Three-phase Contact Formation Research Articles

Three-phase contact formation between an air bubble and solid surfaces with different hydrophobicity degrees in liquid

Interactions between gas bubbles and solid particles are crucial in numerous industrial processes, including froth flotation, in which particle-bubble attachment plays a vital role. The process efficiency depends on differences in the surface wettability (i.e., hydrophobicity, expressed by the contact angle) of separated particles. This study aimed to investigate the influence of surface hydrophobicity and zeta potential on the formation of a three-phase contact (TPC) between a single air bubble and planar glass surfaces. Different degrees of hydrophobicity were achieved by esterification with a range of alcohols and time. The interaction between the bubble and surface was monitored using a high-speed camera with a time resolution of 1 ms, allowing for measurements of the time of thin liquid film drainage, time and diameter of TPC formation, TPC line expansion velocity, and dynamic contact angles. The observations suggest that TPC formation occurs when the solid surface has a sufficient degree of hydrophobicity (critical contact angle) and is dependent on the differences in zeta potential values between the air bubble and the solid surface in water. When both surfaces are negatively charged, the limiting value of the water contact angle is ca. 35°, whereas for oppositely charged surfaces, the value slightly decreases to ca. 31°. The results show that increasing surface hydrophobicity enhances the bubble adhesion effectiveness by increasing both the propagation velocity of the TPC perimeter and its size (measured by the diameter of the formed TPC). The article also discusses the mechanism of bubble-solid adhesion in relation to the hydrophobicity and zeta potential of the solid surface based on the obtained results.

Comparison and Mechanism Analysis of Three-Phase Contact Formation onto Hydrophilic/Hydrophobic Mineral Surfaces in the Presence of Cationic/Anionic Surfactants during Flotation Process

This article presents the dynamic process of the three-phase contact (TPC) formation by colliding bubbles onto muscovite and talc surfaces in water and two types of solutions including cationic CTAC surfactant (cetyltrimethyl ammoniumchloride) and anionic NaOL surfactant (sodium oleate). The TPC formation process was observed through the high-speed camera between bubbles and layered silicate minerals (hydrophilic muscovite and hydrophobic talc). It was found that the rupture of the liquid film between the bubbles and the mineral surface is a prerequisite for TPC formation. In the case of muscovite, TPC was formed only with cationic CTAC, and as the surfactant concentration increased, the time needed for TPC formation was shortened. Due to electrostatic repulsion, TPC did not occur in water and NaOL. However, for talc, TPC occurred both in water and in surfactant solutions. In contrast to muscovite, the time of TPC formation on the talc surface was prolonged with the increase in the surfactant concentration. It was concluded that hydrophobic attraction and electrostatic attraction between mineral surfaces and bubbles can significantly promote the localized foam film rupture, which was the main reason for the TPC appearance in water and surfactants. For the hydrophilic muscovite, CTAC adsorption improved the surface hydrophobicity; I3/I1 in fluorescence spectroscopy increased, and the micro-polarity faded, making TPC formation need more time. However, for the natural hydrophobic talc, the increasing surfactant adsorption decreased I3/I1 values and enhanced the local micro-polarity, causing the extension of time for TPC. Therefore, TPC formation for different minerals resulted from different reasons.

Effect of surface chemical heterogeneity on kinetics of three phase contact formation

Bubble–particle interaction is a major focus for froth flotation research that has widespread use in mineral separation and concentration. The formation of a stable three phase contact (TPC) line is key to successful flotation. In industrial flotation processes, nearly all surfaces of particles are heterogeneous. Thus, the investigation of the influence of surface heterogeneity on the kinetics of TPC formation is important for understanding these processes. In this study, four types of heterogeneous surfaces were prepared by hydrophobization of glass slides followed by partial coverage with different mesh grids and oxidization with UV light. Interactions between bouncing bubbles and heterogeneous surfaces were investigated using a high-speed video camera. The results showed that the interval times between consecutive collisions with the patterned slide surface (before rupture) are almost independent of the surface. However, the behavior of the TPC line differed on various heterogeneous surfaces after bubble rupture. The terminal diameter was measured on different heterogeneous surfaces. It was found that the terminal diameter increased with the decrease in the mesh spacing of the grid. The slope of each curve is used to characterize the TPC line expansion velocity. The TPC line initial expansion velocity is inversely correlated with the grid mesh spacing. The surface heterogeneity had insignificant effect on the collision interaction between the bubble and the surface before the bubble rupture. However, after bubble rupture, the behavior of the expansion of TPC line was affected by the surface heterogeneity.

