Assessment of Flocculation

Rheological techniques are most convenient to assess suspension flocculation without the need of any dilution (which in most cases result in breakdown of the 

A controlled rate instrument is usually used for the above measurements the inner (or outer) cylinder, the cone (or the plate) is rotated at various angular velocities (allowing one to obtain the shear rate y) and the torque is measured on the other element (allowing one to obtain the stress a). 

For a Newtonian system (such as the case of a dilute suspension, with a volume fraction less than 0.1) a is related to y by the equation, 

This is usually the case with weakly flocculated suspensions or those to which a thickener is added. 

The above trend depends on the extent of flocculation relative to Ostwald ripening. Therefore, following ap and with storage time requires knowledge of Ostwald ripening and/or coalescence. Only in the absence of this latter breakdown process can one use rheological measurements as a guide of assessment of flocculation. 

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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]. 

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. 

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

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. 

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 and Prediction of Flocculation Using Rheological Techniques 453... 

Basically, a constant stress cr is applied on the system and the compliance J(Pa ) is plotted as a function of time (see Chapter 20). These experiments are repeated several times, increasing the stress in small increments from the smallest possible value that can be applied by the instrament). A set of creep curves is produced at various applied stresses, and from the slope of the linear portion of the creep curve (when the system has reached steady state) the viscosity at each applied stress, //, can be calculated. A plot of versus cr allows the limiting (or zero shear) viscosity /(o) and the critical stress cr (which may be identified with the true yield stress of the system) to be obtained (see also Chapter 4). The values of //(o) and <7 may be used to assess the flocculation of the dispersion on storage. 

The discussion of chemical and physical pretreatments given in this chapter highlights the potential benefits (and pitfalls) of artificially changing the particle size present in a suspension. Chemical pretreatments in particular can enhance performance in all phases of a filter cycle and provide for improved filtration rates and reduced cake moistures. Some separation devices such as the deep cone thickener and belt filter press rely on the satisfactory performance of flocculants to function correctly and their use is a necessary prerequisite. The use of coagulants and flocculants with other separation devices must be assessed on an individual basis. Their introduction generally increases the operating cost of a separation which must be balanced by the potential improvements in separation characteristics. 

Filtration of flocculated suspensions can be assessed using the filterability test method described in chapter 11. Using the filterabihty number F against the product Gt an optimum flocculation can be determined for filtration through a sample of porous granular material as exemplified in Figure 4.16. 

Bubbles rising through a suspension create velocity gradients due to their motion. These velocity gradients could be used for flocculation in principle, but early assessments of bubble flocculation indicated that the diffusers which existed at that time created bubbles that were too large to be effective... 

Thus, a plot of r versus t gives a straight line, in the initial time of flocculation, and k can be calculated from the slope of the line. Flocculation of emulsions can also be assessed by direct droplet counting using optical microscopy (with image analysis), using the Coulter counter and Photosedimentation methods. 

To fully assess the properties of suspension concentrates, three main types of measurements are required. Firstly some information is needed on the structure of the solid/solution interface at a molecular level. This requires investigation of the double layer properties (for systems stabilised by ionic surfactants and polyelectrolytes), adsorption of the surfactant or polymer as well as the extension of the layer from the interface (adsorbed layer thickness). Secondly, one needs to obtain information on the state of dispersion on standing, such as its flocculation and crystal growth. This requires measurement of the particle size distribution as a function of time and microscopic investigation of flocculation. The spontaneity of dispersion on dilution, i.e. reversibility of flocculation needs also to be assessed. Finally, information on the bulk properties of the suspension on standing is required, which can be obtained using rheological measurements. The methods that may be applied for suspension concentrates are described briefly below. 

This parameter is useful for paints and pigments, giving the covering power of the solids. It is also useful in assessing relative flocculant demand, as this is proportional to the surface area of the particles. 

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