Modifying Rheology

Dispersion and flocculation

The effect of aqueous surface chemistry is most important for small particles, such as those with a diameter less than 10 microns. Typically this occurs in slurries. Weak interparticle bonds in a slurry of flocculated particles make the slurry more viscous than a slurry of dispersed particles.

Therefore, a proportionately smaller number of particles are required to solidify a slurry containing flocculated particles. Conversely, if a slurry contains highly dispersed particles it will have a low viscosity, since the internal structure of a dispersed slurry approaches that of a liquid.

Particles in flocculated slurries settle faster than particles in well dispersed slurries, and this allows faster “dewatering” or solid-liquid separation. However, flocculated particles pack more loosely than dispersed particles, so a larger final volume of sediment is obtained from a flocculated slurry than from a dispersed slurry.

Flocculated systems exhibit shear thinning (plastic behaviour) as the floc structure breaks up. Sometimes this is irreversible. Dispersed systems containing uniformly sized particles and pseudo-spherical particles can show both dilatant and thixotropic behaviour.

Significance of pH

A slurry will be most viscous at a pH when its particles are at their point of zero charge. Mineral surfaces are generally acidic, and the point of zero charge occurs at acidic pH. The point of zero charge for common clays is ~pH 5. For clay mineral slurries, increasing the pH into alkaline regions will increase the effective charge on each particle, and create conditions for particle dispersion. The apparent viscosity of such a clay slurry will markedly reduce. Not all minerals are acidic, however.

The point of zero charge for some alumina minerals is near pH 8. In all cases, a slurry will be least viscous at a pH most distant from its particles’ point of zero charge. This can occur at pH regions both higher and lower than the particles’ isoelectric point.

Effect of polyelectrolytes and multivalent ions

In addition to changing the pH, slurries can be dispersed or flocculated by adding suitable ions and polymers. If the slurry contains dispersed charged particles, these can be neutralised by adding multivalent ions of the opposite charge. An example is flocculation of negatively charged organic and inorganic particles using Al3+, or Fe3+.

If particles in a slurry are not sufficiently dispersed, the particle charge can be increased by adding a polyelectrolyte. The most common polyelectrolytes used as dispersants are polyphosphates and polymers of organic acids. The amounts of ions and polymers required to effect a change are typically very small (20 - 1,000 ppm).

Typical Organic Additives:

Polyacrylamide, which forms a high molecular weight, variable charge polyectrolyte. This is common as a flocculant, which acts to bridge particles. Formula: [-CH₂-CH(CONH₂)-]_n

Typical Inorganic Additives:

• these are negatively charged dispersants for alkaline conditions. The active species are phosphorus polyanions, which are unstable in acid.
  
  
• sodium tripolyphosphate is sometimes known as STPP, and is used in laundry detergent.
  

• in these additives, the multivalent cations are the active species, so these are positively charged coagulants and flocculants.
  

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