Coagulation-flocculation, interaction with

The nature of chemical coagulants are such that the macrofloc may possess certain charges for example lime (CaO), alum (A1203) and flocculating polyvalent cations cany positive charges, which interact with proteins. The interactions are simply illustrated in Figure 7.6. 

Reactions of dissolved species with particulate and colloidal suspended matter include adsorption/desorption, complexation, ion-exchange, precipitation/dissolution, coprecipitation during coagulation and flocculation (Morgan, 1966 Stumm and Morgan, 1981 Parks, 1975). These processes are particularly important at the land-sea boundary in estuaries (Duinker, 1980 Martin et al., this volume). The interaction with particles > 0.45 ym is not discussed here. 

Interaction of Coagulation-Flocculation with Separation Processes... 

Particle-particle interaction is central to a wide range of engineering applications and processing industries. Examples include coagulation, flocculation, dispersion, emulsification, and froth flotation. In these applications, the particle size is small, and the overall particulate behavior is determined by forces associated with the surface properties rather than those related to mass or volume. The surface properties of a particle in a liquid medium are the result of a complex interaction between molecules, atoms, and ions at the particle surface and in the surrounding liquid. If a number of particles are present, interactions also take place between particles at short separation distances, and it is this interaction that is of most interest as it can determine the overall stability or instability of dispersions and/or suspensions. 

Gregory (320) concluded from his studies of the coagulation of latex particles that flocculation and restabilization can be explained simply in terms of charge neutralization and charge reversal. It can only be concluded that the interaction with latex particles is different from that with silica particles. 

Flocculation. The interaction of the cationic PEIs with anionic substrates leads to substrate flocculation. AppHcations of this property include the coagulation of latex (434), commercial appHcation in effluent treatments (435—437), and stabiHzation of highly loaded coal—water mixtures in mining (438). 

The increase in the rheological parameters, ri0> G0 and 1(5 with reduction in surface coverage points towards an increase in particle interaction. This could be the result of either flocculation by polymer "bridging" (which is favourable at coverages <0.5) or as a result of coagulation due to the van der Waals attraction between the "bare" patches on the particles. In the absence of any quantitative relationship between interaction forces and rheology, it is clearly difficult... 

Ideally, it would be desirable to be able to develop quantitative expressions for the interaction energies so that we can deal with coagulation or flocculation, at least in the case of fairly dilute dispersions, the way we did in Sections 13.3-13.4 for electrostatic stabilization. It is possible to develop approximate expressions for interaction energy due to various individual effects such as osmotic repulsion, attraction or repulsion due to the overlap of the tails of the adsorbed (or grafted) polymer layers, interaction of the loops in the layers, and so on (see Fig. 13.15). However, the complicated nature of polymer-induced interactions makes these tasks very difficult. In this section, we merely illustrate some of the issues that need to be considered in developing a fundamental quantitative understanding of polymer-induced forces. In Section... 

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