Froth Flotation

Flotation is the most important concentration method for sulphide ores. Learn surface chemistry (collectors, frothers, modifiers), flotation cells and circuits (rougher, scavenger, cleaner), the two-product formula for recovery, mass pull and enrichment, grade–recovery curves and first-order flotation kinetics.

Invented around 1905, froth flotation made low-grade copper, lead and zinc deposits economic. Chemicals make valuable mineral surfaces water-repellent; air bubbles attach to them and carry them into a froth that overflows as concentrate, while gangue sinks as tailings.

An analogy

Flotation is like fishing with bubbles: collectors put 'hooks' on valuable particles so air bubbles catch them and lift them to the surface.

Reagents

Collectors (xanthates, dithiophosphates) adsorb on sulphides and make them hydrophobic. Frothers (MIBC, glycols) stabilise bubbles. Modifiers: pH regulators (lime), depressants (cyanide, starch, sulphite) and activators (copper sulphate for sphalerite).

Circuits

Rougher cells make a first concentrate at high recovery; scavengers recover more from rougher tails; cleaners upgrade concentrate to saleable grade, often with regrinding. Mechanical tank cells (up to 600 m³) and column cells are used.

Grade–recovery trade-off

Pulling more mass increases recovery but dilutes the concentrate. Smelter contracts pay for metal but penalise low grades and impurities — the economic optimum lies on the grade–recovery curve.

Kinetics and control

Different particles float at different rates (fast, slow, non-floating). Residence time is scaled from lab kinetics (× 2–2.5 for plant). Froth cameras, online XRF analysers and model-predictive control stabilise grade and recovery.

Formulas

• Two-product recovery: R = c (f − t) / [f (c − t)]
• Mass pull: C/F = (f − t) / (c − t)
• Enrichment ratio: ER = c / f
• First-order kinetics: R(t) = R∞ (1 − e^(−k t))

Worked example: Copper rougher balance

Given: f = 1.0 % Cu, c = 25 %, t = 0.1 %

Find: Recovery, mass pull, ER.

• R = 25 × 0.9/(1.0 × 24.9) = 90.4 %
• C/F = 0.9/24.9 = 3.6 %
• ER = 25

Answer: 90 % recovery in 3.6 % of the mass

Worked example: Residence time

Given: R∞ = 95 %, k = 0.5 /min; Target 90 %

Find: Lab time.

• 1 − 90/95 = 0.0526 = e^(−0.5t)
• t = ln(19)/0.5 = 5.9 min

Answer: 5.9 min lab → ≈ 12–15 min in plant

Common mistakes

• Wrong: Maximising recovery regardless of grade. — Right: Optimise the economic point on the grade–recovery curve.
• Wrong: Scaling lab times 1:1 to plant. — Right: Apply a scale-up factor (≈ 2–2.5).

Applications

• Copper, lead, zinc: Sulphide concentrators worldwide.
• Phosphate & potash: Non-sulphide flotation with fatty acids/amines.
• Coal fines: Flotation of fine coal.

Key takeaways

• Collectors, frothers, modifiers.
• R = c(f − t)/[f(c − t)].
• Rougher–scavenger–cleaner circuits.
• R(t) = R∞(1 − e^−kt).

Going further

• Coarse particle flotation: HydroFloat and fluidised-bed cells.
• Froth vision: Cameras measure bubble size and velocity.
• Mass balance reconciliation: Least-squares adjustment of survey data.