High-purity vacuum degassed chromium powder and method for producing the same

Abstract

The present application belongs to the technical field of metal material processing, and particularly relates to a high-purity vacuum degassing chromium powder and a preparation method thereof. The present application effectively removes key impurities such as oxygen, nitrogen, carbon and iron in the chromium powder through a three-stage impurity removal and degassing system of raw material pretreatment, vacuum pre-degassing and vacuum secondary degassing coupled with carbon thermal reduction, and the purity of the prepared chromium powder is ≥99.95%. The chromium powder D50 is stabilized at 28.0-29.5 μm by adopting a ladder type temperature control inertial crushing and two-stage screening combined with online closed-loop regulation and control of a laser particle size instrument, the particle size fluctuation is ≤0.9 μm, and the particle sphericity is ≥0.89 by matching low-speed precise shaping, so that the problems of uneven particle size and many particle edges and corners in conventional crushing are solved, and the subsequent processing adaptability of the chromium powder is improved. High-purity argon inert atmosphere protection is adopted throughout the process, and electromagnetic iron removal is simultaneously performed at each crushing stage, the magnetic field strength is gradually increased, the iron impurities introduced in the crushing process are effectively removed, and the product quality consistency is ensured.

Description

Technical Field

[0001] This invention belongs to the field of metal material processing technology, specifically relating to a high-purity vacuum degassed chromium powder and its preparation method. Background Technology

[0002] Chromium powder, as an important metal powder material, is widely used in high-end fields such as aerospace, electronic devices, high-temperature alloys, and cemented carbide. These applications have extremely high requirements for the purity, particle size distribution, impurity content (especially oxygen, nitrogen, carbon, and iron), and surface condition of chromium powder.

[0003] The existing methods for preparing chromium powder mostly involve mechanical crushing followed by simple degassing, which has the following problems: (1) Incomplete degassing, residual oxygen and nitrogen impurities can cause oxidation and catalysis of chromium powder during high-temperature use, affecting product performance; (2) Improper temperature control during crushing can easily lead to oxidation pollution and uneven particle size distribution; (3) Incomplete removal of impurities, with iron, carbon and other impurities exceeding the standard, which cannot meet the needs of high-end applications; (4) Chromium powder is prone to secondary oxidation on the surface, resulting in poor storage and use stability.

[0004] Therefore, developing a vacuum degassing chromium powder preparation method with high purity, low impurities, uniform particle size distribution, and stable surface has important application value. Summary of the Invention

[0005] The purpose of this invention is to address existing problems by providing a high-purity vacuum degassed chromium powder and its preparation method.

[0006] This invention is achieved through the following technical solution: A method for preparing high-purity vacuum degassed chromium powder includes the following steps: S1. Raw material pretreatment: High-purity chromium sheets are removed of surface oxide scale and oil, cut, and then subjected to ultrasonic cleaning, vacuum drying, and plasma surface treatment to obtain pretreated chromium blocks. S2, Vacuum pre-degassing: The pretreated chromium block is placed in a low-pressure inert atmosphere for pre-degassing treatment, which mainly removes water vapor, oxygen, nitrogen and organic volatiles adsorbed on its surface, as well as adsorbed organic volatiles during processing. At the same time, the extremely thin natural oxide film on the surface is partially decomposed or reduced to achieve surface purification. S3, Stepped temperature-controlled inert pulverization: The pre-degassed material is subjected to multi-stage gradient temperature-controlled pulverization under an inert atmosphere, with simultaneous real-time electromagnetic iron removal. S4. Grading and screening: The pulverized material is subjected to two-stage sieving, and closed-loop control is achieved by combining online particle size detection to obtain chromium powder intermediates; S5, Vacuum secondary degassing coupled with carbothermic reduction for deep deoxidation: After mixing the chromium powder intermediate with a quantitative amount of high-purity carbon powder, the mixture is placed under vacuum for deep deoxidation by carbothermal reduction. The amount of carbon powder added is calculated according to the stoichiometric ratio of the chemical reaction and is 5-10% in excess. No free carbon residue is left after the reaction. S6, Precision Plastic Surgery: The chromium powder intermediate after secondary degassing is placed in a pulverizing and shaping machine for low-speed shaping to remove particle edges and optimize sphericity without changing the core particle size. S7. Plasma reduction and controllable passivation: The shaped chromium powder is placed in a plasma treatment device and treated with argon-hydrogen mixed plasma. First, the residual oxides on the surface are reduced by active hydrogen atoms. Then, a dense Cr2O3 passivation layer with a thickness of 1~3nm is formed under controlled extremely low oxygen partial pressure to obtain high-purity vacuum degassed chromium powder.

[0007] Furthermore, the high-purity chromium sheet mentioned in step S1 has a purity ≥ 99.90%; The side length of the chromium block after cutting is 1~3cm; The cleaning process uses a 1:1 volume ratio of anhydrous ethanol and anhydrous acetone, followed by ultrasonic cleaning for 15-20 minutes at an ultrasonic power of 300-400W. The liquid-to-solid volume ratio of the cleaning solution to the chromium block is (3-5):1. Drying temperature 60~80℃, vacuum degree 100~1000Pa, drying time 2~3h; The plasma treatment uses argon plasma with a plasma power of 80~100W, an argon flow rate of 0.3~0.5L / min, a treatment time of 3~5min, and a treatment vacuum degree of 5~10Pa.

