PNNL

Abstract

Boehmite has proved to be a difficult aluminum phase to dissolve out of high-level waste (HLW), requiring longer residence times and higher temperatures for more complete dissolution than the gibbsite phase. To provide a simple, compact, effective, and proven method to remove aluminum from HLW sludges, HLW feed can be blended with caustic in a reaction vessel while continuously removing permeate from the reactor. This approach allows for a concurrent addition of fresh caustic and HLW feed while simultaneously removing concentrated reacted slurry. Separation in this manner will keep solids in the reactor for longer periods of time, allowing for higher conversions than could be achieved in a normal batch reactor. Following separation from the residual sludge, the supernate is treated through ion exchange to remove cesium. Based on the low potassium and cesium content of the leachate product, a modified process flow sheet has been developed to minimize the equipment requirements for a modular near tank application of this ion exchange process. Advantages for this continuous sludge leaching process are the reduction of the number of high level waste canisters by one third to one half at the Hanford Tank Waste Treatment and Immobilization Plant, resulting in billions of dollars in life-cycle cost savings. Pilot scale tests are expect to confirme that the boehmite removal can be achieved to meet these canister reduction goals while meeting the required cesium decontamination targets.