Direct reduced iron pellets and their uses

Abstract

The present disclosure relates to direct reduced iron (DRI) pellets having an average metallization of 97% or greater. The DRI pellets are essentially carbon-free or contain 2% or less by weight of carbon. The DRI pellets further have (i) a median pore diameter of 1.5 μm or greater; and / or (ii) a median pore diameter of 0.5 μm or greater. 2 / g or less; and / or (iii) an average porosity of 60% or less. The present disclosure further relates to uses of such DRI pellets.

Description

[Technical Field]

[0001] The present disclosure relates to direct reduced iron pellets and uses of such pellets. More particularly, the present disclosure relates to direct reduced iron pellets and uses of such pellets as defined in the introductory parts of the independent claims. [Background technology]

[0002] Steel is the world's most important industrial and construction material. It is difficult to find anything in modern society that does not contain steel or that does not rely on steel for its manufacture and / or transportation. As such, steel is intricately intertwined with nearly every aspect of our modern lives. In 2018, global production of crude steel was 1.81 billion tons, far more than any other metal, and is expected to reach 2.8 billion tons by 2050, 50% of which is expected to come from virgin iron sources.

[0003] Direct reduction is an increasingly popular means of processing iron ore to produce the crude iron needed for steelmaking. In direct reduction, the ore is reduced in a solid-state reduction process at temperatures below the melting point of iron. Shaft-type direct reduction processes utilize pelletized iron ore as a feedstock and produce a porous crude iron product known as sponge iron or direct reduced iron (DRI).

[0004] Most current shaft direct reduction plants are part of integrated steelworks, and the DRI produced is used directly within the steelworks. However, some DRI is also transported to distant steelworks or sold to third parties, and such DRI must be easily handled, transported, and stored. As DRI production becomes more widespread, this general-purpose use of DRI is expected to increase. For example, iron ore producer LKAB recently announced a strategy to switch from ore pellet production to hydrogen-based sponge iron production by the 2030s.

[0005] Cold DRI pellets (cDRI) produced in shaft-type direct reduction plants are typically not well suited for such purposes. Due to their high porosity, low density, large surface area, and low thermal conductivity, cDRI is prone to rapid corrosion and reoxidation reactions. Many of these reactions are exothermic and, if uncontrolled, can self-heat and ultimately lead to autoignition and fire. DRI corrosion and oxidation reactions can also produce hydrogen, an explosive gas lighter than air, and carbon monoxide, a highly toxic gas. These problems are further exacerbated by the fact that DRI is typically relatively weak and tends to crumble during handling, producing dust and fines. DRI dust is even more reactive than bulk DRI and is more prone to self-heating and fire. For example, DRI dust dispersed in the air can ignite in a flash fire or explosion.

[0006] Hot briquetted iron (HBI) was developed to address the transportation and handling challenges of cDRI. HBI is produced by compressing DRI into briquettes at high temperatures (above 650°C). Compressing DRI into high-density briquettes increases strength and reduces reactive surface area, meaning HBI is significantly less reactive and therefore safer and more practical to handle and transport than cDRI.

[0007] Other relevant issues regarding the handling, transportation, and storage of cDRI are detailed in the International Iron Metallics Association's report, "Direct Reduced Iron (DRI): Guide to Shipping, Handling and Storage (April 2022)." The report notes, among other things, that DRI is relatively weak compared to iron oxide pellets and many other common bulk materials, and tends to crumble during handling, producing dust and fines. Furthermore, the report notes that carbon content (in the form of iron carbide / cementite / Fe3C) reduces the reactivity of DRI.

[0008] Kim and Pistorius (Kim G, Pistorius PC, "Strength of Direct Reduced Iron Following Gas-Based Reduction and Carburization," Metallurgical and Materials Transactions B, 2020, Vol. 51, pp. 2628-2641) described a study on the effects of reducing gas composition, degree of reduction, and carburization on the compressive strength of DRI pellets. Various industrially and laboratory-reduced DRI pellets were tested. Carbon monoxide in the reducing gas was found to contribute to improved pellet strength, likely due to the formation of "internal whiskers" in the DRI. DRI reduced using hydrogen alone (or a mixture of hydrogen and steam) was found to have lower strength, and the authors concluded that the lower strength of pellets produced without CO2 may suggest that weaker DRI may be an inherent feature of the fossil-free steelmaking process using H2-based direct reduction. Summary of the Invention [Problem to be solved by the invention]

[0009] There remains a need for a direct reduced iron product that is easy to handle, transport, and store. [Means for solving the problem]

[0010] The inventors of the present invention have identified several drawbacks in prior art means of producing DRI suitable for handling, transportation, and storage. As discussed above, conventional cold-DRI is not particularly suitable for such purposes, and extensive precautions must be taken when transporting such products. Furthermore, cold-DRI typically requires passivation in a controlled atmosphere for several days after production to reduce reactivity to a manageable level, a process that adds additional costs. Briquetting DRI to produce HBI effectively addresses the reactivity issue, but adds an additional step to the production process, resulting in additional costs.

