Among the most difficult of separations of metal ions are the intra- and intergroup separation of Ianthanides and trivalent actinides. Yet environmental concerns related to radioactivity, and new high-tech developments which have increased the demand for pure lanthanides have combined to foster a greater need for effective procedures to attain these separations. The most recent review which concentrated on this problem is now nearly 20 years old. While the methods developed over the preceeding 50-60 years of research are still in use, new understanding and some promising new methods are currently under development. The present manuscript is partly a tutorial of the basic chemical principles leading to successful separations, and partly a review of the results of the past 10-12 years on the separation of trivalent lanthanides and actinides, both intra-, and intergroup.

A REVIEW OF THE BASIC CHEMISTRY AND RECENT DEVELOPMENTS IN TRIVALENT f-ELEMENTS SEPARATIONS

The principal steps in a well-tested procedure for analysis of the rare earths in rocks and minerals are: irradiation of powdered rock samples along with a standard solution mixture of rare-earth nitrates; dissolution of the rock powder in the presence of carrier, using Na2O2 fusion; separation of the rare-earth group from the silicate residue by a sequence of hydroxide and oxalate precipitations; separation of the individual rare earths in samples and irradiated standards by ion exchange; chemical yield determination by EDTA titration; radioassay by beta or gamma counting. The average precision for duplicate analyses by different analysts was found to be better than ±4 percent mean deviation for a shale with a total rare-earth content of 200 ppm.

An accurate procedure for the determination of the rare earths by neutron activation

Lanthanides possess similar chemical properties rendering their separation from one another a challenge of fundamental chemical and global importance given their incorporation into many advanced technologies. New separation strategies combining green chemistry with low cost and high efficiency remain highly desirable. We demonstrate that the subtle bonding differences among trivalent lanthanides can be amplified during the crystallization of borates, providing chemical recognition of specific lanthanides that originates from Ln3+ coordination alterations, borate polymerization diversity and soft ligand coordination selectivity. Six distinct phases are obtained under identical reaction conditions across lanthanide series, further leading to an efficient and cost-effective separation strategy via selective crystallization. As proof of concept, Nd/Sm and Nd/Dy are used as binary models to demonstrate solid/aqueous and solid/solid separation processes. Controlling the reaction kinetics gives rise to enhanced separation efficiency of Nd/Sm system and a one-step quantitative separation of Nd/Dy with the aid of selective density-based flotation. Trivalent lanthanides possess similar chemical properties, making their separation from one another challenging. Here, Wang and colleagues demonstrate that their subtle chemical differences can be greatly amplified during borate crystallization, leading to a low cost and highly efficient separation strategy.

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Rare earth separations by selective borate crystallization

Separations chemistry is at the heart of most analytical procedures to determine the rare earth content of both man-made and naturally occurring materials. Such procedures are widely used in mineral exploration, fundamental geology and geochemistry, material science, and in the nuclear industry. Chromatographic methods that rely on aqueous solutions containing complexing agents sensitive to the lanthanide cationic radius and cation-exchange phase transfer reactions (using a variety of different solid media) have enjoyed the greatest success for these procedures. In this report, we will briefly summarize the most important methods for completing such analyses. We will discuss in some detail the basic aqueous (and two-phase) solution chemistry that accounts for separations that work well and offer explanations for why others are less successful.

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H.Louise Smith

Papers

Ion-exchange separations of the lanthanides and actinides by elution with ammonium alpha-hydroxy-isobutyrate

The new isotopes Pu246 and Am246