Method for producing pig iron in a production line equipped with an 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, - transferring the pig iron 14 to the desulphurization station 15, and - injecting a carbon-containing material into the pig iron 14 at the desulphurization station 15. The present invention also relates to a method for producing steel from said pig iron.

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 when producing coke from coal in a coke plant and when producing hot metal.

[0003] The second main process involves the so-called "direct reduction process". Among them are methods under brands such as 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 steel manufacturers 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, an investment in new molten steel treatment tools is required.

[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.

Summary of the Invention

Problems to be Solved by the Invention

[0006] Therefore, an object of the present invention is to improve the drawbacks of the manufacturing processes of pig iron and steelmaking by providing a new process that efficiently minimizes the environmental impact of such manufacturing.

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 any optional features of claims 2 - 9, considered separately or in any possible technical combination.

[0009] The present invention also addresses a method for producing steel as described in claim 10.

[0010] Such a method can also include the optional features of claim 11.

[0011] Other features and advantages of the present invention will become apparent from the following description of the invention, given by way of indication and with reference to the accompanying drawings, which are in no way limiting.

Brief Description of the Drawings

[0012]

Figure 1

Figure 2

Mode for Carrying Out the Invention

[0013] The elements in the figure are illustrative and may not be drawn to scale.

[0014] Figure 1 shows the steel manufacturing 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.

[0015] 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.

[0016] 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 excrement and biological-derived materials in domestic sewage. In the context of the present invention, biomass may also include plastic residues such as recycled waste plastics like solid waste fuel or SRF.

[0017] Whenever natural gas or biogas is used as the reducing gas, the carbon content of the DRI product can be set to a maximum of 3 wt%, usually in the range of 2 - 3 wt%.

[0018] 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 electricity. CO2-neutral electricity includes, in particular, electricity from renewable sources defined as energy collected from renewable resources. It is naturally replenished on a human time scale, including sources such as sunlight, wind, rain, tides, waves and geothermal heat. In some embodiments, the use of electricity from nuclear sources can be used because it does not emit the CO2 being produced.

[0019] 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.

[0020] The DRI product can be transferred to the smelting furnace in various forms. Preferably, the direct reduced iron product (DRI product) is 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, and is fed to the smelting furnace.

[0021] 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. When hot charging is not possible, for example, when the direct reduction plant 11 and the steelmaking furnace 13 are not in the same location, or when 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 performed.

[0022] The steelmaking furnace 13 uses the electrical energy provided by several electrodes to melt the DRI product 12 and produce pig iron 14. In a preferred embodiment, some or all of the required power is derived from CO2-neutral power. A more detailed description of the steelmaking furnace will be described later with reference to FIG. 2.

[0023] The pig iron 14 is then transferred to a pig iron ladle through at least one tap hole 25 provided with at least one runner 26. Such tap holes 25 are arranged at the lower part of the container 20. They may be arranged on the side wall or the bottom wall of the container. Usually, there are the same number of auxiliary runners as tap holes, and the runners intersect to form a main runner to guide the extracted pig iron to the pig iron ladle.

[0024] This pig iron ladle may be a simple ladle or a torpedo ladle.

[0025] Pig iron 14 is sent to a desulfurization station 15 to carry out a desulfurization step. This desulfurization step can be carried out directly in a dedicated container or, preferably, in a pig iron ladle in order to avoid the transfer of the molten metal and the associated heat losses. 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 wt%. Desulfurization under oxidizing conditions is not effective and is therefore preferably carried out either on the pig iron before oxygen refining or in the steel ladle after steel deoxidation. For very low sulfur contents, for example less than 0.004 wt%, deoxidation and desulfurization are combined for overall higher performance. Thus, the low sulfur grades benefit from carrying out pig iron desulfurization before the conversion step.

[0026] Desulfurization of the pig iron can be carried out by adding reagents based on calcium 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 these reagents into the pig iron ladle. The desulfurized pig iron 16 preferably has a sulfur content of less than 0.03 wt%, preferably less than 0.004 wt%.

[0027] The desulfurized pig iron 16 can then be transferred to a 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 in order to benefit from the heat released by the exothermic reaction resulting from the oxygen injection into the pig iron.