[0011] It would be advantageous to provide a means to overcome or at least mitigate at least some of the above-mentioned drawbacks. In particular, it would be desirable to provide a DRI product that is easy to handle, transport, and store, without the need for the additional expense of DRI briquetting. Furthermore, it would be desirable for such a DRI product to be obtained by a process that is easy to manufacture continuously on a large scale and is more environmentally friendly. In order to better address one or more of these concerns, a direct reduced iron product is provided having the features defined in the independent claims.

[0012] According to a first aspect, there is provided direct reduced iron pellets having an average metallization of 97% or greater. The DRI pellets are essentially carbon-free, or the DRI pellets contain 2% or less by weight of carbon. The DRI pellets further comprise: (i) a median pore diameter of 1.5 μm or greater; and / or (ii) 0.5 m 2 an average BET surface area of ​​less than or equal to 1 / g; and / or (iii) average porosity of 60% or less; The present invention is characterized by having the following.

[0013] That is, the DRI pellets can be further characterized by any of (i); (ii); (iii); (i) and (ii); (i) and (iii); (ii) and (iii); and (i), (ii), and (iii). Preferably, the DRI pellets are characterized by at least (i), i.e., (i); (i) and (ii), (i) and (iii); or (i), (ii), and (iii).

[0014] Such DRI pellets can be obtained by using hydrogen as a reducing gas in an industrial direct reduction process. Surprisingly, it has been found that DRI pellets meeting the above specifications exhibit superior mechanical and aging (reactivity) properties compared to conventional DRI pellets produced using fossil-fuel-based direct reduction. More specifically, such DRI pellets exhibit superior cold compressive strength, superior tumbling index, slower aging in ambient environments, and slower accelerated aging in water compared to conventional DRI pellets produced using fossil-fuel-based reducing gas. Such DRI pellets may already be less reactive upon discharge from the DRI shaft and may not necessarily require further specific passivation procedures.

[0015] According to some embodiments, the DRI pellets may have an average metallization of 98% or greater, such as an average metallization of 99% or greater, such as an average metallization of 99.5% or greater.

[0016] According to some embodiments, the DRI pellets may have a median pore diameter of 2.0 μm or greater, such as 2.5 μm or greater. It has been found that larger median pore diameters result in DRI pellets with reduced reactivity (i.e., superior aging properties) and increased strength (i.e., superior mechanical properties).

[0017] According to some embodiments, the DRI pellets are 0.4 m 2The DRI pellets may have an average BET surface area of ​​0.15 to 0.25 μm / g or less. A lower surface area is expected to correlate with reduced reactivity of the DRI pellets.

[0018] According to some embodiments, DRI pellets may have an average porosity of 58% or less. Low porosity is expected to correlate with reduced reactivity of DRI pellets. This also helps further distinguish DRI pellets from pellets produced on a laboratory scale, which are not suitable for large-scale production and do not necessarily have the same beneficial attributes. DRI pellets may have an average porosity of 56% or less.

[0019] According to some embodiments, the DRI pellets may have an average total iron content of 94 wt.% or more, such as 96 wt.% or more, e.g., 98 wt.% or more. The use of such low-residue DRI allows for the production of low-residue steel, such as bare automotive sheet, while still leaving significant room for the use of high-residue scrap in the melt.

[0020] According to some embodiments, the DRI pellets may contain an average of 3 wt.% or less FeO. A low wustite content has been found to correlate with superior mechanical properties, particularly high DRI compressive strength. The DRI pellets may contain an average of 2 wt.% or less FeO, such as 1 wt.% or less FeO, e.g., 0.5 wt.% or less FeO.

[0021] According to some embodiments, the DRI pellets may contain an average of 0.5 wt.% or less Fe3O4. Low magnetite content has been found to correlate with superior mechanical properties, specifically high DRI compressive strength.

[0022] According to some embodiments, DRI pellets can be obtained by direct reduction in a countercurrent direct reduction shaft in a reducing gas comprising greater than 90% by volume hydrogen and, optionally, steam and an inert gas. The reducing gas can consist essentially of hydrogen and, optionally, steam and an inert gas. Direct reduction in hydrogen under appropriate conditions has been found to provide DRI with superior mechanical and aging (reactivity) properties compared to DRI produced using fossil fuel-based reducing gases such as natural gas or syngas.

