Direct reduced iron pellets and uses thereof

Abstract

The present disclosure relates to direct reduced iron (DRI) pellets, wherein the DRI pellets have an average metallization ratio greater than or equal to 97%. The DRI pellets are substantially free of carbon, or alternatively they comprise less than or equal to 2 wt% carbon. The DRI pellets are further characterized in that they: (i) have a median pore size greater than or equal to 1.5 [mu] m; and / or (ii) has an average BET surface area of less than or equal to 0.5 m2 / g; and / or (iii) has an average porosity of less than or equal to 60%. The present disclosure also relates to the use 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 part of the independent claim. Background Art

[0002] Steel is the world's most important engineering and construction material. In the modern world, it is difficult to find any object that does not contain steel or does not rely on steel for its manufacture and / or transportation. In this way, steel is intricately involved in almost every aspect of our modern lives. In 2018, the total global production of crude steel was 18.1 million tons, far more than any other metal, and is expected to reach 28.0 million tons in 2050, of which 50% is expected to originate from primary iron sources.

[0003] Direct reduction is an increasingly common method of processing iron ore to produce crude iron for steelmaking. In direct reduction, the ore is reduced in a solid-state reduction process at a temperature below the melting point of iron. The shaft furnace-based direct reduction process utilizes pelletized iron ore as a feedstock and produces a porous crude iron product known as sponge iron or direct reduced iron (DRI).

[0004] Most existing direct reduction plants based on shaft furnaces are part of integrated steel mills, and the DRI produced is used directly on-site for steelmaking. However, some DRI is also transported to distant steel mills or sold to third parties, and such DRI must be easily handled, transported and stored. As the production of DRI becomes more common, such commodity uses of DRI are expected to increase. For example, iron ore producer LKAB recently announced a strategy that by the 21st century 30s, they will switch from ore pellet production to hydrogen-based sponge iron production.

[0005] Cold DRI pellets (cDRI) produced by shaft-furnace-based direct reduction plants are generally less suitable for such purposes. Due to their high porosity, low density, large surface area, and low thermal conductivity, they have a tendency to undergo rapid corrosion and reoxidation reactions. Many of these reactions are exothermic, leading to self-heating and ultimately spontaneous combustion and fire if not controlled. The corrosion and oxidation reactions of DRI can also produce hydrogen (an explosive gas that is lighter than air) and carbon monoxide (a highly toxic gas). These problems are exacerbated by the fact that DRI is typically relatively weak and tends to break apart during handling to produce dust and fines. DRI dust tends to be even more reactive than bulk DRI and has a high tendency to self-heat and cause fire. For example, DRI dust dispersed in air may ignite in a flash fire or explosion.

[0006] Hot briquetted iron (HBI) was developed in response to the difficulties in transporting and handling cDRI. HBI is produced by compressing DRI into briquettes at high temperatures (≥650°C). Compacting DRI into dense briquettes increases its strength and reduces its reactive surface area, meaning that HBI has much lower reactivity and is therefore safer and more practical to handle and transport than cDRI.

[0007] Other relevant considerations for the handling, shipping, and storage of cDRI are detailed in the report "Direct Reduced Iron (DRI): Guide to Shipping, Handling and Storage (April 2022)" from the International Iron Metallics Association. The report specifically notes that DRI is relatively weak compared to iron oxide pellets and many other common bulk materials, and tends to break during handling to produce dust and fines. In addition, the report notes that the carbon content (in the form of iron carbides / 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) describe a study of the effects of reducing gas composition, degree of reduction, and degree of carburization on the compressive strength of DRI pellets. Various industrial and laboratory reduced DRI pellets were tested. It was found that carbon monoxide in the reducing gas contributed to the development of pellet strength, possibly by forming "internal whiskers" in DRI. DRI reduced using only hydrogen (or a mixture of hydrogen and steam) was found to have poor strength, and the authors concluded that the low strength of pellets produced without CO may mean that weaker DRI may be the result of using H-based 2 Inherent features of the direct reduction fossil-free ironmaking process.

[0009] There remains a need for a direct reduced iron product suitable for handling, transportation and storage. Summary of the invention

[0010] The inventors of the present invention have recognized many disadvantages of prior art methods of producing DRI suitable for handling, transport and storage. As mentioned above, conventional cold DRI is not particularly suitable for such purposes, and a large number of precautions must be taken when transporting such a product. In addition, in order to reduce the reactivity to a manageable level, cold DRI generally needs to be passivated in a controlled atmosphere for many days after production, which further increases the cost of the process. Briquetting DRI to produce HBI effectively solves the reactivity problem, but at the cost of adding an additional step to the manufacturing process, which results in additional costs.

