FR2674946A1 - Electric furnace for metal production - Google Patents

Abstract

The invention relates to an electric furnace for production of a metal such as steel, comprising a tank (1) limited by a lateral wall (21) surrounding a bottom covered with a floor (2) made of refractory material forming a basin with raised edges, and closed by a removable cover which can be penetrated by at least one electrode, the whole being mounted in a pivoting manner on either side of a transverse median plane to allow the pouring out by overflowing of respectively the steel via a front orifice and the slag via a rear orifice which are each positioned above an overflow threshold (23). The rear part of the floor (20) covering the bottom is prolonged by an essentially vertical wall (24) made of refractory material which rises above the level of the overflow threshold (23), and the rear orifice (4) for discharging the slag is arranged over its entire height through said vertical wall (24) of the floor (20) and is associated with a removable shutting member (41) which is essentially impermeable to air and to liquid metal.

Description

 The subject of the invention is an electric furnace for producing a metal such as steel from a ferrous material such as scrap and also covers the various modes of use of such an furnace.

 In general, an electric arc furnace comprises a melting tank limited by a refractory hearth surrounded by a side wall and closed at its upper part by a removable cover, the tank being associated with one or more connected electrodes to a power source. Each electrode is fixed to the end of an arm extending in overhang over the tank and penetrates vertically therein through the cover through an orifice provided for this purpose. The furnace can be supplied with alternating current by means of three electrodes or alternatively with direct current. In this case, one or more consumable electrodes are used which cross the vault and are associated with one or more fixed electrodes placed in the hearth and connected to the current source, each by a return conductor.

 The raw scrap is supplied in containers called "baskets" of transport, by means of an overhead crane passing over the tank. For loading, the vault constituting the cover as well as the electrodes are separated from the tank and, for this purpose, are mounted either on a gantry horizontally movable, or on arms mounted to rotate about a vertical axis on a base placed at side of the oven.

 The melting of the scrap takes place thanks to the production of electric arcs from the electrodes and the molten metal forms a bath covered with a layer of slag and which accumulates on the sole, the latter having a bottom in hollow bowl shape. Often, the hearth is associated with a pouring spout through which the molten steel is poured through a hole placed at a level higher than that of the bottom of the bowl and whose lower edge constitutes a metal retaining threshold. The tank can tilt while rolling on a horizontal support plane so as to raise the level of the steel which passes above said threshold and is poured by the pouring spout into a pocket placed below the oven.

 On the rear part of the tank, on the side opposite the steel pouring spout, the side wall of the furnace is provided with a second so-called scouring orifice, through which the slag covering the metal bath can be periodically removed.

Indeed, the lower edge of the scouring orifice also forms a threshold placed, sometimes, at a level slightly higher than the steel pouring threshold and the evacuation of the slag is carried out by natural overflow and can be facilitated by tilting the oven backwards, the angle of inclination being adjusted so as to allow the slag to be poured into a pocket placed below the oven while retaining the steel in the bowl.

 The scouring opening generally has a fairly large section and is closed by a removable door which is rarely sealed. This results in air entering the oven which can disturb its operation and, in particular, the thermal balance.

 Furthermore, in the converters for producing steel from cast iron, it is customary to improve their operation by injecting a gas which may be an oxidizing gas into the bath or even inside it. such as air or oxygen or a neutral gas such as nitrogen or argon producing a stirring effect favorable to the metallurgical process.

 The injection of gas can be accompanied by an injection of pulverulent products such as coal, lime, etc.

 Such an injection of air, oxygen or a neutral gas or else of pulverulent products is also advantageous in the case of an electric scrap melting furnace. However, it is more difficult to achieve because, in an electric oven, the depth of the metal bath is more limited than in a converter.

 The invention relates to improvements enabling the operation of an electric oven to be improved, in particular by facilitating the installation of gas injection nozzles or other products.

 However, the invention also has other advantages, in particular that of improving the tightness of the tank by avoiding air inlets.

 The invention therefore applies, in general, to an electric oven comprising a tank bounded by a side wall surrounding a bottom covered with a sole made of refractory material forming a bowl with raised edges substantially symmetrical with respect to a plane. longitudinal median, a removable cover for closing the tank and at least one electrode connected to a current source and capable of entering the tank through an orifice provided in the cover, the assembly being pivotally mounted on either side of a transverse median plane perpendicular to the longitudinal plane of symmetry, to allow the overflow, respectively, of the steel by a front orifice and the slag by a rear orifice, said orifices being placed respectively on either side of said plane median, each above an overflow threshold.

