Explain the terms gangue and slag.

Gangue: the earth impurities around the ore called gangue.
Slag: Flux combines with the gangue and form easily fusible products called slag.
$\text{ Ex: }\underset{\mathrm{Gangue}}{\mathrm{FeO}}+\underset{\mathrm{flux}}{{\mathrm{SiO}}_2}\to \underset{\mathrm{slag}}{{\mathrm{FeSiO}}_3}$