[0011] It would be advantageous to achieve a method that overcomes or at least mitigates at least some of the above mentioned disadvantages. In particular, it would be desirable to provide a DRI product that is readily suitable for handling, transportation and storage without resorting to the additional expense of briquetting the DRI. Furthermore, it would be desirable if such a DRI product could be obtained by a process that is readily suitable for continuous large-scale production and is more environmentally gentle. In order to better address one or more of these problems, a direct reduced iron product having the features defined in the independent claims is provided.

[0012] According to a first aspect, direct reduced iron pellets are provided, wherein the average metallization of the DRI pellets is greater than or equal to 97%. The DRI pellets are substantially free of carbon, or the DRI pellets contain less than or equal to 2 wt. % carbon. The DRI pellets are further characterized in that they:

[0013] (i) having a median pore size greater than or equal to 1.5 μm;

[0014] and / or

[0015] (ii) less than or equal to 0.5 m 2 Average BET surface area of ​​ / g;

[0016] and / or

[0017] (iii) having an average porosity of less than or equal to 60%.

[0018] That is, the DRI pellets may be further characterized by any of the following: (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).

[0019] Such DRI pellets can be obtained by using hydrogen as a reducing gas in an industrial direct reduction process. It was unexpectedly found that DRI pellets meeting the above specifications show excellent mechanical properties and aging (reactivity) properties compared to conventional DRI pellets produced using fossil-based direct reduction. More specifically, such DRI pellets show better cold compressive strength, better tumbling index, slower environmental aging, and slower accelerated aging in water compared to conventional DRI produced using fossil-based reducing gases. Such DRI pellets may already have low reactivity after being discharged from a DR shaft furnace, and may not necessarily require any further specific passivation procedures.

[0020] According to some embodiments, the DRI pellets may have an average metallization greater than or equal to 98%, such as an average metallization greater than or equal to 99%, such as an average metallization greater than or equal to 99.5%.

[0021] According to some embodiments, the median pore size of the DRI pellets may be greater than or equal to 2.0 μm, such as greater than or equal to 2.5 μm. It has been found that larger median pore sizes provide DRI pellets with reduced reactivity (i.e., excellent aging properties) and improved strength (i.e., excellent mechanical properties).

[0022] According to some embodiments, the average BET surface area of ​​the DRI pellets may be less than or equal to 0.4 m 2 / g. The low surface area is expected to be associated with reduced reactivity of the DRI pellets.

[0023] According to some embodiments, the average porosity of the DRI pellets may be less than or equal to 58%. Low porosity is expected to be associated with the reduced reactivity of the DRI pellets. This also serves to further distinguish the 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 properties. The average porosity of the DRI pellets may be less than or equal to 56%.

[0024] According to some embodiments, the average total iron content of the DRI pellets can be greater than or equal to 94 wt.%, such as greater than or equal to 96 wt.%, such as greater than or equal to 98 wt.%. The use of such low-residue DRI allows the production of low-residue steels, such as exposed automotive panels, while still providing a large margin for the use of higher-residue scrap in the melt.

[0025] According to some embodiments, the DRI pellets may contain an average of less than or equal to 3 wt% FeO. Low wüstite content has been found to be associated with excellent mechanical properties, particularly high DRI compressive strength. The DRI pellets may contain an average of less than or equal to 2 wt% FeO, such as less than or equal to 1 wt% FeO, such as less than or equal to 0.5 wt% FeO.

[0026] According to some embodiments, the DRI pellets may contain an average of less than or equal to 0.5 wt % Fe 3 O 4 Low magnetite content was found to be associated with excellent mechanical properties, in particular high DRI compressive strength.

[0027] According to some embodiments, DRI pellets may be obtainable by direct reduction in a countercurrent direct reduction shaft furnace in a reducing gas comprising greater than 90% by volume of hydrogen and optionally steam and inert gases. The reducing gas may consist essentially of hydrogen and optionally steam and inert gases. It was found that direct reduction in hydrogen under appropriate conditions provides DRI having superior mechanical properties and aging (reactivity) properties compared to DRI produced using fossil-based reducing gases such as natural gas or synthesis gas.

[0028] According to some embodiments, the temperature of the reducing gas at the reducing gas inlet of the direct reduction shaft furnace can be greater than or equal to 750° C. It has been found that higher reducing gas temperatures help provide suitable conditions for producing excellent highly metallized DRI. The temperature of the reducing gas at the reducing gas inlet of the direct reduction shaft furnace can be greater than or equal to 800° C., such as greater than or equal to 850° C., such as greater than or equal to 900° C., such as greater than or equal to 950° C.

[0029] The DRI pellets may contain less than or equal to 1.5 wt. % carbon, such as less than or equal to 1.0 wt. % carbon. Since carburization is not an integral part of the direct reduction process, the carbon content may be controlled independently of other properties such as metallization rate. This is an advantage because carbon in DRI is typically lost during subsequent melting processes, and therefore it may be desirable to provide DRI that contains only the carbon strictly required for subsequent processing steps.

