Carburized sponge iron production process

Abstract

The present disclosure relates to a process for producing carburized sponge iron, comprising reducing iron ore pellets using a carbon-lean reducing gas in a direct reduction shaft reactor to provide a sponge iron intermediate, transporting the sponge iron intermediate to a carburizing unit, and carburizing the sponge iron intermediate in the carburizing unit using a carburizing gas to provide the carburized sponge iron. The present disclosure further relates to a system for producing carburized sponge iron, the carburized sponge iron produced by the above-described process, and the sponge iron intermediate obtained during the production of such carburized sponge iron.

Description

Technical Field

[0003]

[0001] The present disclosure relates to a production process of carburized sponge iron. The present disclosure further relates to a production system of carburized sponge iron, the carburized sponge iron produced by the above process, and sponge iron intermediates obtained during the production of such carburized sponge iron.

Background Art

[0002] Steel is the most important engineering technology and construction material in the world. In the modern world, it is difficult to find an object that does not contain steel or whose manufacturing and / or transportation do not rely on steel. Thus, steel is intricately involved in almost all aspects of our modern life.

[0003] In 2018, the world's total crude steel production was 1.81 billion tons, far more than any other metal, and is expected to reach 2.8 billion tons by 2050, 50% of which is expected to be derived from virgin iron sources. Steel is also the most recycled material in the world with a very high recycling grade due to its ability to be repeatedly used after remelting using electricity as a primary energy source.

[0004] Thus, steel is the foundation of modern society and will play an even more important role in the future.

[0005] Steel is mainly produced through three routes. i) An integrated process that uses virgin iron ore in a blast furnace (BF) and reduces iron oxide in the ore with carbon to produce iron. The iron is further processed in a steel mill through oxygen injection in a basic oxygen furnace and subsequent refining to produce steel. This process is also commonly referred to as "oxygen steelmaking". ii) Scrap-based production that uses recycled steel melted in an electric arc furnace (EAF) using electricity as a primary energy source. This process is also commonly referred to as "electric steelmaking". iii) Direct reduction production based on virgin iron ore, which is reduced to sponge iron in a direct reduction (DR) process using carbonaceous reducing gas. The sponge iron is then melted together with scrap in the EAF to produce steel.

[0006] The term "crude iron" is used herein to refer to any iron produced for further processing into steel, whether obtained in a blast furnace (i.e., pig iron) or a direct reduction shaft (i.e., sponge iron).

[0007] While the processes described above have been refined over decades and are approaching theoretical minimum energy consumption, one fundamental problem remains unresolved: the reduction of iron ore using carbonaceous reducing agents produces CO2 as a byproduct. In 2018, an average of 1.83 tons of CO2 were produced per ton of steel produced. The steel industry is one of the largest emitters of CO2, accounting for approximately 7% of global CO2 emissions. As long as carbonaceous reducing agents are used, it is impossible to avoid excessive CO2 generation in the steel production process.

[0008] The HYBRIT initiative was established to address this issue. HYBRIT, an abbreviation for Hydrogen Breakthrough Ironmaking Technology, is a joint venture between SSAB, LKAB, and Vattenfall, with partial funding from the Swedish Energy Agency, aiming to reduce CO2 emissions and decarbonize the steel industry.

[0009] At the heart of the HYBRIT concept is the direct reduction-based production of sponge iron from virgin ore. However, instead of using carbonaceous reducing gases such as natural gas, as in current commercial direct reduction processes, HYBRIT proposes using hydrogen gas as a reducing agent, a process called hydrogen direct reduction (H-DR). Hydrogen gas can be produced by the electrolysis of water using primarily fossil-free and / or renewable primary energy sources, as in the case of electricity production in Sweden. Thus, the crucial step of reducing iron ore can be achieved without the input of fossil fuels, using water as a byproduct instead of CO2. The resulting crude iron is, of course, also deficient in carbon.

[0010] Iron produced today by commercial blast furnaces or direct reduction routes typically contains significant amounts of carbon (usually up to 5% by weight) due to carbon incorporation during the reduction of iron ore. In addition to its use as a reducing agent, carbon plays a more important role in the steelmaking process. Its presence in crude iron from BF or DR processes lowers the melting point of the iron. During subsequent processing of crude iron in EAF or BOF, heat is supplied to the process by the exothermic decomposition of iron carbide and the oxidation of carbon to CO. The gas generation in the EAF due to this CO production provides a foamy slag, which helps in insulating the molten iron and reduces wear on the EAF electrodes. For at least these reasons, the presence of carbon in crude iron can help reduce energy consumption during steel processing. The presence of carbon in the molten material also affects the slag-metal reaction rate and can help in purging gaseous components from the metal. Furthermore, the presence of carbon in directly reduced iron passivates sponge iron, allowing for easier handling and transport. Finally, because the steel industry has a tradition and established practices regarding carbon-rich crude iron, some steel manufacturers may be somewhat resistant to adopting the use of dilute carbon crude iron, regardless of any advantages.

[0011] For these reasons at least, it may be desirable to provide crude iron produced using means that are substantially fossil-free but still contain enough carbon to be usable as a temporary substitute for crude iron today.

[0012] Reference US2015 / 0259760A1 describes a method for producing steel in which iron ore is reduced with hydrogen, and the resulting reduced iron ore intermediate product and possibly its associated materials are subjected to further metallurgical treatment. When reducing the iron ore to produce the intermediate product, a carbon-containing or hydrogen-containing gas is added to the hydrogen to incorporate carbon into the intermediate product. The ratio of hydrogen to the carbon-containing or hydrogen-containing gas can continuously vary as a function of availability. For example, if a very large amount of hydrogen is available, it can be used almost 100% for direct reduction. The remainder consists of the minimum necessary carbon-containing or hydrogen-containing gas flow to adjust the percentage of carbon. The hydrogen for reduction contains at least enough carbon-containing or hydrogen-containing gas to be added to the intermediate product so that the carbon content in the intermediate product is 0.0005% to 6.3% by mass, preferably 1% to 3%. It is stated that this type of intermediate product is particularly suitable for further treatment because it is ideally tuned in terms of carbon content and contributes to the carbon content required for the metallurgical process. Examples of carbon-containing or hydrogen-containing gases include natural gas, biogas, gases obtained from pyrolysis, and renewable resources.

