Solvent lanthanide separation

Baybarz, R. D. (1965). Dissociation contents of the transplutonium chelates with diethylenetriaminepentaacetic acid (DTPA) and the application of DTPA chelates to solvent extraction separations of transplutonium elements from the lanthanide elements, J. Inorg. Nucl. Chem. 27,1831. 

Lanthanides form many types of complexes in both aqueous and non-aqueous solutions and have been studied extensively. In the early stages the main interest was in the development of efficient ligands for the separation of lanthanides by the ion-exchange technique. The second thrust was the study of complexes in non-aqueous solutions which can be used in the solvent extraction separation of lanthanides. 

Diphosphazane dioxide complexes of lanthanides have potential application in the solvent extraction separation of lanthanides. Reaction of lanthanide nitrate with X2P(0)NPr )-P(0)X2(L-L) yields the bis chelate complexes Ln(NC>3)(L-L)2. The structure of the praseodymium complex has been determined by X-ray diffraction and the space group is P32- There are two independent molecules in the unit cell which differ in orientation of the phenyl group. The metal ion is ten-coordinated [264]. 

A well-known example is actinide or lanthanide separation when the individual trivalent metal ion can he isolated from a bombarded target by multistage solvent extraction or ion-exchange chromatography. 

Recently, two how-to manuals have been published which describe in detail useful ion exchange separation procedures for the lanthanides and actinides (Handbook of Ion Exchange Resins, Volumes 1 and 2). As noted above, the separation chemistry of the lanthanides (ion exchange and solvent extraction) has been reviewed in a volume of Gmelin (1983). The latter review discusses very capably the more subtle aspects of lanthanide separation, performing the usual division of the work along the lines of solvent extraction-ion exchange. 

The one review which treats lanthanide/trivalent actinide separation is that of Weaver (1974). Weaver s review is an excellent, if somewhat dated, source for a comprehensive discussion of solvent extraction separations of the lanthanides and trivalent actinides. Weaver discusses many of the historical aspects of lanthanide/ actinide separation, and considers both the successes and failures in the separation of trivalent lanthanides and actinides. 

But whether the result of metal-ligand covalent bonding or a more subtle polarizability effect, extractants and complexants containing soft donor atoms are central to most ion exchange and solvent extraction separations of lanthanides from actinides. To generalize, those materials with the greatest potential for increased covalent interactions provide the most significant opportunity for successful lanthanide/ actinide separations. As discussed below, the sheer multiplicity of reactions involved in separations processes offer many opportunities to exploit this difference in soft donor interactions. 

The most important early investigations of lanthanide separations by solvent extraction were studies of the extraction of the trivalent lanthanide cations by TBP. Work by Peppard and co-workers, and by McKay and co-workers [previously reviewed by Stary (1966)] showed an odd-even effect in the grouping of lanthanide extraction... 

The character of the diluent in a solvent extraction separation scheme generally can have a profound effect on the overall degree of extraction, but the effect of diluent on separation factors is usually much more subtle. Alteration of the diluent modifies metal-ion extraction equilibria primarily by its effect on the solvation of the hydro-phobic metal complex. Although few recent studies have been made of diluent effects in lanthanide/actinide separation (with no truly systematic investigations), enough historical reports exist to make some general observations of the effect of diluent on lanthanide/actinide separation. 

Alyapyshev, M. Y, Babain, V. A., Eliseev, 1.1. J. et al. 2009b. Am-lanthanides separation using new solvents based on 2,2-dipyridyl-6,6-dicarboxylic acid diamides. In Global 2009 p. 1113, Paris, France, September 6-11, 2009. 

Ce(IV) extracts more readily iato organic solvents than do the trivalent Ln(III) ions providing a route to 99% and higher purity cerium compounds. Any Ce(III) content of mixed lanthanide aqueous systems can be oxidi2ed to Ce(IV) and the resultiag solutioa, eg, of nitrates, contacted with an organic extractant such as tributyl phosphate dissolved in kerosene. The Ce(IV) preferentially transfers into the organic phase. In a separate step the cerium can be recovered by reduction to Ce(III) followed by extraction back into the aqueous phase. Cerium is then precipitated and calcined to produce the oxide. 

Yttrium and lanthanum are both obtained from lanthanide minerals and the method of extraction depends on the particular mineral involved. Digestions with hydrochloric acid, sulfuric acid, or caustic soda are all used to extract the mixture of metal salts. Prior to the Second World War the separation of these mixtures was effected by fractional crystallizations, sometimes numbered in their thousands. However, during the period 1940-45 the main interest in separating these elements was in order to purify and characterize them more fully. The realization that they are also major constituents of the products of nuclear fission effected a dramatic sharpening of interest in the USA. As a result, ion-exchange techniques were developed and, together with selective complexation and solvent extraction, these have now completely supplanted the older methods of separation (p. 1228). In cases where the free metals are required, reduction of the trifluorides with metallic calcium can be used. 

The separation of basic precipitates of hydrous Th02 from the lanthanides in monazite sands has been outlined in Fig. 30.1 (p. 1230). These precipitates may then be dissolved in nitric acid and the thorium extracted into tributyl phosphate, (Bu"0)3PO, diluted with kerosene. In the case of Canadian production, the uranium ores are leached with sulfuric acid and the anionic sulfato complex of U preferentially absorbed onto an anion exchange resin. The Th is separated from Fe, A1 and other metals in the liquor by solvent extraction. 

A closely related method does not require conversion of enantiomers to diastereomers but relies on the fact that (in principle, at least) enantiomers have different NMR spectra in a chiral solvent, or when mixed with a chiral molecule (in which case transient diastereomeric species may form). In such cases, the peaks may be separated enough to permit the proportions of enantiomers to be determined from their intensities. Another variation, which gives better results in many cases, is to use an achiral solvent but with the addition of a chiral lanthanide shift reagent such as tris[3-trifiuoroacetyl-Lanthanide shift reagents have the property of spreading NMR peaks of compounds with which they can form coordination compounds, for examples, alcohols, carbonyl compounds, amines, and so on. Chiral lanthanide shift reagents shift the peaks of the two enantiomers of many such compounds to different extents. 

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. 

Several methods have been used to separate the lanthanides chemically solvent extraction, ion exchange chromatography, HPLC using Q-hydroxyisobutyric acid and, in limited cases, selective reduction of a particular metal cation.40-43 The use of di(2-ethylhexyl)orthophosphoric acid (HDEHP) for the separation of various rare-earth elements via solvent extraction has also been reported.44 16 This separation method is based on the strong tendency of Ln3+ ions to form complexes with various anions (i.e., Cl- or N03 ) and their wide range of affinities for com-plexation to dialkyl orthophosphoric acid. When the HDEHP is attached to a solid phase resin, the lanthanides can be selected with various concentrations of acid in order of size, with the smallest ion being the most highly retained. 

Synthesis in liquidAl Al as a reactive solvent Several intermetallic alu-minides have been prepared from liquid aluminium very often the separation of the compounds may be achieved through the dissolution of Al which dissolves readily in several non-oxidizing acids (for instance HC1). For a review on the reactions carried out in liquid aluminium and on several compounds prepared, see Kanatzidis et al. (2005) binary compounds are listed (Re-Al, Co-Al, Ir-Al) as well as ternary phases (lanthanide and actinide-transition metal aluminides). Examples of quaternary compounds (alumino-silicides, alumino-germanides of lanthanides and transition metals) have also been described. As an example, a few preparative details of specific compounds are reported in the following. 

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