Kinetic model of desulphurisation by powder injection and blowing in RH refining of molten steel

Abstract

The desulphurisation by powder injection and blowing in the RH refining of molten steel and its mechanism have been considered and analysed. Based on the two-resistance mass transfer theory and the mass balance of sulphur in the system, a kinetic model for the process has been developed. The related parameters of the model, including the mass transfer coefficients and the effective amount of powder in the molten steel being treated for desulphurisation, have been reasonably determined. Modelling and predictions of the process of injecting and blowing the lime based powder flux under assumed operating modes with the different initial contents of sulphur and amounts of powder injected and blown in a RH degasser of 300 t capacity have been carried out using the model. The relevant circulation rate of the liquid steel and the powder injection and blowing rate were taken to be 100 t min^-1 and 150 kg min^-1, respectively. The initial contents of sulphur in the liquid steel to be treated and the amounts of powder injection and blowing were respectively assumed to be 0·007, 0·006, 0·005, 0·004, 0·003, 0·002 wt-%and 10, 8, 6, 5, 4, 3 kg/t steel. The total treatment time for desulphurisation under each mode was set at 24 min, equivalent to eight circulation cycles of the liquid steel to be treated. The results indicated that the predictions made by this model are in good agreement with data from industrial experiments and practice. By injecting and blowing the lime based powder flux with the composition of 85 wt-% lime (CaO) + 15 wt-%fluorspar (CaF₂ ) of 3–5 kg/t steel, it is possible to decrease the sulphur content in the molten steel to an ultralow level below (5–10) × 10^-4 wt-%from (60–80) × 10^-4 wt-%. The total treatment time needed will be 12–20 min. Intensifying the powder injection and blowing operation and increasing the circulation rate of the liquid steel may effectively increase the rate of the process in RH refining. The model may be expected to offer some useful information and a reliable basis for determining the reasonable process parameters and help in optimising the technology of desulphurisation by powder injection and blowing in the RH refining of molten steel.