Denitriding of sponge iron

Abstract

The invention relates to a method for reducing the content of iron nitrides in sponge iron (1), the temperature of the sponge iron (1) being adjusted to a temperature of at least 450°C. In a method for producing a metal melt comprising melting sponge iron (1), a sponge iron that has been denitrided in this way and has a temperature of 450°C to 1000°C is used. A device for reducing (2) the content of iron nitrides in sponge iron (1) comprises a first container (3) for receiving sponge iron (1) and a heating device (4) for setting a temperature of at least 450°C for sponge iron (1) in the first container (3). A device for feeding (8) sponge iron (1) to a melting device (9) for melting sponge iron (1) comprises a device for reducing (2) the content of iron nitrides in sponge iron (1) and a feed line (10) for feeding sponge iron (1), from the device for reducing (2) the content of iron nitrides in sponge iron (1), into the melting device (9).

Description

[0001]202400243 1 Description Denitridation of Sponge Iron Field of Technology The application relates to a process for reducing the iron nitride content in sponge iron, a process for producing a metallic melt comprising melting such denitrided sponge iron, a device for reducing the iron nitride content in sponge iron, and a device for feeding sponge iron from such a device to a melting device. Prior Art It is known to reduce iron oxide-containing material – such as ores – using reducing gas. This can be done, for example, by direct reduction with reducing gas in a reduction unit, such as a reduction shaft. In currently used, conventional processes on a large industrial scale, the reducing gas is predominantly based on natural gas. Therefore, large quantities of carbon dioxide (CO2) are produced, which is undesirable for environmental reasons, among others.To reduce CO2 emissions during the reduction of iron oxide-containing materials, it is known to use hydrogen (H2) as a reducing gas. Hydrogen (H2) can be used as the sole reducing gas or in combination with other gases, such as natural gas-based or synthesis gas-based reducing gases. The greater the proportion of CO2-neutral hydrogen (H2) in the reducing gas, the less CO2 is emitted. However, storing hydrogen (H2) and transporting it from its point of production to consumers is problematic and costly due to its physical properties. To reduce CO2 emissions during the reduction of iron oxide-containing materials, it is also known to use ammonia (NH3) as a reducing agent. Ammonia (NH3) offers significant advantages over hydrogen (H2) with regard to storage and transport. Ammonia (NH3) can be split into nitrogen (N2) and hydrogen (H2): 2 NH3^N + 3 H2.Hydrogen (H₂) can react as a reducing agent with metal oxides, for example iron oxides: 3 Fe₂O₃ + H₂^ 2 Fe₃O₄ + H₂O Fe₃O₄ + H₂^ 3 FeO + H₂O FeO + H₂^ Fe + H₂O. Ammonia (NH₃) can also act as a reducing agent itself. 3 FeO + 2 NH3^ 3 Fe + N2 + 3 H2O. A reduction gas containing ammonia (NH3) can consist of ammonia (NH3) or a mixture of ammonia (NH3) with one or more other gases – preferably one or more of which can have a reducing effect on iron oxide-containing material. In principle, a reduction gas obtained using ammonia (NH3) can be used to reduce iron oxide-containing material; such a reduction gas can, for example, be ammonia (NH3) or a mixture of ammonia (NH3) with one or more other gases – preferably one or more of which can have a reducing effect on iron oxide-containing material – which would be the case, for example, with a mixture of ammonia (NH3) and its fission products hydrogen (H2) and nitrogen (N2), whereby, of course, other gases could also be present in the mixture.It is known that the presence of ammonia (NH3) during the reduction of iron oxide-containing material can lead to the formation of iron nitrides of the metal. For example, the following reactions with metallic iron lead to the formation of iron nitrides: Similarly, in the presence of ammonia (NH3), reactions can occur even in already reduced material, forming iron nitrides. For example, a layer containing iron nitride or iron nitrides can form on the surface or within the porous porous DRI pellets upon contact with ammonia (NH3). The solid product of a direct reduction of iron oxide-containing material is called direct reduced iron (DRI). If DRI containing iron nitrides is further processed by remelting, the resulting melt will have a nitrogen content influenced by the iron nitride content in the product. The presence of nitrogen (N2) in the melt can lead to undesirable properties of the melt or of the end products obtained from the melt through further processing.Summary of the Invention Technical Problem The object of the present invention is to provide a method for reducing the iron nitride content in sponge iron that is practical for industrial production. Technical Solution The problem is solved by a method for reducing the iron nitride content in sponge iron, characterized in that the temperature of the sponge iron is adjusted to a temperature of at least 450°C, preferably more than 500°C, and particularly preferably more than 550°C. The reduction of the iron nitride content – ​​also called denitridation – also includes the complete removal