Method for producing carburized sponge iron

Abstract

The present disclosure relates to a process for producing carburized sponge iron (208) comprising the steps of charging (s303a) iron ore (207) into a direct reduction shaft furnace (211) and / or discharging (s303b) carburized sponge iron (208) from the direct reduction shaft furnace, whereby a sealing gas (223) is introduced into the direct reduction shaft furnace; removing (s305) a top gas (216) from the direct reduction shaft furnace; recirculating (s307) a proportion of the top gas and mixing it with a make-up gas (215) to form a reducing gas (217); and introducing (s309) the reducing gas into the direct reduction shaft furnace in counter-current flow to the iron ore to reduce the iron ore and produce carburized sponge iron. The sealing gas consists essentially of carbon dioxide, and the make-up gas comprises greater than 80 vol% hydrogen. The present disclosure also relates to a system for producing carburized sponge iron, and to carburized sponge iron produced by the above process.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a process for producing carburized sponge iron. The present disclosure also relates to a system for producing carburized sponge iron and carburized sponge iron produced by the above process. BACKGROUND

[0002] Steel is the most important engineering and construction material in the world. In the modern world, it is difficult to find any object that does not contain steel or whose manufacture and / or transportation does not depend on steel. In this way, steel is intricately involved in almost every aspect of our modern life.

[0003] In 2018, the total production of crude steel in the world was 1810 million tons, far more than any other metal, and is expected to reach 2800 million tons by 2050, of which 50% is expected to come from primary iron sources. Steel is also the most recycled material in the world, with a very high recycling grade, as the metal can be reused repeatedly after remelting using electricity as the primary source of energy.

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

[0005] Steel is produced mainly through three routes:

[0006] i) Integrated production using primary iron ore, where the iron oxides in the ore are reduced by carbon to produce iron. The iron is further processed in a steel plant by blowing oxygen in a basic oxygen furnace (BOF), followed by refining to produce steel. This process is also commonly referred to as “oxygen steelmaking”.

[0007] ii) Scrap-based production using recycled steel, which is melted in an electric arc furnace (EAF) using electricity as the primary source of energy. This process is also commonly referred to as “electric steelmaking”.

[0008] iii) Direct reduced production based on primary iron ore, which is reduced by carbonaceous reducing gas in a direct reduction (DR) process to produce sponge iron. The sponge iron is then melted in an EAF along with scrap steel to produce steel.

[0009] The term crude iron is used herein to denote all iron produced for further processing into steel, irrespective of whether they are obtained from a blast furnace (i.e. pig iron) or from a direct reduction shaft (i.e. sponge iron).

[0010] While the above process has been refined over decades and is approaching the theoretically lowest energy consumption, there is one fundamental problem that has not been solved. The use of carbonaceous reductants to reduce iron ore results in the production of CO2 as a by-product. On average, 1.83 tons of CO2 are produced for every ton of steel produced in 2018. The steel industry is one of the highest CO2 emitting industries, accounting for approximately 7% of global CO2 emissions. As long as carbonaceous reductants are used, the overproduction of CO2 in the steel production process cannot be avoided.

[0011] The HYBRIT initiative has been established to solve this problem. HYBRIT is an acronym for HYdrogen BReakthrough Ironmaking Technology, a joint venture between SSAB, LKAB and Vattenfall, partly funded by the Swedish Energy Agency, and aims to reduce CO2 emissions and decarbonize the steel industry.

[0012] The core of the HYBRIT concept is the production of sponge iron from ore based direct reduction. However, unlike the use of carbonaceous reductant gases such as natural gas in current commercial direct reduction processes, HYBRIT proposes the use of hydrogen gas as a reductant, referred to as hydrogen direct reduction (H-DR). Hydrogen gas can be produced by electrolysis of water using mainly fossil-free and / or renewable primary energy sources, as is the case for example with Swedish electricity production. Thus, the key step of reducing iron ore can be achieved without the need for fossil fuels as input, and water replaces CO2 as a by-product. The resulting crude iron will also naturally lack carbon.

[0013] Due to the incorporation of carbon during the reduction of iron ore, iron produced from today’s commercial blast furnace or direct reduction routes typically contains significant amounts of carbon, often up to 5 wt.%. In addition to being used as a reductant, carbon also plays an important role in the steelmaking process. The presence of carbon in the crude iron from the BF or DR process lowers the melting point of the iron. During the subsequent processing of the crude iron in the EAF or BOF, the exothermic dissociation of the iron carbides and the oxidation of carbon to CO provides heat to the process. The gas evolution in the EAF due to this production of CO provides a foamy slag that helps to insulate the iron melt and helps to reduce the consumption of the EAF electrodes. For at least these reasons, the presence of carbon in the crude iron can help to reduce the energy consumption during processing into steel. The presence of carbon in the melt can also affect the slag-metal reaction kinetics and help to clean the metal from dissolved gaseous elements. Furthermore, the presence of carbon in the directly reduced iron passivates the sponge iron and enables easier handling and transport. Finally, due to the tradition and established practices of the steel industry with respect to carbon-containing crude iron, some steel producers can simply be reluctant to adopt carbon-poor crude iron to the extent that there is any benefit.

[0014] For at least these reasons, there can be a need to provide such a crude iron that is produced using a process that is substantially free of fossil fuels, but still contains carbon to the extent that it can be used as a direct replacement for today's crude iron.

[0015] Document US2015 / 0259760 Al describes a method for producing steel in which iron ore is reduced with hydrogen gas and the resulting intermediate product of the reduced iron ore and possibly accompanying substances are subjected to further metallurgical processing. In reducing the iron ore to produce the intermediate product, a carbon-containing gas or a hydrogen-containing gas is added to the hydrogen gas to incorporate carbon into the intermediate product. Examples of carbon-containing gas or hydrogen-containing gas include CH4, coke oven gas (COG), synthesis gas, natural gas, biogas, gas from pyrolysis, and renewable resources.

[0016] There remains a need for a method of producing carbon-containing crude iron in a more environmentally friendly manner. SUMMARY

[0017] The inventors of the present invention have identified a number of shortcomings of prior art methods of providing carbon-containing crude iron.

