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Particulate interactions in water flotation froths: a micro-mechanistic approach
Int. J. Miner. Process. 74 (2004) 115 – 120 www.elsevier.com/locate/ijminpro
Particulate interactions in water flotation froths: a micro-mechanistic approach M.T. Spyridopoulos, S.J.R. Simons * Colloid and Surface Engineering Group, Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK Received 20 July 2001; received in revised form 10 October 2002; accepted 19 September 2003
Abstract We have constructed a novel apparatus to study fundamental interactions between bubbles and particles in drinking-water flotation froths. The high specifications of the apparatus and its modular construction permit experiments on bubble coalescence and bubble – particle adhesion forces to be carried out. We give a description of the apparatus, along with information on the techniques for its use. We observed the effect of a basic constituent of natural waters, humic acid, on bubble coalescence. Intense collision resulted in immediate bubble coalescence (1 ms), whereas less intense collisions prevented, in many cases, bubble coalescence, even at low-concentrated solutions. D 2003 Elsevier B.V. All rights reserved. Keywords: flotation; froth; apparatus; bubble coalescence; adhesion force
1. Introduction The efficiency of the flotation process in drinkingwater purification plants depends at a high percentage on the same factors that influence mineral flotation. Capture of the solid particles by the rising bubbles, adequate particle –bubble attachment and stability of the froth are only a few of the most crucial parameters. As a consequence, bubble –bubble and bubble – particle interactions have attracted the interest of many researchers throughout the years. However, further investigation of these interactions in the froth layer
* Corresponding author. Tel.: +44-207-679-3805; fax: +44-207383-2348. E-mail address: [email protected] (S.J.R. Simons). 0301-7516/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2003.09.009
of the flotation tank are needed, for better understanding of the process and more efficient application of the method (Ross, 1991). In the froth layer, floating particles can become detached from the bubbles and go back to the bulk liquid. Firstly, due to bubble coalescence the area available for holding particles is reduced, leading the particles back to the bulk. Secondly, particles can be dislodged from the bubbles when shear forces are imposed on them, especially at the bulk – froth interface. Bubble coalescence happens very fast in pure liquids (Oolman and Blanch, 1986). However, it can be delayed or prohibited whenever the liquid contains even small amounts of impurities or surfactants (Chaudhari and Hofmann, 1994). In many cases, the presence of electrolytes has also been found to affect
116
M.T. Spyridopoulos, S.J.R. Simons / Int. J. Miner. Process. 74 (2004) 115–120
bubble coalescence (Christenson and Yaminsky, 1995). Some studies have shown that the presence of the solids in the froth can inhibit or provoke bubble coalescence (Dippenaar, 1982). After a solid particle encounters an air bubble and attaches to it, a three-phase contact line is formed. The solid particle will remain attached on the bubble if the capillary force acting on it overcomes the detaching forces. For a spherical particle, the capillary force, Fc, is given by (Scheludko et al., 1976): Fc ¼ 2prp csina sinðh aÞ
ð1Þ
with rp the radius of the particle, c the surface tension of the liquid, a the angle that defines the position of the particle with respect to the undisturbed liquid surface, and h the contact angle of the liquid on the particle surface. The adhesion is the maximum capillary force, given by (Scheludko and Nikolov, 1975): ha FAd ¼ 2prp ccos2 ð2Þ 2 where ha is the advancing contact angle. Froths in drinking-water treatment flotation tanks contain many constituents that can influence both bubble coalescence and bubble – particle adhesion force. Indeed, chemicals are used to promote flocculation and coagulation of the solids in natural water, which is mainly organic matter. Electrolytes, particles, humic substances (natural organic matter polyelectrolytes that some researchers have found can act as natural surfactants (Beckett, 1990)) are all present in
the froth and it is their effects that we are attempting to investigate. We have designed and constructed a novel apparatus to study the basics interactions in the froth. The apparatus is capable of executing a wide range of measurements, from bubble coalescence to bubble – particle adhesion force-measurements. We describe here the main parts of the apparatus and its basic features. As our study is at an early stage, we report only a few results from a preliminary series of measurements on bubble coalescence in aquatic solutions of humic substances.
