The Role of Polymers in Dispersion Stability

R. Hogg and C. Rattanakawin

DEPARTMENT OF ENERGY AND GEO-ENVIRONMENTAL ENGINEERING, THE PENNSYLVANIA STATE UNIVERSITY, UNIVERSITY PARK, PA 16802, USA

1 Introduction

Soluble polymers are widely used to control the state of dispersion of fine-particle suspensions. Depending on the polymer, and how it is applied, they can serve to enhance stability (dispersants) or to promote aggregation of the particles (flocculants). The topics covered in this chapter are intended as an overview of the use of polymers for stability control in mineral-particle suspensions with particular emphasis on flocculation processes. A brief discussion of stabilisation by polymers is included for completeness.

1.1 Polymeric Dispersants

Both natural and synthetic polymers with molecular weight up to about 20000 are commonly used for stabilisation of fine-particle dispersions. It is generally accepted that these reagents function by presenting a steric barrier to direct contact between particles (steric stabilisation). In effect, adsorption of the soluble polymer provides a lyophilic film on the particle surfaces that prefers contact with the solvent to contact with a similar film on another particle. The basic requirement for steric stabilisation is that the adsorbed layer be thick enough to prevent particles from approaching each other closely enough for attractive forces (e.g. van der Waals) to cause adhesion. More or less complete coverage of the surfaces is generally necessary. Charged polymers (polyelectrolytes) can further enhance the effect by adding an electrostatic repulsion (electrosteric stabilisation). Addition of the electrostatic component may serve to relax the coverage requirements for purely steric stabilisation.

1.2 Polymeric Flocculants

Polymers used to promote flocculation normally have higher molecular weight than the dispersants – usually from about 50 thousand up to about 20 million. Most of those in use today are synthetic, linear-chain molecules, and include non-ionic, anionic and cationic types. Non-ionic flocculants are commonly polyacrylamide, while the anionic reagents are often acrylamide/acrylate copolymers. Typical non-ionic and anionic flocculants have high molecular weights – in the 10 to 20 million range. Cationic flocculants (sometimes referred to as coagulants) are normally of lower molecular weight (<1 million) and commonly owe their charge to the presence of a quaternary ammonium group. The polydiallyldimethylammonium chloride (DMDAAC) types are widely used commercially. Charge densities are usually substantially higher for the cationic reagents than for the anionic types.

Flocculation by polymers, like dispersion, normally involves adsorption at particle surfaces. The high molecular weight flocculants are generally agreed to function through a 'bridging' mechanism whereby a single large molecule can attach to two or more particles simultaneously, providing a physical link – bridge – between them. Smaller, highly charged polyions can adsorb on oppositely charged particle surfaces forming patches of opposite charge. Thus, a cationic polymer can form positively charged patches on the surface of negatively charged particles. Interaction of a patch on one particle with a region of bare surface on another then leads to aggregation. A third mechanism, known as depletion flocculation, does not involve adsorption. Rather, the exclusion of large polymer molecules from small spaces between particles in concentrated suspensions promotes aggregation through osmotic pressure effects.

1.3 Polymer Solutions

The properties of polymer solutions play an important role in the dispersion or flocculation of fine-particle suspensions. Individual molecules in solution generally take on a more-or-less randomly coiled configuration. Repulsion between similarly charged ionic groups in a polyelectrolyte leads to expansion of the coil, increasing the effective size of the molecule in solution. Modification of the charge, e.g. by protonation of acid groups in an anionic polymer, gives rise to changes in molecular conformation. Shielding of the charges in the presence of simple electrolytes generally leads to increased coiling, i.e. smaller effective molecular size.

2 Polymer Adsorption

The adsorption of polymers at solid surfaces is substantially more complicated than that of small molecules. Typically, a small molecule adsorbs by attachment of a functional group on the molecule to a site on the surface. For the case of adsorption from aqueous solution, the process involves displacement of water from sites at the surface. The extent of adsorption is determined by the preference of the surface site and the adsorbing species for contact with water rather than with each other. While the same is generally true for polymers, the presence of a large number of potential attaching groups on each molecule biases the system towards adsorption. A typical polyacrylamide flocculant with a molecular weight of 10 million has about 140000 individual segments, each of which is capable of attaching to the surface. Only one such attachment is necessary for adsorption of the whole molecule. The consequences of this 'multiple adsorption' effect are:

• Polymer adsorption tends to be indiscriminate. The large molecules become attached to essentially any available surface unless strongly repelled from it.

• Adsorption is effectively irreversible. Desorption of the molecule requires simultaneous detachment of all adsorbed segments.

• Since the...