Studies on gas-liquid mass transfer in atmospheric leaching of sulphidic zinc concentrates

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Doctoral thesis (monograph)
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Helsinki University of Technology doctoral theses in materials and earth sciences, 3
In this work, the mass transfer of oxygen in the atmospheric leaching process of zinc sulphide was investigated. Four new experimental apparatus items suitable for this purpose were designed and developed. The experiments conducted with the water model were focused on volumetric mass transfer, gas and liquid flow patterns, gas dispersion and bubble size. The effects of liquid properties and temperature on bubble size were examined with the bubble swarm system. Mass transfer coefficients, kL, between oxygen and different liquids were measured with mass transfer equipment. Modified high-temperature and pressure autoclave was used to determine the oxygen consumption rates in leaching conditions. The experimental set-ups and program carried out are discussed and the errors and problems associated with the techniques reviewed. The results revealed, amongst other occurrences, that the non-coalescence of bubbles occurs and the bubble size is controlled by the formation and breakage close to the impeller. According to the experiments, it seems to be possible to control the foaming and the surface aeration by adjusting the liquid volume and gas flow rate in the process. Too much liquid in the process increases the foaming, while too little increases the surface aeration. Furthermore, increasing the gas flow rate decreases foaming. Gas hold-up increased with mixing speed, while increasing the gas flow rate decreased the power consumption, as expected. Experimentally determined volumetric mass transfer values, kLa, varied between (2.17-12.00)×10−3 1/s and mass transfer values, kL, between (13.81-19.24)×10−5 m/s with oxygen and pure water. On the other hand, kL values between oxygen and process solutions varied between (1.5-11.32)×10−5 m/s. Increasing electrolyte content decreased the mass transfer values notably, sulphuric acid and zinc sulphate additions having a stronger effect than sodium chloride. Both the determined mass transfer parameters were also strongly dependent on the mixing intensity. The oxygen consumption rate in the process solution varied between 0.018-0.075 mmol/(m2s). Increasing the pressure and mixing intensity increased the oxygen consumption rate significantly, but temperature did not have a similar effect. Decreasing the dissolved zinc content in the solution increased the oxygen consumption rate significantly, whereas increasing the amount of concentrate only slightly increased the consumption rate. The experimental results of this work provide additional data for the improvement of existing leaching models, as well as the development of new ones.
leaching, atmospheric, zinc, sulphidic, concentrate, gas, bubble, interface, solubility, dissolution, mass transfer, coefficient, diffusion
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