of iron nitrides. The iron nitride content is, for example, expressed as a weight percent of iron nitrides in a quantity of sponge iron. Up to 100% of the iron atoms in the sponge iron can be present in iron nitride molecules.After reducing the iron nitride content, an iron nitride content of a maximum of 50% by weight, preferably less than 20% by weight, and particularly preferably less than 5% by weight, is desired. The sponge iron, whose iron nitride content is to be reduced, generally has a temperature below 450°C. Sponge iron is solid. It can, for example, be sponge iron that has been transported from one production site to another location where it is to be further processed. The transport can take place, for example, within the steelworks where the sponge iron is produced, or it can take place from production facilities in other countries to the location where the process according to the invention is carried out. The temperature of the sponge iron to be denitrided is adjusted to at least 450°C; for this purpose, the temperature of the sponge iron to be denitrided is increased, i.e., it is heated.Preferably, the temperature is set to more than 500 °C, particularly preferably more than 550 °C. Higher temperatures have the effect of accelerating the reactions leading to the decomposition of iron nitrides, resulting in a higher proportion of iron nitrides being converted into metallic iron or iron oxides. The temperature of the iron sponge can be set to up to 1200 °C. Preferably, the upper limit for setting the temperature of the iron sponge is 1000 °C, particularly preferably 900 °C, and most preferably 800 °C. The gas atmosphere in which the iron sponge is located when its temperature is set to at least 450 °C, preferably more than 500 °C, and particularly preferably more than 550 °C, should be non-nitriding under the prevailing conditions.Non-nitriding means that components of the gas atmosphere do not react with the iron sponge in such a way that the total number of iron nitride molecules per unit mass of iron sponge is increased or remains the same. The total number of iron nitride molecules per unit mass of iron sponge should be decreased. Preferably, the gas atmosphere contains no more than 1 vol% ammonia (NH3), and more preferably, the gas atmosphere contains no ammonia (NH3). Preferably, the iron sponge is held at a temperature of at least 450 °C for at least 15 minutes, more preferably at least 30 minutes, more preferably at least 60 minutes, and most preferably at least 120 minutes. More preferably, the iron sponge is held at a temperature of at least 450 °C for up to 180 minutes.Preferably, the temperature of the sponge iron is increased using a heating gas stream with a temperature of at least 450 °C, preferably more than 500 °C, and particularly preferably more than 550 °C. The gas is passed through the material to be heated. The material to be heated can be a fixed bed or fluidized by a nitrogen stream. The temperature of the heating gas stream can be set up to 1600 °C. Preferably, the upper limit for the temperature of the heating gas stream is 1000 °C, particularly preferably 900 °C, and most preferably 800 °C. Preferably, the heating gas stream contains no more than 1 volume% ammonia (NH3), and particularly preferably, no ammonia (NH3). The heating gas stream can consist of a single gas or a mixture of several gases.For the heating gas stream, gases that are practically inert or reducing with respect to reactions with the sponge iron at the given temperatures are preferred. These gases can be, for example, nitrogen (N₂), argon (Ar), or hydrogen (H₂). Preferably, the heating gas stream comprises nitrogen (N₂), and more preferably, it consists predominantly, and particularly preferably, of more than 50% nitrogen (N₂). The use of nitrogen is advantageous compared to the use of other gases with regard to the achievable denitrification rates and / or the achievable residual iron nitride content. Recirculation of the heating gas stream is also simpler when using nitrogen (N₂). Preferably, the heating gas stream is at least partially recirculated.This means that the gas escaping from the heated sponge after the temperature of the sponge iron has been increased is extracted and, at least partially, possibly after processing – such as increasing the temperature, dedusting, drying, adding components, or removing components – used to provide a heating gas stream. When using nitrogen (N₂) in the heating gas stream, it is advantageous that nitrogen (N₂) is released during denitridation. In practice, this at least partially compensates for gas losses occurring in the recirculation loop of the heating gas stream. Should an excess of nitrogen (N₂) even be generated in the recirculation loop due to denitridation, the nitrogen (N₂) can be removed to be made available for other uses. Removal can be controlled and / or regulated, for example, via a pressure-maintaining valve.The temperature of the heating gas stream is adjusted. Preferably, the temperature of the heating gas stream is adjusted at least partially, and preferably completely, using residual heat from processes involved in the production of the sponge iron or the further processing of the denitrided sponge iron. For example, residual