Theoretical and Practical Aspects of Lance Skulling and Slag Foaming in BOF Vessels

Abstract

In a BOF vessel, the supersonic jet is impinges on the slag and metal surfaces and the lance can become coated with the splashed materials. This leads to an increase in the outer diameter of the lance. This phenomenon of increase in the diameter of the lance due to deposition of slag is called ‘lance skulling’. The skulling can progress layer by layer, leading to a gradual increase in the diameter of the lance. Lance skulling in BOF vessels is undesirable both from the point of view of the control of the slag formation process and for the productivity of the shop. Whereas a lance with a heavy skull has to be replaced and cleaned, heavy slopping results in the loss of material form the vessel. Skulling (due to solid slag formation) precedes slopping. Solid slag formation often results in a high phosphorus and high carbon content of metal and low steel temperature at tap. Lance height and oxygen flow rate, during the progress of the blow, should be so adjusted that the content of FeO and the activity of FeO in the slag are optimally controlled. If the FeO in the slag is reduced to a level below 5-10%, then the slag starts to become solid and the lance skulling begins. This is also manifested by spitting and the consequent changes in exhaust gas composition, and a rise in the temperature and the flow rate of the waste gases. In the present work, the morphological aspects of the lance skulls obtained from five steel plants have been investigated. The phases of varying composition are formed and ejected as droplets from/around the jet impact zone. The study of skulled samples has provided an insight into an alternative mechanism of lime dissolution occurring in the neighborhood of the jet impact region in the BOF process. Skulled samples collected from a co-jet lance have also been investigated. In addition to the morphological studies carried out in the present work, a heat transfer model has been developed to correlate the changes in the lance exit cooling water temperature with the changes in the lance height and the thickness of the lance skull. For the first time, the effect of radiation from hot spot has been considered in the heat transfer model with an objective to predict the start of lance skulling as well as the progress of the formation of skull during the progress blow in a BOF. Post combustion calculations are presented for the actual heats in a 150 ton vessel. The post combustion model can be used to control the slag formation by dynamically changing the lance height and the oxygen flow rate.