Orthokinetic flocculation mechanism

Among the primary collision mechanisms is Brownian flocculation, also termed perikinetic flocculation, which dominates for submicrometer particles at relatively high number densities. The second principal collision mechanism is that of velocity gradient flocculation, also termed orthokinetic flocculation, which dominates for particles of micrometer size and larger. Evidently, the presence of any stabilizer in the solution will reduce the number of particle encounters and subsequent floccing, as discussed in the last section, resulting in slow flocculation. In our discussion we shall separate the transport and stability problems by assuming that the suspension is completely destabilized, so flocculation occurs on encounter rapid flocculation). Our concern here is with the effect of the particle motion alone on the number of encounters between the suspended particles. 

In addition to the above mechanisms orthokinetic flocculation may be induced due to the shear produced inside the porous media. This may encourage entrapment as collection efiddmcy usually inqiroves with increase in suspmded particle size. The collection mechanisms are described in greater detail below. 

Although reduction or elimination of the repulsion barrier is a necessary prerequisite of successful flocculation, the actual flocculation in such a destabilized suspension is effected by particle—particle collisions. Depending on the mechanism that induces the collisions, the flocculation process may be either perikinetic or orthokinetic. 

As an example let us consider the orthokinetic (shear-induced) deposition of colloids on fibers, induced by polymers or polyelectrolytes. Usually, the colloids are stable prior to the addition of polymer. The pulp fibers form floes also in the absence of polymers due to mechanical entanglements. These floes can be broken up by high shear, resulting in a dynamic equilibrium between floe formation and break-up. The higher the shear, the fewer is the number of fibers in floes. Polymers can shift this dynamic equilibrium towards an increased flocculation [12]. Usually at various locations on a paper machine the shear is sufHdently high to break up most fiber floes, which reform in regions of lower shear. Here we will assume that fibers are well-dispersed due to high shear. 

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