Method for producing hot metal in an electric smelting unit

Abstract

The present invention deals with a method for producing hot metal in an electric smelting unit 13. The method comprises the following consecutive steps: - providing a direct reduced iron product 12, - feeding the DRI product 12 into the smelting unit 13, - feeding at least one steel or ironmaking by-product-based material having an iron content exceeding 20% by weight, at least a part of said iron being in an oxidized form, together with the DRI product 13, - melting the DRI product 13 and at least one steel or ironmaking by-product-based material to produce hot metal. The present invention also deals with a steel manufacturing method using 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 the coke plant and in the production of hot metal.

[0003] The second main route includes the so-called "direct reduction process". Among these are methods by 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 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.

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 manufacturing routes of pig iron and steelmaking by providing a new route that efficiently minimizes the impact on such manufacturing environments without undergoing large investments.

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 optional features of claims 2 to 10, considered separately or in any possible technical combination.

[0008] The present invention also addresses a method for manufacturing steel as described in claim 11.

[0009] Such a method can also include the optional feature of claim 12.

[0010] Other features and advantages of the present invention will become apparent from the following description of the invention, given by way of example and with reference to the accompanying drawings, which are in no way limiting.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Embodiments for Carrying Out the Invention

[0012] The elements in the figures are illustrative and may not be drawn to scale.

[0013] 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.

[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 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 bio-derived 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.

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

[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 can be charged at room temperature or a preheating step can 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. A more detailed description of the smelting furnace will be given later with reference to FIG. 2.

[0024] In a preferred embodiment, scrap is also charged into the smelting furnace together with the DRI, and the mass fraction of iron scrap is 1 wt% to 20 wt% based on the amount of the DRI product supplied.

[0025] In a preferred embodiment, the charged scrap is E40 specification scrap according to the EU-27 steel scrap specification of the last update in May 2007.

[0026] 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 related heat losses. This desulfurization step is necessary, for example, for the production of steel grades that require a low sulfur content set at a maximum of 0.03 weight percent. 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, e.g. less than 0.004 weight percent, deoxidation and desulfurization are combined for overall higher performance. Thus, low sulfur grades benefit from performing pig iron desulfurization before the conversion step.

[0027] Desulfurization of the pig iron can be carried out by adding reagents based on calcium compounds 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 wt%, preferably less than 0.004 wt%.

[0028] Subsequently, the desulfurized pig iron 16 can be transferred to the converter 17. A converter basically converts 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 may also be charged into the converter 17 to take advantage of the heat released by the exothermic reaction resulting from oxygen injection into the pig iron.

[0029] Subsequently, the molten steel 19 thus formed can be transferred at any time as required to one or more secondary metallurgy tools 20A, 20B, for example, 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 manufactured. Subsequently, the molten steel having the required composition 21 can be transferred to the casting plant 22, where it can be changed into solid products such as slabs, billets, blooms or strips.

[0030] 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.

[0031] The smelting furnace 13 may be, for example, an open slag bath furnace or an OSBF.

[0032] The container 20 is also provided with at least one tap hole 25 through which the produced pig iron can be tapped. Such tap holes 25 are arranged at the lower part of the container 20. They may be arranged on the side wall or the bottom wall of the container.

[0033] The electrodes 22 provide the electrical energy necessary to melt the charged raw materials to form pig iron. These are preferably Soederberg type electrodes.

[0034] During the melting of the raw materials, two layers are formed: the pig iron 14 layer, which has the highest density and is thus located at the bottom of the vessel 20, and the 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.

[0035] 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.

[0036] In the method according to the invention, at least one steelmaking or ironmaking by-product-based material having an iron content of more than 20% by weight, at least a part of which iron is in an oxidized form, is also charged into the smelting furnace 13.

[0037] Thereby, the iron contained in these materials can be recovered, and thus the overall iron yield of the process can be improved.

[0038] Currently, the recycling of these iron-containing by-products is carried out either in the steelmaking vessel itself (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 invention, the reduction is carried out chemically by carbon and the heat effect is compensated electrically.

[0039] Another advantage is that the inventors have found that the iron recovery rate is very high in the smelting operation, exceeding 90%, much higher than current recycling practices. For example, recycling in current 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.

[0040] The by-products used to form the by-product-based material can be selected from at least one of sintering 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.