[0030] According to some embodiments, in the case where the DRI pellets contain carbon, such pellets may be obtainable by carburizing in a carburizing gas after direct reduction. It was found that carburizing after reduction in hydrogen is not detrimental to the mechanical and aging properties of the DRI compared to simultaneous reduction and carburizing in a carburizing gas (i.e., conventional fossil-based direct reduction). In some cases, carbon-containing DRI may be desirable, for example as a drop-in replacement for DRI produced by conventional fossil-based direct reduction.

[0031] The carburizing gas may comprise or consist essentially of a gas selected from the group consisting of methane, ethane, propane, butane, carbon monoxide, hydrogen, nitrogen, and combinations thereof, provided that it comprises at least 5 volume % carbonaceous components, such as at least 10 volume %, such as at least 20 volume %, such as at least 30 volume %.

[0032] According to some embodiments, the average cold compressive strength of the DRI pellets as measured by the ISO 4700:2015 method may be greater than 160 daN.

[0033] According to some embodiments, the DRI pellets may have a tumble index greater than or equal to 96% as measured by the ISO 3271:2015 method. The tumble index may be greater than or equal to 97%, such as greater than or equal to 98%.

[0034] According to some embodiments, after 28 days of storage at ambient temperature to avoid precipitation, the loss of metallization rate of the DRI pellets can be less than 1%. After such storage, the loss of metallization rate of the DRI pellets can be less than 1%.

[0035] According to another aspect, there is provided the use of the DRI pellets according to the first aspect as a raw material in a smelting furnace for producing steel. Prior to such use, the DRI pellets may not be briquette-formed. The smelting furnace may be located at a distance of at least 100 kilometers from the production site of the DRI pellets. Since the DRI according to the first aspect has excellent mechanical properties and aging characteristics compared to conventional DRI pellets (type (B) DRI), it is very transportable without the need for pre-briquetting into HBI (type (A) DRI). The smelting furnace may be located at a distance of at least 500 kilometers, for example at least 1000 kilometers, from the production site of the DRI pellets. Before being fed to the smelting furnace, the DRI pellets may be stored for a duration of at least 30 days. Since the DRI according to the first aspect has excellent mechanical properties and aging characteristics compared to conventional DRI pellets (type (B) DRI), it is very storable and convenient to handle without the need for pre-briquetting into HBI (type (A) DRI). Prior to being fed to the smelting furnace, the DRI pellets may be stored for a duration of at least 60 days, such as for a duration of at least 90 days, such as for a duration of at least 120 days.

[0036] Additional objects, advantages and novel features of the present invention will become apparent to those skilled in the art from the following detailed description. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] For a more complete understanding of the present invention and additional objects and advantages thereof, the following detailed description should be read in conjunction with the accompanying drawings, in which like reference numerals represent like items in different figures, wherein:

[0038] Figure 1 is a graph showing metallization values, CCS values, and carbon contents (×10) of different exemplary samples;

[0039] Figure 2a The microstructure of the H2-reduced pellets is shown;

[0040] Figure 2b The microstructure of the natural gas reduced pellets is shown;

[0041] Figure 3 is a graph showing the results of a tumble test determined using the ISO 3271:2015 method for some exemplary samples; and

[0042] Figure 4 is a graph showing the weight change of pellets of different samples after aging under different conditions. DETAILED DESCRIPTION

[0043] The present invention is based on the unexpected discovery that highly metallized DRI pellets produced on an industrial scale by continuous shaft-furnace-based direct reduction using hydrogen as the reducing gas have excellent properties that make such pellets very suitable for storage, handling and transportation. These excellent properties are improved mechanical strength and improved resistance to aging compared to DRI pellets produced using conventional fossil-based reducing gases. This is contrary to the accepted wisdom that carbon incorporated into DRI during reduction improves the strength and aging of DRI.

[0044] The present disclosure will now be described with reference to experiments performed in which preferred exemplary embodiments of the present disclosure are described. However, the present disclosure may be presented in other forms and should not be construed as being limited to the embodiments disclosed herein. The disclosed embodiments are provided to fully convey the scope of the present disclosure to the skilled person.

[0045] Synopsis

[0046] Unless otherwise stated, standard methods known in the art were used to determine the properties of the various DRIs tested. Where several methods were routinely used to determine a single property, the variation in the property determined was generally within the limits of experimental error.

