Bubble Attachment Research Articles

Development of a turbulent flotation model from first principles and its validation

A first principle flotation model has been derived from the basic mechanisms involved in flotation. It consists of a set of analytical equations for various sub-processes such as bubble generation, bubble-particle collision, attachment, detachment, bubble coarsening, and froth phase recovery. A computer simulator based on the model can predict flotation from both hydrodynamic and surface chemistry parameters such as bubble size, particle size, energy dissipation rate, ζ-potentials, contact angles, etc. The model predictions are in good agreement with the flotation results obtained in both laboratory and pilot-scale tests. The model-based simulator can be used to design and diagnose flotation circuits. Typical input parameters include the size-by-class mineralogical composition of a feed, flotation cell characteristics, flow rates, and the types and dosages of reagents.

The surface state of hematite and its wetting characteristics

Apart from being a resource for iron/steel production, the iron oxide minerals, goethite and hematite, are used in the paint, cosmetics, and other industries as pigments. Surface characteristics of these minerals have been studied extensively both in resource recovery by flotation and in the preparation of colloidal dispersions. In this current research, the wetting characteristics of goethite (FeOOH) and hematite (Fe2O3) have been analyzed by means of contact angle, bubble attachment time, and Atomic Force Microscopy (AFM) measurements as well as by Molecular Dynamics Simulation (MDS). Goethite is naturally hydroxylated and wetted by water at all pH values. In contrast, the anhydrous hematite surface (001) was found to be slightly hydrophobic at natural pH values with a contact angle of about 50°. At alkaline pH hydroxylation of the hematite surface occurs rapidly and the hematite becomes hydrophilic. The wetting characteristics of the hematite surface then vary between the hydrophobic anhydrous hematite and the completely hydrophilic hydroxylated hematite, similar to goethite. The hydrophobicity can be restored by heating of the hydroxylated hematite surface at 60°C. The hydrophobic character of the anhydrous hematite (001) surface is confirmed by MDS which also reveals that after hydrolysis the hematite (001) surface can be wetted by water, similar to the goethite (001) surface.

Pyrite grain and air bubble attachment kinetics in agitated pulp

Air bubble and pyrite grain attachment is studied. It is found that mineralization area on an air bubble depends on pulp agitation time, potassium butyl xanthate concentration and pyrite grain size. The authors show the relation between mineralization area of air bubble and weight of mineral load. Using experimental data, it is calculated how many pyrite grains from a narrow size range attach to an air bubble during pulp agitation time, and the weight of these grains is estimated. Physical forces on mineral loading exerted by pyrite grains of–0.1 + 0.071,–0.071 + 0.044 and–0.044 + 0 mm in size at an air bubble at different pulp agitation times, as well as the absolute and specific retention forces are calculated, as well.

Cationic flotation of nonsulfide minerals

From the analysis of information on flotation of quartz, barite, hematite and diamond spar using cationic reagents (amines), the authors show deficiency of thermodynamic approach to explain flotation results by one type of adsorption due to ion–electrostatic mechanism. The discussion offers hypothesis that says that at low pH collecting ability of a reagent is connected with hydrophobic attachment of the reagent ions in adsorption layer. In alkaline range of pH, the collecting ability is conditioned by formation and precipitation of ionomolecular associates in the adsorption layer of a mineral. These types of adsorption attach particle surface which is preliminarily made hydrophobic by ion–electrostatic mechanism. These adsorption types are active at bubble–liquid interface and can go to this interface upon rupture of water film between a particle and a bubble. According to the suggested hypothesis, liquid tension in the film becomes nonuniform and a surface force arises and expels kinematic constraint for particle–bubble attachment. The analytical review of the collected test data on cationic reagents proves the suggested hypothesis. The causes of breakdown of correlation between surface pressure and collecting ability for initial conditions of flotation are explained.

Etchant wettability in bulk micromachining of Si by metal-assisted chemical etching

Wet bulk micromachining of Si by metal-assisted chemical etching (MaCE) has successfully been demonstrated. Based on the mechanism of defective etching results from Ag and Au metal catalyst experiments, the wettability of etchant solution, in addition to metal type, has been found to have profound effect on the etching process. Addition of low surface tension co-solvent, ethanol in this work, into conventional etchant formulation has enabled complete wetting of etchant on surface, which prevents hydrogen bubble attachment on sample surface during the etching. The complete elimination of bubble attachment guarantees very uniform etch rate on all over the sample surface, and thus prevents premature fragmentation/rupture of catalyst metal layer. Under the optimized etching conditions, the MaCE could be done for up to 12h without any noticeable film rupture and thus etching defects. Thanks to very smooth surface of the etched patterns, conformal contact and direct bonding of elastomer on such surface has been easily accomplished. The method demonstrated here can pave the way for application of simple, low-cost MaCE process in the bulk micromachining of Si for various applications.

