Low-cost production process for preparing Fe16N2 magnetic material based on hot-rolled low-carbon steel oxide scale

Abstract

The invention relates to the technical field of magnetic material preparation, and discloses a low-cost production process for preparing a Fe16N2 magnetic material based on hot-rolled low-carbon steel oxide scale, which comprises the following steps: (1) raw material pretreatment: sequentially carrying out multi-stage magnetic separation, acid pickling, graded crushing and refining on iron scale; (2) hydrogen reduction: carrying out gradient reduction on the pretreated iron scale powder in a hydrogen atmosphere to generate porous alpha-Fe powder; (3) fluidized bed nitridation: placing the porous alpha-Fe powder in a fluidized bed reactor, and introducing pulse ammonia gas for nitridation; and (4) post-treatment: screening a nitrided product, recovering ammonia gas after passivation, and packaging. By constructing a micron-nano multi-stage pore structure, the contact area of an ammonia diffusion path and a reaction interface is optimized, the mass transfer limitation of a traditional single pore is broken through, the nitridation reaction depth and the target phase purity are synchronously improved, and a material foundation is laid for preparation of a high-performance magnet.

Description

Technical Field

[0001] The present invention relates to the technical field of magnetic material preparation, in particular to the preparation of Fe based on hot-rolled low-carbon steel oxide scale. 16 Low-cost production process for N2 magnetic materials. Background Art

[0002] Magnetic materials (such as Fe 16 N2) is widely used in motors, sensors, high-frequency devices and other fields due to its high saturation magnetization and low coercive force. 16 The N2 preparation process mainly includes powder metallurgy, vapor deposition and high-temperature nitriding. The raw materials are mostly high-purity electrolytic iron or nano-iron powder, which is expensive and complex. In recent years, the iron oxide scale (mainly Fe3O4) produced by hot-rolled low-carbon steel has been explored as an alternative iron source because of its low cost, easy availability and high iron content. However, existing technologies mostly focus on direct reduction or acid leaching purification of iron oxide scale, and have not yet effectively connected to Fe 16 Controllable synthesis of N2 magnetic materials.

[0003] In the prior art, iron oxide is used as raw material to prepare Fe 16 N2 faces multiple bottlenecks: the traditional nitriding process has low nitriding efficiency and high Fe4N impurity phase content in the product due to the single porosity of the raw materials and limited mass transfer; the passivation process relies on silane chemical reagents, which produces wastewater and has poor film stability; the ammonia recovery technology is extensive and the adsorbent selectivity is low, resulting in resource waste and environmental pollution; the product particle size distribution is wide, and the mixture of unnitrided coarse particles leads to uneven density of magnet molding; the nitriding reaction path is not well controlled, and fixed process parameters are prone to side reactions, resulting in high energy consumption and delayed endpoint determination. These problems restrict the development of iron oxide-based Fe 16 Large-scale and low-cost application of N2 materials. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the present invention provides a method for preparing Fe2O3 based on hot-rolled low-carbon steel oxide scale. 16 The low-cost production process of N2 magnetic materials solves the problems in the existing technology of low mass transfer efficiency of nitridation reaction, difficulty in suppressing impurity phase, severe environmental pollution during passivation process, serious waste of ammonia resources, and uneven product particle size leading to degradation of magnet performance.

[0005] To achieve the above objectives, the present invention is implemented by the following technical solutions: Fe 16 The low-cost production process of N2 magnetic materials includes the following steps: (1) Raw material pretreatment: The iron scale is subjected to multi-stage magnetic separation, pickling, graded crushing and refinement in sequence; Iron scale, a mixture of oxides (Fe3O4 / FeO / Fe2O3) that sheds from the surface of hot-rolled steel, often contains non-magnetic impurities (SiO2, Al2O3) and a chromium oxide passivation layer. While conventional processes use high-purity iron powder directly, this new process achieves a balanced balance between raw material activation and impurity removal through a multi-stage magnetic separation, pickling, and crushing process.

[0006] Multi-stage magnetic separation: Through gradient magnetic field intensity separation, non-magnetic impurities are preferentially removed while retaining the magnetic components of iron oxides, avoiding lattice distortion caused by impurity residues in the subsequent reduction stage.

[0007] Pickling and passivation: Use weak acid citric acid solution to selectively dissolve the chromium oxide passivation layer on the surface of the iron scale, exposing the fresh iron oxide surface, improving the reduction reaction activity, and avoiding excessive corrosion of the iron oxide by strong acid.

[0008] Gradual crushing: Through the step-by-step refinement of coarse crushing and nano-sand grinding, the particle size distribution of iron scale powder is controlled, while increasing the specific surface area, the agglomeration problem caused by excessively fine powder is avoided, providing a uniform raw material basis for subsequent reduction.

[0009] (2) Hydrogen reduction: The pretreated iron scale powder is gradient reduced in a hydrogen atmosphere to generate porous α-Fe powder; Traditional processes use high temperatures (>800°C) to reduce iron oxides, which can easily lead to sintering and densification of the iron powder, reducing nitriding efficiency. This invention uses gradient pressure reduction and hydrogen humidity control to directionally generate high-porosity α-Fe powder.

[0010] Gradient pressure reduction: Iron oxides are initially reduced to a porous α-Fe skeleton at atmospheric pressure (500-520°C). High-pressure hydrogen is then used during the pressurized stage (0.5-1.0 MPa) to facilitate pore penetration, forming a three-dimensional interconnected pore structure. This porous structure significantly increases the number of active sites for subsequent nitridation reactions.

[0011] Hydrogen humidity control: Introducing a trace amount of water vapor into the hydrogen, utilizing the local oxidation-reduction cycle of H2O and FeO, etches nanoscale pores on the surface of the iron powder, further increasing the porosity and specific surface area.

[0012] (3) Fluidized bed nitriding: The porous α-Fe powder is placed in a fluidized bed reactor and pulsed ammonia gas is introduced for nitriding; Traditional nitriding processes in fixed-bed reactors are prone to Fe4N impurity formation due to limited ammonia diffusion. The present invention uses pulsed ammonia injection and temperature-flow rate linkage control to achieve low-temperature, high-efficiency nitriding in a fluidized bed.

[0013] Pulse ammonia injection: By intermittently introducing ammonia, the violent movement of particles in the fluidized bed produces a friction effect, continuously removing the passivation layer on the surface of the iron powder and promoting the diffusion of nitrogen atoms into the iron lattice.

[0014] Temperature-flow rate linkage: In the low-temperature section (400-420°C), high-flow ammonia is used to enhance mass transfer and avoid excessive local concentrations that trigger the formation of Fe4N. As the temperature rises, the flow rate is gradually reduced to balance the reaction thermodynamics and kinetics and inhibit the formation of impurity phases.

[0015] (4) Post-processing: Screening of nitridation products, recovery of ammonia after passivation and packaging.

[0016] The traditional process of pickling, passivation and waste gas emissions puts pressure on the environment. This invention achieves green production through in-situ passivation of CO2 and closed-loop recovery of ammonia.

[0017] CO2 passivation: A small amount of CO2 is introduced under argon protection to make it passivate with Fe 16 The active sites on the N2 surface react to form a dense FeCO3 protective film, which replaces the traditional oily passivation agent, avoids oil pollution and does not require subsequent cleaning.

