Method for producing liquid pig iron in electric smelting unit

Abstract

The invention relates to a method for producing liquid pig iron in an electric smelting unit 13. The method comprises the sequential steps of:-providing a direct reduced iron product 12,-feeding the DRI product 12 into a smelting unit 13,-feeding together with the DRI product 13 at least one material based on steelmaking or ironmaking by-products having an iron content higher than 20% by weight, at least a portion of the iron being in the oxidized form,-feeding the DRI product 12 into the smelting unit 13. And melting the DRI product 13 and the at least one material based on the steelmaking or ironmaking by-products in order to produce liquid pig iron. The invention also relates to a steel manufacturing method using the pig iron.

Description

Technical Field

[0001] The present invention relates to a method of making pig iron (also known as 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. The 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] It is therefore an object of the present invention to remedy the disadvantages of pig iron and steelmaking manufacturing by providing a new way to effectively minimize the environmental impact of such manufacturing routes without incurring large investments. Summary of the invention

[0006] This problem is solved by a method for producing pig iron as specified in claim 1 .

[0007] Such a method may also comprise the optional features of claims 2 to 10 considered alone or in any possible technical combination.

[0008] The invention also relates to a method for producing steel according to claim 11 .

[0009] Such a method may further comprise the optional features of claim 12 . BRIEF DESCRIPTION OF THE DRAWINGS

[0010] 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:

[0011] - Figure 1 shows the pig iron and steel making process according to the smelting / BOF route,

[0012] - Figure 2 A smelting furnace is shown.

[0013] Elements in the drawings are illustrative and may not be drawn to scale. DETAILED DESCRIPTION

[0014] 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.

[0015] 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.

[0016] 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.

[0017] 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.

[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 is typically set in the range of 2 wt % to 3 wt %.

[0019] 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.

[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 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.

[0021] 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.

[0022] 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).

[0023] 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.

[0024] 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.

[0025] In a preferred embodiment, scrap is also loaded into the smelting furnace together with DRI, with the mass fraction of iron-containing scrap being 1 wt% to 20 wt% based on the amount of DRI product fed.

[0026] In a preferred embodiment, the scrap loaded is E40 specification scrap according to the EU-27 scrap specification last updated in May 2007.

[0027] The pig iron 14 can optionally be sent to a desulfurization station 15 for a desulfurization step. The desulfurization step can be carried out in a dedicated container or preferably directly in the pig iron ladle to avoid liquid metal transfer and associated heat losses. The 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 the pig iron is preferably desulfurized before oxygen refining, or desulfurized in the ladle after the steel is deoxidized. 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 the 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 these 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 stirring plant, 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 2As 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 smelting furnace 13 may be, for example, an open slag bath furnace or an OSBF.

[0033] 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.

[0034] 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.

[0035] 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.

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

[0037] This allows the recovery of the iron contained in these materials and thus improves the overall iron yield of the process.

[0038] Currently, the recovery of these iron-containing by-products is carried out at the steelmaking vessel itself (BOF / EAF) or back to the sintering. The iron contained in these materials is oxidized and subjected to a strongly endothermic reduction in the vessel where the energy is provided by the combustion of carbon, limiting the environmental benefits of such recovery. With the method according to the invention, the chemical reduction is carried out by carbon and the thermal effects are compensated with electricity.

[0039] Another advantage is that the inventors have found that the iron recovery in the smelting operation is very high, in excess of 90%, which is much higher than in current recycling practices. For example, recycling in current steelmaking vessels may result in partial or low iron reduction, thereby increasing slag mass and oxidation rates, which implies additional costs for energy used for heating and melting without recovering the iron.

[0040] The by-product used to form the by-product-based material may be selected from at least one of sintering dust or sludge, steelmaking dust or sludge, smelting dust or sludge, arc furnace slag, basic oxygen furnace slag, secondary metallurgical slag or mill scale. It may also be a mixture of these different by-products.

