FR2525755A1 - Electric arc furnace with bath stirring system - comprising gas permeable device(s) in furnace hearth - Google Patents

Abstract

Electric arc furnace has one or more permeable devices mounted in the refractory lining of the furnace hearth for introducing gas. Various arrangements of the devices are possible including (i) a single device at the centre of the hearth; (ii) a regular pattern of devices over the hearth area, e.g. on one or more concentric circles; (iii) a device positioned directly below each electrode; and (iv) a device positioned on a hearth dia. passing through the slag taphole. The devices give improved homogeneity of the metal bath and improved heat and mass transfer, allow modification of metallurgical reactions, allow the electrodes to be protected, and can facilitate slag removal from the bath.

Description

 The present invention relates to an arc furnace comprising means making it possible to act on the stirring of the bath, the metallurgical reactions taking place in this latter, and the quality of operation of the furnace.

 Indeed, in melting and refining in an electric oven, the energy of the arcs is dissipated on the surface, while the bottom of the hearth remains cold.

 The natural convection induced by the arcs being weak, there occurs a stratification of the metal, which results in an insufficient agitation of the bath, leading to a limited metallurgical work, a slow, irregular and poorly controlled fusion of the scrap, and a yield overall and a productivity likely to be improved both.

 In order to try to accelerate the convection currents and to allow a better work of the bath, various solutions have been proposed, among which we can cite the electromagnetic stirring of the bath, which is not very powerful and expensive.

 Another possibility lies in the addition of carbon, which does not however allow the control of the movement of the bath, but which nevertheless ensures a certain agitation of the metal and a foaming of the slag.

The present invention aims to provide means making it possible to achieve the following aims:
- acceleration of the connection, which has physical advantages (homogeneity of the metal bath, improvement of the heat transfer conditions, ...), as well as the chemical advantages which are its corollary;
- modification of metallurgical reactions and possibility of producing, under certain conditions, steel grades hitherto inaccessible to the conventional arc furnace;
- Ease of operation of the oven, especially during the slag refining operations.

 The present invention thus relates to an electric arc furnace consisting of a tank lined with a refractory lining, provided at its upper part with a scouring door and a pouring spout: and a styling vault the upper opening of the tank, through which are formed orifices for the passage of the electrodes, characterized in that the bottom of the tank comprises ak; less a permeable device for the introduction of a gas, embedded in the refractory lining and opening into the tank.

 According to various embodiments of the method of the present invention, the permeable device for the introduction of a gas which is new in the refractory lining of the hearth of the furnace, can be located in the center of the hearth; it can also be a set of devices regularly distributed on the floor, for example in one or more concentric circles.

 Furthermore, provision is made to place at least one of these devices vertically with each electrode in the bottom of the oven.

 Finally, one of these devices can be placed in the bottom of the oven, on the diameter passing through the scouring door, in an opposite position relative to the latter.

 The permeable devices for the introduction of a gas must meet a certain number of conditions capable of ensuring a more or less energetic mixing of the metal bath, and which can be modulated as required, a dissemination of the gas in a multitude of small bubbles and an optimization of the insufflation in duration, flow and choice of gas for each of the phases of the operation.

 For this purpose, it is advantageously possible to use, without being limited to the invention, a permeable device for the introduction of gas into a bath of liquid metal comprising a box open at its upper part in which a refractory element is disposed. permeable to gases and the bottom of which is connected to an inlet tube for said gases and comprising an enclosure open at its upper part, housed in the box and in which is placed at least one permeable refractory element retained by a first refractory binder, the bottom of the enclosure comprising an orifice connected to the gas inlet tube which passes through the bottom of the box, and the enclosure being fixed in the box by means of a second refractory binder.

 The enclosure preferably comprises a body of revolution, substantially frustoconical, the smallest diameter of which is at the upper opening and the side wall of which is provided with retaining means projecting inwards and a curved bottom whose convexity is turned towards the bottom of the box.

 The retaining means may consist of one or more circumferential grooves or of a set of lugs formed in the side wall by stamping or any other means.

 This type of permeable device is capable of ensuring a modulated injection of gas and of dispersing it in a multitude of bubbles to the exclusion of any too brutal jet, such as that which could, for example, be obtained using '' an injection nozzle.

 The invention will be set out below in detail with the aid of the appended drawings, which show an embodiment of it.

