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Tracking of particles in the froth phase: An experimental technique
Minerals Engineering 19 (2006) 824–830 This article is also available online at: www.elsevier.com/locate/mineng
Tracking of particles in the froth phase: An experimental technique S. Ata *, S. Pigram, G.J. Jameson Centre for Multiphase Processes, University of Newcastle, Callaghan, NSW 2308, Australia Received 27 July 2005; accepted 18 September 2005 Available online 11 November 2005
Abstract This paper is concerned with the tracking of particles in the froth phase of an aerated water/glycerol mixture. Experiments have been carried out in a specially designed laboratory flotation cell that allows formation of a deep froth. Phosphorescent tracer particles of various sizes were injected into the centre of a froth column where they were excited by ultraviolet lights. The motion of particles was captured on a digital camera with a green filter. The images from the digital camera were then transferred to a computer and an image analysis program was used to convert the color intensity to the concentration of particles at each location within the froth. The tracer technique was used to determine the dispersion of hydrophilic particles and the variation of the concentration of solids with axial and radial positions. 2005 Elsevier Ltd. All rights reserved. Keywords: Froth flotation; Flotation froths; Particle size; Mineral processing
1. Introduction Froth flotation is generally recognized as the most appropriate method for the concentration of mineral particles. It has also found application in fields such as paper deinking and wastewater treatment. In the practice of mineral flotation, a pulp of solid particles in water is conditioned with small quantities of reagents to render one mineral selectively hydrophobic. Air is then pumped into the suspension to form bubbles, and the desired mineral tends to adhere to the bubbles, rising to the froth layer where the concentrate flows over the edge of the cell; the unattached hydrophilic solids remain in the cell to be removed in the tailings. Flotation performance strongly depends on froth phase and its properties. Despite that it is only recently that considerable progress has been made in our understanding of the behaviour of flotation froths. Most works in this area have been done in modelling the gross behaviour of the froth. Moys (1978), Cutting et al. (1981) and Ross *
Corresponding author. Tel.: +61 2 4921 6181; fax: +61 2 4960 1445. E-mail address: [email protected] (S. Ata).
0892-6875/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.09.051
(1991a,b) developed models to predict the overall behaviour of a froth column based on differential elements. The behaviour in each element was described in terms of rate constant for detachment and drainage of particles. The studies by the research groups in the JKMRC (Queensland University) (Gorain et al., 1998; Vera et al., 1999, 2002) and the University of Cape Town (Mathe et al., 1998) have found that the froth phase has a pronounced effect on the overall flotation recovery. The subprocesses operating in the froth (i.e. drainage, detachment, re-attachment) have been lumped into a single parameter to define the handling capacity of the froth. Neethling and Cilliers (1999, 2002a,b) took the approach of adapting two-phase foam models to characterise the mineralized froth phase behaviour. The motion of solid particles and liquid were simulated from the pulp–froth interface to the upper the froth column with no or little experimental verification. While the above-mentioned models have improved our understanding of the froth phase of flotation, currently there is no appropriate model to describe the process as a whole. This is mainly due to incomplete understanding of the behaviour of particles and liquid in the froth zone.
S. Ata et al. / Minerals Engineering 19 (2006) 824–830
Indeed a through understanding of the interactions between various phenomenon occurring in the froth phase and modelling these sub-processes remains a major challenge of current flotation research. This paper describes an investigation that has been undertaken to understand the behaviour of particles in the froth phase. A non-invasive experimental technique has been developed in which the motion of particles could be visually investigated. Phosphorescent particles were injected into a two-dimensional rising froth and their axial and lateral movements were studied. 2. Experimental 2.1. Materials A commercial pigment (Honeywell ‘‘Lumilux Effect Green-N’’) was used for the phosphorescent particles. The particles emit radiation following exposure to light (as opposed to fluorescent particles which only emit whilst being irradiated). The particles consisted of ZnS and Cu and can be used either in a hydrophobic form by using a suitable reagent such as a xanthate collector to allow flotation, or as a hydrophilic gangue tracer. A Warman cyclosizer with five cyclones was used to separate the sample into various size fractions. Three particle sizes were used in the experimental program (22, 34, 57 lm). The size of the samples was determined using a Malvern Mastersizer. The density of the particles, from data supplied by the manufacturer, is 4060 kg/m3. The peak excitation wavelength is 361 nm, and the wavelength of the peak in the emitted phosphorescent light is 535 nm. Analytical grade sodium dodecyl sulphate (SDS) and glycerol were used as reagents in the experiments. 2.2. Experimental procedures The experimental rig is shown schematically in Fig. 1. It consists of a Perspex mixing vessel, connected to a flow cell froth launder
froth phase
UV light
UV light
particle injection system pulp phase glass frit from air supply
peristaltic recirculation vessel pump
Fig. 1. A schematic diagram of the experimental apparatus.
