Method for preventing spontaneous combustion of neodymium iron boron sludge after oil removal

Abstract

This invention relates to the fields of rare earth secondary resource recycling and pyrometallurgical technology, and discloses a method for preventing spontaneous combustion of NdFeB sludge after degreasing. The method includes the following steps: ball milling boron oxide (B2O3) and NdFeB sludge at a mass ratio of 1:1-1:20, kneading them at room temperature to form pellets with an equivalent diameter of 8-35 mm; calcining at 380℃-450℃ in an inert atmosphere for 1-3 hours to thermally decompose the oil; and rapidly cooling to below 100℃ in an inert atmosphere after calcination to obtain stabilized pellets. This invention achieves integrated efficient degreasing, resource recovery, and stabilization treatment through B2O3 + pelletizing + low-temperature pyrolysis + rapid cooling. The degreasing rate of this invention reaches over 96%, and the rare earth leaching rate exceeds 95%.

Description

Technical Field

[0001] This invention relates to the fields of rare earth secondary resource recycling and pyrometallurgical technology, specifically a method to prevent spontaneous combustion of NdFeB sludge after degreasing. Background Technology

[0002] Neodymium iron boron (NdFeB) permanent magnets are widely used in new energy vehicles, wind power generation, consumer electronics, and medical devices due to their excellent magnetic properties. During the machining of NdFeB blanks (such as cutting, grinding, and wire cutting), a large amount of processing waste is generated, of which 15%-30% exists in the form of sludge. This sludge mainly consists of NdFeB fine powder (particle size typically less than 10μm), grinding fluid, cooling oil, iron oxide scale, and a small amount of water. It contains high-value rare earth elements such as neodymium, praseodymium, dysprosium, and terbium, with a total rare earth oxide content reaching 40%-60%, possessing extremely high economic value for recycling. Currently, the recycling and treatment of NdFeB sludge mainly faces the following technical challenges: Limitations of degreasing processes: Oil sludge typically contains 15%-30% oil and organic matter. Direct acid dissolution or pyrometallurgical treatment can lead to organic matter emulsifying the solution, consuming large amounts of acid, or generating significant amounts of flue gas during high-temperature smelting, polluting the environment and reducing product purity. Existing degreasing methods mainly include: 1) High-temperature incineration: The oil sludge is roasted in air at 500℃-800℃ to oxidize and decompose the organic matter. This method is energy-intensive, and at high temperatures, rare earth elements (especially neodymium) in neodymium iron boron are easily oxidized to Nd2O3, leading to difficulties in subsequent acid dissolution and a decrease in rare earth leaching rate. Simultaneously, iron is oxidized to Fe2O3, increasing the difficulty of subsequent separation and purification. 2) Organic solvent extraction: Organic solvents such as n-hexane and trichloroethylene are used for oil extraction. This method is costly, solvent evaporation causes environmental pollution, and solvent residue remains. Furthermore, the degreased material still faces the risk of spontaneous combustion. 3) Vacuum pyrolysis: Heating under vacuum conditions causes the oil to decompose and volatilize. This method requires large equipment investment and has a small processing capacity, making it difficult to achieve continuous industrial production.

[0003] Spontaneous combustion of deoiled materials: Regardless of the deoiling method used, deoiled NdFeB waste exhibits extremely high chemical reactivity due to the following factors: 1) Huge specific surface area: The NdFeB powder in the sludge has an extremely fine particle size (micrometer or even submicrometer). After deoiling, the fresh metal surface is exposed, resulting in a huge contact area with air; 2) High chemical activity: The rare earth elements such as neodymium and praseodymium in NdFeB alloys have extremely high chemical reactivity and can react violently with oxygen at room temperature; 3) Exothermic oxidation reaction: The reaction between rare earth metals and oxygen is a strongly exothermic reaction. The heat of reaction accumulates rapidly, triggering a chain reaction that leads to instantaneous spontaneous combustion of the material. In actual production, once deoiled NdFeB powder comes into contact with air, it usually spontaneously combusts within minutes to tens of minutes, producing a high-temperature flame. This not only causes oxidation loss of rare earth resources but also poses a serious safety hazard, greatly limiting the safe storage, transportation, and continuity of subsequent processes of the deoiled material.

