Geology Reference
In-Depth Information
duced exclusively as a byproduct during the processing of various ores or recovered
from previously processed tailings or residues. Considerable amounts of scandium
oxide Sc
2
O
3
can be obtained as a byproduct of the extraction of uranium. Its only
rich mineral is the rare thortveitite Sc
2
Si
2
O
7
.
Scandium is frequently excluded from the REE list, since its chemical proper-
ties are closer to magnesium than to yttrium and the lanthanides. Its extraction is
di
cult, its global production is in short supply, mostly coming from Bayan Obo
mine and its price reflects that. However it finds application in special alloys with
aluminium and titanium which are used in aerospace industry and for the produc-
tion of highly professional lightweight sporting equipment, such as bicycles - as it
improves the strength, durability and reduces the weight of the base material. Sca-
dium also finds use in fuel cell applications and in high intensity light sources, such
as mercury-vapour lamps, where it is an additive.
B.2.57 Selenium
Selenium is among the rarer elements on the Earth's crust. Whilst occasionally
found in its native form it is usually associated with sulphur, copper, zinc and
lead forming for instance clausthalite CuSe or klockmanite PbSe. Selenium is
recovered commercially as a byproduct of the electrolytic refining of copper where
it accumulates in anode residues.
Selenium is a non-metallic chemical element, resembling sulphur and tellurium
in its chemical activity and physical properties. It has good photovoltaic and pho-
toconductive properties and is used extensively in electronics devices, such as pho-
tocells, light metres and solar cells. The second largest user of selenium is the glass
industry, which employs it to remove the colour from glass. It also finds extensive
application as animal feed and food supplements. Additionally, it can be used in
photocopying, in the toning of photographs, in metal alloys and to improve the
abrasion resistance in vulcanised rubbers.
B.2.58 Silicon
Silicon is the most abundant element in the Earth's crust after oxygen. It never
occurs freely but is invariably combined with oxygen and with few exceptions is
always 4-coordinate in nature. Sand is used as a source of the silicon produced
commercially. Some silicate minerals are mined, e.g. talc and mica. Other mined
silicates are feldspars, nepheline, olivine, vermiculite, perlite and kaolinite.
Silicon is a brittle steel-grey metalloid. It has many industrial uses. It is the
main component of glass, cement, ceramics, most semiconductor devices and sili-
cones. Silicon is also an important constituent of some steels and a major ingredient
in bricks. It is also used as an alloy to provide resistance to aluminium, magne-
sium, copper and other metals. Metallurgic silicon is used as a raw material in the
manufacture of organosilic and silicon resins, seals and oils. Silicon chips are used
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