[0008] Further, step S2 specifically involves: placing the pretreated chromium block in a vacuum sintering furnace, lining the furnace with high-purity graphite lining plates (purity ≥99.99%), and evacuating the furnace to a vacuum level ≤1×10⁻⁶. -3 Pa, refill with high-purity argon gas (purity ≥99.999%) to a furnace pressure of 10~100Pa, and maintain a low-pressure argon atmosphere in dynamic equilibrium inside the furnace by linking the mass flow meter with the vacuum pump (i.e., continuous micro-flow of argon gas and simultaneous evacuation). Heat to 600~700℃ and hold for 4~6 hours for pre-degassing. After treatment, cool to room temperature with the furnace.

[0009] The low-pressure argon atmosphere allows the desorbed gas to be carried away by the dynamic airflow in time, preventing re-adsorption; at the same time, it is conducive to the partial decomposition or reduction of the extremely thin natural oxide film on the surface, achieving surface purification.

[0010] Furthermore, in step S3, the pulverization is carried out under an argon atmosphere of 0.1~0.12MPa, using a three-stage temperature gradient control: First-stage coarse crushing: chamber temperature 10~20℃, rotation speed 2800~3000rpm, crushed to 200~300µm; Second stage medium crushing: chamber temperature 15~25℃, rotation speed 3500~3800rpm, crushed to 50~80µm; Third-stage fine crushing: chamber temperature 10~20℃, rotation speed 4000~4200rpm, crushed to 30~40µm; The feeding speed is 5~8 kg / h, and the single feeding amount is 20~30 kg; Each discharge port is equipped with an electromagnetic iron removal device, with a magnetic field strength of 800~900mT for coarse crushing, 900~1000mT for medium crushing, and 1000~1200mT for fine crushing.

[0011] All material-contacting parts of the crushing equipment are made of high-hardness, wear-resistant materials (such as hard alloys or ceramic coatings). The electromagnetic iron removal device is used to collect trace amounts of iron wear debris that may be generated during the long-term operation of the equipment during the crushing process. Although the wear is extremely low, in order to stably control the iron content at ≤32ppm, the electromagnetic iron removal device serves as a redundancy safeguard, effectively removing any accidentally generated magnetic particles and ensuring that the iron impurities in the product do not exceed the standard.

[0012] Further, step S4 specifically involves: passing the pulverized material through a three-stage ultrasonic stepped sieve for primary sieving, with sieve mesh sizes of 40µm, 30µm, and 27µm respectively, an ultrasonic frequency of 20~25kHz (used to assist in vibration to prevent clogging of the sieve), a sieving time of 10~15min, a feeding speed of 3~5kg / h, and collecting chromium powder intermediates of 27~30μm. The collected chromium powder intermediate was subjected to two-stage sieving using an air classifier with an airflow velocity of 12~15m / s. The grading chamber was temperature-controlled at 20~25℃ using a cooling jacket, with temperature fluctuation ≤±1℃. During the classification process, a laser particle size analyzer is used to detect the particle size in real time at a frequency of 1 time / min. The classifying wheel speed or airflow speed of the air classifier is automatically adjusted to achieve closed-loop control, so that the particle size fluctuation of the intermediate is ≤±1.0μm.

[0013] Further, step S5 specifically involves: mechanically mixing the graded and sieved chromium powder intermediate with high-purity carbon powder (purity ≥99.99%, with an addition amount of 0.1~0.5% of the chromium powder mass) under an inert atmosphere until uniform, and then placing it in a high-purity graphite crucible (purity ≥99.99%, vacuum degassing treatment at 400~450℃ before use) in a vacuum sintering furnace. The amount of carbon powder added is calculated based on the measured oxygen content of the chromium powder intermediate according to the stoichiometric ratio of the chemical reaction (C+O→CO), and is 5-10% in excess to ensure that the carbothermic reduction reaction proceeds fully; the excess carbon powder is extracted in the form of CO under high temperature vacuum and argon carrier gas, and there is no free carbon residue after the reaction. Vacuum up to ≤5×10 -5 The temperature is raised to 850~950℃ and held for 3~4 hours. During this time, high-purity argon gas is introduced at a flow rate of 0.1~0.2L / min as the carrier gas. A vacuum is evacuated every hour to maintain a stable high vacuum (short-term pressure increases to 10 Pa are allowed). -2 ~10 - 1 Pa), after the heat preservation is completed, the furnace is cooled to room temperature.

[0014] Furthermore, the low-speed shaping in step S6 is performed at a speed of 800~1000 rpm for 5~8 min, under argon protection, with an argon flow rate of 0.5~1 L / min.

[0015] Further, in step S7, the volume ratio of the argon-hydrogen mixed plasma is 9:1, the plasma power is 150~200W, the processing temperature is 150~200℃, the processing time is 20~30min, and the vacuum degree is 10~20Pa. The specific process is as follows: First, the surface is pre-cleaned in pure argon plasma for 1-2 minutes. Then, an argon-hydrogen mixture is introduced. Under the high-energy excitation of the plasma, active hydrogen atoms react with residual Cr2O3 and adsorbed oxygen on the surface of the chromium powder to generate H2O(g) and Cr. At the same time, they react with trace amounts of residual nitrogen to generate NH3(g), achieving complete surface reduction. After the treatment, the surface is cooled to below 80°C under argon protection. Then, an argon-oxygen mixture with an oxygen partial pressure of 0.05-0.1 Pa is introduced for surface passivation for 10-15 minutes, forming a dense and continuous Cr2O3 passivation layer with a thickness of 1-3 nm on the chromium surface. This passivation layer can effectively prevent further oxidation of the internal chromium without affecting the conductivity and subsequent processing performance of the powder.

