The reactivity of the metal will have an effect on the method used to extract the metal from the ore.
Carbon (coke) is a good reducing agent because it is cheap and when oxidised releases a gas which escapes easily. Some equations:
ZnO + C→Zn + CO 2Fe2O + 3C→4Fe + 3CO2*
*more correctly the
is reduced by CO in the Blast Furnace
Fe2O3 + 3CO→2Fe + 3CO2
1. Some metals are found as metals, these are the most unreactive. For example silver and gold (and also copper).
2. Some metals are reduced by heating with carbon (coke) or carbon monoxide (
) these have medium reactivity. For example iron, zinc, lead and copper.
3. The most reactive metals can only be extracted effectively using electrolysis. Metals involved include: magnesium, aluminium, all alkali metals, and calcium.
Aluminium is too reactive to be reduced using carbon or carbon monoxide.
The correct process using sodium hydroxide can be summarised in 4 steps:
1. The ore is crushed and heated with the sodium hydroxide solution at 180°C
The aluminium oxide reacts and dissolves to form a solution of
2. The waste solids are filtered off and disposed of.
3. The solution is cooled and aluminium hydroxide crystallises out:
NaOH (aq) + Al(OH)3 (s)
This is filtered off and then heated leaving pure
is then melted and electrolysed – liquid Al collects at the cathode;
Al3+ + 3e
Important details about the aluminium extraction process:
has a very high melting point, so cryolite,
, is added which lowers the melting point to just over 1000°C to conserve energy.
An enormous electrical current is used – maybe 100 000 Amps. So there needs to be a good (and preferably cheap) electrical supply (HEP or Geothermal power).
This enormous current helps keep the the
/ cryolite mixture molten (in liquid form).
The process is carried out in a steel tank, lined with graphite - the tank is the cathode.
The reduction of aluminium ion occurs here :
Al3+ + 3e→AI
The anodes are carbon (graphite) blocks.
The anode reaction is
and at 1000°C the oxygen causes the anodes to burn away, so they need to be replaced regularly. The heat produced also helps to keep the electrolyte molten.
Graphite is used because it conducts electricity well and does not react with, or dissolve in the liquid aluminium.
There is an environmental impact:
Carbon dioxide and some fluorine is released into the atmosphere and large amounts of waste material is left after the initial purification step (see point 2 above).