Mechanical Operation: Classification and Elutriation

The mixture of particles can be separated according to their settling velocity. the settling velocity for a system depends on diameter and density of particles. For two different types of materials having density difference can be separated into 3 fraction. one containing fine particles of lighter material, one containing coarse particles of heavier material and one having mixture of both as shown in figure.

In elutriation, a water stream is passed up to the column through bottom. the particles having settling velocity less than that of water velocity will be carried over and particles having larger settling velocity will be settled. to separate the mixture into more than 2 fraction, 2 or more elutriators can be connected in series with increasing area to reduce the water velocity.

The density of fluid can be taken in between the two fluids. for example in a mixture of quartz and galena with densities 2650 and 7500 kg/m3, if the fluid has density 2650 then quartz particle will have zero settling velocity and galena particles will settle down. But for a given size range we can calculate the minimum density of the fluid that can separate the particles. to find minimum fluid density we can equate the settling velocity for largest particle of lighter particle and settling velocity of smallest particle for heaviest particle. the density of fluid slightly greater than this will separate the particles.