Abstract:
In a direct-current electric-arc furnace, the bottom contact, that is the hearth electrode, must be insulated from the metallic shell of the furnace vessel by insulating material. For this purpose, the bottom plate carrying the bottom contact essentially forms the bottom of the furnace vessel. This bottom contact, with an insulating material, or a material which is at least a poor electrical conductor, inbetween, rests on a part, projecting radially inward, of the metallic vessel shell.

Description:
This application is a continuation of application Ser. No. 07/566,763, filed on Aug. 14, 1990, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a direct-current electric-arc furnace comprising a furnace vessel, which is provided with a metallic shell, at least one electrode connected as cathode, and at least one bottom contact, the bottom of the furnace consisting of a lining layer which has electrically conductive bricks or other identically acting inserts, which lining layer rests on a bottom plate together with which it forms the bottom contact connected as anode, the bottom contact being insulated from the metallic shell of the furnace vessel by insulating material. 
     The invention makes reference here to a prior art as results, for example, from U.S. Pat. No. 4,228,314. 
     2. Discussion of Background 
     In direct-current electric-arc furnaces, both the electrode connected as cathode and the hearth or bottom electrode, which as a rule is constructed as a bottom contact, e.g. according to U.S. Pat. No. 4,228,314, must be insulated from the metallic shell of the furnace vessel. In the known electric-arc furnace, the bottom contact and the adjoining parts of the bottom lining rest on a metal plate. This metal plate in turn rests on a layer of insulating material on the (metallic) vessel bottom. The electrical connection of the bottom contact is made here via contact parts which are passed through openings in the vessel bottom. The insulating layer is highly stressed both by the weight of the charge and the extreme temperatures occurring during the furnace operation. Special cooling of this area is very expensive. 
     SUMMARY OF THE INVENTION 
     Accordingly, one object of this invention is to provide a novel direct-current electric-arc furnace of the generic category mentioned at the beginning whose bottom contact is optimally insulated from the vessel shell and the insulation withstands all operating stresses. 
     This object is achieved according to the invention when the bottom plate essentially forms the vessel bottom, which bottom plate overlaps a part, projecting to the inside, of the metallic shell of the furnace vessel and, with an insulating material, or a material which is at least a poor electrical conductor, inbetween, is supported thereon. 
     The advantage of the invention can be seen in particular from the fact that the entire bottom section of the electric-arc furnace is suspended as it were in a floating manner in the furnace shell, and the insulation of this bottom section from the furnace shell is simple to effect. 
     Exemplary embodiments of the invention as well as the advantages achievable therewith are described in greater detail below with reference to the drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: 
     FIG. 1 schematically shows a first exemplary embodiment of a direct-current electric-arc furnace according to the invention in side view; 
     FIG. 2 shows a detail of the support and insulation of the bottom section in another type of furnace vessel. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts in both of the different views, FIG. 1 shows a direct-current electric-arc furnace having a furnace vessel 1 which is provided with the conventional shell 2 made of metallic material. In the exemplary embodiment, the furnace has only one electrode 3 connected as cathode, but it can also have two, three or more. In the exemplary embodiment, the electrode is of hollow design, i.e. the furnace is suitable for reduction purposes. But the subject matter of the invention can also be used in a melting furnace having a solid electrode connected as cathode, normally a graphite electrode. 
     In the reduction case, concentrates, coke and lime, for example for the manufacture of pig iron, is fed through the central channel 4 in the electrode, and an electrode spot, i.e. a slag-free surface of the melt, is obtained in the conventional manner under the electrode 3. As usual, the furnace has a pouring lip 5 and a door (sic) hole 6. A bottom contact is attached in the base of the furnace. The bottom contact consists of bricks 8 in which metallic conductors 7 are arranged. The conductors 7 penetrate into a bottom lining layer 10 lying underneath which, in the example, consists of electrically conductive carbon bricks. Adjoining to the outside is the conventional furnace brick lining 9. The bricks of the bottom lining layer 10 are arranged in one or more courses and lie on a bottom plate 11 formed like a spherical surface. This bottom plate 11 has a considerable extent and forms the bottom of the furnace. It is made of steel or copper and is connected to the positive pole of the direct-current source 12. The bottom contact is to have a large extent so that spreading of the furnace current over a large part of the bottom is achieved, as a result of which inclination of the electric arc 13 is essentially prevented. The bottom plate 11 is provided with a cooling device 14 so that it can be kept at as low a temperature as possible and is not damaged by the heat of the furnace and 16 is a suction opening for exhaust gases. 
     The bricks 8 provided with conductors 7 serve as current conductors between the charge 15 and the bottom contact consisting of outer lining layer 10 and the bottom plate 11, which bottom contact can of course also be of different construction. Thus other contact-making components can be present instead of the carbon bricks. 
     In this respect, the direct-current electric-arc furnace corresponds to the prior art and is described in detail in U.S. Pat. 4,228,314 mentioned at the beginning or also in German Patent Specification 3,022,566. 
     According to the invention, the shell 2 of the furnace vessel is drawn radially inward and forms a collar 17 projecting to the inside. The bottom plate 11 overlaps the collar 17 in the radial direction. A ring 18 of insulating material is arranged in the overlapping area. In this way, the entire bottom section of the furnace is supported on the collar 17. The bottom section of the furnace floats as it were in the furnace vessel 1. At the same time, the electrical insulation between furnace shell 2 and bottom plate 11 and thus the bottom contact is effected via the insulating material 18. 
     The bent-up end 19 of the collar 17 is used for radial centering, which end 19, however, does not extend up to the bottom plate. In order to pass the forces to the insulating material in a clearly defined manner, the marginal section of the bottom plate 11 is provided with a compensating ring 20 designed roughly in a wedge shape in cross-section. 
     Either prefabricated parts, e.g. bricks or clinkers of refractory material, arranged loosely in layers one above the other or connected to one another by refractory mortar, or a casting or ramming mass can be used as insulating material. The essential factor here, apart from the low electrical conductivity, is that the insulating material withstands the compressive stresses. Furthermore, it need not be an insulator in the actual sense. It is sufficient if the electrical conductivity of the insulating material is less than the conductivity of the bottom contact in the direction of the vertical axis of the furnace by at least a power of ten. 
     The embodiment just mentioned having a casting or ramming mass is illustrated in FIG. 2 in a direct-current electric-arc furnace having a different construction of the furnace shell or bottom. 
     An annular plate 21 is fixed to the cylindrical vessel shell 2. An axially running pipe piece 22 is welded to the inner periphery of this plate 21. With regard to its function, the annular plate 21 corresponds to the collar 17 and the pipe piece 22 corresponds to the bent end 19 of the collar 17 in FIG. 1. In an analogous manner to FIG. 1, the bottom plate 11, in its marginal area, is provided with a compensating ring which is here designed as a welded construction and has a radially running ring 20a and an axially running pipe piece 20b. The pipe piece 20b is extended down beyond the plate 20a and is welded to a supporting ring 23. 
     In the course of manufacture of the furnace, before the furnace brick lining is incorporated, the bottom plate 11 is inserted together with the parts 20a, 20b and 23 and, by means of insulating spacers (not shown in the drawing), is oriented and provisionally fixed relative to the furnace shell 2 and the plate 21. A casting or ramming mass 24, e.g. refractory concrete, is then poured into the space between vessel shell 2 and pipe piece 22 and the area adjoining to the top, and in fact in such a way that the rings 20a and 23 and the pipe piece 20b are completely embedded into this mass. This is followed by the further construction of the furnace in a known manner. 
     Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described herein.