 According to the invention, the rear part of the bottom covering the bottom is extended by a substantially vertical wall of refractory material, which rises above the level of the overflow threshold and the rear slag discharge orifice is formed entirely through said vertical wall of the floor, and is associated with a removable shutter member substantially airtight and liquid metal.

 In a particularly advantageous manner, the tank is provided with means for injecting a fluid and / or pulverulent products passing through the rear wall of the hearth to open below the level of the slag discharge orifice, these means injection may be constituted by at least one nozzle supplied under pressure for the injection of a stirring fluid and / or treatment of the liquid metal or powder.

 Preferably, the angle of inclination towards the rear of the tank can be adjusted so as to give the latter at least two tilting positions towards the rear, respectively a first position for which the injection nozzle opens. inside the metal bath and a second position for which the injection nozzle opens at the interface between the metal and the slag.

 Furthermore, the refractory hearth can be extended on the front of the tank by a raised part forming a platform placed above the upper level of the metal bath, when the axis of the oven is vertical, and surrounded by a substantially wall vertical rising above the level reached by the slag covering the metal bath, said platform being pierced with a pouring orifice with substantially vertical axis, associated with a removable shutter member, airtight and liquid metal .

 Generally, the side wall of the tank has a cylindrical shape of revolution around the axis x ′ x and the elm platform in which the pouring orifice is formed forms a projection covered by a cover. In this case, the pouring orifice is placed outside the cylindrical internal space limited by the side wall.

 However, it is also advantageously possible to place the pouring orifice inside this cylindrical space, the platform then being inclined relative to the axis of the tank so as to be tangentially connected to the bowl of the floor which is shifted backwards. The vertical wall surrounding the elm platform is then in the extension of the side wall.

 Preferably, the removable shutter member of the rear orifice comprises a plug of cross section substantially identical to that of the orifice, made of refractory material at least on its part facing the inside of the tank and of the means for controlling the movement of the plug, at least along the axis of the orifice, between a closed position for which the plug is engaged in the orifice and a withdrawn position, spaced from the tank.

 Advantageously, the means for controlling the movement of the plug determine a first movement of the plug along the axis of the orifice between the closed position and a first position for withdrawing the orifice, then a second movement transversely to the axis of the orifice, between the first withdrawal position and a second position spaced from the passage of the slag.

 In particular, the closure plug can advantageously be mounted at the end of the rod of a jack determining the first axial displacement of the plug and the body of which is fixed to the end of an arm mounted to pivot around a axis spaced from the axis of the orifice and associated with means for controlling the pivoting of said arm between the first position of withdrawal of the plug, placed in the axis of the orifice, and a second position separated laterally from said orifice.

 The invention makes it possible to bring notable improvements to the process of producing steel by melting scrap in an electric furnace.

 Indeed, after having produced by melting a bath of liquid metal covered with a layer of slag and which fills the bottom bowl, the oven is tilted backwards so as to immerse the injection nozzle in the bath metal, the inclination may be such that the discharge orifice, closed by the plug, is also immersed in the slag and, possibly, in the metal. This increases the height of the metal bath which can be stirred and / or treated by injection of a suitable fluid or powder.

 In addition, the tilting of the oven can be adjusted backwards so that the injection nozzle is located just at the interface between the metal and the slag, which makes it possible to inject the fluid and / or the powder in said interface for mixing the metal with the slag.

 The invention also makes it possible, after having produced a bath of liquid metal, to evacuate the slag by overflow by tilting the furnace towards the rear and by opening the scouring orifice, then to close this orifice and to introduce into the tank of products for the formation of a new layer of slag, for the treatment of metal by stirring, the slag can then be evacuated again.

 When the hearth is also extended forward by a raised part, it is possible, after having produced a bath of liquid metal, to tilt the furnace forwards so as to uncover the fluid injection nozzle and thus stop the mixing of the bath which could disturb the flow of steel.

 However, the invention will be better understood from the following description of various embodiments shown, by way of examples, in the accompanying drawings.

 Figure 1 shows, in longitudinal section, a first example of an improved electric oven according to the invention.

 Figures 2 and 3 show, in elevation, other embodiments of such an oven.

 Figures 4, 5, 6, 7 show various positions of the electric oven corresponding to the different phases of the process.

