Flocculation charge patch mechanism

The adsorption of polymers to the surface of particles leads to the two fundamental mechanisms of flocculation by polyelectrolytes. These are commonly referred to as the electrostatic or charge patch mechanism and the bridging mechanism and may act individually or, in some instances, simultaneously. 

Figure 3.7 Schematic representation of the charge patch mechanism of flocculation (cationic polymer and negatively charged particles in the example shown).

Figure 7.5 Typical flocculation, mobility and adsorption data for flocculation by the patch charge neutralisation mechanism.

Simple Charge Neutralization and Charge Patch Neutralization Oppositely charged polyelectrolytes reduce the particle surface charge density such that particles may approach each other sufficiently closely so that the attractive van der Waals force becomes effective. Flocculation caused by this mechanism should not be sensitive to the molecular weight of the polymer. 

A characteristic difference between charge neutralization and patching is that the rate of coagulation for the former mechanism increases with electrolyte concentration. Once an optimal electrolyte concentration has been attained, however, the rate of flocculation by patching will decrease with electrolyte content due to the fact that the electrolyte cations will force the polymer from the particle surface. 

Stock composition, kinetics of adsorption and hydrodynamic shear dictate the point at which a cationic polymer is added to a papermaking furnish in order to induce flocculation. Flocculation of cellulose fibers in turbulent flow proceeds very rapidly and is completed in less than two seconds.120-123 Flocks form due to charge interactions through a patch-type or a bridging-type mechanism. However, these flocks will be sensitive to shear force and deflocculation and reflocculation might occur. 

Charge neutralization (simple and/or patch) is often the mechanism for flocculation by low molecular-weight polyelectrolytes. 

Figure 7.14. Sequence showing schematically (a) charge-stabilized particles, (b) particles flocculated by the patch flocculation mechanism, and (c) particles restabilized by steric repulsive (chain overlap) interactions at higher levels of polymer surface coverage...

The measurement and evaluation of flocculation state is typically carried out over 10 min by means of programmed dosing, in which the flocculation value F is recorded during a stepwise increase in flocculant dosing. Figure 4 shows an example of a flocculation curve. The amount of flocculant required for optimal flocculation is that where F reaches an asymptotic limit (bridging mechanism) or a maximum (patch charge mechanism). 

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