[0041] Sintering or steelmaking dust / sludge or smelting dust / sludge are sludges resulting from the dedusting of the exhaust gases of the furnaces under consideration, such as basic oxygen furnaces, electric arc furnaces, sintering facilities and smelting furnaces. They will be in the form of sludge or dust depending on the treatment applied to the exhaust gases (dry treatment, for example using fabric filters, or wet treatment, for example water spraying). Arc furnace slag and basic oxygen furnace slag or secondary metallurgical slag are slags formed during the production of molten steel. Oxide scale or mill scale is the flaky surface of hot rolled steel, consisting of mixed iron oxides (iron (II) oxide (FeO), iron (III) oxide (Fe2O3) and iron (II, III) oxide (Fe3O4, magnetite)). Mill scale is formed on their outer surface when steel plates, steel sheets or steel profiles are produced by rolling steel semi-finished products in a rolling mill.

[0042] Typical compositions of some by-products are shown in Table 1 below. All percentages are expressed in weight percentages. For iron (Fe), the content covers the content of metallic iron (Fe) or any oxide (FeO, Fe2O3, Fe3O4).

[0043]

[0044]

[0045] Table 1

[0046] In a preferred embodiment, the by-product also contains at least 10% by weight of a slag former. This allows to facilitate the slag control required during the smelting process. The slag former is preferentially CaO or alumina, which allows to obtain a slag composition suitable for use in the cement industry.

[0047] In a preferred embodiment, the by-products are fed to the smelting furnace in the form of briquettes or pellets. Before briquetting or pelletizing, they may first be subjected to preparation steps, including but not limited to crushing and screening of the selected by-products, followed by mixing the screened by-products to obtain the desired material composition, i.e. 20% by weight of iron and optionally at least 10% by weight of slag forming agent. This allows increasing the diversity of the combinations and sources of the different materials mentioned to form the mixed briquettes or pellets.

[0048] In a preferred embodiment, carbonaceous 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 effective stirring of the liquid metal and thus improves the removal of impurities from the steel. The reaction is also exothermic and thus provides additional energy for the scrap melting. The more scrap is used, the smaller the environmental footprint of the process.

[0049] The carbon content of the 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 as possible to 4.5 wt%, which is the saturation level. In a preferred embodiment, the carbon content of the pig iron is set in the range of 4.0 wt% to 4.5 wt% by adding carbonaceous materials.

[0050] The carbonaceous 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 can also advantageously come 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 the calcium atoms can provide a desulfurization effect.

Claims

1. A method for producing liquid pig iron in an electric smelting unit (13), comprising the following steps: i) providing a direct reduced iron product (12), ii) feeding the DRI product (12) into the smelting unit (13), iii) feeding at least one material based on steelmaking or ironmaking by-products having an iron content higher than 20% by weight together with said DRI product (13), at least part of said iron being in oxidized form, iv) melting said DRI product (13) and said at least one steelmaking or ironmaking by-product based material to produce liquid pig iron.

2. The method of claim 1, wherein the steelmaking or ironmaking by-products from which the by-product based material is formed are selected from any one of sintering dust, steelmaking sludge or dust, smelting sludge or dust, secondary metallurgical slag, electric arc furnace slag, basic oxygen furnace slag, mill scale, or combinations thereof.

3. A method according to any one of the preceding claims, wherein the by-product based material further comprises at least 10 wt% of a slag forming agent.

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

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

6. A method according to any one of the preceding claims, wherein the steelmaking or ironmaking by-product based material is fed as briquettes or pellets.

7. The method according to claim 6, wherein the briquettes or pellets are prepared according to the following method: a. crushing the steelmaking or ironmaking by-products, b. Screening of crushed steelmaking or ironmaking by-products, c. mixing the screened steelmaking or ironmaking by-products in appropriate amounts to achieve the target composition of the steelmaking or ironmaking by-product based material, d. Compact or granulate the mixture.

8. The method according to any one of the preceding claims, wherein carbonaceous material is also fed into the smelting unit (13).

9. The method of claim 8, wherein the carbonaceous material is added in an amount sufficient to achieve a final carbon content of 4.0 to 4.5 wt. % 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 comprising at least 50% by volume of hydrogen before the DRI product is loaded into the smelting furnace (13).

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 (18) is added to the pig iron in the converter (17) and melted.

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