Aluminium extraction by electrolysis

Aluminium is extracted industrially by passing an electric current through a molten mixture of aluminium oxide and cryolite. The molten mixture contains ions that can move to the electrodes, where they gain or lose electrons.

Pure aluminium oxide has a very high melting point. Mixing it with cryolite lowers the working temperature needed to keep the electrolyte molten. This makes the process more practical and reduces the energy cost.

Formation of aluminium

Positive aluminium ions, $$\ce{Al^{3+}}$$, move to the negative electrode. Each ion gains three electrons and forms an aluminium atom. The aluminium is produced as a liquid.

Reaction at the carbon anode

Negative oxide ions, $$\ce{O^{2-}}$$, move to the positive electrode, or anode. There they lose electrons and form oxygen:

The anode is made of carbon. Oxygen reacts with the hot carbon to produce carbon dioxide: $$\ce{C + O2 -> CO2}$$. This gradually uses up the carbon, so the positive electrodes must be replaced regularly.

Electrolytic extraction of reactive metals

Some metals cannot be obtained by heating their compounds with carbon. If a metal is more reactive than carbon, carbon cannot remove it from its compound. Some metals also react with carbon instead of being extracted cleanly.

In these cases, the metal can be extracted by electrolysis: passing an electric current through a molten ionic compound of the metal.

Extraction from a molten compound

The compound must first be melted. In the solid compound, the ions are fixed in place. When it is molten, the ions are free to move and carry charge through the liquid.

The positively charged metal ions move towards the negative electrode. There, they gain electrons and become neutral metal atoms. The gain of electrons is called reduction.

Energy needed for electrolytic extraction

Electrolytic extraction uses large amounts of energy. Energy is needed both to melt the compound and to pass an electric current through it.