Aggregation suspensions

Sediment in an aged dispersion may be collected and measured in a crudely quantitative test. To distinguish between a deflocculated, a flocculated, and an aggregated suspension, a weighed amount of solid is uniformly suspended in a small quantity of liquid, the suspension is transferred to a graduate cylinder, the volume of sediment during a stated period of time is measured, and the specific sediment volume (milliliters per gram) vs time is plotted. In... 

FIG. 12 Two-dimensional cluster-cluster aggregation model of an aggregating suspension viewed at various times during the simulation. (From Ref. 50.)... 

Aggregable suspensions were shown (Weitz and Oliveria, 1984 Schaefer et al., 1984) to yield fractal floes, a result which sheds new light on the old subject of coagulation and flocculation. 

The continuous decrease in A1203 incorporation in copper is also explained by a change in particle surface composition.43 Chloride present as an impurity forms CuCl, which adsorbs on the particles and thereby inhibits the adsorption of copper ions on the particles. This is an effect characteristic for a copper sulfate bath and was not found in nickel or cobalt baths. A different type of aging was reported for the codeposition of aggregated polystyrene particles with zinc.54,76 Polystyrene incorporation increased continuously in successive experiments, where the rotation speed of a cylinder electrode was randomly varied. Thixotropic viscous behavior of the aggregated suspension causes changes in aggregate size and suspension viscosity with rotation speed of the same time-scale as the experiments. 

Figure 4. Observed removal efficiencies of specifically aggregated suspensions in varied separation units (sedimentation tanks).

The constants A and B describe characteristic properties of the aggregating suspension A is a measure of the mean settling velocity and B reflects the degree of dispersion (analogous to the standard deviation). The constants are determined by regression analysis from experimental data. 

Flocculation occurs when the net force between the particles is attractive. At low volume fractions, aggregation results in clusters, or floes, which have a fractal structure (7). For most systems, the properties of the aggregating suspension changes drastically at a certain critical particle concentration, 0g, which corresponds to the formation of a space-filling particle network. In dilute suspensions, at 0 < 0g, suspensions have no yield stress and the discrete clusters will settle more or less independently. Above 0g, the suspension can sustain a stress before yielding the elasticity may be significant, and the rate of settling is very slow. 

F. Garcia, N. LeBolay, C. Frances, Rheological behaviour and related granulometric properties of dense aggregated suspensions during an ultrafine comminution process. Powder Tech., 130, 407-414 (2003). 

Colloidal state n. Particular state in which any substance may exist under the proper conditions, determined by fineness of particle subdivision. The colloidal state is defined by a more or less well-marked ul-tramicroscopic zone in the scale of subdivision, the lower extreme of the zone approaching molecular dimensions, and the upper end gradually passing over into molecular aggregates (suspensions) visible under the ordinary microscope. Becher P (1989) Dictionary of colloid and surface science. Marcel Dekker, New York. 

The fractal dimension is a powerful tool to describe and classify particle aggregates because the relevant morphological quantities are condensed in just one parameter. The subsequent sections will show that the fractal dimension is crucial for various physical properties of aggregates. That means that it affects measurable properties of aggregate suspensions and is, thus, accessible by experimental determination. 

Light Scattering from Dilute Aggregate Suspensions in the RDG Limit... 

Zone sedimentation of aggregating suspensions coincides with the formation of a particle network (gel) in the lower part of the sedimentation vessel. This gel is gradually consolidated under its own weight, which reduces its porosity and the sedimentation velocity. The gel is, therefore, called compression or compaction zone, it eventually comprises aU particles of the suspension. The compression zone has a much higher porosity than the sediment of a weU-stabrhsed suspension (Sobisch et al. 2006). For fast aggregation and very fine particles, the gelation can finish even before a visible sedimentation of the dispersed phase occurs. 

Varunio, Prasad and Nur. 2003. Elastic properties of dry clay mmeral aggregates, suspensions and sandstone. 

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