Extraction of Metals

What is a Metal Ore?

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
that is used in the UK come from recycled scrap metal.


How is a Metal Extracted from its Ore?

The method used to extract a metal from its ore
depends on where the metal is in the reactivity series.


Metals
above Carbon in the Reactivity Series.

A metal above carbon in the reactivity series
(potassium, sodium, lithium, calcium, magnesium
and aluminium) can be extracted by electrolysis.

Extraction of the metal from its ore involves reduction of
the metal ions. Electrons are able to reduce any metal ion.

metal ions  +  electrons  arrow  metal atoms (reduction).
non-metal ions  -  electrons  arrow  non-metal atoms (oxidation).

Metals above carbon in the reactivity series could also
be reduced by reaction with a more reactive metal but
this is more expensive than electrolysis and is only
used on a commercial scale for the extraction of titanium.


Metals below Carbon in the Reactivity Series.

A metal below carbon in the reactivity series (zinc to silver)
may be extracted by heating the metal ore with carbon.
During the reaction, the metal in the ore is displaced from
its non-metal anion because carbon is more reactive than
the metal. Carbon is used because it is cheap and readily
available
(coke and charcoal are both carbon). This form
of extraction is less expensive than electrolysis. The metal
in the ore is said to be reduced by reaction with carbon.

Hydrogen can also be used to reduce metals that are
lower than itself in the reactivity series, but since
hydrogen is more expensive than carbon it is only used on
a large scale for the extraction of tungsten. This avoids
tungsten reacting with carbon to form tungsten carbide.

Gold and platinum occur in the Earth as native metal.
This 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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