Bauxite is leached with NaOH (concentrated) to form soluble Na[Al(OH) 4 ] complex and insoluble impurities are filtered off.

Carbonate and hydroxide ores are heated in absence of air below their melting point to convert in to their oxides in reverberatory furnace. This is called calcination. So magnesite, MgCO 3 is subjected to calcination.

This method is commonly used for the concentration of the low grade sulphide ores like galena, PbS (ore of Pb) ; copper iron pyrites Cu 2 S.Fe 2 S 3 or CuFeS 2 (ore of copper) ; zinc blende, ZnS (ore of zinc) etc., and is based on the fact that gangue and ore particles have different degree of wettability with water and pine oil; the gangue particles are preferentially wetted by water while the ore particles are wetted by oil.

Chromite ore (FeO.Cr 2 O 3 ) having magnetic properties is separated from non–magnetic silicious impurities by magnetic separator.