Method for producing hot metal in an electric smelting unit
Patent Information
- Application Number
- JP2025502574
- Authority / Receiving Office
- JP · JP
- Patent Type
- Applications
- Current Assignee / Owner
- Filing Date
- 2022-07-29
- Publication Date
- 2025-09-22
AI Technical Summary
Current steel manufacturing processes, particularly the BF-BOF route, emit significant CO2 and require large investments to process impure scrap in the DRI route, limiting the reduction of CO2 emissions and production of high-quality steel.
A method involving the production of pig iron using direct reduced iron (DRI) from renewable biomass or hydrogen, combined with CO2-neutral power, and utilizing slag formers and carbon-containing materials to minimize environmental impact and energy consumption, while producing high-quality steel.
This method significantly reduces CO2 emissions and energy requirements, enhances iron recovery, and produces high-quality steel with minimal investment, achieving over 90% iron recovery and controlled slag formation.
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Abstract
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 when producing coke from coal in a coke plant and when producing hot metal.
[0003] The second main process involves the so-called "direct reduction method". Among them are methods under brands such as 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 steel manufacturers 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.
Summary of the Invention
Problems to be Solved by the Invention
[0005] Accordingly, an object of the present invention is to improve the drawbacks of the pig iron and steel manufacturing processes by providing a new process that efficiently minimizes the impact on such manufacturing environments without making a large investment.
Means for Solving the Problems
[0006] This problem is solved by a method for manufacturing pig iron as detailed in claim 1.
[0007] Such a method can also include any optional features of claims 2 to 9, considered separately or in any possible technical combination.
[0008] The present invention also relates to a method for manufacturing steel as described in claim 10.
[0009] Such a method can also include the optional features of claim 11.
[0010] Other features and advantages of the present invention will become apparent from the following description of the invention, given by way of example with reference to the accompanying drawings and in no way limiting.
Brief Description of the Drawings
[0011]
Figure 1
Figure 2
Modes for Carrying Out the Invention
[0012] The elements in the figures are illustrative and may not be drawn to scale.
[0013] FIG. 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.
[0014] 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.
[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 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 biologically derived materials in domestic wastewater. 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.
[0017] 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%, usually in the range of 2 - 3 wt%.
[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 vol%, preferably more than 60, 70, 80 or 90 vol% hydrogen, 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.
[0019] In a preferred embodiment, the hydrogen used as the DRI reducing gas is derived from the electrolysis of water, which is preferably powered in part or in whole by CO2-neutral power. CO2-neutral power includes, in particular, power from renewable sources defined as energy collected from renewable resources. It is naturally replenished on a human time scale, including sources such as sunlight, wind, rain, tides, waves, and geothermal heat. In some embodiments, the use of power from nuclear sources can be used because it does not emit the CO2 being produced.
[0020] 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 the iron oxide is completed, and the product is melted to produce pig iron.
[0021] The DRI product can be transferred to the smelting furnace in various forms. Preferably, the direct 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 may be charged at room temperature or a preheating step may be performed.
[0022] Preferably, it 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 may be charged at room temperature or a preheating step may be performed.
[0023] The smelting furnace 13 uses electrical energy provided by several electrodes to melt the DRI product 12 and produce pig iron 14. In a preferred embodiment, some or all of the required power is derived from CO2-neutral power. Further detailed description of the smelting furnace will be described later with reference to FIG. 2.
[0024] The pig iron 14 can optionally be sent to a desulfurization station 15 to perform a desulfurization step. This desulfurization step can be carried out directly in a dedicated container or, preferably, in a pig iron ladle in order to avoid the transfer of molten metal and associated heat losses. This desulfurization step is necessary, for example, for the production of steel grades that require a low sulfur content, for example set at a maximum of 0.03 weight percent sulfur. Desulfurization 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 sulfur contents, for example less than 0.004 weight percent sulfur, deoxidation and desulfurization are combined for overall higher performance. Thus, low sulfur grades benefit from pig iron desulfurization before the conversion step.
[0025] 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 into the pig iron. This can be done, for example, by injecting these reagents into the pig iron ladle. The desulfurized pig iron 16 preferably has a sulfur content of less than 0.03 weight %, preferably less than 0.004 weight %.
[0026] The desulfurized pig iron 16 can then be transferred to 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 to benefit from the heat released by the exothermic reaction resulting from oxygen injection into the pig iron.
[0027] Next, the molten steel 19 thus formed can be transferred to one or more secondary metallurgy tools 20A, 20B such as, for example, a ladle furnace, an RH (Ruhrstahl-Heareus) vacuum vessel, a vacuum tank degassing device, an alloying and stirring station, etc., and processed to reach the steel composition required according to the steel type to be manufactured. Then, the molten steel 21 having the required composition can be transferred to the casting plant 22, where it can be changed into solid products such as slabs, billets, blooms or strips.
[0028] 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.
[0029] 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.
[0030] The electrodes 22 provide the electrical energy necessary to melt the charged raw materials to form pig iron. These are preferably Soederberg type electrodes.
[0031] During the melting of the raw materials, two layers are formed: a layer of pig iron 14 that 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 may be partially covered by a pile of raw materials 24 waiting to be melted.
[0032] 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.
[0033] In the method according to the invention, at least one steelmaking or ironmaking by-product-based material containing more than 10% by weight of a slag former is also charged into the smelting furnace 13.
[0034] Thereby, the formation and chemical properties of the slag in the smelting furnace can be controlled. These slag formers can be selected from at least one of CaO, lime, alumina, magnesia, and aluminosilicate. Those having a preference are CaO or alumina which enable obtaining a slag composition suitable for use in the cement industry.