[0023] According to some embodiments, the reducing gas may have a temperature of 750°C or greater at the reducing gas inlet of the direct reduction shaft. Higher reducing gas temperatures have been found to help provide conditions suitable for the production of superior, high-metallization DRI. The reducing gas may have a temperature of 800°C or greater, such as 850°C or greater, for example 900°C or greater, for example 950°C or greater, at the reducing gas inlet of the direct reduction shaft.

[0024] DRI pellets may contain 1.5 wt.% or less, for example, 1.0 wt.% or less, carbon. Because carburization is not an essential part of the direct reduction process, the carbon content can be controlled independently of other properties, such as metallization. This is advantageous because carbon in DRI is typically lost during the subsequent melting process, and therefore, it may be desirable to provide a DRI containing only the carbon strictly required for subsequent processing steps.

[0025] According to some embodiments, when DRI pellets contain carbon, they may be obtained by direct reduction followed by carburization in carburizing gas. Reduction in hydrogen followed by carburization has been found to not adversely affect the mechanical and aging properties of DRI, as opposed to simultaneous reduction and carburization in carburizing gas (i.e., conventional fossil-fuel-based direct reduction). In some cases, for example, carbon-containing DRI may be desirable as a drop-in replacement for DRI produced by conventional fossil-fuel-based direct reduction.

[0026] The carburizing gas may comprise or consist essentially of a gas selected from methane, ethane, propane, butane, carbon monoxide, hydrogen, nitrogen, and combinations thereof, provided that the carburizing gas comprises at least 5% by volume, such as at least 10% by volume, such as at least 20% by volume, such as at least 30% by volume, of a carbonaceous component.

[0027] According to some embodiments, the DRI pellets may have an average cold crush strength of greater than 160 daN as measured by the method of ISO 4700:2015.

[0028] According to some embodiments, the DRI pellets may have a tumbling index of 96% or greater, as measured by the method of ISO 3271:2015. The tumbling index may be 97% or greater, such as 98% or greater.

[0029] According to some embodiments, the DRI pellets may lose less than 1% of their metallization after 28 days of storage at ambient temperature protected from precipitation. The DRI pellets may lose less than 1% of their metallization upon such storage.

[0030] According to another aspect, there is provided a use of DRI pellets according to the first aspect as a feedstock in a melting furnace for steel production. The DRI pellets do not need to be briquette-formed prior to such use. The melting furnace may be located at a distance of at least 100 kilometers from a production site of the DRI pellets. The DRI according to the first aspect has superior mechanical and aging properties compared to conventional DRI pellets (Type (B) DRI), allowing for easy transport without the need for prior briquetting into HBI (Type (A) DRI). The melting furnace may be located at a distance of at least 500 kilometers, e.g., at least 1000 kilometers, from a production site of the DRI pellets. The DRI pellets may be stored for at least 30 days before being fed to the melting furnace. The DRI according to the first aspect has superior mechanical and aging properties compared to conventional DRI pellets (Type (B) DRI), allowing for easy storage without the need for prior briquetting into HBI (Type (A) DRI), allowing for convenient handling. The DRI pellets may be stored for at least 60 days, such as at least 90 days, such as at least 120 days, before being fed to a melting furnace.

[0031] Additional objects, advantages and novel features of the present invention will become apparent to those skilled in the art from the following detailed description.

[0032] For a more complete understanding of the present invention, together with further objects and advantages thereof, the following detailed description should be read in conjunction with the accompanying drawings, in which like reference characters represent similar items in the various views. [Brief explanation of the drawings]

[0033] [Figure 1] FIG. 1 is a chart showing metallization values, CCS values, and carbon content (x10) for various exemplary samples. [Figure 2a] FIG. 1 shows the microstructure of H2-reduced pellets. [Figure 2b] FIG. 1 shows the microstructure of natural gas reduced pellets. [Figure 3] FIG. 1 is a chart showing the results of the tumbling test determined using the ISO 3271:2015 method for some example samples. [Figure 4] FIG. 1 is a chart showing weight changes of various sample pellets after aging under various conditions. DETAILED DESCRIPTION OF THE INVENTION

[0034] The present invention is based on the surprising discovery that highly metallized DRI pellets produced by industrial-scale continuous shaft direct reduction using hydrogen as the reducing gas possess superior properties that make such pellets highly suitable for storage, handling, and transportation. These superior properties include improved mechanical strength and resistance to aging compared to DRI pellets produced using conventional fossil fuel-based reducing gases. This is in contrast to the commonly accepted belief that carbon incorporated into DRI during reduction enhances the strength and aging of DRI.

[0035] The present disclosure will now be described with reference to experiments performed, and preferred example embodiments of the present disclosure will now be described. However, the present disclosure may be embodied in other forms and should not be construed as limited to the embodiments disclosed herein. The disclosed embodiments are provided so that the scope of the present disclosure will be fully conveyed to those skilled in the art.

