Hall–Héroult process

The Hall-Heroult process is the main process for obtaining aluminium.

In this process, the alumina (Al₂O₃) is dissolved inside an electrolytic tank lined with carbon in an electrolytic bath with cryolite (Na ₃AlF₆) cast. The tank acts as a cathode, while Soldberg graphite electrodes are usually used as the anodes. The total chemical reaction is as follows:

2Al2O3 + 3 C → 4 Al + 3 CO2

Alumina breaks down into aluminum and molecular oxygen. As liquid aluminum is denser than cryolite, it is deposited at the bottom of the tank, so that it is protected from oxidation at high temperatures. The oxygen is deposited on the carbon electrodes, burning and producing CO2.

The process parameters are as follows:

• Voltage: 5-6 V.
• Current density: 1,5-3 A/cm2, which means a current of 150 000 amperios.
• The electrodes must always be at the same height, so we must regulate them as they are broken down during the reaction.
• It is necessary to control that the proportion of alumina is constant during the process, so it will have to be poured more as the process progresses.

This process was independently discovered in 1886 by American Charles Martin Hall and Frenchman Paul Héroult. It is curious that both scientists were born and died in the same years and that they patented their discovery with so little time difference without knowing each other.

This process is used worldwide and is the only one currently used by the industry to produce aluminum.