The Limits of Fine and Coarse Particle Flotation

The flotation behaviour of quartz particles was studied over the particle size range from 0.5 µm to 1000 µm and for advancing water contact angles between 0° and 83°. Flotation was performed in a column and in a Rushton turbine cell. Particle contact angle threshold values, below which the particles could not be floated, were identified for the particle size range 0.5–1000 µm, under different hydrodynamic conditions. The flotation response of the particles, either in a column or in a mechanically agitated cell with a similar bubble size, was comparable. Turbulence plays a role, as does bubble‐particle aggregate velocity and bubble size. The stability of the bubble‐particle aggregate controls the maximum floatable particle size of coarse particles. For fine particles, the flotation limit is dictated by the energy required to rupture the intervening liquid film between the particle and bubble. Flotation of very fine and large particles is facilitated with small bubbles and high contact angles. These results greatly extend our earlier observations and theoretical predictions.     

Contact angle and vacuum floatability of ultrafine size particles

The contact angle of ultrafine size particles has been evaluated using 1 µm monosize SiO₂ particles of various degrees of wettability. The contact angle was determined by film flotation and Zisman plots. Chlorotrimethylsilane (CTS) was used to methylate the SiO₂ particle surface and establish the level of surface wettability. Also, the vacuum floatability of the methylated ultrafine SiO₂ particles was assessed to correlate it to the contact angle. This vacuum floatability was very low below 40º and increased monotonically above this contact angle value because of favorable bubble nucleation and a greater stability of the bubbles on the hydrophobic surface. Free energy of bubble nucleation on the hydrophobic surfaces has been estimated and correlated to the vacuum floatability of the ultrafine particles.     

Adhesion of solid particles to gas bubbles. Part 2: Experimental

In slurry bubble columns, the adhesion of solid catalyst particles to bubbles may significantly affect the G-L mass transfer and bubble size distribution. This feature may be exploited in design by modifying the hydrophilic or hydrophobic nature of the particles used. Previously we have proposed a generalised model, describing the adhesion of particles to G-L interface under stagnant conditions. In this work, we studied the adhesion of particles characterised by different degree of hydrophobicity and porosity: non-porous polystyrene and glass beads, unmodified and hydrophobised mesoporous silica, and activated carbon particles. Images recorded at high optical magnification show the particles adhering to gas bubbles individually or as aggregates. In aqueous media, higher liquid surface tension and particle surface hydrophobicity increase the adhesion strength and the tendency of particles to agglomerate, in agreement with the model. The adhesion of non-porous rough-surface particles to gas bubbles can be characterised by the receding contact angle. The advancing contact angle represents better the adhesion of the same particles to liquid droplets. We found that the “effective” contact angle of porous particles is much lower than an “intrinsic” contact angle calculated from the heat of immersion in water, or measured by sessile drop method. An equivalent contact angle derived from the Cassie rule explains the wetting behaviour of particles having the pores filled with liquid.     

Picobubble Enhanced Fine Coal Flotation

Abstract

Froth flotation is widely used in the coal industry to clean ?28 mesh fine coal. A successful recovery of particles by flotation depends on efficient particle‐bubble collision and attachment with minimal subsequent particle detachment from bubble. Flotation is effective in a narrow size range beyond which the flotation efficiency drops drastically. It is now known that the low flotation recovery of particles in the finest size fractions is mainly due to a low probability of bubble‐particle collision while the main reason for poor coarse particle flotation recovery is the high probability of detachment. A fundamental analysis has shown that use of picobubbles can significantly improve the flotation recovery of particles in a wide range of size by increasing the probability of collision and attachment and reducing the probability of detachment.

A specially designed column with a picobubble generator has been developed for enhanced recovery of fine coal particles. Picobubbles were produced based on the hydrodynamic cavitation principle. They are characterized by a size distribution that is mostly below 1 µm and adhere preferentially to the hydrophobic surfaces. The presence of picobubbles increases the probability of collision and attachment and decreases the probability of detachment, thus enhancing flotation recovery. Experimental results with the Coalberg seam coal in West Virginia, U.S.A. have shown that the use of picobubbles in a 2″ column flotation increased fine coal recovery by 10–30%, depending on the feed rate, collector dosage, and other flotation conditions. Picobubbles also acted as a secondary collector and reduced the collector dosage by one third to one half.     

