Abstract
Oxidative ladle refining (OLR) is the most widely used refining process for removal of dissolved Ca and Al in the industrial production of metallurgical grade silicon (MG-Si). Refining occurs by purging the silicon alloy with an oxygen–air mixture at temperatures preferably between 1823 K and 1873 K, forming a SiO_{2}–CaO–Al_{2}O_{3} slag, which is then separated from the liquid alloy before casting. To meet higher quality demands and customer specifications, it is paramount to achieve greater process control of the industrial system, which requires a deeper understanding of the Ca and Al mass transfer kinetics, and the general refining behavior. In this work, laboratory-scale experiments have been performed, aiming to emulate the industrial process. Five different alloy compositions, with varying initial Ca and Al concentration, were refined over 11 experiments, with samples taken at 0, 5, 7, 10, and 15 minutes at 1873 K. A FactSage-based model was used to explore the underlying thermodynamic system behavior. The results suggest that OLR of MG-Si occurs in three primary steps, initially by the formation of slag by surface oxidation and purge gas, through the depletion of dissolved Ca in the melt, and secondarily by the formation of silica. Towards the end of the refining process, a critical amount of slag has been formed by the previous steps such that a global equilibrium is attained between the SiO_{2}–CaO–Al_{2}O_{3} slag and the silicon alloy.
Highlights
OXIDATIVE ladle refining (OLR) is the most widely used refining process in the industrial production of metallurgical grade silicon (MG-Si, ! 96 wt pct Si[1]), used primarily to reduce and control the amount of Ca and Al in the alloy
Since MG-Si primarily sees use as a raw material or alloying agent in the production of other materials, like aluminum alloys, silicones, steels, and photovoltaics, improvements made to the MG-Si production process, like OLR, will benefit a wide range of industries
In OLR, the molten alloy is purged with an oxygen–air gas mixture from a bottom-mounted porous plug, at temperatures between 1773 K and 1973 K
Summary
OXIDATIVE ladle refining (OLR) is the most widely used refining process in the industrial production of metallurgical grade silicon (MG-Si, ! 96 wt pct Si[1]), used primarily to reduce and control the amount of Ca and Al in the alloy. OXIDATIVE ladle refining (OLR) is the most widely used refining process in the industrial production of metallurgical grade silicon In OLR, the molten alloy is purged with an oxygen–air gas mixture from a bottom-mounted porous plug, at temperatures between 1773 K and 1973 K. This occurs while the alloy is being tapped from the furnace, and may continue for some time after the ladle is full to reach a specific target composition. This work aims to carry out laboratory-scale experiments, focusing on large-scale effects, as these determine the refining path Identifying these large-scale effects are critical for further work on creating mass transfer models to explore the refining kinetics of OLR in detail. The work follows from previous work by Kero et al.,[5] Haaland et al.,[6] and Olsen et al.,[7] who have all explored aspects of the OLR process
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