A steelmaking method

Abstract

A steelmaking method comprising a step of melting a metallic load (L) in a vessel (4) to produce a bath of molten metal (1) topped by a slag layer (3), said slag layer (3) having a density Ds, wherein the method further comprises the loading of an iron-carbon composite material (5) into the vessel (4), said composite material (5) having a density DC inferior or equal to the density DS of the slag layer (3).

Description

A steelmaking method

[0001] The invention is related to a steelmaking method comprising a step of melting a load L with an iron-carbon composite material.

[0002] 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.

[0003] 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.

[0004] 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 iron oxides are reduced to iron by use of a reducing gas comprising carbon monoxide and hydrogen, flowing counter current of the charged oxides. Resulting product is Direct Reduced Iron (DRI) which may take different forms such as HDRI (Hot Direct Reduced Iron), CDRI (cold direct reduced iron), or HBI (hot briquetted iron). The direct reduction process however still produces CO2, resulting from the reactions of reduction of the iron oxides.

[0005] One solution which is currently developed is the progressive increase of the hydrogen content into the reducing gas, in view of reaching a pure hydrogen reducing gas. Following reduction reaction will then occur:Fe2O3 + 3 H2 = 2 Fe + 3 H2O thus releasing harmless H2O instead of the greenhouse gas CO2.

[0006] This however implies that the content of carbon into the reducing gas will be reduced and at some point, no more carbon will be injected into the shaft. Yet this carbon is not only used for the reduction of iron oxides but also to carburize the resulting DRI. Reducing the amount of carbon injected means that the DRI product will have a smaller and smaller carbon content, and even no carbon at all.

[0007] Content of carbon in the DRI product is a key parameter at it plays an important role into the subsequent steps, notably for slag foaming at the Electric Arc furnace (EAF). Slag foaming is important for the energy efficiency of the process but also to protect the electrodes and the refractories of the EAF.

[0008] The aim of the present invention is therefore to remedy the above-mentioned drawbacks and to provide a steelmaking process with reduced environmental impact, while maintaining its productivity and the life of the equipment.

[0009] This problem is solved by a method according to the invention, method comprising a step of melting a metallic load in a vessel to produce a bath of molten metal topped by a slag layer, said slag layer having a density Ds, wherein the method further comprises the loading of an iron-carbon composite material into the vessel, said composite material having a density De inferior or equal to the density Ds of the slag layer.

[0010] The method of the invention may also comprise the following optional characteristics considered separately or according to all possible technical combinations: the composite material consists of lignocellulosic biomass having a moisture content from 5 to 10% by weight and an iron material comprising at least 30% of iron oxides, the molar ratio Fe / C of said composite material being from 3 to 6.5, the molar ratio Fe / C of said composite material is from 3 to 5.9, the composite material comprises biochar and an iron material comprising at least 30% of iron oxides, the molar ratio Fe / C of said composite material being from 3 to 6.5, before its loading into the vessel, the composite material is subjected to a reduction step to reduce the iron oxides to iron, the vessel is part of an electric arc furnace, the composite material density De is inferior or equal to 2.6 g / cm3.

[0011] Other features and advantages of the invention will be apparent from the description of the inventions 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 an equipment to perform a method according to the invention,

[0012] Elements in the figures are illustration and may not have been drawn to scale

[0013] Figure 1 illustrates a steelmaking method according to the invention. A load L is charged into a vessel 4. The vessel 4 is able to melt the load L to produce a bath of molten metal 1 topped by a slag layer 3, said slag having a density Ds.

[0014] The vessel 4 is for example part of an Electrical Arc Furnace (EAF) plant configured for melting the load L by generating electrical arcs.

[0015] The EAF is equipped with two or more electrodes 6 configured for generating electrical arcs between the electrodes 6 and the load L received in the vessel 4.

[0016] Each electrode 6 is for example made of graphite.

[0017] Each electrode 6 is for example connected to an electrical source 8. The electrical source 8 comprises for example an electrical network and / or an electrical power plant using preferably one or several renewable energy sources.

[0018] The electrical power plant is preferably operated using CO2 neutral electricity which includes notably electricity from renewable sources which is defined as energy that is produced from renewable resources, which are naturally replenished on a human timescale, including sources like sunlight, wind, rain, tides, waves, geothermal heat and biogas.

[0019] In some embodiments, the use of electricity coming from nuclear sources can be used as it is not emitting CO2 to be produced.

[0020] A biogas is a renewable energy source that can be obtained by the breakdown of organic matter in the absence of oxygen inside a closed system called bioreactor. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste, food waste or any biodegradable materials.

[0021] The EAF 2 is also equipped with an oxygen injection system configured for injecting oxygen in the vessel during operation. The oxygen injection system 20 is configured for injecting oxygen into the vessel 4 during melting of the load L and / or, preferably, after melting of the load L.

[0022] The load L contains metal materials, in particular steel scrap 10 and, optionally, iron in addition to the steel scrap 10, in particular direct reduced iron DRI 11 and / or pig iron 12.

[0023] In some examples, the load L comprises at least 40% by weight of steel scrap 10.

[0024] In some examples, load L comprises at least 40% by weight of direct reduced iron 11 , preferably from 40 to 60% by weight of DRI 11 .

[0025] In some examples, the load L comprises from 40 to 60% by weight of steel scrap 10, up to 30% by weight of pig iron 12 and from 10% to 60% by weight of DRI 11 .

[0026] For instance, the steel scrap 10 that can be used is referred to, in the EU-21 Steel Scrap specification, as old scraps (category E1 or E3), new scraps (category E8), shredded scraps (category E40) or fragmentized scraps (category E46).

