Gravity-induced flocculation of non-Brownian particles

The theoretical investigation on the rate of gravity-induced flocculation o f non-Brownian particles in quiescent media is presented in this paper. Bas ed on the method of trajectory analysis and incorporating gravitational and interparticle forces (as described by DLVO theory), the stability diagrams for this gravity-induced flocculation of dilute colloidal suspension are p rovided. Depending on the values of the dimensionless parameters of 1 (part icle size ratio), v (electromagnetic retardation parameter), kappa (ionic s trength), N-G (the ratio of gravitational forces to van der Waals attractiv e forces) and N-R (the ratio of electrostatic repulsion forces to van der W aals attractive forces), four distinct regions of flocculation are delineat ed in the stability diagram, (a) flocculation at the primary minimum of the : total interaction energy profile; (b) flocculation at the secondary minim um of the total interaction energy profile: (c) an extremely narrow region of simultaneously flocculation at both the primary minimum and the secondar y minimum of the total interaction energy profile; (d) a region of defloccu lation where colloidal suspension remains stable. Two critical gravitationa l parameters (N-G)(Ci) (i = 1, 2) are successfully applied to explain the o ccurrence of these four flocculation regions. Theoretical calculations of t he capture efficiencies corresponding to these four regions are also presen ted. II is found that the colloidal suspension can be unstable at low and h igh gravitational forces, but stable at intermediate values of gravitationa l forces if the electrostatic repulsion is significant. (C) 2001 Elsevier S cience B.V. All rights reserved.