Yao-de Yan

Aggregate structures formed via a bridging flocculation mechanism

Abstract A high molecular weight cationic polyelectrolyte has been used to flocculate a colloidal dispersion of anionic polystyrene latex particles. The polymer used had a high charge density and the flocculation occurred at a solution pH where both the polymer and the particles were fully charged. Under these conditions, flocculation is expected to occur through a bridging flocculation mechanism. Low angle laser light scattering has been used to follow the flocculation process as a function of time; parameters of interest were the aggregate sizes, size distributions, and aggregate mass fractal dimensions. The light scattering measurements showed that the flocs formed had a mass fractal character. All the systems examined here were overdosed with respect to the optimum flocculation concentration of polymer. Under these conditions, decreasing the polymer concentration was seen to result in an increased flocculation efficiency. A secondary growth process was also observed whereby initially formed fractal aggregates can subsequently aggregate again. These larger aggregates are also expected to be mass fractals although this cannot be determined from the light scattering measurements due to the superposition of Fraunhofer diffraction effects. This type of fractal-in-fractal character is unusual.

The flocculation efficiency of polydisperse polymer flocculants

The flocculation performance of three poly(acrylic acid) (PAA) samples (Mw=9×104, 2.5×105 and 1×106 g/mol) has been investigated. Colloidal alumina particles were used as a model system and tests were performed at pH 5. Using a single-component polyacid, it was found that the optimum dosage required to achieve supernatant clarity was similar between the 9×104 g/mol PAA (23 ppm) and 2.5×105 g/mol PAA (26 ppm), but increased dramatically with the 1×106 g/mol PAA (83 ppm). For the two lower molecular weight samples, flocculation occurs through a charge neutralisation mechanism. In contrast, polymer bridging is inferred to be the dominant flocculation mechanism for the high molecular weight sample. The flocculation performance of a polymer mixture, produced by blending the high and low molecular weight polyacids to give an average molecular weight of 2.5×105 g/mol, was also studied. Supernatant clarity from this system was found to be comparable to that from the single-component polyacid of the same (average) molecular weight. However, the optimum dosage required for the polymer mixture was about twice as much as that for the single-component reference polymer. The results suggest that for the polymer mixture no synergistic effects occur. Instead, analyses of aggregate sizes indicate an independent behaviour for the two polymers in the blend. We also examined re-suspension (under shear) and re-flocculation of the sediment formed in the initial flocculation experiments. For the three single-polymer systems, rapid re-flocculation after shear was seen for the two lower molecular weight samples suggesting a reversible aggregate breakage. For the high molecular weight sample, re-suspension resulted in the formation of a stable dispersion. This result was attributed to breakage of the high molecular weight polymer sample during re-suspension. In the case of the polymer mixture, rapid re-flocculation was again observed despite the presence of a large amount of the high molecular weight sample. This result may have important practical implications.

Bridging flocculation studied by light scattering and settling

Polymers are used increasingly in solid–liquid separation processes. Bridging flocculation is the most common particle aggregation mechanism in these processes. However, little is known about the structure of the aggregates formed. This paper presents a critical comparison of two of the techniques that can be very useful tools for the characterisation of aggregate structures, i.e. static light scattering and settling. Of particular interest was their applicability for bridging flocculated aggregates. Both techniques were tested on two model systems: salt-induced fast coagulation and polymer-induced bridging flocculation of colloidal alumina particles. For diffusion-limited cluster–cluster aggregation at a high salt concentration, aggregate mass fractal dimensions of 1.75 and 1.65 were obtained from the light scattering and settling experiments, respectively. For bridging flocculation whereby flocs were formed using dual polymers, light scattering and settling gave mass fractal dimensions of 2.12 and 1.85, respectively. It was concluded that each of these techniques has certain advantages and disadvantages, therefore, it is best to view them as complementary. The settling method may be better suited for studying aggregates in bridging flocculation where floc sizes can be quite large which may cause the light scattering technique to become inapplicable.

Dewatering properties of dual-polymer-flocculated systems

This work examines the effect of dual-polymer flocculation on the compressive yield stress and hindered settling function of positively charged alumina suspensions as measured by a pressure filtration technique. The primary aggregates were formed using a short-chain poly(acrylic acid) (PAA) under two different shear mixing conditions at pH 5. This resulted in two types of aggregates with different mass fractal dimension and size. These aggregates were further flocculated using either a cationic or an anionic long-chain polymer at the same pH. A filtration study of these dual-polymer-induced aggregates showed that there may exist an optimum dosage for the second polymer, which results in the lowest final cake moisture. Dual-polymer flocculation was found to result in lower compressive yield stresses than that found for the case of single, high-molecular-weight polymer flocculation. This gave rise to lower final cake moistures for the dual-polymer-flocculated systems (28.3% w/w) compared to the case of the single, high-molecular-weight polymer-flocculated system (34.7% w/w). The filtration rate was high for the dual-polymer systems in most cases. In addition, the polymer dosage required to achieve good supernatant clarity was decreased from that of the single-polymer case when dual polymers were used. The use of dual-polymer flocculants of opposite charge was found to give an improvement in final cake moisture over that achieved using dual polymers of like charge. The structural characteristics of the primary aggregates were also found to have a significant influence on the dewatering properties of the dual-polymer-induced aggregates. Under the conditions used, large, open primary aggregates led to final flocs with lower cake moisture and faster filtration rate. It is envisaged that optimisation of industrial dewatering processes in this way can result in considerable savings in transportation and enhanced product value.