[0018] Ammonia recovery: Unreacted ammonia is purified by molecular sieve adsorption and recycled. Combined with the efficient mass transfer characteristics of the fluidized bed, the unit consumption of ammonia is significantly reduced.

[0019] Preferably, in step (1): Multi-stage magnetic separation uses three levels of magnetic field strength, namely 0.5-0.8T, 1.0-1.5T, and 1.8-2.2T; Iron scale, a byproduct of the hot-rolling process, has a complex composition, including Fe₃O₄, FeO, Fe₂O₃, and non-magnetic impurities (SiO₂, Al₂O₃). Traditional magnetic separation processes typically use a single magnetic field strength, resulting in residual weakly magnetic impurities or excessive iron oxide loss.

[0020] This invention utilizes a gradient magnetic field intensity sorting strategy, separating different magnetic components step by step through a three-level magnetic field. The first low magnetic field (0.5-0.8T) prioritizes the removal of non-magnetic impurities (SiO2, Al2O3), preventing them from interfering with the reduction reaction in subsequent processes. The second medium magnetic field (1.0-1.5T) separates weakly magnetic FeO particles while retaining highly magnetic Fe3O3. The third high magnetic field (1.8-2.2T) precisely captures the target iron oxides, ensuring iron scale purity (Fe3O4 ≥ 70wt%). This gradient design improves raw material purity while minimizing iron oxide loss.

[0021] For pickling, use 3-5wt% citric acid solution, solid-liquid ratio 1:5, temperature 50-60℃; A chromium oxide passivation layer often forms on the surface of iron scale, hindering the penetration of hydrogen during the subsequent reduction reaction. Traditional pickling processes often use strong acids (hydrochloric acid, sulfuric acid), which can easily lead to excessive corrosion of iron oxides.

[0022] This invention uses a weakly acidic citric acid solution, whose carboxylic acid groups selectively complex chromium oxide (Cr2O3), dissolving the passivation layer without damaging the main iron oxide structure. The coordinated control of citric acid concentration (3-5wt%) and temperature (50-60°C) ensures a balance between the pickling reaction rate and iron scale surface activation. A solid-to-liquid ratio of 1:5 ensures sufficient contact at the reaction interface while avoiding excessive acid waste.

[0023] The iron scale after pickling is vacuum dried at 80-100℃ for 2-4 hours, and the moisture content is ≤0.5wt%.

[0024] If the residual moisture in the iron scale after pickling is not completely removed, it will consume excessive hydrogen during the subsequent hydrogen reduction process and cause local oxidation. Traditional drying processes often use high temperatures for rapid dehydration, which can easily cause the iron scale powder to agglomerate.

[0025] This method utilizes moderate-temperature vacuum drying (80-100°C) in a low-pressure environment to lower the boiling point of water, achieving gentle dehydration. The drying time (2-4 hours) is carefully matched to the temperature to ensure slow evaporation of water, preventing stress cracking of the powder particles caused by rapid dehydration. The final moisture content is controlled (≤0.5wt%), providing dry and well-dispersed raw materials for the subsequent reduction reaction.

[0026] Preferably, in step (1): Classification, crushing and refinement include: Coarse crushing to 200-300 μm in a tubular ball mill; Nano sand grinding to D 90 ≤60μm.

[0027] Iron scale, a mixture of oxides that shed from the surface of hot-rolled steel, typically has millimeter-sized particles and can occur in a variety of irregular shapes, including flakes and blocks. Traditional crushing processes, which directly utilize a single piece of equipment for ultrafine grinding, can lead to increased equipment wear due to the high hardness of the iron scale. Furthermore, excessively fine powder (less than 100μm) is prone to sintering and agglomeration during subsequent reduction.

[0028] This invention uses a tubular ball mill for coarse crushing, with the rotation of the mill cylinder driving steel balls to impact and grind the scale particles. By controlling the ball mill speed (20-30 rpm) and grinding time, the scale is crushed to a medium particle size range of 200-300 μm. This particle size design reduces energy consumption in subsequent fine grinding while avoiding the premature generation of ultrafine particles that can lead to reduced fluidity. The mechanical forces on the surface of the scale particles after coarse crushing create microcracks, providing a structural foundation for further refinement during nano-sand milling.

[0029] After coarse grinding, the iron scale particles still have an uneven particle size distribution (some fine powder is mixed in the 200-300μm range), which leads to an inconsistent reaction interface when directly entering the reduction stage. While traditional fine grinding (jet milling) can increase fineness, it is prone to causing localized reduction or phase transformation of iron oxides due to transient high temperatures.

[0030] The present invention adopts nano sand mill for secondary refinement, and utilizes the shearing and collision effect of zirconium beads (diameter 0.3-0.5mm) under high-speed rotation (1000-1500rpm) to further crush the iron scale particles to D 90 ≤60μm. By limiting D 90 (90% point of cumulative particle size distribution) rather than a single maximum particle size, ensuring the concentrated distribution of powder particle size and avoiding the risk of agglomeration caused by excessive proportion of extremely fine particles (<10μm). During the sanding process, the microcracks of the iron scale particles expand under the action of continuous mechanical force, eventually forming fine particles with rough surface and porous interior, which significantly increases the specific surface area (10-20m 2 / g), providing sufficient reaction active sites for subsequent hydrogen reduction.

[0031] Preferably, in step (2): Hydrogen reduction includes gradient pressure control: Normal pressure stage: 500-520℃, hydrogen flow rate 10-15L / min, keep warm for 1-2 hours; Pressurization stage: 0.5-1.0 MPa, 540-560°C, hydrogen flow rate 5-10 L / min, and heat preservation for 2-3 hours.

[0032] The reduction process of iron oxides (Fe3O4 / FeO / Fe2O3) involves multiple phase transitions and oxygen removal. Conventional processes employ high-temperature reduction at a single pressure, resulting in rapid sintering and densification of the iron particles, which reduces the activity of the nitriding reaction.

[0033] This invention utilizes a moderate-temperature (500-520°C) reduction strategy during the atmospheric pressure stage, maintaining a moderate reducing atmosphere concentration by controlling the hydrogen flow rate (10-15 L / min). Within this temperature range, Fe₃O₄ is preferentially reduced to FeO, while some FeO further converts to α-Fe. Due to the low temperature, the diffusion and migration of iron particles is limited, resulting in a loosely packed, porous skeleton structure. The holding time (1-2 hours) is designed to ensure sufficient reduction of the iron oxides while avoiding excessive reaction that could lead to pore collapse.

[0034] Although the α-Fe skeleton after atmospheric pressure reduction possesses pores, the pore connectivity is poor and incompletely reduced FeO phase remains. Conventional pressure reduction is often performed at higher pressures (>2 MPa), which can easily induce plastic deformation of the iron particles and lead to pore closure.

[0035] During the pressurization phase, this method utilizes a synergistic effect of mild pressure (0.5-1.0 MPa) and slightly elevated temperature (540-560°C). The increased hydrogen pressure enhances the adsorption and dissociation of hydrogen molecules on the iron particle surface, promoting the deep reduction of residual FeO to α-Fe. Simultaneously, the pressure-driven hydrogen penetrates the pores, expanding pore size and enhancing connectivity through the "hydrogen erosion effect." Reducing the hydrogen flow rate to 5-10 L / min reduces mechanical damage to the pore structure caused by gas turbulence, and extending the holding time to 2-3 hours ensures the stable formation of the pore structure.

