Flocculation pseudoplastic

This equation is based on the assumption that pseudoplastic (shear-thinning) behaviour is associated with the formation and rupture of structural linkages. It is based on an experimental study of a wide range of fluids-including aqueous suspensions of flocculated inorganic particles, aqueous polymer solutions and non-aqueous suspensions and solutions-over a wide range of shear rates (y) ( 10 to 104 s 1). 

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. 

This model was introduced by Neville and Hunter (13,14) for the case of sterically stabilized dispersions which have undergone reversible flocculation. It is assumed that the major contribution to the excess energy dissipation in such pseudoplastic systems comes from the need to provide energy from the shear field to separate contacting particles. Under these conditions, the extrapolated yield value is given by the expression (13,32,33),... 

The turbidity methods are unsatisfactory for lar particles and/or high particle number concentrations when multiple scattering effects intrude. As shown by Hunter et al. (1975), rheological measurements can then be used to detect flcK ulation. Stable dispersions exhibit an Ostwald-type flow curve whereas flocculated systems behave in a pseudoplastic fashion. Bocculation is thus accompanied by a large increase in the Bingham yield value, t, of the dispersion (see Fig. 5.4). 

In some colloidal dispersions, the shear rate (flow) remains at zero until a threshold shear stress is reached, termed the yield stress (ry), and then Newtonian or pseudoplastic flow begins. A common cause of such behaviour is the existence of an inter-particle or inter-molecular network, which initially acts like a solid and offers resistance to any positional changes of the volume elements. In this case, flow only occurs when the applied stress exceeds the strength of the network and what was a solid becomes a fluid. Examples include oil well drilling muds, greases, lipstick, toothpaste and natural rubber polymers. An illustration is provided in Figure 6.13. Here, the flocculated structures are responsible for the existence of a yield stress. Once disrupted, the nature of the floe break-up process determines the extent of shear-thinning behaviour as shear rate increases. 

Flocculated slurries are encountered in flotation cells circuits, thickeners, and various processes in mineral extraction plants. With the formation of floes, the slurry may develop an internal structure. This structure may develop properties leading to a non Newtonian flow, shear thinning behavior (pseudoplastic), and sometimes thixotropic time dependent behavior. When shear stresses are applied to the slurry, the floe sizes may shrirrk and be come less capable of entrapping the carrier slurry. At higher shear stresses, the floes may shrink to the size of particles, and the flow may lose its non Newtonian behavior. 

Rheological characterization of the paste is to measure the relationship between shear stress and shear rate varying harmonically with the time, indicating the level of interparticle force or flocculation in the paste. The ideal paste for thick-film films should have a proper degree of pseudoplastic as well as thixotropic behavior (Wu et al., 2010). The thixotropic effect is a result of aggregation of suspended particles. Aggregation in the system caused by the attraction forces such as Van der Waals and repulsion forces due to steric and electrostatic... 

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