[0016] Furthermore, all argon and hydrogen used in the entire process are dehydrated and deoiled; the argon in the gas flow stage tail gas is filtered, recovered, and recycled.

[0017] The present invention has the following advantages over the prior art: 1. This invention utilizes a three-stage impurity removal and degassing system—raw material pretreatment, vacuum pre-degassing, and vacuum secondary degassing coupled with carbothermal reduction—to effectively remove key impurities such as oxygen, nitrogen, carbon, and iron from chromium powder. The resulting chromium powder has a chromium matrix purity (after deducting metallic impurities and C, O, and N) ≥ 99.95%, with oxygen content ≤ 22 ppm, nitrogen content ≤ 11 ppm, iron content ≤ 32 ppm, and carbon content ≤ 30 ppm. This is far superior to existing conventional methods, and the finished product meets the requirements for high-purity chromium powder in high-end fields such as aerospace and electronic devices.

[0018] 2. This invention employs stepped temperature-controlled inertial pulverization and two-stage sieving, combined with online closed-loop control using a laser particle size analyzer, to stabilize the D50 of chromium powder at 28.0~29.5μm with a particle size fluctuation of ≤0.9μm. Coupled with low-speed precision shaping, the sphericity of the particles is ≥0.89, solving the problems of uneven particle size and numerous angular particles in conventional pulverization, and improving the adaptability of chromium powder for subsequent processing.

[0019] 3. This invention employs a high-purity argon inert atmosphere for protection during key steps such as crushing, sieving, shaping, and degassing, effectively preventing unnecessary oxidation contamination of chromium powder during processing. Simultaneously, electromagnetic iron removal is performed at each crushing stage, with a gradient increase in magnetic field strength to effectively remove trace iron impurities that may be introduced during crushing, ensuring consistent product quality. Finally, plasma reduction and controlled passivation treatment form an atomically dense passivation layer on the chromium powder surface (this passivation layer is artificially and controllably grown, unlike natural oxide films), significantly enhancing the oxidation resistance of the high-purity micro-powder during room temperature storage and subsequent high-temperature processing. Detailed Implementation

[0020] To further explain the present invention, the following specific embodiments are described.

[0021] Example 1

[0022] A method for preparing high-purity vacuum degassed chromium powder includes the following steps: S1. Raw material pretreatment: High-purity chromium sheets with a purity of 99.9% were selected. The surface oxide scale was removed by mechanical grinding, and the oil stains were removed by wiping with anhydrous ethanol. The sheets were then cut into chromium blocks with a side length of 1 cm. The chromium blocks were placed in a mixture of anhydrous ethanol and anhydrous acetone with a volume ratio of 1:1 and ultrasonically cleaned at 300W power for 15 min at a liquid-to-solid volume ratio of 3:1. After cleaning, the blocks were placed in a vacuum drying oven and dried at 60℃ and 100Pa vacuum for 3 h. The dried chromium blocks were then placed in a plasma treatment device, and argon gas was introduced at a rate of 0.3 L / min. The blocks were treated at 5Pa and 80W power for 5 min to obtain pretreated chromium blocks. S2, Vacuum pre-degassing (surface cleaning): The pretreated chromium blocks were placed in a vacuum sintering furnace lined with high-purity graphite (purity ≥ 99.99%), and the furnace was evacuated to a vacuum level of 1 × 10⁻⁶. -3 Pa, refill with high-purity argon (purity ≥99.999%) to furnace pressure 10 Pa, and maintain a low-pressure argon atmosphere in dynamic equilibrium inside the furnace by linking a mass flow meter with a vacuum pump; heat to 600℃ and hold for 6 hours for pre-degassing, and then cool to room temperature with the furnace after treatment; S3, Stepped temperature-controlled inert pulverization: The pre-degassed chromium block is placed into a pulverizing device, and high-purity argon gas at 0.1 MPa is introduced. Pulverization is performed using a three-stage gradient temperature control method. First stage coarse crushing: chamber temperature 10℃, rotation speed 2800rpm, crushed to 300µm; Second-stage medium crushing: chamber temperature 15℃, rotation speed 3500rpm, crushed to 80µm; Third-stage fine crushing: chamber temperature 10℃, rotation speed 4000rpm, crushed to 40µm; The feeding speed is 5 kg / h, and the single feeding amount is 20 kg; Each discharge port is equipped with an electromagnetic iron removal device, with a magnetic field strength of 800mT for coarse crushing, 900mT for medium crushing, and 1000mT for fine crushing. S4. Grading and screening: The pulverized material was subjected to a first-stage sieve using a three-stage ultrasonic stepped sieve with mesh sizes of 40µm, 30µm, and 27µm, an ultrasonic frequency of 20kHz (with an auxiliary anti-clogging screen), a sieve time of 15min, a feed rate of 3kg / h, and chromium powder intermediates of 27~30μm were collected. The collected chromium powder intermediate was subjected to two-stage sieving using an airflow classifier with an airflow velocity of 12 m / s and a cooling jacket to control the temperature of the classification chamber at 20°C. During the classification process, a laser particle size analyzer is used to detect the particle size in real time at a frequency of 1 time / min. The speed of the classifying wheel of the air classifier is automatically adjusted to ensure that the particle size fluctuation of the intermediate is ≤±1.0μm. S5, Vacuum secondary degassing coupled with carbothermic reduction for deep deoxidation: The graded and sieved chromium powder intermediate and high-purity carbon powder (purity ≥99.99%, added at 0.1% of the chromium powder mass) are mechanically mixed evenly under an inert atmosphere, and then placed in a high-purity graphite crucible (purity ≥99.99%, vacuum degassing treatment at 400℃ before use) in a vacuum sintering furnace. The amount of carbon powder added is calculated based on the measured oxygen content (30 ppm) of the chromium powder intermediate according to the stoichiometric ratio (C + O → CO), with an excess of 5%, meaning the actual amount of carbon powder added is 0.0016% of the chromium powder mass (corresponding to 16 mg of carbon powder per kilogram of chromium powder). In this embodiment, for ease of operation, the amount added is 0.002% of the chromium powder mass. Vacuum up to 5×10 -5 Pa, heated to 850℃ and held for 4 hours for deep deoxygenation by carbothermal reduction. During the degassing process, high-purity argon gas was introduced at a flow rate of 0.1 L / min as the carrier gas. Vacuum was replenished every 1 hour. After the holding period, the furnace was cooled to room temperature. After the reaction, the carbon powder was completely converted into CO gas and removed, with no free carbon residue. S6, Precision Plastic Surgery: The chromium powder intermediate after secondary degassing was placed in a pulverizing and shaping machine and shaped at 800 rpm for 8 minutes under argon protection at a speed of 0.5 L / min. S7. Plasma reduction and controllable passivation: The shaped chromium powder was placed in a plasma treatment device, and a 9:1 volume ratio of argon-hydrogen mixed plasma was introduced. The treatment was carried out at 150℃, 10Pa, and 150W for 30 minutes. Under the high-energy excitation of the plasma, active hydrogen atoms reacted with residual Cr2O3 and adsorbed oxygen on the surface of the chromium powder to generate H2O(g) and Cr. At the same time, they reacted with trace amounts of residual nitrogen (≤11ppm) to generate NH3(g), achieving complete surface reduction. After the treatment, the powder was cooled to below 80℃ under argon protection, and then an argon-oxygen mixed gas with an oxygen partial pressure of 0.05Pa was introduced for surface passivation for 10 minutes to obtain high-purity vacuum degassed chromium powder. All argon and hydrogen used in the process are dehydrated and deoiled; the argon in the gas flow stage tail gas is filtered, recovered, and recycled.