Three-Phase Contact Formation and Flotation of Highly Hydrophobic Polytetrafluoroethylene in the Presence of Increased Dose of Frothers

The time of a single bubble attachment to a rough hydrophobic solid surface with contact angle greater than 90° in aqueous solutions at different frothers (n-octanol, α-terpineol and N-octyl-trimethylammonium bromide) in the concentration ranges of 10–6 to 10–3 M was compared with flotation performed under corresponding conditions. A relationship between the time of three-phase contact (TPC) formation at the bubble/liquid/solid interface and flotation rate was obtained. It was found that above a certain frother dose, irrespective of frother type, one could observe significantly longer bubble–solid surface attachment time, which increases from about 2 ms to above 50–60 ms for higher frother concentrations. Accordingly, the flotation rate for the corresponding frothers concentrations decreased from ca. 0.8 s–1 to values below 0.01 s–1. The obtained results confirm that air can be entrapped at rough highly hydrophobic solid surfaces and show the importance of the bubble attachment time for the flotation kine...

Bubble adhesion onto the hydrophobic surface in solutions of non-ionic surface-active agents

Bubbles and drops are entities of enormous practical interest since their interfaces are encountered in numerous industrial processes. Froth flotation is just one of the examples in which bubble–particle attachment and especially fast bubble adhesion play a vital role. This work is focused on the experimental study of the three-phase contact (TPC) line expansion during the bubble adhesion onto the hydrophobic surface in solutions of three non-ionic surface-active agents (Terpineol, Triton X-100 and pentaethylene glycol monododecyl ether), which differ in their structure. The diameters of the TPC line together with bubble dynamic contact angles, dynamic surface tension, bubble rising velocity and dynamic drop contact angles were measured. It was confirmed that the bubble adhesion is fast in solutions of such surfactants whose hydrophobic and hydrophilic parts are small. On the other hand, surfactants with long and flexible hydrophobic tails exhibit more complex behaviour which usually leads to slower bubble adhesion. The molecule steric orientation in the proximity of the interface strongly influences the surfactant motion and thus the predictability of bubble attachment efficiency is low.

Influence of bubble surface fluidity on collision kinetics and attachment to hydrophobic solids

Influence of variations of the bubble surface fluidity on its rising velocity, kinetics of collisions, bouncing and attachment to hydrophobic solid surface was studied experimentally and modeled by means of numerical calculations. A theoretical model was elaborated to describe gradual retardation of the bubble surface fluidity as observed in surface-active substances solutions of increasing concentration. The model is described in details and validated against the experimental results. In experiments the ascending bubble collisions with solid wall, in surface-active substances solutions of gradually increasing concentration, were monitored using high speed photography, while in numerical simulations the bubble behavior (rising velocity, shape deformations, spatial displacement during collisions and bouncing) was determined by solving the governing equations describing conservation of momentum and mass of an incompressible viscous liquid. The results obtained in experiments and in numerical simulations were in a good agreement. Therefore, the model elaborated was used also to confirm that air presence at hydrophobic solid surfaces is responsible for the reported earlier effect of prolongation of time of the bubble attachment at higher concentrations of surface-active substances. The results of numerical simulations showed similar trends as the experimental data, that is, with increasing degree of the bubble surface immobilization the calculated values of the bubble attachment time were prolonged.

Effect of roughness and shape factor on flotation characteristics of glass beads

Abstract The effect of surface roughness and shape factor on behavior of particles in flotation has been investigated. Surface roughness of various degrees was applied on spherical glass beads of −150 +106 μm by means of acid etching. The same procedure was also performed on ground glass beads of the same size interval with different shape factors. The effect of these variations on surface morphology of particles was investigated in terms of flotation recovery, contact angle, and bubble-particle attachment. An Atomic Force Microscope (AFM) was used for surface roughness characterizations and a correction methodology on roughness measurements of spherical particles is proposed. A digital image analyzer was used for shape factor characterizations. It is shown that increase in surface roughness improves the flotation recovery, contact angle, and bubble attachment. Shape factor, however, was found to be more predominant in flotation and bubble attachment. This is attributed to the effect of sharp edges of ground particles which triggers the film rupture process and shortens the attachment time.

From Beetles in Nature to the Laboratory: Actuating Underwater Locomotion on Hydrophobic Surfaces.