 Figure 8 is a side view, on an enlarged scale, of the closure member.

 Figure 9 is a front view of the shutter member.

 Figure 10 is a detail view of the closure plug.

 Figure 11 is an elevational view of another embodiment of the shutter member.

 In Figure 1, there is shown the assembly of an electric scrap melting furnace comprising, conventionally, a tank 1 closed by a cover 11 in which is formed at least one orifice 12 for the passage of an electrode 13 entering the tank 1. The latter consists of a bottom 2 covered with a sole 20 made of refractory material and surrounded by a side wall 21.

 In the example shown, the oven is of the direct current type and can therefore comprise a single consumable electrode 13 forming a cathode associated with a fixed electrode 14 placed in the hearth and forming an anode.

 The assembly is placed in a building (not shown) in which a working floor 15 is formed, making it possible to circulate around the installation.

 The tank 1 is, generally, substantially symmetrical with respect to a longitudinal vertical plane which corresponds to the plane of the Figure, the side wall 21 being substantially cylindrical and centered with respect to a vertical axis x'x.

 The electrode 13 can move vertically and, for this purpose, is mounted on an arm (not shown) extending in overhang above the cover 11, the latter also being carried by another arm or a spanning gantry the oven. The entire installation comprising the tank 1 and the means for supporting the cover 11 and the electrode 13 is placed on a frame 51 resting on a support mass 52 by means of two rolling supports 53 which make it possible to tilt the entire oven on one side or the other of the transverse median plane P passing through the axis x'x and perpendicular to the plane of the Figure so as to pour the liquid metal or the slag into pockets placed in a pit below the floor 15; the tank 1 is provided, for this purpose, with two orifices centered in the longitudinal plane of symmetry and diametrically opposite, respectively a front orifice 3 for casting steel and a rear orifice 4 for casting slag.

 At the start of a melting operation, a load of raw material such as scrap is placed on the hearth 20, which establishes contact between the two electrodes 13 and 14.

When these are supplied with current, there arises between the cathode 13 and the scrap metal electric arcs which cause the melting of the scrap metal. The metal thus melted forms a bath A which accumulates on the bottom of the hearth 20, the latter being hollowed out so as to form a bowl 22 whose edges rise at least up to a threshold 23 above. above which are placed the two orifices 3 and 4.

 The liquid metal bath A is usually covered with a layer of slag or slag B in which the impurities collect and which can be produced from additives introduced into the furnace with the raw material and whose composition may vary depending on the metallurgical process.

 In the example shown in Figure 1, the bowl 22 is surrounded by a raised portion in which is formed the front orifice 3 for pouring the steel which opens onto a spout 30. The threshold 23 of overflow of the steel is therefore placed at the lower edge of the front orifice 3.

 Before recovering the steel A produced by melting the scrap, it is preferable to first remove the slag and, for this, the tank 1 is tilted backwards until the layer of slag B arrives at the level of the scouring orifice 4 which is open so as to allow the slag to be evacuated, the inclination being however limited so that the metal A remains retained below the orifice 4.

 When the slag has been evacuated, the furnace is tilted forward to evacuate by the pouring spout 3 the steel which overflows above the threshold 23.

 The entire bottom 2 of the tank comprising the bottom 20 and rising at least to the bottom of the two orifices 3 and 4, must be made of refractory material to resist contact with steel. On the other hand, the side wall 21 which, normally, is not in contact with the metal, except by splashing, must simply be cooled and can therefore consist of a metal wall in which a circulation of fluid is provided. cooling, such as batten. In conventional ovens, it is therefore necessary to limit the inclination of the tank 1 to avoid contact of the side wall 21 with the slag and, a fortiori, the liquid metal.

 According to a first characteristic of the invention, the rear part of the hearth 20 is extended by a substantially vertical wall 24 also made of refractory material and which rises well above the level of the overflow threshold 23 and the scouring orifice 4 is formed entirely through this refractory wall 24. In addition, the orifice 4 is closed in leaktight manner by a removable closure member 41.

 As a result, the height of the refractory part must be increased towards the rear, but the advantages which arise from such an arrangement are numerous.

 First of all, in the usual installations, the scouring orifice 4 is constituted by an opening having, generally, large dimensions and the largest part of which is formed in the side wall 21, only the lower part serving for evacuation of the slag.

This opening is closed by a simple removable door which does not prevent air from entering.

