SE545624C2 - Process for the production of carburized sponge iron - Google Patents

Abstract

The present disclosure relates to a process for the production of carburized sponge iron (208), comprising the steps of: charging (s303) iron ore into a direct reduction shaft (211); reducing (s305) the iron ore by introducing a reducing gas (217) into the direct reduction shaft in countercurrent flow to the iron ore, wherein the reducing gas comprises reducing make-up gas (215); and concomitantly carburizing (s307a) the iron ore and/or subsequently carburizing (s307b) the reduced ore (208) by introducing a carburizing gas (219). The reducing make-up gas comprises greater than 80 vol% hydrogen gas. The carburizing gas comprises carburizing make-up gas (214) and a hydrogen-rich gas selected from reducing make-up gas, recycled process gas and combinations thereof. The carburizing make-up gas comprises greater than 80 vol% carbon dioxide.The disclosure further relates to a system for the production of carburized sponge iron, as well as a carburized sponge iron produced by the aforementioned process.

Description

TECHNICAL FIELD The present disclosure relates to a process for the production of carburized sponge iron. The disclosure further relates to a system for the production of carburized sponge iron and a carburized sponge iron produced by the aforementioned process.

BACKGROUND ART Steel is the world's most important engineering and construction material. lt is difficult to find any object in the modern world that does not contain steel, or depend on steel for its manufacture and/or transport. ln this manner, steel is intricately involved in almost every aspect of our modern lives. ln 2018, the total global production of crude steel was 1 810 million tonnes, by far exceeding any other metal, and is expected to reach 2 800 million tonnes in 2050 ofwhich 50% is expected to originate from virgin iron sources. Steel is also the world's most recycled material with a very high recycling grade due to the metals' ability to be used over and over again after remelting, using electricity as the primary energy source.

Thus, steel is a cornerstone of modern society with an even more significant role to play in the future. Steel is mainly produced via three routes: i) Integrated production using virgin iron ores in a blast furnace (BF), where iron oxide in the ore is reduced by carbon to produce iron. The iron is further processed in the steel plant by oxygen blowing in a basic oxygen furnace (BOF), followed by refining to produce steel. This process is commonly also referred to as 'oxygen steelmaking'. 2 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 commonly also referred to as 'electric steelmaking'. iii) Direct reduction production based on virgin iron ore, which is reduced in a direct reduction (DR) process with a carbonaceous reducing gas to produce sponge iron. The sponge iron is subsequently melted together with scrap in an EAF to produce steel.

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

Although the above-named processes have been refined over decades and are approaching the theoretical minimum energy consumption, there is one fundamental issue not yet resolved. Reduction of iron ore using carbonaceous reductants results in the production of C02 as a by-product. For every ton steel produced in 2018, an average of 1.83 tonnes of C02 were produced. The steel industry is one ofthe highest CO2-emitting industries, accounting for approximately 7% of C02 emissions globally. Excessive C02-generation cannot be avoided within the steel production process as long as carbonaceous reductants are used.

The HYBRIT initiative has been founded to address this issue. HYBRIT, short for HYdrogen BReakthrough lronmaking Technology -is a joint venture between SSAB, LKAB and Vattenfall, funded in part by the Swedish Energy Agency, and aims to reduce C02 emissions and de- carbonize the steel industry.

Central to the HYBRIT concept is a direct reduction based production of sponge iron from virgin ore. However, instead of using carbonaceous reductant gases, such as natural gas, as in present commercial direct reduction processes, HYBRIT proposes using hydrogen gas as the reductant, termed hydrogen direct reduction (H-DR). The hydrogen gas may be produced by electrolysis of water using mainly fossil-free and/or renewable primary energy sources, as is the case for e.g. Swedish electricity production. Thus, the critical step of reducing the iron ore may be achieved without requiring fossil fuel as an input, and with water as a by-product instead of C02. The resulting crude iron will naturally also be lacking in carbon.Iron produced by present-day commercial blast furnace or direct reduction routes typically comprises significant amounts of carbon (typically up to 5 wt%), due to carbon incorporation during reduction of the iron ore. Besides its use as a reducing agent, carbon plays further important ro|es in the steel-making process. Its presence in the crude iron from the BF or DR process lowers the melting point of the iron. During subsequent processing of the crude iron in an EAF or BOF, the exothermic dissociation of iron carbide and oxidation of carbon to CO supplies heat to the process. The gas evolution in the EAF due to this CO production provides a foamy slag that assists in thermally insulating the iron melt and helps diminish consumption of the EAF electrodes. For at least these reasons, the presence of carbon in the crude iron may assist in reducing energy consumption during processing to steel. The presence of carbon in the melt may also influence slag-metal reaction kinetics, and assist in purging dissolved gaseous elements from the metal. Moreover, the presence of carbon in direct reduced iron passivizes the sponge iron and enables simpler handling and transport. Finally, since the steel industry has a heritage and established practice with respect to carbon-containing crude iron, there may simply be a degree of reluctance among some steelmakers to adopt the use of carbon-lean crude iron, regardless of any benefits.

For at least these reasons, it may be desirable to provide a crude iron produced using substantially fossil-free means, but still containing carbon to an extent that it may be used as a drop-in replacement for present-day crude iron.

Document US 2015/0259760 A1 describes a method for producing steel in which iron ore is reduced with hydrogen and the resulting intermediate product of reduced iron ore and possibly accompanying substances is subjected to further metallurgical processing. ln reducing the iron ore to produce the intermediate product, a carbon-containing or hydrogen-containing gas is added to the hydrogen in order to incorporate carbon into the intermediate product. Examples ofthe carbon-containing or hydrogen-containing gas include CH4, coke oven gas (COG), synthesis gas, natural gas, biogas, gas from pyrolysis, and renewable resources.

There remains a need for a means of producing a carbon-containing crude iron in a more environmentally friendly manner.

SUMMARY OF THE INVENTION The inventors of the present invention have identified a number of shortcomings with prior art means of providing a carbon-containing crude iron.

