Patent Number: 063226103
Section: description

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS With reference to the attached Figures, the device according to the invention is indicated in its entirety by the reference number 10. The device 10 is suitable to be mounted axially fixed on the walls of a furnace for melting metals, or a recipient in general used to perform metallurgical transformations, and has a nozzle, or emission element 23 at the end, the outlet mouth 11 of which is located at a defined distance from the upper level of the liquid bath and above the overlying layer of slag. During the operating step, the outlet mouth is located at a height, with respect to the surface of the bath, of between about 0.5 m and 2.5 m. The angle of incidence of the jet is between about 30.degree. and about 70.degree., advantageously between 40.degree. and 50.degree.. The device 10 consists of a metallic body 12 (FIG. 1), suitable to be inserted into an appropriate aperture, sealed with air-tight sealing means and made on the wall of the furnace, and to cooperate with appropriate equipment of a kind known to the state of the art, to manipulate and possibly to insert, remove, direct, etc. the device 10. With the apertures on the walls of the furnace being air-tight, it is possible to manoeuvre the movable injection organs, drastically reducing the formation of nitrogen oxides or their precursors (the so-called NOx), and thus limiting the dissipation and dispersion of energy from the machine. On the end part of the metal body 12 the emission element 23 is housed, which is innerly defined by a double nozzle configuration. To be more exact, the emission element 23 has a first nozzle 13, inside and substantially coaxial, to emit a supersonic jet of oxygen, gas containing oxygen, or other technological gases, and a second annular nozzle 14 outside and substantially coaxial with the first nozzle 13, to emit a subsonic jet of oxygen or other substances, for example solid fuel in particles or other type of fuel. In a preferential embodiment, the speed at which the flow exits from the second nozzle 14 is between 0.3 and 0.9 Mach. The nozzles, the first 13 and the second 14, are suitably sized geometrically according to mathematical criteria to obtain maximum operational and technological efficiency, according to the method of calculation described and claimed in a parallel application in the name of this Applicant. To be more exact, the geometry of the channel defined by the second nozzle 14 has a profile such as to obtain a desired correlation of the gradient of speed between the supersonic flow, the subsonic flow and the still air inside the furnace. The emission element 23 is attached to the body 12 in such a way that it can be easily and rapidly attached/detached, thus ensuring that it can be replaced in the event of wear or breakage even without interrupting the functioning of the furnace. The nozzles 13 and 14 are advantageously made of copper, stainless steel or other similar metal. According to a variant, the nozzles 13 and 14 are made entirely or partly of ceramic material, so as to reduce the need for cooling also in those steps wherein the device 10 is not working, so as to facilitate the replacement of the nozzles 13 and 14. The two nozzles 13 and 14 are arranged inside a containing shell 15 inside which the channels 16 for the cooling water to circulate in are made. According to the invention, the first nozzle 13, or supersonic nozzle, has a convergent/divergent conformation (Laval-type) defined by a neck 20 made at a position upstream of the terminal section 21 of the nozzle 13; the neck 20 defines a convergent part 13a upstream and a divergent part 13b downstream which in turn forms the terminal section 21. The second nozzle 14, or subsonic nozzle, outside and concentric with the first 13, is convergent in shape wherein the terminal section 22 converges towards the axis 17 of the first nozzle 13. In one form of embodiment, the respective terminal sections 21 and 22 of the nozzles 13 and 14 are arranged inside the outlet mouth 11 of the emission element 23 in such a way that the respective flows interact and expand inside the inner element 23 itself, before they are introduced inside the atmosphere of the furnace. The primary gassy jet emitted by the first nozzle 13 has an outlet speed which can be regulated by acting on the pressure of the gas directly upstream of the nozzle 13 itself. In the embodiment shown in FIG. 1, this pressure is regulated by a throttling valve 18 arranged on the delivery pipe feeding the gas to the first nozzle 13. The throttling valve 18 is regulated in feedback by a control unit 19 according to signals related to the instantaneous pressure of the gas as monitored respectively upstream and in correspondence with the neck 20. This system of regulation in feedback ensures that the characteristics of the jet are maintained irrespective of the conditions of pressure/temperature/density inside the furnace so that the expansion of the supersonic jet takes place entirely inside the emission element 23. As the gas passes through the neck 20, the flow accelerates from subsonic to supersonic in correspondence with the outlet section 21 of the first nozzle 13. The supersonic flow is thermally and operationally protected by the outer ring created by the secondary, subsonic and convergent flow emitted by the second nozzle 14, so that the supersonic flow is less influenced and less able to be influenced by the operating conditions inside the atmosphere of the furnace and the bath. Moreover, the secondary gas flow makes possible to reduce the speed gradients and therefore to reduce energy loss of the primary jet. In this way, the quantity of motion of the primary jet is preserved, simultaneously excluding its interaction with surrounding gases. According to the variant shown with a detail in FIG. 3, in the divergent end part of the first nozzle 13 a plurality of circumferential grooves 24 are made with the function of stabilising that underlayer of the flow leaving the first nozzle 13 which is nearest the wall. The function of the primary jet emitted by the first nozzle 13 is substantially to penetrate the bath to about half of its overall depth and to