Method for producing liquid pig iron in electric smelting furnace

Abstract

The invention relates to a method for producing liquid pig iron in an electric smelting furnace 13, said method comprising the following sequential steps:-providing a direct reduced iron product 12,-providing a material 30 comprising carbon and iron,-alternately feeding the DRI product 12 with the material 30 comprising carbon and iron to at least a part of the smelting furnace, melting the DRI product 12 and the material 30 comprising carbon and iron in order to produce liquid pig iron 14. The invention also relates to the production of steel from said pig iron.

Description

Technical Field

[0001] The present invention relates to a method of making pig iron (also called hot metal), and to a method of producing steel from such pig iron. Background Art

[0002] Steel can be produced by two main manufacturing routes. Today, the most commonly used production route, known as the "BF-BOF route", involves producing hot metal in a blast furnace by using a reducing agent (mainly coke) to reduce iron oxides, which is then converted into steel in a converter process or basic oxygen furnace (BOF). This route releases large amounts of CO2 both when producing coke from coal in a coking plant and when producing hot metal.

[0003] The second major route involves the so-called "direct reduction process". / HYL, In these processes, sponge iron is produced in the form of HDRI (hot direct reduced iron), CDRI (cold direct reduced iron) or HBI (hot briquetted iron) by direct reduction of an iron oxide carrier. Sponge iron in the form of HDRI, CDRI and HBI undergoes further processing in an electric furnace to produce steel.

[0004] Therefore, one of the main options chosen by steelmakers to reduce CO2 emissions is to switch from the BF-BOF route to the DRI route. However, the use of DRI products with ferrous scrap in a classic electric furnace has some limitations. In fact, the scrap contains a lot of impurities and the resulting steel will need to be further processed to produce high-quality steel grades. Therefore, investments in new steel treatment equipment will be necessary.

[0005] Another option is to melt the DRI product using a smelting furnace powered by electrical energy to produce pig iron. This option has the advantage that the pig iron is produced as in a blast furnace, which allows the removal of oxides in the slag and thus the use of classical steel treatment equipment such as basic oxygen furnaces and iron ladles. However, the carbon content of the pig iron obtained by this route is relatively low compared to classical pig iron. This paradoxically reduces the environmental benefits of this route, since the higher the carbon ratio, the higher the possibility of adding recycled scrap metal in the BOF. Summary of the invention

[0006] It is therefore an object of the present invention to remedy the disadvantages of pig iron and steelmaking manufacturing by providing a new approach that effectively minimizes the environmental impact of such manufacturing approaches.

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

[0008] Such a method may also comprise the optional features of claims 2 to 10 considered alone or in any possible technical combination.

[0009] The invention also relates to a method for producing steel according to claim 11 .

[0010] Such a method may also comprise the optional features of claim 12 or 13 considered alone or in any possible technical combination. BRIEF DESCRIPTION OF THE DRAWINGS

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

[0012] - Figure 1 shows the pig iron and steel making process according to the smelting / BOF route,

[0013] - Figure 2 shows a melting furnace,

[0014] - Figure 3 Feedstock loading according to one embodiment of the present invention is shown.

[0015] Elements in the drawings are illustrative and may not be drawn to scale. DETAILED DESCRIPTION

[0016] Figure 1 The steel production route according to the DRI route is shown, from the reduction of iron to the casting of steel into semi-finished products such as slabs, billets, blooms or strips.

[0017] The iron ore 10 is first reduced in a direct reduction facility 11. The direct reduction facility 11 may be designed to implement any type of direct reduction technology, such as Technology or The direct reduction process may be, for example, a conventional natural gas-based or biogas-based process.

[0018] In a preferred embodiment, the DRI product used in the process according to the invention is produced using a reducing gas based on biogas from the combustion of biomass.

[0019] Biomass is a renewable organic material from plants and animals. Biomass sources include wood and wood processing waste (e.g., firewood, wood particles and wood chips, wood and furniture factory sawdust and waste), and black liquor from pulp and paper mills, agricultural crops and waste materials (e.g., corn, soybeans, sugar cane, switchgrass, woody plants and algae), and crop and food processing residues, and also include biogenic materials in municipal solid waste (e.g., paper, cotton and wool products), and food, yard and wood waste, animal manure and human filth. In the sense of the present invention, biomass can also encompass plastic residues, such as recycled waste plastics, such as solid refuse fuel (Solid Refuse Fuel) or SRF.

