composition which normally fluctuates around the following values: radioactive element, and
It is therefore necessary to eliminate all troublesome products from these effluents. In the case of strontium, this can in particular be performed by a coprecipitation with barium sulphate, which entrains between 90 and 95% of the radioactive element. However, this process suffers from the disadvantage of producing a significant quantity of sludge, namely 3.5 l per 1 m.sup.3 of treated effluents.
Another process for the recovery of strontium from acid solutions uses the solvent extraction method, using a crown ether in a diluent able to dissolve a certain quantity of water, e.g. an alcohol such as octanol. This process described in U.S. Pat. No. 5,100,585 is suitable for the extraction of strontium from solutions having a nitric acid concentration up to 6 mole/l, but it does not make it possible to obtain a high strontium extraction selectivity compared with the sodium in the case of effluents having relatively high sodium quantities compared with the strontium quantities.
The strontium elimination process must meet certain industrial demands, particularly with regards to the concentration factor. Thus, the displacement of the strontium in a new volume of the same order of magnitude as that of the effluent, leads to doubling the effluent quantity to be conditioned, which solves no problem at all and even aggravates the situation.
In addition, the concentration of the strontium by selectively separating it from the sodium and its fixing on a matrix compatible with a definitive conditioning constitute the objective to be achieved in a process for the decontamination of effluents containing both strontium and sodium.
SUMMARY OF THE INVENTION
The present invention specifically relates to a process for the decontamination of radioactive nitric aqueous effluents containing strontium and sodium, which permits the selective separation of the strontium and the fixing thereof to a matrix permitting its definitive conditioning by vitrification.
The process according to the invention comprises the following stages: crown ether and a diluent, chosen so as to obtain an extraction selectivity of strontium compared with sodium such that the D.sub.SR /D.sub.Na ratio, in which D.sub.Sr and D.sub.Na respectively represent the strontium and sodium distribution coefficients, is equal to or above 100 and D.sub.Sr is at least equal to 1, solution, and exchange resin.
This process is more particularly applicable to nitric effluents having a nitric acidity of 0.5 to 2N, a strontium concentration up to 1 .mu.g/l and a sodium concentration of 0.01 to 0.2 mole/l, i.e. effluents containing much more sodium than strontium.
Through the choice of an appropriate organic phase favouring the selective extraction of strontium compared with sodium, with the process of the invention it is possible to isolate the strontium from such effluents, then concentrate it on a cation exchange resin with a high concentration factor, e.g. 5000, which permits the definitive conditioning of the strontium under good conditions.
In this process, the high selectivity can be obtained by choosing an appropriate diluent-crown ether pair. It is also possible to act on the selectivity by choosing a part
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Doutreluingne Pierre
Foos Jacques
Guy Alain
Le Buzit Gerard
Leclere Bruno
Compagnie Generales Des Matieres Nucleaires
Nessler Cynthia L.
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