Initial Sulfate Solubility Study for Sludge Batch 4 (SB4)

Field: Chemical Engineering (Physical Sciences)

The objective of this task is to provide the Defense Waste Processing Facility (DWPF) of the Savannah River Site (SRS) with an assessment of the viability of using the current 0.6 wt% SO{sub 4}{sup =} limit (in glass) and/or the possibility of increasing the SO{sub 4}{sup =} solubility limit to account for anticipated sulfur concentrations in Sludge Batch 4 (SB4). The 0.6 wt% SO{sub 4}{sup =} limit was implemented for processing of Frit 418-Sludge Batch 3 (SB3) to avoid the formation of sulfate inclusions in the glass and/or the formation of a molten sulfate-rich phase on the melt pool in the DWPF melter. The presence of such a phase on the surface of the melt pool increases corrosion rates of melter components, enhances the potential for steam excursions in a slurry-fed waste glass melter, and creates the potential for undesirable current paths that could deplete energy delivered to the melter due to the electrical conductivity of the molten salt layer. This suite of sulfate-solubility tests began by testing the 1200-canister, 2nd transfer case for SB4 (as defined by Lilliston and Shah, 2004)--based on this being the most conservative (having the highest predicted viscosity when coupled with specific frits, it could potentially have the greatest impact on SO{sub 4}{sup =} solubility) blending scenario of SB4 with the heel of SB3 for SO{sub 4}{sup =} solubility. Frits 320 and 418 were tested with SB4 and the tests indicated that at the current SO{sub 4}{sup =} limit (in glass) and the tested waste loadings (30% and 40%), neither Frit 320 nor Frit 418 could be utilized with SB4 (for the 1200-canister, 2nd transfer case composition originally provided). More specifically, SO{sub 4}{sup =} was observed on the surface with the SB4 composition and Frit 320 at 40% waste loading (WL) and 0.6 wt% SO{sub 4}{sup =}, and with Frit 418 at 30% and 40% WL and 0.5 wt% SO{sub 4}{sup =}. As alternative frits were being developed--Frits 447, 448, and 449, that contained CaO and/or V2O5 to enhance SO{sub 4}{sup =} solubility based on suggestions of previous studies--testing began of the 1100-canister, 1st transfer case for SB4 (from Lilliston, 2005), which is the baseline flowsheet for the DWPF. The results of the study with the revised compositions have indicated that the SO{sub 4}{sup =} solubility limit in the DWPF of 0.6 wt% can be applicable for the 1100-canister, 1st transfer case of SB4 for certain frits. Five frits were tested in closed-crucible studies--Frits 320, 418, 447, 448, and 449. Tests with Frit 418 showed that SO{sub 4}{sup =} was apparent on the glass surface of tests at 40% WL and 0.6 wt% SO{sub 4}{sup =}. No salt layer formation was evident in any test (30% or 40% WL) with Frits 320, 447, 448, or 449 until SO{sub 4}{sup =} concentrations of 0.8 wt% were targeted. The crucible tests of this study and model predictions (from Jantzen and Smith, 2004) indicated that the SO{sub 4}{sup =} solubility limit for SB4 with those four frits would be similar. However, even with the additions of CaO and V2O5, the solubility of SO{sub 4}{sup =} was not greatly enhanced by Frits 447, 448, and 449 over Frit 320 for the 1100-canister, 1st transfer case. The following recommendation is made regarding the SO{sub 4}{sup =} solubility limit for SB4 in the DWPF: Reinvestigate the solubility of SO{sub 4}{sup =} for SB4 once the final blending and/or washing strategies for SB4 are determined--based on the decisions for the inclusion of Tank 4 and the exact volume and composition of the Np stream--in order to determine if the current SO{sub 4}{sup =} solubility limit (0.6 wt% SO{sub 4}{sup =}) in the DWPF needs to be increased for the processing of SB4.