Method for producing pig iron in an electric smelting furnace and related electric smelting furnace

Abstract

The present invention relates to a method for producing pig iron in an electric smelting furnace 13 comprising a vessel 20, the method comprising the following successive steps: - loading the vessel 20 with a DRI product, - melting the DRI product to form a pig iron layer 14 covered by a slag layer 23, and - directly injecting a carbon-containing material into the pig iron layer 14. The present invention also relates to the production of steel from said pig iron and to the associated electric smelting furnace 13.

Description

Technical Field

[0001] The present invention relates to a method for producing pig iron, also called hot metal, and a method for producing steel from such pig iron.

Background Art

[0002] Currently, steel can be produced through two main manufacturing routes. Today, the most commonly used manufacturing route, called the "BF-BOF route", consists of producing hot metal in a blast furnace by reducing iron oxide using a reducing agent, mainly coke, and then converting the hot metal into steel in a converter process or a basic oxygen furnace (BOF). This route emits a significant amount of CO2 both in the production of coke from coal in a coke plant and in the production of hot metal.

[0003] The second main route includes the so-called "direct reduction process". Among them, there are methods such as those by brands MIDREX(R), FINMET(R), ENERGIRON(R) / HYL, COREX(R), FINEX(R), etc., where sponge iron is produced from the direct reduction of an iron oxide carrier in the form of HDRI (hot direct reduced iron), CDRI (cold direct reduced iron) or HBI (hot briquetted iron). Sponge iron in the form of HDRI, CDRI and HBI is further processed in an electric furnace to produce steel.

[0004] Therefore, one of the main options for steelmakers to reduce CO2 emissions is to switch from the BF-BOF route to the DRI route. However, there are some limitations to using DRI products in a classical electric furnace together with iron scrap. In fact, scrap contains many impurities and the resulting molten steel needs to be further processed to produce high-quality steel grades. Therefore, investment in new molten steel treatment tools is required.

Summary of the Invention

Problems to be Solved by the Invention

[0005] Another option consists of using a smelting furnace powered by electrical energy to melt DRI products for the production of pig iron. This option has the advantage that pig iron is produced as in a blast furnace, enabling the removal of oxides in the molten slag and thus allowing the use of classical steelmaking tools such as basic oxygen furnaces and refining ladles. However, the pig iron obtained by this route has a relatively low carbon content compared to classical pig iron. Since the higher the proportion of carbon, the more recycled scrap metal can be added in the BOF, this paradoxically reduces the environmental benefits of this route.

[0006] Accordingly, it is an object of the present invention to improve the drawbacks of the production routes for pig iron and steelmaking by providing a new route that efficiently minimizes the environmental impact of such production.

Means for Solving the Problems

[0007] This problem is solved by a method for producing pig iron as detailed in claim 1.

[0008] Such a method can also include the optional features of claims 2 - 10, considered separately or in any possible technical combination.

[0009] The present invention also deals with a method for producing steel as described in claim 11.

[0010] Such a method can also include the optional features of claims 12 - 13, considered separately or in any possible technical combination.

[0011] The present invention also deals with the electric smelting furnace described in claim 14.

[0012] Other features and advantages of the present invention will become apparent from the description of the invention given below by way of indication and with reference to the accompanying drawings, which are in no way limiting.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

[0014] The elements in the figures are illustrative and may not be drawn to scale.

[0015] Figure 1 shows the steelmaking route by the DRI route from the reduction of iron to the casting of steel into semi-finished products such as slabs, billets, blooms or strips. Iron ore 10 is first reduced in a direct reduction plant 11. This direct reduction plant 11 can be designed to implement any type of direct reduction technology such as MIDREX(R) technology or Energiron(R). The direct reduction process may be, for example, a process based on traditional natural gas or biogas.

[0016] In a preferred embodiment, the DRI product used in the method according to the present invention is produced using a reducing gas based on biogas generated from the combustion of biomass.

[0017] Biomass is a renewable organic material derived from plants and animals. Biomass sources include, in particular, wood and wood processing wastes such as sawdust, wood pellets and wood chips, sawdust and wastes from sawmills and furniture factories, and black liquor from pulp and paper mills, crops and agricultural wastes such as corn, soybeans, sugarcane, switchgrass, woody plants and algae, and crop and food processing residues, but also municipal solid wastes such as paper, cotton and wool products, and food, yard and wood wastes, animal excreta and bio-derived materials in domestic sewage. In the context of the present invention, biomass may also include plastic residues such as recycled waste plastics such as solid waste fuels or SRF.

