Method for producing iron melts and liquid slag in electric melting furnace

Abstract

The invention relates to a method for producing iron melts and liquid slag in an electric melting furnace (10) having a polygonal horizontal base structure.

Description

Technical Field

[0001] The present invention relates to a method for producing molten iron in an electric melting furnace. Background Art

[0002] The energy supplied by the electrodes is used to melt the charge introduced into the electric furnace. Most of this (total) energy is converted into heat, which causes the charge to melt, while another portion is used to heat the furnace lining. To protect the furnace lining from thermal stress, the furnace walls are (actively) cooled. However, this causes some of the input energy to be lost through cooling. To achieve economical operation of the electric furnace, the so-called sump operation mode (Sumpf-Fahrweise), known in the art, is used. This means that when the furnace is drained, the liquid slag and the underlying molten iron are "discharged" into separate containers, while residual molten iron remains in the furnace (covering the furnace bottom). A new charge to be melted is introduced onto this residual molten iron, and the melting process is restarted by supplying energy to the electrodes. Therefore, methods and devices for producing molten iron in electric furnaces are within the scope of the prior art.

[0003] Furthermore, electric melting furnaces with a polygonal horizontal base structure are known, see for example EP 2 270 239 B1 and EP 3 542 595 B1.

[0004] With a polygonal horizontal base structure, the lower part of the electric furnace is also covered with residual iron melt before the furnace is filled with a new charge. A high energy input through the electrodes is required to melt the charge over the entire polygonal horizontal base structure because, compared to furnaces with a circular horizontal base structure, the distance to the furnace wall varies depending on the design, number of electrodes, and base structure dimensions. This results in a higher energy input required to generate a complete liquid phase over the entire area / volume.

[0005] In addition to high energy consumption, this process also has an adverse effect on the lining of the electric furnace. Summary of the Invention

[0006] The object of the present invention is to further improve this method in order to reduce the energy consumption required for smelting in an electric furnace and at the same time to reduce wear of the lining in the furnace.

[0007] This object is achieved by a method having the features of claim 1. Further embodiments are described in the dependent claims.

[0008] The present invention relates to a method for producing molten iron and liquid slag in an electric furnace having a polygonal horizontal base structure with surrounding walls, wherein an iron-containing charge is introduced and smelted. The electric furnace has a plurality of electrodes that provide the energy required for smelting in order to convert the charge into a liquid phase comprising the molten iron and liquid slag above it. The electrodes are controlled so that the liquid phase only contacts the surrounding walls in certain areas.

[0009] In contrast to the prior art, complete melting of the charge should be avoided, thereby also preventing the resulting liquid phase from contacting the walls of the electric furnace over all areas. The present invention utilizes the fact that less energy is required than for complete melting of the charge introduced into the electric furnace, since only a portion of the charge introduced needs to be melted, and thus only regional contact with the walls is permitted or present. This also has the advantage that only a small portion of the energy input for melting is (directly) dissipated from the liquid phase through regional contact with the cooled walls of the electric furnace. Consequently, the wall lining, or at least a large portion thereof, is subjected to less stress than with complete melting. The liquid phase is generally more aggressive and / or abrasive to the walls or lining than the introduced, hot, activated charge. Wear on the lining can therefore be minimized.

[0010] The linings of metallurgical vessels which come into contact with liquid iron, and therefore also the linings of furnaces, and the corresponding materials, the so-called refractory materials, belong to the state of the art.

[0011] A polygonal horizontal base structure is understood to be the basic shape in a horizontal cross section of the interior space of an electric melting furnace for melting a charge, wherein the corner areas do not necessarily have to be right angles due to the presence of the lining.

