Particle-Bubble Attachment in Mineral Flotation

Soft wetting: Substrate softness- and time-dependent droplet/bubble adhesion

HYPOTHESIS

The droplet/bubble adhesion characteristics depend on the length of the droplet/bubble three-phase contact line. Since the deformation caused by the liquid-gas interfacial tension on the soft substrate, referred as to the wetting ridge, retards contact line spreading and retraction, we conjecture that the droplet/bubble adhesion characteristics depend also on the substrate softness.

EXPERIMENTS

Soft substrates with various shear moduli are prepared and characterized by the spreading and receding dynamics of water droplets and underwater bubbles. Snap-in and normal adhesion forces of droplets/bubbles on such soft substrates are directly measured along with the visualized droplet/bubble shape profiles.

FINDINGS

The droplet/bubble snap-in force, which corresponds to the short-time spreading dynamics, decreases with a decrease in the substrate shear modulus because of the retarded contact line spreading. The droplet maximal adhesion force on a soft substrate can be counterintuitively either smaller or larger than its counterpart on the rigid substrate depending on different dwelling times, i.e., the droplet/bubble-substrate contact time before droplet/bubble-substrate separation. The former is attributed to the retarded contact line spreading, whereas the latter is attributed to the retarded contact line retraction. The substrate softness- and dwelling time-dependent droplet/bubble adhesion reported in this study will benefit various applications related to soft substrates.

Combined microflotation effects in polymetallic ores beneficiation

Producing of heterogeneous concentrates with good recovery in the processing of polymetallic ores is a challenge. Many factors must be taken into account including ore grinding, reagent mode, water composition, pulp density and the volume of supplied bubbles when producing high-quality selective concentrates. Microbubbles smaller than 50 μm in size were produced based on the frother oxal T-92 at different concentrations using a generator. The most optimal number of microbubbles smaller than 50 µm was produced at a T-92 concentration of 0.5 g/dm3. Polymetallic ore of Kazakhstan deposit was used for flotation studies. The studies were conducted in the copper-lead rough concentrate producing cycle. Flotation active minerals chalcopyrite and galena pass into the foam product, while sphalerite and pyrite remain in the chamber product in this cycle. In this paper, the density of pulp (20, 30 and 50%) as one of the main factors that effects the selectivity of flotation is studied. The kinetics of ore flotation in the base mode and with the use of a microbubble generator has been studied at these densities. Test experiments have been performed at the optimum density. The use of a water–air microemulsion generator makes it possible to maintain the quality of the copper-lead concentrate and increase the extraction of copper into the rough concentrate by 7.41%, lead by 5.98%.

Surface interaction mechanisms in mineral flotation: Fundamentals, measurements, and perspectives

As non-renewable natural resources, minerals are essential in a broad range of biological and technological applications. The surface interactions of mineral particles with other objects (e.g., solids, bubbles, reagents) in aqueous suspensions play a critical role in mediating many interfacial phenomena involved in mineral flotation. In this work, we have reviewed the fundamentals of surface forces and quantitative surface property-force relationship of minerals, and the advances in the quantitative measurements of interaction forces of mineral-mineral, bubble-mineral and mineral-reagent using nanomechanical tools such as surface forces apparatus (SFA) and atomic force microscope (AFM). The quantitative correlation between surface properties of minerals at the solid/water interface and their surface interaction mechanisms with other objects in complex aqueous media at the nanoscale has been established. The existing challenges in mineral flotation such as characterization of anisotropic crystal plane or heterogeneous surface, low recovery of fine particle flotation, and in-situ electrochemical characterization of collectorless flotation as well as the future work to resolve the challenges based on the understanding and modulation of surface forces of minerals have also been discussed. This review provides useful insights into the fundamental understanding of the intermolecular and surface interaction mechanisms involved in mineral processing, with implications for precisely modulating related interfacial interactions towards the development of highly efficient industrial processes and chemical additives.

