Using sound to study bubble coalescence

doi:10.1016/j.jcis.2008.12.025

Journal of Colloid and Interface Science

Volume 332, Issue 1, 1 April 2009, Pages 237-245

https://doi.org/10.1016/j.jcis.2008.12.025 Get rights and content

Abstract

Frothers are surfactants used in flotation to aid generation of small bubbles, an effect attributed to coalescence prevention. Studying coalescence at the moment of bubble creation is a challenge because events occur over a time frame of milliseconds. This communication introduces a novel acoustic technique to study coalescence as bubbles are generated at a capillary. The sound signal was linked to bubble formation and coalescence events using high-speed cinematography. The technique has the resolution to detect events that occur within 1–2 ms. The results show that for common flotation frothers and n-alcohols (C₄–C₈) coalescence prevention is not simply related to surface activity. A total stress model is used to give a qualitative explanation to the action observed. Results for salt (sodium chloride) are included for comparison.

Graphical abstract

A novel acoustic technique to study coalescence as bubbles are generated at a capillary is presented. Results of coalescence prevention in presence of surfactants and salt are also included.

Introduction

Flotation is widely used to separate particles according to their hydrophobicity. Separation is achieved by dispersing air into bubbles that collide with and attach to hydrophobic particles. Water is not an easy media in which to generate small bubbles and it is necessary to add reagents called frothers (surfactants) in most flotation systems. Thus, the fine bubble size (typically 0.1–4 mm) results from the interaction of the air delivery system (e.g., through the impeller in a mechanical flotation machine) and frother.

Even though bubbles drive flotation performance, little is known about how frothers act in their production. Analysis of froth formation and stability emphasizes mechanisms that retard coalescence [1], [2]. This coalescence prevention explanation has been extended to bubble generation [3].

At the moment of bubble formation the surface concentration (adsorption density) of surfactant is likely well below the equilibrium value but it is not zero. Forces arising from the presence of surfactant can therefore be anticipated. While frothers are the main reagent used in fine bubble production, some salts at high concentration (>0.4 ionic strength) also produce fine bubbles in flotation systems [4]. A study of both frothers and salts should help identify the mechanism(s).

To study coalescence at the moment of bubble production demands measurements over the time frame of milliseconds. In this paper we introduce a novel approach to the problem using the sound bubbles emit when formed [5] and employ a stress model to qualitatively interpret the findings.

Section snippets

Total stress

The final act of coalescence is thinning to rupture of the liquid (water) film between contacting bubbles. In the presence of surfactants forces opposing film thinning are generated. These forces, or ‘stress,’ may be seen as the resistance of the film to deformation. One component of stress derives from the surface tension gradients that accompany disturbances in the distribution of surfactant, Gibbs–Marangoni elasticity (E) [6], [7]; a second component derives from the reduction in surface

Apparatus

The experimental set-up (Fig. 1) comprises a 30 L acrylic tank where air bubbles are injected through a glass capillary tube. Gas flow rate is measured and regulated with a mass flow meter controller (Sierra, model 840DL1V1). The acoustic emissions are measured with a hydrophone (Lab-core System), which has a wide frequency range, 5 to 85,000 Hz. The signal passes from the hydrophone to an amplifier before being transferred to a computer. The acoustic emissions were recorded with the freeware

Validation

The method was validated by testing the Minnaert equation [Eq. (4)]. Bubbles of diameter 2.4 and 6.0 mm were generated in water. The signals were processed by Fourier analysis to determine the peak frequency corresponding to each bubble size (Minnaert frequency). Fig. 2 shows the result.

The peak frequencies were 2583 Hz for the 2.4 mm bubbles and 1033 Hz for the 6.0 mm bubbles. From Eq. (4) the product ‘ $f \cdot d$ ’ should be constant: this is the case, 6.199 mm/s vs. 6.198 mm/s, respectively. Taking

Reliability

The procedure calls for establishing the gas rate at the transition from non-coalescence to coalescence, i.e., identifying the transition from (a) to (b) in Fig. 9. Three full repeat tests were conducted for pentanol to establish the precision. Fig. 10 shows the results, including error bars representing the standard deviation. The boundary divides coalescence events that occur above the line from the non-coalescence region below the line: the larger the region below the line the more effective

The method

The challenge of recording bubble formation and associated coalescence events over 1–2 ms was met by an acoustic technique. It provided an effective alternative to imaging, reducing storage requirements and avoiding tedious inspection of videos at reduced equipment cost (by about 95% in the present case). The method also lends itself to trying on opaque systems, for example, solid suspensions relevant to flotation studies.

Inducing coalescence by increasing gas rate at a capillary was judged

Conclusions

The characteristic sound trace found when bubbles coalesce at a capillary tip allowed development of a novel technique to study bubble coalescence. The technique uses a hydrophone to record sounds, which are shown to be related to coalescence events. Signal recognition is straightforward and the technique has a resolution high enough to discriminate events that occur within 1 to 2 ms.

