NMT Evaluates Extraction Chromotography for Removal of Pu and Am from HCI Effluents

The Task-Residue Recovery The use of hydrochloric acid (HCl) for aqueous plutonium recovery offers good dissolution of many plutonium residue matrices. Following dissolution an extensive set of chemical separation options such as solvent extraction, ion exchange, extraction chromatography, and several precipitation techniques are presently available. Selection of the best treatment options for residue recovery depends on the type of residue to be recovered, the contaminants to be removed, and the purity and form of the product desired. The HCl processing of plutonium residues is a relatively new method with several potential benefits in process efficiency and waste reduction. This may present itself as a viable process alternative for residues whose disposition cannot be adequately addressed by other processes.

Minimizing the solid wastes and liquid effluents from plutonium processing and making them less hazardous are tasks of fundamental importance. Present goals for residue processing in HCl parallel the NMT Division goals for environmentally benign operations. These goals can be summarized for HCl operations as follows: producing concentrated actinide residues in forms suitable for safe long-term storage, forming stable solid wastes with acceptable disposal routes, and releasing liquid effluents with no radioactivity and low levels of other hazards.

Figure 2. A schematic diagram of the treatment process for the TA-55 high-acid stream. The acid recycle portion was described in the winter 1994 issue of The Actinide Research Quarterly.

Traditional Treatment

Liquid effluents from HCl operations at TA-55 have historically been treated by controlled hydroxide precipitation before they are transferred to the TA-50 Liquid Waste Treatment Facility. This hydroxide neutralization and precipitation operation has been necessary partially because of the corrosive nature of HCl, which requires that all effluent solutions containing significant amounts of chloride be made neutral or basic prior to transfer to TA-50. Hydroxide precipitation, combined with a filtration step, also recovers a fraction of actinides from process solutions in the form of a hydroxide cake. There are several problems, however, with hydroxide precipitation as a generic effluent treatment process: 1) many other metal hydroxides coprecipitate creating large cakes; 2) chloride salts can be entrained in the hydroxide matrix causing corrosion concerns for long-term vault storage of the hydroxide cakes; 3) many metal hydroxides are gelatinous, leading to slow filtration and high gamma exposure from americium (²⁴¹Am) in this hands-on operation; 4) the filtrate from neutralization remains moderately high in radioactivity and requires special treatment at TA-50, producing more transuranic (TRU) solid wastes; and 5) almost all of the chloride from HCl operations is presently lost in the liquid effluent, causing the TA-50 outfall to the environment to approach or exceed recommended National Pollutant Discharge Elimination System limits for chloride concentration.

Hydroxide precipitation remains a good choice for recovery of actinides from some specific processes, but it is not an acceptable choice as a generic treatment for all HCl effluents. Indeed, there is no single technology or unit operation that provides a panacea to resolve all of the residue, liquid, and solid waste issues associated with HCl processing. The problems can be addressed by intelligently developing and applying better treatment technologies, directed toward the specific hazards of individual effluents.

A New Strategy

Recent waste minimization efforts have used the strategy of developing specific treatments for individual waste streams. Greatest effort has been expended on removing actinides from waste streams with high-molar HCl effluents including anion exchange effluents and solvent extraction raffinates. The chemical makeup of these streams includes all the americium in the process solutions (significant amounts in many residues), residual small amounts of plutonium left from purification, about 90% of the total amount of HCl used, and the soluble chloride salts of alkali, alkaline earth, and some transition metals. Americium is often the larger problem in waste streams because of its greater alpha activity compared to most plutonium isotopes and its associated 60-KeV gamma emission. Efficient recovery of the actinides in high-acid effluent streams would have a large impact on wastes from HCl operations.

Extraction chromatography, using ligands such as n-octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (CMPO), diamyl amylphosphonate (DAAP) and/or tributyl phosphate (TBP) loaded on inert supports, has been used by others on an analytical scale to concentrate actinides for analysis. We are evaluating this technology for its adaptation to a much larger scale for removing actinides from high-acid effluent streams.

Recent Results

Our work began with small-scale experiments under carefully controlled conditions, using well-characterized HCl solutions of plutonium(IV), plutonium(III), and americium(III). Commercial resin formulations, (typical resins are formulated by coating inert support polymers with ligands having specific affinity for actinides) performed well at removing plutonium(IV) from high-molar HCl solutions but failed to remove plutonium(III) and americium(III). The preliminary studies also indicated that the resins will function at actinide-loading levels much higher than those recommended for analytical use, making them viable for a practical treatment process.

Work with different resin formulations has generated data indicative of how actinide distribution and kinetics change with various mixtures of extractants. These data have allowed us to tailor new formulations to more effectively remove the trivalent actinides. The more promising formulations have now been prepared in large quantity and tested at full scale with actual waste streams. Decontam-ination of gram quantities of plutonium and americium from anion exchange effluents and solvent extraction raffinates has been achieved with alpha decontamination results varying between 90% and 99.99% for the first ten attempts. The plutonium and americium are readily stripped from the resins with small elutriant volumes, providing relatively pure solutions. The actinides can then be recovered by oxalate or hydroxide precipitation, followed by calcination, to provide concentrated residues suitable for long-term vault storage.

The decontaminated effluent solutions were neutralized before their transfer to TA-50, producing hydroxide cakes of a radioactivity level that should allow fixation and discarding as TRU or low-level waste. Removal of americium from the processing stream as early and as efficiently as possible has the added benefit of reducing exposure to workers in all subsequent handling, treatment, and storage operations. Future plans are to use the decontaminated acid solution as feed for HCl recycling operations, which will further reduce activity, volume, and chloride content of effluents.

Efforts to implement this technology continue as we optimize resin formulations and alter process conditions for our specific applications. Other unique treatment strategies are being developed to dilute HCl process streams including filtrate from oxalate precipitation operations.

Developers and Contributors

Louis D. Schulte, Steven D. McKee and Richard R. Salazar of NMT-6 are the principal developers of this project. They acknowledge a large number of people who have contributed to this work: Mark Dinehart, Keith Fife, Devin Gray, Benjie Martinez, Mike Palmer, Brad Smith, and Wayne Smyth of NMT-2; John FitzPatrick and Brad Schake of CST-4; and Larry Avens, Gordon Jarvinen, and David Romero of NMT-6. Thanks to Mike Gula of EIChrom® Industries for the gift of small samples of resins custom-formulated to our specifications, and to E. Phillip Horwitz of Argonne National Laboratory for helpful discussions.