In the froth flotation process for the purification of minerals, the particles float because:

This method is commonly used for the concentration of the low grade sulphide ores like galena, PbS (ore of Pb); copper pyrites CuFeS2 (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.