Froth flotation is a physical concentration technique used primarily to separate sulphide ores (such as those of copper, lead, and zinc) from the gangue (unwanted rocky/earthy impurities) that surrounds them in the raw ore, based on differences in their surface wetting properties rather than any chemical reaction.

Underlying principle: Sulphide ore particles are naturally (or can be made) preferentially wetted by oil and repel water — i.e., they are hydrophobic in the presence of the collector oil — whereas the gangue particles are preferentially wetted by water — i.e., they are hydrophilic. This difference in surface affinity is exploited to physically separate the two.

Step-by-step process:

1. The ore is finely crushed/powdered and mixed with water in a large tank to form a slurry (pulp).
2. A small quantity of a collector (such as pine oil or a xanthate) is added, which coats the surface of the valuable sulphide mineral particles and makes them water-repellent.
3. A frothing agent is also added to help generate a stable, persistent froth when air is introduced.
4. Air is blown vigorously through the slurry, generating a large number of small air bubbles.
5. The oil-coated, hydrophobic sulphide particles attach themselves to these air bubbles and rise to the surface, forming a mineral-laden froth.
6. The wettable gangue particles, having no affinity for the oil or air bubbles, remain wetted by water and settle/sink to the bottom of the tank.
7. The froth carrying the concentrated ore is skimmed off from the top, dried, and processed further (e.g., roasting, smelting) to extract the metal.

Additional reagents such as depressants (to prevent unwanted minerals from floating, e.g., sodium cyanide used to depress zinc sulphide in the presence of lead sulphide) can also be used to make the separation more selective when the ore contains more than one type of sulphide mineral.

The other listed processes work on different principles and are not what is being described here: electrolytic refining uses an electric current passed through an electrolyte to purify already-extracted impure metal (typically used as a final purification step, not for initial ore concentration); the leaching process dissolves the desired metal compound selectively using a suitable chemical solvent (e.g., cyanide leaching of gold ore, or use of NaOH for bauxite), leaving the solid impurities behind, and does not involve oil, water, or froth; and magnetic separation relies on differences in magnetic susceptibility between the ore and the gangue (using a magnetic roller), which is only effective when either the ore or the impurity is magnetic in nature, unlike the oil–water–air-based mechanism of froth flotation.