Froth Floatation Method

Froth floatation is the concentration method of choice for sulphide ores, and it works on a completely different principle from hydraulic washing or magnetic separation: it exploits wettability, i.e., how selectively the mineral and the gangue attach themselves to oil versus water.

The setup

• A suspension of the powdered ore in water is prepared in a tank.
• Collectors — oils such as pine oil, fatty acids, or xanthates — are added. Their job is to make the mineral particles hydrophobic (non-wettable by water) so they preferentially stick to oil.
• Froth stabilisers (e.g., cresols, aniline) are added to keep the resulting foam stable long enough to be skimmed off.
• A rotating paddle agitates the mixture and simultaneously draws air into it.

Because the mineral particles are coated in oil, they attach themselves to the rising air bubbles and get carried up as a light froth, while the gangue particles — wetted by the surrounding water — stay behind and sink. The froth is skimmed off and dried to recover the mineral (Fig. 6.2). At the level of a single bubble, the picture is one of tiny mineral grains stuck all over its surface, buoyed up purely because oil, not water, coats them.

Separating two sulphide ores from each other

Froth floatation isn't limited to separating mineral from gangue — it can also separate one sulphide mineral from another in the same ore, by:

• adjusting the oil-to-water ratio, or
• adding a depressant — a reagent that selectively stops one mineral from frothing.

A textbook case is an ore containing both ZnS and PbS: adding NaCN as a depressant selectively prevents ZnS from floating to the froth while letting PbS float freely, so the two sulphides can be collected separately.

A method born from a laundry observation

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