Metallurgy of Key Minerals - Thanatia: The Destiny of the Earth's Mineral Resources - page 237

meanwhile can readily be obtained using cadmium cathodes, while separating scan-
dium and samarium necessitates zinc and iron rod cathodes, respectively. Yttrium
likewise is obtained with molten aluminium or magnesium cathodes (Gupta and
Krishnamurthy, 2005).
The fluoride electrolyte bath is also used to overcome the issues associated with
chloride electrolysis (Gupta and Krishnamurthy, 2005). Cells for electrolysing RE
oxide-fluorides use fused salts like REF 3 , LiF and BaF 2 to increase oxide solubility
and conductivity whilst also decreasing the melting point. To provide an example,
the Reno electro-winning cell of around 150 dm 3 , operates at a temperature range
1370-1700 o C and consists of a graphite crucible with graphite anodes, a tungsten
cathode and a molten salt electrolyte composed of an equimolar REF and LiF
mixture, where RE 2 O 3 is added. It is used for the preparation of liquid yttrium,
gadolinium and dysprosium (Morrice et al., 1968). It is also used in the production
of high quality cerium, lanthanum, neodymium and praseodymium in other cell
designs (Gupta and Krishnamurthy, 2005).
Fig. 8.14 shows the preparation of REO metals from chlorides or fluorides.

Metal

La, Ce, Pr, Nd, Gd
ingot metals

Vacuum
melting

Reduction
w/Ca in
Ar atm
1000°C
Reduction
w/Ca+Mg

RECl 3

Hydro-
Chlorination
200-300°C

RE 2 O 3

RE-Mg
Alloy

RE metal
Sponge

Y, Sc
ingot metals

Vacuum
distillation

Arc
melting

REF 3 Vacuum

melting
1800°C

Reduction w/Ca
In Ta crucible
Ar atm, 1000-1800°C

Hydro-
Fluorination
700°C

RE Metal
(high purity)

Slag-Metal
Separation

RE 2 O 3

Fig. 8.14 Preparation of REE metals from chlorides or fluorides

8.13.8 Purification ofREmetals

Purification of RE metals is a complex step. It involves, amongst others, tech-
niques such as vacuum melting and distillation, electro-refining and zone refining.
Vacuum arc melting is used to purify La;Ce;Pr;Nd;Gd;Tb;Y;Lu and Sc which
have vaporization pressures less than 13 Pa. As La;Ce;Pr and Nd have low fu-
sion temperatures and a low vapour pressure, a re-melting process is also applied.
For the case of Gd;Tb;Y and Lu with high fusion temperatures and intermediate
vapour pressures, the RE metal is distilled. Meanwhile for Sc;Er;Ho and Dy, with
high fusion temperatures and high vapour pressures, the RE metal is sublimated.
As explained above, the more volatile RE metals Sm;Eu;Y b and Tm, are obtained