Investigation of complex multiphase flows by advanced optical methods at the example of the flotation process of fluorite
The flotation process is the most important process for the enrichment of valuable minerals. Although the process is now known for approximately 160 years, the amount of investigations on the micro processes is up to now still small. Technological developments in the recent years made optical measurement techniques such as Particle Image Velocimetry (PIV) and Shadowgraphy (SH) available for scientific research. Most of the investigations in the last decades had, however, a practical orientation aiming for an optimisation of process parameters in order to increase the economics. The number of investigations applying these optical methods to the flotation process is surprisingly low. Reasons for this are the high solids and bubble concentrations as well as the high turbulence during the flotation process. The aim of the present work is the investigation of the complex flow structures during the flotation process via PIV and SH. To facilitate the application of the optical methods, a reduced flotation system with lower bubble and solids concentrations is set up. For this purpose, a novel flotation column made from transparent poly(methyl methacrylate) is designed, which allows the formation of single bubble-particle heterocoagulates and simultaneously facilitates a full optical accessibility. Investigations are carried out both in the two-phase as well as the three- phase system. Investigations in the two-phase system focus on the analysis of the bubble behaviour in deionized water and in presence of surface active agents at various concentrations. The aim is to develop a relationship between the bubble characteristics and the induced liquid velocity around the rising bubble. A model three-phase system consisting of glass particles is the starting point for flotation related investigations. The influence of flotation process parameters, e.g. volumetric gas flow rate and particle size, on the flotation outcome in form of maximum recoveries Rmax and flotation kinetics with respect to order n of kinetics as well as flotation rate constant k is investigated. Furthermore, the hydrodynamic structure of the rising single bubble-particle heterocoagulates is revealed via PIV measurements. Afterwards, an industrial fluorite flotation system is investigated using a design of experiment in order to find the optimal process parameters to maximise Rmax, k, as well as the grade of the concentrate with respect to the calcium fluoride weight fraction w(CaF2). Lastly, a fluorescent fluorite mineral is tested for application in optical analysis. The aim is to distinguish valuable fluorescent fluorite mineral particles from gangue particles such as silicon dioxide and barite. The results show that both PIV and SH are applicable for the investigation of multiphase flows. The designed apparatus facilitates both, optical accessibility into the reduced flotation system as well as the possibility to determine important flotation parameters. An automated analysis procedure with a high accuracy has been developed to allow a fast analysis of the obtained image sequences. The reproducibility of consecutive bubbles has been found to be excellent, thus allowing a statistical investigation of different operational parameters on the process performance. The measurements in deionized water have shown a strong relationship between the volumetric gas flow rate and the induced liquid velocity. The surfactants changed the bubble characteristics significantly with increasing concentration towards a spherical form. Furthermore, the rising velocities decreased until a threshold has been reached. Although the rising velocities decreased, an increase in the induced liquid velocity as well as in the induced kinetic energy has been observed. Analysis of the bubble trajectories revealed an oscillating behaviour in horizontal x direction. With increasing surfactant concentration this oscillation frequency increases. Thus, the oscillation has been found to be responsible for this controversial relation. The investigation of the model flotation system has shown the influence of the different operational parameters on both maximum recovery and flotation kinetics. While the order of flotation is best described by the zeroth order model at a high initial solids concentration cp,0, a shift to first order kinetics has been found for lower cp,0. Optical investigations showed large differences in the bubble-particle heterocoagulate rising behaviour for particles with different wettability. The poor flotation performance of hydrophilic particles could be related to a high mobility of the particles on the bubble surface, meaning a poor attachment efficiency. Hydrophobic particles have been found to form larger bubble-particle clusters. The hydrodynamic characterization of bubble-particle heterocoagulates showed an increase of the mean liquid velocity compared to unloaded bubbles. A relation to the particle size has, however, not been found. To investigate the industrial fluorite flotation system with respect to w(CaF2) in the concentrate, a novel quantification method via Fourier Transform (FT) Raman spectroscopy has been developed and cross-validated. This method was then applied in the design of experiment. The optimum process parameters for maximisation of Rmax, k, and w(CaF2) have successfully been determined and correlated to further process characteristics, e.g. Zeta potential. It has been found that the pH value should be slightly alkaline, while both collector concentration and stirrer speed should be high. The volumetric gas flow rate has been found to be insignificant in the investigated parameter range. Lastly, the applicability of the fluorescent fluorite in optical investigations has been proven. A distinction between the fluorescence and reflections at the silicon dioxide surface has been presented under static conditions. Furthermore, first single bubble-particle heterocoagulates have been shown with fluorescent fluorite particles attached to the bubble surface. Thus, further optical investigations are promising.
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