Method for obtaining dust with high iron content from steelworks dust and sludge, and dust obtained

Abstract

The present invention relates to a method for obtaining dust with a high iron content (90% or more), preferably with a pure ferrite composition, from steelworks dust and sludge, the obtained dust being suitable for industrial and environmental applications. The invention also includes a method for obtaining sintered parts / components from the dust with a high iron content, wherein the sintered parts / components are porous, highly ductile and highly plastic.

Description

[0001] METHOD FOR OBTAINING HIGH IRON CONTENT POWDERS FROM STEELWORKS POWDERS AND SLUDGES, AND POWDERS OBTAINED FIELD OF APPLICATION

[0002] The present invention relates to the technical field of extractive metallurgy and powder metallurgy, specifically it refers to a method of purifying and reducing iron oxides from steelmaking powders and sludge from steelmaking processes.

[0003] BACKGROUND

[0004] Modern iron manufacturing processes for the steel industry generate waste that can be found in various formats, and is considered a byproduct of the production process due to its relatively high percentage of iron oxides (25 to 40%), which have been reduced and recovered up to 93.4% (J. Xu, N. Wang, M. Chen, Z. Zhou, and H. Yu, “Comparative investigation on the reduction behavior of blast furnace dust particles during in-flight process in hydrogen-rich and carbon monoxide atmospheres”, Powder Technol, vol. 366, pp. 709-721, Apr. 2020). Among these wastes, those from electric arc furnaces (JGMS Machado, FA Brehm, CAM Moraes, CA dos Santos, ACF Vilela, and JBM da Cunha, “Chemical, physical, structural and morphological characterization of the electric arc furnace dust”, J Hazard Mater, vol. 136, no. 3, pp. 953-960, Aug. 2006) and basic oxygen converters (AKP Singh, MT) can be highlighted.Raju, y U. Jha, “Recycling of Basic Oxygen Furnace (BOF) sludge in iron and steel works”, 2011, E. Siedlecka, “Comprehensive use of products generated during acid leaching of basic oxygen furnace sludge”, J Clean Prod, vol. 264, ago. 2020) y de alto horno (S. Wu, F. Chang, J. Zhang, y H. Lu, “Kinetics and reduction behavior of self-reducing briquettes containing blast furnace dust”, en Minerals, Metals and Materials Series, Springer International Publishing, 2017, pp.

[0005] 591-601; V. V. Tonica, G. Cárác, y V. G. Teodor, “Characterization of blast furnace dust for reintroduction into the agglomeration process”, en E3S Web of Conferences, EDP Sciences, ene 2022).

[0006] Blast furnace dust (BFT) is a residue that can become a pollutant in production processes if left undisturbed, or increase the amount of particulate matter emitted during steel production. It has a composition rich in iron (15-40% w / w) and carbon (20-40% w / w), and its particle size (between 50-200 µm) is suitable for use in sintering processes. Therefore, it is considered a byproduct that can be recovered through recycling, such as into BFT briquettes (S. Wu, F. Chang, J. Zhang, and H. Lu, “Kinetics and reduction behavior of self-reducing briquettes containing blast furnace dust”, in Minerals, Metals and Materials Series, Springer International Publishing, 2017, pp. 591-601), for reincorporation into the agglomeration process in steelmaking (V.V. Tonina, G. Cárác, and VGTeodor, “Characterization of blast furnace dust for reintroduction into the agglomeration process”, en E3S Web of Conferences, EDP Sciences, ene 2022) y como reemplazo para el polvo de coque en procesos de sinterización (C. Lanzerstorfer, B. Bamberger-Strassmayr, y K. Pilz, “Recycling of blast furnace dust in the iron ore sintering process: Investigation of coke breeze substitution and the influence on offgas emissions”, ISIJ International, vol. 55, no 4, pp. 758-764, 2015).

[0007] The iron contained in PAH consists mainly of metallic iron, iron oxides, and intermetallic phases with iron, depending on several factors such as the iron ore extraction zone, the steelmaking process implemented, and the quality of the ore used, among others (JMF Clout and JR Manuel, “Mineralogical, chemical, and physical characteristics of iron ore”, in Iron Ore: Mineralogy, Processing and Environmental Sustainability, Elsevier Inc., 2015, pp. 45-84). PAH also contains, as impurities, Ca (1.40-5.70% w / w), Zn (0.10-0.50% w / w), Pb (0.02-0.07% w / w), Na (0.02-0.47% w / w), K (0.20-0.80% w / w), and Stotai (0.20-1.30% w / w), among other metallic components.

[0008] PCP powder (Conox Converter powder) contains a range of 40 to 70% total iron (Fe), around 20% C, and low contents of oxides of Si (0.71%), Ca (8.90%), Mg (0.38%), Al (0.32%), Zn, along with other components that vary according to the purity of the iron ore used, with total iron representing 64.12% of the total composition of a PCP sample (B. Das, S. Prakash, PSR Reddy, VN Misra, An overview of utilization of slag and sludge from steel industries, Resour Conserv Recycl 50 (2007)).

[0009] In this way, integrated steel mills produce pig iron from basic raw materials (iron ore, coke, and limestone) in a blast furnace. This pig iron is then refined and transformed into steel in oxygen converters (CONOX), guaranteeing products such as wire rod and plates of high purity and controlled quality. During production, exhaust gases are generated that carry metallic dust. The formation of this dust is attributed to the vaporization of iron and other metals present, the ejection of small droplets of molten pig iron, and the blasting of fine particles. The disposal of this dust is limited by cost, space, and environmental regulations. Therefore, integrated steel mills face the challenge of managing this waste and are exploring the use of direct reduction processes with green hydrogen and carbon monoxide to decrease greenhouse gas emissions and improve the sustainability of the process.

[0010] In particular, the steel industry faces a critical challenge in managing the waste generated during steel production, especially steelmaking dust and sludge from blast furnaces and basic oxygen converters (BOFs). This waste, composed of fine particles, is produced in considerable quantities, ranging from 5 to 15 kilograms per ton of pig iron produced. According to data from the World Steel Association, approximately 21 million tons of steelmaking dust were generated globally from BOF converters in 2016. By 2021, with global pig iron production of around 2 billion tons, the generation of this dust reached 28 million tons annually. The high volume of this waste leads to its accumulation and the formation of large deposits, posing a significant challenge for steel companies.Currently, many companies choose to store these powders on their own or third-party land, a practice facing increasing limitations due to associated costs, a scarcity of available space, and growing environmental regulations. In this context, the reuse and valorization of these wastes has become highly relevant, as adding value to them not only helps mitigate environmental impact but can also generate economic benefits for steel companies.

[0011] Furthermore, steel production generates a high emission of combustion gases released into the atmosphere, rich in carbon monoxide (CO) and carbon dioxide (CO2), which contribute significantly to environmental pollution and climate change. Approximately 7% of total global CO2 emissions are attributed to this industry, with an average of 1.9 tons of CO2 per ton of crude steel produced.

[0012] On the other hand, it is known that sintering is a process in which a set of particles, or particles simply contained in a vessel, are compacted under pressure, chemically bonding them together to form a coherent body under the influence of high temperature. In compaction, the powders deposited within a die at room temperature receive an axial press-type load, usually exerted by a punch. A die and punch assembly is designed for each type of part to be manufactured. Temperature, time, and sintering atmosphere are the most relevant factors for the result. The temperature is set below the melting point of the material. The time depends on the degree of sintering achieved by the part. The atmosphere is such that the gases do not diffuse into the material, thus preventing the formation of alloys, and are also not confined within the porous structure.

[0013] En el estudio de A. Abdallah, M. Habibnejad-Korayem, y D. V. Malakhov (Are large particles of iron detrimental to properties of powder metallurgy steels?”, Metals (Basel), vol.

[0014] (10, no. 4, Apr. 2020), concluded that the presence of pearly islands found inside the test specimens was attributable to a lack of uniformity in the size of the constituent grains, and not to the formation of metallic phases resulting from the melting process. Thus, the aforementioned study uses powders from different suppliers with a size between 10 and 400 µm, and also determined that flowability is better the smaller the particles of the powder used, and that the tensile strength exhibits this same behavior. Furthermore, parameters such as those presented by the University of the Basque Country (University of the Basque Country, “Mechanical Technologies: Sintering”, Bilbao) indicate that the metallic powders used in sintering processes range from 10 to 400 µm, with the most common being approximately 100 µm in size.

