Improved process and plant for preheating a metal charge continuously fed into an electric melting furnace

Abstract

A process and a plant for preheating a metal charge (11) continuously fed to an electric melting furnace (12) through a preheating tunnel (16) provided with side walls, a vault and a horizontal conveyor (13), the metal charge (11) being covered in countercurrent with fumes or exhaust gases (17) emerging from the electric melting furnace (12), according to which the intake of air for complete combustion of the fumes or exhaust gases (17) is effected from the surrounding environment through openings along the preheating tunnel, preferably at the interface between the preheating tunnel and the electric melting furnace, and the intake is regulated by acting on the suction fan and / or openings based on measurements revealed by temperature sensors (21) and / or composition of the exit gases at or downstream of the end of the tunnel (16). The metal charge is covered with gas jets injected through a number of nozzles (15) arranged on the vault of the tunnel (16), The gas jets injected by the nozzles 15 are unevenly distributed laterally over the vault of the preheating tunnel 16, with a higher concentration towards the top of the vault of the tunnel 16, and the gas jets are unevenly distributed longitudinally along the preheating tunnel 16, with transverse sections where the nozzles 15 are distributed and longitudinal sections of the tunnel 16 where there are no nozzles 15 interspersed so as to avoid interference phenomena.

Description

Detailed Description of the Invention

[0001] The present invention relates to an improved process for preheating metal charge material that is continuously fed into an electric furnace in which molten metal is produced. The invention also relates to a plant for carrying out the above process.

[0002] Processes and plants for preheating a metal charge (usually scrap) are well known to those skilled in the art, which charge is continuously fed to an electric melting furnace by means of a horizontal conveyor, the preheating process thus favoring the subsequent melting process.

[0003] The charge is preheated in a channel inside the tunnel, using the sensible heat of the exhaust gases from the melting process itself and the heat of combustion (in special cases suitable auxiliary burners have been used to provide this preheating). The emitted fumes are then exhausted from the preheating tunnel and sent to a suitable fume treatment system. The combustion heat used in the preheating process is essentially obtained by the complete combustion of CO (carbon monoxide) and H2 (hydrogen) released during the processes taking place in the melting furnace, while the necessary oxygen is usually obtained from the ambient air supply.

[0004] The process and plant described briefly above are the subject of, for example, U.S. Pat. No. 5,400,358, which describes the injection of air necessary for uniform combustion along the preheating tunnel. This solution teaches injecting oxygen distributed along the preheating tunnel in an amount sufficient to guarantee a 3-5% excess of oxygen to ensure complete combustion of unburned gases, assuming that the seal between the furnace and the tunnel is practically perfect. Plant engineering has shown that this situation is not possible, and the amount of external air that always infiltrates (especially at the interface between the furnace and the tunnel) is considerable, often exceeding the amount sufficient to ensure complete combustion of the process gas entering from the furnace. Furthermore, it has been observed that this air infiltration does not achieve sufficient turbulence; in fact, it tends to flow along the inner walls of the heating tunnel, where it mixes with the process gas and burns slowly. Excessive supply of ambient air into the preheating tunnel must be absolutely avoided, as it would lower the gas temperature too much. Furthermore, if this temperature approaches or falls below the CO flammability limit, there is a risk that complete combustion will not occur, resulting in the release of toxic gases into the surrounding environment and a significant reduction in the efficiency of the charge preheating system.

[0005] In the context of the above-mentioned technical solutions, the amount of air that can be injected through the vault is consequently extremely low, if not zero, which exacerbates the problems caused by the low turbulence, thus effectively preventing optimal utilization of the energy available inside the preheating tunnel.

[0006] The object of the present invention is broadly to overcome the drawbacks of the known art, and with this object in mind, the invention provides an improved utilization of the energy present in the fumes for heating the charge. More particularly, it is an object of the present invention to improve heat exchange between the hot process fumes and the metal charge.

[0007] The above object is achieved by a process and a plant produced according to the independent and dependent claims. The present invention increases turbulence and mixing of the gas stream within the preheat tunnel, improving heat exchange between the hot process fumes and the metal charge, thereby accelerating the combustion process and increasing the rate of convective heat exchange between the combustion gases and the charge material.

[0008] This effect is achieved by using high velocity gas jets. The gas used is usually air, but the possibility of using other gases is not excluded if necessary to control the chemical composition of the atmosphere inside the preheating tunnel, so that this gas can also be effectively preheated.

