The relationship between operating parameters, cell hydrodynamics, flotation response and scale-up of flotation rates has been explored using three geometrically similar Rushton turbine flotation cells with volumes of 2.25, 10 and 50dm³. Mean energy dissipation values measured using Laser Doppler Velocimetry (LDV) and a torque turntable method were in good agreement. As the cell volume was increased, the mean energy dissipation was proportional to N³D, rather than N³D² as may be expected based on dimensional analysis. Possible reasons for this difference are discussed. Aeration resulted in a slight increase in mean energy dissipation. Bubble diameters were measured using a University of Cape Town bubble size analyser to determine the frother concentration at which a constant bubble diameter was achieved for all operating conditions and cell volumes. The critical frother concentration required to achieve this was 20 ppm MIBC. / The mean bubble velocity was estimated by determining the time required to achieve steady state gas holdup in the top part of the cell after commencing gas sparging. For a constant mean bubble diameter, the bubble velocity increased with increasing superficial gas velocity. As the energy dissipation was increased for a given superficial gas velocity, the bubble velocity decreased linearly until a critical energy dissipation was reached. Beyond this value, bubble velocity decreased only slightly. As the cell volume increased, the bubble velocity, at the same superficial gas velocity and energy dissipation, also increased. A series of flotation experiments were carried out in a 2.25 dm³ laboratory-scale Rushton turbine cell using hydrophobic quartz particles to determine the effect of cell hydrodynamics on the flotation rate constant. Flotation was performed at a constant bubble diameter over a range of superficial gas velocities and impeller rotational speeds. / The overall flotation rate constant increased linearly with increasing superficial gas velocity (and hence bubble surface area flux). The rate constant also increased linearly with increasing energy dissipation, until a maximum value was reached. A further increase in energy dissipation had little effect on the rate constant. The dependency of the rate constant on energy dissipation is a reflection of the size range and hydrophobicity of the particles used in this study. The flotation rate constant increased with increasing particle size, except at the highest energy dissipation value examined, for which the flotation rate of the larger particles reached a plateau and, in some cases, decreased. Good agreement was obtained between the experimental results and those predicted by a fundamental flotation model using experimentally measured values for mean energy dissipation and the Sauter mean bubble diameter. The bubble velocity was adjusted to obtain the best fit of the experimental data. / The inferred bubble velocity, based on the flotation model, was found to increase with increasing superficial gas velocity and decrease with increasing impeller rotational speed. While the inferred bubble velocities were significantly lower than experimentally measured bubble velocities, and, except at low superficial gas velocity values, significantly higher than the bubble swarm velocity calculated from gas holdup measurements, similar effects of impeller rotational speed and superficial gas velocity were observed in all cases. / In order to determine a set of scale-up criteria which would produce the same size-by-size flotation rate constants, a series of flotation experiments using hydrophobic quartz particles were conducted in three Rushton turbine flotation cells of volume 2.25, 10 and 50 dm³. The Sauter mean bubble diameter was held constant in all cases, leaving the superficial gas velocity and impeller rotational speed as the only operating parameters to be varied. In all cases, maintaining a constant bubble surface area flux was used as one criterion. This was achieved by keeping the superficial gas velocity constant as the cell volume was increased. Varying the impeller rotational speed to maintain a constant impeller tip velocity (and hence air flow number as suggested by Arbiter and Harris, 1969; and Schubert and Bischofberger, 1998), by keeping ND constant, resulted in decreasing flotation rate constants as the cell volume was increased. / Maintaining N³D² constant (as suggested by Schubert and Bischofberger, 1998), gave higher flotation rate constants than were obtained when the impeller tip velocity was held constant, but also resulted in decreasing rate constants as the cell volume was increased. Successful flotation scale-up was achieved by varying the impeller diameter so that N³D was constant over this cell volume range. This enabled the measured mean energy dissipation for the three cells to be held constant as the cell volume increased. / Thesis (PhDApSc(MineralsandMaterials))--University of South Australia, 2006.
| Identifer | oai:union.ndltd.org:ADTP/267290 |
| Creators | Newell, Ray. |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | copyright under review |
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