What Is Refining of Metals Definition Principles and Types of Refining Methods
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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FAQs on Refining of Metals and Its Important Methods
1. What is refining in chemistry?
Refining in chemistry is the process of removing impurities from raw materials to obtain a pure metal, petroleum product, or chemical substance. It is commonly applied to:
- Metals (e.g., copper, silver, gold)
- Crude oil in petroleum refining
- Industrial chemicals and gases
2. What are the main types of refining processes?
The main types of refining processes are distillation, electrolytic refining, zone refining, liquation, and chemical refining. These include:
- Fractional distillation – used in petroleum refining
- Electrolytic refining – used for metals like copper
- Zone refining – used for ultra-pure semiconductors
- Liquation – separates metals with low melting points
- Oxidative or chemical refining – removes impurities by chemical reactions
3. What is electrolytic refining of metals?
Electrolytic refining is a purification process that uses electrolysis to obtain highly pure metal. In this method:
- The impure metal acts as the anode.
- A thin sheet of pure metal acts as the cathode.
- An aqueous solution of a suitable salt acts as the electrolyte.
Anode: Cu(s) → Cu2+(aq) + 2e-
Cathode: Cu2+(aq) + 2e- → Cu(s)
Pure copper is deposited at the cathode, while impurities settle as anode mud.
4. How does fractional distillation work in petroleum refining?
Fractional distillation separates crude oil based on differences in boiling points of its components. The process involves:
- Heating crude oil in a furnace.
- Vapors rising through a fractionating column.
- Different hydrocarbons condensing at different temperatures.
5. What is zone refining and where is it used?
Zone refining is a purification method used to obtain ultra-pure solids, especially semiconductors like silicon and germanium. In this process:
- A narrow region of the solid is melted using a moving heater.
- Impurities concentrate in the molten zone.
- The molten zone is slowly moved along the solid rod.
6. What is the difference between refining and smelting?
The main difference is that smelting extracts metal from its ore, while refining purifies the extracted metal.
- Smelting involves heating ore with a reducing agent (e.g., carbon) to obtain crude metal.
- Refining removes remaining impurities to increase purity.
7. Why is refining important in industry?
Refining is important because it produces high-purity materials required for industrial and technological applications. Its importance includes:
- Improving electrical conductivity of metals
- Ensuring fuel efficiency in petroleum products
- Producing semiconductor-grade materials
- Enhancing chemical reactivity and reliability
8. What is liquation in metal refining?
Liquation is a refining process that separates metals based on differences in melting points. In this method:
- The impure metal is gently heated.
- The metal with the lower melting point melts and flows away.
- Higher melting impurities remain solid.
9. What is chemical refining of metals?
Chemical refining is a purification process where impurities are removed by chemical reactions. This may involve:
- Oxidation of impurities
- Precipitation reactions
- Selective dissolution
10. What are the products obtained from petroleum refining?
Petroleum refining produces useful fractions such as LPG, petrol, kerosene, diesel, lubricating oil, paraffin wax, and bitumen. These are separated by fractional distillation based on boiling range:
- LPG – cooking fuel
- Petrol (gasoline) – vehicle fuel
- Kerosene – aviation fuel
- Diesel – heavy transport fuel
- Bitumen – road surfacing
