iron briquettes

Abstract

The present disclosure relates to an iron briquette made by providing sponge iron pellets, providing carbon powder, producing a mixture of the sponge iron pellets and the carbon powder, and briquetting the mixture to obtain an iron briquette comprising compressed sponge iron pellets and the carbon powder located in interstitial spaces between the compressed sponge iron pellets, wherein the iron briquette comprises at least 0.2% by weight of the carbon powder and the sponge iron pellets comprise at least 0.5% by weight of iron oxide and are essentially free of carbon. The present disclosure further relates to a method for producing such iron briquettes.

Description

[Technical Field]

[0001] Technical field The present invention relates to iron briquettes and a method for manufacturing such iron briquettes. [Background technology]

[0002] Background technology Steel is the world's most important industrial and construction material. In the modern world, it is difficult to find an object that does not contain steel, or an object that does not depend on steel for its manufacture and / or transportation. Thus, steel is intricately involved in almost every aspect of our modern lives.

[0003] In 2018, the total global production of crude steel was 1,810 million metric tons, far more than any other metal, and is expected to reach 2,800 million metric tons by 2050, with 50% of that expected to come from virgin iron sources. Steel is also the most recycled material in the world, and has a very high recycling grade due to the metal's ability to be reused many times after being remelted using electricity as the primary energy source.

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

[0005] Steel is produced primarily through three routes: i) A standardized production method using virgin iron ore in a blast furnace (BF), where iron oxide in the ore is reduced by carbon to produce iron. The iron is further processed in a steel plant by oxygen blowing in a basic smelting furnace (BOF), followed by smelting to produce steel. This process is also commonly known as "oxygen steelmaking."

[0006] ii) Manufacturing based on scrap using recycled steel, melted in an electric arc furnace (EAF) that uses electricity as the primary energy source. This process is also commonly known as "electric steelmaking."

[0007] iii) Direct reduction production based on virgin iron ore, in which sponge iron is produced by reduction using a direct reduction (DR) process with carbonaceous reducing gas. In a shaft-based process, sponge iron pellets (DRI) are produced, while in another process, sponge iron powder may be produced. Both DRI and sponge iron powder can be compressed to produce hot briquette iron (HBI), a form of sponge iron more suitable for transport. Subsequently, the sponge iron is melted together with scrap in an EAF to produce steel.

[0008] In this specification, the term crude iron is used to refer to any iron produced for further processing into steel, regardless of whether it is obtained from a blast furnace (i.e., pig iron) or a direct reduction shaft (i.e., DRI, HBI, or sponge iron powder).

[0009] The process described above has been refined over several decades, approaching theoretical minimum energy consumption, but one fundamental problem remains unresolved. The reduction of iron ore using carbonaceous reducing agents generates CO2 as a byproduct. In 2018, an average of 1.83 metric tons of CO2 were generated per ton of steel produced. The steel industry is one of the largest emitters of CO2, accounting for approximately 7% of global CO2 emissions. As long as carbonaceous reducing agents are used, it is impossible to avoid excessive CO2 generation within the steel manufacturing process.

[0010] The HYBRIT initiative has been established to address this issue. HYBRIT stands for Hydrogen Breakthrough Ironmaking Technology, and is a joint venture between SSAB, LKAB, and Vattenfall, partially funded by the Swedish Energy Agency, with the goal of reducing CO2 emissions and decarbonizing the steel industry.

[0011] The core of the HYBRIT concept is the production of sponge iron from virgin ore based on a direct reduction shaft. However, instead of using carbonaceous reducing agent gases such as natural gas, as in current commercial direct reduction processes, HYBRIT proposes the use of hydrogen gas as a reducing agent, called hydrogen direct reduction (H-DR). Hydrogen gas can be produced by electrolyzing water using primarily fossil fuel-free and / or renewable primary energy sources, as in the case of electricity production in Sweden. Thus, a crucial step in the reduction of iron ore can be achieved without requiring fossil fuels as input materials, and with water as a by-product instead of CO2.

[0012] However, sponge iron produced by direct hydrogen reduction can sometimes have several drawbacks in the downstream steelmaking steps.

[0013] Sponge iron produced using conventional fossil-based carbonaceous reducing agents typically contains a significant amount of dispersed carbon (typically up to 5% by weight) due to carbon contamination from carbonaceous reducing gases during the reduction of iron ore. This dispersed carbon is primarily in the form of cementite (Fe3C), with a smaller proportion consisting of graphite dispersed throughout the sponge iron. The eutectic (melting) temperature of the iron-cementite system is 1147°C (lower than the melting point of pure iron, 1536°C), and the cementite exothermically separates in the molten metal, accelerating the melting of the sponge iron. Sponge iron produced by direct hydrogen reduction naturally lacks cementite and is therefore more difficult to melt in the EAF. This can lead to increased power requirements in the EAF melting step, excessive wear of the EAF electrodes, and increased molten metal intervals.

[0014] To overcome these drawbacks, a method has been proposed for processing sponge iron produced by direct hydrogen reduction using carburizing gas, in order to obtain sponge iron that is more similar to sponge iron produced by conventional fossil methods.

[0015] International Publication No. 2019 / 238720 discloses a method for producing carburized direct-reduced sponge iron from an iron oxide material. First, direct reduction is carried out with a reducing gas consisting at least primarily of H2. Subsequently, a carburizing gas is supplied to increase the carbon content in the sponge iron, and then the carburizing gas used in this process is removed at least partially, mainly to avoid mixing with the reducing gas. The carburizing gas may be, for example, natural gas, methane, ethane, propane, butane, carbon monoxide, or a mixture of several of these gases.

[0016] U.S. Patent Application Publication No. 2015 / 0259760 A1 describes a method for producing steel, in which iron ore is reduced with hydrogen, and the resulting reduced iron ore and, if any, associated intermediate products are subjected to further metallurgical treatment. In the reduction of the iron ore to produce the intermediate products, a carbon-containing gas or a hydrogen-containing gas is added to the hydrogen to introduce carbon into the intermediate products. Examples of carbon-containing gases or hydrogen-containing gases include CH4, coke oven gas (COG), synthesis gas, natural gas, biogas, gases obtained by pyrolysis, and renewable resources.