The controlled wetting and dewetting of surfaces is a primary mechanism used by beetles in nature, such as the ladybird and the leaf beetle for underwater locomotion.1 Their adhesion to surfaces underwater is enabled through the attachment of bubbles trapped in their setae-covered legs. Locomotion, however, is performed by applying mechanical forces in order to move, attach, and detach the bubbles in a controlled manner. Under synthetic conditions, however, when a bubble is bound to a surface, it is nearly impossible to maneuver without the use of external stimuli. Thus, actuated wetting and dewetting of surfaces remain challenges. Here, electrowetting-on-dielectric (EWOD) is used for the manipulation of bubble-particle complexes on unpatterned surfaces. Bubbles nucleate on catalytic Janus disks adjacent to a hydrophobic surface. By changing the wettability of the surface through electrowetting, the bubbles show a variety of reactions, depending on the shape and periodicity of the electrical signal. Time-resolved (μs) imaging of bubble radial oscillations reveals possible mechanisms for the lateral mobility of bubbles on a surface under electrowetting: bubble instability is induced when electric pulses are carefully adjusted. This instability is used to control the surface-bound bubble locomotion and is described in terms of the change in surface energy. It is shown that a deterministic force applied normal can lead to a random walk of micrometer-sized bubbles by exploiting the phenomenon of contact angle hysteresis. Finally, bubble use in nature for underwater locomotion and the actuated bubble locomotion presented in this study are compared.

Dynamics of dewetting and bubble attachment to rough hydrophobic surfaces – Measurements and modelling

The influence of solid surface roughness (hydrophobic Teflon®) on the timescale of the ascending air bubble (Rb=0.74mm) attachment and the kinetics of the spreading of the three-phase contact (TPC – gas/liquid/solid) line was studied. The moment of the rising bubble collision with a horizontal Teflon® plate immersed in ultrapure water was monitored using fast video recordings (4000fps). It was shown that, depending on the solid surface roughness, the time of the TPC formation was significantly different. Similarly to our previous studies, it was shorter for higher roughnesses. Using high-frequency video acquisition, an additional factor, kinetics of the spreading of the TPC line associated with various bubble shape changes during TPC formation, could be determined. The registered attachment kinetics and bubble shape variations were very reproducible for smooth and very rough Teflon® surfaces, whereas for Teflon® of intermediate roughness, up to five different attachment scenarios were observed, with a relatively large standard deviation of time of TPC formation. Numerical calculations used for simulation of the bubble collisions with a horizontal solid wall with precisely controlled hydrodynamic boundary conditions revealed that the experimentally observed timescales of the bubble attachment and spectacular bubble shape variations can be accurately (qualitatively) reproduced for each roughness of the Teflon® plate studied. Good agreement between experimental and numerical data is, in our opinion, rather strong evidence for air-induced rupture of the liquid film formed between the colliding bubble and the hydrophobic solid plate. This supports the hypothesis that depending on the solid surface roughness, different amounts of air entrapped in solid surface irregularities could drastically change the solid surface hydrodynamic boundary conditions and, consequently, the kinetics of spreading and formation of the TPC.

Study interactions between fine particles and micron size bubbles generated by hydrodynamic cavitation

Interactions between fine subbituminous coal particles and micron size bubbles generated by hydrodynamic cavitation were investigated. The experimental results showed that increasing fluid velocity increases the number counts of micron size bubbles. The number counts of microns size bubbles were found to increase with increasing the dissolved gas content. At a given fluid flow velocity, reducing the dissolved gas content by heating the liquid for 2h at 80°C reduced the number counts of micron size bubbles while saturating the liquid with CO2 led to a significant increase in the number counts of micron size bubbles. However, change in fluid velocity or dissolved gas content had little effect on the bubble size. Introducing micron size bubbles generated by hydrodynamic cavitation into flotation column was found to be a promising method to enhance the recovery of fine coal particles. The micron size bubbles were found to attach to the solids with contact angle of 45° or larger. The micron size bubbles frosted on the solid surface were proven to enhance the bubble–solid attachment. These findings further improved our understanding of the role of hydrodynamic cavitation in the attachment of flotation size bubbles to gas-nucleated fine particles and hence in fine particle flotation. The conclusions derived from this study provide scientific basis for the design of hydrodynamic cavitation assisted flotation devices.