2. Experimental We have designed and constructed a novel apparatus to execute a wide range of interparticulate interactions, such as bubble – bubble and bubble – solid particle interactions. It is based on an earlier device developed in our laboratory to measure the forces between particles and liquid bridges in binderinduced agglomeration (Fairbrother, 1998; Fairbrother and Simons, 1998; Simons and Fairbrother, 2000). A schematic representation of the apparatus is shown in Fig. 1. It comprises the Micro-Force Balance, where the interparticulate interactions take place, a complex visual system, and a computer through which control of the electronic devices is carried out. The Micro-Force Balance is shown in Fig. 2. It is a versatile construction, permitting both bubble coalescence as well as force-measurements between bubbles
Fig. 1. A schematic representation of the experimental apparatus.
M.T. Spyridopoulos, S.J.R. Simons / Int. J. Miner. Process. 74 (2004) 115–120
117
Fig. 2. Schematic representation of the Micro-Force Balance, where particulate interactions take place.
and particles to occur. It has been built around an aluminium-alloy Femto bench, on which the rest of the subsystems are mounted. At the centre of the vertical bench there is an optical-clear glass cell, 20 20 30 mm (width length height), which contains the liquid where the interactions take place. A three-axis micromanipulator (F 1 mm displacement in each axis) is located above the cell; it is used either to hold directly the upper micropipette, at which tip one of the bubbles or a particle is held, or the forcemeasurement device. On the right of the cell, there is a piezo-driven branch, which holds a second micropipette. The piezo-driven branch can move the micropipette in very small steps, down to 0.05 Am. A visual system monitors and records the interactions in the cell. The micropipette screwed onto the piezo-driven branch has one of its edges immersed in the liquidfilled glass cell. The other edge is fitted into a sealed rubber tubing. A bubble is formed at the tip of this micropipette by careful compression of the air in the rubber tubing using a system of two screw clamps, one for coarse and the other for fine squeezing. The size of the bubble depends on the bore of the micropipette. It can be as low as 50 Am, a value comparable with bubble diameters used in drinking-water flotation. By careful handling of the micromanipulator, the other micropipette ‘‘peels’’ the bubble out from the tip
of the other micropipette and keeps it at its edge. Squeezing further the air inside the rubber tubing, a second bubble is formed, making a system of two bubbles. The force-measurement device consists of two stainless steel sheets connected vertically to rigid blocks (see Fig. 3). Any force between the particulates is transferred to the left-hand micropipette, which moves the lower block of the flexure-strip assembly. The latter carries the armature of an LVDT, which measures the displacement (resolution better than 0.1 Am), caused by the bending of the sheets. This depends on the force constant of the sheets which, for a 50 Am thickness, is about 20 N/m. Lower force constants can
Fig. 3. The experimental apparatus fitted with the force-measurement device.
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M.T. Spyridopoulos, S.J.R. Simons / Int. J. Miner. Process. 74 (2004) 115–120
be achieved, either by employing thinner strips, or by making holes at the centre of the sheets. The visual system is comprised of two cameras and their respective displays. A high-speed camera, PHOTRON FASTCAM Super 10 K, capable of recording images between 30 and 3000 frames per second (fps) was purchased from Roper Scientific MASD (California, USA). This camera is used for capturing and recording the events during the experiments. It has its own processor and control unit, and can record images in its built-in RAM memory. The images are transferred to the computer via a SCSI-2 interface card, where they are analysed with an image analysis software, Aphelion, from Amerinex Applied Imaging (Massachusetts, USA). A second, CCD, camera was obtained from Sony (Tokyo, Japan) and is placed at a right angle to the high-speed camera. Each camera is coupled to a single-tube microscope (Edmund Scientific, York, UK) fitted with a plan achromatic objective lens from Olympus Optical (Tokyo, Japan). The mechanism of the microscope permits the movement of the cameras in two dimensions. A M1000 illuminator coupled with a liquid light guide (Stocker & Yale, Salem, USA) provides the necessary illumination. The positioning of the two bubbles is achieved by using the three-axis micromanipulator. The separation between the two bubbles is controlled by two means. Either a coarse adjustment can be made using the micromanipulator, or fine separation control (up to 90 Am) can be obtained by moving the branch attached to the piezoelectric tube (Elliot Scientific, Herts, UK). Due to the inherent hysteresis of piezo tubes, a displacement sensor, LVDT (RDP Electronic, Wolverhampton, UK), is mounted in parallel with the piezo tube, and this follows and records the exact displacement. Instrument control and data acquisition is carried out via a computer through a 16-bit multifunction board (Data Translation, Massachusetts, USA), which sends and receives signals from a user panel, created in the VEE-Pro graphical programming environment (Agilent Technologies, USA).
