Metals always occur in their oxidized state in ores, often as the oxide or sulfide of the metal. In order to convert an ore to its elemental state, therefore, it must be reduced. Reduction is a chemical reaction that is the opposite of oxidation. Metals can be reduced in a variety of different ways.
With ores of iron, for example, reduction can be accomplished by reacting oxides of iron with carbon and carbon monoxide. One of the common devices used for this purpose is the blast furnace. The blast furnace is a tall cylindrical vessel into which is fed iron ore (consisting of oxides of iron), coke (nearly pure carbon) and limestone. The temperature in the blast furnace is then raised to more than 1,832°F (1,000°C). At this temperature, carbon reacts with oxygen to form carbon monoxide, which in turn, reacts with oxides of iron to form pure iron metal. The limestone in the original mixture added to the blast furnace reacts with and removes silicon dioxide (sand), an impurity commonly found with iron ore.
Some metallic oxides do not readily yield to chemical reduction reactions like those in the blast furnace process described above. The reduction of aluminum oxide to aluminum metal is an example. Until 1886, no economically satisfactory method for carrying out this process had been discovered. Then a young college chemistry student, Charles Martin Hall, invented an electrical method for reducing aluminum oxide.
In the first step in this process, aluminum oxide is separated from other oxides (such as oxides of iron) with which it also occurs by the Bayer process. In the Bayer process, the naturally occurring oxide mixture is added to sodium hydroxide, which dissolves out aluminum oxide, leaving other oxides behind. The aluminum oxide is then dissolved in a mineral known as cryolite (sodium aluminum fluoride) and placed in an electrolytic cell. When electric current passes through the cell, molten aluminum metal is formed, sinks to the bottom of the cell, and can be drawn off from the cell.
In some instances, an ore is treated to change its chemical state before being reduced. The most common ores of zinc, for example, are the sulfides. These compounds are first roasted in an excess of air, converting zinc sulfide to zinc oxide. The zinc oxide is then reduced either by reacting it with coke (as in the case of iron) or by electrolyzing it (as in the case of aluminum).