Removal of Dissolved Al and Ca in Si by SiO₂ Additions and Mechanical Stirring

Abstract:

In the oxidative refining of metallurgical grade silicon the loss of Si to the slag in the form of SiO₂ is an economical concern. The purpose of this report is to investigate the possibility of using SiO₂ and mechanical stirring to remove Ca and Al as a substitute to the oxidative refining. In the experiments graphite crucibles were used in a vertical resistance furnace and controlled argon atmosphere. The removal rate of Ca and Al are measured by X-ray fluorescence and the slag is examined in a scanning electron microscope. The slag formation kinetics are examined and a calculation of the activities of Ca, Al and their respective oxides in the slag phase is conducted. The driving forces of creating CaO and Al₂O₃ in this system is calculated to better understand the behavior of the Ca and Al removal. The results show that removal of dissolved Ca and Al by mechanical stirring is possible and in this setup a stirring time of 20 minutes is sufficient since no more refining can be obtained by increasing it.