A method of producing direct reduced iron comprising embedded carbon

Abstract

The invention relates to a method for producing direct reduced iron comprising 5 embedded carbon by reducing iron oxide and / or iron ore in a direct reduction plant. The invention also relates a method for producing an indurated pellet for the direct reduced iron production method.

Description

[0001] A METHOD OF PRODUCING DIRECT REDUCED IRON COMPRISING EMBEDDED CARBON

[0002] Field of the Invention

[0003] The invention relates to a method for producing direct reduced iron comprising embedded carbon by reducing iron oxide and / or iron ore in a direct reduction plant. The invention also relates a method for producing an indurated pellet for the direct reduced iron production method.

[0004] Background of the Invention

[0005] Direct reduced iron (DRI) is produced from the direct reduction of iron ore conglomerates (mainly hematite, Fe2O3) in the form of lumps, pellets, or fines into iron by a reducing gas. Direct reduction refers to a solid-state process which reduce iron oxides to metallic iron at temperatures below the melting point of iron. There are several processes for producing DRI known to the person skilled in the art. A known process relates to a direct reduction plant (DRP) or DRI reactor comprising a direct reduction shaft furnace having a reduction zone and a lower discharge zone from which direct reduced iron (DRI) in solid form is discharged at a regulated rate by means of a suitable discharge mechanism. An example of a direct reduction shaft furnace is disclosed in patent document US-2021 / 0333048-A1. Iron oxide conglomerates in the form of agglomerates, pellets, lumps, or mixtures thereof are fed to the reduction furnace and descend by gravity through the reduction zone where DRI is formed by reaction of said iron oxides with a reducing gas stream at high temperature that is mainly composed of hydrogen, preferably consisting of hydrogen. Although the reducing gas may contain a hydrocarbon, such as a natural gas or a syngas, for the purpose of obtaining some amount of required carbon content in the DRI necessary to produce various grades of steel, the use of a reducing gas consisting of hydrogen is the main goal to be achieved due the environmental concerns.

[0006] The reduction of iron oxides by using 100% hydrogen gas as a reducing gas is carried out through the following net reaction:

[0007] Fe2O3+ 3H22Fe + 3H2O Therefore, the required carbon content cannot be supplied through the reducing gas in case the reducing gas is 100% hydrogen gas. Moreover, the iron ore pellets typically supplied to the DRP do not contain carbon as the carbon added to the iron ore pellet mixture is combusted during the induration step of the producing method of the iron ore pellets.

[0008] The DRI in solid form is further processed on exit from the discharge zone, and optionally also after being compacted into briquettes, in a melt shop typically comprising one or more electric-arc furnaces (EAF) or submerged-arc furnaces (SAF; in the art also known as a reducing electrical furnace or REF) or an open slag bath furnace (OSBF). The furnaces have electrodes and a gas extraction duct to collect the hot gases that are produced during the charging, melting and refining of the DRI.

[0009] Optionally also steel scrap is charged into the furnaces together with the DRI. The melt shop typically further comprises ladle furnaces for metallurgical processing like alloying and refining to produce molten steel or other molten iron containing products. The molten steel or other molten iron containing products are subsequently cast into slabs or coils which are ready for rolling and further heat treatment.

[0010] If no further measures are taken, then the resulting molten iron from the EAF / OSBF / SAF in the melt shop contains almost no carbon. This leads to more energy being required for melting the DRI. On the other hand, iron and carbon form a low melting eutectic, and thus significantly reduce the temperature needed to completely melt the DRI.

[0011] The carbon content in the DRI can be adjusted for its further processing in the melting furnace in a wide range from about 1 .5-4% by injecting a carburizing gas from a suitable source, which may be a hydrocarbon gas, coke oven gas, natural gas, syngas from biomass, or mixtures thereof, or other methane-containing and / or CO-containing syngas or any other carbon-containing gas that may deposit carbon in the DRI. This approach is for example disclosed in patent document EP-1160337-A1. A disadvantage of this approach is the complex handling of hot DRI in combination with a mixture of carburizing gases. The contact time or reaction time is rather short and the subsequent carbonization or increase in carbon content is extremely limited. The use of hydrocarbon gases also leads to the production of CO2and adversely affects the CO2 footprint. Moreover, adding solid carbon to the iron in EAF, SAF, OSBF or REF is also not an effective way to increase the carbon content to the desired level. As the added solid carbon floats on the molten iron due to the density difference, the dissolution of the carbon is limited and / or slow.

