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 adsorption of individual segments is quite reversible, the conformation of the adsorbed molecules can vary continuously and the molecules are free to migrate over the solid surface.

• The strength of the segment-surface site interaction has more effect on the conformation of the adsorbed molecules than on the number of molecules adsorbed.

• The effective 'parking area' of the adsorbed molecule may vary with the extent of adsorption. As more molecules are adsorbed, the area occupied by each may decrease due to a crowding effect.

When the adsorbing polymer acts as a flocculant, the adsorption process is further complicated by concurrent aggregation of the particles. Adsorption of the polymer provides the linkages needed for bridging flocculation while the formation and growth of aggregates reduces the surface area available for adsorption. An interesting consequence is that effective flocculation actually reduces the amount of polymer adsorbed. An example of this inverse relationship is given in Figure 1. Adsorption densities at pH 4.5 and 11, where the dispersion was initially stable and flocculation by the polymer was poor, are more than ten times greater than those on the flocculated suspension at pH 9.

Because polymer adsorption is effectively irreversible, and because adsorption and floc growth occur simultaneously, flocculation is a non-equilibrium process. As a result, performance is largely determined by the kinetics of adsorption and aggregation. Both of these can be regarded as collision processes involving solid particles and polymer molecules. In each case, collisions can arise due to either Brownian motion or agitation of the suspension. The collision frequency v between particles and polymer molecules can be estimated from:

v = Knpn (1)

where n and np are the respective number concentrations for solid particles and polymer molecules and K is a rate constant. For Brownian motion:

k = 2kT/3μ (1 + xp/x + x/xp)

where k is Boltzmann's constant, T is absolute temperature, μ is the fluid viscosity and x and xP are the sizes of the particles and molecules in solution. In the case of collisions due to agitation:

K = G/6 (x + xp)3 (3)

where G is the mean shear rate due to agitation.

The relative importance of the two mechanisms can be seen in Figure 2. The curves indicate, for different shear rates, molecular weight/particle size combinations for which the rates are equal. Combinations to the right of the lines represent domination by shear. It is clear that the rates are dominated by the shear mechanism for most conditions of practical interest.

3 Polymers and Dispersion Stability

Dispersions of fine mineral particles can be stabilised by direct electrical charging of the particles or by steric/electrosteric protection from adsorbed polymers. Stabilisation by direct charging is well described by the classical DLVO theory.

3.1 Steric Stabilisation

Dispersion stabilisation generally requires fairly complete coverage of particle surfaces by adsorbed polymer. Polymer molecular weight should be sufficiently large to prevent close approach of the particles, but not so large as to promote bridging flocculation. Purely steric stabilisation usually involves high polymer dosage – of the order of 1 mg per square metre of solid surface.

3.2 Electrosteric Stabilisation

In many practical cases, stabilisation by polymers involves a combination of steric and charge interactions. Unlike simple electrolytes, multiple adsorption effects permit polyelectrolytes to continue to adsorb well beyond the point where the adsorbed layer charge exceeds that of the particle surface. In this way, the effective charge on particles can be increased substantially at relatively low surface coverage by the polymer.

4 Flocculation by Polymers

In general, flocculation processes involve two basic steps:

• Destabilisation of the dispersion to permit particle–particle collisions and aggregate formation.

• Growth of small aggregates to form large flocs.

Polymers can play a role in both of these.

4.1 Destabilisation

The stability of suspensions of fine mineral-particles usually results from the electrical repulsion forces that prevent association of particles into aggregates. These can be minimised by reducing surface potentials – often by appropriate pH adjustment – or by shielding of the charge using simple electrolytes. In principle, polyelectrolytes can also serve to shield the charge on particle surfaces, but the charge-patch and bridging mechanisms described above are probably more important in practice.

An example of destabilisation by a polyelectrolyte, probably through the charge-patch mechanism, is given in Figure 3. The figure shows the evolution of the floc size distribution with the addition of a cationic polymer (molecular weight about 100000) to a suspension of fine alumina particles in water at pH 11. Initially the dispersion was stable due to the negative charge on the particles at that pH. Upon addition of the polymer, the distribution first becomes bimodal, consisting of small flocs and some residual primary particles. As more polymer is added, the primary-particle mode disappears and the distribution becomes unimodal again at a polymer concentration of about 0.5 mg L-1 (0.0167 mg per gram of solids). However, further polymer addition has very little effect, other than increasing the median floc size from about 6–7 µm at 0.5 mgL-1 to about 9 µm at 5 mgL-1. Apparently, the cationic polymer is effective for destabilising the suspension but not for promoting floc growth.

