Flotation - Lenntech

As opposed to settling, flotation is a solids-liquid or liquid-liquid separation procedure which is applied to particles whose the density is lower than that of the liquid they are in.

There are three types of flotation: natural, aided and induced flotation.

Natural flotation	Aided flotation	Induced flotation
Valid if the difference in density is naturally sufficient for separation.	Occurs when external means are used to promote the separation of particles that are naturally floatable.	It occurs when the density of particles is artificially decrease to allow particles to float. This is based on the capacity for certain solid and liquid particles to link up with gas (usually air) bubble to form « particle-gas » with a density lower than the liquid.

DAF (Dissolved air flotation) is a procedure of induced flotation with very fine air bubble or « microbubbles », 40 to 70 microns.

Mechanical flotation is a term used in the mineral industry to describe the use of dispersed air to produce bubble that measure 0.2 to 2 mm in diameter.

The conditions for the application of the different flotation processes in water treatment are summarized in the following table.

Flotation process	Air flow used Nl.m^-3 water	Size of bubbles	Input power per m³ treated Wh.m^-3	Theoretical retention time min	Hydraulic surface loading mh^-1
Aided flotation (grease removal)	100-400	2-5 mm	5-10	5-15	10-30
Mechanical flotation (froth flotation)	10.000	0.2-2 mm	60-120	4-16
Dissolved air flotation (clarification)	15-50	40-70 μm	40-80	20-40 (excluding flocculation)	3-10

Resource: Water treatment Handbook Degrémont