TRANSACTION PAPER

The effectiveness of current control of submerged arc furnace electrode penetration in selected scenarios

A.C. Mulholland; P.J. Brereton-Stiles; C.J. Hockaday

Mintek, Randburg, South Africa

SYNOPSIS

The electrical control of three-electrode submerged-arc furnaces suffers from a number of complexities, mostly due to the nature of the furnace electrical circuit providing power to the furnace where the three electrodes are interconnected within the furnace through the molten metal bath. This gives rise to what is known as the interaction effect2, where variations in one electrode's current can cause comparable changes to the currents in the other electrodes, particularly in larger furnaces with low power factor.
Resistance-based control of the electrode penetration has largely alleviated these problems, since the resistance encountered by each electrode is predominantly dictated by the length and conductivity of the current path from the electrode's tip to the molten metal bath, which acts as the three-phase circuit's floating neutral point. Hence resistance changes due to tip position or conductivity changes beneath one electrode do not affect the resistances beneath the others, effectively decoupling the control of the individual electrodes¹.
Although resistance-based control is therefore generally accepted as superior to current-based control for the regulation of submerged-arc furnace electrode penetration, a few furnace operators still prefer to use current control under specific furnace conditions. This paper presents the results of analysing the performance of both current and resistance-based control in typical scenarios encountered on industrial furnaces, taking into account a number of factors including electrode penetration, power distribution, efficiency and asymmetry of the electrode currents. In order to accomplish this, three typically encountered scenarios were simulated.
The results obtained show that resistance control provides more benefit in all cases, however, the uneven electrode current distribution generated by resistance control when electrodes are on top stops may cause some concern if baking of the electrodes is required in this scenario.

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References

1. RENNIE, M.S. The Operation, Control and Design of Submerged-arc Ferroalloy Furnaces, Proceedings of Mintek 50: International Conference on Mineral Science and Technology, Sandton, South Africa, March, 1984.         [ Links ]

2. BARKER, I.J., DE WAAL, A., RENNIE, M.S., and KLOPPER, J. The Interaction Effect in Submerged-Arc Furnaces. 49th Electric Furnace Conference, Toronto, Canada. Iron & Steel Soc. of AIME , 1991, pp. 305-310.         [ Links ]

3. ORD, R.J., SCHOFIELD, J.G., and TAN, C.G. Improved Performance of Soderberg Electrodes, CIM Bulletin-Canadian Mining and Metallurgical Bulletin, ISSN: 0317-0926, vol. 88, no. 991, Date 1995.         [ Links ]

4. MOOLMAN, A.L., RENNIE, M.S., and BRERETON-STILES, P.J. Experiences in operation of various electric-arc furnaces under resistance control, Infacon 9, Proceedings of the 9th International Ferroalloys Congress, Quebec, 2001.         [ Links ]

5. NGWENYA, B.A., MABIZA, L., BARKER, I.J., DE WAAL, A., RENNIE, M.S., and MAILER, P. The Performance of a Resistance-Based Furnace Control systemon a Submerged-Arc Furnace. Infacon 7, Proceedings of the 7^th International Ferroalloys Congress. Trondheim, Norway. June 1995.         [ Links ]