Measurements of the surface tension of the aquatic solutions were carried out with a Kru¨ss K-12 tensiometer (Hamburg, Germany), which makes use of the Wilhelmy-plate method. The surface tension of the liquid was calculated by the average of the measurements of five samples from the liquid, and taking at least a hundred measurements from each sample. All the measurements were conducted in room temperature, 20 –25 jC. The diagram of the surface tension of the humic acid under consideration is shown in Fig. 4. As it is shown, the surface tension of the humic acid varies with the concentration and time. Only after 1 h, the surface tension dropped below 65 mN/m, when the concentration of the humic acid was 500 mg/l. The amount of the decrease of surface tension is, roughly speaking, in agreement with data found in literature (Anderson et al., 1995); however, it reveals the strong dependence of the surface tension of the humic acid on time. In order to evaluate the effect of the humic acid on bubble coalescence, the results taken by executing the measurements in AnalaR water are compared with those of the aquatic solutions of the humic acid. Two approaching rates were tried: one very fast and one very slow. Also, four different concentrations of humic acid were tested: 2, 4, 8 and 20 mg/l. When the lower bubble approached the upper bubble rapidly, the coalescence of the bubbles in water, as well as in a solution of humic acid, was almost immediate. Grabbing images at 2000 fps, and comparing the images before and after coalescence,
3. Results We have conducted bubble coalescence in AnalaR water (BDH, Dorset, UK) and in aquatic solutions of humic acid (Aldrich Chemie, Germany).
Fig. 4. Surface tension of aquatic solution of humic acid sodium salt (pH: 5.5 – 6.0).
M.T. Spyridopoulos, S.J.R. Simons / Int. J. Miner. Process. 74 (2004) 115–120
119
Fig. 5. Bubble coalescence in AnalaR water. Upper bubble diameter: 700 Am; lower bubble diameter: 620 Am. From left to right, the images depict the coalescence process: the lower bubble approaches the upper rapidly, the two bubbles come into contact and coalesce immediately after. The time interval between each image is 0.5 ms (recording rate: 2000 fps, resolution: 256 120 pixels).
the coalescence time was found to be around 1 ms (see Fig. 5). When the lower bubble approached the upper bubble at a rather slow rate, 4.0 – 4.5 Am/s, the bubbles that interacted in water coalesced after 0.4 –0.5 s. On the other hand, the bubbles that interacted in all the solutions of humic acid did not coalesce in a high percentage of cases. Only a few pairs lead to coalescence, with a high number of pairs remaining attached and stable throughout the experiment. However, they did coalesce whenever a sudden movement occurred between them.
4. Discussion and conclusions A novel apparatus has been designed and constructed in order to study particulate interactions. The apparatus is capable of conducting bubble coalescence as well as bubble – particle adhesion force-measurements, due to its sophisticated visual system and the force-measurement device. We carried out a series of bubble coalescence experiments in water and in aquatic solutions of humic substances. We found that the humic acid employed in our experiments can prevent bubble coalescence when the approach rate between the two bubbles is in the order of a few microns/s, even at low concentrations, e.g. 2 mg/l. However, fast collision of bubbles will lead to immediate coalescence, for concentrations as high as 25 mg/l. This can be explained from the low decrease of surface tension of the solution for the first few minutes of a typical experiment. Thus, the surface elasticity of the liquid film was negligible and as a consequence, the liquid film between the bubbles ruptured easily. In the near future, we are going to execute further measurements of bubble coalescence in aquatic solutions of different humic acids, in order to have a clear
picture of the influence of these substances on the froth behaviour. Furthermore, we are planning to carry out complex experiments, incorporating bubble –particles (adhesion force-measurements) and bubble – particle – bubble (bubble coalescence in the presence of particles of different surface chemistry) systems.
Acknowledgements The authors would like to thank Dr. Jan Cilliers and Dr. Stephen Neethling (UMIST, UK) for useful discussions and Dr. Leonard Fisher (University of Bristol, UK) for his help in the experimental part of this research. This work was sponsored by the Engineering and Physical Sciences Research Council, Grant GR/M98647, under the Water Infrastructure and Treatment Engineering (WITE) programme, in collaboration with Thames Water, UK.
