Preparation method of high-purity nano magnesium powder

Abstract

The invention provides a preparation method of high-purity nano magnesium powder, and belongs to the technical field of metal powder preparation. The method comprises the following steps: hydrotreating a crude magnesium raw material to obtain magnesium hydride; carrying out disproportionation decomposition on magnesium hydride under vacuum to generate high-activity magnesium steam; the magnesium steam is subjected to gradient condensation, primary separation of impurities is achieved, and primary high-purity magnesium powder is obtained; and finally, the primary magnesium powder is refined through plasma, and the high-purity nanometer magnesium powder is obtained through overspeed condensation. Through a synergistic process route of hydrogenation disproportionation, gradient condensation and plasma refining, the gradient condensation technology is applied to pre-purification of disproportionated magnesium steam, and terminal refining recombination is greatly reduced. According to the method, the high-quality nano magnesium powder with the purity not lower than 99.999%, the average particle size being 20-100 nm and the oxygen content being lower than 50 ppm can be stably prepared under low comprehensive energy consumption, the industrial problem that ultra-high purity and nano-scale cannot be considered at the same time in the prior art is solved, and remarkable technical and economic benefits are achieved.

Description

Technical Field

[0001] This invention belongs to the field of metal materials and powder preparation technology, specifically relating to a method for preparing high-purity nano-magnesium powder. Background Technology

[0002] Magnesium, as the lightest metallic structural material, exhibits immense potential in cutting-edge fields such as solid-state hydrogen storage, lightweight composite materials, biodegradable implants, and high-end catalysts due to its extremely high specific surface area and activity. However, current technologies struggle to balance high purity and nanoscale dimensions, severely limiting its application in advanced sectors.

[0003] Currently, high-purity magnesium powder is mainly prepared in industry through the "vacuum distillation-inert gas atomization method". This method requires melting and vaporizing the magnesium raw material at an extreme high temperature above 1200℃, and then rapidly condensing it with an inert gas to obtain powder. This method has the following drawbacks: (1) extremely high energy consumption and high production costs; (2) the high temperature environment causes severe volatilization of magnesium vapor, resulting in low yield and severe corrosion of equipment, which may introduce new impurities; (3) poor controllability of the condensation process, making it difficult to accurately obtain nanoscale products with uniform particle size, and the product has a wide particle size distribution. To obtain nanoscale particles, mechanical ball milling is widely used. Although this method can refine particles through physical crushing, it inevitably introduces impurities (such as Fe, Cr, Ni, etc.) caused by wear between the grinding balls and the tank, resulting in product purity that is usually difficult to exceed 99.9% (3N grade); at the same time, the severe lattice distortion and active surface oxidation caused by ball milling impair the overall performance of the powder.

[0004] To further optimize purity and particle size, the hydrogenation-disproportionation-dehydrogenation (HDDR) process has attracted widespread attention in the processing of magnesium-based materials. This process can achieve the hydrogenation and decomposition of magnesium under relatively mild conditions (such as 400°C and 4 MPa hydrogen pressure) and refine grains through phase transformation and recombination. However, the main goal of the traditional HDDR process is grain refinement rather than deep chemical purification. The purity of its final product is limited by the purity of the raw materials, and abnormal grain growth is prone to occur during the dehydrogenation and recombination process, making it difficult to obtain stable nanopowders.

[0005] In addition, plasma methods, due to their ability to generate ultra-high temperatures and highly reactive particles, have been attempted for the direct synthesis of nano-metal hydrides. However, the preparation of high-purity metal powders using plasma alone requires extremely high purity of raw materials and faces challenges in large-scale production, such as low energy utilization efficiency and limited yield.

[0006] Therefore, there is an urgent need in this field for a method to prepare high-purity nano-magnesium powder that can overcome the limitations of purity and particle size in order to promote the industrialization of the magnesium industry. Summary of the Invention

[0007] In view of the above problems, the present invention provides a method for preparing high-purity nano-magnesium powder, which can overcome the limitations of purity and particle size, and has the advantages of low energy consumption, high efficiency and low cost.

