Froth flotation is a process in which valuable mineral particles, once their surfaces are rendered hydrophobic, attach to air bubbles and are collected in the froth layer at the top of the cell, while hydrophilic gangue particles remain in the pulp. It is today the dominant method for concentrating copper sulphide ores, particularly chalcopyrite (CuFeS₂), chalcocite and bornite.
Physicochemical principles
Separation relies on differences in surface properties. Copper sulphides have limited natural hydrophobicity; adding a collector increases their contact angle with water. The probability of recovering a particle is the product of three probabilities: particle–bubble collision, stable attachment, and no detachment on the way to the froth.
- The optimum particle size for flotation is typically between 10 and 150 µm.
- Very fine particles (slimes) suffer from low collision probability, and coarse particles from detachment.
- The degree of liberation of the valuable mineral defines the maximum achievable concentrate grade.
Flotation reagents
- Collectors: xanthates (e.g. sodium isopropyl xanthate), dithiophosphates and thionocarbamates, which adsorb selectively on sulphide surfaces.
- Frothers: MIBC and polyglycols to form a stable froth and control bubble size.
- pH modifiers and depressants: lime to raise pH to roughly 10.5–12 and depress pyrite; sodium silicate to disperse silicate gangue.
Circuit configuration
A typical copper circuit comprises rougher, scavenger and cleaner stages. Rougher concentrate is usually reground to about 25–45 µm for further liberation and then upgraded in one or more cleaner stages. Cleaner tailings are recycled to the head of the circuit to minimise losses.
- Mechanical (agitated) cells for high-volume rougher and scavenger duty.
- Column cells for cleaner duty, where froth washing allows higher concentrate grades.
Design criteria
Circuit design is based on batch kinetic tests and locked-cycle tests. Flotation kinetics are commonly described by a first-order model: R = R∞ (1 − e^(−kt)), where R∞ is the ultimate recovery and k the rate constant. Laboratory residence time is converted to plant residence time with a scale-up factor, typically 2 to 3.
- Feed size (P80): typically 75–150 µm for porphyry copper ores.
- Rougher pulp density: typically 30–38% solids by weight.
- Superficial gas velocity (Jg): typically 1–2 cm/s.
- Final chalcopyrite concentrate: typically 24–30% Cu at 85–92% recovery.
Conclusion
The performance of a concentrator depends on the integration of the grinding circuit, correct reagent selection and a well-designed flotation circuit. At the Chah-Firouzeh copper concentrator, the design delivers 100,000 t/y of concentrate at 25% Cu, with tailings dewatered by filtration.