References Anderson, M.A., Hung, A., Mills, D., Scott, M.S., 1995. Factors affecting the surface tension of soil solutions of humic acids. Soil Science 160 (2), 111 – 116. Beckett, R., 1990. The surface chemistry of humic substances. In: Beckett, R. (Ed.), Surface and Colloid Chemistry in Natural Waters and Water Treatment. Plenum, New York, pp. 3 – 20. Chaudhari, R.V., Hofmann, H., 1994. Coalescence of gas bubbles in liquids. Reviews in Chemical Engineering 10 (2), 131 – 190. Christenson, H.K., Yaminsky, V.V., 1995. Solute effects on bubble coalescence. Journal of Physical Chemistry 99, 10420. Dippenaar, A., 1982. The destabilization of froth by solids: I. The mechanisms of film rupture. International Journal of Mineral Processing 9, 1 – 14. Fairbrother, R.J., 1998. A Microscopic Investigation of Particle – Particle Interactions in the Presence of Liquid Binders in Relation to the Mechanisms of ‘‘Wet’’ Agglomeration Processes. PhD thesis, University College London, UK. Fairbrother, R.J., Simons, S.J.R., 1998. Modelling of binder-in-
120
M.T. Spyridopoulos, S.J.R. Simons / Int. J. Miner. Process. 74 (2004) 115–120
duced agglomeration. Particle and Particle Systems Characterization 15, 16 – 20. Oolman, T.O., Blanch, H.W., 1986. Bubble coalescence in stagnant liquids. Chemical Engineering Communications 43, 237 – 261. Ross, V.E., 1991. An investigation of sub-processes in equilibrium froths: 1. The mechanisms of detachment and drainage. International Journal of Mineral Processing 31 (1 – 2), 37 – 50. Scheludko, A., Nikolov, A.D., 1975. Measurement of surface ten-
sion by pulling a sphere from a liquid. Colloid & Polymer Science 253, 396 – 403. Scheludko, A., Toshev, B.V., Bojadjiev, D.T., 1976. Attachment of particles to a liquid surface (capillary theory of flotation). Journal of the Chemical Society. Faraday transactions I 72, 2815 – 2828. Simons, S.J.R., Fairbrother, R.J., 2000. Direct observations of liquid binder – particle interactions, the role of wetting behaviour in agglomerate growth. Powder Technology 110, 44 – 58.
Particulate interactions in water flotation froths: a micro-mechanistic approach M.T. Spyridopoulos, S.J.R. Simons * Colloid and Surface Engineering Group, Department of Chemical Engineering, University College London, Torrington Place, London WC1E 7JE, UK Received 20 July 2001; received in revised form 10 October 2002; accepted 19 September 2003
Abstract We have constructed a novel apparatus to study fundamental interactions between bubbles and particles in drinking-water flotation froths. The high specifications of the apparatus and its modular construction permit experiments on bubble coalescence and bubble – particle adhesion forces to be carried out. We give a description of the apparatus, along with information on the techniques for its use. We observed the effect of a basic constituent of natural waters, humic acid, on bubble coalescence. Intense collision resulted in immediate bubble coalescence (1 ms), whereas less intense collisions prevented, in many cases, bubble coalescence, even at low-concentrated solutions. D 2003 Elsevier B.V. All rights reserved. Keywords: flotation; froth; apparatus; bubble coalescence; adhesion force
1. Introduction The efficiency of the flotation process in drinkingwater purification plants depends at a high percentage on the same factors that influence mineral flotation. Capture of the solid particles by the rising bubbles, adequate particle –bubble attachment and stability of the froth are only a few of the most crucial parameters. As a consequence, bubble –bubble and bubble – particle interactions have attracted the interest of many researchers throughout the years. However, further investigation of these interactions in the froth layer
* Corresponding author. Tel.: +44-207-679-3805; fax: +44-207383-2348. E-mail address: [email protected] (S.J.R. Simons). 0301-7516/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.minpro.2003.09.009
of the flotation tank are needed, for better understanding of the process and more efficient application of the method (Ross, 1991). In the froth layer, floating particles can become detached from the bubbles and go back to the bulk liquid. Firstly, due to bubble coalescence the area available for holding particles is reduced, leading the particles back to the bulk. Secondly, particles can be dislodged from the bubbles when shear forces are imposed on them, especially at the bulk – froth interface. Bubble coalescence happens very fast in pure liquids (Oolman and Blanch, 1986). However, it can be delayed or prohibited whenever the liquid contains even small amounts of impurities or surfactants (Chaudhari and Hofmann, 1994). In many cases, the presence of electrolytes has also been found to affect
116
M.T. Spyridopoulos, S.J.R. Simons / Int. J. Miner. Process. 74 (2004) 115–120
