Direct current electric furnace for melting metal

Industrial electric heating furnaces – Arc furnace device – Power supply system

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Details

373103, 373105, 373108, 373 72, H05B 700

Patent

active

054105641

DESCRIPTION:

BRIEF SUMMARY
FIELD OF ART

The present invention relates to a direct current electric furnace having top and bottom electrodes for melting and refining metal using direct current arcs.


BACKGROUND ART

Electric furnaces are classified into alternating current electric furnaces and direct current electric furnaces. In an alternating current electric furnace, three graphite electrodes are inserted from above into the furnace to form arcs between those electrodes mainly through scraps or molten steel. In a direct current electric furnace, usually one graphite electrode is inserted into the furnace and direct current arcs are allowed to form between the graphite electrode and the furnace bottom as the other electrode.
In the former, since three AC electrodes are used, the upper structure of the furnace is complicated and three-phase arcs are deflected outwards by a mutual electromagnetic force, so that the quantity of radiant heat is large and the thermal efficiency is poor. Besides, the furnace wall is damaged locally due to the deflection of arc. Further, the wear of the electrodes is conspicuous, the noise is loud, and flicker is marked. On the other hand, in the latter, i.e., a DC electric furnace, the construction around the furnace top electrode is simple because the number of electrode used is generally small, and in comparison with an AC electric furnace, the decrease in unit consumption of the graphite electrode and in electric power unit consumption as well as the decrease of flicker can be expected. However, problems are encountered in the service life and safety of the furnace bottom electrode.
As stated in an article entitled "Chokuryu Kanetsuro No Genjyo To Shyorai" ("Present State and Future of DC Arc Furnaces") at page 24-33 of "Kogyo Kanetsu Ro" ("Industrial Heating Furnaces") Vol. 25 (1988), No. 2, published by Nippon Kogyo Ro Kyokai (Japan Industrial Furnace Association), as furnace bottom electrodes in DC electric furnaces there are known a small-diameter multi-electrode air-cooled type having a large number of small-diameter electrodes embedded upright in a refractory provided as a lining on the furnace bottoms and a large-diameter electrode water-cooled type having one to three round steel rods of a large diameter disposed upright in the furnace bottom.
FIG. 6 shows an example of a conventional DC electric furnace of a large-diameter electrode type. In the same figure, three top electrodes 118 made of graphite are inserted into the furnace through a furnace lid 112, while in a furnace bottom 116 three water-cooled type bottom electrodes 130 constituted by steel rods are embedded upright in a molded refractory. The diameter of each bottom electrode 130 is about 250 mm at most. Thyristors 124 constitute electrode control circuits which are each independent together with the three top electrodes 118 and the three bottom electrodes opposed thereto, to control voltage and current. Under this construction, when the total capacity of furnace transformers is 60 MVA, each transformer controls an applied electric power in the range of 20 MVA, so in a steady state there are formed three arcs.
In a DC electric furnace of such a large-diameter bottom electrode type when all of the three bottom electrodes 130 cease to conduct due to the adhesion of slag to the electrodes, one of the top electrodes 118 is connected to an anode side to form arcs between the top electrodes, whereby the scraps in the furnace can be melted. In this case, although the scrap melting time becomes longer because of decrease of the applied electric power, it is possible to cope with the non-conduction of electric current relatively easily.
On the other hand, in the large-diameter electrode type shown in FIG. 6, since three top electrodes 118 are used as in the conventional AC electric furnaces, it is necessary to provide three systems with respect to all of electrode supporting arms, electrode lift devices and electrode control circuits, resulting in that the equipment is complicated and the equipment cost and maintenance cost are increased

REFERENCES:
patent: 941419 (1909-11-01), Keller
patent: 3619464 (1971-11-01), Holzgruber
patent: 3757021 (1973-09-01), Bockman
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patent: 3949151 (1976-04-01), Kerton
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patent: 4612650 (1986-09-01), Stenkuist
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patent: 5138630 (1992-08-01), Suga
2302581 Dec. 14, 1990 Patent Abstracts of Japan, vol. 15, No. 81, M1086 25Fel.

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