Extraction of aluminium by electrolysis (O level Chemistry)
Extraction of aluminium is done by electrolysis because reactive metals such hold on to the element they have combined with strongly so much that it is impossible to extract them by reduction of their oxides. In such cases we extract the metal by electrolysis of the molten ore. During the electrolysis, the metal is produced at the cathode while a non-metal is produced at the anode.
Though the extraction of reactive metals by electrolysis is expensive, its cost is kept low by situating the metal smelters in regions where there is hydroelectric power.
Aluminium is the most abundant metal in the Earth’s crust and it exists mainly in form of its ore called bauxite, which is aluminium oxide mixed with impurities.
The electrolysis of aluminium oxide
The extraction of aluminium by electrolysis involves the following stages:
- Bauxite, impure aluminium oxide, is treated with sodium hydroxide to obtain pure aluminium oxide. This also improves the electrical conductivity of the molten aluminium oxide.
- The pure aluminium oxide is then dissolved in molten cryolite (Na3AlF6, sodium hexafluoroaluminate). Cryolite reduces the melting point of pure aluminium oxide from 2017°C to around 1000°C, providing a considerable saving in the energy requirements of the process.
- The molten mixture of aluminium oxide and cryolite is then electrolysed in a cell similar to the one shown below:
The electrolysis of aluminium oxide is carried out in a large steel tank which is lined with carbon. This carbon lining acts as the cathode (negative electrode). The anodes of this process are blocks of graphite which are lowered into the molten mixture from above
Since aluminium oxide is an ionic compound, when molten its ions become mobile, as the strong electrostatic forces of attraction between the ions are broken by the heat energy provided.
Movement of ions during the electrolysis of aluminium oxide
Aluminium oxide, Al2O3, contains aluminium ions, Al3+ and oxide ions, O2-.
- the negatively charged oxide ions, O2-, are attracted to the anode (the positive electrode), where they lose electrons (oxidation) to form oxygen molecules, O2.
- The anode reaction is 2O2-(l) – 2e → O2(g)
- the positive aluminium ions, Al3+, are attracted to the cathode (the negative electrode) where they gain electrons to form molten aluminium (reduction).
- the cathode reaction is Al3+(l) + 3e → Al(l)
The molten aluminium collects at the bottom of the cell and it is siphoned out at regular intervals. However, at such a high working temperature of the cell, the oxygen liberated reacts with the graphite anodes, producing carbon dioxide. The anodes get burned away and have to be replaced on a regular basis.