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  • 26/09/16 06:03:16
    hoi dai mong may ban giup minh voi
    stages described in Figure 5.1. During the initial period of coagulation, aggregates
    quickly grow and become more convoluted and porous. Later, after about 50 or
    60 min, aggregates appear to be more regular, with a smoother surface. This condition
    corresponds with the later stages described in Figure 5.1, where more loosely
    bound clusters have been broken off and the aggregates become denser as inner pore
    spaces become occupied. The period...
    hoi dai mong may ban giup minh voi
    stages described in Figure 5.1. During the initial period of coagulation, aggregates
    quickly grow and become more convoluted and porous. Later, after about 50 or
    60 min, aggregates appear to be more regular, with a smoother surface. This condition
    corresponds with the later stages described in Figure 5.1, where more loosely
    bound clusters have been broken off and the aggregates become denser as inner pore
    spaces become occupied. The period between about 40 and 60 min shows only minor
    changes, which should correspond with the state indicated around point A in Figure
    5.1. For times greater than about 90 min, the aggregate size seems to decrease
    slightly, again consistent with the later stages described in Figure 5.1.
    5.4.1.2 ParticleSizeand Shape
    Geometric information from the image analysis was used to characterize particles.
    Temporal changes in aggregate size distribution for experiments with latex particles
    mixed with G = 20 sec−1 and G = 10 sec−1 are shown in Figure 5.5, where frequency
    of occurrence (number) in each size class is plotted against the aggregate
    characteristic length (major ellipse axis) using a logarithmic scale. Test results using
    alum (Experiment Set 2) and polymer (Experiment Set 3) are shown. In both cases
    the gradual movement of the peak of the distribution toward larger size is clearly
    seen, indicating that aggregate growth was the primary mechanism during this period
    (also shown in Figure 5.4). In this period (0 to 30 min) conditions are such that
    the model of Equation (5.10) is applicable, that is, breakup is negligible. Compared
    with the results using polymer, alum treatment seems to result in more peaked size
    distributions, with less spread about the mode. The peak size for the polymer treated
    experiment showed a more significant increase with time and the peak had smaller
    magnitudes than with the alum treated test. The higher peak associated with the alum
    FIGURE 5.5 Temporal plot of particle size distribution for latex suspensions mixed at G =
    20 sec−1 using alum, and mixed at G = 10 sec−1 using polymer (Poly).
    treated tests may be related to additional solids added by the alum, and to different
    particle concentrations used in the two sets of experiments. It is difficult to compare
    the two suspensions directly since different coagulants were used and the zeta
    potentials for the two suspensions at charge neutralization were different, as were the
    mixing speeds. Keeping these differences in mind, the polymer treated coagulation
    produced larger (and possibly stronger) particles more quickly than coagulation by
    alum at the charge neutralization stage.
    Temporal changes in the peak of the particle size distribution for Buffalo River suspensions
    mixed with polymer at G = 10 sec−1 are illustrated in Figure 5.6. The peak
    size gradually increases (until about 50 to 60 min) and then it decreases. The decrease
    at later times is thought to be due to the breakup effect, which has been described in
    several previous studies. For example, Williams et al.29 reported a breakup of particle
    size after reaching a peak for silica particles mixed at various speeds. Their study
    suggested that smaller G would induce a larger peak and a relatively smaller decrease
    of peak size over time, compared to a higher mixing rate. A similar observation was
    reported by Selomulya et al.,30 who found that the average aggregate size (for latex
    particles) decreased with time after reaching a peak, using a range of shear rates (40
    to 80 sec−1).
    Further evidence of this type of behavior is seen in Figure 5.7 and Figure 5.8,
    where temporal changes in median aggregate size and D2 are plotted for both latex
    (LT) and Buffalo River (BR) suspensions treated with polymer under three different
    mixing rates. Although not shown here, similar results were found with D3 as with
    D2. In general, slower mixing produced larger aggregates (Figure 5.7) and lower D2
    (Figure 5.8) for both these experiments, and changes in the Buffalo River suspensions
    were relatively more pronounced. This may be due to higher solids concentration for
    the Buffalo River suspension, or to the presence of organic material, which was not
    a factor in the latex tests. In addition, there was greater heterogeneity in aggregate
    size and shape for Buffalo River suspensions. In both cases there is a gradual increase
    in size and decrease in D2, followed by the attainment of approximately steady-state

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      0 · 27/09/16 07:27:21
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