[0023] Example 2

[0024] A method for preparing high-purity vacuum degassed chromium powder includes the following steps: S1. Raw material pretreatment: High-purity chromium sheets with a purity of 99.9% were selected. The surface oxide scale was removed by mechanical grinding, and the oil stains were removed by wiping with anhydrous ethanol. The sheets were then cut into chromium blocks with a side length of 2 cm. The chromium blocks were placed in a 1:1 volume ratio of anhydrous ethanol and anhydrous acetone, and ultrasonically cleaned at 350 W for 18 min at a liquid-to-solid volume ratio of 4:1. After cleaning, the blocks were placed in a vacuum drying oven and dried at 70 °C and 500 Pa for 2.5 h. The dried chromium blocks were then placed in a plasma treatment device, and argon gas was introduced at a rate of 0.4 L / min. The blocks were treated at 8 Pa and 90 W for 4 min to obtain pretreated chromium blocks. S2, Vacuum pre-degassing (surface cleaning): The pretreated chromium blocks were placed in a vacuum sintering furnace lined with high-purity graphite (purity ≥ 99.99%), and the furnace was evacuated to a vacuum level of 5 × 10⁻⁶. -4 Pa, refill with high-purity argon (purity ≥99.999%) to a furnace pressure of 50 Pa, and maintain a low-pressure argon atmosphere in dynamic equilibrium inside the furnace by linking a mass flow meter with a vacuum pump; heat to 650℃ and hold for 5 hours for pre-degassing, and then cool to room temperature with the furnace after treatment; S3, Stepped temperature-controlled inert pulverization: The pre-degassed chromium block is placed into a pulverizing device, and high-purity argon gas at 0.11 MPa is introduced. Pulverization is performed using a three-stage gradient temperature control method. First-stage coarse crushing: chamber temperature 15℃, rotation speed 2900rpm, crushed to 250µm; Second-stage medium crushing: chamber temperature 20℃, rotation speed 3650rpm, crushing to 65µm; Third-stage fine crushing: chamber temperature 15℃, rotation speed 4100rpm, crushed to 35µm; The feeding speed is 6.5 kg / h, and the single feeding amount is 25 kg; Each discharge port is equipped with an electromagnetic iron removal device, with a magnetic field strength of 900mT for coarse crushing, 1000mT for medium crushing, and 1100mT for fine crushing. S4. Grading and screening: The pulverized material was subjected to a first-stage sieve using a three-stage ultrasonic stepped sieve with mesh sizes of 40µm, 30µm, and 27µm, an ultrasonic frequency of 22kHz (with an auxiliary anti-clogging screen), a sieve time of 12min, a feed rate of 4kg / h, and chromium powder intermediates of 27~30μm were collected. The collected chromium powder intermediate was subjected to two-stage sieving using an airflow classifier with an airflow velocity of 13 m / s and a cooling jacket temperature control of 22°C in the classification chamber. During the classification process, a laser particle size analyzer is used to detect the particle size in real time at a frequency of 1 time / min. The speed of the classifying wheel of the air classifier is automatically adjusted to ensure that the particle size fluctuation of the intermediate is ≤±1.0μm. S5, Vacuum secondary degassing coupled with carbothermic reduction for deep deoxidation: The graded and sieved chromium powder intermediate and high-purity carbon powder (purity ≥99.99%, added at 0.3% of the chromium powder mass) are mechanically mixed evenly under an inert atmosphere, and then placed in a high-purity graphite crucible (purity ≥99.99%, vacuum degassing treatment at 420℃ before use) in a vacuum sintering furnace. The amount of carbon powder added is calculated based on the measured oxygen content (26 ppm) of the chromium powder intermediate according to the stoichiometric ratio (C + O → CO), with an excess of 5%, i.e., the amount added is 0.0021% of the mass of the chromium powder. Vacuum up to 3×10 -5Pa, heated to 900℃ and held for 3.5h for deep deoxygenation by carbothermal reduction. During the degassing process, high-purity argon gas was introduced at a flow rate of 0.15L / min as the carrier gas. Vacuum was replenished every 1h. After the holding period, the furnace was cooled to room temperature. After the reaction, the carbon powder was completely converted into CO gas and removed, with no free carbon residue. S6, Precision Plastic Surgery: The chromium powder intermediate after secondary degassing was placed in a pulverizing and shaping machine and shaped at 900 rpm for 6 minutes under argon protection at a speed of 0.7 L / min. S7. Plasma reduction and controllable passivation: The shaped chromium powder was placed in a plasma treatment device, and a 9:1 volume ratio of argon-hydrogen mixed plasma was introduced. The treatment was carried out at 180℃, 15Pa, and 180W for 25 minutes. Under the high-energy excitation of the plasma, active hydrogen atoms reacted with the residual Cr2O3 and adsorbed oxygen on the surface of the chromium powder to generate H2O(g) and Cr. At the same time, they reacted with trace amounts of residual nitrogen to generate NH3(g), achieving complete surface reduction. After the treatment, the powder was cooled to below 80℃ under argon protection, and then an argon-oxygen mixed gas with an oxygen partial pressure of 0.07Pa was introduced for surface passivation for 12 minutes to obtain high-purity vacuum degassed chromium powder. All argon and hydrogen used in the process are dehydrated and deoiled; the argon in the gas flow stage tail gas is filtered, recovered, and recycled.