[0012] The invention provides an additional improvement, an additional advantage, or an alternative to the prior art.

[0013] Objectives of the Invention

[0014] It is an objective of the present invention to provide a method for producing direct reduction iron comprising embedded carbon at a sufficiently high level when the hydrogen-containing reducing gas is composed of at least 90 vol.% of hydrogen.

[0015] It is also an objective of the present invention to provide a method for producing an indurated pellet for the direct reduced iron production method when the hydrogencontaining reducing gas is composed of at least 90 vol.% of hydrogen.

[0016] It is also an objective of the present invention to provide a direct reduced iron comprising embedded carbon.

[0017] It is also an objective of the present invention to provide an indurated pellet comprising a core and a shell covering the core wherein the core consists of a carbonaceous material and the shell comprises iron ore and / or iron oxide.

[0018] Description of the Invention

[0019] One or more of the objectives of the invention are realized by providing a method for producing direct reduced iron in a direct reduction plant comprising the steps of

[0020] - supplying indurated pellets having a core and a shell covering the core to a direct reduction plant wherein the core consists of a carbonaceous material and the shell comprises iron ore and / or iron oxide

[0021] - supplying a hydrogen-containing reducing gas to the direct reduction plant (DRP) wherein the hydrogen-containing reducing gas is composed of at least 90 vol.% of hydrogen

[0022] - subjecting the iron ore and / or iron oxide to direct reduction with the hydrogen from the hydrogen containing reducing gas in the direct-reduction plant so as to obtain direct reduced iron, evacuating the direct reduced iron from the direct-reduction plant.

[0023] By encasing carbonaceous material in the core of the pellets, after the direct reduction of iron ore and / or iron oxide to direct reduced iron in the DRP, the carbonaceous material present in the core of the pellet can be retained without reaction or at least with limited reaction thanks to the layered structure of the indurated pellet. During the induration process of the pellets, a molten layer is formed between the core and the shell. Specifically, during the induration, the partial combustion of the carbonaceous material with carbon dioxide in the core creates a substantial amount of carbon monoxide according to the Boudouard’s equilibrium below.

[0024] C + CO22CO

[0025] The carbon monoxide promotes the formation of higher amounts of molten calcium iron silicate slag in a thin layer around the core. This molten layer closes the pores of the pellets and minimises the access of the oxidising gases to the core. Therefore, the further reaction of the carbonaceous material of the core is substantially prevented during the induration process part of the pellet production. Moreover, as process temperature during direct reduction stays below the melting temperatures of the slag phase, the contact between the reducing gas and carbonaceous material of the core is also mostly prevented during the production of direct reduced iron in a direct reduction plant due to the closed pores of the indurated pellets. The carbonaceous material in the core is not released into the metal until the DRI is melted in the SAF, REF, OSBF, or EAF. Thus, the direct reduced iron contains sufficient carbon for use in the EAF or REF or SAF or OSBF without a need for additional carburization of DRI or at least with limited additional carburization of DRI. As mentioned above, the additional carburization into the molten iron for subsequent steelmaking is incredibly challenging and inefficient due to the significant different densities of carbonaceous materials and the molten iron and slag. As the added carbon floats on the molten iron due to the density difference, the dissolution of the carbon is significantly slow. On the other hand, the indurated pellets having a core allow for better dissolution of the carbon into the molten iron by ensuring more intimate contact between the iron and carbon of the core. Therefore, better submersion of the carbon into the molten iron and more homogeneous mixing can be obtained compared to the additional carburization. In a possible embodiment, the direct reduction plant (DRP) is a rotary hearth furnace (RHF). In a possible embodiment of the invention, the direct reduction plant (DRP) is of the gravitational type, more in particular it is a shaft furnace.

[0026] In a possible embodiment of the invention, the DRP is of the gravitational type and comprises a reduction zone, inside which the iron ore reduction processes occur, feeding means to feed pellets to the reduction zone of said plant, a reducing gas circuit being provided with injection means configured to feed the reducing gas into the reduction plant, a reducing gas heater, an aperture to extract spent reducing gas, and a discharge zone to discharge the directly reduced iron, which is in solid form.

