Ore Concentration: Froth Flotation and Other Separation Methods
Before a metal can be extracted, its ore has to be concentrated — separated from the unwanted rocky gangue that surrounds it. Different separation methods exploit different physical or chemical differences between ore and gangue, and matching the right method to the right ore type is the recurring exam skill here.
Chemistry questions first require the material, particles or reaction condition to be identified before a formula or rule is selected.
Froth flotation exploits surface wettability, and works best on sulfide ores
In froth flotation, powdered ore is mixed with water, a collector (like pine oil), and a stabiliser, then air is blown through the mixture. Ore particles float and are skimmed off in the froth specifically because their surface is NOT easily wetted by water (it is hydrophobic, so the collector coating preferentially attaches and traps air bubbles against the ore particle), while the gangue particles ARE wetted by water and sink. This is not about the particles being light, charged, or simply insoluble — it is specifically a surface-wettability difference. Froth flotation is the standard method for concentrating sulfide ores (like galena, PbS, the classic exam example), because sulfide mineral surfaces are naturally more hydrophobic than typical gangue (silicate/oxide) surfaces. It is NOT generally the right method for carbonate ores like cerussite (PbCO₃) — carbonates do not show the same natural surface hydrophobicity that sulfides do, so a question pairing froth flotation with a carbonate ore like cerussite is flagging an incorrect match.
The collector and the stabiliser play different roles
In froth flotation, the collector (commonly pine oil or a xanthate) coats the ore particle surface to enhance its natural hydrophobicity, while a separate froth stabiliser (like aniline) is added to make the air bubbles at the surface persist long enough to be skimmed off as a stable froth, rather than collapsing immediately. Do not confuse these two roles: without a stabiliser, even well-coated hydrophobic particles would not form a collectible, lasting froth layer.
Leaching and magnetic separation for non-sulfide, chemically distinct mixtures
When ore concentration cannot rely on a sulfide surface-wettability difference, other methods take over. Leaching dissolves one component selectively using a chemical reagent that exploits a difference in reactivity — for example, separating a mixture of Al(OH)₃ and Fe(OH)₃: Al(OH)₃ is amphoteric and dissolves readily in excess NaOH (forming soluble sodium aluminate), while Fe(OH)₃ does not dissolve in NaOH at all and remains as an insoluble residue, so leaching with NaOH cleanly separates the two. The same mixture can also be separated by magnetic separation, since iron compounds are generally more magnetically susceptible than aluminium compounds. Froth flotation and gravity separation are NOT suited to this particular mixture — froth flotation relies on sulfide-type surface hydrophobicity that neither hydroxide has, and gravity separation relies on a density difference that is not the deciding factor here.
OlympIQ exam lens
- ✓Froth flotation works because ore particles are naturally hydrophobic (not easily wetted by water) and gangue particles are hydrophilic — not because of weight, charge, or solubility
- ✓Froth flotation is the standard method for sulfide ores (e.g. galena, PbS); it is a poor match for carbonate ores like cerussite (PbCO₃)
- ✓The collector (pine oil, xanthate) coats and hydrophobises the ore surface; the stabiliser (aniline) keeps the resulting froth stable long enough to skim off
- ✓Al(OH)₃ (amphoteric) dissolves in excess NaOH; Fe(OH)₃ does not — leaching with NaOH separates the two
- ✓Iron compounds are more magnetically susceptible than aluminium compounds, so magnetic separation also separates an Al(OH)3/Fe(OH)₃ mixture
Do not assume froth flotation is a universal ore-concentration method that works on any mineral. It specifically exploits sulfide-type surface hydrophobicity — pairing it with a carbonate ore like cerussite, or applying it to a hydroxide mixture better separated by leaching or magnetism, is exactly the kind of mismatch these questions test.
Ready to try the pattern?
100% coverage across the 5 linked questions.
Practise 5 linked questions