(18e) Effect of Variation in Modular Caustic-Side Solvent Extraction Unit Parameters on Process Performance

A Modular Caustic-Side Solvent Extraction Unit (MCU) is being designed and built at the Department of Energy's (DOE) Savannah River Site (SRS) in Aiken, South Carolina. The Caustic-Side Solvent Extraction (CSSX) process removes cesium from filtered highly alkaline radioactive waste solutions containing high concentrations of sodium and potassium using an organic solvent. In the CSSX process, the solvent is contacted with the cesium-rich aqueous waste solution in an extraction stage to remove the cesium. Following separation of the aqueous and organic phases, the aqueous waste solution exits the MCU as a cesium-depleted effluent stream to be made into a grout. The solvent goes to a scrub stage where it is contacted with 0.05 M nitric acid solution to remove sodium, potassium, and other soluble salts including any trace levels of aluminum, iron, or calcium. Neutralization of any hydroxide carryover from the waste also occurs in the scrub stage. The metals removal and hydroxide neutralization are essential for a robust stripping stage operation. The scrubbed cesium-laden solvent proceeds to a stripping stage, where it is contacted with 0.001 M nitric acid solution to remove the cesium. The nitric acid solution leaves the MCU as a cesium-rich effluent to be made into glass. The solvent from the stripping stage moves to a wash stage, where it is contacted with 0.01 M sodium hydroxide solution to remove residual impurities and any solvent-degradation products before recycling to the extraction stage. The organic solvent comprises four components: (1) an extractant, a calixarene crown ether (Calix[4]arene-bis(tert-octylbenzo-crown-6)) known as BOBCalixC6, that has superior cesium selectivity over sodium and potassium; (2) a modifier, a fluorinated alcohol solvating agent (1-(2,2,3,3-tetrafluoropropoxy)-3-(4-sec-butylphenoxy)-2-propanol) known as Cs-7SB, that improves extraction and helps prevent third-phase formation by increasing the solubility of the extractant; (3) a suppressor, tri-n-octylamine (TOA), that inhibits impurity effects, and improves and stabilizes stripping; and (4) a diluent, Isopar®L, which is a blend of C10 to C12 branched alkanes, that promotes good hydraulics due to its low viscosity and density. The MCU will inevitably have variations in process parameters. The variations will impact the process performance by changing the final cesium concentration. This study evaluated the impact of parameter changes (from the base conditions) on process performance using a SASSE (Spreadsheet Algorithm for Stagewise Solvent Extraction) model to help define acceptable ranges for the MCU process parameters. The SASSE model was developed at Argonne National Laboratory. The SASSE model outputs indicate that broad changes in the MCU process parameters for the extraction, scrub and strip stages (i.e., flow rate, temperature, fraction of interstage carryover, total liquid volume per contactor stage, and efficiency per contactor stage) will not result in the final cesium-137 concentration (Cf) exceeding target, at least for the MCU base conditions. The process parameters ranges are given below.

Flow rate ranges within which Cf < 0.1 Ci/gal Solvent flow rate: nominal = 2.833 gal/min, range is 0.6 to 1.7 times nominal value Aqueous waste flow rate: nominal = 8.5 gal/min, range is < 1.45 times nominal value Aqueous Scrub flow rate: nominal = 0.567 gal/min, range is < 5.5 times nominal value Aqueous Strip flow rate: nominal = 0.567 gal/min, range is > 0.75 times nominal value

Temperature ranges within which Cf < 0.1 Ci/gal Extraction: nominal = 20 ? 26 oC, limit = 28.3 oC maximum Scrub: nominal = 20 ? 26 oC (assumed), limit = 43.7 oC maximum Strip: nominal = 30 ? 36 oC, limit = 30.5 oC minimum

Fraction of interstage carryover ranges within which Cf < 0.1 Ci/gal Extraction: organic carryover = 0.000001 - 0.25, aqueous carryover = 0.00001 - 0.75 Scrub: organic carryover = 0.000001 - 0.95, aqueous carryover = 0.00001 - 0.90 Strip: organic carryover = 0.000001 - 0.90, aqueous carryover = 0.00001 - 0.24 Scrub/Strip: organic carryover = 0.000001 - 0.90, aqueous carryover = 0.00001 - 0.90 Extraction/Scrub/Strip: organic carryover = 0.000001 - 0.23, aqueous carryover = 0.00001 - 0.90

Total liquid volume per contactor stage ranges within which Cf = constant < 0.1 Ci/gal Extraction: organic or aqueous volume = 1/4 - 4 times nominal value Scrub/Strip: organic or aqueous volume = 1/4 - 4 times nominal value Extraction/Scrub/Strip: organic or aqueous volume = 1/4 - 4 times nominal value

Minimum efficiency per contactor stage required for Cf < 0.1 Ci/gal is 65%.

For a given extraction D-value (cesium extraction distribution coefficients between the organic or solvent phase and the aqueous waste phase), Cf and C0 (initial cesium-137 concentration) are linearly related. Also, for a given C0, log (Cf) and log (extraction D-value) are linear with a slope of -1.43. These two relationships allow the quick calculation of Cf at other MCU conditions without using the SASSE model.