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through a tapered transition piece. The flow cell has a rectangular cross-section of internal measurements 70 mm width and 12 mm breadth, and height 800 mm. The vertical sides of the f1ow cell are constructed from Perspex, held apart at the fixed distance of 12 mm by aluminium plate. Bubbles were generated by introducing nitrogen through a sintered glass frit incorporated into the base of the cell. A small hole is drilled in one of the aluminium sides of the f1ow cell 220 mm above the bottom of the f1ow cell and a pressure fitting was installed. This was so that a specially-fabricated injection needle can pass through the column wall without causing any leaks. Four vertical strip lights, available commercially as ‘‘black lighting’’, emitting light in the range 315–380 nm, were used to excite the phosphorescent particles. They were placed 400 mm from the cell, front and back, and maintained in fixed positions. Air bubbles generated in the base of the cell rose into the froth column, over the discharge lip and back to the recirculation vessel. The liquid to the mixing cell consisted of 30% v/v mains water and 70% v/v glycerol with 2.92 g/L analytical grade sodium dodecyl sulphate (SDS) (i.e. 20% above the critical micelle concentration). The effect of the glycerol is to increase the viscosity of the liquid to the range 10–15 cP, similar to that of flotation pulps. A stable froth resulted, in which the size of the bubbles and liquid holdup remained constant through the froth column. The froth–liquid interface was kept 150 mm below the injection point by controlling the flow rate from the peristaltic pump. Altering the rate of the peristaltic pump or the maximum height of the liquid overflow line controls the height of the pulp–froth interface. Once a stable froth was formed the computer and control box were turned on and then the LabView injection program was opened. The injection system was loaded with the desired concentration of particles; the stirrer was then started and operated at a speed that allows the particles to stay in suspension and centrifugal pump was operated at 6 V. The camera and the UV lights were then activated. The commercial software program LabView, installed on a personal computer, was used to control the apparatus. The solenoid was set to open for 1.1 s which corresponds to a pulse of 0.5 ml of suspension. The elapsed time between photos was also set using LabView, to 2 s. The start button in LabView was then activated, and the computer sent a signal to the control box that then opened the solenoid for the desired amount of time and at the same time started the camera. The liquid hold-up profile was computed from the measurements of the hydrostatic-pressure distribution along the froth column at steady state conditions, using a bubble tube apparatus (Cutting et al., 1981; Langberg and Jameson, 1992; Ata et al., 2003). The apparatus consists of a 2 mm ID brass tube through which a very low flow rate of air is passed. The pressure signal generated as the bubbles formed at the tube was monitored using a micromanometer, and the maximum pressure noted at a given height. The local density of the froth column was then
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obtained by fitting a line to the values of the pressure as a function of height above the froth–liquid interface and calculating the derivative. By extending the pressure measurements down into the liquid layer, it was possible to calibrate the micro-manometer. The bubble size in the froth column was measured by taking digital photographs through the wall of the froth column, with front illumination. The digital photos were transferred to a PC and Optimas was used to analyse the average bubble diameter. All experiments were carried out in a dark room of approximately constant ambient temperature of 20. 2.3. Injection system The original injection method involved withdrawing a sample of particles, which are kept in suspension by a magnetic stirrer, from a beaker using a 1 ml syringe. However, it was found that before the particles were injected they would settle in the syringe. To overcome this problem an injection system (Fig. 2) was designed to prevent the particles from settling during the injection process. The particles were kept in suspension in the mixing vessel with a Shelton laboratory stirrer, and a small centrifugal pump which moved the suspension around the injection system. The injection needle was a thin-walled stainless steel capillary tube of length 75 mm and inside diameter 0.8 mm with one end soldered closed. Just before the end, two holes (diameter 0.4 mm) were drilled on opposite sides of the tube, which was placed horizontally in the froth column in such a way that the injected material left the small holes in a horizontal plane, directed towards the vertical persex walls of the column. The injection needle was attached to a 24 V Burkert two way solenoid valve with a 3 mm orifice. The volume of fluid injected into the froth was 0.5 mL and the injection time was 1.1 s, so that the velocity of the liquid through each of the exit holes in the capillary was 0.2 m/s. At this velocity, the injectant was quickly absorbed by the foam, within a distance of 10 hole diameters approximately, or 4 mm from the capillary.
stirrer solenoid injection needle
A PC was connected to a control box which provided the power to open the solenoid and to start the camera. LabView was used to start the injection process and to adjust operating parameters such as the opening time of the solenoid and the time between each photograph. A Nikon D100 digital camera was used in the experimentation process. It has a large (23.7 mm · 15.6 mm) CCD with 6.1 million effective pixels. 2.4. Calibration The froth–liquid interface was set at 150 mm below the injection point by manipulating the flowrate of the recirculation pump. An air flowrate of 0.14 L/min, sufficient to give a superficial air velocity Jg of 0.3 cm/s, was established. The UV lights were activated and all other lighting in the darkroom was extinguished. A black vest was worn by the operator to minimize reflection of light to the foam column. Three photographs were taken of the column in the absence of particles, to establish a background datum, which could later be subtracted from photos taken in the presence of particles. The injection system was charged with a glycerol solution in water (70% v/v), with the desired concentration of particles, initially 750 ppm. The tracer suspension (0.5 mL) was introduced into the froth over a period of 1.1 s at the centre-line of the column. The injection time should be as short as possible so as to approximate an impulsive change in the local concentration of particles in the froth without breaking the froth or incurring large changes in the local liquid fraction. Photographs of the column were taken as the tracer was injected. After the photos were taken, the air supply was stopped and the particles were allowed to settle to the bottom of the column, where they could not influence subsequent experiments. The froth was removed by raising the liquid level. The process was then repeated, with a range of concentrations of the phosphorescent particles from 750 to 5000 ppm (w/w). The photographs were analysed using the Optimas software, which divides the field of view into a grid of 100 · 250 cells, corresponding to a physical area of 70 mm · 175 mm; a single cell corresponds to an area of 0.49 mm2. The cell giving the maximum luminance that could be observed for a given concentration, was found, and the luminance was reported as the average of the value for this cell and those immediately adjacent. It was found experimentally that the luminance varied linearly with the particle concentration in the range from 0 to 3000 ppm, with a correlation coefficient of 0.989, so the experiments were confined to this range. 2.5. Data analysis
Fig. 2. Injection system. The mixing cell was 90 mm high and 50 mm diameter. The horizontal runs were kept as short as practicable to minimise settling of the tracer particles.
