Effects of Slag Composition on Phosphorus Segregation to Dicalcium Silicate‑Tricalcium Phosphate Solid Solution for Recycling of Basic Oxygen Steelmaking Slags

The potential for recycling basic oxygen steelmaking (BOS) slag into a P-rich stream for use as a fertilizer and a Fe-rich stream for on-plant recycling was investigated by studying the segregation of P between dicalcium silicate (C2S)-tricalcium phosphate (C3P) precipitates and the surrounding liquid. Samples of synthetic and industrial BOS slags were put through a homogenization stage at 1600 °C and a separation stage at 1450 °C, then fast cooled and characterized. The segregation and morphology of P were found to be strongly affected by the composition of the slag. Dendrites of C2S–C3P were formed at higher basicities (V-ratio ≥ 2.6) and lower Al2O3 contents (≤ 5%). At lower basicity and higher Al2O3 content, the C2S–C3P dendrites were not formed. Industrial BOS slag samples, after undergoing the same thermal treatment, had similar morphologies and segregation behavior to the synthetic BOS slags. This indicates that despite the simplified compositions of the synthetic BOS slags, they can be used as an analog for studying the industrial BOS slags. Physical separation of the C2S–C3P dendrites and the Fe-rich liquid into distinct layers was demonstrated for fast cooling of samples after the separation stage at 1450 °C. This demonstrates that there is potential for the physical separation of the BOS slag into P-rich and Fe-rich streams.