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NdFeB magnet manufacturing process and waste generation
The production process of NdFeB permanent magnets mainly includes batching, alloy smelting, hydrogen crushing, airflow grinding, magnetic field orientation molding, isostatic pressing, oil stripping and sintering. Afterwards, machining and electroplating are carried out according to the required shape and use occasion.
The raw material metal neodymium is easily oxidized and is usually stored in mineral oil, so it often needs to be properly cleaned and oxide scale removed before smelting. The boron element is added in the form of a boron-iron alloy. In order to improve the various properties of materials, other elements are often added. After the batching is completed, induction melting is performed, and argon gas is used for protection during the melting process.
Due to the high melting point of iron, the iron rod is finally melted into the alloy liquid, refined, and then cast out of the furnace after standing still for a period of time to obtain a NdFeB alloy ingot.
In the production process of NdFeB materials, powdering is an important process step. Commonly used pulverizing processes are hydrogen crushing and jet milling. According to the hydrogen absorption characteristics of rare earth intermetallic compounds, when the NdFeB alloy is placed in a hydrogen atmosphere, a hydrogen absorption reaction will occur, causing the alloy volume to expand. Since the elongation of NdFeB alloy is almost zero and the fracture strength is low, the alloy will break after absorbing hydrogen. The neodymium-rich phase first absorbs hydrogen, and the neodymium-rich phase is distributed at the boundary of the main phase Nd2Fe14B. Therefore, after hydrogenation, intergranular fracture is generally performed, and single crystal particles of Nd2Fe14B are obtained.
After preparing the required NdFeB alloy powder, the NdFeB permanent magnet forming process can be carried out. Select the appropriate mold according to the needs of the product and perform compression molding under a magnetic field. The theoretical density of NdFeB magnet is 7.55g/cm3, so the blank density needs to be 4.5~5.3 g/cm3. In the actual production process, its density is often lower than the required density.
The sintering process is to heat the blank produced by the molding process to the melting point of the main phase (Tsintering=0.70~0.85Tmelting) and keep it warm for a period of time. The density of NdFeB materials will increase significantly during the sintering process. At the same time, the internal components of the grains are further homogenized, the particle gaps are reduced, the contact properties between powder particles are improved, and the strength is increased, allowing the material to achieve microstructural characteristics with high permanent magnetic properties. After sintering, the density of the billet increases and the remanence increases, but the coercive force and magnetic energy product are not enough. Further tempering is usually required. At the effective temperature, the neodymium-rich phase isolates the main phase particles and forms a good microstructure that is conducive to high coercive force. Sintered NdFeB magnets usually require further machining and surface treatment to meet the needs of practical applications.
A certain amount of scrap or waste products will be generated in each link of the production process of NdFeB magnets, mainly including: raw material loss generated in the pretreatment process of raw materials, and severely oxidized NdFeB produced during the induction melting process.
Waste materials, ultrafine powder produced during the pulverizing process, oxidized powder produced due to exposure to air during the pulverizing process, some slightly oxidized NdFeB lumps produced during the sintering process, and machining processes A large amount of leftover materials generated in the process and unqualified products during the surface treatment process, etc.
It can be seen that the recycling of NdFeB materials or waste materials can not only realize the recycling of rare earth elements, but also is a need for actual production. Through the recycling of NdFeB waste, the utilization rate of rare earth elements can be improved, reflecting relatively significant economic benefits and environmental responsibility.
The raw material metal neodymium is easily oxidized and is usually stored in mineral oil, so it often needs to be properly cleaned and oxide scale removed before smelting. The boron element is added in the form of a boron-iron alloy. In order to improve the various properties of materials, other elements are often added. After the batching is completed, induction melting is performed, and argon gas is used for protection during the melting process.
Due to the high melting point of iron, the iron rod is finally melted into the alloy liquid, refined, and then cast out of the furnace after standing still for a period of time to obtain a NdFeB alloy ingot.
In the production process of NdFeB materials, powdering is an important process step. Commonly used pulverizing processes are hydrogen crushing and jet milling. According to the hydrogen absorption characteristics of rare earth intermetallic compounds, when the NdFeB alloy is placed in a hydrogen atmosphere, a hydrogen absorption reaction will occur, causing the alloy volume to expand. Since the elongation of NdFeB alloy is almost zero and the fracture strength is low, the alloy will break after absorbing hydrogen. The neodymium-rich phase first absorbs hydrogen, and the neodymium-rich phase is distributed at the boundary of the main phase Nd2Fe14B. Therefore, after hydrogenation, intergranular fracture is generally performed, and single crystal particles of Nd2Fe14B are obtained.
After preparing the required NdFeB alloy powder, the NdFeB permanent magnet forming process can be carried out. Select the appropriate mold according to the needs of the product and perform compression molding under a magnetic field. The theoretical density of NdFeB magnet is 7.55g/cm3, so the blank density needs to be 4.5~5.3 g/cm3. In the actual production process, its density is often lower than the required density.
The sintering process is to heat the blank produced by the molding process to the melting point of the main phase (Tsintering=0.70~0.85Tmelting) and keep it warm for a period of time. The density of NdFeB materials will increase significantly during the sintering process. At the same time, the internal components of the grains are further homogenized, the particle gaps are reduced, the contact properties between powder particles are improved, and the strength is increased, allowing the material to achieve microstructural characteristics with high permanent magnetic properties. After sintering, the density of the billet increases and the remanence increases, but the coercive force and magnetic energy product are not enough. Further tempering is usually required. At the effective temperature, the neodymium-rich phase isolates the main phase particles and forms a good microstructure that is conducive to high coercive force. Sintered NdFeB magnets usually require further machining and surface treatment to meet the needs of practical applications.
A certain amount of scrap or waste products will be generated in each link of the production process of NdFeB magnets, mainly including: raw material loss generated in the pretreatment process of raw materials, and severely oxidized NdFeB produced during the induction melting process.
Waste materials, ultrafine powder produced during the pulverizing process, oxidized powder produced due to exposure to air during the pulverizing process, some slightly oxidized NdFeB lumps produced during the sintering process, and machining processes A large amount of leftover materials generated in the process and unqualified products during the surface treatment process, etc.
It can be seen that the recycling of NdFeB materials or waste materials can not only realize the recycling of rare earth elements, but also is a need for actual production. Through the recycling of NdFeB waste, the utilization rate of rare earth elements can be improved, reflecting relatively significant economic benefits and environmental responsibility.