Present-day commercial processes require extensive use of carbonaceous fossil fuels such as coa| or natural gas, leading to excessive net C02 emissions. Even in proposed processes to address such issues, substantial amounts of carbon-containing gas are required as input in order to provide a sufficiently carburized iron. The amount of biomass available to produce a fossil-free carburizing agent is limited, and there is an increasing demand for biomass in many sectors, such as for transport fuels, building materials, packaging, etc. Thus it will be difficult to source biomass in sufficiently quantities and at reasonable expense in order to produce a fossil-free carburized iron. lt would be advantageous to achieve a means of overcoming, or at least alleviating, at least some of the above mentioned shortcomings. ln particular, it would be desirable to provide an economic and practicable means of producing a crude iron containing carbon having lower net emissions of C02. To better address one or more of these concerns, a process for the production of carburized sponge iron having the features defined in the appended independent claims is provided. The process comprises the steps of: - charging iron ore into a direct reduction shaft; -reducing the iron ore by introducing a reducing gas into the direct reduction shaft in countercurrent flow to the iron ore, wherein the reducing gas comprises reducing make-up gas; and - concomitantly carburizing the iron ore by introducing a carburizing gas to the iron ore, and/or subsequently carburizing the reduced ore by introducing a carburizing gas to the reduced ore.

The seal gas consists essentially of carbon dioxide. The reducing make-up gas comprises greater than 80 vol% hydrogen gas. The carburizing gas comprises carburizing make-up gas and a hydrogen-rich gas selected from reducing make-up gas, recycled process gas and combinations thereof. The carburizing make-up gas comprises greater than 80 vo|% carbon dioxide.

The present invention utilizes a carburizing gas wherein the carbon source is predominantly carbon dioxide (C02). High concentration carbon dioxide is abundantly available as a by- product from numerous industrial processes. lt is relatively easy to store and transport since it liquefies at relatively low pressures and does not require extreme temperatures extreme temperatures for liquefaction, as required e.g. for LNG. lt is also expected to be much cheaper to buy compared to the corresponding carbon amount of alternative sources, since it generally is an unwanted and unutilized by-product. The use of fossil fuel-derived carbon dioxide in the present invention would typically result in no excess C02 emission overall since the CO2 would otherwise typically be emitted directly by the industrial process in question, without further utilization. However, carbon dioxide derived from renewable sources is also increasingly available, for example as a by-product of bioethanol production for transport fuel. Carburization of iron using such a renewable carbon dioxide source may result in net negative CO2 emissions.

The carbon dioxide introduced as a component part ofthe carburizing gas to the iron ore/reduced ore will ultimately result in carburization of the sponge iron, meaning that the need for further carburizing gasses may be decreased or avoided completely. Without wishing to be bound by theory, the CO2 introduced as a component part ofthe carburizing gas into the direct reduction shaft will form a component part of the process gases, together with hydrogen gas (H2). The CO2 and H2 may react together in situ by reverse water-gas shift reaction to form CO and H20, and/or by Sabatier reaction to form methane (CH4). The CO and/or CH4 in turn may react to provide carburization of the sponge iron produced in the direct reduction process. ln this manner, CO2 introduced into the process is converted into CO and/or CH4. The produced CO/CH4 is taken up by carburization of the sponge iron. The overall result is the production of a carburized sponge iron with a net consumption of CO2 and H The hydrogen ofthe make-up gas may be produced by electrolysis of water. Provided that the electricity used is derived from a renewable source, the result is that carburized sponge iron may be produced with essentially no consumption of fossil fuel. 6 The reducing make-up gas may comprise greater than 80 vo|% hydrogen gas, such as greater than 85 vo|% hydrogen gas, such as greater than 90 vo|% hydrogen gas, such as greater than 95 vo|% hydrogen gas. The reducing make-up gas may consist essentially of hydrogen.

The carburizing make-up gas may consist essentially of carbon dioxide. Thus, a carburized sponge iron may be obtained using carbon dioxide as the sole carbon source.

The carburizing step may be performed in the direct reduction shaft. The step of reducing the iron ore and the carburizing step may be performed concomitantly in the direct reduction shaft, in a combined reduction and carburization zone. Alternatively, the step of reducing the iron ore may be performed in a discrete reduction zone ofthe direct reduction shaft, and the carburizing step may be performed (partially or fully) subsequent to the reducing step in a discrete carburization zone of the direct reduction shaft.

Alternatively, or in addition, the carburizing step may be (partially or fully) performed subsequent to the reducing step in a discrete carburization unit, separate from the direct reduction shaft. The carburization unit may be a carburization shaft. ln such a case, the carburizing step may be performed by carburizing the reduced ore in the carburization shaft wherein the carburizing gas is arranged to flow countercurrent to the reduced ore. lt should be noted that the process disclosed herein is an essentially continuous process, not a batch process, and therefore when it is stated that carburization may be performed subsequently to reduction, it is meant that carburization is performed downstream and essentially separately of reduction, although reduction and carburization steps are ongoing simultaneously in the process on different materials.

The recycled process gas may not be passed through an external reformer. That is to say that under the conditions prevailing for hydrogen direct-reduction, the Sabatier and/or water-gas shift reaction will proceed in the process gas without the need of an external reformer, and use of an external reformer is therefore not required. For example, top gas upon exiting the upper end of the direct reduction shaft may be dried, dedusted, and recycled to the process as a component part ofthe carburizing gas, with essentially no further modification of its composition prior to reintroduction as carburizing gas. 7 The carbon dioxide used in the process may in part or fully be obtained as a by-product of biofuel production. Thus, the process may be performed using only carbon from renewable SOUFCQS.

Alternatively, the carbon dioxide may be produced by oxy-fuel combustion of biomass with oxygen. This means that both heating ofthe process gases and the carbon dioxide used in the process may be fully or partially derived from renewables. The oxygen used in oxy-fuel combustion may be produced by electrolysis of water, meaning that both the hydrogen and oxygen produced in electrolysis are utilized and potentially decreasing the overall cost of oxy- fuel combustion.

