Desulphurisation of iron-carbon melts.

Desulphurisation of iron-carbon melts by CaO-FeO-SiO2 slags (Fayalite to 38 mass % CaO-Fayalite) at 1300 C has been investigated under conditions similar to those encountered in LD steelmaking practice. Techniques were developed to introduce a single metal droplet into a liquid slag contained in an iron crucible. Reaction was terminated by quenching the droplet and slag, the former being analysed to determine sulphur concentration. Equivalent techniques were employed to monitor decarburisation.Results showed an initial efficient desulphurisation followed by a period of slow sulphur removal rate for droplets, initially containing 0.120 mass % S and higher, reacted with the 38 mass % CaO slag. Droplets of lower initial sulphur content experienced sulphur reversion for the first two to five minutes of droplet-slag interaction. Similar trends were established for experiments employing slags containing up to 28 mass % CaO but the reversion phenomenon was mainly restricted to droplets initially containing 0.030 mass % S. Decarburisation data indicated two dominant mechanisms, slag phase control followed by dispersed phase control.The initial desulphurisation period was attributed to conditions imposed at the droplet-slag interface by slag phase controlled decarburisation in conjunction with the presence of FeO.A finite carbon and low oxygen concentration at the droplet surface, coupled with the availability of oxygen anions, created conditions conducive to sulphur removal. The FeO prevented formation of an acid CaO-SiO2 interfacial slag of low oxygen anion concentration.Reversion of lower sulphur concentration droplets was related to gradual depletion of interfacial FeO which moved the slag towards an acid composition and lowered oxygen anion concentration.Replenishment of FeO at the reaction interface due to a transition to mixed or dispersed phase controlled decarburisation enabled desulphurisation to commence.An attempt to determine silicate anion constitutions of synthetic and industrial slags was made using trimethylsilylation GLC techniques. Similarity of results for slags of appreciable FeO concentration irrespective of SiO2 content suggested that massive anionic redistribution had occurred during trimethylsilylation.