Removing Sludge Heels from Savannah River Site Waste Tanks by Oxalic Acid Dissolution

The Savannah River Site (SRS) will remove sludge as part of waste tank closure operations. Typically the bulk sludge is removed by mixing it with supernate to produce a slurry, and transporting the slurry to a downstream tank for processing. Experience shows that a residual heel may remain in the tank that cannot be removed by this conventional technique. In the past, SRS used oxalic acid solutions to disperse or dissolve the sludge heel to complete the waste removal. To better understand the actual conditions of oxalic acid cleaning of waste from carbon steel tanks, the authors developed and conducted an experimental program to determine its effectiveness in dissolving sludge, the hydrogen generation rate, the generation rate of other gases, the carbon steel corrosion rate, the impact of mixing on chemical cleaning, the impact of temperature, and the types of precipitates formed during the neutralization process. The test samples included actual SRS sludge and simulated SRS sludge. The authors performed the simulated waste tests at 25, 50, and 75 deg. C by adding 8 wt % oxalic acid to the sludge over seven days. They conducted the actual waste tests at 50 and 75 deg. C by adding 8 wt % oxalic acid to the sludge as a single batch. Following the testing, SRS conducted chemical cleaning with oxalic acid in two waste tanks. In Tank 5F, the oxalic acid (8 wt %) addition occurred over seven days, followed by inhibited water to ensure the tank contained enough liquid to operate the mixer pumps. The tank temperature during oxalic acid addition and dissolution was approximately 45 deg. C. The authors analyzed samples from the chemical cleaning process and compared it with test data. The conclusions from the work follow. - Oxalic acid addition proved effective in dissolving sludge heels in the simulant demonstration, the actual waste demonstration, and in SRS Tank 5F. - The oxalic acid dissolved ∼100% of the uranium, ∼100% of the iron, and ∼40% of the manganese during a single contact in the simulant demonstration. (The iron dissolution may be high due to corrosion of carbon steel coupons.) - The oxalic acid dissolved ∼80% of the uranium, ∼70% of the iron, and ∼50% of the manganese in the actual waste demonstration for a single contact. - The oxalic acid dissolved ∼3000 kg of uranium, ∼1500 kg of iron, ∼700 kg of manganese, ∼6.2 x 105 Ci of Sr-90, ∼4.1 x 104 Ci of Cs-137, ∼13 Ci of Pu-239/240, and ∼0.27 Ci of Pu-238 in Tank 5F during the first contact cycle. - During the second contact cycle, the oxalic acid dissolved ∼34 kg of uranium, ∼400 kg of iron, ∼50 kg of manganese, ∼2.1 x 104 Ci of Sr-90, ∼1.4 x 103 Ci of Cs-137, ∼0.29 Ci of Pu-239/240, and ∼0.10 Ci of Pu-238 in Tank 5F. With the exception of iron, the amount of the major species dissolved during the second acid strike was significantly lower than in the first strike. Because of this result, SRS Liquid Waste followed the second acid strike with a water spray wash and will attempt mechanical removal of the residual solids. - The demonstrations produced large volumes (i.e., 2 - 14 m3 of gas/m3 of oxalic acid) of gas (primarily carbon dioxide) by the reaction of oxalic acid with sludge and carbon steel. - The reaction of oxalic acid with carbon steel produced hydrogen in the simulant and actual waste demonstrations. The volume produced varied from 0.000006 - 0.00030 m3 hydrogen/m2 carbon steel. The hydrogen production proved higher in unmixed tanks than in mixed tanks. (authors)