Role of Oxygen Potential and Oxygen Ions on Phosphorus Removal from Silicon via Addition of FeO into Slag

Abstract

In present work, the role of the oxygen potential (PO2) and oxygen ion (O2−) concentration for removing phosphorus (P) during CaO–SiO2–Al2O3–FexO slag refining was studied by on-line measurement of oxygen activity in molten silicon (Si), FactSage calculation, Raman spectroscopy and nuclear magnetic resonance (NMR) spectroscopy. The results show that the addition of FeO from 0 to 9.25 wt% in slag can increase the activity of dissolved oxygen (a[O]) in Si and the mole fraction of O2− in slag. Moreover, the increase of O2− concentration leads to the increase of non-bridge oxygen (NBO). The value of LP (the partition ratio of phosphorous between slag and Si shows a first increase and then decrease trend and reaches a maximum value of 1.95 at 5 ± 0.1 wt% FeO. It is believed that the increase of a[O] and NBO can promote the removal of P as FeO content is less than 5 ± 0.1 wt%. the chain structure unit (Q2) of silicate network as the main intermediate structure to capture PO43− from the charge compensation of P2O5 by O2− to form the sheet structure unit Q3(Si and P). When FeO content is increased to more than 5 ± 0.1 wt%, LP value gradually decreases although the values of NBO and a[O] are increasing. NBO plays a leading role in this process, it can be speculated that more NBO can depolymerize the Q3 (Si and P) to destroy the stability of P in silicate network. As a result, a mount of PO43− is present at the interface to prevent the oxidation of phosphorous, which leads to the decrease of LP value.