[0017] There is still a need for more environmentally friendly methods of manufacturing steel. [Overview of the project] [Problems that the invention aims to solve]

[0018] Summary of the Invention Sponge iron pellets (DRI) produced by direct hydrogen reduction are more difficult to melt in a melting furnace than conventional DRI. This difficulty in melting can lead to increased power requirements in the EAF melting step, excessive wear of EAF electrodes, and increased inter-pipe times. If the local rate of DRI input exceeds the furnace's melting capacity, unmelted piles of DRI ("ferrobergs") may accumulate, requiring longer periods of time and greater power consumption to melt and disperse.

[0019] To overcome such disadvantages, means for treating sponge iron produced by direct reduction with hydrogen using carburizing gas to obtain sponge iron similar to that produced by conventional fossil means have been proposed. The inventors of the present invention have identified a number of drawbacks associated with such proposed prior art means for improving the efficiency of downstream processing of sponge iron produced by direct reduction with hydrogen.

[0020] In the proposed prior art means, the sponge iron is converted into a relatively more fusible form using carburizing gas either during or after reduction. Whether the carburizing gas is a fossil gas or not, extensive additional gas treatment and equipment are required, so the proposed process is costly and not very efficient reduction is carried out when carburized during reduction (because the reduction rate of carbon monoxide as a reducing agent is slow compared to hydrogen). Such a process also results in relatively inefficient use of the carbon contained in the carburizing gas, because in order to maintain a stable gas composition, a certain proportion of the carburizing gas is typically purged and burned. If the carburizing gas is fossil-derived, this also results in net CO2 emissions. Furthermore, performing carburization as part of the direct reduction process means that the carbon content cannot be freely optimized because it depends on the settings of the reduction process.

Means for Solving the Problems

[0021] It would be advantageous if means were realized to overcome or at least mitigate at least some of the above disadvantages. In particular, it is desirable to obtain a product derived from sponge iron by direct reduction with hydrogen that is efficiently produced, can be easily applied to efficient downstream processing, and is particularly easily fusible. To adequately address one or more of these problems, a product derived from sponge iron by direct reduction with hydrogen having the features defined in the appended independent claims is provided.

[0022] The product derived from sponge iron by direct hydrogen reduction is an iron briquette. This iron briquette comprises compressed sponge iron pellets and carbon powder located in the intervening space between the compressed sponge iron pellets. This iron briquette contains at least 0.2% by weight of carbon powder. The sponge iron pellets used in the production of the iron briquette contain at least 0.5% by weight of iron oxide and are essentially carbon-free.

[0023] Iron briquettes can be produced by providing sponge iron pellets, providing carbon powder, producing a mixture containing sponge iron pellets and carbon powder, and briquetting the mixture.

[0024] Surprisingly, the inventors have found that the downstream processing advantages typically obtained by having cementite dispersed in sponge iron can also be substantially obtained by briquetting carbon-free sponge iron with carbon powder. That is, the iron briquettes melt in much the same way as the corresponding briquettes produced using conventional fossil-based sponge iron, and efficient reduction of residual iron oxide in the sponge iron occurs. This is done despite the fact that the carbon is not dissolved in the iron and is substantially limited to the intervening spaces between the compressed pellets. However, in order to obtain these advantages, it is important that the iron briquettes contain sufficient amounts of iron oxide and carbon. This is because mechanically added solid carbon does not lower the melting temperature of the iron in the same way as carbon dissolved in the iron as cementite. Instead, although not wishing to be constrained by theory, the inventors have identified a mechanism by which the molten oxide flows out of the compressed sponge iron due to the initial melting of dispersed residual iron oxide (FeO, with a melting point of 1377°C), as the phase expands during melting. This liquid oxide encounters the solid oxide in the intervening space of the briquette, thereby reducing the iron and yielding a carbon-saturated liquid iron phase. This liquid iron phase then dissolves the surrounding solid iron. Thus, the inventors have found that even iron briquettes containing only non-uniformly dispersed solid carbon can have molten properties suitable for further processing in any melting process, such as in an electric arc furnace (EAF) or induction furnace (IF).

[0025] Briquetting carbon-free sponge iron pellets with carbon powder using the method disclosed herein offers several further advantages. It is relatively low-cost in terms of equipment, materials, and operating costs. Less carbon is required, which means reduced emissions. Furthermore, the lower carbon requirements facilitate the use of carbon from renewable resources. The iron oxide content in the molten material can be controlled independently of the reduction stage, and therefore the reduction and post-reduction steelmaking stages can be optimized independently. For example, there is greater freedom in the degree of metallization obtained in the reduction stage. This may allow for the use of less metallization, for example, when electricity costs (and therefore the cost of electrolyzed hydrogen as a reducing gas) are high. The gas produced by the reaction between iron oxide and carbon can also improve the dynamic conditions and energy efficiency in the EAF. Under typical conditions in a melting furnace, the rapid gas generation that occurs when concentrated molten iron oxide encounters concentrated carbon may facilitate the "explosion" of the iron briquette, which may lead to rapid collapse of the briquette and further improve melting.

[0026] Sponge iron pellets used in the manufacture of iron briquettes contain at least 0.5 wt% iron oxide. These may contain at least 1 wt% iron oxide, for example, at least 1.5 wt% iron oxide, for example, at least 3 wt% iron oxide, or at least 4 wt% iron oxide. A higher amount of iron oxide in the sponge iron is associated with improved melting properties, but a corresponding increase in the amount of carbon powder in the iron briquettes may be required to effectively reduce residual iron oxide.

[0027] Iron briquettes can be manufactured by hot briquetting of a mixture. Hot briquetting is a conventional method of processing sponge iron to obtain a stabilized product that is less reactive to air than conventional DRI and therefore easier to transport and handle. The product of hot briquetting is called hot briquette iron, HBI, and is also known as type (A) DRI. By manufacturing iron briquettes using hot briquetting, iron briquettes can be produced using equipment already widely available in the art, requiring only relatively minor modifications. This further reduces the cost of industrial implementation.

[0028] Iron briquettes may contain approximately 95% to 99.5% compressed iron pellets and approximately 0.5% to 5% carbon powder. This should ensure sufficient carbon to optimize performance in the EAF.

[0029] To obtain the appropriate FeO concentration in EAF slag, the carbon content in the iron briquettes can be matched to the degree of metallization of the sponge iron pellets. This helps optimize yield and reduce impurities in the steel.