[0036] Preferably, in step (2): Water vapor is added to the hydrogen to control the dew point from -10°C to 0°C; During the cooling stage, the temperature is lowered to below 200°C at a rate of 5-10°C / min, and the material is discharged under argon protection.

[0037] The traditional hydrogen reduction process uses pure hydrogen. Although it can achieve the reduction of iron oxides, the resulting α-Fe particles have a smooth surface and low porosity.

[0038] The present invention adds a small amount of water vapor to hydrogen and controls the dew point (-10℃ to 0℃, corresponding to a water vapor content of 0.6-1.2g / m 3 ) precisely regulates the vapor phase humidity. At the reduction temperature (500-560°C), water vapor and FeO undergo a localized oxidation-reduction cycle (FeO + H2O → Fe3O4 + H2, followed by Fe3O4 being reduced to Fe by H2). This dynamic process etches nanoscale pores on the surface of the iron particles, forming a multi-level porosity structure (micrometer-scale through-holes + nanometer-scale surface pores). The limited dew point range ensures that the water vapor concentration is sufficient to induce the etching reaction without causing overall oxidation of the iron particles.

[0039] Reduced porous α-Fe powders oxidize rapidly when exposed to air at high temperatures, and rapid cooling can cause the pore structure to collapse. Traditional processes often use natural cooling or direct quenching with inert gas, which carries the risk of oxidation and thermal stress damage.

[0040] This method uses a controlled cooling rate (5-10°C / min) to slowly shrink the α-Fe lattice, preventing deformation or rupture of the pore structure caused by a sudden temperature drop. Argon gas protection (oxygen content ≤ 0.1%) is applied throughout the cooling process, blocking oxygen from contacting the active iron surface and preventing the formation of an oxide layer. Furthermore, argon's low thermal conductivity helps distribute heat evenly and reduces localized thermal stress concentrations. The cooling endpoint temperature is set below 200°C to ensure that the iron powder is thermodynamically stable before being transferred to the nitriding process.

[0041] Preferably, in step (3): The pulse ammonia gas injection cycle is 30-60 seconds to inject and 10-20 seconds to stop; Ammonia flow rate and temperature linkage control: Flow rate 25-30L / min at 400-420℃; Flow rate 15-20L / min at 430-450℃; The flow rate is 10-15L / min at 460-480℃.

[0042] The traditional nitriding process uses continuous introduction of ammonia, which easily forms a dense nitrided layer on the surface of the iron powder, hindering the diffusion of nitrogen atoms into the interior of the particles. In addition, excessive local ammonia concentration will trigger the formation of Fe4N impurity phase.

[0043] This invention employs a pulsed ammonia injection strategy, alternating between periodic injection (30-60 seconds) and pauses (10-20 seconds). This strategy leverages the vigorous fluidization of the iron powder particles within the fluidized bed to achieve surface nitridation during the ammonia injection phase. During the pause phase, frictional collisions between the particles remove the surface passivation layer, exposing fresh active sites. This design shifts the nitridation reaction from traditional "static diffusion" to "dynamic activated diffusion," significantly increasing the penetration depth of nitrogen atoms while preventing the formation of Fe₄N caused by excessive local ammonia concentrations.

[0044] Fe 16 The generation of N2 is extremely sensitive to temperature and ammonia concentration. The traditional process lacks dynamic regulation, causing the reaction path to deviate from the target phase.

[0045] The present invention establishes a temperature-flow rate linkage mechanism based on the thermodynamic and kinetic characteristics of the nitriding reaction: Low temperature section (400-420℃): Use high flow rate ammonia (25-30L / min) to compensate for the insufficient reaction rate at low temperature by enhancing the gas-solid mass transfer efficiency, ensuring that nitrogen atoms are quickly adsorbed to the iron surface; Medium temperature section (430-450℃): moderately reduce the flow rate (15-20L / min) to balance the surface reaction and bulk diffusion rate and inhibit the nucleation of Fe4N; High temperature section (460-480℃): further reduce the flow rate (10-15L / min) to slow down the nitriding reaction rate and avoid Fe 16 Decomposition tendency of N2.

[0046] This linkage mechanism precisely controls the nitridation reaction path through the dynamic matching of temperature and flow rate, allowing nitrogen atoms to gradually penetrate into the α-Fe lattice to form a stable Fe 16 N2 phase.

[0047] Preferably, in step (3): The nitriding endpoint is determined by the N2 / H2 volume ratio in the tail gas, and the reaction is terminated when the ratio reaches 1:3-1:5.

[0048] During the fluidized bed nitriding process, ammonia (NH3) decomposes into nitrogen atoms (N) and hydrogen atoms (H) on the surface of iron powder, among which nitrogen atoms penetrate into the α-Fe lattice to form Fe 16 N2, and hydrogen atoms combine to form H2 gas. As the nitriding reaction proceeds, undecomposed NH3 and the H2 and N2 generated by the reaction together form the exhaust system. The traditional process terminates the reaction by a fixed time or temperature, but it is impossible to accurately determine the degree of nitriding, which can easily lead to residual Fe4N or Fe 16 N2 decomposition.

[0049] The present invention innovatively uses the real-time monitoring of the volume ratio of N2 to H2 in the tail gas (1:3-1:5) as the reaction endpoint criterion. Its mechanism is: Chemical equilibrium correlation: The equilibrium constant of the ammonia decomposition reaction (2NH3→N2+3H2) changes dynamically with the nitriding process. 16 When the formation of N2 phase is nearly complete, the absorption rate of nitrogen atoms by α-Fe lattice decreases, and the proportion of N2 produced by decomposition of unreacted NH3 increases relatively, resulting in an increase in the N2 / H2 ratio in the exhaust gas. Phase change characteristic response: Fe 16 The generation of N2 requires a specific nitrogen atomic solubility (about 11.1 at.%). When the lattice nitrogen is close to saturation, the N2 produced by the decomposition of ammonia cannot be further absorbed. At this time, the N2 / H2 ratio reaches the critical range (1:3-1:5), indicating that the nitridation reaction has shifted from the main reaction stage to the side reaction (Fe 16 N2 decomposition or Fe4N generation) stage.

[0050] By terminating the reaction through this criterion, Fe 16 The criterion is based on the full generation of the N2 phase while avoiding the thermodynamic instability of the nitridation products at high temperatures. Compared with traditional methods, this criterion is directly related to the essential characteristics of the reaction process and has the advantage of dynamic self-adaptation.

[0051] Preferably, in step (4): Passivation is carried out by introducing 100-500 ppm CO2 into argon for 30-60 minutes; Screening was performed using a 325-500 mesh vibrating screen to remove particles larger than 50 μm.

[0052] Fe 16 The surface of N2 particles is highly active after nitridation and easily oxidizes to form Fe3O4 impurity phases when exposed to air. Traditional passivation processes often use oily coatings or strong oxidants, which pose a risk of contamination and require subsequent cleaning.