[0015] There are some alternative solutions for managing steelmaking waste, although with certain limitations:

[0016] - Landfill disposal or storage: This is a common practice, but it faces increasing restrictions due to the costs associated with leasing or acquiring land, transporting, and handling the waste. Furthermore, the availability of space for this purpose is becoming increasingly limited, and environmental regulations are becoming stricter. Storing waste fails to utilize its potential as a raw material, does not mitigate CO2 emissions, and has a negative environmental impact. - Direct recycling in electric arc furnaces: Under this approach, steelmaking dust is loaded directly into electric arc furnaces along with scrap metal and other charge materials. These furnaces operate at temperatures around 1600°C and use graphite electrodes to generate an electric arc that melts the metal charge.These practices allow for the recycling of a significant portion of steelmaking dust, reducing the need for final disposal. They decrease the consumption of virgin raw materials by partially replacing them with steelmaking dust and lower the costs of acquiring raw materials for steel production. However, the amount of dust that can be recycled is limited by its chemical composition and the presence of unwanted impurities in the steel. Furthermore, these practices do not directly address the CO2 emissions associated with steel production.

[0017] - Stabilization / solidification: The powders are mixed with binders and solidified for final disposal. This solution does not utilize the iron content and does not address CO2 emissions.

[0018] In particular, document CN112280973 A (Baosteel Zhanjiang Iron and Steel Co Ltd) refers to a method for cooperatively treating emulsified oil sludges by means of a sintering system, comprising the following steps: uniformly mixing: emulsified oil sludges, water treatment sludges and iron manufacturing dust removal ash according to the mass ratio of 1: (2-4): (5-7) and uniformly mixing to obtain powder and sludge; and then, secondary mixing by uniformly mixing the obtained powder and sludge with coarse OG grains and smaller than pellet size powder according to the mass ratio of (1-3): (2-4): (3-7) and uniformly mixing a second time;Then, sintering and mixing are carried out: the material subjected to secondary mixing is transported to a mixer, mixed with the sintering raw material in proportion to obtain a blended material, granulated using a disc, and then fed into a sintering machine for absorption, where the mass percentage of the material in the blended material is 0.5-1%. The method depends on the existing sintering machine and the facilities of the steel companies, and it consumes the emulsified oil sludge and recycles iron-containing resources.

[0019] Document W02005017216A3 (K & K Tecnologías Ind SA, Yamamoto Mauro Fumio) refers to a process for recycling blast furnace sludge, or dust from dry dust collectors, or fine and coarse sludge from steel mills, or industrial or metallurgical waste by combining the process stages of mineral pulping, ultrasonic treatment, conditioning, gravimetric concentration, hydrochloric acidation, magnetic separation, flotation, and drying for the recovery of carbon, which may be in the form of coke, mineral coal, or charcoal, and metallic iron (Fe), iron oxides (Fe x EITHER y ), silica (SiO2), calcium oxide (CaO) and zinc oxide (ZnO).

[0020] For its part, patent US8540798B2 (Hephaestus Metals Projetos E Equipamentos Ltda) refers to a system and method for processing sludge from a flue gas scrubbing system that cleans the fumes from a steelmaking converter in a way that separates / isolates a significant portion of the metallic iron particles in the sludge and prepares them for convenient handling. The system includes separation equipment that isolates the metallic iron particles in the sludge and forming equipment that forms the isolated particles into briquettes with relatively high mechanical strength, allowing the briquettes to maintain their integrity during handling and storage. The briquettes with a high metallic iron content can be recycled back into the steelmaking process, for example, as charge material for a basic oxygen converter or an electric arc furnace.The water used in the system can be recycled and reused within the system, making the system environmentally friendly.

[0021] BRIEF DESCRIPTION OF THE INVENTION

[0022] The present invention addresses jointly the challenges of waste management and CO2 emissions mitigation in the steel industry, and relates to a method of conditioning, purification and thermochemical reduction of iron oxides present in steelmaking waste dust, thus forming a comprehensive strategy aligned with the principles of circular economy and decarbonization that are currently demanded.

[0023] Given that these wastes contain high levels of iron oxides and carbon, this method harnesses the reducing potential of various gaseous and solid reducing agents, such as carbon monoxide (CO), hydrogen (EL), Ephi / Ar mixtures, and even the carbon present in the powders themselves, to transform the iron oxides into high-purity metallic iron, recovering and recycling a valuable raw material. In this way, by utilizing waste and combustion gases as resources, it reduces the environmental footprint and contributes to global efforts to mitigate climate change and the transition to a more sustainable and environmentally friendly industry. It also allows for the valorization of steelmaking waste.

[0024] The method of the present invention also advantageously uses weak and low-cost acids, such as acetic acid, for selective dissolution instead of strong acids such as HCl. Furthermore, the process generates a supernatant rich in water-soluble calcium and magnesium acetate salts, allowing for their recovery and proper disposal; thus offering the principles of a circular economy by valorizing and recycling waste and contributing to mitigating environmental impact by reducing the need for final waste disposal and utilizing combustion gases; enabling the recovery of a valuable raw material (metallic iron) from waste and transforming it into a higher value-added product (iron powder).

[0025] The present invention relates specifically to a method comprising the purification and reduction of iron oxide particles with H2 gas to generate an iron powder having internal porosity and an iron content of 90% or higher, said iron powder having market value. This iron powder is useful in various products originating via powder metallurgy, among them, the manufacture of sintered components or metal matrix components with lower weight and improved wear resistance; friction materials, such as brake pads, magnetic materials, as well as other non-metallurgy applications, such as a chemical reagent due to its high surface area, a water purification element, soil purification agents, among others.

[0026] BRIEF DESCRIPTION OF THE FIGURES

[0027] Figure 1 shows the variation in chemical composition with the washing processes. Figure 1A and 1C, calcium. Figure 1B and 1D, magnesium.

[0028] Figure 2. Shows the metallic iron particles obtained with the reduction process using an FF / Ar mixture.

[0029] Figure 3. Scanning electron microscopy images with secondary electrons (SEM-SE, Figure 3A), backscattered electrons (SEM-BSE, Figure 3B) for the initial PAH and PCC powders.

[0030] Figure 4. Diffractograms of PAH powders (initial and conditioned, Figure 4A) and PCC powders (initial and conditioned, Figure 4B).

[0031] Figure 5. Precipitate from the neutralization process. Figure 5A: PAH precipitate. Figure 5B: PCC precipitate.

[0032] Figure 6. Scanning electron microscopy images with secondary electrons (SEM-SE, left) and backscattered electrons (SEM-BSE, center) and energy dispersive spectroscopy (EDS, right) for the precipitates of the conditioned PAH and PCC powders.

[0033] Figure 7. Energy dispersive (EDS) spectrum for the precipitates of the conditioned PAH (Figure 7A) and PCC (Figure 7B) powders.

[0034] Figure 8. Percentage composition of powders by SEM-EDS for the different thermochemical treatments with hydrogen. Fe (Figure 8A), O (Figure 8B), Si (Figure 8C), Al (Figure 8D), Mg (Figure 8E), Ca (Figure 8F). Figure 9. PCC image reduced x500; Figure 9A, BSE image, Figure 9B, SE image, Figure 9C, SEM-EDS mapping. A, B, C correspond to cycle 1, P900°C-60min; D, E, F correspond to cycle 2, P1000°C-60min; G, H, I correspond to cycle 3, P900°C-120min; J, K, L correspond to cycle 4, P1000°C-120min; and M, N, Ñ correspond to cycle 5, P950°C-90min.

[0035] Figure 10. SEM-SE reduced PCC with xlOOO magnification. 10A, cycle 1, P900°C-60min; 10B, cycle 2, P1000°C-60min; 10C, cycle 3, P900°C-120min; 10D cycle 4, P1000°C-120min; 10E, cycle 5, P950°C-90min.

[0036] Figure 11. Diffractogram of the reduced powders for each of the thermochemical cycles in an Fb / Ar atmosphere.

[0037] Figure 12. Figure 12A, Reduced PCC, next to a green specimen intended for hardness testing. Figure 12B, Green specimens, intended for compression testing.

[0038] Figure 13. Microstructure with x100 magnification; Figure 13A: polished surface and Figure 13B: chemically etched surface.

[0039] Figure 14. Microstructure at x1000 magnification; Figure 14A, polished surface, and Figure 14B, chemically etched surface.

[0040] Figure 15. Indented microstructural surfaces at x100 magnification; Figure 15A, HV5 test; Figure 15B, HV10 test.

[0041] Figure 16. Stress-strain curves in compression. Figure 16A, curves of the 3 tests. Figures 16B to 16D, determination of the yield strength of each test.

[0042] Figure 17. Sintered specimens, prior to compression test (Figure 17A) and once subjected to the compression test, failure with a barrel shape (Figure 17B).

