FR2691240A1 - Electric furnace for prodn. of metal e.g. steel - comprising vat limited by lateral wall surrounding bottom covered by sill of refractory material forming crucible - Google Patents

Abstract

An electric furnace for the prodn. of a metal such as steel, comprises a vat (1) limited by a lateral wall (21) surrounding a bottom covered by a sill (2) of refractory material forming a crucible (21) and able to rock from one side to the other in a transverse median plane in order to allow the discharge by overflowing, respectively, of the steel via a front taphole (3) with an essentially vertical axis and the slag by a rear taphole. The front taphole (3) is cut into a raised part (26') of the sill (2) surrounded by a wall (25') of refractory material, climbing vertically. The raised part (26') is inclined with respect to the axis (x'x) of the vat (1) and tangentially linked to the crucible (22), the angle of inclination corresponding to the rocking angle of the furnace towards the front of such sort that the platform (26') may become oriented horizontally when the furnace (1) is rocked. USE/ADVANTAGE - The electric furnace is used for the prodn. of a metal, such as steel from a ferrous material such as scrap. It provides an improved furnace performance thanks to the design of its refractory sole with the following advantages: it facilitates the implantation of tyres for the injection of gases or other materials, particularly to foam the slag formed in the furnace; it improves the sealing of the furnace eliminating air ingress with subsequent improvement to the thermal yield of the furnace and the efficiency of the fume exhaust and cleaning system.

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 that can be moved horizontally, or on arms mounted to rotate around a vertical axis on a base placed at side of the oven.

 The scrap is melted 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 bottom, 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.

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

 Generally, the side wall of the tank has a cylindrical shape of revolution around an axis x ′ x and the 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.

 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 furnace to be improved, thanks to a particular shape of the refractory hearth.

 However, the invention also has other advantages, in particular by facilitating the installation of gas injection nozzles or other products and also makes it possible to improve the tightness of the tank by avoiding the entry of air.

 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 substantially symmetrical with respect to a longitudinal median plane, 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 mounted to tilt on either side of a plane transverse median P perpendicular to the longitudinal plane of symmetry, to allow the spillage by overflow, respectively, of the steel by a front orifice with substantially vertical axis and of the slag by a rear orifice, the front orifice of the steel pouring being formed on a raised part of the sole forming a platform surrounded by a wall of refractory material, rising vertically.

 In accordance with the invention, the platform of the hearth, on which the pouring orifice of the steel is formed, is inclined relative to the axis x'x of the tank and is tangentially connected to the bowl, l 'angle of inclination corresponding to the tilting angle of the furnace forward so that said elm platform is oriented substantially horizontally when the furnace is tilted forward for the casting of steel.

 Likewise, the taphole has its axis inclined with respect to the axis x'x of the tank so as to be vertical when the oven is tilted forward for the casting of steel.

 Preferably, the axis y'y of the bowl is offset backwards with respect to the axis x'x of the tank.

 In addition, the wall surrounding the hearth can advantageously have, substantially, the shape of a cylinder of revolution extended upwards by the side wall, the pouring orifice of the steel thus being placed inside the cylindrical internal space of the tank.

 According to another advantageous characteristic, the rear part of the sole covering the bottom is extended by a substantially vertical wall of refractory material, which rises above the level of the slag overflow threshold and that the rear slag discharge orifice is formed over its entire height through said vertical wall of the floor, and is associated with a removable shutter member substantially airtight and liquid metal.

 Furthermore, the tank may be provided with means for injecting a fluid and / or powder, such as nozzles, passing through the vertical wall of the hearth to open below the level of the slag discharge orifice.

In this case, 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 bath A and the slag
B.

 It is thus 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 products for the formation of a new layer of slag, for the treatment of metal by stirring, the slag can then be evacuated again.

 After having produced a bath of liquid metal, the furnace can be tilted forward so as to uncover the fluid injection nozzle and thus stop the stirring of the bath which could disturb the casting of the 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, an example of an electric oven of the so-called "apse" type.

 Figure 2 shows, in elevation, an electric oven according to the invention.

 Figures 3, 4, 5, 6 show various positions of the electric oven corresponding to the different phases of the process.

 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 occurs, 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.

 Before recovering the steel A produced by melting the scrap, it is preferable to first eliminate the slag and, for this, the tank 1 is tilted backwards until the slag layer 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 through the pouring orifice 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 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 water.

 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. Often the front part of the furnace is provided with a steel discharge spout which is poured through an orifice with a horizontal axis.

