Rare earths Separation

The lanthanides form many compounds with organic ligands. Some of these compounds ate water-soluble, others oil-soluble. Water-soluble compounds have been used extensively for rare-earth separation by ion exchange (qv), for example, complexes form with citric acid, ethylenediaminetetraacetic acid (EDTA), and hydroxyethylethylenediaminetriacetic acid (HEEDTA) (see Chelating agents). The complex formation is pH-dependent. Oil-soluble compounds ate used extensively in the industrial separation of rate earths by tiquid—tiquid extraction. The preferred extractants ate catboxyhc acids, otganophosphoms acids and esters, and tetraaLkylammonium salts. 

Liquid—Liquid Extraction. The tiquid—tiquid extraction process for the rare-earth separation was discovered by Fischer (14). Extraction of REE using an alcohol, ether, or ketone gives separation factors of up to 1.5. The selectivity of the distribution of two rare-earth elements, REI and RE2, between two nonmiscible tiquid phases is given by the ratio of the distribution coefficients DI and D2 ... 

Table 7. Commercial Extractants Available for Rare-Earth Separation...

The extent of extraction can be increased by a salting out effect. The selectivity of TBP is very poor compared with HDEHP and it is only useful for light rare-earth separation however, organic phase loadings or REO higher than 100 g/L can easily be achieved. There are a large number of TBP manufacturers in Japan, the United States, and Europe. 

Ion exchange (electrostatic) Equihbrium Deionization Water softening Rare earth separations Recovery and separation of pharmaceuticals (e.g., amino acids, proteins)... 

Huffman, F. H., and R. L. Oswalt A Rare-Earth Separation by Anion... 

Rhone-Poulenc Rare Earths Separation Process... 

One of the first triumphs of ion exchange chromatography was the separation and identification of fission product rare earths in the Manhattan Project in the early 1940s. Initial publication of this work was withheld until 1947 when nine papers from the Oak Ridge National Laboratory and the Ames Laboratory at Iowa State University appeared in the Journal of the American Chemical Society (1—9). The science of rare earth separations was indeed revolutionized. Separations that had taken years could now be done in about a day. 

The transplutonium elements and the rare earths, or lanthanides, are so similar chemically that what is true for one group is generally true for the other. In practice, process development work is usually carried out with lanthanides, and frequently, all the solutions end up as analytical samples. Transplutonium elements, in contrast, are so valuable that the goal is the maximum yield of pure products. Accordingly, the methods and equipment developed with rare earth separations are applied directly to heavy actinide production separations. These may be quite small in scale, but this is "production" for some of these elements. 

Reviews of the biochemical work generally start with Martin and Synge in 1941 (13) and then jump to the work of Cohn on nucleic acid separations by ion exchange chromatography in 1949 (14). It so happens that Waldo Cohn was coauthor of one of those original publications on rare earth separations in 1947. 

Larger-scale separations of americium and curium are based on the pioneering work of Wheelwright and coworkers (24) and the very large-scale use of displacement development for commercial rare earth separations. Application of pressurized ion exchange to displacement development for transplutonium element separations has been pursued at SRL by Hale, Lowe, and coworkers (25, 26), and the work was reviewed in 1972 (27). 

Schadel, Trautmann, and Herrmann compared the HDEHP extraction chromatography system to ion exchange elution development for rare earth separations (32) and found the two methods to be about equally effective under optimum conditions. From their data for separating seven rare earths plus yttrium, they projected that all the rare earths could be separated in about 20 min. However, they did not include in their study those pairs most difficult to separate. 

Rare earths separation has been achieved by formation of species of the formula Ln(NC>3)3 -3TBP, where TBP is tributyl phosphate. This process is solvent extraction which is widely used at present in the production of rare earths in pure form. 

The five-column HPIEC system, because of the sizable scope of its functions, was used by the authors to study those rare earth separations that were too difficult to examine with the three-column system. The largest rare earth oxide sample employed with it was 50 g. 

Use of the three-column system shown in Fig. 2 has been examined to judge its potential for rare earth separations. The purities of Sm, Eu, and Gd that were obtained from the mixed Sm-Eu-Gd sample used for this basic study were Sm20j 99.5%. EUjOj 99.99%, Gd203 99.5%. The composition of the natural Sm-Eu-Gd sample separated in this study was Nd 13%. Tb 4.5%, Sm 25%, Eu 4.8%, Gd 27.0%, and the other RE elements 25%. Gd and Eu alternatively labeled with Gd-(153, 159) and Eu-(152, 154) were used to determine the displacement curves of these two elements while the displacement curve of Sm was determined by using a spectrophotometric method. 

Chen J, Li D-Q (2008) Application of ionic liquids on the rare earth separation. Acta Chem Process (Zl) 54-59 (in Chinese)... 

An impregnated resin prepared from PC-88A and Amberlite XAD7 beads was examined as the stationary column phase for the separation of rare earth [28]. The reagent, PC-88A, is frequently used as an extractant in rare earth separation by solvent extraction. As can be seen in Fig. 27, the distribution ratio at a given pH increases with decreasing ionic radii of lanthanides. The slope analysis for log D versus pH and log D versus concentration of PC-88A indicated that the present extraction mechanism was similar to that of the solvent extraction system. A mutual separation of Y-Gd, La-Pr-Nd, and Ho-Er-Tm was successfully attained with the present resin as the stationary phase and hydrochloric acid as the mobile phase. 

Recovery of nickel and cobalt after the rare earth separation by selective extraction of cobalt over nickel with 25% TOA, followed by precipitation with ammonium oxalate (Zhang et al., 1998), and electrochemical process (Tzanetakis and Scott, 2004b) was also studied. 

Hazardous Decomp. Prods. Heated to decomp., emits toxic fumes of NOx Uses Chelating agent in cosmetics, detergents (stabilizing peroxides) microbiological control for water treatment, cutting fluids, sec. recovery textiles scale control/removal rare earth separations... 

Jan. 23, 1951 Rare-earth Separation by Adsorption and Desorption F.H. Spedding A.F. Voigt... 

Campbell, D.O. Rapid rare earth separation by pressurized ion exchange chromatography. J. Inorg. Nucl. Chem. 1973, 35 (11), 3911-3919. 

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