Effect of microbubbles and particle size on the particle collection in the column flotation

Particle-bubble collection characteristics from microbubble behavior in column flotation have been studied theoretically and experimentally. A flotation model taking into account particle collection has been developed by particle-bubble collision followed by the particle sliding over the bubble during which attachment may occur. Bubble size and bubble swarm velocity were measured as a function of frother dosage and superficial gas velocity to estimate the collision and collection efficiency. Separation tests were carried out to compare with theoretical particle recovery. Fly ash particles in the size range of <38, 38-75, 75-125, >125 mm were used as separation test particles. Theoretical collision and collection efficiencies were estimated by experimental data on the bubble behavior such as bubble size, gas holdup and bubble swarm velocity. Collection efficiency improved with an increase of the bubble size and particle size but decreased in the particle size up to 52 mm. Also, flotation rate constants were estimated to predict the optimum separation condition. From the theoretical results on the flotation rate constant, optimum separation condition was estimated as bubble size of 0.3-0.4 mm and superficial gas velocity of 1.5-2.0 cm/s. A decrease of bubble size improved the collection efficiency but did not improve particle recovery.     

Yoon-Luttrell collision and attachment models analysis in flotation and their application on general flotation kinetic model

We theoretically used the models of Yoon and Luttrell for collision and attachment efficiencies to show the effect of fluid flow condition, the effect of bubble size and velocity and particle surface hydrophobicity in flotation system, and in order to demonstrate the effect of particle density on the attachment behavior we incorporate the correct expression for the maximum collision angle developed by Dukhin collision model in the Yoon-Luttrell attachment efficiency applied for two minerals species such as the quartz and chalcopyrite. Then we used the expression of the analytical model that enables the calculation of the flotation rate constant of particles derived by Pyke et al., developed under turbulent condition and with including the efficiency of collision using the generalized Sutherland equation (GSE), the attachment efficiency using modified Dobby-Finch model, and stability of bubble-particle aggregate includes the various forces acting between the bubble and the attached particle. Some results are obtained revealing the positive inertial effect for the quartz and galena particles under defined flotation data conditions by incorporating in the flotation rate constant mentioned above, the collision and attachment efficiency models of Yoon-Luttrell developed for potential flow condition with assuming that the bubble surface is completely mobile and the particle inertia is ignored. The results show also the influence of the increasing of the bubble velocity to determine the particle size range between the models considering the inertial effect and those who ignored the particle Inertia.     

The bubbling behaviour of fine powders when fluidised

Narrow size cuts of particles in the range 40–260 μm have been examined by X-rays when fluidised by air. No discontinuity in behaviour was observed with decreasing particle size. The bubbles behave the same in all materials except that their velocity increases with decreasing particle size. The visible bubble flow is generally less than the excess over minimum fluidisation flow. Almost all flow occurs interstitially near the bottom of the bed but the proportion decreases with height to approach U_mf near the top of the bed. Appreciable changes in dense phase voidage occur with fine particles and this varies with bed height.     

Demonstration of a minimum in the recovery of nanoparticles by flotation: Theory and experiment

The flotation of nano- and submicron particles does not follow the conventional collection theory based on the interception and collision mechanisms, which predicts extremely low collection efficiency for particles smaller than 10-μm. Brownian diffusion and colloidal forces strongly influence the collection of such particles by air bubbles in flotation. In this paper, a theoretical model is presented for predicting the collection efficiency of nanoparticles. The theory incorporates mass transfer by Brownian diffusion, microhydrodynamics of particles in the vicinity of a slip surface of rising air bubbles, and colloidal interactions that come into effect at small separation distances. The governing equation was solved numerically using the Crank-Nicolson method with variable step size. A finite difference scheme with mesh refinement in the vicinity of the air bubble surface was used to discretise the stiff partial differential equation for the particle concentration. The mesh refinement produced correct numerical solutions without oscillation in the particle concentration distribution, which otherwise occurred due to the stiffness of the differential equation and coarseness of the numerical mesh. Predictions from the model were compared with experimental results obtained with a small laboratory column cell, in which colloidal silica particles with diameters in the range were floated using fine bubbles of typical average diameter . The particle concentration in the pulp was about 1% by weight. Cetyltrimethyl ammonium bromide and Dowfroth 250 were used as the flotation collector and frother, respectively. Both the theory and experiment show significant effect of the electrical double-layer and non-DLVO hydrophobic attractive forces on the collection of nanoparticles by air bubbles. The theoretical and experimental results show the collection efficiency to have a minimum at a particle size in the order of . With larger particles, the interception and collision mechanisms predominate, while the diffusion and colloidal forces control the collection of particles with a size smaller than the transition size.     

