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

Abstract

The present invention relates to a method for producing pig iron in a smelting furnace 13 comprising a vessel 20, the method comprising the following successive steps: - loading said vessel 20 with a DRI product, - melting said DRI product to form a pig iron layer 14 covered with a slag layer 23, and - injecting a desulfurization reagent directly into said 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.

Description

[Technical Field]

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

[0002] Currently, steel can be produced through two main production routes. Today, the most commonly used production 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 basic oxygen furnace (BOF). This route releases significant amounts of CO2, both during the production of coke from coal in the coke plant and during the production of hot metal.

[0003] The second main process involves the so-called "direct reduction processes", among which are processes under brands such as MIDREX®, FINMET®, ENERGIRON® / HYL, COREX®, FINEX®, in which sponge iron is produced from the direct reduction of an iron oxide support in the form of HDRI (hot direct reduced iron), CDRI (ambient direct reduced iron) or HBI (hot briquette iron). Sponge iron in the form of HDRI, CDRI and HBI is further processed in electric furnaces 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, using DRI products in classic electric arc furnaces together with steel scrap has some limitations. Indeed, the scrap contains many impurities, and the resulting molten steel needs to be further processed to produce high-quality steel grades. Therefore, investments in new molten steel processing tools are required. Summary of the Invention [Problem to be solved by the invention]

[0005] It is therefore an object of the present invention to improve upon the shortcomings of the pig iron and steel manufacturing process by providing a new process that efficiently minimizes the environmental impact of such production without requiring large investments. [Means for solving the problem]

[0006] This problem is solved by a method for producing pig iron as detailed in claim 1.

[0007] Such a method may also comprise the optional features of claims 2 to 9 considered separately or in any possible technical combination.

[0008] The invention also relates to a method for producing steel according to claim 10.

[0009] Such a method may also include the optional features of claim 11.

[0010] The invention also relates to a smelting furnace according to claim 12.

[0011] Other characteristics and advantages of the present invention will become apparent from the description of the invention given below by way of indication and which is in no way limiting, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 shows the pig iron and steelmaking process via the smelting / BOF route. [Figure 2] FIG. 1 shows a smelting furnace. [Figure 3] 1 illustrates an embodiment of a method according to the invention; DETAILED DESCRIPTION OF THE INVENTION

[0013] Elements in the figures are illustrative and may not be drawn to scale.

[0014] Figure 1 shows the steel production 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 strip. Iron ore 10 is first reduced in a direct reduction plant 11. This direct reduction plant 11 can be designed to implement any kind of direct reduction technology, such as the MIDREX® technology or Energiron®. The direct reduction process can be, for example, a traditional natural gas or biogas-based process.

[0015] In a preferred embodiment, the DRI product used in the method according to the invention is produced using a reducing gas based on biogas resulting 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 firewood, wood pellets and wood chips, sawdust and waste from sawmills and furniture factories, and black liquor from pulp and paper mills, agricultural crops and wastes, such as corn, soybeans, sugarcane, switchgrass, woody plants and algae, and crop and food processing residues, but also municipal solid waste, such as paper, cotton and wool products, as well as food, yard and wood waste, animal waste, and biological materials in domestic wastewater. In the sense of the present invention, biomass can also include plastic residues, such as solid waste-to-fuel or recycled waste plastics, such as SRF.

[0017] Whenever natural gas or biogas is used as the reducing gas, the carbon content of the DRI product can be set at up to 3% by weight, typically in the range of 2-3% by weight.

[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% by volume, preferably more than 60, 70, 80, or 90% by volume, or is composed entirely of hydrogen. H2-DRI products contain much lower levels of carbon than natural gas or biogas DRI, typically less than 1% by weight or even lower. In a preferred embodiment, the hydrogen used in the DRI reducing gas is derived from water electrolysis, which is preferably powered in part or entirely by CO2-neutral electricity. CO2-neutral electricity specifically includes electricity from renewable sources, defined as energy collected from renewable resources. It is naturally replenished on human timescales, including sources such as sunlight, wind, rain, tides, waves, and geothermal heat. In some embodiments, the use of electricity from nuclear sources is feasible because it does not produce CO2 emissions.