[0053] The present invention introduces a trace amount of CO2 (100-500ppm) under argon protection, and uses CO2 and Fe 16 The selective reaction of N2 surface active sites generates a dense FeCO3 passivation film. This passivation film has the following characteristics: Chemical stability: FeCO3 is difficult to react with oxygen and moisture at room temperature, effectively isolating the external environment from Fe 16 N2 erosion; Structural density: The matching design of CO2 concentration and passivation time (30-60 minutes) ensures that the passivation film evenly covers the particle surface, avoiding stress cracking caused by local excessive thickness; Environmental protection: No need for organic solvents or strong acid cleaning, direct pollution-free passivation is achieved.

[0054] Coarse particles (>50μm) contained in the nitrided product can cause magnetic property fluctuations due to incomplete nitriding or sintering agglomeration. Traditional screening processes use a single-mesh screen, which makes it difficult to achieve a balance between efficiency and precision.

[0055] The present invention uses a 325-500 mesh (corresponding to an aperture of 28-50 μm) vibrating screen for particle size control. Its innovation lies in: Particle size threshold setting: 50μm is the upper limit to accurately remove the insufficiently nitrided α-Fe coarse particles and sintered agglomerates to ensure Fe 16 N2 phase purity; Dynamic screening optimization: The mechanical vibration of the vibrating screen promotes particle dispersion, avoids screen clogging caused by electrostatic adsorption, and improves screening efficiency; Product uniformity guaranteed: the undersize (≤50μm) has a narrow particle size distribution (D 90 ≤60μm), providing consistent raw materials for subsequent magnet forming.

[0056] Preferably, in step (4): Ammonia recovery uses a molecular sieve adsorption tower with an adsorption temperature of 40-60°C and a desorption temperature of 150-200°C.

[0057] Traditional ammonia recovery processes often use water absorption or condensation methods, which suffer from high energy consumption, equipment corrosion, and low ammonia purity. This invention innovatively utilizes molecular sieve adsorption towers to purify and recover ammonia through a physical adsorption-desorption process.

[0058] Selective adsorption by molecular sieve: The pore size of the molecular sieve (0.3-0.5nm) matches the kinetic diameter of ammonia molecules (0.26nm), preferentially adsorbing ammonia (NH3) while excluding small molecules of hydrogen (H2) and nitrogen (N2), ensuring the purity of recovered ammonia (≥99.5%).

[0059] Optimized adsorption temperature (40-60°C): Within this temperature range, ammonia molecules have sufficient kinetic energy to diffuse into the pores of the molecular sieve while avoiding the loss of adsorption capacity caused by excessively high temperatures. This design balances adsorption rate with adsorbent loading, improving ammonia capture efficiency per cycle.

[0060] Desorption temperature control (150-200°C): Through medium-temperature desorption (not traditional high-temperature calcination), the adsorbed ammonia is released by utilizing the thermal expansion effect of the molecular sieve framework, while maintaining the stability of the molecular sieve crystal structure and avoiding pore collapse caused by high-temperature sintering. The desorbed ammonia is condensed and liquefied and can be directly returned to the nitridation reactor for recycling.

[0061] Preferably, the pore size of the molecular sieve is 0.3-0.5 nm, and the ammonia recycling rate is ≥95%.

[0062] The molecular dynamic diameter of ammonia (NH3) is about 0.26nm, while the molecular sizes of hydrogen (H2, 0.29nm) and nitrogen (N2, 0.36nm) accompanying the nitriding exhaust are similar. Traditional adsorption materials (activated carbon) have poor selectivity due to their wide pore size distribution.

[0063] The present invention uses a molecular sieve with a pore size of 0.3-0.5 nm, whose pore size is between that of NH3 and H2 / N2 molecules, and preferentially adsorbs ammonia through the molecular size screening effect. Specifically: Pore diameter lower limit (0.3nm): slightly larger than the diameter of NH3 molecule, allowing it to diffuse freely into the molecular sieve pores; Upper limit of pore size (0.5nm): smaller than the minimum free path of H2 / N2 molecules (considering the intermolecular collision effect), hindering their entry into the pores, thereby achieving physical separation of ammonia and impurity gases at the source.

[0064] The design of this pore size range ensures high adsorption capacity (≥200 mg / g) while significantly improving ammonia selectivity (NH3 / H2>100:1).

[0065] Ammonia recycling efficiency is limited by the adsorbent's regeneration capacity and process stability. Traditional processes suffer from low recycling rates due to coking or incomplete desorption of molecular sieves.

[0066] The present invention achieves the dynamic balance of molecular sieve adsorption and desorption through the coordinated control of adsorption temperature (40-60°C) and desorption temperature (150-200°C): Adsorption stage (40-60°C): In this temperature range, the thermal motion of ammonia molecules is enhanced, which accelerates the diffusion rate into the molecular sieve pores. The molecular sieve framework expands slightly due to moderate temperature increase, further expanding the effective adsorption surface area. Desorption stage (150-200℃): The molecular sieve skeleton shrinks due to heat, and the ammonia adsorbed in the pores is desorbed due to the weakening of van der Waals forces. At the same time, this temperature is far below the thermal decomposition threshold of the molecular sieve (>500℃), ensuring its structural stability and cycle life; Closed-loop circulation design: The high-purity ammonia after desorption is directly reused after condensation and liquefaction, combined with the pulse gas supply mode of the fluidized bed nitriding process to reduce the residual ammonia loss in the system.

[0067] The present invention provides a method for preparing Fe based on hot-rolled low-carbon steel oxide scale. 16 Low-cost production process for N2 magnetic materials. It has the following benefits: 1. The present invention optimizes the ammonia diffusion path and the contact area of the reaction interface by constructing a micron-nano multi-level pore structure, breaking through the mass transfer limitation of traditional single pores, and making the nitridation reaction depth consistent with the target phase (Fe 16 The purity of N2) is improved simultaneously, laying the material foundation for the preparation of high-performance magnets.

[0068] 2. The present invention adopts CO2 in-situ gas-solid reaction to generate a dense passivation film, replacing the traditional chemical reagent passivation process, which not only avoids the introduction of exogenous pollutants, but also realizes the closed-loop utilization of process tail gas, significantly improving the material's antioxidant properties and reducing the environmental load.

[0069] 3. The present invention selectively adsorbs and purifies ammonia in process waste gas through a design based on the precise matching of molecular sieve pore size and gas molecule size, breaking through the technical bottleneck of traditional adsorbents co-adsorbing impurities and promoting the construction of a green production system.

[0070] 4. The present invention eliminates coarse particle defects that have not fully reacted through the coordinated regulation of screening threshold and nitriding kinetics, solves the problem of uneven magnet density caused by residual heterogeneous phases, and improves the batch stability of magnetic properties.

[0071] 5. The present invention establishes a temperature-flow rate linkage mechanism to directionally inhibit the formation of impurity phases and shorten the reaction termination delay time, thereby reducing ineffective energy consumption and raw material waste, and providing a controllable and efficient process paradigm for large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0072] Figure 1 It is a schematic diagram of the process flow of the present invention. DETAILED DESCRIPTION

[0073] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0074] Example 1: Raw material pretreatment: Multi-stage magnetic separation: 0.6T, 1.5T and 2.0T magnetic fields were used in sequence to separate the iron scale and obtain Fe3O4 with a purity of 73wt%.