 In the invention, on the contrary, thanks to the elevation of the refractory wall, the slag can overflow by covering the entire orifice, the entire section of which is entirely useful and can thus be reduced, while maintaining a sufficient discharge flow rate. But above all, the scouring orifice 4 can be closed by a sliding plug 41 having a substantially identical section and producing a practically sealed obturation. The air inlets are thus considerably reduced, which improves the thermal efficiency of the oven and the efficiency of the system usually used for capturing the fumes, thanks to a better balance of gas flows in the oven. In addition, one can reduce the suction rate of the smoke capture device.

 But another very important advantage of the elevation of the rear part of the hearth and of the scouring orifice 4 comes from the fact that one or more can be placed below the level of the orifice 4, one or more nozzles 6 of injection of a fluid, in particular a gas or powder.

 Such a nozzle can, in fact, open above the bath and the slag when the tank is in an upright position and inside the bath when the tank is tilted back, the orifice 4 then being closed.

 In the embodiment shown in Figure 1, the front part of the furnace is provided with a conventional steel discharge spout which is poured through an orifice 3 with a horizontal axis. However, it is also possible, as shown in FIGS. 2 and 3, to pour the steel through a tap hole with a vertical axis formed directly in the bottom of the hearth 20.

 For example, Figure 2 shows a so-called "apse" oven in which the hearth 20 has an ovoid shape comprising an projection 26 which extends on the front of the bowl 22, outside the internal cylindrical space limited by the side wall 21, forming a substantially horizontal platform placed at the threshold 23 of steel overflow and in which is formed an orifice 32 with a substantially vertical axis constituting a taphole of the steel. This platform 26 is surrounded by a raised wall 25, also made of refractory material, the upper edge of which is connected to the side wall 21 by a cover 27.

 The taphole 32 can be closed by a sealing member 33 such as a plain nozzle with drawer or any means of known type.

 Thus, if the oven is tilted forward, the metal passes over the threshold 23 and covers the tap hole 32 over a certain height, which allows the casting of molten steel.

 FIG. 3 represents another particularly advantageous embodiment thanks to the elimination of the projection 26 and, consequently, of the cover 27. The whole of the hearth 20 is then surrounded by a substantially vertical wall 25 ′ which may have a shape of cylinder of revolution extended upwards by the side wall 21. The platform 26 'is inclined relative to the axis x'x by an angle corresponding to the tilting angle of the furnace forwards for casting , and is tangentially connected to the bowl 22, the axis x'x of which is offset backwards with respect to the axis x'x of the furnace. The steel pouring orifice 32 ′ is then placed inside the cylindrical internal space of the tank 1.

 Thus, when the oven is tilted forward, the platform 26 is oriented substantially horizontally and the steel bath submerges the pouring orifice 32 '. By thus giving sole 20 a fully cylindrical shape, better thermal homogeneity of the steel bath is ensured, which, moreover, can have a greater depth, favorable for stirring by nozzles or by electromagnetic effects.

 It will be noted that the taphole 32 ′ may have its axis inclined relative to the vertical, for example around 15, so as to be vertical when the oven is tilted forward.

The rear of the oven is arranged as in the case of
Figures 1 and 2 to allow the slag to flow through the rear orifice 4.

 Thanks to the invention, it is possible, in a particularly advantageous manner, to modulate the height and the position of the metal bath, which makes it possible to adapt to an electric furnace operations and treatments which, until now, s 'were performing in converter.

 By way of example, there is shown diagrammatically, in FIGS. 4 to 7, various positions of the oven corresponding to the different phases of the melting process.

 In FIG. 4, the furnace 1 is in an upright position in the melting phase of the scrap metal C which has been deposited on the hearth 20. As indicated, the melting of the scrap occurs, in a conventional manner, thanks to the electric arcs formed between the electrode 13 and different points of the scrap.

 In this position, the nozzle 6, which passes through the wall 24 and opens out below the scouring orifice 4, is located above the bath and the slag and can be connected by a line 61 to a supply circuit. not shown so as to function as an oxygen, gas or carbon injector, directed towards the scrap or the bath. It is thus possible to increase the temperature and improve the metallurgical process and the efficiency of the operation.

Note that, if the tank has the shape of the
Figure 3, the removal of the apse avoids the risk of scrap metal accumulation in the cold zone formed by the projection 26 of the sole 20.

In the second phase, the molten steel forms a bath
Covered with a layer of slag B and possibly containing not completely melted scrap.

