Aggregates flocculation process

Soluble polymers are widely used to control the state of dispersion of fine-particle suspensions. Depending on the polymer, and how it is applied, they can serve to enhance stability (dispersants) or to promote aggregation of the particles (flocculants). The topics covered in this chapter are intended as an overview of the use of polymers for stability control in mineral-particle suspensions with particular emphasis on flocculation processes. A brief discussion of stabilisation by polymers is included for completeness. 

As lithogenic and biogenic particles are delivered to the coastal zone (via rivers and estuaries), they are exposed to many dynamic processes (e.g., aggregation, flocculation, desorption) that result in steep biogeochemical gradients on the continental shelf. 

See Chapter V of R. Jullien and R. Botet, Aggregation and Fractal Aggregates, World Scientific, Singapore, 1987, for a description of cluster-cluster flocculation processes in the context of computer models. 

Different concentration limits of the filler arise from the CCA concept [22]. With increasing filler concentration first an aggregation limit O is reached. For >+, the distance of neighboring filler particles becomes sufficiently small for the onset of flocculation and clusters with solid fraction A are formed. Dependent on the concentration of filler particles, this flocculation process leads to spatially separated clusters or, for 0>0, a through going filler network that can be considered as a space-filling configuration of fractal CCA-clusters. The different cases for spherical filler particles are shown schematically in Fig. 1. 

Aggregation The process of forming a group of droplets that are held together in some way. For emulsions, this process is sometimes referred to as coagulation or flocculation. 

Flocculation See Aggregation. The products of the flocculation process are referred to as floes. 

The flocculation process is less investigated with respect to emulsions, especially water-in-oil emulsions. According to the DLVO theory [48] the aggregation stability of a disperse system is determined by the sum of energy of ion-electrostatic repulsion (Uj) and Van-der-Waals attraction (Um)... 

It is well known that polymers may serve as bridges between colloidal particles to form floes nonetheless, very little quantitative information is available about their structure and formation, despite the fact that they play key roles in environmental systems [1], Particles may not only be bridged by polymers, but may also facilitate the formation of larger aggregates due to the adsorption of several polymer segments on the same particles. This process can be seen as an example of the CCA model, where polymer conformation, reactivity and total length play important roles. Computer models once again constitute a valuable tool that allows for predictions of flocculation processes. 

Both experiments and computer simulations demonstrated the fractal character of the mixed floes. The optimal schizophyllan biopolymer/hematite concentration ratio obtained by simulation was smaller than that observed in laboratory experiments. The shift in the optimal dose was mainly attributed to a higher than predicted affinity of hematite for the schizophyllan aggregates present in the initial solution in addition to the presence of a large proportion of chains that did not participate in the flocculation process [12]. 

One important conclusion can be made the spatial disposition of particles in floes results from the biopolymer/particle concentration ratio in addition to the biopolymer conformations. In particular, flocculation processes with rigid biopolymers resulted in the formation of a regular network characterized by fractal dimensions that were higher than those obtained on the basis of the classical DLCA or RLCA models (Figure 4.16). Despite the highly loose structure of the aggregate that was formed, the increase in fractal dimension reflected the high order of particles in such networks. 

Stoll, S. and Buffle, J. (1996). Computer simulation of bridging flocculation processes the role of colloid to polymer concentration ratio on aggregation kinetics. J. Colloid Interface ScL, 180,548-563. 

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