According to another aspect of the invention, the objects of the invention are achieved by a system for the production of carburized sponge iron according to the appended independent claims. The production process may be a process as defined in the appended independent claims, The system for the production of carburized sponge iron comprises: - a source of hydrogen gas; - a direct reduction shaft; - optionally a carburization unit; and - a source of carbon dioxide.

The system is arranged to provide hydrogen from the source of hydrogen gas to the direct reduction shaft. Moreover, the system is arranged to provide carbon dioxide from the source of carbon dioxide to the direct reduction shaft and/or carburization unit.

The source of hydrogen gas may be an electrolyser arranged to produce hydrogen from the electrolysis of water. The electrolyser may be arranged in fluid communication with the direct reduction shaft, such that hydrogen produced by the electrolyser may be conveyed to the direct reduction shaft. This may preferably be an indirect fluid communication via treatment of the electrolysis gases and/or via a hydrogen storage facility. 8 The direct reduction shaft may comprise a combined reduction and carburization zone. ln such a case, the system does not necessarily comprise a carburization unit.

The direct reduction shaft may comprise a discrete reduction zone and a discrete carburization zone. ln such a case, the system does not necessarily comprise a carburization unit.

The system may comprise a carburization unit. ln such a case, the system is arranged to provide hydrogen from the e|ectro|yser and/or recyc|ed process gas from the direct reduction shaft to the carburization unit. The carburization unit may be a carburization shaft.

Since the C02 is at least partially consumed in carburization, the system may not require a reformer for reforming of C02. Nor may the system require a device for C02 capture. For example, the system may comprise a conduit for recycling dried, dedusted top gas from the direct reduction shaft and conveying it to be mixed directly with the carburizing make-up gas. The system therefore is considerably simpler than systems requiring reforming or removal of CO According to a further aspect of the invention, the object of the invention is achieved by a carburized sponge iron according to the appended claims. The carburized sponge iron may be produced by a process according to the appended independent claims. The carburized sponge iron has a degree of reduction greater than 90%, such as greater than 94%, and comprises from 0.1 to 5 percent carbon by weight, such as from 0.5 to 3 percent carbon by weight such as about 1 to 2 percent carbon by weight. The carburized sponge iron has a radiocarbon age of less than 1000 years before present, even more preferably less than 90 years before present. This means that the carburized sponge iron must have been carburized using a carbon dioxide containing a significant renewable carbon content, which as described herein is commercially feasible using the process described herein. The carburized sponge iron may be in the form of pellets (i.e. DRI) or briquettes (i.e. HBI).

Further objects, advantages and novel features ofthe present invention will become apparent to one skilled in the art from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS For a fuller understanding ofthe present invention and further objects and advantages of it, the detailed description set out below should be read together with the accompanying drawings, in which the same reference notations denote similar items in the various diagrams, and in which: Pig.Pig. za Pig. zb Fig. 2c Pig.Pig. 4a Pig. 4b schematically illustrates an ore-based steelmaking value chain according to the Hybrit concept; schematically illustrates an exemplifying embodiment of a system suitable for performing a process as disclosed herein; schematically illustrates another exemplifying embodiment of a system suitable for performing a process as disclosed herein; schematically illustrates a further exemplifying embodiment of a system suitable for performing a process as disclosed herein; is a flow chart schematically illustrating an exemplifying embodiment of a process as disclosed herein; schematically illustrates a calculated equilibrium established from an input gas mixture; schematically illustrates an enlargement of a proportion ofthe calculated equilibrium of Figure 4a.

DETAILED DESCRIPTION The present invention is based upon a number of insights by the inventors. The inventors have recognised that in a direct reduction process that utilizes hydrogen as the main component of the reducing gas, carbon dioxide will be converted to carbon monoxide and/or methane by reverse water-gas shift reaction and Sabatier reaction respectively. These gases will be removed from the process gas by carburization ofthe sponge iron. This stands in contrast to typical (fossil) carbon-based reducing systems where C02 is accumulated in the process gas and must be removed and/or reformed.

The term process gas is used herein to denote the gas mixture in the direct reduction process, regardless of stage in the process. That is to say that process gas refers to gas that is introduced to, passes through, leaves, and is recycled back to the direct reduction shaft. More specific terms are used to denote the process gas at various points in the process, or to denote component gases added to the process gas to form part of the process gas. Reducing gas is a gas introduced at a point lower than the in|et of the shaft, and which flows upwards counter to the moving bed of ore in order to reduce the ore. Top gas is partially spent process gas that is removed from an upper end ofthe direct reduction (DR) shaft, in proximity to the ore in|et. After treatment, the top gas may be recycled back to the DR shaft as a component of the reducing gas. Reducing make-up gas is fresh gas added to the reducing gas in order to maintain reducing ability. Thus, the reducing gas comprises reducing make-up gas together with recycled top gas. The reducing make-up gas and recycled top gas may be mixed together prior to introduction into the direct reduction shaft, or may be introduced separately and mixed in the shaft. Seal gas is gas entering the direct reduction shaft from the ore charging arrangement at the in|et of the direct reduction shaft. The outlet end ofthe direct reduction shaft is also sealed using a seal gas, and seal gas therefore may enter the DR shaft from the discharging arrangement at the outlet of the direct reduction shaft. Carburizing gas is a gas introduced to result in production of carburized sponge iron. Reduction and carburization may be performed concomitantly, for example by introducing a combination of reducing and carburizing gases together into the DR shaft, or the carburization step may be performed subsequently to the reduction step. Carburization performed subsequently to reduction may be performed in a discrete carburization zone ofthe direct reduction shaft, or in a discrete carburization unit, such as a carburization shaft. After treatment, carbon-containing process gases may be recycled back to the carburization zone or unit as a component ofthe carburizing gas. Such gases may for example be recycled top gas from the DR shaft, or recycled partially spent off-gas from the carburization zone or unit. Carburizing make-up gas is fresh gas added to the carburizing gas in order to maintain carburizing ability, e.g. by maintaining a certain carbon concentration. The carburizing gas comprises make-up gas together with a hydrogen-rich gas, such as reducing make-up gas, or recycled process gas (e.g. recycled top 11 gas or recycled partially spent carburizing off-gas). The make-up gas and hydrogen-rich gas may be mixed together prior to introduction into the carburization zone or unit, or may be introduced separately and mixed in the reactor.