[0018] Today's commercial methods require excessive use of carbonaceous fossil fuels such as coal or natural gas, resulting in excessive net CO2 emissions. Even in methods proposed to address such issues, a large portion of the carbon-containing gas will still be oxidized to CO2 by contact with the ore. This will be the case regardless of whether the carbon-containing gas is from a renewable source, or whether fossil fuels are used to compensate for fluctuations in the availability of renewable hydrogen, as contemplated by the prior art.

[0019] It would be advantageous to achieve a method that overcomes or at least alleviates at least some of the above-mentioned shortcomings. In particular, it would be desirable to provide a method of producing carbon-containing crude iron with lower net CO2 emissions. To better address one or more of these concerns, there is provided a method for producing carbon-enriched sponge iron having features as defined in the exemplary embodiments discussed herein.

[0020] The method comprises the following steps:

[0021] - charging iron ore into a direct reduction shaft furnace, whereby a sealing gas is introduced into the direct reduction shaft furnace in conjunction with charging the iron ore; and / or discharging carbon-enriched sponge iron (208) from the direct reduction shaft furnace (211), whereby a sealing gas (223) is introduced into the direct reduction shaft furnace in conjunction with discharging the carbon-enriched sponge iron;

[0022] - removing top gas from the direct reduction shaft furnace;

[0023] - recirculating a proportion of the top gas and mixing it with a make-up gas to form a reducing gas; and

[0024] - introducing a reducing gas into the direct reduction shaft furnace countercurrent to the iron ore to reduce the iron ore and produce carbon-enriched sponge iron;

[0025] The sealing gas consists essentially of carbon dioxide, and the make-up gas comprises more than 80 vol% hydrogen.

[0026] When charging the direct reduction shaft furnace with iron ore, and / or discharging the sponge iron produced in the direct reduction shaft furnace, some sealing gas is inevitably introduced into the shaft furnace when charging and / or discharging using methods common in the art. The present invention makes use of a sealing gas consisting essentially of carbon dioxide. The carbon dioxide introduced into the process as sealing gas will eventually lead to carbon enrichment of the sponge iron, which means that the need for further carbon-enriching gas is reduced or avoided completely. Without wishing to be bound by theory, the CO2 introduced into the direct reduction shaft furnace as sealing gas will together with the H2 introduced into the process as make-up gas constitute part of the composition of the process gas. CO2 and H2 can react through a reverse water-gas shift reaction to form CO and H2O. CO can in turn react to provide carbon enrichment of the sponge iron produced in the direct reduction process. The iron-catalysed reaction of hydrogen and carbon dioxide which ultimately provides carbon and steam is commonly known as the Bosch reaction. Alternatively, CO2 and H2 can react through a Sabatier reaction to form methane, and this methane can in turn enrich the sponge iron with carbon. In these ways, the CO2 introduced into the process is converted to CO or CH4, which is removed by carbon enrichment. The overall result is the production of carbon-enriched sponge iron, and the net consumption of CO2.

[0027] A further advantage of the process is that CO2 replaces other inert gases, such as nitrogen, which are normally used as sealing gas. The use of other inert gases normally leads to a build-up of inert gas in the process gas, which means that a proportion of the top gas must be disposed of (for example by burning) in order to maintain an appropriate reducing gas. This is undesirable from both an environmental and an economic point of view. However, in the case of the present invention, there is no build-up of sealing gas, since the CO2 as sealing gas is passively removed from the process gas through carbon enrichment of the sponge iron, so there is no need to dispose of top gas in order to maintain an appropriate reducing gas.

[0028] The hydrogen of the make-up gas can be produced by electrolysis of water.

[0029] The make-up gas can comprise more than 90 vol% hydrogen, for example more than 95 vol% hydrogen.

[0030] The make-up gas can consist essentially of hydrogen. Thus, carbon enrichment can be provided using only carbon dioxide introduced as sealing gas, so that carbon-enriched sponge iron is obtained.

[0031] Alternatively, the make-up gas can consist essentially of hydrogen and carbon dioxide. Thus, carbon dioxide can be used alone to provide carburisation without the use of any typical carburising gas, thereby obtaining carburised sponge iron.

[0032] Alternatively, the make-up gas can consist essentially of hydrogen, an auxiliary carburising gas and optionally carbon dioxide. In such a case, the auxiliary carburising gas can be derived from a renewable source. The high utilisation of carbon present in the carburising gas can allow the use of renewable carburising gases, despite their relative scarcity and high cost compared to fossil equivalents.

[0033] The recirculated top gas can not pass through an external reforming device. That is, under conditions prevalent in direct reduction with hydrogen, the water-gas shift reaction can proceed in the process gas without the need for an external reforming device, and thus the use of an external reforming device can not be required.

[0034] The process can not require the combustion of top gas. As the carbon dioxide seal gas introduced into the process gas is ultimately consumed by carburisation, the seal gas in the process gas can accumulate less, and thus the combustion of top gas to prevent seal gas accumulation can be less or not required.

[0035] The carbon dioxide used in the process can be obtained in part or in whole as a by-product of biofuel production. Thus, only carbon from renewable sources can be used in the process.

[0036] Alternatively, the carbon dioxide can be produced by the oxy-fuel combustion of biomass with oxygen. This means that both the heating of the process gas and the carbon dioxide used in the process can be derived in whole or in part from renewable energy sources. The oxygen used in the oxy-fuel combustion can be produced by the electrolysis of water, which means that both the hydrogen and oxygen produced in the electrolysis are utilised, and potentially reduce the cost of the oxy-fuel combustion.

[0037] Recirculating a proportion of the top gas can include a step of removing water and dust from the top gas.

[0038] The method can further include the steps of:

[0039] - transferring the carburised sponge iron to a carburisation unit; and

[0040] - further carburising the carburised sponge iron in the carburisation unit using a carburising gas to provide further carburised sponge iron.

