Flocculation assessment

An alternative rheological technique for assessing flocculation involves oscillatory measurements which, as noted above, can include two sets of experiments, namely strain and oscillatory sweep measurements. 

The stability of the various dispersions was assessed and compared by determining the critical flocculation conditions (temperature or volume fraction of added non-solvent for the grafted polymer), as a function of particle concentration. 

In common with some other authors (18-20), Napper removed excess stabilizer from the dispersion medium so as to give the dispersed particles full surface coverage, leaving negligible amounts of free polymer in solution. As the solvency was worsened, no more polymer could be adsorbed, so that critical flocculation conditions do not necessarily correspond to surface saturation. In the present work, which may refer more closely with some practical applications, the stabilizer is kept at the plateau adsorption level but at the expense of complicating the system by the presence of free polymer. Clarke and Vincent (21) have reported on the effect of free polystyrene on the stability of silica with terminally-attached sytrene chains, but the very considerable differences to our studies make an assessment of the possible role played by unadsorbed polymer unproductive. 

Even in applications other than dewatering, permeability methods are quite often used to assess the performance of polymeric flocculants, since, in principle, they can give a very sensitive indication of the state of aggregation of particles and are useful in locating optimum polymer concentrations. 

Lf is a quantity of toxoid used for vaccines and is assessed by flocculation (limit of flocculation), which may vary among different products. 

Viscosity. This parameter can be monitored by standard rheological techniques. The rheological properties of emulsions, reviewed by Sherman (1983), can be complex, and depend on the identity of surfactants and oils used, ratio of disperse and continuous phase, particle size, and other factors. Flocculation will generally increase viscosity thus, monitoring viscosity on storage will be important for assessing shelf-life. 

The two primary features of the phenomena are the layer thickness necessary to provide stability and the conditions at which the dispersions flocculate. The first can be quantified by generalizing the potential for terminally anchored chains to interactions between spheres via the Deijaguin approximation, adding the attractive dispersion potential, and then assessing the layer thickness necessary to maintain —fl>mi /fcT < 1 — 2. To illustrate this, consider the small overlap limit of Eq. (122), which transforms into... 

Four types of colloids were considered in the WIPP program intrinsic actinide colloids, mineral colloids, microbes, and humic acid colloids (US DOE, 1996). Intrinsic actinide colloids, consisting of polymerized hydrated actinide hydroxides, are not stable in the neutral to moderately basic pH conditions expected in the WIPP, and were assumed not to contribute to the total actinide concentrations in solution. Mineral colloids are destabilized and tend to flocculate in the high-ionic-strength WIPP brines (Kelly et al., 1999). In the performance assessment calculations for the WIPP, a highly conservative value of 2.6 X 10 mol actinide per liter, for each actinide, was assumed to be bound to mineral colloids and to contribute to the mobile fraction. Actinides sorbed onto microbes and humic acids were estimated to contribute significantly to the concentration of mobile actinides in WIPP brines as discussed above (Section 9.06.3.2.2). 

Sedimentation ratio is often used to assess suspension stability. Byron reported the sedimentation ratios for a 1% sodium fluoresein suspension formulation with different amounts of surfactant (sorbitan trioleate) after standing for 20 days at room temperature. The suspension formulation with the lowest sedimentation ratio had thebest-flocculated system. However, all formulations were easily redispersible one complete revolution of the container was sufficient to produce a homogeneous dispersion. There was no clear difference in the times taken to reach apparent sedimentation equilibrium. Physical stability of the formulation was determined according to ... 

Non-volatile compounds are not removed from water by air stripping. Air stripping may be used in conjunction with another treatment method, such as GAC, flocculation, or chlorination, to achieve treatment levels at an operating cost below that possible by the use of a single technology. Consideration of the factors herein presented allows an accurate assessment of the effectiveness of air stripping for specific water treatment objectives. 

The physical instability of emulsions involves creaming, flocculation, coalescence, or breaking, whereas the chemical instability can be a result of hydrolysis of the stabilizing moieties. In order to assess the stability of the emulsion, heating and freezing cycles and centrifugation and steam sterilization can be employed. 

Two main procedures can be applied for the characterisation of suspensions and assessment of their stability (such as flocculation). The first method depends on the measurement of particle size distribution and the rate of flocculation and/or Ostwald ripening after dilution of the suspension with the dispersion medium, while the second procedure depends on measurement of the state of suspension without dilution, using rheological techniques. As both methods are described in detail in Chapters 19 and 20, only a summary will be provided here. 

In this chapter, a summary of the methods that can be applied to assess the structure of the solid/Uquid interface will first be provided, followed by details of assessing sedimentation, flocculation, and Ostwald ripening. In the latter cases (flocculation and Oswald ripening), information is needed on the particle size distribution, and several techniques are available to obtain this from diluted systems. It is essential to dilute the concentrated suspension with its own dispersion medium in order not to affect the state of the dispersion during examination. The dispersion medium can be obtained by centrifuging the suspension, when the supernatant liquid will be produced at the top of the centrifuge tube in the case... 

Assessment of Flocculation and Ostivald Ripening (Crystal Croivth) 405... 

Assessment of Flocculation and Ostwald Ripening (Crystal Growth)... 

The assessment of flocculation and Ostwald ripening of a suspension requires measurement of the particle size and shape distribution as a function of time. Several techniques may be apphed for this purpose, and these are summarised below [6]. 

The above measurements should be supplemented by particle size distribution measurements of the diluted dispersion (ensuring that no floes are present after dilution) to assess the extent of Ostwald ripening. Another compHcation may arise from the nature of the flocculation which, if it occurs in an irregular fashion (producing strong and tight floes), may cause i (0) to be increased while cr may show some decrease, and this wiU complicate the analysis of the results. Yet, in spite of these complications, constant stress measurements may provide valuable information on the state of the dispersion on storage. 

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