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Aluminum compounds, primarily the oxide in forms of various purity and hydration, are fairly widely distributed in nature. The feldspars, the most common rock-forming silicates, make up nearly 54% of the earth's crust; in these, aluminum has replaced up to half the silicon atoms in SiO2. The major ore of aluminum is bauxite, a hydrated aluminum (III) oxide (Al2O3.xH2O).
In the industrial Bayer process, bauxite is concentrated to produce aluminum hydroxide. When this concentrate is calcined at temperatures in excess of 1000oC, anhydrous aluminum oxide, Al2O3, is formed. Anhydrous aluminum oxide melts at over 2000°C. This is too high to permit its use as a molten medium for electrolytic formation of free aluminum. The electrolytic process commercially used to produce aluminum is known as the Hall process, named after its inventor, Charles M. Hall. The purified Al2O3 is dissolved in molten cryolite, Na3AlF6, which has a melting point of 1012oC and is an effective conductor of electric current. In the following schematic diagram of the electrolysis cell graphite rods are employed as anodes and are consumed in the electrolysis process. The cell electrolytic reaction is:
2Al2O3 + 3C --> 4Al(l) + 3CO2(g)
Typical Hall process electrolysis cell used to reduce aluminum. Because molten aluminum is more dense than the molten mixture of Na3AlF6 and Al2O3, the metal collects at the bottom of the cell.
The cells are ed to use 8,000 A and upwards, and a given cell requires about 5 V although only 2.1 V are theoretically required to decompose aluminum oxide. The excess 2.9 V, plus the heat of combustion of carbon, is used as heat to keep the cell warm. The production of one ton of aluminum requires about 65-70 GJ (18-20 MWh) and about half a ton of carbon. The process is generally nonpolluting but is a heavy consumer of electricity with approximately 36% of the electricity used in the Faradaic process, the rest being lost as heat.