The structure and strength of depletion force induced particle aggregates

Aggregating fine particulate matter is common practice in many industrial solid-liquid separation processes. Data obtained in this work on dilute aqueous dispersions of model colloidal polystyrene latex spheres indicate that depletion flocculation, which uses non-adsorbing polymer, can yield very compact aggregates. Flocculation of the negatively charged latex particles was induced by the addition of a poly(acrylic acid) at pH 10. The structural compactness of the latex flocs formed in the dilute dispersions was characterised using small-angle static light scattering in terms of mass fractal dimensions. Rheological measurements on the concentrated latex dispersions in the presence of the non-adsorbing polyacid showed Bingham yield stress behaviour. Both the compactness and strength of the latex flocs were found to be significantly dependent upon the level of the polyacid, as well as the concentration of the initial particles. In particular, as the level of the polyacid was raised the floc compactness decreased, whereas its strength increased. They were both seen to level off at high polymer concentrations. Atomic force microscopy measurements were made at varying concentrations of the polyacid to provide a qualitative explanation of the observed floc structural behaviour of the dilute dispersions. By combining the fractal dimension and the Bingham yield stress we were also able to estimate the energy required to separate the flocs into single units in the concentrated dispersions. It was concluded that the interparticle interaction energy is the key to understanding the dependence of both the floc structure and strength on the polymer concentration.

Relationship between interaction forces and the structural compactness of depletion flocculated colloids

Abstract The addition of non-adsorbing polymers to a colloidal dispersion can induce particle aggregation through depletion flocculation. Small-angle static light scattering has been used to show that the structure of aggregates formed upon the addition of non-adsorbing poly(acrylic acid) to a dilute aqueous dispersion of colloidal polystyrene latex particles exhibited power-law scattering behaviour. The observed floc mass fractal dimension, d F , was found to be dependent upon the polyacid concentration. Increasing the polyacid concentration resulted in lower fractal dimensions, with d F levelling off at high polymer concentrations. This structural behaviour was attributed to the deepening in the secondary potential energy well and hence higher particle sticking efficiencies upon collisions when the polymer concentration was raised. This is directly supported by atomic force microscopy data, which showed that the interaction force curves between charged surfaces for solutions of the non-adsorbing poly(acrylic acid) exhibited attractive secondary minima, the depths of which increased with increasing polymer concentration, saturating at high polyacid concentrations. In contrast to the much lower values obtained via salt-induced aggregation ( d F =1.78 to 2.20), significantly higher mass fractal dimensions and hence greater structural compactness were observed for these depletion-flocculated flocs.

Effect of aggregate size on sediment bed rheological properties

Three different types of aggregates of submicron alumina particles were produced utilising either polymer, high salt (1.0 M), or low salt (0.075 M) aggregation conditions. All three types of aggregates had similar structural properties (mass fractal dimension = 2.0). The typical size (d[4,3]) of the polymer aggregates was 125 microns, the high salt aggregates 12 microns and the low salt aggregates 4 microns. It was found that smaller aggregates produced higher gel points and higher apparent maximum packing fractions. Larger aggregates produced higher shear and compressive yield stresses at all volume fractions. The relative effect that aggregate size and inter-particle attraction has on the sediment yield stresses was investigated in the salt system. The salt concentration was adjusted so as to produce different size aggregates with the same final salt concentration and thus same level of inter-particle attraction. The size of the aggregates and the magnitude of the inter-particle attraction were found to have similar contributions to the compressive yield stress over the range of size and attraction investigated.

The rheology of concentrated suspensions of depletion-flocculated latex particles

The effect of adding non-adsorbing polymer, poly(acrylic acid) (PAA), on the rheological properties of concentrated electrostatically-stabilised polystyrene latex suspensions was investigated. The extrapolated yield stress of the depletion-flocculated suspensions was studied as a function of the concentration and the molecular weight of the free polymer. Specifically, the Bingham yield value was observed to increase with either an increase in the weight fraction of the polymer or through the use of a longer chain polyacid. Unlike the sterically stabilised colloidal suspensions studied in the literature, at high polymer concentrations these charge-stabilised latex particles exhibited plateauing in the yield stress for each polyacid. The energy of separating particles within an aggregate into single units was calculated using the yield stress and average particle co-ordination number. In contrast to what was commonly done in the literature, the latter was not assumed to be constant but estimated from the aggregate mass fractal dimension as obtained from static light scattering techniques. It was found that the separation energy showed similar trends to those in the yield stress with both varying the polymer concentration and molecular weight. These trends also bear remarkable similarities to those in the depth of the secondary potential energy well as measured using atomic force microscopy (AFM).