 The tank 1 is then tilted backwards (Figure 5) by tilting on the supports 53 and the presence of the raised part 24 of refractory material makes it possible to increase the angle of inclination so as to raise the slag B up to 'above the scouring orifice 4 sealingly closed by the plug 41. This results in an increase in the height h of the steel bath on the rear of the tank, which allows injection, by means of the nozzle 6, either of an oxidizing gas such as air or oxygen, or of a neutral gas such as nitrogen or argon, or of a powdered product.

 Such injections make it possible, on the one hand, to promote the formation of a foaming slag in which the electric arc is embedded, which improves the heat transfer and, on the other hand, by directly opening the nozzle 6 in the bath A, to carry out a stirring thereof which makes it possible to increase the heat exchanges between the liquid steel and the unfused.

 It is also possible to place in the wall 24 several injectors allowing simultaneous injections, for example of carbon and oxygen.

 It will be noted that the particular shape of the sole shown in FIG. 3, with the backward shifting of the bowl 22 makes it possible to further increase the height of the metal bath in the injection phase.

It is also possible to adjust the inclination of the oven in an intermediate position for which the interface
D between the liquid metal A and the slag B is located just at the outlet of the nozzle 6. The injection of gas and / or powder at this level makes it possible to increase the exchanges between the metal and the slag.

 At the end of the treatment, the inclination of the tank 1 is adjusted, as shown in Figure 6, so that the orifice 4 is at the level of the slag layer B. The plug 41 is then removed and the slag B is poured into a pocket 54 placed below the floor 5.

 We can then tilt the tank 1 forward, in the position shown in Figure 7. The metal A rises along the wall 25 and, when the height of liquid steel above the orifice 32 of casting is sufficient, it is open and the steel is poured into a pocket 55.

 After having evacuated the layer of slag B, one can also introduce into the tank of additives such as for example lime, to constitute, above the bath, a layer of a new slag allowing, if necessary, to carry out a particular treatment. In this case, it is possible to reduce the oven to an inclination for which the nozzle 6 opens at the interface between the slag B and the bath A so as to increase the exchanges.

 Other treatments can also be carried out in the position of Figure 7 for which the nozzle (s) 6 open out above the bath.

 The plug 41, which is slidably mounted along the walls of the orifice 4, must be driven by two movements, first of all a first movement along the axis of the orifice between the closed position and a first position removal for which the orifice is open, then a second movement transverse to the axis of the orifice, making it possible to separate the plug and the slag jet mechanism passing through orifice 4.

 In the simplest embodiment, shown in Figure 1, the plug 41 is fixed to the end of the rod 42 of a jack 43 whose body is itself fixed to the end of an arm 7 mounted pivoting about a horizontal axis 71. In this way, the jack 43 controls the withdrawal of the plug 41 and then the arm 7 rotates around the axis 71 to move the whole of the orifice 4 apart.

 In the example shown in Figure 2, the pouring floor 5 is raised from the usual position shown in Figure 1 so as to be only a little below the upper edge of the side wall 21. This arrangement , which is the subject of patent application No. 90.15595 filed on December 12, 1990 by the same company, has many advantages. In the present case, it is possible to place the entire mechanism below the level of the floor 15 and, thus, to completely disengage the periphery of the oven to facilitate the circulation of personnel.

 In Figures 8 and 9, there is shown in detail, respectively in side view and in front view, the mechanism for controlling the movements of the plug.

 The plug 41 is fixed to the end of the rod 42 of a jack 43 by means of a rod 44 slidingly mounted inside a tube 45 centered on the axis of the orifice 4 in the position shown in Figure 8. The tube 45 is fixed to the end of an arm 7 rotatably mounted around an axis 72 parallel to that of the orifice, on a support 55 fixed on an auxiliary floor 54 secured to the frame 51, and extending at the level of the floor 15.

The rotation of the arm 7 is determined by rotary jacks 73 which, after displacement of the plug 41 to the position shown in FIG. 8, make it possible to raise the assembly in the position shown in dotted lines on the
Figure 9. The arm 7 is advantageously provided with an elbow 74 which allows the plug 41 to be placed above the level of the floor 54 passing through an orifice 56 provided for this purpose. It is thus possible to easily intervene on the plug 41 for maintenance or replacement.

