Measurement and modelling of gas dispersion characteristics in a mechanical flotation cell

The gas dispersion characteristics in mechanical flotation cells have a significant effect on the overall flotation performance. Three major properties that can be measured in characterising the gas dispersion in a flotation cell are bubble size, gas hold-up, and superficial gas velocity. Another property that is equally important in flotation is bubble surface area flux which is calculated from bubble size and superficial gas velocity. Despite the importance of gas dispersion in flotation, not much work has been reported previously in this area. Moreover, the study of gas dispersion in flotation has typically considered only a few points in a flotation cell and the average values were assumed to represent the gas dispersion characteristics in the entire volume of the cell. It is known however, that the gas dispersion characteristics are not uniformly distributed in a mechanical flotation cell. This thesis seeks to understand better the gas dispersion characteristics in mechanical flotation cells with a view to optimisation, modelling, cell comparison and selection. The main aim of this thesis was to measure comprehensively the gas dispersion characteristics in a mechanical flotation cell, analyse the behaviour in the entire volume of the cell, and develop a methodology for modelling the gas dispersion characteristics in the whole volume of the cell as well as develop the models themselves. For this purpose, a fully instrumented 3 m³ glass rectangular flotation cell at the Julius Kruttschnitt Mineral Research Centre (JKMRC) at the University of Queensland was used. The cell was fitted with a Dorr-Oliver impeller-stator mechanism and was provided with facilities to change impeller speed and gas flow rate. Sensors to measure the gas dispersion characteristics were also acquired and modified. This cell could only be operated in a two-phase (air-water) system but the opportunity was taken to make some comparative measurements in an operating plant in a three-phase slurry to compare the gas dispersion characteristics in two and three-phase systems. The comprehensive measurements of the gas dispersion characteristics (i.e. bubble size, gas hold-up, and superficial gas velocity, with subsequent calculation of bubble surface area flux) throughout the entire volume of the 3 m³ rectangular flotation cell show that the properties do vary with distance from the impeller, the cell bottom, and the walls. Statistical analysis to test the homogeneity of the properties in the cell confirmed that the differences (variation with distance) were real. It was found that the gas dispersion was poor in the corners of a rectangular flotation cell. These corners are referred to as dead zones. It can be interpreted that in these zones, flotation is less effective compared to other zones in a cell. The results of mapping the gas dispersion characteristics throughout the entire volume of the flotation cell were used to determine the best location to measure gas dispersion characteristics in a flotation cell in order to represent the overall values. It was established that this location in a flotation cell is about halfway between the impeller and the wall, and halfway between the bottom of the flotation cell and the pulp-froth interface. Statistical analysis also showed that there is quarter symmetry, i.e. there is no significant difference between equivalent positions in different quarters in a horizontal plane. In any future work, therefore, measurements of the gas dispersion characteristics need only be made in one quarter, and symmetry in the rest of the cell can be assumed. Following from the statistical analysis that established quarter symmetry in the 3 m³ glass rectangular flotation cell, a methodology to model gas dispersion characteristics in the entire volume of a rectangular mechanical flotation cell was developed, based on an experimental design known as CCRD (central composite rotatable design) which then modified. Using the methodology, models to predict bubble size, gas hold-up, superficial gas velocity and bubble surface area flux in the entire volume of a rectangular flotation cell were developed as a function of air flow rate and impeller speed. The validity of the models was tested using a predictive (cross) validation method, from where it was concluded that the models were valid. These models were then used to analyse the gas dispersion characteristics in detail in the flotation cell, as a function of flow rate, impeller speed, and location in the cell. Finally, a comparison of gas dispersion characteristics in two and three-phase systems in flotation cells was made. Comprehensive measurements of gas dispersion characteristics were performed in a three-phase slurry in an industrial OK 38 m³ rectangular flotation cell at the PT Freeport Indonesia concentrator, and the results were compared to those measured previously in the two-phase system in the 3 m³ rectangular cell. It was found that the profiles of gas dispersion were generally similar in both cells but the magnitude of the gas dispersion properties differed between the two systems. The presence of solid particles had greater effect on the bubble size than on gas hold-up and superficial gas velocity.