[0028] Next, the molten steel 19 thus formed can be transferred to one or more secondary metallurgy tools 20A, 20B such as, for example, a ladle furnace, an RH (Ruhrstahl-Heareus) vacuum vessel, a vacuum tank degassing device, an alloying and stirring station, etc., and processed to reach the steel composition required according to the steel type to be manufactured. Then, the molten steel 21 having the required composition can be transferred to the casting plant 22, where it can be changed into solid products such as slabs, billets, blooms or strips.

[0029] 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 is 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.

[0030] The electrodes 22 provide the electrical energy necessary to melt the charged raw materials to form pig iron. These are preferably Soederberg type electrodes.

[0031] During the melting of the raw materials, two layers are formed: a layer of pig iron 14, which has the highest density and is thus located at the bottom of the container 20, and a slag layer 23 located above the pig iron 14. The slag layer 23 can be partially covered by a pile of raw materials 24 waiting to be melted.

[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 carbon content of the pig iron is preferably as close as possible to 4.5 wt%. This is the saturation level. In a preferred embodiment, the carbon content of the pig iron ranges from 4.0 to 4.5 wt%.

[0034] In fact, carbon is required for the steelmaking process carried out in the converter 17 by blowing oxygen. This is because carbon monoxide gas is generated by the reaction of carbon and oxygen. This vigorously and efficiently stirs the molten metal, thus improving the removal of impurities from the steel. This reaction is exothermic and thus provides additional energy for melting iron scrap, making it possible to incorporate a larger amount of such iron scrap resulting from steel recycling. 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 added to the pig iron during the desulfurization step. This addition can be carried out via an injection device.

[0036] It has been observed by the inventors that by injecting carbon into the pig iron at that stage, complete dissolution of carbon can be achieved with good yield before the pig iron reaches the converter 17. Furthermore, this addition improves the efficiency of the desulfurization process by maintaining strong local deoxidation conditions.

[0037] In a preferred embodiment, the injection is carried out using a lance commonly used for injecting desulfurization reagents.

[0038] Carbon-containing materials can be derived from different sources. This can be selected, for example, from 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 load. In particular, biochar can be used. Adding calcium carbide is particularly advantageous because calcium atoms can provide a desulfurization effect.

[0039] 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.

[0040] 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.

[0041] In a preferred embodiment, the iron source can be selected from sludges from electric furnaces, converters or foundries, slags from electric furnaces or converters, DRI fines, iron fines, or any iron-rich waste from the iron or steel manufacturing route.

[0042] In a preferred embodiment, the silicon-containing material may be injected at the desulfurization station 15 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 the converter. Silicon reacts with oxygen and then contributes to slag formation in the molten steel manufacturing step. This reaction is exothermic and thus provides additional energy for scrap melting. Silicon can further improve the performance of the desulfurization operation itself.

[0043] 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.

[0044] 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 converter 17.

[0045] Adding silicon carbide is particularly advantageous because it enables an increase in the silicon content of the pig iron after adding 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 a production line comprising an electric smelting furnace (13) and a desulfurization station (15) including a container (20), 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); - Transferring the pig iron (14) to the desulfurization station (15); and - Injecting a carbon-containing material into the pig iron (14) at the desulfurization station (15). A method comprising the above steps.

2. The method according to claim 1, 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).

3. The method according to claim 1 or 2, wherein the carbon-containing material is injected together with a carrier gas.

4. The method according to any one of claims 1 to 3, wherein the carbon-containing material is selected from coke, anthracite, silicon carbide, calcium carbide, biomass, carbon resulting from the combustion of biomass, or any mixture of these materials.

5. The method according to any one of claims 1 to 4, wherein the injected carbon-containing material has particles with a particle size of less than 3 mm.

6. The method according to claim 5, 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.

7. The method according to claim 4, 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 (14).

8. The method according to any one of claims 1 to 7, wherein the DRI product is produced using a reducing gas containing at least 50% by volume of hydrogen before being loaded into the smelting furnace (13).

9. The method according to any one of claims 1 to 8, wherein a silicon-containing material is added to the carbon-containing material and injected into the pig iron (14).

10. 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.

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

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