Method for producing pig iron in production line comprising electric smelting furnace

Abstract

The invention relates to a method for producing pig iron in a smelting furnace 13 comprising a vessel 20, said method comprising the following sequential steps:-loading a DRI product into the vessel 20,-melting the DRI product to form a pig iron layer 14, the top of which is a slag layer 23, and-transferring the pig iron 14 to the desulfurization station 15, and injecting a silicon-containing material into the pig iron 14 in the desulfurization station 15. The invention also relates to the production of steel from said pig iron.

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 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. Summary of the invention

[0005] It is therefore an object of the present invention to remedy the disadvantages of pig iron and steelmaking manufacturing by providing a new approach that effectively minimizes the environmental impact of such manufacturing approaches.

[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 7 considered alone or in any possible technical combination.

[0008] The invention also relates to a method for producing steel according to claim 8 .

[0009] Such a method may further comprise the optional features of claim 9. 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. Iron ore 10 is first reduced in a direct reduction facility 11. The direct reduction facility 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.

[0015] 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.

[0016] 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.

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

[0018] In another preferred embodiment, the DRI product used in the method according to the present invention is manufactured by the so-called H2-DRI process, wherein the reducing gas contains more than 50% by volume, and preferably more than 60%, 70%, 80% or 90% hydrogen, or even completely composed of hydrogen. The H2-DRI product will contain carbon at a level far lower than that of natural gas or biogas DRI, and is therefore typically less than 1% by weight, or even lower. In a preferred embodiment, the hydrogen used in the DRI reducing gas comes from the electrolysis of water, which is preferably partially or entirely powered by CO2-neutral electricity. CO2-neutral electricity particularly includes electricity from renewable sources, which is defined as energy collected from renewable sources that are naturally replenished on a human time scale (including sources like sunlight, wind, rain, tides, waves and geothermal). In some embodiments, the use of electricity from nuclear sources can be used because it does not produce CO2 emissions.

[0019] 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.

[0020] 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).

[0021] 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.

[0022] 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.

[0023] The pig iron 14 is then transferred to a pig iron ladle through at least one tap hole 25 provided with at least one iron channel 26. 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. There are usually as many auxiliary iron channels as tap holes, which then intersect to form a main iron channel to guide the extracted pig iron to the pig iron ladle.

[0024] The pig iron ladle may be a simple ladle, but may also be a torpedo ladle.

[0025] The pig iron 14 is sent to a desulfurization station 15 for a desulfurization step. This desulfurization step can be carried out in a dedicated container or preferably directly in the pig iron ladle to avoid liquid metal transfer and related heat losses. This desulfurization step is necessary for the production of steel grades requiring low sulfur content (which is set to a maximum value of 0.03 weight percent sulfur, for example). 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 (for example 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.

[0026] 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 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.

[0027] 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.

[0028] 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 mixing station, 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.

[0029] 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 21, 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.

[0030] Electrodes 22 provide the electrical energy required to melt the loaded raw materials and form pig iron. They are preferably Type electrode.

[0031] 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 layer of slag 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.

[0032] The vessel 20 is also provided with holes called tap holes 25, which are located in the lower part of the vessel 20 and which allow the pig iron 14 to be discharged while keeping most of the slag in the vessel 20. They can be located in the side walls of the vessel or in its bottom wall.

[0033] 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.

[0034] In the framework of the present invention, silicon-containing material is added to the pig iron in the desulfurization station 15. This addition can be done by an injection device. Silicon has a strong deoxidizing ability at high temperatures, and in particular about 1600° C., which is the temperature of the molten steel in the converter. It reacts with oxygen and then contributes to the formation of slag in the converter. The reaction is exothermic and therefore provides additional energy for the melting of the waste. The more waste is used, the smaller the environmental footprint of the process.

[0035] By injecting silicon into the pig iron at this stage, the inventors have observed that it allows complete dissolution of the silicon with good efficiency before the pig iron reaches the converter 17. Furthermore, this addition improves the desulphurization process efficiency by maintaining strong local deoxidation conditions.

[0036] In a preferred embodiment, the injection is done with a lance commonly used for injecting desulfurizing agents. It is then added together with a carrier gas. This gas is preferably inert and may consist of nitrogen, argon, helium or carbon monoxide or any mixture of such gases.

[0037] 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.

[0038] The use of DRI products in the melting furnace 13 will result in an initial amount of silicon generally below 0.2 wt.-% or even below 0.1 wt.-%. The final silicon content of the pig iron is preferably set to a value of 0.1 wt.-% to 0.4 wt.-%, preferably 0.2 wt.-% to 0.4 wt.-%. If necessary, further addition of silicon can be carried out in the converter 17.

[0039] In a preferred embodiment, the carbonaceous material may be injected into the desulfurization station 15 together with the silicon-containing material.

[0040] As explained above, the carbon content of 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%.

[0041] In fact, carbon is essential for the steelmaking process carried out in the converter 17 by blowing oxygen. This is because the reaction of carbon with oxygen produces carbon monoxide gas, which provides an intense and effective stirring of the liquid 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 larger 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.

[0042] 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.

[0043] In another embodiment, the carbonaceous material may also consist of a composite briquette in which an iron source is mixed with one or several of the aforementioned carbon sources.

[0044] In a preferred embodiment, the iron source may be selected from sludge from an electric furnace, a converter or a smelting furnace, slag from an electric furnace or from a converter, or any iron-rich waste from the steel production route.

[0045] The addition of silicon carbide is particularly advantageous as it allows the carbon content of the pig iron to be increased in addition to the addition of silicon. The addition of a mixture of calcium carbide and silicon carbide is even more advantageous as it provides for carbon and silicon addition while ensuring desulfurization.

Claims

1. A method for producing pig iron in a production line comprising an electric smelting furnace (13) and a desulphurisation station (15), the 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), - transferring the pig iron (14) to the desulphurisation station (15), and - Injecting silicon-containing material into the pig iron (14) in the desulphurisation station (15).

2. The method according to claim 1, wherein the silicon-containing material is injected in an amount sufficient to achieve a final silicon content of 0.1% to 0.4% by weight in the pig iron (14).

3. The method according to claim 1 or 2, wherein the silicon-containing material is injected together with a carrier gas.

4. The method according to any one of claims 1 to 3, wherein the silicon-containing material is selected from metallic silicon Si, silicon carbide SiC, silicon manganese SiMn, calcium silicate SiCa, iron silicon alloy FeSi or a mixture of any of those materials.

5. The method according to any one of claims 1 to 4, wherein the injected silicon-containing material has particles with a particle size below 3 mm.

6. The method according to any one of claims 1 to 5, 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).

7. The method according to any one of claims 1 to 6, wherein a carbon-containing material is added to the silicon-containing material to be injected into the pig iron (14).

8. A method for producing steel, wherein the pig iron produced according to any one of claims 1 to 7 is transferred from the smelting 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.

9. The method for manufacturing steel according to claim 8, wherein iron-containing scrap is added to the pig iron in the converter (17) and melted.

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