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For copy machines.
(Gr. Selene, moon) Discovered by Berzelius in 1817, who found it associated with
tellurium, named for the earth.
Selenium is found in a few rare minerals such as crooksite and clausthalite. In years
past it has been obtained from flue dusts remaining from processing copper sulfide ores,
but the anode metal from electrolytic copper refineries now provide the source of most of
the world's selenium. Selenium is recovered by roasting the muds with soda or sulfuric
acid, or by smelting them with soda and niter.
Selenium exists in several allotropic forms. Three are generally recognized, but as
many as that have been claimed. Selenium can be prepared with either an amorphous or
crystalline structure. The color of amorphous selenium is either red, in powder form, or
black, in vitreous form. Crystalline monoclinic selenium is a deep red; crystalline
hexagonal selenium, the most stable variety, is a metallic gray.
Selenium exhibits both photovoltaic action, where light is converted directly into
electricity, and photoconductive action, where the electrical resistance decreases with
increased illumination. These properties make selenium useful in the production of
photocells and exposure meters for photographic use, as well as solar cells. Selenium is
also able to convert a.c. electricity to d.c., and is extensively used in rectifiers.
Below its melting point selenium is a p-type semiconductor and is finding many uses in
electronic and solid-state applications.
Elemental selenium has been said to be practically nontoxic and is considered to be an
essential trace element; however, hydrogen selenide and other selenium compounds are
extremely toxic, and resemble arsenic in their physiological reactions.
Naturally selenium contains six stable isotopes. Fifteen other isotopes have been
characterized. The element is a member of the sulfur family and resembles sulfur both in
its various forms and in its compounds.
Selenium is used in Xerography for reproducing and copying documents, letters, etc. It
is used by the glass industry to decolorize glass and to make ruby-colored glasses and
enamels. It is also used as a photographic toner, and as an additive to stainless steel.
Hydrogen selenide in a concentration of 1.5 ppm is intolerable to man. Selenium occurs
in some solid in amounts sufficient to produce serious effects on animals feeding on
plants, such as locoweed, grown in such soils. Exposure to selenium compounds (as Se) in
air should not exceed 0.2 mg/m3
(8-hour time-weighted average - 40-hour week).
Selenium is priced at about $300/lb. It is also available in high-purity form at a
somewhat higher cost.
Sources: CRC Handbook of Chemistry
and Physics and the American Chemical Society.