Carburization using COZ Adding carbon dioxide to a hydrogen-rich process gas will, as stated above, result in ultimate removal of the carbon from the process gas by carburization of sponge iron. Without wishing to be bound by theory, it is thought that the carbon dioxide will initially be converted in situ to a gaseous component capable of providing carburization of sponge iron by reaction with the hydrogen. The Sabatier reaction and reverse water-gas shift reaction are known mechanisms for the conversion of C02 and H2 to components suitable for carburizing sponge iron. The Sabatier reaction is as follows: CO2+4H2\=*CH4+2 HZO The reverse water-gas shift reaction is as follows: COZ + H2 ä CO + HZO ln a prior art syngas-based process, significant quantities of both hydrogen and carbonaceous gases (CH4, CO, COZ) are already present. This means introduction of small additional quantities of carbon dioxide would not significantly shift any equilibrium. However, in a hydrogen-based direct reduction process, any COZ introduced to the process gas will be converted more-or-less quantitatively to CO and/or CH4 at the elevated temperatures prevailing in the direct reduction shaft. A number of further factors also dictate that conversion of COZ to CO and/or CH4 will be favoured. Iron-based catalysts such as iron oxides (e.g. magnetite) are known to catalyse the Sabatier and water-gas shift reactions, meaning that the reaction will be catalysed by the iron-containing reactants and/or products of direct reduction, and no external reformer will be necessary. ln this regard, it should be noted that the iron ore pellets and sponge iron product are porous (i.e. high catalytic surface area), and that the catalyst bed is continually replenished by throughput ofthe ore through the shaft (thus potentially avoiding problems due to catalyst poisoning). Water is continually removed during recycling ofthe process gases ofthe direct reduction process, thus enhancing production of CO and/or CH4 by Le Chatelier's principle. Finally, as described below, the carbon monoxide and/or methane produced will be continuously removed from the process gas by carburization reactions, further enhancing the conversion of CO2 to CO.

The reverse WGS reaction is endothermic, whereas the Sabatier reaction is exothermic, meaning that, again by Le Chatelier's principle, the equilibrium is shifted towards CO by higher temperatures, and towards CH4 by lower temperatures. This is illustrated in Figures 4a and 4b below.

Once formed, the carbon monoxide and/or methane may partake in a range of further reactions, some ofwhich are shown below.

Methanation CO + 3H2 \=* CH4 + H2CO +2H2 i CH4 + CReduction 3Fe203 (hematite) + CO \=\ 2Fe304 + CO2Fe304 (magnetite) + 2CO \=* 6FeO + 2CO6FeO (wusme) + sco t: ere + scoz Carburization (graphite production) 2CO \=“ CO2+ C CO+H2\=*C+H2O CH4âC+2HCa rburization (cementite production) 3Fe + CH4 \=* Fe3C +2H3Fe +2CO \=* Fe3C + CO3Fe + CO + H2 \=* Fe3C + H2O -205.9 kJ/mol -247.1 kJ/mol -24.9 kJ/mol +45.3 kJ/mol -75.8 kJ/mol -173.7 kJ/mol -131.8 kJ/mol +74.5 kJ/mol +98.3 kJ/mol -148.8 kJ/mol -107.6 kJ/mol Hydrogen also provides reduction of the iron ore by the following reactions:6Fe203 + 2H2 à 4Fe304 + 2H2O +32.7 kJ/mol 2Fe304 + 2H2 à 6FeO + 2H2O +127.6 kJ/mol 6FeO + 6H2 ê 6Fe + 6H2O +171.4 kJ/mol 2Fe304 + 8H2 9 6Fe + 8H2O +299.0 kJ/mol Hydrogen predominates in the process gas and will therefore be the primary reductant. lf reduction and carburization steps are performed separately, i.e. in discrete zones or reactors, the ore will be more-or-less fully reduced prior to contact with any carbon monoxide formed, and it is unlikely that the carbon monoxide will act as reductant to any significant extent. However, where reduction and carburization are performed concomitantly in a combined reduction and carburization zone, the formed carbon monoxide may act as reductant. ln such a case, this however will merely regenerates carbon dioxide that can be re-converted to CO/CH4 by reaction with H2, and therefore the ”primary” reductant can in all cases be considered to be hydrogen. Carbon is only removed from the chemical system by carburization reactions, since a carburized sponge iron product is removed at the outlet of the direct reduction shaft. Thus, the overall chemical system can be conceptualised as hydrogen being consumed in converting ore and carbon dioxide to water and carburized sponge iron, both ofwhich are removed from the system. This stoichiometric boundaries can be illustrated as follows. At a theoretical lower boundary, where no carbon whatsoever is incorporated in the sponge iron, the stoichiometry can be represented as: 3Fe203+9H2ê 6Fe+9H2O At a theoretical complete conversion of the iron ore to cementite (Fe3C, i.e. 6.67 wt% C in Fe), the stoichiometry can be represented as: 3 Fe203 +2CO2 + 13 H2 9 2Fe3C + 13 HDirect Reduction The direct reduction shaft may be of any kind commonly known in the art. By shaft, it is meant a solid-gas countercurrent moving bed reactor, whereby a burden of iron ore is charged at an 14 inlet at the top of the reactor and descends by gravity towards an outlet arranged at the bottom ofthe reactor.