[0009] In particular, the present invention relates to a process and plant for preheating a metal charge continuously fed to an electric melting furnace through a preheating tunnel equipped with a horizontal conveyor, the metal charge being covered in countercurrent flow by fumes or exhaust gases (rich in carbon monoxide and other flammable gases which must be treated in a fume system to be completely oxidized before being released into the atmosphere to avoid the risk of explosion) emerging from the electric melting furnace and from gas jets ejected through a number of nozzles located on the vault of the tunnel, which has side walls and a vault.

[0010] The nozzles, arranged in groups spaced longitudinally along the tunnel, create small-scale turbulence - injecting small, high-velocity gas jets that can penetrate the gas stream - and simultaneously create a "horseshoe vortex" structure with a central downward gas flow ("downwash") and an upward flow ("upwash") near the side walls of the preheat tunnel, providing the necessary gas circulation.

[0011] More particularly, the present invention relates to a process for preheating a metal charge continuously fed into an electric melting furnace through a preheating tunnel having side walls, a vault and a horizontal conveyor, the metal charge being covered in countercurrent flow with fumes or exhaust gases exiting the electric melting furnace; The process comprises the following: the intake of air for the complete combustion of the fumes or exhaust gases (17) from the ambient environment through openings along the preheating tunnel, preferably at the interface between the preheating tunnel and the electric melting furnace, the intake being regulated by acting on the suction fan and / or openings based on measurements detected by an exit gas temperature and / or composition sensor (21) at or downstream of the end of the tunnel (16); The metal charge is impinged upon by gas jets injected through a number of nozzles (15) arranged on the vault of the tunnel (16), the gas jets introduced by the nozzles (15) being unevenly distributed laterally on the vault of the preheating tunnel (16) with a higher concentration towards the roof of the vault of the tunnel (16), and the gas jets are unevenly distributed longitudinally along the preheating tunnel (16), with transverse sections in which the nozzles (15) are distributed and longitudinal sections of the tunnel (16) free of nozzles (15) being interspersed so as to avoid interference phenomena.

[0012] According to the present invention, the nozzles are arranged in groups, with the nozzles in each group spaced apart to correspond to the cross section of the tunnel roof. This allows for the simultaneous generation of small-scale turbulence and large-scale vortex structures. The small-scale turbulence corresponds to the fact that a small amount of high-velocity gas jets can penetrate the main gas flow passing through the tunnel, thereby significantly accelerating gas mixing and combustion. Meanwhile, the large-scale vortex structure, which improves heat exchange between the fumes and the charge, is generally defined as a "horseshoe vortex" and is characterized by a central downward flow ("downwash") that improves heat exchange in the central part of the preheating tunnel, and an upward flow ("upwash") near the tunnel side walls, which allows the necessary gas circulation and limits heat exchange with the tunnel side walls and the horizontal conveyor after imparting part of the thermal energy to the metal charge during the downward phase.

[0013] Contrary to what exists in the prior art, the gas jets mentioned above are not evenly distributed along the preheating tunnel, but rather in at least two groups spaced apart in order to avoid interference with the hydrodynamic properties and to firstly allow good mixing of the gases and rapid progression of combustion (through the action of small-scale turbulence), and then direct the resultant towards the metal charge (through the "horseshoe vortex" effect).

[0014] Contrary to what exists in the prior art, the nozzle is not dimensioned to deliver all the combustion air in a uniformly dispersed state, but rather to deliver a small, high-velocity gas jet whose main function is to provide kinetic energy and adjust the field of motion according to what has been described above, and for this reason said jet can be more accurately defined as a "hydrodynamic turbulence generator" or, more simply, a "hydrodynamic agitator".

[0015] More specifically, the combustion air required for the complete oxidation of the gases emitted by the furnace is drawn from the ambient environment through openings in the preheating tunnel, e.g., the connecting area of the preheating tunnel within the furnace, the interface between the tunnel and the furnace, or along the side edge of the conveyor. The fume extraction plant is adjusted so that the amount of air entering the preheating tunnel from the ambient environment through the openings is sufficient to ensure the complete combustion of the gases leaving the electric furnace, especially carbon monoxide (CO). This is verified by measuring various parameters of the fumes in the end area of the preheating tunnel, such as the temperature, which must exceed a certain threshold, e.g., 700 °C, and the composition of the gas, whose oxygen content must exceed a certain threshold, e.g., 5% by volume, to ensure the complete absence of CO. The adjustment of the extraction plant can be achieved, for example, by acting on the fume suction fan and / or the opening through which the combustion air is drawn from the ambient environment. The opening, for example at the movable end of the preheating tunnel (called the "connecting car") through which the scrap is introduced into the electric melting furnace EAF, is fitted into the furnace itself, i.e., at the interface between the preheating tunnel and the electric melting furnace. In fact, this opening can be easily changed by adjusting the position of the connecting car relative to the furnace.