On these drawings
Fig. 1 is a schematic view in elevation and in partial section of an arc furnace according to the present invention;
Fig. 2 is a schematic plan view and in partial section of the oven of FIG. 1;
the Fia. 3 is a perspective view with partial cutaway of a permeable device for the introduction of gas according to the present invention;
Fig. 4 is a sectional view of the device according to FIG. 3; and
Fig. 5 is a perspective view of a permeable refractory element placed in the enclosure of FIG. 3.

 The electric> arc furnace according to the present invention, shown diagrammatically in FIG. 1, comprises a tank 1 provided with a refractory lining 2, comprising a pouring spout 3 and a scouring door 4 which are diametrically opposed, as is clearly shown in FIG. 2.

This tank is capped with a vault 5 in which are provided orifices for the passage of the electrodes 6 which can be raised or lowered by a suitable horn mechanism not shown,
During operation, the scrap metal 7 initially introduced is gradually melted under the effect of the electric arc generated by the electrodes to form a liquid mutai baL 8 which is in particular steel.

 The bottom 9 of the oven has at its bottom permeable devices 10 for the introduction of a gas which are embedded in the refractory lining 2 and the upper part of which is flush with the surface of the refractory lining facing the interior of the tank.

 These devices can be classified into three main categories, according to the position they occupy in the sole of the fols.

 According to a first category, devices 10a are placed at the center of the bottom of the sole, as shown in FIG. 2, or may be distributed regularly over the circumference of one or more concentric circles having the center of the sole for center.

 Type 1Ob devices, as shown in FIG. i, are placed at the bottom of the hearth, vertical to each electrode. They obviously cover partially the devices of the iOa type previously described and can play the role of the two categories.

 Finally, a third category of device of type 10c is placed at the bottom of the hearth on the diameter defined by the coulper spout 3 and the scouring door, in an opposite position relative to the scouring door 4, that is to say ie in the vicinity of the spout.

 The position of the various permeable devices for the introduction of gas having been described in the hearth of the furnace, a particular embodiment of this device will be described below.

 The permeable device usable in the present invention, shown in FIG. 3, comprises a box 11 of generally parallelepipedal shape open at its upper part and comprising a flat bottom, this box being made of a thin sheet. The bottom of the box 11 is crossed by a gas inlet pipe 12 which is welded in a leaktight manner to the bottom of the box.

 A rigid enclosure 13, which is designed to withstand the pressure of the gas, is housed in the box 11. This enclosure comprises a body 14 of substantially frustoconical shape open at its upper part and the smallest diameter of the frustoconical body corresponding to the opening and a curved bottom 15, the convexity of which faces the bottom of the box. The domed bottom 15 has an orifice 16 which is connected in leaktight manner to the tube 12 by a weld.

 The frustoconical body 14 has along its lateral wall two circumferential grooves 17 which project towards the inside of the enclosure 13.

 According to an Atlantean, the circumferential grooves 17 can be replaced by lugs 17a, or else an appropriate combination of these grooves 17 and lugs 17a can be used.

 Refractory permeable elements 18 are placed in the enclosure 13 on a bed of granular refractory material 19 intended to distribute the stirring gas over the entire section of the permeable element (more clearly visible in FIG. 4).

 The permeable elements 18 are inserted in a first refractory binder 20 which is vibrated (or packed) in the annular space situated between these permeable elements and the enclosure 1, above said bed of granular material 19.

 The refractory material 20 is compatible with the refractory material which constitutes the permeable elements 18 and adheres securely to the latter whose walls are rough and provided with roughness. This reduces the risk of displacement of all the permeable elements inside the enclosure 13, on the one hand, thanks to the frustoconical shape of the latter, and, on the other hand, thanks to the circumferential grooves located in planes parallel to the upper opening and / or to the lugs formed on the internal face of the enclosure, which prevent any displacement of the refractory 20 relative to this enclosure 13. The enclosure 13 is made of a sufficiently thick sheet metal to resist the pressure of the gas and its domed bottom also contributes to increasing the resistance to the pressure of the gas.

 The assembly thus formed of the enclosure 13, in which are placed permeable elements 18 embedded in a refractory binder 20, is housed in the box 11 and made integral with the latter by insertion of a second refractory binder 21 in the space left free between the enclosure 13 and the box 11.

A rigid and monobloc assembly is thus produced between the enclosure 13 and the elements which it contains, and the box 11.

 This box 11 is generally substantially parallelepipedal or pyramidal in shape which is compatible with the format of the brickwork used for the bottom of the oven bottom.

