hoi dai mong may ban giup minh voi stages described in Figure 5.1. During the initial period of coagulation, aggregatesquickly grow and become more convoluted and porous. Later, after about 50 or60 min, aggregates appear to be more regular, with a smoother surface. This conditioncorresponds with the later stages described in Figure 5.1, where more looselybound clusters have been broken off and the aggregates become denser as inner porespaces become occupied. The period... hoi dai mong may ban giup minh voi stages described in Figure 5.1. During the initial period of coagulation, aggregatesquickly grow and become more convoluted and porous. Later, after about 50 or60 min, aggregates appear to be more regular, with a smoother surface. This conditioncorresponds with the later stages described in Figure 5.1, where more looselybound clusters have been broken off and the aggregates become denser as inner porespaces become occupied. The period between about 40 and 60 min shows only minorchanges, which should correspond with the state indicated around point A in Figure5.1. For times greater than about 90 min, the aggregate size seems to decreaseslightly, again consistent with the later stages described in Figure 5.1.5.4.1.2 ParticleSizeand ShapeGeometric information from the image analysis was used to characterize particles.Temporal changes in aggregate size distribution for experiments with latex particlesmixed with G = 20 sec−1 and G = 10 sec−1 are shown in Figure 5.5, where frequencyof occurrence (number) in each size class is plotted against the aggregatecharacteristic length (major ellipse axis) using a logarithmic scale. Test results usingalum (Experiment Set 2) and polymer (Experiment Set 3) are shown. In both casesthe gradual movement of the peak of the distribution toward larger size is clearlyseen, 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 thatthe model of Equation (5.10) is applicable, that is, breakup is negligible. Comparedwith the results using polymer, alum treatment seems to result in more peaked sizedistributions, with less spread about the mode. The peak size for the polymer treatedexperiment showed a more significant increase with time and the peak had smallermagnitudes than with the alum treated test. The higher peak associated with the alumFIGURE 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 differentparticle concentrations used in the two sets of experiments. It is difficult to comparethe two suspensions directly since different coagulants were used and the zetapotentials for the two suspensions at charge neutralization were different, as were themixing speeds. Keeping these differences in mind, the polymer treated coagulationproduced larger (and possibly stronger) particles more quickly than coagulation byalum at the charge neutralization stage.Temporal changes in the peak of the particle size distribution for Buffalo River suspensionsmixed with polymer at G = 10 sec−1 are illustrated in Figure 5.6. The peaksize gradually increases (until about 50 to 60 min) and then it decreases. The decreaseat later times is thought to be due to the breakup effect, which has been described inseveral previous studies. For example, Williams et al.29 reported a breakup of particlesize after reaching a peak for silica particles mixed at various speeds. Their studysuggested that smaller G would induce a larger peak and a relatively smaller decreaseof peak size over time, compared to a higher mixing rate. A similar observation wasreported by Selomulya et al.,30 who found that the average aggregate size (for latexparticles) decreased with time after reaching a peak, using a range of shear rates (40to 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 differentmixing rates. Although not shown here, similar results were found with D3 as withD2. 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 suspensionswere relatively more pronounced. This may be due to higher solids concentration forthe Buffalo River suspension, or to the presence of organic material, which was nota factor in the latex tests. In addition, there was greater heterogeneity in aggregatesize and shape for Buffalo River suspensions. In both cases there is a gradual increasein size and decrease in D2, followed by the attainment of approximately steady-state
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26/09/16 06:03:16hoi dai mong may ban giup minh voi stages described in Figure 5.1. During the initial period of coagulation, aggregatesquickly grow and become more convoluted and porous. Later, after about 50 or60 min, aggregates appear to be more regular, with a smoother surface. This conditioncorresponds with the later stages described in Figure 5.1, where more looselybound clusters have been broken off and the aggregates become denser as inner porespaces become occupied. The period... Xem thêm.
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