[0019] Whatever 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 directly reduced iron product (DRI product) is fed to the smelting furnace in hot form as an HDRI product (so-called Hot DRI), or in ambient form as a CDRI product (so-called Cold DRI), or in hot-briquette form as an HBI product (so-called Hot Briquette Iron), and / or in particulate form, preferably having an average particle size of up to 10.0 mm, more preferably up to 5.0 mm.

[0021] It 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 reducing the amount of energy required for melting. If hot charging is not possible, for example if the direct reduction plant 11 and the smelting furnace 13 are not co-located, or if the smelting furnace 13 is shut down for maintenance and the DRI product therefore has to be stored, the DRI product may be charged at room temperature or a preheating step may be performed.

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

[0023] The pig iron 14 can then be transferred into a converter 17, which essentially converts the molten metal into molten steel by blowing oxygen into the metal to decarburize it. This is commonly referred to as a Basic Oxygen Furnace (BOF). Iron scrap 18 resulting from steel recycling may also be charged into the converter 17 to benefit from the heat released by the exothermic reaction resulting from the injection of oxygen into the pig iron.

[0024] The molten steel 19 thus formed can then be transferred as needed to one or more secondary metallurgical tools 20A, 20B, such as, for example, a ladle furnace, a Ruhrstahl-Heareus (RH) vacuum vessel, a vacuum tank degasser, an alloying and stirring station, and processed to reach the required steel composition depending on the steel grade being produced. The molten steel 21 having the required composition can then be transferred to a casting plant 22, where it can be converted into a solid product such as a slab, billet, bloom, or strip.

[0025] As shown in Figure 2, the smelting furnace 13 consists of a vessel 20 capable of containing molten iron. The vessel 20 may be, for example, circular or rectangular. This vessel 20 is closed by a roof provided with several openings for receiving electrodes 22 to be inserted into the vessel 20 and other openings for allowing the charging of raw materials into the vessel 20.

[0026] The electrodes 22 provide the electrical energy necessary to melt the charged raw materials to form pig iron. They are preferably Soederberg type electrodes.

[0027] During melting of the raw materials, two layers are formed: a layer of pig iron 14, which is the densest and therefore located at the bottom of the vessel 20, and a layer of slag 23, which is 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.

[0028] The vessel 20 also includes openings called tap holes 25 located at its bottom and allowing the discharge of the pig iron 14 while retaining most of the slag within the vessel 20. They may be located in the side walls of the vessel or in its bottom wall.

[0029] The smelting furnace 13 may be an SAF (Submerged-Arc Furnace) where the electrode is immersed in the slag layer 23 or an OSBF (Open Slag Bath Furnace) where the electrode 22 is located above the slag layer 23. It is preferably an OSBF as shown in the figure.

[0030] In the mechanism of the present invention, the desulfurization reagent is added directly into the pig iron layer 14 in the smelting furnace 13. This addition can be done though an injection device.

[0031] In fact, the inventors have observed that the reducing conditions of the smelting furnace 13 favor efficient removal of sulfur.

[0032] By carrying out desulfurization in the smelting furnace, the need for desulfurization treatment between the smelting furnace 13 and the converter 17 can be avoided or at least reduced.

[0033] The final sulfur content of the pig iron is preferably set to a maximum of 0.03% by weight, preferably a maximum of 0.004% by weight.

[0034] In a preferred embodiment, the injection device is a lance inserted into an opening made in the roof of the vessel 20. Such a lance passes through the slag layer 23 and opens into the pig iron layer 14 to allow direct addition.

[0035] In a preferred embodiment, a lance is inserted between the electrodes of the smelting furnace 13 to inject the desulfurization reagent into the region where the temperature of the pig iron 14 reaches its maximum, which is typically located in the center of the vessel 20, near the electrodes.

[0036] In a preferred embodiment, to avoid clogging of the injection equipment, the desulfurization reagent is injected together with a carrier gas, which is preferably inert and can be made of nitrogen, argon, helium or carbon monoxide, or any mixture of such gases.