[0041] Further carbonation of the sponge iron in a separate carbonation unit allows for efficient utilization of the carbon in the carbonation gas, with a greater proportion of the carbon being incorporated into the sponge iron and a smaller proportion of the carbon being oxidized to CO2. Furthermore, performing the further carbonation stage separately can allow for advantageous control of the water-gas shift equilibrium in the reduction shaft.

[0042] The direct reduction shaft can be operated at a pressure greater than 2 bar. In such cases, the sealing gas and the iron ore can be introduced into the direct reduction shaft via a pressurizable charging hopper arranged at the inlet of the direct reduction shaft.

[0043] Alternatively, the direct reduction shaft can be operated at a pressure of 2 bar or less. In such cases, the sealing gas and the iron ore can be introduced into the direct reduction shaft via a sealing tube arranged at the inlet of the direct reduction shaft.

[0044] According to another aspect of the present application, the object of the present application is achieved by a system for producing carbonated sponge iron according to the exemplary embodiments discussed herein. The production method can be the method as defined in the exemplary embodiments discussed herein, and the carbonated sponge iron can be the carbonated sponge iron as defined in the exemplary embodiments discussed herein.

[0045] The system for producing carbonated sponge iron comprises:

[0046] - an electrolysis device arranged to produce hydrogen gas from electrolysis of water;

[0047] - a direct reduction shaft;

[0048] - an iron ore charging device;

[0049] - a carbonated sponge iron discharge device; and

[0050] - a source of carbon dioxide.

[0051] The source of carbon dioxide is arranged in fluid connection with the iron ore charging device, or the source of carbon dioxide is arranged in fluid connection with the carbonated sponge iron discharge device. The source of carbon dioxide can be arranged in fluid connection with both the iron ore charging device and the carbonated sponge iron discharge device.

[0052] The electrolysis device can be arranged in fluid connection with the direct reduction shaft, such that the hydrogen gas produced by the electrolysis device can be transported to the direct reduction shaft.

[0053] This can preferably be indirect fluid connection via treatment of the electrolysis gas and / or via a hydrogen gas storage facility.

[0054] The iron ore charging device can be arranged to transport material, such as iron ore, to the inlet end of the direct reduction shaft.

[0055] Since the CO2 is at least partially consumed in the carbonation, the system can not require a conversion device for converting CO2. The system can also not require a device for CO2 capture. The system is thus much simpler than a system requiring conversion or removal of CO2.

[0056] The iron ore charging device can comprise a pressurizable charging hopper. In such a case, the pressurizable charging hopper can comprise a gas inlet for sealing gas, wherein the gas inlet is arranged in fluid connection with the carbon dioxide source.

[0057] Alternatively, the iron ore charging device can comprise an ore bin and a seal tube. In such a case, the seal tube can comprise a sealing gas inlet, wherein the sealing gas inlet is arranged in fluid connection with the carbon dioxide source.

[0058] According to another aspect of the present application, the objects of the present application are achieved by a carbonated sponge iron according to the exemplary embodiments discussed herein. The carbonated sponge iron can be produced by a method according to the exemplary embodiments discussed herein. The carbonated sponge iron has a degree of reduction of greater than 90%, for example greater than 94%, and comprises 0.1 percent to 5 percent by weight of carbon, for example 0.5 percent to 3 percent by weight of carbon, for example about 1 percent to 2 percent by weight of carbon. The carbonated sponge iron has a radiocarbon age of less than 10,000 years before present, preferably less than 1,000 years before present, even more preferably less than 100 years before present. This means that the carbonated sponge iron must have been carbonated using carbon dioxide comprising a significant renewable carbon content, which is commercially feasible using the methods described herein, as described herein.

[0059] The carbonated sponge iron can be in the form of pellets (i.e. DRI) or briquettes (i.e. HBI).

[0060] Further objects, advantages and novel features of the present application will become apparent to one skilled in the art from the following DETAILED DESCRIPTION. BRIEF DESCRIPTION OF DRAWINGS

[0061] For a more complete understanding of the present application and for further objects and advantages thereof, reference is made to the following DETAILED DESCRIPTION taken in connection with the accompanying drawings, in which like reference characters represent like elements in the different views, wherein:

[0062] Figure 1 A value chain for ore-based steelmaking according to the Hybrit concept is schematically illustrated;

[0063] Figure 2a One exemplary embodiment of a system suitable for carrying out a method as disclosed herein is schematically illustrated;

[0064] Figure 2bAnother exemplary embodiment of a system suitable for carrying out a method as disclosed herein is schematically shown;

[0065] Figure 3 is a flow diagram schematically showing one exemplary embodiment of a method as disclosed herein;

[0066] Figure 4a Schematically shown is a calculated balance established by the input gas mixture.

[0067] Figure 4b Schematically shown is a part of the calculated balance of Figure 4a Schematically shown is a part of the calculated balance of DETAILED DESCRIPTION

[0068] The present invention is based on a number of insights of the inventor. The inventor realized that in processes that make use of hydrogen gas as a major component of the reducing gas, carbon dioxide will be removed from the process gas by the carburization of the sponge iron. This is in contrast to typical (fossil) carbon-based reduction systems, where CO2 accumulates in the process gas and has to be removed and / or converted. The inventor further realized that the relative inertness of CO2 means that it can be introduced into the process as a sealing gas, thus replacing the inert gas that is typically used as a sealing gas. By doing so, a number of advantages are obtained, as described herein.

[0069] The term process gas is used herein to refer to the gas mixture in the direct reduction process irrespective of the stage of the process. That is, process gas refers to the gas that is introduced, passes through, exits and is recycled back into the direct reduction shaft furnace. More specific terms are used to refer to the process gas at different points in the process, or to component gases that are added to the process gas to form part of the process gas. Reducing gas is the gas that is introduced at a point below the inlet of the shaft furnace and which flows upwards against the moving bed of ore to reduce and / or carburise the ore. Top gas is the partially spent process gas that is removed from the upper end of the shaft furnace near the ore inlet. After treatment, the top gas is recycled back into the direct reduction shaft furnace as a component of the reducing gas. Make-up gas is fresh gas that is added to the reducing gas to maintain the reducing capacity. Thus, the reducing gas comprises make-up gas and recycled top gas. The make-up gas and recycled top gas can be mixed together before introduction into the direct reduction shaft furnace, or can be introduced separately into the shaft furnace and mixed in the shaft furnace. Seal gas is the gas that enters the direct reduction shaft furnace from the ore charging device at the inlet of the direct reduction shaft furnace. The outlet end of the direct reduction shaft furnace is also sealed using seal gas, and thus seal gas can enter the DR shaft furnace from the discharge device at the outlet of the direct reduction shaft furnace, although the amount is typically small. The present invention is therefore equally applicable to the use of seal gas that is introduced during the discharge of sponge iron from the direct reduction shaft furnace.

