Influence of polymer charge on the shear yield stress of silica aggregated with adsorbed cationic polymers

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Abstract

Flocs were produced by adding three cationic polymers (10% charge density, 3.0 × 105 g/mol molecular weight; 40% charge density, 1.1 × 105 g/mol molecular weight; and 100% charge density, 1.2 × 105 g/mol molecular weight) to 90 nm diameter silica particles. The shear yield stresses of the consolidated sediment beds from settled and centrifuged flocs were determined via the vane technique. The polymer charge density plays an important role in influencing the shear yield stresses of sediment beds. The shear yield stresses of sediment beds from flocs induced by the 10% charged polymer were observed to increase with an increase in polymer dose, initial solid concentration and background electrolyte concentration at all volume fractions. In comparison, polymer dose has a marginal effect on the shear yield stresses of sediment beds from flocs induced by the 40% and 100% charged polymers. The shear yield stresses of sediments from flocs induced by the 40% charged polymer are independent of salt concentration whereas the addition of salt decreases the shear yield stresses of sediments from flocs induced by the 100% charged polymer. When flocculated at the optimum dose for each polymer (12 mg/g silica for the 10% charged polymer at 0.03 M NaCl, 12 mg/g for 40% and 2 mg/g for 100%), shear yield stress increases as polymer charge increases. The effects observed are related to the flocculation mechanism (bridging, patch attraction or charge neutralisation) and the magnitude of the adhesive force. Comparison of shear and compressive yield stresses show that the network is only slightly weaker in shear than in compression. This is different than many other systems (mainly salt and pH coagulation) which have shear yield stress much less than compressive yield stress. The existing models relating the power law exponent of the volume fraction dependence of the shear yield stress to the network fractal structure are not satisfactory to predict all the experimental behaviour.

Graphical abstract

The shear yield stress is an approximately linear function of the adhesive force between particles.

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Introduction

Solid–liquid separation plays an integral role in many industries, including for example, minerals processing, pharmaceuticals, pulp and paper, as well as the sewage treatment industry. Polymeric flocculants are commonly utilised to hasten the solid–liquid separation step and provide supernatant clarity. The process of polymer addition is known as flocculation and leads to the formation of loose aggregates referred to as flocs [1]. The types of aggregates formed as a result of the flocculation process are dependent upon both chemical (e.g. flocculant type, pH, ionic strength, dose, etc.) and physical factors (e.g. mixing conditions, temperature, etc.) and these aggregates in turn affect the rheological (flow) behaviour of the sediment bed. The rheological behaviour of the sediment bed is very important in the down stream process. For instance, the shear yield stress is an important factor in the pumping of suspensions during processing and tailing disposal.

In many cases, for example in thickeners, aggregation processes are performed at relatively low solids contents. If these aggregates are allowed to settle, they will produce a sediment bed of a higher solids volume fraction. The density of the sediments will increase from the top of the sediment bed to the bottom [2], [3]. The sediment at the top has no particles above it and is thus subjected to no consolidation pressure. The sediment at the top has the minimum volume fraction needed for the aggregates to form a space-filling network that spans the container which is known as the gel-point. At volume fractions above the gel-point the settled bed shows non-Newtonian flow behaviour and in the limit of low shear, a finite yield strength (τY).

The shear yield stress, τY is a measure of the resistance of the particle network to permanent deformation in shear, or simply the minimum stress needed to cause flow to occur. It is an important parameter in the processing of flocculated suspensions and is best viewed as the response of the suspension to an applied load. Suspensions behave in a linear elastic manner to applied stresses less than τY. For stresses above τY, the network fails under the applied load. The maximum stress prior to failure is designated as a yield value. Many researchers measure this yield value by extrapolating curves of shear stress versus shear rate to zero or very low shear along a low shear plateau. In many of such measurements, true equilibrium data are hard to obtain. To overcome many of the artefacts associated with yield stresses measured by extrapolation, Nguyen and Boger [4], [5] adopted the static yield stress measurement technique incorporating the vane–spindle. This technique has now been utilised in a variety of measurements and systems to highlight the interrelationship between inter-particle forces and suspension rheology [6], [7], [8], [9], [10], [11], [12].

It is well known that the strength of the particle network is exhibited in the macroscopic properties of the compressive yield stress, elastic modulus and shear yield stress. The strength of an attractive particle network depends on the strength of bonds between particles and the number of bonds that need to be broken. The number of bonds that need to be broken depends on the network structural factors and particle parameters such as size, shape and volume fraction [13], [14], [15]. The strength of the attractive bond between particles is controlled by the inter-particle attractive forces, which in turn are controlled by the surface chemical and solution conditions such as solution pH, salt and polymer type and concentrations [16], [17], [18], [19], [20]. Several researchers have studied the relationship between the strength of bonds between particles and the shear and compressive yield stress of the particle network, and found that, the greater the particle attraction, the higher the yield stress [21], [22], [23], [24], [25].