[0047] The metallization rate is defined in a conventional manner in the art as (Fe 金属 / Fe 总 )×100. The metallization rate is determined using X-ray diffraction (XRD), but other methods may also be used. Such other methods include:

[0048] ISO 2597-1:2006 (Iron ores — Determination of total iron content — Part 1: Titration after reduction with tin(II) chloride) in conjunction with ISO 5416:2006 (Direct reduced iron — Determination of metallic iron — Bromine-methanol titration method); and

[0049] ISO 10276-1:2000 (Chemical analysis of ferrous materials — Determination of oxygen in steel and iron — Part 1: Sampling and preparation of steel samples for the oxygen determination) was combined with ISO 10276-2:2003 (Chemical analysis of ferrous materials — Determination of oxygen content in steel and iron — Part 2: Infrared method after fusion under inert gas).

[0050] Elemental analysis (LECO analysis) can be used to determine the composition of the DRI tested, such as the carbon content. Relevant standards for such determinations include:

[0051] ISO 15350:2010 (Iron and steel — Determination of total carbon and sulphur contents — Infrared absorption method after combustion in an induction furnace);

[0052] ISO 10036:1989 (Chemical analysis of ferrous materials – Determination of total carbon in iron and steel – Gravimetric method after combustion in a stream of oxygen); and

[0053] ISO 9556:2001 (Iron and steel – Determination of total carbon content – ​​Infrared absorption method after combustion in an induction furnace).

[0054] Device

[0055] Unless otherwise stated, all DRI samples tested were The pilot plant was produced at the Hybrit pilot direct reduction facility. In brief, the pilot plant includes a direct reduction shaft furnace having a total height of about 9.3 meters, a widest diameter of about 1.22 meters, and a total volume of about 7.6 cubic meters. Only the section of the shaft furnace constituting the reduction zone is considered, which has a height of about 3.0 meters and a diameter of about 0.94m. The shaft furnace is of conventional design. That is, the shaft furnace is a solid-gas countercurrent moving bed reactor, in which the iron ore charge is loaded at the inlet at the top of the reactor and falls toward the outlet arranged at the bottom of the reactor due to gravity. In all studies described herein, commercially available KPRS direct reduction pellets from LKAB were used as iron ore charge. However, any suitable iron ore pellets can be used as starting materials to obtain the same or similar results as described herein. The DR shaft furnace includes a reduction zone, an isobaric (transition) zone, and a tapered cooling zone that tapers toward the outlet of the DR shaft furnace. The nominal production capacity of the shaft furnace is about 1 ton of DRI / hour. The operating pressure in the reactor may vary up to about 4 barg.

[0056] A heated reducing gas may be introduced into the reduction zone to reduce the iron ore charge. The reducing gas may, for example, comprise or consist essentially of hydrogen, carbon monoxide, natural gas, and mixtures thereof. The reducing gas flow rate may be about 1500 Nm 3 / hour to about 3000Nm 3 / hour, and the inlet temperature can vary between about 550°C and about 1000°C.

[0057] A cooling gas may be circulated in the cooling zone to cool the DRI after reduction and before discharge. Suitable cooling gases include, for example, nitrogen, hydrogen, or a combination thereof if carbon-free DRI is to be produced, or natural gas (optionally diluted) if carbon-containing DRI is to be produced. The cooling gas flow rate may be about 400 Nm 3 / hour to about 1000Nm 3 / hours varies.

[0058] In some cases, no cooling gas circulates in the cooling zone, but the hot DRI is discharged into a separate shaft furnace where it is cooled and optionally carburized using circulating gas. Such a separate shaft furnace arrangement is disclosed in WO2021 / 225500A1, which is hereby incorporated by reference.

[0059] Study 1 - Cold compressive strength of DRI

[0060] Cold compression strength (CCS) is a measure of the compressive load required to cause the pellet to break. Such compressive loads may occur, for example, during handling, transportation, or storage. Determine the cold compressive strength of some DRI samples produced in a DRI pilot plant under various conditions using natural gas or hydrogen as the reducing gas. The average CCS is determined by measuring 60 pellets of each sample according to the method of ISO4700:2015 "Iron ore pellets for blast furnace and direct reduction feedstocks - Determination of the crushing strength". The results are shown in Table 1 and Figure 1 in.

[0061] Table 1

[0062]

[0063] NG = natural gas; H2 = hydrogen

[0064] The DRI of Examples 1, 2, 4, 5, and 14 was produced in single - shaft furnace operation, whereby cooling gas was provided to the cooling zone of the DRI shaft furnace. The DRI of all other examples was produced in double - shaft furnace operation, whereby cooling and optional carburization were carried out in a separate shaft furnace. Examples 4 and 13 were cooled in natural gas and thus contain carbon. All other examples were cooled in a non - carburizing cooling gas such as nitrogen or hydrogen.

