Bubble Size Analysis Research Articles

The Correlation between Macroscopic Image and Object Properties with Bubble Size in Flotation

This paper studies the correlation between different macroscopic features of image regions and object properties with the Sauter diameter (D32) of bubble size in flotation. Bubbles were sampled from the collection zone of a two-dimensional flotation cell using a McGill Bubble Size Analyzer, and photographed bubbles were processed using image analysis. The Sauter mean diameters were obtained under different experimental conditions using a semiautomated methodology, in which non-identifiable bubbles were manually characterized to estimate the bubble size distribution. For the same processed images, different image properties from their binary representation were studied in terms of their correlation with D32. The median and variability of the shadow percentage, aspect ratio, power spectral density, perimeter, equivalent diameters, solidity, and circularity, among other image or object properties, were studied. These properties were then related to the measured D32 values, from which four predictors were chosen to obtain a multivariable model that adequately described the Sauter diameter. After removing abnormal gas dispersion conditions, the multivariable linear model was able to represent D32 values (99 datasets) for superficial gas rates in the range of 0.4–2.5 cm/s, for four types of frothers and surfactant concentrations ranging from 0 to 32 ppm. The model was tested with 72 independent datasets, showing the generalizability of the results. Thus, the approach proved to be applicable at the laboratory scale for D32 = 1.3–6.7 mm.

Reducing the Presence of Clusters in Bubble Size Measurements for Gas Dispersion Characterizations

This short communication evaluates a new strategy to sample bubbles in gas dispersion characterizations. Bubble size is measured in a bidimensional flotation cell using the McGill bubble size analyzer under different types of frothers, frother concentrations and superficial gas rates. The original design of this bubble viewer is modified, changing the deflecting system to photograph only a fraction of the bubbles entering the device. As a result, the new design increases the ability to successfully identify bubbles by a maximum of 20% using an automated algorithm. This increase is caused by a reduction in the formation of clusters in the visual field. The improvement, which is a function of the operating conditions, is most significant in the transition from ellipsoidal/ellipsoidal–turbulent regimes (no frother or low frother concentrations) to conditions with an over-agglomeration of bubbles in the visual field (high superficial gas rates and high frother concentrations). A comparison of the bubble size parameters obtained from the original and proposed deflecting systems shows that the new design does not distort the estimated bubble size distributions. To complement the research findings, alternative sampling designs, using new or existing segmentation algorithms, are then proposed to improve gas dispersion characterizations at different scales.

Bubble size analysis in a two-phase countercurrent flow in the narrow rectangular column

The flow of bubbles in a two-phase system has great implications in chemical, petrochemical, and biochemical applications. This work enunciates the measurement of bubble size distribution and bubble aspect ratio in three-different axial zones in the countercurrent flow mode with a gas and liquid velocity range of 0.044–0.321 and 0.019–0.058 m/s, respectively. Bubble size is measured by the photographic technique. The bubble aspect ratio and bubble size distribution results reveal that the impact of gas and liquid velocities is significant on the Sauter mean bubble size. The Sauter mean bubble size increases as the gas velocity increases, whereas it decreases with the liquid velocity. The Sauter mean bubble diameter ranges from 2.65 to 6.16 mm. The distribution of bubble sizes follows the LogLogistic probability density function. In addition, a correlation is also proposed for the interpretation of bubble diameter in terms of Reynolds number and Froude number. The bubble aspect ratio changes with axial zones and gas and liquid velocities. Experiments reveal that the bubble aspect increases with liquid velocity while decreasing with gas velocity and axial zones. The bubble aspect ratio correlations are developed in terms of Eötvös and Reynolds numbers. The present results will be helpful for the process intensification of bubble-aided two-phase flow applications.

