WO2024023568A1 - A method of manufacturing molten pig iron into an electrical smelting unit - Google Patents

Abstract

The invention deals with a method of manufacturing molten pig iron into an electrical smelting unit. The method comprises the following successive steps: - providing a directly reduced iron product 12, - feeding the DRI product 12 into the smelting unit 13, - feeding together with the DRI product 13, ferrous scrap having a size lower than 80mm, - melting the DRI product 13 and the ferrous scrap to produce molten pig iron. The invention also deals with a method to produce liquid steel from manufactured pig iron.

Description

A method of manufacturing molten pig iron into an electrical smelting unit

[001 ] The invention is related to a method of manufacturing pig iron, also called hot metal and to a method of producing steel out of such pig iron.

[002] Steel can be currently produced through two mains manufacturing routes. Nowadays, most commonly used production route named “BF-BOF route” consists in producing hot metal in a blast furnace, by use of a reducing agent, mainly coke, to reduce iron oxides and then transform hot metal into steel into a converter process or Basic Oxygen furnace (BOF). This route, both in the production of coke from coal in a coking plant and in the production of the hot metal, releases significant quantities of CO2.

[003] The second main route involves so-called “direct reduction methods”. Among them are methods according to the brands MIDREX®, FINMET®, ENERGIRON®/HYL, COREX®, FINEX® etc., in which sponge iron is produced in the form of HDRI (hot direct reduced iron), CDRI (cold direct reduced iron), or HBI (hot briquetted iron) from the direct reduction of iron oxide carriers. Sponge iron in the form of HDRI, CDRI, and HBI undergoes further processing in electric furnaces to produce steel.

[004] One of the main options chosen by steelmakers to reduce CO2 emissions is therefore to switch from the BF-BOF route towards the DRI route. However, use of DRI products in classical electrical furnaces together with ferrous scraps has some limitations. Indeed, scraps contain a lot of impurities and resulting liquid steel will need to be further processed to produce high quality steel grades. Investment on new liquid steel treatment tools would thus be necessary.

[005] The aim of the present invention is therefore to remedy the drawbacks of the pig iron and steelmaking manufacturing routes by providing a new route efficiently minimizing the environmental impact of such manufacturing without incurring heavy investments. [006] This problem is solved by a method for manufacturing pig iron as detailed in claim 1.

[007] Such method may also comprise the optional characteristics of claims 2 to 7 considered separately or in any possible technical combinations.

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

[009] Such method may also comprise the optional characteristics of claim 9.

[0010] Other characteristics and advantages of the invention will emerge clearly from the description of it that is given below by way of an indication and which is in no way restrictive, with reference to the appended figures in which:

Figure 1 illustrates a pig iron and steelmaking process according to the smelting I BOF route,

Figure 2 illustrates a smelting furnace.

Elements in the figures are illustration and may not have been drawn to scale.

[0011 ] Figure 1 illustrates a steel production route according to the DRI route, from the reduction of iron to the casting of the steel into semi-products such as slabs, billets, blooms, or strips.

[0012] Iron ore 10 is first reduced in a direct reduction plant 11 . This direct reduction plant 11 may be designed to implement any kind of direct reduction technology such as MIDREX® technology or Energiron®. The direct reduction process may for example be a traditional natural-gas or a biogas-based process.

[0013] In a preferred embodiment, the DRI product used in the method according to the invention is manufactured using a reducing gas based on biogas coming from combustion of biomass.

[0014] Biomass is renewable organic material that comes from plants and animals. Biomass sources include notably wood and wood processing wastes such as firewood, wood pellets, and wood chips, lumber and furniture mill sawdust and waste, and black liquor from pulp and paper mills, agricultural crops and waste materials such as corn, soybeans, sugar cane, switchgrass, woody plants, and algae, and crop and food processing residues, but also biogenic materials in municipal solid waste such as paper, cotton, and wool products, and food, yard, and wood wastes, animal manure and human sewage. In the sense of the invention, biomass may also encompass plastics residues, such as recycled waste plastics like Solid Refuse Fuels or SRF.

[0015] Whenever using natural gas or biogas as reducing gas, the carbon content of the DRI product can be set to a maximum of 3 % in weight and usually to a range of 2 to 3% in weight.

[0016] In another preferred embodiment, the DRI product used in the method according to the invention is manufactured through a so called H2-DRI process where the reducing gas comprises more than 50 % and preferably more than 60, 70, 80 or 90 % in volume of hydrogen or is even entirely made of hydrogen. The H2- DRI product will contain a far lower level of carbon than the natural gas or biogas DRI, so typically below 1 % in weight or even lower.

