Question

What is Leaching ? Explain Briefly. (CBSE BOARD EXAMINATION)

Answer 1

Leaching : It is a process in which ore is treated with suitable reagent which reacts with ore but not with the impurities. It is also called chemical separation. Bauxite is purified by this process.

Lets explain how Bauxite ore is purified by this process......

Purification of Bauxite ore by Leaching Bayer's Process : Bauxite ore is treated with caustic soda. $Al_{2} O_{3}$ dissolves in concentrated solution leaving behind impurities.

$Al_{2} O_{3}$ (s) + $2OH^{-}$ (aq) + 3$H^{2} O$ ----------> 2Al$(OH)_{4}$ (aq)

Or

$Al_{2} O_{3}$ + 2NaOH + $3H_{2}O$ --------> 2Na[Al$(OH_{4})]$ (aq)

The solution of sodium meta aluminate is filtered off and cooled and its pH is adjusted dilution or by neutralisation with $CO_{2}.$ White $Al(OH)_{3}$ is precipitated. Seeding with a little freshly precipitated $Al(OH)_{3}$ quickens the process.

$Al(OH)_{4} -(_{aq})$ $\xrightarrow[\text{CO2}]{\text{Dilution}}$ $Al(OH)_{3}$ (s) + $OH^{-}$ (aq)

Or

$2Na[Al(OH)_{4}] + CO_{2} (g) \rightarrow Al_{2} O_{3}.xH_{2}O + 2NaHCO_{3}$

The precipitate of $Al(OH)_{3}$ is filtred, dried and finally heated to about 1473 K to obtain pure $Al_{2} O_{3}.$

$2Al(OH)_{3} (s)$ $\overrightarrow{heat}$ $Al_{2} O_{3} (s) + 3H_{2}O (g)$

Or

$Al_{2} O_{3}.xH_{2}O$ $\overrightarrow{heat}$ $Al_{2} O_{3} (s) + xH_{2}O$

Gold and silver are also concentrated by leaching. In this, the ore is treated with a dilute solution of NaCN or KCN in presence of air to obtain the metal in solution as complex.

Answer 2

Leaching

It is also called chemical separation.It is a process in which ore is treated with suitable reagent. This ore react with ore but didn't react with impurities present in it .

Purification of bauxite ore by leaching :

Purification of Bauxite ore by Leaching is called bayer's Process . Now see this process

Bauxite treated with caustic soda. Al_(2) O_(3) dissolves in concentrated solution leaving behind impurities.

Al_{2} O_{3}Al2O3 (s) + 2OH^{-}2OH− (aq) + 3H^{2} OH2O ----------> 2Al(OH)_{ 4}(OH)4 (aq)

Or

Al_{2} O_{3}Al2O3 + 2NaOH + 3H_{2}O3H2O --------> 2Na[Al(OH_{4})](OH4)] (aq)

The solution of sodium meta aluminate is filtered off and cooled and its pH is adjusted dilution or by neutralisation with CO_{2}.CO2. White Al(OH)_{3}Al(OH)3 is precipitated. Seeding with a little freshly precipitated Al(OH)_{3}Al(OH)3 quickens the process.

Al(OH)_{4} -(_{aq})Al(OH)4−(aq) $\xrightarrow[\text{CO2}]{\text{Dilution}}$Al(OH)_{ 3}Al(OH)3(s) + OH^{-}OH− (aq)

The precipitate of Al(OH)_{3}Al(OH)3 is filtred, dried and finally heated to about 1473 K to obtain pure Al_{2} O_{3}.Al2O3.

2Al(OH)_{3} (s)2Al(OH)3(s) $\overrightarrow{heat}$ Al_{2} O_{3} (s) + 3H_{2}O (g)Al2O3 (s)+3H2O(g)

This is the process of leaching with bauxite ore

Hope it helps !