Desulfurisation of steel during ladle refining is governed by both thermodynamics and kinetics. For
effective desulfurisation, thermodynamically, a low partial pressure of oxygen at the steel-slag
interface and highly basic slag which possess a large sulfide capacity are desired. Kinetically, the
mass transfer coefficient of sulfur in the liquid steel and in the slag determines the rate of
desulfurisation. A fast desulfurisation rate is achieved by maintaining sufficiently strong argon stirring
of the melt.
In the present work, sulfur mass balance for steel and slag was performed after the desulfurisation
treatment for several industrial heats. The decrease in the sulfur quantity of the liquid steel was more
than the increase in the sulfur quantity of the slag. The discrepancy in the sulfur mass balance
suggests that the sulfur could be transferred to: (a) the gas (via slag) phase; or to (b) the refractory
(crucible) phase. The preliminary investigation of the refractory samples taken at slag-refractory and
steel-refractory interface showed no interaction of sulfur with the refractory. Therefore, the possibility
of sulfur transfer to the gas phase was verified by carrying out melting experiments of sulfur
containing slag in an argon/air atmosphere. Sulfur containing slag was initially melted in an MgO-C
crucible under an argon atmosphere followed by creating the air atmosphere just above the melt.
Careful sampling and analysis from the cold spots of the crucible, much above the melt surface,
revealed formation of a fibrous structure. Scanning electron microscopy energy dispersive X-ray
spectroscopy (SEM-EDS) analysis of these fibrous deposits confirmed the presence of oxides and
sulfides. Oxidation of sulfur containing slag under air atmosphere was also performed using a
thermogravimetric analysis (TGA) set-up. It showed a constant sulfur loss in the range of 1300–
1500°C. Thermodynamic calculations carried out using FactSage™ ver 8.3 for these experimental
conditions are in good agreement with the observed results.