[0017] In a preferred embodiment, the hydrogen used in the DRI reducing gas comes from the electrolysis of water, which is preferably powered in part or all by CO2 neutral electricity. CO2 neutral electricity includes notably electricity from renewable source which is defined as energy that is collected from renewable resources, which are naturally replenished on a human timescale, including sources like sunlight, wind, rain, tides, waves, and geothermal heat. In some embodiments, the use of electricity coming from nuclear sources can be used as it is not emitting CO2 to be produced.

[0018] Whatever 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 pig iron.

[0019] The DRI product can be transferred to the smelting furnace in various forms. Preferably, the directly reduced iron product (DRI product) is fed to the smelting furnace in a hot form as HDRI product (so-called Hot DRI), or in a cold form as CDRI product (so-called Cold DRI), or in hot briquette form as HBI product (so-called Hot Briquetted Iron) and/or in particulate form, preferably with an average particle diameter of at most 10.0 mm, more preferably with an average particle diameter of at most 5.0 mm.

[0020] It is preferably charged directly at the exit of the direct reduction plant 11 as a hot product with a temperature from 500°C to 700°C. This allows reducing the amount of energy needed to melt it. When hot charging is not possible, for example if the direct reduction plant 11 and the smelting furnace 13 are not on same location, or if the smelting furnace 13 is stopped for maintenance and thus DRI product must be stored, then the DRI product may be charged cold, or a preheating step may be performed.

[0021 ] The smelting furnace 13 uses electric energy provided by several electrodes to melt the DRI product 12 and produce a pig iron 14. In a preferred embodiment, part of or all the electricity needed comes from CO2 neutral electricity. Further detailed description of the smelting furnace will be given later, based on figure 2.

[0022] The pig iron 14 can be optionally sent to a desulphurization station 15 to perform a desulphurization step. This desulphurization step may be performed in a dedicated vessel or preferentially directly in the pig iron ladle to avoid molten metal transfer and associated heat losses. This desulphurization step is needed for production of steel grades requiring a low Sulphur content, which is, for example set at a maximum of 0.03 weight percent of Sulphur. Desulfurization in oxidizing conditions is not effective and is thus preferentially performed either on pig iron before oxygen refining, or in steel ladle after steel deoxidizing. For very low sulfur contents, for example below 0.004 weight percent of sulfur, deoxidizing and desulphurization are combined for overall higher performance. Low sulfur grades thus benefit from performing pig iron desulfurization before the conversion step.

[0023] Desulphurization of the pig iron can be done by adding reagents, notably based on calcium or magnesium compounds, such as sodium carbonate, lime, calcium carbide, and/or magnesium into the pig iron. It may be done for example by injection of those reagents in the pig iron ladle. The desulphurized pig iron 16 has preferentially a content of Sulphur lower than 0.03 % in weight and preferably lower than 0.004 % in weight. [0024] The desulphurized pig iron 16 can then be transferred into a converter 17. The converter basically turns the molten metal into liquid steel by blowing oxygen through molten metal to decarburize it. It is commonly named Basic Oxygen Furnace (BOF). Ferrous scraps 18, coming from recycling of steel, may also be charged into the converter 17 to take benefit of the heat released by the exothermic reactions resulting from the oxygen injection into pig iron.

[0025] Liquid steel 19 thus formed can then be transferred, whenever needed, to one or more secondary metallurgy tools 20A, 20B such as Ladle furnaces, RH (Ruhrstahl-Heareus) vacuum vessel, Vacuum Tank degasser, alloying and stirring stations, etc.... to be treated to reach the required steel composition according to the steel grades to be produced. Liquid steel with the required composition 21 can then be transferred to a casting plant 22 where it can be turned into solid products, such as slabs, billets, blooms, or strips.

[0026] As shown on figure 2, the smelting furnace 13 is composed of a vessel 20 able to contain hot metal. The vessel 20 may have a circular or a rectangular shape, for example. This vessel 20 is closed by a roof provided with some apertures to receive electrodes 22 to be inserted into the vessel 20 and with other apertures to allow charging of the raw materials into the vessel 20.

[0027] The smelting furnace 13 may be for example an open-slag bath furnace or OSBF.

[0028] The vessel 20 is also provided with at least one tap hole 25 to allow tapping of manufactured pig iron. Such tap holes 25 are located in the lower part of the vessel 20. They may be located in the lateral walls of the vessel or in its bottom wall.