Film entrainment and microplastic particles retention during gas invasion in suspension-filled microchannels

Understanding of microplastics transport mechanism is highly important for soil contamination and remediation. The transport behaviors of microplastics in soils are complex and influenced by various factors including soil and particle properties, hydrodynamic conditions, and biota activities. Via a microfluidic experiments we study liquid film entrainment and microplastics transport and retention during two-phase displacement in microchannels with one end connected to the air and the other connected to the liquid with suspended particles. We discover three transport patterns of microplastic particles, ranging from no deposition to particle entrapment and to particle layering within liquid films, depending on the suspension withdrawal rates and the particle volume fraction in the suspension. The general behavior of particle motion is effectively captured by the film thickness evolution which is shown to be dependent on a modified capillary number Ca₀ taking into account the effects of flow velocity, particle volume fraction, and channel shape. We also provide a theoretical prediction of the critical capillary number Ca₀* for particle entrapment, consistent with the experimental results. In addition, the probability of microplastics being dragged into the trailing liquid film near the gas invading front is found to be proportional to both particle volume fraction and the capillary number. This work elucidates the microplastics transport mechanism during unsaturated flow, and therefore is of theoretical and practical importance to understand the contaminant migration in many natural and engineered systems spanning from groundwater sources to water treatment facilities.

A gateway for ion transport on gas bubbles pinned onto solids

Gas bubbles grown on solids are more than simple vehicles for gas transport. They are charged particles with surfaces populated with exchangeable ions. We here unveil a gateway for alkali metal ion transport between oxygen bubbles and semi-conducting (iron oxide) and conducting (gold) surfaces. This gateway was identified by electrochemical impedance spectroscopy using an ultramicroelectrode in direct contact with bubbles pinned onto these solid surfaces. We show that this gateway is naturally present at open circuit potentials, and that negative electric potentials applied through the solid enhance ion transport. In contrast, positive potentials or contact with an insulator (polytetrafluoroethylene) attenuates transport. We propose that this gateway is generated by overlapping electric double layers of bubbles and surfaces of contrasting (electro)chemical potentials. Knowledge of this ion transfer phenomenon is essential for understanding electric shielding and reaction overpotential caused by bubbles on catalysts. This has especially important ramifications for predicting processes including mineral flotation, microfluidics, pore water geochemistry, and fuel cell technology.

Wrinkling number and force of a particle raft in compression

A particle raft is formed by a layer of small particles floating on a water surface, which has a higher load bearing capacity than pure water. In the present work, we have made a comprehensive study on the wrinkling number and force of the particle raft in planar compression. The wrinkling number during the whole loading process is measured, accompanied with snapshots on the morphologies of the particle raft. The force-displacement curve is given based on the loading system, which has been validated by the numerical simulation. Moreover, the experiment and theoretical results both show that the equivalent Young's modulus is dependent upon the loading displacement. Finally, the maximum wrinkling number of the raft has been analyzed by the scaling law, which agrees well with the experimental result. These findings have deepen our understandings on the mechanical properties of soft materials, which also hold implications on drug delivery, chemical engineering, micro-fluidics, environment protection, petroleum exploitation, mineral flotation, etc.

Hetero-difunctional Reagent with Superior Flotation Performance to Chalcopyrite and the Associated Surface Interaction Mechanism

Surface modification by chemical reagents is of profound importance to modulate the surface characteristic and functionality of materials, which has attracted tremendous interest in many research fields and industrial applications, such as froth flotation of minerals. In this work, a new reagent S-[(2-hydroxyamino)-2-oxoethyl]- O-octyl-dithiocarbonate ester (HAOODE) with heterodifunctional ligands was designed and synthesized to improve the flotability of chalcopyrite (CuFeS₂). Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy results showed the co-adsorption of heterodifunctional ligands (i.e., dithiocarbonate and hydroxamate groups) of HAOODE on chalcopyrite via Cu(I)-S and Cu(II)-O covalent bonds. The bubble probe atomic force microscopy (AFM) technique was employed to quantitatively measure the air bubble-chalcopyrite interactions with and without the reagent adsorption. AFM force results revealed that the bubble could be more readily attached to flat chalcopyrite after HAOODE treatment under different hydrodynamic conditions because of the enhanced hydrophobic interaction, with the decay length D₀ increasing from 0.65 to 1.20 nm. The calculated bubble-particle interaction forces also demonstrated the critical influence of HAOODE treatment, hydrodynamic conditions, and bubble size on the interaction behavior and thin-film drainage process in flotation. In froth flotation, HAOODE exhibited superior recovery for chalcopyrite over pH 3-12 and excellent selectivity for chalcopyrite against pyrite (FeS₂) above pH 10.5, as compared to the conventional reagent sodium isobutyl xanthate. This work provides a useful approach to develop effective reagents that could selectively adsorb on desired mineral surfaces through heterodifunctional ligands and to quantitatively evaluate the role of reagent adsorption in the interactions between air bubbles and mineral surfaces at the nano- and microscale. Our results show implications on developing molecular design principles of novel reagents for surface modifications of materials in a wide range of engineering and biological applications.

