NZ197542A - Arc furnace electrode current supply - Google Patents

Description

Prk?:*:17 Dste^s): PH:h P.°. r c • C .LcL'icntjon Filed cik« W.ejMr, .f*w>. JJ/iq ; 11 4 D£C 2 J r-^ 4-4»• o a a 9 ■ t a s • iTr « i • ^ r» n F*'j5" 15. bb • .V. vJu r .. s I « <0= • eeia«B*aaaataaaaa 1984 Patents Form No. 5 i 26JU«19S, NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION "IMPROVEMENTS IN ELECTRIC ARC FURNACES" :3/WE MANNESMANN AKTIENGESELLSCHAFT a body corporate organised under the laws of the Federal Republic of Germany of Mannesmannufer 2, D-400, Dusseldorf, Germany hereby declare the invention, for which-l/we pray that a patent may l?e granted to me/us, and the method by which it is to be performed, to be particularly described in and by the following statement 1 975 42 This invention relates to a three phase electric arc smelting or reduction furnace.

In connection with the smelting of steel and especially for the manufacture of ferrous and silicon alloys, as well as calcium carbide alloys, considerable use has been made of electric arc furnaces. Such a furnace conventionally . has a vessel of - circular outline when viewed in plan in conjunction with three electrodes arranged at the apices of a triangle and extending downwardly into the vessel. The demand for ferrous alloys constantly increases, but so does the cost of energy, raw materials, and labour. A reduction in production costs and an increase in production efficiency are therefore to be desired.

An attempt to increase the heat supplied to an electric arc furnace for carrying out the metallurgical processes must involve an increased electrical input and eLectrodes and furnace hearths of ever greater cross-sections.

However, the relationship between the quantity of energy transferred through the electrodes and the practicable energy transformation in the hearth with increasing heat requirement leads to a technical/economic performance barrier for the three electrode circular furnace. One barrier is established by the maximum available cross-section of the electrodes (for graphite electrodes about 650 mm, for carbon electrodes about 1400 mm, and for Sflderberg electrodes about 2000 mm). Another barrier O 7 r k 9 - i *_ is set by the maximum practical cross-section of the circular furnace itself, since with increasing furnace size, the usefulness of the hearth surface is reduced. Nhile it is theoretically possible to exceed the limits imposed by these barriers with a circular furnace by doubling the number of electrodes and adjusting the hearth space accordingly, this is not economically justified.

Accordingly, to overcome these problems, the present invention provides a three phase electric arc smelting or reduction furnace including an elongate static furnace vessel of substantially rectangular outline when viewed in plan, and at least one group of six electrodes for the supply of electrical energy to a bath within the furnace vessel, the electrodes of the or each group being arranged in two rows of three electrodes, the rows extending longitudinally of the vessel and being arranged symmetrically with respect to the longitudinal axis thereof, each electrode in one of the rows constituting a pair with a respective transversely aligned electrode in the other row, the electrodes of each row being supplied with three phase current from a respective transformer . means, such that the electrodes of each pair are supplied with respective different phases of current.

With this; construction, it is possible'to process substantially the entire surface of the molten bath without tilting the vessel as is necessary with the conventional circular furnace vessel, whereby the vessel may be static.

The invention will now be described in greater detail by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is a schematic plan view of a first embodiment of the invention, and Figure 2 is a view similar to Figure 1, but of a second embodiment.

Referring to Figure 1, a closed reduction furnace comprises a static furnace vessel 7, side walls of which give the furnace vessel interior an outline which is rectangular when viewed in plan. In a modification, instead of meeting at right angles, the side walls merge by way of arcuate or oblique transition regions. Heat is supplied to a metal bath within the vessel by means of six electrodes passing through a rectangular cover for the vessel and arranged in two rows each including three electrodes, the rows extending parallel to the longitudinal central vertical plane of the furnace. Thus, a first row includes the electrodes numbered 1,2 and 3 and a second row includes the electrodes numbered 4,5 and 6. The electrodes are grouped in pairs, such that each electrode in the first row is aligned with a respective electrode in the second row, the axes of the pair lying in a vertical transverse plane of the vessel. Thus, the electrodes 1 and 6, 2 and 5, and 3 and 4 constitute such pairs. The spacing between the axes of the electrodes of each pair is equal to the spacing between the axes of one pair and the next, so that the electrodes 1, 2, 5 and 6 lie at the corners of a first imaginary square while the electrodes 2, 3, 4 and 5 lie at the corners of a second imaginary square. This spacing as well as the spacing between the electrodes and the side walls of the vessel is such that the areas of the surface of the metal bath encircling the vertical projections of the electrodes and processed by them combines to form a homogenous processed surface extending substantially throughout the extent of the horizontal surface of the bath.

