Methane injection into the arcs of electric arc furnaces has been shown on pilot scale to lead to a remarkable arc voltage increase at constant arc current and arc length. Recent investigations have been concerned with the associated metallurgical effects making use of a gastight 150-kg arc furnace operated with two AC plasma torches. A first test with bored graphite electrodes in this furnace confirmed the power increase observed during methane injection. The carburization slowly occurring when 6 percent CH_4 were injected into the argon atmosphere of the furnace could be avoided by adding minor amounts of CO_2. A slag layer decreased mass transfer rates without noticeably affecting heat transfer. Manganese loss by evaporation was measured to investigate the influence of power increase and slag layers. From the results, an increase of 200 K was concluded for the melt surface temperature when CH_4 was added to pure argon. Methane injection into the arcs proved to accelerate nitrogen removal considerably. Starting with an intentionally high nitrogen content of about 200 ppm, the nitrogen removal rate was found to be slowest with pure argon plasma arcs, faster with 90 percent Ar + 10 percent H_2, and fastest with 95 percent Ar + 5 percent CH_4 reaching final contents of less than 20 ppm of nitrogen. Based on thermodynamic calculations, the denitrogenation reactions appear to take place via atomic nitrogen in pure argon plasma, via NH_2 in Ar + H_2 and via HCN in Ar + CH_4.
展开▼