[0004] The shortcomings of existing improvement technologies: In order to solve the problem of spontaneous combustion, some studies have tried to coat the surface of the material after deoiling with passivating agents or store it in an inert atmosphere, but there are the following shortcomings: 1) Although organic coating agents such as paraffin and silicone oil can isolate air for a short period of time, they need to be removed again in the subsequent recycling process, adding a process; 2) Inorganic coating agents such as phosphate and silicate often need to be sintered at a high temperature (>600℃) to form a dense layer, which can easily lead to oxidation and deterioration of the NdFeB phase; 3) Inert atmosphere storage is only suitable for laboratory scale and cannot meet the needs of large-scale industrial production, and there is still an exposure risk during the transfer process.

[0005] Neglecting material form control: In existing deoiling processes, the control of material form (powder or block) is often neglected. Powdered materials are prone to flying, uneven heat transfer, and spontaneous combustion during the deoiling process; while excessively large block materials have problems such as incomplete deoiling and residual oil in the center.

[0006] Therefore, developing a treatment method that can simultaneously complete surface stabilization during the deoiling process, keep the deoiled material stable and non-spontaneous in air, efficiently recover pyrolysis oil by-products, and not affect the subsequent rare earth recovery efficiency has significant industrial application value. Summary of the Invention

[0007] To address the shortcomings of existing technologies, this invention aims to provide a method for preventing spontaneous combustion of NdFeB sludge after degreasing. By adding boron oxide (B2O3) as an additive, combined with pelletizing, low-temperature inert atmosphere pyrolysis, tail gas condensation recovery, and rapid cooling processes, a dense and stable glassy protective layer is formed in situ on the surface of NdFeB particles while efficiently removing grease. This completely eliminates the risk of spontaneous combustion of degreasing products in air, recovers pyrolysis oil byproducts, and ensures the economic viability of subsequent rare earth recovery. The specific steps are as follows: (1) Mix boron oxide powder with neodymium iron boron sludge to be treated evenly to obtain a mixture, and prepare the mixture into pellets.

[0008] (2) The pellets are roasted and deoiled in an inert atmosphere.

[0009] (3) After roasting, the pellets are quenched and the temperature of the pellets is reduced to below 100°C in 5-10 minutes to obtain the oil-free stabilized NdFeB pellets.

[0010] Preferably, in step (1), the mass ratio of boron oxide powder to neodymium iron boron sludge to be treated is 1:1 to 1:20.

[0011] Preferably, the diameter of the pellet in step (1) is 8mm to 35mm.

[0012] Preferably, the roasting and deoiling conditions in step (2) are to keep the temperature at 380℃~450℃ for 1 hour to 3 hours.

[0013] Preferably, the inert atmosphere in step (2) is one or two of nitrogen or argon mixed in any proportion.

[0014] Preferably, the quenching method in step (3) is forced air cooling, water-cooled jacket cooling, or immersion in a room temperature inert gas fluidized bed for cooling.

[0015] Reaction mechanism and process principle This invention achieves efficient degreasing, resource recovery, and stabilization by adding B2O3, pelletizing, low-temperature pyrolysis, and rapid cooling. After mixing B2O3 powder with oil sludge, the B2O3 particles adsorb moisture and some polar organic matter in the oil sludge, forming capillary bridging effects. This causes the oil sludge powder to aggregate into green pellets with a certain strength. This not only improves the permeability of the material during the roasting process and prevents powder from flying away, but also provides a structural basis for the subsequent uniform coverage of B2O3. Subsequently, through the synergistic effect of low-temperature roasting pyrolysis and quenching, the problem of spontaneous combustion of NdFeB oil sludge after degreasing is finally solved.