[0016] On the other hand, the injected air / gas or gases entering through the nozzles play little or no role in the chemical balance of the gas combustion reaction from the electric furnace, while they perform a hydrodynamic role in increasing turbulence and mixing the fluid flow passing through the preheat tunnel in order to enhance and improve heat exchange with the charge material. In view of this, the nozzle dimensions of the injection plant are not determined using a chemical balance criterion, but purely using a hydrodynamic criterion based on the impact of the air injection on the impact of the fluid flow in the preheat tunnel, as will be explained below.

[0017] The use of "hydrodynamic agitators" is significantly simpler and more user-friendly than alternative solutions that provide for increasing the turbulence inside the preheating tunnel by inserting panel-like baffles, so-called "static agitators." These baffles must operate in a hot gas flow and dusty environment, and are therefore usually made as water-cooled metal panels, which is not an efficient solution from a thermal point of view. Any solution provided by the use of such baffles has in practice been abandoned due to the rapid wear and frequent breakage of the plates.

[0018] The prior art does not take into account that in practical reality, a considerable amount of ambient air always enters the charge preheating tunnel through unavoidable openings, and that the amount of combustion air fluctuates during the process, while for good mixing it must remain approximately constant.

[0019] The advantages provided by the present invention are therefore clear, as it largely decouples the manipulation of the jets to control the turbulence within the preheat tunnel from the control of any available combustion air supply.

[0020] The structural and functional features of the present invention, as well as its advantages over the prior art, will appear more clearly from the following description, taken in conjunction with the accompanying drawings, which illustrate a non-limiting embodiment of the invention itself as applied to an electric arc furnace (EAF) for melting continuously charged metal scrap. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 shows a diagram of a plant according to the prior art. [Figure 2] FIG. 2 illustrates a prior art plant with combustion air injectors evenly distributed along the preheat tunnel. [Figure 3]FIG. 3 shows a diagram of a plant according to the prior art, which is provided with a preheating zone with a burner and a heating zone using off-gas, connected by a fume suction / exhaust zone. [Figure 4] Figures 4a, 4b and 4c show, in side view (4a) and from above (4b and 4c), the gas flow in the preheat tunnel of a plant without high-velocity air injection manufactured in accordance with the prior art, and in particular the top view in Figure 4c shows the absence of turbulence corresponding to the air injectors manufactured and arranged in accordance with the prior art. [Figure 5] 5a and 5b show diagrammatic representations of a plant according to the invention. [Figure 6] FIG. 6 illustrates the arrangement of air injectors in the preheat tunnel of a plant according to the prior art. [Figure 7] FIG. 7 shows a diagram of the nozzle arrangement in the preheating tunnel of a plant according to the invention. [Figure 8] FIG. 8 shows a diagram of the nozzle arrangement in the preheating tunnel of a plant according to the invention. [Figure 9] 9a and 9b illustrate the various gas flows in a plant made according to the invention using a "hydrodynamic agitator" with high velocity jets. [Figure 10] FIG. 10 illustrates a side view of a preheat tunnel with nozzles in each zone according to the present invention. [Figure 11] FIG. 11 shows a cross-sectional view of a portion of a preheat tunnel equipped with nozzles in accordance with the present invention. [Figure 12] FIG. 12 illustrates a group of nozzles according to the present invention that can be placed in the area of the preheat tunnel. DETAILED DESCRIPTION OF THE INVENTION

[0022] [Detailed Description of the Invention] With respect to the figures, Figures 1 to 3 illustrate three plants manufactured in accordance with the prior art: in particular, Figure 1 illustrates a conventional plant with a preheating tunnel without a gas injector, Figure 2 illustrates a plant manufactured and arranged in accordance with the prior art with an air injector in the preheating tunnel, and Figure 3 illustrates a plant with a heating zone and a preheating zone with a burner connected by a fume suction / exhaust zone.

