5. GRAVITY SEPARATION
Separation of two immiscible liquids or slurry containing fine solids can also be carried out by making use of the density difference. In case of these two phase system the continuous phase is a liquid while the disperse phase is solid or other immiscible liquid. Solid-liquid separation processes are generally based on either one or a combination of gravity settling, filtration, and centrifugation principles. In gravity settling separation, solid particles will settle out of a liquid phase if the gravitational force acting on the droplet or particle is greater than the drag force of the fluid flowing around the particle (sedimentation).
The solid phase is assumed to having the higher density and its percentage should not be more than 25% by volume. Because upto the concentration of 1.0 -2.0% (vol.), hindered settling is negligible but above 10% (approx.), hindered settling becomes more dominant. The particle size or droplet to be separated should be less than 500 µm. In this separation, the sedimentation velocity or terminal settling velocity or sedimentation rate can be estimated using:
......................................(5.9)
Above equation shows that if the density difference is negative. The particle will move upwards with a velocity ug toward the surface of the liquid i.e. the particle will move in the opposite direction to that in which the force of gravity. The sedimentation rate depends upon the particle density, density of continuous phase and the viscosity of the continuous phase.
While in case of batch operation (i.e. separation by gravity) time required to achieve complete separation for the particle which is initially at the surface must be sufficient enough to travel through the entire depth of the liquid (figure 5.5a). Time required for the particle sedimentation can be reduced by decreasing the depth of the chamber. Decrease in depth can be achieved by increasing the length of the chamber and keeping same residence time, but with decrease in depth operation may become uncontrollable.