[0027] In a possible embodiment of the invention, the DRI is charged into an electric furnace (SAF, REF, OSBF or EAF), preferably through a hot-connect between the DRP plant and the electric furnace.

[0028] The carbonaceous material is selected from the group: coke, graphite, carbon black, coal, charcoal, biomass, or biochar. Coal may be any grade of coal, including lignite, sub-bituminous coal, bituminous coal, steam coal, or anthracite. The amount of carbon contained in the carbonaceous materials is preferably about 50 wt.% or more, more preferably of at least 60 wt.%; the higher the better. Preferably, the carbonaceous material is selected from the group consisting of anthracite, coke breeze, compressed charcoal, or a combination thereof. These carbonaceous materials contain high amount of carbon content and low porosity, so the volume of the core required for storing sufficient amount of carbon to be needed for the subsequent steelmaking can be reduced. More preferably, the carbonaceous material is compressed charcoal because it is a renewable resource.

[0029] In a possible embodiment of the invention, the core has an average diameter of about 3 to 7 mm, preferably of about 4 to 6 mm. More preferably, the core has an average diameter of about 5 mm. Thus, after the reduction in the DRP, the metallised pellet contains enough carbon content to fulfill the need of the subsequent steelmaking process. In a possible embodiment, the DRI has a carbon content in a range of 1 .0 wt.% to 6.0 wt.%, and preferably in the range of about 3 wt.% to 5 wt.%, more preferably in the range of about 3.5 wt.% to 5.0 wt.%, and most preferably the carbon content in the DRI is about 4.0 wt.%. On the other hand, as the high amount of carbon may decrease the strength of the indurated pellet, the diameter of the cores needs to be limited. In a possible embodiment of the invention, the pellet has an average diameter of about 8 to 20 mm, preferably of about 8 to 18 mm, more preferably of about 10 to 16 mm. Most preferably, the pellet has an average diameter of about 12 mm. By means of the dimensions of the core and the shell, balanced iron-carbon weight ratio can be obtained. Moreover, the shell ensures that the carbon is retained in the core of the pellet until the shell of the pellet is melted in the SAF, REF, OSBF or EAF. In this context, indicated diameter ranges are the ranges before the induration of the pellets.

[0030] In a possible embodiment of the invention, the iron ore is selected from the group consisting of hematite, magnetite, goethite, and mixtures thereof. The iron content in the iron ore should be of about 50 wt.% or more, preferably of about 55 wt.% or more and more preferably of about 62 wt.% or more, the remainder is oxygen and gangue. A decrease in the iron content increases the required energy to melt the same amount of iron and therefore the cost. The gangue is formed mainly by silicon oxide (silica), aluminium oxide (alumina), calcium oxide (lime), magnesium oxide, and other impurities in traces (e.g., sulphur, phosphorus, titanium oxide, manganese oxide, sodium oxide), and for the purpose of this invention gangue is considered to be an inert solid material which does not participate in the reduction reactions.

[0031] In a possible embodiment of the invention, the shell of the pellet contains particulate of iron-oxide containing material formed by iron- or steelmaking reverts, such as for example, steelmaking sludge, rolling scales or blast furnace dust. In a possible embodiment the shell contains a mixture of particulates of iron ore and particulates of iron-oxide containing material formed by iron- or steelmaking reverts such as for example, steelmaking sludge, rolling scales or blast furnace dust. Thus, the circularity of the process can be obtained by using the iron- or steelmaking reverts.

[0032] In a possible embodiment of the invention, the shell comprises a binder. During the agglomeration, the binder holds the iron ore particulates in the shell together. The binder can be an inorganic binder or an organic binder. Alternatively, the shell can comprise a combination of inorganic and organic binders. The inorganic binder may comprise one or more of clay or a salt thereof, lime, calcium aluminates cement, blast furnace cement, Portland cement, or pozzolanic binder. Preferably the clay is bentonite or a salt thereof. In terms of effectiveness and cost, the binder is more preferably bentonite.

[0033] In a possible embodiment of the invention, the shell comprises a fluxing agent. The fluxing agent adjusts the basicity of the pellet. Moreover, it improves the reducibility, melting, porosity properties and sticking behaviour of the shell of the pellet. The fluxing agent is any one of limestone (CaCCh), dolomite (CaMg(CC>3)2), olivine (Mg2SiO4), magnesite (MgCOa), colemanite (Ca2BeOir5H2O), wollastonite (CaSiOs) or combination thereof. Therefore, calcium and magnesium content of the slag can be controlled in an effective way.

