A dephosphorisation technology using CaO-based flux during the tapping process of basic oxygen furnace (BOF) converter was developed by metallurgists. In order to understand the dephosphorisation mechanism during this process, laboratory experiments were carried out using industrial BOF crude steel and some CaO-based fluxes. It was found that the dephosphorisation behaviour of CaO in BOF crude steel differs from that of the dephosphorisation processes of hot metal. Owing to the lack of liquid phase, the efficiency of dephosphorisation using pure CaO is relatively low. The formation of 2CaO·SiO2 phase is also crucial, but the low Si content in BOF crude steel restricts the formation of 2CaO·SiO2–3CaO·P2O5 (C2S–C3P), thus affects the dephosphorisation efficiency. Kinetically, suitable addition of SiO2 and FeOx can accelerate the dissolution of the flux; and thermodynamically, the addition of SiO2 is helpful for the formation of C2S–C3P, resulting in a better dephosphorisation efficiency.
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