bubble coalescence (Christenson and Yaminsky, 1995). Some studies have shown that the presence of the solids in the froth can inhibit or provoke bubble coalescence (Dippenaar, 1982). After a solid particle encounters an air bubble and attaches to it, a three-phase contact line is formed. The solid particle will remain attached on the bubble if the capillary force acting on it overcomes the detaching forces. For a spherical particle, the capillary force, Fc, is given by (Scheludko et al., 1976): Fc ¼ 2prp csina sinðh aÞ
ð1Þ
with rp the radius of the particle, c the surface tension of the liquid, a the angle that defines the position of the particle with respect to the undisturbed liquid surface, and h the contact angle of the liquid on the particle surface. The adhesion is the maximum capillary force, given by (Scheludko and Nikolov, 1975): ha FAd ¼ 2prp ccos2 ð2Þ 2 where ha is the advancing contact angle. Froths in drinking-water treatment flotation tanks contain many constituents that can influence both bubble coalescence and bubble – particle adhesion force. Indeed, chemicals are used to promote flocculation and coagulation of the solids in natural water, which is mainly organic matter. Electrolytes, particles, humic substances (natural organic matter polyelectrolytes that some researchers have found can act as natural surfactants (Beckett, 1990)) are all present in
the froth and it is their effects that we are attempting to investigate. We have designed and constructed a novel apparatus to study the basics interactions in the froth. The apparatus is capable of executing a wide range of measurements, from bubble coalescence to bubble – particle adhesion force-measurements. We describe here the main parts of the apparatus and its basic features. As our study is at an early stage, we report only a few results from a preliminary series of measurements on bubble coalescence in aquatic solutions of humic substances.
2. Experimental We have designed and constructed a novel apparatus to execute a wide range of interparticulate interactions, such as bubble – bubble and bubble – solid particle interactions. It is based on an earlier device developed in our laboratory to measure the forces between particles and liquid bridges in binderinduced agglomeration (Fairbrother, 1998; Fairbrother and Simons, 1998; Simons and Fairbrother, 2000). A schematic representation of the apparatus is shown in Fig. 1. It comprises the Micro-Force Balance, where the interparticulate interactions take place, a complex visual system, and a computer through which control of the electronic devices is carried out. The Micro-Force Balance is shown in Fig. 2. It is a versatile construction, permitting both bubble coalescence as well as force-measurements between bubbles
Fig. 1. A schematic representation of the experimental apparatus.
M.T. Spyridopoulos, S.J.R. Simons / Int. J. Miner. Process. 74 (2004) 115–120
117
Fig. 2. Schematic representation of the Micro-Force Balance, where particulate interactions take place.
and particles to occur. It has been built around an aluminium-alloy Femto bench, on which the rest of the subsystems are mounted. At the centre of the vertical bench there is an optical-clear glass cell, 20 20 30 mm (width length height), which contains the liquid where the interactions take place. A three-axis micromanipulator (F 1 mm displacement in each axis) is located above the cell; it is used either to hold directly the upper micropipette, at which tip one of the bubbles or a particle is held, or the forcemeasurement device. On the right of the cell, there is a piezo-driven branch, which holds a second micropipette. The piezo-driven branch can move the micropipette in very small steps, down to 0.05 Am. A visual system monitors and records the interactions in the cell. The micropipette screwed onto the piezo-driven branch has one of its edges immersed in the liquidfilled glass cell. The other edge is fitted into a sealed rubber tubing. A bubble is formed at the tip of this micropipette by careful compression of the air in the rubber tubing using a system of two screw clamps, one for coarse and the other for fine squeezing. The size of the bubble depends on the bore of the micropipette. It can be as low as 50 Am, a value comparable with bubble diameters used in drinking-water flotation. By careful handling of the micromanipulator, the other micropipette ‘‘peels’’ the bubble out from the tip
of the other micropipette and keeps it at its edge. Squeezing further the air inside the rubber tubing, a second bubble is formed, making a system of two bubbles. The force-measurement device consists of two stainless steel sheets connected vertically to rigid blocks (see Fig. 3). Any force between the particulates is transferred to the left-hand micropipette, which moves the lower block of the flexure-strip assembly. The latter carries the armature of an LVDT, which measures the displacement (resolution better than 0.1 Am), caused by the bending of the sheets. This depends on the force constant of the sheets which, for a 50 Am thickness, is about 20 N/m. Lower force constants can
Fig. 3. The experimental apparatus fitted with the force-measurement device.
