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.