[0036] general principles Unless otherwise indicated, the various DRI properties tested are determined using standard methods known in the art, and for methods traditionally used to determine a single property, the variability in the determined property is typically within experimental error.

[0037] The metallization rate is calculated according to the conventional method in the art (Fe 金属 / Fe 全) × 100. The metallization ratio was determined using X-ray diffraction (XRD), but can also be determined using other methods. Such other methods include: ISO 2597-1:2006 (Iron ores - Determination of total iron content - Part 1: Titrimetric method after reduction with tin(II) chloride) combined with ISO 5416:2006 (Direct reduced iron - Determination of metallic iron - Bromine-methanol titration method); and This method combines ISO10276-1:2000 (Chemical analysis of steel materials - Determination of oxygen in steel and iron - Part 1: Sampling and preparation of steel samples for oxygen determination) with ISO10276-2:2003 (Chemical analysis of steel materials - Determination of oxygen content in steel and iron - Part 2: Infrared method after fusion under inert gas).

[0038] The composition, such as carbon content, of the tested DRI can be determined using elemental analysis (LECO analysis). Specifications relevant to such determinations include: ISO15350:2010 (Steel and iron - Determination of total carbon and total sulfur content - Infrared absorption method after combustion in an induction furnace); ISO10036:1989 (Chemical analysis of iron and steel materials - Determination of total carbon in steel and iron - Gravimetric method after combustion in oxygen flow); and ISO 9556:2001 (Steel and iron - Determination of total carbon content - Infrared absorption method after combustion in an induction furnace).

[0039] set up Unless otherwise indicated, all DRI samples tested were produced at the Hybrit Pilot Direct Reduction Facility in Lulea. Briefly, the pilot facility comprises a direct reduction shaft with an overall height of approximately 9.3 meters, a maximum diameter of approximately 1.22 meters, and a total volume of approximately 7.6 cubic meters. Considering only the portion of the shaft comprising the reduction zone, this zone is approximately 3.0 meters high and approximately 0.94 meters in diameter. The shaft is of conventional design; that is, it is a solid-gas countercurrent moving-bed reactor, whereby the iron ore burden is introduced through an inlet at the top of the reactor and descends by gravity toward an outlet located at the bottom of the reactor. In all studies described herein, KPRS direct reduction pellets, commercially available from LKAB, were used as the iron ore burden. However, the same or similar results as those described herein can be obtained by using any suitable iron ore pellets as the starting material. The DR shaft contains a reduction zone, an isobaric (transition) zone, and a conical cooling zone that tapers towards the outlet of the DR shaft. The nominal production capacity of the shaft is about 1 tonne of DRI / h. The operating pressure in the reactor can be varied up to about 4 barg.

[0040] To reduce the iron ore burden, a heated reducing gas can be introduced into the reduction zone. The reducing gas can comprise or consist essentially of, for example, hydrogen, carbon monoxide, natural gas, and mixtures thereof. The flow rate of the reducing gas is about 1500 Nm 3 / h to approximately 3000Nm 3 / h and the inlet temperature can be varied from about 550°C to about 1000°C.

[0041] A cooling gas can be circulated through the cooling zone to cool the DRI after reduction and before discharge. Suitable cooling gases include, for example, nitrogen, hydrogen, or a combination thereof when producing carbon-free DRI, or natural gas (diluted as needed) when producing carbon-containing DRI. The flow rate of the cooling gas is about 400 Nm 3 / h to approximately 1000Nm3 It can be changed up to / h.

[0042] In some cases, no cooling gas is circulated in the cooling zone, and instead the hot DRI is discharged to a separate shaft where it is cooled and optionally carburized using circulating gas. Such a separate shaft arrangement is disclosed in WO2021 / 225500A1, which is incorporated herein by reference.

[0043] Study 1 - Cold Compression Strength of DRI Cold crush strength (CCS) is a measure of the compressive load required to cause pellet failure. Such compressive loads can occur, for example, during handling, transportation, or storage. Cold crush strength was determined for several DRI samples produced in a DR pilot plant under various conditions, using either natural gas or hydrogen as the reducing gas. The average CCS was determined by measuring 60 pellets for each sample according to the method in ISO 4700:2015, "Iron ore pellets for blast furnace and direct reduction feedstock - Determination of crushing strength." The results are shown in Table 1 and Figure 1.

[0044] [Table 1]

[0045] The DRIs of Examples 1, 2, 4, 5, and 14 were produced in a single-shaft operation, with cooling gas supplied to the cooling zone of the DRI shaft. All other DRIs were produced in a dual-shaft operation, with cooling and optional carburization performed on a separate shaft. Examples 4 and 13 were cooled with natural gas and therefore contain carbon. All other examples were cooled with a non-carburizing cooling gas, such as nitrogen or hydrogen.

