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

In present work, the role of the oxygen potential (P_O2) and oxygen ion (O²⁻) concentration for removing phosphorus (P) during CaO–SiO₂–Al₂O₃–Fe_xO 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 L_P (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%, L_P 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 L_P value.