十二烷基吗啉选择性浮选氯化钠颗粒的作用机理

通过研究十二烷基吗啉（DMP）在饱和卤水中对氯化钠颗粒的浮选行为,揭示了氯化钠颗粒通过增强体系泡沫的稳定性而提高了十二烷基吗啉的浮选性能;运用红外光谱法研究证明,DMP分子以物理作用力吸附于氯化钠颗粒表面;同时,研究了十二烷基吗啉分子在氯化钠颗粒表面的吸附行为及精光卤石颗粒与各种条件下氯化钠颗粒表面的接触角.研究结果表明,十二烷基吗啉选择性浮选分离氯化钠颗粒的作用原理是：十二烷基吗啉分子在饱和卤水介质中由于受到很强的疏水力,选择性吸附于氯化钠颗粒表面;且因精光卤石（KCl&#8226;MgCl₂&#8226;6H₂O）颗粒表面有很强的水合斥力,十二烷基吗啉分子不能吸附于精光卤石颗粒表面;因此,在浮选过程中,吸附在气泡界面的DMP分子因受到疏水作用力,使气泡吸附在氯化钠颗粒表面,氯化钠颗粒被浮选分离.     

Double-layer effects in the flotation of fine particles

Experiments have been conducted in which the charges on particles and bubbles in a flotation process have been measured. The particles were polystyrene latices of diameters between 4 and 20 μm. The bubbles were of mean diameter 53 μm. A cationic surfactant was used to promote flotation, and the charge on the particles and bubbles was controlled by addition of sodium sulphate solution. To measure the charge on bubbles, they were generated electrolytically in a glass electrophoresis cell so that they rose vertically up a “stationary level” in the cell, while at the same time moving sideways under the action of a horizontal potential gradient. The horizontal velocity, taken with the known potential gradient, gave the electromobility. The bubbles were found to carry the same sign as the particles (positive) and under the same electrolyte concentrations, the change on the particles and bubbles was approximately the same. Experimentally determined rate constants for flotation were found to depend strongly on the bubble and particle charge, decreasing by an order of magnitude as the charge increased from 30 to 60 mv. The data were well correlated by the equation: ?1n (k_p/d_p^1.5) = 3.9 + 0.116 U_EU_B where k_p is the rate constant (min^?1), d_p, is the particle diameter (μm) and U_E,U_B are the electromobilities (μm/s/V/cm) of the particle and bubble respectively.     

Experimental testing of the hydrodynamic collision model of fine particle flotation

Flotation rates of glass beads and of latex particles have been measured as a function of particle size using very small bubbles. With glass beads the observed rate versus size relationship agreed quite well with the prediction of a simple hydrodynamic collision model, but that found with latex particles did not. It is suggested that electrical forces may have to be taken into account when the particles have a significant zeta potential. With both types of particle, the relationship between flotation rates measured at two different bubble sizes is consistent with the model's predictions.     

Collection of submicron particles in electro-flotation

A theoretical analysis is presented to describe the deposition of Brownian particles onto hydrogen bubbles under surface interactions. Single collection efficiency has been numerically calculated for zeta potentials, having assumed that the effective Hamaker constant is equal to 3.0 × 10^?14erg though our choice of Hamaker constant is rather arbitrary. From a mass balance, total collection efficiency or the rate of flotation has been determined. In this way, electro-flotation process is quantitatively described.Experimentally, the electro-flotation of polystyrene latices of mean diameter 0.6 μm has been studied to examine the effect of the charge on both particles and bubbles on the total collection efficiency. The bubbles were of mean diameter 20 μm. The electrolyte was AlCl₃. To measure the charge on the bubbles, we directly sampled solution including very small bubbles with a glass tube from a flotation vessel and poured into a micro-electrophoresis cell. The horizontal velocity measured when the bubbles rose up a “stationary level” in the cell under the known potential gradient gave the electromobility. The charge on the latex particles were found to change its sign from negative to positive as flotation time went on.The theoretical total collection efficiency has been in close agreement with the experimentally determined one.     

