The reversibility of flocculation

Several authors, however, have reported a minor caveat in relation to the reversibility (Napper, 1968b Everett and Stageman, 1977 1978a Clarke and Vincent, 1981a Buscall, 1981). This is that if the particles are allowed to remain flocculated for relatively long periods (say, an hour or more), redispersion is no longer complete. The reasons for this aging effect are unknown. One plausible explanation for the irreversibility involves the slow diffusion of the stabilizer away from the stress zones that are created by the close approach of the particles. 

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Hedborg F, Lindstrom T (1996) Some aspects on the reversibility of flocculation of paper stocks. Nordic Pulp Paper Res J 4 254... 

Comparing the time-scales of the various processes involved does not allow a clear description of the behavior of the Stern layer as the particles come closer to one another. It is likely that, if relaxation occurs, it does so—at least partially— during the collision. It is very important to take this behavior into account, since it is likely to influence the reversibility of flocculation. 

The foundations of the theory of flocculation kinetics were laid down early in this century by von Smoluchowski (33). He considered the rate of (irreversible) flocculation of a system of hard-sphere particles, i.e. in the absence of other interactions. With dispersions containing polymers, as we have seen, one is frequently dealing with reversible flocculation this is a much more difficult situation to analyse theoretically. Cowell and Vincent (34) have recently proposed the following semi-empirical equation for the effective flocculation rate constant, kg, ... 

Flocculation. Flocculation means an aggregation of emulsion droplets but, in contrast to coalescence, the films of the continuous phase between the droplets survive. Hence, the process may be partially reversible. Both processes, flocculation and coalescence, speed up the creaming of an emulsion due to the increase of the drop size. The process of flocculation is even more important for dispersions of solids than for emulsions because in this case a coalescence is not possible. 

Hamaker (1937a) has suggested that short-range repulsion is responsible for repeptization (reverse of flocculation). By repeating the analysis of this paper for interactions between unequal spheres, one could calculate both flocculation and repeptization rates and, in principal, thereby predict the distribution of floe sizes, including the limiting equilibrium distribution. 

The phenomenon of soil dispersion with respect to Na+ loads (magnitude of ESP or SAR) appears to be unique to all soils on at least one particular point. As the total salt or Cl- concentration in the water increases, the dispersion index decreases and the saturated hydraulic conductivity increases (Fig. 11.6). When this occurs, the soil-water system becomes toxic to plants and organisms owing to high osmotic pressures. When chloride concentration in solution increases beyond 6000 mg L 1, Na ions near clay surfaces begin to dehydrate because of high osmotic pressure in the surrounding solution. This causes clay particles to flocculate (flocculation is the reverse of dispersion) and, consequently, the saturated hydraulic conductivity of the soil increases. 

The treatment of ADUF by reverse osmosis [13] was found to be useful in concentrating activity in small volume while making a larger volume of the decontaminated effluent for direct disposal after required dilution. Porous cellulose acetate membranes were used in plate module configurations. The concentration of ammonium nitrate in the permeate stream is not very different from that of the contaminated retentate. With the addition of flocculating aids, the decontamination factors in the range of 1000 with VRFs in the range of 100 were achieved. 

In case of reversible flocculation, the temporary existence of monomers and dimers is assumed and the degree of flocculation (D) was calculated from the equation ... 

Another mechanism of flocculation is that involving the secondary minimum (Gmin) which is few kT units. In this case, the flocculation is weak and reversible and hence both the rate of flocculation (forward rate k ) and deflocculation (backward rate k ) must be considered. In this case, the rate or decrease of particle number with time is given by the expression ... 

To illustrate the effect of the reverse process on the rate of flocculation, we solved numerically the set of Equations 5.319, 5.331, and 5.332. To simplify the problem, we used the following assumptions (1) the von Smoluchowski assumption that all rate constants of the straight process are equal to Up (2) aggregates containing more than M particles cannot decay (3) all rate constants... 