[0020] 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 %, and is typically set in the range of 2 wt % to 3 wt %.

[0021] 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, and preferably more than 60%, 70%, 80% or 90% hydrogen, or even consists entirely of hydrogen. The H2-DRI product will contain carbon at a level much lower than that of natural gas or biogas DRI, and therefore typically less than 1 wt.%, or even lower.

[0022] 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 specifically includes electricity from renewable sources, which is defined as energy collected from renewable sources that replenish naturally on a human time scale, including sources like sunlight, wind, rain, tides, waves, and geothermal heat. In some embodiments, the use of electricity from nuclear sources can be used because they do not produce CO2 emissions.

[0023] Regardless of the DRI process used, the resulting direct reduced iron (DRI) product 12 is then charged to a smelting furnace 13 where reduction of the iron oxides is completed and the product is melted to produce pig iron.

[0024] The DRI product may be transferred to the smelting furnace in various forms. Preferably, the direct reduced iron product (DRI product) is fed to the smelting furnace in hot form as a HDRI product (so-called hot DRI), or in cold form as a CDRI product (so-called cold DRI), or in hot briquetted form as a HBI product (so-called hot briquetted iron) and / or in pelletized form (preferably having an average particle size of at most 10.0 mm, more preferably having an average particle size of at most 5.0 mm).

[0025] It is preferably loaded directly at the outlet of the direct reduction facility 11 as a hot product at a temperature of 500 to 700° C. This allows reducing the amount of energy required to melt it. When hot loading is not possible, for example if the direct reduction facility 11 and the smelting furnace 13 are not co-located, or if the smelting furnace 13 is out of operation for maintenance and the DRI product must therefore be stored, the DRI product can be loaded cold or a preheating step can be carried out.

[0026] The smelting furnace 13 utilizes electrical energy provided by several electrodes to melt the DRI product 12 and produce pig iron 14. In a preferred embodiment, part or all of the required electricity comes from CO2 neutral electricity. Figure 2 A further detailed description of the smelting furnace is given.

[0027] The pig iron 14 may optionally be sent to a desulfurization station 15 for a desulfurization step. This desulfurization step may be carried out in a dedicated container or preferably directly in the pig iron ladle to avoid liquid metal transfer and associated heat losses. This desulfurization step is necessary for the production of steel grades requiring low sulfur content (which is set, for example, to a maximum value of 0.03 weight percent sulfur). Desulfurization under oxidizing conditions is ineffective and therefore desulfurization of the pig iron is preferably carried out before oxygen refining or in a ladle after deoxidation of the steel. For very low sulfur contents (e.g., less than 0.004 weight percent sulfur), deoxidation and desulfurization are combined for higher overall performance. Therefore, desulfurization of the pig iron before the conversion step is beneficial for low sulfur grades.

[0028] Desulfurization of pig iron can be accomplished by adding reagents, in particular reagents based on calcium or magnesium compounds, such as sodium carbonate, lime, calcium carbide and / or magnesium, to the pig iron. For example, it can be accomplished by injecting those reagents into a pig iron ladle. The sulfur content of the desulfurized pig iron 16 is preferentially lower than 0.03% by weight, and preferably lower than 0.004% by weight.

[0029] The desulfurized pig iron 16 can then be transferred to a converter 17. The converter essentially transforms liquid metal into molten steel by blowing oxygen through the liquid metal to decarbonize it. It is commonly referred to as a basic oxygen furnace (BOF). Iron-containing scrap 18 from steel recovery can also be charged to the converter 17 to utilize the heat released by the exothermic reaction created by injecting oxygen into the pig iron.

[0030] The molten steel 19 thus formed can then be transferred to one or more secondary metallurgical equipment 20A, 20B, such as a ladle furnace, a RH (Ruhrstahl-Heareus) vacuum vessel, a vacuum tank degasser, an alloying and mixing station, etc., to be processed according to the steel grade to be produced to achieve the desired steel composition. The molten steel 21 with the desired composition can then be transferred to a casting facility 22, where it can be transformed into a solid product, such as a slab, billet, bloom or strip.

[0031] like Figure 2 As shown, the smelting furnace 13 is composed of a container 20 capable of containing molten iron. For example, the container 20 can have a circular or rectangular shape. The container 20 is closed by a top, which is provided with some holes to receive electrodes 22 to be inserted into the container 20, and is provided with other holes to allow raw materials to be loaded into the container 20.

