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,
(ii) Hydrogen as a reducing agent. Because of its inflammable nature, it is used in a very few case
e.g.,
(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.
(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)42– + 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.
In the metallurgical process for the electrorefining of the metal the anode is made of _________ metal.
In the Bessemer converter during the extraction of copper, the substance responsible for forming blister is ___________.
During the extraction, the metallic silver is precipitated by the addition of __________ to ________