Aluminum and Chromium Leaching Workshop Whitepaper

Abstract

SummaryA workshop was held on January [23][24] 2007 to discuss the status of processes to leach constituents from High Level Waste (HLW) sludges at the Hanford and Savannah River Sites. The objective of the workshop was to examine the needs and requirements for the HLW flowsheet for each site, discuss the status of knowledge of the leaching processes, communicate the research plans, and identify opportunities for synergy to address knowledge gaps. The purpose of leaching of non-radioactive constituents from the sludge waste is to reduce the burden of material that must be vitrified in the HLW melter systems, resulting in reduced HLW glass waste volume, reduced disposal costs, shorter process schedules, and higher facility throughput rates. The leaching process is estimated to reduce the operating life cycle of SRS by seven years and decrease the number of HLW canisters to be disposed in the Repository by 1000 [Gillam et al., 2006]. Comparably at Hanford, the aluminum and chromium leaching processes are estimated to reduce the operating life cycle of the Waste Treatment Plant by 20 years and decrease the number of canisters to the Repository by 15,000 -30,000 [Gilbert, 2007]. These leaching processes will save the Department of Energy (DOE) billions of dollars in clean up and disposal costs.The primary constituents targeted for removal by leaching are aluminum and chromium. It is desirable to have some aluminum in glass to improve its durability; however, too much aluminum can increase the sludge viscosity, glass viscosity, and reduce overall process throughput. Chromium leaching is necessary to prevent formation of crystalline compounds in the glass, but is only needed at Hanford because of differences in the sludge waste chemistry at the two sites. Improving glass formulations to increase tolerance of aluminum and chromium is another approach to decrease HLW glass volume. It is likely that an optimum condition can be found by both performing leaching and improving formulations.Disposal of the resulting aluminum and chromium-rich streams are different at the two sites, with vitrification into Low Activity Waste (LAW) glass at Hanford, and solidification in Saltstone at SRS. Prior to disposal, the leachate solutions must be treated to remove radionuclides, resulting in increased operating costs and extended facility processing schedules. Interim storage of leachate can also add costs and delay tank closure. Recent projections at Hanford indicate that up to 40,000 metric tons of sodium would be needed to dissolve the aluminum and maintain it in solution, which nearly doubles the amount of sodium in the entire current waste tank inventory. This underscores the dramatic impact that the aluminum leaching can have on the entire system. A comprehensive view of leaching and the downstream impacts must therefore be considered prior to implementation.Many laboratory scale tests for aluminum and chromium dissolution have been run on Hanford wastes, with samples from 46 tanks tested. Three samples from SRS tanks have been ...

Technical Evaluation of Accelerated Basin De-Inventory Material Addition to Sludge Batch 11

“…The increased hydroxide concentration and higher temperature results in the dissolution of aluminum from the sludge solids. The process preferentially dissolves the aluminum hydroxide phases gibbsite and bayerite, 84 and is designed to ultimately dissolve a significant portion of the aluminum oxyhydroxide phase boehmite. The primary impact is that the solid aluminum hydroxide in the ABD slurry is expected to preferentially dissolve in relation to the aluminum hydroxide and aluminum oxyhydroxide in the legacy Tank Farm sludge that makes up a sludge batch.…”

Solidification of the Hanford Law Waste Stream Produced as a Result of Near-Tank Continuous Sludge Leaching and Sodium Hydroxide Recovery

“…8 Multiple studies have been conducted on the dissolution kinetics of boehmite in caustic solutions. 4,6,8 These studies have shown that the dissolution rate of boehmite is the rate-limiting step in aluminum dissolution and sensitive to temperature changes. One study showed the dissolution rate approximately doubles with every 20 °C temperature increase.…”

“…According to waste dissolution studies on batch processes, 4,6,8 the adequate dissolution of boehmite from Hanford tank waste can be achieved in approximately 24 hours using a 3 -5 M NaOH solution at 100 °C.…”

“…9 Recent projections at Hanford indicate that up to 40,000 metric tons of sodium would be needed to dissolve the aluminum and maintain it in solutions, which nearly doubles the amount of sodium in the entire current waste tank inventory. 6 Therefore, a sodium recovery process must be implemented that allows the caustic to be recycled through the CSL process to minimize the amount of required caustic. 15 3.1.1 Ceramatec Process The first option for sodium recovery after CSL, developed by Ceramatec, electrochemically transfers sodium ions through a Sodium (Na) Super Ion Conductor (NaSICON) ceramic membrane while generating free hydroxide ions and hydrogen at the cathode and oxygen and hydrogen ions at the anode.…”