Alternatives to Nitric Acid Stripping in the Caustic-Side Solvent Extraction (CSSX) Process for Cesium Removal from Alkaline High-Level Waste
- ORNL
Effective alternatives to nitric acid stripping in the Caustic-Side Solvent Extraction (CSSX) solvent have been demonstrated in this work. The CSSX solvent employs calix[4]arene-bis(tert-octylbenzo-18-crown-6) (BOBCalixC6) as the cesium extractant in a modified alkane diluent for decontamination of alkaline high-level wastes. Results reported in this paper support the idea that replacement of the nitrate anion by a much more hydrophilic anion like borate can substantially lower cesium distribution ratios on stripping. Without any other change in the CSSX flowsheet, however, the use of a boric acid stripping solution in place of the 1 mM nitric acid solution used in the CSSX process marginally, though perhaps still usefully, improves stripping. The less-than-expected improvement was explained by the carryover of nitrate from scrubbing into stripping. Accordingly, more effective stripping is obtained after a scrub of the solvent with 0.1 M sodium hydroxide. Functional alternatives to boric acid include sodium bicarbonate or cesium hydroxide as strip solutions. Profound stripping improvement is achieved when trioctylamine, one of the components of the CSSX solvent, is replaced with a commercial guanidine reagent (LIX 79). The more basic guanidine affords greater latitude in selection of aqueous conditions in that it protonates even at mildly alkaline pH values. Under process-relevant conditions, cesium distributions on stripping are decreased on the order of 100-fold compared with current CSSX performance. The extraction properties of the solvent were preserved unchanged over three successive extract-scrub-strip cycles. From the point of view of compatibility with downstream processing, boric acid represents an attractive stripping agent, as it is also a potentially ideal feed for borosilicate vitrification of the separated 137Cs product stream. Possibilities for use of these results toward a dramatically better next-generation CSSX process, possibly one employing the more soluble cesium extractant calix[4]arene-bis(2 ethylhexylbenzo-18-crown-6) (BEHBCalixC6) are discussed.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 961558
- Journal Information:
- Solvent Extraction and Ion Exchange, Vol. 27, Issue 2; ISSN 0736-6299
- Country of Publication:
- United States
- Language:
- English
Similar Records
Evaluation of 5-cm Centrifugal Contactor Hydraulic and Mass Transfer Performance for Caustic-Side Solvent Extraction of Cesium
Caustic-Side Solvent Extraction: Chemical and Physical Properties of the Optimized Solvent
Related Subjects
HIGH-LEVEL RADIOACTIVE WASTES
RADIOACTIVE WASTE PROCESSING
CESIUM
SOLVENT EXTRACTION
NITRIC ACID
MATERIAL SUBSTITUTION
BORIC ACID
PERFORMANCE
CSSX
solvent extraction
cesium
Hanford
tank waste
calixarene
stripping
boric acid