Hello Student
The ores even after concentration contain some earthy matter called gangue which is heated combine with this earthy matter to form an easily fusible material. Such a substance is known as flux and the fusible material formed during reduction process is called slag.
Acidic fluxeslike silica, borax etc are used when the gangue is basic such as lime or other metallic oxides like MnO, FeO, etc
Basic fluxeslike CaO, lime stone (CaCO3), magnesite (MgCO3), haematite (Fe2O3) etc are used when the gangue is acidic like silica, P4O10etc.