The present invention relates to a process for the extraction of cesium ions from aqueous solutions using an adduct compound comprising a macrocyclic polyether (crown ether) and an inorganic heteropoly acid component.
The processing and solidification of medium active aqueous wastes (MAW), developed during the reprocessing of irradiated nuclear fuels and/or nuclear breeder materials, e.g., in form of waste solutions, waste concentrates or waste sludges, is rendered more difficult by the presence of small quantities of the cesium isotopes Cs.sup.134 and Cs.sup.137. During the vitrification of MAW, cesium evaporates to a noticeable degree and is also easily leached out of the solidified products intended for permanent storage, such as glass blocks, cement blocks and bitumen blocks. Selective extraction of the Cs would considerably simplify the treatment of medium active wastes. Furthermore, if Sr.sup.90 were simultaneously extracted from the MAW, low radioactive aqueous wastes (LAW=low active waste) would be obtained since, after a relatively short decay period, practically the entire activity of MAW originates from the relatively long-lived radionuclides Cs.sup.137 (t.sub.1/2 =30 yrs) and Sr.sup.90 (t.sub.1/2 =26 yrs).
According to prior-art processes, the extraction of Cs and Sr was achieved mainly by means of coprecipitation reactions. The coprecipitation reactions, however, did not result in satisfactory DF (decontamination factor) values for Sr and Cs. Therefore, different processes have been investigated which would make possible a selective extraction of the two radionuclides, either jointly or individually.
The extraction processes for Cs.sup.+ ions developed thus far, however, are not suited for the extraction of Cs.sup.+ from a typical MAW with a high concentration of NaNO.sub.3 and free nitric acid. The composition of such a typical MAW is shown in the following Table 1:
TABLE 1 ______________________________________ Components Concentration (strongly acidic MAW) g/dm.sup.3 ______________________________________ Nitric acid 64 (1 mol/dm.sup.3) Sodium nitrate 42.5 (0.5 mol/dm.sup.3) Uranium 1.0 Lead 0.8 Calcium 0.2 Magnesium 0.2 Iron 0.15 Ruthenium 0.07 Potassium 0.05 Zinc 0.05 Copper 0.04 Manganese 0.02 Chromium 0.02 Nickel 0.01 Zirconium 0.005 Cesium 0.004 Cerium 0.004 Strontium 0.002 Antimony 0.002 Niobium 0.001 ______________________________________
The extraction processes for Cs.sup.+ ions that have been developed use mostly organic reagents which are added either to the organic phase or to the aqueous phase to be extracted. These reagents, however, would be destroyed either hydrolytically or by oxidation, or would be made ineffective by protonization if they came into contact with the MAW whose essential components are such as those described in Table 1 above.
Cs.sup.+ selective heteropoly acids are known which are very stable in the highly acid environment provided by an MAW having a composition such as described in Table 1. The Cs.sup.+ selective heteropoly acids are used as extraction agents in the form of an organic phase with nitrobenzene as a solvent. For example, 12-molybdophosphoric acid (HPMo) is a Cs.sup.+ selective heteropoly acid which can be dissolved in nitrobenzene. In this form, the heteropoly acids are equally unsuitable for the continuous Cs.sup.+ extraction from an MAW of the type described in the above Table 1, since they bleed considerably from the nitrobenzene into the aqueous phase due to their good water solubility. They would have to be replaced continuously.
If HPMo is used, the molybdenum would enter into the MAW in large quantities and would furthermore interfere with the subsequent processing of the MAW, such as during vitrification. Finally, depending on the Cs.sup.+ concentration of the MAW and the heteropoly acid concentration in the nitrobenzene phase, there is the danger of precipitate formation which could lead to considerable disruptions in a continuously operating system.
In contrast to the above prior-art water soluble extraction agents for Cs.sup.+, dibenzo crown ethers have a very low water solubility and are thus, at least in part, suitable for use in a continuous extraction process.
Since crown ethers are neutral ligands, the anion in the aqueous solution being treated is also extracted during the extraction. This, however, significantly influences the extraction coefficient. In this way, simple cesium salts, such as chloride and nitrate salts, are very difficult to extract with the use of crown ethers in polar solvents, with the exception of nitromethane solutions and nitrobenzene solutions. For this reason, the prior art started to combine crown ethers with known Cs.sup.+ specific reagents which contain voluminous, polarizable anions. Up to now, this has been done by adding the Cs.sup.+ specific reagents, in most cases picric acid to the aqueous phase to be extracted,, and extracting with the aid of crown ethers, in most cases dibenzo-18-crown-6 (DB-18-C-6) or dicyclo-hexyl-18-C-6, in polar solvents, in most cases nitrobenzene.
For the extraction of Cs.sup.+ from nuclide containing solutions, J. Rais and P. Selucky proposed two extraction systems which use DB-18-C-6. In one method, as described in Czechoslovakian Pat. No. 149403, DB-18-C-6 was employed in an organic phase and dipicrylamine was added to the aqueous phase to be extracted. In a second method, as described in Czechoslovakian Pat. No. 149404, DB-18-C-6 was employed in an organic phase and sodium tetraphenylborate was added to the aqueous phase to be extracted.
These processes, however, are limited to alkaline Cs.sup.+ solutions having a pH of 11 to 13 inasmuch as dipicrylamine is protonized in the acid range, and sodium tetraphenylborate is hydrolyzed in the acid range. Moreover, both processes only function well in the absence of large quantities of Na.sup.+ and K.sup.+.
Furthermore, the same authors, as disclosed in Czechoslovakian Pat. No. 165751, have produced an adduct of DB-18-C-6 with 12-molybdophosphoric acid which permits a column chromatographic extraction of the Cs.sup.+ from waste solutions. Large quantities of Na.sup.+ and K.sup.+, however, also present a problem here. The adduct of DB-18-C-6 with 12-molybdophosphoric acid was prepared by a two phase reaction in which an organic phase comprising a solution of the crown ether in dichloromethane was shaken with an aqueous phase comprising a solution of HPMo in 2 mol/1 HNO.sub.3. This patent also refers generally to adducts with 12-tungstophosphoric acid (HPW). A similar disclosure of an adduct of DB-18-C-6 with molybdophosphoric acid appears in J. Radioanal. Chem. Vol. 35 (1977), pages 351 to 359.