The Nikon D100 camera was focussed manually at the start of the run, and the following camera settings, which were used in all runs, were found to be satisfactory: aperture f3.5, shutter speed 1/6 s, ISO 1600. A yellow–green fil-
S. Ata et al. / Minerals Engineering 19 (2006) 824–830
ter (XO) from HOYA was fixed on the lens to filter out light other than the phosphorescent glow from the particles, thus eliminating the background noise as much as possible. The transmission spectrum of the filter is effectively from 400 to 650 nm, with a peak at 550 nm, which corresponds closely with the emission peak of the particles at 535 nm. Optimas 6.1 software was used to measure the luminance in the photographs. A subroutine was written especially for the purpose of these experiments. A region of interest was chosen, which the subroutine maps into a grid of 100 · 250 cells. Luminance values for each cell were downloaded to a Microsoft Excel spreadsheet for further analysis using the calibration data. 3. Results and discussion The diameters of bubbles were ascertained by photographing them through the transparent wall of the froth column. The measurements were checked by photographing the bubbles as they rose through the liquid towards the froth–liquid interface. There was a negligible increase
in the average bubble size with increasing height above the pulp–froth interface, indicating that the coalescence of bubbles in the foam was negligible. The Sauter-mean diameter (the volume-to-surface area mean bubble diameter), and the arithmetic average bubble size at the injection point were found to be 2.20 and 2.18 mm, respectively. The liquid hold-up in the froth column was measured using the bubble probe inserted from the top of the column. It was found that the liquid fraction was essentially constant and did not change with froth height. The liquid hold-up in the froth was 0.24. This was probably due to the high viscosity of the liquid used in the experiments, which tends to reduce the rate of drainage of liquid back to the liquid phase. It was decided to use a concentration of 2500 ppm of particles in the injected sample, as higher values produced excessive internal reflection of light through the froth, suggesting that particles were present where none were expected. The tracer particles were completely hydrophilic and were not treated with any reagents to render the surface hydrophobic. Therefore, the results given in the paper are
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Fig. 3. Concentration of tracer particles as a function of height above the froth/liquid interface. The injection point was 15 cm above the interface. Particle sizes: (a) 22 lm; (b) 34 lm; (c) 57 lm. The elapsed time between each trace was 10 s.
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related to the behavior of gangue mineral in the froth phase. Note that these experimental conditions were kept constant for all runs presented in this paper. A common observation in all the experiments was that as the pulse of injected particles rises with the rising froth in the column, it elongated and widened for each of the size fractions. However, the pulses containing the particles of diameter 57 lm and 34 lm elongated significantly more than those with the 22 lm particles. This effect is possibly related to the size distribution of each of the particle samples. Analysis with a Malvern Mastersizer showed that the distribution of the largest particles ranged from 20 lm to 200 lm, with the mean at 57 lm, while the range for the smallest particles was from 11 lm to 50 lm with a mean at 22 lm. Fig. 3(a)–(c) shows the concentration of phosphorescent particles as a function of height above the froth–pulp interface for 22, 34 and 57 lm particles, respectively for conditions of 0.14 l/min air flowrate and an injected pulse volume of 0.5 ml, at intervals of 10 s from 10–50 s. As seen, with three particle sizes, the dispersion distance of the particles is initially very low, but quickly spreads out in the
vertical direction with increasing time. The dispersion of the tracer in the foam is due to the action of the liquid as it is dragged upwards by the rising bubbles, influenced also by the terminal velocity of the particles. The concentration peak value decreases with time because of dispersion in the vertical. The particle size clearly affects the nature of the dispersion process, since it can be seen that the 22 lm particles have the lowest dispersion rate. Fig. 4(a)–(c) shows the lateral dispersion of 22, 34 and 57 lm particles, respectively, as they rose in the column. Although tracer particles were injected in the centre of the rising foam, the tracer tended to drift away from the injection point. The horizontal drift of the pulse is due to the effect of the injection needle on the flow of the froth. The injection needle has an outside diameter of 1 mm while the bubbles have a mean diameter of 2.2 mm, and because they are of similar magnitude it is easy to envisage that the needle will affect the movement of the bubbles as they approach the capillary. It was observed that some bubbles were slowed by the needle, therefore to maintain the average throughput of froth, some bubbles to the side of the needle moved at an increased velocity. The velocity profile
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Fig. 4. Horizontal profile of tracer concentration as a function of position across the froth column for various time and particle sizes. (a) 22 lm; (b) 34 lm; (c) 57 lm. The elapsed time between each trace was 10 s.
