Optimisation of hot metal desulphurisation slag in the CaO/Mg co-injection process to improve slag skimming performance

摘要

Co-injection of lime and magnesium is one of the most commonly used hot metal desulphurisation processes in BOF steelmaking plants. The post-desulphurisation slag is comprised of blast furnace slag carryover, the residual from previous heat(s) in the hot metal ladle and the reaction products of the injected CaO and Mg. The formation of MgO and the injected CaO can increase the melting temperature and viscosity of the post-desulphurisation slag. Therefore, the skimming of the post-desulphurisation slag can be more difficult due to the increased slag bulk, altered slag consistency and deteriorated slag conditions. Total iron (TFe) content in the skimmed slag was found to be up to 75 wt% in the form of iron droplets dispersed in the slag, which is one of the big yield losses in the steelmaking process. Efficient slag skimming may not be possible without slag modification for the heats with low hot metal temperature (<1300 °C) and high CaO/Mg injection for producing ultra-low sulphur steel grades. In this study, the effect of the post-desulphurisation slag chemistry on the separation of metal from slag was investigated for the varying hot metal desulphurisation conditions (e.g. BF slag carryover, the amount of CaO/Mg injected, hot metal temperature) via theoretical calculation and laboratory experiments, that is, flow temperature measurement by hemisphere method and metal settling experiments. It was found that slag basicity (%CaO/%SiO2) and MgO content are the two key factors to control. A model for optimising the slag chemistry was proposed and applied successfully in the plant production.