Alkali Treatment of Acidic Solution from Hanford K Basin Sludge Dissolution

Abstract

Nitric acid solutions will be created fiom the dissolution of Hanford K Basin sludge. These acidic dissolver solutions must be made alkaline by treatment with NaOH solution before they are disposed to the Tank Waste Remediation System on the Hanford Site. During the alkali treatments, sodium diuranate, hydroxides of iron and aluminum, and radioelements (uranium, plutonium, and americium) will precipitate from the dissolver solution. Laboratory tests, discussed here, were performed to provide information on these precipitates and their precipitation behavior that is important in designing the . engineering flowsheet for the treatment process. Specifically, experiments were conducted to determine the optimum precipitation conditions; the completeness of uranium, plutonium, and americium precipitation; the rate of sedimentation; and the physico-chemical characteristics of the solids formed by alkali treatment of simulated acidic dissolver solutions. These experiments also determined the redistribution of uranium, plutonium, and americium from the sodium diwanate and iron and alukinum hydroxide precipitates upon contact with carbonate-and EDTA-bearing simulated waste solutions. Note: EDTA is the tetrasodium salt of ethylenediaminetetraacetate. . The results of the experiments indicate that alkali treatment of the acidic dissolver solution is best performed by precipitation and digestion at 40°C with 0.01 M NaOH excess (PH 12) using direct strike addition of NaOH solution to the dissolver solution. These conditions yield the lowest uranium concentration in the supernatant, require the least excess NaOH, and provide the highest apparent partitioning of plutonium to the carbonate-resistant F e O hydroxide solid phase. However, other conditions, such as reverse strike, 8O"C, have little effect on precipitate properties. The resulting supernatant solutions using the range of conditions tested here are non-TRU for both plutonium and americium. Contacting product precipitates having no F e O in the solid phase with carbonate-bearing solutions can produce TRU-level plutonium and americium concentrations in solution. Alkaline solutions containing EDTA also can leach americium to near-TRU levels fi-om iron-free solids.Results from the specific tests are summarized below:The alkali-treated simulated dissolver solutions were found to be non-transuranic for both plutonium and americium. Plutonium concenkations in the alkaline mother solutions were scattered and low (-2x10-* M versus 5 . 5~1 0~ M Pu equivalent to the 100 nCi u9*24!l?u/g TRU waste limit). Americium concentrations in the mother solutions also were low [(5-1 l)xlO-" a versus l . 2~l O '~M Am equivalent to 100 nCi 241Am/g] and not strongly dependent on the precipitate composition. When the NaOH concentration was increased from 0.01 to 1 M, uranium concentrations in the mother solutions increased, accordingly, from 1 . 5~1 0~ to 1 8~1 0 -~ M.The sedimentation rates of precipitates formed by alkali treatment of a simulated dissolver solution were found to be nearly inde...