The gas flow rate marking the boundary between coalescence and non-coalescence was determined as a function of

Acknowledgments

This work was conducted under the Chair in Mineral Processing funded through an NSERC (Natural Sciences and Engineering Research Council of Canada) CRD (Collaborative Research and Development) grant now sponsored by Vale Inco, Xstrata Process Support, Teck Cominco, Agnico-Eagle, COREM, SGS Lakefield Research, Shell Canada and Flottec.

W. Kracht would also like to thank the Chilean Government for the Chilean National Scholarship (Beca Presidente de la República) and the Universidad de Chile for

References (40)

J.J. Quinn et al.
Miner. Eng.
(2007)
U. Hofmeier et al.
J. Colloid Interface Sci.
(1995)
J. Lucassen et al.
Chem. Eng. Sci.
(1972)
E. Dickinson
Colloids Surf. B Biointerfaces
(1999)
H. Schott
J. Pharm. Sci.
(1995)
J.A. Finch et al.
Miner. Eng.
(2006)
L. Wang et al.
Colloids Surf. A Physicochem. Eng. Aspects
(2006)
J.A. Finch et al.
Miner. Eng.
(2008)
P.K. Weissenborn et al.
J. Colloid Interface Sci.
(1996)
P. Dawson
Eur. J. Radiol.
(2002)

G. Drogaris et al.

Chem. Eng. Sci.

(1983)

G. Keitel et al.

Chem. Eng. Sci.

(1982)

B.A. Comley et al.

Int. J. Miner. Proc.

(2002)

S. Gélinas et al.

Miner. Eng.

(2007)

Y.S. Cho et al.

Int. J. Miner. Proc.

(2002)

R. Tuckermann

Atmos. Environ.

(2007)

V. Machon et al.

Trans. IChemE

(1997)

R.A. Grau et al.

Int. J. Miner. Proc.

(2005)

P.J. Harris

R.J. Pugh

Adv. Colloid Interface Sci.

(1996)

Cited by (39)

