Geology Reference
In-Depth Information
by direct reduction-distillation. Consistently, the vacuum sublimation technique is
applied in their refining process (Gupta and Krishnamurthy, 2005).
Vacuum arc melting involves a tantalum crucible which is used to remove the
volatile impurities, obtained in fluoride electrolysis, from the RE metal. Here,
as the name suggests, the unpurified metal is melted at 1800
o
C and at vacuum
pressure so that the unwanted CaF
2
, Ca or H
2
are taken out before the RE metal
begins to melt. During the vacuum melting process, tantalum is dissolved by the
RE metal. As solubility of Ta increases with temperature, those REE with the
highest melting points show a greater dissolving effect. Tantalum crystallises just
holding the temperature close above the melting point of the RE metal and below
the melting point of Ta (3012
o
C) . However, when Ta solubility in REE is high, it
is necessary to resort to a distillation of the RE metal, as in the case of Gd;Tb;Y
and Lu, or to sublimate it off, as in the case of Sc;Dy;Ho and Er. Removing Ta
from a GdTa mixture meanwhile is extremely di
cult, since Gd has a low fusion
temperature and a low vapour pressure. This means that the distillation process
here needs increased refluxing which causes considerable dissolution of the tantalum
constituting the crucible. A tungsten crucible does not significantly improve the
situation either, as it requires the subsequent removal of W derived impurity (Gupta
and Krishnamurthy, 2005).
Electro-refining is an alternative to purification. It is especially applied in the
case of gadolinium with tantalum impurities, by simply replacing the laborious
distillation of the lanthanum-thermic reduction procedure. It may also be used in
refining yttrium and other REE.
The hard to achieve de-oxidation of REE is finally attained at very high tem-
peratures by distilling off any pure metal and leaving part of the oxygenated rare
earth metal as a residue of the higher melting temperature. Oxygen, carbon and
silicon can be removed by chemical/electrochemical means while nitrogen may be
eliminated by degassing (Gupta and Krishnamurthy, 2005).
If ultrapurification of the metal is required, further steps such as zone refining
and electro-transport are used. Both are based on the idea that some impurities
migrate within a RE metal rod via either an application of heat to melt a thin zone
at ultrahigh vacuum conditions, or via employing direct current on the metal in a
noble gas atmosphere. Migration enhances the purity of one end of the metal rod, at
the expense of concentrating the impurities at the other, which is later discarded.
Particularly in electro-transport, metallic impurities migrate to the anode whilst
interstitial impurities such as C;O;N and H move towards the cathode (Gupta
and Krishnamurthy, 2005). Moreover, zone refining is used to eliminate carbon and
metallic impurities such as the Ta or W stemming from the crucibles.
Fig. 8.15 shows a diagram of the processes involved in the purification of RE
metals.
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