[0035] The by-products used to form the by-product-based material can be selected from at least one of sinter dust or sludge, steelmaking dust or sludge, smelting dust or sludge, electric arc furnace slag, basic oxygen furnace slag, secondary metallurgy slag, or mill scale. Also, mixtures of these different by-products may be used.
[0036] Sinter or steelmaking dust / sludge or smelting dust / sludge is sludge resulting from the dust removal of exhaust gas from a furnace under consideration such as a basic oxygen furnace, an electric arc furnace, a sintering plant, and a smelting furnace. These are in the form of sludge or dust depending on the treatment applied to the exhaust gas, for example, a dry treatment such as the use of a cloth filter or a wet treatment such as water spraying. Electric arc furnace slag and basic oxygen slag or secondary metallurgy slag are slags formed during molten steel production. Scale or mill scale is the flaky surface of hot-rolled steel consisting of mixed iron oxides of iron(II) oxide (FeO), iron(III) oxide (Fe2O3), and iron(II,III) oxide (Fe3O4, magnetite). Mill scale is formed on the outer surface of a steel plate, thin steel sheet, or section when the steel semi-finished product is being rolled by a rolling mill.
[0037] The typical compositions of several by-products are shown in Table 1 below. All percentages are expressed as weight percentages. In the case of iron (Fe), the content includes the content of metallic iron (Fe) or any oxide (FeO, Fe2O3, Fe3O4).
[0038]
Table 1
[0039] In a preferred embodiment, the by-product also contains at least 20 wt% iron, and a part of this iron is in an oxidized form.
[0040] Currently, the recycling of iron-containing by-products is carried out in the steelmaking vessels themselves (BOF / EAF) or returned to sintering. The iron contained in these materials is oxidized, and strong endothermic reduction is carried out in a vessel where energy is supplied by carbon combustion, thus limiting the environmental benefits of such recycling. In the method according to the present invention, the reduction is carried out chemically by carbon, and the heat effect is compensated electrically.
[0041] Another advantage discovered by the inventors is that the iron recovery rate in the smelting operation is very high, over 90%. This is much higher than the current recycling methods. For example, the current recycling in steelmaking vessels results in partial or low iron reduction, and thus may increase the slag mass and oxidation rate, which means additional costs for energy to heat and melt without recovering iron.
[0042] In a preferred embodiment, the by-products are supplied to the smelting furnace in the form of briquettes or pellets. Prior to briquetting or pelletizing, these are first, but not limited to, subjected to a production step including the steps of crushing and sieving the selected by-products, followed by mixing the sieved by-products so as to obtain the required material composition, namely, a slag former of 10% by weight and optionally at least 20% by weight of iron. This can enhance the versatility in the supply source and the combination of different materials described above to form mixed briquettes or pellets.
[0043] In a preferred embodiment, a carbon-containing material is also added to the smelting furnace. Carbon reacts with oxygen in the converter to produce carbon monoxide gas. This vigorously and efficiently stirs the molten metal and improves the removal of impurities from the steel. This reaction is also exothermic and thus provides additional energy for scrap melting. The more scrap is used, the smaller the environmental footprint of the process.
[0044] The carbon content of the pig iron 14 produced via the DRI route is generally less than 3% by weight. However, to meet the requirements of the subsequent steelmaking process in the converter, the carbon content of the pig iron is preferably as close as possible to 4.5% by weight. This is the saturation level. In a preferred embodiment, by adding a carbon-containing material, the pig iron carbon content is set in the range of 4.0 - 4.5% by weight.
[0045] The carbon-containing material can be derived from different sources. This can be selected, for example, from among coke, anthracite, silicon carbide, calcium carbide or any mixture of these sources, but can also advantageously be obtained from renewable sources such as biomass for part or all of the carbon load. In particular, biochar can be used. Adding calcium carbide is particularly advantageous as calcium atoms can provide a desulfurization effect.
Claims
1. A method for producing molten pig iron in an electrosmelting unit (13), comprising the steps of: i) providing a direct reduced iron product (12); ii) feeding said DRI product (12) into said smelting unit (13); iii) providing at least one steel or iron making by-product based material containing at least 10 wt. % of a slag former together with said DRI product (13); iv) melting said DRI product (13) and said at least one steel or iron making by-product based material to produce molten pig iron; A method comprising:
2. 10. The method of claim 1, wherein the steel or iron making by-products forming the by-product-based material are selected from among sinter dust, steel making sludge or dust, smelting sludge or dust, secondary metallurgical slag, electric arc furnace slag, basic oxygen furnace slag, mill scale, or any combination thereof.
3. 10. The method of claim 1, wherein the by-product-based material further comprises an iron content of greater than 20% by weight, at least a portion of the iron being in an oxidized form.
4. 10. The method of claim 1, wherein the slag former is selected from at least one of CaO, lime, alumina, magnesia, aluminosilicates, or any combination thereof.
5. 10. The method of claim 1, wherein the steel or iron making by-product based material is provided as briquettes or pellets.
6. The briquettes or pellets are prepared by the following method: a. grinding a steel or iron making by-product; b. screening the pulverized steel or iron making by-product; c. blending the sieved steel or iron making by-products in amounts appropriate to reach a target composition of the steel or iron making by-product based material; d. Briquetting or pelletizing the mixture The method of claim 5 , wherein the polymer is prepared according to the following formula:
7. 2. The method of claim 1, wherein a carbon-containing material is also fed to the smelting unit (13).
8. 8. The method of claim 7, wherein the carbon-containing material is added in an amount sufficient to reach a final carbon content of 4.0 to 4.5 wt. % in the pig iron (14).
9. 2. The method of claim 1, wherein, prior to being charged into the smelting furnace (13), the DRI product is produced using a reducing gas containing at least 50% by volume of hydrogen.
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 (18) is added to the pig iron and melted in the converter (17).