Reduction is typically performed at temperatures of from about 900 °C to about 1100 °C. The temperatures required are typically maintained by pre-heating of the process gases introduced into the reactor, for example using a preheater such as an electric preheater. Further heating ofthe gases may be obtained after leaving the pre-heater and prior to introduction into the reactor by exothermic pa rtial oxidation of the gases with oxygen or air. Reduction may be performed at a pressure of from about 1 Bar to about 10 Bar in the DR shaft, preferably from about 3 Bar to about 8 Bar.

The iron ore burden typically consists predominantly of iron ore pellets, although some lump iron ore may also be introduced. The iron ore pellets typically comprise mostly hematite, together with further additives or impurities such as gangue, fluxes and binders. However, the pellets may comprise some other metals and other ores such as magnetite. Iron ore pellets specified for direct reduction processes are commercially available, and such pellets may be used in the present process. Alternatively, the pellets may be specially adapted for a carbon- lean reduction step, as in the present process. ln contrast to present-day commercial direct reduction processes, the make-up gas used to replenish the reducing gas (reducing make-up gas) comprises little or essentially no carbonaceous substances. lnstead, the main carburizing effect in the present process is achieved by the carbon dioxide introduced as carburizing gas, which is converted to carbon monoxide/methane and subsequently carburizes the sponge iron. The reducing make-up gas may for example comprise, consist essentially of, or consist of, hydrogen. For example, the reducing make-up gas may comprise, consist essentially of, or consist of at least 80 vol%, preferably greater than 90 vol%, even more preferably greater than 95 vol% hydrogen gas (vol% determined at normal conditions of 1 atm and 0 °C).

The top (spent) gas from the DR shaft is at least partially recycled, whereby it may be cleaned and treated to remove by-products such as water and/or dust prior to re-introduction to the process, either in the DR shaft or in the carburizing unit. This recycled top gas may be mixed with fresh reducing and/or carburizing make-up gas prior to reintroduction, or may be introduced separately from any fresh make-up gas supply. The reducing gas may consist essentially of reducing make-up gas and recycled top gas. Carburization Carburization of the sponge iron may be performed concomitantly with the reduction step, e.g. by using a mixed reducing and carburizing gas, or it may be performed as a discrete subsequent step. lf performed as a discrete subsequent step, it may be performed in a discrete zone ofthe direct reduction shaft, or it may be performed in a separate reactor, i.e. a carburization unit. By discrete zone ofthe carburization shaft it is meant that reduction is performed in a reduction zone of the DR shaft having a reducing gas loop, and carburisation is performed in a carburization zone of the DR shaft having a carburizing gas loop. Direct reduction shafts having discrete reduction and carburization zones are known in the art. I\/lixing of gases between the separate zones is preferably avoided, although some degree of mixing is inevitable without physical barriers between the zones. The reduction and carburization steps may also be performed partially sequentially, for example by pre-reducing the iron ore to a partial degree (e.g. 60-90% DoR, such as 70-80% DoR) in a reduction zone of the DR shaft, followed by concomitant final reduction and carburization in a carburization zone of the DR shaft or a separate carburization unit. lf performed in a separate carburization unit, the carburization unit is preferably a carburization shaft, i.e. a solid-gas countercurrent moving bed reactor wherein sponge iron is charged at an inlet at the top of the reactor and descends by gravity towards an outlet arranged at the bottom ofthe reactor.

A potential advantage of performing carburization completely or partially separately from reduction is that it may assist or favour the conversion of C02 to carburizing components such as CO and/or CH4. Without wishing to be bound by theory, when considering the equilibriums in the reverse water-gas shift and Sabatier reactions, the conversion of CO2 to CO and/or CH4 is favoured by high H2 levels and low H20 levels, by Le Chatelier's principle. ln a process gas where reduction and carburization is ongoing concomitantly, a significant proportion of hydrogen will be converted to H2O in reducing the iron ore, thus counteracting the equilibrium formation of CO and/or CH4. ln contrast, in a discrete carburization, little or no H2O is formed, since the ore is pre-reduced, and this favours the equilibrium formation of COand/or CH4. I\/|oreover, in a discrete carburization, the carbon monoxide formed does not come into any significant contact with iron ore and this therefore avoids the CO being consumed in reducing the ore. A further potential advantage of performing the carburization separately is that conditions in the carburization zone or unit (e.g. temperature, pressure, flow rate) may be tai|ored to optimize carburization, without requiring undue consideration ofthe conditions necessary to achieve reduction. ln order to achieve the carburization of sponge iron using C02, it is necessary that the C02 is converted in-situ into carburizing components such as CO and/or CH4. The reverse water-gas shift and Sabatier reactions providing such components from CO2 require the presence of hydrogen. Therefore, the carburizing gas comprises, or consists essentially of, carburizing make-up gas and a hydrogen-rich gas. The carburizing make-up gas comprises greater than 80 vol% carbon dioxide, and may consist essentially, or consist of, carbon dioxide. The hydrogen- rich gas may be any process stream comprising a significant proportion of hydrogen and preferably little H20. The hydrogen-rich gas may for example be reducing make-up gas (i.e. fresh hydrogen), recycled top gas (i.e. spent process gas from the reducing zone), recycled off- gas from the carburization zone or unit, or a combination of such gases. The carburization make-up gas should be introduced in quantities sufficient to achieve the desired level of carburization of the sponge iron, and the hydrogen-rich stream should be introduced in quantities sufficient to achieve suitable equilibrium concentrations of the carburizing components CO and/or CH4. lf reduction and carburization are performed concomitantly, the hydrogen-rich gas may be the reducing gas, and there may be no need for introduction of any further hydrogen-rich gas. The carburization make-up gas may be mixed with the hydrogen- rich gas prior to introduction into the carburization zone or unit, or the gases may be introduced separately.