[0013] There is still a need for more environmentally friendly methods to produce carbon-containing crude iron. [Overview of the project]

[0014] The inventors of this invention have identified many shortcomings in the conventional means of providing carbon-containing crude iron. Today's commercial processes require the excessive use of carbonaceous fossil fuels such as coal or natural gas, leading to excessive CO2 emissions. Even in processes proposed to address these problems, a significant proportion of the carbon-containing gas is still oxidized to CO2 upon contact with the ore. This is true regardless of whether the carbon-containing gas originates from renewable sources or whether fossil fuels are used to compensate for fluctuations in the availability of renewable hydrogen, as assumed in the prior art. Therefore, such processes struggle to utilize the significant proportion of carbon introduced into the process. The produced CO2 is either emitted or accumulated in the process gas if the process gas is reused. To prevent the accumulation of CO2 in the process gas, the gas needs to be treated extensively to separate the CO2. Even if this separated CO2 could be reformed and reused, the cost would be enormous.

[0015] It would be advantageous to achieve means to overcome, or at least mitigate, at least some of, the aforementioned drawbacks. In particular, it is desirable to provide means for producing crude iron containing a sufficient amount of carbon so that it can be used as an immediate substitute for crude iron today, and for more efficiently utilizing any carbon-containing resources required for production. To better address one or more of these concerns, a production process for carburized sponge iron having the features defined in the attached independent claims is provided. [Means for solving the problem]

[0016] This process comprises the following steps: Iron ore is reduced directly in a reduction shaft using a dilute carbon reducing gas to provide a sponge iron intermediate. The dilute carbon reducing gas means that no carbon reducing agent is introduced into the reducing gas circuit. The sponge iron intermediate is transferred to a carburizing unit and carburized using a carburizing gas to provide carburized sponge iron. The carburizing unit may be, for example, a carburizing shaft.

[0017] Performing carburizing separately before reduction yields several advantages. Mainly, because the sponge iron intermediate entering the carburizing unit is already sufficiently reduced, the carburizing gas is not oxidized to CO2 by contact with iron oxide to the same extent. Preventing the carburizing gas from being oxidized to CO2 means that more carbon in the carburizing gas can participate in the carburizing reaction, resulting in a higher utilization rate of carbon in the carburizing gas. The carburizing gas circuit is separated from the reducing gas circuit. Therefore, the carburizing gas circuit can be smaller in size than a combined reducing and carburizing gas circuit. This means that the processing and reuse of used carburizing off-gas is easier, and the carburizing gas is more easily maintained in an optimal state for carburizing. Conversely to reduction, improved utilization of carburizing gas for carburizing enables wider use of carburizing gas from both availability and cost perspectives. Handling of the reducing gas circuit is also simplified. Since reducing gas essentially does not contain CO2, the need for a CO2 separation step in processing used top gas before directly reusing top gas in the reducing shaft is eliminated.

[0018] Carburized gases can be derived from renewable sources. This is possible due to the high utilization rate of carburized gases, which means that net CO2 emissions from the process can be further reduced. For example, carburized gases may contain or consist essentially of biomethane, biogas, gases from the pyrolysis of biomass, or a combination thereof.

[0019] The reducing gas contains or is essentially derived from hydrogen produced by electrolysis; that is, the makeup gas added to the reducing gas circuit contains or can essentially be derived from hydrogen produced by electrolysis. The hydrogen produced by electrolysis can be produced using electricity from renewable and / or fossil-free sources, partly or entirely. Therefore, the direct and / or indirect CO2 emissions of the process can be further reduced. When both the reduction and carburizing steps are carried out using fossil-free and / or renewable sources, net CO2 emissions throughout the entire process are very low.

[0020] Hydrogen can be separated from the off-gas obtained from the carburizing unit. This hydrogen may include hydrogen produced during the carburizing step and hydrogen introduced as a component of the carburizing gas. This helps prevent hydrogen accumulation in the reuse process gas from the carburizing stage. This hydrogen can be directly introduced as a reducing gas into the reduction shaft. This helps reduce the energy requirements for carrying out the reduction step and can help ensure that even the hydrogen content of the carburizing gas is utilized efficiently.

[0021] The carbon dioxide produced in the carburizing step can be separated from the off-gas obtained from the carburizing unit. This prevents the accumulation of CO2 in the reuse process gas during the carburizing stage. This carbon dioxide can be converted to carbon monoxide and reintroduced into the carburizing unit as carburizing gas. Alternatively or additionally, the carbon dioxide can be converted to methane and reintroduced into the carburizing unit as carburizing gas. This helps ensure a very high utilization rate of carbon in the carburizing gas.

[0022] The carburizing gas can be preheated by heat exchange with the off-gas from the carburizing unit. This reduces the overall energy requirements of the process. Alternatively or additionally, the carburizing gas can be mixed with the off-gas from the carburizing unit prior to its introduction into the unit.

[0023] The sponge iron intermediate can be transferred to the carburizing unit at temperatures above approximately 500°C, such as above 600°C or above 700°C. This reduces the need for heating in the carburizing reaction and lowers the overall energy requirements of the process.

[0024] The sponge iron intermediate can have a reduction degree of over 90%. That is, reduction can be carried out almost completely in the reduction shaft directly. As a result, most of the carburized gas is not oxidized to CO2, and potentially, the carbon in the carburized gas can be made to work well without the need to reform the CO2 produced in the carburizing step.

[0025] Alternatively, the sponge iron intermediate may have a reducibility of about 50% to about 90%, preferably about 60% to about 80%, for example, about 70%, etc. That is, the reduction can be carried out to a lower extent in the direct reduction shaft. Thereby, the hydrogen produced as a by-product of the carburization reaction can be consumed as a reducing agent in the carburization unit. Therefore, the need for separation and reuse of hydrogen by-products is reduced, and the plant dimensions can be determined accordingly. This can also be beneficial from an energy perspective when the availability of fossil-free and / or renewable electricity fluctuates, for example, or is limited for a certain period. The reduction in the direct reduction shaft can be carried out to a lower extent, meaning that less hydrogen is required for the production of the intermediate product, and the final reduction and carburization can be carried out in the carburization unit using available fossil-free gas. This leads to a more flexible (and more dynamically responsive) production plant and limits the need for large hydrogen storage to compensate for fluctuations in the production of fossil-free and / or renewable electricity (e.g., due to weather or other reasons).

[0026] According to another aspect of the present invention, the object of the present invention is achieved by a production system for carburized sponge iron according to the appended independent claims. The production process may be the process defined in the appended independent claims, and the carburized sponge iron may be the carburized sponge iron defined in the appended independent claims.

[0027] This system comprises an electrolyzer arranged to produce hydrogen from the electrolysis of water, a direct reduction shaft, and a carburization unit. The carburization unit may be, for example, a carburization shaft.

[0028] Such a system enables the beneficial production of carburized sponge iron as described herein.

[0029] The electrolyzer is arranged in fluid communication with the direct reduction shaft such that the hydrogen produced by the electrolyzer can be transferred to the direct reduction shaft. This may preferably be an indirect fluid communication through the treatment of electrolysis gas and / or through a hydrogen storage facility.

