Oxygen solubility in pure water and in dilute sulfuric acid solution has been studied as a part of atmospheric direct zinc leaching process development. The experimental results were compared with reference data and the Gibbs energy model. The dissolution dynamics of oxygen was further combined with the Gibbs energy minimization method. Together the experimental and model results give useful information and methodology for the developers of industrial processes related to gas solubility. The dynamic Gibbs energy calculation approach presented in this work extends the possibilities of evaluating changes in different process chemistries needed in process design.
[Show abstract][Hide abstract] ABSTRACT: 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. Helsinki University of Technology doctoral theses in materials and earth sciences, ISSN 1795-0074; 3
[Show abstract][Hide abstract] ABSTRACT: This thesis consists of studies of chemical thermodynamics of aqueous electrolytes for industrial and environmental applications. Calculations have been used to represent vapor-liquid-solid equilibria and chemical equilibria for aqueous systems including solubility of gases. Modern simulation methods combined with experiments provide a useful tool for the research and design of new processes as well as evaluating changes in the operational conditions of chemical processes. The Gibbs energy minimization methods ChemSage, and ChemSheet have been used along with activity coefficient models including Pitzer ion interaction model. The calculated results were compared if possible with experiments or with reference data. Further this work consists of studies on oxygen-pressurized peroxide bleaching, which is an important sequence of total chlorine free (TCF) bleaching for environmental reasons. The solution properties like pH have been measured and modelled both in pure H2O2-NaOH-H2O system and in bleaching conditions. Further, the thermodynamic multicomponent model was used with kinetic constraints for pH calculations in reactive solution. The knowledge of the pulp- and solution properties along with the results of the model calculations could be used for optimisation of the bleaching process with respect to reaction time and temperature. Thermodynamics provides a practical tool for the estimation of the chemical states of pulp and paper solutions as well as in hydrometallurgical applications. Such a fundamental approach relates to the chemical energy, chemical reactions, solubility of gases and salts, and an important online process parameter pH. The Gibbs energy approach was further applied to equilibrium and reaction dynamic studies of multiphase CO2-CaCO3-H2O system.
[Show abstract][Hide abstract] ABSTRACT: In atmospheric direct leaching of sulphidic zinc concentrates, oxygen acts as an oxidant in the dissolution. The amount of oxygen in the solution has an important effect on the kinetics of the whole process. In this paper, four laboratory scale equipment are used for studying the gas mass transfer in the leaching process. With each experimental set-up it is possible to analyse some of the factors affecting the kLa, kL and a coefficients. The gas flow rate and mixing intensity increased the kLa as expected. Also the kL value was increased with mixing intensity, but increasing salt concentration had an opposite effect. Lowering the surface tension and increasing the density of liquid decreased the bubble Sauter mean diameter and therefore the total gas–liquid interfacial area a was increased. Oxygen consumption into solution was slightly increased with temperature. On the other hand the effect of pressure and mixing was notable. By combining the results of different equipment enables us to study the mass transfer as a function of parameters such as: temperature, pressure, liquid properties and mixing conditions. Knowing the mass transfer coefficients is essential for process development and design.
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