The Savannah River Plutonium Processing Facility (SRPPF) is planning to treat spent pyrochemical residues from plutonium metal preparation through the Aqueous Recovery System (ARS), a nitrate-based process line. Pyrochemical residues from salt scrub (SS), electrorefining (ER), and molten salt extraction (MSE) may contain significant amounts of chlorides entrained from the calcium chloride (CaCl2) flux. There is a necessity to remove the chlorides to mitigate corrosion of downstream nitrate-based aqueous processing equipment. Reacting CaCl2 with high concentrations of nitric acid (8-12 M) was demonstrated and is a promising means of chloride removal without the use of NO2 gas or mercury, as previously explored at the Savannah River Site. The chloride ions reacted with nitric acid to form gaseous Cl2 and ClNO, which were sparged from the system. A significant amount of chloride from CaCl2 was dissolved and off gassed within six hours at elevated temperatures of 50, 75, and 100°C in high concentrations of nitric acid. Kinetics of chloride removal were dependent on nitric acid concentration, initial chloride content, sparge rates, and temperature, but the trend of chloride removal was predictable. The largest amount of chloride removal occurred within the first hour of the process at the conditions tested and was generally second order with respect to chloride. The reaction mechanism changed over the next few hours to a slower first order rate (i.e., exponential decay) of the chloride. The first order region was influenced by sparge rate, enhanced chrloide removal was observed at higher spage rates. Optimizing sparge rate would enable lower temperature operations while maintaining high efficiency and reducing corrosion rates. The capacity to process high concentrations of chloride salts demonstrates the potential feasibility in processing the salt directly in ARS, completely replacing the salt scrub process.

Chloride to nitrate conversion for SRPPF aqueous recovery

ACS Spring 2022

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