Question

Bauxite is concentrated by:
A) Chemical method
B) Roasting
C) Magnetic separation
D) Froth flotation process

Answer 1

Hint: We know that ores of different metals are concentrated by different methods based on the difference between the physical properties of the desired metal and the undesirables.

Complete step by step answer:
Minerals are found in the earth’s crust and are mined from the same to extract metals from them but all the minerals cannot be used for this. Ores are those minerals which can be used for metal extraction. Examples include bauxite \[\left\{ {Al{O_x}{{(OH)}_{3 - 2x}}} \right\}\left( {0 < x < 1} \right)\] and zincite $\left( {ZnO} \right)$ for aluminum and zinc respectively.
We know that it is rare to find an ore that has only the desired metal. It contains some undesired ones also which are collectively known as gangue. So, the first step involves concentration of ores which is basically removing unwanted materials from the ore. This process is also known to us as dressing or benefaction. All ores cannot be concentrated by using a single method. It depends on how the physical properties of the desired material are different from those of gangue. We can brief some concentration methods as follows:
Magnetic separation: This method can be used when one of the desired metals and gangues shows magnetic behaviour and the other doesn’t. The example includes iron ores.
Froth flotation method: This method is usually employed for sulphide ores. In this method, a suspension is prepared of the powdered ore and water. Collectors and froth stabilizers are added to the suspension. The skimmed froth is dried for further processes.
Chemical method: Here, we treat the ore with suitable chemicals. For example: leaching in case of bauxite. The common impurities found in bauxite are oxides of $Si,\;Fe$ and $Ti$. At first, bauxite is heated with $conc.\;NaOH$ at a temperature of \[{\rm{473}} - 523\;K\] and a pressure of $35 - 36\;bar$. The reaction can be shown as follows:
\[A{l_2}{O_3}\left( s \right) + 2NaOH\left( {aq} \right) + {\rm{ }}3{H_2}O\left( l \right) \to 2Na\left[ {Al{{\left( {OH} \right)}_4}} \right]\left( {aq} \right)\]
The sodium aluminate is then neutralized as follows:
\[2Na\left[ {Al{{\left( {OH} \right)}_4}} \right]\left( {aq} \right) + 2C{O_2}\left( g \right) \to A{l_2}{O_3}.x{H_2}O\left( s \right) + 2NaHC{O_{3{\rm{ }}}}\left( {aq} \right)\]
This hydrated alumina is then heated to get pure alumina as follows:
\[A{l_2}{O_3}.x{H_2}O\left( s \right) \to A{l_2}{O_3}\left( s \right) + x{H_2}O\left( g \right)\]

From the above discussion, we can conclude that option A is the correct one.
Note: Roasting is not used for concentration of ore but oxidation of ore. Calcination involves thermal decomposition of carbonate ores whereas roasting is carried out mostly for sulfide minerals.