Method for producing hot metal in an electric smelting furnace

Abstract

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

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 in the production of coke from coal in a coke plant and in the production of hot metal.

[0003] The second main route includes the so-called "direct reduction processes". Among these are processes such as those by the brands 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 steelmakers 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 to produce pig iron. This option has the advantage that pig iron is produced like 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, the object of the present invention is to improve the drawbacks of the pig iron and steelmaking production routes by providing a new route that efficiently minimizes the environmental impact of such production.

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 - 10 considered separately or in any possible technical combination.

[0009] The present invention also addresses a method for producing steel as described in claim 11.

[0010] Such a method can also include any optional features of claim 12 or 13 considered separately or in any possible technical combination.

[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

Figure 3

Mode for Carrying Out the Invention

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

[0014] Figure 1 shows a 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.

[0015] 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.

[0016] 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.

[0017] Biomass is a renewable organic material derived from plants and animals. Biomass sources include, in particular, wood and wood processing waste, such as sawdust, wood pellets and wood chips, sawdust and waste from sawmills and furniture factories, and black liquor from pulp and paper mills, crops and agricultural waste, 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, and food, yard and wood waste, animal excrement and biological materials in domestic sewage. In the context of the present invention, biomass may also include plastic residues such as recycled waste plastics such as solid waste fuel or 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 wt% and usually in the range of 2 - 3 wt%.

[0019] 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% hydrogen, preferably more than 60, 70, 80 or 90 vol%, 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.

[0020] 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 is defined as electricity from renewable sources, which are naturally replenished on the human time scale, including sources such as sunlight, wind, rain, tides, waves and geothermal energy. In some embodiments, the use of electricity from nuclear sources can be used because it does not emit the CO2 being produced.

[0021] 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 iron oxide is completed, and the product is melted to produce hot metal.

[0022] The DRI product can be transferred to the smelting furnace in various forms. Preferably, the directly reduced iron product (DRI product) is charged directly as a hot product having a temperature of 500°C to 700°C at the outlet of the direct reduction plant 11. This can reduce 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 stopped for maintenance and thus the DRI product has to be stored, the DRI product can be charged at room temperature or a preheating step can be performed.

[0023] 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. 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 stopped for maintenance and thus the DRI product has to be stored, the DRI product can be charged at room temperature or a preheating step can be performed.

[0024] The smelting furnace 13 uses electrical energy provided by several electrodes to melt the DRI product 12 and produce hot metal 14. In a preferred embodiment, some or all of the required power is derived from CO2-neutral power. A more detailed description of the smelting furnace will be given later with reference to Figure 2.

[0025] The pig iron 14 can optionally be sent to a desulphurization station 15 to carry out a desulphurization step. This desulphurization step can be carried out directly in a dedicated container or, preferentially, in the pig iron ladle in order to avoid the transfer of the molten metal and the associated heat losses. This desulphurization step is necessary for the production of steel grades which require a low sulphur content, for example set at a maximum of 0.03 wt%. Desulphurization 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 sulphur contents, for example less than 0.004 wt%, deoxidation and desulphurization are combined for overall higher performance. Thus, low sulphur grades benefit from carrying out pig iron desulphurization before the conversion step.

[0026] Desulphurization 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 ladle. The desulphurized pig iron 16 preferably has a sulphur content of less than 0.03 wt%, preferably less than 0.004 wt%.

[0027] The desulphurized pig iron 16 can then be transferred into 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 can also be charged into the converter 17 in order to benefit from the heat released by the exothermic reaction resulting from oxygen injection into the pig iron.

[0028] Next, the molten steel 19 thus formed can be transferred at any time as required to one or more secondary metallurgy tools 20A, 20B, such as ladle furnaces, RH (Ruhrstahl-Heareus) vacuum vessels, vacuum tank degassing devices, alloying and stirring stations, etc., and can be processed to reach the steel composition required according to the steel grade to be produced. Then, the molten steel having the required composition 21 can be transferred to the casting plant 22, where it can be converted 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 container 20 is also provided with at least one tap hole 25 through which the produced pig iron can be tapped. Such a tap hole 25 is arranged at the lower part of the container 20. These may be arranged on the side wall or the bottom wall of the container.

[0031] The electrodes 22 provide the electrical energy necessary to melt the charged raw materials to form pig iron. These are preferably Soederberg type electrodes.

[0032] During the melting of the raw materials, two layers are formed: a layer of pig iron 14, which is the densest and 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.

[0033] 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.

[0034] As described above, the carbon content of the pig iron 14 produced via the DRI route is generally less than 3% by weight. However, in order to meet the requirements of the subsequent steelmaking process in the converter, the pig iron should preferably have a carbon content as close as possible to the saturation level of 4.5% by weight. In a preferred embodiment, the carbon content of the pig iron is in the range of 4.0 - 4.5% by weight.

[0035] In fact, carbon is required for the steelmaking process carried out in the converter 17 by blowing oxygen. This is because the reaction between carbon and oxygen produces carbon monoxide gas, which provides strong and efficient stirring of 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 such iron scrap resulting from steel recycling in larger quantities. The more iron scrap is used, the smaller the environmental footprint of the steelmaking process.

[0036] In the mechanism of the present invention, during charging into the container 20, the DRI product 12 is charged alternately with the carbon and iron-containing material 30.

