Particle Aggregation by Coagulation and Flocculation

Abstract

This chapter considers particle aggregation. When agglomerated particles in a suspension increase in size they acquire greater sedimentation velocity essential to obtain a good separation by sedimentation. Two methods for increasing the size of solid particles are studied in this chapter, coagulation by reducing inter-particle electrostatic repulsion and flocculation by bridging particles with polymeric agents. Most mineral particles suspended in water in the neutral pH range have negative surface charges. Positive ions in solution are attracted and adsorbed at the negatively charged surface forming the so-called double layer with its Stern plane and the diffuse layer. The Zeta potential measures the difference in the electrical potential of the charged surface and the bulk of the solution in commercial instruments. If particle surfaces come close together, they attract each other by van der Waals force. If there is no counteracting force, the particles will coagulate and settle out of the suspension. The study of orthokinetic coagulation follows. It is generally accepted that polymers used as flocculants in mineral processing plants aggregate fine particle suspensions by bridging mechanisms. Such bridging links the particles into loose flocs and incomplete surface coverage, which ensures that there is sufficient unoccupied surface available on each particle for adsorption during collisions of chain segments attached to the particles. The description of flocs as fractal objects permits a better understanding of their behavior. Flocculation kinetics shows that a short and highly intensive mixing gives the best results for particle aggregation.