MXPA06006011A - Method for pre-heating, transforming and melting a metal charge and relative plant - Google Patents

Abstract

Method and plant for pre-heating, transforming and melting a metal charge comprising metal scrap, in an electric arc furnace (12) associated with a tunnel (11) to transport, pre-heat and discharge the scrap. The furnace (12) comprises a hearth (13) and a roof (14) through which the electrodes (15) pass. The method provides that the furnace (12) is weighed at least periodically in order to detect the quantity of discharged scrap present inside the furnace (12) itself, that the temperature of the liquid bath inside the furnace (12) is detected at least periodically and that at least the discharge delivery of the scrap inside the furnace (12) is detected by weighing and is regulated in order to maintain the temperature of the liquid bath around a pre-determined value.

Description

METHOD FOR PREHEATING, TRANSFORMING AND MELTING A LOAD OF METAL AND RELATED PLANT FIELD OF THE INVENTION The present invention relates to a method and a related plant for preheating, transforming and melting a metal charge.

BACKGROUND OF THE INVENTION [0002] Plants for transforming and melting metal loads, of the prereduced or not type, comprising an electric arc furnace suitable for collaborating with a tunnel for transporting and preheating scrap are known. The electric furnace provides at least one container, or crucible, and a roof covering. The electrodes are introduced into the ceiling through suitable holes. The transport and preheating tunnel cooperates with the electric furnace on one side and, on the other side, both with a scrap loading system and also with a gas elimination system. The gases, as described by IT-B-949.145 (dated 1973), are hot extracted from the interior of the electric arc furnace and run through the entire tunnel in counterflow with the scrap. The same document also teaches to provide a crucible that oscillates by means of supports, in order to provide on each occasion the discharge of the slag or casting of the molten metal. It is also known, for example, from IT-B-1009463 (dated 1974) to provide systems that continuously feed the preheated metal charge into the furnace. This document also discloses that the preheating of the metal charge occurs within a cylinder made of a refractory material that rotates about an axis that is inclined with respect to the horizontal. In this way, the metal load continuously changes its position inside the cylinder, so that by means of the convective movements of the gases emerging from the furnace, a uniform heating of the metal charge is obtained before it is introduced into the cylinder. oven. Document US-A-3, 985, 497 (dated 1974) teaches burning unburnt gases derived from the preheating process of the metal charge, in order to reduce polluting emissions into the environment. Other systems for preheating the scrap in a tunnel before sending it to the furnace are shown in US-A-3,813,209 (dated 1973) and US-A-4,083,675 (dated 1978). The document by Stahl and Eisen 95 (January 1975), describes a method in which the exhaust gases of an electric arc furnace are transported in counterflow through the preheating tunnel, where scrap is transported continuously. baked. This document also provides inside the furnace, after the casting, a head of liquid metal that is maintained between 4 and 6 tons, in order to guarantee a trouble-free start of the subsequent melting. The document IT-B-1201815 (dated 1986), not only provides a continuous feeding of the load to an oven through a preheating plant, but also provides blowing in the bath means of carburation, mixtures of sulfurants, deoxidants and mixtures for foamed slag. Apart from this, the IT'815 also teaches providing cauldron stirrups that collect the cauldron filled with molten steel in the casting station, in order to feed it directly into the emptying zone, so that it functions as a connection element between the melting zone and the continuous casting zone of the molten steel. It also provides an intermediate station to heat the cauldrons in order to maintain the temperature of the molten steel at suitable values for emptying. US-A-4, 543, 124 and US-A-4, 564, 388 respectively describe a device and method for continuously transforming and melting the metal. They are substantially characterized in that in all the charging, melting and refining steps, the electric power supply to the furnace is maintained at maximum power, regardless of the steps in the cycle and the technological characteristics of the material being worked. It also provides that the load with the material that works as the load, it is also continuous during the casting of the molten metal. It also provides that the casting of the molten metal is about 50% of the total volume of the molten metal, and that it occurs by tilting the furnace at most up to 15 °, without removing the electrodes and maintaining the electrical power to the electrodes at maximum power. Patent US-B2-6, 155, 333 substantially returns to the teachings of IT'815, by achieving an integrated plant with a direct connection between the melting zone and the emptying zone, using an intermediate metallurgical vessel. The documents described above are indicated only as examples, since the literature on this subject is extremely extensive. However, those operating in this field are well known for their continuous effort to optimize various technical and technological aspects of the whole process to transform the iron material into molten steel that is sent, for example, for continuous emptying. The purpose of this ongoing investigation is to ensure, at the same time: optimum quality characteristics of the molten material, wear and tear and risk of reduced damage to the parts and components, and therefore, reduced need for maintenance and replacement, particularly of the covers and / or the refractory material, reduced energy consumption for the same amount of molten metal, - reduced cycle times, - lower incidence of labor cost. In view of this, and based on long and extensive studies and experiments, the Applicant has made significant improvements to the existing technologies, including those described in the aforementioned documents, has overcome some disadvantages of the state of the art and incorporated the present invention.

