Elutriation tank , Stroke's Law , Construction , Working , Advantages

Elutriation :

it is a process of separating particles based on their size shape and density , using a steam of gas or liquid flowing in a direction usually opposite to the direction of sedimentation .

the smaller or lighter particle rise to the top ( Overflow ) because their terminal sedimentation velocities are lower then the velocity of the rising fluid . the terminal velocity of any particle in any medium can be calculated using Strokes Law  .

Strokes' Law :

V =  2 / 9  * ( ( P - f )  /  U  ) *g*R*R 

where :

P =  density of the sphere .

f = density of the fluid .

g =  acceleration of gravity 

R = radius of particle  .

U =  Viscosity of the fluid .

Construction :

An elutriation tank consist of simply a vertical column with an inlet near the bottom for feed  ,  an outlet at the base for coarse particles , and an overflow near the op for fluid and fine particles  .

- single elutriators  .

- multi -stage  elutriators .

Working :

1. the Particle ( sample ) are mixed with large quantities of water . the solid particles are uniformly distributed in the liquid by stirring and then allowed to settle down .

2. Depending upon the density of particles will settle down or remain suspended in the tank .

3. sample is withdrawn at different height through the outlets .

4. In case of multistage elutriator the overflow of preceding column act as a feed for the next . the different relative velocities are obtained due to column of different diameter .

5. the velocity of the stream in the first column is highest and last column is the lowest .

6. thus the coarser and denser particles are separated at first column and the fine product is obtained at the last .

Advantages :

1. Continuous Process

2. separation is quick .

Disadvantages :

1. suitable only for the insoluble particles  .

2. Fluid velocity is not constant across the column error may occur  .