Extraction and Separation of Uranium and Lanthanides with Supercritical Fluids

Abstract

Uranyl ions in nitric acid solutions can be effectively extracted by supercritical CO 2 containing tributyl phosphate (TBP) or organophosphine oxides. The form of the extracted uranyl nitrate-TBP complex and the kinetics of the supercritical extraction are similar to those reported for the conventional solvent extraction of uranyl nitrate with TBP. On-line back-extraction of uranium in supercritical CO 2 with an aqueous solution has also been demonstrated. The results suggest that supercritical CO 2 could be as effective as the organic solvents used in the PUREX process. Supercritical CO 2 containing organophosphinic acids such as Cyanex 301 and Cyanex 302 has been shown to extract heavy lanthanides selectively from the light lanthanides in aqueous solutions. This in situ chelation-SFE technique is also capable of removing leachable uranium from solid samples such as mine tailings as indicated by the EPA Toxicity Characteristics Leaching Procedure.There has been considerable interest in the past two decades to utilize supercritical fluids as solvents for chemical separations (i). The reasons for developing supercritical fluid extraction (SFE) technologies are mostly due to the environmental regulations and waste disposal costs for conventional solvents. Supercritical fluids have both gas-like and liquid-like properties. The solvation power of a supercritical fluid depends on pressure and temperature; thus, one can achieve the optimum conditions for a particular separation process by manipulating the temperature and pressure of the fluid phase. The high diffusivity and low viscosity of supercritical fluids enable them to penetrate and transport solutes from solid matrices. Carbon dioxide is the most widely used gas for SFE because of its moderate critical constants (T c = 31.3 °C, P c = 72.9 atm), nontoxic nature, and availability in pure form. In SFE processes, solutes dissolved in supercritical carbon dioxide are separated by reducing 390 In Metal-Ion Separation and Preconcentration; Bond, A., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999. 391the pressure of the fluid phase causing precipitation of the solutes. The fluid phase is usually expanded into a collection vessel to remove the solutes and the gas is recycled for repeated use. Typical examples of large-scale industrial applications of the SFE technology using supercritical C0 2 include the preparation of decaffeinated coffee and hop extracts (1).Until recently, little information was available in the literature regarding SFE of metal species. Direct extraction of metal ions is highly inefficient because of the charge neutralization requirement and the weak solute-solvent interactions. However, when metal ions are chelated with organic ligands, they may become quite soluble in supercritical C0 2 (2). This in situ chelation-SFE technique appears to have a wide range of applications including the treatment of metal contaminated or radioactive waste materials and mineral processing.Background

New Directions in Inorganic and Metal-Organic Coordination Chemistry in Supercritical Fluids

“…10% without increasing T c significantly . Alkyl phosphates and related compounds show particular promise as modifiers for metal complexes, especially in extraction and separation processes (Section III; Part 1). − An interesting variation described by Cooper et al uses a fluorinated dendrimer to extract a dye which is normally insoluble in scCO 2 .…”

“…Supercritical fluid extraction and chromatographic separation of organic complexes from matrices such as soil samples, biomaterials, and natural products are well-proven and established techniques. ,− In comparison, the extraction of metal complexes using SCFs (primarily scCO 2 ) is relatively unexplored but it is gaining interest for a variety of applications such as replacement solvents for cleanup of hazardous solids or dissolved metal wastes . By far the greatest amount of work in the area of SCF extraction (SFE) of metal complexes has been carried out by Wai and co-workers, who have written a number of excellent articles and comprehensive reviews on this subject. ,,, …”

“…Extraction can be conducted in static or flowing (dynamic) modes or a combination of these. Of course solid matrices can be either wet or dry, while the pH of the metal containing solutions can vary. ,, Sometimes the extraction vessels contain a filter in the outlet (such as a glass wool plug) to prevent finely divided solid being blown out of the vessel by the SCF.

11 Three basic types of SFE experiments for the removal of metal ions directly from (a) a solid matrix, (b) an aqueous solution, and (c) the solid itself.

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Extraction of Actinides and Lanthanides

“…The extraction is enhanced if the system also contains fluorinated ketones. [103][104][105] Other phosphates that have been used for the extraction of lanthanides include sodium metaphosphate with a Dowex I-X4 anion exchange column,l06 and ~-ketophosphonates. The ~-ketophosphonates that have been used are dibutyl phenacylphosphonate 26 and two of its derivatives.…”

Investigation of the uranium(Vl)-nitric acid-tributyl phosphate-supercritical carbon dioxide system

“…Meguro et al [44] used supercritical fluid leaching, and Shimada et al used a "direct extraction column" (Super-DIREX) [45] in the decontamination process of radioactive wastes using SCCO2, TBP and H N O 3 to separate uranium and plutonium [46] . and Cyanex 302 [49]. with the liquid phase are required to describe pressure quenching [53].…”