Question

What is the significance of leaching in the extraction of aluminium?

Answer 1

Hint: As we know the Bayer Process was developed by Carl Josef Bayer in which there is the refining of bauxite to produce alumina. Bauxite ore is a mixture of aluminium oxides and other elements such as iron.

Complete step by step answer:
During the process of extraction of aluminium, the significance of leaching is to concentrate pure alumina $A{{l}_{2}}{{O}_{3}}$ which is from Bauxite $A{{l}_{2}}{{H}_{2}}{{O}_{4}}$ ore.
During the process of leaching, aluminium is concentrated by dissolving the powdered ore with a concentrated solution of sodium hydroxide at 473-523 K at 35-36 bar.
Now let us see the chemical equation to get pure alumina from bauxite ore:

& A{{l}_{2}}{{O}_{3\left( s \right)}}+2NaO{{H}_{\left( aq \right)}}+3{{H}_{2}}{{O}_{\left( l \right)}}\xrightarrow[35-36bar]{473-523K}2Na{{\left[ Al{{\left( OH \right)}_{4}} \right]}_{\left( aq \right)}} \\\ & \\\ \end{aligned}$$ Now under this condition, aluminium is dissolved as sodium meta aluminate $$NaAlO_2$$ and silica $$Si{{O}_{2}}$$ will dissolve as sodium silicate leaving impurities behind. $$\begin{aligned} & SiO{}_{2\left( aq \right)}+2NaO{{H}_{\left( aq \right)}}\xrightarrow[35-36bar]{473-523K}NaSi{{O}_{3\left( aq \right)}}+{{H}_{2}}{{O}_{\left( l \right)}} \\\ & \\\ \end{aligned}$$ Now, the solution is neutralized by passing $$C{{O}_{2}}$$ gas and thus impurities are filtered. And hence, hydrated alumina $$A{{l}_{2}}{{O}_{3}}x{{H}_{2}}{{O}_{\left( s \right)}}$$ gets precipitated leaving behind sodium bicarbonate $$NaHC{{O}_{3}}$$ in the solution. $$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)}}$$ At the end of equation, hydrated alumina obtained which is then filtered, dried to give concentrated and pure form of alumina $$A{{l}_{2}}{{O}_{3}}_{\left( s \right)}$$ by heating back at temperature 1470K. $$A{{l}_{2}}{{O}_{3}}.x{{H}_{2}}{{O}_{\left( s \right)}}\xrightarrow{1470K}A{{l}_{2}}{{O}_{3\left( s \right)}}+x{{H}_{2}}{{O}_{\left( g \right)}}$$ Additional information: Few aluminium hydroxides which are produced during the leaching process are used in the manufacture of water treatment chemicals like aluminium sulfate, PAC (poly aluminium chloride). Sodium aluminate solution which is left over during the extraction process is then recycled. Note: Remember, the extraction process which converts the aluminium oxide in the ore to soluble sodium aluminate $$NaAl{{O}_{2}}$$ & this treatment also dissolve silica $$Si{{O}_{2}}$$ while some other components of bauxite do not dissolve, and therefore sometimes lime is added at this stage to precipitate the silica as calcium silicate $$C{{a}_{2}}Si{{O}_{4}}$$.