[0023] In each figure, 1 represents the entire plant in which a metal charge consisting of scrap 11 is continuously fed to an electric arc furnace (EAF) 12 in which a bath of molten metal is present in the liquid phase.

[0024] The flow 17 of fumes coming from the furnace 12 follows a substantially straight path that tends towards the wall of the preheat tunnel, and therefore away from the metal charge 11. Similarly, in the configuration of Figure 2, by using the combustion air injectors 19, the flow 17 is not significantly deflected, and the penetration of cold air 18 at the interface between the furnace 12 and the tunnel is sufficient to completely combust the fumes 17 coming from the furnace 12, so the additional combustion air introduced through the injectors 10 according to the teachings of the prior art is very small and does not create significant turbulence.

[0025] The nozzles or injectors provided in the prior art plants do not have any particular spatial arrangement and are therefore not characterized by any particular spatial arrangement within the preheat tunnel described according to the present invention.

[0026] A plant 1 of this kind is described, for example, in US Pat. No. 5,183,143. The plant 1 comprises at least one horizontal conveyor 13 suitable for continuously moving a metal charge consisting of scrap 11 onward towards the electric melting furnace EAF 12, the horizontal conveyor 13 defining a continuous horizontal line of feed of the charge 11 towards the charging section IV, which is the furnace 12 itself.

[0027] As can be clearly seen in the figures, the horizontal conveyor 13 forms the base of a preheating tunnel 16 for the metal charge consisting of scrap 11. Specifically, the plant 1 is comprised of a preheating zone III, which introduces a metal charge consisting of scrap 11 into an electric melting furnace EAF 12 from a fume discharge zone II and from zone I. The fume discharge zone II is located within the plant 1 and is provided upstream of the preheating zone III, taking into account the direction of movement of the scrap 11. Zone I receives the metal charge consisting of scrap 11 using a conventional scrap 11 receiving system.

[0028] The horizontal conveyor 13 vibrates and transports the metal charge consisting of scrap 11 from the preheating section III to a movable end called the "connecting car" 20, which introduces the scrap 11 into the electric melting furnace EAF 12.

[0029] According to the invention, a nozzle is present in the vault of the tunnel of the preheating section III (preheating tunnel 16) and provides for the injection of gas 15. In particular, these nozzles are nozzles 15 which provide high velocity injection of gas.

[0030] The nozzles 15 are distributed to obtain a turbulent swirling motion inside the preheating tunnel 16, which improves the heat exchange between the off-gas 17 and the metal charge consisting of scrap 11.

[0031] As shown in Figures 9a and 9b, the nozzles 15 provided in the vault of the tunnel 16 of the preheating section III increase the turbulence of the off-gas 17, which allows the following to be obtained:

[0032] Increased speed of mixing reactive gases and their combustion Improving conditions for complete combustion of CO, H2 and other gases, as well as carbonaceous dust coming from the electric melting furnace EAF12 Improved and uniform temperature distribution inside the preheating tunnel 16 Improved heat exchange between the combustion gases and the metal charge 11 on the horizontal conveyor 13 inside the preheating tunnel 16 In prior art plants without nozzles or "hydrodynamic agitators" 15, the air entering the plant through the connections is not controlled and the limited turbulence and vorticity results in insufficient mixing of the gases (Figures 4a, 4b and 4c), which results in slow and sometimes incomplete combustion in the preheat tunnel 16.

[0033] On the other hand, the presence of the "hydrodynamic agitator" 15 inside the preheating tunnel 16 results in a greater mixing of the gases and at the same time a higher flame intensity, which also serves to limit cooling from outside the plant, especially within the preheating tunnel 16, due to the intrusion of air in particular.

[0034] As shown in Figures 7 and 9a, 9b, the arrangement of nozzles or "hydrodynamic agitators" 15 makes it possible to concentrate the so-called downwash part of the movement area in the center of the horizontal conveyor 13, so that in this area maximum heat exchange is obtained between the metal charge 11 and the gases / fumes 17 present in the preheating tunnel 16.

[0035] In order to obtain the above-mentioned vortex structure, the nozzles 15, and therefore the incoming jets, are distributed unevenly laterally on the vault of the preheating tunnel 16, with more concentration at the top of the vault of the tunnel 16.

