Method for directly preparing neodymium iron boron powder from neodymium oxide and application

Abstract

The invention discloses a method for directly preparing neodymium iron boron powder from neodymium oxide and application, and the method comprises the following steps: S1, neodymium oxide, an iron source, a boron source and a carbon reducing agent are obtained, and the molar ratio of neodymium to iron to boron to carbon is 1: (13-15): 1: (2.8-3.5); s2, mixing the raw materials obtained in the step S1, and refining the raw materials until the granularity is 1-10 microns to obtain a mixture; s3, granulating or pre-pressing the mixture; and S4, putting the mixture treated in the step S3 into a high-temperature-resistant container, raising the temperature to 200-400 DEG C, preserving heat for 0.5-2 hours, raising the temperature to 1100-1400 DEG C, preserving heat for 0.5-6 hours, cooling and taking out. According to the technical scheme, the effects of remarkably reducing or avoiding dependence on strong-activity reducing agents such as Ca / CaH2 and the treatment cost of byproducts caused by dependence on strong-activity reducing agents such as Ca / CaH2 and improving the purity and the yield of the Nd2Fe14B phase are achieved.

Description

Technical Field

[0001] This invention relates to the field of neodymium iron boron permanent magnet material preparation technology, specifically to a method for directly preparing neodymium iron boron powder from neodymium oxide. Background Technology

[0002] Neodymium iron boron (Nd–Fe–B) sintered permanent magnets are typically manufactured industrially using the powder metallurgy route. The typical conventional process includes several steps such as alloy melting / ingot casting (vacuum induction melting), hot working (e.g., hot rolling or strip casting), pulverization / hydrogen embrittlement (HD) or atomization powdering, ball milling / refining, magnetic field directional pressing, degreasing, sintering (approximately 1000–1100℃), heat treatment and surface protection. The entire process involves many steps, complex equipment, and high requirements for the composition and particle size distribution of the powder.

[0003] To reduce costs or utilize oxidized raw materials and waste, academia and industry have explored various alternative routes using oxides as starting materials. Key representative directions include: 1. Reduction-Diffusion (RD) / Chemical Reduction Route (using strong reducing agents such as Ca, CaH2, Mg): First, an oxide precursor (e.g., prepared by sol-gel, hydrothermal, etc.) is mixed with a strong reducing agent, and then Nd2Fe is obtained through reduction-diffusion and post-treatment. 14 Phase B. This type of method can produce highly coercive microparticles on a laboratory scale, but it usually relies on a large amount of active metal reducing agent and produces a large number of by-products (such as CaO), requiring complex post-treatment such as acid washing / water washing.

[0004] 2. Mechanochemical / High-Energy Ball Milling Combined with Activated Reducing Agent: Mechanochemical reactions (mechanical reduction / activation) are achieved through high-energy ball milling, followed by Nd₂Fe production via subsequent heat treatment or with the aid of reducing agents such as Ca during the ball milling process. 14 B. This method can promote phase formation at lower temperatures, but it still requires the treatment of reducing agent residues and oxide byproducts, and scale-up is difficult.

[0005] 3. High-Temperature Carbothermic Reduction / Metallurgical Recovery Route: Commonly used in recycling and smelting, this method involves separating or recovering rare earth / iron components from oxides or waste magnets through high-temperature carbothermic reduction (typically requiring 1300–1500℃ or higher). While feasible for recovery, the carbothermic method is energy-intensive and prone to rare earth volatilization, carbide formation, and the generation of other byproducts. The resulting product is difficult to directly yield suitable high-purity Nd₂Fe for pressing and sintering. 14 B powder.

[0006] One-step direct synthesis: for example, using calcium to directly reduce Nd2O3 and synthesize Nd2Fe. 14 B, from neodymium oxide to Nd2Fe in one step14 While B is feasible in the laboratory, these methods still present challenges in terms of process stability, raw material compatibility (industrial neodymium oxide), by-product treatment, and industrial scale-up.

[0007] The main shortcomings of existing technologies (regarding "one-step preparation of industrially usable Nd2Fe from Nd2O3") 14 (Regarding the issue of "B powder") 1. Numerous process steps and complex flow: Most existing routes break down "oxide → metal Nd → alloying → powder preparation" into multiple operations (smelting / reduction, pickling / separation, re-alloying, etc.), which is not conducive to cost reduction and industrial scale-up.