[0065] From the CCS values ​​obtained, the following conclusions can be drawn. Higher CCS values ​​can be obtained by reduction in hydrogen compared to reduction in natural gas. Comparison of Examples 1 and 2 shows that higher carbon levels in DRI have a seemingly negative effect on CCS when reduced in natural gas. Compared with the pilot production examples, the DRI samples purchased for reference in industrial production have higher carbon and even lower CCS (85daN). However, as shown by the comparison of Examples 4, 5 and 13, when the pellets are reduced in hydrogen and then carburized during cooling, the carbon level in DRI seems to have almost no effect at the levels tested in the examples. Comparison of Examples 13 and 14 shows that the selection of single shaft furnace operation or double shaft furnace operation does not seem to affect the CCS value. Although not shown in this example, it was observed in other experiments that the source and characteristics of the iron ore also have an effect on the CCS of the DRI obtained. Finally, the results show that a higher metallization rate increases the CCS value of hydrogen-reduced DRI.

[0066] Looking more closely at the effect of metallization rate on CCS, Figure 1 A graph of metallization values, CCS values, and carbon content (×10) for various examples is shown. Line 101 marks the metallization value of 97%. Line 103 marks the average CCS value of 160 daN. It can be seen that for metallizations greater than about 97%, high average CCS values ​​are obtained, which are greater than about 160 daN in this example. Conversely, metallizations below about 97% produce lower average CCS, which are less than about 160 daN in this example. The metallization rate is related to the reduction temperature to a certain extent, and higher reduction temperatures generally produce higher metallization rates, but, as demonstrated by Example 7, high reduction temperatures do not guarantee high metallization rates.

[0067] Study 2 - Mining Research

[0068] To investigate the characteristics of the pellets / DRI at various points through the direct reduction shaft, excavation was performed. This involved running the DR shaft for a determined period of time at a selected steady state, then quenching the shaft to stop the reduction, and subsequently excavating to remove samples at various depths within the shaft.

[0069] The selected operating point before quenching used hydrogen as the reducing gas, with a reducing gas temperature of the reactor of 935°C, a system pressure of 3 barg and a pressure of 2450 Nm 3 / hour of reducing gas flow. DRI is cooled in the cooling zone at 775 Nm 3 / hour nitrogen flow rate. The operating point before quenching has a stabilization period of about 28 hours. The key quality parameters of the DRI obtained at the reactor outlet are shown in Table 2 below (as determined by XRD). It can be seen that in the pilot direct reduction shaft furnace as described under "Apparatus", H2-based direct reduction under these specified conditions allows the production of DRI with very high total Fe and metallization. Such DRI has favorable mechanical properties and aging properties as disclosed herein.

[0070] Table 2

[0071]

[0072] The quenching was carried out using nitrogen. After the reactor was quenched and cooled, excavation was carried out. Excavation mainly consisted of descending into the shaft furnace to take samples in both radial and vertical directions along the shaft furnace. The target layer thickness in the reduction zone was set to 150 mm, the layers in the isobaric zone and the cooling zone were thicker. 13 layer samples were taken for each layer. Each layer sample weighed about 1200 g. The composition and cold compressive strength (CCS) of the pellets in each layer sample were determined.

[0073] It was found that with iron ore from hematite (Fe 2 O 3 ) through magnetite (Fe 3 O 4 ) is gradually reduced to pyrite (FeO), and the compressive strength of the pellets decreases, reaching a minimum of about 85 dN at a depth of 3m to 3.5m into the DR shaft furnace. It was found that after most of the oxides were reduced, somewhere near the transition zone between the reduction zone and the isobaric zone, the strength recovered to about 150 dN to 170 dN. This shows that the oxides magnetite and pyrite play a core role in determining the compressive strength of DRI pellets.

[0074] For comparison purposes, a similar quenching and tapping was performed for a natural gas (NG) based direct reduction. The reduction gas flow rate was 2500 Nm 3 / hour, and the reducing gas temperature was 1080 ° C, but the process parameters were similar in other respects. The key 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 higher end of the typical range of industrial DRI obtained by fossil-based processes, and there are still relatively large amounts of residual oxides, especially wüstite.

[0075] Table 3

[0076]

[0077] After quenching and excavation, the excavated samples were subjected to CCS and composition analysis. It was found that when a natural gas based reducing gas was used, the drop in compressive strength was more pronounced in the reduction zone, reaching a minimum of about 70 dN at a depth of 1m to 1.5m. Although the strength appeared to recover subsequently with increased reduction, reaching about 120 daN to 150 daN in the isobaric zone, it can be seen that the reduction never proceeded fully to completion, and even the pellets in the isobaric zone showed significant residual amounts of oxide magnetite and wüstite. This agrees well with the quality of the DRI obtained before quenching.

[0078] Figure 2a and Figure 2b The microstructures of H2-reduced (2a) and NG-reduced (2b) pellets are shown. It can be seen that the H2-reduced pellets contain very little residual oxide, and any residual oxide is mainly located between the grains. However, the NG-reduced pellets still contain a large amount of wüstler located inside each grain, indicating that the NG-based reducing gas has difficulty penetrating into the center of the grain.

