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

Abstract

The present invention deals with 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 vessel 20 with a DRI product, - melting said DRI product to form a pig iron layer 14 covered with a slag layer 23, - tapping said pig iron 14 into a ladle, and - adding silicon-bearing material directly to said pig iron 14 in at least one runner of said smelting furnace taphole 25. The present invention also deals with the production of steel from said pig iron and 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 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 MIDREX1®, 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. Summary of the Invention [Problem to be solved by the invention]

[0005] 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]

[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 7 considered separately or in any possible technical combination.

[0008] The invention also deals with a method for producing a steel according to claim 8.

[0009] Such a method may also comprise the optional features of claim 9 or 10 considered separately or in any possible technical combination.

[0010] The invention also deals with a smelting furnace as claimed in claim 11.

[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] Figure 1 shows the pig iron and steelmaking process via the smelting / BOF route. [Figure 2] Figure 2 shows the smelting furnace. 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 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 is feasible because it does not emit the CO2 produced.

[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 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.

[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 is then transferred to the pig iron ladle through at least one tap hole 25 provided with at least one runner 26. Such tap holes 25 are located in the lower part of the vessel 20. They may also be located in the side walls of the vessel or in its bottom wall. There are usually as many auxiliary runners as there are tap holes, which intersect to form a main runner that directs the extracted pig iron to the pig iron ladle.

[0024] This pig iron ladle may be a simple ladle, but may also be a torpedo ladle.

[0025] The pig iron 14 can optionally be sent to a desulfurization station 15 to perform a desulfurization step. This desulfurization step can be performed in a dedicated vessel or, preferentially, directly in the pig iron ladle to avoid molten metal transfer and associated heat losses. 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 therefore is 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 and / or magnesium carbide, to the pig iron, for example by injecting these reagents into the pig iron ladle. The desulfurized pig iron 16 has a sulfur content preferentially less than 0.03% by weight, preferably 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 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.

[0033] In the present invention, silicon-containing material is added to the pig iron in at least one of the runners 26 of the smelting furnace taphole 25. 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 in the converter. This reaction is exothermic, thus providing additional energy for scrap melting. The more scrap is used, the smaller the environmental footprint of the process.

[0034] This addition can be by top-feed addition, which is a low-cost operation, or via injection devices such as submerged lances, which provide high yields of up to 90% or more.

[0035] The inventors have observed that by adding silicon to the pig iron at that stage, the gradual addition during tapping allows for good mixing with the pig iron, and then benefits from strong natural mixing when the pig iron is tapped into the ladle.

[0036] 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.

[0037] 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.

[0038] In a preferred embodiment, the carbon-containing material may be injected into the pig iron along with the silicon-containing material.

[0039] 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. %.

[0040] 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.

[0041] 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.

[0042] The addition of silicon carbide is particularly advantageous since it allows for the addition of silicon while still allowing for an increase in the carbon content of the pig iron. The addition of a mixture of calcium carbide and silicon carbide is even more advantageous since it provides the addition of carbon and silicon while ensuring desulfurization.

Claims

1. 1. A method for producing pig iron in an electric smelting furnace (13) comprising a vessel (20) provided with a taphole (25), the method comprising the steps of: - loading said container (20) with DRI product, - melting said DRI product to form a pig iron layer (14) covered with a slag layer (23); - tapping the pig iron (14) into a ladle; and - adding silicon-containing material directly to the pig iron (14) in at least one runner of the smelting furnace taphole (25), A method comprising:

2. 10. The method of claim 1, wherein the silicon-containing material is injected in an amount sufficient to reach a final silicon content of 0.1 to 0.4 wt. % in the pig-iron layer (14).

3. 3. The method of claim 1 or 2, wherein the silicon-containing material is injected through a submerged lance.

4. 4. The method according to any one of claims 1 to 3, wherein the silicon-containing material is selected from among silicon metal Si, silicon carbide SiC, silico-manganese SiMn, calcium silicate SiCa, ferrosilicon alloy FeSi or a mixture of any of these materials.

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

6. 6. The method according to any one of claims 1 to 5, 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).

7. The method of any one of claims 1 to 6, wherein the carbon-containing material is added to the silicon-containing material and injected into the pig iron (14).

8. 8. A method for producing steel, wherein pig iron produced according to any one of claims 1 to 7 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.

9. 9. A method for producing steel according to claim 8, wherein iron scrap is added to the pig iron and melted in the converter (17).

10. 10. The method according to claim 8 or 9, wherein the pig iron is transferred from the smelting furnace (13) to a desulfurization station (15) and then to the converter (17).

11. 1. An electric smelting furnace for producing pig iron (14), comprising a vessel (20) provided with a taphole (25) coupled to a runner (26) that allows the produced pig iron to be tapped into a pig iron ladle, said furnace (13) further comprising injection means that allows the direct injection of silicon-bearing material into the pig iron (14) at at least one runner of said smelting furnace taphole (25).

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