HIGH TEMPERATURE PREMELTING APPARATUS

Abstract: This invention is a premelting vessel (1) and a method for premelting charge (12) before the charge (12) is introduced into a melting vessel (50) or furnace (50) in the steelmaking and metallurgical industry. The invention uses chemical energy from fuel sources layered within the charge (12) and from available process off-gases within a plant to premelt the charge (12) before it is transferred to the melting vessel (50), thereby reducing the melting phase time and increasing the production rate. This invention also eliminates the use of a post-combustion chamber.

Description: HIGH TEMPERATURE PREMELTING APPARATUS CLAIM OF PRIORITY This application claims the benefit of a co-pending provisional application entitled "High Temperature Premelting Apparatus" that was filed in the United States Patent and Trademark Office having Application No. 60/153,775 and filing date September 14, 1999. TECHNICAL FIELD This invention relates to a device, used mainly in the steelmaking and metallurgical industry, for preheating the charge to be fed into a furnace, converter or melting vessel by combusting combustible materials available in the charge or from secondary gases recovered from related sources in the process. BACKGROUND OF THE INVENTION It is a well known fact that in the production of steel in steelmaking plants approximately 75 to 80% of the total energy required is used to bring the charge to a temperature close to the melting point. The remaining 20 to 25% is sufficient to complete the melting process and to assure adequate overheating in accordance with the technological requirements. It follows that the greater part of the energy supplied to the melting volumes commonly used is needed to heat the charge and not to melt it. A common practice to make up for this drawback is the use of various mechanical and electrical machines to increase the power usable during the initial charge preheating phase (such as natural gas burners, oxygen tuyeres, oxygen lances, injection of coal dust, etc) so as to reach the melting point in the shortest possible time. This involves two types of drawbacks: the sequential nature of the phases implemented in the same technological unit prevents a drastic reduction of the operation time and, from a mechanical point of view, the control and management of the process becomes very complex. Several methods are known in the steelmaking industry to preheat a solid charge to produce steel. Among the known preheating methods is the preheating of scrap in a charging bucket at low temperature (400 to 450° C). This method requires a lot of space, consumes energy for preheating fans, can pollute the workplace, has low yield and generally does not recover energy from the melting process. Another known preheating method is the preheating of scrap in a charging bucket at high temperature (800 to 850 C). This is generally not an acceptable practice, in part because it requires high-temperature heat-resistant buckets (often involving pressure water cooling), it generates a considerable amount of pollutants, and it requires a combustion chamber capable of thermally destroying pollutants (mainly dioxin) and of completing the combustion of certain gases (mainly carbon monoxide). This method is costly because thermal losses must be regained by using auxiliary burners. The process can be very complicated, and the very short melting time may so reduce the time available for preheating such that there is insufficient time for processing all scrap buckets, thereby limiting production. Another known preheating method is the use of shaft preheating plants, in which a charge volume is directed toward the melting volume arranged in correspondence with the roof section of the furnace where the gases are exhausted. This method has the disadvantage of making it difficult or impossible to operate if there is welding between the scrap pieces in the shaft (because the shaft is an integral part of the melting volume), thus preventing normal charging. Other disadvantages include containing only limited charge quantities, the impossibility of independently managing gases generated in the melting volume from those generated in the preheating volume, and the need to create a downstream post-combustion chamber. In the same way, preheating and continuous charging processes enable only a limited and poorly efficient regeneration of energy from the process gases, because the exposed exchange surface of the scrap is but a just a fraction of the total surface. Furthermore, when these processes are charged, the yield of additions and of coal is very low as these materials are conveyed by the gases of the plant itself. Another disadvantage of this charging method consists in the dimensions of the charging and preheating plant and in the systematic maintenance required by the frequent mechanical problems. Moreover, the continuous charging operation requires a constant flat bath during the melting process, with consequential energy losses. Our invention overcomes the above-described limitations in known preheating devices and processes by heating the entire charge in an adequate premelting volume (separated from the melting location in a furnace or other melting vessel), in a fast and homogenous manner at temperatures ranging from 800 to 1200° C. in parallel with the melting phase which occurs in an independent vessel. A key concept of the invention is the use, in the premelting volume, of all energy sources having a