A Method to Detect Abnormal Gas Dispersion Conditions in Flotation Machines

This short communication presents a methodology to detect abnormal gas dispersion conditions in flotation machines. These abnormal conditions are characterized by the significant presence of cap-shaped bubbles. The approach considers the use of a bubble size analyzer to measure gas dispersion at industrial scale. The detection of abnormal conditions is critical when estimating bubble size by automated software. Otherwise, the estimates are significantly biased, since irregular bubbles are typically removed from the analysis. From the recorded images and the respective black and white representation, the variability of the shadow percentage caused by the bubbles in the vision field, abruptly increases in the presence of cap-shaped bubbles. Experimental conditions with coefficients of variation lower than 60–70% in the shadow percentage represent spherical and spherical-ellipsoidal regimes. In the former, automated bubble sizing software has proved to be sufficiently effective in obtaining reliable results. In the latter, different segmentation techniques have been proposed to obtain satisfactory results. Abnormal conditions are detected under coefficients of variation greater than 80% in the shadow percentage. The presence of cap-shaped bubbles causes inefficiencies in the collection of hydrophobic minerals in the pulp zone as well as disturbances in the separation stage (froth zone). Therefore, the detection of these irregular bubbles is suitable to provide feedback to flotation processes, allowing gas dispersion to be driven towards normal operating conditions.

Frother Characterization Using a Novel Bubble Size Measurement Technique

Bubble size measurement is a vital part of flotation system analysis and diagnostics. This work evaluates a commercial camera probe as a novel method for in situ bubble size measurement. This device is compared to the conventional Anglo Platinum Bubble Sizer (Stone ThreeTM). It was found that, in laboratory applications, the in situ bubble size analysis technology appears to be a more user-friendly and reliable option for determining bubble size in flotation, whereas the Anglo Platinum Bubble Sizer is more applicable for full scale industrial work. This probe was then used to conduct a rigorous comparison of the behavior of different frother chemistries at a variety of background solution ionic strength conditions. The critical coalescence concentrations and the minimum Sauter mean bubble diameters were determined. Five frothers were compared in terms of their ability to reduce bubble size and sensitivity to salinity. In order to adjust plant recipe and control strategy accordingly, it is recommended that the plant would need to use less frother during periods of the high salinity of process water to achieve the minimum Sauter mean bubble size.

Proving the existence of nanobubbles produced by hydrodynamic cavitation and their significant effects in powder flotation

Selective attachment of nanobubles (NBs) generated in different conditions on the surface of valuable minerals during flotation separation was a challenge that needs to be addressed. This investigation filled this gap and proved the existence of NBs on the target mineral's surface and their selective effectiveness through the process. The bubble size analysis results showed that the mean diameter of bubbles was between 60 and 70 nm; thus, they could be correctly called “nanobubbles”. Flotation test results showed a significant increase in the flotation recovery (by 37%) and grade (more than 1%) of fine phosphate ore sample (d80: 37 µm) using NBs that generated in the presence of collector. Interestingly, surface analyses of flotation products showed that the amounts of flotation collector adsorbed onto the surface of floated particles was decreased in the presence of NBs compared with their absence. The change in the particle surface (zeta) potential in the presence of NBs also provides additional evidence of NBs “adsorbed” (i.e., the surface NBs) onto the particle surface. These results indicated that NBs produced by hydrodynamic cavitation could adsorb onto the target mineral particles. This adsorption could change their surface properties, improve their hydrophobicity and surface potentials, and enhanced the bubble-particle attachment in flotation.

Compound analysis of gas bubble trajectories with help of multifractal algorithm

Three-dimensional trajectories of gas bubbles were the subject of study in this article. Authors carried out experiments in two glass tanks with dimensions of 300 × 300 × 1500 mm and 300 × 150 × 500 mm, respectively, filled with distilled water with a temperature equal to T = 21 ± 1 °C. The tests were carried out for an air volume flow rate, values from 0.00492 l/min to 0.0645 l/min. Basic characteristics of recorded measurement data like: analysis of the bubble detachment frequency from the nozzle, gas bubble size analysis have been shown. Based on the conclusions from those characteristics the multifractal algorithm analysis has been presented as one that allows comprehensive analysis of such a process. The results as a comparison of the main points on the air volume flow rate showed that: the trajectories of gas bubbles in liquid are multifractal and the width of the multifractal spectrum, and thus the complexity of the process being examined, depends on the air flow, the diameter of the bubble generating nozzle and the height of the liquid column in which the phenomenon is examined. The results of the analysis show that the key impact on the overall observed phenomenon is the deformation of the bubble surface, oscillation of the bubble column and observation of the two-phase flow spatially (three-dimensional).

