The charge patch mechanism

Figure 3.7 Schematic representation of the charge patch mechanism of flocculation (cationic polymer and negatively charged particles in the example shown).

The adsorption of polymers to the surface of particles leads to the two fundamental mechanisms of flocculation by polyelectrolytes. These are commonly referred to as the electrostatic or charge patch mechanism and the bridging mechanism and may act individually or, in some instances, simultaneously. 

Figure 7.5 Typical flocculation, mobility and adsorption data for flocculation by the patch charge neutralisation mechanism.

For filter belt presses and centrifuges, resistance to shear and mechanical pressure is the most important parameter. In general, floes produced by charge patch neutralization are stronger than those produced by inorganic salts alone. 

Simple Charge Neutralization and Charge Patch Neutralization Oppositely charged polyelectrolytes reduce the particle surface charge density such that particles may approach each other sufficiently closely so that the attractive van der Waals force becomes effective. Flocculation caused by this mechanism should not be sensitive to the molecular weight of the polymer. 

The 29si MAS NMR spectra suggest the occurrence of substitution of Si atoms for P and A1 atom pairs, a mechanism which doesnot create net negative framework charges. However, if present, the silica patches cannot be large as it is expected that micropores in siliceous domains of the crystals are filled with organics. 

The surface coagulation mechanism appears to be associated with at least partial de-wetting of the particle at the interface, either as a consequence of desotption of the stabilizing surfactant or as a consequence of a patchy surface with some of the patches being hydrophobic. FIFE particles, which have a low surface energy (c. 12 mN m ) and frequently a low concentration of charged... 

The elecirochemical mechanism, also called the mixed-potential mechanism [477], assumes charge transfer within a particle from the cathodic patch, at which oxygen is reduced, to the anodic one, at which the sulfide itself and/or xanthate anion are oxidized. This mechanism describes a broad spectrum of interfacial phenomena involving, as an intermediate step, a redox reaction in which the anodic and cathodic processes are spatially separated. Some examples of this include electrocatalytic chemisorption of xanthate and synthesis of dixanthogen and precipitation of xanthate-metal complexes (nncleation of a microphase of metal xanthate). In the latter reaction, the anodic sulfide dissolution is initiated with the ionization of surface metal atoms, and the metal ions thus produced on the surface are transferred into aqueous solution to form hydrated metal ions or metal-ion complexes associated with anions [478]. The ionization of surface metal atoms is an electrochemical oxidation, whereas the hydration or complex-ation of metal ions is a chemical process (an acid-based reaction). 

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