Bubble interactions with chalcopyrite and molybdenite surfaces in seawater

The detrimental effect of seawater on Cu-Mo flotation has attracted much attention in recent years, which has mainly been focused on altering the detrimental effect of seawater on the floatability of molybdenum ore. The interaction between bubbles and particles during flotation is a key factor in understanding the detrimental effect of seawater. Therefore, this study aimed to investigate the effect of seawater on bubble-particle interactions with chalcopyrite and molybdenite surfaces. Moreover, the effect of emulsified kerosene, a typical molybdenite collector, on bubble-particle interactions in seawater was investigated. Artificial seawater was used as a seawater model solution in this study. Flotation tests using pure chalcopyrite and molybdenite showed that the addition of emulsified kerosene to artificial seawater at a specific pH could selectively improve the floatability of molybdenite while maintaining the low floatability of chalcopyrite. A study of the bubble-particle interactions was then performed to analyze the phenomenon. It was found that the kerosene adsorbed at the air/liquid interface of the bubble improved the bubble aspect ratio and reduced the bubble rise velocity in artificial seawater. Moreover, kerosene could accelerate the formation of three-phase contact between the bubble and both mineral surfaces at the natural pH of artificial seawater. Additionally, this study showed that seawater colloidal precipitate formed under high pH conditions might be adsorbed on the mineral surfaces and might improve the stability of the intervening liquid film on the surface, thus preventing bubble-particle attachment and decreasing mineral recovery. Under these high pH conditions, the emulsified kerosene and seawater precipitate might compete in terms of adsorption on the mineral surfaces and the flotation results would most likely depend on the kerosene and hydrophilic adsorbate coverage on the mineral surfaces.

Effect of surface roughness of Chinese sub-bituminous coal on the kinetics of three-phase contact formation

In this paper, terminal diameter and expansion velocity of the three-phase contact (TPC) line on the low-rank coal surfaces with different surface roughness were investigated. To eliminate the heterogeneity effect on the hydrophilic surfaces of low-rank coal particles, experimental samples with low ash content were adopted. Moreover, the analysis result of XPS indicated that there were few hydrophilic mineral particles on the low-rank coal surface. Furthermore, from the analyses of SEM and AFM measurements there are many crevices (pillars, protrusions) of different sizes (height and width) in different local areas of low-rank coal surfaces. Therefore, the three-phase contact lines on the low-rank coal surfaces presented different wetting behaviors. It demonstrated that the wetting film diameter became smaller and the expansion velocity of the three-phase contact line was faster while the surface roughness of low-rank coal samples increased.

A relationship between time of three-phase contact formation and flotation kinetics of naturally hydrophobic solids

Flotation and single bubble tests were performed to compare the kinetics of flotation and time of a bubble attachment to naturally hydrophobic solids with different degree of hydrophobicity that is highly hydrophobic polytetrafluoroethylene, moderate hydrophobic fluorite and slightly hydrophobic quartz. The paper identifies an important relationship between the flotation kinetics, expressed as the inversed first order rate constant (1/k), and time of three-phase contact formation (tTPC) determined from the single bubble tests. It was shown that flotation kinetics is mainly determined by the time of three-phase contact formation, that is the time from the first bubble collision to its attachment to the solid surface. Despite the complexity of flotation process, which includes vast number of sub-processes, the kinetics of drainage of liquid film separating the bubble and solid surfaces seems to be a factor of crucial importance for the final flotation outcome.

Effect of kerosene emulsion in MgCl2 solution on the kinetics of bubble interactions with molybdenite and chalcopyrite

Abstract The formation of stable bubble–particle aggregates is essential to the froth flotation process. To form such aggregates, bubbles and particles must collide, the intervening liquid film must be drained below its critical rupture thickness, and three-phase contact (TPC) formation must occur. A good understanding of the interaction mechanism between bubbles and particles during collision and attachment is important. We herein investigated the effects of emulsified kerosene in a 0.01 M MgCl2 solution (a model seawater component) on bubble interactions with pure molybdenite (MoS2) and chalcopyrite (CuFeS2) surfaces. In 0.01 M MgCl2 at pH 6 and 11, kerosene retarded the bubble surface mobility and reduced the bubble rise velocity. In the presence of kerosene at pH 6, the TPC formed more rapidly on both mineral surfaces. This was due to the increase in surface hydrophobicity caused by kerosene. In addition, TPC formation was more rapid on the molybdenite surface than on the chalcopyrite surface due to the effect of the adsorbed kerosene and the low surface homogeneity of molybdenite. Finally, floatability tests demonstrated that the separation of molybdenite and chalcopyrite should be possible by adding emulsified kerosene in a 0.01 M MgCl2 solution at pH 9.