[0016] Compared with the prior art, the present invention has the following beneficial effects: (1) Completely eliminate the risk of spontaneous combustion: B2O3 softens and uniformly coats NdFeB particles during the roasting process. After rapid cooling, a dense glassy protective layer is formed. This protective layer is chemically stable and does not react or crack in air at room temperature, allowing the deoiled NdFeB pellets to be safely stored in air for a long time (experiments have verified that there is no spontaneous combustion phenomenon after more than 72 hours), thus solving the long-standing safety hazard of spontaneous combustion in the industry.

[0017] (2) Significantly improves the subsequent rare earth leaching rate: The B2O3 glass layer dissolves during the acid dissolution process, and the released borate ions help to break the dense structure of the NdFeB alloy and promote the reaction of rare earth elements with acid. Experiments show that after degreasing using the method of the present invention, the rare earth leaching rate can reach more than 95%, which is significantly improved compared with the traditional high-temperature incineration method (about 88%) and direct acid leaching method (about 72%).

[0018] (3) Protecting the NdFeB phase: Since B2O3 can form a protective layer at low temperature, there is no need for high temperature (>500℃) treatment, thus avoiding the rare earth elements in NdFeB from being oxidized into insoluble oxides, providing phase protection for subsequent high leaching rate.

[0019] (4) Low cost and simple process: B2O3 is a common industrial raw material with low price; the amount added is small (mass ratio 1:5~1:20), and no complicated post-processing is required; the whole process is short and easy to implement industrially.

[0020] (5) Balancing environmental and economic benefits: The introduction of B2O3 does not generate additional pollutants, and the pyrolysis oil can be recovered as a by-product after condensation and reuse; compared with the traditional incineration method, carbon dioxide emissions are reduced. Attached Figure Description

[0021] Figure 1 The image shows the XRD pattern of the NdFeB pellets after degreasing in Example 1, after being left in air for 2 hours.

[0022] Figure 2 XRD pattern of NdFeB pellets from Comparative Example 1 after deoiling without B2O3 and leaving them in air for 2 hours. Detailed Implementation

[0023] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0024] Example 1 Raw material preparation: Take 5 kg of NdFeB oily sludge waste generated by a NdFeB processing enterprise. After testing, its water content is 0.2%, oil content (determined by organic solvent extraction method) is 24.8%, total rare earth (REO) is 25.6%, the main rare earth elements are Nd, Pr, Dy and Tb, the powder particle size D50 is 8.2μm, and the boron oxide (B2O3) powder is commercially available industrial grade with a purity ≥98% and a particle size of -200 mesh.

[0025] A method for preventing spontaneous combustion of NdFeB sludge after degreasing, the specific steps of which are as follows: (1) Mixing and molding: Add 2kg of boron oxide powder to 5kg of neodymium iron boron sludge (the mass ratio of B2O3 to neodymium iron boron sludge is 2:5), put it into a double planetary mixer, and stir at 60r / min for 30 minutes at room temperature to make B2O3 and neodymium iron boron sludge fully and evenly mixed. Transfer the mixture to a disc pelletizer and roll it into spherical green pellets with a diameter of 20±2mm at room temperature. The green pellets are tested and the drop strength (free fall from a height of 1m to a steel plate) is 5 times without breaking, which meets the requirements for subsequent calcination.

[0026] (2) Calcination and deoiling: The raw pellets are evenly spread in a heat-resistant steel sagger with a thickness not exceeding 50 mm. The sagger is pushed into a continuous mesh belt atmosphere furnace. High-purity nitrogen (99.99% purity) is introduced into the furnace to maintain a slight positive pressure (10-20 mm water column) to ensure that air cannot enter. The furnace temperature curve is set: the temperature is increased from room temperature to 400℃ at a rate of 5℃ / min, and then held at 400℃ for 2 hours.

[0027] (3) After the heat preservation is completed, the sagger containing the pellets automatically enters the cooling section of the mesh belt furnace. The cooling section is equipped with a water-cooled jacket and nitrogen gas is continuously introduced for protection. The material is detected to drop below 100°C in 5 minutes, and the oil-free stabilized NdFeB pellets are obtained.