[0018] Whenever natural gas or biogas is used as the reducing gas, the carbon content of the DRI product can always be set to a maximum of 3 wt%, usually in the range of 2 - 3 wt%.

[0019] In another preferred embodiment, the DRI product used in the method according to the invention is produced by a so-called H2-DRI process in which the reducing gas contains more than 50 vol%, preferably more than 60, 70, 80 or 90 vol% hydrogen, or consists entirely of hydrogen. H2-DRI products contain much lower levels of carbon, typically less than 1 wt% or even lower, than natural gas or biogas DRI. In a preferred embodiment, the hydrogen used in the DRI reducing gas is derived from the electrolysis of water, which is preferably powered in part or in whole by CO2-neutral power. CO2-neutral power is defined as power from renewable sources, which includes energy collected from renewable resources that are naturally replenished on a human time scale, including sources such as sunlight, wind, rain, tides, waves, and geothermal. In some embodiments, the use of power from nuclear sources can be used because it does not emit the CO2 being produced.

[0020] Regardless of the DRI process used, the resulting direct reduced iron (DRI) product 12 is then charged into a smelting furnace 13 where the reduction of the iron oxide is completed and the product is melted to produce pig iron.

[0021] The DRI product can be transferred to the smelting furnace in various forms. Preferably, the directly reduced iron product (DRI product) is supplied to the smelting furnace in a hot form as an HDRI product (so-called Hot DRI), or in a room temperature form as a CDRI product (so-called Cold DRI), or in a hot formed form as an HBI product (so-called hot briquette iron), and / or preferably in a particulate form having an average particle size of up to 10.0 mm, more preferably up to 5.0 mm.

[0022] This is preferably charged directly as a hot product having a temperature of 500°C to 700°C at the outlet of the direct reduction plant 11. Thereby, the amount of energy required for melting can be reduced. If hot charging is not possible, for example, if the direct reduction plant 11 and the steelmaking furnace 13 are not in the same location, or if the steelmaking furnace 13 is stopped for maintenance and thus the DRI product has to be stored, the DRI product may be charged at room temperature or a preheating step may be carried out.

[0023] The steelmaking furnace 13 uses the electrical energy provided by several electrodes to melt the DRI product 12 and produce pig iron. In a preferred embodiment, some or all of the required electric power is derived from CO2-neutral power. A more detailed description of the steelmaking furnace will be described later with reference to FIG. 2.

[0024] The pig iron can then optionally be transferred to a desulfurization station 15 to carry out a desulfurization step. This desulfurization step is necessary for the production of steel grades that require a low sulfur content, for example set to a maximum of 0.03 weight percent sulfur. Desulfurization under oxidizing conditions is not effective and is therefore preferably carried out either on the pig iron before oxygen refining or in the ladle after steel deoxidation. For very low sulfur contents, for example less than 0.004 weight percent sulfur, deoxidation and desulfurization are combined for overall higher performance. Thus, low sulfur grades benefit from pig iron desulfurization before the conversion step.

[0025] Desulfurization of the pig iron can be carried out by adding reagents based on calcium compounds or magnesium compounds such as sodium carbonate, lime, calcium carbide and / or magnesium, in particular, into the pig iron. This can be done, for example, by injecting those reagents into the pig iron previously transferred to the ladle. This ladle may be a simple ladle as shown in FIG. 1 or a torpedo ladle. The desulfurized pig iron 16 preferably has a sulfur content of less than 0.004 weight %.

[0026] Subsequently, the desulfurized pig iron 16 can be transferred into the converter 17. The converter basically converts the molten metal into molten steel by blowing oxygen into the molten metal to decarburize it. This is generally called a basic oxygen furnace (BOF). Iron scrap 18 resulting from steel recycling may also be charged into the converter 17 to take advantage of the heat released by the exothermic reaction resulting from oxygen injection into the pig iron.

[0027] Subsequently, the molten steel 19 thus formed can be transferred at any time as required to one or more secondary metallurgy tools 20A, 20B, for example, ladle furnaces, RH (Ruhrstahl - Heareus) vacuum vessels, vacuum tank degassing devices, alloying and stirring stations, etc., and can be processed to reach the steel composition required according to the steel grade to be produced. Subsequently, the molten steel having the required composition 21 can be transferred to the casting plant 22, where it can be changed into solid products such as slabs, billets, blooms or strips.

[0028] As shown in FIG. 2, the smelting furnace 13 is composed of a container 20 that can accommodate hot metal. The container 20 may be, for example, circular or rectangular. This container 20 may be closed by a roof provided with several openings for receiving the electrodes 22 inserted into the container 20 and other openings for enabling the charging of raw materials into the container 20.