spread inside the bath, ensuring an efficiency of use which is substantially 100%. This injection substantially takes place without splashing, since the penetration of the jet is determined only by the quantity of motion possessed by the gas delivered, and not by chemical reactions. The supersonic jet also has the function of creating a depression in the bath, suitable to increase the speed of decarburation, and also to promote stirring in the bath with exchange of mass and of energy, encouraging the homogenization and uniformity of the bath. At the same time, an increase is obtained in the foaming effect and in the homogenization of the overlying slag. The secondary flow emitted by the second convergent nozzle 14 creates an outer protective ring, concentric to the jet emitted by the first nozzle 13, and has the main function of surrounding the supersonic flow, protecting it thermally and fluido-dynamically from the surrounding disturbing agents; this increases the independence of the supersonic jet from the conditions in the atmosphere of the furnace. The shape of the outlet section 22 of the second nozzle 14 can be suitably chosen, for example, circular, elliptic or other, according to the desired position and direction of the flow. The secondary flow emitted from the second nozzle 14 reaches the overlying layer of slag, starting and encouraging the combustion of the CO emerging from the bath and giving an extra energy contribution for the melting process. With the nozzles 13 and 14 shaped according to the invention, the supersonic jet emerging from the first nozzle 13 maintains the fluid threads substantially parallel for a greater length than what happens in traditional systems, without there being any dispersion of the tubular flow caused by any other gas entering inside the volume of the jet itself. Moreover, when the jet is introduced into high density fluid systems (for example water, liquid metal or other), the supersonic jet of the first nozzle 13 reaches greater depths, since this jet is equipped with a greater quantity of motion and is completely surrounded by the subsonic jet emitted by the second nozzle 14. This is completely different from what happens in traditional systems, where the primary flow of gas, already turbulent as it leaves the lance, generates a cavity in correspondence with the zone where it penetrates into the bath, thus causing a large part of the oxygen injected to leave the injection area without exerting the desired effect in the bath of liquid metal and hence causing a reduction in efficiency. The device 10, thanks to its emission characteristics described above, allows to work at a greater distance from the bath, and does not necessarily require a manipulator as used at present, with a consequent reduction in wear and consumption of its mechanical parts. According to the invention, the device 10 can operate in burner mode with a variable stechiometric ratio and variable flame length, wherein the first convergent/divergent nozzle 13 is used as a Venturi tube to mix a combustible substance and a comburent substance, such as for example oxygen or air enriched with oxygen (FIG. 4). When the device 10 is used as a burner, the second nozzle 14 can be employed, according to a variant, to emit a jet of oxygen, or air enriched with oxygen, in order to obtain a combustion in stages, and therefore to maintain the fuel/comburent ratio in the primary flow in sub-stechiometric conditions and to use the secondary comburent to complete combustion. When the device 10 is used as a burner, the double nozzle configuration 13 and 14 allows to obtain a plurality of advantages, and also to reduce the formation of NO.sub.X. In the first place, it allows to increase the efficiency of the transfer of convective heat, and minimizes the excess of total comburent needed to complete combustion. Moreover, it guarantees a high level of stability for the flame in a wide range of operating conditions, allowing to regulate the characteristics of the flame itself both in terms of length and in terms of diameter according to the type of furnace and the required processing parameters. FIG. 4 shows an operating mode with a long flame, with the outer secondary annular jet 25 consisting of oxidant-rich gas which surrounds the inner primary jet 26 which is rich in combustible gas; this functioning is particularly useful for dissolving the scrap, in the initial steps of the cycle, which are in front of the outlet mouth 11 of the device 10. According to the operating conditions of the process of metallurgical transformation, the performance of the burner can be regulated to modify the length of the flame and also the stechiometric ratios in the different zones of the flame. FIGS. 5 and 6 show a variant wherein, in correspondence with the terminal section 22 of the second nozzle 14 there are deflector elements 27. These elements 27 rotate around a pin 29 and can assume a first, substantially horizontal position (shown by a line of dashes in FIG. 6), wherein they do not interact with the jets 25 and 26, allowing the long-flame configuration in burner mode, and a second position at least partly inclined (shown by a continuous line), wherein they reduce the outlet section of the second nozzle 14, generating a swirling movement of the second, outer jet (shown by the arrows 28). According to the greater or lesser inclination of the deflector elements 27, and to the consequent greater or lesser partial closing of the outlet, the length and shape of the flame are regulated according to the desired result. According to a further variant, at least the first nozzle 13 can be axially positioned with respect to the outlet mouth 11 of the device 10, which remains fixed, however, with respect to the wall of the furnace, for example retractable, in order to ensure the stability of the flame irrespective of the conditions which are established inside the furnace. Moreover, the retractability of the first nozzle 13, if combined with the retractability of the second nozzle 14, allows to create a pre-combustion chamber of variable volume inside the emission element 23 which guarantees an efficient mix of the two gassy jets before they are introduced into the atmosphere of the furnace. In a further