Gravity separations free-settling velocity

Gravity separations depend essentially on rite density differences of rite gas, solid, or liquids present in the mix. The particle size of the dispersed phase and the properties of die continuous phase are also factors with rite separation motivated by die acceleration of gravity. The simplest representation of this involves rite assumption of a rigid spherical panicle dispersed in a fluid with rite terminal or free-settling velocity represented by... 

Elutriation differs from sedimentation in that fluid moves vertically upwards and thereby carries with it all particles whose settling velocity by gravity is less than the fluid velocity. In practice, complications are introduced by such factors as the non-uniformity of the fluid velocity across a section of an elutriating tube, the influence of the walls of the tube, and the effect of eddies in the flow. In consequence, any assumption that the separated particle size corresponds to the mean velocity of fluid flow is only approximately true it also requires an infinite time to effect complete separation. This method is predicated on the assumption that Stokes law relating the free-falling velocity of a spherical particle to its density and diameter, and to the density and viscosity of the medium is valid... 

Equations 20 and 21 apply only to free, particulate separation not affected by flocculation or aggregation and for all particles having the same density. Under gravity field, few shear forces exist and have little effect on floes. Flocculation may be strong to change behavior of settling suspension to that of zone settling (i.e., all particles in slurry settle en masse at the same velocity irrespective of their size). 

---