SUMMARY OF THE INVENTION The present invention is set forth and characterized essentially in the main claims, while the dependent claims describe other innovative features of the invention. The invention relates to a method and a relative plant for preheating, transforming and melting a metal charge. The plant according to the invention comprises an electric arc furnace associated with a tunnel that transports and preheats the metal charge that is introduced into the furnace. The electric furnace comprises a containment crucible that can be oscillated by means of supports, an angle of between ± 4 ° during normal operation, for example, during the loading, melting and refining steps of the liquid metal. In addition, it can be oscillated at a greater angle, comprised between -15 ° and + 25 °, respectively, during the discharge steps of the slag (deslagging), and straining the liquid metal. For example, during the step to discharge the slag (deslagging), the crucible can be tilted as much as -15 °, while during the pouring step, it tilts opposite to + 25 °. According to a feature of the present invention, the passage of the melt provides maintaining within the crucible an amount of molten metal equal to about 30-40% of the total capacity of the molten metal of the crucible. This amount has been identified by the Applicant as the optimal compromise between the need to ensure a rapid start of the subsequent melting, without a break and without risks arising, for example, from the sudden drops of scrap towards the bottom of the crucible and the normal working productivity of the furnace. For example, in the case of a crucible suitable for a molten metal head of 80 tons, the amount removed in the pouring step is approximately 55 tons, while a base of approximately 25 tons of molten metal is left inside the oven. According to the invention, just before starting to make the furnace oscillate, during the casting step, the method provides that the electrodes rise from the bath, so that they do not come into contact with the bath in any situation, At the moment when the electrodes begin to withdraw from the liquid bath, the power supply is interrupted. In addition, in the event that the loading of the material occurs discontinuously by means of baskets with preheated scrap, the invention provides that the roof of the oven is opened and that the electrical energy to the electrodes is interrupted, that the basket is emptied inside of the furnace, that the ceiling is replaced and that the electrical power is restored to the electrodes. In the event that the load occurs by means of a preheating tunnel directly connected to the furnace, the invention provides that the load can take place in a specific step that follows the casting of the molten metal and can continue partially during the cycle of cast. However, the load is interrupted during the final refining stage of the metal, which precedes the casting, and also during the passage of the casting. The Applicant has verified in fact, that the loading of the scrap during the refining determines a deterioration in the quality of the molten metal and in any case, lengthens the required times, thus canceling the benefits derived from the initial introduction of the scrap. In addition, when the load occurs with the furnace in its inclined condition, for example, in the passage of the laundry, the insertion of the scrap can cause bumps and damage to the furnace walls, with the elimination of the refractory material and the possible breakage of the cooling panels. According to the invention, between the end of the casting operation and the beginning of the loading of the scrap for the subsequent cycle, the electrical supply to the electrodes is interrupted and the following operations are carried out: a) the furnace is rotates from the casting position to the deslagging position, in order to interrupt the flow of liquid steel; b) the cleaning of the pouring orifice is controlled; c) the pouring channel is filled with granular material with a high melting point; d) the furnace returns to the horizontal position and the vertical blocks are inserted (mechanical stops that limit the rotation to reduced values during normal operation); e) it is only at this point that the electrodes are allowed to descend (and therefore, provide electrical power). The presence of the vertical blocks allows to limit the oscillation of the electric arc furnace in order to avoid the interference of the crucible with the last (retractable) segment of the scrap conveyor entering the furnace. Then, when the electrodes are inserted into the roof and the furnace has stabilized in its normal operating position, the loading of the scrap is started and the electrical energy is progressively supplied until the desired value is reached. According to the invention, before proceeding with the raising of the electrodes in order to start the casting, the electric power fed to the furnace is reduced with respect to its normal working value, by means of a reduction step. According to the invention, the electrical power is fed to the furnace according to the amount of scrap present inside the furnace. In other words, power supplied not always and in no case, is the maximum power available to the plant, but on each occasion is regulated according to the actual amount of scrap in that specific melt cycle and / or in accordance with the detected temperature of the liquid metal bath. This strategy comprises an obvious first advantage in terms of energy consumption, since the power supplied is always the optimum power for the specific amount. In addition, the quality of the material obtained is also improved, since the power is supplied for the melt and the refining is adjusted according to the actual amount of scrap. To measure the amount of scrap, the invention provides that the entire "kiln system" is weighed continuously, by removing the tare, known in advance, in order to find how much scrap is present inside. According to another characteristic of the invention, at least one periodic control is carried out for the temperature of the molten metal. To be more exact, according to a first characteristic of the invention, the value of the temperature of the molten metal is used to regulate the melting profile and the loading of the scrap inside the furnace, to maintain the temperature always around a value pre-established According to another characteristic, as mentioned above, the value of the temperature is used as another parameter, apart from the weight of the scrap inside the furnace, in order to regulate the electric power supplied and more particularly, to maximize the power according to the with the load and the stability signal of the arc, detected for example, by means of a harmonic analyzer. This means that the power fed to the electrodes will oscillate steadily from the maximum value that the system can supply to the electrodes, at a lower value according to the amount of scrap that in each case remains so that it also melts with the temperature of the already melted metal. According to the invention, the nozzles that introduce oxygen, carbon, carburetion medium, desulphurisation mixtures, deoxidants and mixtures for foamed slag, and also the oxygen lances, are placed in a position so that they remain as far as possible below the level of the liquid metal. To be more exact, in the furnace according to the invention, there are specific nozzles for the introduction of carbon and specific nozzles for the introduction of oxygen. The transport gas is passed through the nozzles for the introduction of the carbon during the passage of the casting and the transient passages, while the nozzles for the introduction of oxygen are kept active during these steps as well, with a minimum amount of oxygen; in both cases, this is in order to prevent the oxygen outlet holes from blocking. According to another feature of the present invention, the characteristics of the gases as they exit the transport and preheat tunnel, are controlled in order to verify that no combustible gases are present. In order to ensure that all the combustible gases as they leave the electric furnace and are channeled to the preheating tunnel of the scrap are burned, the method according to the invention provides, at the beginning of the tunnel, ie, near the electric furnace , a burner that has the sole purpose of igniting, under any operating condition, the combustible gases that arise from the electric furnace, so that they burn naturally. In addition, the method always provides some residual oxygen (> 6-8%), possibly fed by means of air injectors, sufficient to guarantee under any operating condition, that unburned and potentially explosive gases are absent.