[0070] Sealing gas

[0071] In a direct reduction shaft furnace, the charge of iron ore must be safely introduced into the shaft furnace. Since the process gas that passes through the shaft furnace is highly flammable (typically comprising hydrogen, carbon monoxide and hydrocarbons), it is essential that an explosive air / process gas mixture is not formed when the ore is introduced into the shaft furnace. This is typically achieved by ensuring that only inert seal gas, and not air, is introduced into the shaft furnace when the ore is being charged, and that no process gas escapes the shaft furnace from the charging device. How this is achieved depends on the operating pressure of the direct reduction shaft furnace.

[0072] Generally, in a DR shaft furnace that is operated at low pressure (e.g. < 2 bar) (e.g. Midrex process), a dynamic gas seal is arranged at the connection of the ore charging hopper to the seal leg of the direct reduction shaft furnace. At one or more points in the seal leg, seal gas is introduced at a pressure that exceeds the operating pressure of the DR shaft furnace. This higher pressure seal gas prevents air from being introduced into the DR shaft furnace with the iron ore charge, and prevents process gas from escaping the DR shaft furnace through the seal leg.

[0073] Generally, in a DR shaft furnace operating at higher pressures (e.g. > 2 bar) (e.g. Hyl ZR process), a pressurizable charging hopper is arranged at the inlet of the direct reduction shaft furnace. Ore is charged into the pressurizable hopper, which is then first flushed with inert sealing gas to exclude air, and then pressurized with the sealing gas to approximately the operating pressure of the DR shaft furnace. After pressurization, the valves separating the charging hopper and the DR shaft furnace are opened to allow the iron ore charge and the sealing gas to be introduced into the shaft furnace together. After the ore in the charging hopper is emptied, the charging hopper is resealed and flushed again with sealing gas to expel process gas from the charging hopper. Finally, the charging hopper can be opened to the atmosphere and refilled with ore. Typically, several pressurizable charging hoppers (e.g. 4 hoppers) are arranged in parallel to supply the DR shaft furnace with ore.

[0074] It should be noted that when the reduced sponge iron product is discharged from the outlet of the direct reduction shaft furnace, sealing gas is also used, and sealing gas can typically also be introduced into the shaft furnace at this point. However, the amount of sealing gas introduced at the outlet is typically small compared to the amount introduced at the inlet.

[0075] Regardless of the technique used for the ore charge, it is unavoidable that sealing gas is introduced into the process gas. As mentioned above, the sealing gas typically used, e.g. nitrogen, must be inert (i.e. not form an explosive mixture with the process gas), and thus remains in the process gas. However, other components of the process gas are typically either consumed by reaction (e.g. H2, CO, CH4) or removed from the cycle (e.g. H2O, CO2). This means that if no action is taken, the sealing gas will gradually accumulate in the process gas, and constitute an increasing proportion. The end result will be a process gas with reduced reducing capacity for the ore. To avoid such a situation, a proportion of the process gas is typically removed from the cycle (as top gas) and combusted to maintain a suitable concentration of inert components in the process gas. This is at least economically detrimental, and depending on the source of the process gas (e.g. fossil fuels), can also have negative environmental impacts.

[0076] However, in the process of the present invention, carbon dioxide is used as the sealing gas. As will be described below, under the conditions prevailing in hydrogen-based direct reduction, the carbon dioxide participates in the carburization of the sponge iron produced in the direct reduction shaft, and is passively removed from the process gas. This has several advantages. A carburized sponge iron product is obtained with low attendant CO2 emissions. Depending on the source of the carbon dioxide used, the total CO2 emissions can be close to zero, or even negative. Since the carbon dioxide sealing gas is largely passively removed from the process gas, there is no necessity for any over-accumulation of the sealing gas in the process gas. This means that it can not be necessary to dispose of the top gas to control the inert gas levels in the process gas. It can also not be necessary for any separate means to remove or convert the carbon dioxide, such as an external conversion means or a carbon capture means. Another advantage is that the cost of producing the sealing gas can be reduced, since nitrogen, which is commonly used in the sealing gas, is usually produced by an expensive air separation unit.

[0077] To ensure that sufficient carbon dioxide is introduced into the process, more sealing gas than is strictly required for sealing purposes can be introduced into the process gas. For example, the sealing gas can be introduced into the burden pipe or pressurizable charging hopper at a higher pressure. The pressurizable charging hopper arrangement can also be configured to allow a continuous (leakage) flow of carbon dioxide to the process gas, even between charging of the iron ore burden into the shaft.

[0078] Carbon addition using CO2

[0079] As mentioned above, the addition of carbon dioxide to a hydrogen-based direct reduction process will result in the eventual removal of carbon from the process gas by carburization of the sponge iron. Without wishing to be bound by theory, it is believed that the carbon dioxide will initially be converted to carbon monoxide in the hydrogen-rich process gas by a reverse water-gas shift reaction.

