ABSTRACT
This chapter is concerned with processes in which small particles in water form larger aggregates that can be more easily removed by physical separation processes such as sedimentation, flotation, and filtration. Assuming that the particles are initially stable in the colloidal sense (see Chapter 4), then there are two essential steps in the aggregation process, which are shown schematically in Figure 6.1:
• Destabilization of particles • Collisions of particles to form aggregates
We shall be dealing mainly with particles that are stable by virtue of their surface charge and hence electrical double-layer repulsion. In that case, destabilization involves either an increase in ionic strength or a neutralization of the particle charge, as discussed in Chapter 4. Adding salts simply to increase ionic strength is almost never a practical option and other additives have to be used, as described later in this chapter. The purpose of the destabilization step is to make the collision efficiency, α, as high as possible, ideally with α = 1, so that every collision leads to aggregation.
