An investigation of the effect of physical and chemical variables on bubble generation and coalescence in laboratory scale flotation cells
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Doctoral thesis (article-based)
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Helsinki University of Technology doctoral theses in materials and earth sciences, 4
AbstractA new technique for measuring bubble size in laboratory scale flotation cells was developed. A method of sampling and photographing bubbles from a flotation cell was combined with modern methods of image processing and analysis. The technique is capable of sizing accurately a large number of bubbles by exposing a stream of bubbles to a progressive scan camera. The technique was used to study the effect of several physical and chemical variables on bubbles size in laboratory scale flotation cells. Three mechanically agitated Outokumpu flotation cells were used in the tests, the size of the cells being 50 dm3, 70 dm3 and 265 dm3, respectively. The last mentioned was especially designed and constructed for this study. The cells were operated under batch conditions. The hydrodynamic conditions prevailing in the cells were modified mainly by altering the impeller speed and aeration conditions, as well as the frother concentration. An extensive study of the effect of frother concentration on the bubbles generated in flotation cells was carried out. A series of common flotation frothers, DF-200, DF-250 and DF-1012, was chosen to test the effect of frothers on bubble coalescence and the bubble break-up process. The experimental tests revealed that bubble size strongly depends on frother concentration. With increasing frother concentration, the degree of bubble coalescence decreases, while at a particular frother concentration, known as the Critical Coalescence Concentration (CCC), bubble coalescence is totally hindered. The experimental results also indicate that frothers appear to affect the break-up process or bubble generation. While the DF-200 frother, characterized by much larger CCC values than DF-1012 and DF-250, has the ability to produce finer bubbles at concentrations exceeding the CCC value, the bubbles generated in the DF-1012 solutions at concentrations exceeding CCC are much larger. The aeration rate has a profound impact on bubble generation; bubble size increases with an increase of the air flow rate entering the flotation cell. The aeration rate seems to determine to a large extent the size, shape and behaviour of the aerated cavities formed behind the blades of the rotor of the cell. These gassed cavities appear to control the mechanism of bubble generation in a flotation cell. The formation and behaviour of aerated cavities behind the Outokumpu rotor was examined using a high-speed camera. While the maximum stable bubble diameter seems to characterize the bubble break-up process adequately, the Sauter mean bubble diameter and the number bubble mean diameter turned out not to be very sensitive to the changes in impeller speed. These two diameters are not always able to reveal adequately differences between bubble-size distributions. Since bubble coalescence can be entirely prevented in the cell at frother concentrations exceeding the CCC values, it was possible to assess the impact of two commercial rotor/stator mechanisms on bubble generation. With the aid of a new sensor developed for measuring continuously local gas velocity, a series of tests was conducted to study how efficiently the incoming air is dispersed throughout the volume of the cell by the rotor/stator mechanism.
flotation, bubble size, gas dispersion, flotation frothers, critical coalescence concentration
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