Carburization is suitably performed at temperatures sufficiently high to ensure that the various reactions, such as the reverse water-gas shift, Sabatier and carburization reactions proceed adequately. Lower temperatures favour CH4 formation and higher temperatures favour CO formation. Carburization may for example be performed at temperatures of from about 500 °C to about 900 ° C, such as from about 600 °C to about 800 °C. 17 lf a separate carburization zone or unit is utilized, off- gas from the zone or unit is at least partially recycled back to the carburization zone or unit as a component part of the carburizing gas. lt may be cleaned and treated to remove by-products such as water and/or dust prior to re-introduction to the process.

Conversion of the C02 to CO and/or CH4 proceeds in-situ within the carburization zone or unit, and there therefore is typically no need for an external reformer, i.e. an external unit for performing the reverse water-gas shift and/or Sabatier reactions as detailed herein. However, under some conditions it may be favourable to provide an external reformer in order to provide greater freedom in tailoring the conditions prevailing in the carburization zone or unit. Thus, it is envisaged that in some embodiments the process may entail passing carburization off-gas through an external reformer.

Sponge iron The term crude iron is used herein to denote all irons produced for further processing to steel, regardless ofwhether they are obtained from a blast furnace (i.e. pig iron), or a direct reduction shaft (i.e. sponge iron). The sponge iron obtained at the outlet of the DR shaft is typically predominantly in the form of pellets, due to the structural integrity of the direct reduction pellets, as well as the conditions prevailing in the DR shaft. Such sponge iron is typically referred to as direct reduced iron (DRI). Depending on the process parameters, it may be provided as hot (HDRI) or cold (CDRI). Cold DRI may also be known as Type (B) DRI. DRI may be prone to re-oxidation and in some cases is pyrophoric. However, there are a number of known means of passivating the DRI. One such passivating means commonly used to facilitate overseas transport ofthe product is to press the hot DRI into briquettes. Such briquettes are commonly termed hot briquetted iron (HBI), and may also be known as type (A) DRI.

The sponge iron product obtained by the process herein may be an essentially fully metallized sponge iron, i.e. a sponge iron having a degree of reduction (DoR) greater than about 90%, such as greater than about 94% or greater than about 96%. Degree of reduction is defined as the amount of oxygen removed from the iron oxide, expressed as a percentage of the initial amount of oxygen present in the iron oxide. lt is often not commercially favourable to obtain sponge irons having a DoR greater than about 96% due to reaction kinetics, although such sponge irons may be produced if desired. 18 By carburized sponge iron it is meant carbon-containing sponge iron. The carbon present in the sponge iron product may typically be in the form of cementite (Fe3C) and/or graphite. Graphite tends to dust and to be lost from the sponge iron prior to reaching the melt of the EAF. For this reason, it may be preferable if carbon is present in the sponge iron as cementite.

The carburized sponge iron may comprise from 0.1 to 5 percent carbon by weight, such as from 0.5 to 3 percent carbon by weight such as about 1 to 2 percent carbon by weight. lt is typically desirable for further processing that the sponge iron has a carbon content of from 0.5 to 5 percent carbon by weight, preferably from 1 to 4 percent by weight, such as about 3 percent by weight, although this may depend on the ratio of sponge iron to scrap used in a subsequent EAF processing step. lf desired, the carburized sponge iron product of the present process may subsequently be further carburized by other means prior to further processing. Gases The hydrogen gas may preferably be obtained at least in part by electrolysis of water. lf the water electrolysis is performed using renewable energy then this allows the provision of a reducing gas from renewable sources. The electrolytic hydrogen may be conveyed by a conduit directly from the electrolyser to the DR shaft, or the hydrogen may be stored upon production and conveyed to the DR shaft as required.

The present invention requires a source of carbon dioxide. lt is preferable that the source of carbon dioxide is essentially pure carbon dioxide, e.g. 95 vol% carbon dioxide or greater, preferably 98 vol% or greater. The source of carbon dioxide may preferably be from a high- concentration source, preferably a high-concentration biogenic source. For example, concentrated ”green” C02 may be obtained as a by-product of bio-gas production by anaerobic digestion, or as a by-product of bioethanol production. |fthe carbon dioxide used in the process is from a renewable source then the process may be net negative with regard to C02 emissions. However, even use of a source of carbon dioxide from a fossil source that otherwise would have been directly emitted means that the process may not result in any excess emission of C02. An alternative means of providing carbon dioxide is to preheat the reducing gas prior to introduction into the direct reduction shaft using oxy-fuel combustion of biomass. The principle of oxy-fuel combustion is simple: the biomass is combusted using essentially pure oxygen as the oxidant. The resulting flue stream consists essentially of carbon 19 dioxide and steam. The steam may be removed by simple condensation, providing an essentially pure source of carbon dioxide. Conventionally, the provision of essentially pure oxygen is an economic impediment to the utilization of oxy-fuel combustion. However, in the present case there may be a ready supply of oxygen available at low additional cost from water electrolysis, making oxy-fuel preheating of the reduction gas economically feasible. lt is preferred that the carbon dioxide used is derived from a renewable source, and in such case, the carbon in the sponge iron product will also derive from a renewable source. lt can be determined whether the carbon in the sponge iron derives from a renewable source or a fossil source by radiocarbon dating of the sponge iron. Methods for sample preparation and radiocarbon dating of iron products are known in the art. For example, an appropriate method is disclosed in Cook, A., Wadsworth, J., & Southon, J. (2001). AMS Radiocarbon Dating of Ancient Iron Artifacts: A New Carbon Extraction Method in Use at LLN L. Radiocarbon, 43(2A), 221-227, the methods ofwhich are incorporated by reference herein.

Carbon derived from fossil resources typically has a radiocarbon age of in excess of 35 000 years, whereas carbon derived from renewable sources 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 may range from about 35 000 years (if the carbon is exclusively fossil-derived) to ”modern” (if the carbon is exclusively renewable- derived). A list of radiocarbon dated iron objects 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 process described herein, due to its excellent utilization of carbon, is capable of being performed in a commercially viable manner using carbon dioxide and optionally carburizing gas derived predominantly or essentially from renewable sources. Thus the resulting sponge iron product may have a radiocarbon age of less than 1 000 years before present, such as less than 90 years before present. Embodiments The invention will now be described in more detail with reference to certain exemplifying embodiments and the drawings. However, the invention is not limited to the exemplifying embodiments discussed herein and/or shown in the drawings, but may be varied within the scope ofthe appended claims. Furthermore, the drawings shall not be considered drawn to scale as some features may be exaggerated in order to more clearly illustrate certain features.