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...

Influence of non-ionic and ionic surfactants on kinetics of the bubble attachment to hydrophilic and hydrophobic solids

Abstract The paper is devoted to kinetics and mechanism of the three-phase contact (TPC) formation by the colliding bubble at model hydrophobic (polytetrafluoroethylene – Teflon) and hydrophilic (muscovite mica) solid surfaces. To determine influence of type of the surfactant polar group the experiments were carried out in solutions of non-ionic (n-hexanol and n-octanol), cationic (n-octyl-(OTABr), n-dodecyl-(DDTABr) and n-hexadecyl-trimethylammonium (CTABr) bromides, and anionic (sodium hexadecyl sulfate (SHS)) surface active substances (SAS). The time of the TPC formation (tTPC) and timescale of rupture of the thin liquid film separating the colliding bubble and a solid surface was determined using high-speed video registrations. The tTPC was defined as the time span from the moment of the bubble first collision up to the liquid film rupture and formation of the TPC (dewetted) hole. It was shown that the influence of SAS on the kinetics of the TPC formation could be completely different depending on the hydrophilic/hydrophobic properties of the solid surface. At the Teflon surface the TPC was formed always, both in distilled water and in SAS solutions of various composition and concentration. Moreover, the tTPC was significantly prolonged at higher SAS concentrations, independently on the SAS type (ionic, non-ionic). At the mica surface the wetting film was stable and the TPC was never registered in distilled water as well as in solutions of non-ionic and anionic surfactants. However, the TPC was formed at the mica surface in solutions of cationic surfactants, but the tTPC values were decreasing with increasing cationic surfactants concentration. Mechanism of the TPC formation is presented and reasons of the completely different influence of SAS on kinetics of the TPC formation at the hydrophilic and hydrophobic solid surfaces are explained.

Influence of n-octanol and α-terpineol on thin film stability and bubble attachment to hydrophobic surface

We have investigated the influence of concentration of surfactants typically used as flotation frothers (α-terpineol and n-octanol) and roughness of the solid surface on phenomena occurring during rising bubble collisions with a model hydrophobic Teflon surface. The time of three-phase contact (TPC) formation (t(TPC)) and the time of drainage (t(D)) of the film formed between the colliding bubble and Teflon surfaces were determined using a high-speed camera working with a frequency 1040 Hz. The Teflon surface roughness was varied on a microscopic scale, within a roughness ranging between 1 and 100 μm. We have found that the roughness of the Teflon surface is a crucial factor of the kinetics of the TPC formation, both in the absence and in the presence of the surfactants. With the surface roughness increasing from ca. 1 to 80 μm the t(TPC) can be shortened by an order of magnitude, i.e. from 105 ms down to a few milliseconds. We have demonstrated that bouncing of the colliding bubble is responsible for the large differences in the times of TPC formation at the Teflon surfaces of different roughness. Low concentrations of α-terpineol and n-octanol caused a decrease in the t(TPC) with respect to distilled water. However, at high concentrations the t(TPC) was prolonged. The prolongation of the time of the TPC formation was dependent on the Teflon surface roughness and we have attributed this effect to different amounts of air present in the cavities and scratches of hydrophobic surfaces of different roughness. The mechanism of prolongation of the t(TPC) at high concentration of surface-active substances (frother overdosage) is proposed.

Role of Surface Charge and Hydrophobicity in the Three-Phase Contact Formation and Wetting Film Stability under Dynamic Conditions

Determination and control of the parameters influencing three-phase contact formation and wetting film stability under dynamic conditions is important for both industrial applications and fundamental studies. Hydrophobicity profoundly affects not only the critical thickness of rupture but also the subsequent rate of the three-phase contact expansion. On the other hand, surface charge has a significant impact on the wetting film stability.In the present study, by comparing collision of air bubbles of different sizes with hydrophobized amorphous titanium dioxide (TiO2) surfaces immersed into solutions at different pH values, we demonstrated that for an intermediate degree of hydrophobicity (advancing contact angle ≤ 60°), the surface charge becomes a decisive factor determining the time of the three-phase contact formation, tTPC, and the time of wetting film drainage, td. For pH values below the isoelectric point (IEP) of TiO2, the tTPC and td were significantly shorter than for pH values above the IEP. Mor...