[0041] Sintering or steelmaking dust / sludge or smelting dust / sludge is the sludge generated from the dust removal of exhaust gas from considered furnaces such as basic oxygen furnaces, electric arc furnaces, sintering plants, and smelting furnaces. These are in the form of sludge or dust depending on the treatment applied to the exhaust gas, either a dry treatment such as the use of cloth filters or a wet treatment such as water spraying. Electric arc furnace slag and basic oxygen slag or secondary metallurgy slag are the slags formed during molten steel production. Scale or mill scale is the flaky surface of hot-rolled steel consisting of a mixed iron oxide 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 steel plates, thin steel sheets, or profiles when the semi-finished steel product is being rolled in a rolling mill.

[0042] The typical compositions of some 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).

[0043]

Table 1

[0044] In a preferred embodiment, the by-product also contains at least 10 wt% of a slag former. This can contribute to the slag control required in the smelting process. This slag former is preferably CaO or alumina which enables obtaining a slag composition suitable for use in the cement industry.

[0045] In a preferred embodiment, the by-products are fed to the smelting furnace in the form of briquettes or pellets. Prior to briquetting or pelletizing, these are first subjected to a preparation step that includes, but is not limited to, crushing and screening the selected by-products, and subsequently mixing the screened by-products so as to obtain the required material composition, namely, 20 wt% iron and optionally at least 10 wt% slag former. This enhances the versatility in the source and combination of different materials described above, and enables the formation of mixed briquettes or pellets.

[0046] In a preferred embodiment, a carbon-containing material is also added to the smelting furnace. The reaction of carbon with oxygen in the converter 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 also exothermic and thus provides additional energy for scrap melting. The more scrap is used, the smaller the environmental footprint of the process.

[0047] The carbon content of the hot metal 14 produced via the DRI route is generally less than 3 wt%. However, in order to meet the requirements of the subsequent steelmaking process in the converter, the hot metal should preferably have a carbon content as close as possible to the saturation level of 4.5 wt%. In a preferred embodiment, by adding a carbon-containing material, the hot metal carbon content is set in the range of 4.0 - 4.5 wt%.

[0048] The carbon-containing material can be derived from different sources. This 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 part or all of the carbon load. In particular, biochar can be used. Adding calcium carbide is particularly advantageous because calcium atoms can provide a desulfurization effect.

Claims

1. A method for producing hot metal in an electric smelting unit (13), comprising: i) providing a direct reduced iron product (12); ii) feeding said DRI product (12) into said smelting unit (13); iii) feeding at least one steelmaking or ironmaking by-product-based material having an iron content of more than 20% by weight, at least a part of said iron being in an oxidized form, together with said DRI product (13); iv) melting said DRI product (13) and said at least one steelmaking or ironmaking by-product-based material to produce hot metal. A method as described above.

2. The method according to claim 1, wherein the steelmaking or ironmaking by-product forming the by-product-based material is selected from any one of sinter dust, steelmaking slag or dust, smelting slag or dust, secondary metallurgy slag, electric arc furnace slag, basic oxygen furnace slag, mill scale, or a combination thereof.

3. The method according to claim 1 or 2, wherein said by-product-based material further comprises at least 10% by weight of a slag former.

4. The method according to claim 3, wherein said slag former is selected from at least one of CaO, lime, alumina, magnesia, aluminosilicate, or a combination thereof.

5. The method according to any one of claims 1 to 4, wherein iron scrap is also fed into the electric smelting unit (13), and the mass fraction of the iron scrap is 1% to 20% by weight based on the amount of the supplied DRI product.

6. The method according to any one of claims 1 to 5, wherein the steelmaking or ironmaking by-product-based material is supplied as briquettes or pellets.

7. Said briquettes or pellets are produced by the following method: a. pulverizing a steelmaking or ironmaking by-product; b. screening the pulverized steelmaking or ironmaking by-product; c. mixing the screened steelmaking or ironmaking by-product in an amount appropriate to reach the target composition of said steelmaking or ironmaking by-product-based material; d. briquetting or pelletizing the mixture. The method according to claim 6.

8. The method according to any one of claims 1 to 7, wherein a carbon-containing material is also fed into said smelting unit (13).

9. The method according to claim 8, wherein the carbon-containing material is added in an amount sufficient to reach a final carbon content of 4.0 to 4.5% by weight in the pig iron (14).

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 charged into the smelting furnace (13).

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 of less than 2.1% by weight by blowing oxygen to obtain molten steel.

12. The method for producing steel according to claim 11, wherein iron scrap (18) is added to and melted with the pig iron in the converter (17).

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