Concentrated alkali (like $\mathrm{NaOH}$ ) is used for the concentration of bauxite ore because it dissolves bauxite forming soluble sodium metal aluminate while impurities remain insoluble and precipitate as red mud. $\mathrm{Al}_2\mathrm{O}_3 \cdot 2\mathrm{H}_2\mathrm{O}+2\mathrm{NaOH} \rightarrow 2\mathrm{NaAlO}_2$ $+3\mathrm{H}_2\mathrm{O}$