Colloid mobilization and heavy metal transport in the sampling of soil solution from Duckum soil in South Korea

Colloid mobilization is a significant process governing colloid-associated transport of heavy metals in subsurface environments. It has been studied for the last three decades to understand this process. However, colloid mobilization and heavy metal transport in soil solutions have rarely been studied using soils in South Korea. We investigated the colloid mobilization in a variety of flow rates during sampling soil solutions in sand columns. The colloid concentrations were increased at low flow rates and in saturated regimes. Colloid concentrations increased 1000-fold higher at pH 9.2 than at pH 7.3 in the absence of 10 mM NaCl solution. In addition, those were fourfold higher in the absence than in the presence of the NaCl solution at pH 9.2. It was suggested that the mobility of colloids should be enhanced in porous media under the basic conditions and the low ionic strength. In real field soils, the concentrations of As, Cr, and Pb in soil solutions increased with the increase in colloid concentrations at initial momentarily changed soil water pressure, whereas the concentrations of Cd, Cu, Fe, Ni, Al, and Co lagged behind the colloid release. Therefore, physicochemical changes and heavy metal characteristics have important implications for colloid-facilitated transport during sampling soil solutions.

Foams: From nature to industry

This article discusses different natural and man-made foams, with particular emphasis on the different modes of formation and stability. Natural foams, such as those produced on the sea or by numerous creatures for nests, are generally stabilised by dissolved organic carbon (DOC) molecules or proteins. In addition to this, foam nests are stabilised by multifunctional mixtures of surfactants and proteins called ranaspumins, which act together to give the required physical and biochemical stability. With regards to industrial foams, the article focuses on how various features of foams are exploited for different industrial applications. Stability of foams will be discussed, with the main focus on how the chemical nature and structure of surfactants, proteins and particles act together to produce long-lived stable foams. Additionally, foam destabilisation is considered, from the perspective of elucidation of the mechanisms of instability determined spectroscopically or by scattering methods.

Armoring confined bubbles in the flow of colloidal suspensions

We study the process of coating the interface of a long gas bubble, which is translating in a horizontal circular capillary tube filled with a colloidal suspension. A typical elongated confined bubble is comprised of three distinct regions: a spherical front cap, a central body that is separated from the tube wall by a thin liquid film, and a spherical cap at the back. These three regions are connected by transitional sections. Particles gradually coat the bubble from the back to the front. We investigate the mechanisms that govern the initial accumulation of the particles and the growth of the particle-coated area on the interface of the bubble. We show that the initial accumulation of particles starts at the stable stagnation ring on the rear cap of the bubble, and the particles will completely coat the spherical cap at the back of the bubble before accumulating on the central body. Armoring the central interface of the bubble with particles thickens the liquid film around the bubble relative to that around the particle-free interface. This effect creates a rather sharp step on the interface of the bubble in the central region, which separates the armored region from the particle-free region. After the bubble is completely coated, the liquid film around the body of the bubble will adjust again to an intermediate thickness. We show that the three distinct thicknesses that the liquid film acquires during the armoring process can be well described analytically.

