The effect of polyacrylamide charge density and molecular weight on the flocculation and sedimentation behaviour of kaolinite suspensions

The effects of the surface charge and molecular weight of anionic and cationic polyacrylamide (PAM) on the surface chemistry, settling rates, floc sizes and sediment bed compactness of kaolinite suspensions has been investigated at pH 7. At optimum polymer concentrations, the kaolinite floc sizes were larger and the settling rates 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 results show that the magnitude of the compressive yield stress Py(ϕ) is strongly dependent upon the floc structure; with greater compressive yield stress being observed for the cationic PAM than for the anionic PAM. The difference in the compression sensitivity 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 strong and less compressible 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 less resistance to compression loads and, subsequently, produce compact sediments having greater gel points ϕg following the application of some compression load. It is found that the compressive yield stress and gel point are important factors that should be considered when selecting the type of polyelectrolyte for use in a particular solid liquid separation. A strong correlation between the polymer type and flocculation, compression sensitivity of flocs and sedimentation behaviour of kaolinite is established.