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Explain Leaching of: (i) Alumina from Bauxite (Bayer's Process) (ii) Gold and Silver (Cyanide Process) |
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Answer» Solution :Principle : It works on the difference in the solubilities of ore and impurities in a suitable solvent. (i) LEACHING of Alumina from Bauxite (Bayer.s process): Bauxite is the principal ore of aluminium. It has the impurities of ferric oxide `(Fe_2O_3)`, Silica `(SiO_2)` and titanium oxide `(TiO_2)`. The powdered ore is digested with concentrated NAOH solution at 473-523 K temperature and 35-36 bar pressure. The alumina gets dissolved as aluminate while `Fe_2O_3, TiO_2` are left behind. `SiO_2` gets dissolve as sodium silicate. `Al_2O_(2(s)) + 2NaOH_((aq)) + 3H_2O_((aq)) underset("473-523 K")overset("35-36 bar pressure")(rarr) 2Na[Al(OH)_4]_((aq))`sodium aluminate `SiO_(2(s)) + 2NaOH_((aq)) overset("473-823K"(rarr)Na_2SiO_(3(aq)) + H_2O` The sodium aluminate present in solution is neutralised by passing `CO_2` gas and hydrated `Al_2O_3` is precipitated. At this stage a small amount of freshly prepared sample of hydrated `Al_2O_3` is added to a solution. This is called seeding. It induces precipitation. `2Na[Al(OH_4)]_((aq)) + 2CO_(2(G)) to Al_2O_3 cdot xH_2O + 2NaHCO_(3(g))` Sodium silicate remains in the solution and hydrated alumina is filtered to give BACK pure `Al_2O_3.` `Al_2O_3 cdot xH_2O overset(1470 K)(rarr)Al_2O_(3(s)) + xH_2O_((g))` (ii) Cyanide Process (Leaching of Gold and Silver) : In the metallurgy of gold and silver, the respective metal is leached with KCN or NaCN in the presence of air which supplies `O_2`. The metal is obtained later by replacement reaction. `4M_((s)) + 8CN_((aq))^(-)+ 2H_2O_((aq)) + O_2_((g)) to 4[M(CN)_2]_((aq)) + 4OH_((aq))^(-)` `M = Ag or Ar`. Ag and Au is recovered from solution by addition of electropositive elements such as zinc. `2[M(CN)_(2)]_((aq))^(-) + Zn_((s)) to [Zn(CN)_(4)]_((aq))^(2-) + 2M_((s))`. |
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