[0030] Iron briquettes weigh approximately 4000 kg / m³. 3 For example, exceeding approximately 5000 kg / m 3 It can have an effective density exceeding [a certain value]. Such densities are typical in the case of HBI, which ensures that the sponge iron has adequate compression to become properly passive.

[0031] Iron briquettes can have a minimum dimension exceeding approximately 20 mm, preferably exceeding approximately 30 mm. Iron briquettes can have a maximum dimension of less than approximately 150 mm. Such dimensions are typical for HBI, which helps ensure that iron briquettes can be used interchangeably with conventional HBI.

[0032] Sponge iron pellets can have a median diameter exceeding approximately 7 mm, preferably exceeding approximately 10 mm, before briquetting. That is, the sponge iron is DRI with standard dimensions and is not a fine powder. Sponge iron pellets may be essentially spherical. The large diameter of the sponge iron pellets to be briquetized means that the carbon powder added to the briquette will inevitably be unevenly dispersed within the pellet.

[0033] Sponge iron pellets weigh approximately 1500 kg / m³ before briquetting. 3 ~About 2000kg / m 3 It can have a bulk density of over 85%, preferably over 90%, and is typical for DRI, meaning the final briquette is significantly compressed compared to the initial pellets. Sponge iron pellets can have a metallization of over 85%, preferably over 90%, and for example, over 95%. A higher degree of metallization means less carbon is needed for the reduction of residual iron oxide in the iron briquette, and therefore, in some cases, carbon consumption and overall emissions during the steelmaking process are reduced. Sponge iron pellets can contain over 85% by weight of total iron, preferably over 90% by weight of total iron. Such a total iron content is the same as that of conventional DRI, and this allows for the production of a final product of good quality.

[0034] The carbon powder may contain more than about 80% by weight of carbon, preferably more than about 90% by weight of carbon. This helps ensure that sufficient volatile matter is removed from the carbon powder to prevent the deterioration of iron briquettes at high temperatures, which is typically common during briquetting. The carbon powder may consist of, for example, crushed anthracite, coke, graphite, or biocoal, which has a composition substantially corresponding to anthracite, coke, graphite, or any combination thereof. For example, the carbon powder may consist of, for example, biocoal derived from biomass, such as lignocellulosic biomass, such as high-temperature pyrolysis of forest residues.

[0035] When carbon powder is derived from renewable resources, it can have a radiocarbon age dating to less than 10,000 years ago, preferably less than 1,000 years ago, and even more preferably less than 100 years ago. Since relatively small amounts of carbon are required in iron briquettes, the use of relatively rare renewable carbon sources is facilitated. Using renewable carbon can further reduce the environmental impact of the steelmaking process.

[0036] Iron briquettes may further contain additive flux. For example, iron briquettes may further contain preferably about 0.1% to about 4% by weight of additive flux. The flux can be added in appropriate amounts to obtain the optimal slag composition in the EAF. Thus, by adding both carbon for reducing iron oxide and flux to control other slag oxides, the slag composition can be controlled independently of the direct reduction parameters, which means that both the reduction step and the subsequent steelmaking step can be optimized independently. This is not possible in conventional processes where flux is added mainly during ore pelletization and carbon is introduced into sponge iron during the reduction step.

[0037] According to a further aspect of the present invention, the object of the present invention is achieved by a method for producing iron briquettes according to the appended independent claims.

[0038] The above method comprises the steps of: providing sponge iron pellets containing at least 0.5% by weight of iron oxide and essentially free of carbon; providing carbon powder; producing a mixture containing sponge iron pellets and carbon powder, wherein the mixture contains at least 0.2% by weight of carbon powder; and briquetting the mixture.

[0039] By the method described above, the iron briquettes described herein are obtained, and thus their corresponding advantages are obtained.

[0040] The step of briquetting the mixture can be carried out at a temperature above 500°C, for example above 600°C, and preferably above 650°C. Such temperatures are conventional in the production of HBI.

[0041] Sponge iron pellets and carbon powder can be added to the briquetting apparatus separately and then mixed within the apparatus. This can help avoid the problem of the mixture separating due to the significant size difference between the carbon powder and the sponge iron pellets. Alternatively, the mixture of sponge iron pellets and carbon powder can be prepared before being introduced into the briquetting apparatus.

[0042] Further objectives, advantages, and novel features of the present invention will become apparent to those skilled in the art from the following detailed description.

[0043] Brief explanation of the drawing For a better understanding of the present invention and its further objectives and advantages, the detailed description below should be read in conjunction with the accompanying drawings, where the same reference numerals in various figures indicate similar items. [Brief explanation of the drawing]

[0044] [Figure 1] This diagram schematically illustrates one exemplary embodiment of an ore-based steelmaking value chain under the Hybrid concept. [Figure 2a] This diagram schematically shows the equipment used to manufacture HBI. [Figure 2b] This specification schematically shows an iron briquette manufactured by the method disclosed herein. [Figure 3] The images show the progress of melting three briquettes made from sponge iron pellets at various time points (a) to (e). [Figure 4] This image shows a microscopic photograph of an unreduced KPRS pellet. [Figure 5] This image shows a micrograph of a reduced (90% DoR) KPRS pellet. [Figure 6](a) Micrographs of hydrogen-reduced KPRS pellets before heating and (b) after heating at 1500°C for 240 seconds are shown. [Figure 7] Microscopic images of hydrogen-reduced KPRS pellets with a 99% reduction degree after heating at 1600°C for various times (a) to (f) are shown. [Figure 8] This shows the relative carbon mass loss of iron briquette samples B2-B5 with respect to time and temperature. [Figure 9] This shows the microstructure of an iron briquette after heating at 1500°C for 300 seconds. [Modes for carrying out the invention]

[0045] Detailed explanation The present invention will now be described in more detail with reference to some exemplary embodiments and drawings. However, the present invention is not limited to the exemplary embodiments discussed herein and / or shown in the drawings, and may be modified within the scope of the appended claims. Furthermore, some features may be exaggerated in order to more clearly illustrate certain features, so the drawings should not be considered to be drawn to scale.

[0046] This invention is based on the inventors' discovery that briquetting carbon-free sponge iron pellets (H-DRI) with carbon powder surprisingly yields briquettes that exhibit far superior melting properties than H-DRI briquettes briquetted without carbon, and that exhibit melting properties equivalent to those of briquettes produced using conventional fossil-based DRI. This is unexpected because iron briquettes do not have uniformly dispersed carbon like conventional DRI, but instead consist of large fragments of carbon-free compressed sponge iron pellets and carbon powder substantially located in the gaps between such compressed pellets.