S. Ata et al. / Minerals Engineering 19 (2006) 824–830
created effectively moves the pulse to the right of the injection point. Nevertheless, as the froth rises away from the needle, it stabilises. During the experiments, as soon as the tracer particles were injected, particles were observed to disperse in the horizontal direction. The velocity of the dispersion front is quite high, since some particles reach the side walls of the column in less than 10 s. With increasing time, the peak values decrease, because particles are continually being removed from the region of observation by the convective flows within the froth, which are moving vertically in both directions. The lateral dispersion of particles occurs due to the geometric layout of the plateau borders, which can be likened to the quincunx experiment. Such a behavior was previously described by Saffman (1959) to model the fluid flows through the pores medium and more recently by Neethling and Cilliers (2002a) to describe the motion of the unattached particles within a flotation froth. As a pulse moves upwards in the froth column, the particles spread horizontally as well as vertically. Fig. 5 shows the magnitude of the maximum concentrations of particles as a pulse moves upwards in the column, for particles of diameter 22, 34 and 57 lm respectively. For plotting purposes, the concentrations for a given particle size have been normalized with respect to the initial concentration at the point of injection in each case. It can be seen that the concentration of tracer initially decreases rapidly as the pulse moves up in the column, but the rate of decrease slows markedly with increasing height. From about 5 cm onwards, the peak concentration appears to decrease linearly. With the larger particles, there is considerable scatter, but the smallest particles behave quite regularly. The decrease in the peak concentration is due to the lateral and axial dispersion of particles in the froth. Eventually of course, the particles will become evenly distributed across the cross-section of the column.
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An observation of considerable practical consequence can be inferred from Fig. 5. The results show that if gangue particles are introduced into a froth, a significant fraction will find their way into the concentrate. Equally, wash water that enters the froth will also find its way into the flotation product, having displaced some of the liquid that had been entrained when the froth-forming bubbles rose out of the liquid layer. However, because of the dispersion characteristics of the froth, not all of the entrained liquid can be replaced, so some gangue will always find its way into the product. Although the present experiments were carried out under idealized conditions, i.e. constant liquid hold-up and bubble size vertically up the froth column, a similar phenomenon has been observed in flotation experiments on a mixture of hydrophobic and hydrophilic particles. Ata et al. (2002, 2004) carried out experiments in a special flotation cell that separates the froth phase from the pulp zones. Hematite particles were floated in the pulp whilst glass particles were introduced in wash water added to the froth. It was found that a small fraction of hydrophilic particles (glass beads) always reported to the product even though washwater was applied from the top of the column. Thus the elimination of entrainment of glass particles to the concentrate was not achievable in their system. 4. Conclusions An experimental technique has been developed in which the motion of particles in a rising foam could be visually observed. Phosphorescent particles were used as a tracer, and were injected into the centre of a froth column with a specially designed injection system. Particles of different sizes (22, 34, 57 lm) were used. Particles were excited by ultraviolet lights, and the light emitted over the region of interest was captured on a digital camera, the output then being transferred to a computer. An image analysis program was used to convert the colour intensity to the concentration of particles at each location within the froth. The advantage of the technique is that it allows particle behaviour to be studied without causing any disturbance to froth dynamics. The results showed that when the tracer particles were injected into the column, they quickly spread both horizontally and vertically, due to dispersion mechanisms associated with the flow of the liquid and the internal geometry of the bubble assembly in the froth. Acknowledgements
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The authors acknowledge the financial support of the Australian Research Council for the Centre for Multiphase Processes.
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Fig. 5. Variation of peak tracer concentration with froth height for various particle sizes. The concentrations have been normalized with respect to the initial concentration at the injection point.
References Ata, S., Ahmed, N., Jameson, G.J., 2002. Collection of hydrophobic particles in the froth phase. Int. J. Miner. Process. 64, 101–122.
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Ata, S., Ahmed, N., Jameson, G.J., 2003. Gangue drainage in flotation froths. In: Proc. XXII IMPC, 28 September–3 October, Cape Town. Ata, S., Ahmed, N., Jameson, G.J., 2004. The effect of hydrophobicity on the drainage of gangue mineral in flotation froths. Minerals Eng. 17 (7–8), 897–901. Cutting, G.W., Watson, D., Whitehead, A., Barber, S.P., 1981. Froth structure in continuous flotation cells: relation to the prediction of plant performance from laboratory data using process models. Int. J. Miner. Process. 7, 347–369. Gorain, B.K., Harris, M.C., Franzidis, J.P., Manlapig, E.V., 1998. The effect of froth residence time on the kinetics of flotation. Minerals Eng. 11 (7), 627–638. Langberg, D.E., Jameson, G.J.J., 1992. The coexistence of the froth and liquid phases in a flotation column. Chem. Eng. Sci. 47 (17/18), 4345– 4355. Mathe, Z.T., Harris, M.C., OÕConnor, C.T., Franzidis, J.P., 1998. Review of froth modeling in steady state flotation systems. Minerals Eng. 11 (5), 397–421. Moys, M.H., 1978. A study of a plug-flow model for flotation behavior. Int. J. Miner. Process. 5, 21–38.
