Gravity separation

Gravity separation

Gravity separation is an industrial method of separating two components from a suspension or any other homogeneous mixture where separating the components with gravity is sufficiently practical. All of the gravitational methods are common in the sense that they all use gravity as the dominant force. Often other methods are applied to make the separation faster and more efficient, such as flocculation, coagulation and suction. The most notable advantages of the gravitational methods are their cost effectiveness and in some cases excellent reduction.


Clarification is a name for the method of separating fluid from solid particles. Often clarification is used along with flocculation to make the solid particles sink faster to the bottom of the clarification pool while fluid is obtained from the surface which is free of solid particles.

Thickening is the same as clarification except reverse. Solids that sink to the bottom are obtained and fluid is rejected from the surface.

The difference of these methods could be demonstrated with the methods used in waste water processing: in the clarification phase, sludge sinks to the bottom of the pool and clear water flows over the clear water grooves and continues its journey. The obtained sludge is then pumped into the thickeners, where sludge thickens farther and is then obtained to be pumped into digestion to be prepared into fertilizer.

Theory of Clarification and Thickening

The duration it takes for the solid particles to set to the bottom can be illustrated with Stokes' Law: v = (ρ' - ρ) g x² / (18 μ)

*v = velocity of sinking / floatation (cm/s)
*ρ' = density of the solid particle (g/cm²)
*ρ = density of the liquid (g/cm²)
*g = acceleration of earth's gravitational pull (9.81 m/s²)
*x = diameter of the solid particle (cm)
*μ = viscosity of the liquid (g/cms)

Cyclonic Separation

Cyclonic separation relies on both, gravitational and centrifugal forces in separation of components. Cyclonic separation can be used to clear gases and fluids that contain rough particles.

When fluid or gas enters the cyclone with high enough pressure, a spiral flow is created. Rough particles, being heavier, are thrown into the wall of the separator where they sink down into the bottom of the separator where they are removed and the fluid flows out from the outlet pipe that is situated at the center of the cyclone.

inking chamber

When clearing gases, an often used and mostly working method for clearing large particles is to blow it into a large chamber where the gas's velocity reduces and the solid particles start sinking to the bottom. This method is used mostly because of its cheap cost.


*Prosessitekniikan Yksikköprosessit (Finnish (Unit processes of process technics)) by Juhani Pihkala
**(Book's website

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