DEFINITION
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.
The zone refining method is based on the principles of fractional crystallization. When an impure metal is melted and allowed to solidify, the impurities will prefer to be in the molten region. i.e., impurities are more soluble in the melt than in the solid-state metal.
In this process the impure metal is taken in the form of a rod. One end of the rod is heated using a mobile induction heater which results in melting of the metal on that portion of the rod. When the heater is slowly moved to the other end the pure metal crystallises while the impurities will move on to the adjacent molten zone formed due to the movement of the heater. As the heater moves further away, the molten zone containing impurities also moves along with it. The process is repeated several times by moving the heater in the same direction again and again to achieve the desired purity level. This process is carried out in an inert gas atmosphere to prevent the oxidation of metals.
Elements such as germanium (Ge), silicon (Si) and gallium (Ga) that are used as semiconductor are refined using this process.
This technique is based on the fact that impurity has more solubility in molten state. Because, when a molten metal crystallizes on cooling, impurities are automatically excluded as they do not form part of the pure crystal.
In zone refining process, 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
Process of zone refining
In this process, a circular mobile heater is fixed at one end of the metal rod which is made up of the impure metal. Now, the circular mobile heater is moved slowly across the metal rod.
The metallic impurities melt at the temporary position of this heater. The melt containing the impurities moves forward along with the heater through the entirety of the metal rod. The pure metal is left to solidify as the heater moves along the rod.
As the heater moves forward, the concentration of the impurities in the melt increases. This is because the impurities are more soluble in their corresponding melt state. Finally, the impurities are accumulated at one end of the metal rod.
The process described above is repeated many times in the same direction. The end of the rod in which the impurities have now accumulated in is cut off, leaving behind the pure metal. Thus, the required to be purified form of the metal is obtained. This process is very effective in the removal of impurities from semiconducting elements such as Germanium, Gallium, and Silicon. This process is also used in refining high-purity metals
Application of zone refining process:
- Zone refining process is used to obtained the purest form of elements.
- It is used to remove the impurities from semiconductors such as Ge, Ga and Si.
- Float zone refining process is used in solar cells.
- Zone refining process is used to form a single layer of crystal.
- It is used for organic and inorganic compounds as well. This process is highly useful for scientists involved in preparing organic chemical standards for HPLC,
- This zone process is used in 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.
- Zone refining process is an expensive process.
- These applications are limited to laboratory reagents and valuable chemicals.
- In this zone refining process, the solid -liquid equilibria is not favourable for all impurities. For this time, this process has to combine zone refining with other techniques to achieve ultrahigh purity.
