INVESTIGATION OF THE FLOCCULATION OF COLLOIDAL SUSPENSIONS BY CONTROLLING ADSORBED LAYER MICROSTRUCTURE AND POPULATION BALANCE MODELLING P. SOMASUNDARAN 1 and V. RUNKANA 1,2 1 NSF Industry/University Cooperative Research Center for Advanced Studies in Novel Surfactants, School of Engineering and Applied Science, Columbia University, New York, USA 2 Tata Research Development and Design Centre, Pune, India T he stability of colloidal suspensions is determined primarily by the interactions among suspended particles, which in turn depends on pH, electrolyte concentration, tempera- ture and so on. In most practical systems, flocculation or stabilization is controlled by adsorbing polymers, surfactants or their mixtures. In this paper, the role of adsorbed layer microstructural properties, particularly polymer conformation at solid – liquid interface, in controlling stability and efficiency of flocculation is examined. When polymers are used, their conformation can be manipulated by changing solution conditions such as pH and/or by the addition of a secondary polymer or surfactant. A multi-pronged approach involving the use of fluorescence, ESR, Raman and NMR spectroscopic techniques along with measure- ments of surface charge and hydrophobicity was employed to explore the structure of the adsorbed layer. A detailed population balance model for coagulation and flocculation of col- loidal suspensions by inorganic salts and polymers is then presented incorporating the modern theories of surface forces. In particular, the classical DLVO theory is modified for flocculation by polymers and integrated in a population balance framework for the kinetics of flocculation. The open and irregular structure of flocs is accounted for by embedding the mass fractal dimension of flocs in the model. For demonstration, the evolution of mean floc size with time is simulated for flocculation of hematite and polystyrene latex suspensions. The model predictions are in reasonable agreement with experimental data. As it is computationally less intensive, the proposed model can be utilized for online optimization and control of solid – liquid separation processes that are widely encountered in water treatment, mineral processing, waste management, and so on. Keywords: colloidal suspensions; flocculation; polymer; conformation; population balance modelling; surface forces. INTRODUCTION Flocculation of colloidal suspensions is a critical step in solid – liquid separation operations in many industrial pro- cesses dealing with particulate solids, for example, pulp and papermaking (Pelton, 1999), mineral processing (Somasundaran et al., 1996) and water treatment (Thomas et al., 1999). Inorganics, polymers/polyelectrolytes and surfactants are commonly used as coagulants or flocculants. Flocculation by polymers is a complex phenomenon, which involves several steps or sub-processes such as colloid aggregation, polymer adsorption and reconformation, and floc fragmentation and restructuring, occurring sequentially or concurrently. Nonetheless, polymers and polyelectro- lytes are employed as flocculating agents in industrial oper- ations, presumably because it is possible to control stability as well as rate of flocculation of flocculating suspensions by controlling polymer adsorption and conformation at the solid-solution interface by manipulating variables such as pH, ionic strength, polymer concentration and temperature. Flocculation by adsorbing polymers occurs by any one or more of the three well-known mechanisms: simple charge neutralization, charge patch neutralization and polymer bridging (Levine and Friesen, 1987). Polymer conformation at the solid – liquid interface is one of the most critical para- meters in stabilization and flocculation. The conformation  Correspondence to: Professor P. Somasundaran, NSF Industry/University Cooperative Research Center for Advanced Studies in Novel Surfactants, School of Engineering and Applied Science, Columbia University, New York, NY 10027, USA. E-mail: ps24@columbia.edu 905 0263–8762/05/$30.00+0.00 # 2005 Institution of Chemical Engineers www.icheme.org/journals Trans IChemE, Part A, July 2005 doi: 10.1205/cherd.04345 Chemical Engineering Research and Design, 83(A7): 905–914