In this method, the concentrated ore is oxidised by heating it with excess of oxygen in a suitable furnace below the melting point of the metal.

The conversion of ore into metal oxide (oxidation) is called calcination. It is the process in which the ore is subjected to the action of heat at high temperature in the absence of air below its melting point. Example
\({ CaCO }_{ 3 }(limestone)\longrightarrow CaO+{ CO }_{ 2 }\uparrow \)
\({ MgCO }_{ 3 }\left( Magnesite \right) \longrightarrow { MgO }+{ CO }_{ 2 }\uparrow \)

(i) The standard free energy for the formation of CuFeS₂ (copper pyrites) is greater than those of CS₂ and H₂S.
(ii) Hence, CuFeS₂ cannot be reduced carbon or hydrogen.
(iii) However, the free energy of copper oxide less than that of CO₂,
(iv) That is the reason, extraction of copper easier from its oxide through reduction.

(i) Gold is usually found in native state
(ii) The leaching process is intended to concentrate the gold metal
\(4Au_{ (s) }+8Na{ CN }_{ (aq) }^{ - }+2{ H }_{ 2 }O_{ (aq) }+{ O }_{ 2(g) }\longrightarrow 4Na[Au\left( CN \right) _{ 2 }]^{ - }_{ (aq) }+4NaO{ H }^{ -1 }_{ (aq) }\)
\(2Na\left[ Au(CN)_{ 2 } \right] +Zn\longrightarrow { Na }_{ 2 }\left[ Zn\left( CN \right) \right] +2Au\downarrow \)
(iii) In this reaction, Zn acts as a reducing agent.

(i) Copper is extracted form copper matte which contains iron as impurity.
(ii) Silica is added to remove this impurity as iron silicate in the form of Fusible slag.
\(\underset { Impurity }{ FeO } +{ SiO }_{ 2 }\longrightarrow \underset { Ironsilicate(slag) }{ { FeSiO }_{ 3 } } \)

(i) Conversion of ores into oxides.
(ii) Reduction of metal oxides.

Copper, Silver, Gold and Platinum

Ellingham diagram for the formation of ZnO and CO₂ intersects around 1200 K. Above this temperature ZnO lies above Carbon which indicates Carbon is the better reducing agent CO is more effective reducing agent only below 983 K.

In this extraction, a basic flux, quick lime (CaO) is used, since the silica gangue present in the ore is acidic in nature. The quick lime combines with it to form calcium silicate (slag).
CaO_(s) + Sio_2(s) ⟶ CaSio_3(s)
Flux       Gangue    Slag

(i) Concentration of the ore
(ii) Extraction of crude metal
(iii) Refining of crude metal