Floes structure

R. Chanamai, N. Herrmann, D. J. McClements 2000, (Probing floe structure by ultrasonic spectroscopy, viscom-etry, and creaming measurements), Langmuir 16, 5884. 

Torres, F. E., Russel, W. B., and Schowalter, W. R., Floe structure and growth kinetics for rapid shear coagulation of polystyrene colloids. J. Colloid Interface Sci. 142, 554-574 (1991a). Torres, F. E., Russel, W. B., and Schowalter, W. R., Simulations of coagulation in viscous flows. J. Colloid Interface Sci. 145, 51-73 (1991b). 

The next problem is to find an expression for Asg. This entropy difference is a function of the particle volume fractions in the dispersion ( ) and in the floe (<(> ). As a first approximation, we assume that Ass is independent of the concentration and chain length of free polymer. This assumption is not necessarily true the floe structure, and thus < >f, may depend on the latter parameters because also the solvent chemical potential in the solution (affected by the presence of polymer) should be the same as that in the floe phase (determined by the high particle concentration). However, we assume that these effects will be small, and we take as a constant. 

Some further light may be shed on carbon floe structure by the electron micrographs of Figure 16 showing an appreciable decrease in floe size with increase in dispersant content (for the suspensions tumbled for 150 hours). One might conclude that the 0L0A-1200 was an efficient "grinding aid"... 

Figure 16. Electron micrographs showing floe structure of carbon black dispersed in odorless kerosene after 150 hours of agitation. Parts OLOA-1200 per 100 parts of carbon black (a)-0, (b)-0.A, (c)-2.0, (d)-A.O. These are from samples (a), (c), (i) and (k) of Figure 15. Reproduced with permission from Ref. (1A). Copyright 1983, Elsevier Science Publishers.

It has been assumed that the total surface area of a floe is proportional to the number of singlets in that floe, a reasonable assumption for small floes and open floe structures. The effective fractional surface coverage, 0k, is not necessarily equal to the fractional surface coverage at equilibrium for a given amount of adsorbed polymer. This will be discussed in some detail below. 

M.F. Dignac, V. Urbain, D. Rybacki, A. Bruchet, D. Snidaro, R Scribe, Chemical description of extracellular polymers implications on activated sludge floe structure, Water Sci. Technol. 38 (1998) 45-53. 

In general, in the academic literature on Colloid Science, the latter states are regarded as an end-product and the main emphasis in research has been directed towards understanding the factors which control stability. However, in the technological application of latices, it is often material in the flocculated state which is utilised in order to obtain the desired flow properties. In fact, in order to focus attention on the question of floe structure and pursue this topic further, we must ask a series of questions -... 

Without doubt the quantity of Al2(SO g used in the process is by far the most important factor which influences the sludge volume of the freshly formed hydroxide floes. The effect on sludge production of a change of floe structure because of a change of the experimental conditions is thus subsidiary to the influence of the dosage. Of course the floe structure (19 0) plays an important role in the rate of reduction of the sludge because of floe ageing phenomena (21). 

In the screen tests, each coagulant or flocculant is added to the beaker samples of representative slurry or hquor in a dropwise fashion, while the sample is mixed with a spatula, stirrer, or 3-6 jar stirrer mechanism. The amount of coagulant or flocculant required to initiate floe particle formation is noted along with relevant notes as to the size of the floe, capture of fines, resultant liquor clarity, and stabihty of the floe structure. The dosage is typically noted in g/t solids if the sample is primarily solids (thickener design), or in mg/L liquor if the sample is primarily for clarification and the solids concentration is low. 

It is usually difficult to observe visually a change in floe structure in a concentrated slurry. The two best indications that an effective quantity of chemical has been added is a sudden thickening or increase in viscosity of the slurry and the formation of riveriets on the surface of a spatula when treated slurry is shaken from it. It is generally necessary to exceed a threshold quantity of chemical before there is a measurable improvement. The proper dosage becomes an economic balance between the cost of additional chemicals and the savings resulting from a reduction in filter area. 

The above processes of attraction may continue for long periods of time such that, ultimately, the suspoemulsion becomes physically unstable. Any flocculation will result in an entrapment of the liquid between the particles in the floe structure, and this causes a significant increase in the viscosity of the system. 

Figure 16. Cryo-SEM image of a similar oil sands sludge sample as in Figure 15. After mild vibration of the sample, the frozen, fractured, and sublimed sample shows disturbance of the structuring observed with a settled sample. The utility of microscopic methods in probing the important floe structure of suspensions rather than simply characterizing the component particles is illustrated.

Figure 28. Yield stress and gel strength as a function of shear rate. This oil sands fine tailings sample was sheared in rheometer to disrupt the floe structure before the run. The hysteresis in the curve is indicative of the thixotropy or shear thinning behavior of the suspension. This behavior is related to the structure changes observed using microscopy (Figures 16 and 17).

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