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

6 Rare Earth Elements and Materials

strong base such as NaOH (sodium hydroxide) is an often light beam of uniform wavelength is applied to an aqueous
preferred alternative in the case of monazites and xeno- solution of Eu 3+ and Eu 2+ results as shown below:
times because the phosphate is removed more efficiently light
+
than by the H 2 SO 4 treatment. H 2 O + Eu 3+ −−−→ Eu 2+ + H + OH.
190 nm
Subsequent treatment depends on the intended use of 2−
Again, SO 4 can be present simultaneously with the pho-
the rare earths. For some applications, particularly the
toreduction allowing precipitation as EuSO 4 .
older ones, it is not necessary to achieve a separation of
It is now time to separate the remaining 14 elements.
the elements. For example, a mixture of the cerium group
Most chemical properties such as solubility of a salt or
metals, called “mischmetal,” has been used for decades for
stability of a complex ion depend to some extent on the
lighter flints. Thus from a bastnasite or monazite ore base, 3+
size of the species involved. As the sizes of the RE
which contains predominantly the cerium group elements,
ions are very similar, differences in solubilities or stabil-
little further separation work is necessary. However, for
ities will be slight but will at least be systematic due to
basic research into the properties of the elements and their
the lanthanide contraction. Separation techniques preva-
compounds and in applications involving increasingly so-
lent before the 1950s relied on the differences in solubili-
phisticated technology the availability of the individual
ties of various rare earth salts such as the double nitrates.
elements in a high purity form is essential.
RE(NO 3 ) 3 · 2N–H 4 NO 3 · 4H 2 O, and were known as frac-
tional crystallization or precipitation methods. These are
exceedinglylaboriousmethodsand,sinceabout1950,ion-
2. Modern Separation Methods
exchange methods have become the rule for separating the
First, it is relatively easy to separate cerium and europium rare earths.
(andinsomecasessamariumandytterbium)fromanaque- To perform an ion-exchange separation, a solution of
ous mixture of RE 3+ ions. Here one exploits the fact that in rare earth ions is introduced at the top of a column con-
an aqueous environment only these elements can have sta- taining cation-exchange resin, a polymeric material typ-
ble oxidation states different from the prevailing 3+ state, ically in the form of sodium polystyrene sulfonate. The
4+
that is, Ce and Eu . Cerium is normally removed first RE 3+ ions readily undergo ion exchange displacing Na +
2+
by a process of oxidation, an increase in valence or ox- ions, thus forming a band of the lanthanide ions bound
idation state from 3+ to 4+. In fact, for bastnasite ores to the top of the column. Affinity of metal ions for most
this oxidation is often carried out before the acid treat- resins is based loosely on ionic size and charge. To move
ment by heating the mineral in air, which contains oxy- these rare earth ions down the column and effect a sep-
gen, to 650 C producing CeO 2 (in which Ce is 4+) which aration, a solution consisting of a negatively charged or-
◦
is insoluble in the acid leach. For large-scale operations, ganic species (a ligand) is slowly passed through the col-
other oxidation methods can be used such as electrolysis umn. The ligand, typically having a number of sites that
or chlorine gas. On the laboratory scale, chemicals called are capable of metal binding (a chelating ligand), have
−
oxidizing agents such as permanganate (MnO ) can be greater affinities for the RE 3+ ions than the resin by form-
4
used according to the equations below: ing stable metal–ligand complex. Because the complexes
formed between the ligand and the RE 3+ possess a lower
+
−
3Ce 3+ + MnO + 4H 2 O → 3CeO 2 ↓ + MnO 2 + 8H ,
3+
4 positive charge than the initial RE , they are less tightly
held by the resin, and are displaced from the ion-exchange
where CeO 2 is insoluble and is removed from solution. To
material into the surrounding solution. The RE 3+ cations
remove europium its oxidation state must be reduced from
with smallest radius are most strongly bound to the lig-
3+ to 2+ and this requires a chemical called a reducing
and and so these ions have the greatest tendency to be
agent. On the laboratory scale, zinc (Zn) is used according
eluted first. The size difference between different RE 3+
to the following equations:
is small, but enough to produce an effective separation.
Among the useful ligands are α-hydroxyisobutyric acid
2Eu 3+ + Zn → 2Eu 2+ + Zn 2+
and ethylenediamine tetraacetate (EDTA) (Fig. 2). The
Eu 2+ + SO) 2− → EuSO 4 ↓.
4
Addition of sulfate (SO ) following the reduction results
2−
4
in precipitation of EuSO 4 . For large-scale operations, re-
duction by sodium amalgam (sodium dissolved in mer-
cury) removes europium, samarium, and ytterbium. As a
recent development, the separation of Eu can be achieved FIGURE 2 Molecular structures of two commonly used chelating
by photochemical means using lasers. That is, an intense ligands for ion-exchange separation of rare earth ions.