[0043] DETAILED DESCRIPTION OF THE INVENTION

[0044] The present invention relates to a treatment method for the recovery and valorization of steelmaking dust and sludge, where said steelmaking dust and sludge preferably originate from two main sources: the blast furnace (PAH) and the basic oxygen converter (BOF or Conox, PCC), and have a complex composition that varies according to the production process and the raw materials used, and which basically comprise a composition rich in iron oxides in the form of hematite (Fe2O3) and magnetite (FesCh), and also contain a mixture of metallic oxides, carbon and other impurities (Ca, Zn, Pb, Na, K, S). The present treatment method for the recovery and valorization of steelmaking dust and sludge as high-purity iron powder comprises the following steps: a) purification of said steelmaking dust and sludge, preferably from the blast furnace (PAH), the basic oxygen converter (BOF or Conox, PCC) or both,by means of selective chemical dissolution processes with a weak carboxylic acid, preferably a weak carboxylic acid selected from acetic acid, citric acid, tartaric acid, oxalic acid, lactic acid, formic acid, malic acid, benzoic acid, phosphoric acid or a mixture of one or more of the same, at a concentration between 1-70% w / v, preferably between 30-50% w / v and even more preferably 33% w / v, to dissolve metal oxides other than iron oxide that are present in steelmaking dust or slurry, including calcium oxide, magnesium oxide, silicon oxide, aluminum oxide, zinc oxide, among others, wherein the dissolution is carried out in a closed reactor with side arm and heating jacket, with stirring and temperature controlled in the range of 30-90°C, preferably 70°C, the ratio (w / v, g / ml) of steelmaking dust or slurry to weak acid being approx. 1-10 to 10-50; and,

[0045] b) thermochemical reduction, according to the carbon content of the steelmaking dust and slurries resulting from step a), to transform the iron oxides into metallic iron by heating to a temperature between 700-1200°C, preferably 900-1000°C, even more preferably 950°C, and a reducing atmosphere, in a furnace, preferably a tubular furnace, even more preferably a Carbolite Gero tubular furnace, under a reducing atmosphere comprising noble gases, including N2; Ar; He; H2; CO; water vapor (H2O) and mixtures thereof, preferably an H2 / Ar mixture atmosphere, preferably an H2 / Ar mixture of v / v ratio 5:95, then centrifuging, drying and vacuum filtering.

[0046] Steps a) and b) of the method of the invention may be repeated one or more times, separately or sequentially, if required. That is, separately means that step a) is repeated but not step b), or vice versa. Sequentially means that steps a) and b) are repeated in cycles.

[0047] Optionally, prior to stage a), stage a0), of drying and sieving of the steelmaking powders and slurries, can be carried out to ensure a homogeneous distribution of particles and to eliminate the moisture present in them, where the drying is carried out at a temperature of 200°C, and then the dried steelmaking powder and slurries are subjected to grinding and mechanical separation with agitation, by sieving using meshes between 50 and 200 mesh, corresponding to cut sizes between 74 - 297 pm.

[0048] Hydroxides formed during conditioning generally do not present major environmental risks, but they can pose risks to the environment under certain conditions, especially if released in large quantities without adequate control. Therefore, to ensure their safe handling and compliance with environmental regulations, they can be additionally and optionally encapsulated in inert solid matrices, such as cement or similar materials, to form stable blocks or structures that prevent dispersion into the environment (E. Sobiecka, “Investigating the chemical stabilization of hazardous waste material (fly ash) encapsulated in Portland cement,” International Journal of Environmental Science and Technology, vol. 10, no. 6, pp. 1219-1224, Nov. 2013).This stage allows the final disposal of the conditioning stage by-products that have an acidic pH, and through acid-base titration these wastes are neutralized, generating liquid waste with a neutral pH for compliance with regulations, and a solid precipitate enriched to its hydroxide form.

[0049] The present method then optionally comprises, after step a) and separately from step b), the encapsulation of the hydroxides formed and discarded in step a), in selected inert solid matrices of cement or similar materials.

[0050] The calcination process is also an alternative for converting hydroxides into more stable compounds (oxides) and, consequently, safer ones. Once stabilized, the material can be disposed of in landfills or solid waste disposal facilities, in accordance with current regulations.

[0051] The present method then optionally comprises, after step a) and separately from step b), calcining the hydroxides formed and discarded in step a), to transform them into oxides, and then optionally discarding them in landfills or solid waste disposal facilities.

[0052] In the selective dissolution stage, a technique involving the use of a specific chemical agent to selectively attack and dissolve certain components of a mixture without dissolving others, acetic acid (CEECOOH) was used. This was due to its ability to selectively dissolve calcium and magnesium oxides, facilitating the efficient recovery of valuable metals such as iron (Fe) present in steelmaking dust. Iron oxides are present as a precipitate, enriched with iron oxides, which remain in solid form due to their low solubility in acetic acid. Thus, treating steelmaking dust and sludge with weak acids removes calcium and magnesium oxides, consequently increasing purity and enriching the iron content of the dust for subsequent thermochemical reduction treatment, thereby enabling its effective reuse.

[0053] The present invention provides a standardized and reproducible method for conditioning steelmaking powders, which mainly contain hematite (Fe₂O₃) and magnetite (FesCu) as iron sources, in addition to quartz, calcite, and amorphous carbon. For example, chemical etching of iron powders with 33% w / v acetic acid at 70°C for 2 hours is effective in reducing Ca and Mg species in steelmaking powder, PAH, and PCC, generating water-soluble acetate salts. This method reduces the presence of calcium by 90% in both powders and magnesium by approximately 50% in PAH powders and approximately 80% in PCC powders (Figure 1). The presence of quartz in the samples is also reduced.

[0054] Experiments were also conducted varying the concentration of acetic acid (33% and 50% v / v), temperature (30°C and 70°C), and time (2 and 5 h) to confirm the effective removal of 80–100% of unwanted oxides without significantly affecting iron content. After conditioning, the powders were characterized by SEM / EDS and XRD. The oxides present in the selectively solubilized, dry, and sieved powders allow the precipitation of calcium (CaO), aluminum (Al₂O₃), silicon (SiO₂), and magnesium (MgO) oxides without affecting the iron oxides present (hematite, Fe₂C₃; and magnetite, Fe₃O₄). This chemical treatment is carried out while maintaining constant agitation, preferably at 80 rpm, for a predefined time.

[0055] The purification / conditioning stage significantly increases the purity of the conditioned powders, which is crucial for improving the efficiency of iron (Fe) conversion in the subsequent gas reduction heat treatment stage. A reduction of over 90% and 80% was observed for calcium and magnesium, respectively, confirming the effectiveness under conditions of a 33% v / v acetic acid concentration at 70°C for two hours (T3C3-2H treatment, see Table 2). Furthermore, the selective dissolution of calcium and magnesium oxides as a purification strategy for the iron powders (PAH and PCC) offered a significant advantage by generating a supernatant rich in calcium and magnesium acetate salts, which are water-soluble due to the resulting acid-base substitution reaction.These salts were readily precipitated by a neutralization reaction with sodium hydroxide (NaOH), producing a supernatant with a neutral pH and a precipitate rich in calcium and magnesium oxides, confirming the dissolution of these components with a percentage composition of Ca (0.11% and 1.01% for PAH and PCC, respectively) and Mg (2.28% and 0.11% for PAH and PCC, respectively). Furthermore, the presence of other oxides, such as aluminum and silicon, was detected, thus demonstrating the effective purification of the steelmaking dust and highlighting its potential application in industrial processes.

[0056] The reduction stage with hydrogen and / or blast furnace gases with an H2 / Ar mixture of 5:95 and a temperature in the range of 700 to 1200°C, preferably 900-1000°C, even more preferably at 900°C, 950°C and 1000°C, allows obtaining characteristic metal powders (high purity; high iron content, 90% or higher), useful in the manufacture of sintered components.

[0057] The present invention also relates to a method for obtaining a part or component synthesized from the high iron content (90% or greater) powder described above, comprising sintering a test specimen of the high iron content powder described above at a temperature of 1150 °C and obtaining a sintered part or component with a porous, ductile, and highly plastically deformable ferritic matrix, comparable to that obtained from commercial pure iron powder.

[0058] The present invention also relates to an iron powder intermediate or prior to step b) comprising a weight ratio of Fe:Al:Ca:Mg:Si in the range of 70-50:5, 0-2.0:0, 1-2.0:0, 1-1:0, 5-6.0. Preferably comprising a weight ratio of Fe:Al:Ca:Mg:Si of 50:3.3:1.4:0.6:4.9; 50:4.0:0.6:0.3:4.8; 50:2.6:0.5:0.5:5.8; 59:0.4:0.1:0.1:0.6.

[0059] The present invention relates to an iron powder intermediate or prior to step b), comprising a weight ratio of hematite: magnetite: quartz: amorphous forms of 30-35: 10-9: 12-18: 40-45. Preferably, comprising a weight ratio of hematite: magnetite: quartz: amorphous forms of 31.1: 9.5: 16.4: 43.0.

[0060] The present invention relates to an iron powder intermediate or prior to step b), comprising a magnetite to hematite weight ratio of 10-9: 30-35. Preferably, comprising a magnetite to hematite weight ratio of 9.5: 31.1.

