Thursday, November 6, 2008

The Extraction of Aluminium-igcse

Aluminium is obtained from mining the mineral bauxite.
The purified bauxite ore of aluminium oxide is continuously fed in. Cryolite is added to lower the melting point and dissolve the ore.

Ions must be free to move to the electrode connections 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.

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 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: for electrolysis

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

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.

The redox details of the electrode processes:
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
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!

anode graphite react with oxigen to form carbodioxide. so the it should be replaced time to time

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