heat from a direct reduction unit in which the sponge iron to be denitrided is produced, or residual heat from a melting unit – for example, an EAF, converter, or smelter – used for melting the denitrided sponge iron. The use of residual heat can be achieved, for example, by extracting heat from process gases using heat exchangers – for example, operated with thermal oil or salts – and transferring it to the heating gas stream. The heat can also be buffered with steam before being transferred to the heating gas stream.The temperature of the heating gas stream can also be adjusted partially or completely by using energy from other sources. For example, gas heaters supplied by other energy sources – such as gas-powered or, preferably, electrically powered gas heaters – can be used. Preferably, the temperature of the sponge iron is increased using induction heating. A combination of the aforementioned methods for increasing the temperature of the sponge iron, optionally with other temperature-increasing methods, is also possible. A further subject matter of the present application is a process for producing a metallic melt comprising melting sponge iron, characterized in that at least one denitrided sponge iron according to the invention with a temperature of 450°C to 1000°C is used as the feed material. Sponge iron is typically melted during further processing into steel.As explained at the outset, denitridation is carried out to avoid a high nitrogen content in a melt. Melting requires an energy input. The denitridation process according to the invention provides a denitridated sponge iron, which, due to the process, already has an elevated temperature. In the case of further processing of a sponge iron still heated by the denitridation process by melting, the energy required for melting is lower compared to melting cold sponge iron. The heat energy still present in the sponge iron – which was supplied to it for denitridation – does not need to be supplied separately, unlike when melting cold sponge iron. The process according to the invention thus enables the energy-efficient production of a metallic melt with a low nitrogen (N₂) content, even when sponge iron with a high iron nitride content is used.Furthermore, the melting process is faster, resulting in higher productivity and lower consumption of the melting unit. A nitrogen content of no more than 200 ppm (parts per million) of dissolved nitrogen (N2) is preferred for the metallic melt. The nitrogen content in a steel melt from an electric arc furnace is typically between 50 and 200 ppm. In practice, attempts are made to keep the nitrogen content as low as possible, as a high nitrogen content can negatively affect the mechanical properties of the steel, for example, by increasing its brittleness. The metallic melt can be either pig iron or steel. The process for producing a metallic melt can include, in addition to melting sponge iron, other steps – possibly running in parallel – such as melting other components like scrap metal.For example, scrap and sponge iron can be charged into an EAF or a converter vessel and melted together. Alternatively, only sponge iron can be melted, for example in a smelter that produces pig iron. Preferably, in the inventive process for producing a metallic melt comprising melting sponge iron, a sponge iron denitrided according to a process according to one of claims 1 to 7 is produced in a device for reducing the iron nitride content in sponge iron, then fed to a storage device, and from there to a melting device for melting sponge iron. This allows, for example, a continuously operated denitriding process to be coupled with a batch-operated process for melting sponge iron. For example, an EAF is operated batchwise.The melting device is, for example, a member of the group consisting of: - electric arc furnace (EAF); - submerged arc furnace (SAF); - open slag bath furnace (OSBF); - melting unit, also called smelter; - converter vessel. A melting unit melts at least partially using electrical energy. EAF, SAF, and OSBF are not to be understood as a melting unit within the scope of this application. A converter vessel is, for example, a steelworks converter used for steel production. Exhaust gas from a melting device can contribute to the heating gas flow.Another subject matter of the present application is a device for reducing the iron nitride content in sponge iron, characterized in that it comprises: - a first container for receiving sponge iron, - a heating device for setting a temperature of at least 450 °C, preferably more than 500 °C, particularly preferably more than 550 °C, for sponge iron in the first container. Preferably, the heating device comprises a supply line for a heating gas stream with a temperature of at least 450 °C, preferably more than 500 °C, particularly preferably more than 550 °C. Preferably, the device for reducing the iron nitride content in sponge iron comprises an outlet for the heating gas stream, which leads into a recirculation line connected to the supply line for the heating gas stream.Preferably, the supply line and / or recirculation line comprises a heating device for heating gas to a temperature of at least 450 °C, preferably more than 500 °C, and particularly preferably