Performance and hydrodynamic characterisation of laboratory batch flotation cells

Abstract

The purpose of this study was to compare the Denver and Leeds laboratory flotation cells by evaluating their performance in the flotation of quartz of four sizes (−25 μm, +25–45 μm, +45–75 μm, and +75–106 μm). This study was conducted at the Materials Science and Metallurgical Engineering laboratory of the University of Pretoria. The impeller diameters of the Denver and Leeds cells were measured to be 0.07 m and 0.074 m, respectively. The impeller speeds of the cells were calibrated for identical flotation performance (assessed using quartz recoveries without interfering with the cell design), with the Denver cell set at 1200 rpm and the Leeds cell at 1400 rpm. The reagent regime was kept constant, using 25 g/t Flotigam EDA ether amine as a collector, no frother, and NaOH to modify the pH to 9.5. The air flow rate was kept constant at 2 L/min in each of the 3.5 L cells. Flotation kinetics tests were conducted at the optimal rotation speeds, and the results were similar. Both cells achieved similar quartz recoveries of over 70 % for the three +25 μm fractions, but only 15 % for the −25 μm fraction. An additional collector was required to improve the recovery of the −25 μm fraction significantly. These findings demonstrated the effect of particle size on flotation recovery, and the finer particles requiring more reagent due to their larger surface areas. The performance of these cells was further evaluated using dimensionless numbers and with a chemical tracer. The use of dimensionless numbers, such as Power and Reynolds numbers, allowed for a detailed analysis of the cells' hydrodynamics. Additionally, a chemical tracer (NaOH), was used to assess the mixing efficiency of the impellers. The Denver flotation cell exhibited superior performance compared to the Leeds cell. It managed to achieve higher recovery rates while consuming less power. This can be attributed to the effective design of its impeller and stator, which enabled it to overcome the resistance posed by the slurry, allowing it to operate at optimal levels that surpassed the capabilities of the Leeds cells. The performance of the Leeds cell was found to be inferior to that of the Denver cell, and this is attributed to several factors. One of the main reasons is the slightly larger bubble size of 3.5 mm in the Leeds cell, compared to the Denver cell's average of 2.5 mm. Therefore, the surface area available for particle attachment was still low for the Leeds cell, even at higher impeller speeds. Additionally, the power numbers for the Leeds cell were higher, averaging at 1.03 between 1000 iii and 1500 rpm in the presence of solids, while the Denver cell averaged 0.77 under the same conditions. This indicates that the Leeds cell requires more power to create the necessary flow. All this leads to deterioration in particle collection efficiency and an overall reduction in performance.