 However, it is also possible, as shown in FIG. 1, to pour the steel through a tap hole 32 with a vertical axis, formed directly in the bottom of the hearth 20. By way of example, FIG. 1 shows an oven of the so-called "apse" type in which the hearth 20 has an ovoid shape comprising a projection 26 which extends on the front of the bowl 22, outside the cylindrical internal 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 closure member 33 such as a spool nozzle or any known type of means.

 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.

 Figure 2 shows an oven tank according to the invention. In this case, the elm platform 26 ′ is inclined with respect to the axis x′x by an angle corresponding to the tilting angle of the oven forwards for casting, and is tangentially connected to the bowl 22.

 Thus, when the oven is tilted forward, the platform 26 is oriented substantially horizontally and the steel bath submerges the pouring orifice 3.

 It will be noted that the tap hole 32 ′ may have its axis inclined relative to the vertical, for example around 150, so as to be vertical when the oven is tilted forward.

 According to another advantageous arrangement, the axis y'y of the bowl 22 is offset rearward relative to the axis x'x of the oven. In addition, the substantially vertical wall 25 which surrounds the whole of the hearth 20 can have the shape of a cylinder of revolution, extended upwards by the side wall 21. The orifice 3 for casting the steel is then placed at inside the cylindrical internal space of the tank 1.

 Such an embodiment is particularly advantageous thanks to the elimination of the projection 26 and, consequently, of the cover 27. By thus giving the sole 20 a fully cylindrical shape, better thermal uniformity of the steel bath is ensured. , in addition, may have a greater depth, favorable to stirring by nozzles or by electromagnetic effects.

 The rear of the oven is arranged to allow the slag to flow through the rear orifice 4.

 Advantageously, 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.

 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 advantage of the elevation of the rear part of the hearth and of the scouring orifice 4 comes from the fact that it is thus possible to place 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.

 Thanks to these provisions, 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, have been were performing in converter.

 By way of example, diagrammatically shown in FIGS. 3 to 6 are various positions of the furnace corresponding to the different phases of the melting process.

 In FIG. 3, 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.

 It will be noted that the removal of the apse avoids the risk of accumulation of scrap metal 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 4) 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 the Figures, with an offset towards the rear 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 5, 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 6. 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 trade.

 Other treatments can also be carried out in the position of Figure 6 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 mechanism, from the slag jet passing through the orifice 4.

 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. .

 In particular, 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 (9)

 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) of refractory material forming a bowl (22), substantially symmetrical with respect to a longitudinal median plane, a removable cover (11) for closing the tank (1) and at least one connected electrode (13) to a current source and capable of entering the tank through an orifice (12) formed in the cover (11), the assembly being mounted tilting on either side of a transverse median plane P perpendicular to the longitudinal plane of symmetry, to allow the spillage by overflow, respectively, of the steel by a front orifice (3) and the slag by a rear orifice (4), the front orifice (3) for discharging the steel being formed on a raised part (26) of the sole (20) surrounded by a wall (25), made of material refractory, rising vertically and opening into a tap hole which crosses the hearth (20),  characterized by the fact that the raised part (26) of the hearth (20), on which the steel pouring orifice (3) is formed, is inclined relative to the axis (x'x) of the tank (1) and tangentially connects to the bowl (22), the angle of inclination corresponding to the tilting angle of the oven forward so that said raised part (26) is oriented substantially at horizontal when the furnace (1) is tilted forward for the casting of steel.  2. Electric oven according to claim 1, characterized in that the taphole (32 ') has its axis inclined relative to the axis (x'x) of the tank (1) so as to be vertical when the oven (1) is tilted forward for the casting of steel.  3. Electric oven according to one of claims 1 and 2, characterized in that the angle of inclination is about 15 degrees.  4. Electric oven according to one of the preceding claims, characterized in that the axis (y'y) of the bowl (22) is offset rearward relative to the axis (x'x) of the tank (1).  5. Electric oven according to one of the preceding claims, characterized in that the wall (25) surrounding the hearth (20) has substantially the shape of a cylinder of revolution extended upwards by the side wall (21), the orifice (3) for casting the steel being placed inside the cylindrical internal space of the tank (1).  6. Electric oven according to one of the preceding claims, characterized in that the rear part of the hearth (20) covering the bottom is extended by a substantially vertical wall (24) of refractory material, which rises above the level of the slag overflow threshold and that the rear orifice (4) for discharging the slag is formed over its entire height through said vertical wall (24) of the hearth (20), and is associated with a removable member (41 ) obturation substantially airtight and liquid metal.  7. Electric oven according to one of the preceding claims, 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 sole (20) to open below the level of the slag discharge orifice (4).  8. Electric oven according to claim 7, 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.  9. Electric oven according to claim 7, 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).