Oxidation and fume formation during production of Si and Mn-based alloys
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Thermally generated fumes from the production process of manganese ferroalloys are generated due to the reaction between high temperature liquid- or metal vapor and ambient oxygen in the air, and are mostly composed of metal oxides. Thermally generated fumes are difficult to capture and reduce, thus they are released to the working environment. These emissons are not only a concern for workers health in the plant, but also contribute to fugitive emissions from the plant to surrounding communities. However, when reviewing the relevant literature, there is a general lack of fume formation mechanism understanding, fume properties and kinetic data for oxidation of liquid manganese alloys, in particular silicomanganese alloys. Such understanding, in addition to information on how characteristic properties of the formed fume particles depend on process parameters and process type, is essential to design effective fume mitigation and capture strategies. In the current thesis work, three experimental scales have been utilized to investigate fume formation, i.e. laboratory (kg) scale, pilot (100´s of kg) scale and industrial scale. In the laboratory scale experiments, the oxidation process of silicomanganese and silicon alloys under various oxidation parameters (temperature, gas flow rate, gas composition) was studied and collected particulate matter was characterized in order to obtain a fundamental understanding of the mechanisms governing the rate of fume formation and fume composition. In a pilot scale SAF SiMn alloy production experiment, minor and trace element contents in raw materials, and their subsequent distributions between resulting product phases (metal, slag, fume), were investigated. Finally, an industrial fume measurement campaign at tapping and casting stations of the Eramet Kvinesdal plant was carried out in order to compare the rate and composition of fume formed with that in laboratory scale experiments. While Mn exhibits high vapor pressure at elevated temperatures, Si forms SiO gas at low oxygen partial pressures. For SiMn alloys, both gas species oxidise in air and produce complex Si, Mn oxide fumes. The mass flux / fuming rate, morphology and compositions of the SiMn fume are dependent on the melt temperatures. The mass flux of SiMn fume increases exponentially with increasing melt temperature, following an Arrhenius-type relationship. With the same amount of oxygen delivered to the system, increasing the total gas flow rate enhances the rate of fume formation, which may be explained by a decrease in the effective diffusion boundary layer thickness of both O2, diffusing towards the metal surface and Mn vapor, diffusing from the metal surface. The injection of water vapor decreases the SiMn mass flux at a melt temperature of 1450oC where Mn vapor is the main fume forming gas species. Meanwhile, the injection of water vapor increases the Si mass flux, and the prompted oxidation may be explained by the formation of Si-H species and Si-O-H species. However, the water vapor does not significantly affect the total SiMn mass flux at a melt temperature of 1600oC, where both Mn vapor and SiO gas are main fume forming gases. In the oxidation of liquid SiMn alloy, the phase distribution of trace elements is generally in accordance with the boiling-point-model. In a SAF SiMn production process, the distribution of elements between the product condensed phases (metal and slag) and dust is also generally in good accordance with the boiling point model. The distribution of elements between metal and slag mainly follows the Gibbs energies of oxidation of the elements. The industrial measurement work has contributed to a better understanding of fume formation during the SiMn alloy tapping and casting processes. With the metal tapping temperature at approximately 1500OC, fume samples show both spherical/amorphous and irregular/crystalline particle shapes, containing Si, Mn and O as main elements, along with minor/trace level elements K, Na, S, Mg and Al. At a metal casting temperature below 1400oC, the fume samples show mainly irregular/crystalline particle shapes, also containing Si, Mn and O as main compositions, with presence of K, Na and S. This observation is in accordance with the laboratory scale experiments, in which the fume morphology is dependent on the melt temperatures.