(366f) Precipitation Kinetics of Aluminum Solids Formation During the Caustic Side Solvent Extraction (CSSX) Process

The Caustic Side Solvent Extraction Process (CSSX) process is used for cesium separation at the Savannah River Site (SRS).  The CSSX process is comprised of 4 steps:  1) extracting cesium into the solvent; 2) scrubbed of the loaded solvent to remove co-extracted sodium and potassium; 3) stripping to transfer the cesium out of the solvent; and 4) washing of the stripped solvent to remove solvent degradation products if present.  During the scrubbing process, aluminum precipitation has been observed.  This is a concern as solids formation may have undesirable consequences during operation.  The work reported was undertaken to identify conditions under which solids formation may occur and to identify an operating region where solids formation is avoided or minimized.

Experiments were conducted at room temperature using simulants based on different dissolution fractions of SRS Tank 25F contents.  Combination of a given amount of simulant with a 0.05 M nitric acid solution mimicked the carryover during the CSSX process.  Aluminum concentration and solution pH were measured over time for the prepared samples.  Simulation of these experiments were also completed using the Environmental Simulation Program (ESP, OLI Systems, Inc.) and comparisons made.  Solids formation was observed for a select subset of samples both experimentally and through ESP predictions.  For these samples, a second set of samples was prepared.  The temporal variation of aluminum concentration was measured, until solids were visually observed in each sample.  The resulting data were analyzed using a first order reversible rate expression, and forward and backward reaction rate constants determined.  Solids formed were isolated, dried and identified using polarized light microscopy.