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. 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 iron scrap having a size of less than 80 mm together with the DRI product 13, - melting the DRI product 13 and the iron scrap to produce hot metal. The present invention also deals with a method for producing molten steel from the produced 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 a coke plant and in the production of hot metal.

[0003] The second main route includes the so-called "direct reduction method". Among them, there are methods by brands such as MIDREX (registered trademark), FINMET (registered trademark), ENERGIRON (registered trademark) / HYL, COREX (registered trademark), FINEX (registered trademark), etc., and 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, 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 for pig iron and steel by providing a new route that efficiently minimizes the impact on such manufacturing environments without requiring 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 7 considered separately or in any possible technical combination.

[0008] The present invention also deals with a method for manufacturing steel as described in claim 8.

[0009] Such a method can also include the optional feature of claim 9.

[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] Figure 1 shows the 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 (registered trademark) technology or Energiron (registered trademark). 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 biological materials in domestic sewage. In the context of the present invention, biomass may also include plastic residues such as recycled waste plastics like 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 in particular as power from renewable sources, which includes energy collected from renewable resources that are naturally replenished on the 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 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] This is preferably charged directly as a hot product having a temperature of 500 °C to 700 °C at the outlet of the direct reduction plant 11. Thereby, the amount of energy required for melting can be reduced. 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 provided 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 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, e.g. less than 0.004 weight percent sulfur, deoxidation and desulfurization are combined for overall higher performance. Thus, low sulfur grades benefit from performing 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 at any time as required to one or more secondary metallurgy tools 20A, 20B, such as 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 produced. Then, 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.

[0028] As shown in FIG. 2, the smelting furnace 13 is composed of a container 20 capable of accommodating 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 smelting furnace 13 may be, for example, an open slag bath furnace or an OSBF.

[0030] 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.

[0031] The electrodes 22 provide the electrical energy necessary to melt the charged raw materials to form pig iron. These are preferably Söderberg type electrodes.

[0032] During the melting of the raw materials, two layers are formed: a layer of pig iron 14, which has the highest density and is 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.

[0033] 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.

[0034] In the method according to the present invention, iron scrap typically of a size smaller than 80 mm is also charged into the smelting furnace 13 to be melted together with the DRI product 12. The scrap is preferentially subjected to a shredding step before charging.

[0035] This has the advantage of first increasing the scrap consumption in the ESF / BOF route and thus reducing the overall carbon footprint of the steelmaking process. Furthermore, the addition of scrap can increase the iron yield.

[0036] Another advantage is that due to the reducing conditions in the smelting furnace 13, the iron oxide present on the scrap is reduced in the furnace, and thus low-quality scrap can be used without specific pretreatment other than size reduction.

[0037] When the size of the scrap is small, it can be charged through the same opening as in the case of the DRI product 12.

[0038] In a preferred embodiment, the mass fraction of the iron scrap is 1 wt% to 20 wt% based on the amount of the DRI product supplied.

[0039] In a preferred embodiment, the charged scrap is E40 specification scrap according to the EU-27 steel scrap specification of the final update in May 2007.

[0040] In a preferred embodiment, a carbon-containing material is also added to the smelting furnace. The reaction between carbon and 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.

[0041] The carbon content of the pig iron 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 pig iron should preferably have a carbon content as close as possible to 4.5 wt%, which 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 wt%.

[0042] The carbon-containing material can be derived from different sources. This can be selected, for example, from 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 some 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 (14) within an electric steelmaking unit (13), comprising: i) providing a direct reduced iron product (12); ii) feeding said DRI product (12) into said steelmaking unit (13); iii) feeding iron scrap having a size of less than 80 mm together with said DRI product (13); iv) melting said DRI product (13) and said iron scrap to produce hot metal. A method as described above.

2. The method according to claim 1, wherein the mass fraction of the iron scrap is 1 wt% to 20 wt% based on the amount of the supplied DRI product.

3. The method according to claim 1 or 2, wherein said iron scrap is E40 scrap.

4. The method according to any one of claims 1 to 3, wherein said iron scrap is subjected to a shredding step and then fed into a steelmaking furnace (13).

5. The method according to any one of claims 1 to 4, wherein a carbon-containing material is also fed into the steelmaking furnace.

6. The method according to claim 4, wherein said carbon-containing material is added in an amount sufficient to reach a final carbon content of 4.0 to 4.5 wt% in the hot metal (14).

7. The method according to any one of claims 1 to 5, wherein the provided DRI product is produced using a reducing gas containing at least 50% by volume of hydrogen.

8. A method for producing steel, comprising transferring the hot metal produced according to any one of claims 1 to 7 from said steelmaking furnace (13) to a converter (17), and then reducing the carbon content of said hot metal to a value of less than 2.1 wt% by blowing oxygen to obtain molten steel.

9. The method for producing steel according to claim 8, wherein iron scrap (18) is added to and melted with said hot metal in said converter (17).

Images