[0075] Pickling: 4.5wt% citric acid, solid-liquid ratio 1:5, reaction at 60℃ for 3.5 hours.

[0076] Drying: vacuum drying at 100°C for 2 hours, moisture content 0.5wt%.

[0077] Hydrogen reduction: Normal pressure stage: 510℃, H2 flow rate 12L / min, keep warm for 1.5 hours.

[0078] Pressurization stage: 0.8 MPa, 550°C, H2 flow rate 8 L / min, and heat preservation for 2.5 hours.

[0079] Fluidized bed nitriding: Pulse ammonia: 45 seconds on / 15 seconds off, flow rate linkage control: 410℃: 27L / min; 440℃: 17L / min; 470℃: 12L / min.

[0080] End point determination: terminate when the exhaust gas N2 / H2 ratio reaches 1:4.

[0081] Post-processing: Passivation: 300 ppm CO2 was introduced into argon for 45 minutes.

[0082] Screening: 400 mesh vibrating screen to remove particles larger than 50 μm.

[0083] Ammonia recovery: molecular sieve adsorption temperature 50℃, desorption temperature 175℃.

[0084] Example 2: Raw material pretreatment: Multi-stage magnetic separation: 0.7T, 1.8T, 2.2T magnetic field separation, Fe3O4 purity 75wt%.

[0085] Pickling: 5.0wt% citric acid, solid-liquid ratio 1:6, reaction at 65℃ for 4 hours.

[0086] Drying: vacuum drying at 110℃ for 1.5 hours, moisture 0.3wt%.

[0087] Hydrogen reduction: Normal pressure stage: 520℃, H2 flow rate 15L / min, keep warm for 1 hour.

[0088] Pressurization stage: 1.0 MPa, 560°C, H2 flow rate 10 L / min, and heat preservation for 2 hours.

[0089] Fluidized bed nitriding: Pulse ammonia: 60 seconds on / 10 seconds off, flow rate linkage control: 420℃: 30L / min; 450℃: 20L / min; 480℃: 15L / min.

[0090] End point determination: terminate when the exhaust gas N2 / H2 ratio reaches 1:3.

[0091] Post-processing: Passivation: 500ppmCO2, passivation for 30 minutes.

[0092] Screening: 325 mesh vibrating screen to remove particles larger than 50 μm.

[0093] Ammonia recovery: adsorption temperature 60℃, desorption temperature 200℃.

[0094] Example 3: Raw material pretreatment: Multi-stage magnetic separation: 0.5T, 1.2T, 1.8T magnetic field separation, Fe3O4 purity 70wt%.

[0095] Pickling: 4.0wt% citric acid, solid-liquid ratio 1:4, reaction at 55℃ for 3 hours.

[0096] Drying: vacuum drying at 90℃ for 3 hours, moisture 0.6wt%.

[0097] Hydrogen reduction: Normal pressure stage: 500°C, H2 flow rate 10L / min, keep warm for 2 hours.

[0098] Pressurization stage: 0.5 MPa, 540°C, H2 flow rate 5 L / min, and heat preservation for 3 hours.

[0099] Fluidized bed nitriding: Pulse ammonia: 30 seconds on / 20 seconds off, flow rate linkage control: 400℃:25L / min; 430℃:15L / min; 460℃:10L / min.

[0100] End point determination: terminate when the exhaust gas N2 / H2 ratio reaches 1:5.

[0101] Post-processing: Passivation: 100ppmCO2, passivation for 60 minutes.

[0102] Screening: 500 mesh vibrating screen to remove particles larger than 50 μm.

[0103] Ammonia recovery: adsorption temperature 40℃, desorption temperature 150℃.

[0104] Comparative Example 1: Compared with Example 1, the difference is that water vapor is not added during the hydrogen reduction stage (no dew point control), and only pure hydrogen is used for reduction. The remaining steps and parameters are the same.

[0105] Comparative Example 2: Compared with Example 1, the difference is that: the hydrogen reduction stage only adopts a single pressure mode (0.8 MPa pressurization throughout the process), the normal pressure-pressurization stage control is cancelled, and the remaining steps and parameters are the same.

[0106] Comparative Example 3: Compared with Example 1, the difference is that continuous ammonia gas is introduced (without pulse start and stop) in the fluidized bed nitriding process, the flow rate is fixed at 20 L / min, and the other steps and parameters are the same.

[0107] Comparative Example 4: Compared with Example 1, the difference is that: in the passivation process, immersion in a silane coupling agent solution for 30 minutes is used instead of the passivation step of passing CO2 into argon gas, and the other steps and parameters are the same.

[0108] Comparative Example 5: Compared with Example 1, the difference is that an activated carbon adsorption tower (pore size 1-2 nm) is used instead of 0.3-0.5 nm molecular sieve in the ammonia recovery process, the adsorption temperature is 60°C, and the desorption temperature is 250°C. The other steps and parameters are the same.

[0109] Comparative Example 6: Compared with Example 1, the difference is that a 200-mesh sieve (pore size 75 μm) is used to remove particles in the screening process, and the other steps and parameters are the same.

[0110] Comparative Example 7: Compared with Example 1, the difference is that the linkage control of ammonia flow rate and temperature is cancelled in the fluidized bed nitriding process, the flow rate of the whole stage is fixed at 20 L / min, and the other steps and parameters are the same.

[0111] Comparative Example 8: Compared with Example 1, the difference is that the concentration of CO2 introduced in the passivation process is adjusted to 800 ppm (outside the range of 100-500 ppm), and the other steps and parameters are the same.

[0112] Comparative Example 9: Compared with Example 1, the difference is that the pore size of the molecular sieve adsorption tower is adjusted to 0.6 nm (beyond the range of 0.3-0.5 nm in the claim), and the remaining steps and parameters are the same.

[0113] Comparative Example 10: Compared with Example 1, the difference is that the vibration screening process is eliminated and the nitrided product is directly collected. The remaining steps and parameters are the same.

[0114] Test Example 1: Nitriding Efficiency and Product Purity Test Experiment Description Experimental procedures Sample preparation: Take 5 g of each of the nitrided products of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3, grind them to a size of less than 200 mesh and set aside.

[0115] XRD test (Fe 16 N2 phase purity analysis): Instrument: X-ray diffractometer (Cu-Kα radiation, scanning range 20°-80°, step size 0.02°).

[0116] Method: The sample was scanned in full spectrum and the Fe 16 The mass fraction of the N2 phase is quantitatively determined based on the integrated intensity of the characteristic peaks (200) and (211).

[0117] Microhardness test (nitriding depth analysis): Instrument: Vickers hardness tester (load 200g, holding time 15 seconds).

[0118] Method: After the sample was mounted and polished, the hardness value was measured every 10 μm along the cross-section direction. The position where the hardness value dropped to 90% of the matrix hardness (≈120 HV) was defined as the depth of the nitriding layer.

[0119] Data Records: Each sample was tested 3 times and the average value was taken.