[0047] While not intending to be constrained by theory, the inventors have discovered that the favorable melting of iron briquettes is due to a previously undisclosed mechanism in which iron oxide initially melts in the briquette, flows through the pores in the compressed pellets, and accumulates in the gaps between the pellets. In these gaps, carbon dissolves into the liquid oxide, simultaneously reducing the oxide to liquid iron. As a result, a carbon-saturated liquid iron phase is obtained even at temperatures below the melting point of pure iron. This pool of liquid iron dissolves the surrounding solid iron. Compared to the initial porous structure of iron briquettes, the storage of liquid metal in the pores and gaps in this method increases the effective thermal conductivity of the briquette, thereby promoting and accelerating the melting process. For comparison, the thermal conductivity of air is 0.113 Wm². -1 K -1 On the other hand, liquid iron is approximately 40 Wm². -1 K -1 Therefore, a sufficient amount of residual iron(II) oxide (FeO) in the iron briquette is important for this mechanism to proceed.

[0048] In addition to the favorable melting obtained, further advantages are gained from the carbon monoxide produced when residual oxides are reduced or when carbon reacts with FeO in the slag. These include the formation of foamy slag due to gas generation, which helps in the separation of the molten material and the protection of the EAF electrodes. This, in turn, reduces energy consumption and less wear on the EAF electrodes. Furthermore, gas generation facilitates the purging of dissolved gaseous elements such as nitrogen from the metal bath.

[0049] This disclosure facilitates an ore-based steelmaking value chain that is more energy-efficient, requires less carbon, and generates fewer emissions. Figure 1, based on the Hybrid concept, schematically illustrates an exemplary embodiment of the ore-based steelmaking value chain into which this disclosure is incorporated. The ore-based steelmaking value chain begins at an iron ore mine 101. After mining, the iron ore 103 is collected and processed at a pelletizing plant 105 to produce iron ore pellets 107. These pellets are converted into sponge iron pellets 108 by being reduced in a direct reduction shaft 111, with hydrogen gas 115 as the primary reducing agent and water 117 as the primary by-product. Hydrogen gas 115 is mainly produced by electrolyzing water 117 in an electrolytic cell 119 using electricity 121 from non-fossil or renewable resources 122. The hydrogen gas 115 can be stored in a hydrogen storage facility 120 before being introduced into the direct reduction shaft 111. According to this disclosure, it is desirable that the sponge iron be readily meltable in the subsequent EAF treatment step. Therefore, sponge iron pellets 108 obtained from the direct reduction shaft 111 are supplied to a briquetting unit 113, preferably together with carbon powder 114 from renewable resources. In the briquetting unit 113, the sponge iron pellets 108 are briquetized together with the carbon powder 114 to obtain iron briquettes 109. The iron briquettes 109 are then melted using an electric arc furnace 123, optionally together with a certain proportion of scrap iron 125 or another iron source, to obtain a molten product 127. The electricity 121 used in the electric arc furnace 123 is preferably derived from renewable resources 122. Further downstream secondary metallurgical processes 129 are carried out on the molten product 127 to produce steel 131.

[0050] Iron briquettes are manufactured using a mixture that essentially consists of or comprises carbon-free sponge iron pellets, carbon powder, and optionally further additives such as additive flux. The mixture may contain about 95% to about 99.5% by weight of sponge iron pellets. The mixture may contain about 0.5% to about 5% by weight of carbon powder. Optionally, the mixture may contain about 0.1% to about 4% by weight of additive flux.

[0051] Carbon-free sponge iron pellets Sponge iron pellets used in iron briquettes are essentially carbon-free. Such pellets can be obtained as the product of a shaft-based direct reduction process using only a reduction gas that is essentially carbon-free. The reduction gas can essentially consist of, for example, hydrogen and, optionally, a gas that is inert in the process (e.g., nitrogen, argon). A pilot plant capable of producing such carbon-free sponge iron pellets using hydrogen as the reduction gas is currently operating in Lulea, Sweden.

[0052] Essentially carbon-free means that carbon is not intentionally introduced into the sponge iron pellets, for example, by using carburizing gas. However, small amounts of carbon may be present in the pellets due to residual carbon-containing components from unreduced pellets. For example, to prevent aggregation and adhesion in direct reduction shafts, iron ore pellets are typically coated with carbonate-containing inorganic materials (e.g., lime or cement), and carbon derived from such carbonates may remain in the sponge iron pellets. Sponge iron pellets can contain less than about 0.1% by weight of carbon, preferably less than about 0.05% by weight. For comparison, DRI produced by conventional fossil means typically contains about 1% to about 5% by weight of carbon.

[0053] Sponge iron pellets can have a metallisation of more than 85%, preferably more than 90%, for example more than 95%. However, in order to obtain the advantageous melting properties described herein, it is important that at least 0.5% by weight of residual iron oxide is maintained in the sponge iron.

[0054] In addition to the above considerations, the sponge iron pellets used in the production of iron briquettes may be very similar to conventional sponge iron pellets, also known as DRI or type (B) DRI. These can have a median diameter of more than about 7 mm, preferably more than about 10 mm, before briquetting. These can have a median diameter of less than about 25 mm, preferably less than about 20 mm, before briquetting. These can have a bulk density of from about 1500 kg / m 3 ~ about 2000 kg / m 3 and preferably from about 1750 kg / m 3 ~ about 1900 kg / m 3 They can contain more than about 85% by weight total iron, preferably more than about 90% by weight total iron.

[0055] Carbon powder Any suitable carbon powder can be used in the production of iron pellets. Suitable carbon powder means a powder having a suitable high carbon content, for example more than about 80% by weight carbon, preferably more than about 90% by weight carbon. It is preferred that the carbon powder does not release an excessive amount of volatiles at typical temperatures during briquetting, because this can interfere with the briquetting or cause a reduction in the integrity of the briquettes. Thus, when a hot briquetting process is used, the carbon powder is suitably a carbon powder having low volatiles and a constant high carbon content, such as, for example, pulverised anthracite, coke or graphite, or a biochar having a composition substantially corresponding to such carbon. For example, the carbon powder can be a biochar derived from the high temperature pyrolysis of biomass such as lignocellulosic biomass.

