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 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 carbon-containing material directly into said pig iron layer 14. The present invention also relates to the production of steel from said pig iron and 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 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 in the production of coke from coal in the coke plant and in the production of hot metal.

[0003] The second main route involves the so-called "direct reduction processes", among them those under the brands MIDREX®, FINMET®, ENERGIRON® / HYL, COREX®, FINEX®, etc., 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.

[0005] Another option is to use an electrically powered smelting furnace to melt the DRI product to produce pig iron. This option has the advantage that pig iron is produced in a blast furnace, allowing for oxide removal in the molten slag, thus allowing for the use of classic steel processing tools such as basic oxygen furnaces and refining ladles. However, the pig iron obtained via this route has a relatively low carbon content compared to classic pig iron. This paradoxically reduces the environmental benefits of this route, since the higher the carbon content, the more recycled scrap metal can be added to the BOF. Summary of the Invention [Problem to be solved by the invention]

[0006] It is therefore an object of the present invention to ameliorate the shortcomings of the pig iron and steel production route by providing a new route that efficiently minimizes the environmental impact of such production. [Means for solving the problem]

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

[0008] Such a method may also comprise the optional features of claims 2 to 11 considered separately or in any possible technical combination.

[0009] The invention also deals with a method for producing steel according to claim 12.

[0010] Such a method may also comprise the optional features of claims 13-14 considered separately or in any possible technical combination.

[0011] The invention also relates to an electric smelting furnace according to claim 15.

[0012] 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]

[0013] [Figure 1] Figure 1 shows the pig iron and steelmaking process via the smelting / BOF route. [Figure 2] Figure 2 shows the smelting furnace. [Figure 3] FIG. 3 shows an embodiment of the method according to the invention. DETAILED DESCRIPTION OF THE INVENTION

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

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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% by weight, typically in the range of 2-3% by weight.

[0019] In another preferred embodiment, the DRI product used in the process according to the invention is produced by a so-called H2-DRI process in which the reducing gas contains more than 50% by volume of hydrogen, 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.

[0020] In preferred embodiments, the hydrogen used for 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 specifically includes electricity from renewable sources, which is defined as energy collected from renewable resources that are 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 can be used because no CO2 emissions are produced.

[0021] 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.

[0022] 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.

[0023] 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 in the same location, 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.

[0024] 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.

[0025] The pig iron 14 can then optionally be transferred to a desulfurization station 15 to perform a desulfurization step. This desulfurization step is necessary for the production of steel grades requiring a low sulfur content, e.g., a maximum of 0.03 weight percent sulfur. Desulfurization under oxidizing conditions is ineffective and is therefore preferentially performed either on the pig iron before oxygen refining or in the steel ladle after steel deoxidation. For very low sulfur contents, e.g., less than 0.004 weight percent sulfur, deoxidation and desulfurization are combined for overall higher performance. Therefore, low sulfur grades benefit from pig iron desulfurization before the conversion step.

[0026] The 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, to the pig iron. This can be done, for example, by injecting these reagents into the pig iron that has previously been transferred to a ladle. This ladle can be a simple ladle, such as the one shown in FIG. 2, but can also be a torpedo ladle. The desulfurized pig iron 16 preferentially has a sulfur content of less than 0.004% by weight.

[0027] The desulfurized pig iron 16 can then be transferred into a converter 17, which essentially converts the molten metal into molten steel by blowing oxygen into it 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.

[0028] The molten steel 19 thus formed can then be transferred whenever necessary to one or more secondary metallurgical tools 20A, 20B, such as ladle furnaces, Ruhrstahl-Heareus (RH) vacuum vessels, vacuum tank degassing units, alloying and stirring stations, etc., and processed to reach the required steel composition according to the steel grade to be produced. The molten steel having the required composition 21 can then be transferred to a casting plant 22, where it can be converted into solid products such as slabs, billets, blooms, or strip.

[0029] 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.

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

[0031] 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.

[0032] 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.

[0033] 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.

[0034] As explained above, 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, the pig iron should preferentially 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. %.