[0034] In a possible embodiment of the invention, the shell preferably comprises a binder or binders up to about 2 wt.%, a carbonaceous material up to about 2 wt.% and fluxing agent or agents up to about 5 wt.% with the remaining material being iron ore, iron oxide, iron oxide containing reverts or a combination thereof. More than 5% the fluxing agent content will significantly reduce iron content in the pellet and increase the slag content. Therefore, energy cost for melting the same amount of iron content will be increased.

[0035] In a possible embodiment of the invention, the hydrogen-containing reducing gas is composed of at least 95 vol.% of hydrogen, preferably of at least 97 vol.% of hydrogen, more preferably of at least 99 vol.% of hydrogen. As the ultimate goal, the hydrogencontaining reducing gas is composed of 100 vol.% of hydrogen because the higher the hydrogen content the lower the CO2 footprint.

[0036] In a possible embodiment of the invention, direct reduction of the iron ore and / or iron oxide realized at a temperature of 750°C or above.

[0037] Moreover, one or more of the objectives of the invention are realized by a method for producing an indurated pellet for the above-mentioned direct reduced iron production method, comprising the steps of

[0038] Providing a core consisting of a carbonaceous material,

[0039] Optionally screening the core for obtaining a core within a predetermined average diameter range,

[0040] Providing a pellet mixture comprising iron ore and / or iron oxide,

[0041] - Agglomerating the pellet mixture to form a shell covering the core thereby obtaining a green pellet,

[0042] Screening a green pellet for obtaining a green pellet within a predetermined average diameter range,

[0043] Indurating the green pellet to obtain an indurated pellet, wherein the maximum temperature of the pellet during induration is 1260°C, preferably 1250°C.

[0044] During the induration process, a molten layer is formed between the core and the shell due to the initial reaction between the carbon of the core and the pellet mixture, so the pores in this molten layer are substantially eliminated. Thus, this molten layer minimises the further contact of the carbon of the core with oxidising gases in induration, and with reducing gases in the DRP. Therefore, the amount of the carbonaceous material kept in the core without reduction during the producing of DRI in the DRP can be increased. As a result, the carbonaceous material in the core is not released into the metal until the DRI is melted in the SAF, REF, OSBF or EAF. In addition, excessive melting in the pellets can occur at higher temperatures than 1260°C, and this would cause the exposure of the cores. Therefore, the temperature is dramatically increased and eventually it causes the pellets to fuse together and form a connected mass. It is prevented by controlling the maximum temperature of the pellet during induration.

[0045] In a possible embodiment of the invention, the cores having predetermined size can be obtained only by screening the carbonaceous material. On the other hand, the cores can also be formed by means of agglomeration in an agglomeration device. The agglomeration is realized by pelletising. In a possible embodiment of the invention, the cores can be formed by extrusion. Particularly, the carbonaceous material is extruded into short cylinders of predetermined size. The cores having predetermined size can usually be obtained directly by agglomeration or extrusion. Nevertheless, after the agglomeration or extrusion, the cores are preferably screened to separate the unwanted smaller or larger pellets. The cores are screened by sieving. In a possible embodiment of the invention, the core has an average diameter of about 3 to 7 mm, preferably of about 4 to 6 mm. More preferably, the core has an average diameter of about 5 mm. In a possible embodiment of the invention, the green pellet has an average diameter of about 8 to 20 mm, preferably of about 8 to 18 mm, more preferably of about 10 to 16 mm. Most preferably, the green pellet has an average diameter of about 12 mm. Preferably, the green pellet having predetermined size are directly obtained by the agglomeration. Nevertheless, after the agglomeration, the green pellets are optionally screened to separate the unwanted smaller or larger pellets. In a possible embodiment of the invention, the green pellet is agglomerated by pelletising. In a possible embodiment, the core is wetted before the core and the pellet mixture come into contact during agglomeration. The core is preferably wetted with water. The wetted core facilitates the layering of the pellet mixture on the surface of the core.