 The second refractory binder 21 is for example a refractory clay compatible both with the first refractory binder 20 and the refractory material which constitutes the bricks of the refractory lining covering the bottom of the oven bottom.

 By way of example, the permeable elements 18 can be in the form of a monolithic mass produced from a grainy, sintered, porous refractory mixture, for example as described in US Pat. No. 4,230,931, or under the form of a stack of wafers made of a refractory material, these wafers being placed side by side and arranged vertically in the enclosure 13.

 The plates 18, one of which is shown in detail in FIG. 5, in particular have a parallelepipedic or preferably prismatic shape generated on a trapezoidal base and are arranged vertically along their largest dimension, such that the large base 22 of the main faces 23 of the plates rests on the bed of refractory material gra nucleus 19 .

 It is possible to provide on the lateral faces 24 of the plates 18 with anchoring means in the form of protruding parts 2 or in hollow (not shown) with respect to these faces which compete with the trapezoidal shape of the main faces of the plates at the solid anchoring of the latter in the refractory binder 20, thus preventing route ejection of the wafers 18 by the stirring gas.

 The mass r = actuary, constituted by the adjoining plates, is imparted to the desired permeability for the passage of gases, by producing substantially vertical microchannels 16 which promote rolling of the gas and ensure the permeability and the flow of gas through the permeable elements. .

 To produce these microchannels, it is possible, for example, to longitudinally streak one or both main faces 23 of the plates which are then joined together, so that the ridges, when the plates are stacked, constitute these longitudinal microchannels which give the permeable element excellent permeability and allow large quantities of gas to be delivered.

 It is thus possible, for example, to produce plates having a thickness of approximately 8 mm, a length of 650 to 750 mm obtained in one piece, or possibly in two pieces and a width of 150 to 250 mm. The ridges which are formed on the surface of the plates, on one or both main faces, have a depth of a few tenths of a millimeter, and at most X, 3 mm, so that the open gap between two plates juxtaposed in no case exceeds 0.5 mm. This limitation of the size of the microchannels eliminates the risk of metal infiltration which would cause the gradual loss of the permeability of the permeable element constituted by the stacking of platelets.

 The microchannels can also be obtained by introducing, during the pressing of these plates at the bottom of the mold, an aerated organic fabric whose wire diameter will be between 1 / 10th of a millimeter to 0.3 mm. When these parts are cooked, the organic fiber fabric will burn and will therefore give the platelets the microchannels necessary for good permeability.

 The refractory material constituting the permeable element 18 is chosen to resist the thermal shocks to which it is subjected when, preheated by the atmosphere of the furnace, it is suddenly cooled when the brewing gas is admitted at full flow. This material must resist the action of gases which pass through it and which can be, for example: neutral, such as nitrogen or argon; reducing agents, such as carbon monoxide, hydrogen; oxidants, such as oxygen or water vapor; or carbonating agents, such as carbon dioxide, or compatible mixtures of these gases. These materials must therefore resist the corrosive action of the oxidized liquid metal, as well as that of the basic slag charged with iron oxide with which they are in contact during the brewing operations.

 These materials can in particular be based on grains of magnesia, corundum, sintered and / or electrofused mullite, bonded together by clay, alumina and / or chromium oxide, then refritted at high temperature. All of these wafers could be made entirely of silicon nitride or a variety of sialons.

 To improve the corrosion resistance of this refractory element with surface or porous ridges, it is also possible to add to the above compounds, graphite or boron nitride which further reduce the wettability of these elements relative to the molten metal.

 The first refractory binder 20 in which are embedded permeable elements 18 must obviously be compatible with the latter and is a conventional material used for this purpose.

 The second refractory binder material is also a conventional material such as a basic rammed earth chosen for its compatibility both with the first refractory binder and with the basic refractory material which constitutes the briquetting of the bottom of the hearth.

 In fact, by contacting with the liquid metal, the metal envelopes constituting the box 11 and the envelope 13 are liquefied and eliminated at least over a certain height so that the basic refractory material constituting the brick face of the bottom of the floor comes in contact with the second refractory binder 11 which is itself in contact with the first refractory binder 20.

The various categories of permeable devices 10 for the introduction of a gas exercise, depending on their position, different functions which cover the following three distinct aspects.
- a physical aspect linked to the acceleration of the connection movements in the bath;
- an aspect relating to metallurgical improvements and the protection of electrodes;
- an aspect which concerns the ease of operation and, in particular, of the cleaning operation.

