Flocculation higher molecular weight polymers

Higher molecular weight polymers (bridging flocculation)... 

Flocculants and surfactants (qv) are used frequently as filter aids, particularly when slimes are present or when the particles to be filtered are very fine and difficult to filter. Low molecular weight polymers are more commonly used. These form small, dense floes which provide higher cake porosity. 

Polyacrylamides are nonionic polymers, usually with much higher molecular weights (MW from 100,000 up to 12 or 15 M). They often are copolymerized with polyacrylates. Depending on the MW ratios employed, they may act as colloidal dispersants, sludge conditioners, or flocculants. Nonionics such as polyacrylamides (and isobutylenes) are particularly useful at dispersing uncharged particles. 

Adsorption rates were not significantly affected by molecular weight, but flocculation was about 25% faster for the high molecular weight polymer. Two shear rate levels were tested 1800 s-1 and 8000 s-. The absolute adsorption and flocculation rates increased with shear rate as expected. The "pseudo" OFC appeared to be shifted to a higher value for the higher shear rate. Collision efficiencies were affected by both molecular weight and shear rate, as discussed below. 

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. 

Depletion flocculation theories all predict qualitatively the observed dependence of the critical concentration of free polymer required for flocculation on both the molecular weight of the free polymer and the particle radius. All of the observations published to-date point to (i) higher molecular weight free polymer being a more potent flocculant than lower molecular weight species, and (ii) larger particles being more readily flocculated than smaller ones. 

This purer acrylamide can be polymerized to a higher molecular weight than is possible with the chemical product Since much of the polyacylamide is used as a flocculant whose effectiveness increases with its molecular weight, smaller amounts of polymer prepared from the enzymatic acrylamide are needed for those precipitation processes in which it is the preferred flocculant. Some of these processes are found in the water-treatment industry, where a reduced input of a purer reagent is preferred. It therefore seems likely that the cleaner enzymatic process, which currently produces some 30,000 tonnes of acrylamide annually, will take a progressively larger share of the current annual production of about 200,000 tonnes. 

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). 

In the polyelectrolyte precipitation of protein, an important but less-studied polyelectrolyte parameter is molecular weight. In non-protein systems, the effect of polymer molecular weight on ffocculation has been studied, but these results do not address the precipitation mechanism because, for flocculation, insoluble particles are already present. Observations directly from protein precipitation are that precipitation yields are higher with larger molecular weight polymers [4] and that the binding of poly(acrylic acid) with bovine serum albumin [5] and chymotrypsin [6] increases with PAA molecular weight. 

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