Assessment of two automated image processing methods to estimate bubble size in industrial flotation machines

This Short Communication presents a comparison between two automated methodologies to estimate bubble size in industrial flotation equipment (mechanical cells and columns). The studied database includes 106 conditions in flotation machines from 10 to 300 m3, which were sampled by means of the McGill bubble size analyser. The first methodology (conventional image processing) uses circularity, ellipse detection, segmentation and reprocessing of separated objects to estimate the Sauter mean diameter D32. The second methodology evaluates the binary images as trains of pulses, whose spectral bandwidth is correlated with the D32. Both techniques have been compared to a semi-automatic approach, which manually complete the conventional image analysis to obtain the total bubble size distribution. For D32 < 2 mm (spherical regime), the first approach shows better performance than the spectral analysis, with significantly higher precision. For D32 ≥ 2 mm (ellipsoidal regime), the conventional image processing presents significant bias, high dispersion as well as a cliff trend for D32 > 3.5 mm. On the other hand, the spectral method showed a better sensitivity with an approximately linear trend up to 5 mm. Thus, a combination of these two methodologies is suggested for the D32 characterization in industrial flotation machines.

Development of a laboratory-scale flotation column with inlet bubble size measurement

This paper presents the development of a small-scale (1 in.) laboratory flotation column with online measurement of bubble size. In contrast to the systems that use sampling-for-image techniques and have the viewing chamber located at the top of the flotation device, the system employed here measures the bubble size via image analysis using a bubble size analyzer located at the bottom of the column using a well-referenced sloped viewing chamber. The main advantage of this configuration is that it allowed the measurement and monitoring of the bubble size during flotation experiments (three-phase system). This was possible because the sizes of the bubbles were determined before they entered the column; moreover, the “cloudy effect” caused by fine particles in the bubble viewing chamber was minimized owing to the use of an expansion chamber (named the bubble injection chamber). The image capture, processing, and analysis were automated, which rendered it possible to determine the bubble size distribution and mean bubble size (D32) of consecutive images every 5 s using a simple image analysis procedure. Two-phase experiments conducted to evaluate the system showed that the mean bubble size (D32) and gas holdup were within the ranges found in the literature. As expected, the holdup was dependent on the bubble size, which was strongly dependent on the frother concentration. Finally, some exploratory experiments were conducted to test the apparatus in a three-phase system. Recovery-particle size curves for different bubble sizes showed that mass recovery increases as the mean bubble size decreases, and the coarser the particles, the greater is the effect of the bubble size on the recovery.

Aqueous foams stabilized with silica nanoparticle and alpha olefin sulfonates surfactant

Carbon dioxide (CO2) foams have been introduced to improve mobility of CO2 in CO2 flooding. However, using surfactant alone to stabilize CO2 foam has potential weaknesses such as high surfactant retention in porous media and the foam is thermodynamically unstable for a long-term. Nanoparticle has been an alternative in stabilizing CO2 foam longer. This study aims to analyze CO2 foam stability at varying concentrations of surfactant, silica nanoparticle (SNP) and brine. The additions of SNP in anionic surfactant of alpha olefin sulfonates (AOS)-water and in AOS-brine towards foam stability were demonstrated in this study. CO2 foam stability was measured through the foam height observation and bubble size analysis. The performance of SNP and AOS suspensions in stabilizing foam were observed at different concentrations of AOS (0.1, 0.3 and 0.5 wt%), SNP (0.1, 0.3 and 0.5 wt%) and brine (0.1, 1 and 5 wt%). The results revealed that the CO2foams were most stable at 0.3 wt% SNP suspension in 0.5 wt% AOS-water. It was found that the most stable foams formed at concentration of 1 wt% of brine. Smaller and uniform bubble size has been produced at 0.3 wt% SNP in 0.5 wt% AOS solution. Thus, concentrations of surfactant, SNP and brine have significant effects on CO2 foam stability.

