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Pasolini, Pier Paolo, - 1922-1975

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Pyrometallurgy is an attractive process widely used to extract metals such as cobalt and nickel from industrial residues at high temperatures. During this treatment, a liquid oxide slag is generated. The final slag contains small cobalt and nickel particles, which are consequently lost after slag tapping, resulting in lower metal yields. This thesis aims to investigate the surface tension of cobalt and nickel, as it is one of the main parameters affecting the separation of the metal particles from the slag. The influence of oxygen and sulphur, both known as surface-active elements, on the surface tension of liquid cobalt and nickel was experimentally studied. Firstly, several alloys of cobalt and nickel containing different concentrations of oxygen and sulphur were prepared using a vertical high temperature tube furnace (GERO). These include Co-O, Ni-O, Co-S-O, and Ni-S-O alloy systems. To verify the levels of targeted oxygen and sulphur, these alloys were subjected to chemical analysis using a LECO analyzer. The alloys were successfully prepared with different oxygen and sulphur contents for the present surface tension study. A Confocal Laser Scanning Microscope (CLSM) apparatus, connected to an external camera for video acquisition, was employed for the in-situ measurement of the contact angle between molten alloys droplets and an alumina substrate. The captured photos were read through an image analysis software (SCA20) to retrieve the contact angle values. The surface tension of the alloys was determined through Laplace’s equation, modified for a sessile drop experiment. The surface tension was measured to be 1715 to 1013 mN/m for Co alloys at 1540 °C, and 1840 to 1097 mN/m for Ni alloys at 1480 °C, depending on the oxygen and sulphur content. It was confirmed that both the contact angle and surface tension of the alloy systems decreased with oxygen and sulphur content. The maximum oxygen adsorption on the liquid alloy surface was estimated via Gibbs isotherm equation. It was determined to be 16.9 x 10−6 mol/m2 at 1480 °C and 12.6 x 10−6 mol/m2 at 1540 °C for Ni-O and Co-O systems, respectively. Positive values indicate that the surface of the drop is being enriched in oxygen, with respect to the bulk liquid. These values are comparable with similar works from the literature. The work of adhesion Wa between the liquid alloys and the alumina substrate was obtained to be 695-718 mN/m and 692-719 mN/m for the nickel and cobalt alloy systems, respectively. The Wa was similar for both alloys. Little influence of oxygen and sulphur was observed on the work of adhesion. Positive values of Wa confirm that bonding occurred between the two surfaces. The microstructure of the liquid metal/alumina substrate interface was analysed using SEM-EDAX. The formation of a complex AlNiOx layer with a thickness of 3 to 30 micrometers was identified at the Ni-O alloy/alumina substrate interface due to their interaction. This finding implies a reactive wetting of liquid Ni-O alloy on solid alumina, for which the final degree of wetting and the spreading kinetics are not controlled by the original liquid Ni drop/solid alumina substrate system, but instead by the newly formed compound (i.e. the complex AlNiOx layer) at the interface.