functioning mode, the second nozzle 14 is used to inject material in particles or in powder form mixed with a vector gas or a transporter gas (FIG. 7). The material injected can also be a combustible material of a solid type in powder form or particles, or the atomized liquid type, or of the gassy type. In this case, the primary nozzle 13 can be used to inject secondary comburent. In the embodiment shown in FIG. 8, the second nozzle 14 is used to inject solid material, such as carbon powder or lime, on a fluid vehicle, for example an inert gas or similar. This embodiment is particularly useful in order to increase the foamy slag effect and the recarburization of the liquid steel. Moreover, a contribution of chemical energy is given, with a consequent saving in electric energy, and the composition of the slag is adjusted to values more suitable for the desired operating conditions inside the furnace. In this functioning mode, the emission element 23 can be replaced to modify the shape of the second nozzle 14 from a convergent configuration (FIG. 8), to non-convergent configurations (FIGS. 9, 12) with an outlet jet which is more or less parallel to the primary jet emitted by the first nozzle 13. FIG. 11 shows a further configuration wherein the second nozzle 14 has characteristics of great convergence in order to encourage the transport of the solid fuel in powder form by means of the supersonic jet emitted by the first nozzle 13. According to a further variant, the solid fuel in powder form is injected on a fluid vehicle through the first nozzle 13 (FIG. 10), while the second nozzle 14 can be used to emit a subsonic jet to protect the primary jet delivered by the first nozzle 13. The first nozzle 13 can be used for the whole of its section or, as shown in FIG. 12, a thin axial channel 30 may be made inside it, for solid fuel to be injected; in this case, the thin axial channel 30 extends substantially as far as the outlet mouth 11 of the device 10. The gassy jets emitted by the first nozzle 13 and the second nozzle 14 form annular crowns which protect and convey the jet of fuel delivered through the axial channel 30. According to a preferential embodiment of the invention, the walls of the nozzles or channels used to inject solid fuel are lined, at least in correspondence with the bends, with wear-resistant and erosion-resistant material, for example, high resistance resins, ceramic linings or specific protective varnishes. In the configurations shown in FIGS. 8-12, it is therefore possible to inject carbon powders (to produce foamy slag and to limit the power of the furnace), or lime powders or other material of a basic nature (to passivate the slag), at the same time as O.sub.2 or other technological gases which are needed for the metallurgical treatment of the melting baths are injected. It is clear from the above description what the characteristics and advantages of using the device 10 according to the invention are. The device 10 is mounted fixed to the wall of the furnace, also with its outlet mouth 11 at a distance from the liquid bath, so that it does not require manipulators or the substitution of parts which are progressively consumed. If the device 10 is used alternately in burner mode and simple oxygen lance mode, it is possible to open the road to the supersonic jet, for example dissolving the scrap and creating a direct passage towards the bath of liquid metal without deviations and reflections which cause energy losses and a slow down in the jet. According to the specific effect to be obtained, the inclination of the lance can be varied, for example maintaining a lesser inclination during the pre-heating step, the descent of the scrap step, and the melting step, and a greater inclination in the decarburation step and the bath stirring step. Apart from this, the quantity of motion in the primary jet, together with the protection effect caused by the secondary jet emitted by the second nozzle 14 causes the primary supersonic jet emitted by the first nozzle 13 to penetrate into the bath without dispersions and without loss of speed, maintaining a high level of efficiency, in the region of 100%, and preventing dangerous and harmful splashes of liquid metal. The depression which is created in the liquid bath, due to the pressure and dynamic impulse created by the supersonic jet, causes an increase in the speed of decarburation due to the increase in local and overall stirring of the bath and the consequent increased exchange of mass and energy. This improved stirring and uniformity of the bath deriving from the turbulence caused by the supersonic jet increases the spreading process in correspondence with the interface between the slag and the metal, which entails a reduced demand for electric energy and an increase in the speeds of decarburation. The increase in decarburation then allows a greater use of the device 10 for the high efficiency injection of carbon powder, causing a further input of chemical energy with an improved slag foaming effect and a consequent greater efficiency of the arc, reduction of electrode consumption, and reduction of energy losses through the cooling elements of the furnace. Moreover, the quantity of motion of the primary supersonic jet emitted by the first nozzle 13 generates a zone wherein the post-combustion caused by the second subsonic jet delivered by the second nozzle 14 can be carried out without contact with the slag or the metal, but in a zone in close proximity with the slag itself, thus increasing the efficiency of the reaction from a value of around 35% to a value of 75% and more. The energy deriving from the post-combustion process is transferred to the drops of metal trapped between the slag by conduction, instead of by radiance as traditionally happens. When the drops of metal return to the liquid bath, they transfer their energy content to it, further reducing the demand for electric energy. The fact that it is possible to inject carbon or lime through the device 10, even autonomously and independently with respect to the supersonic jet of oxygen, increases the flexibility and versatility of the device 10 with respect to the variation in the conditions of the furnace to obtain the desired quality of the final product.