[0080] The reverse water-gas shift reaction is as follows:

[0081]

[0082] In prior art syngas based processes, there is already a significant amount of both hydrogen and carbon containing gases (CO, CO2). This means that the introduction of a small amount of additional carbon dioxide as a sealing gas will not significantly change any of the balances. However, in a hydrogen based direct reduction process, any CO2 introduced to the process gas will more or less quantitatively convert to CO at the elevated temperatures prevailing in the direct reduction shaft. Many additional factors also conspire to make the conversion of CO2 to CO advantageous. Iron based catalysts such as iron oxides (e.g. magnetite) are known to catalyze the water-gas shift reaction, which means that this reaction will be catalyzed by the iron containing reactants and / or direct reduction products and will not require an external conversion unit. The continuous removal of water from the top gas of the direct reduction process enhances the production of CO according to Le Chatelier’s principle. The reverse WGS reaction is endothermic, which means that again according to Le Chatelier’s principle, the equilibrium is shifted towards CO at high temperatures. Finally, as described below, the produced carbon monoxide will be continuously removed from the process gas by the carbonation reaction, which further enhances the conversion of CO2 to CO.

[0083] After formation, the carbon monoxide can participate in a series of additional reactions, some of which are shown below.

[0084] Methanation

[0085]

[0086] Reduction

[0087]

[0088] Carbon addition (graphite production)

[0089]

[0090]

[0091] Carbon addition (cementite production)

[0092]

[0093] Hydrogen also provides reduction of iron ore by the following reaction:

[0094] 6Fe2O3+ 2H2→ 4Fe3O4+ 2H2O + 32.7 kJ / mol

[0095] 2Fe3O4+ 2H2→ 6FeO + 2H2O + 127.6 kJ / mol

[0096] 6 FeO + 6 H2→ 6 Fe + 6 H2O + 171.4 kJ / mol

[0097] 2 Fe3O4+ 8 H2→ 6 Fe + 8 H2O + 299.0 kJ / mol

[0098] The reaction of hydrogen and carbon dioxide to produce methane is commonly referred to as the Sabatier reaction. The reaction of hydrogen and carbon dioxide to produce carbon and steam is commonly referred to as the Bosch reaction.

[0099] Hydrogen is dominant in the process gas and will therefore be the primary reducing agent. However, since carbon monoxide is formed from the reaction of hydrogen with carbon dioxide, and since even carbon monoxide acts as a reducing agent, this only regenerates carbon dioxide, the "primary" reducing agent can be considered to be hydrogen in all cases. Carbon is only removed from the chemical system through the carburization reaction, as the carburized sponge iron product is removed at the outlet of the direct reduction shaft. The entire chemical system can therefore be conceptualized as: in the conversion of ore and carbon dioxide to iron and carbon monoxide, respectively, hydrogen is consumed, followed by carbon monoxide through carburization, and the final products are water and carburized sponge iron, both of which are removed from the system.

[0100] Direct reduction

[0101] The direct reduction shaft can be any kind known in the art. By shaft it is meant a solid-gas countercurrent moving bed reactor, wherein the iron ore burden is charged at the inlet at the top of the reactor and descends by gravity towards an outlet arranged at the bottom of the reactor.

[0102] The reduction is typically carried out at a temperature of about 900 °C to about 1100 °C. The required temperature is typically maintained by preheating the process gas introduced into the reactor, for example using a preheater such as an electric preheater. After leaving the preheater and before being introduced into the reactor, further heating of the gas can be obtained by exothermic partial oxidation of the gas with oxygen or air. The 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.

[0103] The iron ore burden typically consists mainly of iron ore pellets, but some lump iron ore can also be introduced. The iron ore pellets typically comprise mainly hematite, as well as additional additives or impurities such as gangue, fluxes and binders. However, the pellets can comprise some other metals and other ores such as magnetite. Iron ore pellets designated for direct reduction processes are commercially available, and such pellets can be used in the process of the present invention. Alternatively, the pellets can be specifically adapted for the carbon lean reduction step, as in the process of the present invention.

[0104] The make-up gas for make-up of reducing gas contains little or essentially no carbon-adding gas compared to today's commercial direct reduction processes. By carbon-adding gas it is meant a carbon-containing gas that is able to directly add carbon to the sponge iron, such as all incompletely oxidized carbon compounds, such as hydrocarbons or carbon monoxide. The make-up gas can for example contain hydrogen, consist essentially of hydrogen or consist of hydrogen. In contrast, the main carbon-adding effect in the process of the present application is achieved by the introduction of carbon dioxide as a sealing gas, the conversion of carbon dioxide to carbon monoxide and subsequent carbon addition to the sponge iron. However, in order to achieve a suitable level of carbon addition to the sponge iron, a small amount of additional carbon can be included in the make-up gas as carbon dioxide and / or auxiliary carbon-adding gas. For example, the make-up gas can contain at least 80 vol.%, preferably more than 90 vol.%, even more preferably more than 95 vol.% hydrogen (vol.% determined at standard conditions of 1 atm and 0°C); consist essentially of at least 80 vol.%, preferably more than 90 vol.%, even more preferably more than 95 vol.% hydrogen; or consist of at least 80 vol.%, preferably more than 90 vol.%, even more preferably more than 95 vol.% hydrogen. The balance can contain, consist essentially of or consist of carbon dioxide and / or auxiliary carbon-adding gas. If carbon dioxide and / or auxiliary carbon-adding gas forms part of the make-up gas, these can be introduced to the direct reduction shaft together with the hydrogen make-up gas. Alternatively, a certain proportion or all of the carbon dioxide and / or auxiliary carbon-adding gas that forms part of the make-up gas can be added to the direct reduction shaft separately from the main volume of make-up gas. For example, the auxiliary carbon-adding gas can be added to the carbon-adding zone or the cooling zone of the direct reduction shaft.

[0105] The process gas is at least partially recycled whereby the top (off) gas from the DR shaft can be cleaned and treated to remove by-products such as water and / or dust before being reintroduced to the DR shaft. This recycled top gas can be mixed with fresh make-up gas before being reintroduced to the reactor or can be introduced separately from any fresh make-up gas supply.

[0106] The sponge iron is obtained as a reduction product of iron ore and is discharged from an outlet at the bottom end of the direct reduction shaft. The shaft can have a cooling and discharge hopper arranged at the bottom to cool the sponge iron before being discharged from the outlet.