Bubble-Driven Detachment of Bacteria from Confined Microgeometries

Moving air-liquid interfaces, for example, bubbles, play a significant role in the detachment and transport of colloids and microorganisms in confined systems as well as unsaturated porous media. Moreover, they can effectively prevent and/or postpone the development of mature biofilms on surfaces that are colonized by bacteria. Here we demonstrate the dynamics and quantify the effectiveness of this bubble-driven detachment process for the bacterial strain Staphylococcus aureus. We investigate the effects of interface velocity and geometrical factors through microfluidic experiments that mimic some of the confinement features of pore-scale geometries. Depending on the bubble velocity U, at least three different flow regimes are found. These operating flow regimes not only affect the efficiency of the detachment process but also modify the final distribution of the bacteria on the surface. We organize our results according to the capillary number, [Formula: see text], where μ and γ are the viscosity and the surface tension, respectively. Bubbles at very low velocities, corresponding to capillary numbers Ca < 5 × 10^-5, exhibit detachment efficiencies of up to 80% at the early stage of bacterial adhesion. In contrast, faster bubbles at capillary numbers Ca > 10^-3, have lower detachment efficiencies and cause significant nonuniformities in the final distribution of the cells on the substrate. This effect is associated with the formation of a thin liquid film around the bubble at higher Ca. In general, at higher bubble velocities bacterial cells in the corners of the geometry are less influenced by the bubble passage compared to the central region.

Formation and influence of the dynamic adsorption layer on kinetics of the rising bubble collisions with solution/gas and solution/solid interfaces

BACKGROUND

The DAL (dynamic adsorption layer) formation, that is, the establishment of uneven distribution of adsorption coverage over the rising bubble surface, with significantly diminished coverage at the upstream pole, is the factor of crucial importance for the bubble motion parameters and kinetic of the bubble collisions with various interfaces. The DAL presence can influence the stability of the thin liquid films formed by the colliding bubble at solution/gas and solution solid interfaces.

AIM

The purpose of this paper is to critically review the existing state of art regarding the influence of the DAL formation and existence on the bubble motion parameters as well as kinetics of coalescence at free solution surface and three phase contact (TPC) formation at solid/liquid interfaces of different hydrophilic/hydrophobic properties.

CONCLUSIONS

Despite the fact that up to now there is no direct experimental evidence showing DAL existence, it is documented by experimental data showing clear correlation between bubble local velocity variations and shape pulsations as well as lifetimes of the liquid film formed by the colliding bubble at gas/liquid and gas/solid interfaces.

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.

Direct observation of individual particle armored bubble interaction, stability, and coalescence dynamics

The interactions between two individual particle-stabilized bubbles were investigated, in the absence of surfactant, using a combination of coalescence rig and high-speed video camera. This combination allows the visualization of bubble coalescence dynamics which provide information on bubble stability. Experimental data suggested that bubble stability is enhanced by both the adsorption of particles at the interface as indicated by the long induction time and the increase in damping coefficient at high surface coverage. The interaction between an armored bubble and a bare bubble (asymmetric interaction) can be destabilized through the addition of a small amount of salt, which suggested that electrostatic interactions play a significant role in bubble stability. Interestingly, the DLVO theory cannot be used to describe the bubble stability in the case of a symmetric interaction as coalescence was inhibited at 0.1 M KCl in both the absence and presence of particles at the interfaces. Furthermore, bubbles can also be destabilized by increasing the particle hydrophobicity. This behavior is due to thinner liquid films between bubbles and an increase in film drainage rate. The fraction of particles detached from the bubble surface after film rupture was found to be very similar within the range of solution ionic strength, surface coverage, and particle hydrophobicity studied. This lack of dependence implies that the kinetic energy generated by the coalescing bubbles is larger than the attachment energy of the particles and dominates the detachment process. This study illuminates the stability behavior of individual particle-stabilized bubbles and has potential impact on processes which involve their interaction.