[0025] Example 3

[0026] A method for preparing high-purity vacuum degassed chromium powder includes the following steps: S1. Raw material pretreatment: High-purity chromium sheets with a purity of 99.9% were selected. The surface oxide scale was removed by mechanical grinding, and the oil stains were removed by wiping with anhydrous ethanol. The sheets were then cut into chromium blocks with a side length of 3cm. The chromium blocks were placed in a 1:1 volume ratio of anhydrous ethanol and anhydrous acetone, and ultrasonically cleaned at 400W power for 20 minutes at a liquid-to-solid volume ratio of 5:1. After cleaning, the blocks were placed in a vacuum drying oven and dried at 80℃ and 1000Pa vacuum for 2 hours. The dried chromium blocks were then placed in a plasma treatment device, and argon gas was introduced at a rate of 0.5L / min. The blocks were treated at 10Pa and 100W power for 3 minutes to obtain pretreated chromium blocks. S2, Vacuum pre-degassing (surface cleaning): The pretreated chromium blocks were placed in a vacuum sintering furnace lined with high-purity graphite (purity ≥ 99.99%), and the furnace was evacuated to a vacuum level of 1 × 10⁻⁶. -4 Pa, refill with high-purity argon (purity ≥99.999%) to furnace pressure 100Pa, and maintain a low-pressure argon atmosphere in dynamic equilibrium inside the furnace by linking a mass flow meter with a vacuum pump; heat to 700℃ and hold for 4 hours for pre-degassing, and then cool to room temperature with the furnace after treatment; S3, Stepped temperature-controlled inert pulverization: The pre-degassed chromium block is placed into a pulverizing device, and high-purity argon gas at 0.12 MPa is introduced. Pulverization is performed using a three-stage gradient temperature control method. First-stage coarse crushing: chamber temperature 20℃, rotation speed 3000rpm, crushed to 200µm; Second-stage medium crushing: chamber temperature 25℃, rotation speed 3800rpm, crushing to 50µm; Third-stage fine crushing: chamber temperature 20℃, rotation speed 4200rpm, crushed to 30µm; The feeding speed is 8 kg / h, and the single feeding amount is 30 kg; Each discharge port is equipped with an electromagnetic iron removal device, with a magnetic field strength of 900mT for coarse crushing, 1000mT for medium crushing, and 1200mT for fine crushing. S4. Grading and screening: The pulverized material was subjected to a first-stage sieve using a three-stage ultrasonic stepped sieve with mesh sizes of 40µm, 30µm, and 27µm, an ultrasonic frequency of 25kHz (with an auxiliary anti-clogging screen), a sieve time of 10min, a feed rate of 5kg / h, and chromium powder intermediates of 27~30μm were collected. The collected chromium powder intermediate was subjected to two-stage sieving using an air classifier with an airflow velocity of 15 m / s and a cooling jacket to control the temperature of the grading chamber at 25°C. During the classification process, a laser particle size analyzer is used to detect the particle size in real time at a frequency of 1 time / min. The speed of the classifying wheel of the air classifier is automatically adjusted to ensure that the particle size fluctuation of the intermediate is ≤±1.0μm. S5, Vacuum secondary degassing coupled with carbothermic reduction for deep deoxidation: The graded and sieved chromium powder intermediate and high-purity carbon powder (purity ≥99.99%, added at 0.5% of the chromium powder mass) are mechanically mixed evenly under an inert atmosphere, and then placed in a high-purity graphite crucible (purity ≥99.99%, vacuum degassing treatment at 450℃ before use) in a vacuum sintering furnace. The amount of carbon powder added is calculated based on the measured oxygen content (22 ppm) of the chromium powder intermediate according to the stoichiometric ratio (C + O → CO), with an excess of 5%, that is, the amount added is 0.0018% of the mass of the chromium powder; Vacuum up to 1×10 -5 Pa, heated to 950℃ and held for 3 hours for deep deoxygenation by carbothermal reduction. During the degassing process, high-purity argon gas was introduced at a flow rate of 0.2 L / min as the carrier gas. Vacuum was replenished every 1 hour. After the holding period, the furnace was cooled to room temperature. After the reaction, the carbon powder was completely converted into CO gas and removed, with no free carbon residue. S6, Precision Plastic Surgery: The chromium powder intermediate after secondary degassing was placed in a pulverizing and shaping machine and shaped at 1000 rpm for 5 minutes under argon protection at a speed of 1L / min. S7. Plasma reduction and controllable passivation: The shaped chromium powder was placed in a plasma treatment device, and a 9:1 volume ratio of argon-hydrogen mixed plasma was introduced. The treatment was carried out at 200℃, 20Pa, and 200W for 20 minutes. Under the high-energy excitation of the plasma, active hydrogen atoms reacted with the residual Cr2O3 and adsorbed oxygen on the surface of the chromium powder to generate H2O(g) and Cr. At the same time, they reacted with trace amounts of residual nitrogen to generate NH3(g), achieving complete surface reduction. After the treatment, the powder was cooled to below 80℃ under argon protection, and then an argon-oxygen mixed gas with an oxygen partial pressure of 0.10Pa was introduced for surface passivation for 15 minutes to obtain high-purity vacuum degassed chromium powder. All argon and hydrogen used in the process are dehydrated and deoiled; the argon in the gas flow stage tail gas is filtered, recovered, and recycled.

