2020 Volume 60 Issue 5 Pages 922-929
In order to clarify the effect of the lime (CaO) dissolution rate in slag on hot metal dephosphorization, a 150 kg scale hot metal dephosphorization experiment was carried out. The rate of decrease of free CaO and the dephosphorization rate were measured while varying the ratio of CaO and SiO2, which was defined as basicity.
The dephosphorization rate showed its maximum value at basicity of around 1.0. At basicity higher than 1.0, the dephosphorization rate decreased due to poor dissolution of CaO in the flux.
A thermodynamic calculation revealed that crystallization of 2CaO·SiO2(-3CaO·P2O5) in the liquid slag deteriorated CaO dissolution when basicity was higher than 1.5.
The mass transfer coefficient of CaO in slag was calculated assuming that the interface between the CaO and liquid slag is saturated with CaO. High basicity showed a low mass transfer coefficient.
The apparent slag viscosity was calculated in terms of the solid phase and showed a correlation with the CaO diffusion rate. The CaO diffusion rate in slag decreased with higher values of not only the liquid slag viscosity, but also the solid-liquid coexistent slag viscosity. These results suggest the existence of an optimum basicity for effective CaO dissolution.