The Dissolution of Metal Decontamination Sludges Stored in Tanks and Their Management

Abstract

The decontamination of stainless steel components is accomplished by the use of alkaline permanganate solutions, followed by an application of solutions of complexing agents such as citric acid or oxalic acid. Spent decontamination solutions comprising residues from both steps were combined in several waste storage tanks, where they have been in storage for several years. In those tanks, a reaction between residual permanganate and unreacted complexing agents produced sludges, consisting mainly of manganese dioxide, that reside in the tanks along with supernatant liquid. In a campaign that was conducted a few years ago, the accumulated waste solution was partially treated and disposed. This treatment consisted of decanting only the supernatant liquid and transporting it to a liquid waste treatment facility that employed a Thin Film Evaporator (TFE) to concentrate the liquid and ultimately produce a bitumen-encapsulated solidified waste form for storage. A study of treatment options for the remaining sludge is reported here. The requirement was to determine a simple means of treating the sludge using existing routine processes and equipment. This will be a significant step toward the decommissioning of the decontamination waste storage tanks. The available equipment at the liquid waste treatment facility was not designed to process sludge or slurries containing a large volume fraction of solids. Laboratory testing was carried out to find a means of dissolving the decontamination waste sludges, preferably in situ, and filtering undissolved solids to meet the feed requirements of the TFE in the liquid waste treatment facility. A concentrated citric acid solution was applied to sludge samples, without the use of externally applied mixing of the reagent and sludge. In all of the samples of actual decontamination waste sludge that were tested, a quantity of undissolved material remained after treatment with citric acid. The quantities were relatively small in volume, and easily removed by coarse filtration. Tests of these residues indicated that, while they were still radioactive, the radioactivity had been decreased relative to that of untreated sludges. Processing of the sludges, following their treatment with citric acid, involved extraction from the storage tanks and filtration to remove the small quantity of undissolved residue. The conditioned liquid stored in a holding tank is currently being processed through the TFE where its volume will be reduced and it will be mixed with a bitumen emulsion to produce a solid waste form suitable for storage in 205 L drums.