Illustration : Write short notes on :
(c) Self reduction
(d) Electrolytic reduction
(a) This process is used for the extractioin of noble metals like gold and silver from the native ores. The finely crushed ore is brought into contact with mercury which combines with the metal forming its amalgam. The metal is then recovered by distilling the amalgam.
(b) Hydrometallurgy is the process of bringing the metal into solution by the action of suitable chemical reagents (e..g., sodium cyanide solution or chlorine in presence of water etc.) followed by recovery of the metal by the use of a proper precipitating agent which is a more electropositive metal. For example, poor ores of silver are suspended in a dilute solution of sodium cyanide and air is blown through it when the silver present goes into solution as the argentocyanide complex. From this solution the metal is precipitated by adding zinc turnigs.
4Ag + 8NaCN + O2 (air) + 2H2O —> 4Na [Ag(CN)2] + 4NaOH
or AgCl + 2NaCN —> Na[Ag(CN)2] + NaCl
2Na [Ag(CN)2] + Zn —> Na2[Zn(CN)4] + 2Ag
(c) In some cases no extra reducing agent is required. A part of the sulphide ore is changed to oxide or sulphate which then reacts with the remaining part to give the metal and sulphur dioxide. Copper and lead are obtained by this method.
2Cu2S + 3O2 —> 2Cu2O + 2SO2
Cu2S + 2Cu2O —> 6Cu + SO2
2PbS + 3O2 —> 2PbO + 2SO2
PbS + 2O2 —> PbSO4
PbSO4 + PbS —> 2Pb + 2SO2
(d) The carbon reduction methods are not applicable for the highly electropositive and chemically active metals such as alkali metals, alkaline earth metals, aluminium etc. The oxides of these metals are very stable and have to be heated very strongly with carbon in order to reduce them to metals. But at high temperatures these metals combine with carbon to form carbides, thus creating problem. Hence these metals are extracted by the electrolytic reduction of their fused halides. The metal is liberated at the cathode. Aluminium is however obtained by the electrolytic reduction of alumina (Al2O3 ) dissolved in molten cryolite (Na3AlF6).