Outline the principles of refining of metals by the following methods:
(i) Zone refining
(ii) Electrolytic refining
(iii) Vapour phase refining
Answers
Explanation:
1. Zone refining refers to the method of purifying a crystal wherein a thin region of the crystal undergoes melting. This ‘molten zone’ is now moved across the crystal.
The impurities in the metal are melted at the forward edge by the molten zone and move through the block of metal, leaving the solidified pure element behind.
As they move through the block of metal, the impurities in the metal are concentrated in the melt and are transported to one end of the metal block. An illustration for such a process is provided below.
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Zone Refining Process
In the zone refining 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. An illustration of this process is provided below.
Thus, the required 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.
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2. Electrolytic Refining
What is Electrolytic Refining?
Electrolytic refining is a process of refining a metal (mainly copper) by the process of electrolysis. As far as the mechanism of the process is concerned, during electrolysis, a large chunk or slab of impure metal is used as the anode with a thin strip of pure metal at the cathode. In this setup, an electrolyte (metal salt aqueous solution) depending on the metal is often used.
The clean or pure metal is formed at the cathode when the electrical current of a sufficient voltage is applied by dissolving impure metal at the anode. Electrolytic refining is also sometimes referred to as Electrorefining.
Also Read: Electrolysis
Electrolytic Refining of Metals
The below-given table outlines the methods used to refine five metals. It is necessary to choose the electrolyte and other conditions so that both anodic dissolution and metal deposition proceed with high efficiency while none of the impurity metals can move from the anode to the cathode. Clearly, there must be no passivation of the anode and the aim is to produce a deposit at the cathode of good quality, sometimes extremely crystalline. Additives are applied to the electrolyte when necessary to impose the right behaviour on both electrodes.

Electrolytic Refining of Copper
Here we will take an example of electrolytic copper refining to understand the process more clearly. Copper is usually mined from its coal known as blister copper. It is about 98 to 99 per cent pure. However, the electro-refining process can easily make it 99.95% pure which makes it a good product to be used in electrical components.
A block of impure copper is taken as an anode or positive electrode. Copper sulfate which is acidified with sulfuric acid is used as a graphite-coated electrolyte along with pure copper tubes, as a cathode or negative electrode. In this phase of electrolysis copper sulfate divides into a positive ion of copper (Cu++) and a negative