ROASTING : PYROMETALLURGY:

The ore is heated strongly below its melting point in presence of excess of air which removes impurities of non – metals at their volatile oxides.

(1) Roasting is exothermic process once started it does not required additional heating.

(2) Roasting employed for Sulphide ores.

(3) Chemical Conversion of ore takes place.

(4) Roasting is carried out in Reverbatory furnace

The process of roasting required the following:

(1) Conversion of sulphide ores to their respective oxides.

(2) Conversion of sulphide ores to their Sulphate.

Note: Some time roasting may not bring about complete oxidation

(3) Roasting at high temperature: the sulphide ore of some of the metal like Cu, Pb, Hg, Sb, etc when heated strongly in the free supply of air or O₂ are reduced directly to the metal rather than to the metallic oxide for example.

The reduction of the sulphide ore directly into metal by heating it in air or O₂ is called  Self reduction, Auto reduction, Air reduction etc and the SO₂ produced is utilised  for manufacturing of Sulphuric acid.

Consequence of Roasting:

(1) Sulphide Ore is converted into oxide/sulphate which may be further decomposed into metal oxide, example Sulphur dioxide

(2) Organic Matter is burnt away.

(3) Impurities of sulphure, phosphorous, arsenic and antimony are oxidised into the respective volatile oxide.

(4) When concentrated tine Stone ore (SnO₂) is heated strongly in a free supply of air (roasting), the impurities of CuS and FeS present in the ore are converted into CuSO₄ and FeSO₄ respectively.

CALCINATION : PYROMETALLURGY:

Ore is heated strongly in absence or limited supply of air (air is used here as carrier of heat and moisture).Calcination process temperature is below melting point of treated ore. This process of calcination brings about the following changes.

(1) The Carbonate ore gets decomposed to form the oxide of the metals for example

(2) Water of crystallisation present in the hydrated oxide ores are gets lost as moisture as

Consequence of Calcination:

(1) Volatile products like water vapour / CO₂ are removed and ore become porous

(2) Organic impurities decomposes into smaller volatile products

Note:  Both calcination and Roasting are generally carried out in a Reverbatory furnace. In case of roasting, the air holes are kept open while they are partially or completely closed during calcination.

SMELTING : PYROMETALLURGY:

When roasting forms a liquid product, which makes separation easier, it is called smelting. Metal is extracted by heating calcined or roasted ore with powdered coke in presence of a flux.

Consider, for example, the smelting of zinc oxide:

The gaseous carbon monoxide separates from the liquid zinc, allowing the metal to be readily recovered. Other examples are

Slag:

In some cases, a flux must be added to the mixture during smelting to help separate the two materials. The flux is a material that will react with the gangue to form a substance with a low melting point. For example, oxides of silicon within gangue can be liquefied by reaction with calcium carbonate according to the reaction:

The waste liquid solution that forms from the flux and gangue is usually a silicate material called a slag. The liquid metal and the liquid slag have different densities and therefore separate. Holes tapped at different heights into the side of the container holding the liquid metal and slag allow the more dense liquid to flow out of the lower tap holes and the less dense liquid to flow out of the higher tap holes.

Principle of Slag formation:

The principle of slag formation is essentially the following:

Nonmetal oxide (acidic) + Metal oxide (basic) to form a Fusible (easily melted) slag

Removal of unwanted basic and acidic oxides: For example, FeO is the impurity in extraction of Cu from copper pyrite.

Matte also contains a very small amount of iron (II) sulphide.

To remove unwanted acidic impurities like sand and P₄O₁₀, smelting is done in the presence of limestone.

Properties of a Slag:

(1) Slag is a fusible mass.

(2) It has low melting point.

(3) It is lighter than and immiscible with the molten metal. It is due to these impurities that the slag floats as a separate layer on the molten metal and can thus be easily separated from the metal. The layer of the slag on the molten metal prevents the metal from being oxidised.