 According to the first aspect, the stirring of the bath will be obtained by injecting the gas by a device of type 10a, that is to say either through a single permeable device disposed in the center of the hearth, or through several smaller devices distributed concentrically at the bottom of the floor, either by a combination of the two previous formulas.

 It should be noted that, apart from purely physical agitation of the bath, the injection of gas by these devices of type 10a already results in a modification of the metallurgical reactions.

 According to the second aspect, devices of the 1Ob type are placed vertically on each electrode tip, at the bottom of the bottom, and allow a low argon flow injection with the triple aim of stabilizing the arc, protecting each electrode from oxidation and protect the metal bath from nitrogen recovery.

 According to the third aspect, the installation, at the bottom of the hearth, opposite the scouring door of an additional permeable device of small flow rate of the type 10c, makes it possible to facilitate the scouring operations. In fact, the injection of gas through this device is only carried out when the slag is cleaned and has an essentially physical effect which resides in the creation of an intumescence on the surface of the metal bath and thus drives out the slag to the scrub door. This effect will be possibly reinforced by the complementary admission of gas by all or part of the other elements previously described.

 The arc furnace according to the present invention provided with these permeable devices 10 makes it possible to obtain a certain number of advantages which are highlighted below.

 these are, first of all, advantages arising from the acceleration of the connection currents.

 The acceleration of the rate of melting of scrap and the regular descent of the charge lead to a reduction in "tap-to-tap". The scrap subjected to the radiation of the arc is, thanks to the stirring of the bath, permanently licked at its base by currents of hot molten metal, as indicated by the arrows on the z. 1 Their fusion is regulated and accelerated, in particular, the transitional period of end of fusion of scrap is much better controlled.

 A better lobai yield of the entire operation is obtained. n effect, as the yield of the arc is above all optimized on a completely molten bath and that this stage is more rapidly reached ((thanks to the quantity cos Y), this results in a better overall performance of the entire operation, which is characterized by a lower proportion of reactive energy (Ul # 3 sin #) and therefore better use of the transformer power.

 A better homogeneity of the bath in composition and temperature is obtained.

 It results from stirring and leads to greater ease in dissolving additions to the oven in the bath, a more precise adjustment of the temperature, hence a reduction in the risk of pocket wolf formation, and possibly, in diameter. sheet metal of the determined furnace tank, increasing the height of the metal bath and therefore the capacity of the furnace, without fear of the disadvantages of the heterogeneity of the bath.

 There is also a gain on the charging of coke, coal or fuel products.

 Such products are placed in the oven for two purposes, namely firstly to ensure a certain agitation of the bath by the release of carbon monoxide bubbles coming from the oxidation of carbon, moreover, to form a foaming slag on the surface of the metal, which coats the arcs and the electrode tips and protects the refractories from the radiation of the arcs.

 The part "agitation of the bath" and the foaming effect being ensured by the insufflation of gas, it follows a corresponding saving on the furnaces of coke, coal or fuel products.

 The reproducibility of agitation and better control of the operation also facilitate the automation of driving.

A second series of advantages results from the modification of metallurgical reactions and can be classified according to four points
- better resistance of the electrodes.

 The injection of argon by the devices located vertically on the electrodes has the effect of constituting a neutral atmosphere at the tip of the electrodes where the arcs burst and therefore of limiting the wear of the electrodes by oxidation.

 - limitation of nitrogen uptake by the metal. In a conventional furnace, the absorption of nitrogen on the metal occurs mainly in the area of the arcs where the nitrogen from the ambient air passes into the steel under the effect of the very high temperature.

The neutral atmosphere which is created in this zone by the injection of argon thus limits the nitrogen uptake;
- possibility of producing steel grades hitherto inaccessible to the conventional arc furnace.

 Indeed, one of the pitfalls of conventional arc furnaces lies in the stratification of the bath which limits the evolution of chemical reactions and, in particular, the decarburization of the metal.

The injection of C02 by the permeable devices has the effect of promoting the reaction
CO2 + C of the bath -n 2 Co (gas) and therefore makes it possible to produce steels with a lower carbon content.

 Furthermore, the injection of an argon-oxygen mixture (to a few a of oxygen) in a carburetted metal bath containing chromium, has the effect of lowering the partial pressure of CO and making possible the selective oxidation of the carbon.

