gangue, iron(III) oxides/hydroxides and aluminosilicate. Alkali leaching In this method, the ore is treated with aqueous alkali to form a soluble complex. For example, bauxite, an important ore of aluminum is heated with a solution of sodium hydroxide or sodium carbonate in the temperature range - K at atm to form soluble sodium meta-aluminate leaving behind the impurities, iron oxide and titanium oxide. Al O (s) + 2NaOH (aq) + 3H O (l) 2Na[Al(OH) ] (aq) The hot solution is decanted, cooled, and diluted.
This solution is neutralised by passing CO gas, to form hydrated Al O precipitate. 2Na[Al(OH) ] (aq) + 2CO (g) Al O .3H O (s) + 2NaHCO (aq) The precipitate is filtered off and heated around K to get pure alumina Al O . Acid leaching Leaching of sulphide ores such as ZnS, PbS etc., can be done by treating them with hot aqueous sulphuric acid. 2ZnS (s) + 2H SO (aq) + O (g) 2ZnSO (aq) + 2S (s) + H O In this process the insoluble sulphide is converted into soluble sulphate and elemental sulphur.
Evaluate yourself . Write the equation for the extraction of silver by leaching with sodium cyanide and show that the leaching process is a redox reaction. XII U1 Metallurgy - XII U1 Metallurgy - - - - - . .
Magnetic separation This method is applicable to ferromagnetic ores and it is based on the difference in the magnetic properties of the ore and the impurities. For example tin stone can be separated from the wolframite impurities which is magnetic. Similarly, ores such as chromite, pyrolusite having magnetic property can be removed from the non magnetic siliceous impurities. The crushed ore is poured on to an electromagnetic separator consisting of a belt moving over two rollers of which one is magnetic.
The magnetic part of the ore is attracted towards the magnet and falls as a heap close to the magnetic