[0008] 2. Reliance on highly active reducing agents or ultra-high temperature conditions: Although Ca / CaH2, Mg, etc. can complete the reduction, they produce a large number of solid by-products and bring environmental and treatment costs; the high-temperature carbothermal method consumes a lot of energy and has high equipment requirements.

[0009] 3. Difficulty in controlling Nd volatilization and phase purity: Under high temperature or strong reducing conditions, rare earth elements are prone to volatilization or the formation of undesirable carbides / oxides, leading to Nd2Fe... 14 The yield and magnetic properties of the B phase decrease, and additional magnetic separation / acid washing / heat treatment are required to improve them.

[0010] 4. The product is difficult to use directly in conventional pressing-sintering processes: even if Nd2Fe can be obtained... 14 The particle size, surface condition, impurities, and residual by-products of the B phase powder often require extensive post-processing to adapt it to industrial pressing and sintering. Summary of the Invention

[0011] To overcome the shortcomings of the aforementioned related technologies, this application provides a method for directly preparing NdFeB powder from NdO2, which directly converts Nd2O3 into high-content Nd2Fe. 14 A process route for obtaining B-phase magnetic powder suitable for pressing and sintering significantly reduces or avoids dependence on highly active reducing agents such as Ca / CaH2 and the associated byproduct treatment costs, thereby improving the Nd2Fe production capacity. 14 B-phase purity and yield.

[0012] This application provides a method for directly preparing neodymium iron boron powder from neodymium oxide, comprising the following steps: S1 obtains neodymium oxide, an iron source, a boron source, and a carbon reducing agent, wherein the molar ratio of neodymium, iron, boron, and carbon is: 1:(13–15):1:(2.8–3.5); S2. The raw materials obtained in step S1 are mixed and the particle size is refined to 1–10 μm to obtain a mixture; S3 granulates or pre-presses the mixture; S4. Place the mixture treated in step S3 into a high-temperature resistant container, heat it to 200–400°C, keep it at that temperature for 0.5–2 hours, then heat it to 1100–1400°C, keep it at that temperature for 0.5–6 hours, cool it, and remove it. Preferably, it also includes S5 post-processing: S5.1 Crushing: The product obtained in step S4 is coarsely crushed and ground under the protection of an inert gas. S5.2 Magnetic separation: Removes unreacted iron; S5.3 Low-temperature annealing: Anneal at a temperature of 600–800℃ for 0.5–2 hours; S5.4 Surface treatment: Phosphate coating, alkanolamine passivation, or vacuum drying followed by sealing under an inert atmosphere.

[0013] Preferably, the iron source in step S1 is metallic iron powder with a particle size of 1–20 μm; the boron source is FeB, metallic B, or a B-containing compound; and the carbon reducing agent is carbon black or microcrystalline graphite with a particle size of <5 μm.

[0014] Preferably, step S1 further includes an auxiliary agent, which is 1–3 wt% of CaO or MgO of the total raw material.

[0015] Preferably, in step S2, the raw materials are mixed and the particle size is refined by ball milling or planetary ball milling. The ball milling parameters are: ball-to-material ratio of 5:1-15:1, ball milling time of 6-12h, and rotation speed of 150-350rpm. The planetary ball milling parameters are: ball-to-material ratio of 5:1-15:1, ball milling time of 12-24h, and rotation speed of 200-600rpm.

[0016] Preferably, ethanol or isopropanol is added during ball milling or planetary ball milling, wherein the amount of ethanol or isopropanol added is ≤5wt%, and after mixing, the ball-milled product is dried at 60–120°C for 0.5–2h.

[0017] Preferably, step S3 specifically involves lightly granulating or isostatically pressing the mixed powder. Granulation: spray granulation followed by drying of the granules; isostatic pressing: pressing into small blocks 5–20 mm thick at a pressure of 50–300 MPa.

[0018] Preferably, in step S4, the high-temperature resistant container is a graphite crucible or a high-temperature resistant metal container, and the reaction atmosphere is a high-purity inert atmosphere or a medium-high vacuum. The high-purity inert atmosphere is high-purity Ar with O2 ≤ 100 ppm, and the medium-high vacuum is 10 ppm. -2 -10 -3 Pa, the heating rate is 5–20℃ / min, and the cooling rate is controlled at 5–20℃ / min.

[0019] An application of NdFeB powder involves directional pressing, degreasing, sintering at 1000–1100℃, and conventional annealing of the powder obtained by the above method to produce NdFeB sintered permanent magnets.