[0079] In summary, the excavation experiments showed that the presence of oxides such as wüstite and magnetite is detrimental to the compressive strength of DRI, and that reduction in a carbonaceous reducing gas may exacerbate the detrimental effects of these oxides. Furthermore, the experiments demonstrated that DRI with very high metallizations and virtually no residual oxides can be obtained by using hydrogen as the reducing gas, while comparative experiments using natural gas as the reducing gas resulted in DRI with more typical values ​​of metallization and residual oxides.

[0080] Study 3 - Drum Test

[0081] In order to further study the effect of reducing gas composition and temperature on the mechanical properties of DRI, the tumble index and anti-wear index of some DRI samples were obtained. The tumble index provides an indication of the sensitivity of DRI pellets to breakage due to wear during handling and transportation. The tumble index and anti-wear index of the tested DRI and iron ore pellets were determined using the method of ISO 3271:2015 "Iron ores for blast furnace and direct reduction feedstocks-Determination of the tumble and abrasion indices".

[0082] Figure 3The results of these tests are shown, as well as the metallization and carbon content of the individual test samples. The exact metallization and carbon content of sample A (industrial reference), sample C (NG excavation) and sample D (H2 excavation) are unknown. The natural gas-based DRI is presented on the left. The lowest value of 90.4%>6,3 mm after drumming (TTH) is for sample A, which is a purchased industrial DRI reference (exact composition unknown) produced using fossil-based direct reduction and having a fairly high degree of metallization and carbon content. This value can be compared with a pilot-produced fossil-based reference sample B with a TTH value of 95.3% after drumming. From the excavation carried out after furnace age K2, an incompletely reduced NG-DRI sample (sample C) was also tested as a comparison. It can be seen that a lower degree of metallization seems to be associated with a lower drum index.

[0083] For the hydrogen-reduced DRI presented on the right side of the figure, an excavated sample with incompletely reduced H2-DRI is included (sample D). This result is lower than the remaining hydrogen-reduced samples (samples E to L) with metallization rates exceeding 98% and produced using different process conditions. Such different process conditions include, in particular, reducing gas temperatures and carbon contents (carbon-free or post-carburized with natural gas to a carbon content of 1% C) varying between 800°C and 900°C. The results from the drums of highly metallized hydrogen-reduced DRI are drum indices TTH of 98% to 99% TTH in all cases. By comparison, these values ​​are far superior to those obtained from natural gas-reduced DRI (samples A to C) and are even superior to iron ore pellets for direct reduction.

[0084] Also shown is the Anti-Abrasion Index (ATH), representing the percentage of samples less than 0.5 mm after tumbling. It can generally be said that the Anti-Abrasion Index is negatively correlated to the Tumbling Index.

[0085] As can be seen from the drum testing, the hydrogen reduced DRI has significantly improved mechanical properties compared to the natural gas reduced DRI reference. The H2-DRI achieved excellent mechanical properties over a range of reduction temperatures and was independent of whether the DRI was subsequently carburized. However, the incompletely reduced H2-DRI samples from the excavation study were found to have poor drum indices compared to the fully reduced H2-DRI samples with metallization greater than or equal to 98%.

[0086] Study 4 - Aging Study

[0087] In order to investigate the reactivity and reoxidation of the produced DRI batches during storage, a number of aging studies were performed both under ambient conditions as well as under conditions where accelerated aging is expected.

[0088] Batch Aging Studies

[0089] By filling large bags (volume about 1m 3 ), and then aging studies were performed on various DRI batches by storing the bags under protection at ambient temperature. The batches studied were:

[0090] NG-reduced DRI (metallization rate 87.6%);

[0091] Highly metallized H2-reduced DRI (metallization 99%, reduction temperature 900°C); and

[0092] Carburized highly metallized H2 reduced DRI (metallization 99%, C 1.4%, reduction temperature 900°C).

[0093] The change in composition and metallization is determined by periodically sampling some of the pellets from each bag and analyzing the pellets using XRD and LECO elemental analysis. The average change in composition and metallization over each time period can then be determined.

[0094] It was found that NG-reduced DRI exhibited a fairly rapid loss of metallization, losing about 1.6% of metallization within the first four weeks (28 days) of storage. However, after an additional 4 weeks of storage (56 days in total), no further significant decrease in metallization was observed.

[0095] The highly metallized H2-reduced DRI was not found to have any detectable decrease in metallization rate, even after extended storage for a total of 180 days. This was true whether the DRI was carbon-free (cooled in nitrogen) or carburized (cooled in natural gas).

