4.2 Concentration of Ores

• Magnetic Separation: This method is based on differences in magnetic properties of the ore and the impurities. The two can be separated from each other by means of magnetic separator. By this method chromite (an ore of chromium) and haematite (ore of iron) being magnetic can be separated from the siliceous impurity.

• Froth Floatation Process: This method is commonly used for the concentration of the low grade sulphide ores like galena, $PbS$ (ore of $Pb$); copper iron pyrites $Cu_2S.Fe_2S_3$ or $CuFeS_2$ (ore of copper); zinc blende $ZnS$ (ore of $Zn$) and is based on the fact that gangue and ore particles have different degree of wettability with water and pine oil. In this process, a suspension of powdered ore is made with water.
  
  To it, collectors and froth stabilisers are added. Collectors (eg. pine oils, fatty acids, xanthates, etc.) enhance non-wettability of the mineral particles and froth stabilisers (eg. cresols, aniline) stabilise the froth. The mineral particles become wet by oils while the gangue particles by water. A rotating paddle agitates the mixture and draws air in it.
  
  As a result, froth is formed which carries the mineral particles. The froth is light and is skimmed off. It is then dried for recovery of the ore particles. Sometimes it is possible to separate two sulphide ores by adjusting proportion of oil to water or by using ‘depressants’. For example, in case of an ore containing $ZnS$ and $PbS$, the depressant use is $NaCN$. It selectively prevents $ZnS$ from coming to the froth but allows $PbS$ to come with the froth.
  
  
  
  

Chemical methods include

• Leaching: It involves treatment of the ore with suitable reagent (acid, bases or other reagents) which can selectively dissolve the ore but not the impurities.
  
  
  
  
  1. Leaching of alumina from bauxite
  
  The principal ore of aluminium, bauxite, usually contains $SiO_2$, iron oxides and titanium oxide ($TiO_2$) as impurities. Concentration is carried out by digesting the powdered ore with a concentration solution of $NaOH$ at $473-523$ $K$ and $35-36$ bar pressure. As a result, $Al_2O_3$ is leached out as sodium aluminate (and $SiO_2$ too as sodium silicate) leaving the impurities behind: $$A{l_2}{O_3}\left( s \right) + 2NaOH\left( {aq} \right) + 3{H_2}O\left( l \right) \to 2Na\left[ {Al{{\left( {OH} \right)}_4}} \right]\left( {aq} \right)$$ The aluminate in solution is neutralized by passing $CO_2$ gas and hydrated $Al_2O_3$ is precipitated. At this stage, the solution is seeded with freshly prepared samples of hydrated $Al_2O_3$ which includes the precipitation: $$2Na\left[ {Al{{\left( {OH} \right)}_4}} \right]\left( {aq} \right) + C{O_2}\left( g \right) \to A{l_2}{O_3}.x{H_2}O\left( s \right) + 2NaHC{O_3}\left( {aq} \right)$$ The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give back pure $Al_2O_3$: $$A{l_2}{O_3}.x{H_2}O\left( s \right)\mathop \to \limits^{1470K} A{l_2}{O_3}\left( s \right) + x{H_2}O\left( g \right)$$
  
  2. Other Examples
  
  In the metallurgy of silver and gold, the respective metal is leached with a dilute solution of $NaCN$ or $KCN$ in the presence of air (for $O_2$) from which the metal is obtained later by replacement: $$\begin{equation} \begin{aligned} 4M\left( s \right) + 8C{N^ - }\left( {aq} \right) + 2{H_2}O\left( {aq} \right) + {O_2}\left( g \right) \to 4{[M{\left( {CN} \right)_2}]^ - }\left( {aq} \right) + 4O{H^ - }\left( {aq} \right)\quad \left( {M = Ag{\text{ }}or{\text{ }}Au} \right) \\ 2\left[ {M{{\left( {CN} \right)}_2}{]^ - }\left( {aq} \right) + Zn\left( s \right) \to } \right[Zn{\left( {CN} \right)_4}{]^{2 - }}\left( {aq} \right) + 2M\left( s \right) \\\end{aligned} \end{equation} $$