A method for producing molten pig iron into an electrical smelting unit

Abstract

A method of manufacturing molten pig iron into an electrical smelting unit. The method includes the following successive steps: providing a directly reduced iron product, feeding the DRI product into the smelting unit, feeding together with the DRI product at least one steel or ironmaking by-product-based material including at least 10% in weight of slag forming agents, and melting the DRI product and the at least one steel or ironmaking by-product-based material to produce molten pig iron. A steel manufacturing method using the pig iron is also provided.

Claims

1-11. (canceled)

12. A method for manufacturing molten pig iron in an electrical smelting unit, the method comprising the following steps of: providing a directly reduced iron product; feeding the DRI product into the smelting unit; feeding, together with the DRI product, at a least one steel or ironmaking by-product-based material comprising at least 10% in weight of slag forming agents; and melting the DRI product and the at least one steel or ironmaking by-product-based material to produce molten pig iron.

13. The method according to claim 12 wherein the steel or iron-making by-products forming the by-product-based material are chosen from at least one of the group consisting of: sinter dust, steelmaking sludges or dusts, smelting sludges or dusts, secondary metallurgy slag, electric arc furnace slag, basic oxygen furnace slag, mill scale and any of their combinations.

14. The method according to claim 12 wherein the by-product-based material further includes an iron content upper than 20% in weight, at least a part of the iron being in an oxidized form.

15. The method according to claim 12 wherein the slag forming agents are chosen from at least one of the group consisting of: CaO, lime, alumina, magnesia, aluminosilicate and any of their combinations.

16. The method according to claim 12 wherein the steel or ironmaking by-product-based material is fed as briquettes or pellets.

17. The method according to claim 16 wherein the briquettes or pellets are prepared according to the following methods: crushing of steel or ironmaking by-products; sieving of the crushed steel or ironmaking by-products; mixing of the sieved steel or ironmaking by-products in appropriate amounts to reach a targeted composition of the steel or ironmaking by-product-based material; and briquetting or pelletizing of the mixture.

18. The method according to claim 12 wherein a carbon-containing material is also fed to the smelting unit.

19. The method according to claim 18 wherein the carbon containing material is added in an amount sufficient to reach a final carbon content of 4.0 to 4.5% in weight in the pig iron.

20. The method according to claim 12 wherein the DRI product is manufactured using a reducing gas containing at least 50% in volume of hydrogen before being loaded in said smelting furnace.

21. A method for manufacturing steel employing the method as recited in claim 12 comprising the steps of: transferring the pig iron from the smelting furnace to a converter; and lowering a carbon content of the pig iron to a value below 2.1 percent in weight by blowing oxygen to obtain liquid steel.

22. The method for manufacturing steel according to claim 21 wherein ferrous scraps are added to the pig iron in the converter, the ferrous scraps being melted.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] Other characteristics and advantages of the present 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:

[0013] FIG. 1 illustrates schematically a pig iron and steelmaking process according to the smelting/BOF route, and

[0014] FIG. 2 illustrates schematically a smelting furnace.

[0015] Elements in the figures are illustrations and may not have been drawn to scale.

DETAILED DESCRIPTION

[0016] FIG. 1 illustrates schematically 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.

[0017] 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.

[0018] In a preferred embodiment, the DRI product used in the method according to the present invention is manufactured using a reducing gas based on biogas coming from combustion of biomass.

[0019] 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 present invention, biomass may also encompass plastics residues, such as recycled waste plastics like Solid Refuse Fuels or SRF.

[0020] 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.

[0021] In another preferred embodiment, the DRI product used in the method according to the present invention is manufactured through a so called H.sub.2-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 H.sub.2-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.

[0022] 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 CO.sub.2 neutral electricity. CO.sub.2 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 CO.sub.2 to be produced.

[0023] Whichever DRI process is 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.

[0024] 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.

[0025] 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.

[0026] 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 or all of the electricity needed comes from CO.sub.2 neutral electricity. Further detailed description of the smelting furnace will be given later, based on FIG. 2.

[0027] The pig iron 14 can be optionally sent to a desulphurization station 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.

[0028] Desulphurization of the pig iron 14 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 14. 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.

[0029] 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.

[0030] 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, et cetera 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 122 where it can be turned into solid products, such as slabs, billets, blooms, or strips.

