Interparticle bridging mechanisms

The interparticle bridging mechanism was affected by the charge [7], adsorption sites [5] of particles extending in the flocculation system, the physical characteris-... 

Polyacrylamide (30% hydrolysed) is an anionic polymer which can induce flocculation in kaolinite at very low concentrations. Restabilisation occurs by overdosing, probably by the mechanism outlined in Fig. 7.32. Dosages of polymer which are sufficiently large to saturate the colloidal surfaces produce a stable colloidal system, since no sites are available for the formation of interparticle bridges. Under certain conditions, physical agitation of the system can lead to breaking of polymer-suspension bonds and to a change in the state of the system. 

Flocculation of alumina suspensions obtained by the sequential addition of polystyrene sulfonate (M j, = 4600) and cationic polyacrylamide (M, = 4,000,000) at pH 4.5 is compared in Figure 7.33 with that obtained using single polymers. While the anionic polystyrene sulfonate had only a minor effect, cationic polyacrylamide did not produce any flocculation. However, when used together, both polymers adsorb completely. This coadsorption is attributed to the interaction of complexes between cationic polyacrylamide and the polystyrene sulfonate at the solid-liquid interface. The mechanism of the superior flocculation obtained with the dual polymer system is illustrated schematically in Figure 7.34. The anionic polystyrene sulfonate adsorbs on alumina surface and acts as an anionic anchor for the adsorption of the long-chain cationic polymer, which ultimately provides interparticle bridging and excellent flocculation. 

Nystrom et al. [92, 93] correlated the observed flocculation behavior of calcium carbonate, induced by mixtures of cationic starch and anionic poly(sodium acrylate) (NaPA) at various electrolyte concentrations, with the complex properties. A strong correlation exists between the properties of the PEL mixture, primarily the amount of complexes formed, and the flocculation behavior. Several mechanisms are involved in this flocculation process induced by the two polymers. However, interparticle bridging by the PECs and charge neutralization induced by the deposition of the complexes were found to be the main reasons for the enhanced flocculation. 

Moisture can significantly affect loose materials, particularly their flowability. Low temperatures, particle bridging, and caking can alter interparticle void fractions and cause dramatic changes in bulk density. Moisture becomes bound to solids because of mechanical, physicochemical, and chemical mechanisms. Moisture retained... 

Mechanical compressibility in compression tests is, at least partly, a result of the collapse of an initial open bed structure (supported by cohesive interparticle forces or interparticle liquid bridges), which is totally and irreversibly destroyed. In the case of vibrational tapping tests, the openness of the bed structure can be recovered at least to some extent. Thus, different mechanisms yield different tendencies in the compressibility measured by tapping and mechanical compressibility. 

Weakly flocculated systems have been studied by Buscall et al. (165), Heath and Tadros (166), Goodwin et al. (167), Patel and Russel (168, 169), Otsubo (170-172), Buscall et al. (51,173), Woutersen and de Kruif (174), and Nakai et al. (175). The weak flocculation can be obtained by several means, including secondary-minimum flocculation, depletion flocculation, polymer-bridging flocculation, and incipient flocculation. Details of the various mechanisms of interparticle attraction can be found in Russel et al. (27) and Somasundaran and Yu (176). Normally, flocculated systems have a solid volume fraction of no less than 0.2. When 0 < 0.2, an attractive system will settle down quickly. When 0 > 0.2, a flocculated suspension can be maintained easily for a period of time for a steady shear measurement to be completed. 

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