Abstract
M.Ing. ( Civil Engineering)
The use of dissolved-air flotation has shown a marked increase in the past decade. The practise is however still relatively unknown, and plants are still being constructed using empirical guidelines. Probably the most important aspect that should be highlighted, is the successful generating of microbubbles in the flotation tank. The microbubbles are generated when the saturator water pressure is released via a reduction nozzle. These bubbles should be homogeneous, and in the region of 100 /Lm in diameter. If these bubbles are too big, they will rise too fast, and disturb the float layer. If they rise too slowly, the retention time would be too long. The purpose of this work was to try to refine nozzle design to such a level that microbubbles can be generated with chosen size and size.distribution. Very little is however done at this stage to understand the design of nozzles. The result of this lack of knowledge, is that many flotation plants fail because of ineffective nozzles and nozzle design. Before experimenting could be started, it was necessary to acquire a better understanding of the fundamental behaviour of bubbles. A large amount of literature is available on this subject, but very little is specifically for dissolved-air flotation. It was attempted to give an overview of all the relevant literature regarding bubbles and their behaviour. Further insight is also given regarding the generation of bubbles in relation to physical parameters such as temperature and pressure. A short summary is also given of all the most commonly used commercially available nozzles, together with simple section drawings. The bubble size constitutes the most important parameter in this study, and therefore a purpose made flotation column was set up, that would simulate real life situations. Photographs were taken with different nozzle configurations, chosen such that they would contribute meaningfully to the experimental results. The complete experimental setup and procedures are discussed extensively in the document. The results of the study led to the formulation a bubble growth model to explain the behaviour of bubbles resulting from these nozzles. The use of this model, together with a couple of other assumptions, spread new light on the generation of microbubbles. This newly found information could lead to the development of a rational design procedure for injection nozzles for the production of microbubbles.