Video Transcript
Aluminum is commonly extracted by
electrolysis of aluminum oxide, Al2O3. Before it can be electrolyzed, the
aluminum oxide must be molten. However, aluminum oxide has a
melting point of over 2,000 degrees Celsius, which is very expensive to
maintain. What is added to lower the melting
point of the aluminum oxide before electrolysis?
The Hall–Héroult process uses
electrolysis to extract aluminum. Electrolysis is a chemical process
where electricity is passed through an electrolyte, causing the substances within
the electrolyte to decompose into their constituent elements. The substance we are hoping to
decompose in this electrolysis is alumina, which is a form of purified aluminum
oxide.
However, in order for alumina to be
a usable electrolyte, the ions must be able to move freely as part of a solution or
a liquid. While we may think the simplest
solution would be to just melt the alumina, it has a melting point of over 2,000
degrees Celsius. This is a very high temperature and
is very expensive to maintain on an industrial scale. Because of this, the alumina is
mixed with another substance.
Molten cryolite, Na3AlF6, serves
this purpose. When added together, cryolite and
alumina produce an electrolyte solution, which melts around 900 degrees Celsius. This lowers the energy costs
related to producing the mobile ions necessary for the electrolysis used to extract
aluminum.
Therefore, the substance that is
added to lower the melting point of the aluminum oxide before electrolysis is
cryolite.