The yielding behaviour of the particle network in shear is expected to be related to that under compression given that the fundamental basis of both types of deformations is the ability of the network to accommodate elastic strain up to the yield point. Recent analysis by Buscall [26] based on poroelastic material theory suggests that the ratio of the shear yield stress to the compressive yield stress should be on the order of the elastic strain which is typically on the order of 1%. Consequently, exploring the links between shear and compressive yield stress has been the subject of a number of studies [26], [27], [28], [29], [30], [31], [32], [33]. The magnitude of the compressive yield stress always exceeds that of the shear yield stress as expected. However, the observed relationship between shear and compressive yield stress varies significantly across these studies and the shear yield stress is usually found to be much less than half of the compressive yield stress. Shin and Dick [33] compared the compressive yield stress and shear yield stress for water treatment sludges and observed a linear relationship between the two. Buscall et al. [27] compared compressive yield stress and shear yield stress for flocculated latex and observed an approximately constant ratio of 55 and later observed a ratio of 100 for silica particles. Meeten [29] made early investigations into the area and Green and Boger [32] obtained a constant ratio of 15 for zirconia. Zhou et al. [31] also investigated the ratio of the compressive yield stress to the shear yield stress for titania and different particle sized alumina and obtained a constant ratio of 29 and 14, respectively, at low volume fractions but observed that the ratio then increased linearly with volume fraction above a critical volume fraction of around 0.4 for both materials. In all these studies the suspensions for shear yield stress measurements were prepared at the final volume fraction by batching the suspension to that volume fraction. In some cases the flocculated suspensions were stirred or sheared prior to shear yield stress measurements. The suspensions for compressive yield stress measurements were typically prepared at volume fraction near the gel-point and the volume fraction increased by the application of pressure via a centrifuge or filter press. When suspensions are consolidated by pressure, a strong touching particle network (like a skeleton) can result which produces a structurally stronger network than when the suspension is sheared [34]. The strength of a particle network thus depends upon the pressure history of the suspension. We contend that many of the measurements of compressive and shear yield stresses published to date have not been conducted on the same network. Since the network used to measure the compressive yield stress has been subject to compressive pressure, it is stronger than the network used to measure the shear yield stress which has not been subjected to consolidation pressure.

Rheological properties such as the steady shear viscosity, the elastic modulus, the shear yield stress and the compressive yield stress have been found to increase dramatically (with power law behaviour) with increasing particle volume fraction (ϕ) [27], [30], [32], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44]. In the case of the shear yield stress (τY), and compressive yield stress (Py) for example,τy=AϕmPy=Bϕnwhere A, B, m and n are constants and determined from empirical fits to the experimental data. m usually has values of between 3 and 5 in the volume fraction ranging from 0.15 to 0.40.

Although these earlier studies identify some chemical and physical factors that affect the yield stress, there is very little information available in the literature, which systematically describes how polymer charge densities, under various aggregation conditions, including polymer dose, initial solid concentration, background electrolyte concentration, and shear intensity during aggregation influence the as-consolidated shear yield stress of the sediment beds from the directly settled flocs. A previous report [45] described the influence of polymer charge densities, under various aggregation conditions, including polymer dose, initial solid concentration, background electrolyte concentration and shear intensity during aggregation on the polymer conformation at the particle surface, flocculation mechanisms and aggregate properties. In this previous paper, we determined that the difference in charge density was significantly more important than the molecular weight for the three polymers investigated in controlling aggregation mechanism and aggregate properties [45].

Our initial investigation [45] on the system studied in the current paper which included zeta potential, polymer adsorption and aggregate property measurements of the flocculated particles provides important information to understand the conformations of the polymers on the particle surface and dominant flocculation mechanism. The conclusions of these analyses are as follows: the 10% charged polymer at 0.03 M NaCl concentration induces bridging flocculation, moreover, the bridging is enhanced by an increase in the polymer dose due to the increase in the number of polymer loops and tails protruding from the particle surfaces and extending beyond the influence of the electric double layer. The 40% charged polymer at the dose of 12 mg/g silica also brings about bridging flocculation, since the polymer molecules can only adopt extended conformations with loops and tails protruding away from the particle surface. At the dose of 4, 6 and 8 mg/g silica, the lower surface coverage of the polymer molecules results in a combination of charge neutralisation and bridging. However, it is worth noting that the bridging mechanism increases with the increase in the polymer dose relative to the charge neutralisation mechanism. The 100% charged polymer molecules are adsorbed with flat conformations at the dose of 1 and 2 mg/g silica, causing the electrostatic patch mechanism while 100% charged polymer at the polymer dose of 8 and 12 mg/g silica adopts extended conformations due to the formation of loops and tails which lead to bridging flocculation.