[0046] The following conclusions can be drawn from the obtained CCS values: Higher CCS values ​​can be obtained by reduction with hydrogen compared to reduction with natural gas. Comparing Examples 1 and 2, higher carbon levels in the DRI when reduced with natural gas seemingly have a negative effect on CCS. A reference sample of industrially produced DRI purchased for comparison had higher carbon and lower CCS (85 daN) compared to the pilot-produced example. However, when the pellets were reduced with hydrogen and subsequently carburized during cooling, the carbon level in the DRI appears to have little or no effect at the levels tested in these examples, as can be seen from a comparison of Examples 4, 5, and 13. A comparison of Examples 13 and 14 shows that the selection of single- or dual-shaft operation does not appear to affect the CCS values. Although not shown in this example, other experiments have observed that the source and characteristics of the iron ore also affect the CCS of the resulting DRI. Finally, the results suggest that for hydrogen-reduced DRI, a higher metallization rate leads to improved CCS values.

[0047] Looking more closely at the effect of metallization on CCS, Figure 1 shows plots of metallization, CCS, and carbon content (x10) for various examples. Line 101 indicates a metallization value of 97%. Line 103 indicates an average CCS value of 160 daN. It can be seen that metallization rates above about 97% result in high average CCS values, in this example above about 160 daN. Conversely, metallization rates below about 97% result in low average CCS, in this example below about 160 daN. Metallization is somewhat correlated with reduction temperature; higher reduction temperatures typically result in higher metallization, although as shown in Example 7, a high reduction temperature does not guarantee high metallization.

[0048] Research 2-excavation research Excavations were conducted to investigate pellet / DRI characteristics at various points during their passage through the direct reduction shaft. This involved operating the DR shaft at selected steady-state conditions for a period of time, followed by quenching the shaft to stop reduction, and then excavating to retrieve samples at various depths within the shaft.

[0049] At the selected operating point before quench, hydrogen was used as the reducing gas, the reducing gas temperature to the reactor was 935°C, the system pressure was 3 barg, and the reducing gas flow rate was 2450 Nm 3 The DRI was 775 Nm / h in the cooling zone. 3 The reactor was cooled with a nitrogen flow rate of 1000 kJ / h. At the operating point before quenching, the stability period was approximately 28 hours. The main quality parameters of the DRI obtained at the reactor outlet are shown in Table 2 below (determined by XRD). As explained in the "Setup," it can be seen that H-based direct reduction under these specified conditions in the pilot direct reduction shaft allows the production of DRI with very high total Fe and metallization. Such DRI has advantageous mechanical and aging properties, as disclosed herein.

[0050] [Table 2]

[0051] Quenching was performed using nitrogen gas. After the reactor was quenched and cooled, mining was carried out. Mining consisted mainly of radial and vertical sampling along the shaft while descending the shaft. The target bed thickness in the reduction zone was set at 150 mm, with thicker layers in the isobaric and cooling zones. 13 bed samples were taken for each bed. Each bed sample weighed approximately 1200 g. The composition and cold crushing strength (CCS) of the pellets from each bed sample were determined.

[0052] As the iron ore is gradually reduced from hematite (Fe2O3) to magnetite (Fe3O4) to wüstite (FeO), the compressive strength of the pellets decreases, reaching a minimum of approximately 85 dN at depths of 3-3.5 m in the DR shaft. After most of the oxides are reduced, the strength recovers to approximately 150-170 dN near the transition between the reduced and isobaric zones. This suggests that the oxides magnetite and wüstite play a central role in determining the compressive strength of DRI pellets.

[0053] For comparison, the same quench and excavation were carried out using natural gas (NG)-based direct reduction. The reduction gas flow rate was 2500 Nm 3 / h, and the reducing gas temperature was 1080°C, but otherwise the process parameters were similar. The main quality parameters of the DRI obtained before quenching are shown in Table 3 below. It can be seen that the total Fe and metallization are at the upper end of the typical range for industrial DRI obtained from fossil-fuel based processes, and that relatively large amounts of residual oxides, especially wüstite, are still present.

[0054] [Table 3]

[0055] After quenching and excavation, CCS and compositional analysis were performed on the excavated samples. When natural gas-based reducing gas was used, the decrease in compressive strength was found to be more pronounced in the reduced zone, reaching a minimum of approximately 70 dN at a depth of 1–1.5 m. Subsequently, strength appears to recover with increasing reduction, reaching approximately 120–150 daN in the isobaric zone. However, reduction is never fully complete, and even pellets in the isobaric zone exhibit significant residual amounts of magnetite and wüstite oxides. This is in good agreement with the quality of the DRI obtained before quenching.