 - Finally, the insufflation of gas through the permeable devices and the dissemination of gas in the metal bath in a multitude of small bubbles promotes the settling of inclusions and their absorption by the slag.

 A final series of advantages derives from the ease of scouring.

 Indeed, the possibility of carefully descaling the slag before pouring the metal into the ladle excludes any subsequent re-pollution of the metal by the slag when the metal in the ladle is poured. So, for example, the rephosphoration is almost zero.

 One can avoid a subsequent and always expensive transfer operation from pocket to pocket.

 Finally, after cleaning the slag before pouring the metal, operations previously impossible to perform can be carried out in the furnace, such as, for example, desulfurization of the metal which involves the absence of slag, the total deoxidation of the steel and a basic refractory lining.

 The type of gas injected by the permeable devices 10 varies as a function of the aim pursued and of the moment of insufflation.

 We can use a neutral gas such as nitrogen or argon, a reducing gas, such as carbon monoxide or natural gas, an oxidizing gas, such as air, carbon dioxide, oxygen, optionally as a mixture, or a compatible mixture of these various gases.

 The gases will be injected by the various devices 10 at an appropriate time following the progress of the production of the steel. The central device 10a, the function of which is more especially the mixing, is not triggered until the moment when there is liquid metal. The gas flow through this device is increased in successive stages until the end of fusion.

Type 1Qb devices placed vertically above the electrodes are used simultaneously with or as a relay to the central device.

 Finally, the device 10c, the function of which is more specifically intended to facilitate cleaning, is only used at the time of this operation.

Claims (14)

 1. Electric arc oven consisting of a tank (1) lined with a refractory lining (2), provided at its upper part with a scouring door (4) and a pouring spout (3), and a vault (5) covering the upper opening of the tank, through which are formed orifices for the passage of the electrodes (6), caramel in that the bottom (9) of the tank comprises in g! oi; .s a permeable device (10a; 10b; 10c) for the introduction; -on a gas embedded in the refractory lining and outlets, in the tank.  2. Oven according to claim 1, characterized in that the device (10a) is located in the center of the hearth.  3. Oven according to reve.dicatS3n 1, characterized in that it comprises a set of devices regularly distributed on the hearth.  4. Oven according to claim 3, characterized in that the devices are distributed in one or more concentric circles.  5. Oven according to any one of the preceding claims, characterized in that devices (10b) are placed in the bottom of the pan vertically of each electrode.  6. Oven according to any one of the preceding claims, characterized in that a device (10c) is placed in the hearth of the oven on the diameter passing through the scouring door, in an opposite position relative to the latter.  7. Oven according to any one of the preceding claims, characterized in that the permeable device for the introduction of yaz comprises a box (il) open at its upper part in which is disposed a refractory element permeable to gases (18) and the bottom of which is connected to an inlet tube (12) for said gases, and an enclosure (13) open at its upper part, housed in the box (11) and in which is placed at least one permeable refractory element (18) retained by a first refractory binder (20), the bottom (15) of the enclosure comprising an orifice (16) connected to the gas intake tube (12) which passes through the bottom of the box, and the enclosure being fixed in the box by means of a second refractory binder (21).  8. Oven according to claim 7, characterized in that the enclosure comprises a body (14) substantially frustoconical whose smallest diameter is at the upper opening and whose side wall is provided with retaining means (17; 17a) projecting inwards and a domed bottom (15) whose convexity is turned towards the bottom of the box.  9. Oven according to any one of claims 7 and 8, characterized in that the box is substantially parallelepiped.  10. Oven according to any one of claims 7 to 9, characterized in that the refractory element consists of a monolithic mass in a grainy, sintered and porous refractory mixture.  11. Oven according to any one of claims 7 to 10, characterized in that the permeable element (18) consists of a set of straight parallelepipedic or prismatic plates arranged vertically along their largest dimension, the lateral faces of which are provided with anchoring means (15) and comprising longitudinal microchannels (16), placed on a bed of granular refractory material (9).  12. Oven according to claim 11, characterized in that the plates are made of a material based on grains of magnesia, corundum, sintered and / or electrofused mullite, bonded together by clay, alumina , and / or chromium oxide, then refritted at high temperature.  13. Oven according to any one of claims 11 and 12, characterized in that the plates are made of silicon nitride or a variety of sialons.  14. Oven according to claim 12 or 13, characterized in cr ciue the plates further comprise as constituent material of graphite or boron nitride.