Separating the Lanthanides

Diagrammatic representation of the system of fractional crystallization used to separate salts of the rare-earth elements (reproduced with permission from D.M. Yost, H. Russell and C.S. Garner, The Rare Earth Elements and their Compounds, John Wiley, 1947.) 

It was subsequently found that amine polycarboxylates such as EDTA gave stronger complexes and much better separations. In practice, some Cu + ions ( retainer ) are added to prevent precipitation of either the free acid H4EDTA or the lanthanide complex 

HLn(EDTA).xH20 on the resin. The major disadvantage of this method is that it is a slow process for large-scale separations. 

For a lanthanide Lha distributed between two phases, a distribution coefflcent T a is defined  

For two lanthanides Lha and Lns in a mixture being separated, a separation factor can be defined, where 

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There are few principal lanthanide deposits, and there are no minerals that are sources for cerium alone. All the lighter lanthanides occur together in any potential deposit, and processes separating the lanthanides are necessary to obtain pure cerium products. 

The classical methods used to separate the lanthanides from aqueous solutions depended on (i) differences in basicity, the less-basic hydroxides of the heavy lanthanides precipitating before those of the lighter ones on gradual addition of alkali (ii) differences in solubility of salts such as oxalates, double sulfates, and double nitrates and (iii) conversion, if possible, to an oxidation state other than -1-3, e g. Ce(IV), Eu(II). This latter process provided the cleanest method but was only occasionally applicable. Methods (i) and (ii) required much repetition to be effective, and fractional recrystallizations were sometimes repeated thousands of times. (In 1911 the American C. James performed 15 000 recrystallizations in order to obtain pure thulium bromate). 

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. 

Monazite or Xenotime. The rare earth phosphate containing ores are attacked with either concentrated sulfuric acid or sodium hydroxide solution. The processing involves cracking the ore, removing the thorium, and separating the lanthanides. 

Monazite is usually treated with NaOH at 150 °C to remove phosphate as Na3P04, leaving a mixture of the hydrated oxides, which are dissolved in boiling HCl at pH 3.5, separating the lanthanides from insoluble Th02. Sulfuric acid can also be used to dissolve the lanthanides. 

Strictly, the rare-earth metals are oxides of the lanthanide elements (lanthanide plus the 14 metals in the lanthanide series). They are chemically related and difficult to separate. The lanthanides are also members of the rare metals, a larger group of the less commonly occurring metals. Some rare metals (e.g., plutonium and promethium) occur only as a result of nuclear fission. 

Like any other instrumental technique used to analyze solid samples, XRF spectrometry is subject to matrix effects and the problems derived from differences in grain size when samples are used in pellet form. Both shortcomings can be circumvented by dissolving the material and separating the lanthanides by using one of the above-described procedures. After separation, the diflPerent fractions containing the analytes are placed on solid supports for analysis [23]. This procedure is usually employed with some geological samples that are attacked by... 

Lucy et al. (1993) report on a multicolumn method for chromatographic analysis of irradiated nuclear fuels. The first stage is characterized as a semi-preparative reversed-phase separation that removes the uranium matrix. A second column concentrates and separates the lanthanides prior to colorimetric detection of the ions using Arsenazo III. Instead of the 0.5—100 g of uranium required for conventional analysis of lanthanide content in such samples, these authors indicate a detection limit of 20ng/g (uranium) for Sm, Gd, Eu, and Dy from a 20mg uranium sample. They indicate no interferences in the analysis from transition or alkaline-earth metals. The uranium solution (containing the lanthanides) is initially dissolved in 0.025 M hiba. The reverse-phase column passes the lanthanides in a band and retains Th and U. The lanthanide band from the precolumn is channeled to the analysis column and sq)arated with an hiba gradient elution sequence. 

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