Friday, February 25, 2011

Elutriation

The word “Elutriation” is derived from the Latin word “elutriare” meaning “to wash out”.

Definition:
It is the separation, purification or removal of something from a mixture by decanting, straining or washing.

Process of elutriation:
In the process of elutriation, the movement of the fluid, generally water or air, is in the opposite direction to that of the sedimentation process.

Types of elutriation:
According to direction:

Vertical elutriation:
In the gravitational process, the larger particles present in water (or any other liquid) will move vertically downwards with the affect of gravity while the small particles in the fluid travels straight up with the fluid. This is a type of vertical elutriation.

Horizontal elutriation:
If a water current of suspended particles is flowed through a settling chamber. The particles that move out of the water current are collected in the bottom of the chamber. This is a type of horizontal elutriation.

Centrifugal elutriation:
In this case the water current is caused to spin with some force resulting in the large centrifugal force on the suspended particles. The heavier particles will settle to the walls of the elutriator or to the bottom.

The DorrClone is an example of a centrifugal-type of classifier.

According to the type of fluid:

• Air elutriation

• Water elutriation

If the velocity of the fluid is smaller than the velocity of setting down of the particles then the particles will settle downwards. On the other hand, if the velocity of the fluid is larger than the velocity of setting down of the particles then the particles will be carried up along with the fluid.

Air elutriation will give precise separation of the particles and in less time than water elutriation.

Factors affecting elutriation:

Elutriation is affected by the

• velocity of the fluid

• the particle size : As the small sized particles will flow (upward) along the fluid while the large sized particles will move downwards (against the velocity of the fluid).

• position of the particle in the (tube containing) fluid

• density of the particle

In a tube, there exist different velocities i.e. the velocity is largest in the centre and is smallest along the walls of the tube. So the small sized particles move upward, when in the centre and in the meantime they are also pushed towards the wall of the tube. Where the velocity is smaller and here the small sized particles start to move downwards.

Process of removal of particles:

If the upward flow of the water (or any other liquid) is slightly increased, the small sized particles (which move down slowly) will move along the movement of the water (i.e. upward) and will be removed from the water. In this process, the medium sized particles will remain immobile and the heavier particles will continue to move downward.

The upward flow of water will then again be increased and the next smallest size particles will be removed. And in this way, particles of different sizes will be separated and obtained.
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