Figure 1 schematically illustrates a prior art embodiment of the ore-based steelmaking value chain according to the Hybrit concept. The ore-based steelmaking value chain starts at the iron ore mine 101. After mining, iron ore 103 is concentrated and processed in a pelletizing 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 gas 115 as the main reductant and producing water 117 as the main by- product. The hydrogen gas 115 is produced by electrolysis of water 117 in an electrolyser 119 using electricity 121 that is preferably primarily derived from fossil-free or renewable sources 122. The hydrogen gas 115 may be stored in a hydrogen storage 120 prior to introduction into the direct reduction shaft 111. The sponge iron 109 is melted using an electric arc furnace 123, optionally together with a proportion of scrap iron 125 or other iron source, to provide a melt 127. The melt 127 is subjected to further downstream secondary metallurgical processes 129, and steel 131 is produced. lt is intended that the entire value-chain, from ore to steel may be fossil-free and produce only low or zero carbon emissions.

Figure 2a schematically illustrates an exemplifying embodiment of a system suitable for performing the process as disclosed herein.

A direct reduction shaft 211 is arranged with an inlet 211a for iron ore 207, an outlet 211b for discharging sponge iron 208, an inlet for reducing gas 211c and an outlet for top gas 211d. ln use, iron ore 207 is introduced into inlet 211a and progressively passes through the reactor to be discharged at outlet 211b. During its passage through the reactor 211 the ore 207 is reduced by reducing gas 215 in a counter-current flow, such that the ore 207 is reduced to sponge iron 208 at the discharge outlet 211b of the reactor 211. Since the reducing gas in this exemplifying embodiment comprises no carbonaceous components, the sponge ironobtained at the outlet 211b of the direct reduction shaft will be essentially carbon-free.

Reducing make-up gas 215 is supplied from a source of reducing make-up gas 220, such as a hydrogen gas store or water electrolyser. The make-up gas 215 is mixed with recycled top gas 218 to form reducing gas 217. The reducing gas 217 is passed through a pre-heater 241 prior to introduction into the direct reduction shaft 211. The top gas 216 exiting outlet 211d ispassed through a plurality of treatment apparatuses 243 in order to prepare the gas for re- introduction to the DR shaft 211. The plurality of treatment apparatuses may include a cleaning step, such as passage through an electrostatic precipitator to remove solids from the gas, heat exchange with other process gases such as the reducing gas 217, and separation of water. The recycled top gas 218 is mixed with the make-up gas 215 and passed through the pre-heater 241 prior to reintroduction into the direct reduction shaft 211 through in|et 211c. The temperature ofthe gases entering in|et 211c may be further increased by partia| oxidation. ln such a case, a supply of oxygen (not shown) will be arranged between the pre- heater 241 and in|et 211c.

A carburization unit, herein i||ustrated as a carburization shaft 213 is arranged with an in|et 213a for sponge iron 208, an out|et 213b for discharging carburized sponge iron 209, an in|et for carburizing gas 213c and an out|et for off-gas 213d. ln use, the sponge iron 208 from direct reduction shaft 211 is introduced into carburization shaft 213 via carburization shaft in|et 213a. During its passage through the reactor 213 the sponge iron 208 is carburized by carburizing gas 219 in a counter-current flow, such that carburized sponge iron 209 is obtained at the discharge out|et 213b of the reactor Carburizing make-up gas 214 is supplied from a source of carburizing make-up gas 245, such as a C02 storage facility or oxyfuel burner flue. The carburizing make-up gas 214 is mixed with reducing make-up gas 215 and recycled off-gas 250 to form carburizing gas 219. The carburizing gas 219 is passed through a pre-heater 247 prior to introduction into the carburization shaft 213. The off-gas 248 exiting out|et 213d is passed through a plurality of treatment apparatuses 249 in order to prepare the gas for re-introduction to the carburization shaft 213. The plurality of treatment apparatuses may include a cleaning step, such as passage through an electrostatic precipitator to remove solids from the gas, heat exchange with other process gases such as the carburizing gas 219 or reducing gas 217, and separation of by- products such as water. The treatment apparatuses may further include an apparatus arranged to convert any C02 from the off-gas to CO and/or CH4. Such an apparatus may be for example a reformer utilizing the reverse water-gas shift reaction to convert C02 and H2 to CO and H20, or utilizing co-electrolysis of C02 and H20 as a feed to provide C0 and H2. The treated off-gas 250, together with any reformed C02 from the off-gas, is mixed with the carburizing make-up gas 214 and passed through the pre-heater 247 prior to reintroduction 22 into the carburization shaft 213 through inlet 213c. The temperature ofthe gases entering inlet 213c may be further increased by partial oxidation. ln such a case, a supply of oxygen (not shown) will be arranged between the pre-heater 247 and inlet 213c.

The carbon dioxide 214 introduced to the carburization shaft as carburizing make-up gas will circulate in the process gases where, without wishing to be bound by theory, it will first be converted by reaction with hydrogen to carbon monoxide and then be taken up as carbon (e.g. graphite or cementite) in the sponge iron. Thus, the carbon dioxide 214 will be passively removed from circulation and will not accumulate in the process. lnstead, carbon will be removed from the process in the form of carburized sponge iron Figure 2b schematically i||ustrates an exemplifying embodiment similar to that of Figure 2a. However, in this embodiment, reduction is performed in a discrete reduction zone 211e of the direct reduction shaft 211, and carburization is performed in a discrete carburization zone 211f. Although separate gas loops are provided for the reduction and carburization zones, there are no physical barriers to prevent process gases passing from one zone to the other, and therefore the reducing and carburizing process gases will inevitably be mixed to some degree.

