Method for producing pig iron in electric melting furnace and associated electric melting furnace

Abstract

The invention relates to a method for producing pig iron in a smelting furnace 13 comprising a container 20, said method comprising the following sequential steps:-loading a DRI product into the container 20,-melting the DRI product to form a pig iron layer 14, the top of which is a slag layer 23, and-directly injecting a carbonaceous material into the pig iron layer 14. The invention also relates to the production of steel from said pig iron and to a related electric smelting furnace 13.

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 currently be produced by two main manufacturing routes. Today, the most commonly used production route, known as the "BF-BOF route", involves producing hot iron 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 in the production of coke from coal in the coking plant and in the production of hot iron.

[0003] The second major route involves the so-called "direct reduction process". 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 tools will be necessary.

[0005] Another option is to use a melting furnace powered by electrical energy to melt the DRI product 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 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. This paradoxically reduces the environmental benefits of this route, since the higher the carbon ratio, the greater 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 routes by providing a new route that effectively minimizes the environmental impact of such manufacturing routes.

[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 11 considered alone or in any possible technical combination.

[0009] The invention also relates to a method for producing steel according to claim 12 .

[0010] Such a method may also comprise the optional features of claims 13 to 14 considered alone or in any possible technical combination.

[0011] The invention also relates to an electric smelting furnace according to claim 15 . BRIEF DESCRIPTION OF THE DRAWINGS

[0012] 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:

[0013] - Figure 1 shows the pig iron and steel making process according to the smelting / BOF route,

[0014] - Figure 2 Shows the melting furnace

[0015] - Figure 3 An embodiment of the method according to the invention is shown.

[0016] Elements in the drawings are illustrative and may not be drawn to scale. DETAILED DESCRIPTION

[0017] 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. Iron ore 10 is first reduced in a direct reduction plant 11. The direct reduction plant 11 can be designed to implement any kind 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 the 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, thus 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 resources 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] The DRI product is preferably loaded directly at the outlet of the direct reduction unit 11 as a hot product at a temperature of 500° C. 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 unit 11 and the smelting furnace 13 are not co-located, or if the smelting furnace 13 is stopped for maintenance and the DRI product must therefore be stored, the DRI product can be cold loaded, 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 then optionally be transferred to a desulfurization station 15 for a desulfurization step. This desulfurization step is necessary for the production of steel grades requiring low sulfur contents, which are set, for example, at a maximum of 0.03 weight percent sulfur. Desulfurization under oxidizing conditions is ineffective, and therefore it is preferred to desulfurize the pig iron before oxygen refining, or to desulfurize in a ladle after the steel has been deoxidized. For very low sulfur contents, for example, less than 0.004 weight percent sulfur, deoxidation and desulfurization are combined to obtain higher overall performance. Therefore, low sulfur grades benefit from desulfurization of the pig iron prior to the conversion step.

[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 the pig iron previously transferred to a hot metal ladle. The hot metal ladle can be, for example, Figure 2 A simple ladle is shown, but a torpedo ladle may also be used. The sulfur content of the desulfurized pig iron 16 is preferably less than 0.004 weight percent.

[0029] The desulfurized pig iron 16 can then be transferred to a converter 17. The converter essentially transforms molten metal into molten steel by blowing oxygen through the molten metal to decarburize 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 tools 20A, 20B, such as a ladle furnace, a RH (Ruhrstahl-Heareus) vacuum vessel, a vacuum tank degasser, an alloying and stirring station, etc., if necessary, to be processed according to the type of steel to be produced so as to achieve the desired steel composition. The molten steel 21 with the desired composition can then be transferred to a casting device 22, where it can be transformed into a solid product, such as a slab, a billet, a bloom or a strip.

[0031] like Figure 2 As shown, the smelting furnace 13 is composed of a container 20 capable of containing hot 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] Electrodes 22 provide the electrical energy required to melt the loaded raw materials and form pig iron. They are preferably Type electrode.

[0033] During melting of the raw material, two layers are formed, a layer of pig iron 14 which is the densest and therefore located at the bottom of the vessel 20, and a slag layer 23 located above the pig iron 14. The slag layer 23 may be partially covered by a pile 24 of raw material waiting to be melted.

[0034] The container 20 is also provided with holes called tap holes 25, which are located in the lower part of the container 20 and allow the pig iron 14 to be discharged while keeping most of the slag in the container 20. They can be located in the side walls of the container or in its bottom wall.

[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 accompanying drawings.

[0036] As explained above, 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 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%.

[0037] In fact, carbon is essential for the steelmaking process performed by oxygen blowing in the converter 17. This is because the reaction of carbon with oxygen produces carbon monoxide gas, which provides an intense and efficient stirring of the molten 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 higher 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.

[0038] In the framework of the present invention, carbonaceous material is added to the smelting furnace 13, directly to the pig iron layer 14. This addition can be done by injection means.

[0039] By injecting carbon directly into the pig iron layer 14, the inventors have observed that the carburizing process can achieve very high yields of more than 80%. In fact, the slag layer 23 has a high thickness that can be more than 50 cm, and the density of the carbon source is usually lower than the density of the slag itself. This induces physical limitations for carbon to pass through the slag into the pig iron layer 14.

[0040] Furthermore, direct injection of carbon 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.