Neethling, S.J., Cilliers, J.J., 1999. A visual kinematic model of flowing foams incorporating coalescence. Powder Technol. 101 (3), 249–256. Neethling, S.J., Cilliers, J.J., 2002a. The entrainment of gangue into a flotation froth. Int. J. Miner. Process. 64, 123–134. Neethling, S.J., Cilliers, J.J., 2002b. Solids motion in flowing froths. Chem. Eng. Sci. 57, 607–615. Ross, V.E., 1991a. An investigation of sub-processes in equilibrium froths (I): the mechanisms of detachment and drainage. Int. J. Miner. Process. 31, 37–50. Ross, V.E., 1991b. An investigation of sub-processes in equilibrium froths (II): the effect of operating conditions. Int. J. Miner. Process. 31, 51– 71. Saffman, P.G., 1959. A theory of dispersion in a porous medium. J. Fluid Mech. 6 (3), 321–349. Vera, M.A, Franzidis, J.P., Manlapig, E.V., 1999. Simultaneous determination of collection zone rate and froth zone recovery in mechanical flotation environment. Minerals Eng. 12 (10), 1163–1176. Vera, M.A., Mathe, Z,T., Franzidis, J.P., Harris, M.C., Manlapig, E.V., OÕConnor, C.T., 2002. The modelling of froth zone recovery in batch and continuously operated laboratory flotation cells. Int. J. Miner. Process. 64, 135–151.
Tracking of particles in the froth phase: An experimental technique S. Ata *, S. Pigram, G.J. Jameson Centre for Multiphase Processes, University of Newcastle, Callaghan, NSW 2308, Australia Received 27 July 2005; accepted 18 September 2005 Available online 11 November 2005
Abstract This paper is concerned with the tracking of particles in the froth phase of an aerated water/glycerol mixture. Experiments have been carried out in a specially designed laboratory flotation cell that allows formation of a deep froth. Phosphorescent tracer particles of various sizes were injected into the centre of a froth column where they were excited by ultraviolet lights. The motion of particles was captured on a digital camera with a green filter. The images from the digital camera were then transferred to a computer and an image analysis program was used to convert the color intensity to the concentration of particles at each location within the froth. The tracer technique was used to determine the dispersion of hydrophilic particles and the variation of the concentration of solids with axial and radial positions. 2005 Elsevier Ltd. All rights reserved. Keywords: Froth flotation; Flotation froths; Particle size; Mineral processing
1. Introduction Froth flotation is generally recognized as the most appropriate method for the concentration of mineral particles. It has also found application in fields such as paper deinking and wastewater treatment. In the practice of mineral flotation, a pulp of solid particles in water is conditioned with small quantities of reagents to render one mineral selectively hydrophobic. Air is then pumped into the suspension to form bubbles, and the desired mineral tends to adhere to the bubbles, rising to the froth layer where the concentrate flows over the edge of the cell; the unattached hydrophilic solids remain in the cell to be removed in the tailings. Flotation performance strongly depends on froth phase and its properties. Despite that it is only recently that considerable progress has been made in our understanding of the behaviour of flotation froths. Most works in this area have been done in modelling the gross behaviour of the froth. Moys (1978), Cutting et al. (1981) and Ross *
Corresponding author. Tel.: +61 2 4921 6181; fax: +61 2 4960 1445. E-mail address: [email protected] (S. Ata).
0892-6875/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2005.09.051
(1991a,b) developed models to predict the overall behaviour of a froth column based on differential elements. The behaviour in each element was described in terms of rate constant for detachment and drainage of particles. The studies by the research groups in the JKMRC (Queensland University) (Gorain et al., 1998; Vera et al., 1999, 2002) and the University of Cape Town (Mathe et al., 1998) have found that the froth phase has a pronounced effect on the overall flotation recovery. The subprocesses operating in the froth (i.e. drainage, detachment, re-attachment) have been lumped into a single parameter to define the handling capacity of the froth. Neethling and Cilliers (1999, 2002a,b) took the approach of adapting two-phase foam models to characterise the mineralized froth phase behaviour. The motion of solid particles and liquid were simulated from the pulp–froth interface to the upper the froth column with no or little experimental verification. While the above-mentioned models have improved our understanding of the froth phase of flotation, currently there is no appropriate model to describe the process as a whole. This is mainly due to incomplete understanding of the behaviour of particles and liquid in the froth zone.
