What is flux ? What is itz role in metallurgy of Iron n Copper ?

Flux,  in metallurgy, is any substance introduced in the smelting of ores to promote fluidity and to remove objectionable impurities . When it is heated with ore, it combines with the earthly impurities present in ore to form easily fusible mass known as slag. The slag is lighter and insoluble in the molten metal. So it can be easily removed from the surface of the molten metal.

[Smelting, process by which a metal is obtained, either as the element or as a simple compound, from its ore by heating beyond the melting point, ordinarily in the presence of oxidizing agents, such as air, or reducing agents, such as coke. ]

There are two types of fluxes.

Acidic flux

Acidic flux like silica (SiO₂) is used to remove basic earthly impurities (gangue) such as lime (CaO) or MgO.

Basic flux

Basic fluxes like lime (CaO), magnesium oxide (MgO) are used to remove acidic gangue such as SiO₂.

Limestone(CaCO₃) is commonly used as basic flux for this purpose in smelting iron ores.  Limestone is decomposed to CaO which removes silicate impurity of the iron ore as slag.

CaCO₃ ------------>  CaO + CO₂

CaO + SiO₂ ------>  CaSiO₃

      (Slag)

Silica (SiO₂) is commonly used as acidic flux for this purpose in smelting copper ores. It reacts with impurity (FeO) of copper ore and removes it as slag (FeSiO₃).

FeO + SiO₂ ------>   FeSiO₃

      (Slag)