Oxidative Refining of Metallurgical Grade Silicon: Lab-Scale Measurements on the Overarching Refining Behavior of Ca and Al

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 SiO2–CaO–Al2O3 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 SiO2–CaO–Al2O3 slag and the silicon alloy.