S. Ata et al. / Minerals Engineering 19 (2006) 824–830
Indeed a through understanding of the interactions between various phenomenon occurring in the froth phase and modelling these sub-processes remains a major challenge of current flotation research. This paper describes an investigation that has been undertaken to understand the behaviour of particles in the froth phase. A non-invasive experimental technique has been developed in which the motion of particles could be visually investigated. Phosphorescent particles were injected into a two-dimensional rising froth and their axial and lateral movements were studied. 2. Experimental 2.1. Materials A commercial pigment (Honeywell ‘‘Lumilux Effect Green-N’’) was used for the phosphorescent particles. The particles emit radiation following exposure to light (as opposed to fluorescent particles which only emit whilst being irradiated). The particles consisted of ZnS and Cu and can be used either in a hydrophobic form by using a suitable reagent such as a xanthate collector to allow flotation, or as a hydrophilic gangue tracer. A Warman cyclosizer with five cyclones was used to separate the sample into various size fractions. Three particle sizes were used in the experimental program (22, 34, 57 lm). The size of the samples was determined using a Malvern Mastersizer. The density of the particles, from data supplied by the manufacturer, is 4060 kg/m3. The peak excitation wavelength is 361 nm, and the wavelength of the peak in the emitted phosphorescent light is 535 nm. Analytical grade sodium dodecyl sulphate (SDS) and glycerol were used as reagents in the experiments. 2.2. Experimental procedures The experimental rig is shown schematically in Fig. 1. It consists of a Perspex mixing vessel, connected to a flow cell froth launder
froth phase
UV light
UV light
particle injection system pulp phase glass frit from air supply
peristaltic recirculation vessel pump
Fig. 1. A schematic diagram of the experimental apparatus.
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through a tapered transition piece. The flow cell has a rectangular cross-section of internal measurements 70 mm width and 12 mm breadth, and height 800 mm. The vertical sides of the f1ow cell are constructed from Perspex, held apart at the fixed distance of 12 mm by aluminium plate. Bubbles were generated by introducing nitrogen through a sintered glass frit incorporated into the base of the cell. A small hole is drilled in one of the aluminium sides of the f1ow cell 220 mm above the bottom of the f1ow cell and a pressure fitting was installed. This was so that a specially-fabricated injection needle can pass through the column wall without causing any leaks. Four vertical strip lights, available commercially as ‘‘black lighting’’, emitting light in the range 315–380 nm, were used to excite the phosphorescent particles. They were placed 400 mm from the cell, front and back, and maintained in fixed positions. Air bubbles generated in the base of the cell rose into the froth column, over the discharge lip and back to the recirculation vessel. The liquid to the mixing cell consisted of 30% v/v mains water and 70% v/v glycerol with 2.92 g/L analytical grade sodium dodecyl sulphate (SDS) (i.e. 20% above the critical micelle concentration). The effect of the glycerol is to increase the viscosity of the liquid to the range 10–15 cP, similar to that of flotation pulps. A stable froth resulted, in which the size of the bubbles and liquid holdup remained constant through the froth column. The froth–liquid interface was kept 150 mm below the injection point by controlling the flow rate from the peristaltic pump. Altering the rate of the peristaltic pump or the maximum height of the liquid overflow line controls the height of the pulp–froth interface. Once a stable froth was formed the computer and control box were turned on and then the LabView injection program was opened. The injection system was loaded with the desired concentration of particles; the stirrer was then started and operated at a speed that allows the particles to stay in suspension and centrifugal pump was operated at 6 V. The camera and the UV lights were then activated. The commercial software program LabView, installed on a personal computer, was used to control the apparatus. The solenoid was set to open for 1.1 s which corresponds to a pulse of 0.5 ml of suspension. The elapsed time between photos was also set using LabView, to 2 s. The start button in LabView was then activated, and the computer sent a signal to the control box that then opened the solenoid for the desired amount of time and at the same time started the camera. The liquid hold-up profile was computed from the measurements of the hydrostatic-pressure distribution along the froth column at steady state conditions, using a bubble tube apparatus (Cutting et al., 1981; Langberg and Jameson, 1992; Ata et al., 2003). The apparatus consists of a 2 mm ID brass tube through which a very low flow rate of air is passed. The pressure signal generated as the bubbles formed at the tube was monitored using a micromanometer, and the maximum pressure noted at a given height. The local density of the froth column was then
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obtained by fitting a line to the values of the pressure as a function of height above the froth–liquid interface and calculating the derivative. By extending the pressure measurements down into the liquid layer, it was possible to calibrate the micro-manometer. The bubble size in the froth column was measured by taking digital photographs through the wall of the froth column, with front illumination. The digital photos were transferred to a PC and Optimas was used to analyse the average bubble diameter. All experiments were carried out in a dark room of approximately constant ambient temperature of 20. 2.3. Injection system The original injection method involved withdrawing a sample of particles, which are kept in suspension by a magnetic stirrer, from a beaker using a 1 ml syringe. However, it was found that before the particles were injected they would settle in the syringe. To overcome this problem an injection system (Fig. 2) was designed to prevent the particles from settling during the injection process. The particles were kept in suspension in the mixing vessel with a Shelton laboratory stirrer, and a small centrifugal pump which moved the suspension around the injection system. The injection needle was a thin-walled stainless steel capillary tube of length 75 mm and inside diameter 0.8 mm with one end soldered closed. Just before the end, two holes (diameter 0.4 mm) were drilled on opposite sides of the tube, which was placed horizontally in the froth column in such a way that the injected material left the small holes in a horizontal plane, directed towards the vertical persex walls of the column. The injection needle was attached to a 24 V Burkert two way solenoid valve with a 3 mm orifice. The volume of fluid injected into the froth was 0.5 mL and the injection time was 1.1 s, so that the velocity of the liquid through each of the exit holes in the capillary was 0.2 m/s. At this velocity, the injectant was quickly absorbed by the foam, within a distance of 10 hole diameters approximately, or 4 mm from the capillary.
