Caustic Leaching of SRS Tank 12H Sludge With and Without Chelating Agents

Abstract

The primary objective of this study was to measure the effect of adding triethanolamine (TEA) to caustic leaching solutions to improve the solubility of aluminum in actual tank-waste sludge. High-level radioactive waste sludge that had a high aluminum assay was used for the tests. This waste, which originated with the processing of aluminum-clad/aluminum-alloy fuels, generates high levels of heat because of the high {sup 90}Sr concentration and contains hard-to-dissolve boehmite phases. In concept, a chelating agent, such as TEA, can both improve the dissolution rate and increase the concentration in the liquid phase. For this reason, TEA could also increase the solubility of other sludge components that are potentially problematic to downstream processing. Tests were conducted to determine if this were the case. Because of its relatively high vapor pressure, process design should include methods to minimize losses of the TEA. Sludge was retrieved from tank 12H at the Savannah River Site by on-site personnel, and then shipped to Oak Ridge National Laboratory for the study. The sludge contained a small quantity of rocky debris. One slate-like flat piece, which had approximate dimensions of 1 1/4 x 1/2 x 1/8 in., was recovered. Additional gravel-like fragments with approximate diameters ranging from 1/8 to 1/4 in. were also recovered by sieving the sludge slurry through a 1.4-mm square-pitch stainless steel mesh. These particles ranged from a yellow quartz-like material to grey-colored gravel. Of the 32.50 g of sludge received, the mass of the debris was only 0.89 g, and the finely divided sludge comprised {approx}97% of the mass. The sludge was successfully subdivided into uniform aliquots during hot-cell operations. Analytical measurements confirmed the uniformity of the samples. The smaller sludge samples were then used as needed for leaching experiments conducted in a glove box. Six tests were performed with leachate concentrations ranging from 0.1 to 3.0 m NaOH, 0 to 3.0 m TEA and 0 to 2.9 m NaNO{sub 3}. Figure ES.1 illustrates the leaching of aluminum in all six tests. One test was performed at an operating temperature of 80 C to obtain baseline data, and the remaining five tests were all performed at 60 C. A leaching solution of 3.0 m NaOH was used for the test performed at 80 C and for one of the tests performed at 60 C. These results indicated that more aluminum entered the solution at the higher temperature, though equilibrium was achieved at both temperatures within {approx}10 days. The addition of TEA significantly increased the concentration of aluminum in the leachate, and the concentration continued to increase even after 11 days of processing. The fraction of aluminum dissolved at 60EC increased from {approx}35% using 3.0 m NaOH alone to {approx}87% using a combination of 3.0 m NaOH and 3.0 m 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 had no effect on this process. The use of TEA also increased the solubility of some other sludge components. The fractions of copper, nickel, and iron that were dissolved increased to 72, 13, and 52%, respectively. However, the original fractions of these metals were only 0.055, 0.72, and 3.1%, respectively, of the dry mass of the sludge and therefore represent minor constituents. The presence of nickel in the leachate did have a dramatic effect on its color, which changed from light yellow to deep green as the nickel concentration increased. By comparison, the baseline leaching with 3.0 m NaOH at 60 C removed {approx}14% of the copper; iron and nickel were below the detectable limit.