Flocculation effective size

The proper measure of flocculation effectiveness is the performance of subsequent solids separation units in terms of both effluent quality and operating requirements, such as filter backwash frequency. Effluent quality depends greatly on the reduction of residual primary size particles during flocculation, while operating requirements relate more to the floe volume applied to separation units. 

The polymer radius has to be larger than 80% of the particle radius to avoid adsorption limitation under orthokinetic conditions. As a rule of thumb a particle diameter of about 1 pm marks the transition between perikinetic and orthokinetic coagulation (and flocculation). The effective size of a polymeric flocculant must clearly be very large to avoid adsorption limitation. However, if the polymer is sufficiently small, the Brownian diffusion rate may be fast enough to prevent adsorption limitation. For example, if the particle radius is 0.535 pm and the shear rate is 1800 s-, then tAp due to Brownian motion will be shorter than t 0 for r < 0.001, i.e., for a polymer with a... 

The particles in a sound field, however, behave in a peculiar manner which is not quite so simple. St. Clair points out that at a frequency of 5000 cycles, particles of 0.5 n or smaller (density = 1.5) have an amplitude and velocity the same as that of the surrounding gas, while particles of 10 p will scarcely vibrate at all. The intermediate sizes will pulsate out-of-phase with the pulsations of the gas stream. Thus particles 2 u will have an amplitude of 0.87 that of the gas and will be about 30 deg out-of-phase. It has been suggested by Brandt and Hiedemann that the flocculating effect is due to the increased number of collisions between the particles due to the kinetic energy imparted to them. However, as St. Clair points out, this cannot be the sole factor since flocculation is observed at a few hundred cycles for which the suspended particles remain stationary. 

Suspensions of liposomes, microspheres and microcapsules, and nanospheres and nanocapsules formed from a variety of polymers or proteins, as discussed in section 8.6.3 form a new class of pharmaceutical suspension in which physical stability is paramount. It is important that on injection these carrier systems do not aggregate, as this will change the effective size and the fate of the particles. The exception to this is the deliberate flocculation of latex particles administered to the eye, where aggregation leads to agglomerated... 

The effect of different flocculants particle size on flocculation properties... 

Sedimentation in emulsions may also be accompanied by the aggregation of emulsion droplets, referred to as flocculation. Flocculation leads to an increase in the effective size of settling aggregates, and as a result, leads to a higher sedimentation velocity. In dilute finely dispersed emulsions in which electrostatic stabilization is of primary importance, the major laws governing flocculation are close to those of coagulation of hydrosols, and are given by the DLVO theory (see Chapter VII, 4). In such systems flocculation may be reversible. 

In HEC-thickened formulations, low-shear-rate viscosities increase with decreasing latex particle size. This effect has been a major limitation in formulating small-particle latices. The phenomenon appears to arise from electro viscous, hydration, or flocculation effects, not a depletion layer mechanism. Associative thickeners achieve efficient viscosity in coating formulations via participation in synthesis and formulation surfactant micelles to form pseudo macromolecules and via an ion-dipole interaction between the cations of surface carboxylate groups on the latex and the ether linkages of the associative thickener. Generally, an excess of synthesis surfactant is found in the production of small-particle latices. The achievement of lower viscosities in small-particle ( 100 nm) latex coatings thickened with associative thickener appears to occur by extensive disruption of the polymer hydrophobe s participation in intermicellar networks. 

Compared to macro- and miniemulsions, the water uptake is lower in microemulsions. By virtue of their smaller sizes and, subsequendy, the more efficient packing of the surfactant at the interface, microemulsions are not subject to flocculation effects. In the field of microemulsions, the water uptake is defined as the molar ratio of water to surfactant, W. The corrected Wq (called taking account of the solubility of water in CO2 is defined as ... 

Water soluble polymers can influence the stability of emulsions to gravitational separation in a variety of ways. Non-adsorbed polymers may either increase or decrease stability depending on their effective size and concentration in solution. Increasing the concentration of a polymer in solution causes an increase in continuous phase viscosity (and may even lead to gelation), which should slow down droplet movement. On the other hand, the presence of non-adsorbed polymer also increases the magnitude of the depletion attraction between droplets, which may cause flocculation and therefore accelerate droplet movement. The presence of an adsorbed polymer may also influence stability to gravitational separation in a number of ways. For example, the size of the droplets produced... 

Generally, measurable (directly or indirectly) physical properties of the solids and the water in which these solids are suspended effectively determine the rate at which particles settle and whether or not hydraulic shear forces are sufficient to keep the particles suspended in the water column. The chemical properties of the solids and the water can also influence the deposition process through particle aggregation (flocculation) and the effect this has on the effective size, density, and shape of the suspended material. 

Thus, at a concentration of 0.95 g Na2S /100 g solution, the solubihty of mercuric sulfide has increased to 2100 ppm. It is customary to use no greater than a 20% excess of the alkah sulfide. Because the particle size of the precipitated mercuric sulfide is so small, it is helpful to add a ferric compound such as ferric chloride or ferric sulfate to effect flocculation. Sometimes other flocculating agents (qv) may also be added, eg, starch or gum arabic. 

Another type of flocculation results from particle—particle collisions caused by differential settlement. This effect is quite pronounced in full-size plants where large rapidly falling particles capture small particles that settle more slowly. 

The measured diameters of particles shoiild as nearly as possible represent the effective particle size of a dust as it exists in the gas stream. When significant flocculation exists, it is sometimes possible to use measurement methods based on gravity settling. 

Solids separation based on density loses its effectiveness as the particle size decreases. For particles below 100 microns, separation methods make use of differences in the magnetic susceptibility (magnetic separation), elec trical conductivity (electrostatic separation), and in the surface wettability (flotation and selec tive flocculation). Treatment of ultrafine solids, say smaller than 10 microns can also be achieved by utilizing differences in dielectric and electrophoretic properties of the particles. 

---