[0056] Figures 2a and 2b show the microstructures of the H-reduced (2a) and NG-reduced (2b) pellets. The H-reduced pellets contain almost no residual oxides, and any remaining oxides are primarily present between the grains. However, the NG-reduced pellets still contain a significant amount of wüstite within each grain, indicating that the NG-based reducing gas has difficulty penetrating to the grain center.

[0057] In summary, the mining experiments suggest that the presence of oxides such as wustite and magnetite adversely affect the compressive strength of DRI, and that reduction with a carbon-containing reducing gas can further exacerbate the adverse effects of these oxides. Furthermore, the experiments demonstrate that by using hydrogen as the reducing gas, it is possible to obtain DRI with very high metallization and few residual oxides, while comparative experiments using natural gas as the reducing gas result in DRI with more typical metallization and residual oxide values.

[0058] Study 3 - Tumbling Test To further investigate the effect of reducing gas composition and temperature on the mechanical properties of DRI, the tumbling index and abrasion index of several DRI samples were obtained. The tumbling index provides an indication of the susceptibility of DRI pellets to breakage due to abrasion during handling and transportation. The tumble index and abrasion index of the tested DRI and iron ore pellets were determined using the method of ISO 3271:2015, "Iron ore for blast furnace and direct reduction feedstock - Determination of tumble index and abrasion index."

[0059] Figure 3 shows these test results, as well as the metallization and carbon content of various test samples. The exact metallization and carbon content of Sample A (industrial reference), Sample C (NG mining), and Sample D (H2 mining) are unknown. On the left, natural gas-based DRIs are shown. The lowest value of 90.4% after tumbling (TTH) above 6.3 mm is for Sample A, a purchased industrial DRI reference produced using fossil-fuel-based direct reduction and with significantly higher metallization and carbon content (exact composition unknown). This value can be compared to the 95.3% TTH value after tumbling for Sample B, a pilot-produced fossil-fuel-based reference. A sample of NG-DRI that was not fully reduced, Sample C, from a mining conducted after campaign K2, was also tested as a comparison. Lower metallization appears to correlate with a lower Tumbling Index.

[0060] The hydrogen-reduced DRI is shown on the right side of the graph, which also includes a mined sample of H-DRI that was not fully reduced (Sample D). This result has a metallization rate of over 98%, which is lower than other hydrogen-reduced samples (Samples E–L) produced using different process conditions. These different process conditions include, among others, various reduction gas temperatures from 800–900 °C and carbon contents (no carbon or post-carburized with natural gas to a carbon content of 1% C). Tumbling results for the highly metallized hydrogen-reduced DRI show a tumbling index (TTH) of 98–99% TTH in all cases. By comparison, these values ​​are far superior to those obtained from the natural gas-reduced DRI (Samples A–C) and even superior to the iron ore pellets used for direct reduction.

[0061] Also shown is the abrasion threshold (ATH), which represents the percentage of samples with diameters less than 0.5 mm after tumbling. In general, it can be said that the abrasion threshold has an inverse correlation with the tumbling threshold.

[0062] Tumbling tests show that hydrogen-reduced DRI exhibits significantly improved mechanical properties compared to the natural gas-reduced DRI reference. H2-DRI exhibits excellent mechanical properties across a range of reduction temperatures, regardless of whether the DRI is subsequently carburized. However, the incompletely reduced H2-DRI samples from the excavation studies were found to have inferior tumbling indices compared to fully reduced H2-DRI samples with metallization rates of 98% or greater.

[0063] Study 4 - Time-course study To study the reactivity and reoxidation of manufactured DRI batches during storage, several aging studies were performed both at ambient conditions and under conditions expected to accelerate aging.

[0064] Bulk aging studies For various DRI batches, large bags (volume approx. 1 m 3 ) with each batch and then storing these protected bags at ambient temperature. The batches studied were as follows: NG-reduced DRI (metallization rate 87.6%); High metallization H2-reduced DRI (metallization rate 99%, reduction temperature 900℃); and Carburized, highly metallized H2 reduced DRI (metallization rate 99%, C 1.4%, reduction temperature 900°C).

[0065] Changes in composition and metallization were determined by periodically extracting a few pellets from each bag and analyzing the pellets using XRD and LECO elemental analysis, after which the average changes in composition and metallization over each period could be determined.

[0066] The NG-reduced DRI was found to exhibit a fairly rapid decline in metallization, with a decrease of approximately 1.6% in metallization over the first 4 weeks (28 days) of storage, but no significant decrease in metallization was observed after an additional 4 weeks (56 days total) of storage.