[0012] In other words, in the polygonal horizontal base structure of the electric melting furnace, the charge is not melted at the corners, thus maintaining a distribution of approximately columns of charge in the corners. These columns gradually shrink due to the thermal load of the adjacent liquid phase, as portions of them transform into the liquid phase. Since the charge is added continuously or, preferably, intermittently, depending on the operating mode, until the desired charge level or molten iron output is reached, the corners are repeatedly "refilled." During discharge, these columns can gradually collapse as the liquid phase in the central region of the electric melting furnace decreases. This has the advantage that charge that was exposed to certain temperatures in this region / corner during the previous melting cycle and did not transform into the liquid phase can fall into the region to be melted during the new melting cycle, thus no longer requiring the full energy required to melt the newly added (cold) charge. Part of the collapsed column enters the molten pool, allowing it to reach the liquidus temperature more quickly and also melt.

[0013] The electrodes can be controlled in such a way that in the region where no (pure) liquid phase is present or should not be present, the electrodes are arranged below TL The charging temperature, where T L Corresponds to the liquidus temperature. Therefore, in the corners of the electric furnace, there will be areas where, depending on the temperature, a slurry phase will be generated, which is similar to a solid-liquid mixture phase, and therefore corresponds to, for example, T S and T L temperature, or maintain its solid phase, especially when the temperature can be set at the solidus temperature T S or below. The formation of these regions depends on control, for example also on the local arrangement of the electrodes in the furnace and on the cooling power of the walls. In addition, the temperature setting in the furnace, especially in the corners, can also be influenced by introducing cold charge. The temperature in the liquid phase is above T L Depending on the composition of the iron melt to be produced, which composition results in particular from the iron-containing charge introduced, the temperature T can be derived from the so-called iron-carbon diagram. S and T L The temperature in the corresponding area can be set to or at T S -300K and less than T L between, especially less than T L -5K, preferably less than T L -20K. The electrodes are controlled in such a way that a temperature higher than T is established around the electrodes. L , for example, at least T L +30K, especially at least T L The liquidus temperature is approximately +50K. The theoretical liquidus temperature of pure iron is approximately 1538°C. This temperature essentially refers to the temperature of the molten iron, which can be measured by known means. Known measurement methods include thermocouples directly immersed in the molten iron or non-contact pyrometers. Reasonable assumptions can also be made, following standard practice, which allow inferences about the temperature in the furnace, such as through visual monitoring of the molten bath and determination of different radiation intensities, different flow behaviors of the liquid slag / molten iron, any changes in the shape and melting of the charge cone, or measurement of wall temperatures.

[0014] The smelting process (cycle) can be monitored using an imaging device, such as one or more cameras, for example, in the form of a top-down temperature profile. Thus, for example, bright areas, representing a high-temperature liquid phase, and dark, cooler mixed or solid phases can be identified. During the smelting process, the charge and optional additives are converted into a liquid phase comprising molten iron and liquid slag. The liquid slag, due to its lower density than the molten iron, forms above the molten iron. Therefore, the liquid phase temperature detected in the top-down view does not correspond to the temperature of the molten iron, but rather to the temperature of the liquid slag above it. Especially when energy is input into the furnace via the liquid slag, the temperature of the liquid slag can deviate by up to 200 K (or more) from the actual temperature of the molten iron. Therefore, in the image processing software, a correction factor can be set for the temperature profile in the liquid phase region to provide an approximate representation of the molten iron temperature in the image as light / dark.

[0015] The temperature distribution in the interior of the electric melting furnace can be in the following form: from the outside to the inside, based on the base structure, at least in the corners, preferably completely below T S The area, then T S To T L The mixing zone is followed by a zone with a temperature higher than T L The dimensions of these regions, in particular in order to influence the gradient of the temperature distribution so that the liquid phase only contacts the surrounding wall in certain areas, can be controlled not only by the electrodes but also by the cooling power of the wall and / or by the introduction of a cold charge.

[0016] Preferably, reduced iron ore is used as the iron-containing charge in the form of sponge iron blocks or sponge iron pellets, with a carbon content between 0 and 4.5% by weight, in particular >0% by weight, and a metallization of at least 85%. The metallization reflects the ratio of the metallic iron content to the total iron content in the sponge iron. The carbon content also affects the T S and T L , which decreases with increasing carbon content, which is advantageous because the energy required for melting can also be reduced. If the iron melt cannot be provided with a defined carbon content, which can be between 0, in particular >0 wt. %, and 4.5 wt. %, by the iron-containing charge, it is necessary to consider adding carbon-containing additives in an amount that achieves the desired carbon content in the iron melt.

