Floe layers

These results give rise to a picture of the drying process like that drown in Figure 14.6 [12c]. In this picture one sees an ordered dispersion in the bulk with a particle separation of 13 run. and a flocculated phase at the air-water intoface. According to van Tent, as water is lost, fire particle density in the bulk remains constant, but the floe layer at the surface becomes tbidmr until it reaches the substrate. 

By supplying the ions electrolytically, the cathodic reaction (3) generates hydrogen gas micro bubbles, which capture the flocculated pollutants and float them to the surface. Under the right conditions, this combination of processes captures the pollutants as a stable surface floe layer that is easily separated from the treated water. Reaction (3) also means that the water s pH will increase with greater treatment dosing, and its pH must be pre-adjusted such that it is always near neutral at discharge. 

What happens is here that the floes evidently move into the thin boundary layers adjaeent to the hydraulieally smooth surfaees, where mueh of the energy is dissipated, with the result that the partieles are subjeeted to strong stresses beeause of the small volume of the boundary layers. This hypothesis is supported by the good eorrelation of the results for the smooth disc with the results for various impellers in Fig. 6 it was assumed here that in the case of the disc, the majority of the power is dissipated in the boundary-layer volume V5, and the relationship nj/e V/V5 is approximately valid. The volume of the boundary layer (Eq. (21)) was obtained by integration from the theoretical solution [65] for the thickness of the boundary layer (Eq. (21)) of a smooth disc with turbulent flow. 

As in boundary-layer flows, smaller reference floe diameters are found with gas sparging than with the same average power input in a baffled stirred tank 127] or [44,45]. This can be explained if it is assumed that the floes come into close contact with the gas phase and find their way into the zones of higher stress. 

Much higher shear forces than in stirred vessels can arise if the particles move into the gas-liquid boundary layer. For the roughly estimation of stress in bubble columns the Eq. (29) with the compression power, Eq. (10), can be used. The constant G is dependent on the particle system. The comparison of results of bubble columns with those from stirred vessel leads to G = > 1.35 for the floccular particle systems (see Sect. 6.3.6, Fig. 17) and for a water/kerosene emulsion (see Yoshida and Yamada [73]) to G =2.3. The value for the floe system was found mainly for hole gas distributors with hole diameters of dL = 0.2-2 mm, opening area AJA = dJ DY = (0.9... 80) 10 and filled heights of H = 0.4-2.1 m (see Fig. 15). 

These include electrostatic interaction between the particles and interaction of particles with the fluid governed by their wettability, morphology and density (17-19) the extent of adsorption of the polymer and its influence on the interaction of particles, the orientation or configuration of the adsorbed polymers (and surfactant when it is present) and resultant interaction of adsorbed layers the hydrodynamic state of the system and its influence on the interaction of floes themselves. 

Among the properties measured here, the settling rate is mainly a measure of the size of the floes and in later stages the compressibility of floes and floe networks, and the supernatant clarity is a measure of the size distribution of floes and size dependent capture of the particles and floes by the polymer. The sediment volume and the pulp viscosity on the other hand, are direct measures, not only of floe size and structure but also of adsorbed polymer layers. It is to be noted in this regard that it is this latter aspect which makes it possible to estimate the thickness of adsorbed polymer layers by measuring the viscosity of the medium and the suspension in the presence of polymers (20,21). This combination of effects is another reason one cannot always expect correlation between various flocculation responses. 

The floe rupture model may also be used to explain the maximum observed in versus temperature (figure 5). According to equation (4) Tg = f (< > H) Eg, where f(2H) is the collision frequency term. Although Eg increases with increase of temperature, f(< > H) is a decreasing function of temperature as a result of decrease of solvency of the dispersing medium which leads to the contraction of the adsorbed layer (13). The increase of Eg with increase of temperature initially outweighs any reduction of f (< > H), but at higher temperatures, the reduction in f (4> j ) as a result of chain contraction may exceed the increase in Eg and this results in reduction in the measured Tg. 

XPS can be used to quantify HPAM adsorption onto minerals at various polymer bulk concentrations. It is seen here that kaolinite has twice the affinity for HPAM than feldspar at pH 9.0 and 50 ppm. Little or no adsorption was monitored on the surface of quartz or mica. Imaging XPS to monitor selective adsorption of mineral mixes proved difficult. Flocculating a mineral mixture of kaolinite, mica and quartz caused the kaolin floes to encapsulate the other minerals. This created a layer of kaolin on the quartz and mica prohibiting polymer mapping on their surfaces. It is shown that the effectiveness of the kaolin recovery is more strongly affected by encapsulation of other minerals during flocculation rather than the selective adsorption process. 

Shepson, P. B., A.-P. Sirju, J. F. Hopper, L. A. Barrie, V. Young, H. Niki, and H. Dryfhout, Sources and Sinks of Carbonyl Compounds in the Arctic Ocean Boundary Layer Polar Ice Floe Experiment, J. Geophys. Res., 101, 21081-21089 (1996). 

A biofilm is commonly visualized as a two-dimensional matrix layered on a solid surface. However, aggregates of exopolymer, detritus, and cells also form in the water column through a variety of physical, chemical, and biotic processes (Ward et al., 1994 Grossart et al., 1997 Chapter 12). These aggregates are variously described as floes or snow . A type of aggregate... 

---