Which of the reaction is suitable for concentrating ore by leaching...

Leaching: It involves the treatment of the ore with a suitable reagent as to make it soluble while impurity remains insoluble and after this, the ore is recovered from the solution by suitable methods.

The principal ore of aluminium and bauxite usually contain ${SiO}_{2}$, iron oxides and titanium oxides as impurities.

${Al}_{2}{O}_{3}\left(s\right) + 2NaOH\left(aq\right) + 3{H}_{2}O \to 2Na\left[Al{\left(OH\right)}_{4}\right]\left(aq\right)2Na\left[Al{\left(OH\right)}_{4}\right]\left(aq\right) + {CO}_{2}\left(g\right) \to {Al}_{2}{O}_{3} \cdot {xH}_{2}O\left(s\right) + 2{NaHCO}_{3}\left(aq\right)$

The sodium silicate remains in the solution and hydrated alumina is filtered, dried and heated to give pure ${Al}_{2}{O}_{3}$.

${Al}_{2}{O}_{3} \cdot {xH}_{2}O\left(s\right)\overset{1470K}{\to}{Al}_{2}{O}_{3}\left(s\right) + {xH}_{2}O\left(g\right)$