[0030] The outlet of the direct reduction shaft may be connected to the inlet of the carburizing unit so that sponge iron intermediates can be supplied directly from the outlet to the inlet. The system may further include means for transporting sponge iron from the direct reduction shaft to the carburizing unit. The exact means of transport used depends on parameters such as the operating pressure of the direct reduction shaft and the requirements regarding the degree to which the various units used in the process must be airtight. The means of transport may be passive, such as a chute positioned between the outlet of the direct reduction shaft and the inlet of the carburizing unit, or active, such as a conveyor.

[0031] According to a further aspect of the present invention, the object of the present invention is achieved by carburized sponge iron according to the appended claims. Carburized sponge iron can be produced by the process according to the appended independent claims. Carburized sponge iron has a reduction degree of more than 90%, such as more than 94%, and contains 0.5 to 5% by weight of carbon, such as 1 to 4% by weight of carbon, such as about 3% by weight of carbon. Carburized sponge iron has a radiocarbon age of less than 10,000 years to date, preferably less than 1,000 years to date, and more preferably less than 100 years to date. This means that the carburized sponge iron must be carburized using a carburizing gas containing a considerable amount of renewable carbon, which is commercially feasible using the process described herein, as described herein.

[0032] The mass fraction of carbon present as cementite may be greater than 50%, preferably greater than 70%, and more preferably greater than 90%. A high degree of cementite is preferable because cementite usually reaches the molten material during further processing and is not lost as dust before reaching the molten material, unlike the tendency of graphite.

[0033] Carburized sponge iron can be in the form of pellets (i.e., DRI) or briquettes (i.e., HBI).

[0034] According to yet another aspect of the present invention, a sponge iron intermediate is provided. The sponge iron intermediate may be an intermediate in a process defined in the appended independent claims. The sponge iron intermediate is in the form of pellets. It has a degree of reduction of about 50% to about 100%, such as about 50% to about 90% (preferably about 60% to about 80%) or more than about 90% (preferably more than 94%). It has a carbon content of less than 0.5% by weight, such as less than 0.2% by weight, preferably less than 0.05% by weight, and more preferably less than 0.0005% by weight. For example, the sponge iron intermediate may have a degree of reduction of about 60% to about 90% (preferably about 60% to about 80%) and a carbon content of less than 0.05% by weight (preferably less than 0.0005% by weight). The very low carbon content is due to the carbon dilute reduction process in which it is produced.

[0035] Those skilled in the art will find the following detailed description to reveal further objectives, effects, and novel features of the present invention. [Brief explanation of the drawing]

[0036] To fully understand the present invention and its further objectives and effects, the detailed description below should be read in conjunction with the accompanying drawings. In the drawings, the same reference numerals indicate similar items in various drawings.

[0037] [Figure 1] This outlines the ore-based steelmaking value chain related to the HYBRIT concept. [Figure 2a] This specification schematically illustrates exemplary embodiments of systems suitable for carrying out the processes disclosed herein. [Figure 2b] Another exemplary embodiment of a system suitable for carrying out the processes disclosed herein is schematically shown. [Figure 2c] Further exemplary embodiments of systems suitable for carrying out the processes disclosed herein are schematically shown. [Figure 3] This flowchart schematically illustrates exemplary embodiments of the processes disclosed herein. [Figure 4] This provides a schematic representation of the model system on which the calculations are based. [Modes for carrying out the invention]

[0038] In conventional commercial direct reduction processes, iron ore is reduced and carburized directly in a reduction shaft. The gases used for reduction and carburization are typically synthesis gas obtained from hydrocarbons reformed using an external reformer, as in the Midrex process, and / or natural gas converted by internal reforming, as in the Hyl ZR process. The resulting sponge iron typically contains about 2% to 4.5% by weight of carbon, depending on the process used. The carbon exists as a mixture of graphite and iron carbide, with cementite (Fe3C) being the dominant iron carbide. For reference, pure cementite contains 6.69% by weight of carbon. A high proportion of iron carbide in sponge iron is desirable because the graphite contained in sponge iron is easily lost as dust during handling and processing of the sponge iron.

[0039] Carbonaceous gases introduced directly into a reduction shaft can be involved in many competing reforming, reduction, and / or carburizing reactions. Examples of such reactions are given below.

[0040] Modification CH4 + H2O → CO + 3H2 + 205.9 kJ / mol CH4+CO2→2CO+2H2 +247.1kJ / mol

[0041] reduction 3Fe2O3 (hematite) + CO → 2Fe3O4 + CO2 - 24.9 kJ / mol 2Fe3O4 (magnetite) + 2CO → 6FeO + 2CO2 + 45.3 kJ / mol 6FeO (würstite) + 6CO → 6Fe + 6CO2 - 75.8 kJ / mol

[0042] Carburizing (graphite production) 2CO → CO2 + C -173.7 kJ / mol CO + H2 → C + H2O -131.8 kJ / mol CH4 → C + 2H2 + 74.5 kJ / mol

[0043] Carburizing (cementite production) 3Fe+CH4→Fe3C+2H2 +98.3kJ / mol 3Fe+2CO→Fe3C+CO2 -148.8kJ / mol 3Fe+CO+H2→Fe3C+H2O -107.6kJ / mol

[0044] Whether hydrogen gas is introduced directly into the reduction shaft or formed by the reaction described above, it also contributes to the reduction of iron ore through the following reaction.

[0045] 6Fe2O3+2H2→4Fe3O4+2H2O +32.7kJ / mol 2Fe3O4+2H2→6FeO+2H2O +127.6kJ / mol 6FeO+6H2→6Fe+6H2O +171.4kJ / mol 2Fe3O4+8H2→6Fe+8H2O +299.0kJ / mol

[0046] Efforts to reduce the environmental impact of conventional direct reduction processes have typically focused on reducing the overall energy consumption of the process, and consequently, the amount of natural gas required as input per ton of sponge iron production. Proposals to reduce the amount of fossil fuels used, and consequently CO2 emissions, have focused on replacing some of the natural gas used in the process with renewable hydrogen and / or biofuels, leaving the overall process substantially unchanged. However, this has failed to recognize that most of the carbon present in the process gas is not incorporated into the sponge iron, but instead acts as a reducing agent in the direct reduction process, leading to CO2 formation. This CO2 is typically captured or released, or reformed in an energy-intensive external reforming step heated by the combustion of fossil fuels. In any case, the overall process consequently reduces the utilization rate of carbon introduced into the process, regardless of whether this carbon originates from fossil or renewable sources. This is a crucial consideration for any process that attempts to replace fossil fuels with renewable fuels, given that renewable fuels are typically less readily available and considerably more expensive than their fossil counterparts. Availability and fuel costs are critical in global steelmaking processes.

