Flux is a substance which combines with gangue (impurities) present in roasted ore to form easily fusible material called slag.
 \(Flus+\underset { (impurities }{ Gangue } \longrightarrow Slag\)
Thus, flux helps to remove impurities from the roasted or calcined ore. Slag is not soluble in molten metal Being lighter, it can be easily skimmed off from the surface of the molten metal.
The choice of flux depends upon the nature of impurities present in the ore. For example,
(a) If the ore contains acidic impurities such as SiO₂ , P₂O₅ , etc., then the basic flux like lime (CaO) is used.
 \(\underset { Acidic\quad impurity }{ { SiO }_{ 2 } } +\underset { Basic\quad flux }{ CaO } \longrightarrow \underset { Calcium\quad silicate(slag) }{ { CaSiO }_{ 3 } } \) 
(b) If basic impurities are present in the ore such as CaO, FeO, MgCO₃ , etc., then acidic flux like sand (SiO₂) is used. 
\(FeO+{ SiO }_{ 2 }\longrightarrow \underset { Ferrous\quad silicate }{ { FeSiO }_{ 3 } } \)
 \(\underset { Basic\quad impurity }{ CaO } +\underset { acidic\quad flux }{ { SiO }_{ 2 } } \longrightarrow \underset { Calcium\quad silicate\quad (slag) }{ Ca{ SiO }_{ 3 } } \) 

(i) It prevents certain sulphides like ZnS to enter the froth in presence of PbS, therefore, helps in their separation. Sodium cyanide is used as depressant in the separation of ZnS from PbS.
(ii) It acts as flux.
(iii) Graphite rods act as anode in the extraction of aluminium.

Aluminium is very useful metal. It is very light and good conductor of both heat and electricity. Although it is placed high up in the series but layer of its oxide deposited on the surface of the metal checks its corrosion.

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