Patent Number: 047909609
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a process for the stripping of cesium ions from aqueous solutions in which a precipitation agent is added to the aqueous solution and the resulting precipitate, containing the Cs.sup.+ ions, is stripped from the solution. Cs-137 in its property as hard gamma ray emittor, is a particularly undesirable fission product in medium radioactive aqueous waste products (MAW), and renders more difficult the processing and solidification of MAW. A prior selective stripping of the Cs-137 would considerably simplify the further processing of medium radioactive waste products. After stripping the Cs-137 from the MAW, the shielding of the concentrate and/or the solidified ultimate waste package could be totally or at least partially omitted. In addition, such a process could also be profitably used for obtaining or stripping of Cs isotopes from highly active waste solutions as they occur, for example, in the reprocessing of nuclear fuels in the first extraction cycle. Here, the extraction of pure isotopes or isotope mixtures of cesium would be of practical importance for radio-chemical use and as radiation or heat source. In the past, attempts have been made to precipitate Cs.sup.+ ions with sodium tetraphenylborate (commercial name Kalignost), but it has been determined that such a precipitation cannot be done either selectively or in an acid milieu. The stripping of cesium is done, according to a known process, mainly by coprecipitation reactions. However, the coprecipitation does not supply satisfactory decontamination factors for Cs (DF values). Thus, other processes were locked for which would permit a selective stripping of the cesium radionuclides. The extraction procedures which have so far been developed for Cs.sup.+ ions are not suitable for stripping Cs.sup.+ from a typical MAW with its high content of NaNO.sub.3 and free nitric acid. J. Rais and P. Selucky proposed an extraction system for stripping Cs.sup.+ from aqueous solutions, which uses 2,3,11,12-dibenzo-1,4,7,10,13,16-hexa-oxo-cyclo-octadeca-2,11-dien (dibenzo-18-crown-6DB-18-C-6 for short) in an organic phase and sodium tetraphenylborate was added to the aqueous phase which forms a Cs.sup.+ containing adduct to be extracted. (Czechoslovakian Patent CS-PS No. 149,404). However, the process is limited to alkaline Cs.sup.+ solutions (pH 11 to 13) because sodium tetraphenylborate is hydrolyzed in the acid range. In addition, the process only works well in the absence of substantial amounts of Na.sup.+ and K.sup.+. SUMMARY OF THE INVENTION A primary object of the present invention is to provide a process of the type stated above in which cesium can be stripped selectively, with high effectiveness from aqueous solution, as compared to other alkaline metal cations, such as Li.sup.+, Na.sup.+ and K.sup.+. Additional objects and advantages of the present invention will be set forth in part in the description which follows and in part will be obvious from the description or can be learned by practice of the invention. The objects and advantages are achieved by means of the processes, instrumentalities and combinations particularly pointed out in the appended claims. To achieve the foregoing objects and in accordance with its purpose, the present invention provides a process for the stripping of cesium ions from aqueous solution in which a precipitation agent is added to the aqueous solution and the resulting precipitate, containing the Cs.sup.+ ions, is separated from the solution, comprising adding a sodium or lithium tetraphenylborate having electron-attracting substituents on the phenyl rings as a precipitation agent. Preferably, the compounds which are used as precipitation agents are compounds in which the phenyl rings are substituted one to five times. Particularly good results are obtained with a compound which is disubstituted, in each of its phenyl rings in the 2,4 positions of the phenyl rings. However, compounds which are fourfold substituted in each of its phenyl rings in the 2,3,5,6 positions of the phenyl rings, or fivefold substituted in each of its phenyl rings in the 2,3,4,5,6 positions of the phenyl rings can also be successfully used. A particularly advantageous version of the process according to the present invention occurs when the substituents on the phenyl rings are fluorine atoms. An effective embodiment of the process occurs when the addition of the precipitation agent and/or the precipitation reaction, as such, takes place or is done at a temperature of between 239.degree. K. and 303.degree. K. Preferably, the precipitation agent is added to the solution at a slight excess with regard to the cesium content, e.g. between 1.2 times to 5 times the stoichiometrically-needed amounts. The stripping of the precipitates, can be done, for example, either through filtration, liquid extraction, centrifugation or flotation. A particularly good stripping is attained with the process according to the invention when the solution containing the cesium ions (a) is adjusted to a Cs.sup.+ concentration in the range of between 10.sup.-1 and 10.sup.-3 mol/l and (b) the precipitation agent is added to the solution from step (a) and the resulting precipitate is stripped off. Steps (a) and (b) can be repeated at least once to achieve a desired decontamination of existing Cs-137 with the repetition of step (a) being conducted with inactive cesium (as carrier). Thus, in the repetition of step (a) inactive cesium is added to adjust the solution to the concentration in the range of between 10.sup.-1 and 10.sup.-3 mol/l. The precipitation reaction preferably takes place in the presence of an acid concentration in the range of between 0 and 6 mol/l. In one preferred embodiment of the present invention, the separation of the precipitate from the solution occurs by means of extraction with an organic solvent. For example, chloroform; diethyl ether ligroine (b.p. 40.degree.-60.degree. C.) 2:1 [vol./vol.]; 4-methyl-2-pentanone (5% by volume in chloroform); or 4-methyl-2-pentanone (5% by volume in toluol) can be employed as organic solvent. It is understood that both the foregoing general description and the following detailed description are exemplary and explanatory, but are not restrictive of the invention.