[0047] This invention is based on the inventors' insight that carrying out a carburizing reaction as a specific process step offers many advantages in the specific context of developing a steelmaking process that dramatically reduces CO2 emissions.

[0048] The initial reduction of iron ore is carried out in a dilute carbon reducing gas. This ensures that little to no CO2 is produced in the initial reduction step, avoiding the need for CO2 capture or reforming in connection with the reduction step. This inevitably requires significant simplification within the plant, as large amounts of CO2 are usually produced during the reduction step. The sponge iron intermediate produced in the reduction step is essentially carbon-free and is significantly reduced. In the carburizing unit, since the iron is already significantly reduced upon introduction, the balance of the reaction shifts towards reforming and / or carburizing. This means that less of the carburizing gas is oxidized to CO2, and much of the carbon can be utilized, mainly by being incorporated into the sponge iron as cementite. By removing water from the carburizing gas, the balance of the reaction can be further shifted towards carburizing by reducing the possibility of hydrocarbon reforming reactions. Furthermore, the off-gas from the carburizing reactor is relatively concentrated, as it is not diluted with all the gas required for reduction. This means that the off-gas can be easily handled to remove any CO2 produced during the carburizing step and reuse the gas used. A further advantage is that, since carburizing is carried out as a specific process step, greater flexibility is gained regarding the selection of the carburizing gas and the degree of carburizing. The composition of the carburizing gas and the degree of carburizing can be freely adjusted without affecting the reduction process steps.

[0049] A direct reduction shaft and a carburizing unit are required to carry out the process described herein.

[0050] reduction The direct reduction shaft may be any type commonly known in the art. A shaft refers to a solid-gas counter-flow moving-bed reactor, where a load of iron ore is introduced into the inlet at the top of the reactor and descends by gravity toward the outlet at the bottom of the reactor. The reducing gas is introduced at a point below the reactor inlet and flows upward against the moving bed of ore to reduce the ore to metallized iron. Reduction is typically carried out at a temperature of about 900°C to about 1100°C. The required temperature is usually maintained by preheating the process gas introduced into the reactor, typically using a preheater, such as an electric preheater. Further heating of these gases may be achieved by partial exothermic oxidation of the gases with oxygen or air after they have left the preheater and before they are introduced into the reactor. Reduction can be carried out in the DR shaft at a pressure of about 1 bar to about 10 bar, preferably about 3 bar to about 8 bar. The reactor may have a cooling discharge cone located at the bottom to cool the sponge iron intermediate prior to discharge from the outlet.

[0051] Iron ore loads typically consist mainly of iron ore pellets, although some massive iron ore may also be introduced. Iron ore pellets usually consist mostly of hematite, with further additives or impurities such as gangue, flux, and binder. However, pellets may contain other metals and other ores such as magnetite. Iron ore pellets specifically designed for direct reduction processes are commercially available, and such pellets may be used in this process. Alternatively, pellets may be specially adapted to the carbon dilute reduction step, as in this process.

[0052] The reducing gas is carbon-sparse. The reducing gas means that a combination of fresh makeup gas and reused process (top) gas is directly introduced into the reduction shaft. Carbon-sparse means that no carbon reducing agent is introduced into the reducing gas circuit; that is, the makeup gas introduced into the reducing gas circuit does not contain a carbon reducing agent. A carbon reducing agent means any carbon or carbon-containing compound that can act directly as a reducing agent, such as elemental carbon, hydrocarbons, carbon monoxide, or any incomplete carbon oxide compound. For example, any makeup gas added to replenish the reducing gas may essentially consist of hydrogen gas. However, some amount of carbon-containing gas may be present in the reducing gas. For example, if the outlet of the direct reduction shaft is connected to the inlet of a carburizing unit, a relatively small amount of carbon-containing gas may inadvertently seep from the carburizing unit directly into the reduction shaft. Another example is when carbonates present in iron ore pellets volatilize and appear as CO2 in the top gas of the DR shaft, resulting in an amount of CO2 that can be returned to the DR shaft and reused. Since hydrogen gas is dominant in the reducing gas circuit, any CO2 present can be converted to CO by the reverse water-gas conversion reaction. The dilute carbon reducing gas may contain less than about 10 vol% (determined under normal conditions of 1 atm and 0°C), preferably less than 5 vol% of carbon-containing gas. The reducing gas may be mainly hydrogen gas. The reducing gas entering the direct reduction shaft may consist of more than 80 vol% hydrogen gas, preferably more than 90 vol% hydrogen gas (volume %) determined under normal conditions of 1 atm and 0°C. The reducing gas may essentially consist of hydrogen gas together with gaseous byproducts formed in the direct reduction shaft. The hydrogen gas is preferably obtained at least in part by the electrolysis of water. If the water electrolysis is carried out using fossil-free, optionally renewable energy, this allows the reducing gas to be supplied from such a source. Electrolyzed hydrogen can be transported directly from the electrolytic cell to the DR shaft by conduit, or hydrogen can be stored during production and transported to the DR shaft as needed. As described later, hydrogen separated from the off-gas of the carburizing unit can also be introduced into the DR shaft as a reducing gas.The reducing gas is reusable, and the top (spent) gas from the DR shaft can be cleaned and treated to remove by-products such as water and / or dust prior to its reintroduction into the DR shaft. This reused gas can be mixed with fresh makeup gas prior to its reintroduction into the reactor, or it can be introduced separately from any fresh makeup gas supply.

[0053] Sponge iron intermediates The sponge iron intermediate obtained at the outlet of the DR shaft is usually in pellet form, due to the structural integrity of the direct reduction pellets and the prevailing conditions within the DR shaft. The degree of reduction of the obtained sponge iron intermediate depends on the processing conditions used within the DR shaft. It may be desirable to obtain a substantially completely metallized sponge iron intermediate, i.e., sponge iron with a DoR of over 90%, such as over 94%. Due to reaction dynamics, obtaining sponge iron with a DoR of over 96% is often not commercially viable. A sponge iron intermediate with a high DoR ensures that very little CO2 is produced in the subsequent carburizing step, potentially simplifying the treatment of off-gas from the carburizing reactor. Alternatively, it may be desirable to obtain a sponge iron intermediate with a lower degree of reduction, such as about 50% to about 90%, preferably about 60% to about 80%. This offers a double advantage. Since shorter residence times are required in the DR shaft, the reactor can be made smaller in size. Furthermore, since at least some of the hydrogen gas generated during carburizing is consumed to reduce the sponge iron to its final high DoR (higher than 90%), it potentially reduces hydrogen accumulation during carburizing and simplifies the treatment and reuse of carburizing off-gases. Regardless of the degree of reduction of the sponge iron intermediate, it has a very low carbon content because no carbon is introduced during the reduction stage. It has a carbon content of less than 0.5% by weight, preferably less than 0.05% by weight, and more preferably less than 0.0005% by weight.