Dynamic adsorption layer formation and time of bubble attachment to a mica surface in solutions of cationic surfactants (C nTABr)

The paper presents the influence of the dynamic adsorption layer (DAL) formation on time of the bubble attachment to mica surface in n-dodecyltrimethylammonium bromide (DDTABr) and n-hexadecyltrimethylammonium bromide (CTABr) solutions. Phenomena occurring during collisions of the single rising bubbles with mica plates, located at different distances (“close” and “far”) from the point of the bubble detachment, were monitored using high-speed camera. It was found that when the distance from the capillary to the mica plate was L = 3 mm (location “close”) then the time of the three-phase contact formation ( t TPC), was significantly shorter than for the location “far” ( L = 100 mm). The differences between the t TPC for the locations “close” and “far” were the largest at lowest concentration. For example in 1 × 10 −6 M CTABr solution the t TPC was 162 ± 97 ms for L = 3 mm and was prolonged to 578 ± 121 ms for L = 100 mm. Similarly, in 1 × 10 −6 M DDTABr solution the t TPC was 543 ± 79 and 1840 ± 440 ms for L = 3 and 100 mm, respectively. Longer the t TPC values at the mica location “far” were attributed to the formation of the dynamic adsorption layer (DAL), with the top pole of the rising bubble almost devoid of the adsorbed cationic surfactants. The bubble attachment to mica was due to the electric charge reversal by the cationic surfactant molecules adsorbed at the bubble surface, which led to the wetting film rupture due to electrostatic attractive interactions between oppositely charged interacting interfaces. Therefore, at the location “far” the time of the TPC formation ( t TPC “far”) was longer because there was needed an additional time to restore such degree of cationic surfactant coverage at the bubble top pole, which caused the electric charge reversal from negative to positive. It was showed that the adsorption coverages needed for the electric charge reversal at the liquid gas interface can be restored during the Δ t TPC = t TPC “far” − t TPC “close” time intervals.

An investigation into the mechanism of the three-phase contact formation at fluorite surface by colliding bubble

Mechanism of the three-phase contact (TPC) formation and phenomena occurring during collisions of the ascending bubble with a solid and flat surface were studied. Three fluorite samples of different origins and surface roughness and smooth mica plates were investigated. It was found that the time-scale and the mode of air bubble/flat fluorite surface bouncing and attachment significantly depended on the surface roughness. For smooth fluorite surfaces the time of the TPC formation and the bubble attachment was over 150 ms while for rough surfaces it was 5 to 45 ms. It was also shown that the TPC formation at fluorite and mica surfaces was determined mainly by the electrical charge of the interacting interfaces. A small amount of cationic surfactant ( n-octyltrimethylammonium bromide) was used to reverse the sign of electrical charge of the bubble surface. In n-octyltrimethylammonium bromide presence there was no TPC formation at the fluorite surface due to repulsions between positively charged interfaces, but the bubble attachment to negatively charged mica surface was observed due to attractive electrostatic interactions. It was also determined that there was no TPC formation at pHs when the signs of zeta potentials of the solution/gas and solution/fluorite interfaces were the same.

Air-Facilitated Three-Phase Contact Formation at Hydrophobic Solid Surfaces under Dynamic Conditions

The paper presents results documenting the mechanism of facilitation of the three-phase contact (TPC) formation due to gas entrapped during immersion of hydrophobic (Teflon) plates into distilled water and n-octanol solutions. Collisions, bouncing, the time scale of the TPC formation, and bubble attachment to Teflon plates of different surface roughness were studied using a high-speed camera. Processes occurring during the microscopic wetting film formation at the Teflon plates were monitored using the microinterferometric method (Scheludko-Exerowa cell). A strong relation between the time necessary to form a stable TPC and the roughness of the Teflon was observed. The higher the Teflon roughness was the shorter the time for the TPC formation. This effect can be attributed to two factors: (i) local differences in the thickness of the thinning intervening liquid layer (quicker attainment of rupture thickness at pillars of rough surface) and/or (ii) the presence of gas at the hydrophobic surface. Experimental findings, that (i) prolongation of the plate immersion time resulted in quicker TPC formation, (ii) white irregular and disappearing spots (air pockets) were recorded during the wetting film formation, and (iii) high n-octanol concentration caused prolongation of the time of the TPC formation, show that attachment (TPC formation) of the colliding bubble to hydrophobic surfaces was facilitated by air entrapped at the Teflon plates (and re-distributed) during their immersion into water phase. Thus, on collision instead of solid/gas wetting liquid film a thin gas/liquid/gas foam film was formed which facilitated the TPC formation.