[0061] The present invention relates to a high-purity iron powder, having an iron content of 90% or greater. The following examples are intended to illustrate the invention and its preferred embodiments, but in no circumstances should they be considered to restrict the scope of the invention, which will be defined by the terms of the appended claims.

[0062] EXAMPLES

[0063] Example 1: Selective dissolution

[0064] The selective dissolution of the steelmaking dust was carried out at 30°C (TI) for 2 hours (ti) at an acetic acid (CH3COOH) concentration of 33% (Cl); and at 70°C (T2) for 5 hours (t2) at an acetic acid concentration of 50% (C2). The steelmaking dust consisted of iron ore collected from the blast furnace (PAH) and the CONOX converter (PCC), which underwent mechanical separation by sieving using 50-200 mesh screens, corresponding to cut sizes between 74 and 297 µm, with agitation maintained for 15 minutes. The oxides present were selectively solubilized, precipitating calcium (CaO), aluminum (Al2O3), silicon (SiO2), and magnesium (MgO) oxides without significantly affecting the iron oxides present (Fe2O3, ferrite, and Fe3O4, magnetite).

[0065] The acetic acid washes were performed in a three-necked flask, positioned on a heating mantle, equipped with a mechanical stirrer and thermocouple for temperature monitoring. After reaching a constant temperature, the steelmaking powder (PCC or PAH) was added to the acetic acid solution, maintaining a constant stirring speed of 80 rpm for a period of time. The steelmaking powder was then subjected to vacuum suction filtration using Munktell 393 grade filter paper for both types of powder, PAH or PCC. The reddish PCC was further filtered using a 0.45 µm MFS cellulose nitrate filter. The resulting precipitate was dried in a Memmert oven at 70°C for 3 hours, while the supernatant was reserved for further processing. The dried steelmaking powder was then subjected to an additional wash in triplicate with the same acetic acid solution used initially.After each wash, the treated powder was centrifuged for 15 minutes to facilitate the separation of residues. Subsequently, the resulting powder was subjected to further drying in an oven at a temperature of 50°C for 2 hours.

[0066] Example 2: Characterization of conditioned powders

[0067] Morphological and compositional analysis was performed using a Tescan Vega3 SBU scanning electron microscope equipped with energy-dispersive X-ray spectroscopy (SEM / EDS). Images were obtained at 500x magnification using secondary electron (SE) and backscattered electron (BSE) imaging, along with distribution mapping of the same scanned area. The crystalline phases present in the steelmaking powders (PAH and PCC) before and after conditioning were characterized using X-ray diffraction (XRD) with a Bruker AXS GmbH D4 ENDEAVOR X-ray diffractometer. Operating parameters included a voltage of 40 kV and a current of 40 mA, using Cu Kal radiation and a 2θ angle range between 20° and 80°.The XRD patterns were adjusted using Rietveld refinement for quantifying the crystalline phases using MAUD software. This involved adjusting the structural model parameters and those related to the nonlinear least-squares diffraction profile to minimize the differences between the calculated and experimentally obtained diffraction patterns. This iterative purification process, followed by filtration, drying, washing, and centrifugation, and then further separation and drying, was performed until convergence was achieved, as assessed by the sigma value and the Rwp value.

[0068] The initial PAH powder showed a composition rich in Fe (50.0 ± 1.0%) with high amounts of Al (3.3 ± 0.2%), Ca (6.9 ± 0.1%), Mg (0.8 ± 0.1%), and Si (4.9 ± 0.2%). Regarding the PCC powders, EDS analysis showed a composition rich in Fe (59.0 ± 0.8%) with low amounts of Al (0.4 ± 0.1%), Ca (4.1 ± 0.1%), Mg (0.9 ± 0.1%), and Si (0.5 ± 0.1%). Analysis of the conditioned powders revealed a significant decrease in Ca content after treatment compared to the untreated powder (p < 0.05). In particular, through a treatment at 70°C and 33% (T2Cl-2h), the decrease of Ca and Mg was favored with a reduction of more than 90% in Ca levels and a decrease of more than 80% in Mg levels, confirming the effectiveness of the proposed treatment.

[0069] The results obtained reflect a significant increase in the purity of the conditioned powders, crucial for improving the efficiency of iron (Fe) conversion in the subsequent gas reduction heat treatment stages.

[0070] Furthermore, the selective dissolution of calcium and magnesium oxides as a purification strategy for iron powders (PAH and PCC) offered a significant advantage by generating a supernatant rich in water-soluble calcium and magnesium acetate salts due to the resulting acid-base substitution reaction. These salts were readily precipitated by neutralization with sodium hydroxide (NaOH), producing a supernatant with a neutral pH and a precipitate rich in calcium and magnesium oxides. This confirmed the dissolution of these components, with a percentage composition of Ca (0.11% and 1.01% for PAH and PCC, respectively) and Mg (2.28% and 0.11% for PAH and PCC, respectively). Additionally, the presence of other oxides, such as aluminum and silicon, was detected, demonstrating the effective purification of the steelmaking powders and highlighting their potential application in industrial processes.

[0071] Scanning electron microscopy with electron dispersive spectroscopy (SEM-EDS) analysis of conditioned powders

[0072] The scanning electron microscopy analysis with electron dispersive spectroscopy (SEM-EDS) of the conditioned powders is shown in Figures 3A and 3B, and show the results of the morphological and compositional analysis, by scanning electron microscopy with secondary (SE) and backscattered (BSE) electrons for the initial PAH and PCC powders and under chemical treatment with acetic acid at different concentrations, reaction times and process temperatures (See Table 1).

[0073] Table 1. Experimental conditions

[0074]

[0075] According to the SEM-SE images, the initial PAH and PCC powders showed an irregular morphology with average particle sizes of 208.7 ± 90.0 pm and 31.0 ± 19.0 pm, respectively. Likewise, the SEM-BSE images showed particles with different shades of gray depending on the atomic weight of their constituent components. A high distribution of large, individual, and brighter particles was observed, corresponding to silicon (Si) compounds; less bright particles corresponding to Fe-rich compounds in which Ca, Mg, and Al oxide particles were embedded; and dark particles corresponding to Carbon (C) (Figures 3A and 3B).

[0076] Following the purification process, both powders showed a decrease in particle size under all treatments evaluated, with a marked trend toward decreasing particle size as reaction time increased (Table 2). According to the SEM-SE images, the particles exhibited an irregular morphology with a reduced presence of Ca and Mg particles. Similarly, the SEM-BSE images for the conditioned powders showed a significant reduction in bright areas in all cases, with individual C and Fe particles and, in some cases, smaller silicon oxide particles.

[0077] Table 2. Particle size for PAH and PCC powders purified under treatments T1C3 (30°C, 33%), T3C3 (70°C, 33%) and T3C4 (70°C, 50%) evaluated for 2 and 5 hours.

[0078]

[0079] Regarding the percentage composition analysis using EDS, the initial PAH powder showed a composition rich in Fe (50.0 ± 1.0%) with high amounts of Al (3.3 ± 0.2%), Ca (6.9 ± 0.1%), Mg (0.8 ± 0.1%), and Si (4.9 ± 0.2%), with the weight ratio Fe:Al:Ca:Mg:Si being 50:3.3:6.9:0.8:4.9. Analysis of the conditioned powders revealed that all processes showed a significant decrease in Ca content compared to the untreated PAH (p<0.05). Under the first treatment at 30°C and 33% for 2 hours (TlC3-2h), the Ca and Mg content decreased drastically (1.4 ± 0.2% and 0.6 ± 0.0%, respectively). However, Al and Si remained in the sample without a significant decrease. The weight ratio Fe:Al:Ca:Mg:Si is approximately 50:3.3:1.4:0.6:4.9. Increasing the process temperature to 70°C (T3C3-2h) favored a decrease in Ca and Mg, with percentage values ​​of 0.6 ± 0.1% and 0.3 ± 0.2%, respectively.However, Al and Si contents were found without a significant decrease compared to process T1C3, with values ​​of 4.0 ± 1.7% and 4.8 ± 0.8%, respectively. The weight ratio Fe:Al:Ca:Mg:Si is approximately 50:4.0:0.6:0.3:4.8. Similarly, chemical treatments of steelmaking powders with higher concentrations of acetic acid at 50% and 70°C for 2 h (T3C4-2h) showed Ca, Mg, Al, and Si contents of 0.5 ± 0.1%, 0.5 ± 0.1%, 2.6 ± 0.4%, and 5.8 ± 0.8%, respectively, with no significant differences compared to process T3C3-2h. The weight ratio Fe:Al:Ca:Mg:Si is approximately... 50: 2.6: 0.5: 0.5: 5.8. On the other hand, no significant changes were observed in the Ca and Mg contents between the conditioning processes T1C3, T3C3 and T3C4 with the increase in reaction time.