A comparison of methods for measuring the induction time for bubble–particle attachment

Froth flotation is an exceedingly complex physicochemical process. The convenience of distilling much of the complexity of the particle–bubble interactions into a single parameter has led to the continuing popularity of the classical ‘induction time’ to quantify the threshold for particle–bubble attachment to occur. Despite this popularity and the simplicity of the concept, there is no single universal method of evaluating the induction period.In this paper, we begin with a critical review of the available techniques for estimating the induction period. These are: back-calculation from experimental (micro)flotation tests; pushing a particle toward a stationary bubble (or vice versa) using an atomic force microscope (AFM); pushing a bubble toward a stationary bed of particles in the ‘Induction Timer’; pushing a bubble toward a stationary solid surface using the ‘integrated thin film drainage apparatus’ (ITFDA); and dropping particles onto a submerged stationary bubble using the ‘Milli-Timer’ device. Each one of these methods has advantages and disadvantages, and the best choice depends on the application.In the experimental section, we present quantitative comparison of the induction periods estimated using two different techniques, namely the Induction Timer and the Milli-Timer. The same particles were tested in each device, under the same conditions. It was found that by tuning the operation of the particle pick-up device, similar estimates of induction period could be obtained to the estimates made by direct observation with the Milli-Timer. In the former device a bubble is driven toward a particle bed at a controlled rate, whereas in the latter a particle’s motion is governed by the hydrodynamics. The potential to match these presents an intriguing prospect for better understanding the bubble–particle interaction, and the possibility to ‘calibrate’ the simpler Induction Timer against direct observations.

Interaction between Air Bubbles and Superhydrophobic Surfaces in Aqueous Solutions.

Superhydrophobic surfaces are usually characterized by a high apparent contact angle of water drops in air. Here we analyze the inverse situation: Rather than focusing on water repellency in air, we measure the attractive interaction of air bubbles and superhydrophobic surfaces in water. Forces were measured between microbubbles with radii R of 40-90 μm attached to an atomic force microscope cantilever and submerged superhydrophobic surfaces. In addition, forces between macroscopic bubbles (R = 1.2 mm) at the end of capillaries and superhydrophobic surfaces were measured. As superhydrophobic surfaces we applied soot-templated surfaces, nanofilament surfaces, micropillar arrays with flat top faces, and decorated micropillars. Depending on the specific structure of the superhydrophobic surfaces and the presence and amount of entrapped air, different interactions were observed. Soot-templated surfaces in the Cassie state showed superaerophilic behavior: Once the electrostatic double-layer force and a hydrodynamic repulsion were overcome, bubbles jumped onto the surface and fully merged with the entrapped air. On nanofilaments and micropillar arrays we observed in addition the formation of sessile bubbles with finite contact angles below 90° or the attachment of bubbles, which retained their spherical shape.

Effect of solvent addition on bitumen–air bubble attachment in process water

Bitumen–air bubble attachment is an essential element for bitumen recovery from oil sands by flotation. Effect of solvent addition to bitumen on bitumen–air bubble attachment in an industrial process water was investigated by determining the minimum time required to achieve the bitumen–air bubble attachment, known as induction time. Proper addition of solvent to bitumen significantly reduced the induction time of bitumen–air bubble attachment, more so for naphtha than for toluene and reaching optimal at 10wt% solvent added to bitumen. The measurement of bubble coalescence time and total organic carbon content in the resulting process water revealed a critical role in bitumen–air bubble attachment of air bubble interfacial properties controlled primarily by surfactants in the process water and water chemistry. Increasing the solvent addition to bitumen led to an increased migration of natural surfactants to the bitumen–water interface. At a given solvent dosage, more surfactants adsorbed at toluene–diluted bitumen–water interface than at naphtha–diluted bitumen–water interface. Interestingly, zeta potentials of diluted bitumen droplets in the process water exhibited a maximum at about 10wt% of solvent addition to bitumen for both toluene and naphtha. At an identical dosage of solvent, toluene–diluted bitumen in the tailings water possesses a more negative zeta potential than naphtha–diluted bitumen. Finally, a possible adsorption mechanism of ions and surfactants accumulating at the diluted bitumen–water interface under the impact of solvent addition is proposed.

Water Model Study on Removing Inclusion from Molten Steel by Bubble Attachment in RH Degasser

In this paper, a water model is established, at geometric similarity ratio of 1:4, to simulate a real 180-t RH vacuum refining device. Removing inclusion from molten steel by bubble attachment in RH degasser is analyzed using the water model. The effects of variables such as bubble size, treatment time, life-gas flow rate, amount and time of NaHCO3 addition are investigated by using high speed video and image-process software.

