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Thulium
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(Thule, the earliest
name for Scandinavia) Discovered in 1879 by Cleve. Thulium occurs in
small quantities along with other rare earths in a number of minerals.
It is obtained commercially from monazite, which contains about 0.007%
of the element.
Thulium is the least
abundant of the rare earth elements, but with new sources recently
discovered, it is now considered to be about as rare as silver, gold, or
cadmium.
Ion-exchange and solvent
extraction techniques have recently permitted much easier separation of
the rare earths, with much lower costs.
Only a few years ago,
thulium metal was not obtainable at any cost; in 1985 the oxide sold for
$3400/kg. Thulium metal costs $50/g.
Thulium can be isolated
by reduction of the oxide with lanthanum metal or by calcium reduction
of a closed container. The element is silver-gray, soft, malleable, and
ductile, and can be cut with a knife. Twenty five isotopes are known,
with atomic masses ranging from 152 to 176.
Natural thulium, which
is 100% 169Tm, is stable. Because of the relatively high price of the
metal, thulium has not yet found many practical applications. 169Tm
bombarded in a nuclear reactor can be used as a radiation source in
portable X-ray equipment. 171Tm is potentially useful as an energy
source. Natural thulium also has possible use in ferrites (ceramic
magnetic materials) used in microwave equipment. As with other
lanthanides, thulium has a low-to-moderate acute toxic rating. It should
be handled with care.
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