Anh V. Nguyen

Colloidal science of flotation

INTRODUCTION Flotation and the Flotation Process Colloidal Aspects of Flotation DYNAMICS OF PARTICLES AND BUBBLES IN FLOTATION Hydrodynamics Particle Settling Velocity Bubble Rise Velocity Water Velocity around Air Bubbles Motion of Particles in Flotation Bubble-Particle Interaction BUBBLE-PARTICLE ENCOUNTER INTERACTION Determination of Encounter Efficiency Motion of Particles Diverted by Bubbles Theory of Encounter Efficiency Experimental Investigations of Encounter Interaction BUBBLE-PARTICLE ATTACHMENT INTERACTION Contact Time Theories Properties of the Intervening Liquid Films van der Waals Forces Electrostatic Double-Layer Forces Non-DLVO Surface Forces Thinning of Intervening Liquid Films Rupture of the Intervening Liquid Thinning Film Experimental Studies of Thinning Liquid Films Three-Phase Contact Expansion Modeling of Attachment Efficiency STABILITY OF BUBBLE-PARTICLE AGGREGATES Force Analysis of Particle-Meniscus Stability Force Analysis of Bubble-Particle Stability Energy Analysis of Bubble-Particle Stability Strength of Bubble-Particle Aggregates FLOTATION FROTH Froth Structure Froth Drainage Macroscopic Processes in Froth FLOTATION KINETICS Kinetic Models for Flotation Effects of Particle Size in Flotation INDUSTRIAL APPLICATIONS Industrial Applications Index

A review of factors that affect contact angle and implications for flotation practice.

Contact angle and the wetting behaviour of solid particles are influenced by many physical and chemical factors such as surface roughness and heterogeneity as well as particle shape and size. A significant amount of effort has been invested in order to probe the correlation between these factors and surface wettability. Some of the key investigations reported in the literature are reviewed here. It is clear from the papers reviewed that, depending on many experimental conditions such as the size of the surface heterogeneities and asperities, surface cleanliness, and the resolution of measuring equipment and data interpretation, obtaining meaningful contact angle values is extremely difficult and such values are reliant on careful experimental control. Surface wetting behaviour depends on not only surface texture (roughness and particle shape), and surface chemistry (heterogeneity) but also on hydrodynamic conditions in the preparation route. The inability to distinguish the effects of each factor may be due to the interplay and/or overlap of two or more factors in each system. From this review, it was concluded that: Surface geometry (and surface roughness of different scales) can be used to tune the contact angle; with increasing surface roughness the apparent contact angle decreases for hydrophilic materials and increases for hydrophobic materials. For non-ideal surfaces, such as mineral surfaces in the flotation process, kinetics plays a more important role than thermodynamics in dictating wettability. Particle size encountered in flotation (10-200 microm) showed no significant effect on contact angle but has a strong effect on flotation rate constant. There is a lack of a rigid quantitative correlation between factors affecting wetting, wetting behaviour and contact angle on minerals; and hence their implication for flotation process. Specifically, universal correlation of contact angle to flotation recovery is still difficult to predict from first principles. Other advanced techniques and measures complementary to contact angle will be essential to establish the link between research and practice in flotation.

Nanobubbles and the Nanobubble Bridging Capillary Force

Interactions between hydrophobic surfaces at nanometer separation distances in aqueous solutions are important in a number of biological and industrial processes. Force spectroscopy studies, most notably with the atomic force microscope and surface-force apparatus, have found the existence of a long range hydrophobic attractive force between hydrophobic surfaces in aqueous conditions that cannot be explained by classical colloidal science theories. Numerous mechanisms have been proposed for the hydrophobic force, but in many cases the force is an artifact due to the accumulation of submicroscopic bubbles at the liquid-hydrophobic solid interface, the so called nanobubbles. The coalescence of nanobubbles as hydrophobic surfaces approach forms a gaseous capillary bridge, and thus a capillary force. The existence of nanobubbles has been highly debated over the last 15 years. To date, experimental evidence is sound but a theoretical understanding is still lacking. It is the purpose of this review to bring together the many experimental results on nanobubbles and the resulting capillary force in order to clarify these phenomena. A review of pertinent nanobubble stability and formation theories is also presented.

Particle interactions in kaolinite suspensions and corresponding aggregate structures

The surface charge densities of the silica face surface and the alumina face surface of kaolinite particles, recently determined from surface force measurements using atomic force microscopy, show a distinct dependence on the pH of the system. The silica face was found to be negatively charged at pH>4, whereas the alumina face surface was found to be positively charged at pH<6, and negatively charged at pH>8. The surface charge densities of the silica face and the alumina face were utilized in this study to determine the interaction energies between different surfaces of kaolinite particles. Results indicate that the silica face-alumina face interaction is dominant for kaolinite particle aggregation at low pH. This face-face association increases the stacking of kaolinite layers, and thereby promotes the edge-face (edge-silica face and edge-alumina face) and face-face (silica face-alumina face) associations with increasing pH, and hence the maximum shear-yield stress at pH 5-5.5. With further increase in pH, the face-face and edge-face association decreases due to increasing surface charge density on the silica face and the edge surfaces, and decreasing surface charge density on the alumina face. At high pH, all kaolinite surfaces become negatively charged, kaolinite particles are dispersed, and the suspension is stabilized. The face-face association at low pH has been confirmed from cryo-SEM images of kaolinite aggregates taken from suspension which show that the particles are mostly organized in a face-face and edge-face manner. At higher pH conditions, the cryo-SEM images of the kaolinite aggregates reveal a lower degree of consolidation and the edge-edge association is evident.

