Method for operating an electric melter

Abstract

The invention relates to a method for operating an electric melter (10).

Description

[0001] The invention relates to a method for operating an electric melter.

[0002] The energy supplied by electrodes is used to melt the feedstocks placed in electric melters. A large portion of the total energy is converted into thermal energy, which melts the feedstocks, while another portion heats the melter's lining. To protect the lining from thermal stress, the melter wall is actively cooled. This cooling process results in a loss of some of the supplied energy. To operate an electric melter economically, the melting process is implemented using the so-called "sump" operating method, a technique well-known in the industry.This means that when the electric melter is emptied, the liquid slag and the molten iron beneath it are "tapped" into separate containers, leaving a (bottom-covering) residual molten iron layer with a layer of slag on top in the melter. The raw materials to be melted are then added to this slag layer, and the process is continuously continued by applying energy to the electrodes. Therefore, the methods and equipment for producing molten iron in electric melters are considered state of the art.

[0003] Furthermore, electric melters, for example with a rectangular (horizontal) base structure, are known; see, for example, EP 2 270 239 B1 and EP 3 542 595 B1. Due to the known sump operation, even with a rectangular horizontal base structure, the lower part of the electric melter is covered with residual iron molten metal and slag while the melter is being filled with new feedstock. To melt the feedstock across the entire rectangular horizontal base structure, a high energy input via the electrodes is required, since the distance to the melter wall is higher compared to a melter with a circular (horizontal) base structure, depending on the design, number of electrodes, and size of the base structure, and consequently the energy input is higher, in order to produce a complete liquid phase across the entire surface / volume of the melter.

[0004] Furthermore, it is known from the prior art to provide gas lances on / in electric arc furnaces through which oxygen is injected, either to combust the carbon monoxide present in the atmosphere to carbon dioxide (see DE 197 44 151 C5) or to adjust a specific carbon-oxygen ratio in combination with a carbon-containing material (see EP 4 029 954 B1), in both cases where a complete liquid phase is already present. Oxygen lances are also used to generate CO bubbles by means of substoichiometric combustion in order to froth the slag.

[0005] The introduction of electrical energy can lead to the formation of localized cold spots, known as "cold spots," in the melter due to slow heat transfer. This reduces the effective melter volume, which can result in a decrease in production output.

[0006] The object of the present invention is to further develop this method and electric melters in such a way that the formation of "cold spots" can be substantially reduced or prevented.

[0007] This problem is solved by a method having the features of claim 1. Furthermore, the problem is solved by an electric melter having the features of claim 10. Further embodiments are described in the dependent claims.

[0008] According to a first teaching, the invention relates to a method for operating an electric melter in which molten iron and liquid slag are produced by introducing and melting iron-containing feedstocks, wherein the electric melter comprises several electrodes which provide the energy required for melting in order to transfer the feedstocks into a liquid phase comprising the molten iron and the liquid slag arranged on the molten iron, wherein a reducing atmosphere is set and / or maintained in the melter during operation, wherein additional means are arranged which are controlled during operation to support locally heating the feedstocks and optionally transferring them into the liquid phase.

[0009] The invention takes advantage of the fact that additional means are provided which supply additional energy to reduce or even eliminate "cold spots" in the melter. These means, which can be controlled during operation, serve to locally heat the feedstock, meaning that the areas supplied with additional energy are not yet liquid. These areas can optionally be supplied with energy until the feedstock melts, but this is not necessary if the heat transfer from the electrodes is sufficient to also convert the still-solid areas into the liquid phase.

[0010] The advantage of the additional resources essentially leads to a reduction in the temperature gradient between the solid and liquid phases, an increase in the effective melting volume, a homogenization of the liquid slag and molten iron composition, and thus ultimately to an effective increase in production. The additional resources are primarily deployed temporarily and therefore as needed.

[0011] According to one embodiment, the additional means are arranged in the area of ​​the melter wall and are controlled such that the temperature of the feedstock is maintained at least 200 K below its liquidus temperature TL. Controlling the means is not strictly necessary, meaning the feedstock does not necessarily have to reach its liquidus temperature TL. The temperature of the feedstock can be controlled to at least 150 K, preferably at least 100 K, and preferably at least 50 K below its liquidus temperature TL. Alternatively, the means can, of course, be controlled such that the feedstock reaches a temperature equal to or above its liquidus temperature TL, thus effectively melting it.

[0012] According to one embodiment, the means are gas lances through which an oxygen-containing or oxidizing gas is passed, and the introduction is controlled in such a way as to maintain the reducing atmosphere in the melter. Advantageously, an oxygen-containing or oxidizing gas is introduced, which premixes with the gas in the melter's atmosphere or mixes at the gas lance outlet and ignites. The gas lance, and thus the flame, is directed toward the cold spot to heat and optionally melt the feedstock. The atmosphere in the melter contains ignitable components, including carbon monoxide and hydrogen. The gas lances are preferably directed or arranged toward the "cold spots" during operation to achieve a faster, particularly temporary, temperature equalization between the liquid and solid phases.

