Xiang Yu

Enhanced flocculation with double flocculants

Abstract Flocculation of alumina was investigated in this study with polystyrene sulfonate and cationic polyacrylamide as polymers, alone and in combinations with each other. When used alone, polystyrene sulfonate showed some limited flocculation, while cationic polyacrylamide did not produce any flocculation of the alumina powder. Electrokinetics and adsorption results suggest this to be due to charge neutralization in the former case and electrostatic repulsion in the latter case. When these two polymers were added together, flocculation, measured in terms of settling rate, was enhanced markedly. This is attributed to the bridging by cationic polyacrylamide of alumina particles with preadsorbed polystyrene sulfonate. Viscosity results for the polymer combinations support the governing role of the polymer—polymer complexation. It was also found that flocculation obtained with the two polymers premixed was lower than that obtained with the polymers introduced individually. This suggests that polymer complex formation in bulk during mixing is detrimental to the flocculation process since the interparticle bridging capacity of the polymer is thus reduced.

Role of conformation and orientation of surfactants and polymers in controlling flocculation and dispersion of aqueous and non-aqueous suspensions

Abstract Flocculation and dispersion of both aqueous and non-aqueous suspensions using surfactants and polymers have widespread applications. In this work, the role of adsorbed layer microstructural properties, namely their conformation and orientation at the solid-liquid interface, in controlling dispersion properties is examined. A multi-pronged approach, involving the use of fluorescence and ESR techniques along with measurements of surface charge and hydrophobicity, was used to explore the structure of the adsorbed layer. The adsorption isotherm of sodium dodecylsulfate on alumina in aqueous solution interface showed different regions corresponding to different adsorption mechanisms. The suspension stability also displayed significant changes concomitant with changes in the structure of the adsorbed layer. When polymers are used their conformation can be manipulated by changing solution conditions such as pH and/or by the addition of a secondary polymer or surfactant. Such manipulation can be used to obtain desired levels of flocculation or stabilization. In non-aqueous media, adsorption was found to depend on the solid-solvent-solute interactions as expressed by an effective interaction parameter δ eff . The suspension stability was dependent on the amount adsorbed and the packing of molecules in the adsorbed layer. The presence of even trace polar impurities was found to have a critical effect on the suspension stability. For example, trace amounts of water were found to cause a significant enhancement of stabilization of alumina cyclohexane suspension in the presence of adsorbed surfactant layers. However, at higher concentrations water induced rapid flocculation in these systems. Hydrophobic polymers were found to be effective stabilizers for alumina in both aqueous and non-aqueous media. In this case the polymer adsorbed at the interface in different orientations with the lyophilic side chains dangling into the solution to provide steric repulsion and thus stabilization.

Stabilization of alumina suspensions in aqueous and non-aqueous media by a hydrophobically modified polymer

Abstract A hydrophobically modified polymer, DAPRAL GE 202, is shown to stabilize alumina both in water and in toluene. The fluorescence technique, utilizing pyrene for the aqueous system and 7-dimethylamino-4-methylcoumarin for the non-aqueous system as probes, was used to investigate the polymer association and orientation in the bulk solution and at the solid—liquid interface. It is proposed that while in the aqueous system the stabilization is caused by both electrostatic repulsion and steric hindrance, the steric hindrance provided by the adsorbed polymer is responsible for the stabilization in toluene.

Effect of a comb-like amphiphilic polymer on the stability of alumina dispersions

Abstract The effect of a hydrophobically modified anionic polymer (a maleic anhydride α-olefin copolymer. DAPRAL), which has a comb-like structure, on the stability of alumina suspensions was studied. In the absence of the polymer the suspension stability is controlled by the electrostatic repulsion between particles. Addition of DAPRAL was found to have a significant effect only when the electrostatic repulsion was not sufficient to maintain a stable suspension. The zeta potential of alumina particles and the adsorption density of DAPRAL on alumina were also measured in order to develop the underlying mechanisms for the stability changes. The results suggest that interactions between hydrocarbon chains on the dangling loops and tails do reduce the effect of electrostatic repulsion between alumina particles at low polymer concentrations. At higher polymer concentrations, the hydrocarbon chains on such loops and tails form micellar aggregates; the stability of the suspension is drastically enhanced under such conditions by both steric and electrostatic repulsion.

Effect of hydrophobically modified comb-like polymer on interfacial properties of coal

Abstract Hydrophobically modified polymers arc known to exhibit interesting interfacial properties and here surface modification of coal by a comb-type hydrophobic-hydrophilic copolymer (DAPRAL) was studied. Floatability tests revealed the coal to become more hydrophobic with increasing DAPRAL concentration regardless of its original floatability, suggesting that free segments of adsorbed polymer molecules can reorient with the hydrocarbon chains protruding into the gas phase. Zeta potential measurements it pH 3.5 and 7 showed the minimum DAPRAL concentrations needed to mask completely the surface charge of coal to correspond to those required to increase the coal floatability to the maximum. Hydrophobicity measured using a new reliable film-levitation technique also showed the coal to become more hydro-phobic with increasing polymer concentration but only until a plateau is reached. Further increase in DAPRAL concentration caused a decrease in the hydrophobicity which is suggested to be due to the consumption of the hydrocarbon chains on unadsorbed polymer segments by the formation of micellar aggregates.

Dispersions: Progresses and prospects

The papers presented in this session on dispersion, rheology and mixing cover a broad spectrum of topics ranging from instrumentation for better characterization, to process development for improved stabilization, selective flocculation and dense packing, to modeling of colloidal dispersions. There are also some, but limited, efforts to understand the mechanisms involved in these processes. Importantly, there is a clear trend to adapt, modify and develop techniques and instrumentations for even the difficult to characterize, highly concentrated dispersions. While the reported works deal with different systems, they all address a common problem: control of interfacial properties via chemical or physical means to achieve the desired stability or flocculation of suspensions and in some cases selectively.