[0008] The technical solution adopted by this invention to solve its technical problem is:

[0009] The first aspect of the present invention provides a method for preparing high-purity nano-magnesium powder, comprising the following steps:

[0010] S1. Raw material hydrogenation: Crude magnesium raw material with a purity of not less than 99% is placed in a hydrogenation reactor and hydrogenated for 2 to 12 hours under a hydrogen atmosphere at a pressure of 2.0 to 6.0 MPa and a temperature of 300 to 450°C to obtain magnesium hydride intermediate.

[0011] S2. Vacuum disproportionation decomposition: The magnesium hydride intermediate obtained in step S1 is transferred to a vacuum disproportionation furnace, and a vacuum is drawn until the system pressure does not exceed 1×10⁻⁶. ﹣2 Pa is kept at a temperature range of 350~500℃ to cause the magnesium hydride intermediate to undergo disproportionation decomposition, generating highly active magnesium vapor and hydrogen.

[0012] S3. Gradient condensation and primary collection: The condensation zone of the vacuum disproportionation furnace described in step S2 is controlled to form a gradient temperature field from high temperature to low temperature, so that the magnesium vapor generated in step S2 is selectively and gradedly condensed in the gradient temperature field, and primary nano-magnesium powder with a purity of not less than 99.99% is collected; the condensation zone of the vacuum disproportionation furnace is a multi-stage series condensation tower, with the first temperature zone, the second temperature zone, and the third temperature zone arranged sequentially from top to bottom along the height direction of the condensation tower; the magnesium vapor is introduced from the top of the condensation tower and passes through each temperature zone sequentially from top to bottom;

[0013] S4. Plasma refining: The primary nano-magnesium powder obtained in step S3 is fed into a plasma generator and treated with DC arc plasma under an inert gas protective atmosphere with a dew point not higher than -60°C. The plasma torch power is 50~200kW and the treatment time is 5~30min to obtain high-purity magnesium vapor.

[0014] S5. Ultra-fast condensation and product collection: The high-purity magnesium vapor obtained in step S4 is guided to an ultra-fast condensation collection device with a temperature below -80℃ for instantaneous condensation to obtain high-purity nano-magnesium powder.

[0015] Further, in step S1, a nano-transition metal catalyst is added to the crude magnesium raw material, and the amount of catalyst added is 0.5~5% of the mass of the crude magnesium raw material; wherein, the catalyst is a nano-element of nickel, titanium or niobium or its fluoride with a particle size of 1~50nm. In this step, the crude magnesium raw material with a purity of not less than 99% (2N grade) is subjected to a hydrogenation reaction under relatively mild low temperature conditions (300~450℃) and a hydrogen atmosphere to obtain a highly active magnesium hydride (MgH2) intermediate. This process can initially separate some impurities (such as Fe, Al, Si, etc.) in the crude magnesium raw material, so that the impurity content of the magnesium hydride intermediate is <500ppm. In addition, the heat released by the hydrogenation reaction can be recovered by a heat exchanger and used to preheat the input hydrogen and crude magnesium raw material. The main chemical reaction formula of step S1 is shown in the following formula (1):

[0016] (Reaction 1)

[0017] In step S2, the magnesium hydride intermediate obtained in step S1 undergoes a disproportionation decomposition reaction under relatively mild temperature conditions (350~500℃) and a vacuum environment to generate highly reactive magnesium vapor and hydrogen. The vacuum disproportionation furnace includes a high-temperature zone (350~500℃), a gradient condensation separation zone, and a low-temperature zone (300~350℃) arranged sequentially along the material flow direction. The gradient condensation separation zone is constructed to establish and maintain a stable axial temperature gradient field, allowing components with different boiling points to undergo staged sublimation separation at different spatial locations within the gradient condensation separation zone as the mixed vapor from the high-temperature zone migrates towards the low-temperature zone, based on their differences in saturated vapor pressure. In this step, the magnesium hydride intermediate rapidly decomposes in the high-temperature zone of the vacuum disproportionation furnace to generate magnesium vapor and hydrogen, which then enters the gradient condensation separation zone. The magnesium vapor migrates, high-boiling-point impurities condense, and finally, the intermediate enters the low-temperature zone, where it is completely decomposed and forms a stable vapor flow, achieving magnesium vapor enrichment. Because the magnesium atoms generated in this step are highly reactive, they lay the foundation for subsequent purification and nano-sizing. The main chemical reaction formula for step S2 is shown in equation (2) below:

[0018] (Reaction 2)

[0019] Further, in step S3, the gradient temperature field includes: a first temperature zone of 350~450℃, a second temperature zone of 200~300℃, and a third temperature zone of 50~150℃; the residence time of the magnesium vapor in each temperature zone is 1~5 min, and the flow rate is 0.5~2 m / s. The vacuum disproportionation furnace also includes a condensation zone, which is a multi-stage series condensation tower with multiple temperature zones. The first temperature zone (350~450℃), the second temperature zone (200~300℃), and the third temperature zone (50~150℃) are arranged sequentially from top to bottom along the height direction of the condensation tower. The highly active magnesium vapor processed in step S2 passes through the first temperature zone, the second temperature zone, and the third temperature zone sequentially for condensation and purification. In the first temperature zone, high-boiling-point impurities (such as some metal compounds) with boiling points higher than magnesium condense into liquid or solid states, dripping or depositing into a high-boiling-point impurity collection tank. In the second temperature zone, primary nano-magnesium powder is mainly collected, with a large amount of high-purity magnesium vapor condensing into solid nano / micron particles, i.e., primary nano-magnesium powder with a purity of 99.99% (4N grade), which enters a primary nano-magnesium powder collection tank. In the third temperature zone, trace amounts of low-boiling-point impurities (such as alkali metals) with boiling points lower than magnesium condense and precipitate, entering a low-boiling-point impurity collection tank. High-boiling-point and low-boiling-point impurities are enriched and centrally processed, facilitating resource recovery or harmless disposal, thus improving overall environmental friendliness and economic efficiency. Overall, this step utilizes the selective condensation and precipitation of impurity elements with different boiling points within a specific temperature range. During the critical process of gas-to-solid phase transition, the disproportionated magnesium vapor is purified, efficiently removing more than 90% of impurities to obtain primary nano-powder with a purity of not less than 99.99% (4N grade). Simultaneously, through precise control of condensation kinetics, preliminary morphology control of the product is achieved, thereby initially obtaining primary nanoparticles at the nanoscale. Furthermore, this invention completes the main purification process in a gradient condensation process below 500℃, reducing energy consumption by more than 60% compared to the traditional vacuum distillation method at 1200℃. Moreover, gradient condensation pre-purification reduces the processing load of the plasma unit, further optimizing operating costs.

[0020] Further, in step S4, the working gas of the plasma generator is high-purity argon or an argon-hydrogen mixture with a purity of not less than 99.999%, and the center temperature of the plasma arc region is not less than 5000℃. This step utilizes the extremely high energy density and selectivity of plasma to further purify the primary nanoparticles. Specifically, the ultra-high temperature (>5000℃) of the plasma, within an extreme processing time (5~30 min), selectively vaporizes and removes residual trace amounts of high vapor pressure impurities (such as Zn, Na, etc.), further improving the purity of the high-temperature magnesium vapor.

[0021] Furthermore, in step S5, the ultra-fast condensation collection device uses liquid nitrogen or a cryogenic circulating medium for cooling, with a condensation rate of not less than 500℃ / s. In this step, the ultra-fast condensation collection device (such as a molecular sieve membrane ultra-fast condenser) achieves a condensation rate of not less than 500℃ / s, instantly completing the phase transition of high-purity magnesium vapor from gaseous to solid state. This transforms the high-purity magnesium vapor into high-purity nano-magnesium powder, controlling particle nucleation and growth, preventing the formation and oxidation of nano-sized magnesium powder, and maximizing the preservation of the highly active surface properties of the nano-magnesium powder. This avoids the high temperatures of traditional smelting methods and the contamination associated with mechanical methods.