[0013] Alternatively or additionally, the means can also be burners supplied with a fuel gas and an oxygen-containing gas, such as air. Fuel gas can, for example, contain hydrogen, which can preferably be produced by electrolysis using green energy (wind, water, solar). The use of other gases, especially combustible blast furnace gases, is also conceivable. The air-fuel ratio is set, for example, substoichiometrically to maintain a reducing atmosphere in the melter.

[0014] According to an alternative or additional embodiment, the means can be electrodes. The additional electrodes are arranged in the areas where "cold spots" can occur during operation, in order to achieve a faster, especially temporary, temperature equalization between the liquid and solid phases electrically.

[0015] The detection of "cold spots" can be determined either through preliminary tests or by simulation during the design of the melter, so that the person skilled in the art knows in which area(s) the additional means are to be provided or arranged.

[0016] According to one embodiment, the iron-containing feedstock preferably used is reduced iron ore in the form of sponge iron pieces and / or sponge iron pellets with a carbon content between 0 and 4.5 wt.%, in particular > 0 wt.%, and a metallization degree of at least 70%, in particular up to 100%, preferably up to 98%. The metallization degree reflects the ratio of the metallic iron content to the total iron content in the sponge iron. The carbon content also influences the liquidus temperature TL, so that it decreases with increasing carbon content, which is advantageous because the energy required for melting can also be reduced. If the iron melt has a defined carbon content, which is between 0, in particular > 0, and 4.5 wt.%, the process can be further refined.Since the carbon content cannot be supplied via the iron-containing feedstocks, carbon-containing additives must be taken into account in such a quantity that the desired carbon content in the iron melt can be achieved.

[0017] The invention could also be used, for example, in electric smelters that are operated with scrap and / or crude steel as iron-containing feedstocks.

[0018] Additionally, additives, particularly metallurgical waste and / or (blast furnace) dusts, which may contain iron, for example, can be added to the process medium. In the case of gas lances, the additives, such as iron-containing powder and / or dusts, can be injected into the gas stream of the oxygen-containing or oxidizing gas.

[0019] The preferred use of sponge iron as an iron-containing feedstock also entails slag-forming components, which are naturally present in the iron ore and cannot be driven off in a prior reduction process; these are referred to as gangue. If the gangue provided by the sponge iron is insufficient, further slag formers can be added as additives to produce a liquid slag suitable for further processing. According to one embodiment, slag formers are preferably added such that a basicity B3 in the liquid slag is established between 0.7 and 1.8. B3 can be at least 1.0, preferably at least 1.1, and particularly at most 1.7, preferably at most 1.6. The basicity B3 corresponds to the ratio (CaO+MgO) to (SiO₂+Al₂O₃), whereby the determination of the characteristic values ​​in the slag in the solid state is familiar to those skilled in the art.The slag former comprises at least one or more of the elements from the group (CaO, MgO, SiO2, Al2O3).

[0020] To increase reactivity in the melter, the slag formers can alternatively or additionally be blown in with the oxygen-containing or oxidizing gas via the gas lances, which are preferably present as additional means in the melter. An advantage would be, for example, accelerated heating and thus instantaneous liquefaction of the introduced slag formers.

[0021] To improve or increase the recycling rate, scrap metal can be added to the iron-containing feedstocks, preferably to the sponge iron pieces and / or sponge iron pellets. This can be done, for example, by adding > 0 kg, in particular at least 20 kg, preferably at least 50 kg, preferably at least 80 kg up to 200 kg of scrap metal per ton of molten iron produced.

[0022] To melt the iron-containing solids, the electric melter has several electrodes that can be energized with electricity, thus providing the necessary energy to convert the solids into a liquid phase comprising molten iron and liquid slag. Depending on the size / dimensions of the electric melter, three, four, five, six, or more than six electrodes can be used. The energy required for melting is preferably supplied from renewable energy sources (solar, wind, water, biomass), or at least partially from renewable sources. This allows the electric melter to be operated in a more environmentally friendly manner.

[0023] In the event that additional electrodes are used as a means of reducing or avoiding "cold spots", these additional electrodes used during operation can also be powered by regeneratively generated energy, at least partially from renewable energy.

[0024] In the event that additional gas lances are used as a means of reducing or avoiding "cold spots", oxygen can preferably be supplied as a gas, which preferably originates from a regeneratively operated electrolysis or air separation process.

[0025] According to a preferred embodiment, the basic structure of the electric melter is essentially rectangular with two opposite sides of the melter wall of equal length, wherein the electrodes, for example four or more, are preferably arranged in a row.