[0080] Regarding the PCC powders, EDS analysis showed a composition rich in Fe (59.0 ± 0.8%) with low amounts of Al (0.4 ± 0.1%), Ca (4.1 ± 0.1%), Mg (0.9 ± 0.1%), and Si (0.5 ± 0.1%), with the Fe:Al:Ca:Mg:Si ratio being approximately 59:0.4:4.1:0.9:0.5. Analysis of the conditioned powders revealed a significant decrease in Ca content compared to the initial PCC (p < 0.05). The T3C3-2h and T3C3-5h treatments showed the greatest Ca dissolution, with values ​​of 0.1 ± 0.1% in both cases. However, the percentage amounts of Al, Mg, and Si remained largely unchanged with values ​​of: 0.4 ± 0.3% and 0.5 ± 0.1% for Al, 0.1 ± 0.0% and 0.2 ± 0.1% for Mg, and 0.7 ± 0.1% and 0.6 ± 0.1% for Si. The Fe:Al:Ca:Mg:Si ratio is approximately 59:0.4:0.1:0.1:0.6.

[0081] Table 3. Percentage decrease / elimination of Ca and Mg in conditioned powders

[0082]

[0083] The results obtained reveal a significant increase in the purity of the treated material after conditioning the steelmaking powders and slurries. This purity improvement is fundamental to increasing the efficiency of iron (Fe) conversion in subsequent gas reduction thermal processing stages. According to the data (Table 3), a reduction of over 90% in Ca levels was achieved in both types of powders, and an average decrease of approximately 80% in Mg levels. These results confirm that an 80% to 100% reduction in CaO and MgO content was achieved, and it was also possible to eliminate all SiCh present in the PCC powder. Accordingly, the conditioning treatment is defined as washing with acetic acid at a concentration of 33% at a temperature of 70°C for two hours (treatment designated T3C3-2H).

[0084] X-ray diffraction (XRD) analysis with Rietveld refinement

[0085] X-ray diffraction analysis of the initial and purified powders is shown in Figure 4. The analyses revealed that the iron oxides present in the initial PAH samples are mostly in the form of hematite (FezCL, 37.1%), with some magnetite (FesCL, 7.4%). Regarding the hematite content, a variation of 31.1% was observed after powder conditioning. Similarly, an increase in magnetite content was evident after the conditioning process, reaching 9.5%. Figure 4A also shows the characteristic diffraction peaks of calcium carbonate (CaCCL, calcite, 14.5%), present only in the original powder, whose signal disappears in the diffractograms of the PAH conditioned under the specified conditions (Figure 4A). This suggests that the conditioning stage was effective in removing calcite from the PAH powders.Furthermore, characteristic peaks confirming the presence of quartz (SiCh) were found in both the original powders (18.4%) and the conditioned powders (16.4%). However, these signals showed greater intensity in the initial PAH powders. This difference in intensity and percentage composition obtained through refinement confirms that the concentration or relative proportion of this mineral phase was slightly modified in the conditioned sample compared to the initial sample due to the decrease in the Ca phase content. Thus, by reducing the contribution of these phases, the relative signal of quartz, hematite, and magnetite remained the predominant phase in both powders.

[0086] On the other hand, no signals associated with the crystalline structure of magnesium oxide (MgO) or silicate- and aluminate-based compounds were observed. The goodness-of-fit values ​​obtained showed a sigma of 1.34 and a Rwp of 4.38%, thus accepting the refinement. Furthermore, it confirms that the composition of the conditioned PAH powder does not contain significant amounts of magnesium, aluminum, or their oxides, and therefore they are not detected by XRD. Finally, broad, low-intensity peaks were observed around the 20:26° angle, associated with amorphous forms of carbon present in the original (22.6%) and also in the conditioned (43.0%) PAH powders. The quantification of carbon and quartz by this technique is limited because both carbon in the form of graphite and quartz exhibit a diffraction peak at a very close position (26.6°).Therefore, the original powder has a weight ratio of hematite: magnetite: quartz: amorphous forms of carbon of 37.1: 7.4: 18.4: 22.6. While the conditioned powder has a ratio of hematite: magnetite: quartz: amorphous forms of carbon of 31.1: 9.5: 16.4: 43.0.

[0087] For the initial PCC powder, the analyses revealed three main crystalline phases: SiCh (5.7%), FezCl (7.6%), and FesCu (86.7%). Comparing the diffraction pattern of the initial powder with the conditioned powder showed a decrease in the intensity of the peaks corresponding to hematite, resulting in a decrease in the percentage values ​​of these phases to 6.5%. Meanwhile, the magnetite diffraction peaks present in the unpurified PCC remained unchanged in diffraction angles and showed a percentage value of 93.4%. Therefore, the original powder has a SiCh:hematite:magnet weight ratio of 6-7:7-9:85-90, preferably a weight ratio of 5.7:7.6:86.7.Conversely, the quartz diffraction peaks decreased in intensity in the diffractogram of the conditioned powder (Figure 4B), with a percentage value of 0.002%, indicating a significant decrease in the content of this phase with acceptable values ​​for sigma of 1.11 and a water-weight ratio (WWR) of 2.98%. Furthermore, this technique did not reveal any signals attributable to the presence of calcite (CaCCh) or magnesium and aluminum oxides in the samples. However, SEM-EDS did identify the presence of these metals, possibly because this characterization technique involves analyzing a region of the material associated with the scan in the micrograph. Therefore, these metals are present in concentrations below 3-5% w / w and are thus undetectable by XRD.

[0088] Example 3. Final disposal of generated by-products

[0089] Following the conditioning treatment of the steelmaking dust, a supernatant with a pH below 3 was obtained, making it impossible to dispose of it down the drain without prior treatment. This supernatant was neutralized, and the precipitated salts were collected as described below:

[0090] Neutralization: A 1.0 M sodium hydroxide (NaOH) solution (Merck, Germany) was used for the acid-base titration. pH was continuously monitored using a pH meter (Hanna Instruments pH 211 model), adjusting the volume of NaOH added until a neutral pH was reached (pH = 7.0 ± 0.5). The molar ratio between the acid solution and NaOH was 2:1.

[0091] After 24 hours of titration, two phases were identified: a neutralized supernatant and a precipitate rich in metallic compounds. Subsequently, vacuum suction filtration was performed using Munktell grade 393 filter paper (Buchi brand, model V-100). Finally, the dried precipitate was calcined at 800°C for 2 hours, and scanning electron microscopy / electron dispersive spectroscopy (SEM / EDS) analysis was performed to determine its morphology and chemical composition.

[0092] The major advantage of selectively dissolving calcium and magnesium oxides as a purification strategy for iron powders (PAH and PCC) is the ability to form a supernatant rich in water-soluble calcium and magnesium acetate salts through an acid-base substitution reaction. In this reaction, acetic acid reacts with calcium carbonate as a base to form the salt, water (H₂O), and carbon dioxide (CO₂). In the case of magnesium oxides (MgO), these act as a base, reacting with acetic acid (CH₃COOH) to form magnesium acetate and H₂O.

[0093] After achieving the formation of calcium and magnesium hydroxide precipitates along with sodium acetate salts in solution, 24 hours after the titration, two phases were identified: a neutralized supernatant and a precipitate rich in metallic compounds, as shown in Figures 5A and 5B. The precipitate in the PAH phase was yellowish, unlike the PCC phase, which had a reddish hue. Furthermore, the PCC powder yielded four times the amount of precipitate obtained compared to the PAH phase. Both precipitates showed a viscous consistency, which confirms that gelation was indeed generated by polymerization of the Si, Mg and Fe particles (K. Song, S. Park, W. Kim, CW Jeon, and JW Ahn, “Effects of experimental parameters on the extraction of silica and carbonation of blast furnace slag at atmospheric pressure in low-concentration acetic acid,” Metals (Basel), vol. 7, no. 6, Jun. 2017).

[0094] Characterization of the phases present in the generated by-products

[0095] Figure 6 shows the results of morphological and compositional analysis, using SEM-BSE, SEM-SE and energy dispersive spectroscopy (EDS) for the precipitation of the supernatant from the chemical washing performed on PAH and PCC.

[0096] According to the SEM-SE and SEM-BSE images, the powder precipitate presented an irregular morphology and in relation to the percentage composition analysis by EDS that is observed in Figure 7A and 7B corresponding energy dispersive x-ray spectroscopy for PAH and PCC, in which the characteristic peaks for Ca at 0.3 keV in a spectral line of Lal and Mg at 1.3 keV in a spectral line of Ka 12 can be observed. It was observed that the precipitates do indeed contain calcium and magnesium which indicates the dissolution of these components with a percentage composition of Ca (0.11 and 1.01% for PAH and PCC respectively) and Mg (2.28 and 0.11% for PAH and PCC respectively). In addition, the presence of other components such as Si (0.81 and 2.30% for PAH and PCC respectively), Al (0.24 and 1.58% for PAH and PCC respectively) and O (29.91 and 64.09% for PAH and PCC respectively) is observed.