[0036] In a preferred embodiment, the nozzles are located laterally in the center of the tunnel vault, leaving the sides of the vault free of nozzles. This gas injection arrangement is thus intended to obtain, for example, a well-defined vortex structure (FIGS. 9a, 9b) inside the preheating tunnel 16, characterized by the following regions:

[0037] A downflow region in the central region immediately downstream of the area where the nozzles 15 are located, to improve heat exchange with the metal charge of scrap 11 in that region. · Upflow areas at the sides to limit heat exchange with the walls of the preheating tunnel 16 and the horizontal conveyor 13.

[0038] This swirling structure of the gas within the preheat tunnel 16 is commonly referred to as a "horseshoe vortex" and is obtained in accordance with an embodiment of the present invention by positioning the nozzle 15, and therefore the jet, on approximately two-thirds of the cross section of the preheat tunnel 16 and leaving two side walls near the side walls of the tunnel 16 vault open.

[0039] The arrangement of the nozzles on the vault of the preheat tunnel 16 can be varied to accommodate specific plant engineering problems (see, for example, the solution of the embodiment shown in Figure 8), but the requirement remains that the relatively high velocity jet always blocks the central portion of the off-gas 17 flow, leaving the side portions free to favor the establishment of upward gas circulation and the formation of a horseshoe vortex within the entraining flow.

[0040] The high-velocity jets, which act as "hydrodynamic agitators", are distributed laterally on the vault of the preheating tunnel 16, but not evenly, according to the "injection zones" that are appropriately spaced from each other, in order to avoid hydrodynamic interference phenomena, the distance between two adjacent injection zones should be 4-6 m depending on the velocity of the gas passing through the preheating tunnel. The space between two adjacent injection zones is intended to allow time for the high-intensity flame generated by the upstream zone to develop before being pushed into the charge by the injection zone directly below it.

[0041] The initial injection zone is located as close as possible to the electric melting furnace EAF 12 in order to make the most of the length of the preheating tunnel 16 in completely burning off CO, H2 and any pollutants present in the process gas in the tunnel and in improving heat recovery.

[0042] The first set of high velocity gas jets is located near the electric melting furnace 12, at a distance of between 7 and 10 meters from the furnace. The gas jets are arranged with increasing velocity and / or flow rate between successive "gas injection zones." The number of injection zones varies from two to four, depending on the amount of combustible gases generated by the envisaged melting process.

[0043] As shown in Figure 11, along the gas flow (gas flow in the opposite direction to the movement of the metal charge 11) starting from the electric melting furnace EAF 12 and heading towards the fume extraction plant, a number of nozzles 15 constituting the first injection zone can be provided above the connecting car 20 (first water-cooled hood). Meanwhile, the nozzles 15 constituting the other injection zone can be positioned at the beginning of each segment of the heat-resistant zone (heat-resistant lining hood) of the preheating tunnel 16.

[0044] In the example shown, three injection zones are used, each consisting of four nozzles 15 . The gas injected is typically air at room temperature, but other gases or mixtures of gases are also possible, and the gases used may be preheated.

[0045] Each zone may be provided with means for controlling the operating conditions of the nozzles 15 . The jet emitted from the nozzle 15 is small and fast, as it must be able to both mix and deflect the gas flow passing through the preheat tunnel 16. This jet activates a "downwash" motion of the hot gas 17 towards the metal charge 11, arriving with sufficient velocity to penetrate into the interstices of the material (the so-called "impingement" effect, as seen in Figure 5, where the hot gas 17 flow is pushed downwards towards the metal charge 11), improving convective heat exchange.

[0046] This effect is obtained by evaluating the flow conditions and gas flow velocities involved in the process: defining the impact of a fluid flow as the product of the mass flow rate and the velocity of the flow itself, the size of a single jet must be determined so that the group of jets has an impact equivalent to that of the main stream of fumes passing along the tunnel 16 from the furnace 12 to the suction plant.

[0047] Therefore, the conditions for determining this size are as follows:

number

[0048] Within the scope of the proposed technical solution, the injected gases result in a progressive increase in the gas flow rate through the preheat tunnel 16, which may require consideration of more impactful injections for injection zones located further away from the furnace 12.

[0049] After the gas flow exits the melting furnace 12 and enters the preheat tunnel 16, a condition exists where the first injection zone utilizes a lower flow rate and velocity than subsequent injection zones due to the overall increased gas flow rate through the preheat tunnel.