Determination of contact angles, silane coverage, and hydrophobicity heterogeneity of methylated quartz surfaces using ToF-SIMS

Methylated quartz surfaces are extensively used in colloid science for wettability studies and the control and impact of hydrophobicity in key physicochemical processes. In this study, time-of-flight secondary ion mass spectrometry (ToF-SIMS) has been used to correlate the surface chemistry of trimethylchlorosilane-methylated quartz surfaces with the contact angle. Models have been developed for the calculation of both advancing and receding contact angles based on measurements of the ToF-SIMS signals for SiC(3)H(9)(+) (TMCS) and Si(+) (quartz). These models enable the contact angle across surfaces and, more importantly, that of individual particles to be determined on a micrometer scale. Distributions of contact angles in large ensembles of particles, therefore, can now be determined. In addition, from the ToF-SIMS analysis, the surface coverage of the methylated species can be quantitatively determined, in line with the Cassie equation. Moreover, advancing and receding contact angle maps can be calculated from ToF-SIMS images, and hence the variation in microscopic hydrophobicity (e.g., at the particle level) can be extracted directly from the images.

Detachment of deposited colloids by advancing and receding air-water interfaces

Moving air-water interfaces can detach colloidal particles from stationary surfaces. The objective of this study was to quantify the effects of advancing and receding air-water interfaces on colloid detachment as a function of interface velocity. We deposited fluorescent, negatively charged, carboxylate-modified polystyrene colloids (diameter of 1 μm) into a cylindrical glass channel. The colloids were hydrophilic with an advancing air-water contact angle of 60° and a receding contact angle of 40°. After colloid deposition, two air bubbles were sequentially introduced into the glass channel and passed through the channel at different velocities (0.5, 7.7, 72, 982, and 10,800 cm/h). The passage of the bubbles represented a sequence of receding and advancing air-water interfaces. Colloids remaining in the glass channel after each interface passage were visualized with confocal microscopy and quantified by image analysis. The advancing air-water interface was significantly more effective in detaching colloids from the glass surface than the receding interface. Most of the colloids were detached during the first passage of the advancing air-water interface, while the subsequent interface passages did not remove significant amounts of colloids. Forces acting on the colloids calculated from theory corroborate our experimental results, and confirm that the detachment forces (surface tension forces) during the advancing air-water interface movement were stronger than during the receding movement. Theory indicates that, for hydrophilic colloids, the advancing interface movement generally exerts a stronger detachment force than the receding, except when the hysteresis of the colloid-air-water contact angle is small and that of the channel-air-water contact angle is large.

Air at hydrophobic surfaces and kinetics of three phase contact formation

This review focuses on the importance of air presence at hydrophobic solid surfaces for wetting film rupture and kinetics of three phase contact formation. Affinity to air is a typical feature of hydrophobic surfaces, but it has been often either overlooked or not taken into consideration. When the hydrophobic surface, contacted earlier with air, is immersed into water then air can stay attached to the surface. The origin of long range hydrophobic forces and data showing that these interactions were due to the bridging of nanobubbles attached to the hydrophobic surfaces are discussed. A major part of the review is devoted to the description and analysis of data showing that air (nano-, micro-bubbles and/or air film) present at a hydrophobic surface facilitated rupture of the liquid film and three phase contact formation during bubble collisions with flat Teflon plates of different surface roughness. Although all Teflon plates were highly hydrophobic (contact angles ca. 100 degrees -130 degrees ) the time of the three phase contact (TPC) formation and attachment of the colliding bubble was strongly affected by the plate surface roughness. The time of the TPC formation was shortened from over 80 down to 2-3 ms when the roughness was increased from below 1 microm to over 50 microm. Higher surface roughness means that larger amounts of air was entrapped during the Teflon plates' immersion in water. Additional experimental evidence is given, showing that facilitation of the TPC formation and the bubble attachment was due to air presence and re-distribution over the Teflon surfaces: i) prolonging the plate immersion time resulted in quicker attachment; ii) irregular and disappearing air pockets were recorded at a Teflon surface; iii) a satellite bubble left at a Teflon surface during the first collision facilitated the attachment; iv) attachment always occurred during the first collision in the case of a very rough "Teflon V" surface, but in highly concentrated n-octanol and n-heptanol solutions there was bouncing and attachment occurred during the second collision, moreover; v) the degree of bubble kinetic energy transferred into surface energy was significantly smaller during collisions with hydrophobic (Teflon) surfaces than with the hydrophilic ones. The mechanism of air entrapment and redistribution over Teflon plates immersed in water is presented.