Figure 2c schematically i||ustrates an exemplifying embodiment similar to that of Figure 2b. However, in this embodiment, there are no separate gas loops for reduction and carburization, only a single gas loop into which reducing make-up gas 215 and carburizing make-up gas 214 is introduced. The steps of reduction and carburization therefore take place concomitantly in an integrated reduction and carburization zone ofthe direct reduction shaft 211. The carburizing gas 219 in this case may comprise a higher proportion of hydrogen in order to account for the hydrogen consumed in reducing the iron ore to sponge iron.

Figure 3 is a flow chart schematically illustrating an exemplifying embodiment of the process disclosed herein. Step s301 denotes the start of the process. ln step s303 iron ore is charged into a direct reduction shaft. ln step s305 the iron ore is reduced by introducing a reducing gas into the direct reduction shaft in countercurrent flow to the iron ore. ln step s307a the iron ore is concomitant to step s305 carburized by introducing a carburizing gas. Alternatively, or in addition, in step s307b the reduced ore resulting from step s305 is subsequently carburized by introducing a carburizing gas. Step s 309 denotes the end of the process. The reducing gas 23 comprises reducing make-up gas, and the reducing make-up gas comprises greater than 80 vo|% hydrogen gas. The carburizing gas comprises carburizing make-up gas and a hydrogen- rich gas selected from reducing make-up gas, recycled process gas and combinations thereof.

The carburizing make-up gas comprises greater than 80 vo|% carbon dioxide.

Figures 4a and 4b illustrate an equilibrium composition formed using an initial gas composition of: H2: 91 %; C02: 4%; and H20: 5% (by volume) at a pressure of4 Bar. The equilibrium is calculated using HSC Equilibrium software from Outotec. Figure 4a illustrates all components, whereas Figure 4b focusses on the carbonaceous components of the mixture. lt can be seen that at relatively low temperatures (< approx. 600 ° C) methane dominates, at intermediate temperature (approx. 600 - 900 °C) carbon (graphite formation) dominates, whereas at temperatures in excess of approx. 900 °C carbon monoxide dominates. Carbon dioxide is not favoured at any ofthe equilibrium conditions investigated, although at low temperature it can be assumed that it will be kinetically favoured since it is a component ofthe initial gas mixture. lt should be noted that the prevailing temperature at the reducing gas inlet of a direct reduction shaft is typically in excess of 950 °C, and therefore carbon monoxide will be favoured by equilibrium. Once formed, the carbon monoxide will participate in carburization and be removed from the process in the manner previously described.

Claims (17)

1. Claims A process for the production of carburized sponge iron (209) from iron ore (207), the process comprising the steps: charging (s303) iron ore into a direct reduction shaft (211); reducing (s305) the iron ore by introducing a reducing gas (217) into the direct reduction shaft in countercurrent flow to the iron ore, wherein the reducing gas comprises reducing make-up gas (215); and concomitantly carburizing (s307a) the iron ore and/or subsequently carburizing (s307b) the reduced ore (208) by introducing a carburizing gas (219) to the iron ore and/or reduced ore; characterized in that the reducing make-up gas comprises greater than 80 vo|% hydrogen gas; the carburizing gas comprises carburizing make-up gas (214) and a hydrogen-rich gas selected from reducing make-up gas, recyc|ed process gas and combinations thereof; and the carburizing make-up gas comprises greater than 80 vo|% carbon dioxide. A process according to claim 1, wherein the carburizing make-up gas (214) consists essentially of carbon dioxide. A process according to any one ofthe preceding claims, wherein the reducing make-up gas (215) consists essentially of hydrogen, and wherein the hydrogen is preferably produced by electrolysis of water. A process according to any one of the preceding claims, wherein the carburizing step (s307a/s307b) is performed in the direct reduction shaft. A process according to claim 4, wherein the step of reducing the iron ore and the carburizing step are performed concomitantly in the direct reduction shaft, in a combined reduction and carburization zone. A process according to claim 4, wherein the step of reducing the iron ore is performed in a discrete reduction zone (211e) of the direct reduction shaft, and the carburizing step is performed subsequent to the reducing step in a discrete carburization zone (211f) of the direct reduction shaft. A process according to any one of claims 1-3, wherein the carburizing step is performed subsequent to the reducing step in a discrete carburization unit (213), separate from the direct reduction shaft. A process according to claim 7, wherein the carburizing step is performed by carburizing the reduced ore in a carburization shaft (213) wherein the carburizing gas is arranged to flow countercurrent to the reduced ore. A process according to any one of the preceding claims, wherein the recycled process gas is not passed through an external reformer. A process according to any one of the preceding claims, wherein the carbon dioxide is obtained as a by-product of biofuel production. A system for the production of carburized sponge iron, the system comprising: a source of hydrogen gas (220); a direct reduction shaft (211); optiona||y a carburization unit (213); and a source of carbon dioxide (245); wherein the system is arranged to provide hydrogen from the source of hydrogen gas to the direct reduction shaft; and wherein the system is arranged to provide carbon dioxide from the source of carbon dioxide to the direct reduction shaft and/or carburization unit. A system according to claim 11, wherein the direct reduction shaft comprises a combined reduction and carburization zone, and wherein the system does not comprise a carburization unit. A system according to claim 11, wherein the direct reduction shaft comprises a discrete reduction zone (211e) and a discrete carburization zone (211f), and wherein the system does not comprise a carburization unit (213). A system according to claim 11, wherein the system comprises a carburization unit (213), and wherein the system is arranged to provide hydrogen (215) from the e|ectro|yser and/or recycled process gas (218) from the direct reduction shaft to the carburization unit. A system according to claim 14, wherein the carburization unit is a carburization shaft. A system according to any one of claims 11-15, wherein the system does not comprise a reformer. A system according to any one of claims 11-16, wherein the system does not comprise a device for C02 capture.