Particle-bubble encounter

Reay also considered the viscous interaction between a bubble and particle as the particle approaches the bubble. In effect, he allowed the particle to rotate as it gets close to the bubble. This model predicts, as G becomes very small, that particles will attach to bubbles as a result of hydrodynamic forces alone (for small particles and large bubbles). In effect, the vacuum induced in the wake behind a rising bubble can trap particles in spite of mtcrfacial repulsion due to electrostatic effects. This model suggests that for flotation of oily water another mechanism (hydrodynamic capture), in addition to collision, may contribute to the overall removal rate. Evidence is presented in this paper that hydrodynamic capture is an operative mechanism tor the bubble and particle sizes encountered in flotation of oil drops using bubbles from 0.2 to 0.7 mm in diameter. 

Two main operational variables that differentiate the flotation of finely dispersed coUoids and precipitates in water treatment from the flotation of minerals is the need for quiescent pulp conditions (low turbulence) and the need for very fine bubble sizes in the former. This is accompHshed by the use of electroflotation and dissolved air flotation instead of mechanically generated bubbles which is common in mineral flotation practice. Electroflotation is a technique where fine gas bubbles (hydrogen and oxygen) are generated in the pulp by the appHcation of electricity to electrodes. These very fine bubbles are more suited to the flotation of very fine particles encountered in water treatment. Its industrial usage is not widespread. Dissolved air flotation is similar to vacuum flotation. Air-saturated slurries are subjected to vacuum for the generation of bubbles. The process finds limited appHcation in water treatment and in paper pulp effluent purification. The need to mn it batchwise renders it less versatile. 

The flow patterns for single phase, Newtonian and non-Newtonian liquids in tanks agitated by various types of impeller have been repotted in the literature.1 3 27 38 39) The experimental techniques which have been employed include the introduction of tracer liquids, neutrally buoyant particles or hydrogen bubbles, and measurement of local velocities by means of Pitot tubes, laser-doppler anemometers, and so on. The salient features of the flow patterns encountered with propellers and disc turbines are shown in Figures 7.9 and 7.10. 

Certain three-dimensional electrodes, also known as slurry or fluidized-bed electrodes, are sometimes used as well in order to have a strongly enhanced working surface area. Electrodes of this type consist of fine particles of the electrode material (metal, oxide, carbon, or other) kept in suspension in the electrolyte solution by intense mixing or gas bubbling. A certain potential difference is applied to the system between an inert feeder elecnode and an auxiliary electrode that are immersed into the suspension. By charge transfer, the particles of electrode material constantly hitting the feeder electrode acquire its potential (fully or at least in part), so that a desired electrochemical reaction may occur at their surface. In this reaction, the particles lose their charge but reacquire it in subsequent encounters with the feeder electrode. 

The gas fluidization and bubble characteristics have been defined in the literature (4-10). They are affected by the properties of the materials being fluidized and the design characteristics of the equipment being used, which vary with equipment vendors. Only the fundamentals of these phenomena will be described because of this dependence. Figure 1 illustrates typical fluidization characteristics for substrates of various particle sizes and densities encountered in air suspension processing. 

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