RHEOLOGY OF SETTLED SOLIDS IN THE SMALL COLUMN ION EXCHANGE PROCESS

Abstract

The Small Column Ion Exchange (SCIX) process is being developed to remove cesium, strontium, and actinides from Savannah River Site (SRS) Liquid Waste using an existing waste tank as process housing. This method includes the addition of monosodium titanate (MST) to a waste tank containing salt solution and entrained sludge solids, followed by tank mixing and filtration. The filtrate is then processed through in-tank ion exchange columns containing crystalline silicotitanate (CST) media. While the process is operating, it is known that solid particles begin to settle in the tank and temperatures may reach beyond 45 C. Previous testing has shown that sludge-MST slurries that sit for extended periods at elevated temperatures can develop large shear strengths, making them difficult to resuspend and remove from the tank. The authors conducted rheological testing of mixtures containing various concentrations of sludge simulant, MST, and CST (three preparations) that were aged at different times (i.e., 0 to 13 weeks) and isothermally maintained to 30, 45, or 60 C. Two types of grinding methodologies were employed to prepare CST for this testing, herein called Savannah River National Laboratory (SRNL) and Vitreous State Laboratory (VSL) ground materials. Unground CST particles were also tested. A small number of samples were irradiated prior to 4 week settling and 60 C temperature treatment, with exposures ranging from 0 to 100 MRad. Additional tests are also being conducted that will allow the solid particles to settle at 45 C for 6, 12, and 24 months. The objectives of this task are to determine the impact of feed composition, settling time, and temperature on the shear strength, yield stress, and consistency of the slurries and to determine the impact of radiation on slurry rheology. The testing will determine the relative impact of these parameters rather than predict the shear strength, yield stress, and consistency as a function of feed and operating conditions. This document describes the rheology of slurries containing MST, CST, and simulated sludge that sat at indicated temperatures for up to 13 weeks. A previous SRNL report described preliminary rheology data of slurries containing MST and sludge. Preliminary results of the irradiation tests are also presented in this report, though additional data are still being collected. Rheology of the long term settling samples (6, 12, and 24 months) and additional irradiation test results will be reported at a later date. Conclusions from this analysis are as follows: (1) Slurries containing MST and unground CST have the largest shear strength. Due to the high shear strengths measured in slurries containing unground CST, evaluations of specific tank contents and mixing capability should be performed prior to any addition of this material into a waste tank. Experimentally determined shear strengths indicate mixing could be problematic in mixtures containing unground CST. (2) Increasing the ground CST fraction in the slurry increases the slurry shear strength, yield stress, and consistency. (3) Increasing the sludge fraction in the slurry decreases the slurry shear strength, yield stress, and consistency. (4) Slurries containing VSL ground CST have larger shear strength, yield stress, and consistency than slurries containing SRNL ground CST. (5) The effects of settling time and temperature on slurry shear strength are slurry dependent. (6) No effects of settling time and temperature on slurry yield stress or consistency were observed. (7) Radiation up to 100 MRad does not appear to affect properties of shear strength, yield stress, or consistency of process feeds.