Investigation of Rheological Impacts on Sludge Batch 3 as Insoluble Solids and Wash Endpoints are Adjusted

The Defense Waste Processing Facility (DWPF) is currently processing and immobilizing radioactive sludge slurry into a durable borosilicate glass. The DWPF has already processed three sludge batches (Sludge Batch 1A, Sludge Batch 1B, and Sludge Batch 2) and is currently processing the fourth sludge batch (Sludge Batch 3). A sludge batch is defined as a single tank of sludge slurry or a combination of sludge slurries from different tanks that has been or will be qualified before being transferred to DWPF. As a part of the Sludge Batch 3 (SB3) qualification task, rheology measurements of the sludge slurry were requested at different insoluble solids loadings. These measurements were requested in order to gain insight into potential processing problems that may occur as the insoluble solids are adjusted up or down (by concentration or dilution) during the process. As a part of this study, a portion of the ''as received'' SB3 sample was washed with inhibited water (0.015 M NaOH and 0.015 M NaNO2) to target 0.5M Na versus a measured 1M Na in the supernate. The purpose of the ''washing'' step was to allow a comparison of the SB3 rheological data to the rheological data collected for Sludge Batch 2 (SB2) and to determine if there was a dependence of the yield stress and consistency as a function of washing. The ''as received'' SB3 rheology data was also compared to SB3 simulants prepared by the Simulant Development Program in order to provide guidance for selecting a simulant that is more representative of the rheological properties of the radioactive sludge slurry. A summary of the observations, conclusions are: (1) The yield stress and plastic viscosity increased as the weight percent insoluble solids were increased for the ''as received'' and ''washed'' SB3 samples, at a fixed pH. (2) For the same insoluble solids loading, the yield stress for the SB2 sample is approximately a factor of three higher than the ''as received'' SB3 sample. There also appears to be small difference in the plastic viscosity. This difference is probably due to the different Na concentrations of the slurries. (3) The yield stress for the SB2 sample at 17.5 wt. % insoluble solids loading is four times higher than the ''washed'' SB3 sample at 16.5 wt. % insoluble solids. There also appears to be small difference in the plastic viscosity. The differences for the yield stress and consistency can be explained by the differences in the Fe and Na concentrations of the sludge slurry and the anion concentrations of the resulting supernates. (4) The rheological properties (i.e. yield stress and plastic viscosity), as the insoluble solids are adjusted, for the ''as received'' and ''washed'' SB3 samples are different. The plastic viscosity curve for the ''as received'' SB3 sample was higher than the plastic viscosity curve for SB3 ''washed'' sample. The yield stress curve for the ''washed'' SB3 sample is slightly lower than the ''as received'' SB3 sample up until {approx}19 wt. % insoluble solids. The ''washed'' SB3 sample then exceeds the yield stress curve for the ''as received'' SB3 sample. This rheological behavior is probably due to the difference in the Na concentration of the supernate for the samples. (5) No unusual behavior, such as air entrainment, was noted for the ''as received'' SB3 sample. (6) The observed physical properties of the SB3 sample changed after washing. The ''washed'' SB3 sample entrained air readily at higher insoluble solids loadings (i.e. 14.1, 16.5, 19.5 wt. %) as it did for SB2. The air entrainment appeared to dissipate for the SB3 sample at the lower insoluble solids loadings (i.e. 9.7 and 11.7 wt. %). (7) The physical behavior of SB3 can be influenced by controlling the Na concentration in the supernate and the wt. % insoluble solids. The cause for the air entrainment in the ''washed'' SB3 sample could be due to a change in the particle size during the washing step. (8) The SB3 simulants prepared for the Simulant Development Program were approximately a factor of 1.6 to 4 times higher for yield stress and 2.6 to 4 times higher for the plastic viscosity over a similar range of insoluble solids loadings. The difference noted between the radioactive and simulant samples could be due to several factors including particle size, thermal treatment (i.e. aging of the sludge), shear history, etc