[0054] The sponge iron obtained from the reduction step is referred to herein as an intermediate, but this sponge iron does not necessarily need to be subsequently carburized and can instead be used as is in further metallurgical processes, such as in an electric arc furnace or for wrought iron production.

[0055] Carburizing The sponge iron intermediate is supplied as input to the carburizing unit. The sponge iron intermediate may be supplied to the carburizing reactor at a high temperature, for example, by directly discharging the output of the DR shaft to the carburizing unit. This helps conserve energy and potentially reduces or avoids the need for heating associated with the carburizing reaction. Alternatively, the sponge iron may be supplied to the carburizing reactor after being cooled, for example, by storing the sponge iron intermediate prior to its introduction into the carburizing unit.

[0056] The carburizing unit is preferably a carburizing shaft. As mentioned above, a shaft refers to a solid-gas countercurrent moving bed reactor. In this case, the sponge iron intermediate is introduced at the reactor inlet, and the carburizing gas flows countercurrently toward the moving sponge anvil to carburize the sponge iron and optionally further reduce it. Carburized sponge iron is obtained at the reactor outlet.

[0057] Alternatively, the carburizing unit may be a conveyor unit or a batch reactor. However, a continuous reactor such as a carburizing shaft is preferred.

[0058] The DR shaft and the carburizing unit may be connected such that the outlet of the DR shaft is directly connected to the inlet of the carburizing unit, provided that an arrangement is provided to prevent significant penetration of the carburizing gas into the DR shaft. Such an arrangement may include a pressure difference between the reactors that prevents the carburizing gas from directly penetrating into the reduction shaft, and / or a lock or discharge device that provides a physical barrier to gas transport into the reduction shaft. Alternatively, the DR shaft and the carburizing unit may be connected by a shaft or chute, or further means for transporting the sponge iron intermediate, such as one or more transport crucibles, may be utilized.

[0059] The carburizing gas may be any gas known or anticipated in the art that provides carburizing. In this regard, gas means a substance that is gaseous at high temperatures and predominant in the carburizing reactor, but may be liquid or solid at room temperature. Suitable carburizing gases include hydrocarbons such as methane, natural gas, LPG or petroleum, or other carbonaceous substances such as synthesis gas, lower (C1-C6) alcohols, esters, and ethers. The carburizing gas may be of fossil origin, but is preferably obtained from a partially or entirely renewable source in order to reduce net CO2 emissions. Renewable means a resource that is naturally replenished on a human timescale. The high utilization rate of carbon present in the carburizing gas makes it possible to use renewable carburizing gases even if they are relatively scarce and costly as fossil equivalents. Suitable renewable carburizing gases include biomethane, biogas, gases obtained by the thermal decomposition or partial combustion of biomass, lower alcohols or ethers such as methanol, DME or ethanol derived from renewable raw materials, or combinations thereof. Sulfur-containing carburizing gases can be used because sulfur is known to prevent graphite nucleation and passivate sponge iron products.

[0060] The composition of the carburizing gas can be selected to match the desired final carburized sponge iron. Carburizing reactions with hydrocarbons are relatively endothermic, leading to relatively low-temperature final products, while reactions with CO-containing carburizing gases are more exothermic, leading to higher-temperature final products. This effect can be used to adjust the temperature of the final product. For example, if a high-temperature briquette product (HBI) is desired, a gas containing partially oxidized carbon (e.g., CO, ketones, aldehydes) may be used, and if a low-temperature sponge iron (CDRI) is desired, biomethane may be used.

[0061] The carburizing step may be configured to proceed in order to provide a sponge iron product having any desired carbon content. As will be discussed below, the desired carbon content can typically be in the range of about 1% to 3% by weight. This can be achieved by wisely selecting carburizing process parameters, including, but not limited to, residence time in the reactor, reaction temperature, reaction pressure, flow rate of the carburizing gas, and composition of the carburizing gas. The required temperature is usually maintained by preheating the process gas introduced into the reactor, for example, using a preheater such as an electric preheater. Further heating of these gases may be achieved by partial exothermic oxidation of the gas with oxygen or air after it has left the preheater and before it is introduced into the reactor. However, if a high-temperature sponge iron intermediate is introduced as the feed into the carburizing unit and a low-temperature sponge iron product is desired, a preheater or partial oxidation may not be necessary. The carburizing unit may have a cooling discharge cone located at the bottom so that the sponge iron intermediate is cooled prior to discharge from the outlet.

[0062] The spent carburizing gas, or off-gas, may be treated to remove undesirable components and returned to the carburizing and / or reduction reactors for reuse. For example, hydrogen may be separated from the carburizing off-gas and stored or transported directly to the DR shaft for use as a reducing gas. Such separation may be carried out, for example, using membrane separation techniques or pressure swing adsorption. The off-gas may be subjected to reforming steps to reform any CO2 formed during carburizing into CO and / or CH4. Such reforming steps may include, for example, utilizing reverse water-gas conversion reactions to convert CO2 and H2 into CO and H2O, utilizing the Sabatier reaction to convert CO2 and H2 into CH4 and H2O, utilizing co-electrolysis with CO2 and H2O as a feed for providing CO and H2, and combinations thereof. Alternatively, any CO2 formed during carburizing may be captured and stored (CCS), reformed, released, or utilized for other purposes (CCU). Water and / or dust in the carburizing gas may be removed. The remaining gas, which consists mostly of unreacted carburized gas and CO, can be returned to the carburizing reactor and reused.

[0063] To improve the utilization of resources used in the process, the carburizing and reduction stages can be integrated in various ways. For example, the hydrogen formed in the carburizing stage may be used in the reduction stage as described above, or the CO2 formed in the carburizing stage may be reformed into CO for further carburizing. The off-gas obtained in the carburizing stage and / or the top gas obtained in the reduction stage may be supplied through one or more heat exchangers to preheat the gas introduced into the reactor.

[0064] Sponge iron The term crude iron is used herein to refer to any iron produced for further processing into steel, whether obtained in a blast furnace (i.e., pig iron) or a direct reduction shaft (i.e., sponge iron). Sponge iron exiting a carburizing unit is usually in the form of pellets, and such sponge iron is usually referred to as directly reduced iron (DRI). Depending on the process parameters, this may be supplied as high-temperature (HDRI) or low-temperature (CDRI). Low-temperature DRI is also known as type (B) DRI. DRI is readily reoxidized and, in some cases, spontaneously combustible. However, several known means of passivating DRI exist. One such passivation method, commonly used to facilitate the overseas transport of products, is the compression of high-temperature DRI into briquettes. Such briquettes are generally referred to as high-temperature briquetteed iron (HBI) and are also known as type (A) DRI.

