Aluminium is more reactive than carbon so it must be extracted from its compounds using electrolysis. Even though aluminium is more abundant than iron in the Earth's crust, aluminium is more expensive than iron. This is mainly because of the large amounts of electrical energy used in the extraction process.
Aluminium oxide is insoluble in water, so it must be molten to act as an electrolyte. However, the melting point of aluminium oxide is high. A lot of energy must be transferred to break its strong ionic bonds, and this is expensive. To reduce costs, powdered aluminium oxide is dissolved in molten cryolite. Cryolite melts at a lower temperature than aluminium oxide, reducing costs. However, significant amounts of energy are still required to melt cryolite.
The diagram shows an electrolysis cell used to extract aluminium. Both electrodes are made of graphite, a form of carbon with a high melting point and which conducts electricity.
The oxygen reacts with the carbon anodes, forming carbon dioxide. So the anodes gradually wear away. They must be replaced frequently, adding to the cost of producing aluminium.