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An investigation of gas/liquid mass transfer in mechanically agitated pressure leaching systems DeGraaf, Kenneth Brant


Oxygen pressure leaching rates have been accelerated to the point where oxygen consumption rates of 0.75 moles of O₂ per litre of leach solution are obtained within a residence time of the order of 1 hour. At these high oxygen consumption rates the mass transfer of dissolved oxygen at the gas/liquid interface may become rate-determing. The purpose of this study has been to examine gas/liquid mass transfer rates in mechanically agitated pressure leaching systems. Using an O₂-Na₂SO₃ system to measure the oxygen mass transfer rates, the effect of a number of process variables on the mass transfer rates, impellor gas pumping rates, and the volumetric power requirements has been studied. The experimental work was done using both bench-scale (2 litres and 20 litres) and pilot-scale (2100 litres) equipment. It is demonstrated that dimensionless mixing correlations previously applied are not useful for extrapolating or scaling-up a mechanically agitated gas/liquid mass transfer system. It is shown that gas/liquid systems are more appropriately described and scaled-up in terms of the impellor tip velocities and gas pumping characteristics. The existence of a critical impellor velocity, which corresponds to the point when bubbles first form at the impellor is confirmed experimentally and discussed theoretically. The gas/liquid mass transfer rate is found to be directly related to the gas pumping capacity of the impellor. The practical Implications of this principle in the design of agitation for gas/liquid systems are shown experimentally to have great potential for improving gas/liquid mass transfer rates and reducing the power consumption of an agitator.

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