[0020] Compared with related technologies, the present invention has the following advantages: Using Nd₂O₃, Fe, FeB (or other quantifiable boron sources) and carbon as raw materials, Nd₂Fe is directly generated in a single heat treatment process through carbothermic reduction-alloying. 14 B significantly simplifies the process compared to multi-step RD / recycling routes.

[0021] By using carbon (carbon black / microcrystalline graphite) as the main reducing agent and combining it with mechanical activation, airtightness and atmosphere control, the dependence on active metals such as Ca / CaH2 is reduced, and the amount of acid washing / waste treatment is reduced.

[0022] By optimizing parameters such as the C / O ratio, Fe / B ratio, ball milling activation level, heating rate, holding temperature and time, and crucible sealing / filler protection, Nd volatilization is effectively reduced, the formation of carbides and unreacted oxides is controlled, and Nd2Fe2+ is improved. 14 Selective formation of phase B.

[0023] The product can be directly adapted to industrial processes: the target particle size and surface condition of the obtained powder can be directly entered into the magnetic field directional pressing-degreasing-sintering process, reducing subsequent magnetic separation, acid washing or complex processing steps, and making it easy to connect with existing powder metallurgy production lines.

[0024] Economic and environmental advantages: Fewer process steps, avoidance of large amounts of active reducing agents and complex waste treatment, reduced energy consumption and environmental protection costs, resulting in better cost-effectiveness and sustainability.

[0025] Industrial scale-up friendly: The process parameters can be replicated in conventional high-temperature furnaces (belt / rotary furnaces or vacuum / inert atmosphere furnaces), facilitating large-scale production and technology scale-up. Attached Figure Description

[0026] Figure 1 This is a scanning electron microscope image from Example 1; Figure 2 This is a scanning electron microscope image from Example 2; Figure 3 This is a scanning electron microscope image from Example 3. Detailed Implementation

[0027] First, those skilled in the art should understand that these embodiments are merely used to explain the technical principles of the embodiments of this application and are not intended to limit the scope of protection of the embodiments of this application. Those skilled in the art can make adjustments as needed to adapt to specific application scenarios.

[0028] The present invention will be further described in detail below with reference to specific embodiments.

[0029] Raw materials and proportions (molar basis and examples) Types of raw materials: Nd2O3 (technical or industrial grade, ≥99% preferred); Metallic iron powder (Fe, particle size 1–20 μm, ≥99%) Boron source: FeB is preferred (the B content should be clearly indicated for measurement purposes), but metallic B or B-containing compounds can also be used; Carbon reducing agent: carbon black or microcrystalline graphite, particle size <5μm, high specific surface area preferred; (Optional) Additives / protectants: a small amount of inert oxides (such as CaO, MgO) or excess Nd2O3 as a "protective pad".

[0030] Molar ratio (theoretical stoichiometry, preferred range): Theoretical formula: Nd2O3:Fe:FeB:C≈1:13:1:3 (molar ratio). To compensate for losses or adjust the phase, Fe can be taken as 13–15 mol, C as 2.8–3.5 mol, and FeB is adjusted by equivalent amount according to its actual B content (see mass example below).

[0031] Example of mass calculation (based on 1 mol Nd₂O₃, used for engineering conversion and batching calculation): Nd₂O₃: 336.481 g / mol; 13molFe: 13×55.845=725.985g; 1 mol FeB (Fe+B): 55.845 + 10.81 = 66.655 g (Note: The B content of commercially available FeB materials varies and must be corrected according to the actual product instructions). 3molC: 3 × 12.011 = 36.033g.

[0032] (Total approximately 1165.15g; industrial formulations are calculated based on batch size and may allow for a slight surplus of Fe or slight adjustment of C to avoid excessive carbides.) Mixing and mechanical activation Mixing method: ball mill (box / tube ball mill) or mixer (planetary ball mill for high-energy activation).

[0033] Ball milling parameters (preferred range): Ball-to-powder ratio (BPR): 5:1 to 15:1, preferably 8:1–12:1; Ball milling time: 2–24 hours (6–12 hours is preferred in the laboratory); high-energy ball milling may require 12–24 hours to achieve mechanical activation; Speed / Energy: Planetary ball mill 200–600 rpm (depending on equipment); Ball mill 150–350 rpm; Grinding aids: A small amount of ethanol or isopropanol (≤5wt%) can be added to reduce agglomeration and dust; Objective: To refine the particle size to d50≈1–10μm, increase the specific surface area, and induce defects / activation to reduce the synthesis temperature and shorten the synthesis time.