[0026] The electrodes are controlled in such a way that a liquid phase with a temperature greater than TL, for example at least TL + 30 K, and in particular at least TL + 50 K, is established around them. The liquidus temperature of pure iron is theoretically around 1538 °C. The temperature essentially refers to the temperature of the molten iron, which can be measured using known methods. Known measuring devices include, for example, thermocouples immersed directly in the molten iron or non-contact pyrometers. It can also be quite common to make qualified assumptions that allow conclusions to be drawn about the temperature in the melter, such as optically monitoring the melt pool and observing differences in radiation intensity, changes in the flow behavior of the liquid slag / molten iron, changes in morphology, melting of the feedstock, or measuring the temperature(s) of the melter wall.Determining / measuring temperature is familiar to the expert.

[0027] The melting process (cycle) can be monitored using imaging devices, such as cameras, for example, in the form of a top-down temperature profile. This allows for the identification of bright areas, hot and liquid phases, and dark areas, which represent slightly cooler mixed or solid phases, as well as "cold spots." During the melting and transfer of the feedstocks and optional additives into a liquid phase comprising the molten iron and the liquid slag, the liquid slag forms on top of the molten iron due to its lower density. The temperature of the liquid phase captured in the top-down view therefore does not correspond to that of the molten iron, but rather to the liquid slag that forms on top of it. This liquid slag temperature can deviate from the actual temperature of the molten iron by up to 200 K (plus), especially when energy input in the melter occurs via the liquid slag.The temperature profile can therefore be corrected in the image editing software with a correction factor in the liquid phase area in order to be able to represent the approximate temperature of the molten iron graphically in light / dark.

[0028] According to a second teaching, the invention relates to an electric melter comprising several electrodes with a melter wall surrounding a rectangular base structure, wherein additional means are arranged in the area of ​​the melter wall.

[0029] A rectangular (horizontal) basic structure refers to the fundamental shape, particularly of the interior of the electric melter, for heating and ultimately melting the input materials and optional additives in a horizontal (cross-sectional) view, whereby the corner areas of the interior do not necessarily have to have a right angle.

[0030] The electric melter can preferably be a furnace of the OSBF (Open Slag Bath Furnace) type. This includes electric reduction furnaces, especially SAF (Submerged Electric Arc Furnace), which are melting furnaces with arc resistance heating that form electric arcs between the electrode and the feedstock and / or the liquid phase, or which heat the feedstock and / or the liquid phase by means of the Joule effect. In SAF furnaces, the electrodes are immersed in the feedstock and / or the liquid phase, particularly in the liquid slag. Depending on the operating principle / mode, the electric reduction furnaces can be designed as alternating current arc reduction furnaces (SAFac) or direct current arc reduction furnaces (SAFdc).

[0031] Alternatively, melting furnaces with direct arc contact, which differ from the operating principle / mode described above, so-called EAF (Electric Arc Furnace), can also be used, which form electric arcs between the electrode and the liquid phase. This includes the alternating current electric arc melting furnace (EAFac), the direct current electric arc melting furnace (EAFdc), and the ladle furnace (LF).

[0032] However, this requires the establishment and / or maintenance of a reduced atmosphere within the electric melter, essentially inside the melter's interior. If this is not possible with melters using direct arc heating (EAF), electric reduction furnaces with arc resistance heating (SAF) are preferred.

[0033] According to one embodiment, the additional means are arranged in the corners of a rectangular base structure. Since the distance to the melting chamber wall varies from the centrally located electrodes used for heating and melting the feedstock, the corners or corner areas are typical "cold spots" that can be thermally and / or electrically, particularly temporarily, supplied with energy by the appropriate means in order to achieve the aforementioned advantages and thus also an increase in production output.

[0034] The invention is explained in more detail with reference to the following exemplary embodiments in conjunction with the drawing.

[0035] The invention is implemented using the Figures 1 and 2 This is explained in more detail using the example of an electric melter (10) with a rectangular (horizontal) basic structure. The Figures 1 and 2The following is a sketchy example with an essentially rectangular basic structure; however, a square, circular, or elliptical basic structure (not shown here) could also be considered in an electric melter.