[0027] Comparative Example 1 Compared with Example 2, the specific steps of Comparative Example 1 are basically the same as those of Example 2, except that step S2, vacuum pre-degassing, is omitted, and the pretreated chromium block is directly subjected to step-controlled temperature inert pulverization. The remaining steps and parameters are the same as those of Example 2.

[0028] Comparative Example 2 Compared with Example 2, the specific steps of Comparative Example 2 are basically the same as those of Example 2, except that step S5, vacuum secondary degassing coupled with carbothermal reduction, is omitted, and the chromium powder intermediate after classification and sieving is directly and precisely shaped. The remaining steps and parameters are the same as those of Example 2.

[0029] Comparative Example 3 The specific steps of Comparative Example 3 are basically the same as those of Example 2, except that: in step S3, the pulverization is carried out at a single rotation speed and a single temperature, without three-stage gradient temperature control. Specifically, the pulverization is carried out under an argon atmosphere of 0.11 MPa, a chamber temperature of 20°C, and a rotation speed of 3650 rpm, directly pulverizing to 35 μm without graded pulverization and without synchronous electromagnetic iron removal. The remaining steps and parameters are the same as those in Example 2.

[0030] Comparative Example 4 Compared with Example 2, the specific steps of Comparative Example 4 are basically the same, except that step S7, plasma reduction and controllable passivation, is omitted, and the precisely shaped chromium powder is used directly as the finished product. The remaining steps and parameters are the same as in Example 2.

[0031] Comparative Example 5 Chromium powder was prepared using a conventional mechanical pulverization + simple vacuum degassing method: High-purity chromium sheets of the same purity as in Example 2 were selected, mechanically ground to remove oxide scale and oil, and then directly subjected to single mechanical pulverization (3650 rpm, 20°C). After pulverization, the powder was simply sieved (only one stage of ultrasonic sieve, 30 μm mesh size), and then subjected to a vacuum degassing process at 800°C. -3 Degassing under a vacuum of Pa for 2 hours yields chromium powder, without electromagnetic iron removal, precision shaping, plasma reduction, or controllable passivation steps.

[0032] The chromium powder products prepared in Examples 1-3 and Comparative Examples 1-5 were tested according to the following method. All tests were conducted under the same environmental conditions (temperature 25°C, humidity 50% RH). Each group of samples was tested in parallel 3 times, and the average value was taken as the final result.

[0033] (1) Purity and metal impurity test: According to the series of standards GB / T 4702.1~4702.16 "Methods for chemical analysis of metallic chromium", the content of Fe, Si, Mn, Al, Cu, Ni, Co, Pb and Sn metal impurity elements in chromium powder was determined by inductively coupled plasma optical emission spectrometry (ICP-OES), and the total purity was calculated.

[0034] The "purity" mentioned in this invention is calculated according to the following formula: Purity = 100% – (total mass fraction of all the above metallic impurity elements) – (mass fraction of C + O + N) (2) Oxygen, nitrogen and carbon content test: According to GB / T 4702.17-2016 "Determination of oxygen, nitrogen and hydrogen content of metallic chromium by inert gas melting infrared absorption method and thermal conductivity method", the oxygen and carbon content were determined by infrared absorption method and the nitrogen content was determined by thermal conductivity method.

[0035] (3) Particle size distribution test: According to GB / T 19077-2024 "Particle size analysis by laser diffraction", the particle size D10, D50, D90 and particle size fluctuation range of chromium powder were determined by laser particle size analyzer.