[0097] Example 4: Thermochemical Reduction

[0098] In the thermochemical reduction stage, the purified powders were reduced in a tubular furnace with an alumina tube (FHA 13 / 50 / 200 Carbolite Gero), using a heating rate of 5°C / minute with temperatures ranging from 700 to 1200°C and residence times between 1 and 2 minutes. H₂ / Ar mixtures (5 / 95, 60 / 40, 70 / 30, 100 / 0), and even the carbon present in the purified powders themselves, were used to transform the iron oxides into high-purity metallic iron or ferrite, recovering and recycling a valuable raw material. See Figure 2

[0099] Specifically, the H2 / Ar mixture generates a reducing atmosphere composed of 5% hydrogen and 95% argon. The target temperature and residence time are defined for each cycle. Furthermore, the process is configured to be carried out at an EE / Ar inlet flow rate of 15 L / min. For each cycle, 4.0 ± 0.010 g of purified PCC were weighed using a Boeco Germany BAS 31 plus balance; these powders were evenly distributed into alumina crucibles.

[0100] Table 4 summarizes the temperature and time conditions used in the thermochemical cycles. A response surface methodology derived from a simplified 2-level (minimum and maximum) factorial design with a center point was used, along with the Stat Ease software. The maximum and minimum temperatures were defined as 1000°C and 900°C, respectively, and the maximum and minimum times as 120 minutes and 60 minutes, respectively, with a center point temperature of 950°C and a center point time of 90 minutes.By setting the gas flow at 15 L / min, constant, and a low concentration of EL (5%) and Ar (95%), the ANOVA or analysis of variance that accompanies the design of the response surface is simplified, since it studies the differences between the means of the groups of a sample, performing statistical analysis, identifying causes or their randomness, and several executions are carried out for a better application, and it is possible to design a model capable of generating a response surface based on the input of iron recovery and mass loss results of each thermochemical cycle studied, to facilitate the identification of trends and interaction between factors, not fixed.

[0101] Table 4. Temperature and time conditions of thermochemical cycles

[0102]

[0103] At the end of each cycle, the crucibles were removed and the amount of recovered powder was measured. The variation between the initial and obtained mass in each cycle was expressed as a loss, as in the purification stage, and based on this variation a value was calculated that was subsequently used as one of the criteria for selecting the thermochemical cycle and as an input value for generating the response surface.

[0104] Table 5 presents a summary of the dust variation in the samples after the thermochemical cycles, along with the conditions under which each cycle was performed. Initially, it can be concluded that both temperature and time have a positive effect on the reduction of the washed CCP (Critical Point of Concentration), since the dust loss recorded in the process is related to the reduction of oxides in hydrogen atmospheres, which, upon changing phase, release oxygen in the form of water. The loss value recorded in the P950°C-90min sample (27.88%) is considered inconsistent when compared to the minimum time and temperature condition studied (900°C and 60 minutes), which resulted in a 29.08% loss. This difference could be explained, for example, by insufficient drying.

[0105] The greatest mass variation occurred at a temperature of 1000°C and a time of 120 minutes, with a 36.02% loss. When compared to sample P1000°C-60min, there was a difference of 5.13%, which implies a greater impact of time on the process. This is compared to increasing the temperature by 100°C when the reduction time is 60 minutes, between samples P.900°C-60min and P1000°C-60min, which only shows a difference in mass loss of 1.82%.

[0106] Table 5. Variation in powder mass after thermochemical cycles

[0107]

[0108] SEM-EDS analysis

[0109] SEM-EDS analysis applied to the washed and reduced PCC allowed for the identification of differences in the composition of the reduced powder components, which are summarized in Table 6 and, along with those of the original and washed PCC, in Figures 8A to 8F. This allowed for the determination of iron recovery in each cycle, the decrease in oxygen content, and the variation in the impurity content of magnesium, silicon, aluminum, and calcium. Sample P900°C-60min registered the lowest iron content, with 84.730% iron recovery, and the highest oxygen content, at 12.660%. The percentage content for the samples from the other cycles was similar, with values ​​above 90% iron and values ​​below 5% oxygen. Sample P950°C-90min showed the highest iron recovery, at 92.920%.

[0110] The reduction process of PCC with hydrogen releases oxygen when it changes phase, that is, from hematite to magnetite, then to wüstite, finally from wüstite to metallic iron, which produces a decrease in the mass of the samples and a change in the percentage composition of the reduced powder, as evidenced in Figure 8A to Figure 8F, however, using EDS analysis it is not possible to identify the phases of the components in the reduced PCC samples, so the characterization must be complemented with other analyses, such as X-ray diffraction.

[0111] Table 6. Results of reduced PCC percentage composition using SEM-EDS

[0112]

[0113] Figures 9A to 9J show the morphological changes of the powder after the thermochemical cycles, and present the morphological analysis using BSE and SE SEM. The morphology of spherical particles is found in the P900°C-60min sample of Figures 9A and 9B, along with the presence of small porosities in irregularly shaped particles and iron whiskers; the latter is more clearly seen in Figure 10A, which shows the magnification of the demarcated area.

[0114] Figures 9C to 9J show a more marked change to a porous and thicker morphology, highlighting the formation of channels and the grouping of whiskers for the formation of an iron layer on the surface of the particles. This phenomenon is observed in more detail in Figures 10B and 10E, for samples P.1000°C-60min and P950°C-90min. Finally, in the magnified area of ​​sample P.1000°C-120min and a particular area in sample P900°C-120min, particles with a unique morphology are observed, as shown in Figures 10C and 10D, where the iron whiskers do not group together, revealing the interior of the particle nucleus. The above is consistent with the findings of J. Xu, N. Wang, M. Chen, Z. Zhou, H. Yu, Comparative investigation on the reduction behavior of blast furnace dust particles during inflight process in hydrogen-rich and carbon monoxide atmospheres, Powder Technol 366 (2020), on the thermochemical reduction behavior of blast furnace dust particles.

[0115] The temperature variation between 900 and 1000°C for a reduction time of 120 minutes resulted in a difference in iron content of only about 1%. A similar result was obtained by maintaining a temperature of 1000°C and varying the time from 60 to 120 minutes, yielding a 1% difference, which could be attributed to the impurity content of the sample used under each condition. The difference in the percentage of iron obtained under each condition exceeded 90%, surpassing the range commercially used for reduced iron powders, which is around 85% (JFE Steel Corporation, Reduced iron powders, (n.d.)). V. San Martin, in his study on the PCC reduction process in FE (5%) and Ar (95%) atmospheres under optimal conditions, obtained an iron percentage of 62% with a total impurity content of 16% in the composition (V.San Martín, Valorization process of steelmaking dust generated in a BOF converter from reduction with an Ar / H2 atmosphere, University of Bío-Bío, 2024). The above demonstrates the impact of the PCC impurity pretreatment on the effectiveness of the reduction process, where a higher percentage of iron in the composition was achieved at a lower temperature. In this way, the reduction occurs efficiently with less interference from impurities in the process. XRD analysis.

[0116] Figure 11 shows XRD diffractograms, where the iron present in the reduced powder samples is detected, mainly in the ferrite phase for P900°C-120min and P1000°C-120min. It was also identified that for the P900°C-60min sample, there is a coincidence of 6 characteristic peaks of wüstite phase iron, similarly for the P1000°C-60min sample, but with less intensity, and finally a coincidence of only 3 wüstite phase peaks for the P950°C-90min sample.

[0117] For P900°C-60min and P1000°C-60min, it can be concluded that the change in temperature from 900 to 1000°C did not represent a significant change in the phases present in the reduced powder, since in both conditions the presence of wüstite was detected and therefore incomplete reduction. Then, when the time was increased to 120 minutes, both diffractograms of the conditions at 900 and 1000 °C reached complete reduction, with the presence purely of ferrite phase, which implies that the reduction time has a greater significance for the thermochemical process.

[0118] A. Vásquez, in Development of a thermochemical process for obtaining metallic powder from the reduction of sludge generated in a blast furnace carried out in a tubular furnace, (2023), performed an XRD analysis to study the phases present in the reduced powder in his thermochemical process study of blast furnace sludge in an Argon atmosphere, resulting in the identification of a metallic iron phase, and the compounds stishovite (O2SÍ) and akermanite (Ca2Mg(SÍ2O?)) for a reduction temperature of 1200°C.

[0119] Example 5. Characterization of reduced powders

[0120] Scanning electron microscopy (SEM) analyses were performed using secondary electrons (SE) and backscattered electrons (BSE), along with energy-dispersive X-ray spectroscopy (EDS). A TESCAN VEGA 3 EASYPROBE SBU scanning electron microscope was used to characterize the initial PCC powders, purified PCC powders, and powders resulting from the thermochemical reductions of each of the five thermochemical cycles. Magnifications of 500x and 1000x were taken for morphological analysis. The corresponding micrographs for BSE and SE, the map of the analyzed area, and associated frequency spectrum tables were obtained. These results were analyzed using graphs to represent the percentage present in the samples.

