Entrainment
- Also known as
- mechanical entrainment, hydraulic entrainment, entrained water
Entrainment is the carrying of fine particles into the flotation froth with water, rather than by attaching to air bubbles. Because it is not selective, it brings unwanted gangue into the concentrate and lowers its grade.
What is entrainment?
In froth flotation, particles can reach the froth in two ways. The intended route is true flotation: a water-repellent particle attaches to an air bubble and is lifted up. The other route is entrainment: particles suspended in the water between bubbles are carried up with that water as the froth forms, whether or not they are attached to anything.
True flotation is selective, because only particles made water-repellent by the collector attach to bubbles. Entrainment is not. Any fine particle in the water, valuable or gangue, can be entrained.
Why entrainment matters
Entrainment is a main reason concentrates carry unwanted minerals. Fine gangue that reaches the concentrate by entrainment lowers concentrate grade, which can reduce the price paid for the concentrate, increase smelter penalties, and add to transport costs. Because entrained water carries particles with it, entrainment also adds water to the concentrate, which then has to be removed by dewatering and filtration.
What controls entrainment
- Water recovery. The amount of gangue entrained is closely related to how much water reports to the froth. More water in the froth means more entrainment.
- Particle size. Entrainment mainly affects fine particles, which settle slowly and follow the water. Coarse particles drain back out of the froth.
- Froth structure. A deep, well-drained froth gives entrained particles time to fall back. A wet, overly stable froth holds them; see froth stability.
- Frother and air. Frother type and dosage and air rate affect bubble size, froth wetness and water recovery.
- Slimes and clays. Very fine clay particles are easily entrained and can also coat valuable minerals.
Entrainment is sometimes described as mechanical or hydraulic entrainment to distinguish it from true flotation.
How entrainment is reduced
- Froth washing, in which clean water is sprayed onto the froth to wash entrained particles back into the pulp. It is a key feature of flotation columns.
- Deeper froths and cleaner stages, which give entrained material more chances to drain.
- Reagent adjustments, including frother choice and depressants that keep gangue from floating.
- Controlling fines and slimes, for example by desliming or dispersing clays.
- Operating conditions, such as air rate and froth depth, tuned to reduce water recovery without losing valuable mineral.
These measures shift the balance between metallurgical recovery and grade, so they are usually evaluated on the grade-recovery curve.
Entrainment and Locus biosurfactants
Locus Mining describes its biosurfactant-based flotation reagents as reducing entrainment through chemistry, lowering water recovery and improving concentrate quality, rather than relying only on mechanical changes. Results are evaluated in structured laboratory and plant trials.
Frequently asked questions
In true flotation, particles attach to air bubbles because their surfaces are water-repellent, which makes the process selective. In entrainment, particles are simply carried up with the water in the froth, so gangue is recovered along with valuable minerals.
Fine particles settle slowly and move with the surrounding water, so they are carried into the froth with it. Coarser particles tend to drain back into the slurry.
Usually by comparing the recovery of gangue, or of a non-floating tracer mineral, with the recovery of water to the concentrate. The closer gangue recovery tracks water recovery, the more it is entrained rather than floated.
Sources
- Wills' Mineral Processing Technology, 8th edition, B. A. Wills and J. A. Finch, Butterworth-Heinemann, 2016
- Peer-reviewed literature on flotation froth behavior and entrainment, e.g. Minerals Engineering and International Journal of Mineral Processing