Very similar effects can be seen for higher molecular weight (about 15 million) non-ionic and anionic polymers. Some results for the addition of a non-ionic polyacrylamide to the same alumina under the same conditions are given in Figure 4. The unimodal-bimodal-unimodal progression can be seen again. The principal differences from the cationic polymer are that more polymer is needed to eliminate the primary-particle mode (about 3 mg L-1) but continued polymer addition leads to significant growth of the flocs – up to about 80 µm at 5 mg L-1of the polymer. For the anionic polymer, even more polymer is required to eliminate the primary particles (10mg L-1) and floc growth continues up to more than 200 µm. It seems that the higher molecular weight polymers are less effective for destabilisation of the primary particles but more effective for promoting floc growth. The anionic polymer is able to induce flocculation despite the similar charge on the polymer and the particles. Apparently, the high molecular weight permits adsorption and bridging to occur.

It is interesting to compare destabilisation by polymers with equivalent results for simple electrolytes. The effects of adding calcium chloride to the alumina suspension at pH 11 are shown in Figure 5. At a salt concentration of about 10-4 moles L-1 (11 mg L-1), the size distribution changes abruptly from that of the primary particles to a unimodal floc size distribution centred at about 6 µm.

Destabilisation of the alumina at pH 5 where the particles are positively charged shows quite similar trends, the principal difference being that the cationic polymer is completely ineffective. Presumably, the positive charges on both particles and polymer are sufficient to prevent adsorption of the relatively small, highly charged polyions. The behaviour of the non-ionic and anionic polymers is essentially the same as at pH 11. Destabilisation by the addition of divalent sulfate ions at pH 5 is virtually identical to that observed for calcium at pH 11. Adjustment of the pH to around the point of zero charge (pH 9) leads to the same floc size distributions as the simple salt additions.

4.2 Floe Growth

Collisions and adhesion between particles/flocs lead to floc growth. The collision mechanisms described previously for polymer adsorption apply equally to floc formation and growth. Collisions due to shear can be expected to dominate for most practical conditions. In order for growth to continue, it is necessary for the collisions to lead to adhesion and for the resulting flocs to be resistant to degradation and breakage in the agitated system. Destabilisation of the dispersion should ensure that particles adhere on contact, but the forces involved will not necessarily be sufficient to withstand disruption of a large floc subjected to shear. In processes for deaggregation or breakage of particles under applied stress, it is generally observed that breakage rates increase with increasing particle or aggregate size. It follows that simultaneous growth and breakage in flocculation processes can be expected to lead to the approach to a limiting floc size, at which growth and breakage rates are equal. Stronger flocs should grow to larger limiting sizes.

It is postulated that polymers can perform a dual function in flocculation. They can aid in destabilisation as noted above and also serve as binders in the growing flocs. The results shown in Figures 3–5 are clearly consistent with this concept. Destabilisation by pH control or simple salt addition is sufficient to allow individual particles to adhere on contact, but the resulting flocs are weak and can only grow up to a relatively small limiting size (about 6 µm for the conditions applying in Figure 5). The use of a polymer produces stronger flocs as reflected in the larger limiting sizes shown in Figures 3 and 4: over 100 µm for the high molecular weight non-ionic and anionic polymers. The low molecular weight cationic polymer appears to provide only a minor increase in floc strength.

It is apparent that pH control or the addition of simple salts or low molecular weight polyelectrolytes is typically sufficient for destabilising fine-particle dispersions, while high molecular weight polymers are necessary to permit floc growth to large sizes. It follows that the use of reagent combinations should offer considerable potential for efficient flocculation. Some examples of this approach are illustrated in Figures 6 and 7. The effects of pH control and salt addition can be seen in Figure 6. Addition of a non-ionic polymer directly to the stable dispersion requires 9–10 mg L-1 to produce flocs with a median size of about 50 µm. By first adding 10-2 moles L-1 of sulfate ions or adjusting the pH to about 8, the same floc size can be obtained with a polymer addition of less than 2 mg L-1.