[0056] Because relatively small amounts of carbon are required in iron briquettes, the use of relatively rare renewable carbon sources is encouraged. Using renewable carbon can further reduce the environmental impact of the steelmaking process. Carbon powder derived from renewable resources, such as from the high-temperature pyrolysis of biomass, has a much younger radiocarbon age than carbon derived from fossil sources. For example, carbon derived from fossil resources typically has a radiocarbon age of over 35,000 years, while carbon derived from renewable resources is known to be "modern." Depending on the ratio of renewable carbon to fossil carbon in the carbon powder, the radiocarbon age of carbon powder can range from approximately 35,000 years (when the carbon powder is derived exclusively from fossils) to "modern" (when the carbon powder is derived exclusively from renewable sources). It is preferable that the carbon powder be derived at least partially, or entirely, from renewable resources. Therefore, carbon powders can have radiocarbon ages dating back less than 10,000 years from the present, preferably less than 1,000 years from the present, and even more preferably less than 100 years from the present. Highly reliable methods for radiocarbon dating of carbon powders such as biocoal and coal, using methods such as accelerator mass spectrometry (AMS), are well known in the art.

[0057] To integrate into the iron briquette and dissolve efficiently in the liquid iron, the carbon powder should be ground to a sufficiently fine consistency. However, it should not be ground to the point of causing dusting and handling problems. In addition to these general considerations, the particle size of the carbon powder has not been found to be important in experiments conducted to date. An average particle size of approximately 0.01 mm to 2 mm (D) is less than approximately 3 mm. 50 Powders containing MMD may be suitable.

[0058] Further additives Depending on the composition of the sponge iron pellets, it may be desirable to add further flux to the iron briquettes to produce self-fluxing briquettes. Flux refers to a substance added to the briquettes to facilitate the removal of impurities in the form of slag during melting. This can reduce or avoid the need to add slag-forming agents during melting in the EAF, which can help ensure an optimal slag composition. This, in turn, can help optimize the quality and yield of the steel products obtained from the briquettes. Furthermore, current direct reduction processes can typically use self-fluxing iron ore pellets in which flux has already been introduced into the pellets during the iron ore pelletization stage prior to direct reduction. The ability to introduce flux, referred to herein as “added flux,” at or in addition to the briquetting step, provides further flexibility in the design of optimal iron ore pellets and processes from the direct hydrogen reduction of iron ore.

[0059] Suitable fluxes are well known in the art and include, but are not limited to, lime, dololime, calcined lime, calcined dololime, silica, and combinations thereof.

[0060] Flux can be added to iron briquettes in any appropriate amount. For example, iron briquettes can contain approximately 0.1% to 4% by weight of added flux.

[0061] Briquetting Briquetting can be carried out using any suitable apparatus and comprises the steps of: providing sponge iron pellets as specified herein; providing carbon powder as specified herein; producing a mixture comprising sponge iron pellets and carbon powder, wherein the mixture comprises at least 0.2% by weight of carbon powder; and briquetting the mixture.

[0062] The mixture can be prepared before being introduced into the briquetting unit. Alternatively, the sponge iron pellets and carbon powder can be introduced into the briquetting unit separately and mixed within the unit. This helps to avoid excessive separation of the mixture before briquetting, thereby preventing large variations in briquette composition.

[0063] Briquetting can be carried out using hot briquetting to obtain hot briquette iron (HBI) briquettes, which is a well-established technique in the art. Apparatus suitable for the production of HBI is schematically shown in Figure 2 and includes a briquetting press 233 having two synchronously counter-rotating rollers 235, a screw feeder 237, and a material supply section 239. At any appropriate point before briquetting, a mixture of hot sponge iron pellets 208 directly from the reduction shaft and carbon (not shown) is formed. This mixture is fed between the rollers 235 and compressed in pockets formed by the rollers to produce a continuous string of briquettes 241. Downstream of the briquetting press is a briquette string separator 243, such as a roller with an impact bar, for separating the briquettes of the formed briquettes into individual briquettes 209.

[0064] Hot briquetting can be carried out at temperatures above 600°C, for example, between approximately 600°C and 800°C, or between approximately 650°C and 750°C, for example, at approximately 700°C.

[0065] Alternatively, briquetting can be carried out using any other suitable technique.

[0066] A schematic diagram of an enlarged cross-section of an iron briquette formed by this process is shown in Figure 2b. It can be seen that the briquette contains compressed sponge iron pellets 245 and has intervening spaces 247 between the compressed pellets. The carbon powder 214 is mainly present in these intervening spaces, although some small proportions of the powder may penetrate into the sponge iron before briquetting after mixing, due to the highly porous nature of sponge iron.

[0067] In other words, iron briquettes produced by the method of this disclosure may be similar to conventional HBI. These have a yield of approximately 4000 kg / m³. 3 For example, exceeding approximately 5000 kg / m 3 They can have an effective density exceeding [a certain value]. This allows the HBI to easily penetrate the slag and reach the molten material when introduced into the EAF. They can have a minimum diameter of approximately 20 mm or more, for example, approximately 30 mm or more. Typical HBIs are roughly the same size and resemble the shape of a standard bar of soap. [Examples]

[0068] experiment Preparation of sponge iron pellets To obtain pellets with different reduction levels, commercially available KPRS hematite pellets are subjected to a vertical tubular furnace in which pure hydrogen (2 L min) is added. -1 The reduction was performed at 900°C using the following formula:

number

[0069] Furthermore, the pellets were reduced at 900°C in a 20% CO-80% H2 atmosphere. The estimated carbon potential was 0.7 relative to graphite. Therefore, it is presumed that all carbon dissolves in the metallic phase. The pellets were then sent to SSAB Oxeloesund for LECO analysis to determine the carbon content. After subtracting the mass increased by carbon, the degree of reduction was calculated using equation (1).

[0070] Preparation of iron briquettes The reduced iron ore pellets were briquetized at room temperature using a steel die and a hydraulic press. The pressure during briquetting was 300 bar. Two different sized steel dies were used. Approximately 6.6 grams of reduced iron ore pellets were pressed using an 11 mm diameter steel die. This yielded 5.34 g / cm³. -3 Briquettes with dimensions of φ11 mm × height 13 mm, corresponding to the effective density, were obtained, while a 95-gram sample was used with a 30 mm diameter steel die. Carbon as graphite powder was added unevenly to the pellet, meaning that virtually no mixing was performed before briquetting.