[0028] Performance testing: Oil removal rate determination: The weight of the NdFeB pellets after oil removal was 5.75 kg. The pellets were crushed and the residual oil rate was determined by Soxhlet extraction. The result was 0.3% residual oil rate. The oil removal rate was calculated as 99.53% according to the formula: oil removal rate = (initial oil content - residual oil content) / initial oil content.

[0029] Self-ignition performance test: After cooling, the pellets were taken out and spread flat on a stainless steel tray. They were left to stand in an air environment with room temperature (25℃) and relative humidity of 60%. The surface temperature of the pellets was continuously monitored using thermocouples, and it was observed whether there was smoke or combustion. The results showed that the surface temperature of the pellets remained consistent with the ambient temperature for 72 hours, and there was no heating, smoke or combustion.

[0030] Rare earth leaching rate test: The deoiled pellets were ground to -200 mesh and leached in 1.5 mol / L hydrochloric acid solution with a solid-liquid ratio of 1:5 between the deoiled pellet powder and the hydrochloric acid solution at 80°C for 2 hours. The total rare earth leaching rate was 99.5% as determined by ICP, which is significantly higher than that of direct acid leaching of untreated oil sludge (72.3%) and acid leaching after high-temperature incineration deoiling (88.2%).

[0031] XRD analysis was performed on the deoiled and cooled pellets, such as... Figure 1 As shown, neither Fe nor rare earth elements were oxidized and remained in their elemental state.

[0032] Example 2 Raw material preparation: Take 5 kg of NdFeB sludge from another batch produced by a NdFeB processing enterprise. The oil content is 25.5%, the water content is 1%, and the total rare earth (REO) content is 32%. The B2O3 powder is the same as in Example 1.

[0033] A method for preventing spontaneous combustion of NdFeB sludge after degreasing, the specific steps of which are as follows: (1) Mixing and molding: Add 0.25 kg of boron oxide powder to 5 kg of neodymium iron boron sludge (the mass ratio of B2O3 to neodymium iron boron sludge is 1:20), put it into a double planetary mixer, and stir at 60 r / min for 40 minutes at room temperature to make B2O3 and neodymium iron boron sludge fully and evenly mixed. Transfer the mixture to a disc pelletizer and roll it into spherical green pellets with a diameter of 15±2 mm at room temperature. The green pellets are tested and the drop strength (free fall from a height of 1 m to a steel plate) is 5 times without breaking, which meets the requirements for subsequent calcination.

[0034] (2) Calcination and deoiling: The raw pellets are evenly spread in a heat-resistant corundum crucible. The corundum crucible is placed in a tube furnace and argon gas is introduced (flow rate 3L / min). The temperature is increased to 450℃ at 8℃ / min and held for 1.5 hours.

[0035] (3) After the heat preservation is completed, the corundum crucible containing the pellets is quickly pulled from the center of the furnace tube to the cooling zone at the end of the furnace tube (still in the argon atmosphere). The outer wall of the cooling zone is equipped with circulating water cooling. It is detected that the material temperature drops below 100°C in 5 minutes, and the oil-free stabilized NdFeB pellets are obtained.

[0036] Performance testing: Oil removal rate determination: The weight of the NdFeB pellets after oil removal was 3.925 kg. The pellets were crushed and the residual oil rate was determined by Soxhlet extraction. The result was 0.5% residual oil rate. The oil removal rate was calculated as 98.46% according to the formula: oil removal rate = (initial oil content - residual oil content) / initial oil content.

[0037] Self-ignition performance test: After cooling, the pellets were taken out and spread flat on a stainless steel tray. They were left to stand in an air environment with room temperature (25℃) and relative humidity of 60%. The surface temperature of the pellets was continuously monitored using thermocouples, and it was observed whether there was smoke or combustion. The results showed that the surface temperature of the pellets remained consistent with the ambient temperature for 48 hours, and there was no heating, smoke or combustion.