Online bubble size analysis in micro flotation

Measuring the size of bubbles is of fundamental interest in process engineering, especially in flotation. The size of a bubble influences its rising velocity, coalescence tendency and interaction with particles and surrounding liquid. This paper presents an online Bubble Size Analyzer (BSA) with automated image analysis software. The importance of isokinetic sampling with short residence time in the sampling system is shown. Key parameters of the image analysis software are pointed out. The BSA was used to characterize a laboratory dissolved air flotation setup. In accordance with literature, bubble size reduction with increasing saturation pressure and recycle flow was measured. It was discovered that bubble generation in the flotation setup happened by classical dissolved air flotation only when it was operated with pure water. With sodium chloride saline and complex protein solution a mixed dispersed/dissolved air flotation was observed. Maximum bubble sizes were found in water, reduced sizes in saline and minimum size in protein solution. Due to its flexible online installation, the BSA is recommended as a suitable tool for bubble size measurements in laboratory and real processes, where inline analysis is not possible.

Comparison of two analysis methods on bubble size distribution

The object of this work was to analyze foam bubbles’ diameter distribution. Firstly, Rosin-Rammler distribution function was used in bubble size analysis in some studies. This Rosin-Rammler distribution function should be modified to fit foam bubbles distribution curve. Secondly, coefficient of variation value was introduced in our study. This analysis method could be carried out directly without any modification. Thirdly, these two methods were compared whose characteristics were analyzed. Both analysis methods were effective to analyze foam bubbles figures with relatively obvious difference.

Bubble size analysis generated on the submerged orifice, depending on the gas supply mode

A numerical analysis of an algebraic equation of the fifth degree describing the achievement of the diameter of a gas bubble at an aperture is made, the results obtained earlier for limiting cases by simplifying the initial equation and solving it by an analytical method are refined and generalized. With the help of the ratio of the Froude criterion to the Weber criterion, the boundaries of realization of the static, transient and dynamic regimes are established. Approximating formulas are proposed for these regimes, which, with an allowable error in the performance of applied calculations, make it possible to find the value of the detachment diameter of the bubble.

A simple technique for the automation of bubble size measurements

An increasing number of research topics and applications ask for a precise measurement of the size distribution of small bubbles in a liquid - and hence for reliable and automated image analysis. However, due to the strong mismatch between the refractive index of a liquid and a gas, bubbles deform strongly the path of light rays, rendering automated bubble size analysis a challenging task. We show here how this challenge can be met using the fact that bubbles act like inverted, spherical lenses with a curvature which is the inverse of the bubble radius. The imaging properties of each bubble can then be used to accurately determine the radius of the bubble upon imaging an object which can be filtered easily by a computer. When bubbles are large enough to be deformed under the influence of gravity, it is more appropriate to measure their size after squeezing them between two narrowly spaced glass plates. We therefore show here, how the analysis can be extended to this case; and how both approaches can be combined to measure the size distributions of strongly polydisperse foams containing simultaneously small (several 10s of micrometres) and large bubbles (several 100s of micrometres).

A novel approach to prevent bubble coalescence during measurement of bubble size in flotation

Effect of frothers in preventing bubble coalescence during flotation of minerals has long been investigated. To evaluate the performance of a frother, an apparatus to measure the bubble size is a basic necessity. McGill Bubble Size Analyzer (MBSA) or bubble viewer that has been developed and completed by McGill University’s Mineral Processing Group during the last decade is a unique instrument to serve this purpose. Two parameters which are thought to influence the bubble size measurements by McGill bubble viewer include water quality and frother concentration in the chamber. Results show that there is no difference in Sauter mean (D32) when tap or de-ionized water was used instead of process water. However, the frother concentration, in this research DowFroth 250 (DF250), inside the chamber exhibited a pronounced effect on bubble size. Frother concentration below a certain point can not prevent coalescence inside the chamber and therefore caution must be taken in plant applications. It was also noted that the frother concentration which has been so far practiced in plant measurements (CCC75-CCC95) is high enough to prevent coalescence with the bubble viewer.