[0107] Sponge iron

[0108] The term crude iron is used herein to mean all iron produced for further processing into steel, regardless of whether they are obtained from a blast furnace (i.e. pig iron) or from a direct reduction shaft (i.e. sponge iron). Due to the structural integrity of the direct reduced pellets and the conditions prevailing in a DR shaft, the sponge iron obtained at the outlet of a DR shaft is typically predominantly in the form of pellets. Such sponge iron is commonly referred to as direct reduced iron (DRI). Depending on the process parameters, it can be provided hot (HDRI) or cold (CDRI). Cold DRI can also be referred to as (B) type DRI. DRI can be prone to re-oxidation and in some cases is pyrophoric. However, there are many known methods of passivating DRI. One such passivation method commonly used to facilitate overseas transportation of the product is to press the hot DRI into briquettes. Such briquettes are commonly referred to as hot briquetted iron (HBI) and can also be referred to as (A) type DRI.

[0109] The sponge iron product obtained by the process herein can be substantially fully metallized sponge iron, i.e. sponge iron having a degree of reduction (DoR) of greater than about 90%, for example greater than about 94% or greater than about 96%. The degree of reduction is defined as the amount of oxygen removed from the iron oxides, expressed as a percentage of the initial amount of oxygen present in the iron oxides. Due to reaction kinetics, it is not typically commercially advantageous to obtain sponge iron having a DoR of greater than about 96%, but such sponge iron can be produced if desired.

[0110] By carbon-enriched sponge iron, it is meant sponge iron containing carbon. The carbon present in the sponge iron product can typically be in the form of cementite (Fe3C) and / or graphite. Graphite tends to become dust and tends to be lost from the sponge iron before reaching the melt in the EAF. For this reason, it can be preferred if the carbon is present in the sponge iron as cementite.

[0111] The carbon-enriched sponge iron can comprise between 0.1 and 5 percent by weight of carbon, for example between 0.5 and 3 percent by weight of carbon, for example between about 1 and 2 percent by weight of carbon. For further processing, it is typically desirable for the sponge iron to have a carbon content of between 0.5 and 5 percent by weight, preferably between 1 and 4 percent by weight, for example about 3 percent by weight, but this can depend on the ratio of sponge iron to scrap steel used in the subsequent EAF processing step. If desired, the carbon-enriched sponge iron product of the process of the present invention can be further carbon-enriched by other means prior to further processing.

[0112] Gas

[0113] The hydrogen gas can preferably be obtained at least in part by electrolysis of water. If the water electrolysis is performed using renewable energy, this allows the reducing gas to be provided from a renewable source. The electrolytic hydrogen gas can be delivered directly from the electrolyser to the DR shaft via a conduit, or the hydrogen gas can be stored after production and delivered to the DR shaft as required.

[0114] The present invention requires a source of carbon dioxide as the sealing gas. Some carbon dioxide can also be introduced as a make-up gas to provide increased carburisation of the sponge iron. In order to avoid build-up of inert gases in the process gas, the sealing gas consists essentially of carbon dioxide, i.e. carbon dioxide and impurities in the gas source. For example, it is preferred that the carbon dioxide source is essentially pure carbon dioxide, for example 95% or more by volume, preferably 98% or more by volume carbon dioxide. The carbon dioxide source can preferably be from a high concentration source, preferably a high concentration biological source. For example, concentrated "green" CO2 can be obtained as a by-product of biogas production by anaerobic digestion or as a by-product of bioethanol production. If the carbon dioxide used in the process is from a renewable source, the process can be net negative in terms of CO2 emissions. However, even if the carbon dioxide source is from a fossil source that would otherwise be directly emitted, it means that the process can not result in any excess CO2 emissions. An alternative method of providing carbon dioxide is to preheat the reducing gas using oxy-fuel combustion of biomass prior to introduction into the direct reduction shaft. The principle of oxy-fuel combustion is simple: biomass is combusted using essentially pure oxygen as the oxidant. The resulting flue gas stream consists essentially of carbon dioxide and steam. The steam can be removed by simple condensation, thereby providing an essentially pure source of carbon dioxide. Conventionally, providing essentially pure oxygen is an economic barrier to the use of oxy-fuel combustion. However, in the present case, the available oxygen can be supplied at low additional cost from water electrolysis at all times, thereby making oxy-fuel preheating of the reducing gas economically viable.

[0115] The auxiliary carburizing gas can be any gas known or desired in the art for providing carburization. Gas in this respect refers to a substance that is gaseous at the high temperatures prevailing in the carburization reactor, but which can 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 can be of fossil origin, but it is preferred that it is obtained partly or entirely from renewable sources to reduce the net CO2 emissions. By renewable it is meant a resource that is replenished naturally on the human timescale. The high utilization of carbon present in the carburizing gas allows the use of renewable carburizing gases, despite their relative scarcity and high cost compared to fossil equivalents. Suitable renewable carburizing gases include biomethane; biogas; gases obtained from pyrolysis or partial combustion of biomass; lower alcohols or ethers derived from renewable feedstocks such as methanol, DME or ethanol; or combinations thereof. Carburizing gases containing sulfur can be used, as sulfur is known to prevent graphite nucleation and passivate the sponge iron product.

[0116] The composition of the carburizing gas can be selected to suit the final carburized sponge iron to be obtained. The carburization reaction with hydrocarbons is relatively endothermic, resulting in a relatively cold final product, whereas the reaction with CO-containing carburizing gases is more exothermic, resulting in a hotter final product. This effect can be used to adjust the temperature of the final product obtained. For example, if a hot product is desired for briquetting (HBI), a gas containing some partially oxidized carbon (e.g. in the form of CO, ketones, aldehydes) can be used, whereas if a cold sponge iron (CDRI) is desired, biomethane can be used.

[0117] In considering the total amount of carbon dioxide and / or auxiliary carburizing gas suitable for addition to the process gas, or as sealing gas (carbon dioxide) or in the make-up gas (carbon dioxide and / or auxiliary carburizing gas), factors to be considered are the degree of carburization desired, the effect of the added carbon on the water-gas shift equilibrium, and the effect that the added carbon has on the reducing power of the reducing gas. For example, an excess of carbon addition can lead to a substantial build-up of carbon dioxide in the process gas, potentially reducing the reducing power of the reducing gas.

