Zone Refining- The Metallurgical Process, Principle, Applications, Practice Problems, FAQs

Metallurgical processes to obtain pure metals involves several steps of physical and chemical changes. The processes are selected on the basis of metal activity, ore nature and the purity required etc. Zone refining is one such metallurgical process employed in end stages of separation of the very pure matel.

John Desmond Bernal first invented this method of Zone refining while he was at Bell Labs, which was further improvements upon by William Gardener Pfann.

Table of Content

• Zone Refining Process
• Principle
• Application
• Practice Problems
• Frequently Asked Questions

Zone refining

This is a technique for purifying metallic crystals and is variously called as zone refining, zone melting and floating-zone method. The zone melting involves the melting a small area (zone) of a metallic crystal ore and moving the heated, molten area along the ore concentrate. While the molten stream advances, the molten zone melts and carries away the impurities at the forward border, leaving behind,.the pure crystal to cool.As the pure crystal is produced in its wake, the impurities continue to accumulate in the molten zone and are gathered at the other end.

The exceedingly pure semiconductors produced by this approach were subsequently doped to produce conducting semiconductors, which were then used in a variety of circuits.

The first semiconductor to be purified using this technique was germanium, which was later utilised to create transistors. This process can produce semiconductors with a purity of one impure atom per ten billion pure atoms, which is an astoundingly high purity. This is an extremely high level of purity that cannot be attained using other conventional purifying methods.

Zone refining can be used to purify any system of solute and solvent when the difference in concentration at equilibrium is high enough, as opposed to simply semiconductors. A high concentration difference speeds up the removal of contaminants.

Principle

It is based on the principle that impurities are more soluble in the molten state than in the solid metal. A circular mobile heater is fixed at one end of a rod of impure metal. The molten zone moves along with the heater that moves forward.

High concentration differences cause contaminants to infiltrate into the molten ore when their concentration is lower than that of the crystal we are trying to cleanse. The impurities will be collected at the end of the crystal by the time the procedure is complete because the area where this diffusion occurs at the solid-liquid border is so small. By placing a crystal "seed" at its base, the cleaned area can be consolidated once more in the shape of a crystal. This is done to regulate the direction that crystallisation occurs. Repeating this procedure repeatedly after removing the contaminated region each time will significantly increase the cleanliness of this sample.

The semiconductors are also doped using this theory. Zone levelling is the term used when doping is present. Zone levelling involves planting a predetermined doping substance in the molten zone and allowing it to travel through it at a specific rate. By doing this, the crystal is doped and the doping agent (such as antimony) is distributed evenly throughout the crystal's body. Integrated circuits and computer chips can now be produced using this doped crystal.

Numerous heaters are used during zone refining. The capacity of any heater used in zone refining to locally heat the substance is its most crucial feature. The procedure will be wasted if the heat spreads to the other portions of the ore and causes them to melt all at once. It should be strong enough to resist the heat of the process to which it is subjected. It should also be able to move along the crystal as it melts it. While the entire ore is surrounded by a magnetic field, the heater sends a current to the molten area. The ensuing Lorentz force prevents the molten ore from leaking onto the lower portions of the block by keeping it suspended in the crystal. Although optical heaters are an alternative, their potential is constrained by their relatively modest power. They are therefore mostly employed in small-scale operations that aim to create a tiny quantity of crystal.

Applications

Creating silicon for solar panel application typically involves zone refining. Due to its superior ability to utilise the photoelectric effect to transform solar energy into electrical energy, the silicon used in solar panels is the single crystal variant of the semiconductor. In comparison to other varieties, single crystal silicon has the most charge carriers, making it suitable for use in delicate systems like solar panels. The number of charge carriers does not change much over the course of its lifetime, making it the most lasting of all the variations. This is a crucial characteristic since a high charge carrier durability material is required because the panel will be utilised repeatedly during its operating cycle. The panel's efficiency has also been greatly improved.

Practice Problems

1. The correct statement is:

A). Zone refining is used for the refining of titanium
B). Zone refining is used for the refining semiconductors like Ge, Si. etc.
C). Both of these
D). None of these

Answer: B

Solution: Semiconductors like Ge, Si, B, Ga, In, etc. and other metals can be purified using zone refining.

2. The refining method used when the metal and the impurities have low and high melting temperatures, respectively is:

A). Liquation
B). Vapour phase refining
C). Zone refining
D). Distillation

Answer: A

Solution: The liquation method is used when the melting point of metal is less in comparison to the melting point of the associated impurity.

Frequently Asked Questions

1. In poling process, which stick is used?
Answer: Bamboo stick which is rich in organic matter (hydrocarbon) acts as a reducing agent and reduces the oxide impurities.

2. What is a fused metal?
Answer: It is that state of metal, in which it can be melted easily at lower temperatures.

3. Why there is a need to make fusible ore?
Answer: Metal to be extracted in their pr state are present in combined form. The process of extraction of metal from its ore often occurs at a very temperature. This adds to the high cost at the industrial level. In order to reduce the high expenditure, compounds of metals and other impurities are formed such that metal can be extracted at low temperatures.

4. What is the need to form slag?

• Slag is not soluble in molten metal, thus helps in easy separation of metal.
• It is because of the lower density of slag, it gets lighter and can be easily removed from the surface of the fused metal.
• It forms a layer on the surface of molten metal thus preventing further oxidation of the metal.

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