[0071] Experimental Techniques Different experimental techniques were used within the scope of this study. These techniques were used to investigate (a) how different DRI samples (1. without carbon, 2. with dissolved carbon, and 3. with mechanically added carbon powder) melt; (b) the effect of carbon on the melting rate of DRI; and (c) the mechanism for understanding the behavior of carbon during melting and reduction of FeO. Both horizontal and vertical furnaces were used.

[0072] The main feature of a horizontal furnace is that the sample can be observed through a quartz glass window as it melts. Therefore, a horizontal furnace was used to observe the melting behavior in situ and to compare the melting rates of different samples.

[0073] To investigate the carbon-induced melting mechanism of DRI and the reduction of FeO, a vertical furnace capable of faster cooling was used. The experimental technique is briefly described below, and a more in-depth explanation can be found here: A. Vickerfalt, J. Martinsson and D. Sichen, “Effect of Reduction Degree on Characteristics of Slag Formed by Melting Hydrogen-Reduced DRI and Partitions of P and V between Slag and Metal”, Steel Research International, 2021, 92, pp. 1-11.

[0074] A vertical tubular furnace with an alumina reaction tube was used to melt the sample. The alumina reaction tube was connected to a water-cooled aluminum cooling chamber at its upper end and to a water-cooled aluminum cap at its lower end. All connections were sealed with O-rings. A steel rod was inserted from the top of the cooling chamber. Depending on the sample system size, the sample was connected to the steel rod at its lower end using either a 40 cm Mo wire or Mo rod. Next, the steel rod was connected to a lifting system at its upper end. This lifting system allowed for rapid vertical movement of the sample. A thermocouple was inserted into the aluminum cap at the lower end of the tube. The temperature within the uniform temperature zone was uniform over a length of 5 cm.

[0075] Each sample, either a pellet or a briquette, was placed in an MgO crucible. A small basket of Mo wire was woven to hold the crucible in experiments with one pellet and a small briquette, while a Mo holder was used to hold larger sample sizes. The masses of the pellet / briquette, crucible, and basket were kept the same in all experiments within the same system size range to ensure that the heat capacity of the sample remained unchanged.

[0076] The furnace was heated to either 1500°C or 1600°C. During the heating procedure, the sample was placed in a cooling chamber. Once the target temperature was reached, the sample was lowered to a preheating position of 1200°C or 1300°C (lower than the melting point of FeO).

[0077] The sample was held in a preheating position for 10 minutes, then rapidly cooled to a uniform temperature zone, where it was maintained for a predetermined time range of 60–1800 seconds. To stop any reactions and fix the microstructure, after the predetermined time, the sample was raised to the cooling chamber for a few seconds, while simultaneously increasing convection by blowing in a large amount of argon. The same procedure was used for experiments with small briquettes, except that preheating was omitted to eliminate undesirable reactions with carbon.

[0078] The total weight of the samples was measured before and after the experiment. Some iron samples were sent to SSAB Oxeloesund for carbon analysis using LECO. Phase composition was determined using electron dispersion spectroscopy (EDS) in a scanning electron microscope (SEM). Composition data was used to determine the presence of different phases. The actual composition should be analyzed using semi-quantitative methods, considering the limitations of EDS and the very small size of some phases (approximately 1 μm). For a larger system size of 95 g, XRF analysis to obtain the overall slag phase composition and analysis of the metallic phase by OES were possible.

[0079] Melting behavior of different briquettes To observe the in-situ melting behavior of reduced iron ore pellets, a horizontal furnace equipped with a quartz window was used. The sample moved horizontally along a graphite track and could be viewed through the quartz window. A video camera placed in front of the window recorded the melting in real time. Three samples of briquetteed sponge iron pellets were placed in a row on an alumina substrate. This substrate was placed on top of a graphite holder. After the furnace reached the target temperature of 1600°C, the sample was moved from the cooling chamber to the uniform temperature zone. Recording began when the sample came to a standstill within the uniform temperature zone.

[0080] Figure 3(a-e) shows the melting process of three briquettes made from sponge iron pellets. The three samples had the same DRI mass, i.e., 6.6 grams, and the same degree of reduction (99.5% metallization). The sample on the far right (Sample 1) consisted of a pellet with 0.9 wt% dissolved carbon (reduced with a CO-H2 gas mixture). The sample in the center (Sample 2) consisted of a carbon-free DRI pellet briquetteed with 0.06 grams of graphite powder (reduced in pure hydrogen). This corresponds to 0.9 wt% carbon. Therefore, the total carbon content of Sample 1 and Sample 2 was the same. The sample on the far left (Sample 3) contained no carbon and consisted only of a carbon-free DRI pellet.

[0081] Figure 3(a) shows the initial state of the briquette. This image was taken at the time of insertion into the uniform temperature zone of the horizontal furnace. By observing the sample in the center of Figure 3(a), the non-uniform distribution of carbon can be seen. During briquetting, graphite was pushed into the voids between the pellets. As a result, graphite pockets were formed within the briquette, which can be seen as black areas on the surface.

[0082] Figure 3(b) shows the samples 70 seconds after insertion into the uniform temperature zone. Sample 1 (far right) and Sample 3 (far left) show no effect. However, gas generation is evident, as can be seen from the bubble formation on the surface of Sample 2 (center). This indicates that reduction of iron oxide by mechanically added carbon occurs during the melting process of this briquette.

[0083] Figures 3(c) and 3(d) show the appearance of the samples 155 seconds and 156 seconds after insertion, respectively. Sample 3 remains unchanged, while samples 1 and 2 show morphological changes from their initial state. A liquid film of slag and molten iron has formed on the surface of the central sample (sample 2). This is confirmed by the movement of spherical slag droplets across the surface of the briquette. Furthermore, rapid vibration of sample 2 was observed due to the gas produced by the reaction shown in Equation 2. FeO(l) + C(s) → Fe(l) + CO(g) (2)

[0084] This oscillation is shown by comparing the relative positions of sample 2 in Figures 3(c) to 3(d). No oscillation was observed in sample 1, indicating that no reduction occurred in this sample. The mechanically added graphite showed higher reactivity than the dissolved carbon. This is understandable, as the carbon activity in sample 2 is 1, which is much higher than the carbon activity in sample 1. Although we do not wish to be constrained by theory, it is thought that the rapid generation of gas due to the reaction between molten iron oxide and mechanically added carbon may cause an "explosion" of iron briquettes under typical conditions in a melting furnace, which may further accelerate the rapid collapse and melting of the briquettes.