Gas Dispersion Studies at Highland Valley Copper

This paper discusses three case studies conducted at Highland Valley Copper mine (HVC) that illustrate operational improvements and insights which resulted from using the McGill University gas dispersion sensors. In the first study, the superficial gas velocity (Jg) sensor was used to implement a gas profiling control strategy for the rougher, scavenger and first cleaner cells in the Cu/Mo separation circuit. The data showed a 40% reduction of down-the-bank Cu recovery at constant Mo recovery which resulted from operating the cells at the target profiles. In the second study, the bubble size analyzer was used to evaluate the effect of controlling the size of the bubbles generated in one of two flotation columns that make up the final Mo cleaning stage. The results showed that a large proportion of small (< 1 mm) bubbles increased the metallurgical response of the test column significantly compared to the reference column (typical operation) with the Mo recovery increasing 3–4 fold while the concentrate grade increased 1–2% Mo (absolute). In the third study, real-time measurements obtained from the gas hold-up and bulk density sensors were used to determine the pulp density (% solids) in a rougher cell of the bulk flotation circuit. Validation tests demonstrated that this approach provides reliable pulp density measurements.RésuméCet article discute de trois études de cas, effectuées à la mine de Highland Valley Copper (HVC), qui illustrent les améliorations opérationnelles et la connaissance résultant de l'utilisation des détecteurs de dispersion du gaz de l'université McGill. Dans la première étude, on a utilisé le détecteur de vélocité superficielle du gaz (Jg) pour mettre en place une stratégie de contrôle du profilage de gaz des cellules de dégrossissage, d'épuisement et pour la première cellule de finition dans le circuit de séparation du Cu/Mo. Les données ont montré une réduction de 40% de la récupération de cuivre en aval, à récupération constante de Mo, ce qui résultait de l'opération des cellules aux profils cibles. Dans la deuxième étude, on a utilisé l'analyseur de taille de bulle pour évaluer l'effet du contrôle de la taille des bulles engendrées dans l'une des deux colonnes de flottation qui constituent l'étape finale de nettoyage du Mo. Les résultats ont montré qu'une grande proportion de petites bulles (< 1 mm) augmentait significativement la réponse métallurgique de la colonne d'essai par rapport à la colonne de référence (opération typique), la récupération du Mo augmentant de 3–4 fois alors que la qualité du concentré augmentait de 1–2% Mo (absolu). Dans la troisième étude, on a utilisé les mesures en temps réel obtenues à partir des détecteurs de rétention de gaz et de densité apparente pour déterminer la densité de la pulpe (%de solides) dans une cellule de dégrossissage du circuit de flottation collective. Les essais de validation ont démontré que cette approche fournissait des mesures fiables de la densité de la pulpe.

Suppression Performance Comparison for Aspirated, Compressed-Air and In Situ Chemically Generated Class B Foams

This contribution describes the experimental program undertaken to test the performance of a novel firefighting foam against the current Class B foam technology. The novel foam involved the use of a chemical reaction between dissolved species to generate inert nitrogen gas in situ; hence in situ generated nitrogen foams (ISNF). The experimental program encompassed the utilisation of the DEF(AUST)5706 test standard for measuring the suppression and burnback performance of the ISNF as well as that of compressed air foams (CAF). All tests were performed using the same two types of surfactant concentrates (one telomer and one PFOS-based) in order to accurately determine the effect of the generation technology on the foam performance. The three kinds of foams (i.e., aspirated, CAF and ISNF) were characterised in terms of their bubble size distributions and drainage rates. Bubble size analysis placed the size distribution of the ISNF midway between those of aspirated and compressed air foams, whilst drainage performance was found to be only slightly superior to the aspirated foam. The results of the experiments showed that in situ nitrogen foams are able to achieve good performance, similar to that of aspirated and CAF technologies. Further work is needed to optimise foam formulations for ISNF so that surfactants are not affected by the chemistry of nitrogen generation and by the presence of salts formed during chemical reactions.

