Modeling Research Of The Electrical Arc Furnace For The MgO Crystal Production

Liaoning Province is well known in the world for its enormous and superior quality of natural magnesite deposit. MgO is produced from the process of heating of natural magnesite. When calcined or dead-burned MgO is heated in excess of 2800℃in an electric arc furnace, MgO crystals are produced. MgO single crystal is used widely as substrate for high temperature superconductor (HTS) thin films and also a kind of an important optical material.Due to the hostile environment for observing the process occurring in the inner zone of the furnace, direct measurements on the arc plasma or the molten bath by conventional diagnostic method are impractical. The problems including low productivity of high qualified crystals, high power consumption and poor control strategy still exist in the arc fused method to grow MgO crystals. Nowadays, depending on the advangtages of the low-cost resources, the electric arc furnace for MgO production has been widely used in China. Howerver, because the impracticability of the measurements inside the furnace leads to the lack of the fundamental understanding of the underlying physical mechanisms in the furnace, the improvement of the control system has been mostly developed by the semi-empirical or empirical models. A dramatic technology improvement is not possible without automatic operations; the thermal-electric analysis could not describe the heat and mass transfer phenomena in the furnace; Modern control theory could not be successfully applied to the furnace control system without the efficient simulation support. So, based on the above considerations, the computational fluid dynamics models are used to better understand the physical mechanisms in the furnace and to give decision support for the control system.In order to estimate the heating effect of the arc plasma and obtain equivalent parameters, a three dimension magnetohydrodynamic model based on finite element method has been developed for the arc plasma in a DC electric arc furnace. Setting proper boundary conditions, the SIMPLEF algorithm is used to analyze the characteristics of the fluid flow and temperature field of the arc plasma. The calculated results show good agreements with the published measurements in a pilot-scale furnace. The behavior of arcs for different current levels and different arc lengths has been studied. Much of the energy from the arc is delivered to a localised area directly beneath the arc. The distribution of the arc pressure on the bath surface shows that the arc plasma impingement is large enough to cause a crater-like depression in the surface of the bath. It is also found that for a constant arc current the pressure on the bath surface increases with the decreasing arc length, but it does not keep growing. For example, for the arc plasma with 10kA current, the critical length is 3cm. The model can also be used to calculate the arc power, arc voltage, and arc efficiency, and the results are important for the boundary condition settings of the molten bath in the DC arc furnace.A twin-electrode DC submerged arc furnace has been designed for MgO production and this technique has been found to be another effective method to grow high quality MgO crystals. In order to describe the environment of crystal growth and estimate some important parameters, we present a three-dimensional magnetohydrodynamic model of the DC furnace to investigate the heat and fluid flow phenomena in the bath. It is assumed that the flow direction is dominated by the Lorentz Forces. The shape of the melt-solid interface is found to be significantly affected by the electromagnetic stirring. It is observed that the environment is more suitable for the crystal growth between each electrode bottom and the shell, and the stable environment may be affected by the variation of the flow field which is determined by the current. For example, the shape of the bath is significantly affected with a large current of 10kA, and the volume of the bath becomes much smaller when the current is 6kA. The predicted shape of the molten bath shows good agreement with the experiments. Other detailed information including the electric power of arc, the arc efficiency, the voltage drop of arcs, the resistance of the bath and the Joule heating power is also given approximately by the model to improve the operation strategy.AC submerged arc furnace designed for MgO production is still universally used nowadays. For this method, a three-dimensional finite element method based model of the furnace is presented. It is assumed that the furnace is three-phase balanced and is in a steady state. The arc efficiency, voltage drop of the arcs, arc heating power and the Joule heating power of the bath are calculated approximately. The results reveal that the temperature control of the furnace begins to more depend on the Joule heating power rather than arc heating power if the current becomes small. The Joule heating power may prolong the crystal growth time in the second and last stages. It is also found that the formation of the molten bath is significantly affected by the electromagnetic stirring which is determined by the current. The predicted bath shape agrees well with the measurements. In experiments the high quality MgO crystals were mainly around the bottom of the bath but the largest ones always appeared between each electrode bottom and the shell. Results calculated by the model are from three cases with currents of 12kA, 10kA and 6kA. It is found that the crystal growth environment will not significantly affected by the current fluctuation. The radial distance of the bath's boundary is about 0.75m. Comparing with the DC method, a stable temperature field and a small disturbance of the electromagnetic stirring allow larger crystals to be formed in these locations.