[0065] The sponge iron products obtained by the processes described herein may, in essence, be fully metallized sponge iron, i.e., sponge iron with a degree of reduction (DoR) of over 90%, such as over 94% or over 96%. The degree of reduction is defined as the amount of oxygen removed from the iron oxide and is expressed as a percentage indicating the initial amount of oxygen present in the iron oxide. Due to reaction kinetics, obtaining sponge iron with a DoR of over 96% is often not commercially desirable, but such sponge iron can be produced if necessary.

[0066] The carbon present in the sponge iron product is usually in the form of cementite (Fe3C) and / or graphite. Graphite is prone to becoming a dust and is easily lost from the sponge iron before reaching the molten EAF. For this reason, a high proportion of cementite in the sponge iron is preferable. Due to the control provided by performing carburizing as a separation step, this method can yield sponge iron with a high cementite / graphite ratio. A high cementite / graphite ratio means that the mass fraction of carbon present as cementite in the sponge iron product is greater than 70%, preferably greater than 90%, and so on, exceeding 80%.

[0067] When carbon is also present in sponge iron as graphite, sponge iron with a desired carbon content can be produced by the process described herein, which includes a carbon content exceeding the theoretical carbon content of cementite (6.69%). However, for further processing, it is generally desirable for sponge iron to have a carbon content of 1 to 4% by weight, such as 0.5 to 5% by weight, preferably 3% by weight, which may depend on the ratio of sponge iron to scrap used in the subsequent EAF processing step.

[0068] The carburizing gas is preferably derived from a renewable source, in which case the carbon in the sponge iron product is also derived from a renewable source. Whether the carbon in the sponge iron is derived from a renewable source or a fossil source can be determined by radiocarbon dating of the sponge iron. Methods for sample preparation of iron products and radiocarbon dating are known in the art. For example, suitable methods are disclosed in Cook, A., Wadsworth, J., & Southon, J. (2001) AMS radiocarbon dating of ancient iron artifacts: Novel carbon extraction methods used in LLNL, Radiocarbon, 43(2A), 221-227, and these methods are incorporated herein by reference.

[0069] Carbon derived from fossil resources typically has radiocarbon ages exceeding 35,000 years, while carbon from renewable sources is known to be "modern." Depending on the ratio of renewable carbon to fossil carbon in sponge iron, and consequently the ratio of renewable carbon to fossil carbon in the carburized gas, the radiocarbon age of sponge iron can range from approximately 35,000 years (when the carburized gas is exclusively fossil-derived) to "modern" (when the carburized gas is exclusively renewable-derived). A list of radiocarbon-dated iron materials is provided in Cook, AC, Southon, JR & Wadsworth, J., *The Use of Radiocarbon Dating for Dating Iron-Based Artifacts*, JOM55, 15-22 (2003). The processes described herein can be carried out in a commercially viable manner using carburized gas derived primarily or essentially from renewable sources due to their excellent carbon utilization. Therefore, the resulting sponge iron products may have a radiocarbon age of less than 10,000 years, preferably less than 1,000 years, for example, less than 100 years.

[0070] Embodiment The present invention will now be described in more detail with reference to specific exemplary embodiments and drawings. However, the present invention is not limited to the exemplary embodiments described herein and / or shown in the drawings, and may vary within the scope of the appended claims. Furthermore, since some features are emphasized for the purpose of more clearly illustrating certain features, the drawings should not be considered to represent a scale.

[0071] Figure 1 schematically illustrates an exemplary embodiment of an ore-based steelmaking value chain relating to the HYBRIT concept. The ore-based steelmaking value chain begins at an iron ore mine 101. After mining, the iron ore 103 is concentrated and processed at a pelletizing plant 105 to produce iron ore pellets 107. These pellets, along with any of the massive ore used in the process, are converted to sponge iron intermediates 108 by reduction in a direct reduction shaft 111 using hydrogen gas 115 as the primary reducing agent, producing water 117 as the primary byproduct. The hydrogen gas 115 is produced primarily by the electrolysis of water 117 in an electrolytic cell 119 using electricity 121 from a fossil-free or renewable source 122. The hydrogen gas 115 is stored in a hydrogen storage facility 120 prior to its introduction into the direct reduction shaft 111. According to this disclosure, it is desirable that the sponge iron contains carbon, preferably renewable carbon. Therefore, the sponge iron intermediate 108 obtained directly from the reduction shaft 111 is supplied to a carburizing unit, indicated herein as a carburizing shaft 113. In the carburizing shaft 113, the sponge iron intermediate 108 is treated with carburizing gas 114 to provide carburized sponge iron 109. The carburized sponge iron 109 is then melted using an electric arc furnace 123, along with optionally a certain proportion of scrap iron 125 or other iron sources, to provide a molten product 127. The electricity 121 used in the electric arc furnace 123 is preferably obtained from a fossil-free, optionally renewable source 122. The molten product 127 is further subjected to a downstream secondary metallurgical process 129 to produce steel 131.

[0072] Figure 2a schematically shows an exemplary embodiment of a system suitable for carrying out the process disclosed herein.

[0073] The direct reduction shaft 211 is provided with an inlet 211a for iron ore 207, an outlet 211b for discharging sponge iron intermediate 208, an inlet for reducing gas 211c, and an outlet for top gas 211d. During use, iron ore 207 is introduced into the inlet 211a, gradually passes through the reactor, and is discharged at the outlet 211b. While passing through the reactor 211, the ore 207 is reduced by the countercurrent reducing gas 215, and the ore 207 is reduced to sponge iron intermediate 208 at the outlet 211b of the reactor 211.

[0074] The reducing gas 215 is supplied from a source of reducing gas 220, such as a hydrogen gas storage or a water electrolytic cell. The reducing gas 215 passes through a preheater 241 prior to its direct introduction into the reduction shaft 211. The top gas 216 exiting outlet 211d passes through a plurality of processing devices 243 to prepare the gas for reintroduction into the DR shaft 211. The plurality of processing devices may include cleaning steps such as passing through an electrostatic precipitator to remove solids from the gas, heat exchange with other process gases such as the reducing gas 215, and separation of water. The processed top gas 218 is mixed with the reducing gas 215 and passes through a preheater 241 prior to its direct reintroduction into the reduction shaft 211 through inlet 211c. The temperature of the gas entering inlet 211c may be further increased by partial oxidation or electric heating. In such cases, an oxygen supply or an electric gas heater (not shown) is placed between the preheater 241 and inlet 211c.

