Analysis of flow patterns inside the stirred tank used in alumina refinery

In this study, the turbulent flow patterns inside two model tanks were analysed using video camera in order to determine possible causes of undesirable formations of gibbsite sediments that are occurring at the bottom of precipitation tanks in the alumina refinery. The two tanks were geometrically similar and were of 1500L and 200L respectively each with a height equal to 1.2D and agitated with three pitched blade impellers of diameter 0.5D. The flow patterns were highlighted by the addition of polystyrene particles and were analysed through video camera recordings. The tanks revealed complex flow patterns that were not able to be fully analysed through the use video cameras. However, flow patterns were observed and analysed on a visual basis. Of notice was a complex region of flow that contained erratic fluctuations between radial and axial flow patterns. The erratic behaviour of this complex region of flow frequently amplified out to the tank walls and bottom causing brief stagnation periods and periods of uncharacteristic flow patterns. These results were verified with both tank sizes.