[0118] It is preferred that the carbon dioxide and any auxiliary carburizing gas used originates from a renewable source, and in such cases the carbon in the sponge iron product will also originate from a renewable source. By radiocarbon dating of the sponge iron, it can be determined whether the carbon in the sponge iron originates from a renewable source or a fossil source. Methods for sample preparation and radiocarbon dating of iron products are known in the art. Suitable methods are disclosed in, for example, Cook, A., Wadsworth, J., & Southon, J. (2001). AMS Radiocarbon Dating of Ancient Iron Artifacts: A New Carbon Extraction Method in Use at LLNL. Radiocarbon, 43(2A), 221-227, the method of which is incorporated herein by reference.

[0119] Carbon originating from a fossil source typically has a radiocarbon age exceeding 35,000 years, while carbon originating from a renewable source is found to be “modern”. Depending on the proportion of renewable carbon to fossil carbon in the sponge iron, which in turn depends on the proportion of renewable carbon to fossil carbon in the carbon dioxide and carburizing gas, the radiocarbon age of the sponge iron can range from about 35,000 years (if the carbon originates solely from fossil) to “modern” (if the carbon originates solely from renewable). A list of iron objects that were radiocarbon dated is provided in Cook, A. C., Southon, J. R. & Wadsworth, J. Using radiocarbon dating to establish the age of iron-based artifacts. JOM 55, 15-22 (2003). The method described herein is able to be performed in a commercially viable manner using carbon dioxide and optionally carburizing gas that originates predominantly or essentially from renewable sources due to its superior carbon utilization. Thus, the radiocarbon age of the resulting sponge iron product can be less than 10,000 years old, preferably less than 1,000 years old, for example less than 100 years old.

[0120] Embodiment

[0121] The present application will now be described in more detail, with reference to certain example embodiments and the accompanying drawings. However, the application is not limited to the example embodiments discussed herein and / or shown in the accompanying drawings, but can vary within the scope of the example embodiments discussed herein. Furthermore, the drawings should not be considered to be drawn to scale as some features can be exaggerated for the sake of clarity.

[0122] Figure 1One prior art embodiment of a mine-based steelmaking value chain according to the Hybrit concept is schematically illustrated. The mine-based steelmaking value chain starts with iron ore mine 101. After mining, the iron ore 103 is beneficiated and processed in a pellet plant 105 and iron ore pellets 107 are produced. These pellets, together with any lump ore used in the process, are converted to sponge iron 109 by reduction in a direct reduction shaft 111 using hydrogen 115 as the main reducing agent and producing water 117 as the main by-product. Hydrogen 115 is produced by electrolysis of water 117 in an electrolysis plant 119 using electricity 121, preferably mainly derived from fossil-free or renewable sources 122. Hydrogen 115 can be stored in a hydrogen storage 120 before being introduced to the direct reduction shaft 111. The sponge iron 109 is melted together with optionally a proportion of scrap iron 125 or other iron sources using an electric arc furnace 123 to provide a melt 127. The melt 127 is subjected to further downstream secondary metallurgical processes 129 and steel 131 is produced. The entire value chain from ore to steel is intended to be fossil-free and produces only low or zero carbon emissions.

[0123] Figure 2a One exemplary embodiment of a system suitable for carrying out a method as disclosed herein is schematically illustrated. Figure 2a The system shown in Fig. 2 is particularly suitable for a direct reduction process operating at elevated pressure (e.g. > 2 bar).

[0124] The direct reduction shaft 211 is arranged with an inlet 211a for iron ore 207, an outlet 211b for discharge of sponge iron 208, an inlet 211c for reducing gas and an outlet 211d for top gas. A pressurizable charging hopper 213 is arranged in communication with the inlet 211a of the direct reduction shaft 211. The pressurizable charging hopper 213 comprises an inlet 213a for iron ore 207, an outlet 213b for iron ore 207, a gas inlet 213c and a gas outlet 213d. The inlet 213a of the pressurizable charging hopper 213 is arranged in connection with the ore bin 227. The gas inlet 213c of the pressurizable charging hopper 213 is arranged in connection with the carbon dioxide source 221.

[0125] In use, iron ore 207 from ore bin 227 is introduced into pressurisable charging hopper 213. After loading with ore, ore inlet 213a and outlet 213b are sealed and hopper 213 is purged with a volume of carbon dioxide 223 from carbon dioxide source 221 to expel air from hopper 213. The air mixed with carbon dioxide exits hopper 213 as purged gas 225. If required, the purged gas mixture can be recycled to recover carbon dioxide. Gas outlet 213d is then sealed and hopper 213 is pressurised with carbon dioxide 223 to a pressure close to the operating pressure of direct reduction shaft furnace 211. After pressurisation, outlet 213b of hopper 213 is opened to allow iron ore 207 to be charged to enter inlet 211a of direct reduction shaft furnace 211. Carbon dioxide 223 is inevitably introduced into the direct reduction shaft furnace with the charge. Iron ore 207 charged to direct reduction shaft furnace 211 is progressively passed through the shaft to be discharged at outlet 211b. During its passage through shaft 211, ore 207 is reduced by reducing gas 217 flowing in counter current, such that sponge iron 208 is obtained at discharge outlet 211b of reactor 211.

[0126] Make-up gas 215 is supplied from make-up gas source 220, for example a hydrogen store or a water electrolyser. Make-up gas 215 is mixed with treated top gas 218 to form reducing gas 217. Reducing gas 217 passes through preheater 241 before being introduced into direct reduction shaft furnace 211. Top gas 216 exiting outlet 211d passes through a plurality of treatment devices 243 to prepare the gas for reintroduction to DR shaft furnace 211. The plurality of treatment devices can include cleaning steps, for example by electrostatic precipitators to remove solids from the gas, heat exchange with other process gases, for example reducing gas 217, and separation of water. Treated top gas 218 is mixed with make-up gas 215 and passes through preheater 241 before being reintroduced into direct reduction shaft furnace 211 through inlet 211c. The temperature of the gas entering inlet 211c can be further increased by partial oxidation. In such a case, a supply of oxygen (not shown) would be arranged between preheater 241 and inlet 211c.