[0120] The final experimental data are shown in Table 1 below: Table 1 - Nitriding efficiency and product purity test results The present invention induces a dynamic oxidation-reduction cycle in the α-Fe matrix by incorporating water vapor and controlling the dew point during the hydrogen reduction phase, forming a micron-nano multi-level pore structure. The test results of Example 1 show that this structure significantly improves the nitridation efficiency and product purity (Fe 16 The purity of N2 phase reaches 95.3%, and the nitriding depth is 51.2μm. In contrast, the porosity of Comparative Example 1 is insufficient due to the lack of water vapor regulation, which hinders the diffusion of ammonia and Fe 16The purity of the N2 phase dropped to 82.7%, and the nitridation depth was only 28.5μm. This difference confirms the role of in-situ multi-level pore construction in promoting the nitridation reaction: the micron-scale through-holes provide a fast diffusion channel for ammonia, while the nanoscale surface pores increase the reaction contact area. The two work together to overcome the mass transfer limitations of traditional single pore structures.

[0121] In Example 1, hydrogen reduction was controlled by stepwise control from atmospheric pressure to pressurized pressure. The penetration and etching of hydrogen were regulated by gradient pressure, which increased the pore connectivity to 80%. In Comparative Example 2, a single pressurized mode (0.8 MPa) was used. Although the pore size was enlarged, the pore distribution uniformity was reduced due to the lack of step-by-step reduction in the atmospheric pressure stage. The pores were blocked in some areas, resulting in a nitriding depth of only 35.8 μm. 16 The purity of the N2 phase dropped to 88.4%. This result proves that the staged pressure strategy achieves precise control of pore morphology by optimizing the solid-gas reaction path between hydrogen and Fe3O4, providing a uniform reaction interface for subsequent nitridation.

[0122] In addition, Example 1 adopts pulsed ammonia gas injection and flow rate-temperature linkage control to suppress the formation of Fe4N impurity phase through periodic interface refresh. Comparative Example 3 adopts continuous ammonia gas injection (flow rate is fixed at 20L / min). Due to insufficient reaction interface refreshment, ammonia is excessively adsorbed on the particle surface to form a passivation layer, resulting in Fe 16 The purity of the N2 phase dropped sharply to 76.9%, and the nitriding depth was only 23.1μm. This shows that the dynamic interface management technology maintains the continuous penetration of active nitrogen atoms by balancing the ammonia supply and reaction consumption rate, thereby ensuring the Fe 16 Efficient directional growth of N2 phase.

[0123] Test Example 2: Passivation Film Stability Test Experiment Description Experimental procedures Sample preparation: 3 g of each of the final products of Example 1, Comparative Example 4 (silane passivation), and Comparative Example 8 (CO2 800 ppm passivation) were taken, and the surfaces were cleaned and set aside.

[0124] Oxidation weight gain rate test: Equipment: Constant temperature and humidity chamber (temperature 40℃±1℃, humidity 85%±3%).

[0125] Method: The sample was placed in a crucible and weighed (accuracy 0.1 mg). After continuous exposure for 72 hours, the sample was re-weighed and the oxidation weight gain per unit surface area (mg / cm²) was calculated.

[0126] Electrochemical impedance spectroscopy: Equipment: Electrochemical workstation (frequency range 10 5 -10 -2Hz, amplitude 10mV).

[0127] Methods: The sample was used as the working electrode, platinum sheet as the counter electrode, and Ag / AgCl as the reference electrode. The impedance spectrum was measured in 3.5wt% NaCl solution, and the passive film resistance (Ω·cm 2 ).

[0128] Data Records: Each sample was tested three times, and the median was taken after removing outliers.

[0129] The final experimental data are shown in Table 2 below: Table 2 - Passivation film stability test results The present invention uses CO2 in-situ passivation technology. By precisely controlling the CO2 concentration (300ppm) in argon, a gas-solid reaction between the surface of α-Fe particles and CO2 is induced, forming a dense FeCO3 film with uniform thickness (about 5nm). The test results of Example 1 show that the passivation film exhibits excellent antioxidant properties in a hot and humid environment (oxidation weight gain rate of 0.42mg / cm 2 ), the passivation film resistance is as high as 8.7×10 4 Ω·cm 2 In contrast, in Comparative Example 4, silane solution was used for passivation. However, due to the weak interfacial bonding between the silane molecules and the iron matrix, microporous defects existed in the passivation film, which led to a significant increase in the oxidation weight gain rate to 2.15 mg / cm 2 , the resistance value is only 1.2×10 4 Ω·cm 2 This difference demonstrates the dual advantages of CO2 gas-solid reaction passivation in terms of film density and environmental friendliness: the FeCO3 film tightly covers the substrate surface through chemical bonding, blocking water and oxygen penetration, while traditional silane passivation relies on physical adsorption and is easily ineffective due to thermal stress or humidity changes.

[0130] Comparative Example 8 increases the CO2 concentration to 800ppm, which exceeds the preferred range, resulting in uneven passivation film thickness (local area>15nm) and microcracks on the surface. The test results show that its oxidation weight gain rate (1.83mg / cm 2 ) is better than Comparative Example 4, but still significantly worse than Example 1, and the passivation film resistance (3.9×10 4 Ω·cm 2 ) is only 45% of that in Example 1. This phenomenon reveals the nonlinear relationship between CO2 concentration and the growth dynamics of the passivation film: at low concentrations, the reaction rate is controllable, which is conducive to uniform film formation; at high concentrations, CO2 rapidly adsorbs and reacts on the particle surface, causing local over-thickness and stress concentration, which damages the integrity of the film layer.

[0131] The performance advantages of the passivation film in Example 1 further demonstrate the synergistic design of the present invention: CO2 passivation eliminates the need for exogenous chemical reagents, directly utilizing CO2 from process exhaust gas for surface encapsulation, and, together with molecular sieve ammonia recovery technology, creates a green closed-loop system. The shortcomings of Comparative Examples 4 and 8 demonstrate that traditional passivation methods or uncontrolled parameters can undermine this synergistic effect, validating the irreplaceable nature of the present invention in terms of material stability and process sustainability.

[0132] Test Example 3: Ammonia Recovery Efficiency Test Experiment Description Experimental procedures Sample preparation: 10 L of tail gas from each of the ammonia recovery systems of Example 1, Comparative Example 5 (activated carbon adsorption), and Comparative Example 9 (0.6 nm molecular sieve) (standard operating conditions) were collected and stored in air bags for future use.

[0133] Adsorption-desorption cycle test: Equipment: fixed bed adsorption tower (diameter 50 mm, filling height 200 mm), temperature control system (accuracy ±1°C).

[0134] method: Adsorption stage: the tail gas is passed into the adsorption tower at a flow rate of 5 L / min, and the adsorption temperature is respectively 50°C in Example 1, 60°C in Comparative Example 5, and 40°C in Comparative Example 9, until the outlet NH3 concentration is ≥100 ppm; Desorption stage: switch to nitrogen purge (flow rate 2 L / min), the desorption temperatures are respectively as follows: Example 1 (175°C), Comparative Example 5 (250°C), and Comparative Example 9 (150°C), the desorbed gas is collected and the volume is measured.

[0135] Ammonia purity analysis: Instrument: Gas chromatograph (TCD detector, PorapakQ column).

[0136] Method: 1 mL of desorbed gas sample was injected and the NH3 purity was calculated by peak area normalization method.

[0137] Data Records: Each sample was subjected to 10 repeated adsorption–desorption cycles, and the cumulative recovery and average purity were calculated.

