Effects of Sodium Hydroxide and a Chelating Agent on the Removal of Aluminum from Radioactive Sludge

Abstract:

An essential step during the remediation of nuclear waste by the U.S. Department of Energy involves the separation of nonradioactive components such as aluminum from high-level waste sludges to minimize the ultimate volume to be stored in a nuclear waste repository. Plans for waste treatment at Hanford and the Savannah River Site include the use of 1 to 3m sodium hydroxide (NaOH) at an elevated temperature to leach the aluminum from the sludge. Triethanolamine (TEA) was added to caustic leaching solutions in an effort to improve the solubility of aluminum from authentic tank-waste sludge. High-level radioactive waste sludge with significant amounts of gibbsite and hard-to-dissolve boehmite phases was used in these tests. In concept, a chelating agent such as TEA can both improve the dissolution rate and increase the aluminum concentration in the liquid phase. However, TEA could also increase the solubility of other sludge components that are potentially problematic to downstream processing.

Six tests were performed with leachate concentrations ranging from 0.1 to 3.0m NaOH, 0 to 3.0m TEA, and 0 to 2.9 m NaNO₃. One test was performed using 3.0m NaOH at 80°C in order to simulate the baseline process, while the other tests were performed at 60°C. As expected, more aluminum entered the solution at 80 C than at 60°C when other test conditions were held constant. With caustic alone, equilibrium was achieved at both temperatures within 10 days. The addition of TEA significantly increased the concentration of aluminum in the leachate, and the aluminum concentration continued to increase even after 11 days of processing. The fraction of aluminum dissolved at 60°C increased from 35% using 3.0 in NaOH alone to 87% using a combination of 3.0 m NaOH and 3.0m TEA. The high-nitrate, low-hydroxide solutions did not significantly dissolve the aluminum because aluminate ion could not be produced. A small addition of TEA to these low-caustic solutions had no effect on aluminum removal.

The use of TEA also increased the solubility of copper, nickel, and iron, which are only minor constituents. The TEA also had a significant effect on the solubility of the radionuclides ¹³⁷Cs and ⁶⁰Co. The significant presence of ¹³⁷Cs in the leachates was expected with and without TEA. The high-nitrate leaches, which were the least effective of the leaching solutions, removed 69% of the ¹³⁷Cs from the washed sludge, while a combination of 3.0m NaOH and 3.0m TEA removed 96%. Very little ⁶⁰Co was removed from the sludge except with the use of the 3.0/h NaOH-3.0m TEA solution, which removed 53%. These results indicate that only TEA and ⁶⁰Co need to be examined for potential chemical and radiological impacts, respectively, on downstream processes.