[0029] The electrodes 22 provide the electrical energy necessary to melt the charged raw materials to form pig iron. These are preferably Soederberg type electrodes.

[0030] During the melting of the raw materials, two layers are formed: a layer of pig iron 14, which is the densest and thus located at the bottom of the container 20, and a slag layer 23 located above the pig iron 14. The slag layer 23 may be partially covered by a pile of raw materials 24 waiting to be melted.

[0031] The vessel 20 also comprises openings called tap holes 25, which are located at its lower part and enable the tapping of the pig iron 14 while retaining most of the slag within the vessel 20. They may be arranged in the side wall or the bottom wall of the vessel.

[0032] The smelting furnace 13 may be a SAF (Submerged-Arc Furnace) in which the electrodes are immersed in the slag layer 23 or an OSBF (Open Slag Bath Furnace) in which the electrodes 22 are located above the slag layer 23. This is preferably an OSBF as shown in the figure.

[0033] As described above, the carbon content of the pig iron 14 produced via the DRI route is generally less than 3 wt%. However, in order to meet the requirements of the subsequent steelmaking process in the converter, the pig iron should preferably have a carbon content as close as possible to the saturation level of 4.5 wt%. In a preferred embodiment, the carbon content of the pig iron is in the range of 4.0 - 4.5 wt%.

[0034] In fact, carbon is necessary for the steelmaking process carried out in the converter 17 by blowing oxygen. This is because the reaction of carbon with oxygen produces carbon monoxide gas, which provides a strong and efficient stirring of the molten metal, thus improving the removal of impurities from the steel. This reaction is exothermic and thus provides additional energy for the melting of iron scrap, enabling a larger amount of such iron scrap resulting from steel recycling to be incorporated. The more iron scrap is used, the smaller the environmental footprint of the steelmaking process.

[0035] In the mechanism of the present invention, the carbon-containing material is directly added into the pig iron layer 14 in the smelting furnace 13. This addition can be carried out through an injection device.

[0036] The inventors have observed that by injecting carbon directly into the pig iron layer 14, the carbonization process can reach a very high yield exceeding 80%. In fact, the slag layer 23 can have a layer height of more than 50 cm, and the density of the carbon source is usually lower than the slag density itself. This causes physical limitations for carbon to enter the pig iron layer 14 through the slag.

[0037] Furthermore, carbonization requires a large amount of energy, and since it can be optimally provided by electric heating in the smelting furnace rather than an additional heating station, the direct injection of carbon ensures the optimal energy efficiency of the smelting process.

[0038] Finally, increasing the carbon content of the pig iron at the smelter reduces the liquidus temperature of the pig iron, allowing for a lower tapping temperature.

[0039] In a preferred embodiment, the injection device is the tuyere 26 inserted at the bottom of the container 20. Such a bottom tuyere opens into the pig iron layer 14 to allow for direct addition.

[0040] The use of this bottom tuyere 26 avoids the injection of carbon-containing materials from the upper part of the smelting furnace 13 where the available space may be insufficient due to the presence of electrodes and charging devices for DRI products.

[0041] In a preferred embodiment, to avoid clogging of the injection device, carbon is injected together with a carrier gas. This gas is preferably inert and can be made of nitrogen, argon, helium or carbon monoxide, or any mixture of such gases.

[0042] The carbon-containing material can be derived from different sources. This can be selected, for example, from among coke, anthracite, silicon carbide, calcium carbide, or a mixture of any of these sources, but can also advantageously be obtained from renewable sources such as biomass for part or all of the carbon loading. In particular, biochar can be used. Adding calcium carbide is particularly advantageous because calcium atoms can provide a desulfurization effect. Adding silicon carbide is particularly advantageous because it enables an increase in the silicon content of the pig iron.

[0043] The carbon-containing material injected through the injection device preferably has a particle size of less than 3 mm. In a preferred embodiment, the material has a particle size of 75 μm or less, and the remaining particles have a particle size of 2 mm or less.

[0044] In another embodiment, the carbon-containing material may also be made of composite briquettes in which an iron source is mixed with one or several of the aforementioned carbon sources.

[0045] In a preferred embodiment, the iron source can be selected from among sludge from an electric furnace, a converter or a smelter, slag from an electric furnace or a converter, DRI fines, or any waste rich in iron from the production route of pig iron or steel.

