Flocculation values

Flocculation values achieved from turbidity measurements using the light scattering technique showed improvement with nonylphenol ether carboxylic acid (4 mol EO) in particular. The oil solubilization rate has been found to be proportional to the surfactant micellar size [190]. 

The flocculation value is the minimum concentration required for turbidity in a definite time. 

The sensitivity of the stability ratio to chemical or particle interaction factors can be illustrated by an examination of the model expression for Wn in Eq. 6.75. For example, if temperature and the particle interaction parameters are fixed, then Wn will vary with the concentration, c (also included in /c), of Z-Z electrolyte. At low values of c, k is also small, and the first equality in Eq. 6.75 indicates that Wu will take on its largest values. (Decreasing c also provokes an increase in dm because of Eq. 6.73, but this effect is dominated by that of k.40) Conversely, as c increases, the value of Wu will drop until it achieves its minimum, Wn = 1.0, when Z dm = 2 (Eq. 6.75). At this concentration, termed the critical coagulation concentration (ccc), or flocculation value, the flocculation process has become transport-controlled and therefore is rapid. Thus in general... 

The effects of various electrolytes are usually compared in terms of the flocculation values, or minimal concentrations (expressed in millimoles per liter), required to bring about coagulation. Flocculation values for nonspecific electrolytes can be interpreted in terms of electrostatic repulsion and van der Waals attraction. The van der Waals attractive forces vary with size and shape of the particles, but roughly speaking the force is appreciable between colloidal particles at distances of the order of magnitude of their own radii. The significant feature is that flocculation occurs before the zeta potential reaches zero, that is, when it reaches a small critical value. 

Flocculation values (F.V.) of copoly-aniline-aldehyde with different molecular weights used for coal suspension (mM)... 

Figure 14. Effects of a mixture of sodium chloride and different potential peptizing agents on the NaCl flocculation value (suspension stability). (Reproduced with permission from reference 1. Copyright 1977 Wiley.)...

The flocculation values united in Tables I and II clearly demonstrate the well-known rule of Schulze and Hardy stating that the flocculation value is first of all determined by the valency of the ions which are oppositely charged to the particles of the sol, whereas the spvecific nature of these ions is far less important. The influence of the valency and. the nature of the ions bearing the same charge as the particles is of subordinate importance. 

FLOCCULATION VALUES IN MILLIMOLS/LITER FOR NEGATIVELY CHARGED SOLS... 

In Part I the principles were discusseicl on which the interaction of colloidal particles may be understood, and by, way of application the case of the interaction of two parallel flat plates was dealt with in Part II. Starting from this case it was possible to form a picture of the behaviour of colloidal particles and to come to an understanding of such fundamental properties as stability, flocculation value and the rule of Schulze and Hardy. 

The curves for a = 10 , which may be considered as representative for the particle dimensions. usually found in colloid chemistry, show a very satisfactory course. For low concentrations of electrolyte x. (yr> lO " — 10 ), the potential must Iiave at least a minimum value of about 50 millivolts in order to ensure reasonable stability, whereas for higher concentrations of electrolyte x. > 10 ) must be larger. This increase of occurs the sooner, the higher the value of A, just as in the case of flat plates. The very steep upward turn of the curves indicates that in this region the stability is very sensitive to small changes in the concentration of electrolyte. This explains why it was possible to determine flocculation values with fairly great accuracy, and why this quantity played such a preponderant part in the experiments on stability. 

Again, taking a potential of 100 millivolts and a radius of 10 we find the flocculation values at 47, 2.6, and 0.3 miUi,-mols/hter. These flocculation values do not differ very much from those found in Part II and based on the model of flat plates. For larger particles the flocculation values are shifted to somewhat smaller concentrations. We return to the influence of the particle radius further on in this section. 

These curves, especially those of Figure 47, are another remarkable confirmation of the facts of colloid chemistry. It is well known that flocculation values can be determined with fairly great accuracy, once the stability criterion has been chosen. Usually this criterion is chosen in such a way that a practically complete flocculation has to result in times varying from a few minutes to 24 hours. This would correspond to... 

It is well known that many flocculated colloidal systems (e.g., different sulphides) may be. made to form stable sols again simply by washing out the flocculating electrolyte. In this, it is not sufficient to decrease the concentration of electrolyte just below the flocculation value, but it must be decreased to a very low value by carefully washing out the precipitate before repeptization occurs. 

The influence of the valency and the concentration of the electrolytes agrees with experimental data as expressed by the rule of Schulze and Hardy. As a first approximation following from the theory we find that the flocculation values for monovalent, bivalent and trivalent electrolytes are in the proportion... 

The hydrophobic colloids (suspensoids) can be flocculated easily by additkm of small quantities of electrolytes. When a dilute electrolyte solution is added drt wise to a Colloidal solution and mixed thoroughly after each addition, it is observed that a pc t comes wdien after addition of a single drop, rapid aggregation takes place leading to the formation of a coarse flocculate. The volume of the added electrol3fte at this point gives the flocculation value of the colloidal system. 

The measurement and evaluation of flocculation state is typically carried out over 10 min by means of programmed dosing, in which the flocculation value F is recorded during a stepwise increase in flocculant dosing. Figure 4 shows an example of a flocculation curve. The amount of flocculant required for optimal flocculation is that where F reaches an asymptotic limit (bridging mechanism) or a maximum (patch charge mechanism). 

The time of day, the actual flocculation aid concentration, the change in the flocculation value in comparison with the previous concentration step (dF), and the absolute flocculation value (F) are given the bar chart corresponds to the flocculation curve shown before. The optimal flocculant concentration is defined as that at which increasing of the Sedipur concentration twice no longer increases the flocculation value by more than 20%. On the basis of this Sedipur concentration, which is transmit-... 

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