Chemical separation, lanthanides

The lanthanide elements are very difficult to separate because of their highly similar chemistry, but the earlier actinide elements have sufficiently different redox chemistry to allow easy chemical separations. This is important in the nuclear power industry, where separations have to be made of the elements produced in fuel rods of nuclear power stations as fission products, and of the products Np and Pu, which arise from the neutron bombardment of the uranium fuel. 

RARE-EARTH ELEMENTS AND METALS. Sometimes referred to as the fraternal fifteen," because of similarities in physical and chemical properties, the rare-earth elements actually are not so rare. This is attested by Fig. 1, which shows a dry lake bed in California that alone contains well in excess of one million pounds of two of die elements, neodymium and praseodymium. The world s largest rare earth body and mine near Baotou, Inner Mongolia, China is shown in Fig. 2. It contains 25 million tons of rare earth oxides (about one quarter of the world s human reserves. The term rare arises from the fact that these elements were discovered in scarce materials. The term earth stems from die tact that the elements were first isolated from their ores in the chemical form of oxides and that the old chemical terminology for oxide is earth. The rare-earth elements, also termed Lanthanides, are similar in that they share a valence of 3 and are treated as a separate side branch of the periodic table, much like die Actinides. See also Actinide Contraction Chemical Elements Lanthanide Series and Periodic Table of the Elements. 

Hudson, M.J. 2003. Some new strategies for the chemical separation of actinides and lanthanides. Czech. J. Phys. 53 (Suppl. 1) A305-A311. 

Mathews, S.E. Parzuchoski, P. Garcia-Carrera, A. Griittner, C. Dozol, J.F. Bohmer, V. Extraction of lanthanides and actinides by a magnetically assisted chemical separation technique based on CMPO-calix[4]arenas, Chem. Commun. 5 (2001) 417—418. 

In effect, the lanthanide contraction has shrunk these elements. Since electronic configuration, charge, and ionic size generally determine chemical behavior, we should expect that Zr+4 and Hf+4 have extremely similar behaviors and that the same should be true for Nb+5 and Ta+5. Such similarities are indeed observed and prove exceedingly troublesome in the course of chemical separations. 

Treatment of irradiated targets. The chemical operations relative to the production of transplutonium elements (americium 243, curium 244) are all performed using a nitric acid medium. The highly corrosive nature of the solutions concentrated with Cl" ions, which were used in the USA for the development of the Tramex process (JO, and the instability of SCN" ions to radiation (12), led us to select nitric acid solution to perform the chemical separations. Once the medium was selected, it was necessary to find an adequate additive which, in combination with a suitable extractant, would allow solution of the main problem namely separation of the trivalent actinides from triva-lent lanthanides. 

This paper reports the results of investigations of the complex formation between actinide or lanthanide ions and azide or orthophenanthroline. The aim of this work was first to confirm whether these relatively soft ligands give complexes of different stabilities with the trivalent lanthanide and actinide ions, as a consequence of the broader extension of 5f orbitals as compared with 4f. Secondly, we attempted to use the results in actinide chemical separation processes. 

Fig. 3 compares the accuracy of INAA, ICP-MS, ETAAS, ICP-AES and EDXRF in the analysis of plant reference materials (NIST-SRM-1575 Pine needles, NIST-SRM-1572 Citrus leaves and Bowen s kale). Twenty-three elements are determined by INAA, 13 by ICP-MS, 17 by ICP-AES - lanthanides are determined after chemical separation and preconcentration (Markert, 1996) - 10 by ETAAS and 9 by EDXRF. The method displaying greatest problems with accuracy is ICP-MS where accuracy for Cr is 126%, for Cu 45%, for Mn 30%, for Mo 94% and for Sb 50%. EDXRF faces difficulties in the determination of Ca and Fe and ICP-AES for Ce and Na. ETAAS is the only method permitting determination of Cd in plants at background levels and again as in the other cases Ni is problematic for all methods. 

Depending on the detection technique employed and the purpose of the analysis, it is occasionally sufficient to conduct a relatively simple group separation to isolate the rare earths from the matrix. Neutron activation analysis (NAA), inductively coupled plasma/atomic emission spectroscopy (ICP/AES), and mass spectrometry (ICP/MS) are examples of techniques that have been applied for simultaneous detection/quantitation of individual lanthanides in a mixture of lanthanides. Chemical separation techniques are often required prior to application of these methods because of the susceptibility of element-specific techniques to interferences that may compromise the analysis. 

Figure 12 illustrates an anion transport system with a lanthanide tris(p-diketonate) as the carrier. When the lipophilic lanthanide complex is present in Membrane, a highly coordinated complex is formed with the anion guest at the interface between Aq. I and Membrane, and K(I) cation is extracted into Membrane as the counter-cation. The resulting ternary complex moves across Membrane. At the interface between Membrane and Aq. II, the guest anion is released into Aq. II together with its counter-cation. Crown ether carrier mediates anion transport in a In Chemical Separations with Liquid Membranes Bartsch, R., et al. ... 

This discovery completed the sequence of 14 elements occurring between the atomic numbers 58 and 71 inclusive. The group did not fit into the original periodic tables and was included as the separate lanthanide series . Lanthanum itself, together with two other group 3A elements, yttrium and scandium (and sometimes thorium), is often included with the lanthanide series elements in discussions of the REE. This is because they frequently occur together in rare earth minerals, having similarities in ionic radii and chemical activity. 

Although rare-earth ions are mosdy trivalent, lanthanides can exist in the divalent or tetravalent state when the electronic configuration is close to the stable empty, half-fUed, or completely fiUed sheUs. Thus samarium, europium, thuUum, and ytterbium can exist as divalent cations in certain environments. On the other hand, tetravalent cerium, praseodymium, and terbium are found, even as oxides where trivalent and tetravalent states often coexist. The stabili2ation of the different valence states for particular rare earths is sometimes used for separation from the other trivalent lanthanides. The chemicals properties of the di- and tetravalent ions are significantly different. 

Extraction by carboxyUc acids (qv) is carried out in a neutral or weaMy acidic medium. The most widely used carboxyUc acid is RR (CH2)CCOOH, where Rplus represents seven carbon atoms. Trade names are Versatic 10 (Shell Chemicals) and Neodecanoic acid (Exxon Chemicals). CarboxyUc acids can be used either in chloride or in nitrate media and have a better selectivity for light lanthanides than for heavy lanthanide separation. 

There is no single best form of the periodic table since the choice depends on the purpose for which the table is used. Some forms emphasize chemical relations and valence, whereas others stress the electronic configuration of the elements or the dependence of the periods on the shells and subshells of the atomic structure. The most convenient form for our purpose is the so-called long form with separate panels for the lanthanide and actinide elements (see inside front cover). There has been a lively debate during the past decade as to the best numbering system to be used for the individual... 

The most important minerals of the lanthanide elements are monazite (phosphates of La, Ce, Pr, Nd and Sm, as well as thorium oxide) plus cerite and gadolinite (silicates of these elements). Separation is difficult because of the chemical similarity of the lanthanides. Fractional crystallization, complex formation, and selective adsorption and elution using an ion exchange resin (chromatography) are the most successful methods. 

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. 

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