Numerical and experimental study of a hydrodynamic cavitation tube
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I. INTRODUCTION Flotation is the most versatile physical separation process for minerals and is being rapidly extended to other separation and environment related problems. Flotation of ions and precipitates has been used to remove toxic species and recover metals from industrial effluents.[1–7] Paper recycling is a growing practice in which flotation is the preferred technology for ink removal.[8] A low flotation rate of fine particles is encountered in many of these applications, which can be attributed to low particle inertia. It has been established that below a certain size, particles follow the liquid streamlines around a bubble, resulting in a low collision frequency. To improve fine particle flotation, theoretical analysis suggests the following:[9–13] (a) the use of small bubbles to improve the particle-bubble collision frequency; and (b) the adoption of high turbulence or high shear conditions to increase the frequency of particle-bubble collision and the kinetic energy for rupturing the intervening liquid films between a particle and a bubble. Exploiting small bubbles and high turbulence has been the focus of several recent developments in flotation machinery. A survey of flotation devices, including the Jameson cell, pneumatic cell, contact cell, Microcell, etc.[13–17] showed that the flotation of fine particles can be improved significantly by introducing high shear or intense mixing in the cell, which has been attributed to the production of fine bubbles and high energy dissipation.[18] The actual mechanism of generating the bubble may also be a factor. Bubbles may form by cavitation, defined as the generation of gas nuclei, under the high energy dissipation conditions encountered in many of these new flotation cells (and around the impeller in conventional mechanical machines). Further, cavitation could promote in situ bubble formation on hydrophobic particles (i.e., nucleation), thus eliminating the collision step altogether. In a recent study, we have demonstrated[19] bubble formation by cavitation when forcing a liquid flowing
H. HU, Research Assistant, and J.A. FINCH, Professor, are with the Department of Mining and Metallurgical Engineering, McGill University, Montreal, PQ, Canada H3A 2A7. Z. ZHOU, Postdoctoral Student, and Z. XU, Associate Professor, are with the Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB, Canada T6G 2G6. Manuscript submitted July 11, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS B
through a nozzle at velocities in excess of 5 to 15 m/s, depending on the experimental conditions (importantly, the amount of dissolved gas). This velocity range corresponds to the feed stream velocities in several of the aforementioned new flotation cells. Experimentally, we show that by incorporating a cavitation tube in the feed line, flotation recovery of fine silica (d50 5 1.6 mm) and ZnS precipitates (d50 5 1.2 mm) can be improved significantly. These results suggest that bubbles formed by hydrodynamic cavitation play a role in increasing flotation k
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