Robustness of the cssx process to feed variation: Efficient cesium removal from the high potassium wastes at hanford

This contribution finds the Caustic-Side Solvent Extraction (CSSX) process to be effective for the removal of cesium from the Hanford tank-waste supernatant solutions. The Hanford waste types are more challenging than those at the Savannah River Site (SRS) in that they contain significantly higher levels of potassium, the chief competing ion in the extraction of cesium. By use of a computerized CSSX thermodynamic model, it was calculated that the higher levels of potassium depress the cesium distribution ratio (D_Cs), as validated to within ±11% by the measurement of D_Cs values on various Hanford waste-simulant compositions. A simple analog model equation that can be readily applied in a spreadsheet for estimating the D_Cs values for the varying waste compositions was developed and shown to yield nearly identical estimates as the computerized CSSX model. It is concluded from the batch distribution experiments, the physical-property measurements, the equilibrium modeling, the flowsheet calculations, and the contactor sizing that the CSSX process as currently formulated for cesium removal from alkaline salt waste at the SRS is capable of treating similar Hanford tank feeds, albeit with more stages. For the most challenging Hanford waste composition tested, 31 stages would be required to provide a cesium decontamination factor (DF) of 5000 and a concentration factor (CF) of 2. Commercial contacting equipment with rotor diameters of 10 in. for extraction and 5 in. for stripping should have the capacity to meet throughput requirements, but testing will be required to confirm that the needed efficiency and hydraulic performance are actually obtainable. Markedly improved flowsheet performance was calculated based on experimental distribution ratios determined for an improved solvent formulation employing the more soluble cesium extractant BEHBCalixC6 used with alternative scrub and strip solutions,respectively 0.1M NaOH and 0.010M boric acid. The improved solvent and flowsheet can meet minimum requirements (DF 5 5000 and CF 5 2) with 15 stages or more ambitious goals (DF 5 40,000 and CF 5 15) with 19 stages. Thus,a modular CSSX application for the Hanford waste seems readily obtainable with further short-term development.