[0121] X-ray diffraction studies were performed using a Bruker D4 Endeavour instrument with copper radiation, ranging from 20 to 90°2 theta, with a step size of 0.02 degrees at 1-second intervals and a reading area of ​​10 mm². Prior to measurements, the samples were prepared by reducing the powder particle size in an agate mortar in 15-minute cycles. The analysis was performed using X'pert HighScore Plus software to identify the characteristic peaks of the phases present in each sample; these were then associated with the main peaks, evaluating their overlap and the minimum percentage required for inclusion. Finally, the diffractograms were generated using OriginPro 8.5 software. The results of these analyses allowed for both the creation of the response surface and the selection of the thermochemical reduction conditions for the powder to be used in subsequent mechanical tests.

[0122] Using EDS, it was possible to identify the percentage composition of the reduced samples and thus determine the amount of iron recovered in each sample. However, it is not possible to identify the phases corresponding to each component, and because SEM-EDS analysis is a semi-quantitative analysis, it cannot be stated that the analyzed sample is representative of all the powder reduced in each reduction cycle. From the micrographs obtained using SEM-SE, it is possible to observe and compare the changes in particle morphology under different time and temperature conditions.

[0123] XRD analysis is a reliable and viable quantitative study for the identification of the phases present in a sample, and allowed the recording of the diffraction of x-rays on the crystalline structure of the phases present in the sample, where the peaks recorded are representative of each phase.

[0124] The selected reduction cycle was at a temperature of 1000°C with a residence time of 2 hours, corresponding to cycle 4. With this information, a greater quantity of reduced raw material was generated until the minimum amount necessary for the production of metal parts and application in mechanical tests. Example 6. Density

[0125] From test specimens produced from the obtained powders, a value of 4.799 ± 0.097 g / cm³ is obtained for the densities. 3 in green and 5.087 ± 0.124 g / cm 3 sintered. These values ​​mark the beginning of the study of the properties of the material resulting from the sintering of the washed and reduced PCC, and allow the first comparisons to be made.

[0126] Figure 12A shows the metal powder used in this stage after being ground and sieved, processed under the selected reduction conditions of 1000°C and 120 minutes. The first compacted specimen intended for hardness testing is also shown. Figure 12B shows green specimens, intended for compression testing, which were used to calculate the densities in Tables 7 and 8.

[0127] Commercial iron powder with an iron purity of over 98% has green density ranges between 6.5 and 6.7 g / cm³ 3 This is well above the reduced PCC. However, this is an expected value due to the difference in iron content and the nature of the powders used commercially, which come from mill scale, iron ore, cast iron, among others (JFE Steel Corporation, Reduced iron powders, (n.d.); Yiyunyingliping, Iron Powder Material, (2023)).

[0128] Table 7. Dimensions and green density of pressed specimens.

[0129]

[0130] Table 8. Dimensions and sintered density of test specimens.

[0131]

[0132] Table 9 summarizes the calculation of the densification of the specimens due to sintering and the relative density when compared with the theoretical density (pt) corresponding to 7.87 g / cm 3 of iron.Table 9. Densification and relative density of the sintered specimens.

[0133]

[0134] V. San Martín, in his work "Process for the valorization of steelmaking dust generated in a BOF converter from reduction with an Ar / FL atmosphere," at the University of Bío-Bío in 2024, produced metal parts from the reduction of PCC in an EL (5%) and Ar (95%) atmosphere under optimal reduction conditions at a temperature of 1200°C and a time of 2 hours, which yielded an iron content of 62%. The resulting specimens had a green density of 4.23 ± 0.29 g / cm³. 3 and a sintered density of 4.4 ± 0.28 g / cm³ 3 The values ​​in Tables 7 to 9 show an improvement in the properties of the PCC when it undergoes a purification process using acetic acid washes.

[0135] Microstructural analysis

[0136] Figures 13A and 13B show the surface of the metal piece before and after chemical etching at x100 magnification. The ferritic matrix, indicated by white arrows, is visible, as are the porosity and grain boundaries. The chemical etching process helped to reveal and identify the phases and grain boundaries present on the specimen surface.

[0137] Figures 14A and 14B show another area of ​​the surface in greater detail at a magnification of x1000. In this view, only the grain boundaries are shown, providing a clearer view of the ferritic matrix and the material's porosity.

[0138] Metallographic analysis can be enhanced by using more precise characterization methods such as SEM analysis, which allows for a more detailed identification of the phases, inclusions, and porosities in the microstructure of the studied pieces. Example 7. Cold compaction of metallic specimens

[0139] The prepared specimens were initially compacted in an Across MP24A hydraulic press using iron powder obtained as described in the previous examples, i.e., based on the present method. A 10 mm diameter die was used, with a compaction pressure of 500 MPa, a value used for iron powders in the powder metallurgy industry, and a residence time of 10 seconds, to obtain four 10 mm diameter specimens: one for hardness testing and three for compression testing. Prior to pressing, the powder was ground in an agate mortar for 15 minutes, followed by manual sieving through an 80 mesh sieve, corresponding to a particle size smaller than 177 µm. The die, the lower punch, and the upper punch were lubricated with zinc stearate before the powder was poured in, which facilitated the separation of the specimen from the die after pressing.

[0140] Finally, the mass of each specimen and its diameter and height dimensions were recorded for subsequent calculation of the green and sintered density of the specimens. The powder mass-to-height ratio obtained from the first compacted specimen was used to provide an approximate estimate of the powder needed to produce taller specimens for compression testing. The mass required to obtain specimens with a minimum height of 8 mm was determined, with the minimum amount of metallic powder needed per specimen being 3.776 g, and 11.3 g for the three specimens required for compression testing.

[0141] A traditional sintering process was carried out on the compacted parts in a tubular furnace (FHA 13 / 50 / 200 Carbolite Gero) under a reducing atmosphere of 5% H2 and 95% Ar, with a flow rate of 15 L / min. Sintering was performed at a temperature of 1150°C for 1 hour, with a heating rate of 5°C / min.

[0142] A microstructural analysis and preparation of the specimen for metallographic study were also performed. The process began with embedding the sintered metal specimen in Bakelite, followed by mechanical grinding of the surface using SiC abrasive paper until a particle size of 8 µm was achieved. During this procedure, water was added and the specimen was rotated 90° each time the abrasive paper was changed. The second stage consisted of coarse polishing with 5-micron alumina, followed by fine polishing with 1-micron diamond suspension and subsequently 0.5-micron.

[0143] Finally, a chemical etching was performed using 3% nital (a solution of alcohol and nitric acid) according to standard E407-99. This process lasted 3 seconds and was stopped by applying alcohol and water, respectively. Subsequently, the piece was dried under a flow of hot air (ASTM, Standard Practice for Microetching Metals and Alloys, n.d.).

[0144] The microstructural analysis of the piece was carried out using a Unitron optical microscope, belonging to the materials laboratory. Magnifications of x100 and x200 were used to identify porosity, phases, and grain boundaries in the microstructure of the metallographic surface. When comparing V.San Martín (V San Martín, Valorization process of steelmaking dust generated in a BOF converter from reduction with an Ar / H2 atmosphere, University of Bío-Bío, 2024), it is possible to conclude that washing PCC with acetic acid improves the reduction process, and would be related, without adhering to any theory, to the decrease in impurities that hinder the contact of the reducing gas with the surface of the PCC particles, resulting in a higher percentage of iron recovered and a complete reduction of the oxides. In this way, a test specimen made with a lower amount of impurities and less presence of iron oxides achieves better compaction during pressing and then a decrease in the size of the porosities when going through the sintering process.

[0145] Example 8. Mechanical Analysis of Metal Parts

[0146] Vickers Hardness Test

[0147] To apply the Vickers hardness (HV) test to parts manufactured using powder metallurgy with metallic powders, a Zwick / Roell ZHU8187.5 LKV durometer was used to determine HV10 and HV5 hardness, using loads of 10 kgf and 5 kgf, respectively, both with a duration of 10 seconds. The ISO 6507-1:2023 standard was used to obtain a representative value. Eight indentations were made per test, the Vickers hardness value for each indentation was recorded, and finally, the minimum and maximum values ​​for each test were excluded, and an average and standard deviation were calculated (ISO, ISO 6507:2023 Metallic materials - Vickers hardness test, 2023).

[0148] The same metal piece used in the microstructural analysis was used for the Vickers hardness test, as it requires a pre-prepared surface free of irregularities that could affect the indentations. Figures 15 and 15B show the marks left by the indenter on the surface of the specimens, along with the diagonal measurements for the HV5 and HV10 tests. Tables 10 and 11 summarize the measurements and resulting values ​​from the Vickers HV5 and HV10 hardness tests, respectively, of the sintered specimen. The lower and upper hardness values ​​for each test were discarded, and a Vickers hardness value for the sintered specimen was obtained equal to 32,173 ± 0.615 HV5 and 29,807 ± 0.844 HV10, classifying this material as having low hardness and high ductility. This can be explained by both the porosity and the nature of the phase identified in the microstructural analysis.

