US2959630A - Electric arc reduction furnace - Google Patents

Description

Nov. 8, 1960 c.v RESCHKE ELECTRIC ARC REDUCTION FURNACE 3 Sheets-Sheet 1 Filed Nov. 12, 1957 INVENTOR.

Nov. 8, 1960 c. RESCHKE 2,959,630

ELECTRIC ARC REDUCTION FURNACE Filed Nov. 12, 1957 3 Sheets-Sheet 2 INVE R.

Carl /ec e BYWJW Nov. 8, 1960 c. RESCHKE ELECTRIC ARC REDUCTION FURNACE 3 Sheets-Sheet 3 Filed Nov. 12, 1957 United States atent O ELECTRIC ARC REDUCTION FURNACE Carl Reschke, Duisburg, Germany, assignor to Demag- Elektrometallurgie G.m.b.H., Duisburg, Germany, a corporation of Germany Filed Nov. 12, 1957, Ser. No. 695,752

Clams priority, application Germany Nov. 14, 1956 5 Claims. (Cl. 13-9) Thisinvention relates to electric arc furnaces, and in particular, to polyphase smelting furnaces adapted primarily to produce pg iron.

Heretofore in the production of pig iron, electric arc furnaces have not been competitive with blast furnaces because operational economy of arc furnaces depends somewhat upon their size and it has not been feasible to construct an arc furnace of such size as to develop optimum operational economy. For example, it is common for blast furnaces to produce between 800 and 1200 metric tons of pig iron per day, whereas in the case of arc furnaces an output of 200 metric tons per day has not as yet been surpassed.

' It is known to produce pig iron in a circular electric furnace whose three electrodes are arranged triangularly, but the capacity of such a furnace is limited by the load capacity of the individual electrodes and the impracticability of spacing the electrodes at greater distances in accordance with an increase in furnace diameter. This situation is aggravated somewhat by the increase in electric inductive losses that accompanies spacing three-phase conducto rs at greater distances and is accompanied by other disadvantages that will be evident later.

, Accordingly, the best Construction for large arc furnaces is the oblong form and consequently the invention is based on this construction. In an oblong furnace, it is possible to arrange the electrodes in close triangular spacing in independent or dependent electric systems or electrode groups, and to connect the groups in such a way so that all adjacent electrodes are in two difierent phases. By so arranging the electrodes in what amounts to staggered, parallel rows, it is assured that between each electrode pair, constant voltage exists and the density of electric energy, and therefore heat, is equally distributed over the entire furnace. V With the electrodes arranged in the manner above indi- (fated a number of diificulties are brought about which make it understandable why hitherto existing suggestions for the Creation of large scale electric furnaces remained a paper technique. For example, when an initial investment is made in an electric furnace large enough to lead to operational economy there will be times when it is desirable to operate such furnace at less than full load. It has already been suggested that it is uneconomical to operate several smaller arc furnaces comparable to a bank of blast furnaces, since the efficiency of the arc furnace depends somewhat upon its size and upon operating at full capacity. One purpose here is to disclose a flexible design and other features of novelty for an electric furnace which enables expansion and contraction of its capacity in accordance with demand for its output and to enable production of pig iron on a basis competitive with blast furnaces.

A further object is to provide an are furnace made of easily expandable sections that are adapted for independent or joint operation with each other.

' Amore specific object of this invention is to provide an are furnace structure with a cover comprising a plurality of similar cover parts that are adapted to be assembled in the nature of building blocks when it is desired to expand the capacity of the furnace over that of the original installation.

A further object of this invention is the provision of an arc furnace whose electrodes are so arranged that their inductive losses are minimized by virtue of the electrodes being spaced triangularly with respect to each other and with respect to adjacent groups. An important adjunct of this object is that the busses supplying the electrodes of the same phase are easily placed in close proxmity with each other with a view toward reducing the inductive losses incident to transmitting heavy currents over relatively long circuits.

Yet another object is to provide an electric arc furnace that requires low initial investment in transformers, controls and other auxiliary equipment because no reserve capacity is needed, but still allows for increasing its capacity with repetitive installation of similar auxiliaries and furnace parts accompaned by increased economy on incremental investment.