[0046] In a possible embodiment, the core is agglomerated on a first balling device. Then the core is transferred to a second balling device. The core is added to a second balling device before the pellet mixture is added. The shell is agglomerated to cover the core on the second balling device. In another possible embodiment, the core and the shell can be agglomerated on the same balling device in two stages. In this case, the pellet mixture forming the shell is added to the balling device after the core is formed. Optionally, the core is wetted before adding it into the balling device.

[0047] In a possible embodiment, the balling device is a balling drum or a balling disc. In a possible embodiment, temperature of the pellets during induration is kept between 1200°C and 1260°C, preferably between 1200°C and 1250°C. More preferably, temperature of the pellets during induration is kept between 1230°C and 1250°C. In a possible embodiment, temperature of the pellets during induration is kept between 1200°C and 1260°C, preferably between 1200°C and 1250°C for at least 2 minutes. More preferably, temperature of the pellets during induration is kept between 1230°C and 1250°C for at least 2 minutes. In a possible embodiment of this invention, the induration furnace preferably comprises interconnecting zones with different gas flow rates. These interconnecting zones are the zones where the pellets are dried, preheated, indurated, and cooled. In this case, a bed of the green pellets is carried by a grate through the zones. The bed height of the green pellets is arranged as a maximum of 20 cm in order to help keep the temperature of the pellets within the optimum temperature range. One of the possible induration furnaces is a straight grate pellet induration furnace.

[0048] In a possible embodiment, the pellet mixture comprises a binder. The binder is an inorganic binder or an organic binder or a combination of inorganic and organic binders. Preferably, the binder is bentonite. In a possible embodiment, the pellet mixture comprises a fluxing agent. The fluxing agent is one of limestone (CaCOs), dolomite (CaMg(CO3)2), olivine (Mg2SiO4), magnesite (MgCOs), colemanite (Ca2BeOir5H2O), wollastonite (CaSiOs) or combination thereof. In a possible embodiment of the invention, the iron ore is selected from the group consisting of hematite, magnetite, goethite, and mixtures thereof. In a possible embodiment, the pellet mixture is obtained by mixing the iron ore, optionally iron oxide, water, at least one binder and at least one fluxing agent.

[0049] In a possible embodiment of the invention, the carbonaceous material is any one of anthracite, coke breeze, charcoal, or a combination thereof.

[0050] Moreover, one or more of the objectives of the invention are realized by the direct reduced iron (DRI) produced by the above-mentioned direct reduced iron production method comprising embedded carbon. In possible embodiments, the direct reduced iron comprises one or more of the features described with the possible embodiments of the methods above. In a possible embodiment, the DRI has a carbon content in a range of 1.0 wt.% to 6.0 wt.%, and preferably in the range of about 3 wt.% to 5 wt.%, more preferably in the range of about 3.5 wt.% to 5.0 wt.%, and most preferably the carbon content in the DRI is about 4.0 wt.%.

[0051] Moreover, one or more of the objectives of the invention are realized by an indurated pellet produced by the above-mentioned indurated pellet production method comprising a core and a shell covering the core. The core consists of a carbonaceous material and the shell comprises iron ore and / or iron oxide. The indurated pellet is suitable for use in the above-mentioned direct reduced iron production. In possible embodiments, the indurated pellet comprises one or more of the features described with the possible embodiments of the methods above. In a possible embodiment, the indurated pellet has a carbon content at least 1.5 wt.%, preferably at least 2.0 wt.% and more preferably at least 2.5 wt.%, most preferably at least 3.0 wt.%.

[0052] Test Results

[0053] Pellets having a core consisting of carbonaceous material and a shell comprising iron ore were prepared within the scope of laboratory scale study. The carbonaceous material was selected as coke breeze and the cores having average diameter of between 5 mm and 6.3 mm were obtained by sieving. The pellet mixture for the shell contained iron ore as remainder, 8 wt.% water, 2.5 wt.% of limestone, 0.05 wt.% of olivine and 0.45 wt.% of bentonite. The ore was wetted to 6% moisture in the mixer with the bentonite and limestone. The basicity of the mixture is 0.85%. The iron ore contained 85% magnetite and 15% hematite.

[0054] In order to obtain green pellets, the cores consisting of coke breeze were placed in a rotating balling drum, and the iron ore mixed with water, bentonite, olivine and limestone was added slowly to a drum via a vibratory feeder. The green balls having average diameter of between 10 mm and 13 mm were obtained.