Equation 5.320. We see that in an initial time interval all cnrves in Figure 5.55 touch the von Smoluchowski distribution (corresponding to h = 0), bnt after this period we observe a redaction in the rate of flocculation which is larger for the curves with larger values of b (larger rate constants of the reverse process). These S-shaped curves are typical for the case of reversible coagulation, which is also confirmed by the experiment. 

Deflocculation The reverse of aggregation (or flocculation or coagulation). Peptization means the same thing. 

Most colloidal stable suspensions show more or less reversible response to compression and decompression. However, in the case of flocculated suspensions, the compressive properties are irreversible. In concentrated flocculated suspensions, a continuous particle network forms. The particle network can support some stress up to a critical value. Once this critical stress, also called the compressive yield stress Fy, is exceeded, the network consolidates to a higher volume fraction with a higher critical stress. 

As distinct from uncharged droplets, flocculation in the range of micrometer-sized droplets is possible. As seen in Fig. 9, even rather large droplets (4 pm) aggregate reversibly if file electrolyte concentration is lower than (1-5) X10 M and the Stem potential is higher than 25 mV. For smaller droplets the domain of flocculation will extend while the domain of coagulation will shrink. For submicrometer droplets, flocculation takes place even at high electrolyte concentrations (0.1 M). 

Genz et al. (163) found that the conductive and dielectric properties of suspensions of carbon black in mineral oil was highly dependent on shear. When a shear force was applied to the suspensions, flocculated aggregates were tom apart and a reduction in the permittivity levels was observed. When the shear stopped, the permittivity rose to previous levels. Thus, the reversibility of the shear-induced floe disintegration could be followed by means of dielectric spectroscopy. 

Consider the use of flocculants or deep cone. Flocculant dosage should be related to feed inlet concentration, see also Section 9.3. Include high pressure water purge lines for both forward and reverse flow at > 1 m/s. Raise and lower the rake once per shift. For startup, pump feed into the empty tank and recycle underflow until the design underflow densities are achieved. 

Chapter ix, entitled Reversal of Charge phenomena, Equivalent Weight, and Specific properties of the ionised groups, not only concerns the phase boundary of coacervates, but also the boundary of floccules and even the boundary of adsorbed colloid films on particles (e.g., on SiOe). 

If such a colloid and such salts are chosen, that form favourable combinations for viscosimetric investigation, — favourable here meaning that no flocculation or coacervation accompanies the reversal of charge — then it may be expected that, similar to the combination amylum solubile + hexol nitrate minima in the( / — o )/>/ curves must also occur. 

In general flocculation (or coacervation) sets in already before the reversal of charge point is reached. We shall later see an example in which equivalent amounts of 6 and 3 valent ions are indeed fixed at the reversal of charge point (see p. 265 Fig. 4). 

As no flocculation, occurs the reversal of charge concentrations needed for the calculation of R.H.N. were determined on suspended particles (for instance Si02) which become covered with a complete colloidal film. For particulars see p. 277 2 b. 

Egg lecithin sol gave with 6—1 a slight turbidity only very slowly and only in the neighbourhood of the reversal of charge point, therefore in the Fig. symbol 5—1 has been chosen for its flocculability. 

This apparent equivalent weight plays a great part a) in determining the spread of cations in the reversal of charge spectrum ( 21, p. 295) b) in the extent of the antagonism CaCIg — NaCl ( 5 b, p. 314-315) c) in complex flocculation or complex coacervation with positive protein sols (see p. 374 Ch. X 2r). 

In many cases the salts used flocculate or coacervate the sol in a concentration range around the reversal of charge point. In these cases the electrophoretic velocity (at 1/5 depth of the cuvette) is determined as a function of the salt concentration on small flocculi or small coacervate drops. 

The supposed mechanism might also give an explanation, why at sufficiently long chain length or at sufficient complexity of structure of the organic ion not only the reversal of charge concentration becomes small but also why then flocculation of the colloid is a frequent phenomenon. 

Interaction between oppositely charged sols is a general phenomenon in colloid science, occurring both in hydrophobic and "hydrophylic colloids. This interaction usually manifests itself in flocculation, the latter being a maximum at or neat to the mixing ratio of the sols corresponding to the reversal of charge point of the floccules. 

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