chemical origin and of process gases from the plant. By using these sources in the premelting volume of the charge, rather than the melting volume, and by arranging the premelting phase to occur contemporaneously (to the maximum degree possible) with the melting phase being conducted in a separate vessel, the transfer of thermal energy is much greater, resulting in a reduction in melting phase time and consequent production increases; simplification of the melting process; energy recovery from and the upgrading of the process gases; improved input energy efficiency with consequent reduction in the specific consumption of the integrated plant; and elimination of the post-combustion chamber as opposed to conventional preheating methods. This principle is implemented through this invention by achieving a premelting process in an independent volume separated from the melting volume, in parallel with the melting phase of the previous charge. SUMMARY OF THE INVENTION This invention comprises a process for premelting charge before the charge is introduced into a melting furnace in the steelmaking and metallurgical industry, comprising the steps of: premelting the charge in a premelting vessel, defining a premelting volume, to an average temperature near the melting temperature using essentially chemical energy; discharging the premelted charge from the premelting vessel into a melting furnace; and completing the melting process in the melting furnace using essentially electrical energy. This invention further comprises a melting apparatus in the steelmaking and metallurgical industry comprising: a premelting vessel equipped with devices for the utilization of chemical energy; a melting furnace equipped with devices for the utilization of electrical energy; and a discharging device utilized for transferring the premelted charge from the premelting vessel to the melting vessel. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a diagrammatic cut-away view of the premelting apparatus, in longitudinal section. Figure 2 illustrates a bottom-up embodiment of the premelting apparatus in conjunction with a melting furnace. Figure 3 illustrates a top-down embodiment of the premelting apparatus in conjunction with a melting furnace. Figure 4 depicts the top-down embodiment illustrated in Figure 3, having a carbon dispersed in the charge. Figure 5 depicts the top-down embodiment illustrated in Figure 4, in which off-gases from the furnace are fed through the body of the premelting apparatus. Figure 6 depicts the top-down embodiment illustrated in Figure 4, having burners in the shell of the premelting apparatus. Figure 7 depicts the top-down embodiment illustrated in Figure 4, having liquid fuel dispersed in the charge. DETAILED DESCRIPTION OF THE INVENTION Referring now to the drawings and particularly to Figure 1 , the premelting apparatus is comprised of a heat-resistant shell 1 , which in turn defines an enclosed interior space known as the premelting volume 2 in which occurs the actual premelting of the charge 20 utilizing chemical energy. Shell 1 preferably has at least one off-gas feeding duct 3, into which are directed off-gases 30 from a melting furnace or other suitable source within the plant, and at least one exhaust duct 4 for exhausting gases 40 generated within the premelting volume 2. The preferred embodiment illustrated in Figure 1 shows one off-gas feeding duct 3 and one exhaust duct 4. These ducts can be connected to shell 1 by a suitable means, including ring connectors. Preferably, ducts 3 and 4 are cooled by an appropriate means, and gas flow through them is regulated by a suitable means such as valves or regulators. Shell 1 must be heat resistant and be capable of maintaining its integrity during the premelting process. This can be accomplished in various possible solutions, such as by providing interior panels 7 which can be lined with refractory materials and/or fitted with welded pipes for cooling by pressurized water. In another solution, interim panels 7 can be thinly arranged in such a way as to form a serpentine into which refractory material is gunned. This material protects against excessive heat dispersion and must be replaced less frequently than in a solution with just refractory material. A mixed solution can also be installed in shell 1, with some parts of the inner wall of shell 1 being made of refractory material and other parts made of water-cooled panels, depending on the heat stress resulting from different zones of the premelting volume 2. Shell 1 also has an outlet channel 6 for discharging pre-melted charge 20 from premelting volume 2 into the melting furnace or vessel. In a preferred embodiment of the invention, there is a system of valves 5 within shell 1 at the base of the premelting volume 2 that controls the flow of pre-melted charge 20 through outlet channel 6. Such valves open through lateral rotation, like universally adopted scrap buckets, to discharge pre-melted charge 20 into a bucket, ladle or other suitable carrying device. Alternatively, pre-melted charge 20 can be discharged through outlet channel 6 through a chute, revolving tunnel, or any other system capable of transferring the charge from the premelting volume 2 directly into a melting furnace or vessel. This system minimizes energy loss via radiation from the melting furnace or vessel that would otherwise occur when the melting furnace or vessel cover is opened. Shell 1 also has moving roof or a port 9 for charging scrap into premelting volume 