[0036] (4) Sphericity test: Referring to the image analysis principle in YS / T 1297-2019 "Method for Determination of Sphericity of Titanium and Titanium Alloy Powder", the morphology of chromium powder particles was observed using a scanning electron microscope (SEM) and the sphericity was calculated.

[0037] (5) Antioxidant performance test: Chromium powder was placed in an air atmosphere at 200℃ for 24 hours and the mass change rate was measured. This condition was set reasonably based on the preheating process temperature of the product in the high-end electronic packaging field. The test results are shown in Tables 1 and 2 below.

[0038] Table 1

[0039] Table 2

[0040] As shown in Tables 1 and 2 above, in Examples 1-3, with the optimization of process parameters, the oxygen, nitrogen, and carbon contents decreased sequentially, and the purity increased accordingly. The D50 was controlled at 28.0~29.5μm, the particle size fluctuation was ≤0.9μm, the sphericity was ≥0.89, and the 24h antioxidant quality change rate was ≤0.015%, showing obvious gradient optimization, which is in line with the law of improving raw material purity and fine control of process parameters. Comparative Example 1 (omitting pre-degassing) resulted in oxygen and nitrogen contents increasing to 68 ppm and 43 ppm, respectively, approximately 2.6 times and 3.1 times that of Example 2. This is because water vapor, air, and volatile organic compounds adsorbed on the chromium block surface directly enter the subsequent high-temperature process without being removed. Some of these substances are adsorbed on the surface or react with chromium at high temperatures to form oxides / nitrides, leading to increased impurity content. This demonstrates that pre-degassing is crucial for controlling gaseous impurities.

[0041] In Comparative Example 2 (carbothermic reduction deep deoxidation omitted), the carbon content increased to 67 ppm, and the oxygen content increased to 74 ppm. Since no carbon powder was added for carbothermic reduction, the residual oxygen in the chromium powder could not be effectively removed; simultaneously, the original carbon content of the chromium powder (approximately 30 ppm) remained unchanged. This comparative example, compared to Example 2, confirms the necessity of the carbothermic reduction step for deep deoxidation. Notably, the nitrogen content in this comparative example is similar to that of Example 2 (16 ppm), indicating that nitrogen is mainly removed through pre-degassing and a vacuum environment.

[0042] In Comparative Example 3 (without gradient grinding and without electromagnetic iron removal), the iron content significantly increased to 88 ppm, mainly due to iron filings introduced by wear of the grinding equipment. Gradient magnetic field iron removal effectively adsorbed these magnetic particles. Simultaneously, the oxygen content slightly increased (55 ppm), which is related to localized overheating and oxidation caused by single-grinding. Furthermore, single-grinding resulted in a wide particle size distribution (fluctuation ±1.8 μm), a larger D50 (32.6 μm), and more angular particles (sphericity 0.75).

[0043] Comparative Example 4 (passivation omitted) had a bulk composition essentially the same as Example 2 (purity 99.96%), demonstrating that the plasma passivation step does not change the internal impurity content of the chromium powder, but only affects the surface condition. The weight gain of Comparative Example 4 was as high as 0.260%, indicating that the unpassivated chromium powder surface has a large number of high-energy active sites (fresh surface), which gradually adsorb oxygen in the air and undergo surface oxidation, leading to an increase in mass. This verifies the crucial role of the passivation step in improving storage and usage stability.

[0044] Comparative Example 5 (conventional process) had the highest levels of all impurities: oxygen 115 ppm, nitrogen 80 ppm, iron 125 ppm, and carbon 92 ppm, with a purity of only 99.70%. Compared to the raw material (99.90%), the purity decreased by 0.20%, mainly due to oxidation and oxygenation during the crushing process, iron gain from equipment wear, and nitrogen gain from ineffective degassing. This fully demonstrates the advantages of the multi-stage impurity removal system of this invention. Furthermore, Comparative Example 5, using a conventional process without gradient crushing and shaping, had the worst particle size distribution (fluctuation ±2.5 μm) and a sphericity of only 0.68. Comparative Example 5 had the highest weight gain (0.380%), which is directly related to its high impurity content, large specific surface area, and lack of passivation treatment.

[0045] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing high-purity vacuum degassed chromium powder, characterized in that, Includes the following steps: S1. Raw material pretreatment: High-purity chromium sheets are removed of surface oxide scale and oil, cut, and then subjected to ultrasonic cleaning, vacuum drying, and plasma surface treatment to obtain pretreated chromium blocks. S2, Vacuum pre-degassing: The pretreated chromium block is placed in a low-pressure inert atmosphere for pre-degassing treatment to achieve surface purification; S3, Stepped temperature-controlled inert pulverization: The pre-degassed material is subjected to multi-stage gradient temperature-controlled pulverization under an inert atmosphere, with simultaneous real-time electromagnetic iron removal. S4. Grading and screening: The pulverized material is subjected to two-stage sieving, and closed-loop control is achieved by combining online particle size detection to obtain chromium powder intermediates; S5, Vacuum secondary degassing coupled with carbothermic reduction for deep deoxidation: The chromium powder intermediate was mixed with high-purity carbon powder and then subjected to deep deoxidation treatment by carbothermal reduction under vacuum. S6, Precision Plastic Surgery: The chromium powder intermediate after secondary degassing was placed in a pulverizing and shaping machine for low-speed shaping. S7. Plasma reduction and controllable passivation: The shaped chromium powder is placed in a plasma treatment device and treated with argon-hydrogen mixed plasma. First, the residual oxides on the surface are reduced by active hydrogen atoms, and then a dense Cr2O3 passivation layer is formed under an oxygen partial pressure of 0.05~0.1 Pa to obtain high-purity vacuum degassed chromium powder.

