AU2022471041B2 - A method for manufacturing pig iron in an electrical smelting furnace and associated electrical smelting furnace - Google Patents

Abstract

The invention deals with a method for manufacturing pig iron in an electrical smelting furnace 13 comprising a vessel 20, said method comprising the following successive steps: − loading DRI product in said vessel 20 − melting said DRI product to form a pig iron layer 14 topped by a slag layer 23 and − injecting a carbon containing material directly in said pig iron layer 14. It also deals with the manufacturing of steel from said pig iron and to the associated electrical smelting furnace 13.

Description

A METHOD FOR MANUFACTURING PIG IRON IN AN ELECTRICAL SMELTING FURNACE AND ASSOCIATED ELECTRICAL SMELTING FURNACE

[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. 2022471041

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[001a] The discussion of the background to the invention herein is intended to facilitate an understanding of the invention. However, it should be appreciated that the discussion is not an acknowledgement or admission that any aspect of the discussion was part of the common general knowledge as at the priority date of the 10 application.

[002] Unless the context requires otherwise, where the terms “comprise”, “comprises”, “comprised” or “comprising” are used in this specification (including the claims) they are to be interpreted as specifying the presence of the stated features, integers, steps or components, but not precluding the presence of one or more other 15 features, integers, steps or components, or group thereof.

[003] 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 20 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.

[004] The second main route involves so-called “direct reduction methods”. Among them are methods according to the brands MIDREX®, FINMET®, 25 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.

[005] 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 5 need to be further processed to produce high quality steel grades. Investment on new liquid steel treatment tools would thus be necessary. 2022471041

[006] Another option consists in using smelting furnaces powered by electric energy to melt the DRI products to produce pig iron. This option has the advantage that pig iron is produced, as in the Blast Furnace, which allows oxides removal in 10 molten slag and thus classical liquid steel treatment tools such a Basic Oxygen Furnace and refining ladles may be used. However, the pig iron obtained by this route has a carbon content which is relatively low compared to classical pig iron. This paradoxically reduces the environmental interest of this route because the higher the carbon rate, the more it will be possible to add recycled scrap metal in 15 the BOF.

[007] It would therefore be desirable 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.

[008] A first aspect of the present invention provides a method for manufacturing 20 pig iron in an electric smelting furnace comprising a vessel, said method comprising the following successive steps: loading DRI product in said vessel melting said DRI product to form a pig iron layer topped by a slag layer and injecting a carbon containing material directly in said pig iron layer in an amount sufficient to reach a final carbon content of 4.0 to 4.5% in weight in the pig iron layer, said injection being 25 done by a tuyere inserted in the bottom of the vessel and said carbon-containing material being injected together with a carrier gas which is inert.

[009] Such method may also comprise the optional characteristics of claims 2 to 8 considered separately or in any possible technical combinations.

[0010] A second aspect of the present invention provides a method for 30 manufacturing steel wherein pig iron manufactured according to the first aspect of the present invention is transferred from said smelting

2a 23 Feb 2026

furnace to a converter 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.

[0011] Such method may also comprise the optional characteristics of claims 10 to 11 considered separately or in any possible technical combinations. 5

[0012] Other characteristics and advantages of the invention will emerge clearly 2022471041

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 10 smelting / BOF route, - Figure 2 illustrates a smelting furnace - Figure 3 illustrates an embodiment of the method according to the invention

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

[0013] Figure 1 illustrates a steel production route according to the DRI route, from 15 the reduction of iron to the casting of the steel into semi-products such as slabs, billets, blooms, or strips. 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

[0014] 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.

[0015]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.

[0016] 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.

[0017] 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. 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 pig iron. In a preferred embodiment, part or

all of 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 may then optionally be transferred to a desulphurization station

15 to perform a desulphurization step. 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 previously transferred in a ladle. This ladle

may be a simple one as illustrated one figure 1 but could also be a torpedo ladle.

The desulphurized pig iron 16 has preferentially a content of Sulphur lower than

0.004 weight percent.

[0024] The desulphurized pig iron 16 can then 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 the 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 may be 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 electrodes 22 provide the required electric energy to melt the charged

raw materials and form pig iron. They are preferably Söderberg-type electrodes.

[0028] 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.

[0029] The vessel 20 is also provided with apertures named tap holes 25 located in

its lower part and allowing to discharge the pig iron 14 while keeping most of the

slag into the vessel 20. They may be located in the lateral walls of the vessel or in

its bottom wall.

[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] As explained above, 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 in the range of 4.0 to 4.5% in weight.

[0032] Indeed, carbon is necessary for the steelmaking process performed in the

converter 17 through oxygen blowing. This is because the reaction of carbon with

oxygen 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 exothermic and therefore provides additional energy for ferrous scraps

melting, allowing to incorporate a higher amount of such ferrous scraps coming from

steel recycling. The more ferrous scraps used, the smaller the environmental

footprint of the steelmaking process.

[0033] In the frame of the invention, a carbon containing material is added in the

smelting furnace 13, directly in the pig iron layer 14. This addition can be done

though an injection device.

[0034] By injecting carbon directly in the pig iron layer 14, it has been observed by

the present inventors that the carburization process can reach a very high yield,

above 80%. Indeed, the slag layer 23 has a high height, that can be above 50 cm

and the density of carbon sources is usually lower than the slag density itself. This triggers physical limitations for carbon to go through the slag into the pig iron layer

14.

[0035] Moreover, the direct injection of carbon ensures an optimal energy efficiency

of the smelting process as carburization requires a high amount of energy that can

be optimally provided by electric heating in the smelting furnace rather than by an

additional heating station.