[0038] Rare earth leaching rate test: The deoiled pellets were ground to -200 mesh and leached in 1.5 mol / L hydrochloric acid solution with a solid-liquid ratio of 1:5 between the deoiled pellet powder and the hydrochloric acid solution at 80℃ for 2 hours. The total rare earth leaching rate was 95.8% as determined by ICP, which is significantly higher than that of direct acid leaching of untreated oil sludge (72.3%) and acid leaching after high-temperature incineration deoiling (88.2%).

[0039] Example 3 Raw material preparation: Take 5 kg of NdFeB oily sludge waste generated by a NdFeB processing enterprise. After testing, its water content is 2.5%, oil content (determined by organic solvent extraction method) is 24.8%, total rare earth (REO) is 25.6%, the main rare earth elements are Nd, Pr, Dy and Tb, the powder particle size D50 is 8.2μm, and the boron oxide (B2O3) powder is commercially available industrial grade with a purity ≥98% and a particle size of -200 mesh.

[0040] A method for preventing spontaneous combustion of NdFeB sludge after degreasing, the specific steps of which are as follows: (1) Mixing and molding: Add 1.0 kg of boron oxide powder to 5 kg of neodymium iron boron sludge (the mass ratio of B2O3 to neodymium iron boron sludge is 1:5), put it into a double planetary mixer, and stir at 60 r / min for 20 minutes at room temperature to make B2O3 and neodymium iron boron sludge fully and evenly mixed. Transfer the mixture to a disc pelletizer and roll it into spherical green pellets with a diameter of 25±2 mm at room temperature. The green pellets are tested and the drop strength (free fall from a height of 1 m to a steel plate) is 5 times without breaking, which meets the requirements for subsequent calcination.

[0041] (2) Calcination and degreasing: The raw pellets are evenly spread in a heat-resistant steel sagger, with a sagger thickness not exceeding 50 mm. The sagger is then pushed into a continuous mesh belt atmosphere furnace. High-purity nitrogen (99.99% purity) is introduced into the furnace to maintain a slight positive pressure (10-20 mm water column) to ensure that air cannot enter. The furnace temperature profile is set: the temperature is increased from room temperature to 380°C at a rate of 4°C / min, and held at 380°C for 3 hours.

[0042] (3) After the heat preservation is completed, the sagger containing the pellets automatically enters the cooling section of the mesh belt furnace. The cooling section is equipped with a water-cooled jacket and nitrogen gas is continuously introduced for protection. The material is detected to drop below 100°C in 5 minutes, and the oil-free stabilized NdFeB pellets are obtained.

[0043] Performance testing: Oil removal rate determination: The weight of the NdFeB pellets after oil removal was 4.635 kg. The pellets were crushed and the residual oil rate was determined by Soxhlet extraction. The result was 0.1% residual oil rate. The oil removal rate was calculated as 99.6% according to the formula: oil removal rate = (initial oil content - residual oil content) / initial oil content.

[0044] Self-ignition performance test: After cooling, the pellets were taken out and spread flat on a stainless steel tray. They were left to stand in an air environment with room temperature (25℃) and relative humidity of 60%. The surface temperature of the pellets was continuously monitored using thermocouples, and it was observed whether there was smoke or combustion. The results showed that the surface temperature of the pellets remained consistent with the ambient temperature for 72 hours, and there was no heating, smoke or combustion.

[0045] Rare earth leaching rate test: The deoiled pellets were ground to -200 mesh and leached in 1.5 mol / L hydrochloric acid solution with a solid-liquid ratio of 1:5 between the deoiled pellet powder and the hydrochloric acid solution at 80℃ for 2 hours. The total rare earth leaching rate was 96.5% as determined by ICP, which is significantly higher than that of direct acid leaching of untreated oil sludge (72.3%) and acid leaching after high-temperature incineration deoiling (88.2%).

