SULFATE SOLUBILITY LIMIT VERIFICATION FOR DWPF SLUDGE BATCH 7B

The objective of this study was to determine a sulfate solubility limit in glass for Sludge Batch 7b (SB7b). The SB7b composition projection provided by Savannah River Remediation (SRR) on May 25, 2011 was used as the basis for formulating glass compositions to determine the sulfate limit. Additions of Na{sub 2}O to the projected sludge composition were made by the Savannah River National Laboratory (SRNL) due to uncertainty in the final concentration of Na{sub 2}O for SB7b, which is dependent on washing effectiveness and the potential need to add NaOH to ensure an acceptable projected operating window. Additions of 4, 6, and 8 wt % Na{sub 2}O were made to the nominal May 25, 2011 composition projection. An updated SB7b composition projection was received from SRR on August 4, 2011. Due to compositional similarities, no additional experimental work using the August 4, 2011 compositions was considered to be necessary for this study. Both Frit 418 and Frit 702 were included in this study. The targeted sulfate (SO{sub 4}{sup 2-}) concentrations of the study glasses were selected within the range of 0.6 to 0.9 wt % in glass. A total of 52 glass compositions were selected based on the compositional variables of Na{sub 2}O addition, Actinide Removal Process (ARP) stream addition, waste loading, frit composition, and sulfate concentration. The glasses were batched, melted, and characterized following SRNL procedures. Visual observations were recorded for each glass after it cooled and used as in indicator of sulfur retention. Representative samples of each of the glasses fabricated were subjected to chemical analysis to determine whether the targeted compositions were met, as well as to determine the quantity of sulfate that was retained after melting. In general, the measured composition data showed that there were only minor issues in meeting the targeted compositions for the study glasses, and the measured sulfate concentrations for each study glass were within 10% of the targeted values. The results for the SB7b glasses fabricated with Frit 418 showed an apparent trend of increasing sulfate retention with increasing Na{sub 2}O additions to the 5/25/11 sludge projection. This trend appears contradictory to other recent studies of sulfate retention in Defense Waste Processing Facility (DWPF) type glasses. Additional apparent contradictions to this trend were found in the data collected in the present study. Overall, the results for the SB7b sulfate study glasses with Frit 418 and the 5/25/11 projection with Na{sub 2}O additions showed that subtle changes in this complex glass composition impacted the degree of sulfate retention. These results do however provide confidence that a 0.6 wt % sulfate limit in glass is warranted for Frit 418 with the SB7b compositions evaluated in this study. The results for the SB7b glasses fabricated with Frit 702 are consistent with those of the previous SB7a study in that Frit 702 allowed for higher sulfate retention as compared to Frit 418 for the same sludge compositions. It is recommended that the DWPF implement a sulfate concentration limit of 0.6 wt % in glass for SB7b processing with Frit 418. If a higher than projected sulfate concentration is measured when SB7b processing begins (i.e., if a sulfate concentration higher than 0.6 wt % becomes necessary to achieve targeted waste loadings), DWPF should consider a transition to Frit 702. The sulfate limit could likely be raised to 0.8 wt % by transitioning to this frit. However, if DWPF considers transitioning from Frit 418 to Frit 702, additional glasses should be fabricated to confirm this higher limit due to the issues with incorrect B{sub 2}O{sub 3} concentrations for some of the glasses made with Frit 702 in this study. There are several factors other than sulfate retention that must also be carefully considered prior to changing frit compositions