Oxidative dissolution of chromium from Hanford Tank sludges under alkaline conditions

Abstract

Because of the expected high cost of vitrifying and disposing of high-level waste at the U.S. Department of Energy's Hanford Site, pretreatment processes are being developed to reduce the anticipated volume of borosilicate glass. Sludge washing and caustic leaching, the baseline sludge pretreatment process, is expected to leach out a substantial portion of the 137C.9, possibly other radionuclides, and a significant portion of such major nonradionuclides as A I or P. The decontaminated solution will be routed to the low-level waste stream, where it will be immobilized in a glass matrix. The leached solids, which will contain the transuranic elements and W r , will be handled as high-level waste. Previous studies indicate that poor removal of chromium in the +3 oxidation state [Cr(III)] occurs during baseline pretreatment. Because the concentration of Cr allowed in high level waste glass is low, a relatively small amount of Cr in the sludge can have a relatively large impact on the volume of high level waste glass produced. For this reason, additional leach steps to remove Cr would be desirable, and oxidative alkaline leaching has been proposed as a simple addition to the baseline sludge pretreatment. This report describes small-scale screening tests on the oxidative alkaline leaching of Cr performed with actual Hanford tank sludges. from Tanks B-1 1 1 and SY-103 was evaluated. Two types of oxidants, KMnO4 and 0 3 , were investigated as a function of time, temperature, and (for permanganate) Mn to Cr ratio. Reaction times were rapid with both oxidants, and very little additional Cr dissolved upon further heating of the solution to 80°C. Analysis of the reaction of sludge with either 1:l or excess permanganate to Cr indicates that insoluble MnO2 will be produced under any practical processing conditions. The oxidation of Cr(II1) to soluble CrO42-allows for good dissolution of Cr during oxidative alkaline leaching with high Cr(II1) sludges. Calcium is the only other element that dissolves significantly, probably due to changes in the supernatant pH rather than any direct oxidant-Ca reaction. Under the test conditions, less than a third of the available Cr dissolved with B-1 1 1 sludge, but 90% dissolved with SY-103 sludge. Heating the dried sludges in air at 80°C for several hours oxidized some Cr(II1) to Cr(V1) and may have oxidized a small portion of PU. Some enhanced dissolution of Pu was observed when ozone was contacted with SY-103 sludge, although the concentrations should not prohibit the formation of low-level-waste glass if the oxidative leaching supernatant is combined with the caustic leach supernatant. For B-1 1 1, oxidative alkaline leaching did not reduce the amount of high-level glass required because no further dissolution of the glasslimiting component, P, was observed. For SY-103, however, it reduced the amount of high-level glass required by 90%. Either permanganate or ozone appears to be about equally effective. However, the nature of the glass-limiting component can change. If ozone is used Cr re...

Oxidative Alkaline Leaching of Americium from Simulated High‐Level Nuclear Waste Sludges

“…In fact, it has been reported that the concentration of plutonium in the leachate was increased in oxidative leaching. [6] A preliminary study from this group also shows that the presence of K 2 S 2 O 8 could increase the concentration of americium in the leachate by 10-20% at 85 o C. [7] Clearly this is an adverse outcome, as the purpose of sludge washing is to remove the non-radioactive and/or problematic constituents while concentrating the actinides in the sludge. These preliminary and limited data [6,7] indicate that a comprehensive understanding of the chemical behavior of actinides (americium and plutonium in particular) in oxidative alkaline leaching is critical to the development of the strategies for HLW sludge pretreatment.…”

Partitioning of U(VI) and Eu(III) between Acidic Aqueous Al(NO3)3and Tributyl Phosphate in n‐Dodecane

“…The extraction behavior of the CrO 4 2-ion was examined to determine the tendency of Cr (III or VI) to partition into the 60% TBP phase. After phase mixing (contact times ranged from 0.5 -40 minutes), aqueous phase samples were diluted 10 fold and distribution ratios, D Cr , were determined by mass deficit between [Cr] init and that remaining in the aqueous phase after equilibration.…”

“…Recognizing that CrO 4 2-could be present in acid-leached Al(NO 3 ) 3 and that Cr(VI) extraction would have a deleterious effect on the stability of the extractant, we elected in subsequent experiments to maintain the presence of Cr in our simulated leachates but to suppress the extraction of CrO 4 2-. To accomplish this adjustment, the HNO 3 /Al(NO 3 ) 3 solutions containing CrO 4 2-were further adjusted with the addition of 2.0 Â 10 23 M ascorbic acid which rapidly and quantitatively reduced Cr(VI) to Cr(III).…”

“…The aqueous HNO 3 /Al(NO 3 ) 3 solutions used in this study were adjusted to pH 1 and contained 1.0 Â 10 23 M Cr(VI). Under these conditions the dominant aqueous species are HCrO4 2 and H 2 CrO 4(17,18).…”

Oxidative Leaching of Plutonium from Simulated Hanford Tank‐Waste Sludges

“…Rapko, Lumetta, and coworkers (PNNL) have conducted the most extensive series of studies of oxidative leaching for Cr removal from actual Hanford tank sludges and simulants (4,(6)(7)(8). They have reported that:…”