Froth flotation

What is froth flotation?

Froth flotation separates minerals by exploiting differences in how their surfaces interact with water. Crushed and ground ore is mixed with water to form a slurry, chemicals called flotation reagents are added, and air is bubbled through it. Particles of the valuable mineral, made water-repellent by the reagents, attach to the bubbles and are carried up into a froth layer on top of the cell, which overflows as the concentrate. The rest, the gangue, stays in the slurry and leaves as tailings.

Flotation was developed in the early twentieth century and made it economic to process low-grade and complex ores. It is used for copper, lead, zinc, nickel, gold-bearing sulfides, phosphate, potash, coal, lithium minerals and, by reverse flotation, iron ore.

How flotation works

1. Grinding. Ore is ground finely enough that grains of the valuable mineral are freed from the rock around them, called mineral liberation.
2. Conditioning. Reagents are added and given time to adsorb on particle surfaces.
3. Bubble attachment. Air bubbles collide with particles. Water-repellent particles attach; water-attracting particles do not.
4. Froth formation. Bubbles carrying particles rise into the froth, which must be stable enough to hold them but drain enough to release unwanted material and water.
5. Collection. The froth overflows as concentrate. Circuits usually include several stages, such as roughers, scavengers and cleaners, to balance recovery and grade.

Flotation reagents

• Collectors adsorb on the valuable mineral and make it water-repellent, so it attaches to bubbles.
• Frothers stabilize small bubbles and the froth.
• Modifiers tune selectivity: depressants keep unwanted minerals from floating, activators help specific minerals respond to the collector, and pH regulators control surface chemistry.

The combination and dosage of reagents is called the reagent scheme; see flotation reagent.

Recovery and grade

Flotation performance is judged by two measures that pull against each other:

• Metallurgical recovery: the share of the valuable mineral that reports to the concentrate.
• Concentrate grade: how pure that concentrate is.

Pushing for higher recovery tends to pull in more gangue and lower the grade, and the reverse. This trade-off is described by the grade-recovery curve, and improving flotation selectivity is what shifts the whole curve.

What limits flotation performance

• Particle size. Flotation works best in an intermediate size range. Very fine particles collide with bubbles less efficiently and are easily carried by water; coarse particles detach from bubbles.
• Entrainment. Fine gangue carried into the froth with water, not attached to bubbles, lowers grade.
• Froth stability. A froth that is too unstable loses particles; one that is too stable carries too much water and gangue.
• Ore variability. Changes in mineralogy, oxidation and clay content alter reagent response.
• Water chemistry. Dissolved ions in recycled process water change surface chemistry.

Froth flotation at Locus

Locus Mining develops biosurfactant-based flotation reagents that it describes as improving recovery and selectivity and reducing entrainment, evaluated through laboratory and pilot testing on customers’ ores.

Frequently asked questions