Flocculation structures

The method is apphcable for unflocculated pulps or those in which the ionic characteristics of the solution produce a flocculent structure. If polymeric flocculants are used, the floccule size will be highly dependent on the feed concentration, and an approach based on the Kynch theoiy is preferred. In this method, the test is carried out at the expec ted feed solids concentration and is continued until underflow concentration is achieved in the cyhnder. To determine the unit area, Talmage and Fitch (op. cit.) proposed an equation derived from a relationship equivalent to that shown in Eq. (18-45) ... 

Air bubbles becoming physically trapped in the insoluble solids original or flocculated structure... 

Air bubbles being chemically adsorbed to the insoluble solids in their original form or their flocculated structure... 

For the intepretation of the rheological results, using the elastic floe model, it is necessary to have a model for the flocculated structure. For the present case, flocculation probably takes place by interpenetration of PVA tails under worse than 9- conditions for the chain. A typical floe may be assumed to consist of strings of particles linked together in a more-or-less three-dimensional network. The compactness of the floe (as measured by Cpp) is related to its strength by the number of chains, n, which pass through unit cross sectional area of the floe (29,31). n can be calculated from the total number of bonds per floe (36), i.e. 

The interaction between gas bubbles and oil drops has been described as follows (I) absorption of an oil drop to a gas bubble due to precipitation of a bubble on (he oil surface and collision between the drop and bubble (2) entrapment of a gas bubble in a flocculated structure of oil drops as it rises and (3) absorption of bubbles into a flocculated structure as it forms. 

Wu G, Asai S, Sumita M, Hattori T, Higuchi R, Washiyama J (2000) Estimation of flocculation structure in filled polymer composites by dynamic rheological measurements. Colloid Polym Sci 278 220-228... 

Examples include oil-well drilling muds, greases, lipstick, toothpaste, and natural rubber polymers. An illustration is provided in Figure 6.13. Here, the flocculated structures are responsible for the existence of a yield stress. Once disrupted, the nature of the floe break-up process determines the extent of shear thinning behaviour... 

Clearly, depends on the volume fraction of the dispersion, as well as the particle size distribution (which determines the number of contact points in a floe). Therefore, for quantitative comparison between various systems, it must be ensured that the volume fraction of the disperse particles is the same, and that the dispersions have very similar particle size distributions. also depends on the strength of the flocculated structure - that is, the energy of attraction between the droplets - and this in turn depends on whether the flocculation is in the primary or secondary minimum. Flocculation in the primary minimum is associated with a large attractive energy, and this leads to higher values of when compared to values obtained for secondary minimum flocculation (weak flocculation). For a weakly flocculated dispersion, as is the case for the secondary minimum flocculation of an electrostatically stabilised system, the deeper the secondary minimum the higher the value of (at any given volume fraction and particle size distribution of the dispersion). 

St e 3 Finally, at very high electrolyte concentrations, interlayer spacings diminish even further and extensive flocculated structures build up, again raising the viscosity (Figure 8.6). 

However, if the ionic strength is enhanced, the thickness of the electrolyte cushion is rendered thinner. A larger fraction of particles may then collide per unit time, leading to a coagulation of the particles to a dense sediment. For a O.l-mol/dm 1 1 electrolyte solution, the repulsive electrostatic barrier (Tables 8.3 and 8.4) may be considered to be effectively removed and the particle coagulation is controlled by diffusion alone. Sometimes a balance is reached at intermediate distances (3 to 5 nm), which is characterized by a weak negative minimum. Then, the particles form a loose flocculated structure that is easily redispersed. To a great extent the association kinetics follows the same principles as for the ions discussed above, but at a slower rate. - - " ... 

Direct observation of particles in suspension using microscopic techniques is especially useful because it makes it possible to characterize interparticle associations and flocculated structures. Suspension bulk behavior can often be better understood when related to microscopic observations of the interparticle structuring. 

The crucial factor seems to be the formation of the (firmly) adsorbed phase (i.e. the reduction in flowable volume ), the phase separation ( seams ) and flocculation structures (chain formation) together with the interaction between the adsorbed phase and the matrix. The formation of the seams and chains, which are extremely thin and very extended structures that have the effect of increasing viscosity, appears to be particularly important. At the same time it must be remembered that the dispersed phase does not interact directly with the matrix, but only through the shell of matrix material firmly adsorbed on it (i.e. in a very intense fashion). 

Because of these considerations it is often necessary to deliberately reflocculate the particles but now in a controlled fashion. This can be brought about by the addition of a low level of an adsorbed bridging polymer (see above) or else the use of a fully dissolved nonattached polymer that brings about depletion flocculation. This latter phenomenon occurs due to the exclusion of dissolved polymer from between the particles, resulting in a net attraction between particles. The use of both mechanisms of polymer induced flocculation have been employed by Tadros et al. to promote resuspendability of dispersed particles by forming a loosely flocculated structure. 

Figure 13.8 Depletion flocculation of a latex. Left the water-soluble polymer concentration is depleted in a region around each particle (soluble polymer coils ate shown only near the region of depletion layer overlap for clarity). Right qrstem flee energy is minimized when aqueous phase voIuiik unavailable to the soluble polymo is minimized, in a flocculated structure...

In some colloidal dispersions, the shear rate (flow) remains at zero until a threshold shear stress is reached, termed the yield stress (ry), and then Newtonian or pseudoplastic flow begins. A common cause of such behaviour is the existence of an inter-particle or inter-molecular network, which initially acts like a solid and offers resistance to any positional changes of the volume elements. In this case, flow only occurs when the applied stress exceeds the strength of the network and what was a solid becomes a fluid. Examples include oil well drilling muds, greases, lipstick, toothpaste and natural rubber polymers. An illustration is provided in Figure 6.13. Here, the flocculated structures are responsible for the existence of a yield stress. Once disrupted, the nature of the floe break-up process determines the extent of shear-thinning behaviour as shear rate increases. 

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