[0050] Each injection zone can be managed, controlled and adjusted independently of the others, depending on the progress of the process and the characteristics of the charge 11 present in the preheating tunnel 16 and the gases 17 leaving the furnace. In the simplest embodiment, all nozzles 15 are identical and located at the top of the preheat tunnel 16. Their number is determined primarily by the width of the preheat tunnel 16 itself, taking into account that the available area is approximately two-thirds of the central area (the area where the "downwash" zone is located) and the distance between each jet is approximately 450-500 mm. To achieve an effective impingement effect, the top of the preheat tunnel 16 must be located approximately 800-1200 mm from the load present on the conveyor (this may require redesign of the preheat tunnel when applying the present invention to an existing plant). If the preheat tunnel 16 has a special configuration, for example, if plant constraints prevent the nozzles 15 from being equidistant from each other, different jet arrangements and dimensions can be used to achieve the same hydrodynamic effect.

[0051] Contrary to known plants and processes in which air injection leads to control of the combustion process from a stoichiometric point of view, in the present invention, the injection of air or other gases is generally, if not exclusively, used to obtain certain turbulent flow conditions inside the preheat tunnel 16.

[0052] Even in the most common cases where air injection is used, the overall injection volume of the described system is always smaller than the air flow rate required for complete combustion of the process gas 17 coming from the furnace 12, since the primary purpose of the described system is to stabilize the turbulence. Control of the air supply for the combustion of the process gas inside the preheating tunnel 16 is essentially left to adjusting the suction pressure and the width of the gap between the end of the preheating tunnel and the furnace (which can never be completely eliminated from a plant engineering perspective). In this way, the air injection as a turbulence generator is clearly separated from the ambient air supply for complete combustion of the process gas.

[0053] Specifically, decoupling is performed as follows: In a first step, e.g., plant start-up, the nozzle 15 is released and the plant is put into operation to start the melting process in the EAF furnace. The fumes are drawn through discharge zone II along preheat tunnel III, and the temperature of the fumes generated in the furnace is measured using one or more sensors 21 (located in fume discharge zone II or in other suitable areas of the fume system) and, if necessary, their chemical composition, in particular their oxygen and / or carbon monoxide content, is analyzed. Complete combustion of the CO is usually considered to have been reached when the measured temperature remains above 700°C and, if the chemical composition is also being measured, when the oxygen content is above 5% by volume.

[0054] These conditions are achieved by adjusting the suction plant, in particular by adjusting the speed of the suction fan. Furthermore, by adjusting the position of the connecting car 20, the opening at the interface between the connecting car 20 and the furnace can be widened or narrowed, thereby increasing or decreasing (by the same adjustment as the suction system) the flow of ambient air 18 sucked through said opening due to the internal pressure reduction.

[0055] In addition to or as an alternative to adjusting the position of the connecting wheel 20, there may be an adjustable element 22 (as shown in Figure 5b) that allows for the variation or adjustment of the opening at the interface between the connecting wheel 20 and the furnace. For example, this adjustable element 22 may be a connecting flange with an adjustable opening. As mentioned above, once the desired temperature conditions and possibly the chemical composition of the fumes have been reached, the plant is in a safe state with respect to the combustion of the fumes.

[0056] At this point, nozzle 15 comes into operation, creating turbulent conditions, as shown, that increase both the combustion of the fumes generated in the furnace and the heat exchange with the scrap 11. The air (or other gas or gas mixture) injected through nozzle 15 obviously does not play a significant role from the point of view of chemical balance, since, in the sequence of events described, all the combustion air is drawn in through the opening of the preheat tunnel. The role of the air (or other gas or gas mixture) injected through nozzle 15 is, on the contrary, essentially of the hydrodynamic type, since it favors the mixing of the fluid flow and thereby aids in better combustion of the fumes.

[0057] As evidence of its role in the chemical balance of the reaction, the amount of air introduced through nozzle 15 is typically about 5-15% of the amount of air in stream 18, i.e., the amount of air drawn in through the opening of the preheat tunnel.

[0058] The speed of the air introduced by the nozzle 15 can therefore be easily adjusted regardless of the criteria related to the combustion of fumes emitted by the electric melting furnace, which, as stated, is left to the air drawn in through the opening of the preheating tunnel. As a result, the operation of the nozzle 15 can be adjusted based on other parameters, such as the type of process, the charge material, or any transient phenomena in the plant. For example, in the event of a conveyor stoppage, the air drawn in by the nozzle 15 can be completely closed to avoid localized melting of the charge material, while at the same time ensuring the normal combustion of fumes emitted by the furnace, which, as already indicated, is achieved by the air drawn in through the opening of the preheating tunnel.

