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Enhancement of gas-liquid mass transfer in hydrometallurgical leaching systems Kimweri, Henry Tegani Hosea

Abstract

The influence of gas-liquid reactions on oxygen mass transfer in three hydrometallurgical leaching systems under typical operating conditions has been investigated and assessed in terms of enhancement factors and Hatta numbers. The gas-liquid reactions considered were: Fe(II) oxidation under zinc pressure leaching conditions, Cu(I) oxidation in sulphate media under conditions used for the total oxidation of copper sulphides, and Cu(I) oxidation in ammoniacal media under conditions used for ammonia leaching of copper sulphides. Mass transfer rates and interfacial areas were determined simultaneously in order to obtain the parameters necessary for the evaluation of the enhancement factors. Performing oxygen absorption tests via surface aeration under a flat interface with a single impeller situated at the bottom of the stirrer shaft rotating at moderate speeds fulfilled this requirement. An attempt was also made to determine directly the gas-liquid reaction rate constant for Hatta number calculations in tests carried out in a well-agitated reactor fitted with dual impellers. The progress of Fe(II) oxidation was followed by sampling and titration , whereas the progress of Cu(I) oxidation was followed by measuring the rate of oxygen consumption using gas mass flowmeters. Reactant concentrations in the two copper systems were estimated using a speciation routine based on the minimization of Gibbs free energy. Density and viscosity of all solutions were also measured at low temperatures , and extrapolated to elevated temperatures using available correlations and curve fitting. Hatta numbers obtained from Fe(II) oxidation tests under a flat interface were greater than 3, situating the reaction in the very fast regime. However, similar values evaluated from tests carried out in a well-agitated reactor fell between 0.3 and 3, categorizing the reaction in the intermediate regime. An instantaneous reaction model in the two copper systems successfully modeled experimental results from well-agitated reactor tests. Results of speciation calculations indicate that metallic copper existed prior to the introduction of oxygen when the metallic copper charge exceeded 5 g/L in sulphate media. However, all of the metallic copper was converted to Cu(I) in ammoniacal media. Enhancement factors estimated from flat interface data indicated that the reactions in the two systems fall under the very fast reaction regime, complementing the well-agitated reactor test results. Based on the results, it is concluded that oxygen mass transfer rates may be determined using the gas-liquid reactions typically taking place in certain hydrometallurgical systems. The Hatta numbers and enhancement factors obtained indicate that all solutions are depleted of oxygen at the high concentrations of Fe(II) and Cu(I) in both sulphate and ammoniacal media, and therefore, the use of oxygen solubility in solving the kinetic equations appears to be suspect. The nature of the gas-liquid interface (stagnant film versus surface renewal) has a notable effect on both the kinetics of Fe(II) oxidation and oxygen mass transfer. In the two Cu(I) systems, oxygen mass transfer is enhanced several times over the maximum possible rate of physical absorption under the same conditions. Finally, with some modifications it would be possible to evaluate mass transfer parameters directly from the rate of Cu(I) oxidation in either sulphate or ammoniacal media. It is recommended that, whenever possible, Hatta numbers or enhancement factors be evaluated for all gas-liquid reactions and incorporated into the design of hydrometallurgical reactors. Further studies are recommended to determine the solubility of ferrous sulphate under zinc pressure leaching conditions, the bulk concentration of oxygen under test conditions , and oxygen solubility in solutions containing Fe(II).

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