Structures in Flocculation

Clay colloids provide a good example of the kinds of structures that can be formed upon flocculation. The association of plate-like clay particles is complicated by the fact that minerals such as montmorillonite, illite, and kaolinite can exhibit different surface charges at different particle surfaces. 

The rates of coagulation for the different types of association differ from each other. The EE aggregation is generally the fastest, since it involves the largest collision diameter (the platelet diameter). The FF aggregation is generally the slowest, since it involves the smallest collision diameter (the platelet thickness). Also, the three types of associations will not necessarily occur together or at the same rates when the day flocculates [49]. 

Example 5.4 The clay minerals are composed [46, 89] of sheets of tetrahedrally coordinated silica and sheets of octahedrally coordinated alumina or magnesia. These sheets occur stacked upon one another forming plate-like layers or particles. Montmorillonite, for example, clay occurs as relatively broad, thin platelets, typically about 0.3 pm across and about 2 nm thick. When dispersed in aqueous solutions of near-neutral pH to alkaline pH, the particles carry a net negative 

The different kinds of particle associations in aggregation have an important consequence in that they can also strongly influence sediment volumes. The 

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R Hogg, R C Klimpel. D T Ray. Agglomerate structure in flocculated suspensions and its effect on sedimentation and dewatering. Minerals and Metallurgical Processing May (1987) 108... 

Kose A and Flachisu S 1976 Ordered structure in weakly flocculated monodisperse latex J. Colloid Interface Sol. 55 487-98... 

Plastic fluids are Newtonian or pseudoplastic liquids that exhibit a yield value (Fig. 3a and b, curves C). At rest they behave like a solid due to their interparticle association. The external force has to overcome these attractive forces between the particles and disrupt the structure. Beyond this point, the material changes its behavior from that of a solid to that of a liquid. The viscosity can then either be a constant (ideal Bingham liquid) or a function of the shear rate. In the latter case, the viscosity can initially decrease and then become a constant (real Bingham liquid) or continuously decrease, as in the case of a pseudoplastic liquid (Casson liquid). Plastic flow is often observed in flocculated suspensions. 

The typical viscous behavior for many non-Newtonian fluids (e.g., polymeric fluids, flocculated suspensions, colloids, foams, gels) is illustrated by the curves labeled structural in Figs. 3-5 and 3-6. These fluids exhibit Newtonian behavior at very low and very high shear rates, with shear thinning or pseudoplastic behavior at intermediate shear rates. In some materials this can be attributed to a reversible structure or network that forms in the rest or equilibrium state. When the material is sheared, the structure breaks down, resulting in a shear-dependent (shear thinning) behavior. Some real examples of this type of behavior are shown in Fig. 3-7. These show that structural viscosity behavior is exhibited by fluids as diverse as polymer solutions, blood, latex emulsions, and mud (sediment). Equations (i.e., models) that represent this type of behavior are described below. 

Dirican, C. "The Structure and Growth of Aggregates in Flocculation" M.S. Thesis, The Pennsylvania State University, University Park, PA, 1981. 

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... 

Figure 6.13 Schematic representation of the disruption of structure of flocculated particles accompanying an increase in flow rate. From Yariv and Cross [49].

The spontaneous shrinking of a colloidal dispersion due to the release and exudation of some liquid frequently occurs in gels and foams but also occurs in flocculated suspensions. Mechanical syneresis refers to enhancing syneresis by the application of mechanical forces. Micro-syneresis is a special case of syneresis in which the polymer molecules cluster together while retaining some of the original bulk gel structure. This process creates regions of free liquid within the gel network. 

The second model introduced by Hunter and cowor)ters (20,21) is the elastic floe model. In this case, the structural units (which persist at high shear rates) are assumed to be small floes of particles (called floccules) which are characterised by the ability of the particle structure to trap some of the dispersion medium. In this energy dissipation is considered to arise from two processes, namely the viscous flow of the suspension medium around the floes (which are the basic flow units) and the energy involved in stretching the floes to brealc the floe doublets apart so that the amount of structure in the system is preserved inspite of the floc-floc collision. This model gives the following expression for the yield value. 

Redispersion of the flocculate and other evidences for the hydrophilic character of the support coated with the adsorbed surfactant in the neighbourhood of the cmc indicate that bilayer coverage represents complete saturation of the surface. Commonly, the term bilayer is applied to an adsorbed structure in which the surfactant molecules are oriented perpendicular to the surface and fully extended [5,9,20,37,81,89]. The hydrocarbon tails of both layers form a hydrophobic core between the heads. At both sides counterions accumulate between the ionic head-groups. The result looks like a lamellar micelle. For certain physical regimes, the adsorbed amount is only a fraction of what is expected for a tightly packed bilayer [37,48] the structure which best fits the experimental data can... 

Spicer P.T., Pratsinis S.E., Raper )., Amae R., Bushell G., Maestehs G., Eflect of shear schedule on particle size, density, and structure during flocculation in stirred tanks, Powder Technol. Laus. 97 (1998) 1, p. 26-34... 

Figure 8.10. The boundary between structural stability (flocculation zone) and instability (dispersion zone) as influenced by chemical factors in soils.

The restoring force for a dispersion to return to a random, isotropic situation at rest is either Brownian (thermal fluctuations) or osmotic. The former is most important for submicrometer particles and the latter for larger particles. Changing the flow conditions changes the structure, and this leads to thixotropic effects, which are especially strong in flocculated systems. 

Wessel and Ball " and Kanai et al. studied in detail the effects of shear rate on the fractal structure of flocculated emulsion drops. They showed that the size of the floes usually decreases with the increase of the shear stress often the floes are split to single particles at high shear rates. As a result, the viscosity decreases rapidly with the increase of the shear rate. 

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