[0055] The green balls were placed in a stainless-steel mesh basket placed into the pot-grate, surrounded by the alumina balls. The pot, including the basket was then fired in an induration furnace. The temperature of the pellets was measured by thermocouples disposed above and below the basket. The maximum temperature recorded at the thermocouple was 1230°C. The temperature of the pellets was maintained between 1200°C and 1230°C for between 200 seconds and 250 seconds. The carbonaceous material was retained in the core after the induration. After the induration, pellets were subsequently reduced in a pure hydrogen atmosphere at 900 °C for 8 hours. The carbonaceous material was also kept in the core after the reduction. 3.39 wt.% carbon was retained in the reduced pellets. Chemical analyses of indurated and reduced pellets are shown in detail in Table 1.

[0056] Table 1. Chemical analyses of indurated and reduced pellets

Claims

CLAIMS1. A method for producing direct reduced iron comprising embedded carbon in a direct reduction plant comprising the steps of- supplying indurated pellets having a core and a shell covering the core to a direct reduction plant wherein the core consists of carbonaceous material and the shell comprises iron ore and / or iron oxide,- supplying a hydrogen-containing reducing gas to the direct reduction plant, wherein the hydrogen-containing reducing gas is composed of at least 90 vol.% of hydrogen,- subjecting the iron ore and / or iron oxide to direct reduction with the hydrogen from the hydrogen containing reducing gas in the direct- reduction plant so as to obtain direct reduced iron,- evacuating the direct reduced iron from the direct-reduction plant.

2. The method according to claim 1 , wherein the carbonaceous material is selected from the group consisting of anthracite, coke breeze, charcoal, or a combination thereof.

3. The method according to claim 1 or claim 2, wherein the iron ore is selected from the group consisting of hematite, magnetite, goethite, and mixtures thereof.

4. The method according to any one of the preceding claims, wherein the core has an average diameter of about 3 to 7 mm, preferably of about 4 to 6 mm, more preferably of about 5 mm.

5. The method according to any one of the preceding claims, wherein the pellet has an average diameter of about 8 to 20 mm, preferably of about 8 to 18 mm, more preferably of about 10 to 16 mm, most preferably of about 12 mm.

6. The method according to any one of the preceding claims, wherein the shell comprises a binder and / or a fluxing agent.

7. The method according to any one of the preceding claims, wherein the hydrogen-containing reducing gas is preferably composed of at least 95 vol.% of hydrogen, more preferably of at least 97 vol.% of hydrogen, most preferably of at least 99 vol.% of hydrogen.

8. A method for producing an indurated pellet for the direct reduced iron production method according to any one of the claims 1-7 comprising the steps of- Providing a core consisting of a carbonaceous material,- Optionally screening the core for obtaining a core within a predetermined average diameter range,- Providing a pellet mixture comprising iron ore and / or iron oxide,- Agglomerating the pellet mixture to form a shell covering the core thereby obtaining a green pellet,- Screening a green pellet for obtaining a green pellet within a predetermined average diameter range,- Indurating the green pellet to obtain an indurated pellet, wherein the maximum temperature of the pellet during induration is 1260°C, preferably 1250°C.

9. The method according to claim 8, wherein the temperature of the pellet during induration is kept between 1200°C and 1260°C, preferably 1200°C and 1250°C, for at least 2 minutes.

10. The method according to claim 8 or 9, wherein the core is obtained by agglomeration or extrusion.

11. The method according to any one of the claims 8-10, wherein the core is wetted before the core and the pellet mixture come into contact with each other.

12. The method according to any one of the claims 8-11 , wherein the core has an average diameter of about 3 to 7 mm, preferably of about 4 to 6 mm, more preferably of about 5 mm.

13. The method according to any one of the claims 8-12, wherein the green pellet has an average diameter of about 8 to 20 mm, preferably of about 8 to 18 mm, more preferably of about 10 to 16 mm, most preferably of about 12 mm.

14. A direct reduced iron produced by the method according to any one of the claims 1-7 comprising embedded carbon.

15. An indurated pellet produced by the method according to any one of the claims 8-13 comprising a core and a shell covering the core wherein the core consists of a carbonaceous material and the shell comprises iron ore and / or iron oxide.

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