Surfactant Recovery in Adsorbing Colloid Flotation

Abstract

The displacement of surfactants from floc–water interfaces by salts is examined by statistical mechanical methods. The effect of added salts on the adsorption isotherm is exhibited, and it is found that surfactant condensed films can readily be displaced. This may markedly improve the economics of adsorbing colloid flotation by facilitating surfactant recovery. Preliminary experimental results supporting the theory are presented; Na₂CO₃ is used to displace sodium lauryl sulfate from Fe(OH)₃. The viscous drag forces on floc particles attached to rising bubbles are calculated for bubbles having diameters in the range 0 to 1 mm. At the upper end of this range these forces appear to be large enough to reduce the efficiency of foam flotation.     

Determination of the collision frequency between bubbles and particles in flotation

Collision efficiency for a spherical bubble rising in a uniform concentration of small non-inertial particles is studied by direct numerical simulations (DNS). The Stokes number of the particles is negligibly small so that the particle trajectories follow the streamlines. The effect of the bubble interface contamination is studied for the flow surrounding the bubble using the spherical cap model. Numerical results are obtained for a wide range of bubble Reynolds number (based on bubble diameter d_b) ranging from 0.01 to 1000 and for different angles of contamination ranging from 0^° to 180^°. The collision efficiency is found to be increased with the Reynolds number and significantly decreased with the level of contamination. Correlations of the numerical results are proposed for efficiencies versus d_p/d_b (d_p being the particle diameter), bubble Reynolds number and interface contamination degree. For clean (respectively, fully contaminated) spherical bubbles, the efficiency evolves as d_p/d_b (respectively (d_p/d_b)²) whatever the bubble Reynolds number and the particle size. For partially contaminated bubbles, efficiency can be scaled with d_p/d_b or (d_p/d_b)² depending on both the level of contamination and the particle size.     

Bubble–particle collision and attachment probability on fine particles flotation

Particle size is an important parameter in flotation and has been the focus of flotation research for decades. The difficulty in floating fine particles is attributed to the low probability of bubble–particle collision. In this research, the influence of hydrodynamic parameters on collision probability of fine particles was investigated. Collision probability was obtained using Stokes, intermediate I and intermediate II and potential equations. Maximum collision probability was 5.65% obtained with impeller speed of 1100 rpm, air flow rate of 30 l/h and particle size of 50 μm. Also, attachment probability under Stokes flow, turbulent and potential flow conditions was calculated 100, 99.49 and 81.87% respectively. Maximum attachment probability was obtained with impeller speed of 700 rpm, contact angle of 90°, particle size of 20 μm and air flow rate of 15 l/h. Collision angles were obtained between 60.71° and 60.18° and attachment angles were obtained between 9.15° and 59.83°.     

A new experimental method for determining particle capture efficiency in flotation

A single bubble experiment has been developed for the determination of the capture efficiency of particles by bubbles in flotation under well-controlled hydrodynamics and physico-chemical conditions. In a glass column, small single bubbles (d_b=0.22−1.16 mm) are produced in pure water and then rise at their terminal velocity through a suspension consisting of spherical glass particles where bubble–particle capture takes place. The capture efficiency E_capt is calculated as the ratio of the number of particles captured by one bubble to the number of particles present in the volume swept out by this bubble. Images recorded at high optical magnification show that particles slip on the interface, then adhere to air bubbles individually or as aggregates and cover the rear part of bubble surface. The bubble's effective density and interface contamination level are increased by captured particles. As a result, bubble's rising velocity U_b is reduced along the experimental device. By establishing the relationship between capture efficiency E_capt, bubble rise velocity U_b and bubble clean angle θ_clean, a new approach to measure particle–bubble capture efficiency is proposed. This new experimental technique is applied to provide a new set of data for capture efficiency in the case of bubbles with a clean interface. E_capt is found to grow as d_b decreases and d_p increases, within the range between 0.02 and 0.20, which is in the order of magnitude of experimental results of Ralston and Dukhin (1999) as well as of numerical results of Sarrot et al. (2005). These data are favorably compared to numerical modeling of collision efficiency.     

