The Extraction of Aluminium

The process of electrolysis uses of large amounts of energy in the extraction of a reactive metals and makes aluminium expensive to produce.

Aluminium is a very useful metal but expensive to produce.

Because its position in the reactivity series of metals, aluminium cannot be extracted using carbon because it is above carbon in the reactivity series ie more reactive than carbon in the series, carbon is not reactive enough to displace aluminium from its compounds such as aluminium oxide.
So, if aluminium is too reactive to be obtained by carbon reduction of its oxide another method must be employed which is called electrolysis.
Aluminium is obtained from mining the mineral bauxite which is mainly aluminium oxide (Al2O3) and bauxite must be purified prior to electrolysis, adding to the manufacturing costs.
The purified bauxite ore of aluminium oxide is continuously fed in. Cryolite is added to lower the melting point and dissolve the ore.
The ore–compound containing the aluminium must be molten so the ions are free to move to the electrodes. The conducting melt is called the electrolyte, so extracting aluminium this way involves the electrolysis of molten aluminium oxide.
Ions must be free to move to the electrodes called the cathode (–, negative), attracting positive ions e.g. Al3+, and the anode (+, positive) which attracts negative ions e.g. O2–.
When the d.c. current is passed through aluminium forms at the negative cathode (metal*) and sinks to the bottom of the tank where it can tapped off, collected and run into moulds to cool down before transportation to it will be used to make things.
At the positive anode, oxygen gas is formed (non–metal*). This is quite a problem. At the high temperature of the electrolysis cell it burns and oxidises away the carbon electrodes to form toxic carbon monoxide or carbon dioxide. So the carbon–graphite electrode is regularly replaced and the waste gases dealt with!
It is a costly process (6x more than Fe!) due to the large quantities of expensive electrical energy needed for the process.
* Two general rules:
Metals and hydrogen (from positive ions), form at the negative cathode electrode.
Non–metals (from negative ions), form at the positive anode electrode.
Raw materials for the electrolysis process:

Bauxite ore of impure aluminium oxide [Al2O3 made up of Al3+ and O2– ions]

Carbon (graphite) for the electrodes.

Cryolite reduces the melting point of the ore and saves energy, because the ions must be free to move to carry the current and less energy is needed to melt the aluminium oxide obtained from the bauxite ore.

Electrolysis means using d.c. electrical energy to bring about chemical changes e.g. decomposition of a compound to form metal deposits or release gases. The electrical energy splits the compound!

At the electrolyte connections called the anode electrode (+, attracts – ions) and the cathode electrode (–, attracts + ions). An electrolyte is a conducting melt or solution of freely moving ions which carry the charge of the electric current.

ELECTRODE EQUATIONS: redox details of the electrode processes
Electrolysis reminders – the negative electrode (–) is called the cathode and attracts positive ions or cations e.g. Al3+, and the positive electrode (+) is called the anode and attracts negative ions or anions e.g. O2–.
The negative cathode electrode attracts positive ions, the aluminium ion.
At the negative (–) cathode, reduction occurs (electron gain) when the positive aluminium ions are attracted to it. They gain three electrons to change to neutral Al atoms.
Al3+ + 3e– ==> Al
The positive anode attracts negative ions, the oxide ion.
At the positive (+) anode, oxidation takes place (electron loss) when the negative oxide ions are attracted to it. They lose two electrons forming neutral oxygen molecules.
2O2– ==> O2 + 4e–
or
2O2– – 4e– ==> O2
Note: Reduction and Oxidation always go together!
The overall electrolytic decomposition is ...
aluminium oxide ==> aluminium + oxygen
2Al2O3 ==> 4Al + 3O2
and is a very endothermic process, lots of electrical energy input!
Note that the aluminium oxide loses its oxygen, therefore in this electrolytic process the compound aluminium oxide is reduced to the metal aluminium.