Introduction to 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, the purification of metals was carried out by progressive freezing.
Zone refining is also known as zone melting, floating-zone process, or traveling 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 (especially 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 that moves along the crystal.
Principle of Zone Refining
Zone refining technique is based on the fact that impurity has more solubility in a 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 the molten zone through a 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
The zone refining process is used to get the purest form of elements.
It is a very effective process in the removal of impurities from semiconductors such as Ge, Ga, and Si.
The 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 ultra-high purity.
FAQs on Zone Refining
1. What are the other ways of purification or refining of crude metals other than the Zone Refining (Fractional crystallization)?
The metals we got after the various processes of extraction are not completely pure and hence are called Crude metal. For the purification or refining of the metals, various procedures are developed and are employed, One of these procedures is the Zone Refining method or zone melting procedure, about which the above blog has discussed in detail. Let us talk about other procedures employed for the refining of the metals from the crude metals, which are -
Liquation Process: This method is helpful in the purification of metals with low melting. i.e. Bi (Bismuth), Pb (lead), Hg (mercury) etc.
Distillation: This method uses the same principle as the process of Liquation. And is helpful in the refining of metals having low boiling point-like. i.e. Bi (Bismuth), Cd (Cadmium) and Hg (mercury), etc.
Electrolytic Refining: This procedure is the most commonly used method for the refining of metals, and is based on the concept of Electrolysis. Elements such as Mg (Magnesium) and Copper (Cu) are refined with this method
2. Who was the creator of the Zone refining method, that is used to refine various metals?
A famous U.S. scientist by the name of “W.G. Pfann” had described the intensive procedure of Zone refining, which is also known as the process of Zone melting. William Gardner Pfann (commonly known as W.G. Pfann) was an inventor and materials scientist with Bell Labs. He invented this process to obtain the purest form of metals for the use of semiconductors. He himself first used this procedure in the late 1950s to purify germanium for the manufacturing of transistors. The purity achieved with this method was just one part of detectable impurity in 10,000,000,000 parts of germanium. This level of purity was previously unheard of, but this method achieved the unachievable for its time.
3. Why is the process of zone purification (or Zone refining) used even after it is relatively slower than many other methods?
The process of zone purification (zone refining) is relatively very slow, but this procedure is still widely used because of the high level of purification one can attain by using this method. Zone refining is considered as the ultimate purification method for hundreds of metals. The process is used when the purest form of a metal is required like in the case of semiconductors, where you need the metal of very high quality or in capacitors. With the help of this procedure, these major elements are purified Ge (Germanium), Ga (Gallium), and Si (Silicon).
4. Talk a bit about the limitations that are found in the procedure of zone refining (or Float zone refining)?
Even if the zone refining provides the purest form of metals and has been doing so since 1950, it still has many issues that need to be resolved.
The procedure of zone refining can be very costly for small-scale production, hence is very costly suitable for the production at a small-scale production.
It is required to combine with other procedures of purification in order to get the purest form of metals.
The whole process of zone refining is also very slow relative to the other procedures available for the purification of metals
5. Talk a little about the procedure zone refining and also tell how this procedure impacts our everyday lives.
Zone refining is also known as Float zone refining or zone melting is the procedure of refining or purifying various metals. There are various procedures in existence for the purification of metals but this process is famous because it can provide the user with the purest form of the metals. Hence, this process is irreplaceable for the purification of tens or hundreds of elements.
The purest form of metal produced by this method is necessary for the manufacture of semiconductors. And these semiconductors are what powers most of the electronic devices (processors), hence without the invention of this procedure, would not have looked like what they look now. Due to all of these The invention of this process gravely impacts our everyday lives.