Experimental determination of particles capture efficiency in flotation

A single bubble experiment is developed for the determination of the capture efficiency by rising bubbles in a uniform concentration of small inertialess glass particles under carefully controlled hydrodynamics and physico-chemical conditions. Air bubbles (0.35-1.3 mm in diameter) rise and reach their terminal velocity in clean water before passing through a suspension of particles (15- in size), where capture takes place. After passing through another zone containing pure water to remove particles trapped in their wake, bubbles release captured particles at the surface from where the particles are collected and counted. A capture efficiency is then calculated as the ratio of the number of particles captured by one rising bubble to the number of particles present in the volume swept out by this bubble. Capture efficiencies range between 10^-3 and 5×10^-1 and are in the order of magnitude of the experimental results presented by Ralston and Dukhin [1999. The interaction between particles and bubbles. Colloids and Surfaces A: Physicochemical and Engineering Aspects 151, 3-14] as well as of numerical results for collision efficiency presented by Sarrot et al. [2005. Determination of the collision frequency between bubbles and particles in flotation. Chemical Engineering Sciene 60 (22), 6107-6117].     

The effect of particle chemical composition on the activation probability in n-butanol condensation particle counters

The effect of particle chemical composition on the counting efficiency of a commercially available n-butanol condensation particle counter (CPC) was theoretically investigated. The activation probability of particles soluble in n-butanol or covered by a soluble coating was determined by Köhler theory, whereas the activation of insoluble particles was determined by heterogeneous nucleation theory. The theoretically predicted counting efficiencies were fit to experimental data to infer the n-butanol microscopic contact angle on insoluble particles or the volume of a soluble layer coating the particle. The calculated microscopic contact angles were found to depend on particle chemical composition, particle diameter, and the CPC saturator-to-condenser temperature difference. The average n-butanol microscopic contact angle on diesel exhaust and CAST soot was determined to be 5–10°, on Emery oil particles close to 0°, on thermally pre-treated tetracontane (C₄₀H₈₂) particles 25°, and on dry sodium chloride particles 15–20° for CPCs operated at a temperature difference of approximately 7 °C (low saturation ratios). The counting efficiencies were very sensitive to particle contamination, as determined by the particle generation method and treatment, an effect that could be reproduced by modified Köhler theory. The dependence of the counting efficiency on particle chemical composition was found to be stronger at lower CPC temperature differences.     

Modeling of particle interaction dynamics in standing wave acoustic field

Interactions between fine particles (PM_2.5) suspended in flue gas can be promoted via application of acoustic fields. This may lead to particle collision and agglomeration, thereby facilitating possible realization of PM_2.5 abatement. The dynamic behavior of particle interactions in standing wave acoustic fields, however, is not well understood, and this severely restricts the development of practical acoustic agglomeration devices. Availability of limited information concerning PM_2.5 interactions, insufficient consideration of interaction mechanisms, and neglect of spatial variation in acoustic velocity under the standing wave condition are a few limitations of previous studies performed in this regard. To address these concerns, a theoretical model capable of accurately describing the interaction between two neighboring particles in a standing wave acoustic field was developed in this study. Experimentally obtained parameters, such as particle velocity due to acoustic entrainment, interaction pattern, and collision time, were reproduced via numerical simulations performed using the proposed model. Additionally, the influences of model improvements on PM_2.5 interaction dynamics were analyzed. Finally, the proposed model was adopted to investigate the effect of particle size on collision time. Results demonstrate that in cases involving identically sized particles, the collision time significantly reduces with increase in particle size. Maintaining the size of a particle constant whilst increasing that of the other particle causes the collision time to decrease. This is coupled with reduction in initial orientation angle range corresponding to particle collision. Consequently, no collisions occur when a substantial difference exists between particle sizes.

Copyright © 2019 American Association for Aerosol Research     

纳米气泡在微细粒矿物浮选中的应用研究现状

我国矿石资源禀赋差,很大一部分微细粒矿物资源难以回收.提高微细粒矿物资源的综合利用率是解决我国现阶段面临的矿产资源匮乏问题最有效的途径之一.文中主要对微细粒矿物的分选现状、纳米气泡的发展历程、形成方法、稳定性研究现状及在矿物浮选中的应用现状进行讨论与分析.纳米气泡浮选是针对微细粒矿物粒度小、质量轻、比表面积大、表面能高...