Early projections of the Sludge Batch 4 (SB4) composition predicted relatively high concentrations of alumina (Al2O3, 23.5 wt%) and sulfate (SO4 2-, 1.2 wt%) in the sludge. A high concentration of Al2O3 in the sludge, combined with Na2O additions in the frit, raises the potential for nepheline crystallization in the glass. However, strategic frit development efforts at the Savannah River National Laboratory (SRNL) have shown that frits containing a relatively high concentration of B2O3 can both suppress nepheline crystallization and improve melt rates. A high sulfate concentration is a concern to the DWPF as it can lead to the formation of sulfate inclusions in the glass and/or the formation of a molten, sulfate-rich phase atop the melt pool. To avoid these issues, a sulfate concentration limit of 0.4 wt% SO4 2- in glass was originally set in the Product Composition Control System (PCCS) used at DWPF. It was later shown that this limit could be increased to 0.6 wt% SO4 2- in glass for the Frit 418, Sludge Batch 3 (SB3) system. Two frits have been evaluated for use with the early projections of SB4. Frit 418, which has been used previously with SB3, has been shown to produce glasses with SB4 that have an acceptable and predictable durability. Frit 503, a high boron concentration frit, has also been shown to produce durable glasses with SB4, and has advantages in melt rate over Frit 418. The applicability of the current 0.6 wt% sulfate limit to the SB4/Frit 418 or Frit 503 system, and/or the possibility of increasing the sulfate limit for SB4 are the subjects of this study. Ten glasses were fabricated in the laboratory using the May 2006 SB4 composition projection and Frits 418 and 503 at 38% waste loading.
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