[0096] Accelerated aging in water

[0097] To further investigate the effects of metallization and carbon content on the aging characteristics of hydrogen-reduced DRI, some H2-reduced DRI were subjected to accelerated aging testing in water. The batches tested were highly metallized H2-reduced DRI (both carbon-free and carburized) as described in the aging experiments above, and another batch:

[0098] Moderately metallized H2-reduced DRI (metallization ratio about 96%, reduction temperature 800°C).

[0099] About 200 g of each DRI was placed in a separate bucket, which was then filled with water until the DRI was completely covered. Samples were taken after 3 days, 2 weeks and 4 weeks and analyzed by XRD and LECO elemental analysis as previously described. Before the water-wet samples could be prepared for analysis, they were dried at 105°C for 24 hours.

[0100] It was found that after 28 days, each of the highly metallized H2-DRIs lost about 1% to 1.5% in metallization. This was true whether the DRI was carburized or carbon-free. The moderately metallized H2-DRI showed an even greater reduction in metallization after 28 days, about 2% to 4%.

[0101] Single Pellet Aging Study

[0102] To further investigate the effect of metallization rate on aging, single pellet studies were conducted on carbon-free (nitrogen cooled) highly metallized and moderately metallized pellets. The individual pellets were stored indoors or outdoors to prevent settling. At regular intervals, the pellets were weighed using a high precision scale. The total test period was approximately 1 month. It was assumed that all weight gain was due to reoxidation of the iron. The results are shown in Figure 4 middle.

[0103] The highly metallized H2-DRI was found to have very little tendency to gain weight over the test period, whether it was stored indoors or outdoors. The moderately metallized H2-DRI stored outdoors showed a linear gain in weight throughout the test period, resulting in a total weight gain of about 0.4% to 0.5% at the end of the test period. The moderately metallized H2-DRI stored indoors was found to gain in weight relatively quickly by about 1.2% (after about 1 week), but did not gain any further weight after this initial gain.

[0104] In summary, hydrogen-reduced DRI was found to age more slowly than natural gas-reduced DRI. For hydrogen-reduced DRI, increases in metallization were found to cause less rapid aging in both ambient and accelerated (water) testing. The carbon content of the hydrogen-reduced DRI was not found to have any significant effect on aging, at least for the highly metallized H2-DRI tested.

[0105] Study 5 - Porosity and surface area measurements

[0106] The porosity and BET surface area of ​​some Examples from Study 1, along with some other Examples, were determined.

[0107] The porosity of the tested DRI 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-Part 1: Mercury porosimetry". The mercury temperature was 20.0°C, and the pressure range was 0.10 psia to 61,000.00 psia.

[0108] 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—BET method.” Krypton at an analysis bath temperature of 77 K was used in the BET surface area determination.

[0109] At least two pellets were tested for each example and the values ​​shown are the average of all pellets for each example. A compilation of the results is shown in Table 4.

[0110] Table 4

[0111]

[0112] The examples marked Ln were obtained by laboratory scale reduction of iron ore pellets in a stream of relevant reducing gas heated to the relevant temperature. The iron ore pellets used were of the same type as used in the pilot scale studies. Clear differences can be observed between the properties of the laboratory produced DRI produced by the batch process and the pilot scale DRI obtained by the large scale continuous process in a pressurized shaft furnace. The pilot scale DRI typically has a lower BET surface area and a lower porosity than the laboratory produced DRI, typically a porosity of ≤ 60% and a BET surface area of ​​≤ 0.5 m 2 / g. In contrast, laboratory-produced DRI typically has a porosity of >60% and a void volume of >0.6 m 2 / g of BET surface area.

[0113] No significant differences were found between NG-reduced DRI and H2-reduced DRI regarding % porosity and BET surface area.

[0114] However, in subsequent testing, the median pore size and total pore area of ​​NG-reduced DRI samples and H2-reduced DRI samples were also determined by the method of ISO 15901-1: 2016. The samples tested were the same or similar to the industrial-scale samples listed in Table 4 above.

[0115] The NG-reduced DRI samples tested were found to have a median pore size ranging from 1.1 μm to 1.4 μm. The H2-reduced samples had a median pore size ranging from 1.5 μm to 4.0 μm. It was found that improved strength and aging were associated with larger median pore sizes. That is, the median pore size of the H2-reduced samples with excellent mechanical properties and aging characteristics was greater than or equal to 2.0 μm, preferably greater than 2.5 μm. It was found that such a difference in median pore size was independent of whether the H2-reduced DRI was carburized. That is, the H2-reduced DRI that was subsequently carburized had a significantly larger median pore size than the NG-reduced DRI, and could be distinguished from the NG-reduced DRI in this way. It was also found that the H2-reduced samples with excellent mechanical properties and aging characteristics had a median pore size less than or equal to 0.5 μm. 2 / g total pore area.

