Electroflotation: its application to water treatment and mineral processing

Research Doctorate - Doctor of Philosophy (PhD)Flotation of particles of diameter less than 10 μm is important economically yet recovery is very poor in conventional flotation machines where the bubble diameter is typically greater than 600 μm. Many studies have reported that flotation recovery of fine particles increases with decreased bubble size. Electroflotation can create very fine hydrogen and oxygen bubbles and may be a viable option to recover very fine particles. This study aims to develop an increased understanding of the principles of electroflotation and to use this knowledge to float very fine particles. The interaction between the gas phase with the mineral surface may bring about changes in the surface properties of the mineral, which can be either beneficial or detrimental in improving the flotation recovery. To investigate this interaction, flotation recovery of silica between air and molecular hydrogen was performed in a laboratory Denver, type D12, flotation machine. For both gases, the pH of the suspension, gas flow rate, concentration of collector and frother, solids concentration, particle size and speed of impeller were kept constant. Almost identical recoveries were obtained for both gases, suggesting that gas composition played no significant role in silica flotation. There is wide variation in the reported measurements of bubble size in electroflotation, and uncertainty with the influence of electrode curvature, surface preparation and current density on bubble size have made it difficult to effectively design an efficient electroflotation system for fine particle recovery. Experiments were performed in a viewing cell that allowed direct visualization of hydrogen bubbles being generated and transported away from platinum wire electrodes of 90, 120 and 190 μm in diameter. It was found that the detached bubble diameter varied between 15-23 μm in diameter, and for each wire diameter, was little influenced by the applied current in the range 150-350 A/m². The measurements were consistent with those predicted from a simple force-balance analysis based on a H2-Pt-0.2M Na₂SO₄ contact angle of 0.18°. Interestingly, upon detachment the bubble size increased rapidly, recording up to an 8-fold increase in volume in the first few millimetres of rise, before approaching a steady state diameter of between 30-50 μm in the bulk. This increase in bubble size was found to be mostly due to the transfer of dissolve hydrogen into the growing bubble while moving through the electrolyte that was super-saturated with dissolved hydrogen. The equilibrium bulk diameter was found to be a function of the rate of hydrogen production, bubble nucleation rate, and dissolved gas concentration field. Consequently, it was concluded that in order to optimise electroflotation performance the cell geometry needed to be designed to optimise the contact between the supersaturated liquid and the rising bubble plume. By doing this, the volumetric flux of bubbles will be maximised leading to improved flotation performance. The influence of electrolyte flowrate past the electrode surface on resultant bubble size was also investigated. A peristaltic pump was used to a create a flow of electrolyte past 90 and 190 μm diameter platinum wire electrodes operating at a constant current density of 354 A/m². The superficial upward liquid velocity ranged from 1.5-7.1 mm/s. It was observed directly that the detachment diameter varied between 8-15 and 15-22 μm for the 90 and 190 μm diameter cathode wires, respectively. The corresponding bubble diameters in the bulk were found to be 14-31 and 30-43 μm, respectively. Both detachment and bulk bubble diameter decreased with increased superficial liquid velocity. Both bulk bubble size and electroflotation recovery are functions of the fraction of generated hydrogen that results in gas bubbles. Experimentally it was found that approximately 98 percent of the (theoretical) hydrogen produced by the electrolysis resulted in gas bubbles. This is a positive result, in that almost all of the electrical power is being converted to hydrogen (and oxygen) bubbles that can be used for flotation recovery. For a given current density, the rate of hydrogen gas production was largely independent of the concentration of the suspended solids. There was a very small increase in the hydrogen bubble production rate with the introduction of mechanical agitation, while the opposite trend was observed for the degassed electrolyte solution. Flotation of 3-15 μm diameter silica particles was carried out with electrolytically generated hydrogen bubbles with mean diameters of 30 and 40 μm. Fractional recoveries after two minutes of flotation were found to be 0.82, 0.90, 0.96 and 0.88 for the 3.1, 5.3, 12.3 and 14.7 μm diameter particles, respectively. The relatively high recoveries were directly attributed to the very small bubbles generated by the electrolysis process, which are known to increase flotation recovery for very fine particles. Finally, a flotation recovery model was developed which included a layered packing structure when estimating the fractional surface coverage of the bubble. Whilst it was not possible to measure surface coverage directly, the observed maximum plateau in flotation recovery appeared to occur when the bubble-particle aggregate projected area, based on a single layer particle packing, approached that of just the bubble. Moreover, flotation recovery was also found to be negatively influenced with a reduction in the bubble rise velocity due to attached particles