Sulfate Solubility Limit Verification For DWPF Sludge Batch 7A

During processing at the Defense Waste Processing Facility (DWPF), high sulfate concentrations in the feed are a concern to DWPF as it can lead to the formation of a detrimental, sulfate-rich, molten salt phase on the surface of the glass melt pool. To avoid these issues, a sulfate concentration limit was implemented into the Product Composition Control System (PCCS). Related to SB7a frit development efforts, the Savannah River National Laboratory (SRNL) assessed the viability of using the current 0.6 wt % SO42- limit set for SB6 (in glass) and the possibility of increasing the SO42- solubility limit in PCCS to account for anticipated sulfur concentrations, targeted waste loadings, and inclusion of secondary streams (e.g., Actinide Removal Process (ARP)) with two recommended frits (Frit 418 and Frit 702) for SB7a processing. For a nominal SB7a blend with a 63 inch SB6 heel remaining in Tank 40 (projection SB7a-63), a 0.60 wt% SO42- in glass limit was determined for waste loadings of 34 wt% up to 40 wt% with Frit 418 based on crucible melts with batched chemicals. SRNL also examined the inclusion of ARP for the same blending scenario (SB7a-63-ARP) with Frit 418 and at least a 0.6 wt% SO42- level, and waste loadings of 34 wt% to 40 wt% were also acceptable. When a visible yellow and/or white sulfate salt layer was visible on the surface of any cooled glass, it was assumed to have surpassed the solubility limit of SO42- for that particular composition. All of the glasses fabricated at these concentrations did not exhibit a sulfate rich salt layer on the surface of the glass melt and retained the majority of the batched SO42-. At higher levels of SO42- 'spiked' into the projected sludge compositions over the aforementioned interval of waste loadings, with Frit 418, low viscosity sulfur layers were observed on the surface of glass melts which confirm exceeding the solubility limit. The same sludge scenarios were also tested with Frit 702 and all glasses did not exhibit sulfur layers on the surfaces of the glass melts at spiking levels up to 0.80 wt% SO42-. An ultimate SO42- limit was not defined with Frit 702, but if projected SO42- concentrations are expected to increase with the onset of SB7a processing, a higher limit is achievable with Frit 702 than is achievable with Frit 418. Given the anticipated concentration of SO42- for SB7a, a SO42- limit of 0.6 wt % SO42- is recommended for processing using Frit 418. Once the confirmed SB7a composition is known and should a higher limit be needed, SRNL can re-evaluate the limit based on the actual composition and provide an updated recommendation. It has been observed that higher levels of SO42- in glass can be retained with compositional changes to the frit, as was demonstrated by the glasses fabricated using Frit 702. SRNL also recommends the continuation of studies to define a more 'global' sulfate concentration limit to account for future sludge batch composition uncertainties.