Explain different methods of reduction of the roasted/calcined ore to the metallic form.

Various reduction process are described under the following main categories, viz.,
(a) Chemical reduction
(b) Auto-reduction
(c) Displacement method
(d) Electrolytic reduction.

(a) Chemical reduction:

(i) Carbon as a reducing agent. Carbon and carbon monoxide reduces many metal oxides into the respective metals. Examples,

  $$\begin{gathered} \underset{\left(\mathrm{cassiterite}\right)}{{\mathrm{SnO}}_2} + 2\mathrm{C} \stackrel{1473-1573\mathrm{K}}{\to } \mathrm{Sn} + 2\mathrm{CO} \\ \mathrm{ZnO} + \mathrm{CO} \stackrel{1600 \mathrm{K}}{\to } \mathrm{Zn} +{\mathrm{CO}}_2 \\ {\mathrm{Fe}}_2{\mathrm{O}}_3+ \mathrm{CO} \stackrel{823 \mathrm{K}}{\to } 2\mathrm{FeO} + {\mathrm{CO}}_2 \\ \mathrm{FeO} + \mathrm{CO} \stackrel{1123 \mathrm{K}}{\to } \mathrm{Fe} + {\mathrm{CO}}_2 \end{gathered}$$

(ii) Hydrogen as a reducing agent. Because of its inflammable nature, it is used in a very few case
e.g.,

${\mathrm{MO}}_3 +3{\mathrm{H}}_2 \to \underset{(\mathrm{where} \mathrm{M} = \mathrm{Mo} \mathrm{or} \mathrm{W})}{\mathrm{M} + 3{\mathrm{H}}_2\mathrm{O}}$

(iii) Aluminium as a reducing agent. The oxides of chromium and manganese are reduced by aluminothermic reduction process/or Goldschmidt thermic process.

Cr₂O₃ + 2Al Al₂O₃ + 2Cr 3M₃O₄ + 8Al → 4Al₂O₃ + 9Mn

(iv) Other metals as reducing agents. Sodium and magnesium metals are used in certain cases such as titanium oxide TiO₂. Thus,

TiO₂ + 2Mg → Ti + 2MgO or TiO₂ + 4Na → Ti + 2Na₂O

(b) Auto-reduction: In certain cases no reducing agent is required. The metal is obtained either by simple roasting or by the reduction of its partly oxidized form. Thus, mercury is directly obtained by roasting its ore cinnabar (HgS) in air.

$\mathrm{HgS}\left(\mathrm{s}\right) + {\mathrm{O}}_2\left(\mathrm{g}\right) \to \mathrm{Hg}\left(\mathrm{g}\right) + {\mathrm{SO}}_2\left(\mathrm{g}\right)$
$$\begin{gathered} \mathrm{or} \mathrm{HgS}\left(\mathrm{s}\right) + 3{\mathrm{O}}_2\left(\mathrm{s}\right) \to 2\mathrm{HgO} + 2{\mathrm{SO}}_2 \\ 2\mathrm{HgO}\left(\mathrm{s}\right) \to \mathrm{Hg}\left(\mathrm{g}\right) + {\mathrm{O}}_2 \\ \mathrm{also} 2\mathrm{HgO}\left(\mathrm{s}\right) + \mathrm{HgS}\left(\mathrm{s}\right) \to 3\mathrm{Hg}\left(\mathrm{g}\right) + {\mathrm{SO}}_2\left(\mathrm{g}\right) \\ {\mathrm{CU}}_2\mathrm{S} \mathrm{is} \mathrm{also} \mathrm{reduced} \mathrm{by} \mathrm{partly} \mathrm{converted} {\mathrm{Cu}}_2\mathrm{O} \mathrm{in} \mathrm{this} \mathrm{way} \mathrm{during} \mathrm{smelting}. \end{gathered}$$
                              $$\begin{gathered} {\mathrm{Cu}}_2\mathrm{S} + 2\mathrm{CuO} \to 6\mathrm{Cu} + {\mathrm{SO}}_2 \\ \mathrm{also} \mathrm{PbS} + {\mathrm{PbSO}}_4 \to 2\mathrm{Pb} + 2{\mathrm{SO}}_2 \end{gathered}$$

(c) Displacement method: Some metals are reduced by displacement by more reactive metal, from their complexes. Siver and gold, for example, are obtained from their complex cyanides when more reactive zinc metal displaces them.

2M(CN)₂ + Zn → Zn(CN)₄^2– + 2M
(where M = Ag or Au).

(d) Electrolytic reduction: Highly electropositive elements of groups 1 and 2 are reduced by electrolytic method. These metals which occur as chlorides or oxo-salts are converted into their chlorides. When electric current is passed through a fused chloride the Mⁿ⁺ ions are discharged at cathode and deposited. In the electrolysis of brine (NaCl) using Hg cathodes, Na⁺ is discharged at cathode and forms an amalgam. This takes place in preference to the liberation of H₂ due to high H₂ over voltage at Hg cathode.

$\mathrm{Na} \underset{(\mathrm{at} \mathrm{cathode})}{\overset{\mathrm{discharged} {\mathrm{e}}^{+}}{\leftarrow }} {\mathrm{Na}}^{+} \leftarrow \underset{\mathrm{brine}}{\mathrm{NaCl}} \to {\mathrm{Cl}}^{-} \underset{(\mathrm{at} \mathrm{anode})}{\overset{\mathrm{discharged} {\mathrm{e}}^{-}}{\to }} 1/2 \mathrm{Cl}$