Transcriptome for Photobiological Hydrogen Production Induced by Sulfur Deprivation in the Green Alga Chlamydomonas reinhardtii

ABSTRACT Photobiological hydrogen production using microalgae is being developed into a promising clean fuel stream for the future. In this study, microarray analyses were used to obtain global expression profiles of mRNA abundance in the green alga Chlamydomonas reinhardtii at different time points before the onset and during the course of sulfur-depleted hydrogen production. These studies were followed by real-time quantitative reverse transcription-PCR and protein analyses. The present work provides new insights into photosynthesis, sulfur acquisition strategies, and carbon metabolism-related gene expression during sulfur-induced hydrogen production. A general trend toward repression of transcripts encoding photosynthetic genes was observed. In contrast to all other LHCBM genes, the abundance of the LHCBM9 transcript (encoding a major light-harvesting polypeptide) and its protein was strongly elevated throughout the experiment. This suggests a major remodeling of the photosystem II light-harvesting complex as well as an important function of LHCBM9 under sulfur starvation and photobiological hydrogen production. This paper presents the first global transcriptional analysis of C. reinhardtii before, during, and after photobiological hydrogen production under sulfur deprivation.

A review of induction and attachment times of wetting thin films between air bubbles and particles and its relevance in the separation of particles by flotation

Bubble-particle attachment in water is critical to the separation of particles by flotation which is widely used in the recovery of valuable minerals, the deinking of wastepaper, the water treatment and the oil recovery from tar sands. It involves the thinning and rupture of wetting thin films, and the expansion and relaxation of the gas-liquid-solid contact lines. The time scale of the first two processes is referred to as the induction time, whereas the time scale of the attachment involving all the processes is called the attachment time. This paper reviews the experimental studies into the induction and attachment times between minerals and air bubbles, and between oil droplets and air bubbles. It also focuses on the experimental investigations and mathematical modelling of elementary processes of the wetting film thinning and rupture, and the three-phase contact line expansion relevant to flotation. It was confirmed that the time parameters, obtained by various authors, are sensitive enough to show changes in both flotation surface chemistry and physical properties of solid surfaces of pure minerals. These findings should be extended to other systems. It is proposed that measurements of the bubble-particle attachment can be used to interpret changes in flotation behaviour or, in conjunction with other factors, such as particle size and gas dispersion, to predict flotation performance.

Elementary steps in particle-bubble attachment

Abstract Elementary steps in particle—bubble attachment interaction are critically analysed. Three elementary steps are of great importance: (1) thinning of the intervening liquid film to a thickness of film rupture (critical thickness); (2) rupture of the intervening liquid film and formation of a three-phase contact nucleus (a hole of a critical wetting radius); and (3) expansion of three-phase contact line from the critical radius to form a stable wetting perimeter. Some preliminary theoretical and experimental results are presented.

Attraction between hydrophobic surfaces studied by atomic force microscopy

Abstract Attraction between hydrophobic surfaces, known as the hydrophobic force, is critically important for attachment of particles to air bubbles in flotation. However, the origins and models for this attractive force between hydrophobic surfaces have been a source of debate since the first direct measurements of this force in the early 1980s. Using an atomic force microscope (AFM) we studied the attraction between an AFM hydrophobic probe and a flat hydrophobic surface in water, in water–ethanol mixtures, and in water saturated by gases with different solubility. The strong attractive force with long-range jump-in attachment positions decreases with an increase in the ethanol content and disappears in pure ethanol. The size of steps on the force curves depends on the gas solubility. However, the measured forces do not depend on the gas solubility significantly. The influence of surface roughness and heterogeneity appear to be significant. Experimental results indicate the role of surface stabilized submicron-sized bubbles in the hydrophobic attraction. This is in line with recent direct and indirect evidences for the presence of gaseous bubbles at hydrophobic surfaces as well as with the early insights of flotation scientists.

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

New method and equations for determining attachment tenacity and particle size limit in flotation

Abstract A new method and simple, yet accurate, equations for determining the tenacity of particle attachment and the particle size limit in flotation were developed by applying the force analysis of the gravity–capillarity coupling phenomena controlling the bubble–particle stability and detachment. Approximate solutions to the Young–Laplace equation were used to develop simple equations for the tenacity of attachment of particles with diameter up to 20 mm. Simple equations for the maximum size of floatable particles were derived as explicit functions of the particle contact angle, the surface tension, the particle density and the mean centrifugal acceleration of turbulent eddies. For the typical particle size and contact angle encountered in flotation, the analysis showed that the bubble size has little effect on the tenacity of particle attachment. The prediction for the largest size of floatable particles is compared with the experimental data and signifies influence of turbulence on the particle detachment.