[0075] In this specification, the carburizing unit, referred to as the carburizing shaft 213, is provided with an inlet 213a for the sponge iron intermediate 208, an outlet 213b for discharging the carburized sponge iron intermediate 209, an inlet for the carburizing gas 213c, and an outlet for the off-gas 213d. During use, the sponge iron intermediate 208 from the direct reduction shaft 211 is introduced into the carburizing shaft 213 via the carburizing shaft inlet 213a. As it passes through the reactor 213, the intermediate 208 is carburized by the counterflowing carburizing gas 214, and the carburized sponge iron 209 is obtained at the outlet 213b of the reactor 213.

[0076] The carburizing gas 214 is supplied from a source of carburizing gas 245, such as a biomass vaporizer. The carburizing gas 214 passes through a preheater 247 prior to its introduction into the carburizing shaft 213. The off-gas 248 exiting the outlet 213d passes through a number of processing devices 249 to prepare the gas for reintroduction into the carburizing shaft 213. The processing devices may include cleaning steps such as passing through an electrostatic precipitator to remove solids from the gas, heat exchange with other process gases such as the carburizing gas 214 or reducing gas 125, and separation of by-products such as hydrogen, carbon dioxide, and / or water. The processing devices may further include devices configured to convert CO2 from the off-gas to CO and / or CH4. Such devices may be, for example, reformers utilizing a reverse water-gas conversion reaction to convert CO2 and H2 to CO and H2O, or co-electrolytic units that utilize CO2 and H2O as feedstocks to provide CO and H2. The treated off-gas, along with reformed CO2, 250 from the off-gas, is mixed with carburizing gas 214 and passes through a preheater 247 prior to its reintroduction into the carburizing shaft 213 through inlet 213c. The temperature of the gas entering inlet 213c can be further increased by partial oxidation or electric heating. In such cases, an oxygen supply and an electric gas heater (not shown) are placed between the preheater 247 and inlet 213c.

[0077] Figure 2b schematically shows a process system similar to that shown in Figure 2a, the difference being that the hydrogen gas 215 separated from the carburizing off-gas 248 is supplied to the preheater 241 for use as a reducing gas directly within the reduction shaft 211.

[0078] Figure 2c schematically illustrates a process system similar to that shown in Figure 2a, the difference being a somewhat different relative ratio between the direct reduction shaft 211 and the carburizing shaft 213. The reduction process parameters within the direct reduction shaft 211 are adapted to provide a less reduced sponge iron intermediate 208, such as between 60% and 80%. The carburizing shaft 213 is relatively large, having dimensions that allow for a higher reduction of the sponge iron intermediate beyond the usual purpose of carburizing. Reduction within the carburizing shaft 213 consumes at least some of the hydrogen gas formed as a byproduct of carburizing, potentially reducing or eliminating the need for hydrogen separation from the off-gas 248.

[0079] In all illustrated embodiments, the preheater 247 is used to preheat the carburizing gas entering the carburizing shaft 213. However, depending on the desired type of sponge iron, the preheater 247 may not be necessary. For example, when producing DRI, relatively low-temperature carburized sponge iron may be desirable. This can be achieved using a carburizing gas that provides an endothermic carburizing reaction, such as (bio)methane, and without preheating the carburizing gas. In such cases, the preheater 247 is not a necessary device.

[0080] Figure 3 is a flowchart schematically illustrating an exemplary embodiment of the process disclosed herein. Step s301 indicates the start of the process. In step s303, the iron ore is reduced using a dilute carbon reducing gas in a direct reduction shaft to provide a sponge iron intermediate. The reduction may be carried out almost to completion to provide a highly reduced intermediate, i.e., more than 90% reduction, or only partially to provide a less reduced intermediate, i.e., 60-90% DoR. In step s305, the sponge iron intermediate is transferred to a carburizing unit. In step s307, the sponge iron intermediate is carburized in a carburizing unit using a carburizing gas to provide carburized sponge iron. If a less reduced intermediate is used as feed to the carburizing unit, the reduction is carried out almost to completion in the carburizing unit to provide a highly reduced carburized sponge iron. Step s309 indicates the end of the process.

[0081] In one exemplary embodiment of this process, DRI is produced as carburized sponge iron by using methane or biomethane as the carburizing gas. In another exemplary embodiment of the process, HBI is produced by using a biogas containing partially oxidized carbon, after which a briquetting step is performed in which the high-temperature carburized sponge iron is briquetized.

[0082] Examples Using a proprietary model, we modeled the sponge iron production system shown in Figure 4 and calculated the balance of mass and energy. The components shown are as follows:

[0083] 407 Iron Ore Pellets 408 Intermediate Sponge Iron 409 Carburized sponge iron 411 Direct Reduction Shaft 413 Carburized shaft 420 Electrolytic cell 441 Preheater 445 Biomethane / Biogas Sources 451 Heat exchanger 453 Top gas refining 455 Compressor 459 Off-gas purification 461 Hydrogen Separation 463 CO2 separation 465 Compressor 467 Preheater

[0084] The calculations were based on the input of 1400 kg of standard LKAB DR pellets (including moisture) per ton of DRI produced. Unless otherwise noted, DRI was calculated to have a reduction degree of 95.8% and a metallization degree of 94%. The carburization degree of the final sponge iron product was 2%.

[0085] We calculated three different scenarios. Scenario 1 involves using H-DR followed by a separate carburizing process using biomethane. Scenario 2 involves using H-DR followed by a separate carburizing process using a biomethane / biogas blend. Scenario 3 involves using only 75% of the DoR from H-DR, followed by separate carburizing / reduction using biomethane.

[0086] A reference scenario based on a conventional natural gas-based direct reduction process was also calculated for comparison. The reference scenario is a fully natural gas-based process and was calculated using a adapted version of the model described above, combined with key parameters found in published literature.

[0087] The total amount of gas directly entering the reduction shaft is nearly identical for all calculation scenarios. The only difference, depending on whether the reduction is performed by hydrogen gas or natural gas, is the composition of the gas.

[0088] Scenario 1: Hydrogen reduction by separate carburizing using biomethane Hydrogen gas is used as the sole reducing agent. Since hydrogen is consumed in the reduction of iron ore, it is necessary to continuously supply hydrogen to the reducing gas circuit from the electrolytic cell 420. The water produced as a result of the reduction reaction is removed from the top gas in the heat exchanger 451. This calculation shows that the top gas of the reduction shaft consists only of hydrogen, nitrogen, and water, and that 72% can be recirculated so that it returns to the reduction shaft after the water has been removed. Since no CO2 is produced in the reduction, a CO2 removal system from the reduction circuit is not necessary. The reducing gas is initially preheated in the heat exchanger 451, then electrically heated to 900°C, and further heated to 1050°C by partial oxidation using oxygen from the electrolytic cell.

