Fundamentals of flocculation

Flocculation (aggregation) of particles is a vital stage in many solid‐liquid separation operations. The process usually involves some form of chemical destabilization and a step in which particles collide and form aggregates (flocs). Destabilization may involve simply overcoming any repulsive forces between particles, as with simple salts, “bridging” between particles by polymeric flocculants, or the precipitation of metal hydroxides leading to “sweep flocculation” as with aluminum and iron salts. Recent work on interparticle forces, the action of polymeric flocculants, and the nature of species produced by hydrolyzing metal salts is reviewed and the relevance to practical flocculation processes is discussed. The kinetics of particle aggregation, aggregate size distribution, and the morphology of aggregates have received a great deal of fundamental attention recently. Most of this work is concerned with rather ideal systems, but conclusions of much wider applicability can be drawn. In particular, the relationship between floc density and floc size, a subject of much empirical observation, can be rationalized on the basis of these new ideas.