Actinides extraction methods

Nuclear Energy Research Initiative (NERI) Final Report. 2005. Advanced extraction methods for actinide/lanthanide separations. Submitted by University of Florida, PI Scott, M.J., collaborating organization Argonne National Laboratory. PNNL-13221. 

Schultz, M.K., Burnett, W.C. and Inn, K.G.W. (1998) Evaluation of a sequential extraction method for determining actinide fractionation in soils and sediments. /. Environ. Radioactivity, 40, 155-174. 

The tributyl phosphate extraction method, where control of the concentration of nitric acid from 6 to 16 M allows control of the transfer of various actinide ions into kerosene containing 30% (C O O. 

The cost induced by the off-shoring of rare earth mining and separation to China is generating a renewed interest in liquid-liquid extraction methods for these strategic elements. Moreover, recovery of actinides, in particular... 

The problems of practical separation of these elements are (1) the separation of actinide elements fi om the lanthanide group (trivalent actinide ions behave like lanthanide ions), and (2) the separation of the actinide elements from each other. Ion-exchange and solvent extraction methods have so far been extensively studied. 

There are two basic types of solvent extraction reactions which are applied to metal-ion separations, those based on metal-complex solvation and those based on metal-ion complexation. A third extraction method, observed only with quaternary amine extractants, involves ion-pair formation between an anionic, aqueous complex and the positively charged, organic-soluble quaternary amine. Extraction by quaternary amines incorporates aspects of both basic processes. The development and use of extractants for actinide separations have been discussed by Shoun and McDowell (1980). 

Ion exchange (qv see also Chromatography) is an important procedure for the separation and chemical identification of curium and higher elements. This technique is selective and rapid and has been the key to the discovery of the transcurium elements, in that the elution order and approximate peak position for the undiscovered elements were predicted with considerable confidence (9). Thus the first experimental observation of the chemical behavior of a new actinide element has often been its ion-exchange behavior—an observation coincident with its identification. Further exploration of the chemistry of the element often depended on the production of larger amounts by this method. Solvent extraction is another useful method for separating and purifying actinide elements. 

This latter situation affords a good method for separating uranium from plutonium. Hydroxylammonium formate (HAF) and hydrazium formate (NHF) were added to the formic acid to reduce Pu(IV) to Pu(III) to aid in plutonium recovery, although formic acid alone will strip tetravalent actinides, e.g., Th(IV) from 0D[IB]CMP0, once excess HNO3 present in the organic phase is removed. Thus, formic acid with HAF and NHF affords an excellent method for stripping all the actinides from these very powerful CMP extractants. Under the above conditions Am(III) and Cm(III) present in... 

The application of these methods is described in some detail for recovery of base metals and platinum group metals in Sections 9.17.5-9.17.6 focusing mainly on solution-based hydrometal-lurgical operations, largely those involving solvent extraction, because the nature of the metal complexes formed is usually best understood in such systems. NB. Extraction of lanthanides and actinides is not included as this subject is treated separately in Chapters 3.2 and 3.3. 

We have considered typical examples of lanthanide and actinide solvent extraction by chelate formation, involving complexes with citric acid and with TTA, to prove that the labelling of a stable element by one of its radioactive isotopes can help to produce accurate data on the stability constants for complex formation. The method is applicable to elements with radioisotopes having a half-life allowing an ion concentration of 10 6m or less. Other methods of partition such as radiopolarography and radio-coulometry also result in accurate thermodynamical data when the same procedure of labelling is used (29). 

Often the products of nuclear reactions have very short half-lives. This is especially true for the heaviest elements obtained by bombardment of heavy targets with heavy ions. To identify and characterize such short-lived nuclides, fast separations are required solvent extraction techniques are well suited to provide the required fast separations. For example, the SISAK method [68] has been successfully used in conjunction with in-line gas jet separators at heavy ion accelerators to identify short half-life actinide isotopes produced by collision of heavy atoms. The Sisak method involves use of centrifugal contactors, with phase residence times as low as tenths of a second, in conjunction with in-line radiometric detection equipment. 

Americium may be separated from other elements, particularly from the lanthanides or other actinide elements, by techniques involving oxidation, ion exchange and solvent extraction. One oxidation method involves precipitation of the metal in its trivalent state as oxalate (controlled precipitation). Alternatively, it may be separated by precipitating out lanthanide elements as fluorosilicates leaving americium in the solution. Americium may also he oxidized from trivalent to pentavalent state by hypochlorite in potassium carbonate solution. The product potassium americium (V) carbonate precipitates out. Curium and rare earth metals remain in the solution. An alternative approach is to oxidize Am3+ to Am022+ in dilute acid using peroxydisulfate. Am02 is soluble in fluoride solution, while trivalent curium and lanthanides are insoluble. 

Americium and other actinide elements may be separated from lanthanides by solvent extraction. Lithium chloride solution and an eight to nine carbon tertiary amine are used in the process. Americium is then separated from curium by the above methods. 

Organic extractants facilitate the transfer of the metal ions from the aqueous phase to the organic phase in solvent extraction. Based on the nature of the organic extractant, the metal ion, and the diluent, effective separation methods can be devised. Uranium extraction into diethyl ether from nitrate medium by salting out is perhaps one of the first uses of solvent extraction for large-scale actinide processing (9). In this case, ether not only acts as the diluent, it also acts as the extractant, which works according to the solvation mechanism (discussed below). 

Studies on the solvent extraction of actinide ions by different combinations of extractants have been reviewed. Various equilibria involved in the extraction processes and the formation of the extract-able complexes have been considered along with their equilibrium constant data. Various methods which are useful in establishing the composition and the nature of the extractable complexes are presented. The data on isolation and structural studies of some complexes, involved in synergic extraction, are also included. A brief description of the different areas in which synergic extraction is finding application is also given. Many combinations of extractants, where the studies conducted are very few but, which are likely to yield enhanced extractions are indicated. Areas of research, both from the academic and applied points of view, which require attention are suggested. 

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