Characteristics of flocculation of mineral suspensions by polymers

Abstract

The mechanism of flocculation by polymers has been examined by a study of fluorite suspensions. The "bridging" theory accounts qualitatively for the data-e.g., the influence of mol. wt., the much stronger mutual adhesion of particles (and hence the more open structure of flocs) than those resulting from electrolytic coagulation, etc. However, the quantitative theory of refiltration curves developed by Smellie and La Mer5 is shown to be invalid, because it is based on erroneous assumptions as to the origin of the permeability maximum. The mechanism of adsorption of polymers by ionic crystals such as calcium fluoride has been considered, and the possibility of dipole interaction of non-ionic groups with the electrostatic field of the crystal lattice is suggested.Water-soluble polymers are widely used for flocculating suspensions of finelydivided minerals, either for clarification of the water or for recovery of the solids.1 It is generally accepted that such flocculants act by adsorption and " bridging " and this theory is in accord with the following evidence. (i) Polymers produce larger, " tougher " flocs than do simple electrolyte coagulants. (Hydrolyzing coagulants, such as alum must, however, be classed as polymeric bridging agents?) (ii) Effectiveness of polymers of a given chemical type increases greatly with increase of molecular weight .3 (iii) Linear macromolecules are more effective flocculants than highly branched compounds of comparable molecular weight and chemical type.4 (iv) At high concentrations, flocculants become protective colloids. Smellie and La Mer 5 suggested that optimum flocculation is obtained when about half the available adsorption sites are occupied by polymer. (v) Highly charged particles are not flocculated by polymer until their zeta-potential has been reduced to a low value, either by use of a polyelectrolyte of opposite sign or by addition of suitable supplementary electrolyte.laThe selection of a flocculant for a particular mineral suspension is largely by trial and error, the main obstacle to a theoretical approach being inadequate knowledge of the surface chemistry of the minerals in water and the specific mechanisms of adsorption of the polymers.Kuzkin and Nebera 1 have concluded that the following adsorption mechanisms can be distinguished. (a) General electrostatic interaction; where a surface and a polyelectrolyte are of opposite sign, flocculation can always be obtained. (b) Local electrostatic adsorption-e.g., on to individual charged sites on a, more-or-less, iso-electric surface or by a more-or-less iso-electric protein.6 (c) Hydrogen-bonding adsorption (e.g., of uncharged polyacrylamide on to alumina or freshly prepared silica 7). ( d ) Chemisorption of more specific type, such as co-ordinate bonding. In addition, it should be possible to attach polymeric ions to surfaces of like sign of charge by use of di-or tri-valent ions of opposite sign-by analogy with the use of " activators " to promote the sorption of collector ions in flotation of minerals (e.g., Pbzf with c...

Flocculation

“…The data (56) provided in Figure 2 showed, that filter-cake height depends upon concentration, that optimum permeability and maximum settling rate do not coincide, and that particle size cannot be the same in the stirred suspension and the filter cake since the settling rate, ie, measure of floc size, depends on concentration. Although these results did not support the predictive theory (56), several observations were consistent with the bridging theory of flocculation, including stronger floc formation and the deterioration of flocculation with increasing polymer concentration.…”

The mechanisms of glutaraldehyde-fixed sarcoma 180 ascites cell aggregation

“…In the present investigation, organic solvents and electrolytes are employed to assess the role of various nonbonding interactions in the aggregation of glutaraldehyde-fixed S 180 cells. Since, when a suspension of particles is allowed to settle by gravity the final sediment height is proportional to the tendency of the particles to aggregate while settling (Slater & Kitchener, 1966;Harding, 1972;Dollimore &Horridge, 1972, sediment height analysis was used to investigate the aggregation of glutaraldehydefixed S180 cells.…”

“…Sediment height analysis of polydisperse colloidal suspensions has been a commonly employed technique for several decades. Its utility arises from the fact that the final height of a sediment is proportional to the extent of particle aggrega-tion independently determined by such methods as light transmission, sedimentation velocity, fluid filtration rate, viscosity, subsidence rate, and electrophoretic mobility (Slater & Kitchener, 1966;Harding, 1972, Dollimore & Horridge, 1972. The more extensively a suspension of particles aggregates as it settles out of solution, the greater will be its final sediment height in a column of specified geometry.…”

Flocculation of Sulfamerazine Suspensions by a Cationic Polymer

“…Subsequent investigations confirmed the validity of a bridging mechanism in polymer flocculation (5-8). Charge neutralization may be involved in the attachment of the polymer to the particle surface (5), although a nonionic mechanism also may be operative (6). The first study of polymer flocculation in pharmaceutical systems (7) concerned the effect of xanthan gum in several suspensions.…”