In further work on the same systems [46], we studied the compressive yield stresses from flocs formed by the addition of the three charged polymers (10%, 40% and 100%) to 90 nm diameter silica particle suspensions. It was found that the polymer charge density plays an important role in influencing the compressive yield stresses of sediment beds. The compressive yield stress of sediment beds from flocs induced by the 10% charged polymer was observed to increase with an increase in polymer dose, initial solid concentration and background electrolyte concentration at all volume fraction. In comparison, polymer dose has a marginal effect on the compressive yield stresses of sediment beds from flocs induced by 40% and 100% charged polymers. Initial solid concentration has no influence on the compressive yield stresses of sediment beds from flocs induced by either 40% or 100% charged polymers at the polymer doses used. The compressive yield stresses of sediments from flocs induced by the 40% charged polymer is independent of salt concentration whereas the addition of salt decreases the compressive yield stresses of sediments from flocs induced by the 100% charged polymer. It is found that the shear intensity only affects the compressive yield stresses of sediments from flocs induced by the 10% charged polymer. When flocculated at the optimum dose for each polymer (12 mg/g silica for the 10% charged polymer at 0.03 M NaCl, 12 mg/g for 40% and 2 mg/g for 100%), compressive yield stress increases as polymer charge increases. The effects observed are related to the flocculation mechanism (bridging, patch attraction or charge neutralisation) and the magnitude of the attractive force.

More recently our investigations [47] focussed on the force between silica surfaces for the above three charged polymers (10%, 40% and 100%) in aqueous NaCl solutions. The atomic force microscope (AFM) colloidal probe technique was used to determine silica inter-particle interaction forces which were compared to macroscopic information of the strength of interactions such as compressive yield stress measurements. It was found that in 30 mM NaCl solution the 10% charged polymer produced steric repulsion upon approach, and long ranged adhesion with multiple pull off events upon retraction at the optimum flocculation concentration. This suggests that the polymer was adsorbed in a conformation where segments extend from the surface resulting in bridging flocculation. The 40% and 100% charged polymers produced attraction upon approach and strong adhesion with snap out from contact upon separation at optimum polymer dosages. This suggests that these polymers are adsorbed with flat conformations and is typical of charge neutralisation or patch attraction. The attractions for 40% and 100% charged polymers measured with the AFM are significantly larger than for the 10% charged polymer. The polymer dose which produced the optimum flocculation and the maximum compressive yield stress typically corresponded to the polymer concentration which produced the maximum adhesion for each polymer. It was found that the magnitude of the adhesive force was more significant in determining the compressive yield stresses of the silica particle sediments than the aggregate size and structure.

In this study, a series of flocs were produced by adding the three cationic polymers with various charge densities used in the previous studies [45], [46], [47] to 90 nm diameter silica particle suspensions. The objective of this research is to systematically investigate the effect of polymer charge density, under various aggregation conditions, including polymer dose, initial solid concentration, background electrolyte concentration and shear intensity during aggregation on the as-consolidated shear yield stresses of the sediment beds from the directly settled flocs. The relationship between shear and compressive yield stress will be compared and discussed. The experimental results are then compared with theoretical models that are currently available.

Section snippets

Materials and aggregate characterisation

Details of the materials used in this study, the size and structure of the aggregates and the effect of polymer charge on flocculation mechanism have been described elsewhere [45]. Monodisperse spherical silica particles were obtained from Nissan Chemical America Corporation, with a BET surface area of 30 m2 g−1, a mean particle diameter of 90 nm and a density of 2.2 g cm−3. Two cationic copolymers of acrylamide/diallyldimethylammonium, chloride (D6010 and D6040) and one cationic homopolymer of

Effect of polymer dose on the shear yield stresses of sediments

The optimum polymer doses in terms of the best supernatant clarity for the 10% charged polymer at 0.03 M NaCl concentration, 40% and 100% charged polymers were visually determined to be 12, 12 and 2 mg/g silica, respectively [45]. The addition of 0.03 M NaCl was found to be necessary to produce clear supernatants for flocculation with the 10% charged polymer. The addition of 0.03 M NaCl alone did not produce coagulation since the suspension remained stable to sedimentation for at least 10 days

Conclusions

The shear yield stresses of sediment beds from flocs induced by weakly charged polymers depend more strongly on the polymer dose than highly charged polymers over the range of polymer doses investigated. The shear yield stresses of sediments increase with increasing polymer charge when compared at each polymer’s optimum dose. This is consistent with the increase in adhesion measured previously [47]. The changeover of the flocculation mechanism from charge neutralisation and patch attraction to

Acknowledgments

Thanks to Richard Buscall for the enlightening discussions. Ying Zhou thanks the Australian government for the award of an IPRS and the University of Newcastle, Australia, for the award of an UNRS Central scholarship. Thanks are also extended to SNF for the provision of the cationic polymers. The authors wish to acknowledge financial support from the Australian Research Council (in particular, Discovery Grant 0209669 and the Special Research Centre for Multiphase Processes).

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