Task 20 - Prevention of Chloride Corrosion in High-Temperature Waste Treatment Systems (Corrosives Removals from Vitrification Slurries)

摘要

With assistance from GTS Duratek of Columbia, Maryland, the Energy and Environmental Research Center (EERC) in Grand Forks, North Dakota, has developed a three-step process to remove corrosives (primarily chloride and sulfate) from low-level radioactive, hazardous, and mixed wastes destined for vitrification. The EERC is optimizing the process for incorporation into a commercial system for remediation of tank wastes at the Hanford site in southeast Washington. Sulfate in vitrification feedstocks is undesirable because it can damage the vitrifier electrode, and chloride is undesirable because it can damage components of the vitrifier emission control system., Removal of corrosives from tank wastes is difficult because the presence of up to 10-molar concentrations of sodium hydroxide along with significant levels of nitrate, nitrite, and other anions render standard ion-exchange, membrane filtration, and other separation technologies relatively ineffective. The EERC corrosives removal system comprises: (1) initial chloride and sulfate removal via ion-selective membrane filtration; (2) secondary sulfate removal via sulfate-specific ion exchange with zirconium-, silver-, or other metal-intercalated cation exchange resin; and (3) secondary chloride removal via chloride-specific ion exchange with silver-, or other metal-intercalated exchange resin. Process systems developed to date have demonstrated 69% chloride removal (from 15,100 parts per million (ppm) to 4700 ppm) and 68% sulfate removal (from 39,200 to 12,600 ppm) from 10-molar sodium hydroxide solutions. Hanford tank waste chloride and sulfate concentrations can range as high as 15,000 and 40,000 ppm, respectively, and preliminary Duratek-specified target maximum chloride and sulfate concentrations for vitrification feedstocks are 5000 and 2000 ppm, respectively.