[0017] The invention is also conceivable for use in electric melting furnaces which are operated with scrap and / or crude steel as ferrous charge.

[0018] The preferred use of sponge iron as an iron-containing charge also brings about slag-forming components, which are naturally present in iron ore and cannot be removed in the previous reduction process and are called gangue. If the gangue provided by the sponge iron is insufficient, other slag-forming agents can be introduced as additives as needed to produce liquid slag that can be further processed. Therefore, slag-forming agents are preferably added so that the basicity B3 of the liquid slag is set between 0.9 and 1.8. B3 can especially be at least 1.0, preferably at least 1.1, in particular a maximum of 1.7, preferably a maximum of 1.6. The basicity B3 corresponds to the ratio of (CaO+MgO) to (SiO2+Al2O3), wherein the determination of characteristic quantities in the slag in the solid state is known to those skilled in the art. The slag-forming agent comprises at least one or more elements selected from the group consisting of (CaO, MgO, SiO2, Al2O3).

[0019] To improve or increase the recovery rate, scrap can be added to the iron-containing charge, preferably to the sponge iron blocks 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, and more preferably 80 kg to 200 kg, of scrap per ton of molten iron produced.

[0020] To smelt iron-containing solid materials, electric furnaces have multiple electrodes to which an electric current can be applied, thereby providing the energy required to transform the solid material into a liquid phase comprising molten iron and liquid slag. Depending on the size of the furnace, three, four, five, six, or even six electrodes may be used. The energy required for smelting is preferably derived from renewable energy sources (solar, wind, hydro, or biomass). This allows the operation of the electric furnace to be more environmentally friendly.

[0021] Depending on the design of the polygonal base structure of the electric furnace and the arrangement of the electrodes within it, the electrodes can be controlled in various ways so that the iron-containing charge in each region is not completely converted to the liquid phase, and the liquid phase only regionally contacts the wall and melts accordingly. Thus, for example, the respective openings for discharging liquid slag and molten iron are located in the wall in regions of regional contact with the liquid phase. The opening for discharging liquid slag is slightly higher than the opening for discharging molten iron. Thus, the two openings can be arranged one above the other in the region of regional contact. To achieve sequential discharge, the openings for discharging can be arranged on different sides of the wall, for example, opposite each other. In an electric furnace with a polygonal horizontal base structure, these openings are each located substantially centrally on one side of the wall. The base structure can be substantially square, with four equally long wall sides. For example, three or more electrodes can be controlled so that a substantially circular liquid phase forms, resulting in regional contact with the wall only in four discrete regions, in this case, the middle regions of the four respective sides.

[0022] Alternatively, the base structure can also be essentially rectangular and designed with opposite wall sides of equal length, wherein, for example, four or more electrodes, preferably arranged in a row, are controlled so that an essentially elliptical liquid phase is formed, thereby making regional contact with the wall only in four discrete areas, here in the middle areas of the corresponding four sides.

[0023] The electric melting furnace is preferably an OSBF (Open Slag Bath Furnace) type furnace. This also includes reducing arc furnaces, in particular SAF (Submerged Electric Arch Furnace), which are melting furnaces using arc resistance heating, which form an arc between the electrode and the charge and / or liquid phase, or heat the charge and / or liquid phase by the Joule effect. In a SAF furnace, the electrodes are immersed in the charge and / or liquid phase, in particular liquid slag. Depending on the working principle / operating mode, the reducing arc furnace can be implemented as an AC submerged arc reduction furnace (SAFac) or a DC submerged arc reduction furnace (SAFdc). Alternatively, a direct arc action melting furnace different from the working principle / operating mode can also be used, namely the so-called EAF (Electric Arch Furnace), which forms an arc between the electrode and the liquid phase.

