This type of method is usually used for the metal ores that are lighter than the gangue particles, especially the sulphide ores. For example Copper pyrite (ore of copper) and Argentite (ore of silver).
In this method the preferential wetting properties of the ore with the frothing agent and the water is utilised.
The sulphide ore is easily join with the oil droplets and floats with oil, where as the gangue particles join with the water and become heavy and settle at the bottom. this way the metal ore is separated from the gangue particles.