Operation and control of vacuum circulation (RH) process with lance oxygen input

Additional oxygen supply via an RH top lance is mainly used for forced decarburisation in case of a lack of dissolved oxygen, and for chemical heating. To develop a dynamic control system for lance oxygen input, existing RH process models of BFI and IRSID were extended to the effect of oxygen supply on decarburisation and steel temperature. Within this model development, the effect of other oxygen sources like the reduction of slag or vessel skull oxides was investigated and considered in determining the lance oxygen efficiency. Furthermore, the influence of operation parameters like lance height and vessel pressure during oxygen blowing was investigated, and a new lance tip designed by Val Technometal for an increased oxygen yield at the voestalpine plant was tested. The extended process models were validated with process data from the RH plants of Voestalpine Linz and Sollac Méditerranée. The behaviour of decarburisation and steel temperature during RH treatment without and with oxygen lance operation can be predicted with good accuracy. Based on the process models, a dynamic setpoint calculation was developed and implemented for online control at the RH/1 plant of voestalpine. It calculates the required oxygen input for forced decarburisation and chemical heating, using a prediction of the oxygen input from other sources, and of the temperature losses and gains up to the end of RH treatment. With the model-based control, the aim values of steel carbon, oxygen and temperature can be adjusted more accurately, which results in a decreased consumption of lance oxygen and aluminium for deoxidation.