[0085] 195 seconds after insertion, sample 1 is completely melted, as shown in Figure 3(e). Substantially partial melting of sample 2 (center) is evident, while sample 3, which contains no carbon, shows no change from its initial state. Therefore, it is clear that these samples have not reached the melting temperature of pure iron after 195 seconds. Furthermore, sample 2 shows partial melting. This indicates that during melting, the mechanically added carbon dissolves into the iron, forming a liquid metal phase. The liquid metal has a significantly increased effective thermal conductivity, thereby accelerating the melting process. Thus, the melting time of a briquette consisting of carbon-free DRI and mechanically added carbon is equivalent to the melting time of a briquette consisting of DRI with dissolved carbon (i.e., conventional fossil-based DRI).

[0086] Changes in the microstructure of carbon-free sponge iron pellets To understand the melting mechanism of DRI pellets, it is important to investigate the microscale changes during melting. Therefore, the changes in the microstructure of KPRS pellets with various reduction degrees during melting were investigated in detail. For this purpose, 18 samples were examined. The experimental conditions for these samples are listed in Table 1 below.

[0087] [Table 1]

[0088] First, unreduced KRPS pellets were examined using SEM-EDS to observe the present phases. An example of a microscopic image is shown in Figure 4. Four phases were observed: (1) hematite phase, (2) calcium silicate phase, (3) a phase containing both MgO (approximately 10% by weight) and iron oxide, and (4) apatite phase.

[0089] Furthermore, two reduced pellets were examined: one with a 90% reduction degree and the other with a 99% reduction degree. Figure 5 shows an example of a micrograph of the 90% reduced sample. The main phases present in the reduced pellets are metallic iron, iron oxide (FeO) present as islands surrounded by an iron matrix, and the CaO-SiO2 phase. All of these phases are also found in the material reduced to 99%, although the iron oxide phase is far less abundant in the 90% metallized pellets.

[0090] Microstructure of a pellet with a DoR of 90% at 1500°C (Sample B1) A pellet with a 90% reduction degree was heated to 1500°C to investigate the formation of slag before the melting of the metal phase (Sample B1). The microstructure before and after heating is shown in Figures 6(a) and (b), respectively. Figure 6(a) shows the microstructure of the pellet with a 90% reduction degree. Figure 6(b) shows the microstructure of the pellet with the same reduction degree after being maintained at 1500°C for 240 seconds.

[0091] A comparison of Figures 6(a) and 6(b) reveals the following: (1) The microstructure becomes coarser, while the iron phase remains solid after being maintained at 1500°C for 240 seconds. (2) Slag has already formed in the pores of the solid Fe. As shown in Figure 6(b), the formed slag contains a matrix liquid phase and a wustite phase of two different phases. The liquid phase contains CaO, SiO2, FeO, Al2O3, MgO, TiO2, and P2O5 (ignoring all compounds <1 wt%). The wustite phase contains FeO and small amounts of MgO and V2O3. It is clear that FeO reacts with the CaO and SiO2-rich phase to form the slag phase.

[0092] FeO melts at 1377°C and has a viscosity of 0.3 poise at 1377°C. For this reason, the flow from FeO to the CaO and SiO2-rich phase is likely to initiate slag formation. The pores within the pellet allow for the flow of FeO. Regarding the coarsening of the microstructure shown in Figures 6(a) and (b), rearrangement of the grain structure is also included in this process.

[0093] Microstructure of pellets with 99% DoR at 1600℃ (Samples A1-A5) Pellets with a 99% reduction degree were maintained at 1600°C for different durations (60-600 s). After 60 s at 1600°C, the iron was not yet liquid. However, a slag phase had already formed and was distributed within the pores of the pellet (Figure 7(b)). Comparing Figure 7(a), which shows the reduced pellet before heating, with Figure 7(b), it can be seen that the microstructure is coarser in this case as well. This supports the observations made in Figures 6(a) and (b). The micrograph in Figure 7(c) has a higher magnification and shows the phase present after 60 s at 1600°C.

[0094] The dominant phase is the iron phase. The slag consists of a liquid phase and two precipitate phases, wustite and spinel. The spinel phase is not observed at a reduction degree of 90%, which is consistent with previous experience. This slag formation is consistent with findings shown in earlier reports that discuss in detail the phases of bulk lag obtained from self-formed slag by melting KPRS pellets. This indicates that, despite the large difference in FeO content, both pellets reduced to 90% and pellets reduced to 99% form FeO, CaO, and SiO2 (and more) slag very rapidly. In fact, slag formation is completed during melting.

[0095] After 90 seconds, the iron is liquid. As shown in Figure 7(d), several spherical slag droplets, up to 1 mm in size, are present in the molten iron. On a micrometer scale, most slag droplets are much smaller than this. A slag layer is formed on the surface of the iron. The same type of slag phase as after 60 seconds is present.

[0096] After 120 seconds, most of the slag phase has separated from the liquid metal phase. However, several slag particles, approximately 30 μm in size and containing multiple slag phases, still remain in the metal bath, see Figure 7(e).

[0097] After 240 seconds, the largest slag particles were approximately 10 μm in size. Most of the slag particles were homogeneous, close to pure FeO, with other oxides dissolved at a certain percentage value; see Figure 7(f).

[0098] Mechanism of reduction (decarburization) and melting of FeO by carbon Carbon was added as graphite powder to briquettes of carbon-free DRI. The weight of the samples was measured before and after the experiment to evaluate the degree of decarburization. Next, the relative carbon loss was calculated from the weight difference using a stochiometric mass balance based on the decarburization reaction shown in equation (2).

[0099] Figure 8 shows the relative carbon mass reduction for samples B2-B5 with respect to time and experimental temperature. One sample, B5, was sent for carbon analysis using LECO. This result is also shown in Figure 8, where the carbon concentration has been recalculated to reflect the relative carbon mass reduction. It can be seen that 70-85% of the added carbon mass reacted after 240-300 seconds at 1500°C, and no carbon remained after 180 seconds at 1600°C.

[0100] Figure 8 clearly shows that decarburization occurs in two distinct stages: (a) before the melting of the metallic phase, and (b) after the melting of the metallic phase. This supports the observations made in Figure 3(b). Decarburization occurs during the melting process, and in fact, the majority of decarburization takes place in this step. The predicted amount of residual carbon using mass balance calculations was found to be in agreement with the LECO analysis. This means that equation (2) represents the decarburization reaction, and the efficiency of the carbon added during briquetting is close to the stochastic (theoretical maximum).