118
M.T. Spyridopoulos, S.J.R. Simons / Int. J. Miner. Process. 74 (2004) 115–120
be achieved, either by employing thinner strips, or by making holes at the centre of the sheets. The visual system is comprised of two cameras and their respective displays. A high-speed camera, PHOTRON FASTCAM Super 10 K, capable of recording images between 30 and 3000 frames per second (fps) was purchased from Roper Scientific MASD (California, USA). This camera is used for capturing and recording the events during the experiments. It has its own processor and control unit, and can record images in its built-in RAM memory. The images are transferred to the computer via a SCSI-2 interface card, where they are analysed with an image analysis software, Aphelion, from Amerinex Applied Imaging (Massachusetts, USA). A second, CCD, camera was obtained from Sony (Tokyo, Japan) and is placed at a right angle to the high-speed camera. Each camera is coupled to a single-tube microscope (Edmund Scientific, York, UK) fitted with a plan achromatic objective lens from Olympus Optical (Tokyo, Japan). The mechanism of the microscope permits the movement of the cameras in two dimensions. A M1000 illuminator coupled with a liquid light guide (Stocker & Yale, Salem, USA) provides the necessary illumination. The positioning of the two bubbles is achieved by using the three-axis micromanipulator. The separation between the two bubbles is controlled by two means. Either a coarse adjustment can be made using the micromanipulator, or fine separation control (up to 90 Am) can be obtained by moving the branch attached to the piezoelectric tube (Elliot Scientific, Herts, UK). Due to the inherent hysteresis of piezo tubes, a displacement sensor, LVDT (RDP Electronic, Wolverhampton, UK), is mounted in parallel with the piezo tube, and this follows and records the exact displacement. Instrument control and data acquisition is carried out via a computer through a 16-bit multifunction board (Data Translation, Massachusetts, USA), which sends and receives signals from a user panel, created in the VEE-Pro graphical programming environment (Agilent Technologies, USA).
Measurements of the surface tension of the aquatic solutions were carried out with a Kru¨ss K-12 tensiometer (Hamburg, Germany), which makes use of the Wilhelmy-plate method. The surface tension of the liquid was calculated by the average of the measurements of five samples from the liquid, and taking at least a hundred measurements from each sample. All the measurements were conducted in room temperature, 20 –25 jC. The diagram of the surface tension of the humic acid under consideration is shown in Fig. 4. As it is shown, the surface tension of the humic acid varies with the concentration and time. Only after 1 h, the surface tension dropped below 65 mN/m, when the concentration of the humic acid was 500 mg/l. The amount of the decrease of surface tension is, roughly speaking, in agreement with data found in literature (Anderson et al., 1995); however, it reveals the strong dependence of the surface tension of the humic acid on time. In order to evaluate the effect of the humic acid on bubble coalescence, the results taken by executing the measurements in AnalaR water are compared with those of the aquatic solutions of the humic acid. Two approaching rates were tried: one very fast and one very slow. Also, four different concentrations of humic acid were tested: 2, 4, 8 and 20 mg/l. When the lower bubble approached the upper bubble rapidly, the coalescence of the bubbles in water, as well as in a solution of humic acid, was almost immediate. Grabbing images at 2000 fps, and comparing the images before and after coalescence,
3. Results We have conducted bubble coalescence in AnalaR water (BDH, Dorset, UK) and in aquatic solutions of humic acid (Aldrich Chemie, Germany).
Fig. 4. Surface tension of aquatic solution of humic acid sodium salt (pH: 5.5 – 6.0).
M.T. Spyridopoulos, S.J.R. Simons / Int. J. Miner. Process. 74 (2004) 115–120
119
Fig. 5. Bubble coalescence in AnalaR water. Upper bubble diameter: 700 Am; lower bubble diameter: 620 Am. From left to right, the images depict the coalescence process: the lower bubble approaches the upper rapidly, the two bubbles come into contact and coalesce immediately after. The time interval between each image is 0.5 ms (recording rate: 2000 fps, resolution: 256 120 pixels).
the coalescence time was found to be around 1 ms (see Fig. 5). When the lower bubble approached the upper bubble at a rather slow rate, 4.0 – 4.5 Am/s, the bubbles that interacted in water coalesced after 0.4 –0.5 s. On the other hand, the bubbles that interacted in all the solutions of humic acid did not coalesce in a high percentage of cases. Only a few pairs lead to coalescence, with a high number of pairs remaining attached and stable throughout the experiment. However, they did coalesce whenever a sudden movement occurred between them.