2. The preparation method according to claim 1, characterized in that, The high-purity chromium sheet mentioned in step S1 has a purity of ≥99.90%; The side length of the chromium block after cutting is 1~3cm; The cleaning process uses a 1:1 volume ratio of anhydrous ethanol and anhydrous acetone, followed by ultrasonic cleaning for 15-20 minutes at an ultrasonic power of 300-400W. The liquid-to-solid volume ratio of the cleaning solution to the chromium block is (3-5):

1. Drying temperature 60~80℃, vacuum degree 100~1000Pa, drying time 2~3h; The plasma treatment uses argon plasma with a plasma power of 80~100W, an argon flow rate of 0.3~0.5L / min, a treatment time of 3~5min, and a treatment vacuum degree of 5~10Pa.

3. The preparation method according to claim 1, characterized in that, Step S2 specifically involves placing the pretreated chromium block in a vacuum sintering furnace, lining the furnace with high-purity graphite plates, and evacuating the furnace to a vacuum level of ≤1×10⁻⁶. -3 Pa, refill with high-purity argon gas to a furnace pressure of 10~100Pa, and maintain a low-pressure argon atmosphere in dynamic equilibrium inside the furnace by linking a mass flow meter with a vacuum pump; heat to 600~700℃ and hold for 4~6 hours for pre-degassing, and then cool to room temperature with the furnace after treatment.

4. The preparation method according to claim 1, characterized in that, In step S3, the pulverization is carried out under an argon atmosphere of 0.1~0.12MPa, using a three-stage temperature gradient control. First-stage coarse crushing: chamber temperature 10~20℃, rotation speed 2800~3000rpm, crushed to 200~300µm; Second stage medium crushing: chamber temperature 15~25℃, rotation speed 3500~3800rpm, crushed to 50~80µm; Third-stage fine crushing: chamber temperature 10~20℃, rotation speed 4000~4200rpm, crushed to 30~40µm; The feeding speed is 5~8 kg / h, and the single feeding amount is 20~30 kg; Each discharge port is equipped with an electromagnetic iron removal device, with a magnetic field strength of 800~900mT for coarse crushing, 900~1000mT for medium crushing, and 1000~1200mT for fine crushing.

5. The preparation method according to claim 1, characterized in that, Step S4 is as follows: The pulverized material is subjected to a three-stage ultrasonic stepped sieve for primary sieving. The sieve mesh sizes are 40µm, 30µm and 27µm respectively. The ultrasonic frequency is 20~25kHz to assist in vibration to prevent clogging of the sieve. The sieving time is 10~15min and the feeding speed is 3~5kg / h. The chromium powder intermediate with a size of 27~30μm is collected. The collected chromium powder intermediate was subjected to two-stage sieving using an airflow classifier with an airflow velocity of 12~15m / s. The classifier chamber was temperature-controlled at 20~25℃ using a cooling jacket, with the temperature fluctuation range within ±1℃ of the set temperature. During the grading process, a laser particle size analyzer is used to detect the particle size in real time at a frequency of 1 time / min. The speed of the classifying wheel of the air classifier is automatically adjusted to achieve closed-loop control, so that the particle size fluctuation of the intermediate is within ±1.0μm of the target particle size.

6. The preparation method according to claim 1, characterized in that, Step S5 specifically involves: mechanically mixing the graded and sieved chromium powder intermediate with high-purity carbon powder under an inert atmosphere until uniform, and then placing it in a high-purity graphite crucible in a vacuum sintering furnace. The amount of high-purity carbon powder added is calculated based on the measured oxygen content of the chromium powder intermediate according to the stoichiometric ratio of the chemical reaction, and is 5% in excess to ensure that the carbothermic reduction reaction is fully carried out; the excess carbon powder is extracted in the form of CO under the action of high temperature vacuum and argon carrier gas. Vacuum up to ≤5×10 -5 Pa, heat to 850~950℃ and hold for 3~4h for deep deoxygenation by carbothermal reduction. During the degassing process, high-purity argon gas is introduced as the carrier gas at a flow rate of 0.1~0.2L / min. Vacuum is replenished every 1h to maintain a stable vacuum. After the holding period, the furnace is cooled to room temperature.

7. The preparation method according to claim 1, characterized in that, The low-speed shaping in step S6 is performed at a speed of 800~1000 rpm and a shaping time of 5~8 min, under argon protection, with an argon flow rate of 0.5~1 L / min.

8. The preparation method according to claim 1, characterized in that, In step S7, the volume ratio of the argon-hydrogen mixed plasma is 9:1, the plasma power is 150~200W, the processing temperature is 150~200℃, the processing time is 20~30min, and the vacuum degree is 10~20Pa. The specific process is as follows: first, pre-clean in argon plasma for 1-2 minutes, then introduce argon-hydrogen mixed gas for reduction treatment. After the treatment, cool to below 80°C under argon protection, and then introduce argon-oxygen mixed gas with an oxygen partial pressure of 0.05-0.1 Pa for surface passivation for 10-15 minutes.

9. A high-purity vacuum degassed chromium powder, characterized in that, The chromium powder is prepared by the preparation method according to any one of claims 1 to 8.

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