[0035] In fact, carbon is necessary for the steelmaking process, which is carried out in converter 17 with oxygen blowing. This is because the reaction between carbon and oxygen produces carbon monoxide gas, which results in a powerful and efficient stirring of the molten metal and therefore improves the removal of impurities from the steel. This reaction is exothermic and therefore provides additional energy for the melting of scrap iron, making it possible to incorporate larger amounts of such scrap iron coming from steel recycling. The more scrap iron is used, the smaller the environmental footprint of the steelmaking process.

[0036] In the mechanism of the present invention, the carbon-containing material is added directly into the pig iron layer 14 in the smelting furnace 13. This addition can be done though an injection device.

[0037] It has been observed by the inventors that by injecting carbon directly into the pig iron layer 14, the carbonization process can reach very high yields of over 80%. In fact, the slag layer 23 has a thick layer thickness that can exceed 50 cm, and the density of the carbon source is usually lower than the slag density itself. This causes physical limitations for carbon to pass through the slag and enter the pig iron layer 14.

[0038] Furthermore, direct injection of carbon ensures optimal energy efficiency of the steelmaking process, since carbonization requires a large amount of energy, which can best be provided by electrical heating in the smelting furnace rather than by additional heating stations.

[0039] Finally, increasing the carbon content of the pig iron at the smelter reduces the liquidus temperature of the pig iron, allowing for lower tap temperatures.

[0040] 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.

[0041] In a preferred embodiment, a lance is inserted between the electrodes of the smelting furnace 13 to inject carbon 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.

[0042] In a preferred embodiment, to avoid clogging of the injection equipment, the carbon 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.

[0043] 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.

[0044] 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 size of 2 mm or less.

[0045] 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.

[0046] In a preferred embodiment, the iron source can be selected from among dust or sludge from an electric furnace, converter or smelter, slag from an electric furnace or converter, or any iron-rich waste material from the steel manufacturing route.

[0047] In a preferred embodiment, silicon-containing materials may be injected into the pig iron layer 14 together with the carbon-containing materials. Silicon has a strong deoxidizing ability at high temperatures, especially 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 during the conversion of pig iron to steel. This reaction is exothermic, thus providing additional energy for scrap melting in the converter. The presence of some silicon can further improve the performance of the desulfurization operation.

[0048] 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.

[0049] The use of DRI products in the smelting furnace 13 typically results in natural amounts of silicon of 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 additions of silicon can be made in the desulfurization station 15 and / or the converter 17.

[0050] In a preferred embodiment, desulfurization reagents, with or without silicon addition, may also be injected together with the carbon-containing material. Such reagents may be based in particular on calcium compounds such as sodium carbonate, lime and / or calcium carbide.

[0051] 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.

[0052] 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.

[0053] It should be noted that the addition of calcium carbide is particularly advantageous because the addition of calcium provides a desulfurization effect in addition to the addition of carbon. The addition of silicon carbide is also particularly advantageous because it allows the silicon content of the pig iron to be increased in addition to the action of carbon. The addition of 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) 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 carbon-containing 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 carbon-containing material directly into the pig iron layer (14).

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

4. The method of any one of claims 1 to 4, 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).

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

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

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

8. 8. The method of claim 7, wherein 70-80% 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.

9. 7. The method of claim 6, wherein the carbon-containing material is premixed with an iron source and formed into a composite briquette that is poured into the pig iron layer (14).

10. 10. The method according to any one of claims 1 to 9, 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).

11. The method according to any one of the preceding claims, wherein silicon-containing material and / or desulfurization reagents are added to the carbon-containing material and injected into the pig-iron layer (14).

12. 11. A method for producing steel, wherein pig iron produced according to any one of claims 1 to 10 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.

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

14. 13. The method according to claim 11 or 12, wherein the pig iron is transferred from the smelting furnace (13) to a desulfurization station (15) and then to the converter (17).

15. 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 direct injection of a carbon-containing material into a layer of pig iron (14) contained in the vessel (20).

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