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Encyclopedia of Physical Science and Technology EN014A-653 July 28, 2001 20:55

Rare Earth Elements and Materials 7

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elutes are then treated with oxalate ion ( O 2 C–CO ) and ease in implementing counter-current techniques in the
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the precipitated RE oxalates ignited to the oxides. The decanting mixers with a very high degree of automation.
low solubility of chelating ligands in aqueous solution, Solvent extraction technology is the most efficient and
their high prices, and delicate and expansive recovery are economical separation method presently available and to-
some of the limitations of ion-exchange technique. When day all large-scale commercial production is done in this
coupled with the disadvantages of using resins, namely, way.
process discontinuity and low kinetics of ion exchange,
this results in high cost for large-scale production of rare
earths by this method. Presently only a few heavy rare II. CHEMICAL PHYSICS
earths are purified commercially on a small scale by ion OF THE RARE EARTHS
exchange.
The rare earths have been the object of intensive studies
For large-scale production the chosen path is liquid–
by scientists from many disciplines, including condensed
liquid extraction. The technique, using counter-current
matter physics, solid-state chemistry, solution chemistry,
two-phase extraction procedure, relies on the differential
biochemistry, and materials sciences. The scientific and
partitioning of soluble rare earth complexes between im-
technological interests in the rare earths are mainly due to
miscible aqueous and organic phases. A component will
their unique optical and magnetic properties. These inter-
have a distribution coefficient, measured at equilibrium:
esting properties are a consequence of the atomic structure
D = (concentration in organic phase)/ (also called electronic configuration) of the rare earth el-
ements.
(concentration in aqueous phase).
For two components, RE A and RE B , both distributed be-
A. Atomic Structure: Electronic Configuration
tween the organic and aqueous phases, a separation factor
can be defined as β A/B = D A /D B , where D A and D B are Each atom of a given element contains, among other ele-
the distribution coefficients of RE A and RE B , respectively. mentary particles, equal numbers of electrons and protons
The closer the separation factor approaches 1 the more in quantities given by the atomic number of the element.
difficult it will be to separate those two components. The Each electron in an atom occupies a state of well-defined
degree of separation is maximized by optimization of op- energy which is characterized by a set of four indices
erating conditions and increase of separation stages (ex- called quantum numbers. To put it another way, each state
traction and washing cycles). A simplified liquid–liquid can be thought of as a box or container, labeled by a unique
counter-current extraction circuit is illustrated in Fig. 3. set of four quantum numbers. If the number and relative
Continuity of operations is the predominant factor in the energies of the available boxes are known for a given ele-
rapid expansion of liquid–liquid extraction, especially its ment, its electronic configuration can be easily determined

FIGURE 3 A schematized liquid–liquid counter-current extraction circuit for large-scale separation of rare earth ions.
RE A and RE B are the two components of the mixture.