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General Description.
A solid element or compound which occurs
naturally
in the Earth's crust is called a
mineral.
A mineral which contains a
high enough percentage of
a metal
for economic extraction is called a
metal ore.
Economic extraction means that the
cost of getting the metal out of the
ore
is sufficiently less than the amount
of money made by selling the metal.
The most
common metal ores are oxides and
sulfides.
Sulfides are the oldest ores, formed in the Earth's history
when there was a lot of sulfur from
volcanic
activity.
Oxides formed later when photosynthesis in
plants
released large amounts of oxygen into the atmosphere.
Metal
ore deposits are a finite
resource
(there are only a certain amount of
them)
and non-renewable (once used,
they are gone and will not be replaced).
Many metals are obtained today from recycling
(melting and refining) scrap
metals.
About half of the aluminium, copper,
lead, steel and tin
which are used in the UK come from
recycled scrap metal.
A metal above carbon in
the reactivity
series
may be extracted from its ore by electrolysis.
A metal below carbon in
the reactivity series (zinc to silver)
may be extracted from its ore by heating with
carbon.
The metal is displaced from its
non-metal anion
by the more reactive carbon.
Carbon is used because it is
readily available and cheap
(coke or charcoal are both carbon).
The metal in the ore is said to be reduced by reaction with carbon.
Hydrogen may be used to reduce metals
which are lower than itself in the
reactivity series,
but since it is more expensive than
carbon
it is only used on a large scale for the extraction of tungsten,
to avoid the formation of
tungsten carbide.
Gold
and platinum occur in the Earth as native
metal,
which means that they are found as the element, not the compound,
and so do not need to be reduced.
Silver and copper may also be
found as native metal.
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