[0067] None of the highly metallized H-reduced DRIs showed any detectable loss of metallization, even after long-term storage for a total of 180 days. This was true whether the DRI was carbon-free (nitrogen-cooled) or carburized (natural gas-cooled).

[0068] Accelerated aging in water To further investigate the effect of metallization and carbon content on the aging characteristics of hydrogen-reduced DRI, accelerated aging tests in water were conducted on several H-reduced DRI. The batches tested included the highly metallized H-reduced DRI (both carbon-free and carburized) described in the aging experiments above, as well as the following batches: Medium metallization H2 reduced DRI (metallization rate approximately 96%, reduction temperature 800℃).

[0069] Approximately 200 g of each DRI was placed in a separate bucket and then filled with water until the DRI was completely covered. After 3 days, 2 weeks, and 4 weeks, samples were removed and analyzed by XRD and LECO elemental analysis as described above. Before preparing the water-soaked samples for analysis, they were dried at 105°C for 24 hours.

[0070] Each of the high-metallization H2-DRIs was found to have a metallization loss of approximately 1-1.5% after 28 days. This was true whether the DRI was carburized or carbon-free. The medium-metallization H2-DRI showed an even greater loss of metallization, approximately 2-4% after 28 days.

[0071] Single pellet aging study To further investigate the effect of metallization on aging, single pellet studies were conducted on carbon-free (nitrogen-cooled) highly and medium-metallized pellets. Individual pellets were stored indoors or outdoors protected from rain. At regular intervals, the pellets were weighed using a high-precision balance. The total test period was approximately one month. Any weight increase was assumed to be due to iron reoxidation. The results are shown in Figure 4.

[0072] Highly metallized H2-DRI showed little tendency to gain weight over the test period, regardless of whether it was stored indoors or outdoors. Medium-metallized H2-DRI stored outdoors showed a linear weight gain throughout the test period, with a total weight increase of approximately 0.4-0.5% at the end of the test period. Medium-metallized H2-DRI stored indoors showed a relatively rapid weight gain of approximately 1.2% (after approximately one week), but no further weight gain occurred after this initial increase.

[0073] In summary, hydrogen-reduced DRIs were found to age more slowly than natural gas-reduced DRIs. Hydrogen-reduced DRIs were found to age more slowly in both ambient and accelerated (water) tests as metallization increased. The carbon content of hydrogen-reduced DRIs was found not to significantly affect aging, at least for the highly metallized H2-DRIs tested.

[0074] Study 5 - Porosity and Surface Area Measurements Porosity and BET surface area were determined for some examples from Study 1 as well as for some other examples.

[0075] The porosity of the tested DRIs was determined by the method of ISO 15901-1:2016, "Evaluation of pore size distribution and porosity of solid materials by mercury porosimetry and gas adsorption methods - Part 1: Mercury porosimetry." The mercury temperature was 20.0°C, and the pressure ranged from 0.10 to 61,000.00 psia.

[0076] The BET surface area of ​​the tested DRI was determined by the method of ISO 9277:2010 "Determination of the specific surface area of ​​solids by gas adsorption methods - BET method." Krypton was used for the determination of the BET surface area at an analytical bath temperature of 77 K.

[0077] At least two pellets were tested for each example, and the values ​​shown are the average of all pellets for each example. A summary of the results is shown in Table 4.

[0078] [Table 4]

[0079] Examples labeled Ln were obtained by laboratory-scale reduction of iron ore pellets in a stream of the relevant reducing gas heated to the relevant temperature. The iron ore pellets used were of the same type as those used in the pilot-scale studies. Clear differences can be observed between the properties of laboratory-produced DRI produced by a batch process and those of pilot-produced DRI obtained by a large-scale continuous process in a pressurized shaft. Pilot-produced DRI generally have a lower BET surface area and lower porosity than laboratory-produced DRI, typically with a porosity of 60% or less and a BET surface area of ​​0.5 m 2 In contrast, laboratory-produced DRI typically has a porosity greater than 60% and a BET surface area of ​​0.6 m 2 / g or more.

[0080] No significant differences were observed between the NG-reduced and H2-reduced DRIs in terms of % porosity and BET surface area.

[0081] However, in subsequent testing, the median pore diameter and total pore area of ​​the NG-reduced and H-reduced DRI samples were also measured by the method of ISO 15901-1:2016. The samples tested were the same as or similar to the industrial-scale samples listed in Table 4 above.

