Sometimes, we require metals in a stare high purity. For example, Uranium metal to be used in nuclear reactors as fuel should not contain boron more than one part per million (1 ppm) because it can capture neutrons. If the amount of the boron is more in uranium it would stop the nuclear reaction by absorbing neutrons released by disintegration of uranium.
Similarly, germanium used to prepare semiconductor devices should be of high purity. It should not contain more than one part of copper per ten millions parts. Special methods have been developed to obtain metals of high purity. Now let us study two methods used to obtain highly pure metals.
1. Van Arkel Methods:
In this method, a volatile halide is formed by direct reaction between metal and a halogen at a lower temperature. The volatile halide is purified b some suitable method such as distillation and the halide is then decomposed to the metal and halogen at a higher temperature. For example. Pure titanium can be obtained by this method,. Impure titanium is heated with iodine. Titanium reacts with iodine to form titanium tetraiodide. The volatile tetra iodide thus formed is decomposed by bringing in contact with hot tungsten filament. The regenerated iodine can be reused to repeat the process.
2. Zone refining:
This method is particularly used when metals are required in high degree of purity. IN this method, a metal rod is placed inside a small high frequently induction furnace. A narrow zone of metal is melted. The furnace is now slowly moved along the rod. The pure metal crystallises out of the melt while impurities remain in the melt and move along with the molten zone of the rod with movement of the furnace. The process is repeated several times. The end of the rod where the impurities have collected is cut off. This method is employed for the purification of germanium, which is used in semiconductors.