Rapid flocculation

It can be observed that with an increase in solvent polarity the dispersion stability displays a maximum, which corresponds to a minimum in the normalized setthng rate the normalization is done to account for differences in the density and the viscosity of the solvents. The normalized setthng rate equals the observed settling rate times the solvent viscosity/(particle density minus the solvent density). The maximum stabihty in this case is observed in moderately polar solvents (20 < e < 45). Bare particles suspended in a hquid medium are in constant Brownian motion and can flocculate rapidly on colhsion if the 1 is larger than about 15 kT. Stabilization can usually be achieved by decreasing the van der Waals attractive forces. The potential energy due to the van... 

Figure 7.24 shows the effect of water on the stability of alumina suspensions that have been dispersed in cyclohexane by the adsorption of a monolayer of AOT." As the water content in the system increases, the suspension goes through a series of flocculated, stabilized, and flocculated stages. In an extremely dry state the dispersions flocculate rapidly, and the addition of trace amounts of water stabilizes the suspensions quite significantly. Thereafter, the suspensions remain stable over a range of water concentrations and then flocculate very rapidly at higher water concentrations. 

Two limiting cases can be distinguished the rate of coales-eence is much greater than that of flocculation (rapid coalescence) ... 

The cholesterol (like for that matter numerous other organic non-electrolytes such as triolein) sensitises the sol, i. e., so changes it that it flocculates rapidly with electrolytes. 

In the absence of repulsion, flocculation rapidly leads to large clusters. To counteract the van der Waals attraction, it is necessary to create a repulsive force. The two main types of repulsion can be distinguished, depending on the nature of the emulsifier used, are electrostatic (due to the creation of double layers) and steric (due to the presence of adsorbed surfactant or polymer layers). 

For example, van den Tempel [35] reports the results shown in Fig. XIV-9 on the effect of electrolyte concentration on flocculation rates of an O/W emulsion. Note that d ln)ldt (equal to k in the simple theory) increases rapidly with ionic strength, presumably due to the decrease in double-layer half-thickness and perhaps also due to some Stem layer adsorption of positive ions. The preexponential factor in Eq. XIV-7, ko = (8kr/3 ), should have the value of about 10 " cm, but at low electrolyte concentration, the values in the figure are smaller by tenfold or a hundredfold. This reduction may be qualitatively ascribed to charged repulsion. 

Most synthetic latices contain 5—10 wt % of nonelastomeric components, of which more than half is an emulsifier or mixture of emulsifiers. One reason for this relatively high emulsifier concentration as compared with natural latex is that emulsifier micelles containing solubiHzed monomer play a principle role in the polymerization process. A high emulsifier concentration is usually necessary to achieve a sufficiently rapid rate of polymerization. Secondly, a considerable fraction of the surface of the polymer particles must be covered by adsorbed soap or equivalent stabilizer to prevent flocculation... 

Another type of flocculation results from particle—particle collisions caused by differential settlement. This effect is quite pronounced in full-size plants where large rapidly falling particles capture small particles that settle more slowly. 

Although filtratioa rates can be much faster with flocculants, the final cake moisture is often higher ia a flocculated cake (60—63). Ia coatrast, usiag flocculants optimized for filtration, coal, and other mineral slurries can be dewatered to moisture contents significantly lower than the untreated cake (64—68). The advantages of rapid filtration rates can also be preserved. Flocculants that provide better filtration tend to form floes having the foUowiag characteristics (65) ... 

With polymer flocculation of slurries, however, extended agitation after the addition of the polymer may be detrimental. The reagent should be added to the slurry under conditions which promote rapid... 

The factors which may make CCD a preferred choice over other separation systems include the following rapidly settling solids, assisted by flocculation relatively high ratio of solids concentration between underflow and feed moderately high wash ratios allowable (2 to 4 times the volume of hquor in the thickened underflows) large quantity of sohds to be processed and the presence of fine-size sohds that are difficult to concentrate by other means. A technical feasibihty and economic study is desirable in order to make the optimum choice. 

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

It should be noted that the total loss of head of a filter bed is in inverse ratio to the depth of penetration of the matter in suspension. In a normal wastewater treatment plant, the water is brought onto a series of rapid sand filters and the impurities are removed by coagulation-flocculation-filtration. Backwashing is typically performed in the counterfiow mode, using air and water. One type of common filter is illustrated in Figure 6, consisting of closed horizontal pressurized filters. 

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