The gases eliminated at the beginning of the tunnel, that is, in the part where the scrap is loaded, enter at a speed of approximately 20 m / s and a temperature of approximately 800 ° C in a sedimentation chamber in order to sediment the particulates; the sedimentation chamber is thermally insulated. From the sedimentation chamber, through an uninsulated tube, the gases enter the cooling tower. The invention provides that the gases are cooled rapidly in the cooling tower at a controlled temperature of at least 250 ° C, or less lower, with a cooling rate of not less than 250 ° C / second, advantageously 400 ° C /second. The device provided for this rapid cooling consists of a system for injecting water, mixtures of water and air, or also mixtures of water and powders in an appropriate and controlled amount. The injection means are made, constructed and handled in such a manner, to produce a nebulized flow characterized by a fine dispersion of the water droplets, so that they are capable of varying, as desired, their speed, diameter and opening angle. of the cone. Thanks to this treatment, the reduction of dioxin and furan pollutant emissions is guaranteed, completely and in line with the strictest international guidelines in this field.

BRIEF DESCRIPTION OF THE DRAWINGS These and other features of the present invention will become apparent from the following description of a preferred embodiment form, given as a non-restrictive example, with reference to the accompanying Figure 1, which is a schematic illustration of a plant to preheat, transform and melt a metal charge.

DETAILED DESCRIPTION OF A PREFERRED MODE OF THE INVENTION With reference to the attached figure, a plant for preheating, transforming and melting a metal charge is denoted in its entirety by the reference number (10) and comprises, as essential elements, a tunnel (11) for transporting and preheating the scrap and an electric arc furnace (12). The electric furnace (12) comprises a crucible (13) for containing the scrap metal and a roof covering (14). The roof (14) has holes through which the electrodes (15) are introduced, which are supported by and mounted on related arms (16). The electrodes (15) are connected in a known manner to a power transformer and can be powered regardless of whether it is with direct current (DC) or alternating current (C?). The crucible (13) is supported by supports (17) that allow it to oscillate according to the coherent methods, with the various stages of the cycle, particularly a reduced oscillation, ± 4 ° during the cycle to introduce and melt the scrap, and a greater oscillation, between -15 ° and + 25 ° during the steps of deslagging and casting. The shape of the crucible, in relation to the inclination that it adopts in the pouring step of the liquid metal, is such to maintain a liquid deposit of approximately 30-40% of its capacity. The furnace (12), in a known manner, is equipped with burners, oxygen spears and injectors of carbon and lime powders, capable of promoting foaming of the slag, but these are not shown here. To be more exact, the injection nozzles are positioned to have the outlet nozzles placed below the level of the slag. The furnace system is also mounted in load cells (18), suitable for providing the information with respect to the weight supported therein, with the methods and for the purpose that will be clarified hereinafter. Within the furnace, temperature probes are provided in order to measure the temperature of the liquid bath for the purposes indicated hereinafter.