The electrodes are energised by three phase electric current in such a way that the electrodes of each pair are subjected to respective different phases, while each electrode in each row is at a phase different from that of the next adjacent electrode or electrodes in that row. 1 9754 Thus, assuming the phases to be designated R, S and T, then as shown in Figure 1, the pair of electrodes 1 and 6 are at phases R and S, respectively; the pair of electrodes 2 and 5 are at phases S and T, respectively; and the electrodes 3 and 4 are at phases T and R, respectively. To achieve this phase distribution, each of the rows of three electrodes forms part of a three phase system separate from that including the other electrodes and supplied from a respective transformer means, or set M in the case of the first row and N in the case of the second row of electrodes. Each transformer set comprises three separate but aligned individual transformers, the primary windings jof which are connected to the three phase grid system such that the primary winding of each transformer has applied to its terminals the two phases indicated in the Figure.

First, considering, for example, transformer set M,, the terminals of the primary winding of the first transformer of this set, i.e. that shown uppermost in the Figure, receive phases S and R, those of the second or intermediate transformer receive phases T and R, and \ those of the third transformer, i.e. that shown lowest in the Figure, receive the phases T and S. The terminals of the secondary windings are connected to supply conductors leading to the vessel, and connected by flexible high current conductors with the electrodes. The electrodes, conductors, and secondary transformer windings form a delta connection, thus terminals U and X of the first transformer of transformer set M are connected to electrodes 1 and 2 respectively, terminals Z and W of the second transformer are connected to electrodes 1 and 3 respectively, and terminal V and Y of the third transformer are connected to electrodes 2 and 3, respectively. The arrangement of the second row of electrodes and transformer set N will be self-evident given this explanation.

The tranformers with the associated supply and high current conductors are arranged at the longer side of the furnace vessel in a symmetrical manner as shown by the Figure.

Thus, in each three phase current system, the high current conductors between the supply conductors and the electrodes are symmetrical to the transverse vertical plane including the axes of,the electrodes 2 and 5, centrally of the rows. The supply conductors extending between the transformer and the flexible conductors extending to the electrodes themselves are laid closely adjacent one another and form a bifilar conductor system which by virtue of the direction of the electro-magnetic field substantially suppresses the inductive reactance.

Each transformer set is provided with a load control switch and the supply to each electrode may be individually controlled according to its effective performance.

The arrangement shown in Figure 2 is similar to that shown in Figure 1, but makes use of a furnace vessel 1 97542 of extended length equipped with 12 electrodes in all arranged in two rows of six, namely a first row including the electrodes 7 to 12 and a second row including the electrodes 13 to 18. The electrodes 7 to 9 and 16 to 18 constitute a first group supplied in the manner described with reference to Figure 1 from transformers M and N, while the electrodes 10 to 12 and 13 to 14 form a second but identical group supplied from transformers and .

Compared with a circular furnace with three electrodes of comparable output, the furnace described and illustrated herein with six electrodes, operates with smaller electrodes, but nevertheless exhibits a greater electrical efficiency. By making possible the use of smaller electrode cross-sections, the conductors in the region of the electrodes may likewise, be of reduced cross-section. Since the hearth height of the furnace depends to some extent upon the electrode cross-section, the furnace in accordance with the invention may be of smaller hearth height compared with an equivalent circular furnace. The reduction in hearth height results in smaller radiation surfaces, leading to a better use of hearth space and to better electrical conditions and above all to improved electro-thermal efficiency. The rectangular furnace described•herein is also characterised by a reduced c-7 ff-.- v./' y length of the system conductors in the region of the electrode. Since the high current conductors may be shorter with reduced cross-sections, the inductance of the conductor system is essentially reduced, leading to a reduced reactance. Through the considerable reduction in impedance now made possible, the possibility arises of equipping the furnace with less highly rated transformers and reactance compensation equipment.

The construction of a rectangular furnace vessel is simpler and cheaper compared with a circular furnace and it is easier to provide with thermal insulation.

Because the whole hearth surface is utilised in carrying out the process, pivoting or agitation of the furnace vessel necessary with circular furnaces in order to distribute the energy to the hearth is unnecessary and, as described, the furnace is static.. By means of the reduction in induction, the bifilar arrangement leads to a reduction in the eddying magnetic field and avoids the bath being set in rotation and segregating the metal and slag especially in the border region, as occurs in conventional circular furnaces. Likewise, the division of the whole performance into three pairs of electrodes in connection with the symmetrical arrangement about the furnace longitudinal axis, together with the partial bifiliarility of the whole 1 975 4 2 system, diminshes the disturbing influences of a geometrical electrical asymmetry.

Because the transformers are arranged alongside the longer sides of the furnace vessel, one of the narrow sides may be kept free for the tapping equipment and the like.

