Sedimentation of Small Particles

Abstract

This lecture is too theoretical, so let’s start with some practical examples. The sedimentation of particles is one of the basic techniques available in chemical engineering to unmix things — precisely the opposite of the subject of this volume! Sometimes the purpose of the sedimentation is the recovery of the liquid as in the purification of water in sewage treatment plants, while sometimes it is the collection of the particles as in the recovery of cream (fat globuls) from milk. Different types of particles can be separated from one another as in the enrichment of mineral ores. In the atmosphere rain drops, dust and pollution in ash fall out under the action of gravity. Where a fresh water river meets the saline sea water, the repulsive electrical forces between clay platelets become screened by the counter-ions from the salt, so that the platelets can aggregate under van der Waals forces, and the aggregates sediment thus silting up the estuary with mud. In all these applications, one is interested in the time for the sedimentation, and also the structure of the sediment.