[0149] In both tests, a standard deviation of less than 1 is observed, which for hardness ranges represents a low value. This can be attributed to a good phase distribution and porosity in the metallic specimen, in addition to the absence of other iron-carbon phases, as observed in the microstructural analysis, which increase the hardness of the materials.

[0150] Table 10. Vickers hardness values ​​at a load of 5kgf for the sintered specimen

[0151]

[0152] Table 11. Vickers hardness values ​​at a load of 10 kgf for the sintered specimen

[0153]

[0154] V. San Martín, in his work "Process for the valorization of steelmaking dust generated in a BOF converter from Ar / EE atmosphere reduction," University of Bío-Bío, 2024, obtained a Vickers hardness of 91.75 HV10 in his specimens sintered from PCC. For comparison, he performed hardness tests on specimens made with commercial iron, averaging 65.69 HV10. He concluded that the oxides present in the microstructure of his specimens impart greater hardness to the material. F. Beltrán, in his work "Development of self-lubricating iron matrix materials reinforced with Ti2SnC MAX phase prepared by powder metallurgy," (2024), similarly compared pure iron matrix specimens with iron specimens reinforced with MAX phase particles using commercial iron. He obtained a hardness of 71.89 HV10 for the pure commercial iron specimens and 96.14 HV10 for the best condition reinforced with MAX phases.

[0155] Compression Test: The ASTM E9 standard was applied to perform compression tests on three specimens using a ZWICKROELL BT1-FB100TN mechanical testing machine. The specimens were carefully sanded and cleaned with acetone prior to testing. Because the specimens were composed primarily of iron but lacked prior testing records, a preload of 20 N and a speed of 1 mm / min were set for the test up to 45% deformation (ASTM INTERNATIONAL, E 9 - Standard Test Methods of Compression Testing of Metallic Materials at Room Temperature, (n.d.)). The compressive stress values ​​obtained in the test varied as the load was applied and the specimen deformed.

[0156] The 0.2% method was applied to determine the yield strength. This involved drawing a trend line over the elastic region on the stress-strain curve, where deformations are not permanent and Hooke's Law applies. The line was then shifted by 0.2%, and the point of intersection with the strain curve defined the yield strength value for each tested specimen.

[0157] Table 12 presents the yield strength results, determined by the YS deviation method (offset 0.2%) and compressive strength at 45% deformation, where for the specimens made with washed and reduced PCC (P1000°C-120min) and sintered at 1150°C for one hour, a value of 216.9 ± 37.9 MPa is obtained for the yield strength and 736.63 ± 12.4 MPa for the compressive strength at 45% deformation.

[0158] Figures 16A to 16D show the compression strain curves along with the lines drawn to determine the yield strength. At first glance, a consistent deformation pattern is observed in the specimens. Figures 17A and 17B show the three specimens tested before and after the compression test, respectively, from which we can conclude the material's high ductility, capable of withstanding high plastic deformations.

[0159] Table 12. Compression test results

[0160]

[0161] The yield strength obtained from the compression tests was 216 MPa, which is consistent with the 191 MPa recorded for commercial iron powder. Furthermore, the compression strain curve proved to be equivalent to that of ductile materials. The failure mechanism, characterized by a barrel shape without fractures, allowed us to conclude that the material possesses a high capacity to withstand plastic deformation. This establishes commercial iron as a good benchmark for reduced PCC and opens opportunities for improvement in alloys that use iron powder as the primary matrix to enhance their mechanical properties.

Claims

CLAIMS 1. Method for obtaining high iron content powders from steelmaking dusts and sludge, CHARACTERIZED in that it comprises the following steps: a) Dissolve, by selective dissolution, metal oxides other than iron oxide present in the steelmaking powder or sludge with a weak carboxylic acid at a concentration between 1-70% w / v, with stirring and controlled temperature in the range of 30-90°C, the ratio (w / v, g / ml) of steelmaking powder or sludge to weak acid being approximately 1-10 to 10-50; b) separating the iron powders resulting from step a) from the solution containing them; c) thermochemical reduction of the separated iron powders, to transform the iron oxides into metallic iron by heating, at a temperature between 700-1200°C, under a reducing atmosphere, using pure reducing gases or gases mixed with inert gases, selected from N2, Ar, He, H2, CO, H2O, and mixtures thereof.

2. The method of claim 1, CHARACTERIZED in that said steps a) and b) can be repeated one or more times sequentially.

3. The method of claim 1, CHARACTERIZED in that said steps a) and b) can be repeated one or more times separately.

4. The method of claim 1, CHARACTERIZED in that prior to step a), a step a0) is carried out which includes drying the steelmaking powders and slurries at a temperature of 200°C, grinding and mechanical separation with agitation of said powders and sieving of the same using meshes between 50 and 200 mesh, corresponding to cut sizes between 74 - 297.

5. The method of claim 1, CHARACTERIZED in that said weak carboxylic acid is selected from the group consisting of acetic acid, citric acid, tartaric acid, oxalic acid, lactic acid, formic acid, malic acid, benzoic acid, phosphoric acid or a mixture of one or more of the same.

6. The method of any of claims 1 to 5, CHARACTERIZED in that said weak carboxylic acid has a concentration between 30-50% w / v.

7. The method of any of claims 1 to 6, CHARACTERIZED in that said weak carboxylic acid has a concentration of 33% w / v.

8. The method of claim 1, CHARACTERIZED in that the metal oxides other than iron oxide are selected from the group consisting of calcium oxide, magnesium oxide, silicon oxide, aluminum oxide, zinc oxide 9. The method of claim 1, CHARACTERIZED in that the controlled temperature in step a) is 70°C.

10. The method of claim 1, CHARACTERIZED in that the thermochemical reduction at a temperature between 700-1200°C under a reducing atmosphere is carried out in a tubular furnace.

11. The method of claim 10, CHARACTERIZED in that the temperature is in the range of 900-1000°C.

12. The method of claim 1, CHARACTERIZED in that said reducing atmosphere is an atmosphere of an FF / Ar mixture with a v / v ratio of FE to Ar of 5:

95.

13. The method of claim 1, CHARACTERIZED in that it optionally comprises, after step a) and separately from step b), encapsulating the hydroxides formed and discarded in step a), in inert solid matrices selected from cement or similar materials.

14. The method of claim 1, CHARACTERIZED in that it optionally comprises, after step a) and separately from step b), calcining the hydroxides formed and discarded in step a), to transform them into oxides.

15. Iron powder, CHARACTERIZED in that it is obtained by the method of any of claims 1 to 14 and comprises 90% or more iron.

16. The iron powder of claim 15, CHARACTERIZED in that it comprises a weight ratio of Fe:Al:Ca:Mg:Si in the range of 70-50:5, 0-2.0:0, 1-2.0:0,1-1:0, 5-6.

0.

17. The iron powder of claim 16, CHARACTERIZED in that it comprises a weight ratio Fe:Al:Ca:Mg:Si of 50:3.3:1.4:0.6:4.

9.

18. The iron powder of claim 16, CHARACTERIZED in that it comprises a weight ratio Fe:Al:Ca:Mg:Si of 50:4.0:0.6:0.3:4.

8.

19. The iron powder of claim 16, CHARACTERIZED in that it comprises a weight ratio Fe:Al:Ca:Mg:Si of 50:2.6:0.5:0.5:5.

8.

20. The iron powder of claim 16, CHARACTERIZED in that it comprises a weight ratio Fe:Al:Ca:Mg:Si of 59:0.4:0.1:0.1:0.

6.

21. The iron powder of claim 16, CHARACTERIZED in that it comprises a weight ratio of hematite: magnetite: quartz: amorphous forms of 30-35: 10-9: 12-18: 40-45.

22. The iron powder of claim 21, CHARACTERIZED in that it comprises a weight ratio of hematite: magnetite: quartz: amorphous forms of 31.1: 9.5: 16.4: 43.

0.

23. The iron powder of claim 22, CHARACTERIZED in that it comprises a magnetite to hematite weight ratio of 10-9: 30-35.

24. The iron powder of claim 23, CHARACTERIZED in that it comprises a magnetite to hematite weight ratio of 9.5:31.

1.

25. Sintered, porous part or component with high ductility and plastic deformation, CHARACTERIZED in that it comprises iron powder in accordance with any of claims 15 to 24.

26. Use of iron powder of any of claims 15 to 24, CHARACTERIZED in that it serves in the manufacture of sintered components or metal matrix components of less weight and improved wear resistance; friction materials, including brake pads, magnetic materials; and also as a chemical reagent, element for water purification and soil purifying agent.

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