Finally, with the electrical arrangements -described, the alloying bunkers may be arranged in two parallel rows and with two adjacent furnaces, only three rows of alloying bunkers are necessary, each row running parallel to the electrode rows.

Research and evaluation in experimental plants have demonstrated that a 40 mw furnace in accordance with the invention as described above is especially favourable in comparison with a 40 mw circular furnace with three electrodes, and for that matter compared with a 40 mw rectangular furnace having six electrodes arranged in a single row, other than in accordance with the invention.

Claims (9)

I o 7 r a o « /t w- - r c. - li - WHAT HE CLAIM IS

1. A three phase electric arc smelting or reduction furnace, including an elohgite furnace vessel of substantially rectangular outline-when viewed in plan, and at least one group of six electrodes for the supply of electrical energy to a bath within the furnace vessel, the electrodes of the or each group being arranged in two rows of three electrodes, the rows extending longitudinally of the vessel and being arranged symmetrically with respect to the longitudinal axis thereof, each electrode in one of the rows constituting a pair with a respective transversely aligned electrode in the other row, the electrodes of each row being supplied with three phase current from a respective transformer means, such that the electrodes of each pair are supplied with respective different phases of current.

2. A furnace as claimed in claim 1, wherein two adjacent pairs of electrodes are disposed at the corners of a square.

3. A furnace as claimed in claim 1 or claim 2, i6S£P1984 - 12 - 19754 2 wherein a tap hole is arranged adjacent one of the shorter sides of the vessel.

4. A furnace as claimed in any preceding claim, wherein the electrodes in each row of the or each electrode group are connected with secondary transformer winding in such a way that the electrodes form the connecting points of three phase delta connections.

5. A furnace as claimed in any preceding claim, wherein high current conductors between the transformer secondary winding and the electrodes of the or each group are arranged symmetically with respect to an axis extending transversely of the furnace and passing through the axes of the central pair of electrodes.

6. A furnace as claimed in any preceding claim, wherein the current supply to each electrode is individually controllable.

7. A furnace according to any preceding claim, wherein the ratio between the number of rows of electrodes and the number of electrodes in each row is substantially equal to the ratio of the shorter side of the vessel to >*$£?1984 +i - 13 - the longer side of the vessel.

8. A three phase arc smelting or reduction furnace, especially for the production of ferrous or silicon alloys or the like, comprising an elongate furnace vessel, having side walls which meet at right angles or in insubstantial arcuate transition portions, electrical energy being supplied to a bath within the vessel by at least six electrodes which in the direction parallel to the longitudinal axis of the vessel are spaced apart by equal distances, a predetermined number of adjacent electrodes being included in a circuit independently of the remaining electrodes, each such electrode group being associated with a. transformer or transformer group connected with the electrodes by way of high current conductors with closely adjacent bifilar run cables, wherein: (a) the number of electrodes amounts to six or a whole number multiple of six, \ (b) the electrodes are so arranged in two rows running parallel to the longitudinal axis of the untippable furnace vessel, that each row contains A half the number of all the electrodes provided, \ (c) two adjacent electrodes, as well as two electrodes lying opposite these adjacent electrodes, lie at 1 O 7 r .< 2 -Tithe corners of an imaginary square, (d) the mutual spacing of the electrodes as well as the spacing of the electrodes from the vessel wall is such that the activated surfaces of the bath extending around the projected surfaces of the electrodes are transformed into a homogenous surface which covers as great as possible horizontal cross-section of the vessel, (e) the electrodes are related to one phase of the three phase system, such that each phase corresponds to a third of the number of electrodes provided, and three electrodes arranged along the vessel longitudinal side form a three phase current system, (f) in each of such three phase systems, the high current conductors are connected with the transformer secondary windings by way of the , electrodes in such a way that the electrodes form the connecting points of a three phase delta connection, (g) the electrodes form respectively a star connected three phase system, . WSEPi984 1 o 7 r o i / / h ^ d. - 15 - (h) the respective three phase current system formed through the uniting of three electrodes is arranged symmetrically to the vessels longitudinal axis, (i) each three phase current system is supplied with a different phase sequence, such that related to the furnace longitudinal axis, the opposite electrodes are supplied with different phases of three phase current, (j) in each three phase current system, the high current conductors between the transformers and the electrodes are arranged symmetrically to an axis passing through the axis of the central electrode of the respective three phase current system and extending parallel to the transverse axis of the furnace, and ;EP^984 713^ (k) the arrangement of the high current conductors ,between the transformers and the electrodes is symmetrical about the longitudinal axis of the furnace vessel.

9. A furnace substantially as hereinbefore described with reference to and as illustrated in Figure 1 or Figure 2 of the accompanying drawings BALDWINysON & CAREY