[0031] As shown schematically in FIG. 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.

[0032] 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.

[0033] The electrodes 22 provide the required electric energy to melt the charged raw materials and form pig iron. They are preferably Sbderberg-type electrodes.

[0034] 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.

[0035] The smelting furnace 13 may be a SAF (Submerged-Arc Furnace) wherein the electrodes 22 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.

[0036] In the method according to the present invention, at least one steel or ironmaking by-product-based material containing more than 10% in weight of slag forming agents are also charged in the smelting furnace 13.

[0037] This allows to control the slag formation and chemistry in the smelting furnaces. These slag forming agents may be chosen among at least one of CaO, lime, alumina, magnesia, aluminosilicate. They are preferentially CaO or Alumina which allows to get a slag composition suitable for usage in cement industry.

[0038] The by-products used to form the by-product-based material may be chosen among at least one of sintering dust or sludges, steelmaking dust or sludges, smelting dust or sludges, electric arc furnace slag, basic oxygen furnace slag, secondary metallurgy slag or mill scale. It may also be a mixture of those different by-products.

[0039] Sintering or steelmaking dust/sludges or smelting dust/sludges are sludges resulting from the dedusting of exhaust gases from the considered furnaces, such as Basic Oxygen Furnace, Electric Arc furnaces, sintering plants and smelting furnaces. They will be in form of sludge or dust depending on the treatment applied to the exhaust gas, either a dry treatment, such as use of fabric filters or a wet treatment such as water spraying. Electric arc furnace slag and basic oxygen slag or secondary metallurgy slags are slag formed during the liquid steel production. Scale or Mill scale is the flaky surface of hot rolled steel, consisting of the mixed iron oxides iron(II) oxide (FeO), iron(III) oxide (Fe2O3), and iron(II,III) oxide (Fe3O4, magnetite). Mill scale is formed on the outer surfaces of steel plates, sheets or profiles when they are being produced by rolling steel semi-products in rolling mills.

[0040] Typical compositions of some by-products are indicated in table 1 below. All percentages are expressed in weight percent. For iron (Fe) the content encompasses content of metallic iron (Fe) or any oxides (FeO, Fe2O3, Fe3O4).

TABLE-US-00001 TABLE 1 Fe P S Mg SiO2 Al Zn Al2O3 TiO2 CaO MgO BOF Slag 10-35 0-2.5 <0.15 8-24 1-6 0.4-2 30-55 5-15 EAF Slag 20-63 <2 <0.2 9-26 2-16.5 28-64 5-15.5 EAF sludge 20-40 0.1-2 5-14 2-30 BOF sludge 48-80 <3 3.0-17 Scale 33-72 0.4-16 Sinter dust 44-50 0.01-0.25 0.2-4.1 0-1 0.4-2.2 Secondary 0.1-20 1-40 20-65 2-20 metallurgy slag

[0041] In a preferred embodiment the by-product contains also at least 20% in weight of iron, part of this iron being under an oxidised form.

[0042] Currently recycling of iron-bearing by-products is done at the steelmaking vessels themselves (BOF/EAF) or back to sintering. The iron contained in these materials is oxidized and the strongly endothermic reduction is done in vessels where energy is provided by carbon combustion, thus limiting the environmental benefit of such recycling. With the method according to the present invention the reduction is done chemically by carbon and the thermal impact is compensated with electricity.

[0043] Another advantage is that the inventors have discovered is that the iron recovery rate is very high in smelting operation, more than 90%, which is much higher than in current recycling practices. For example, recycling in current steelmaking vessels may lead to a partial or low iron reduction thus increasing slag mass and oxidation rate which imply extra cost in energy for heating and melting without recovering iron.

[0044] In a preferred embodiment the by-products are fed to the smelting furnace in form of briquettes or pellets. Before the briquetting or the pelletizing, they may first be subjected to preparation steps, including, but not limited to crushing and sieving of the chosen by-products followed by the mixing of the sieved by products so as to obtain the requited material composition, namely a 10% in weight of slag forming agents and optionally at least 20% in weight of iron. This allows to increase versatility in the sources and the combination of the different mentioned materials to form mixed briquettes or pellets.

[0045] 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.

[0046] 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.

[0047] 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.