Lee, Shun Yin; (1991) The flocculation of charged particles in aqueous solutions by cationic polyelectrolytes. Doctoral thesis (Ph.D.), University College London.

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Abstract

The flocculation of a dilute suspension of silica particles (size range 1-3μm) has been studied using six cationic polyelectrolytes. The flocculation process is monitored by an optical method which measures the fluctuations of light intensity transmitted through the flowing suspension. An increase in light intensity fluctuations indicates aggregation of particles. The reduction in particle concentrations due to aggregation can be confirmed by Coulter counting. The charge neutralization effect of the adsorbed polymer is the main mechanism for flocculation. The amount of positive charge adsorbed at optimum dosage is about the same for all of the polymers used. The reversal of particles charge is also found to occur at a dosage very close to the optimum flocculation dosage. However, the charge density and the molecular mass of the polymer can affect the rate of flocculation. The polymer adsorption rate can be slow compared to the particle collision rate in a dilute suspension, especially when the polymer is of low charge density and low molecular mass. A high molecular mass polymer of 30% charge density is found to be the most effective flocculant. The range of effective flocculation dosages is less critical in the presence of added electrolyte but the highest flocculation rate always occurs in the absence of salt. As the particles are neutralized by both the added electrolyte and the adsorbed polymer, a lower polymer dosage is able to bring about effective flocculation in salt solution. The charge screening effect of the added electrolyte allows higher polymer dosages to give effective flocculation before particles become restabilized due to excess polymer. The need to displace adsorbed ions is thought to be responsible for the lowering of the optimum flocculation rate. The larger the size of counter-ions to the particles the greater is the effect on the lowering of the ability of polymer to cause flocculation. The size, degree of hydration and adsorbability of ions are interdependent. The stability of the dispersion is found to be dependent on the ionic strength, the type of electrolytes and the characteristics of the polymers.

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