[0024] This includes AC arc melting furnace (EAFac), DC arc melting furnace (EAFdc) and ladle furnace LF (Ladle Furnace).

[0025] The advantage of using a reducing arc furnace (SAF) with arc resistance heating is that it operates in a reducing atmosphere, while a direct arc furnace (EAF) operates in an oxidizing atmosphere. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] The present invention is described in detail through the following embodiments with reference to the accompanying drawings.

[0027] The present invention takes an electric melting furnace (10) having a polygonal horizontal base structure as an example. Figures 1 to 3 Elaborate in detail. Figure 1 and Figure 2 Schematically shows an example of a base structure having a substantially square shape, Figure 3 An example of a substantially rectangular base structure is schematically shown. DETAILED DESCRIPTION

[0028] The invention provides a method for producing an iron melt and liquid slag in an electric melting furnace (10) having a polygonal horizontal base structure with a surrounding wall (12). An iron-containing charge is introduced and smelted. The required charge is supplied by means not shown. The iron-containing charge contains or consists of sponge iron blocks / pellets. In addition, in order to further increase the recovery rate, other iron-containing charges, such as iron-containing scrap, can be added. In particular, when the so-called gangue of the sponge iron preferably used is insufficient to adjust the desired basicity of the liquid slag to be discharged, other additives, such as slag-forming agents such as lime, silicon dioxide, magnesium oxide and / or aluminum oxide can also be introduced. Adjusting the desired basicity by corresponding mixing / addition is known to those skilled in the art. The amount of iron-containing charge charged is calculated according to the desired output of the iron melt. The electric melting furnace (10) has a plurality of electrodes (11), for example Figure 1 and 2 The three shown in Figure 3 The six electrodes (shown in FIG) are arranged in a row and provide the energy required for smelting to convert the charge into a liquid phase (L) comprising molten iron and liquid slag above. The electrical energy required for smelting preferably comes from renewable energy sources (solar, wind, hydro). Unlike the prior art, the electrodes (11) are controlled so that the liquid phase (L) only partially contacts the surrounding wall (12).

[0029] The electrode (11) is controlled so that the charge temperature is controlled to be below T in the region (S) where the liquid phase (L) is not present. L -50K, of which T L corresponds to the liquidus temperature, and a temperature of at least T is set around the electrode (11) L +30K liquid phase (L).

[0030] Depending on the design of the polygonal base shape of the electric melting furnace (10) and the arrangement of the electrodes (11) in the electric melting furnace (11), the electrodes can also be controlled in different ways so that the iron-containing charge in the area (S) is not completely converted into the liquid phase (L), and the liquid phase (L) is only in regional contact with the furnace wall (12) and melts accordingly. Thus, for example, the corresponding openings (13, 14) for discharging liquid slag and molten iron are located in the area of ​​the wall (12) in regional contact with the liquid phase (L). The opening (13) for discharging liquid slag is slightly higher in height than the opening (14) for discharging molten iron. Thus, the two openings (13, 14) can be arranged one above the other in the area of ​​regional contact, as shown in the example in the figure. In order to achieve sequential discharging, the openings (13, 14) for discharging can be arranged on different sides of the wall (12), for example, arranged opposite each other. In an electric melting furnace (10) having a polygonal horizontal base structure, openings (13, 14) are each arranged substantially centrally on one side of a wall (12).

[0031] The basic square base structure is shown in Figure 1 and Figure 2 , designed with four equally long wall (12) sides, wherein, for example, the three electrodes (11) can be controlled so that a substantially circular liquid phase (L) is formed, so that the wall (12) is only contacted regionally in four discrete areas, here in the middle areas of the respective four sides, see Figure 1 . Figure 2 The difference is that the upper electrode (11) in the figure is subjected to a higher power, whereby its area of ​​influence and thus the liquid phase (L) extends further than in the case of two adjacent electrodes (11) subjected to a lower power. The liquid phase (L) can assume a trilobal ("trefoil") shape, in which regional contact with the wall (12) must take place in at least one discrete area, here in the middle area of ​​the upper side shown, on which the openings (13, 14) for the discharge are arranged.