[0034] Post-mixing treatment: Dry the ball-milled product (60–120℃, 0.5–2h), and load it in a glove box or under an inert atmosphere to minimize secondary oxidation.

[0035] Granulation / Pre-compression To facilitate high-temperature processing and increase bulk density, the mixed powder can be lightly granulated or isostatically pressed into small round cakes / blocks: Granulation method: spray granulation / drying granules or pressing into small blocks 5–20 mm thick; Pre-compression density: Compression pressure 50–300 MPa (depending on the equipment) to ensure that the sample obtains a stable contact interface during heat treatment, which is conducive to solid-phase diffusion.

[0036] Carbothermic reduction-alloying heat treatment Loading and protection: The dried mixture is placed in a graphite crucible or a high-temperature resistant metal container (e.g., lined with Mo, Ta, or stainless steel, but care must be taken to avoid reaction with carbon). A graphite crucible is preferred, and a small amount of Nd2O3 protective pad is placed on the outer layer of the crucible to reduce Nd volatilization. For large-scale production, sealed metal containers (with a short period of local vacuum followed by inert gas filling) or graphite / heat-resistant plugs can be used to reduce evaporation losses.

[0037] Atmosphere and pressure: Preferred atmospheres include high-purity inert atmospheres (high-purity Ar, O2 ≤ 100 ppm) or medium-high vacuum atmospheres (10 ppm). -2 -10 -3 Pa) conditions.

[0038] Gas flow rate (inert gas): 0–200 mL / min (depending on furnace type and sample volume); static inert gas can be used if a closed crucible is used.

[0039] Heating program: Pre-degassing: 200–400℃, keep warm for 0.5–2h (to remove moisture and solvent residue); Heating rate: 5–20℃ / min (preferably 10℃ / min); Reaction holding temperature: 1100–1400℃ (preferably 1150–1350℃); Insulation time: 0.5–6h (preferably 1–4h); Note: Higher temperatures are conducive to phase formation, but increase the probability of Nd volatilization and carbonization of secondary phases; 1200–1300℃ and holding for 1–3 hours is preferred.

[0040] Cooling: Allow to cool naturally to room temperature or allow to cool in a controlled manner at 5–20 °C / min; remove under an inert atmosphere.

[0041] Safety and Emissions: Carbothermic reactions produce CO / CO2, so combustion or absorption devices and proper ventilation should be provided to avoid harmful emissions.

[0042] Post-processing and powder conditioning After the product is removed, it is coarsely crushed and ground in an inert gas or glove box (ground to the required particle size). Magnetic separation: removes large, unreacted Fe or other metallic impurities; Chemical or thermal treatment (optional): Low-temperature annealing (600–800℃, 0.5–2h) to improve crystal order; avoid strong acid washing to prevent loss of rare earth elements; Surface treatment: To prevent oxidation, phosphate coating, alkanolamine passivation, or vacuum drying followed by sealing in an inert atmosphere can be performed.

[0043] For use in sintered permanent magnets: the powder is oriented, pressed, degreased, sintered at 1000–1100°C, and then annealed according to conventional methods.

[0044] Equipment: Planetary ball mill or tubular ball mill, graphite crucible and lid, vacuum or inert gas tubular high-temperature furnace (up to 1400℃), vacuum pump, inert gas (Ar) cylinder and flow controller, glove box (inert gas), magnetic separator, mortar or grinder, balance, weighing paper or weighing boat, carbon / oxygen analyzer (manufacturer LECO), inductively coupled plasma optical emission spectrometer / mass spectrometer (ICP-OES / MS), X-ray diffractometer (XRD), scanning electron microscope-energy dispersive X-ray spectrometer (SEM / EDS), vibrating sample magnetometer (VSM).

[0045] Safety / Exhaust Gas: Exhaust gas burner or oxidizer, online CO or CO2 detector, local exhaust hood (LAF) and protective clothing / gloves / eyewear.

[0046] Example 1 raw material: Nd2O3: 336.48g Fe: 725.99g FeB: 66.66g C (carbon black): 36.03g.

[0047] Detailed steps Weighing: Accurately weigh each raw material in a drying oven or glove box, and record the batch number and quality of each material.

