Effect of Oxygen Partial Pressure and B₂O₃ on Crystallization Behavior of Phosphorus- and Iron-Containing Phases in a CaO–SiO₂–Fe₂O₃–P₂O₅ Melt

To help maintain the sustainability of the steel industry by improving slag utilization, we are developing a novel process to recover P and Fe from steelmaking slag by controlling oxygen partial pressure and adding slag modifier B₂O₃, to control the precipitation of targeted phases. In this paper, the precipitated phases of the molten synthetic CaO–SiO₂–FeO–P₂O₅(–B₂O₃) slags were predicted through thermodynamic calculations using FactSage 8.1. A confocal laser scanning microscope (CLSM) was used to in situ observe the crystallization behavior of the targeted metal oxides in the slags. It was found that iron and phosphorus could be recovered in the form of magnetite (Fe₃O₄) and calcium phosphate (Ca₁₀P₆O₂₅) phases by controlling oxygen partial pressure and adding slag modifier B₂O₃. By changing oxygen partial pressure from 0.21 to 10^–6 atm, the iron-containing phase transformed from Ca₂Fe₂O₅ to magnetite (Fe₃O₄). Through the introduction of B₂O₃ from 2 to 8 pct, the amount of calcium phosphate (Ca₁₀P₆O₂₅) first increased and then decreased, with the maximum of 28 pct (e.g. 28/100 g slag studied) at 6 pct B₂O₃.