According to the invention, a large electric arc furnace is characterized by the electrodes being arranged in rows along the longitudinal direction of the furnace wherein the electrodes are joined in groups to electrically closed systems that may be connected or disconnected individually. Thus, the ease of disconnectng the individual systems or adding to them enables alterng the furnace capacity with the greatest facility and there is no compulsion to keep the furnace capacity limited, which would be the case if the capacity were to be increased over the original plant with prior art constructions. The invention, therefore, largely obviates the prejudce against large electric are furnaces for the production of pig iron.

It is known how to operate individual circular furnaces using closed, three-phase alternating current systems including a common neutral electrode that may be in or out of circuit according to whether it is desired to op erate the furnace at full or approximately half capacity. However, this sole alternative exhausts the possib ility for varying the capacity of a circular furnace. It is here a concept of great moment that only the oblong furnace, providing for a plurality of tap openings along the longitudinal walls, is suitable for enabling expansion and contraction of productive capacity as indicated above. It may be further commented, that oblong electric furnaces heretofore encountered difliculties insofar as practical dimensions of the furnace cover are concerned, for it is frequently desirable to gain access .to the furnace' in order to remove foreign objects and broken electrode fragments, for example. Formerly, under such circumstances the entire furnace had to be disconnected and it is easy to see that access to the furnace interior would be rather d fficult where an enormous cover had to be removed for this purpose. In accordanee with the invention, this difficulty has been obviated by making the cover in a number of individual, geometrically similar parts which are adapted to interfit in the manner of building blocks and which are provided with such suitable marginal comple mentary recesses as to enable embracng the charging tubes, the exhaust gas tubes and the electrodes. Suitable sleeves and glands are provided for eifecting a substantially gas proof seal wherever any of these items enter the furnace through its top.

The idea of making the furnace top in parts further facilitates increasing the capacity of the furnace since the cover parts may be premanufactured, and, adopting the building block principle, the furnace may be extendcd to any desired size. There are no significant ditficulties iusofar as enlarging the furnace shell or body is concerned as the latter may be easily opened at its end since itcon sists in nothing but a metallic coverng and refractory brickw'ork.

A convenient arrangement of the electrodes, from an electrical viewpoint as well as facilitating uniform formation of the cover parts, consists of two rows of electrodes which are located at" the ap'i'ces of' e'quilateral 'triangles, whereby the cover' parts have a sole ent'rance for one electrode in their symmetry axis. parts are adapted to rest on a longitudinal wall of the furnace shell and at a distancefrom' this longitudinal wall the cover part is Suspended or supported' by means of a supporting arm carried on turnbuckle rods', for example.

The furna'ce represented in' the drawing's ex'emplifies the'` const'ru'ctio'n outlin'ed' above and a more specific' description will be set forth in reference to the following drawings in which Fig. 1 represents' a vertical' cross section taken through an electric arc furnace constructed according to the invention;

Fig. 2 is a plan view of the furnace, emphasiz'ing the arrangement of the electrodes and cover parts, and with parts omitted; and,

Fig. 3 is a schematic plan view of the furnace showing the electrical conneotions to the electrodes.

Referring to Fig. l, one sees that the furnace comprises a refraotory body 1 defining a cavity for receiving a charge 2, of material to be melted, and into which a double row of electrodes 3 extends. Refractory furnace body 1 is surrounded and supported by a metal shell framework 12 on a suitable foundation. The shell 12 may consist of structural members and plate as is well known.

Current is supplied to the electrodes 3 through the agency of bus bar groups designated R', S' and T' that run in a horizontal direction laterally across the furnace top. A suitable connector assembly 13` constitutes termination of the busses R', S', T' and serves as a means for attaching fiexible leads 14 to electrode clamps 15. The electrode connectors and clamps are merely symbolized in this description, but those versed in the art know of their more detailed construot'ion.

Referring particularly to Fig. 3, it will be seen that the electrodes R, S, T are arranged in groups of three, constituting a del ta connected group in this case, and which groups and their associated furnace sections are designated respectively by the letters A, B and C. The various electrode groups are supplied from their associated transformers 4a, 4b and 4c, respectively. The primary connections to the transformers have been omitted for the sake of brevity, but it will be understood that all transformers may be connected to a common high voltage bus. In Fig. 3 the phases of the electrodes are designated by the letters R, S and T, and it will be noted that each electrode group is arranged triangularly to bring about minimal inductive losses and to facilitate the electrode connections coming fromopposte sides of the furnace for a purpose to be explai'ned more fully hereinafter.

