12 - SEDIMENTATION AND CONSOLIDATION

Abstract

This chapter discusses the sedimentation and consolidation of particles in multiphase flow. The sedimentation of particles is a physically simple, common example of multi-phase flow. To obtain the concentration of particles in the sedimentation column, the particle balance must be supplemented by a momentum balance for the velocity ν . In the case of slow, dilute sedimentation of noncolloidal particles, the particle velocity very quickly becomes the velocity of fall of a single particle in an infinite Newtonian fluid. Hence, there exists a constant settling zone having a constant particle concentration. In the top zone, this concentration is zero as it has been implicitly assumed that zero time in this problem is the time when the particles acquired the terminal velocity and just began to move away from the top boundary. In the free settling zone, the particle concentration stays at its initial value. This elementary analysis can be applied to the settling of particles having a wide particle-size distribution. If some particles are lighter than the fluid, they will move to the top, while the denser particles descend. The principal effect observed is used in the separation of minerals by froth flotation, which works on this principle, where air bubbles, acting as particles, carry fine tailings to the top of a flotation unit. The sediment concentration varies from a maximum at the bottom of the column to that present initially in the free settling zone.