 Of course, the plug 41 must be formed so as to withstand the temperature of the bath in the same way as the sole 20. This is why, as shown in FIG. 10, the plug 41 will advantageously include a wall of refractory material 47 covering the face facing the interior of the oven and fixed on a support plate 48 cooled by water circulation.

 Furthermore, the plug 41 is advantageously associated with a cleaning tool 46 consisting for example of a rotating or vibrating drill bit and which makes it possible, if necessary, to clean or unblock the orifice 4 before or after evacuation of the slag, these two functions can be performed by the same body or by two different devices.

 As can be seen, in fact, in FIG. 9, the cleaning tool 46 is fixed, like the plug 41, at the end of the arm 7 so as to be at the same distance from the axis of rotation 72 , which makes it possible to place, in the axis of the orifice 4, either the plug 41 or the cleaning tool 46, by adjusting the angular position of the arm 7.

 Of course, the invention is not only limited to the details of the embodiments which have just been described by way of example, other equivalent provisions which can be used without departing from the protective framework defined by the claims. .

 For example, in Figure 10, the sliding guide tube 45 of the plug 41 and on which the cylinder 43 is fixed, is articulated around a horizontal axis at the end of the arm 7 and is associated with an auxiliary cylinder 75 which , after removal of the plug 41, makes it possible to pivot the assembly of the tube 45 and of the control mechanism of the plug 41. Thus, during the pivoting of the arm 7, the tube 45 remains in a plane perpendicular to the axis 45, at a fixed distance from the wall 21 of the tank and therefore less likely to hinder movement around the oven.

 Finally, the invention has been described in the context of a direct current furnace and a single consumable electrode, but all of its provisions could be applied, with the same advantages, to an furnace provided with several electrodes and operating with alternating current.

 The reference signs, inserted after the technical characteristics mentioned in the claims, have the sole purpose of facilitating the understanding of the latter and in no way limit their scope.

Claims (16)