2. The premelting apparatus can be charged using well-known methods such as charging buckets, a belt conveyor or other charging system. The premelting apparatus can be fitted with industrial burners 8 which can be arranged along the containing walls and/or roof of the shell and which protrude through shell 1 and into premelting volume 2. Figure 1 illustrates one embodiment of the invention showing two industrial burners fitted into shell 1 near the point of connection with an off-gas feeding duct 3. When placed at this location, the industrial burners increase the temperature of off-gases 30, preferably to 1600 to 2000° C, by both completing the combustion of the unburned fractions in off-gases 30 and by providing an independent source of heat to mix with off- gases 30. Figure 6 illustrates another embodiment of the invention showing a plurality of supplemental burners 8 fitted into shell 1. Burners situated in this position use fossil fuel (such as methane, oil, fuel oil, coal) to directly heat charge 20 in premelting volume 2. Figure 5 shows another embodiment in which an high power burner 8 can be fitted on off-gas feeding duct 3. The premelting apparatus also can be fitted with a means 10 for introducing, into premelting volume 2, gases holding oxygen (O 2 ) or air, or air enriched with O 2 and a means for introducing, into premelting volume 2, partially unburned gases from other sources in the plant. Preferably, said means directs the gaseous effluents through the charge-fuel combination inside premelting volume 2. Means 10 may take the form of a ring header consisting of a hollow space obtained in shell 1 and may be fed by fans or by an oxygen system to allow a part of the air or comburent mixture to be directed to the intermediate or final part of premelting volume 2. Means 10 for introducing O 2 or air or air enriched with O 2 provides a very flexible control of the gaseous mixture burning temperature, of the oxygen enrichment, and of the temperature of the exhaust gases. The temperature of the gases inside premelting volume 2 should be maintained as uniformly as possible and sufficiently high, but not so high as to melt, the solid charge over its whole stay time inside premelting volume 2. A means to further heat gaseous effluents to a temperature between 1500 and 1800° C, such as one or more industrial burners, may be fitted within the roof or side walls of premelting volume 2. The partial melting of the charge that percolates downwards homogenizes the charge temperature by transferring energy from hotter to colder regions in the charge. The premelting apparatus can have different configurations, such as those illustrated exclusively but not exhaustively in the figures. As discussed above. Figure 1 shows the basic features of the invention. Figure 2 depicts a "bottom-up" embodiment of the premelting apparatus in relation to an electric arc furnace. In this embodiment, the premelting apparatus is positioned with respect to the melting furnace 50 such that exhaust gases from melting furnace 50 enter the premelting apparatus through feeding duct 3 near the bottom of premelting volume 2. Figures 3 through 7 depict a "top-down" embodiment of the invention, wherein exhaust gases from melting furnace 50 enter the premelting apparatus through feeding duct 3 at the top of premelting volume 2. The premelting process can be carried out using the following elements, indiscriminately, either independently or combined in suitable combinations: partially unburned gaseous effluents, which may be arranged so as to mix with the comburent gases (O 2 or air or air enriched with O 2 ); industrial burners acting directly on the contents of premelting volume 2; fuels included in the charge 20 that improve combustion; and particular fuel in the charge 20 such as aluminothermic powders. The gaseous effluents may be selected from a suitable group such as carbon monoxide (CO), hydrogen (H2), hydrocarbons, or a combination thereof. The source of said gaseous effluents can be melting furnace 50 when that melting process is complete or can be another part of the steelmaking or metallurgical plant which is dedicated to producing gases rich in one or more of CO, H 2 . or hydrocarbons. In overview, essentially chemical energy (from fuels within charge 20 and/or from suitable gaseous effluents produced in melting furnace 50 or elsewhere in the plant) is utilized within premelting vessel 2 to premelt charge 20 to an average temperature near its intended melting temperature. A desired average temperature near the melting temperature is between 800 and 1200° C. Preferably, this premelting process is shorter than 40 minutes. When the premelted charge reaches the desired temperature, the premelted charge is discharged from premelting volume 2 into melting furnace 50. in which the melting process is completed using essentially electrical energy within melting furnace 50, accomplished by devices for the utilization of electrical energy within melting furnace 50. such as electrodes to generate an electric current. As a variation of this process, premelting of a first charge may be conducted in premelting volume 2, while the melting of a second charge is being conducted in melting furnace 50, said second charge having been previously premelted in premelting volume 2. When the melted charge is discharged from melting furnace 50, the premelted charge is discharged from premelting volume 2 into melting furnace 50, and then a third charge is charged into premelting volume 2. These