more than 550 °C. The gas in the supply line or the gas in the recirculation line is heated. A further object of the present application is a device for supplying sponge iron to a melting device for melting sponge iron, comprising: - a device according to the invention for reducing the iron nitride content in sponge iron, - a supply line for supplying sponge iron from the device for reducing the iron nitride content in sponge iron to the melting device. Preferably, the supply line comprises a storage device for intermediate storage of sponge iron between the device for reducing the iron nitride content in sponge iron and the melting device for melting sponge iron.Another subject matter of the present application is a signal processing device with a machine-readable program code, characterized in that it includes control and / or regulation commands for carrying out a method according to the invention. A further subject matter is a signal processing device for carrying out a method according to any one of claims 1 to 9. A further subject matter of the present application is a machine-readable program code for a signal processing device, characterized in that the program code includes control and / or regulation commands which cause the signal processing device to carry out a method according to the invention.Another object is a computer program product comprising instructions for a signal processing device which, upon execution of the program for the signal processing device, cause it to carry out the method according to any one of claims 1 to 9. Another object of the present application is a storage medium with machine-readable program code according to the invention stored thereon. Another object is a storage medium with a computer program stored thereon for carrying out a method according to any one of claims 1 to 9. Brief descriptions of the drawings: The invention will now be explained in more detail with reference to exemplary embodiments. The drawing is exemplary and is intended to illustrate the concept of the invention, but in no way to restrict it or even to present it exhaustively. Fig. 1 shows a schematic representation of a process sequence according to the invention. 202400243 14 Fig.Figure 2 shows a schematic representation of another variant of a process sequence according to the invention. Brief description of the embodiments Examples Figure 1 schematically shows the process sequence according to the invention. Iron sponge 1 containing iron nitrides is fed to a device 2 for reducing the iron nitride content in the iron sponge. This can be done, for example, via an input line. The device 2 for reducing the iron nitride content in the iron sponge comprises the first container 3 and a heating device 4 for setting a temperature of at least 450 °C for the iron sponge in the first container 3. In the illustrated example, the heating device 4 comprises a supply line 5 for supplying a heating gas stream that has a temperature of at least 450 °C. The iron sponge 1 located in the container 3 is heated to a temperature of at least 450 °C.The device for reducing the iron nitride content 2 in sponge iron comprises an outlet 6 for the discharge of the heating gas stream. This outlet leads into a recirculation line 7, which is connected to the supply line 5 for the heating gas stream. This allows the heating gas stream to be at least partially recirculated after leaving the first container 3. An outlet from the recirculation line 7 is schematically represented by a wavy arrow. For clarity, the illustration of any processing devices or compressors that may be present in the recirculation line 7 is omitted. Figure 1 also schematically shows an example of a process for producing a metallic melt, comprising melting sponge iron. In this illustration, a denitrided sponge iron is used as the feedstock that is melted.A device for supplying sponge iron 8 to a melting device 9 for melting sponge iron – for example, EAF – comprises the device for reducing the iron nitride content 2 in sponge iron and a feed line 10 for supplying sponge iron from the device for reducing the iron nitride content 2 in sponge iron to the melting device 9. In the illustrated variant, the feed line 10 includes a storage device 11 – which is generally optional – for temporarily storing sponge iron between the device for reducing the iron nitride content 2 in sponge iron and the melting device 9 for melting sponge iron. Sponge iron from the device for reducing the iron nitride content 2 in sponge iron enters the storage device 11, and from the storage device 11, the sponge iron is added to the melting device. Figure 2 shows a variant largely analogous to Figure 1.A heating device 12a / 12b is provided in the recirculation line 7 and / or the supply line 5; shown with a dashed outline, as it is optional. 202400243 16 Sponge iron device for reducing (the iron nitride content in sponge iron) First container Heating device Supply line (for supplying a heating gas stream) Outlet (for discharging the heating gas stream) Recirculation line Device for supplying sponge iron (to a melting device 9) Melting device Supply line (for supplying sponge iron from the reduction device 2 to the melting device 9) Storage device 12a / 12b Heating device.