[0059] The ultimate aim of the present invention is therefore to increase the combustion intensity of the process gases coming from the furnace and the heat exchange between these and the charge, thereby increasing the overall energy efficiency of the melting process.

[0060] The improved mixing and combustion of the process gases discharged from the furnace achieved by application of the present invention makes it possible to obtain better thermal destruction of the contaminants (and related precursors) present therein. These multiple benefits are substantiated by experimental results showing a reduction in the residual CO content by up to 30%, in addition to electrical energy savings of approximately 5-10 kWh / tonne in the process and an overall reduction in other unwanted elements such as VOCs (volatile organic compounds), PAHs (polycyclic aromatic hydrocarbons) and PCBs (polychlorinated biphenyls).

[0061] The invention is also applicable to plants such as those described in WO 2012 / 007105, in which a preheating tunnel and a heating tunnel are provided for a metal charge consisting of, for example, scrap.

[0062] Thanks to the present invention, the heating of the scrap charge is achieved by turbulence created in the preheating tunnel 16. This differs from the method used in known plants, where the introduction of air is only performed in accordance with the requirements of the chemical combustion process and there is no logic for managing the movement zone inside the preheating tunnel.

[0063] Thanks to the present invention, it is also possible to avoid the use of deflectors inside the preheating tunnel 16. These deflectors have the following drawbacks: they create a significant pressure drop in the fume suction, they operate in a very hot and dusty gas flow, which requires cooling and frequent maintenance (which not only increases the complexity of the plant but also represents a source of risk for leaks and leads to unnecessary losses of thermal energy with the gas), they are difficult to adjust and control from a practical point of view, since the angle of incidence cannot be easily changed, and their effectiveness is limited at low treated velocities, i.e. low flow rates.

[0064] Thus, the objects of the invention as set out in the preamble of this specification have been achieved. The scope of protection of the present invention is determined based on the recitation of each claim.

Claims

1. A process for preheating a metal charge (11) continuously fed into an electric melting furnace (12) through a preheating tunnel (16) having side walls, a vault and a horizontal conveyor (13), wherein the metal charge (11) is surrounded in countercurrent by fumes or exhaust gases (17) flowing out of the electric melting furnace (12), the intake of air for the complete combustion of the fumes or exhaust gases (17) from the ambient environment is carried out through openings along the preheating tunnel (16), said intake being regulated by acting on the suction fan and / or said openings on the basis of measurements revealed by temperature sensors (21) and / or composition of the exit gases at or downstream of the end of the preheating tunnel (16); the metal charge is covered with gas jets injected through a plurality of nozzles (15) arranged on the vault of the preheating tunnel (16), the gas jets injected by the nozzles (15) being unevenly distributed laterally on the vault of the preheating tunnel (16) with a higher concentration towards the top of the vault of the preheating tunnel (16) and the gas jets being unevenly distributed longitudinally along the preheating tunnel (16), the nozzles (15) being distributed in transverse sections and the preheating tunnel (16) being interspersed with longitudinal sections free of the nozzles (15) so as to avoid interference phenomena; A process characterized by:

2. 2. The process according to claim 1, characterized in that the gas jets emitted from the nozzles (15) are distributed over approximately two-thirds of the cross section of the preheat tunnel (16), leaving at least a portion of the side wall of the vault of the preheat tunnel (16) open.

3. A process as described in claim 1 or 2, characterized in that the first group of gas jets is located in the vicinity of the electric melting furnace (12) at a distance from the furnace within a range of 7 to 10 meters.

4. A process described in any one of claims 1 to 3, characterized in that the gas injection is arranged so that the velocity and / or flow rate increases between areas where consecutive nozzles (15) are provided, called ``gas injection areas.''

