Elutriation, also known as air classification, is a process for separating lighter particles from heavier ones using a vertically-directed stream of gas or liquid (usually upwards). This method is profoundly used for particles with size (>1ųm) The smaller or lighter particles rise to the top (overflow) because their terminal velocities are lower than the velocity of the rising fluid. The terminal velocites of any particle in any media can be calculated using Stoke's Law given by u₀A =d²Ag (ρA − ρ) /18ü

An air elutriator is a simple device which can separate particles into two or more groups.

Air Elutriation

Material may be separated by means of an elutriator, which consists of a vertical tube up which fluid is passed at a controlled velocity. When the particles are introduced, often through a side tube, the smaller particles are carried over in the fluid stream while the large particles settle against the upward current. If we start with low flow rates small less dense particle attain terminal velocites, and flow with the stream, the particle from the stream is collected in overflow and hence will be separated from the feed. Flow rates can be increased to separate higher size ranges. Further size fractions may be collected if the overflow from the first tube is passed vertically upwards through a second tube of greater cross-section, and any number of such tubes can be arranged in series

It is used in mineral processing for size classification.

The Elutriation Dust Value is also a usual measure for quantification of dust, generated by testing wherein mechanical forces such as vibration are applied to granules of e.g. a detergent agent.

Elutriation is a common method used by biologists to sample meiofauna. The sediment sample is constantly agitated by a flow of filtered water from below, the action of which dislodges interstitial organisms embedded between sediment grains. A very fine filter at the top captures these organisms from the overflow.

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Source: adapted by the editor from Wikipedia, the free encyclopedia; from the article "Elutriation". Image Credit.

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