Fast flotation of coal at low pulp density using the Reflux Flotation Cell

Fast particle flotation is accomplished by maximising three fundamental aspects: the kinetics of particle–bubble attachment, the bubble interfacial flux for particle extraction, and the rate of bubble–liquid segregation. In practice, it has been impossible to extend all three aspects simultaneously using conventional flotation devices. Hence, significantly higher processing rates using a single flotation cell has not been possible. Here, the Reflux Flotation Cell has been used in this work to address all three aspects in unison in a single stage of separation. This novel system permits throughput rates well beyond conventional flotation standards. Stable operation using extreme gas and feed fluxes is accomplished using a system of parallel inclined channels located below the vertical portion of the cell. In this paper a highly diluted coal feed comprised of well-liberated coal particles at 0.35wt% solids, was prepared from hydrocyclone overflow. The volumetric feed flux was increased to nearly 10 times the typical conventional level, achieving an extremely low cell residence time, in the order of 25s. Very good combustible recoveries were obtained, with the +38μm portion increasing from 92.3% to 98.5% with increasing gas flux. The partitioning of particles below 38μm decreased with decreasing particle size until separation became governed by hydraulic entrainment, clearly evident at a particle diameter of ∼1.65μm.

Flocculation/flotation of hematite fines with anionic temperature-responsive polymer acting as a selective flocculant and collector

A novel approach to the recovery of valuable fines is the use of temperature-responsive polymers such as poly (N-isopropyl acrylamide) (PNIPAM). These polymers act as dual-function flocculants and collectors to form hydrophobic aggregates improving particle–bubble collision and attachment. The aim of this study is to investigate the flotation performance of anionic PNIPAM for an iron ore sample containing fines compared to sodium oleate, an industrial collector for hematite. PNIPAM conditioned at room temperature (25°C, below the lower critical solution temperature (LCST)) and floated at 50°C (above the LCST) was found to provide improved hematite grade and recoveries for particles above 20μm, compared to sodium oleate. This was attributed to the increased selectivity and hydrophobicity of PNIPAM. Turbidity testing confirmed the flocculation of fines with PNIPAM, which deslimes the surface of the coarser particles. Below 20μm, the hematite fines were almost completely recovered with PNIPAM. However, this recovery was not selective, attributed to the entrapment of gangue in the hydrophobic aggregates. Furthermore, conditioning of the polymer above the LCST resulted in significant losses in grade and selectivity, as the polymer self-aggregates hydrophobically and precipitates unselectively onto the closest surface.

Rebound and attachment involving single bubble and particle in the separation of plastics by froth flotation

Bubble–particle interaction is critical to the separation of particles by flotation which can be used in plastic recycling. The two particular processes of the interaction (rebound and attachment) are examined visually using high-speed photography in a laboratory scale flotation column. The effects of surfactant (concentration and type) and plastic material on these two stages are also studied quantitatively. The considered surfactants are Tea Saponin, SDBS and CTMAB, while the plastic materials are polyethylene, polypropylene and Teflon. A simple model of bubble attachment time is developed to analyse and interpret the experimental data. The results show that the presence of surfactant significantly affects the rebound process of a gas bubble, while the type of surfactant and plastic material seem to have slight effect. The bubble attachment process is a strong function of the surfactant and plastic material, which can be associated with the changing of bubble size and its hydrodynamic fluid resistance, contact angle on solid surface, as well as surface tension and viscosity of the solution. Such results are helpful for evaluation and analysis of the plastic flotation process, especially for the selection of reagents and objects at an early stage of design.

Probing the interaction between air bubble and sphalerite mineral surface using atomic force microscope.

The interaction between air bubbles and solid surfaces plays important roles in many engineering processes, such as mineral froth flotation. In this work, an atomic force microscope (AFM) bubble probe technique was employed, for the first time, to directly measure the interaction forces between an air bubble and sphalerite mineral surfaces of different hydrophobicity (i.e., sphalerite before/after conditioning treatment) under various hydrodynamic conditions. The direct force measurements demonstrate the critical role of the hydrodynamic force and surface forces in bubble-mineral interaction and attachment, which agree well with the theoretical calculations based on Reynolds lubrication theory and augmented Young-Laplace equation by including the effect of disjoining pressure. The hydrophobic disjoining pressure was found to be stronger for the bubble-water-conditioned sphalerite interaction with a larger hydrophobic decay length, which enables the bubble attachment on conditioned sphalerite at relatively higher bubble approaching velocities than that of unconditioned sphalerite. Increasing the salt concentration (i.e., NaCl, CaCl2) leads to weakened electrical double layer force and thereby facilitates the bubble-mineral attachment, which follows the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory by including the effects of hydrophobic interaction. The results provide insights into the basic understanding of the interaction mechanism between bubbles and minerals at nanoscale in froth flotation processes, and the methodology on probing the interaction forces of air bubble and sphalerite surfaces in this work can be extended to many other mineral and particle systems.