Browsing by Author "Taskinen, Pekka;"

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    The dissolution kinetics of sphalerite concentrates
    (Teknillinen korkeakoulu, 2005) Aaltonen, Mari; Taskinen, Pekka;; Department of Chemical Technology; Kemian tekniikan osasto; Kontturi, Kyösti; Prof.
    In the literature part of the thesis, an overview of the mechanisms and kinetics of oxidative sphalerite dissolution was given. The factors affecting the rate of dissolution and some methods of study were briefly described. In the experimental part various methods were used. In batch dissolution experiments with manganese dioxide and ferric ions as the oxidising species, it was found that MnO2 affects the dissolution of a sphalerite concentrate by two mechanisms: by directly oxidising the sulphur in the mineral and by regenerating the active ferric species. The results also suggested further oxidation of the elemental sulphur layer by the manganese species. The dissolution behaviour of two sphalerite concentrates was studied by batch dissolution experiments using ferric ions as the oxidant. Fractions with particle sizes of < 37 µm and > 37 µm were used. The measurement parameters were chosen to match those in industrial scale dissolution. The average zinc conversions for the fractions were from 45 to 74 mass percent after 3 hours dissolution. The conversions were strongly effected by the size fraction of the concentrate, temperature and the concentration of iron in the solution. The effects of the sulphuric acid and zinc concentrations were on average negligible. As expected, the concentrate fraction with the largest surface area per gram had the highest conversion and the conversions decreased with decreasing surface area. The average conversion divided by the surface area of the concentrate was 15 m-% higher for the concentrate with a higher concentration of iron in the lattice. A rotating ring disc electrode was constructed, in which a pressed concentrate pellet was used as the disc. The Fe2+ ions formed by the dissolution of ZnS in the disc were detected on the platinum ring by oxidising them back to the ferric form. The measured current was used to determine the rate of dissolution. The method is fast and thus can be used for rapid comparison of the dissolution characteristics of different concentrate fractions under varying conditions. In addition, the method allows for the effects of kinetics and transport processes on the current to be separated, allowing a closer study of the factors affecting the kinetics of the reaction. Theoretical calculations of the rate of mass transfer showed, that kinetics are rate limiting under these conditions. Increasing the mass transport in the solution will not increase the rate of the dissolution process; the conditions have to be made more favourable for faster kinetics. A mineral-carbon paste electrode was constructed by mixing sphalerite concentrate with fine carbon powder and oil. Cyclic voltammetric measurements showed the carbon-oil paste electrode to be inert under the conditions used. The voltammetric and potentiostatic measurements conducted on the modified mineral electrode showed the effect of the electrode potential on the rate of dissolution. The oxidation of the sulphide began above 0.8 V vs. Ag/AgCl and the rate was increased with increasing potential until a maximum was reached at 1.2 V vs. Ag/AgCl.