Zone melting is a method of removing impurities from solid materials that are used in industry and in research. Germanium metal was the first material to be refined commercially using this procedure. Germanium can be purified by zone melting until it contains only 1 atom of an impurity in every 10 billion atoms. If a boxcar of sugar were this pure, it would contain only one grain of impurity. Extremely pure germanium and other substances are used in making semiconductor electronic devices, such as transistors and integrated circuits.
The apparatus used for zone melting consists of a row of ring-shaped heaters that move slowly along a tube containing the solid to be purified. Each heater melts a narrow band of the material, forming a liquid “zone” that moves along with the heater. After each heater passes, the liquid cools and freezes. The impurities tend to stay in the liquid zone and are carried to one end of the tube. The material melts again when the next heater passes, and it becomes purer with each melting and freezing.