[0046] Example 4 Raw material preparation: 5 kg of oily sludge waste from a NdFeB processing enterprise was taken. Testing revealed a water content of 2.5%, an oil content (determined by organic solvent extraction) of 24.8%, and a total rare earth element (REO) content of 25.6%. The main rare earth elements were Nd, Pr, Dy, and Tb, and the powder particle size D50 was 8.2 μm. The boron oxide (B2O3) powder was commercially available industrial grade, with a purity ≥98% and a particle size of -200 mesh.

[0047] A method for preventing spontaneous combustion of NdFeB sludge after degreasing, the specific steps of which are as follows: (1) Mixing and molding: Add 0.5 kg of boron oxide powder to 5 kg of neodymium iron boron sludge (the mass ratio of B2O3 to neodymium iron boron sludge is 1:10), put it into a double planetary mixer, and stir at 60 r / min for 30 minutes at room temperature to make B2O3 and neodymium iron boron sludge fully and evenly mixed. Transfer the mixture to a disc pelletizer and roll it into spherical green pellets with a diameter of 20±2 mm at room temperature. The green pellets are tested and the drop strength (free fall from a height of 1 m to a steel plate) is 5 times without breaking, which meets the requirements for subsequent calcination.

[0048] (2) Calcination and deoiling: The raw pellets are evenly spread in a heat-resistant steel sagger with a thickness not exceeding 50 mm. The sagger is pushed into a continuous mesh belt atmosphere furnace. High-purity nitrogen (99.99% purity) is introduced into the furnace to maintain a slight positive pressure (10-20 mm water column) to ensure that air cannot enter. The furnace temperature curve is set: the temperature is increased from room temperature to 400℃ at a rate of 5℃ / min, and then held at 400℃ for 2 hours.

[0049] (3) After the heat preservation is completed, the sagger containing the pellets automatically enters the cooling section of the mesh belt furnace. The cooling section is equipped with a water-cooled jacket and nitrogen gas is continuously introduced for protection. The material is detected to drop below 100°C in 5 minutes, and the oil-free stabilized NdFeB pellets are obtained.

[0050] Performance testing: Oil removal rate determination: The weight of the NdFeB pellets after oil removal was 4.135 kg. The pellets were crushed and the residual oil rate was determined by Soxhlet extraction. The result was 0.3% residual oil rate. The oil removal rate was calculated as 98.6% according to the formula: oil removal rate = (initial oil content - residual oil content) / initial oil content.

[0051] Self-ignition performance test: After cooling, the pellets were taken out and spread flat on a stainless steel tray. They were left to stand in an air environment with room temperature (25℃) and relative humidity of 60%. The surface temperature of the pellets was continuously monitored using thermocouples, and it was observed whether there was smoke or combustion. The results showed that the surface temperature of the pellets remained consistent with the ambient temperature for 72 hours, and there was no heating, smoke or combustion.

[0052] Rare earth leaching rate test: The deoiled pellets were ground to -200 mesh and leached in 1.5 mol / L hydrochloric acid solution with a solid-liquid ratio of 1:5 between the deoiled pellet powder and the hydrochloric acid solution at 80℃ for 2 hours. The total rare earth leaching rate was 96.5% as determined by ICP, which is significantly higher than that of direct acid leaching of untreated oil sludge (72.3%) and acid leaching after high-temperature incineration deoiling (88.2%).

[0053] Example 5 The purpose of this embodiment is to compare the direct impact of different pellets on the results. The process method in this embodiment is the same as in Example 1, the only difference being the diameter of the prepared pellets. Group A: Diameter 5mm ± 1mm; Group B: Diameter 10mm ± 1mm; Group C: Diameter 20mm ± 2mm; Group D: Diameter 30mm ± 2mm; Group E: Diameter 40mm ± 2mm.

[0054] The performance test results for different groups are shown in Table 1.

[0055] Table 1. Performance test results of different groups of pellets Results analysis: When the pellet diameter is in the range of 10mm-30mm, the oil removal rate is higher than 98%, and the anti-self-ignition performance is excellent, and the B2O3 glass layer can form a complete coverage.

