Higher molecular weight polymers bridging flocculation

3 Higher molecular weight polymers (bridging flocculation) 

A large number of commercial coagulants and synthetic flocculants are available with which to pretreat suspensions. While newer products have displaced the use of more traditional chemicals in recent years, few have been eliminated completely from the marketplace. The more important and widely used pretreatment chemicals are described here. 

Although some inorganic salts can act as indifferent (non-adsorbing) electrolytes, the more important types of coagulant from a process engineering viewpoint are the salts of multivalent metal ions such as Ca, Fe +, Fe and Al +. These ions hydrolyse or specifically adsorb to particle surfaces to induce coagulation. 

As alternatives to aluminium, the salts of iron can make very effective coagulants. The most widely used is the trivalent ferric chloride (FeCl3). This is more effective than the equivalent hydrated sulphate compound Fe2(S04)3-(H20)g. 

As non-ionic flocculants carry either no charge or a very low charge in aqueous media they need to function via the bridging mechanism in order to produce flocculation. Thus, for the polymer chains to stretch sufficiently far from the particle surfaces, the non-ionic flocculant must have a high or very high 

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The joining of particles by polymers as part of the process of flocculation. Usually polymers of higher molecular weight produce higher levels of molecular bridging. 

The molecular weight of the adsorbed polymer chains around the particle may also have an influence on the colloidal stability. Intuitively, the higher the polymer molecular weight, the thicker particle surface layer of the adsorbed polymer chains and, therefore, the better the colloidal stability. However, if a very-high-molecular-weight polymer with multiple potential points for adsorption is used to stabilize the colloidal dispersion, the polymer chain may become adsorbed onto several particles and thereby lead to bridging flocculation [40]. This is especially true when the population of colloidal particles is relatively small (Figure 2.11). 

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