[0048] Preliminary mixing and ball milling: Load all powders into a ball mill jar (stainless steel or graphite liner optional), and add stainless steel or alumina balls (BPR10:1).

[0049] Add a small amount of anhydrous ethanol (about 2–3 wt% of the total material mass) as a grinding aid to reduce dust.

[0050] Planetary ball milling: 300 rpm, intermittent operation (e.g., 30 min run every 10 min stop), for a total of 10 h (or conventional ball milling 12 h); after completion, place in an inert gas glove box to dry (60–80℃ vacuum drying for 2 h) and cool to room temperature.

[0051] Loading the crucible: Transfer the dried powder into the graphite crucible and gently press the surface flat (avoiding excessive porosity). Place approximately 5 wt% Nd₂O₃ protective pads on the outer layer of the crucible (i.e., sprinkle a ring of Nd₂O₃ powder between the crucible lid and the crucible) or place a thin graphite cap on top of the crucible and press it lightly to seal. Record the loading mass.

[0052] Furnace preparation: Place the crucible in the tubular furnace sample stage and connect the vacuum and inert gas systems; check the sealing, exhaust gas route, and online CO or CO2 detector.

[0053] Displacement and vacuuming: Vacuuming to 10 -2 -10 -3 Pa, backfill with high-purity Ar (displacement three times) or directly at 1×10 -2 Heating under vacuum (Pa) (depending on equipment capabilities). If Ar gas is selected, set the flow rate to 100 mL / min.

[0054] Heat treatment process: 300℃, keep warm for 1 hour (to remove adsorbed moisture and volatile residues); Increase the temperature to 1250℃ at a rate of 10℃ / min (or you can first increase it to 800℃ to observe the gas evolution before continuing). Hold at 1250℃ for 2 hours (as shown in the example, this can be optimized within the 1150–1350℃ range). When the furnace is shut off and cooled to 600°C, the temperature is reduced to a slow cooling rate (5–10°C / min) or allowed to cool naturally to room temperature, while maintaining an inert atmosphere.

[0055] Removal and preliminary treatment: After cooling to room temperature, open the crucible under glove box or inert conditions, and first weigh the product to complete the material balance. If there are lumps of unreacted material in the crucible, crush them lightly with a mortar and pestle.

[0056] Post-processing: coarse crushing → magnetic separation (removal of free iron blocks) → sieving (75μm) → low-temperature annealing at 700℃ for 1h (Ar) to improve microstructure → surface coating (such as phosphate or epoxy film) to prevent oxidation.

[0057] Waste gas treatment: The exhaust gas from the furnace is discharged after passing through an exhaust gas burner or catalytic oxidation; the peak values ​​of CO and CO2 are recorded.

[0058] Characterization methods XRD (CuKα, 2θ 10–90°, step size 0.02°) was used to quantify Nd₂Fe. 14 B main phase content (Rietveld); ICP-OES was used to detect Nd / Fe / B content and perform mass balance. LECO measures C content; Br, Hc, and (BH)max were measured using VSM (room temperature, maximum field 1.5–2T).

[0059] Specific effects: Phase composition (XRD Rietveld quantitative analysis): Nd2Fe 14 B main phase content ≈ 72wt% (secondary phase: Nd2O3, Fe, small amount of carbides).

[0060] Particle size (laser particle size or sieving, target after grinding): d50≈6μm.

[0061] Residual carbon (LECO): C≈0.8wt%.

[0062] The NdFeB powder obtained in this embodiment was subjected to directional pressing, degreasing, sintering at 1000–1100℃, and conventional annealing to obtain NdFeB sintered permanent magnets. The scanning electron microscope images of the NdFeB sintered permanent magnets are shown below. Figure 1 As shown.

[0063] Magnetic properties: Remanence Br≈0.95T; Intrinsic coercivity Hcj≈900kA / m; Maximum magnetic energy product (BH)max≈14MGOe≈111.4kJ / m 3 .

[0064] Usable powder yield (mass of powder that can enter the conventional sintering process after magnetic separation and sieving / initial total charge mass): approximately 78%.

[0065] Example 2 Planetary ball milling is longer (higher energy) → finer powder with more dislocations / defects (which is beneficial for solid-state reactions); Reduce the insulation temperature to 1220–1230℃ and extend the insulation period, or use a two-stage insulation process (first a short-term high temperature followed by medium-temperature aging). An extra FeB layer and protective pad are placed inside the crucible to compensate for B loss and suppress volatilization.

