Which of the following methods is best suited for the concentration of bauxite ore?
The principal ore of aluminium - bauxite - usually contains SiO2, iron oxides, and TiO2 as impurities. Concentration is carried out by digesting the powdered ore with a concentrated solution of NaOH at 473-523 K and 35-36 bar pressure. Thus, Al2O3 is leached out as sodium aluminate leaving the impurities behind.
Al2O3 (s) +2NaOH(aq) +3H2O(l) → 2Na[Al(OH)4](aq)
Of course, the SiO2 impurities get dissolved in NaOH to form sodium silicate. This exists in the solution along with sodium aluminate, from which Al(OH)3 is seeded out by using a seed crystal. This is the famous Bayer’s process. Before recovering the ore from this solution, what do you think should be done with other insoluble impurities like TiO2 and Fe2O3?
Hall’s process and Serpeck’s process can also be used for the concentration of bauxite ore. Even after concentration, we only get pure alumina Al2O3.