stirrer solenoid injection needle
A PC was connected to a control box which provided the power to open the solenoid and to start the camera. LabView was used to start the injection process and to adjust operating parameters such as the opening time of the solenoid and the time between each photograph. A Nikon D100 digital camera was used in the experimentation process. It has a large (23.7 mm · 15.6 mm) CCD with 6.1 million effective pixels. 2.4. Calibration The froth–liquid interface was set at 150 mm below the injection point by manipulating the flowrate of the recirculation pump. An air flowrate of 0.14 L/min, sufficient to give a superficial air velocity Jg of 0.3 cm/s, was established. The UV lights were activated and all other lighting in the darkroom was extinguished. A black vest was worn by the operator to minimize reflection of light to the foam column. Three photographs were taken of the column in the absence of particles, to establish a background datum, which could later be subtracted from photos taken in the presence of particles. The injection system was charged with a glycerol solution in water (70% v/v), with the desired concentration of particles, initially 750 ppm. The tracer suspension (0.5 mL) was introduced into the froth over a period of 1.1 s at the centre-line of the column. The injection time should be as short as possible so as to approximate an impulsive change in the local concentration of particles in the froth without breaking the froth or incurring large changes in the local liquid fraction. Photographs of the column were taken as the tracer was injected. After the photos were taken, the air supply was stopped and the particles were allowed to settle to the bottom of the column, where they could not influence subsequent experiments. The froth was removed by raising the liquid level. The process was then repeated, with a range of concentrations of the phosphorescent particles from 750 to 5000 ppm (w/w). The photographs were analysed using the Optimas software, which divides the field of view into a grid of 100 · 250 cells, corresponding to a physical area of 70 mm · 175 mm; a single cell corresponds to an area of 0.49 mm2. The cell giving the maximum luminance that could be observed for a given concentration, was found, and the luminance was reported as the average of the value for this cell and those immediately adjacent. It was found experimentally that the luminance varied linearly with the particle concentration in the range from 0 to 3000 ppm, with a correlation coefficient of 0.989, so the experiments were confined to this range. 2.5. Data analysis
Fig. 2. Injection system. The mixing cell was 90 mm high and 50 mm diameter. The horizontal runs were kept as short as practicable to minimise settling of the tracer particles.
The Nikon D100 camera was focussed manually at the start of the run, and the following camera settings, which were used in all runs, were found to be satisfactory: aperture f3.5, shutter speed 1/6 s, ISO 1600. A yellow–green fil-
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ter (XO) from HOYA was fixed on the lens to filter out light other than the phosphorescent glow from the particles, thus eliminating the background noise as much as possible. The transmission spectrum of the filter is effectively from 400 to 650 nm, with a peak at 550 nm, which corresponds closely with the emission peak of the particles at 535 nm. Optimas 6.1 software was used to measure the luminance in the photographs. A subroutine was written especially for the purpose of these experiments. A region of interest was chosen, which the subroutine maps into a grid of 100 · 250 cells. Luminance values for each cell were downloaded to a Microsoft Excel spreadsheet for further analysis using the calibration data. 3. Results and discussion The diameters of bubbles were ascertained by photographing them through the transparent wall of the froth column. The measurements were checked by photographing the bubbles as they rose through the liquid towards the froth–liquid interface. There was a negligible increase
in the average bubble size with increasing height above the pulp–froth interface, indicating that the coalescence of bubbles in the foam was negligible. The Sauter-mean diameter (the volume-to-surface area mean bubble diameter), and the arithmetic average bubble size at the injection point were found to be 2.20 and 2.18 mm, respectively. The liquid hold-up in the froth column was measured using the bubble probe inserted from the top of the column. It was found that the liquid fraction was essentially constant and did not change with froth height. The liquid hold-up in the froth was 0.24. This was probably due to the high viscosity of the liquid used in the experiments, which tends to reduce the rate of drainage of liquid back to the liquid phase. It was decided to use a concentration of 2500 ppm of particles in the injected sample, as higher values produced excessive internal reflection of light through the froth, suggesting that particles were present where none were expected. The tracer particles were completely hydrophilic and were not treated with any reagents to render the surface hydrophobic. Therefore, the results given in the paper are
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Fig. 3. Concentration of tracer particles as a function of height above the froth/liquid interface. The injection point was 15 cm above the interface. Particle sizes: (a) 22 lm; (b) 34 lm; (c) 57 lm. The elapsed time between each trace was 10 s.