[0138] The final experimental data are shown in Table 3 below: Table 3 - Ammonia recovery efficiency test results The present invention uses a molecular sieve with a pore size of 0.3-0.5nm as an ammonia adsorbent, and achieves selective adsorption and efficient recovery based on the precise matching of the kinetic diameter of the NH3 molecule (0.26nm) and the pore size of the molecular sieve. The test data of Example 1 shows that under the coordinated control of the adsorption temperature of 50°C and the desorption temperature of 175°C, the cumulative recovery rate of ammonia reaches 95.2% and the purity is 99.6%. In contrast, Comparative Example 5 uses activated carbon with a pore size of 1-2nm. Due to the large pore size, impurity gases such as CO and H2O are co-adsorbed, and the recovery rate drops sharply to 78.4%, and the purity is only 89.3%. This difference shows that the pore size selectivity of the molecular sieve is the core of ensuring the efficiency of ammonia recovery: when the pore size is close to the size of the NH3 molecule, the van der Waals force is enhanced, and the target gas is preferentially adsorbed, while adsorbents with larger pores cannot effectively eliminate impurity interference due to the lack of size screening effect.

[0139] In Comparative Example 9, the molecular sieve pore size was adjusted to 0.6 nm. Although it was still able to partially adsorb NH3, the pore size was slightly larger than the optimal range, resulting in a decrease in adsorption capacity (cumulative recovery rate of 83.7%) and a decrease in cycle stability (the purity of the 5th cycle dropped to 87.5%). This phenomenon reveals the dynamic equilibrium relationship between the molecular sieve pore size and the diffusion rate of gas molecules: when the pore size is too small, gas diffusion is hindered; when the pore size is too large, the effective surface area of the adsorbent is reduced. In Example 1, the 0.3-0.5 nm molecular sieve achieves a balance between adsorption kinetics and thermodynamics by optimizing the pore size distribution, so that high-purity NH3 can be released at only 175°C during the desorption stage, avoiding the problem of molecular sieve structure collapse caused by high temperature (250°C in Comparative Example 5).

[0140] Furthermore, the coordinated adsorption-desorption temperature design (50°C for adsorption and 175°C for desorption) in Example 1 further strengthens the closed-loop process: moderately elevated adsorption temperatures enhance the migration rate of NH3 within the molecular sieve pores, while moderate desorption temperatures protect the adsorbent crystal structure and ensure cycle stability. The degradation results of Comparative Examples 5 and 9 demonstrate that deviation from the coordinated temperature-pore size control mechanism directly undermines the economic viability and sustainability of the ammonia recovery system.

[0141] Test Example 4: Particle Size Control and Impurity Phase Test Experiment Description Experimental procedures Sample preparation: 10 g of each of the nitrided products of Example 1, Comparative Example 6 (200 mesh sieve), and Comparative Example 10 (no sieving) were dispersed in ethanol and ultrasonically treated for 5 minutes to remove agglomerates.

[0142] Laser particle size analysis: Instrument: Laser diffraction particle size analyzer (measuring range 0.1-1000 μm, dispersion pressure 0.5 MPa).

[0143] Method: The sample was suspended in deionized water and ultrasonically dispersed for 30 seconds before testing. The D 90 (Particle size corresponding to 90% of the cumulative distribution).

[0144] Coarse particle impurity analysis: Magnetic separation: The sample was magnetically separated using a 0.5 T permanent magnet to enrich the coarse unnitrided α-Fe particles (>50 μm) with weak magnetic properties. Chemical titration: Dissolve the coarse particles in dilute hydrochloric acid and determine Fe by EDTA titration. 2+ Content, converted into α-Fe residual content (wt%).

[0145] Data Records: Each sample was tested 3 times, and the maximum and minimum values were taken.

[0146] The final experimental data are shown in Table 4 below: Table 4 - Particle size control and impurity phase test results The present invention uses a vibration screening process (325-500 mesh screen combination) to accurately intercept the insufficiently nitrided α-Fe coarse particles (> 50 μm), ensuring the uniformity of the product particle size distribution (D 90 ≤60μm). The test data of Example 1 shows that the residual α-Fe after screening is only 1.8wt%, indicating that the nitriding reaction is almost complete. However, in Comparative Example 6, a 200-mesh screen (pore size 75μm) is used. Due to the large pore size, the coarse particles cannot be effectively removed, resulting in an increase of the residual α-Fe content to 12.4wt%. 90 This difference reveals the intrinsic relationship between the screening threshold and the kinetics of the nitriding reaction: coarse particles, due to their low specific surface area and long mass transfer path, are prone to forming unreacted α-Fe nuclei during the nitriding process. Precise screening can remove such defective particles in advance, preventing them from entering the subsequent process and interfering with magnet forming.

[0147] Comparative Example 10 After the screening process was eliminated, a large amount of unnitrided coarse particles (D 90 =105.2μm, α-Fe residual content 23.6wt%). Such particles have poor plastic deformation ability during the magnet pressing stage, causing local density unevenness and deterioration of magnetic properties. Example 1 uses screen classification control to limit the particle size to a range where the nitriding reaction is fully completed (≤50μm), ensuring that Fe 16 The high purity of the N2 phase and the consistency of magnet molding. Comparative results show that screening is not a simple physical separation step, but a key step in indirectly regulating the nitridation reaction process through particle size screening: fine particles have a large specific surface area and a developed pore structure, which makes it easier for nitrogen atoms to penetrate deeply and complete the phase transformation.

[0148] Further analysis found that the coarse particles that were not screened out would form a "shell-core" structure (surface nitrided, internal residual α-Fe) during the nitriding process, and its magnetic characteristics were similar to those of Fe 16 The N2 phase exhibits significant differences, resulting in a decrease in the magnet's coercivity and remanence. The screening strategy of Example 1, by eliminating these heterogeneous particles, reduces the density fluctuation of the magnet molding process to less than 5%, while the density fluctuations of Comparative Examples 6 and 10 reach 12% and 18%, respectively. This demonstrates the closed-loop synergy between the screening process, nitriding kinetics, and magnet processing performance, demonstrating the innovative nature of this invention in process integration design and quality control.

[0149] Test Example 5: Reaction Path Control Test Experiment Description Experimental procedures Sample preparation: 3 g of each of the nitrided intermediate products (nitrided to the theoretical endpoint time) of Example 1 and Comparative Example 7 (with a fixed ammonia flow rate) were taken and ground into 300 mesh for later use.

[0150] XRD semi-quantitative analysis (Fe4N impurity phase detection): Instrument: X-ray diffractometer (scanning range 30°-60°, step size 0.02°, integration time 0.5 s / step).

[0151] Method: Through the (111)Fe4N peak and (200)Fe 16 The intensity ratio of the N2 peak was combined with the standard curve to calculate the mass fraction of the Fe4N phase (±0.5% error).

[0152] Determination of nitriding endpoint delay time: Equipment: Online temperature monitoring system (thermocouple accuracy ±1°C).

[0153] method: Record the time interval from when the temperature in the nitriding furnace reaches the set value (450°C) to when the ammonia flow is actually stopped; The NH3 concentration was monitored online by gas chromatography, and the termination signal was when the concentration dropped to 5% of the initial value.