[0116] Summary of experimental studies

[0117] Therefore, in summary, it is found that the highly metallized hydrogen-reduced DRI produced on a pilot scale has excellent mechanical properties and aging characteristics compared to conventional natural gas-reduced DRI and compared to hydrogen-reduced DRI with a lower metallization rate. The pilot-scale DRI can be distinguished from the laboratory-scale DRI by porosity and BET surface area. The pilot-scale highly metallized hydrogen-reduced DRI is mainly distinguished from the pilot-scale natural gas-reduced DRI by its high metallization rate and the corresponding lack of oxide magnetite and wüstite. The pilot-scale highly metallized hydrogen-reduced DRI is also distinguished from the pilot-scale natural gas-reduced DRI by its relatively large median pore size (≥1.5, preferably ≥2.5). If the pilot-scale highly metallized hydrogen-reduced DRI is not carburized, it can also be easily distinguished from the pilot-scale natural gas-reduced DRI due to its lack of carbon.

[0118] Those skilled in the art recognize that the present disclosure is not limited to the preferred embodiments described above. Those skilled in the art also recognize that modifications and variations are possible within the scope of the appended claims. For example, the skilled person understands that, under appropriate conditions, it may be possible to produce DRI pellets having specific combinations of metallization rate, carbon content (or lack thereof), median pore size, porosity, and BET surface area that are not specifically disclosed in the examples. In addition, the skilled person understands that the favorable results obtained herein may be obtained using iron ore pellets that are more suitable than the iron ore pellets and DR shaft furnaces specifically disclosed herein and using DR shaft furnaces of more appropriate sizes. In addition, the skilled person, in practicing the claimed disclosure, can understand and implement changes to the disclosed embodiments from a study of the drawings, the disclosure, and the appended claims.

Claims

1. A direct reduced iron (DRI) pellet, wherein the average metallization rate of the DRI pellet is greater than or equal to 97%, in: - the DRI pellets contain substantially no carbon; or - the DRI pellets contain less than or equal to 2 wt% carbon; and wherein said DRI pellets are further characterized in that they: (i) having a median pore size greater than or equal to 1.5 μm; and / or (ii) less than or equal to 0.5 m 2 Average BET surface area of ​​ / g; and / or (iii) having an average porosity of less than or equal to 60%.

2. The DRI pellets according to claim 1, wherein the median pore size of the DRI pellets is greater than or equal to 2.0 μm, preferably greater than or equal to 2.5 μm.

3. The DRI pellets according to any one of the preceding claims, having an average BET surface area of ​​less than or equal to 0.4 m 2 / g.

4. The DRI pellets according to any one of the preceding claims, having an average porosity of less than or equal to 58%.

5. The DRI pellets according to any one of the preceding claims, wherein the average total iron content of the DRI pellets is greater than or equal to 94 wt%.

6. The DRI pellets according to any one of the preceding claims, wherein the DRI pellets comprise an average of less than or equal to 3 wt% FeO.

7. The DRI pellets according to any one of the preceding claims, wherein the DRI pellets contain an average of less than or equal to 0.5 wt % Fe 3 O 4 .

8. DRI pellets according to any one of the preceding claims, obtainable by direct reduction in a countercurrent direct reduction shaft furnace in a reducing gas consisting essentially of hydrogen and optionally steam and inert gases.

9. The DRI pellets according to claim 8, wherein the temperature of the reducing gas at the reducing gas inlet of the direct reduction shaft furnace is greater than or equal to 750°C.

10. The DRI pellets according to any one of claims 8 to 9, wherein the DRI pellets contain less than or equal to 2 wt% of carbon, and wherein the DRI pellets are obtainable by carburizing in a carburizing gas after direct reduction.

11. The DRI pellets according to claim 10, wherein the DRI pellets are obtainable by carburizing in a carburizing gas selected from the group consisting of methane, ethane, propane, butane, carbon monoxide, hydrogen, nitrogen and combinations thereof, provided that the carburizing gas contains at least 5 volume % of carbonaceous components.

12. The DRI pellets according to any one of the preceding claims, having an average cold compressive strength greater than 160 daN as measured by ISO 4700:2015 method.

13. The DRI pellets according to any one of the preceding claims, having a tumble index greater than or equal to 96% as measured by ISO 3271:2015 method.

14. Use of DRI pellets according to any one of claims 1 to 13 as a raw material in a melting furnace for producing steel.

15. The use according to claim 14, wherein the DRI pellets are not briquette before use in the smelting furnace.

16. Use according to any one of claims 14 to 15, wherein the smelting furnace is located at a distance of at least 100 km from the production site of the DRI pellets.

17. Use according to any one of claims 14 to 16, wherein the DRI pellets are stored for a duration of at least 30 days before being fed to the smelting furnace.

Images