[0036] Finally, increasing the carbon content of the pig iron in the smelter leads to a

decrease of the liquidus temperature of the pig iron, allowing a lower tapping

temperature.

[0037] In a preferred embodiment, the injection device is a tuyere 26 inserted in the

bottom of the vessel 20. Such bottom tuyere opens in the pig iron layer 14 to allow

direct addition.

[0038] The use of this bottom tuyere 26 avoids injecting the carbon-containing

material from the top of the smelting furnace 13 where the available space may be

scarce due to the presence of electrodes and charging devices for the DRI product.

[0039] In a preferred embodiment, the carbon is injected together with a carrier-gas

to avoid clogging the injection device. This gas is preferably inert and may be made

of nitrogen, argon, helium or carbon monoxide or any mixtures of such gases.

[0040] 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 load. In particular, biochar can be

used. Adding calcium carbide is particularly advantageous as the calcium atoms

can provide a desulphurizing effect. Adding silicon carbide is also particularly

advantageous as it allows increasing the silicon content of the pig iron.

[0041] The carbon containing material to be injected through the injection device

preferably has a particle size below 3mm. In a preferred embodiment, said material

has a particle size less than or equal to 75um, remaining particles having a particle

size less than or equal to 2 mm.

[0042] In another embodiment, the carbon containing material may also be made of

composite briquettes of an iron source mixed with one or several of the previously

mentioned carbon sources.

[0043] In a preferred embodiment, iron source can be chosen among sludges from

electric furnaces, converters or smelters, slags from electric furnaces or from

converters, DRI fines or any waste rich in iron from pig iron or steel production route.

[0044] In a preferred embodiment, silicon containing material may be injected

together with the carbon containing material in the pig iron layer 14. Silicon has a

strong deoxidizing power at high temperature and notably around 1600°C which is

the temperature of the liquid steel in the converter. It reacts with oxygen and

contributes then to the formation of the slag. The reaction is exothermic and

therefore provides additional energy for scrap melting in the converter. It can also

improve the performance of the desulphurization operation, if any.

[0045] Such silicon can be added under different forms. It may be metal Silicon Si,

silicon carbide SiC, silicomanganese SiMn, calcium silicate SiCa or a ferro silicon

alloy FeSi such as FeSi75 or FeSi65.

[0046] The use of DRI products in the smelting furnace 13 will lead to a natural

amount of silicon usually below 0.2 or even below 0.1 % in weight. The final silicon

content of the pig iron is preferentially set at a value of 0.1 to 0.4% in weight,

preferably of 0.2 to 0.4 % in weight. Further additions of silicon in the desulphurization station 15 and/or the converter 17 may be performed if required.

[0047] In a preferred embodiment, desulphurization reagents can also be injected

together with the carbon containing material, with or without silicon addition. Such

reagents can notably be based on calcium compounds, such as sodium carbonate,

lime, and/or calcium carbide.

[0048] The final sulphur content of the pig iron is preferentially set at a maximum

value of 0.03 weight percent and preferably at a maximum value of 0.004 weight

percent.

[0049] Performing desulphurization in the smelting furnace can allow suppressing

the need for a desulphurizing treatment between the smelting furnace 13 and the

converter 17 or at least reducing such treatment.

[0050] It has to be noted that adding calcium carbide is particularly advantageous

as the calcium addition can provide a desulphurizing effect on top of adding carbon.

Adding silicon carbide is also particularly advantageous as it allows increasing the silicon content of the pig iron on top of acting carbon. Adding a mix of calcium carbide and silicon carbide is even more advantageous as it provides carbon and silicon addition, while ensuring desulphurization.

Claims (11)

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:

1) A method for manufacturing pig iron in an electric smelting furnace comprising a vessel, said method comprising the following successive steps: 2022471041

− loading DRI product in said vessel − melting said DRI product to form a pig iron layer topped by a slag layer and − injecting a carbon containing material directly in said pig iron layer in an amount sufficient to reach a final carbon content of 4.0 to 4.5% in weight in the pig iron layer, − said injection being done by a tuyere inserted in the bottom of the vessel and said carbon-containing material being injected together with a carrier gas which is inert.

2) A method according to claim 1, wherein said smelting furnace comprises at least one bottom tuyere provided on such vessel, through which is injected said carbon containing material directly in said pig iron layer.

3) A method according to any one of claims 1 or 2 wherein said carbon containing material is chosen among coke, anthracite, silicon carbide, calcium carbide, carbon coming from the combustion of biomass or a mixture of any of those materials.

4) A method according to any one of claims 1 to 3, wherein said carbon containing material is injected has particles having a particle size below 3mm.

5) A method according to claim 4, wherein 70 to 80% of the particles have a particle size less than or equal to 75µm, remaining particles having a particle size less than or equal to 2 mm.

6) A method according to claim 5, wherein said carbon containing material is previously mixed with an iron source and formed into composite briquettes which are injected in the pig iron layer.

7) A method according to any one of claims 1 to 6 wherein, before being loaded in said smelting furnace, said DRI product is manufactured using a reducing gas containing at least 50 % in volume of hydrogen.

8) A method according to any one of claims 1 to 7, wherein silicon containing material and/or desulphurizing reagents are added to the carbon containing 2022471041

material, to be injected in the pig iron layer.

9) A method for manufacturing steel wherein pig iron manufactured according to any one of claims 1 to 7 is transferred from said smelting furnace to a converter 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.

10) A method for manufacturing steel according to claim 9, wherein ferrous scraps are added to said pig iron in said converter and melted.

11) A method according to claim 9 or 10 wherein said pig iron is being transferred from said smelting furnace to a desulphurization station before being transferred to said converter.