Zone Refining

Purest forms of silicon and germanium metals were required for semiconductors, so scientists were looking for methods to get the purest form of metals. That is when in 1951, William G. Pfann developed a zone refining method to get the purest form of metal. It was a remarkable step for the development of the transistor and for the entire electronics revolution. Before the invention of the zone refining process, purification of metals was carried out by progressive freezing. 

Zone refining also known as zone melting, floating -zone process or travelling melting zone. Zone refining process was first invented by John Desmond Bernal. Zone refining was first commercially used for germanium. As zone refining by using electrical heating coils did not work well with silicon because of its high melting point. 

What is Zone Refining?

Zone refining is a technique to get highly pure crystals of impure elements (specially metals) by using melting and crystallization processes. In this a narrow region of the crystal is heated so that impurities get melted and forms a molten zone which moves along the crystal. 

Principle of Zone Refining 

Zone refining technique is based on the fact that impurity has more solubility in molten state. Because of this when a molten metal crystallizes on cooling, impurities are automatically excluded as they do not form part of the pure crystal. 

In zone refining, impurities are removed by moving molten zone through moving heater and recrystallized pure metal is left behind in solid form. In this, the zone should be moving as slowly as possible to get highly pure metal. 

Zone Refining Process 

In the zone refining process, a mobile heater is fixed at one end of the impure metal rod which we want to purify. Now an impure metal rod fitted with a circular mobile heater is fixed in a column filled with inert gas. This circular heater heats the rod radially and produces a region in which the temperature is uniformly elevated to the melting point in a plane perpendicular to the axis of the rod. As the heater moves along the rod, the molten zone also propagates down the rod. We make the heater move very slowly. Because of this impurities and pure metal both get melted but atoms of pure metal recrystallize while molten impurities move with heater or molten zone. Thus, after the process impurities get collected at one end while solid pure metal remains at another end. The process is repeated many times to get pure metal.

Applications of Zone Refining Process 

  • Zone refining process is used to get the purest form of elements. 

  • It is a very effective process in removal of impurities from semiconductors such as Ge, Ga and Si. 

  • Float zone refining process is used in solar cells. 

  • It is used to form a single layer of crystal.

  • It is used for organic and inorganic compounds as well. 

  • It is highly useful for scientists involved in preparing organic chemical standards for HPLC, absorbance or fluorescence spectroscopy.

  • It is useful for all those analytical methods that require the highest purity for standardizing or calibrating instrumentation. 

  • It is used for crystallization to concentrate an enzyme, peptide, antibiotic or any other thermally unstable substance. 

  • It is used in the concentration of heat labile biological materials in aqueous solutions. 

Limitations of Zone Refining process 

Zone refining process is an expensive process, because of this its applications are limited to laboratory reagents and valuable chemicals. Sometimes when solid -liquid equilibria is not favorable for all impurities that time we have to combine zone refining with other techniques to achieve ultrahigh purity.