Investigation of acoustic spectral variations in the pool boiling regimes of water on wire heater
2023, Applied Thermal Engineering
This paper presents the experimental investigation of acoustic spectral variations in saturated pool boiling regimes of water on a heated wire. In the current study, two different wires of standard wire gauge (SWG) 36 and 42 are considered to investigate the acoustic spectral variations through the pool boiling curve. The amplitude and frequency changes are evaluated for each regime of pool boiling. In a single regime, amplitude rise is observed with respect to the heat flux without any significant change in dominant frequencies. On the other hand, frequency shifts are observed in regime transitions. A change in the diameter of the heater wire has no significant effect on the boiling acoustic spectra. However, the number of high-frequency components increased for the SWG – 42 than the SWG – 36 wire. A frequency peak near 2000 Hz is found to be crucial for boiling regime identification. The sound pressure level (SPL) for SWG – 36 is higher than the SWG – 42, and it is further noted that SPL follows an ‘N’ shaped pattern for both wires owing to the frequency shifts and variation of mean bubble departure diameter at that heat flux.
Review on research progress in boiling acoustics
2022, International Communications in Heat and Mass Transfer
Ever-growing miniaturization of electronic devices and space-conserving endeavours of heat transfer systems pose a challenging task to the current cooling strategies. Boiling acoustics is one of the most potent and efficacious methodologies to reliably predict the boiling regime inside the cooling systems to assay the safety and address the emergency conditions. Boiling acoustics is gripping attention out of the few foreseeable technologies for the future cooling requirements. Though the potentiality of boiling acoustics was unravelled in the late 1990s, the research data present in this arena is however lacking. This paper presents a comprehensive review of the literature reported from the 1970s to the present date. Furthermore, this paper also details the evolution of boiling acoustics from the initial application of boiling incipience detection to regime identification. Much focus is given on salient features of boiling acoustic characterization dealing with the regime detection. Effects of various parameters such as thermos-physical properties of the heater surface and the boiling liquid that directly or indirectly influence the acoustic spectra are also presented. The prediction of the boiling regime constitutes the first necessary step in producing autonomous cooling systems. Hence, the detection and characterization of boiling noise under various conditions such as pressure, heat flux, and flow rate is essential.
Froth stabilities and iron ore flotation of collectors 3-dodecyloxypropanamine and 3-tetradecyloxypropylamine: An experimental and molecular dynamics study
2022, Colloids and Surfaces A: Physicochemical and Engineering Aspects
In the mineral processing industry, froth properties are essential. To separate iron minerals and gangue minerals in a flotation mill, froths that are both too stable and fragile are useless for obtaining qualified concentrate in the mining industry. We tried to develop two collectors to adjust the froth stability during flotation. In this study, we first researched the froth stabilities of collectors 3-dodecyloxypropanamine (DOPA) and 3-tetradecyloxypropylamine (TOPA). Contact angle measurements and surface tension measurements were used to research the hydrophobicity of DOPA and TOPA and to predict its flotation performance. Froth stability measurements and molecular dynamics (MD) were conducted to investigate the froth properties of DOPA, TOPA, and dodecyl amine (DDA), and to reveal the froth mechanism of flotation. The film interface models were built using software Materials Studio 2017 software, and MD was used to simulate the flotation system. The interaction formation energy (IFE) was calculated to compare the froth film strength of the three collectors. Surface tension measurements indicated that DOPA and TOPA were more robust collectors in decreasing water surface tension than DDA. The froth stability measurements showed that the DDA froth was the most stable of the froths with the three collectors, consistent with the MD simulations of the foam models. Furthermore, the adsorption configuration of the interface of DOPA or TOPA and solution was drawn, and the numbers and the average length of different kinds of hydrogen bonds of the three configurations were counted and calculated. We can infer that the hydrogen bonds between the O atom of water and the —NH₂ of DOPA or TOPA are not the main force of their IFE, and the main force is van der Waals force. Finally, the micro-flotation of the artificial mixed mineral was conducted to verify the previous prediction of DOPA and TOPA. This paper provided a new case to adjust the flotation performance by developing a collector and predicting its froth stability.
Numerical study of surfactant effects on the rise of a single bubble and two coaxial bubbles
2022, International Communications in Heat and Mass Transfer
A hybrid lattice Boltzmann and finite difference method is presented for two-phase flows with unequal densities and insoluble surfactants. This method simulates two-phase flows through an improved lattice Boltzmann color-gradient model, which is developed from the previous density ratio model but considers surfactant effect by incorporating dynamic surface tension and Marangoni stress, and solves surfactant transport by a finite difference method. We first use it to simulate a single bubble rise and explore the surfactant roles on bubble motion and deformation for varying Bond (Bo) and Galilei (Ga) numbers. Results show that the addition of surfactants always retards bubble motion and deformation but the retarding effect decreases with increasing Bo. As Ga increases, the retarding effect of surfactants increases due to the increased Marangoni stress and a wake vortex would arise behind the surfactant-laden bubble, which opposes the surfactant accumulation at bubble bottom. We then simulate the rise of two coaxial bubbles and study the surfactant roles on bubble coalescence for varying Ga and radius ratios of leading to trailing bubbles. It is found that the presence of surfactants delays bubble coalescence when Ga or radius ratio is relatively low, but promotes coalescence when Ga or radius ratio is high.
Total petroleum hydrocarbon removal from oil-sand by injecting charged nanobubbles
2022, Environmental Nanotechnology, Monitoring and Management
The objective of this study is to demonstrate oil-sand separation in the injection of positively charged nanobubbles (NBs) with respect to the effects of the NB/oil-sand ratio, treatment time, and NB injection regimes. Utilizing total petroleum hydrocarbon (TPH) removal, an assessment of the oil-sand separation efficiency is presented. The resulting injection of NBs was shown to be an effective treatment process with a TPH removal ≥85%. A greater TPH removal was obtained with the intermittent injection of positively charged NBs for an NB/oil-sand ratio ≥2/1 (v/v) and separation times from 5 to 20 min. The buoyancy force played a major role in detaching the oil. The larger external contact angle between the bubble and oil layer and the creation of micro- and macro-bubbles via NB coalescence in the oil-sand reactor attached to the oil layer were fundamental in producing a stronger buoyancy force for enhanced TPH removal.
Passive acoustic emission monitoring to detect bubble coalescence in the presence of solid particles
2017, Minerals Engineering
Citation Excerpt :
Unlike the work of Kracht and Finch who found no partial coalescence in the presence of frother the present study reveals a small region of partial coalescence in the MIBC system. At MIBC concentrations below 0.1 mM (10 ppm), the Kracht and Finch (2009) data show a lower transition flow rate while at higher concentrations the literature transition data generally fall in the partial coalescence region. Fig. 5 shows the results for increasing sodium chloride concentration.
Passive acoustic emission monitoring was used to study the role of solid particles on bubble coalescence. The effect of 1–10% w/w talc (hydrophobic) or silica (hydrophilic) particles on air bubble formation and coalescence at a capillary in the presence of MIBC or sodium chloride was determined. Both solids slightly inhibited bubble coalescence while silica created a larger region of partial coalescence compared to talc. At 10% w/w the silica particles appeared to promote coalescence at high MIBC concentration.

View all citing articles on Scopus

View full text

Using sound to study bubble coalescence

Abstract

Graphical abstract

Introduction

Section snippets

Total stress

Apparatus

Validation

Reliability

The method

Conclusions

Acknowledgments

Miner. Eng.

J. Colloid Interface Sci.

Chem. Eng. Sci.

Colloids Surf. B Biointerfaces

J. Pharm. Sci.

Miner. Eng.

Colloids Surf. A Physicochem. Eng. Aspects

Miner. Eng.

J. Colloid Interface Sci.

Eur. J. Radiol.

Chem. Eng. Sci.

Chem. Eng. Sci.

Int. J. Miner. Proc.

Miner. Eng.

Int. J. Miner. Proc.

Atmos. Environ.

Trans. IChemE

Int. J. Miner. Proc.

Adv. Colloid Interface Sci.