[0056] When the diameter is too small (5mm), although the deoiling rate is acceptable, the high pellet density makes it easy for B2O3 to accumulate excessively on the pellet surface during the roasting process, resulting in a lack of protection for the internal particles. At the same time, the small gaps between small-diameter pellets make it difficult for the central heat to dissipate during quenching, affecting the glass phase curing effect and ultimately leading to spontaneous combustion.

[0057] When the diameter is too large (40mm), heat transfer to the center is slow, the internal oil is not fully decomposed, and the oil removal rate decreases; at the same time, B2O3 is difficult to migrate to the core area of ​​the pellet, and the central NdFeB particles are exposed, becoming the initiation point for spontaneous combustion.

[0058] Comparative Example 1 As a comparison, this comparative example differs from Example 1 in that B2O3 is not added; otherwise, it is the same as Example 1. The final oil removal rate was 98.4%.

[0059] Spontaneous combustion test: After being exposed to air for about 15 minutes, the cooled pellets began to smoke. Subsequently, the surface of the pellets became red-hot and spontaneously combusted violently. The pellets were completely burned, and the residue was a mixture of reddish-brown iron oxide and white rare earth oxides.

[0060] XRD analysis showed that all samples were oxides of Fe and rare earth elements (e.g., ...). Figure 2 ).

[0061] Comparative Example 2 In comparison, the only difference between this comparative example and Example 1 is that the pellets were allowed to cool naturally to 100°C after roasting; all other steps were the same as in Example 1.

[0062] Performance testing: Oil removal rate: residual oil rate 0.4%, oil removal rate 98.4%.

[0063] Spontaneous combustion test: After being exposed to air for about 2 hours after cooling, the pellets showed localized smoke and temperature rise, and some pellets burned. Analysis suggests that during the slow cooling process, the B2O3 glass phase crystallized to form metaboric acid crystals, which caused microcracks in the protective layer, allowing air to penetrate.

[0064] Comparative Example 3 As a comparison, this comparative example differs from Example 1 only in that the calcination temperature is 600°C, while all other aspects are the same as in Example 1.

[0065] Results test: Oil removal rate: residual oil rate 0.2%, oil removal rate 99.2%.

[0066] Spontaneous combustion test: The pellets were stable in the air and showed no spontaneous combustion.

[0067] Rare earth leaching rate: Under acid leaching conditions, the total rare earth leaching rate was only 68.5%. Analysis showed that most of the Nd2Fe in the pellets... 14 B has been oxidized to Nd2O3 and Fe2O3, severely damaging the rare earth phases and making acid dissolution difficult.

Claims

1. A method for preventing spontaneous combustion of NdFeB sludge after degreasing, characterized in that: The specific steps are as follows: (1) The boron oxide powder and the neodymium iron boron sludge to be treated are ball-milled and mixed evenly to obtain a mixture, and the mixture is prepared into pellets; (2) The pellets were roasted and deoiled in an inert atmosphere; (3) After roasting, the pellets are quenched and the temperature of the pellets is reduced to below 100℃ in 5-10 minutes to obtain the oil-free stabilized NdFeB pellets; In step (1), the mass ratio of boron oxide powder to NdFeB sludge to be treated is 1:1 to 1:20; In step (1), the diameter of the pellet is 8mm~35mm; In step (2), the conditions for roasting and degreasing are to keep the temperature at 380℃~450℃ for 1 to 3 hours.

2. The method for preventing spontaneous combustion of NdFeB sludge after degreasing according to claim 1, characterized in that: In step (2), the inert atmosphere is one or a mixture of nitrogen or argon in any proportion.

3. The method for preventing spontaneous combustion of NdFeB sludge after degreasing according to claim 1, characterized in that: In step (3), the quenching method is forced air cooling, water-cooled jacket cooling, or cooling in a room temperature inert gas fluidized bed.

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