The connection and arrangement of the conduotor pars comprising the various busses R', S', T' is such that each triangle of electrodes R, S, T uses all three phases of the three-phase alternating current system and each furnace section A, B or C can be activated or inactivated separately so that, when in use, each electrode is opposite an electrode of a different phase in two directions. Electrical symmetry is thereby assured to' the highest possible degree. The center triangle of triode R, S, T in section B, for example, is supplied from the three paralleled transformers b in such manner that the electrodes R, S and T constitute a delta connected load upon the transformer tb. The situation respecting other groups is similar.

It will. be observed in. Fig. 3 that the lowermost in the 4-b group of transformers has secondary busses designated by the letters S' and R' which respectively connect to electrodes S and R in the electrode group. Shifting attention laterally, it will be noted that the uppermost transformer in the :ta group has secondary busses designated by the One end of the cover letters R' and T', and that the R' bus connects to the R electrode in group A; Thus, where the R' bus crosses to connect to its corresponding; electrode in group B, it is paralleled by an R' bus extending diagonally across the furnace to connect the R electrode in group A. The proximity of the R busses in this case exemplifies what happens with respect to connecting the other electrodes and the arrangement tends to reduce inductive losses by virtue of busses in the same phase relationship to each other being. closely adjacent.

From the description given thus far it is evident that any of the phase groups of electrodes A, for example, may be operated 'ind'ependently or jointly with adjacent groups B and C. Moreover, the arrangement is such that when the furnace is expanded or contracted, the electrical parts will not be afiected so as to produce different inductive losses between the electrodes R, S, T or between the busses R', S', T' thatsupply them.

Moreover, the transformers for the various groups may be initially of small size, without reserve capacity or wasted. investment, since when the furnace is expanded only sufficient transformer capacity for supplying the added turnace section need be added.

As indicated earlier, in order to gain the greatest advantage from the invention, it is necessary that the cover or furnace top be made up of a plurality of cover sections 5, each of which is interfitting and similar so as to enable practice of the building block principle when expansion or contraction of the furnace capacity is desired. Each cover part is made of suitable refractory material and provided with holes 16 for receiving charging tubes 7 after the cover is located. In this way the charging Capacity of the furnace is easily expanded in accordance with increases in` the furnace melting capacity.

In Fig. 2 it will be seen that each cover part 5 is further provided at its corners and triangular apex with. an` arcuately shaped notch 17 that complements other similar notches to define a substantially round opening when they are interfitted for receiving additional charging tubes 7 along the zig-zag longitudinal center line of the furnace. This arrangement enables staggering of the charging tubes and causes the material being charged to disperse equally between the electrodes R, S, T on the interior of the furnace. The cover parts are further provided with a large aperture in which there is disposed a cup gasket 6 of re-entrant cross section and adapted to seal the electrodes R, S, T with respect to the furnace cover, see Fig. 1 particularly. Of course, the cover part aperture in which the gasket resides is larger than the electrodes and the gasket 6 is easily removable.

Only at the head ends of the furnace are the cover parts 5' of a different shape than the others 5 in order to maintain the rectangular form of the urnace. The various cover parts rest in cantilever suspension where one end of each of'them bears on the metal shell 12 at 19, see Fig. l, and another end extends over the furnace charnber while it is supported on turnbuckle rods 9 that are suitably anchored.

Cover parts 5 of refractory material may be bounded by a metallic framework such as an angle iron frame whose one leg protndes upwardly so that adjacent interfitting parts 5 join in a metal-to-metal abutment that enables welding the cover parts at their seams if desired. The welded seams are suggested by broken lines designated 20. The metallic adjoining seams may be covered by strips such as 8 which may be welded to the angle frame to further effect a secure sealed joint. In the alternative, the abutting angle irons with one leg extending upward may be covered by overlapping inverted channel irons, not shown. This construction permits abandonment of the usual curved non-selfsupporting cover structure and allows development of the furnace top as an uninterrupted refractory plate.

