A slag is an easily fusible material which is formed when gangue still present in the roasted or the calcined are combines with the flux. For example, in the metallurgy of copper, the sulphide ore (i.e., copper pyrites) is roasted in a reverberatory furnace and converted into a mixture of FeO and Cu2O. 2CuFeS2+O2‌
∆
⟶
Cu2S+2‌FeS+SO2 Copper pyrites 2Cu2S+3O2‌
∆
⟶
2Cu2O+2SO2 2‌FeS+3O2‌
∆
⟶
2‌FeO+2SO2 To remove FeO (basic impurity), an acidic flux silica is added during smelting, FeO then combines with SiO2 to form ferrous silicate (FeSiO3) slag which floats area molten matte. FeO+‌
SiO2
‌ flux ‌
⟶‌
FeSiO3
‌ slag ‌
Thus, the role of silica in the matellurgy of copper is to remove iron oxide as slag.