[0066] The raw materials are the same as in Example 1. Detailed steps High-energy ball milling: BPR=12:1, planetary ball milling at 400rpm, intermittent operation, total time 14–18h (completed in batches over 2–3 days to prevent overheating); grinding aid ethanol 2–3wt%; vacuum drying at 80℃ for 2h after ball milling.

[0067] Crucible loading: Load the powder into a graphite crucible, sprinkle a layer of 3wt% FeB (excess layer) at the bottom of the crucible, cover with powder, and then place a protective ring of 3wt% FeB and 3wt% Nd2O3 on top to ensure "local enrichment" of B and Nd at high temperatures to compensate for volatilization. Seal with a graphite cap, and fill the gaps in the cap with graphite paper or graphite clay to increase the airtightness.

[0068] Heat treatment process: Vacuum pumped to 10 -2 Pa → Backfill Ar (100mL / min); Preheat at 300℃ for 1 hour; The temperature was increased to 600℃ at a rate of 8℃ / min and held for 0.5h (to promote initial solid solution and degassing). Increase the temperature to 1220–1230℃ at a rate of 5–8℃ / min and hold for 2.5–3 hours (to encourage phase change and control Nd volatilization). Controlled cooling: 5℃ / min to 600℃, then naturally cooled to room temperature.

[0069] Extraction and post-processing: Same as in Example 1, but first use metal sieve and magnetic separation to separate large pieces of Fe, and then anneal at 700℃ for 1 hour.

[0070] Specific effects: Phase composition (XRD Rietveld): Nd2Fe 14 B content ≈ 82wt% (significantly better than Example 1).

[0071] Nd recovery rate: ≈95% (volatilization loss ≈5%).

[0072] Particle size: d50≈3.5μm (fineer, larger specific surface area, which is beneficial for densification and improving remanence).

[0073] Residual carbon (LECO): C≈0.6wt%.

[0074] The NdFeB powder obtained in this embodiment was subjected to directional pressing, degreasing, sintering at 1000–1100℃, and conventional annealing to obtain NdFeB sintered permanent magnets. The scanning electron microscope images of the NdFeB sintered permanent magnets are shown below. Figure 2 As shown.

[0075] Magnetic properties: Remanence Br≈1.05T; Intrinsic coercivity Hcj≈1300kA / m; Maximum magnetic energy product (BH)max≈14MGOe≈159.2kJ / m 3 .

[0076] Usable powder rate: ≈85% (ball milling activation improves reaction completion and reduces large unreacted particles).

[0077] Example 3 Adding 1–3 wt% CaO (based on total material mass) to the baseline formulation improves solid-phase reduction kinetics, allowing the insulation temperature to be lowered to 1180–1200℃.

[0078] A relatively short acid wash (weak acid dilution) is used to remove residual Ca compounds (but the acid wash should be controlled to avoid loss of Nd).

[0079] Detailed steps Weighing: Add 2wt% CaO, which is 23g, and the rest is the same as in Example 1.

[0080] Ball milling: Same as in Example 1 or slightly shorter (8–12 h).

[0081] Crucible loading: Use a standard graphite crucible that is sealed, with a small amount of Nd2O3 protective pad on top.

[0082] Heat treatment process: Vacuum evacuation and Ar replacement → Preheat at 300℃ for 1 hour; Increase the temperature by 10℃ / min to 1180–1200℃, and hold for 2.5–3 hours; Natural or controlled cooling.

[0083] Post-treatment (including residue removal): Magnetic separation and screening; If significant chemical residue (CaO) is present, rapidly rinse with dilute hydrochloric acid (e.g., 0.1–0.5 M) for 1–5 minutes (weak acid, short time), followed by thorough rinsing with deionized water until neutral (pH≈7), and then dry under an inert atmosphere (60–80°C, vacuum drying for 1–2 hours). Note: Acid washing carries a risk of Nd loss; concentration and time must be strictly controlled and monitored with ICP.

[0084] Low-temperature annealing at 700℃ for 1 hour (Ar).

[0085] Characterization: Same as in Example 1, except that the residual Ca content (ICP / EDS) was specifically tested.

[0086] Specific effects: Phase composition (XRD Rietveld): Nd2Fe 14 B content ≈ 78 wt% (better than Example 1, slightly worse than Example 2).

[0087] Nd recovery rate: ≈91%.