[0037] In a preferred embodiment, silicon-containing materials may be injected into the pig iron layer 14 along with desulfurization reagents. Silicon has strong deoxidizing capabilities at high temperatures, particularly at temperatures of about 1600°C, which is the temperature of molten steel in a converter. Silicon reacts with oxygen and then contributes to the formation of slag in the converter. Furthermore, this reaction is exothermic, thus providing additional energy for scrap melting in the converter. The more scrap is used, the smaller the environmental footprint of the process.

[0038] Such silicon can be added in different forms: it can be silicon metal Si, silicon carbide SiC, silicomanganese SiMn, calcium silicate SiCa, or ferrosilicon alloys FeSi such as FeSi75 or FeSi65.

[0039] The use of DRI products in the smelting furnace 13 results in natural amounts of silicon that are typically less than 0.2% by weight, or even less than 0.1% by weight. The final silicon content of the pig iron is preferentially set to a value of 0.1-0.4% by weight, preferably 0.2-0.4% by weight. If necessary, further addition of silicon can be made in the converter 17.

[0040] In a preferred embodiment, the carbon-containing material may be injected together with a desulfurization reagent, with or without silicon addition.

[0041] The carbon content of pig iron 14 produced via the DRI route is generally less than 3 wt. %. However, to meet the requirements of the subsequent steelmaking process in the converter, it is preferred that the carbon content of the pig iron be as close as possible to 4.5 wt. %. This is the saturation level. In a preferred embodiment, the pig iron carbon content is set in the range of 4.0-4.5 wt. % by addition of carbon-containing materials.

[0042] The carbon-containing material can come from different sources. It can be selected from, for example, coke, anthracite, silicon carbide, calcium carbide or a mixture of any of these sources, but it can also be advantageous to obtain part or all of the carbon load from renewable sources such as biomass. In particular, biochar can be used. The addition of calcium carbide is particularly advantageous, since calcium atoms can provide a desulfurization effect. The addition of silicon carbide is particularly advantageous, since it allows the silicon content of the pig iron to be increased.

[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, with the remaining particles having 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 sludge from an electric furnace, a converter or a smelter, slag from an electric furnace or a converter, or any iron-rich waste material from the steel manufacturing route.

[0046] It should be noted that the addition of calcium carbide is particularly advantageous because the carbide addition provides carbon addition to the pig iron while also desulfurizing it. The addition of a mixture of calcium carbide and silicon carbide is even more advantageous because it provides carbon and silicon addition while ensuring desulfurization.

Claims

1. A method for producing pig iron in an electric smelting furnace (13) comprising a vessel (20), the method comprising the following successive steps: - loading said container (20) with DRI product, - melting said DRI product to form a pig iron layer (14) covered with a slag layer (23); and - injecting a desulfurization reagent material directly into said pig iron layer (14); A method comprising:

2. 2. The method of claim 1, wherein the smelting furnace (13) is provided with a roof fixture on such vessel (20) through which a lance (26) is inserted, the lance being used as an injection device for injecting the desulfurization reagent directly into the pig iron layer (14).

3. 3. The method of claim 1 or 2, wherein the smelting furnace comprises an electrode (22) for melting the DRI product, and the lance (26) is inserted adjacent to the electrode (22).

4. 4. The method of any one of claims 1 to 3, wherein the desulfurization reagent is injected in an amount sufficient to reach a final sulfur content in the pig iron layer (14) of less than 0.02 wt.%.

5. The method according to any one of claims 1 to 4, wherein the desulfurization reagent is injected with a carrier gas.

6. 6. The method according to any one of claims 1 to 5, wherein the desulphurization reagent is selected from sodium carbonate, lime, calcium carbonate, magnesium or a mixture of any of these materials.

7. 7. The method according to any one of claims 1 to 6, wherein the injected desulfurization reagent comprises particles having a particle size of less than 3 mm.

8. 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 silicon-containing and / or carbon-containing materials are added to the desulfurization reagent and injected into the pig iron layer (14).

10. 10. A method for producing steel, wherein 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 below 2.1 wt. % by oxygen blowing to obtain molten steel.

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

12. 1. An electric smelting furnace (13) for producing pig iron (14), comprising a vessel (20) provided with a roof fixture thereon through which a lance (26) is inserted, the lance being designed to allow for the direct injection of a desulfurization agent into a layer of pig iron (14) contained in the vessel (20).

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