SLUDGE BATCH 6/TANK 51 SIMULANT CHEMICAL PROCESS CELL SIMULATIONS
Qualification simulant testing was completed to determine appropriate processing conditions and assumptions for the Sludge Batch 6 (SB6) Shielded Cells demonstration of the DWPF flowsheet using the qualification sample from Tank 51 for SB6 after SRNL washing. It was found that an acid addition window of 105-139% of the DWPF acid equation (100-133% of the Koopman minimum acid equation) gave acceptable Sludge Receipt and Adjustment Tank (SRAT) and Slurry Mix Evaporator (SME) results for nitrite destruction and hydrogen generation. Hydrogen generation occurred continuously after acid addition in three of the four tests. The three runs at 117%, 133%, and 150% stoichiometry (Koopman) were all still producing around 0.1 lb hydrogen/hr at DWPF scale after 42 hours of boiling in the SRAT. The 150% acid run reached 110% of the DWPF SRAT limit of 0.65 lb H{sub 2}/hr, and the 133% acid run reached 75% of the DWPF SME limit of 0.223 lb H{sub 2}/hr. Conversely, nitrous oxide generation was subdued compared to previous sludge batches, staying below 25 lb/hr in all four tests or about a fourth as much as in comparable SB4 testing. Two other processing issues were noted. First, incomplete mercury suspension impacted mercury stripping from the SRAT slurry. This led to higher SRAT product mercury concentrations than targeted (>0.45 wt% in the total solids). Associated with this issue was a general difficulty in quantifying the mass of mercury in the SRAT vessel as a function of time, especially as acid stoichiometry increased. About ten times more mercury was found after drying the 150% acid SME product to powder than was indicated by the SME product sample results. Significantly more mercury was also found in the 133% acid SME product samples than was found during the SRAT cycle sampling. It appears that mercury is segregating from the bulk slurry in the SRAT vessel, as mercury amalgam deposits for example, and is not being resuspended by the agitators. The second processing issue was significant ammonium ion formation as the acid stoichiometry was increased due to the high noble metal-high mercury feed conditions. Ammonium ion was found partitioned between the SRAT product slurry and the condensate from the lab-scale off-gas chiller downstream of the SRAT condenser. The ammonium ion was produced from nitrate ion by formic acid. Formate losses increased with increasing acid stoichiometry reaching 40% at the highest stoichiometry tested. About a third of the formate loss at higher acid stoichiometries appeared to be due to ammonia formation. The full extent of ammonia formation was not determined in these tests, since uncondensed ammonia vapor was not quantified; but total formation was bounded by the combined loss of nitrite and nitrate. Nitrate losses during ammonia formation led to nitrite-to-nitrate conversion values that were negative in three of the four tests. The negative results were an artifact of the calculation that assumes negligible SRAT nitrate losses. The sample data after acid addition indicated that some of the initial nitrite was converted to nitrate, so the amount of nitrate destroyed included nitrite converted to nitrate plus some of the added nitrate from the sludge and nitric acid. It is recommended that DWPF investigate the impact of SME product ammonium salts on melter performance (hydrogen, redox). It was recommended that the SB6 Shielded Cells qualification run be performed at 115% acid stoichiometry and allow about 35 hours of boiling for mercury stripping at the equivalent of a 5,000 lb/hr boil-up rate.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 979410
- Report Number(s):
- SRNL-STI-2010-00173; TRN: US201010%%820
- Country of Publication:
- United States
- Language:
- English
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