[0127] Carbon dioxide 223 introduced to the direct reduction shaft furnace with ore 207 will be recycled in the process gas, wherein, without wishing to be bound by theory, carbon dioxide 223 will first be converted to carbon monoxide by reaction with hydrogen and then absorbed as carbon (for example graphite or cementite) in the sponge iron. Thus, carbon dioxide 223 will be passively removed from the cycle and will not accumulate in the process.

[0128] Figure 2b schematically shown in Figure 2aThe process system shown is similar to that of a conventional process system, except that it is particularly suitable for direct reduction processes operating at near atmospheric pressure (e.g., ≤2 bar). In this system, iron ore 207 is charged into the inlet 211a of the direct reduction shaft furnace 211 via a sealing pipe 229. Carbon dioxide 223 is supplied to the sealing pipe 229 at a pressure exceeding the operating pressure of the direct reduction shaft furnace 211 at the sealing gas inlet 229c, and the mixture of carbon dioxide and air is removed as a purging gas 225. In this way, the iron ore 207 charged into the inlet 211a of the direct reduction shaft furnace 211 is de-aired, and the process gas from the direct reduction shaft furnace 211 is prevented from leaving the shaft furnace via the sealing pipe 229. However, such an arrangement inevitably results in carbon dioxide 223 being introduced into the direct reduction shaft furnace 211 along with the ore 207. In other aspects, the operation of the system and process is similar to... Figure 2a Similar to those described in [the text]. Again, the purged gas 225 can be recirculated.

[0129] Although not shown, Figure 2a and 2b The process system shown also includes a carburized sponge iron discharge device arranged at the discharge outlet 211b of the direct reduction shaft furnace. Such a discharge device is known in the art and may include a pressurized discharge hopper, or a sealing pipe and a discharge bin. Such a discharge device is designed to be combined with… Figure 2a and 2b The ore loading device described operates in a similar manner.

[0130] Figure 3 This is a flowchart schematically illustrating an exemplary embodiment of the method disclosed herein. Step s301 indicates the start of the process. In step s303a, iron ore is charged into the direct reduction shaft furnace, thereby inevitably introducing carbon dioxide, as a sealing gas, into the direct reduction shaft furnace along with the iron ore. In step s305, the top gas is removed from the direct reduction shaft furnace. In step s307, a certain proportion of the top gas is recirculated and mixed with makeup gas to form a reducing gas. In step s309, the reducing gas is introduced into the direct reduction shaft furnace countercurrently with the iron ore to reduce the iron ore and produce carbonized sponge iron. Step s311 indicates the end of the process. Alternatively or additionally, step s303b (an additional step) may replace or supplement step s303a. In step s303b, carbonized sponge iron is discharged from the direct reduction shaft furnace, thereby introducing carbon dioxide, as a sealing gas, into the direct reduction shaft furnace along with the discharge of the carbonized sponge iron.

[0131] Figure 4a and 4bThe equilibrium composition formed at a pressure of 4 bar using an initial gas composition of H2: 91%; CO2: 4%; and H2O: 5% (by volume) is shown. The equilibrium was calculated using HSC Equilibrium software from Outotec. Figure 4a All components are shown, whereas Figure 4b The carbonaceous components of the mixture are concentrated. It can be seen that at relatively low temperatures (

Claims

1. A method for producing carbonized sponge iron (208) from iron ore (207), the method comprising the following steps: - Iron ore is charged into (s303a) a direct reduction shaft furnace (211), and / or carbonized sponge iron (208) is discharged from (s303b) the direct reduction shaft furnace (211), thereby introducing a sealing gas (223) into the direct reduction shaft furnace along with the charging of the iron ore and / or the discharge of the carbonized sponge iron. -Remove (s305) furnace top gas (216) from the direct reduction shaft furnace; - A portion of the furnace top gas is recirculated (s307) and mixed with the makeup gas (215) to form a reducing gas (217); and - The reducing gas is introduced into the direct reduction shaft furnace in a countercurrent manner with the iron ore (s309) to reduce the iron ore and produce carbonized sponge iron; The method is characterized by: The sealing gas contains 95% by volume or more carbon dioxide, and The supplemental gas contains more than 80% by volume hydrogen, and the hydrogen is generated by water electrolysis. The direct reduction shaft furnace operates at a pressure greater than 2 bar, and the sealing gas and the iron ore are introduced into the direct reduction shaft furnace via a pressurized charging hopper (213) arranged at the inlet (211a) of the direct reduction shaft furnace.

2. The method of claim 1, wherein the supplementary gas comprises more than 90% by volume hydrogen.

3. The method according to claim 1, wherein the supplementary gas consists of hydrogen and carbon dioxide.

4. The method according to any one of claims 1 to 3, wherein the recirculated top gas does not pass through an external CO2 conversion unit.

5. The method according to any one of claims 1 to 3, wherein the furnace top gas is not burned.

6. The method according to any one of claims 1 to 3, wherein the carbon dioxide is obtained as a byproduct of biofuel production.

7. The method according to any one of claims 1 to 3, wherein recirculating a portion of the top gas includes the step of removing water and dust from the top gas.

8. A system for producing carbonized sponge iron, the system comprising: - An electrolysis device (220) is arranged to produce hydrogen gas by electrolysis of water; -Direct reduction shaft furnace (211); -Iron ore charging device; -Carbonized sponge iron discharge device; and - Carbon dioxide source (221), The carbon dioxide source is arranged to be fluidly connected to the iron ore charging device and / or the carbon-enriched sponge iron discharge device. The iron ore charging device includes a pressurized charging hopper (213), wherein the pressurized charging hopper includes a gas inlet (213c) for sealing gas, and wherein the gas inlet is arranged to be fluidly connected to the carbon dioxide source.

9. The system according to claim 8, wherein the system does not include a CO2 conversion device.

10. The system according to claim 8 or 9, wherein the system does not include a device for CO2 capture.

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