Froth Flotation

Froth flotation is the standard concentration method for SULPHIDE ores -- for example galena (PbS) and zinc blende (ZnS) -- and works on a completely different principle from gravity separation: it exploits the fact that the metallic sulphide particles are PREFERENTIALLY WETTED BY OIL, while the gangue particles are preferentially wetted by water.

Procedure. The crushed ore is first suspended in water in a flotation cell (Figure 1.1) and mixed with a frothing agent such as pine oil or eucalyptus oil. A small quantity of a COLLECTOR -- sodium ethyl xanthate is the example given -- is also added; the collector molecules attach themselves to the surface of the ore particles and render them water-repellent (hydrophobic). Air is then blown through the mixture from below via the air supply, generating a froth. Because the collector-coated ore particles are water-repellent and oil-wetted, they attach to the rising air bubbles and rise with the froth to the surface, while the gangue particles, being preferentially wetted by water, remain in the bulk liquid and settle towards the bottom. The froth layer, now carrying the concentrated ore, is continuously skimmed off at the top of the cell and dried to recover the concentrate; the denser residue that sinks and is drawn off at the base is called the tailings. …