Nitrogen removal from liquid steel during plasma arc heating by injecting methane into the argon arc

In view of the rising quality requirements for steels produced in the electric arc furnace mainly from scrap, the effect of hydrogen and methane additions to the furnace atmosphere during arc-heating of the steel melt was investigated with regard to the rate and the extent of nitrogen removal. Steel qualities for deep-drawing applications require nitrogen contents as low as 40 ppm, while the normal scrap-based electric arc furnace leads to nitrogen contents of 80 to 120 ppm. The tests were carried out with 150 kg steel melts in a pilot melting furnace equipped with two plasma torches containing thoria-doped tungsten electrodes. Argon was used as the plasma gas. Molecular gases were added to the furnace from the periphery. 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 % Ar + 10 % H₂, and fastest with 95 % Ar + 5 % CH₄ 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 NH₃ in Ar + H₂, and via HCN in Ar + CH₄. Plasma is needed to make atomic hydrogen available at the reaction interface.