[0101] To support the above findings for larger masses, briquettes consisting of approximately 95g of reduced pellets were examined. Seven experiments were conducted, and samples C1-C7 in Table 1 are referenced.

[0102] The carbon content in the metallic phase of the sample was determined by OES analysis (see Table 1). The amount of added carbon varied based on the degree of reduction and mass balance calculations targeting a slag FeO content of 10–50 percent. The melting time for larger system sizes was 900 seconds. The amount of added carbon had a slight effect on the final dissolved carbon concentration after complete decarburization of the steel following melting, ranging from 0.47 to 1.14 wt%.

[0103] Microstructure of carbon-free pellets and iron briquettes made of graphite at 1500°C (Sample B2) Furthermore, the microstructure of sample B2 (90% metallized, 0.084g of carbon added to 6.6g of briquette) containing added carbon, maintained at an experimental temperature of 1500°C for 300 s, was examined by SEM. Figure 9 shows that the microstructure of the briquette containing added carbon (sample B2) shows a liquid metal phase (white) near the voids (black).

[0104] A liquid metal phase (white) is formed around large voids (black). These voids are then connected to the surface of the sample. Unaffected areas show a structure similar to that in Figure 6(b), which supports the observations made in Figure 3(e). Carbon is dissolved in the iron forming the liquid metal phase at temperatures below the melting point of pure iron. Note that the dissolution of carbon into Fe is a necessary condition down to lower melting temperatures. These results clearly show that the reduction of FeO by carbon and the dissolution of carbon into the produced metal occur simultaneously. These also indicate that it is desirable to optimize the amount of carbon added by considering the process, such as the required melting temperature, the carbon content in the steel, and the FeO content in the slag.

[0105] conclusion As shown in Figure 6(b), during heating, the iron oxide remaining after reduction flows out into the porous structure of the pellet as the iron grains ingest gangue, melting the oxides and forming slag. As the slag flows through the pores, it encounters and readily reacts with solid carbon particles in the pockets between the pressed pellets. Decarburization occurs in two stages. Decarburization begins during the melting process (Fe is still solid) and generates gas, as is evident from the formation of bubbles on the surface (seen in Figure 3(b)) and the vibration of the briquette in Figures 3(c) and (d). The reaction between FeO and carbon produces a liquid metal phase in which carbon is dissolved during melting, see Figure 9. The initially formed liquid metal continues to dissolve the remaining carbon particles and simultaneously dissolves the solid Fe (which does not contain carbon). Thus, the melting process of the entire briquette is accelerated. The liquid metal phase shortens the melting time of the briquette to a degree comparable to that of pellets carburized with the CO-H2 gas mixture in Figure 3(e). Decarburization continues even after the complete melting of the metallic phase, see Figure 8. The reaction between carbon and iron oxide is shown to approach stochasticity, see Equation (2) and Figure 8. Briquetting of hydrogen-reduced iron ore pellets with carbon yields minimal carbon release while simultaneously providing efficient melting for crude steel production.

Claims

1. Iron briquettes (109, 209) are produced by providing sponge iron pellets (108, 208), providing carbon powder (114, 214), producing a mixture containing the sponge iron pellets and the carbon powder, and briquetting the mixture to obtain iron briquettes containing compressed sponge iron pellets (245) and carbon powder (214) located in the intervening space (247) between the compressed sponge iron pellets, wherein the iron briquettes contain at least 0.2% by weight of carbon powder, and the sponge iron pellets contain at least 0.5% by weight of iron oxide and are essentially carbon-free.

2. The iron briquette according to claim 1, which is produced by hot briquetting of the mixture.

3. An iron briquette according to claim 1 or 2, comprising approximately 95% to approximately 99.5% by weight of compressed sponge iron pellets and approximately 0.5% to approximately 5% by weight of carbon powder.

4. Approximately 4000kg / m ３ An iron briquette according to claim 1 or 2, having an effective density exceeding [a certain value].

5. An iron briquette according to claim 1 or 2, having a minimum dimension exceeding approximately 20 mm.

6. The iron briquette according to claim 1 or 2, wherein the sponge iron pellet has a median diameter of about 7 mm or more before briquetting.

7. The aforementioned sponge iron pellets weigh approximately 1500 kg / m³ before briquetting. ３ ~Approx. 2000kg / m ３ An iron briquette according to claim 1 or 2, having a bulk density.

8. The iron briquette according to claim 1 or 2, wherein the sponge iron pellet has more than 85% metallization.

9. The iron briquette according to claim 1 or 2, wherein the sponge iron pellets contain more than approximately 85% by weight of total iron.

10. The iron briquette according to claim 1 or 2, wherein the carbon powder contains more than about 80% by weight of carbon.

11. The iron briquette according to claim 1 or 2, wherein the carbon powder has a radiocarbon age less than 10,000 years from the present.

12. The iron briquette according to claim 1 or 2, further comprising an additive flux.

13. A method for manufacturing iron briquettes according to claim 1 or 2, A step of providing sponge iron pellets containing at least 0.5% by weight of iron oxide and essentially free of carbon, A step of providing carbon powder, A step of producing a mixture comprising the sponge iron pellets and the carbon powder, wherein the mixture comprises at least 0.2% by weight of carbon powder, The step of briquetting the mixture, Methods that include...

14. The method according to claim 13, wherein the step of briquetting the mixture is performed at a temperature exceeding 500°C.

15. The method according to claim 13, wherein the sponge iron pellets and carbon powder are supplied separately to a briquetting apparatus and mixed within the briquetting apparatus.

16. Iron briquettes (109, 209) are produced by providing sponge iron pellets (108, 208), providing carbon powder (114, 214), producing a mixture containing the sponge iron pellets and the carbon powder, and briquetting the mixture to obtain iron briquettes containing compressed sponge iron pellets (245) and carbon powder (214) located in the intervening space (247) between the compressed sponge iron pellets, wherein the iron briquettes contain at least 0.2% by weight of carbon powder, and the sponge iron pellets have a median diameter greater than about 7 mm before briquetting, contain at least 0.5% by weight of iron oxide, are essentially carbon-free, and contain less than about 0.1% by weight of carbon.

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