4. Discussion and conclusions A novel apparatus has been designed and constructed in order to study particulate interactions. The apparatus is capable of conducting bubble coalescence as well as bubble – particle adhesion force-measurements, due to its sophisticated visual system and the force-measurement device. We carried out a series of bubble coalescence experiments in water and in aquatic solutions of humic substances. We found that the humic acid employed in our experiments can prevent bubble coalescence when the approach rate between the two bubbles is in the order of a few microns/s, even at low concentrations, e.g. 2 mg/l. However, fast collision of bubbles will lead to immediate coalescence, for concentrations as high as 25 mg/l. This can be explained from the low decrease of surface tension of the solution for the first few minutes of a typical experiment. Thus, the surface elasticity of the liquid film was negligible and as a consequence, the liquid film between the bubbles ruptured easily. In the near future, we are going to execute further measurements of bubble coalescence in aquatic solutions of different humic acids, in order to have a clear
picture of the influence of these substances on the froth behaviour. Furthermore, we are planning to carry out complex experiments, incorporating bubble –particles (adhesion force-measurements) and bubble – particle – bubble (bubble coalescence in the presence of particles of different surface chemistry) systems.
Acknowledgements The authors would like to thank Dr. Jan Cilliers and Dr. Stephen Neethling (UMIST, UK) for useful discussions and Dr. Leonard Fisher (University of Bristol, UK) for his help in the experimental part of this research. This work was sponsored by the Engineering and Physical Sciences Research Council, Grant GR/M98647, under the Water Infrastructure and Treatment Engineering (WITE) programme, in collaboration with Thames Water, UK.
References Anderson, M.A., Hung, A., Mills, D., Scott, M.S., 1995. Factors affecting the surface tension of soil solutions of humic acids. Soil Science 160 (2), 111 – 116. Beckett, R., 1990. The surface chemistry of humic substances. In: Beckett, R. (Ed.), Surface and Colloid Chemistry in Natural Waters and Water Treatment. Plenum, New York, pp. 3 – 20. Chaudhari, R.V., Hofmann, H., 1994. Coalescence of gas bubbles in liquids. Reviews in Chemical Engineering 10 (2), 131 – 190. Christenson, H.K., Yaminsky, V.V., 1995. Solute effects on bubble coalescence. Journal of Physical Chemistry 99, 10420. Dippenaar, A., 1982. The destabilization of froth by solids: I. The mechanisms of film rupture. International Journal of Mineral Processing 9, 1 – 14. Fairbrother, R.J., 1998. A Microscopic Investigation of Particle – Particle Interactions in the Presence of Liquid Binders in Relation to the Mechanisms of ‘‘Wet’’ Agglomeration Processes. PhD thesis, University College London, UK. Fairbrother, R.J., Simons, S.J.R., 1998. Modelling of binder-in-
120
M.T. Spyridopoulos, S.J.R. Simons / Int. J. Miner. Process. 74 (2004) 115–120
duced agglomeration. Particle and Particle Systems Characterization 15, 16 – 20. Oolman, T.O., Blanch, H.W., 1986. Bubble coalescence in stagnant liquids. Chemical Engineering Communications 43, 237 – 261. Ross, V.E., 1991. An investigation of sub-processes in equilibrium froths: 1. The mechanisms of detachment and drainage. International Journal of Mineral Processing 31 (1 – 2), 37 – 50. Scheludko, A., Nikolov, A.D., 1975. Measurement of surface ten-
sion by pulling a sphere from a liquid. Colloid & Polymer Science 253, 396 – 403. Scheludko, A., Toshev, B.V., Bojadjiev, D.T., 1976. Attachment of particles to a liquid surface (capillary theory of flotation). Journal of the Chemical Society. Faraday transactions I 72, 2815 – 2828. Simons, S.J.R., Fairbrother, R.J., 2000. Direct observations of liquid binder – particle interactions, the role of wetting behaviour in agglomerate growth. Powder Technology 110, 44 – 58.