Polymer concentration effect flocculation

Polymer Concentration. The effect of polymer dosage on additional flocculation responses was measured together with adsorption... 

To obtain a suitable concentrate, the flocculated particles must be separated from the suspension. The usual method is sedimentation of the floes combined with elutriation of the dispersed particles. Flotation of the flocculated particles is a possible method to achieve that separation. The effect of polymers used as flocculants on the flotation of a few minerals has received... 

At moderate to high polymer concentrations, the free polymer chains in the solution may begin to exercise an influence. One such effect is the so-called depletion flocculation caused by the exclusion of polymer chains in the region between two particles when the latter are very close to each other (i.e., at surface-to-surface distances less than or equal to approximately the radius of gyration of the polymer chains). The depletion effect is an osmotic effect and is discussed further in Section 13.6. 

Because of restrictions on the number of possible configurations, non-adsorbing polymers tend to stay out of a region near the surfaces of the particles, known as the depletion layer. As two particles approach, the polymers in the solution are repelled from the gap between the surfaces of the particles. In effect the polymer concentration in the gap is decreased and is increased in the solution. As a result, an osmotic pressure difference is created which tends to push the particles together. The resulting attractive force is the reason for depletion flocculation. In contrast to this, depletion stabilisation has been mentioned above. 

Addition of a surfactant can further enhance the flocculation at the ultralow polymer dosage regime. As shown in Figure 7.37, the addition of anionic surfactant (SDS) significantly improves the flocculation at very low polymer concentrations. At higher polymer levels, the surfactant addition has no effect on flocculation. This is probably due to a significant fraction of adsorption sites already occupied by polymer, making them unavailable for adsorption of the surfactant molecules. 

Water-soluble cellulose derivatives themselves adsorb onto solid particles and may for instance affect the suspension properties of these insolubles. The mechanisms involved are quite complex and depend on the polymer concentration. At low concentrations macromolecules influence the electrophoretic mobility and the flocculation of the particles. At higher concentrations, surface coverage by the adsorbed polymer is sufficient to prevent particle-particle interaction and thus to stabilize the suspension sterically. As an example, the effect of NaCMC (among other polymers) on the zeta potential, flocculation and sedimentation properties of sulfadimidine has been investigated by Kellaway and Najib [115,116],... 

The polyacrylate polymers and a derivative of a vinyl acetate maleic anhydride copolymer cause V30 to decrease monotonically with increasing polymer concentration, similar to the CMC polymers (Figure 46). The polymers PVA and poly(vinyl pyridinium) (PVP) hydrochloride markedly increased V30 at low concentration at concentrations above 1 g of polymer per gram of added bentonite PVA functions as a static fluid loss additive. The maximum in the API fluid loss at low PVA concentrations approximately coincides with the maximum in the yield stress and plastic viscosity found by Heath and Tadros (75). The increased static fluid loss is consistent with Heath and Tadros s conclusion that bentonite is flocculated by low concentrations of PVA. The concentration of PVA required to decrease V30 below that of the neat bentonite suspension is significantly larger than the concentration of CMC, where effective static fluid loss control can be achieved at polymer bentonite weight ratios of about 0.1 g/g. More effective fluid loss control has been achieved with other synthetic polymers such as poly(vinyl sulphonate)-poly(vinyl amide) copolymer (40) and other sulphonated polymers (39). 

Macromolecular colloid solutions also play an important role in ensuring the stability of disperse systems (e.g. suspensions, emulsions). In the case of emulsions the polymer decreases the rate of separation by increasing viscosity on the one hand, and it has an enthalpy stabilizing effect by adsorption on the surface of the droplets on the other hand [3, 4, 7]. Depending on the concentration of the polymer, a protecting and flocculating effect can be observed during the interaction between suspensions and polymers. If the polymer concentration is low, the polymer adsorbed on the surface of the particles connects the particles into loose floccules. Thereby, the rate of... 

At very low polymer concentrations (say several ppm), the predominant effect, if an effect is indeed observed, is bridging flocculation. This can be observed not only with electrostatically stabilized particles but also with certain sterically stabilized particles (Evans and Napper, 1973d). It has been shown that even quite low molecular weight poly(oxyethylene) can... 

The polymer emulsions can be concentrated by headng imder vacuum to remove excess water and organic solvent by evaporation [43]. In this method, it is possible to reduce the water down to 2% and to increase the polymer crmtent in the emulsion up to 70% [30]. To be effective flocculants, the polymer emulsions must exhibit excellent freeze-thaw properties [11]. Another important requirement concerns their ability to invert wiA excess water to yield a highly viscous dilute polymer solution used as such for applications. A wetting ageru like a surfactant with a high HLB value (10-14) may be used to facilitate inversion. Typical surfactants are oxyethylated alkylphenols, fatty alcohols or fatty acids. They are added (up to 5% by weight of the total formulation) to water or to polymer emulsion prior to dissolution [22,43]. 

The formation of floes due to bridging flocculation has a dramatic effect on sedimentation rates, sediment volumes and on the ease of filtration. Effective flocculation may occur over a narrow range of polymer concentration because too little polymer will not permit floe formation, while too much polymer adsorption will eliminate the fraction of free particle surface needed for the bridging action (i.e. the polymer molecules will adsorb onto single particles in preference to bridging several particles). It has been proposed that the optimum degree of bridging flocculation may occur when particle surfaces are half covered with adsorbed polymer. 

Figure 1. Effect of Polymer Concentration on Settling Rate of Flocculated Gypsum.

The range over which depletion attraction operates equals 2R. In particular, for highly swollen polymers, R may reach values of some tens of nanometer and, hence, the depletion forces may be effective over separation distances between particles that exceed the range of dispersion and double layer forces (cf Section 16.1). On the other hand, the osmotic forces are relatively weak. Depletion flocculation occurs when the molar polymer concentration is sufficiently high, which is more readily achieved by using polymers of a relatively low degree of polymerization. 

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