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Experimental residence time distribution (RTD) measurement and computational fluid dynamics (CFD) simulation are the best methods to study the hydrodynamics of process flow systems. However, CFD approach leads to better understanding of the flow structure and extent of mixing in stirred tanks. In the present study, CFD models were used to simulate the flow in an industrial gold leaching tank. The objective of the investigation was to characterize the flowfield generated within the tank after process intensification. The flow was simulated using an Eulerian-Eulerian multi-fluid model where the RANS standard -ε mixture model and a multiple reference frame approach were used to model turbulence and impeller rotation respectively. The simulated flowfield was found to be in agreement with the flow pattern of pitched blade axial-flow impellers that was used for mixing. The leaching tank exhibited good "off-bottom suspension" which reveals minimum deposition of gold ore particles on the bottom of the leaching tanks. Simulation results were consistent with experimental results obtained from a radioactive tracer investigation. CFD approach gave a better description of the flow structure and extent of mixing in a leaching tank. Hence it could be a preferred approach for flow system analysis where the cost of experimentation is high.

Gel precipitation IPolyacrylamide /Fast reactor fuel /Mixed actinide oxides /Ammonium uranate/Gel structure SummaiyThe physical and chemical changes invoWed in the foimation of spherical particles of (U,Th)Oj and (U, 30% Pu)Oj by a gel piecipitation loute aie desciibed, with emphasis being placed on the role of the polymer. In addition to Controlling the overall shape of the gel particle, the polymer is found to control the internal structure through the formation of bonds with ammonium uranate and other hydrous metal oxides in the precipitated gel. A common unit of structure is observed over the ränge of (U, Th) compositions studied and for the (U, 30% Pu) system. Removal of the polymer can be achieved without any significant disruption of the structure.

The performance of the California Puff (CALPUFF) modelling system has been evaluated using a case study in Ghana. The performance evaluation consisted of a quantitative comparison of dispersion simulation results of SO 2 and NO 2 with measurements at the Tema Oil Refinery, and meteorological simulation results with observations from the Tema Meteorological Station, both in the Greater Accra region of Ghana. Four statistical indicators-Index of Agreement (IOA), Fractional Bias (FB), Normalized Mean Square Error (NMSE) and the Pearson correlation coefficient(R) employed in the assessment indicate sufficient reliability of both CALPUFF and its meteorological simulator, CALMET. IOA values of 0.73 and 0.67 and FB values of 1.65 and 1.42 were obtained for SO 2 and NO 2 respectively. IOA between measured and modelled emissions were 0.72 and 0.69 for SO 2 and NO 2 respectively. The correlations between the simulated and observed emission were 0.66 and 0.08 for SO 2 and NO 2 respectively. An IOA value of 0.66 was obtained for both wind speed and wind direction with correlations of 0.29 and 0.58 in comparison with observations from the meteorological station.

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