Modelling of AC Arcs in submerged-arc furnaces for production of silicon and ferrosilicon

Three-phase submerged-are furnaces for production of silicon metal and ferrosilicon have are currents around 100 kA, the phase voltages are typically 100 V and the total furnace power 10-60 MW. The area burn in gas filled cavities or "craters", where the main atomic components of the plasma mixture are silicon, oxygen and carbon. Two quite different simulation models for high-current AC arcs have been developed: the simple PC based Channel Are Model (CAM), and the more sophisticated Magneto-Fluid-Mechanic (MFD) model, which is here described in some detail. Simulation results obtained with the channel are model has previously been reported at this conference [I]. The coupling between the arcs and the AC power source is described by a complete three-phase Electric Circuit Model. A three-phase model is required to predict correctly the overharmonics spectra. Modelling results for similar to1 kA AC arcs in a laboratory plasma reactor agree satisfactorily with electrical measurements. In the industrial similar to 100 kA case the simulations clearly show that the maximum possible are length is 5 - 10 cm, which is much less than previously assumed. Preliminary results with a Cathode Sub-Model for high-current AC arcs indicate that the cathode current density varies considerably during an AC period, while the spot radius remains almost constant. Model simulations further show that the influence of the easily ionizable contaminants Ca and Al on are behaviour is much less than expected. Preliminary studies of the effect of Fe vapour on the plasma properties suggest that modelling results obtained for silicon metal furnaces are also applicable to ferrosilicon furnaces.