Nitrogen Removal in a Vacuum Tank Degasser : An Investigation on the Nitrogen Removal Performance

Sammanfattning: The impending change of processes at SSAB Oxelösund due to the HYBRIT project, where the blast furnace and LD converter are to be replaced with an EAF will have a significant impact on the manufacturing of steel in Oxelösund. One issue that will arise is the nitrogen content in the steel. Sources claim that the nitrogen content in steel from an EAF route is substantially larger, 60-70 ppm, than in steel from a blast furnace and LD converter route, which have a nitrogen content of around 25 ppm. Therefore, the nitrogen removal capabilities of SSAB Oxelösund’s vacuum tank degasser were to be examined. Industrial trials were performed where the amount of slag during vacuum treatment was lowered. The intention was that half the amount of slag removed prior to vacuum treatment and later completely slag free. This was performed in an attempt to increase the effective reaction area, where nitrogen removal occurs. Due to some practical problems with the steel mill, the industrial trial were unfortunately cut short. Consequently, only trials with half the amount of slag were performed and compared to existing process data for standard praxis. Also, nitrogen removal calculations based on the industrial data were performed. A parameter representing the overall reaction rate, which is dependent on effective reaction area was obtained, validated and subsequently applied to a future case scenario. The results indicate that the reduction in slag amount does have the desired effect, increasing the said area and increasing the rate of nitrogen removal. However, the sample size is not nearly sufficient enough to determine this definitively. The conclusions reached were that the facility does have the possibility to decrease the increased nitrogen content down to reasonable levels, around 20-30 ppm. Albeit, an increase in vacuum treatment time is probably required. Another conclusion was that surface active elements, such as oxygen and sulphur greatly reduce the nitrogen removal. Therefore, efforts should be taken to remove these elements prior to vacuum treatment. In addition, it was established that the effective reaction area is of great importance for a successful nitrogen removal. Therefore, actions to maximise this area should be taken. Finally, it was stated that further research is necessary in order to fully understand nitrogen contamination prevention- and removal techniques