Bubble Size Analysis and Boundary Conditions for Automatic Control of Industrial Flotation Cells

The gas dispersion has a high impact on flotation processes performance. To evaluate this effect, the bubble surface area flux (Sb) must be evaluated from superficial gas rate (Jg) and Sauter mean bubble diameter (D32) local measurements at industrial scale. Gas rate sensors and visual bubble caption techniques (video, photograph) have been developed successfully, but the commercial software for automated image analysis still has troubles when irregular big size bubbles and bubble clusters appears in industrial measurements, which prevents an accurate and more reliable bubble size estimation. Also, a criteria for bubble size distribution (BSD) diagnostic and control at industrial scale, has not been developed yet. In this work, measurements of BSD were carried out in large flotation cells, of different size, operating in several flotation circuits in Chile. Also, new semi-automatic software for image analysis was developed to overcome the troubles before mentioned and permitting more reliable bubble size estimations. Experimental results showed that under normal operating conditions BSD matched well with a lognormal distribution. From this reference two types of operating problems has been identified and the corresponding diagnosis criteria were established, which is a powerful tool for automatic control purposes.

Statistical analysis of bubble and crystal size distributions: Formulations and procedures

Bubble and crystal size distributions have previously been described only by either exponential or power law functions. Within this limited framework, it has not been possible to characterize size distributions in a fully quantitative manner. We have developed an analytical and computational formulation with which to characterize and study crystal and bubble size distributions (BSD). This formulation demonstrates that all distributions known to date belong to the logarithmic family of statistical distributions. Four functions within the logarithmic family are best suited to natural bubbles and crystals (log normal, logistic, Weibull, and exponential). This characterization is supported by the fact that the power law function widely used for crystal and bubble size analysis is not a statistical distribution function, but rather represents an approximation of the upper regions (larger bubbles/crystals) of the logistic distribution, whose sizes are much larger than the mode. The coefficients for each of the four logarithmic functions can be derived by 1) best fit exceedance function of the logarithmic distribution, and 2) best fit of the linear transformation of the distribution probability density. A close match of the coefficients derived by the above two methods can be used as an indicator of correct function fitting (choice of initial values). Function fitting by exceedance curves leads to the most accurate statistical results, but has certain strict limitations, including 1) a requirement to rescale the base distribution function; 2) a higher failure rate for function fitting than that for distribution density; 3) uncertainty in observational data error estimates; and 4) unsuitability for visual interpretation. The most productive approach to visualization and interpretation of size distributions is through linear transformation of logarithmic distributions on the basis of probability densities. This also makes it possible to 1) clearly discern bimodal distributions; 2) assess the range of observed objects relative to the full range of the indicated distribution; 3) determine number densities for each mode directly; and 4) integrate to obtain total volume fraction for comparison with available observations. The latter could, in some cases, provide more accurate results than many measurement methods. Unambiguous definition of Bubble Number Density (BND) must be based on the number of bubbles per melt volume (not number of bubbles per bulk volume), so that like is done with crystals, it can be directly used as an indicator of basic vesiculation processes such that: a) nucleation leads to increase of BND, b) diffusive or decompressive bubble growth keeps BND constant, and c) coalescence decreases BND.

Hydrodynamics and scale up in Rushton turbine flotation cells: Part 1 — Cell hydrodynamics

The effect of operating parameters on the hydrodynamics of three geometrically similar Rushton turbine flotation cells with volumes of 2.25, 10 and 50 dm 3 was determined. The operating parameters investigated were superficial gas velocity ( J g), impeller rotational speed ( N), and frother (methyl isobutyl carbinol, MIBC) concentration. Mean energy dissipation values measured using Laser Doppler Velocimetry (LDV) and a torque turntable method were in good agreement. As the cell volume was increased, the mean energy dissipation was proportional to N 3 D, rather than N 3 D 2 as may be expected based on dimensional analysis. Possible reasons for this difference are discussed. Aeration resulted in a slight increase in mean energy dissipation. Bubble diameters were measured using a University of Cape Town bubble size analyzer to determine the frother concentration at which a constant bubble diameter was achieved for all operating conditions and cell volumes. The critical frother concentration was 20 ppm MIBC. The mean bubble velocity was estimated by determining the time required to achieve steady state gas holdup in the top part of the cell after commencing gas sparging. For a constant mean bubble diameter, the bubble velocity increased with increasing superficial gas velocity. As the energy dissipation was increased for a given superficial gas velocity, the bubble velocity decreased linearly until a critical energy dissipation was reached. Above this value, bubble velocity decreased only slightly. As the cell volume increased, the bubble velocity, at the same superficial gas velocity and energy dissipation, also increased.