steps of parallel premelting (in premelting volume 2) and melting (in melting furnace 50) can be repeated as desired. Charge preparation, distribution of combustion gases, combustion control in steps temperature control, and the timing of premelting with respect to melting in the main melting vessel, are key components of the operation of the premelting apparatus. First, depending on the technology and type of fuel and charge available, the charge needs to be stratified with the fuels included in the charge, such as anthracite, coke, oil, fuel oil or other suitable fuel. The fuels must be suitably arranged to form a fluid bed through which flow the gases rich in oxidization potential to improve combustion. Preferably, layers of solid or liquid fuel are interspaced between layers of the charge so that the fuel is burned in two successive stages, as described below. Figures 3 and 7 show the use of coke 12 and heavy fuel oil 13, respectively. as the fuel layers. Gases for preheating burn layer by layer inside the prepared charge, pre- heating it with greater heat exchanging efficiency. This arrangement allows for controlled combustion within the charge. Second, combustion gases must be properly distributed to maximize the operational efficiency of the premelting apparatus. Combustion gases must reach the fuel directly to form and keep a desired reducing atmosphere in the entire premelting volume. Gas inlet ports 11 provide for the proper circulation of combustion gases within premelting volume 2 to accomplish this. Maintenance of a reducing atmosphere prevents a heavy formation of iron oxides, which would require more energy and lower the yield of the charge. The formation of iron oxide in the premelting process can also be controlled by increasing the speed of the energy transfer when the charge temperature reaches and exceeds 800° C. which is the condition, which thermodynamically facilitates iron oxidation. Third, temperature control is vital to the proper operation of the premelting apparatus. The method to reduce the formation of iron oxide during the premelting process is to achieve a pluristage or multistage combustion, keeping the reducing conditions over the whole charge volume so as to complete combustion in the terminal part of the charge before removing the exhaust gases. This means that the combustion must be reduced so that inlet gases are preferably kept at an average temperature between 1600 and 1800° C by use of, when necessary, industrial burners 8 fitted in the roof and/or walls of shell 1 of the premelting volume 2. This average temperature will promote a gradual and continuous oxidation of fuel in the charge. In this way, the oxidation potential of the charge is reduced, in spite of the fact that very high temperatures are reached and, in these conditions, oxidation is a very fast process. In the burning of fuel in two successive stages, the first stage produces carbon monoxide, which promotes the reducing atmosphere in the first half or two-thirds of the charge through which flow the gases and where the temperature causes the iron oxide to react. The second phase produces carbon dioxide (the species supplies approximately two-thirds of the total transformation energy) in the remaining part of the charge, thus favoring a uniform temperature as mentioned above. Fourth, the timing of melting in premelting volume 2 vis a vis melting in the melting furnace 50 is an important feature of the invention. The advantageous thing to do is to arrange a maximum degree of contemporaneity between the charge heating phase and the real melting phase. This principle is implemented through this invention by achieving the premelting process in the independent premelting volume 2 separate from the melting furnace 50, in parallel with the melting phase of the previous charge. This strategy to control the charge premelting process assures the maximum heat exchange from the off-gases (through radiation and convection) in a short time and with minimum energy losses. Even though it is an independent unit, the premelting apparatus and its method of operation cooperate with the melting furnace or vessel, the most adaptive (but not exclusive) being an electric arc furnace. Using the premelting apparatus with the melting process in a furnace considerably simplifies the operations performed in the furnace to fast melting by means of an electric arc, decarburization and refining, and slight post-combustion above the slag to protect against heat losses in the charge. In summary, the premelting apparatus is a high efficiency pre-heating technology, because fuel burns progressively inside the charge with the special design of the invention. A main aim of the invention is to bring the charge, for example an electric arc furnace charge, to a high (about 1000° C) and as homogenous temperature as possible, and close to the melting temperature. This is essentially achieved using the energy derived from the direct combustion of combustible components in the gases and materials already available in the charge or from secondary gases recovered from other plant components and which are thermally upgraded by use of the burners. As a non-limiting example, the following table shows the performance achieved by the premelting apparatus in the case of a 100 ton charge of scrap in two different processes having a different duration, one using carbon and the other using oxymethane burners:

Claims: CLAIMS What is claimed is: 1. A process for melting charge in the steelmaking and metallurgical industry, comprising the steps of: (a) premelting the charge in a premelting vessel, defining a premelting volume, to an average temperature near the melting temperature using essentially chemical energy; (b) discharging the premelted charge from the premelting vessel into a melting furnace: and (c) completing the melting process in the melting furnace using essentially electrical energy. 2. A process for melting charge in the steelmaking and metallurgical industry, comprising the steps of: (a) premelting a first charge in a premelting vessel, defining a premelting volume, to an average temperature near the melting temperature using essentially chemical energy, while in a melting furnace is completed the melting process of a second charge using essentially electrical energy, said second charge being previously premelting in the premelting vessel; (b) discharging the melted second charge from the melting furnace; (c) discharging the premelted first charge from the premelting vessel to the melting furnace; (d) charging a third charge to the premelting vessel; (e) repeating steps from (a) to (d). 3. The process of claims 1 or 2 wherein said average temperature near the melting temperature is between 800 and 1200° C. . The process of claims from 1 to 3 wherein the premelting step (a) is carried out using either independently or combined: (a) industrial burners arranged along the containing walls and/or roof of the premelting vessel and directly acting on the premelting volume; (b) fuels included in the charge suitably arranged to form a fluid bed, said fuels being burnt in a controlled way in presence of gases holding O 2 ; (c) gaseous effluents at least partially unburned coming from a source in the plant, wherein said gaseous effluents are upgraded to a temperature between 1500 to 1800° C by completing the combustion of the unburned fractions in proximity of the premelting volume; (d) particular fuel in the charge such as aluminothermic powders 5. The process of claim 4 wherein the industrial burners are fossil fuel burners. 6. The process of claim 4 wherein the fuels are selected form the group consisting of anthracite, coke, oil, fuel oil or a combination of them. 7. The process of claims 4 or 6 wherein the fuels are advantageously stratified within the charge. 8. The process of claim 4 wherein the gaseous effluents contain a gas selected from the group consisting of CO, H 2 or other hydrocarbons or a combination of them. 9. The process of claims 4 or 8 wherein the gaseous effluents are the gases coming from the melting furnace where the melting process is completed. 10. The process of claims 4 or 8 wherein the gaseous effluents are the bases coming from a dedicated plant for the production of gas rich in CO or H 2 or other hydrocarbons or a combination of them. 11. The process of claims from 1 to 10 wherein the premelting step (a) is preferably shorter than 40 minutes. 12. A melting apparatus in the steelmaking and metallurgical industry comprising: (a) a premelting vessel equipped with devices for the utilization of chemical energy; (b) a melting furnace equipped with devices for the utilization of electrical energy; (c) a discharging device utilized for transferring the premelting charge from the premelting vessel to the melting furnace. 13. The melting apparatus of claim 12 wherein the premelting vessel comprises a shell defining an enclosed interior premelting volume wherein the utilization of chemical energy takes place, said shell having heat resistant interior walls. 14. The melting apparatus of claim 13 wherein said heat resistant interior walls are selected from the group consisting of refractory linings, water cooled panels or both. 15. The melting apparatus of claim 12 wherein the premelting vessel devices for the utilization of chemical energy are selected from the group consisting of: (a) industrial burners arranged along the containing walls and/or roof of the premelting vessel; (b) means for introducing in the premelting vessel gaseous effluents containing O 2 or air or air enriched with O 2 ; (c) means for introducing in the premelting vessel gaseous effluents partially unburned coming from a source in the plant; or a combination of them. 16. The melting apparatus of claim 15 wherein the industrial burners are fossil fuel burners. 17. The melting apparatus of claim 15 wherein said means for introducing in the premelting vessel gaseous effluents directs the gaseous effluents through the charge-fuel combination inside the premelting volume. 18. The melting apparatus of claim 15 wherein the melting apparatus comprises further means for additionally heating said gaseous effluents to a temperature between 1500 and 1800° C. 19. The melting apparatus of claim 18 wherein the melting apparatus' further means are one or more industrial burners. 20. The melting apparatus of claim 15 or 19 wherein said industrial burner is a ring header fed by fans. 21. The melting apparatus of claim 15 wherein said source of gaseous effluents is the melting furnace. 22. The melting apparatus of claim 15 wherein said source of gaseous effluents is a dedicated plant for the production of gas rich of CO or H 2 or other hydrocarbons or a combination of them.

Inventors: BENEDETTI GIANPIETRO, DELLA NEGRA ANGELICO, PAVLICEVIC MILORAD, POLONI ALFREDO, BENEDETTI, GIANPIETRO, DELLA NEGRA, ANGELICO, PAVLICEVIC, MILORAD, POLONI, ALFREDO
Applicants: DANIELI CORP, DANIELI CORPORATION
Application year: 2000
Main classification: C21B7
IPC codes: C21C5