Aluminum is easily oxidized, and so its ore, Al2O3, is difficult to reduce. In fact water is reduced rather than Al3+(aq), and so electrolysis must be carried out in a molten saltAn ionic compound that can be formed by replacing the hydrogen ion of an acid with a different cation.. Even this is difficult because the melting pointThe temperature at which a solid becomes a liquid. Also called freezing point. of Al2O3 is above 2000°C—a temperatureA physical property that indicates whether one object can transfer thermal energy to another object. which is very difficult to maintain.The first successful method for reducing Al2O3 is the one still used today. It was developed in the United States in 1886 by Charles Hall (1863 to 1914), who was then 23 years old and fresh out of Oberlin College. Hall realized that if Al2O3 were dissolved in another molten salt, the meltingThe process of a liquid forming from a solid. point of the mixtureA combination of two or more substances in which the substances retain their chemical identity. would be lower than for either pure substanceA material that is either an element or that has a fixed ratio of elements in its chemical formula.. The substance Hall used was cryolite, Na3AlF6, in which the Al2O3 can be dissolved at just over 1000°C. electrolytic cellAn electrochemical cell used to drive a reaction that would otherwise be non-spontaneous (reactant-favored). used for the Hall process. (Fig. 1) consists of a steel box lined with graphite. This contains the molten Na3AlF6 and Al2O3 and also serves as the cathodeThe electrode in an electrochemical cell where reduction occurs; the negatively charged electrode in a vacuum tube.. The anodeThe electrode in an electrochemical cell where oxidation occurs. The positively charged electrode in a vacuum tube. is a large cylinder of carbon. Passage of electrical current maintains the high temperature of the cell and causes the following half-equations to occur:
Al3+ + 3e– → Al(l) (1a)
2O2– + C(s) → CO2(g) + 4e– (1b)
Since the carbon anode is consumed by the oxidationThat part of a chemical reaction in which a reactant loses electrons; simultaneous reduction of a reactant must occur. half-equation, it must be replaced periodically.
Aluminum production requires vast quantities of electrical energyA system's capacity to do work., both to maintain the high temperature and to cause half-equations (1a) and (1b) to occur. Currently about 5 percent of the total electrical energy produced in the United States goes into the Hall process. Much of this energy comes from combustionVigorous combination of a material with oxygen gas, usually resulting in a flame. of fossil fuels and hence consumes a valuable, nonrenewable resource. Since Al is protected from oxidation back to Al2O3 by a surface coating of oxide, it is a prime candidate for recycling, as well as for applications such as house siding, where it is expected to remain for a long time. Throwing away aluminum beverage cans, on the other hand, is a tremendous waste of energy.
Several other easily oxidized metals are currently produced by electrolysis, but not in such large quantities as Al. Mg is obtained by electrolyzing molten MgCl2 which is derived from seawater, and Na and Ca are produced together from a molten mixture of NaCl and CaCl2.