 1. Electric furnace for producing a metal such as steel from a ferrous material such as scrap, comprising a tank (1) limited by a side wall (21) surrounding a bottom (2) covered a sole (20) made of refractory material forming a bowl (22) with raised edges, substantially symmetrical with respect to a longitudinal median plane, a removable cover (11) for closing the tank (1) and at least one electrode ( 13) connected to a current source and capable of entering the tank through an orifice (12) formed in the cover (11), the assembly being mounted to tilt on either side of a transverse median plane P perpendicular to the longitudinal plane of symmetry to allow the overflow, respectively, of the steel through a front orifice (3) and the slag through a rear orifice (4), said orifices being placed respectively on either side of said median plane, each above an overflow threshold (23),  characterized in that the rear part of the sole (20) covering the bottom is extended by a substantially vertical wall (24) of refractory material, which rises above the level of the overflow threshold (23) and that the orifice rear (4) evacuation of the slag is formed over its entire height through said vertical wall (24) of the sole (20), and is associated with a removable shutter member (41) substantially airtight and with liquid metal. 2. Electric oven according to claim 1, characterized in that the tank (1) is provided with means (6) for injecting a fluid and / or powder, passing through the vertical wall (24) of the hearth ( 20) to open below the level of the slag discharge orifice (4).  3. Electric oven according to claim 2, characterized in that the injection means (6) consist of at least one nozzle (6) connected to a circuit (61) for supplying pressure under pressure in a fluid such as oxygen, a combustible or brewing gas, or a powder.  4. Electric oven according to claim 3, characterized in that the angle of inclination towards the rear of the tank (1) can be adjusted so as to give the latter at least two tilting positions towards the rear respectively a first position for which the injection nozzle (6) opens out inside the metal bath and a second position for which the injection nozzle opens out at the interface between the metal bath (A) and the slag (B).  5. Electric oven according to one of the preceding claims, characterized in that the refractory hearth is extended forward by a raised part (26) forming a platform placed above the upper level of the steel bath (A ) when the axis (x'x) of the furnace is vertical, and surrounded by a substantially vertical wall (25) rising above the level reached by the slag (B) covering the metal bath (A) and the platform ( 26) is pierced with an orifice (32) with a substantially vertical axis, associated with a removable shutter member (33) airtight and liquid metal.  6. Electric oven according to claim 5, characterized in that the side wall (21) of the tank (1) having substantially the shape of a cylinder of revolution, the platform (26) in which the orifice of the casting forms a projection covered by a cover (27), the pouring orifice being placed outside the cylindrical side wall.  7. Electric oven according to claim 5, characterized in that the side wall (21) of the tank (1) having substantially the shape of a cylinder of revolution, the pouring orifice (32 ') is placed at the 'interior of said cylinder and the elm platform (26) is inclined relative to the axis (x'x) to connect tangentially to the bowl (22), the latter being off-center backwards with respect to said axis (x' x).  8. Electric oven according to claim 1, characterized in that the removable member for closing the rear orifice (4) comprises a plug (41) of cross section substantially identical to that of the orifice (4), made of refractory material at least on its part (47) facing the interior of the tank (1) and means (7) for controlling the movement of the plug (41), at least along the axis of the orifice ( 4), between a withdrawal position, spaced from the tank (1), and a closed position for which the plug (41) is engaged in the orifice (4).  9. Electric oven according to claim 8, characterized in that the means (7) for controlling the movement of the plug (41) determines a first movement of the plug (41) along the axis of the orifice (4) between the closed position and a first position for withdrawing the orifice (4) then a second movement transverse to the axis of the orifice (4) between the first withdrawal position and a second position spaced from the slag passage.  10. Electric oven according to claim 9, characterized in that the plug (41) for closing the orifice (4) is mounted at the end of the rod (42) of a jack (43) determining the first movement of the plug (41) and whose body (43) is fixed to the end of an arm (7) pivotally mounted about an axis (72) spaced from the axis of the orifice (4) and associated with means (73) for controlling the pivoting of said arm (7) between the first position for withdrawing the plug (41), placed in the axis of the orifice (4), and a second position separated laterally from said orifice ( 4).  11. Electric oven according to claim 8, characterized in that the plug (4) consists of a metal plate (48) of section substantially identical to that of the orifice (4) and covered with a refractory lining ( 47) on a face facing the interior of the tank (1), said plate (48) being provided with channels for circulation of a refractory lining cooling fluid (47) connected to a supply and fluid evacuation.  12. A method of producing a metal such as steel from a ferrous material such as scrap, in an electric oven comprising a tank (1) limited by a side wall (21) surrounding a bottom ( 2) covered with a sole (20) of refractory material forming a bowl (22) with raised edges substantially symmetrical with respect to a longitudinal median plane, a removable cover (11) for closing the tank (1) and at least one electrode (13) connected to a current source and capable of entering the tank through an orifice (12) formed in the cover (11), the assembly being mounted to tilt on either side of a transverse median plane ( T) perpendicular to the longitudinal plane of symmetry to allow the overflow, respectively, of the steel through a front opening (3) and the slag through a rear opening (4), said openings being placed respectively on either side of said median plane, each above a s overflow eye (23),  characterized by the fact that the rear orifice (4) for evacuating the slag (B) is housed in a raised wall (24) of refractory material extending upwards the rear part of the sole (20) and rising to the above said orifice (4), for evacuating the slag, the latter being associated with a removable member (41) for tight sealing and that, after having produced by melting the ferrous material a bath of liquid metal (A) covered with a layer of slag (B), the oven (1) is tilted backwards so as to immerse the injection nozzle (6) in the metal bath (A), the discharge orifice, closed by the plug (41), also being immersed in the slag (B) and, optionally the metal, and the metal is mixed and / or treated by injection of a fluid and / or an adequate powder .  13. A method of producing metal according to claim 12, characterized in that the tilting of the furnace is adjusted backwards so that the nozzle (6) for injecting fluid or powder is at the interface between the metal (A) and the slag (B) and that the fluid and / or powder is injected into said interface so as to mix the metal with the slag.  14. A method of preparing metal according to one of claims 12 and 13, characterized in that, after having produced a bath of liquid metal (A), the oven is tilted backwards to make the slag arrive ( B) at the rear orifice (4), the cap (41) is opened and the slag (B) is evacuated by overflow, then the orifice (4) is closed and introduced into the tank (1) of the products for the formation of a new layer of slag, the latter being subsequently removed after treatment of the metal (A).  15. A method of preparing metal in an electric oven according to one of claims 5, 6, 7, characterized in that, after having produced a bath of liquid metal, the oven is tilted forward so as to uncover the fluid injection nozzle (6).  16. A method of preparing metal according to one of claims 12 to 15, characterized in that, the oven being in the straight position, an oxidizing fluid and / or a fluid or a powdered product is injected through the nozzle (6 ) in the ferrous material during the melting phase thereof.