Method for producing hot metal in an electric smelting furnace

Abstract

The present invention relates to a method for producing hot metal in an electric smelting furnace 13, comprising the following consecutive steps: - providing a direct reduced iron product 12, - providing a carbon-containing material 30, - alternately feeding the DRI product 12 and the carbon-containing material 30 into at least a part of the smelting furnace, - melting the DRI product 12 and the carbon-containing material 30 to produce hot metal 14. The present invention also relates to the production of steel from 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 a 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 steel manufacturers 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.

[0005] Another option is to use an electrically powered smelting furnace to melt the DRI product to produce pig iron. This option has the advantage that pig iron is produced in a blast furnace, allowing for oxide removal in the molten slag, thus allowing for the use of classic steel processing tools such as basic oxygen furnaces and refining ladles. However, the pig iron obtained via this route has a relatively low carbon content compared to classic pig iron. This paradoxically reduces the environmental benefits of this route, since the higher the carbon content, the more recycled scrap metal can be added to the BOF. Summary of the Invention [Problem to be solved by the invention]

[0006] It is therefore an object of the present invention to ameliorate the shortcomings of the pig iron and steel production route by providing a new route that efficiently minimizes the environmental impact of such production. [Means for solving the problem]

[0007] This problem is solved by a method for producing pig iron as detailed in claim 1.

[0008] Such a method may also comprise the optional features of claims 2 to 7 considered separately or in any possible technical combination.

[0009] The invention also deals with a method for producing a steel according to claim 8.

[0010] Such a method may also comprise the optional features of claim 9 or 10, obtained separately or in all possible technical combinations.

[0011] Other characteristics and advantages of the present invention will become apparent from the description of the invention given below by way of indication and which is in no way limiting, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1 is a diagram showing the pig iron and steelmaking process by the smelting / BOF route. [Figure 2] FIG. 2 is a diagram showing a smelting furnace. [Figure 3] FIG. 3 is a diagram illustrating raw material charging according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

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

[0014] Figure 1 shows the steel production route via the DRI route 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 plant 11. This direct reduction plant 11 can be designed to implement any kind of direct reduction technology, such as MIDREX® technology or Energiron®. The direct reduction process can be, for example, a traditional natural gas or biogas based process.

[0016] 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.

[0017] 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 such as solid waste 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%, usually in the range of 2 - 3 wt%.

[0019] 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.

[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 is defined as electricity from renewable sources, which are naturally replenished on a human time scale, including in particular sources such as sunlight, wind, rain, tides, waves and geothermal energy. In some embodiments, the use of electricity from nuclear sources is possible because it does not emit the CO2 being produced.

[0021] 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 iron oxide is completed, and the product is melted to produce hot metal.

[0022] 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 may be charged at room temperature or a preheating step may be performed.

[0023] 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 may be charged at room temperature or a preheating step may be performed.

[0024] The smelting furnace 13 uses electrical energy provided by several electrodes to melt the DRI product 12 and produce hot metal 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 Figure 2.

[0025] The pig iron 14 can optionally be sent to a desulfurization station 15 to carry out 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 the molten metal and the 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 wt%. 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, for example less than 0.004 wt%, deoxidation and desulfurization are combined for overall higher performance. Thus, low sulfur grades benefit from carrying out pig iron desulfurization before the conversion step.

[0026] The 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, in particular, 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%.

[0027] 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 the recycling of steel can also be charged into the converter 17 in order to benefit from the heat released by the exothermic reaction resulting from the oxygen injection into the pig iron.

[0028] The molten steel 19 thus formed can then be transferred whenever necessary to one or more secondary metallurgical tools 20A, 20B, such as ladle furnaces, Ruhrstahl-Heareus (RH) vacuum vessels, vacuum tank degassing units, alloying and stirring stations, etc., and processed to reach the required steel composition according to the steel grade to be produced. The molten steel having the required composition 21 can then be transferred to a casting plant 22, where it can be converted into solid products such as slabs, billets, blooms, or strip.

[0029] As shown in Figure 2, the smelting furnace 13 consists of a vessel 20 capable of containing molten iron. The vessel 20 may be, for example, circular or rectangular. This vessel 20 is closed by a roof provided with several openings for receiving electrodes 22 to be inserted into the vessel 20 and other openings for allowing the charging of raw materials into the vessel 20.

[0030] The vessel 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 located in the lower part of the vessel 20. They may also be located in the side walls of the vessel or in its bottom wall.

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

[0032] During melting of the raw materials, 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, which is 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] As described above, the carbon content of the hot metal 14 produced via the DRI route is generally less than 3% by weight. 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% by weight. In a preferred embodiment, the carbon content of the hot metal is in the range of 4.0 to 4.5% by weight.

[0035] In fact, carbon is required for the steelmaking process carried out in the converter 17 by blowing oxygen. This is because the reaction between carbon and oxygen 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 exothermic and thus provides additional energy for melting iron scrap, making it possible to incorporate a larger amount of such iron scrap resulting from steel recycling. The more iron scrap is used, the smaller the environmental footprint of the steelmaking process.

[0036] In the mechanism of the present invention, during charging into the vessel 20, the DRI product 12 is charged alternately with the carbon-containing material 30.

