[Chemistry Class Notes] on Refining Pdf for Exam

Refining is defined as the process of reducing the impurities of a substance. Metals that are extracted from its ore are usually impure in nature. The extracted impure metal is called crude metal. Refining of metals is the final process. After the extraction process the metal is free from impurities. The refining method to be chosen will depend on the physical and chemical properties of a particular metal. There are different methods of making a metal pure :

• Distillation

• Liquation

• Poling

• Electrolysis

• Zone refining

• Vapour Phase refining

• Chromatographic Method

Distillation

Metals such as Zinc and Mercury consist of a very low boiling point. So zinc and mercury readily vaporise while heating them, leaving behind their impurities. The leftover impure of metal is heated beyond its melting point in a furnace and the vapours are reconverted to metals once the impurities are separated.

Liquidation 

Liquidation is suitable for metals having low melting points like tin. In Liquidation process, the impure metal is heated and then it flows on a sloped surface. While flowing, the impurities will remain behind and the pure metal will collect at the bottom of the slope.

Poling 

Poling is a method used for metals that have oxidised impurities to purify them. Poling is mainly used to purify metals that are in the impure form like copper or tin of copper oxide or tin oxide. In the poling method a log of wood is taken that is still green and then used to stir the liquid metal. The hydrocarbons of the green wood reduce the metal, and the oxygen leaves as CO₂ gas.

Electrolytic Refining 

Electrolytic Refining is the most common and widely used method because it is applicable to most metals. In Electrolytic Refining, different electrochemical properties of the metals and the impurities are used. In Electrolytic Refining, the impure metal is the anode and the pure metal is cathode. The electrolyte is the type of the salt solution of the same metal through which an electric current is passed. The pure metal extracted from the anode will dissolve in the electrolyte bath and then collect at the anode. The impurities found will either dissolve or be found in a heap at the base of the anode.

Vapour-Phase Refining 

In Vapour-phase refining, For releasing a metal from impurities, it is first converted into a suitable volatile compound by heating it. And the heating requirements are:

• The metal used for this process should form a volatile compound with a suitable reagent.

• The volatile compound that is to be used should be easily decomposable so that the recovery is easy.

Ni + 4CO Ni(CO)₄ Ni + 4CO.

Zone Refining 

It is a special method used to purify metals. In this method, metals are purified to a very high degree. Impure metal rod is placed in a container filled with inert gas. Then a circular heater is placed around the rod at the top for heating the impure metal. And when the heater shifts to the next zone, the pure metal cools and crystallises. The impurities that are melted will move along with the movement of the heater and shift to the next zone. All these impurities are then collected to the last zone and then it can be separated.

Chromatographic Method 

This method depends on the chromatography. Chromatography mainly deals with the movements of components at different rates in a mixture and/or differential absorption of an absorbent. In the chromatographic method, impure metal is put in either liquid or a gas medium. Then the medium is moved through an absorbent. Different components will absorb the impure metal at different levels. And then those components are removed by using a suitable solvent. There are different forms of chromatographic methods, like Column Chromatography, Thin Layer Chromatography, Gas-liquid chromatography etc.

Did You Know?

The distillation using a terracotta apparatus was prevalent in Indus valley civilization since 3000 B.C. Also, Babylonians practised this method in Mesopotamian civilisation. 

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