[0046] In a preferred embodiment, the silicon-containing material may be injected into the pig iron layer 14 together with the carbon-containing material. Silicon has a strong deoxidizing ability at high temperatures, particularly at about 1600 °C, which is the temperature of the molten steel in a converter. Silicon reacts with oxygen and then contributes to the formation of slag. This reaction is exothermic and thus provides additional energy for scrap melting to the converter. Even a small amount of silicon can further improve the performance of the desulfurization operation.

[0047] Such silicon can be added in different forms. The silicon may be metallic silicon Si, silicon carbide SiC, silicomanganese SiMn, calcium silicate SiCa, or a ferrosilicon alloy FeSi such as FeSi75 or FeSi65.

[0048] When using DRI products in the smelting furnace 13, a natural amount of silicon of usually less than 0.2% by weight, and further less than 0.1% by weight is brought about. The final silicon content of the pig iron is preferentially set to a value of 0.1 to 0.4% by weight, preferably 0.2 to 0.4% by weight. If necessary, further addition of silicon can be carried out in the desulfurization station 15 and / or the converter 17.

[0049] In a preferred embodiment, the desulfurization reagent can also be injected together with the carbon-containing material, regardless of the presence or absence of silicon addition. Such reagents can be based in particular on calcium compounds such as sodium carbonate, lime and / or calcium carbide.

[0050] The final sulfur content of the pig iron is preferably set to a maximum value of 0.03% by weight, preferably a maximum value of 0.004% by weight.

[0051] By carrying out desulfurization in the smelting furnace, the necessity for desulfurization treatment between the smelting furnace 13 and the converter 17 can be suppressed, or such treatment can at least be reduced.

[0052] It should be noted that the addition of calcium carbide is particularly advantageous because the addition of calcium can provide a desulfurization effect after adding carbon. Adding silicon carbide is also particularly advantageous because it enables an increase in the silicon content of the pig iron after the action of carbon. Adding a mixture of calcium carbide and silicon carbide is even more advantageous because it provides the addition of carbon and silicon while ensuring desulfurization.

Claims

1. A method for producing pig iron in an electric smelting furnace (13) equipped with a container (20), comprising the following continuous steps: - Loading a DRI product into the container (20); - Melting the DRI product to form a pig iron layer (14) covered with a slag layer (23); and - Directly injecting a carbon-containing material into the pig iron layer (14). A method including the above steps.

2. The method according to claim 1, wherein the smelting furnace (13) is provided with at least one bottom tuyere (27) provided in such a container (20), through which the carbon-containing material is directly injected into the pig iron layer (14).

3. The method according to claim 1 or 2, wherein the carbon-containing material is injected in an amount sufficient to reach a final carbon content of 4.0 to 4.5 wt% in the pig iron layer (14).

4. The method according to any one of claims 1 to 3, wherein the carbon-containing material is injected together with a carrier gas.

5. The method according to any one of claims 1 to 4, wherein the carbon-containing material is selected from coke, anthracite, silicon carbide, calcium carbide, carbon resulting from the combustion of biomass, or any mixture of these materials.

6. The method according to any one of claims 1 to 5, wherein the injected carbon-containing material has particles with a particle size of less than 3 mm.

7. The method according to claim 6, wherein 70 to 80% of the particles have a particle size of 75 μm or less, and the remaining particles have a particle size of 2 mm or less.

8. The method according to claim 7, wherein the carbon-containing material is pre-mixed with an iron source and formed into a composite briquette, which is injected into the pig iron layer (14).

9. The method according to any one of claims 1 to 8, wherein the DRI product is produced using a reducing gas containing at least 50 vol% hydrogen before being loaded into the smelting furnace (13).

10. The method according to any one of claims 1 to 9, wherein a silicon-containing material and / or a desulfurization reagent is added to the carbon-containing material and injected into the pig iron layer (14).

11. A method for producing steel, wherein the pig iron produced according to any one of claims 1 to 9 is transferred from the smelting furnace (13) to a converter (17), and then the carbon content of the pig iron is reduced to a value of less than 2.1 wt% by oxygen blowing to obtain molten steel.

12. A method for producing steel according to claim 10, wherein iron scrap is added to the pig iron in the converter (17) and melted.

13. The method according to claim 10 or 11, wherein the pig iron is transferred from the steelmaking furnace (13) to the desulfurization station (15) and then to the converter (17).

14. An electric steelmaking furnace (13) for producing pig iron (14), comprising a container (20), wherein the container (20) is provided with a bottom tuyere (26) designed to directly inject a carbon-containing material into the pig iron contained in the container (20).

Images