[0037] The result of this alternating supply is schematically shown in FIG. 3. The raw material pile 24 is composed of alternating layers of the carbon-containing material 30 and the DRI product 12. This is only a schematic diagram, and it is obvious to those skilled in the art that the pile of raw materials is not so clear and the boundary between different layers is more ambiguous. During melting, FeC droplets are formed and transported towards the layer of hot metal 14 by the circulation of the natural slag 23.

[0038] As an example, the smelting furnace 13 is provided with charging means comprising at least two hoppers connected to at least one opening designed on the roof of the container 20. At least one hopper contains the DRI product 12, and at least one other hopper contains the carbon-containing material 30. When the level of the molten metal is low, the first hopper opens to charge DRI into the container 20, and the second hopper remains closed. Next, the first hopper is closed and the second hopper is opened, and an amount of carbon-containing material necessary to reach the required carbon content in the pig iron close to 4.5 wt% as much as possible as described above is added.

[0039] This method of adding carbon enables the overall density of the raw materials to be close to 100% DRI charge, and thus the presence of the carbon-containing material does not impair the downward movement of this charge during melting.

[0040] Next, the carbon is transported through the slag layer to the liquid metal by gravity supply in the DRI pile. The slag layer 23 can have a layer thickness exceeding 50 cm, and the density of the carbon source is usually lower than the slag density itself. This causes a physical limitation for the carbon to enter the pig iron layer 14 through the slag. In the method according to the present invention, it is possible to overcome this limitation.

[0041] Furthermore, carbonization requires a large amount of energy, and since it can be optimally provided by electric heating in the smelting furnace rather than an additional heating station, the addition of carbon in the smelting stage ensures the optimal energy efficiency of the steelmaking process.

[0042] The carbon-containing material can be derived from different sources. This can be selected, for example, from among coke, anthracite, silicon carbide, calcium carbide or any mixture of these sources, but it can also be advantageously 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.

[0043] The carbon-containing material to be added preferably has the same size as the DRI product in order to use the same feeding device.

[0044] As an example, for DRI products obtained from the natural gas reduction process, 30 kg of anthracite is added per ton of pig iron produced.

[0045] In a preferred embodiment, a desulfurization reagent can also be fed together with the carbon-containing material. Such reagents can be based in particular on calcium compounds such as sodium carbonate, lime and / or calcium carbide.

[0046] The final sulfur content of the pig iron is preferably set to a maximum of 0.03 wt%, preferably a maximum of 0.004 wt%.

[0047] By performing desulfurization in the smelting furnace, the need for desulfurization treatment between the smelting furnace 13 and the converter 17 can be suppressed, or such treatment can be at least reduced.

[0048] In a preferred embodiment, the silicon-containing material can be charged together with the carbon-containing material regardless of the presence or absence of the desulfurization reagent. Silicon has a strong deoxidizing ability at high temperatures, particularly at about 1600 °C, which is the temperature of the molten steel in the converter. Silicon reacts with oxygen and then contributes to the formation of slag in the converter. This reaction is exothermic and thus provides additional energy for scrap melting. The more scrap is used, the smaller the environmental footprint of the process.

[0049] Such silicon can be added in different forms. The silicon may be metallic silicon Si, silicon carbide SiC, silicomanganese SiMn, calcium silicate SiCa, or ferrosilicon alloy FeSi such as FeSi75 or FeSi65.

[0050] When using DRI products in the smelting furnace 13, a natural amount of silicon of usually less than 0.2% by weight, and further less than 0.1% by weight, is brought about. The final silicon content of the pig iron is preferentially 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 converter 17.

[0051] Adding silicon carbide is particularly advantageous because it enables an increase in the silicon content of the pig iron after adding carbon. Adding a mixture of calcium carbide and silicon carbide is even more advantageous because it provides the addition of carbon and silicon while ensuring desulfurization.

Claims

1. A method for producing hot metal in an electric smelting furnace (13), comprising: i) providing a direct reduced iron product (12); ii) providing a carbon-containing material (30); iii) alternately feeding said DRI product (12) and said carbon-containing material (30) to at least a part of said smelting furnace; iv) melting said DRI product (12) and said carbon-containing material (30) to produce hot metal (14). A method as described above.

2. The method according to claim 1, wherein said carbon-containing material (30) is added in an amount sufficient to reach a final carbon content of 4.0 to 4.5 wt% in the hot metal (14).

3. The method according to claim 1, wherein said carbon-containing material is selected from coke, anthracite, silicon carbide, calcium carbide, biomass, carbon resulting from the combustion of biomass, or a mixture of any of these materials.

4. The method according to claim 1 or 2, wherein said DRI product is provided at a temperature of 500 to 700 °C.

5. The method according to any one of claims 1 to 4, wherein said DRI product is produced using a reducing gas containing at least 50 vol% hydrogen before being charged into said smelting furnace (13).

6. The method according to any one of claims 1 to 5, wherein a silicon-containing material is added to said carbon-containing material and fed into said smelting furnace.

7. The method according to any one of claims 1 to 6, wherein said carbon-containing material (30) and said DRI product (12) are provided as briquettes having the same size.

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

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

10. The method according to claim 8 or 9, wherein said hot metal is transferred from said smelting furnace (13) to a desulfurization station (15) before being transferred to said converter (17).

Images