Graeme J. Jameson

The effect of bubble size on the rate of flotation of fine particles

Abstract Flotation experiments have been carried out in a cell in which bubbles of known size could be generated independently of the turbulence levels, which could be controlled by varying the impeller speed. The mean bubble size ranged from 75 μm to 655 μm. Results are reported for the flotation rates of fine particles, less than 50 μm diameter. The materials floated were polystyrene latex particles, quartz and zircon. These were chosen to give a wide range of specific gravity. It was found that the flotation rate was very strongly affected by the bubble size, there being an increase of up to one hundred-fold when the bubble size was reduced from 655 μm to 75 μm. Effects of particle density and impeller speed are also reported.

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.

Bubble size distribution and coarsening of aqueous foams

This paper characterises the bubble size distribution of aqueous foams, produced by a compressed-air foam generator. The time evolution of bubble size distribution in aqueous foams is experimentally measured using a CCD video camera. A computer model, which predicts the change in bubble size distribution with time, is used to simulate the coarsening and disproportionation of aqueous foams. As an extension of previous computer simulations, our model incorporates the variation in liquid fraction during the foam aging process, thereby enabling the simulation of both wet and dry foams. It is found that a Weibull-type distribution best approximates the narrow bubble-size distribution produced by a compressed-air foam generator. The modelled predictions show good agreement with the experimental measurements and a sensitivity analysis indicates a significant dependence of the model on drainage (hence on the thickness of the lamellae), the Henry constant, the gas diffusivity and the surface tension of the foam solution. This paper characterises the bubble size distribution of aqueous foams, produced by a compressed-air foam generator. The time evolution of bubble size distribution in aqueous foams is experimentally measured using a CCD video camera. A computer model, which predicts the change in bubble size distribution with time, is used to simulate the coarsening and disproportionation of aqueous foams. As an extension of previous computer simulations, our model incorporates the variation in liquid fraction during the foam aging process, thereby enabling the simulation of both wet and dry foams. It is found that a Weibull-type distribution best approximates the narrow bubble-size distribution produced by a compressed-air foam generator. The modelled predictions show good agreement with the experimental measurements and a sensitivity analysis indicates a significant dependence of the model on drainage (hence on the thickness of the lamellae), the Henry constant, the gas diffusivity and the surface tension of the foam solution.

Polyelectrolyte adsorption at the solid/liquid interface: Interaction forces and stability

Abstract The forces between negatively charged surfaces in the presence of an adsorbing cationic copolymer of acrylamide and 2(methacryloyloxy)ethyltrimethylammonium chloride have been investigated using an atomic force microscope. The results were compared with measurements from adsorption isotherm, electrophoretic mobility, stability, and light scattering experiments. The adsorbed amount of polyelectrolyte and adsorbed layer conformation at the solid/liquid interface were found to be strongly dependent on the polymer concentration from which initial adsorption takes place. At low polyelectrolyte concentrations unstable silica suspensions were observed from stability tests; light scattering experiments indicate a large aggregate size under equivalent conditions. The adsorbed amount was also seen to be low, well less than monolayer coverage, and force measurements indicated that the polymer was adsorbed in a flat conformation. At high polyelectrolyte concentrations, an increase in the adsorbed amount was observed which resulted in a higher surface coverage, a higher mobility and a stable suspension. Direct force measurements indicated the presence of an electrosteric barrier.

A study of bubble coalescence in flotation froths

Abstract This paper is concerned with changes in bubble size and bubble size distribution in the froth phase of a flotation column. A continuous flotation cell of special design is used in which deep froths can be formed. The effect of parameters such as degree of hydrophobicity, gangue concentration (entrained solids), initial bubble size (pulp bubble size) and froth height has been investigated. Special attention has been given to the use of particles of well-defined hydrophobicities so that their effect on the behaviour of the froth phase could be assessed more accurately. Glass particles of different hydrophobicities (as reflected in the contact angle) are prepared using controlled silanation. Contact angles chosen are 50°, 66° and 82°. The results suggest that the size of the bubbles strongly depends on the degree of hydrophobicity of particles. In the presence of entrained solids, the bubble coalescence rate substantially decreases mainly due to reduced liquid drainage in the bubble films.

Effect of ultrasound on surface cleaning of silica particles

Abstract One substantial benefit of ultrasonics in mineral processing can be the removal of surface coatings of clay and iron oxides from mineral surfaces. This is mainly achieved through the large, but very localised, forces produced by cavitation. The application of sonication to the reduction of iron oxide in a silica sand from 0.025% to less than 0.012% Fe2O3 — the difference between material suitable for clear glass container ware and that suitable for table ware — is presented. The ultimate reduction in iron oxide contamination is dependent on the sonication power, whereas increasing the concentration of Na2CO3 in the suspension, up to 0.2%, shortens the sonication time to approach this limit.

The application of power ultrasound to the surface cleaning of silica and heavy mineral sands.

Power ultrasound may be used in the processing of minerals to clean their surfaces of oxidation products and fine coatings, mainly through the large, but very localised, forces produced by cavitation. Results of the application of power ultrasound to remove iron-rich coatings from the surfaces of silica sand used in glass making and to improve the electrostatic separation of mineral sand concentrates through lowering the resistivity of the conducting minerals (ilmenite and rutile) are presented. Parameters affecting ultrasonic cleaning, such as input power and levels of reagent addition, are discussed. In particular, we present data showing the relationship between power input and the particle size of surface coatings removed. This can be explained by the Derjaguin approximation for the energy of interaction between a sphere and a flat surface.

PREDICTION OF THE BUBBLE SIZE GENERATED BY A PLUNGING LIQUID JET BUBBLE COLUMN

Abstract In this paper a model is presented to predict the maximum bubble size generated within the mixing zone at the top of a plunging liquid jet bubble column. The model uses a critical Weber number, where the energy dissipation rate per unit volume is derived from the theory of liquid-jet gas ejectors. The length of the mixing zone, and hence its volume, was determined experimentally from the vertical axial pressure profile along the wall of the column. The model was tested experimentally for a range of column and jet diameters, jet velocities, and liquid physical properties, and it was found that the measured maximum bubble diameter was in good agreement with the model predictions based on a critical Weber number of 1.2. It was also found that the bubble diameter distribution was fitted by a log-normal distribution, with a Sauter-mean-to- maximum-diameter ratio of 0.61 which is consistent with reported literature values.

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.

Investigations of bubble-particle interactions

Bubble-particle interaction during flotation comprises of collision, attachment and detachment. This paper presents a review of our investigations into these microprocesses. Analysis of collision phenomenon focuses on the physicochemical hydrodynamics of water flow passing the rising bubbles. The influence of the fore-and-aft asymmetry of water streamlines and of the mobility of the bubble surface on collision efficiency is quantified. In the case of attachment, the analysis considers contact and attachment times and reveals that the available models for contact times are far from satisfactory. It may be necessary to include short-range hydrodynamic interactions for the modeling of contact times. At present, the actual attachment time is difficult to predict from first principles. Finally, the examination of detachment focuses on models for predicting the tenacity of attached particles. The influence of the bubble size on tenacity is also analyzed. Simplified equations describing the maximum particle size for stable attachment to air bubbles are derived