[0032] The electric melting furnace (10) with a polygonal horizontal base structure is preferably implemented as a fixed type and cannot be tilted. The operation mode of the electric melting furnace (10) is also known to those skilled in the art.

[0033] Not shown is that in the area where the liquid phase (L) is in at least regional contact and where the openings (13, 14) for discharge are also arranged, the wall (12) or its lining has a greater thickness than the wall (12) or lining of the rest of the wall, preferably at least 10%, preferably at least 20%, more preferably at least 25%.

[0034] The size ratio of the electrodes to the furnace (container) is not shown. Furthermore, it is also not shown that, in addition to the (standard) electrodes, at least one electrode can also be (additionally) arranged in the furnace bottom, see EP 3 542 595 B1.

[0035] Also not shown is how the molten iron is removed and transported to subsequent processing steps. The molten iron is preferably transported to a treatment station for reducing the carbon content in the molten iron to the desired level. This is achieved, for example, with the aid of oxygen in the so-called oxygen steelmaking process, particularly preferably in a converter. The discharged liquid slag is then preferably transported to a granulation process to produce slag particularly suitable for the construction industry.

Claims

1. A method for producing molten iron and liquid slag in an electric melting furnace (10) having a polygonal horizontal base structure with a surrounding wall (12), wherein: An iron-containing charge is charged and smelted, wherein the electric melting furnace (10) has a plurality of electrodes (11) which provide the energy required for smelting in order to convert the charge into a liquid phase (L) comprising an iron melt and liquid slag arranged above the iron melt, characterized in that the electrodes (11) are controlled in such a way that the liquid phase (L) only contacts the surrounding wall (12) in certain areas.

2. The method according to claim 1, wherein the electrode (11) is controlled so that in the region (S) where no liquid phase (L) is present, a temperature below T L The charging temperature, where T L corresponds to the liquidus temperature.

3. The method according to claim 1 , wherein the electrode ( 11 ) is controlled in such a way that a temperature higher than T L of the liquid phase (L).

4. The method according to claim 1, wherein only regional contact of the liquid phase (L) with the surrounding wall (12) is controlled additionally by the cooling power of the wall and / or by the introduction of a cold charge.

5. A method according to any one of the preceding claims, wherein reduced iron ore is used as the iron-containing charge, the reduced iron ore being in the form of sponge iron chunks or sponge iron pellets, having a carbon content between 0 and 4.5 wt. % and a metallization of at least 85%.

6. The method according to any one of the preceding claims, wherein a slag former is added such that the basicity B3 of the liquid slag is set between 0.9 and 1.8, the slag former comprising at least one or more elements selected from the group consisting of (CaO, MgO, SiO2, Al2O3).

7. A method according to any one of the preceding claims, wherein the energy required for smelting comes from renewable energy sources.

8. The method according to any of the preceding claims, wherein the electrodes (11) are controlled in different ways.

9. The method according to any of the preceding claims, wherein respective openings (13, 14) for discharging liquid slag and iron melt are arranged in the region of the wall (12) in regional contact with the liquid phase (L).

10. The method according to any of the preceding claims, wherein the base structure of the electric melting furnace (10) is implemented in a square shape with four wall (12) sides of equal length, wherein three or more electrodes (11) are controlled so that a substantially circular liquid phase (L) is formed, thereby making regional contact with the wall (12) only in four discrete areas, here in the middle areas of the respective four sides.

11. The method according to any one of claims 1 to 9, wherein the base structure of the electric melting furnace (10) is implemented in a rectangular manner with two opposite sides of the wall (12) of equal length, wherein: Four or more electrodes (11), preferably arranged in a row, are controlled so that a substantially elliptical liquid phase (L) is formed, making regional contact with the wall (12) only in four discrete areas, here in the middle areas of the respective four sides.

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