Flocculation of suspensions

By using the equations for G i and G, the critical coagulation concentration (ccc) could be calculated, as will be shown below. Theory predicts that the ccc is directly proportional to the surface potential y/, and is inversely proportional to the Hamaker constant A and the electrolyte valency Z. As will be shown below, the ccc is inversely proportional to Z at high surface potential, and inversely proportional to Z at low surface potential. 

As discussed above, the condition for kinetic stabihty is 25 kT, but when 

Gjnax 5 IcT then flocculation will occur. Two types of flocculation kinetics may be distinguished (i) fast flocculation with no energy barrier and (ii) slow flocculation, where an energy barrier exists. 

The fast flocculation kinetics was investigated by Smoluchowski [12], who considered the process to be represented by second-order kinetics and the process to be simply diffusion-controlled. The number of particles n at any time t may be related to the initial number (at t = 0) by the following expression  

The half-life t 2 (w = (1/2) can be calculated at various values of or volume fraction cp, as listed in Table 7.4 

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Flocculating agents can be simple electrolytes that are capable of reducing the zeta potential of suspended charged particles. Examples include small concentrations (0.01-1%) of monovalent ions (e.g., sodium chloride, potassium chloride) and di- or trivalent ions (e.g., calcium salts, alums, sulfates, citrates or phosphates) [80-83], These salts are often used jointly in the formulations as pH buffers and flocculating agents. Controlled flocculation of suspensions can also be achieved by the addition of polymeric colloids or alteration of the pH of the preparation. 

J. Phys. Chem., 1962, 66, 1835 see also Audsley, A., Mineral Processing Information Note No. 5 "Flocculation of Suspensions of Solids with Organic Polymers ---- A Literature Sur-... 

Gregory, J. Stability and Flocculation of Suspensions in Processing of Solid-Liquid Suspensions, Shamlou, P.A. (Ed.), Butterworth-Hei-nemann Boston, 1993, pp. 59-92. 

Zatz JL, Lue R-Y. Flocculation of suspensions containing nonionic surfactants by sorbitol. J Pharm Sci 1987 76 157-160. 

Lower molecular weight polymers are generally better suited to the flocculation of suspensions which subsequently undergo vacuum or pressure filtration ... 

The above rheological techniques can be used to assess sedimentation and flocculation of suspensions. This will be discussed in detail below. 

In one of the earliest attempts, Schuetz and Piesche [73] calculated the flow field in a stirred tank first to determine the local energy dissipation rate and then solved the PBEs using the finite volume method [74] to predict the local aggregate size distribution. Heath and Koh [75] have solved the population balances as scalar equations in the commercial CFD software CFX for simulating flocculation of suspensions by polymers. They employed 35 discrete sectional equations to represent the aggregate size distribution. 

Often the van der Waals attraction is balanced by electric double-layer repulsion. An important example occurs in the flocculation of aqueous colloids. A suspension of charged particles experiences both the double-layer repulsion and dispersion attraction, and the balance between these determines the ease and hence the rate with which particles aggregate. Verwey and Overbeek [44, 45] considered the case of two colloidal spheres and calculated the net potential energy versus distance curves of the type illustrated in Fig. VI-5 for the case of 0 = 25.6 mV (i.e., 0 = k.T/e at 25°C). At low ionic strength, as measured by K (see Section V-2), the double-layer repulsion is overwhelming except at very small separations, but as k is increased, a net attraction at all distances... 

Comprehension of the interactions among microstructures composed of tethered chains is central to the understanding of many of their important properties. Their ability to impart stability against flocculation to suspensions of colloidal particles [52, 124, 125] or to induce repulsions that lead to colloidal crystallization [126] are examples of practical properties arising from interactions among tethered chains many more are conceivable but not yet realized, such as effects on adhesion, entanglement or on the assembly of new block copolymer microstructures. We will be rather brief in our treatment of interactions between tethered chains since a comprehensive review has been published recently of direct force measurements on interacting layers of tethered chains [127]. 

In a series of experiments on the flow of flocculated kaolin suspensions in laboratory and industrial scale pipelines(26-27-2Sl, measurements of pressure drop were made as a function of flowrate. Results were obtained using a laboratory capillary-tube viscometer, and pipelines of 42 mm and 205 mm diameter arranged in a recirculating loop. The rheology of all of the suspensions was described by the power-law model with a power law index less than unity, that is they were all shear-thinning. The behaviour in the laminar region can be described by the equation ... 

A highly concentrated suspension of flocculated kaolin in water behaves as a pseudo-homogeneous fluid with shear-thinning characteristics which can be represented approximately by the Ostwald-de Waele power law, with an index of 0.15. It is found that, if air is injected into the suspension when in laminar flow, the pressure gradient may be reduced even though the flowrate of suspension is kept constant, Explain how this is possible in slug flow and estimate the possible reduction in pressure gradient for equal volumetric flowrates of suspension and air. 

JL Zatz, L Schnitzer, P Sarpotdar. Flocculation of sulfamerazine suspensions by a cationic polymer. J Pharm Sci 68(12) 1491-1494, 1979. 

An improved suspension has been developed, which controls the flocculation of the insoluble active ingredient particles, such that they will remain sub-... 

In part II of the present report the nature and molecular characteristics of asphaltene and wax deposits from petroleum crudes are discussed. The field experiences with asphaltene and wax deposition and their related problems are discussed in part III. In order to predict the phenomena of asphaltene deposition one has to consider the use of the molecular thermodynamics of fluid phase equilibria and the theory of colloidal suspensions. In part IV of this report predictive approaches of the behavior of reservoir fluids and asphaltene depositions are reviewed from a fundamental point of view. This includes correlation and prediction of the effects of temperature, pressure, composition and flow characteristics of the miscible gas and crude on (i) Onset of asphaltene deposition (ii) Mechanism of asphaltene flocculation. The in situ precipitation and flocculation of asphaltene is expected to be quite different from the controlled laboratory experiments. This is primarily due to the multiphase flow through the reservoir porous media, streaming potential effects in pipes and conduits, and the interactions of the precipitates and the other in situ material presnet. In part V of the present report the conclusions are stated and the requirements for the development of successful predictive models for the asphaltene deposition and flocculation are discussed. 

Precipitation and accumulation of clay. Precipitation of clay particles takes place at some depth in the soil as a result of 1. flocculation of clay particles, or 2. (mechanical) filtration of clay in suspension by fine capillary pores. 

Steric Stabilization. Steric stabilization was a term first introduced by Heller to explain how adsorbed polyethylene oxide polymers increased the salt concentration required for flocculation of negatively charged aqueous suspensions.(6) Heller s systems were stabilized by both mechanisms, as are most commercial dispersions today, aqueous and non-aqueous. Much of the more recent literature on steric stabilizers has been preoccupied with solubility requirements, for the solubility of polymers is a delicate matter and very sensitive to temperature and solvent... 

Therefore, a systematic examination of the influences of pH and electrolyte concentration on the state of flocculation of both silica types seemed promising. For a quantitative examination of the state of flocculation a Laserphoto Sedimentometer was used to follow the turbidity and the sedimentation of the suspension as a function of time. This method was described in detail in our earlier work (1,10). 

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