4.4.3.3 - Extracting aluminium by electrolysis

Extraction choice and energy demand

Electrolysis can extract a metal when reduction of its oxide with carbon is unsuitable.

Aluminium lies above carbon in the reactivity series. Carbon cannot remove oxygen from aluminium oxide by the ordinary carbon-reduction route used for less reactive metals. Aluminium must instead be obtained through a process such as electrolysis. In other extraction cases, a metal may react with carbon and become contaminated, which is another reason for choosing electrolysis.

Aluminium oxide is a giant ionic substance. Strong electrostatic attractions act between oppositely charged aluminium and oxide ions in all directions. A large amount of energy is needed to overcome sufficient attractions to make the ions mobile in a melt. Solid aluminium oxide has charged particles but cannot carry current because the ions are fixed in the lattice.

The extraction has two distinct energy requirements. Heat is needed to maintain a molten electrolyte, and electrical energy is needed to drive the electrode reactions. A lower operating temperature can reduce heating costs, but it does not remove the need for electrical current. This is why electrolytic extraction can be expensive even if the starting oxide is abundant.

When comparing extraction routes for an unfamiliar metal, use the stated reactivity and supplied information about carbon reactions, energy use, impurities and costs. A metal below carbon may be suitable for carbon reduction, but the evidence may still favour electrolysis for a particular purity requirement. Reactivity alone is not a complete economic evaluation.

Aluminium oxide and cryolite

The industrial electrolyte is a molten mixture of aluminium oxide and cryolite.

Aluminium oxide is also called alumina. Pure aluminium oxide has a very high melting point, so using it alone as a melt would require much more heating. It is dissolved in molten cryolite, producing a mixture that can operate at a lower temperature than pure molten aluminium oxide. Aluminium and oxide ions are mobile in this electrolyte.

The important statement is that the mixture has a lower melting point than pure aluminium oxide. Cryolite does not change the intrinsic melting point of a pure sample of aluminium oxide, and it is not a catalyst speeding an otherwise unchanged reaction. It is part of the molten electrolyte mixture.

Working at the lower temperature reduces energy used for heating and helps reduce operating cost. Heat is still required to keep the mixture molten, and current is still required for discharge. Water would not serve the same purpose: the process requires an appropriate molten electrolyte, not an aqueous aluminium-salt solution with competing ions from water.

Aluminium oxide is replenished as it is used up. The cryolite-containing mixture provides the environment for conduction while the aluminium oxide supplies the ions from which aluminium and oxygen are made. Distinguishing the additive's role from the reactant's role avoids predicting that cryolite is simply converted into the desired aluminium product.

Electrode products in aluminium extraction

Aluminium forms at the negative cathode, while oxygen forms at the positive carbon anodes.

The cell has a carbon-lined container acting as the cathode and carbon anodes connected to the positive terminal above it. Aluminium ions move towards the negative cathode and are discharged to form aluminium atoms. At the operating temperature, the aluminium collects as a liquid and can be tapped off. Oxide ions move towards the positive anodes and are discharged, forming oxygen.