[0089] Carburizing is carried out using biomethane as the reducing gas. The amount of biomethane required for carburizing is less than one-quarter of the amount of natural gas used in the reference scenario. Hydrogen gas is produced as a byproduct in the carburizing reactor and separated in unit 461. This hydrogen gas is added to the reducing gas circuit prior to compressor 455, resulting in at least an 11% reduction in the amount of hydrogen required for electrolysis. Since the carburizing off-gas is inherently free of CO2, there is no need for CO2 separation unit 463 in this scenario.

[0090] Scenario 2: Hydrogen reduction by separate carburizing using biogas. This scenario is similar to Scenario 1 described above, but instead of using biomethane as the carburizing gas, a blend of biomethane and biogas is used. For calculation purposes, this blend is assumed to be 50 / 50 biogas and biomethane. However, the exact composition of the carburizing gas may vary. The main difference compared to Scenario 1 lies in the carburizing circuit. The carburizing reaction with methane is an endothermic reaction, while the carburizing reaction with CO (from biogas) is an exothermic reaction, so a higher flow rate of carburizing gas is required to obtain a low-temperature DRI product. Furthermore, since some CO2 is produced in the carburizing reaction, CO2 separation is necessary. Therefore, approximately 5% of the off-gas exiting the carburizing shaft is separated in unit 463.

[0091] However, although CO2 separation is required, the flow rate of off-gas passing through the CO2 separation unit 463 is less than 20% of the corresponding volume of top gas requiring CO2 separation in the natural gas-based reference scenario. This simplifies the CO2 separation step and reduces energy requirements. Furthermore, the amount of CO2 produced in the reference scenario is at least nine times greater than the amount of CO2 produced in scenario 2.

[0092] Scenario 3: After performing a lower degree of reduction in the reduction shaft, perform separate carburizing / reduction in the carburizing shaft using biomethane. Scenario 3 is similar to Scenario 1, except that the degree of reduction in the reduction shaft is set to 75% instead of 95.8%. This means that the hydrogen gas requirement for the reduction circuit is reduced by 17%. Final reduction and carburizing are carried out in the carburizing shaft 413. This requires that the carburizing gas be heated before it enters the carburizing shaft. The model used cannot account for further reduction in the carburizing shaft. However, from Scenario 1, it is known that carburizing of sponge iron in the carburizing shaft produces hydrogen gas equivalent to 11% of the total hydrogen acceptor in the reduction circuit. In Scenario 3, this hydrogen gas could instead react with less reduced sponge iron in the carburizing shaft to provide final reduction simultaneously with carburizing. The advantage of such a process is that increased productivity in the reduction circuit can be achieved, allowing for a reduction in the size of the reduction shaft or improved throughput. As mentioned above, such scenarios are also advantageous in that they can provide increased flexibility and responsiveness with respect to the supply of fossil-free / renewable energy. A disadvantage is that carbon dioxide is produced within the carburizing shaft, which requires separation by unit 463. However, the separated CO2 can be converted back to CO and reintroduced into the carburizing shaft.

[0093] Summary of Modeling Scenarios Therefore, in summary, Scenario 1 provides carburized sponge iron without associated CO2 production. The amount of biomethane required for carburizing is less than one-quarter of the amount of natural gas used in the reference scenario. Even Scenarios 2 and 3, although resulting in some CO2 production requiring the separation of CO2 from the carburizing off-gas, are preferable to the natural gas-based reference scenario. The amount of CO2 produced in the reference scenario is at least nine times more than the amount of CO2 produced in Scenario 2, and the amount of gas requiring treatment in the reference scenario is more than five times greater.

[0094] In each of scenarios 1-3, coal is not introduced into the process, and carburizing is achieved using biomethane / biogas from a bio-based source. Therefore, the produced sponge iron has a radiocarbon age equal to the radiocarbon age of the bio-based source of biomethane / biogas. The exact radiocarbon age, while depending on the biomass used to produce the biomethane / biogas, is most likely to be less than 100 years to the present (due to rotational aging), and almost certainly less than 1000 years to the present. In contrast, in the reference scenario, only natural gas, i.e., fossil fuels, was used for carburizing the sponge iron. In this scenario, the radiocarbon age of the carburized sponge iron is equal to the radiocarbon age of the carburizing gas, but since the carburizing gas in this scenario is natural gas, the sponge iron has a radiocarbon age of approximately 30,000-35,000 years to the present.

Claims

1. The production process for carburized sponge iron (109, 209), The process involves reducing iron ore (107, 207) using a carbon-dilute reducing gas (115, 215) containing less than 10 volume percent of carbon-containing gas in a direct reduction shaft (111, 211) to provide a sponge iron intermediate (108, 208) having a reduction degree of 60% to 80% (s303), The step (s305) is to transfer the sponge iron intermediate to the carburizing unit (112, 213), The process includes the step (s307) of using carburizing gas (114, 214) to carburize the sponge iron intermediate in the carburizing unit to provide carburized sponge iron, A process in which the carburizing gas contains hydrocarbons, and hydrogen produced as a byproduct of the carburizing reaction is consumed as a reducing agent in the carburizing unit, and the final reduction is carried out in the carburizing unit.

2. The process according to claim 1, wherein the carburizing gas is derived from a renewable source.

3. The process according to claim 1 or 2, wherein the carburizing gas includes biomethane, biogas, gas obtained from biomass pyrolysis, or a combination thereof.

4. The process according to any one of claims 1 to 3, wherein the reducing gas contains hydrogen produced by electrolysis.

5. The process according to any one of claims 1 to 4, wherein hydrogen is separated from the off-gas (248) from the carburizing unit, and the hydrogen separated from the off-gas is optionally introduced into the direct reduction shaft as a reducing gas.

6. The process according to any one of claims 1 to 5, wherein the carbon dioxide produced in step (s307) is separated from the off-gas (248) from the carburizing unit, the carbon dioxide is optionally converted to carbon monoxide and introduced into the carburizing unit as a carburizing gas.

7. The process according to any one of claims 1 to 6, wherein the carburizing gas is preheated by heat exchange with an off-gas (248) from the carburizing unit, and / or the carburizing gas is mixed with an off-gas from the carburizing unit prior to its introduction into the carburizing unit.

8. The process according to any one of claims 1 to 7, wherein the sponge iron intermediate is transferred to the carburizing unit at a temperature exceeding approximately 500°C.

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