Method for producing pig iron in an electric smelting furnace and related smelting furnace

Abstract

The present invention deals with a method for producing pig iron in an electric smelting furnace 13 comprising a vessel 20, the method comprising the following successive steps: - loading said vessel 20 with a DRI product, - melting said DRI product to form a pig iron layer 14 covered with a slag layer 23, - tapping said pig iron 14 into a ladle, and - adding silicon-bearing material directly to said pig iron 14 in at least one runner of said smelting furnace taphole 25. The present invention also deals with the production of steel from said pig iron and the associated electric smelting furnace.

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 major 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 major route includes the so-called "direct reduction process". Among them, there are methods such as those by brands MIDREX1 (registered trademark), FINMET (registered trademark), ENERGIRON (registered trademark) / HYL, COREX (registered trademark), FINEX (registered trademark), 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 in 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] Therefore, an object of the present invention is to improve the shortcomings of the production routes for pig iron and steel by providing a new route that efficiently minimizes the environmental impact of such manufacturing. [Means for solving the problem]

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

[0007] Such methods may also include optional features of claims 2 to 7, which are considered separately or in any possible technical combination.

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

[0009] Such methods may also include optional features of claim 9 or 10, which are considered separately or in any possible technical combination.

[0010] The present invention also deals with the smelting furnace described in claim 11.

[0011] Other features and advantages of the present invention will be apparent from the description of the invention, which is given below as reference to the accompanying drawings and is not in any way limiting. [Brief explanation of the drawing]

[0012] [Figure 1] Figure 1 shows the ironmaking and steelmaking processes via the smelting / BOF route. [Figure 2] Figure 2 shows a smelting furnace. [Modes for carrying out the invention]

[0013] The elements in the diagram are illustrative and may not be drawn to scale.

[0014] Figure 1 shows the steel production route via the DRI route, from iron reduction to the casting of steel into semi-finished products such as slabs, billets, blooms, or strips. The 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® technology or Energiron® technology. 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 present invention is produced using a reducing gas based on biogas generated from the combustion of biomass.

[0016] Biomass is renewable organic material derived from plants and animals. Biomass sources include, in particular, wood and wood processing waste, such as firewood, wood pellets and wood chips, sawdust and waste from sawmills and furniture factories, as well as black liquor from paper and pulp mills, crops and agricultural waste, such as corn, soybeans, sugarcane, switchgrass, woody plants and algae, and crop and food processing residues, but also include municipal solid waste, such as paper, cotton and wool products, as well as bio-derived materials in food, garden and wood waste, animal excrement and domestic wastewater. In the sense of the present invention, biomass may also include solid waste fuel or plastic residues such as recycled waste plastics like 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% by weight, and typically in the range of 2-3% by weight.

[0018] In another preferred embodiment, the DRI product used in the method according to the present invention is produced by a so-called H2-DRI process in which the reducing gas contains more than 50 volume% of hydrogen, preferably more than 60, 70, 80, or 90 volume% of hydrogen, or is composed entirely of hydrogen. The H2-DRI product contains much lower levels of carbon than natural gas or biogas DRI, typically less than 1 wt% or even lower. In a preferred embodiment, the hydrogen used in the DRI reducing gas is derived from the electrolysis of water, which is preferably partially or entirely powered by CO2-neutral power. CO2-neutral power includes power from renewable sources, defined as energy collected from renewable resources that are naturally replenished on a human timescale, including sources such as sunlight, wind, rain, tides, waves, and geothermal energy. In some embodiments, the use of power derived from nuclear sources is possible because it does not emit CO2 produced.

[0019] Regardless of the DRI process used, the resulting directly 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 pig iron.

[0020] DRI products can be transferred to the smelting furnace in various forms. Preferably, the directly reduced iron product (DRI product) is supplied to the smelting furnace in a hot form as an HDRI product (so-called Hot DRI), in a room temperature form as a CDRI product (so-called Cold DRI), or in a hot-formed form as an HBI product (so-called Hot Bricket Iron), and / or preferably in a particle form having an average particle size of up to 10.0 mm, more preferably up to 5.0 mm.

[0021] 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, when the direct reduction plant 11 and the steelmaking furnace 13 are not in the same location, or when the steelmaking 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 carried out.

[0022] The steelmaking furnace 13 uses the electrical energy provided by several electrodes to melt the DRI product 12 and produce hot metal 14. In a preferred embodiment, part or all of the required power is derived from CO2-neutral power. Further detailed description of the steelmaking furnace will be described later based on Figure 2.

[0023] Next, the hot metal 14 is transferred to a hot metal ladle through at least one tap hole 25 provided with at least one runner 26. 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. Usually, there are the same number of auxiliary runners as the tap holes, and the runners intersect to form a main runner to guide the extracted hot metal to the hot metal ladle.

[0024] This hot metal ladle may be a simple ladle or a torpedo ladle.

[0025] 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 molten metal and related 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% by weight. 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% by weight, 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] Desulfurization of 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 those reagents into the pig iron ladle. The desulfurized pig iron 16 preferably has a sulfur content of less than 0.03% by weight, preferably less than 0.004% by weight.

[0027] The desulfurized pig iron 16 can then be transferred into a converter 17. A converter basically converts 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 in order to benefit from the heat released by the exothermic reaction resulting from oxygen injection into the pig iron.