[0037] The result of this alternate supply is schematically shown in FIG. 3. The raw material pile 24 is composed of alternate layers of the carbon and iron-containing material 30 and the DRI product 12. This is only a schematic view, and it is obvious to those skilled in the art that the actual pile of raw materials is not so clear and the boundary between different layers is more ambiguous. During melting, FeC droplets are formed and transported towards the layer of pig iron 14 by the circulation of the natural slag 23.

[0038] As an example, the smelting furnace 13 is provided with charging means comprising at least two hoppers connected to at least one opening designed in the roof of the container 20. At least one hopper contains the DRI product 12 and at least one other hopper contains the carbon and iron-containing material 30. When the level of the molten metal is low, the first hopper is opened to charge the DRI into the container 20 and the second hopper remains closed. Next, the first hopper is closed and the second hopper is opened, and an amount of carbon and iron-containing material necessary to reach the required carbon content in the pig iron close to 4.5 wt% as much as possible as described above is added.

[0039] This method of adding carbon enables the overall density of the raw materials to be close to 100% DRI charge, and thus the presence of the carbon and iron-containing material does not impair the downward movement of this charge during melting. Furthermore, the presence of iron protects the carbon from pre-melting combustion, thus increasing the carbon yield and reducing carbon emissions.

[0040] Next, the carbon is transported through the slag layer to the liquid metal by gravity supply in the DRI pile. The slag layer 23 can have a layer thickness exceeding 50 cm, and the density of the carbon source is usually lower than the slag density itself. This causes a physical limitation for the carbon to enter the pig iron layer 14 through the slag. In the method according to the present invention, it is possible to overcome this limitation.

[0041] Furthermore, carbonization requires a large amount of energy and can be optimally provided by electric heating in the smelting furnace rather than an additional heating station, so the injection of carbon in the smelting stage ensures the optimal energy efficiency of the steelmaking process.

[0042] The carbon and iron-containing material is preferably a mixture of a carbon source and an iron source.

[0043] The carbon source 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 some or all of the carbon load. In particular, biochar can be used. Adding calcium carbide is particularly advantageous since calcium atoms can provide a desulfurization effect.

[0044] The iron source can be selected from ferrosilicon, iron fines, DRI fines, shredded scrap, steelmaking slag, steelmaking dust, scale, or steelmaking sludge.

[0045] In a preferred embodiment, the composite carbon and iron-containing material 30 is formed such that more than 50% of the outer surface is made of iron material. This configuration enables improving the protection of carbon from combustion.

[0046] The added carbon and iron-containing material preferably has the same size as the DRI product in order to use the same feeding device.

[0047] In a preferred embodiment, a desulfurization reagent can also be fed together with the carbon and iron-containing material. Such reagents can be based in particular on calcium compounds such as sodium carbonate, lime and / or calcium carbide.

[0048] The final sulfur content of the hot metal is preferably set to a maximum value of 0.03 wt%, preferably a maximum value of 0.004 wt%.

[0049] By performing desulfurization in the smelting furnace, the need for desulfurization treatment between the smelting furnace 13 and the converter 17 can be suppressed, or such treatment can at least be reduced.

[0050] In a preferred embodiment, the silicon-containing material can be charged together with the carbon and iron-containing materials, regardless of the presence or absence of a desulfurization reagent. 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 the formation of slag in the converter. This reaction is exothermic and thus provides additional energy for scrap melting. The more scrap is used, the smaller the environmental footprint of the process.

[0051] 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.

[0052] When using DRI products in the smelting furnace 13, a natural amount of silicon of usually less than 0.2 wt%, and further less than 0.1 wt% is brought about. The final silicon content of the hot metal is preferably set to a value of 0.1 - 0.4 wt%, preferably 0.2 - 0.4 wt%. If necessary, further addition of silicon can be carried out in the converter 17.

[0053] Adding silicon carbide is particularly advantageous because it enables an increase in the silicon content of the hot metal 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 hot metal in an electric smelting furnace (13), comprising: i) providing a direct reduced iron product (12); ii) providing a carbon and iron-containing material (30); iii) alternately feeding the DRI product (12) and the carbon and iron-containing material (30) to at least a part of the smelting furnace; iv) melting the DRI product (12) and the carbon and iron-containing material to produce hot metal (14). A method comprising the above steps.

2. The method according to claim 1, wherein the carbon and iron-containing material is injected in an amount sufficient to reach a final carbon content of 4.0 to 4.5 wt% in the hot metal (14).

3. The method according to claim 1 or 2, wherein the carbon and iron-containing material 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 resulting from the combustion of biomass, or a mixture of any of these materials.

5. The method according to claim 3 or 4, wherein the iron source is selected from ferrosilicon, iron fines, sinter dust, DRI fines, crushed scrap, steelmaking slag, steelmaking dust, scale, or steelmaking sludge.

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

7. The method according to any one of claims 1 to 6, wherein the DRI product is provided at a temperature of 500 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 containing at least 50 vol% hydrogen before being charged 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 and iron-containing material (30) and fed into the smelting furnace.

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

11. A method for producing steel, wherein the hot metal 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 hot metal is reduced to a value less than 2.1 wt% by oxygen blowing to obtain molten steel.

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

13. The method according to claim 11 or 12, wherein the hot metal is transferred from the steelmaking furnace (13) to the desulfurization station (15) before being transferred to the converter (17).

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