5. The size of one jet of the nozzle (15) is determined by a conditional formula that determines the size so that a group of jets has an impact equivalent to the impact of the main stream of fumes passing through the preheating tunnel (16). [Equation 1] where Wgas = mass flow rate of the fumes in the preheating tunnel (16) at a given injection zone [kg / s] Wjet = mass flow rate of one jet in the same injection zone [kg / s] Vgas = velocity of the fumes in the preheating tunnel (16) corresponding to the same injection zone [m / s] Vjet = velocity of one jet [m / s] Njets = Number of jets on the given injection area [#] The process according to any one of claims 1 to 4, characterized in that the temperature is determined according to:

6. 1. A plant for carrying out a process for preheating a metal charge (11) that is continuously fed into an electric melting furnace (12) through a preheating tunnel (16) provided with side walls, a vault and a horizontal conveyor (13), the metal charge (11) being covered in countercurrent flow by fumes or exhaust gases emerging from the electric melting furnace (12) and by gas jets emitted by a number of nozzles (15) arranged on the vault of the preheating tunnel (16), the plant comprising a suction fan and openings along the preheating tunnel (16), the suction fan and / or the openings being adjustable to draw air from the surrounding environment to obtain complete combustion of the fumes or exhaust gases (17), the complete combustion of the fumes or exhaust gases (17) being verified on the basis of measurements revealed by temperature sensors (21) and / or composition of the exit gases at or downstream of the end of the preheating tunnel (16), On the vault of the preheating tunnel (16), a group of at least two nozzles (15) is provided at a longitudinal distance relative to the preheating tunnel (16), the nozzles (15) are distributed unequally across the vault of the preheating tunnel (16), with a higher concentration towards the top of the vault of the preheating tunnel (16), and the nozzles (15) are distributed unequally along the longitudinal direction of the preheating tunnel (16), with transverse sections in which the nozzles (15) are distributed and longitudinal sections in the preheating tunnel (16) free of the nozzles (15) being interspersed so as to avoid interference phenomena; A plant characterized by:

7. 7. A plant according to claim 6, characterized in that the nozzles (15) are distributed over approximately two-thirds of the cross section of the preheating tunnel (16), leaving at least part of the side wall of the vault of the preheating tunnel (16) open.

8. 7. A plant according to claim 6, characterized in that the plant provides 2 to 4 sections in which the nozzles (15) or "gas injection zones" are provided, spaced apart from one another in the longitudinal direction relative to the preheating tunnel (16) by a distance of about 4 to 6 meters.

9. 9. The plant according to claim 8, characterized in that the first "gas injection zone" is located in the vicinity of the electric melting furnace (12), at a distance of 7 to 10 meters from the furnace.

10. 10. A plant according to any one of claims 6 to 9, characterized in that there are three "gas injection zones" along the preheating tunnel (16), each zone having four nozzles (15).

11. A plant as described in any one of claims 6 to 9, characterized in that the nozzles (15) are arranged so that the velocity and / or flow rate increases between successive "gas injection zones".

12. 12. Plant according to any one of claims 6 to 11, characterized in that the nozzles (15) are all identical and are all arranged at the top of the preheating tunnel (16), in a number determined by the width of the preheating tunnel (16) itself, with a centre distance of about 450 to 500 mm between each jet.

13. 6. The process according to claim 1, wherein the opening is an opening present at the interface between the preheat tunnel (16) and the electric melting furnace (12).

14. 13. The plant according to claim 6, wherein the opening is an opening present at the interface between the preheating tunnel (16) and the electric melting furnace (12).

15. 1. A plant for carrying out a process for preheating a metal charge (11) that is continuously fed into an electric melting furnace (12) through a preheating tunnel (16) provided with side walls, a vault and a horizontal conveyor (13), the metal charge (11) being covered in countercurrent flow by fumes or exhaust gases emerging from the electric melting furnace (12) and by gas jets emitted by a number of nozzles (15) arranged on the vault of the preheating tunnel (16), the plant comprising a suction fan and an opening at the interface between the preheating tunnel (16) and the electric melting furnace (12), the suction fan and / or the opening being adjustable to draw air from the surrounding environment to obtain complete combustion of the fumes or exhaust gases (17), the complete combustion of the fumes or exhaust gases (17) being verified on the basis of measurements revealed by temperature sensors (21) and / or composition of the exit gases at the end of the preheating tunnel (16) or downstream of said end, On the vault of the preheating tunnel (16), a group of at least two nozzles (15) is provided at a longitudinal distance relative to the preheating tunnel (16), the nozzles (15) are distributed unequally across the vault of the preheating tunnel (16), with a higher concentration towards the top of the vault of the preheating tunnel (16), and the nozzles (15) are distributed unequally along the longitudinal direction of the preheating tunnel (16), with transverse sections in which the nozzles (15) are distributed and longitudinal sections in the preheating tunnel (16) free of the nozzles (15) being interspersed so as to avoid interference phenomena; A plant characterized by:

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