[0027] The percentage of DRI and / or of pig iron in the load L is highly dependent on the quality of the steel scrap 10 which is used and of the steel grade to be produced. If the level of impurities, such as copper, chromium, molybdenum, nickel, tin, antimony, zinc and / or arsenic in the scrap 10 is low then the quantity of scrap 10 to be charged may be increased and thus the quantity of DRI and / or pig iron PI decreased.

[0028] However, DRI and / or pig iron are not always available in sufficient quantity and can themselves bring impurities. In the method according to the invention an ironcarbon composite material 5 is also loaded into the vessel 4. This composite material 5 has a density De which is lower than the density Ds of the slag 3.

[0029] Density of the slag Ds will vary according to the composition of said slag and notably its content of iron oxides, limestone and silica. This density Ds may be either estimated by sampling of the slag and measurement of the density of the sample or by thermodynamical calculations taking into account the chemistry of the load L. The density De of the composite material is preferably lower than 2.6 g / cm3.

[0030] By having a density De lower than the density Ds of the slag 3, the composite material 5 will remain in the slag layer 3.

[0031] The density De of the composite material may be controlled by adapting the respective amounts of iron and carbon in the material. Indeed, iron being denser than carbon, increasing the iron content will increase the density of the composite.

[0032] The composite material 5 may consist of lignocellulosic biomass having a moisture content from 5 to 10% by weight and an iron material comprising at least 30% of iron oxides, the molar ratio Fe / C of said composite material being from 3 to 6.5.

[0033] The composite material 5 may also consist of biochar and an iron material comprising at least 30% of iron oxides, the molar ratio Fe / C of said composite material being from 3 to 6.5.

[0034] The molar Fe / C ratio of the composite is set in a range from 3 to 6.5 in order to provide enough carbon for the reduction of the iron oxides while bringing sufficient iron.Below 3, the Fe / C ratio will be too low to have a reducing effect. Above 6.5, the iron content would be too low.

[0035] The molar Fe / C ratio is preferably set in a range from 3 to 5.9 in order to reduce its density De.

[0036] In both cases the iron material comprising at least 30% by weight of iron oxides maybe a concentrate of iron ore, direct reduction fines, oily mill sludge, iron mine tailings, electric arc furnace dust, sintering fines, or a mixture of any of those materials. The iron oxides may be hematite Fe2O3, magnetite Fe3O4, goethite FeO(OH), limonite FeO(OH)n(H2O), siderite FeCO3 or a mixture of those different oxides. In a preferred embodiment the iron material is a low-quality material comprising less than 67% by weight of iron oxides.

[0037] Prior to its loading into the vessel 4, the composite material 5 may be subjected to a reduction step to reduce the iron oxides to iron.

[0038] According to its size and to the configuration of the EAF, the composite material 5 may be charged together with the scrap 10 in a bucket, through a dedicated aperture in the roof of the vessel or injected as powder with a dedicated lance or with the oxygen injection device 20.

[0039] When loaded into the vessel 4 the composite material 5 will melt, providing both carbon and iron to the slag layer 3. Iron, due to its density will flow in the bath of molten metal 1 while the carbon will react with the iron oxides present in the slag or with the injected oxygen to form gaseous carbon monoxide. This gaseous carbon monoxide will cause foam bubbles to form on top of the slag layer 3, and a foaming slag will be generated.

[0040] During the process, the foaming slag protects the graphite electrodes from wear and covers the arcs. This allows for a higher productivity in the furnace since it increases the heat transfer between electrode and the molten metal. It also reduces radiation losses since the slag isolates the light beams. This in turn protects the refractories from wear, which reduces the amount of down-time for maintenance work. The foaming slag also contributes to stabilization of the arc, ensuring a higher efficiency.

[0041] The use of biochar or biomass as carbon source in the iron-carbon composite material has further advantage to generate more gases than standard DRI which will enhance slag foaming. This increased gas generation is due to the fact that biochar and biomass have a higher proportion of volatile matter VM compared to fossil carbon. VM is the component of coal / biomass which is driven off when the coal / biomass is rapidlyheated out of contact of air. The gases released are usually a mixture of short- and long-chain hydrocarbons, aromatic hydrocarbons, and sulfur. VM is the percentage loss in mass, adjusted for moisture, when coal is heated out of contact with air under standard conditions.

Claims

CLAIMS1 ) A steelmaking method comprising a step of melting a metallic load (L) in a vessel (4) to produce a bath of molten metal (1 ) topped by a slag layer (3), said slag layer (3) having a density Ds, wherein the method further comprises the loading of an iron-carbon composite material (5) into the vessel (4), said composite material (5) having a density De inferior or equal to the density Ds of the slag layer (3).2) A steelmaking method according to claim 1 wherein the composite material (5) consists of lignocellulosic biomass having a moisture content from 5 to 10% by weight and an iron material comprising at least 30% of iron oxides, the molar ratio Fe / C of said composite material being from 3 to 6.5.3) A steelmaking method according to claim 2 wherein the molar ratio Fe / C of said composite material is from 3 to 5.9.4) A steelmaking method according to claim 1 wherein the composite material (5) comprises biochar and an iron material comprising at least 30% of iron oxides, the molar ratio Fe / C of said composite material being from 3 to 6.5.5) A steelmaking method according to claim 4 wherein the molar ratio Fe / C of said composite material is from 3 to 5.9.6) A steelmaking method according to anyone of claims 2 to 5, wherein, before its loading into the vessel (4), the composite material (5) is subjected to a reduction step to reduce the iron oxides to iron.7) A steelmaking method according to anyone of claims 1 to 6 wherein the vessel (4) is part of an electric arc furnace.8) A steelmaking method according to anyone of claims 1 to 7 wherein the composite material (5) density De is inferior or equal to 2.6 g / cm3.

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