Electrolysis

The production of aluminum by the Hall process was one of the earliest applications of electrolysis on a large scale, and is still the major method for obtaining that very useful metal. The process was discovered in 1886 by Charles M. Hall, a 21-year-old student at Oberlin College in Ohio, who had been searching for a way to reduce aluminum oxide to the metal. Aluminum was a rare and expensive luxury at that time, because the metal is very reactive and therefore difficult to reduce from its compounds by chemical means. On the other hand, electrolysis of a molten aluminum salt or oxide is difficult because the salts are hard to obtain in anhydrous (dry) form and the oxide, Al₂O₃, does not melt until 3,762°F (2,072°C).

Hall discovered that Al₂O₃, in the form of the mineral bauxite, dissolves in another aluminum mineral called cryolite, Na₃AlF₆, and that the resulting mixture could be melted fairly easily. When an electric current is passed through this molten mixture, the aluminum ions migrate to the cathode, where they are reduced to metal:

At the anode, oxide ions are oxidized to oxygen gas:

The molten aluminum metal sinks to the bottom of the cell and can be drawn off.

Notice that three moles of electrons (three faradays of electricity) are needed to produce each mole of aluminum, because there are three positive charges on each aluminum ion that must be neutralized by electrons. The production of aluminum by the Hall process therefore consumes huge amounts of electrical energy. The recycling of beverage cans and other aluminum objects has become an important energy conservation measure.