[0032] The container 20 is also provided with at least one tap hole 25 to allow the produced pig iron to be tapped. Such tap holes 25 are located in the lower part of the container 20. They can be located in the side walls of the container or in its bottom wall.

[0033] Electrodes 22 provide the electrical energy required to melt the loaded raw materials and form pig iron. They are preferably During the melting of the raw materials, two layers are formed, the layer of pig iron 14 which is the densest and therefore located at the bottom of the container 20, and the slag layer 23 located above the pig iron 14. The slag layer 23 can be partially covered by the pile of raw materials 24 waiting to be melted.

[0034] The smelting furnace 13 may be a SAF (Submerged-Arc Furnace) in which the electrodes are immersed in a slag layer 23, or an OSBF (open-slag bath furnace) in which the electrodes 22 are located above the slag layer 23. It is preferably an OSBF as shown in the drawings.

[0035] As explained above, the carbon content of pig iron 14 produced by the DRI route will generally be less than 3 wt%. However, in order to meet the requirements of the subsequent steelmaking process at the converter, the carbon content of the pig iron should preferably be as close to 4.5 wt%, which is the saturation level, as possible. In a preferred embodiment, the pig iron carbon content is in the range of 4.0 wt% to 4.5 wt%.

[0036] In fact, carbon is essential 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 an intense and effective stirring of the liquid metal and thus improves the removal of impurities from the steel. The reaction is exothermic and thus provides additional energy for melting the iron-containing waste, allowing the incorporation of larger amounts of such iron-containing waste from steel recycling. The more iron-containing waste is used, the smaller the environmental footprint of the steelmaking process.

[0037] In the framework of the invention, during the loading of the container 20, the DRI product 12 and the material 30 containing carbon and iron are alternately loaded.

[0038] The result of this alternating feeding is schematically shown in Figure 3 The raw material pile 24 consists of alternating layers of carbon and iron containing material 30 and DRI product 12. This is only a schematic diagram, as it is clear to a person skilled in the art that the actual raw material pile is not so flat and the boundaries between the different layers are blurred. During melting, FeC droplets will be formed and transported towards the pig iron 14 layer by the natural slag 23 circulation.

[0039] As an example, the smelting furnace 13 is provided with a loading device comprising at least two hoppers connected to at least one hole designed into the top of the vessel 20. At least one hopper contains the DRI product 12, and at least another hopper contains the material containing carbon and iron 30. When the level of liquid metal is low, the first hopper is opened to charge DRI into the vessel 20, and the second hopper remains closed. The first hopper is then closed, and the second hopper is opened to add the required amount of material containing carbon and iron to reach the required carbon content in the pig iron, which, as previously explained, is as close as possible to 4.5% by weight.

[0040] This method of adding carbon allows the overall density of the feedstock to be brought close to that of a 100% DRI charge, so the presence of material containing carbon and iron will not impair the downward movement of the charge during melting. In addition, the presence of iron will protect the carbon from burning before it melts, thereby increasing carbon yield and reducing carbon emissions.

[0041] The carbon is then transported in the DRI stack by gravity feeding through the slag layer and into the liquid metal. The slag layer 23 has a high thickness which can be greater than 50 cm and the density of the carbon source is usually lower than the density of the slag itself. This causes physical limitations for the carbon to pass through the slag into the pig iron layer 14. With the method according to the invention this limitation can be overcome.

[0042] Furthermore, injecting carbon during the melting phase ensures optimal energy efficiency of the steelmaking process, since carburization requires a large amount of energy which can be optimally provided by electrical heating in the melting furnace rather than by an additional heating station.

[0043] The material containing carbon and iron is preferably a mixture of a carbon source and an iron source.

[0044] The carbon source can be selected from, for example, coke, anthracite, silicon carbide, calcium carbide or a mixture of any of those sources, but can also advantageously come from a renewable source, such as biomass, for part or all of the carbon load. In particular, biochar can be used. Adding calcium carbide is particularly advantageous because the calcium atom can provide a desulfurization effect.

[0045] The iron source may be selected from iron-silicon, iron fines, DRI fines, shredded scrap, steelmaking slag, steelmaking dust, iron oxide scale or steelmaking sludge.

