Oxidative alkaline dissolution of chromium from Hanford tank sludges: Results of FY 98 studies

Plans for disposing of the high-level radioactive wastes at the Hanford Site call for retrieving, pretreating, and finally immobilizing the wastes in a glass matrix. Since the cost for vitrifying and disposing of high-level wastes will be very great, pretreatment processes are being developed to reduce their volume. The baseline method for pretreating Hanford tank sludges is caustic leaching. Earlier studies with Hanford tank-sludge simulants and with actual Hanford tank sludges have indicated that treating water-washed and caustic-leached solids with oxidants can significantly increase the removal of Cr. Permanganate and ozone have been shown to be generally the most rapid and effective chemical agents for this purpose. The work described in this report continues to examine the effectiveness of solubilizing additional Cr from Hanford tank wastes by oxidation of the water-insoluble solids from tanks U-108, U-109, and SX-108 under alkaline conditions. The current study confirms that permanganate is highly effective at removing chromium from water solids under alkaline conditions, with Cr removals of up to 99+%. Elemental oxygen can also be highly effective, with removals up to 97+%