[0041] Finally, increasing the carbon content of the pig iron in the furnace results in a decrease in the liquidus temperature of the pig iron, allowing for lower tapping temperatures.

[0042] In a preferred embodiment, the injection means is a lance inserted into a hole made in the top of the vessel 20. Such a lance penetrates the slag layer 23 and opens in the pig iron layer 14 to allow direct addition.

[0043] In a preferred embodiment, the lance is inserted between the electrodes of the smelting furnace 13 to inject carbon into the area where the temperature of the pig iron 14 reaches its maximum. This area is usually located in the center of the vessel 20, near the electrodes.

[0044] In a preferred embodiment, the carbon is injected together with a carrier gas to avoid clogging of the injection device. This gas is preferably inert and may consist of nitrogen, argon, helium or carbon monoxide or any mixture of such gases.

[0045] 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 those 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. Adding silicon carbide is also particularly advantageous because it allows the silicon content of the pig iron to be increased.

[0046] The particle size of the carbonaceous material to be injected by the injection device is preferably below 3 mm. In a preferred embodiment, the particle size of the material is less than or equal to 75 μm, and the particle size of the remaining particles is less than or equal to 2 mm.

[0047] In another embodiment, the carbon-containing material may also be composed of a composite briquette in which an iron source is mixed with one or more of the aforementioned carbon sources.

[0048] In a preferred embodiment, the iron source may be selected from dust or sludge from an electric furnace, a converter or a smelter, slag from an electric furnace or from a converter, or any iron-rich waste from the steel production route.

[0049] In a preferred embodiment, silicon-containing material may be injected into the pig iron layer 14 together with the carbon-containing material. Silicon has a strong deoxidizing ability at high temperatures, and in particular about 1600° C., which is the temperature of molten steel in the converter. It reacts with oxygen and then contributes to the formation of slag during the conversion of pig iron into steel. The reaction is exothermic and thus provides additional energy for the melting of the scrap in the converter. If anything, it may also improve the performance of the desulfurization operation.

[0050] 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.

[0051] The use of DRI products in the smelting furnace 13 will result in a natural amount of silicon generally below 0.2% by weight or even below 0.1% by weight. The final silicon content of the pig iron is preferably set to a value of 0.1% to 0.4% by weight, preferably 0.2% to 0.4% by weight. If necessary, further addition of silicon can be carried out in the desulfurization station 15 and / or the converter 17.

[0052] In a preferred embodiment, a desulfurization agent may also be injected together with the carbonaceous material, with or without silicon addition. Such an agent may in particular be based on calcium compounds, such as sodium carbonate, lime and / or calcium carbide.

[0053] 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.

[0054] Carrying out the 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.

[0055] It must be noted that the addition of calcium carbide is particularly advantageous, since calcium addition can provide a desulfurization effect on top of the addition of carbon. The addition of silicon carbide is also particularly advantageous, since it allows the silicon content of the pig iron to be increased on top of the carbon. Adding a mixture of calcium carbide and silicon carbide is even more advantageous, since it provides the addition of carbon and silicon while ensuring desulfurization.

Claims

1. A method for producing pig iron in an electric smelting furnace (13) comprising a vessel (20), the method comprising the following sequential steps: - loading the container (20) with a DRI product, - melting the DRI product to form a pig iron layer (14) topped by a slag layer (23), and - Injecting carbonaceous material directly into the pig iron layer (14).

2. The method according to claim 1, wherein the smelting furnace (13) comprises a top fitting on such a vessel (20), through which a lance (26) is inserted, the lance being used as an injection device to inject the carbonaceous material directly into the pig iron layer (14).

3. A method according to claim 1 or 2, wherein the smelting furnace comprises an electrode to melt the DRI product, and wherein the lance (26) is inserted near the electrode.

4. The method according to any one of claims 1 to 4, wherein the carbon-containing material is injected in an amount sufficient to achieve a final carbon content of 4.0% to 4.5% by weight in the pig iron layer (14).

5. The method according to any one of claims 1 to 4, wherein the carbonaceous material is injected together with a carrier gas.

6. The method according to any one of claims 1 to 5, wherein the carbonaceous material is selected from coke, anthracite, silicon carbide, calcium carbide, carbon from biomass combustion, or a mixture of any of those materials.

7. A method according to any one of claims 1 to 6, wherein the injected carbonaceous material has particles with a particle size below 3 mm.

8. The method according to claim 7, wherein 70% to 80% of the particles have a particle size less than or equal to 75 μm and the remaining particles have a particle size less than or equal to 2 mm.

9. The method according to claim 6, wherein the carbonaceous material is pre-mixed with an iron source and formed into a composite briquette, and the composite briquette is injected into the pig iron layer (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 charged into the smelting furnace (13).

11. The method according to any one of claims 1 to 10, wherein a silicon-containing material and / or a desulfurizing agent is added to the carbon-containing material to be injected into the pig iron layer (14).

12. A method for producing steel, wherein the 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 below 2.1 weight percent by oxygen blowing in the converter to obtain molten steel.

13. 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.

14. 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).

15. An electric smelting furnace (13) for producing pig iron (14) comprising a vessel (20), said vessel (20) being provided with a top fitting on such vessel (20), through which a lance (26) is inserted, said lance being designed to allow the direct injection of carbonaceous material into a layer of pig iron (14) contained in said vessel (20).

Images