[0082] The NG-reduced DRI samples tested were found to have median pore diameters ranging from 1.1 μm to 1.4 μm. The H-reduced samples had median pore diameters ranging from 1.5 μm to 4.0 μm. Improved strength and aging were found to correlate with increased median pore diameters. That is, H-reduced samples with superior mechanical and aging properties had median pore diameters of 2.0 μm or greater, preferably greater than 2.5 μm. This difference in median pore diameter was observed regardless of whether the H-reduced DRI was carburized. That is, H-reduced DRI that was subsequently carburized had a significantly larger median pore diameter than NG-reduced DRI and could be distinguished from NG-reduced DRI in this manner. The H-reduced samples with superior mechanical and aging properties also had median pore diameters greater than 0.5 μm. 2 It was found that the total pore area was less than 1 / g.

[0083] Overview of experimental research Therefore, in summary, pilot-scale high-metallization hydrogen-reduced DRI has been found to have superior mechanical and aging properties compared to conventional natural-gas-reduced DRI and to hydrogen-reduced DRI with lower metallization rates. Pilot-scale DRI can be distinguished from laboratory-scale DRI by its porosity and BET surface area. Pilot-scale high-metallization hydrogen-reduced DRI is distinguished from pilot-scale natural-gas-reduced DRI primarily by its high metallization rate and the associated lack of magnetite and wustite oxides. Pilot-scale high-metallization hydrogen-reduced DRI is also distinguished from pilot-scale natural-gas-reduced DRI by its relatively large median pore diameter (1.5 or greater, preferably 2.5 or greater). Pilot-scale high-metallization hydrogen-reduced DRI, if not carburized, may also be easily distinguished from pilot-scale natural-gas-reduced DRI by the lack of carbon.

[0084] Those skilled in the art will recognize that the present disclosure is not limited to the preferred embodiments described above. Those skilled in the art will further recognize that modifications and variations are possible within the scope of the appended claims. For example, those skilled in the art will recognize that DRI pellets having particular combinations of metallization, carbon content (or lack thereof), median pore diameter, porosity, and BET surface area not specifically disclosed in the examples may be capable of being produced under appropriate conditions. Furthermore, those skilled in the art will recognize that the suitable results obtained herein may be obtained using more suitable iron ore pellets and more suitable DR shaft dimensions than those specifically disclosed herein. Furthermore, variations of the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed disclosure, from a study of the drawings, the disclosure, and the appended claims.

Claims

1. Direct reduced iron (DRI) pellets having an average metallization of 97% or greater, - essentially carbon-free; or - contains less than or equal to 2% by weight of carbon; (i) a median pore diameter of 1.5 μm or greater; and / or (ii) 0.5 m ２ / g or less average BET surface area; and / or (iii) Average porosity of 60% or less The DRI pellet further characterized by having:

2. 2. The DRI pellet of claim 1 having a median pore diameter of 2.0 μm or more, preferably 2.5 μm or more.

3. 0.4 m ２ 3. The DRI pellets of claim 1 or 2, having an average BET surface area of ​​less than or equal to 1 / g.

4. 4. The DRI pellet of claim 1, having an average porosity of 58% or less.

5. 5. The DRI pellets of any one of claims 1 to 4, having an average total iron content of 94 wt% or greater.

6. 6. The DRI pellet of claim 1, comprising an average of 3 wt. % or less FeO.

7. Average of 0.5 wt% or less Fe ３ O ４ 7. The DRI pellet of claim 1, comprising:

8. 8. DRI pellets according to any one of claims 1 to 7, obtainable by direct reduction in a countercurrent direct reduction shaft in a reducing gas consisting essentially of hydrogen and optionally water vapor and an inert gas.

9. 9. The DRI pellets of claim 8, wherein the reducing gas has a temperature of 750°C or greater at the reducing gas inlet of the direct reduction shaft.

10. 10. DRI pellets according to claim 8 or 9, containing less than 2% by weight of carbon and obtainable by direct reduction followed by carburization in carburizing gas.

11. 11. The DRI pellet of claim 10, obtainable by carburizing in a carburizing gas selected from methane, ethane, propane, butane, carbon monoxide, hydrogen, nitrogen, and combinations thereof, with the proviso that said carburizing gas comprises at least 5% by volume of a carbonaceous component.

12. 12. The DRI pellets of any one of claims 1 to 11, having an average cold crush strength of more than 160 daN as measured by the method of ISO 4700:2015.

13. 13. The DRI pellets of any one of claims 1 to 12, having a tumbling index of 96% or greater when measured by the method of ISO 3271:2015.

14. 14. Use of the DRI pellets according to any one of claims 1 to 13 as a feedstock in a melting furnace for the production of steel.

15. 15. The use of claim 14, wherein the DRI pellets are not briquettered prior to use in the melting furnace.

16. 16. The use according to claim 14 or 15, wherein the melting furnace is located at a distance of at least 100 kilometers from the production site of the DRI pellets.

17. 17. The use according to any one of claims 14 to 16, wherein the DRI pellets are stored for at least 30 days before being fed into the melting furnace.

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