Flocculated systems

Fig. 5. Effect of polymer dosage on different observed properties of flocculated slurry (40). Comparison of five parameters in a flocculation system (8%fluorite suspension + polyacrylamide Cyanamer P250). A, Rate of settling of floe boundary, in cm/s B, height of settled bed, cm C, height of consoHdated filter-cake, cm D, refiltration rate, arbitrary units and E, clarification, % optical transmission of 1 cm of supernatant Hquid after 3 min settling...

If there is particle—particle interaction, as is the case for flocculated systems, the viscosity is higher than in the absence of flocculation. Furthermore, a flocculated dispersion is shear thinning and possibly thixotropic because the floccules break down to the individual particles when shear stress is appHed. Considered in terms of the Mooney equation, at low shear rates in a flocculated system some continuous phase is trapped between the particles in the floccules. This effectively increases the internal phase volume and hence the viscosity of the system. Under sufficiently high stress, the floccules break up, reducing the effective internal phase volume and the viscosity. If, as is commonly the case, the extent of floccule separation increases with shearing time, the system is thixotropic as well as shear thinning. 

The parameter used to design rapid mix and flocculation systems is the root mean square velocity gradient G, which is defined by equation... 

Kashiki, I. and Suzuki, A., 1986. Flocculation system as a particulate assemblage A necessary condition for flocculants to be effective. Industrial and Engineering Chemistry Fundamentals, 25, 444-449. 

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

The degree of flocculation, therefore, is an expression of the increased sediment volume resulting from flocculation. For example, if ft has a value of 5.0, this means that the volume of sediment in the flocculated system is five times that in the deflocculated state. As the value of ji approaches unity, the degree of flocculation decreases. 

At higher coverages a sedimenting flocculated system was developed which produced a final boundary layer. 

Two models may be used to interpret the rheological results of the present flocculating system. Both models have been introduced by Hunter and his coworkers (13,14,29-32). ... 

Flocculation systems should be capable of reducing the concentration of most common electroplated metals to less than 1 ppm, and suspended solids to below 20 ppm. If lower... 

For situations i) and ii) the coagulated state, i.e. with the particles in intimate contact, is desirable. For other purposes, the flocculated state is required, i.e. with the particles still essentially individual and separated by a thin layer of liquid, thus giving control of the rheological properties of the sytem. Frequently, the secondary minimum plays a significant role in flocculated systems. 

The dispersion of an aggregated (coagulated or flocculated) system. Same meaning as Deflocculation. 

Monosilicic acid is stable in aqueous solution only at low pH and very low concentration (Her, 1979, p. 209). The rate of condensation-polymerization of silicic acid is dependent on pH, concentration and temperature. At a certain stage, the polysilicic acid sol is converted into either a precipitate (i.e. a flocculated system) or a hydrogel. 

Flocculation comes from the Latin word flocculate meaning loose and woolly. Flocculated systems result in rapid rate of settling because each individual unit is composed of many particles and is therefore larger. However, due to the loose packing of floes they are easily dispersible on shaking. Deflocculated systems on the other hand are made up of smaller particles whose settling rate is slower, but the settled particles tend to form an irreversible compact and are difficult to redisperse. This phenomenon is called caking. For coarse suspensions, a deflocculated suspension will have better uniformity of dose but poorer stability... 

Sedimentation ratio is often used to assess suspension stability. Byron reported the sedimentation ratios for a 1% sodium fluoresein suspension formulation with different amounts of surfactant (sorbitan trioleate) after standing for 20 days at room temperature. The suspension formulation with the lowest sedimentation ratio had thebest-flocculated system. However, all formulations were easily redispersible one complete revolution of the container was sufficient to produce a homogeneous dispersion. There was no clear difference in the times taken to reach apparent sedimentation equilibrium. Physical stability of the formulation was determined according to ... 

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