[0154] Data Records: Each sample was tested 5 times, and the average value was taken after removing the extreme values.

[0155] The final experimental data are shown in Table 5 below: Table 5 - Reaction Path Control Test Results The present invention uses a linkage control strategy of temperature and ammonia flow rate to dynamically reduce the ammonia supply rate (gradually from 15L / min to 8L / min) in the later stage of the nitridation reaction, effectively suppressing the formation of Fe4N impurity phase. The test results of Example 1 show that the Fe4N content is only 2.3wt%, and the reaction endpoint delay time is shortened to 4.2 minutes, indicating that the reaction path accurately guides Fe4N to the target phase. 16 The main direction of the formation of N2 phase. In contrast, the fixed flow rate (15L / min) was used in Comparative Example 7, which resulted in local supersaturation of ammonia in the high temperature stage, prompting the Fe4N phase to undergo the solid solution decomposition path (Fe→Fe4N→Fe 16 Large amounts of N₂ (8.7 wt%) were generated. Furthermore, because the flow rate was not reduced promptly, residual ammonia continued to react during the cooling phase, extending the endpoint delay to 31.5 minutes. This discrepancy reveals a coordinated regulation mechanism between reaction kinetics and thermodynamic equilibrium: dynamically reducing the flow rate can match the temperature dependence of the nitrogen atomic diffusion rate, preventing nitrogen accumulation caused by excessive NH₃ dissociation at the reaction interface, thereby blocking the formation of the metastable Fe₄N phase.

[0156] The defects of Example 7 further verify the process design logic of the present invention: in the early stage of the nitridation reaction (300-400 ° C), a higher flow rate (15 L / min) can quickly establish the NH3 partial pressure and drive nitrogen atoms to penetrate into the α-Fe lattice; when the temperature rises to above 450 ° C, the nitrogen diffusion rate increases significantly. At this time, reducing the flow rate (8 L / min) can maintain the reaction driving force and prevent the surface nitrogen concentration from exceeding the Fe 16 Thermodynamic stability range of N2 phase. Example 1 matches the flow rate and temperature change in real time to keep the reaction in the Fe 16 The N2 phase is the dominant growth zone, while a fixed flow rate breaks this balance, causing the side reaction path to dominate.

[0157] In addition, the shortening of the endpoint delay time (only 4.2 minutes in Example 1) reflects the optimization effect of linkage control on the reaction termination criterion: when the temperature reaches the set value, the system reduces the NH3 supply by adjusting the flow rate, and quickly determines the reaction endpoint in conjunction with online concentration monitoring to avoid ineffective energy consumption. However, since Comparative Example 7 does not establish a flow rate-temperature correlation mechanism, it needs to rely on a lagging temperature signal to terminate the reaction, resulting in ammonia waste and impurity accumulation. The present invention realizes efficient directional control of the nitridation reaction path through the coordinated regulation of multiple parameters, which is a good solution for Fe 16 It provides key technical support for the large-scale preparation of N2 phase.

[0158] While embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions, and variations may be made to these embodiments without departing from the principles and spirit of the invention, and that the scope of the invention is defined by the appended claims and their equivalents.

Claims

1. Preparation of Fe based on hot-rolled low-carbon steel oxide scale 16 A low-cost production process for N2 magnetic materials, characterized in that: The following steps are involved: (1) Raw material pretreatment: The iron scale is subjected to multi-stage magnetic separation, pickling, graded crushing and refinement in sequence; (2) Hydrogen reduction: The pretreated iron scale powder is gradient reduced in a hydrogen atmosphere to generate porous α-Fe powder; (3) Fluidized bed nitriding: The porous α-Fe powder is placed in a fluidized bed reactor and pulsed ammonia gas is introduced for nitriding; (4) Post-processing: Screening of nitridation products, recovery of ammonia after passivation and packaging.

2. The method for preparing Fe based on hot-rolled low-carbon steel oxide scale according to claim 1 16 A low-cost production process for N2 magnetic materials, characterized in that: In step (1): Multi-stage magnetic separation uses three levels of magnetic field strength, namely 0.5-0.8T, 1.0-1.5T, and 1.8-2.2T; For pickling, use 3-5wt% citric acid solution, solid-liquid ratio 1:5, temperature 50-60℃; The iron scale after pickling is vacuum dried at 80-100℃ for 2-4 hours, and the moisture content is ≤0.5wt%.

3. The method for preparing Fe based on hot-rolled low-carbon steel oxide scale according to claim 1. 16 A low-cost production process for N2 magnetic materials, characterized in that: In step (1): Classification, crushing and refinement include: Coarse crushing to 200-300 μm in a tubular ball mill; Nano sand grinding to D 90 ≤60μm.

4. The method for preparing Fe based on hot-rolled low-carbon steel oxide scale according to claim 1 16 A low-cost production process for N2 magnetic materials, characterized in that: In step (2): Hydrogen reduction includes gradient pressure control: Normal pressure stage: 500-520℃, hydrogen flow rate 10-15L / min, keep warm for 1-2 hours; Pressurization stage: 0.5-1.0 MPa, 540-560°C, hydrogen flow rate 5-10 L / min, and heat preservation for 2-3 hours.

5. The method for preparing Fe based on hot-rolled low-carbon steel oxide scale according to claim 1 16 A low-cost production process for N2 magnetic materials, characterized in that: In step (2): Water vapor is added to the hydrogen to control the dew point from -10°C to 0°C; During the cooling stage, the temperature is lowered to below 200°C at a rate of 5-10°C / min, and the material is discharged under argon protection.

6. The method for preparing Fe based on hot-rolled low-carbon steel oxide scale according to claim 1 16 A low-cost production process for N2 magnetic materials, characterized in that: In step (3): The pulse ammonia gas injection cycle is 30-60 seconds to inject and 10-20 seconds to stop; Ammonia flow rate and temperature linkage control: Flow rate 25-30L / min at 400-420℃; Flow rate 15-20L / min at 430-450℃; The flow rate is 10-15L / min at 460-480℃.

7. The method for preparing Fe based on hot-rolled low-carbon steel oxide scale according to claim 6. 16 A low-cost production process for N2 magnetic materials, characterized in that: In step (3): The nitriding endpoint is determined by the N2 / H2 volume ratio in the tail gas, and the reaction is terminated when the ratio reaches 1:3-1:

5.

8. The method for preparing Fe based on hot-rolled low-carbon steel oxide scale according to claim 1 16 A low-cost production process for N2 magnetic materials, characterized in that: In step (4): Passivation is carried out by introducing 100-500 ppm CO2 into argon for 30-60 minutes; Screening was performed using a 325-500 mesh vibrating screen to remove particles larger than 50 μm.

9. The method for preparing Fe based on hot-rolled low-carbon steel oxide scale according to claim 1 16 A low-cost production process for N2 magnetic materials, characterized in that: In step (4): Ammonia recovery uses a molecular sieve adsorption tower with an adsorption temperature of 40-60°C and a desorption temperature of 150-200°C.

10. The method for preparing Fe based on hot-rolled low-carbon steel oxide scale according to claim 9. 16 A low-cost production process for N2 magnetic materials, characterized in that: The pore size of the molecular sieve is 0.3-0.5 nm, and the ammonia recycling rate is ≥95%.

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