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related to the behavior of gangue mineral in the froth phase. Note that these experimental conditions were kept constant for all runs presented in this paper. A common observation in all the experiments was that as the pulse of injected particles rises with the rising froth in the column, it elongated and widened for each of the size fractions. However, the pulses containing the particles of diameter 57 lm and 34 lm elongated significantly more than those with the 22 lm particles. This effect is possibly related to the size distribution of each of the particle samples. Analysis with a Malvern Mastersizer showed that the distribution of the largest particles ranged from 20 lm to 200 lm, with the mean at 57 lm, while the range for the smallest particles was from 11 lm to 50 lm with a mean at 22 lm. Fig. 3(a)–(c) shows the concentration of phosphorescent particles as a function of height above the froth–pulp interface for 22, 34 and 57 lm particles, respectively for conditions of 0.14 l/min air flowrate and an injected pulse volume of 0.5 ml, at intervals of 10 s from 10–50 s. As seen, with three particle sizes, the dispersion distance of the particles is initially very low, but quickly spreads out in the
vertical direction with increasing time. The dispersion of the tracer in the foam is due to the action of the liquid as it is dragged upwards by the rising bubbles, influenced also by the terminal velocity of the particles. The concentration peak value decreases with time because of dispersion in the vertical. The particle size clearly affects the nature of the dispersion process, since it can be seen that the 22 lm particles have the lowest dispersion rate. Fig. 4(a)–(c) shows the lateral dispersion of 22, 34 and 57 lm particles, respectively, as they rose in the column. Although tracer particles were injected in the centre of the rising foam, the tracer tended to drift away from the injection point. The horizontal drift of the pulse is due to the effect of the injection needle on the flow of the froth. The injection needle has an outside diameter of 1 mm while the bubbles have a mean diameter of 2.2 mm, and because they are of similar magnitude it is easy to envisage that the needle will affect the movement of the bubbles as they approach the capillary. It was observed that some bubbles were slowed by the needle, therefore to maintain the average throughput of froth, some bubbles to the side of the needle moved at an increased velocity. The velocity profile
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created effectively moves the pulse to the right of the injection point. Nevertheless, as the froth rises away from the needle, it stabilises. During the experiments, as soon as the tracer particles were injected, particles were observed to disperse in the horizontal direction. The velocity of the dispersion front is quite high, since some particles reach the side walls of the column in less than 10 s. With increasing time, the peak values decrease, because particles are continually being removed from the region of observation by the convective flows within the froth, which are moving vertically in both directions. The lateral dispersion of particles occurs due to the geometric layout of the plateau borders, which can be likened to the quincunx experiment. Such a behavior was previously described by Saffman (1959) to model the fluid flows through the pores medium and more recently by Neethling and Cilliers (2002a) to describe the motion of the unattached particles within a flotation froth. As a pulse moves upwards in the froth column, the particles spread horizontally as well as vertically. Fig. 5 shows the magnitude of the maximum concentrations of particles as a pulse moves upwards in the column, for particles of diameter 22, 34 and 57 lm respectively. For plotting purposes, the concentrations for a given particle size have been normalized with respect to the initial concentration at the point of injection in each case. It can be seen that the concentration of tracer initially decreases rapidly as the pulse moves up in the column, but the rate of decrease slows markedly with increasing height. From about 5 cm onwards, the peak concentration appears to decrease linearly. With the larger particles, there is considerable scatter, but the smallest particles behave quite regularly. The decrease in the peak concentration is due to the lateral and axial dispersion of particles in the froth. Eventually of course, the particles will become evenly distributed across the cross-section of the column.
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An observation of considerable practical consequence can be inferred from Fig. 5. The results show that if gangue particles are introduced into a froth, a significant fraction will find their way into the concentrate. Equally, wash water that enters the froth will also find its way into the flotation product, having displaced some of the liquid that had been entrained when the froth-forming bubbles rose out of the liquid layer. However, because of the dispersion characteristics of the froth, not all of the entrained liquid can be replaced, so some gangue will always find its way into the product. Although the present experiments were carried out under idealized conditions, i.e. constant liquid hold-up and bubble size vertically up the froth column, a similar phenomenon has been observed in flotation experiments on a mixture of hydrophobic and hydrophilic particles. Ata et al. (2002, 2004) carried out experiments in a special flotation cell that separates the froth phase from the pulp zones. Hematite particles were floated in the pulp whilst glass particles were introduced in wash water added to the froth. It was found that a small fraction of hydrophilic particles (glass beads) always reported to the product even though washwater was applied from the top of the column. Thus the elimination of entrainment of glass particles to the concentrate was not achievable in their system. 4. Conclusions An experimental technique has been developed in which the motion of particles in a rising foam could be visually observed. Phosphorescent particles were used as a tracer, and were injected into the centre of a froth column with a specially designed injection system. Particles of different sizes (22, 34, 57 lm) were used. Particles were excited by ultraviolet lights, and the light emitted over the region of interest was captured on a digital camera, the output then being transferred to a computer. An image analysis program was used to convert the colour intensity to the concentration of particles at each location within the froth. The advantage of the technique is that it allows particle behaviour to be studied without causing any disturbance to froth dynamics. The results showed that when the tracer particles were injected into the column, they quickly spread both horizontally and vertically, due to dispersion mechanisms associated with the flow of the liquid and the internal geometry of the bubble assembly in the froth. Acknowledgements
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The authors acknowledge the financial support of the Australian Research Council for the Centre for Multiphase Processes.
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Fig. 5. Variation of peak tracer concentration with froth height for various particle sizes. The concentrations have been normalized with respect to the initial concentration at the injection point.
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