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Particle Bubble Interaction
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Schematic of a flotation system. The bubbles are usually much larger than the particle so that the water-air interface is almost planar at a length scale similar to the size of the particle. |
Flotation is a method to separete various kinds of solid particles from each other. It is of enormous importance to the mining industry where it is used for large scale processing of crushed ores. The desired mineral is separated from the gangue or non-mineral containing material. Originally the procedure was applied only to some sulfides and oxides (iorn oxides, rutile, quartz). Meanwhile many minerals such as gold, borax, pyrite, phosphate minerals, fluorite, calcite, apatite are separated by flotation. Another large application is the removal of unwanted material for water purification and to clean industrial waste products. The deinking of paper is a very similar process with growing importance. In flotation of ore first the material is crushed to under 0.1 mm particle size. The particles are mixed with water. This sol is called pulp. The pulp flows into a container and air bubbles are passed through. The mineral rich particles bind to the air bubbles by hydrophobic forces and are carried to the surface. A stable foam, also called froth, is formed. With the froth the mineral rich particles can be skimmed off and removed. |
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The contact angle plays a crucial role in flotation. Hydrophobic particles are most likely to attach to a bubble and be incorporated into the froth. Some minerals naturally have a hydrophobic surface and thus a high flotation efficacies. For other minerals surfactants are used to improve the separation. These are collectors, which adsorb selectively on the mineral and make its surface hydrophobic. Activators support the collectors. Depressants reduce the collectors' effect. Frothing agents increase the stability of the foam.
Due to the economic importance of this process, a detailed understanding of the underlying mechanisms of bubble particle interaction is desirable. Still today, much of the knowledge in this field is empirical and improvements strategies follow a trial and error approach. Using the colloidal probe technique, one can measure the interaction between single particles and an air bubble with high precision. For this purpose, a small air bubble is attached to the hydrophobic bottom of a liquid cell. Force-distance curves (link) between this bubble and a small particle attached to the end of an AFM cantilever can be measured. From this, one can learn about the forces contributing to the interaction and test the influence of different surfactants on the interaction. If the spherical particle is at least partly hydrophobic (contact angle > 0°), one can also deduce the contact angle of the single particle from the force distance curve. When the particle is still far from the bubble surface (1), no deflection of the cantilever is observed (equilibrium deflection). As soon as the particle touches the surface of the bubble, it is drawn into the bubble by the attractive interaction (2). Since the gradient of the attractive force is stronger than the spring constant of the cantilever, this will lead to a jump in of the particle and formation of a three phase contact line. At this point, the cantilever is bent downwards. When the particle moves further downwards, the cantilever will be bent upwards (3). Upon retraction, the particle reaches a point where the deflection of the cantilever becomes equal to the equilibrium deflection (4). At this point, the angle of the three phase contact is equal to the equilibrium contact angle of the particle. This angle can be calculated from the radius of the sphere and the jump in distance. Retracting further, the particle will first adhere to the bubble and finally detach (5). |
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| From such a force-distance-curve, the contact angle of the single particle can be calculated from simple geometric considerations. | |
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| The following figure show the measured contact angles for different micrometer sized spheres compared to the contact angles measured on bulk planar surfaces of the same materials. | |
| Contact: M. Kappl / H.J. Butt | |