Claims

202400243 17 Claims 1. A method for reducing the iron nitride content in sponge iron (1), characterized in that the temperature of the sponge iron (1) is adjusted to a temperature of at least 450°C, preferably more than 500°C, particularly preferably more than 550°C.

2. A method according to claim 1, characterized in that the sponge iron (1) is held at a temperature of at least 450°C for a duration of at least 15 minutes, preferably at least 30 minutes, particularly preferably at least 60 minutes, and most preferably at least 120 minutes during temperature adjustment.

3. A method according to claim 1 or 2, characterized in that during temperature adjustment, the temperature of the sponge iron (1) is increased using a heating gas stream with a temperature of at least 450°C, preferably more than 500°C, particularly preferably more than 550°C. 4.A method according to any one of claims 1 to 3, characterized in that the heating gas stream comprises nitrogen N2, preferably consisting predominantly, particularly preferably more than 50%, of nitrogen N2.

5. A method according to any one of claims 1 to 4, characterized in that the heating gas stream is at least partially recirculated.

6. A method according to any one of claims 1 to 5, characterized in that the temperature of the heating gas stream is at least partially, preferably completely, adjusted. 202400243 18. This process utilizes residual heat from processes involved in the production of the sponge iron (1) or the further processing of the denitrided sponge iron (1).

7. A process according to any one of claims 1 to 6, characterized in that the temperature of the sponge iron (1) is increased by induction heating when adjusting the temperature.

8. A process for producing a metallic melt comprising melting sponge iron (1), characterized in that at least one sponge iron denitrided according to any one of claims 1 to 7 at a temperature of 450°C to 1000°C is used as the feedstock. 9.Method according to claim 8, characterized in that a denitrided iron sponge (1) according to a method according to any one of claims 1 to 7 is produced in a device for reducing (2) the iron nitride content in the iron sponge (1), is then fed to a storage device (11), and from there is fed to a melting device (9) for melting the iron sponge (1).

10. Device for reducing (2) the iron nitride content in the iron sponge (1), characterized in that it comprises: - a first container (3) for receiving the iron sponge (1), - a heating device (4) for setting a temperature of at least 450 °C, preferably more than 500 °C, particularly preferably more than 550 °C, for the iron sponge (1) in the first container (3). 202400243 19 11. Device according to claim 10, characterized in that the heating device (4) comprises a supply line (5) for supplying a heating gas stream with a temperature of at least 450 °C, preferably more than 500 °C, particularly preferably more than 550 °C.

12. Device according to claim 10 or 11, characterized in that the device for reducing (2) the iron nitride content in sponge iron (1) comprises an outlet (6) for discharging the heating gas stream, which opens into a recirculation line (7) which is connected to the supply line (5) for supplying a heating gas stream.

13. Device according to any one of claims 10 to 12, characterized in that the supply line (5) and / or recirculation line (7) comprises a heating device (12a, 12b) for heating gas to a temperature of at least 450 °C, preferably more than 500 °C, particularly preferably more than 550 °C. 14.Device for supplying (8) iron sponge (1) to a melting device (9) for melting iron sponge (1), comprising - a device for reducing (2) the iron nitride content in iron sponge (1) according to one of claims 10 to 13, - a supply line (10) for supplying iron sponge (1) from the device for reducing (2) the iron nitride content in iron sponge (1) to the melting device (9). 202400243 20 15. Device according to claim 14, characterized in that the supply line (10) comprises a storage device (11) for intermediate storage of sponge iron (1) between the device for reducing (2) the iron nitride content in sponge iron (1) and the melting device (9) for melting sponge iron (1).

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