[0028] The molten steel 19 thus formed can then be transferred to one or more secondary metallurgical tools 20A, 20B, such as a ladle furnace, an RH (Ruhrstahl-Heareus) vacuum vessel, a vacuum tank degasser, an alloying and stirring station, etc., as needed, and processed to achieve the required steel composition according to the steel grade to be manufactured. The molten steel having the required composition 21 can then be transferred to a casting plant 22, where it can be transformed into solid products such as slabs, billets, blooms, or strips.

[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. The vessel 20 is closed by a roof which has several openings for receiving electrodes 22 inserted into the vessel 20, and other openings for allowing raw materials to be charged into the vessel 20.

[0030] Electrode 22 provides the electrical energy necessary to melt the charged raw material and form pig iron. These are preferably Soederberg type electrodes.

[0031] During the melting of the raw materials, two layers are formed: a layer of pig iron 14, which is densest and therefore 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.

[0032] The smelting furnace 13 may be a Submerged Arc Furnace (SAF) in which the electrodes are immersed in the slag layer 23, or an Open Slag Bath Furnace (OSBF) in which the electrodes 22 are located on top of the slag layer 23. Preferably, it is an OSBF as shown in the figure.

[0033] In the mechanism of the present invention, a silicon-containing material is added to pig iron in at least one of the runners 26 of the smelting furnace tap 25. Silicon has a strong deoxidizing ability at high temperatures, especially at about 1600°C, the temperature of 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 therefore provides additional energy for scrap melting. The more scrap used, the smaller the environmental footprint of the process.

[0034] This addition can be performed by top-feed injection, which is a low-cost operation, or by injection devices such as immersion lances, which provide high yields of up to 90% or more.

[0035] The inventors observed that by adding silicon to pig iron at that stage, the stepwise addition during tapping allows for good mixing with the pig iron, and that the pig iron then benefits from strong spontaneous mixing when it is tapped into the ladle.

[0036] 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.

[0037] When DRI products are used in the smelting furnace 13, a natural amount of silicon is typically obtained that is less than 0.2% by weight, and even less than 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, and more preferably 0.2 to 0.4% by weight. If necessary, further silicon can be added in the converter 17.

[0038] In a preferred embodiment, the carbon-containing material may be injected into the pig iron together with the silicon-containing material.

[0039] As described above, the carbon content of pig iron 14 produced via the DRI route is generally less than 3% by weight. However, 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 the saturation level of 4.5% by weight. In a preferred embodiment, the carbon content of the pig iron is in the range of 4.0 to 4.5% by weight.

[0040] In fact, carbon is necessary for the steelmaking process carried out in the converter 17 by blowing in oxygen. This is because the reaction between carbon and oxygen produces carbon monoxide gas, which leads to strong and efficient stirring of the molten metal and thus improves the removal of impurities from the steel. This reaction is exothermic and therefore provides additional energy for melting iron scrap, making it possible to incorporate larger quantities of such iron scrap resulting from steel recycling. The more iron scrap is used, the smaller the environmental footprint of the steelmaking process becomes.

[0041] Carbon-containing materials can originate from different sources. These can be selected from, for example, coke, anthracite, silicon carbide, calcium carbide, or a mixture of these sources, but some or all of the carbon load can also be advantageously obtained from renewable sources such as biomass. Biochar, in particular, can be used. Adding calcium carbide is especially advantageous because the calcium atoms can provide a desulfurization effect.

[0042] Adding silicon carbide is particularly advantageous because it allows for an increase in the carbon content of pig iron while also providing silicon. Adding a mixture of calcium carbide and silicon carbide is even more advantageous because it provides carbon and silicon addition while ensuring desulfurization.

Claims

1. A method for manufacturing steel, comprising the following steps: - A step of loading the DRI product into the container (20) of an electric smelting furnace (13) which is equipped with a tapping port (25), - The step of melting the DRI product to form a pig iron layer (14) covered with a slag layer (23), - The step of tapping the pig iron (14) into a ladle, and - A step of directly adding a silicon-containing material to the pig iron (14) in at least one runner of the smelting furnace tap (25), - The pig iron (14) is transferred to a converter (17), and then molten steel is obtained in which the carbon content of the pig iron is reduced to less than 2.1% by weight by blowing in oxygen. including, method.

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

3. A method for producing steel according to claim 1, wherein the silicon-containing material is injected through an immersion lance.

4. A method for producing steel according to claim 1, wherein the silicon-containing material is selected from among metallic silicon Si, silicon carbide SiC, silicomanganese SiMn, calcium silicate SiCa, ferrosilicon alloy FeSi, or a mixture thereof.

5. A method for producing steel according to claim 1, wherein the silicon-containing material added has particles having a particle size of less than 3 mm.

6. A method for producing steel according to claim 1, wherein the DRI product is produced using a reducing gas containing at least 50 volume percent of hydrogen before being loaded into the smelting furnace (13).

7. A method for producing steel according to claim 1, comprising adding a carbon-containing material to a silicon-containing material and injecting it into pig iron (14).

8. A method for producing steel according to claim 1, wherein iron scrap is added to the pig iron in the converter (17) and melted.

9. A method for producing steel according to claim 1, wherein the pig iron is transferred from the smelting furnace (13) to a desulfurization station (15), and then to the converter (17).

Images