The metal load used consists of at least 80% iron, which can be sponge iron that comes from a direct reduction process or granules, briquettes or scrap in general. Apart from using the conveyor system associated with the tunnel (11), the furnace can be loaded by means of a basket system, particularly in the case of a cold start or after interruptions in the operation of the same. According to the invention, the loading by means of the tunnel is activated after a quantity of metal has melted, at least equal to the normal amount that the liquid deposit forms and which is maintained inside the crucible. Another condition for initiating charging by means of the tunnel, according to the invention, is that the temperature of the molten metal is at least about 1500 ° C. According to the invention, the speed at which the scrap or other material constituting the load is introduced, is regulated to maintain in any case, the temperature of the bath in close proximity with a pre-established value. In other words, after adjusting a desired value of the bath temperature, for example, around 1500-1600 ° C, the temperature is periodically detected as the melting cycle proceeds and the following cases may occur: if a decrease is detected of the temperature with respect to the desired value, this means that the loading speed of the scrap is too high, and therefore should be decreased, while - if a temperature above the desired value is detected, this means that the load has place very slowly and therefore should accelerate. The transport and preheating tunnel (11) serves to bring the temperature of the metal charge to an average value of around 200-270 ° C, before it is introduced to the electric furnace (12). The tunnel (11) consists, in this case, of a plurality of modules (22), of which the first two, oriented towards the electric oven (12), are equipped with burners (19) in order to achieve self-combustion desired of the gases that arise with respect to the advancing material. At least some of the modules (22) are equipped with air injectors (20) to achieve the post-combustion of CO and other combustible gases contained in the exhaust gases. In the initial part of the tunnel (11), in a position adjacent to the outlet pipe of the preheated vapors (24), there is a compensation chamber (23) that acts as a dynamic seal for the exhaust vapors that are thrown into the medium ambient. The purpose of the chamber (23) is to control the amount of external air that can be sucked into the vapor circuit of the loading side of the scrap conveyor. The chamber (23) has ventilation means inside it, its function is to ensure that the depression inside the chamber (23) is slightly smaller than the depression in the tube (24). This strategy allows to keep the amount of air that enters from the outside towards the vapor outlet pipe at a reduced value, while at the same time preventing the vapors from escaping into the atmosphere. Upstream of the vapor outlet pipe (24), there are also probes (25) to analyze the vapors and temperature detectors (26). Before being discharged into the atmosphere, the vapors are also sent to a cooling tower (27). The heating temperature inside the tunnel (11), is continuously verified by thermocouples (21). According to the invention, a typical cycle time between two consecutive casting operations is approximately 40-45 minutes. A cycle begins with the discharge of the scrap in the furnace, at the bottom of which is present a quantity of metal comprising between 30% and 40% of its total capacity, for example, approximately 25 tons in the case of a capacity of approximately 80 tons. In the initial step, various additives are introduced into the bath by means of lances and / or nozzles, in order to carburize the bath and encourage rapid foam formation of the slag. If the slag foam as quickly as possible, it provides protection to the cooling panels of the electric arc radiation and allows a good part of the heat to be contained in the bath.; this allows to increase quickly the loading speed of the scrap and also the progressive increase of the electric power. This electrical power increases progressively as the amount of scrap that is discharged into the furnace increases, up to a maximum working value. The value supplied can grow from the initial value, equal to approximately 40-50% of the work value to its maximum value. Through the charging cycle, the weight of the furnace system and the temperature of the bath are checked periodically, or also continuously, in order to regulate the discharge of the scrap, to keep the bath temperature substantially constant.

When the desired amount of scrap has been reached, the discharge is interrupted, and the bath temperature is increased until the desired value for casting is reached. According to the invention, at least the discharge of the scrap is interrupted before casting during a range of between approximately 8 and 12% of the total cycle time. The power supply is also interrupted before casting for a time of up to 5% of the total cycle time. When the scrap is completely melted, and after the refining step has been completed, the casting is carried out, by actuating the supports (17) so that they tilt the crucible (13) and discharge the molten metal into suitable containers or cauldrons . Modifications and variants to the method and device can be made as described heretofore, all of which fall within the scope of the present invention, as defined by the appended claims.