[0046] In a preferred embodiment, the combined carbon and iron containing material 30 is formed such that greater than 50% of the outer surface is composed of the iron material. This configuration allows for improved protection of the carbon from combustion.

[0047] The material containing carbon and iron to be added is preferably of the same size as the DRI product in order to use the same feeding equipment.

[0048] In a preferred embodiment, a desulfurization agent can also be fed together with the material comprising carbon and iron. Such an agent can be based in particular on calcium compounds, such as sodium carbonate, lime and / or calcium carbide.

[0049] The final sulfur content of the pig iron is preferably set to a maximum of 0.03 weight percent and preferably to a maximum of 0.004 weight percent.

[0050] Performing desulfurization in the smelting furnace may allow suppressing the need for a desulfurization treatment between the smelting furnace 13 and the converter 17, or at least reducing such treatment.

[0051] In a preferred embodiment, the silicon-containing material can be loaded with the iron-containing material containing carbon, with or without a desulfurizing agent. Silicon has a strong deoxidizing ability at high temperatures, and in particular about 1600° C., which is the temperature of molten steel in a converter. It reacts with oxygen and then contributes to the formation of slag in the converter. The reaction is exothermic and therefore provides additional energy for the melting of the waste. The more waste used, the smaller the environmental footprint of the process.

[0052] Such silicon can be added in different forms, such as metallic silicon Si, silicon carbide SiC, silicon manganese SiMn, calcium silicate SiCa or iron silicon alloy FeSi, such as FeSi75 or FeSi65.

[0053] The use of DRI products in the smelting furnace 13 will result in an initial amount of silicon generally below 0.2 wt.-% or even below 0.1 wt.-%. The final silicon content of the pig iron is preferably set to a value of 0.1 wt.-% to 0.4 wt.-%, preferably 0.2 wt.-% to 0.4 wt.-%. If necessary, further addition of silicon in the converter 17 can be carried out.

[0054] The addition of silicon carbide is particularly advantageous, as it allows to increase the silicon content of the pig iron in addition to the addition of carbon. The addition of a mixture of calcium carbide and silicon carbide is even more advantageous, as it provides for carbon and silicon addition while ensuring desulfurization.

Claims

1. A method for producing liquid pig iron in an electric smelting furnace (13), comprising the following steps: i) providing a direct reduced iron product (12), ii) providing a material (30) comprising carbon and iron, iii) feeding the DRI product (12) alternately with the material comprising carbon and iron (30) into at least a portion of the smelting furnace, iv) melting the DRI product (12) and the material comprising carbon and iron to produce liquid pig iron (14).

2. The method according to claim 1, wherein the material comprising carbon and iron is injected in an amount sufficient to achieve a final carbon content of 4.0 to 4.5 wt. % in the liquid pig iron (14).

3. The method according to claim 1 or 2, wherein the material containing carbon and iron is a mixture of a carbon source and an iron source.

4. The method according to claim 3, wherein the carbon source is selected from coke, anthracite, silicon carbide, calcium carbide, biomass, carbon from biomass combustion, or a mixture of any of those materials.

5. The method according to claim 3 or 4, wherein the iron source is selected from iron-silicon, iron fines, sintered ore dust, DRI fines, shredded scrap, steelmaking slag, steelmaking dust, iron oxide scale or steelmaking sludge.

6. The method according to any one of claims 1 to 5, wherein the material (30) comprising carbon and iron is formed such that more than 50% of the outer surface consists of iron material.

7. The process according to any one of claims 1 to 6, wherein the DRI product is provided at a temperature of 500°C to 700°C.

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

9. The method according to any one of claims 1 to 8, wherein silicon-containing material is added to the material (30) comprising carbon and iron to be fed into the smelting furnace.

10. The method according to any one of claims 1 to 9, wherein the material comprising carbon and iron (30) and the DRI product (12) are provided as briquettes having the same size.

11. A method for manufacturing steel, wherein the pig iron manufactured according to any one of claims 1 to 10 is transferred from the melting furnace (13) to a converter (17), wherein the carbon content of the pig iron is then reduced to a value below 2.1 weight percent by blowing oxygen to obtain molten steel.

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

13. A method according to claim 11 or 12, wherein the pig iron is transferred from the smelting furnace (13) to a desulphurisation station (15) before being transferred to the converter (17).

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