On modelling of bubble–particle attachment probability in flotation

We have developed the theoretical basis for experimental determination of bubble–particle attachment probability in single-bubble flotation experiments. We use these experimentally determined data to simulate the dependence of induction time on particle size. Using these simulated results, we verify the bubble–particle attachment probability prediction developed on the basis of sliding time and induction time (the Sutherland approach). We have calculated the induction times of fine quartz particles of different diameters less than 80 μm and different hydrophobicity, collected by single bubbles with diameters of 0.75 mm, 1.2 mm and 2.0 mm. The simulation reported in this present paper shows that the calculated induction times increase with decreasing particle size, except for particles of strong hydrophobicity (advancing water contact angle of 65° and 88°) collected by the small bubbles with a diameter of 0.75 mm. These results for induction time indicate that extant attachment probability models, based on sliding time and induction time, cannot adequately describe the elementary steps in the bubble–particle attachment microprocess in flotation. A detailed analysis of induction time, as originally defined and measured by Sven-Nilsson and others was performed. The necessity of the elementary step `rupture of liquid film and formation of three-phase contact of a critical radius (TPC nuclei)' in bubble–particle attachment has been demonstrated. It is suggested that attachment probability can be adequately described by the product of the probabilities of three elementary steps: (1) thinning of intervening liquid film to a critical film thickness; (2) rupture of intervening liquid film and formation of TPC nuclei; (3) expansion of three-phase contact line from the critical radius to form a stable wetting perimeter. The attachment probability model developed according to the Sutherland approach describes only the first elementary step. The poor response of fine particle flotation may be attributed to the decreasing probability of the second elementary step. A more detailed investigation of these elementary steps is required for understanding the attachment microprocess in flotation.

On the kinetics of froth flotation

The problem of froth flotation kinetics is treated as a transport phenomenon. Two coupled equations of balance are formulated which concern the particles attached upon the bubbles and the free particles in the pulp. The combined equations are solved and an analytical expression for the recovery as a function of time is obtained. The correctness of the theoretical derivation is checked by way of laboratory flotation experiments with glass ballotini (160–200 μm) in a wide pH range (pH 3–13) and collector dodecylamine hydrochloride (C 12H 25NH 2, HCL). The authors demonstrate that the recovery of glass possesses a well-defined maximum at pH 11.2. The cause of this maximum is sought for in the strong dependence of the three-phase contact (TPC) rate of expansion on pH. The role of the kinetic characteristics: time of TPC formation and TPC expansion rate are discussed with the help of the well-known criterion of floatability “attachment forces greater than the detachment forces” which is applied locally in any moment of the particle-bubble interaction.

New theoretical and experimental investigations on stability of bubble/particle aggregates in flotation: A theory on the upper particle size of floatability

During the last three decades flotation has been developed into the most important separating method for minerals. Nowadays more than two thousand million tons of more than 100 different minerals are annually floated in the world. In the last years the field of applicability of flotation has been considerably extended, above all to the operations of process engineering, such as waste-water purification, the paper industry and inorgano-chemical process engineering. Today a new field of application is developing — the flotation of microorganisms for protein recovery in biotechnological processes. Likewise, the minerals to be processed are becoming poorer on the one hand and on the other it is necessary to reduce the energy consumption of the process considerably. Furthermore, automation of the operations is required. All these requirements cannot be adequately met as long as there is no physically based flotation model allowing for the calculation of probabilities of single process stages and the simulation of the process as a whole. Therefore, investigation of the process, aiming at clearing up the single fundamental phenomena, is a task to be urgently solved today. Recently, we succeeded in presenting the three essential and important fundamental stages of flotation: 1. (1) Approach of solid particles and gas bubbles upon formation of a thin liquid film between them. 2. (2) Formation of three-phase contact (tpc). 3. (3) Stability of the formed aggregates. The present paper deals, above all, with the latter phenomenon. It is shown that by considering the kinetic energy of aggregates in the turbulent field of a flotation machine, their stability, i.e. the upper particle size limit of floatability, can be calculated. The behaviour and the forces acting upon the single particles in the fluid interface are the preconditions. The calculations are based on numerical integration of the Laplace differential equation. Experiments were carried out to confirm the theoretical considerations.