Metallurgy

Generally, metals occur as compounds in the earth crust. The compounds of metals which occur in earth crust, along with the earthy impurities associated with them, are called minerals.

The mineral from which the metal can be extracted very easily and economically is known as an ore. All ores are minerals, but all minerals are not ores.

Metallurgy

Metals generally occur as their oxides, carbonates, sulphides, chlorides, silicate minerals. The term metallurgy includes:

1. Concentration of the ore
2. Reduction of the ore
3. Refining of metal

The removal of the gangue from the ore is called ore dressing or concentration of the ore. The separating of metal in crude form (impure form) from the ore is called reduction of the ore. The separation of a metal in pure state from the crude metal is called refining of metal.

Concentration of Ore

The concentration of the ore can be done by

1. Hand picking
2. Gravity method (washing with water)
3. Froth floatation 
4. Magnetic separation
5. Liquation
6. Leaching

In washing method, ore is made into fine powder by using ball mills. Ores like cassiterite and alluvial sand (contains Gold) are concentrated by washing with water.

Gravity method is based on the difference between the densities of the ore particle and gangue. Gravity process is used for the concentration of denser ores from the water soluble and lighter impurities (Gangue). Oxide ores and carbonate ores are concentrated by this method.

Froth Floatation Process

This method is used for the concentration of Sulphide ores.

The mineral particles become wet by oils while the gangue particles by water. A rotating paddle agitates the mixture and draws air in it. As a result, froth is formed which carries the mineral particles. The froth is light and is skimmed off. It is then dried for recovery of the ore particles.

Electromagnetic Process

This method is used if the ore or gangue particles are magnetic in nature. For example, Haematite or Magnetite ore is magnetic. It contains silica as impurity. The silica is non magnetic.

Reduction of Ores

Generally oxide and halide ores are reduced.

The commonly used reducing agents in metallurgy are H₂ gas, CO gas, water gas, Al, Mg, Ca metals, Coke.

Metal oxide ore on reduction with carbon or coke gives metal. This method is used for the extraction of metals like Zn and Sn.

Roasting

The process of heating an ore alone (or) mixed with other substances in air below its melting point is known as roasting. The roasting process removes volatile impurities from the ore. The roasting process is used for sulphide ores.

Calcination

The process of heating an ore in the absence of air below its melting point is known as Calcination. The calcination process removes volatile impurities from the ore. The calcination process is used for oxide, carbonate and bicarbonate ores.

Refining

The removal of impurities from crude metal is called refining of metal. The methods used for refining of metals are liquation, distillation, cupellation, poling electrolysis, zone refining.