Because of the cover parts 5 being designedfor practicing the' building blockprinciple, an enlargement of the furnace top, increasing the number of electrodes, and thereby increasing the furnace capacity is easily possible. The various furnace sections A, B and C are provided with one of more pouring spouts located on one longitudinal side of the furnace and arranged in such way that they permit tapping of the melt according to whether the furnace is adapted for full or partial Capacity. Two gas and fume removal conduits 11 are placed in the endmost cover sections for a purpose well understood by those practicing the art.

The building block principle or modular construction of the furnace and its top has advantages beyond enabling enlargement of a plant. It is no longer necessary, according to the invention, to design a special furnace for all given conditions, for uniformity results where the electrodes, their holding and adjusting devices, the regulating devices, arrangement of the conductors, etc., and finally location of the transformers is merely repetitive regardless of the furnace size. Hence, design effort is 'educed appreciably and each new plant can be projected and constructed within the shortest time through stocking of similar parts for different possible furnace sizes. This results in considerable cost reduction for plant expansion and improves the competitive position of an arc furnace as compared with a blast furnace.

In laying out a plant using a furnace according to the invention, it is advisable to keep the floor space adjacent an end of the furnace clear of the usual auxiliary equipment so that expansion can be carried with the greatest dispatch whenever circumstances dictate. Moreover, this enables the furnace capacity to be altered without interrupting production of the existing plant except that the furnace will have to be cooled when an end is removed in order to facilitate connection of the added part to the furnace.

In Summary, there has been disclosed a furnace construction that enables most eflicient arrangement of the electrodes and their supply busses. Furthermore, it has been shown how the furnace top may be made of interfitting refractory sections that cooperate with each other to extend the cover area and facilitate acceptance of additional charging tubes and electrodes without exertion of serious design effort. Although a preferred form of the invention has been described, it will be understood that the invention may take a number of different forms and may be variously embodied. It is, therefore, to be construed according to the claims which follow.

It is claimed:

1. An electric furnace comprisng a furnace body having a melting cavity therein, a pluralty of electrodes projecting into the melting cavity, said electrodes being staggered with respect to each other in parallel rows running in the longitudinal direction of the furnace, a cover for said furnace cavity comprisng a multiplicity of refractory cover parts disposed adjacent each other and extendng laterally across the furnace from one side thereof, said cover parts being of similar configuration and including a symmetrical end portion extending longitudinally of the part, said end portions complementing and interfitting with ofiset end portions extending from the other furnace side, said cover parts having apertures for receiving said electrodes therethrough at intervals along the longitudinal direction of the furnace.

2. The invention set forth in claim 1 wherein the one symmetrcal end portion of said cover part includes diverging margins defining an apex and opposite corners, said apex and said corners each being notched for defining part of the boundary of an opening, the rotches of one said cover part complementing comparable notches in other endwise located cover parts to define apertures for receiving charging tubes or the like.

3. The invention set forth in claim 1 wherein the electrode aperture has a larger diameter than required for the electrode and said aperture is occupied by an easily removable ring that seals the electrode With respect to the cover part.

4. An electric furnace having a melting cavity therein, a pluralty of electrodes projecting into said cavity, said electrodes being arranged in discrete groups spaced at the corners of an equilateral triargle, adjacent groups of electrodes having their apices oppositely directed and said groups being disposed to define stagge'ed electrode rows running along the longitudinal direction of the furnace, said furnace cavity being enclosed at its top by a multiplicity of substantially similar and abutting cover parts generally comprisng one-half of the total cover and whose axes of symmetry are parallel with each other, similar cover parts generally comprisng the other half of the total cover being in endwise relation to the first mentioned cover parts and with their aXes of symmetry in endwise alignment With lines of abutment.

5. The invention set forth in claim 4 wherein the cover parts include a metal frarne and said frames are welded to each other to form an integral furnace cover.

References Cited in the file of this patent UNITED STATES PATENTS 1,113,778 Gray Oct. 13, 1914 1,127,475 Marshall Feb. 9, 1915 2,368,998 Missim Feb. 6, 1945 2,752,410 Olsson June 26, 1956 FOREIGN PATENTS 28,536 Great Britain of 1910 519,437 Belgium May 15, 1953 557,654 Germany Aug. 26, 1932

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