[0088] Particle size: d50≈4.0μm.

[0089] Residual carbon: C≈0.5–0.6wt%.

[0090] The NdFeB powder obtained in this embodiment was subjected to directional pressing, degreasing, sintering at 1000–1100℃, and conventional annealing to obtain NdFeB sintered permanent magnets. The scanning electron microscope images of the NdFeB sintered permanent magnets are shown below. Figure 3 As shown.

[0091] Magnetic properties: Remanence Br≈1.00T; Intrinsic coercivity Hcj≈1100kA / m; Maximum magnetic energy product (BH)max≈14MGOe≈135.3kJ / m 3 .

[0092] Usable powder rate: ≈82% (including compromise value of pickling loss).

[0093] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A method for directly preparing NdFeB powder from NdO, characterized in that, Includes the following steps: S1 obtains neodymium oxide, an iron source, a boron source, and a carbon reducing agent, wherein the molar ratio of neodymium, iron, boron, and carbon is: 1:(13–15):1:(2.8–3.5); S2. The raw materials obtained in step S1 are mixed and the particle size is refined to 1–10 μm to obtain a mixture; S3 granulates or pre-presses the mixture; S4. Place the mixture treated in step S3 into a high-temperature resistant container, heat it to 200–400°C, hold it at that temperature for 0.5–2 hours, then heat it to 1100–1400°C, hold it at that temperature for 0.5–6 hours, cool it, and remove it.

2. The method for directly preparing NdFeB powder from NdO according to claim 1, characterized in that, Also includes S5 post-processing: S5.1 Crushing: The product obtained in step S4 is coarsely crushed and ground under the protection of an inert gas. S5.2 Magnetic separation: Removes unreacted iron; S5.3 Low-temperature annealing: Anneal at a temperature of 600–800℃ for 0.5–2 hours; S5.4 Surface treatment: Phosphate coating, alkanolamine passivation, or vacuum drying followed by sealing under an inert atmosphere.

3. The method for directly preparing NdFeB powder from NdO according to claim 1, characterized in that, In step S1, the iron source is metallic iron powder with a particle size of 1–20 μm; the boron source is FeB, metallic B, or a B-containing compound; and the carbon reducing agent is carbon black or microcrystalline graphite with a particle size of <5 μm.

4. The method for directly preparing NdFeB powder from NdO according to claim 1, characterized in that, Step S1 also includes an auxiliary agent, which is 1–3 wt% of CaO or MgO in total raw material mass.

5. The method for directly preparing NdFeB powder from NdO according to claim 1, characterized in that, In step S2, the raw materials are mixed and the particle size is refined by ball milling or planetary ball milling. The ball milling parameters are: ball-to-material ratio of 5:1-15:1, ball milling time of 6-12h, and rotation speed of 150-350rpm. The planetary ball milling parameters are: ball-to-material ratio of 5:1-15:1, ball milling time of 12-24h, and rotation speed of 200-600rpm.

6. The method for directly preparing NdFeB powder from NdO according to claim 5, characterized in that, Ethanol or isopropanol is added during ball milling or planetary ball milling, wherein the amount of ethanol or isopropanol added is ≤5wt%. After mixing, the ball-milled product is dried at a temperature of 60–120℃ for 0.5–2h.

7. The method for directly preparing NdFeB powder from NdO according to claim 1, characterized in that, Step S3 specifically involves lightly granulating or isostatically pressing the mixed powder. Granulation: spray granulation followed by drying of the granules; isostatic pressing: pressing into small blocks 5–20 mm thick at a pressure of 50–300 MPa.

8. The method for directly preparing NdFeB powder from NdO according to claim 1, characterized in that, In step S4, the high-temperature resistant container is a graphite crucible or a high-temperature resistant metal container, and the reaction atmosphere is a high-purity inert atmosphere or a medium-high vacuum. The high-purity inert atmosphere is high-purity Ar with O2 ≤ 100 ppm, and the medium-high vacuum is 10 ppm. -2 -10 -3 Pa, the heating rate is 5–20℃ / min, and the cooling rate is controlled at 5–20℃ / min.

9. An application of neodymium iron boron powder, characterized in that: The powder obtained by the method of directly preparing NdFeB powder from NdFeB as described in any one of claims 1-8 is oriented, pressed, degreased, sintered at 1000–1100°C, and annealed according to conventional methods to obtain NdFeB sintered permanent magnets.

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