[0029] The electrodes 22 provide the required electric energy to melt the charged raw materials and form pig iron. They are preferably Soderberg-type electrodes.

During the melting of the raw materials, two layers are formed, a pig iron 14 layer which is the densest and is thus located at the bottom of the vessel 20 and a slag layer 23 located above the pig iron 14. The slag layer 23 can be partially covered by piles of raw materials 24 waiting to be melted.

[0030] The smelting furnace 13 may be a SAF (Submerged-Arc Furnace) wherein the electrodes are immersed into the slag layer 23 or an OSBF (open-slag bath furnace) wherein the electrodes 22 are located above the slag layer 23. It is preferentially an OSBF as illustrated in the figures.

[0031 ] In the method according to the invention small size ferrous scraps, typically below than 80mm, are also charged into the smelting furnace 13 to be melted with DRI product 12. Scrap is preferentially subjected to a shredding step before charging.

[0032] This has first the advantage to increase the scrap consumption into the ESF/BOF route and thus to reduce the overall carbon footprint of the steelmaking process. Moreover, addition of scrap allows to increase the iron yield.

[0033] Another advantage is that thanks to the reducing conditions within the smelting furnace 13, oxidized iron present on scrap is reduced in the furnace and thus lower quality scrap may be used without specific pre-treatment besides size reduction.

[0034] Small size of the scrap allows charging through the same aperture as for the DRI Products 12.

[0035] In a preferred embodiment the mass fraction of ferrous scrap is from 1 % to 20% by weight, based on the amount of DRI products fed in.

[0036] In a preferred embodiment charged scrap is E40 specification scrap according to Ell-27 steel scrap specification, last update of May 2007.

[0037] In a preferred embodiment a carbon-containing material is also added to the smelting furnace. Reaction of carbon with oxygen in the converter creates carbon monoxide gas, which provides intense and efficient stirring of the molten metal and thus improves the removal of impurities from the steel. This reaction is also exothermic and therefore provides additional energy for scrap melting. The more scrap is used, the smaller the environmental footprint of the process.

[0038] The carbon content of the pig iron 14 produced through the DRI route will generally be lower than 3 % in weight. However, to fulfil the requirements of the subsequent steelmaking process at the converter, the pig iron should preferentially have a carbon content as close as possible to 4.5% in weight, which is the level of saturation. In a preferred embodiment, the pig iron carbon content is set in the range of 4.0 to 4.5% in weight through the addition of carbon containing material. [0039] The carbon containing material may come from different sources. It may be chosen, for example, among coke, anthracite, silicon carbide, calcium carbide, or a mixture of any of those sources, but can also advantageously come from renewable sources like biomass for part or all the carbon loads. In particular, biochar can be used. Adding calcium carbide is particularly advantageous as the calcium atoms can provide a desulphurizing effect.

Claims

1 ) A method of manufacturing molten pig iron (14) into an electrical smelting unit (13), comprising the steps of: i) providing a directly reduced iron product (12), ii) feeding the DRI product (12) into the smelting unit (13), iii) feeding together with the DRI product (13) ferrous scrap having a size lower than 80mm, iv) melting the DRI product (13) and the ferrous scrap to produce molten pig iron.

2) A method according to claim 1 wherein the mass fraction of ferrous scrap is from 1 % to 20% by weight, based on the amount of DRI product fed in.

3) A method according to claim 1 or 2 wherein said ferrous scrap is E40 scrap.

4) A method according to anyone of the previous claims wherein said ferrous scraps are subjected to a shredded step before being fed to the smelting furnace (13).

5) A method according to anyone of the previous claims wherein a carbon-bearing material is also fed to the smelting furnace.

6) A method according to claim 4, wherein said carbon containing material is fed in an amount sufficient to reach a final carbon content of 4.0 to 4.5% in weight in the pig iron (14).

7) A method according to anyone of claims 1 to 5 wherein the provided DRI product is manufactured using a reducing gas containing at least 50 % in volume of hydrogen.

8) A method for manufacturing steel wherein pig iron manufactured according to anyone of claims 1 to 7 is transferred from said smelting furnace (13) to a converter (17) wherein the carbon content of said pig iron is then lowered to a value below 2.1 percent in weight by oxygen blowing, so as to obtain liquid steel.

9) A method for manufacturing steel according to claim 8, wherein ferrous scraps

(18) are added to said pig iron in said converter (17) and melted.