High-Power AC Arcs in Metallurgical Furnaces

Abstract

A typical three-phase submerged-arc furnace for production of silicon metal and ferrosilicon has electrode currents &sim;100 kA, phase voltages &sim;100 V and total furnace power &sim;10 - 60 MW. The arcs bum in gas filled cavities or "craters", where the main atomic components of the plasma mixture are <i>silicon</i>, <i>oxygen</i> and <i>carbon</i>. Two quite different simulation models for high-current AC arcs have been developed: the simple PC based <i>Channel Arc Model (CAM)</i> &#91;1&#93;, and the more sophisticated <i>Magneto-Fluid-Dynamic (MFD)</i> model, which is here described in some detail. The coupling between the arcs and the AC power source is described by a complete three-phase <i>Electric Circuit Model</i>.<br> Modelling results for &sim;1 kA laboratory AC arcs agree satisfactorily with electrical measurements. In the industrial &sim;100 kA case the simulations clearly show that the maximum possible arc length is 5 - 10 cm, which is much less than previously assumed.<br> Preliminary results with a <i>Cathode Sub-Model</i> for high-current AC arcs indicate that the cathode current density varies considerably during an AC period, while the spot radius remains almost constant.<br> Model simulations further show that the influence of the easily ionised contaminants <i>Ca</i> and <i>Al</i> on arc behaviour is much less than expected. Preliminary studies of the effect of <i>Fe</i> vapour on the plasma properties suggest that modelling results obtained for silicon metal are also applicable to ferrosilicon furnaces.<br> Arc splitting - i.e. several parallel arcs appearing simultaneously - may also playa role in the furnace craters.