The effect of polyacrylamides on the flocculation and settling behaviour of kaolinite dispersions
Advancing the chemical engineering fundamentals
Interfacial & Colloidal Phenomena (T2-6P)
Keywords: kaolinite, Polyacrylamide, floc size, settling rate, floc density
M. S. Nasser* and A. E. James
School of Chemical Engineering and Analytical Science,
The University of Manchester, PO Box 88, Manchester M60 1QD, U.K.
Abstract
The effects of the surface charge and molecular weight of anionic and cationic polyacrylamide (PAM) on the floc size, effective floc density and settling rate of kaolinite suspensions has been investigated at pH 7. The experimental setup consists of two main parts: the flocculation and photographic units which are used to measure the floc size. At optimum polymer concentrations, the kaolinite floc sizes were larger and the settling rates were greater in the presence of anionic PAM than cationic PAM. Optimum flocculation for these anionic flocculants was linked to a small reduction in the magnitude of the zeta potential. In the case of cationic polymer, the optimum flocculation was linked to the reduction of the magnitude of the zeta potential to zero by charge neutralization. The difference in the floc sizes of the flocculated slurries may be attributed to floc structure-related adsorption. Cationic polymer chains adsorb via hydrogen bonding interactions between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, these electrostatic attractions between the positively charged polymer and the negatively charged kaolinite produce dense and close floc structures. For anionic PAM, however, although the adsorption still occurs through hydrogen bonding between the silanol and aluminol OH groups at the particle surface and polymer’s primary amide functional groups, the interactions appear to be weakened as a consequence of electrostatic repulsion between the negatively charged polymer and negatively charged kaolinite surface. These repulsive forces allow the polymer molecules to be extended and produce loops and tails, which lead to the formation of large open-structure flocs having low density.
Corresponding author. Tel: + 44 (0) 161 306 4368; Fax: + 44 (0) 161 306 4399;
E-mail: Mustafa.Nasser@postgrad.manchester.ac.uk
Presented Thursday 20, 13:30 to 15:00, in session Interfacial & Colloidal Phenomena (T2-6P).