Claims (17)

1. A method for pre-screening, transforming and melting a metal charge, comprising scrap metal, into an electric arc furnace (12) associated with a tunnel (11) which transports, preheats and discharges the scrap, the furnace (12) comprises a crucible (13) and a roof (14), through which the electrodes (15) pass, which provides: that the furnace (12) weighing at least periodically in order to detect the amount of discharged scrap present inside the furnace (12) itself; that the temperature of the liquid bath inside the oven (12) is detected at least periodically, and - that at least the supply of the scrap discharge into the furnace (12) is detected by weighing and regulated in order to maintain the temperature of the liquid bath around a predetermined value.

The method according to claim 1, wherein the electric power supplied to the furnace (12) is continuously varied from a minimum value, which corresponds to the first scrap unloading step and a maximum value corresponding to the step of melted, according to the amount of scrap present inside the furnace (12), as detected by weighing the furnace (12).

The method according to claim 1 or 2, wherein the charge of the furnace (12) with the scrap is interrupted before the casting of the liquid metal during a range of between about 8 and 12% of the total cycle time.

4. The method according to any preceding claim, wherein the power supply to the furnace (12), is interrupted before casting for a time of up to 5% of the total cycle time.

The method according to claim 2, wherein the minimum value of the electric power supplied increases from an initial value of about 40-50% of the working value to its maximum value.

The method according to any preceding claim, wherein an amount of liquid metal of about 30-40% of the total capacity is always left in the furnace (12), to constitute a reserve of liquid for the subsequent cycle.

The method according to any preceding claim, wherein in the step of casting the electrodes (15) are raised at least from the liquid bath and the power supply to them is interrupted.

The method according to any preceding claim, wherein between the end of the casting operation and the beginning of the loading of the scrap for the subsequent cycle, the electrical supply to the electrodes (15) is interrupted and the following operations: a) the furnace (12) is rotated from the cast position to the deslagging position, in order to interrupt the flow of liquid steel; b) the cleaning of the pouring orifice is controlled; c) the pouring channel is filled with granular material with a high melting point; d) the furnace is returned to the horizontal position and stops are inserted that limit the rotation to the reduced values during normal operation; e) the electrodes are lowered and the power supply is re-established.

9. A plant for preheating, transforming and melting a metal charge consisting of scrap metal, comprising a tunnel (11) that transports and preheats the scrap and an electric arc furnace (12), the electric furnace (12) comprises a crucible (13) to contain the scrap and a roof cover (14), through which pass the electrodes (15), held and moved by related arms (16), comprising weighing means (18) for weighing the electric oven (12), means for detecting the temperature of the liquid metal bath inside the oven (12), and means capable of regulating the speed of discharge inside the oven (12), in accordance with the detections made by the means of heavy, in order to keep the temperature of the liquid metal bath around a predetermined value.

The plant according to claim 9, wherein the weighing means comprises load cells (18).

11. The plant according to claim 9, wherein the crucible (13) is mounted on supports (17), capable of oscillating it at a limited angle, comprised between ± 4 °, during normal operation, for example, during the loading, melting and refining steps of the liquid metal , and a greater angle, comprised between -15 ° and + 25 °, during the discharge steps of the slag and the casting of the liquid metal.

The plant according to claim 9, wherein the crucible (13) has a shape, in relation to the inclination that it adopts in the passage of the casting of the liquid metal, to maintain a liquid reservoir equal to approximately 30-40% of your capacity.

The plant according to claim 9, wherein the transport and preheating tunnel (11) comprises a plurality of injection systems, wherein at least the one closest to the electric oven (12) has at least one burner (19). ), and wherein at least in a position adjacent the burner (19), there is at least one air injector (20).

14. The plant according to claims 9 and 13, wherein in the initial part of the tunnel (11), in a position adjacent to the outlet pipe of the preheated vapors (24), there is a compensation chamber (23) capable of functioning as a sealing system, in order to prevent the exit vapors arising from the tunnel (11), leaking into the atmosphere.

The plant according to any of claims 9 to 14 inclusive, wherein in cooperation with the transport and preheating tunnel (11), there is a sedimentation chamber, for the purpose of depositing the particulates, with which a cooling tower is associated. (27), to bring the temperature of the vapors coming in at about 20 m / s, to about 250 ° C or less.

16. The plant according to any claim 9 a 15, where the vapors enter the settling chamber at approximately 800 ° C.

17. The plant according to any claim 9 a 16, where the vapors are cooled in the cooling tower at a cooling rate not less than 250 ° C / second, advantageously 400 ° C / second.