[0036] The invention provides a method for operating an electric melter (10) with a melter wall (12) preferably surrounding a rectangular base structure, in which molten iron and liquid slag are produced by introducing and melting iron-containing feedstocks (S). The required feedstocks can be supplied by means not shown. The iron-containing feedstocks preferably comprise or consist of pieces and / or pellets of sponge iron. Additional iron-containing feedstocks, such as iron-containing scrap, can also be supplied to increase the recycling rate.Further additives, such as slag formers like lime, silicon dioxide, magnesium oxide, and / or aluminum oxide, can be added, particularly if the gangue of the preferably used sponge iron is insufficient to achieve the desired basicity B3 of the liquid slag to be tapped. Adjusting the desired basicity by appropriate mixing / addition is familiar to those skilled in the art. The amount of iron-containing feedstock added is determined by the desired yield of the molten iron. The electric melter (10) comprises several electrodes (11), for example, six arranged in series, cf. Figures 1 and 2, which provide the energy required for melting in order to transfer the feedstocks (S) into a liquid phase (L) comprising the molten iron and the liquid slag arranged on the molten iron, whereby a reducing atmosphere is established and / or maintained during operation in the melter (10).

[0037] The electrical energy required for melting can preferably be generated from renewable energy sources (sun, wind, water).

[0038] To substantially reduce or even prevent the formation of "cold spots," additional means (E, O) are arranged which are controlled during operation to assist in heating the feedstock (S) and optionally converting it into the liquid phase (L). The additional means (E, O) are arranged in the region of the melter wall (12), preferably in the region of the corners of the base structure, and are controlled such that the temperature of the feedstock (S) is maintained at least 200 K below the liquidus temperature TL of the feedstock (S).

[0039] The difference between the Figures 1 and 2 is that in Figure 1Gas lances (O) are provided as a means through which an oxygen-containing gas is passed and the introduction is controlled in such a way as to maintain the reducing atmosphere. Light areas in the corners symbolize the alignment of the gas lances (O) with the feedstocks (S). In Figure 2In particular, additional electrodes (E) are provided as an alternative means, which are represented, for example, by electrodes (E) in the corners that are smaller in size compared to the (main) electrodes (11). The additional electrodes (E) can be accessed from above, and heating of the feedstock (S) is achieved either by arc resistance heating or direct arc exposure. Alternatively or additionally, the additional electrodes (E) can also be incorporated / integrated into the base of the melter (10). Alternatively or additionally, burners not shown can also be used. More than one electrode (E) per "cold spot" can also be used, not shown here.

[0040] Not shown are corresponding openings in the melter wall (12) through which liquid slag and molten iron can be tapped / drained.

[0041] The electric melter (10), with its preferably rectangular base structure, is stationary and not pivotable. The operation of electric melters (10) is familiar to those skilled in the art.

[0042] It is also not shown how the molten iron is extracted and fed into a further processing step. Preferably, the molten iron is treated to reduce the carbon content to a desired level. This is done, for example, using oxygen in a so-called oxygen blowing process, particularly preferably in a converter. The tapped liquid slag is also preferably fed into a granulation process to produce slag, especially for the construction industry.

Claims

1. A method for operating an electric melter (10) in which molten iron and liquid slag are produced by introducing and melting iron-containing feedstocks (S), wherein the electric melter (10) comprises a plurality of electrodes (11) which provide the energy required for melting in order to convert the feedstocks (S) into a liquid phase (L) comprising the molten iron and the liquid slag arranged on the molten iron, wherein a reducing atmosphere is set and / or maintained in the melter (10) during operation, characterized in that additional means (E, O) are arranged which are controlled during operation to support heating of the feedstocks (S) and optionally to transfer them into the liquid phase (L).

2. Method according to claim 1, wherein the additional means (E, O) are arranged in the region of the melter wall (12) and are controlled in such a way that a temperature of the feed materials (S) is established which is at least 200 K below the liquidus temperature T L of the input materials (S).

3. Method according to one of the preceding claims, wherein the means (O) are gas lances through which an oxygen-containing gas is passed and the introduction is influenced in such a way that the reducing atmosphere is maintained.

4. Method according to one of the preceding claims, wherein the means (E) are electrodes.

5. Process according to one of the preceding claims, wherein reduced iron ore in the form of sponge iron pieces and / or sponge iron pellets with a carbon content between 0 and 4.5 wt.% and a degree of metallization of at least 70% are used as iron-containing feedstocks.

6. Process according to one of the preceding claims, wherein slag formers are added which comprise at least one or more of the elements from the group (CaO, MgO, SiO2, Al2O3) so that a basicity B3 in the liquid slag of between 0.7 and 1.8 is established.

7. Method according to one of the preceding claims, wherein the energy required for melting is provided at least in part from renewable energy.

8. Method according to one of the preceding claims, wherein the basic structure of the electric melter (10) is rectangular with two equally long opposite sides of the melter wall (12), wherein the electrodes (11), four or more, are preferably arranged in a row.

9. Electric melter (10) comprising a plurality of electrodes (11) with a melter wall (12) surrounding a rectangular basic structure, in particular, characterized in thatadditional means (E, O) are arranged in the region of the melter wall (12).

10. Electric melter (10) according to claim 9, wherein the additional means (E, O) are arranged in the region of the corners of a rectangular basic structure.

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