Reductive Heating Experiments on BOF-Slag: Simultaneous Phosphorus Re-Distribution and Volume Stabilization for Recycling

Basic-oxygen furnace (BOF) slag is the second abundant by-product from the steel-making process. It is not readily recycled for high phosphorus content and volume instability. To eliminate its phosphorus-hosting dicalcium silicate (C₂S) and volumetrically unstable free-lime, reductive heating experiments on BOF-slag are carried out at 1300–1600 °C using quartz-sand or serpentine as a basicity modifier. In the products, C₂S and free-lime are consumed through reacting with CaO and MgO, which are produced by Ca-ferrite and Mg-wustite reduction, respectively, to form merwinite and akermanite. The products are also characterized by silicate–metal segregation and the extent of that increases with increasing temperature and decreasing particle size of starting mixtures. Thermodynamic calculations show that phosphorus in C₂S is mostly converted to Fe₂P in the metal domain. Compared to the slag–serpentine experiments, the slag–quartz experiments resulted in higher extents of silicate–metal segregation with lower phosphorus contents of ≈0.1% in the silicate domain, which therefore is recyclable. To optimize silicate–metal segregation for recycling BOF-slag, it is suggested (i) bringing the BOF-slag composition within the low-temperature side of the C₂S–merwinite–akermanite Alkemade triangle by adding quartz-sand; (ii) reductively heating to 1500–1600 °C; and (iii) cooling slowly then quenching to suppress C₂S crystallization from melts.