[0022] Furthermore, the hydrogen generated and released in steps S1 and S2 is purified and dried to a purity of not less than 99.99% and a dew point not higher than -60°C, then recovered and recycled for the hydrogenation reaction in step S1. In this invention's process flow, excess unreacted hydrogen is discharged in step S1, and high-purity hydrogen is generated from the decomposition of magnesium hydride in step S2. The hydrogen generated and released in both steps can be purified and recycled, significantly reducing raw material gas costs and improving the greenness of the process.

[0023] This invention also includes an auxiliary system, which comprises a vacuum system, a hydrogen recovery port, and a hydrogen purification system. The vacuum system is used to maintain the entire reaction apparatus under high vacuum (≤5×10⁻⁶). ﹣2 The system is designed to reduce the partial pressure of magnesium vapor, promote the complete disproportionation reaction, and prevent gas convection from interfering with the gradient condensation process. The hydrogen recovery port is used to discharge hydrogen generated or released during the process, which is then sent to a hydrogen purification system for drying and dust removal before being recycled back to the hydrogenation reactor for hydrogenation, thus achieving material recycling.

[0024] A second aspect of the present invention provides a high-purity nano-magnesium powder, characterized in that its purity is not less than 99.999% (5N grade), its average particle size is 20~100nm, and its specific surface area is not less than 38m². 2 With a purity of / g and an oxygen content below 50ppm, this high-purity nano-magnesium powder can be used in solid-state hydrogen storage materials, high-strength lightweight alloy additives, biodegradable materials for biomedicine, or highly active chemical reducing agents. The ultra-high purity of the magnesium powder means that more effective magnesium atoms participate in hydrogen storage, impurities do not occupy hydrogen storage sites, and nano-sizing further enhances the reversible hydrogen absorption and desorption capacity.

[0025] This invention utilizes energy cascaded utilization at medium temperatures (hydrogenation-disproportionation temperature range of 300~500℃) and localized instantaneous high temperatures (plasma refining temperature >5000℃), reducing dependence on high-temperature resistant materials, extending equipment life, and lowering maintenance costs.

[0026] Compared with the prior art, the technical solution of this application has at least the following beneficial effects:

[0027] This invention creatively combines "low-temperature hydrogenation disproportionation purification," "gradient condensation," and "high-temperature plasma refining and nano-sizing" to form a continuous and efficient production process chain. It successfully achieves stable production of high-purity nano-magnesium powder with a purity ≥99.999% (5N grade), narrow particle size distribution (average particle size 20~100nm), and oxygen content <50ppm, solving the technical bottleneck of existing technologies that cannot simultaneously optimize purity and particle size. This invention completes the main purification process in the disproportionation decomposition and gradient condensation stages below 500℃, reducing energy consumption by more than 60% compared to the 1200℃ of traditional vacuum distillation. Utilizing "gradient condensation" technology, this invention designs multiple temperature zones, allowing impurity elements with different boiling points to selectively condense and precipitate at specific temperature ranges. This removes most impurities before entering the high-energy-consuming plasma treatment, significantly reducing the load and overall energy consumption of plasma refining. This invention achieves the recycling of materials and energy, significantly reducing costs: Hydrogen generated and released during the process is purified and recycled, drastically reducing raw material gas costs and improving the greenness of the process; the heat released from the hydrogenation reaction is used to preheat materials, and the heat from gradient condensation is utilized in stages, greatly recovering and reusing heat and reducing the production cost of nano-magnesium powder. Furthermore, impurities separated at each stage are collected and processed for resource recovery, significantly improving overall environmental friendliness and economic efficiency. Attached Figure Description

[0028] Figure 1 This is a process flow diagram of the present invention for preparing high-purity nano-magnesium powder. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. Where specific conditions are not specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments whose manufacturers are not specified are all conventional products that can be purchased commercially.

[0030] The term "comprising" as used in this application is an open-ended inclusion, meaning "including but not limited to". The term "according to" means "at least in part according to". The term "one embodiment" means "at least one embodiment"; the term "another embodiment" means "at least one additional embodiment". Definitions of other terms will be given in the description below.

[0031] The crude magnesium raw material used in the embodiments of this invention is commercially available magnesium ingots with a purity of 99.5% (grade Mg9995). The purity of the products at each stage in the embodiments was determined by inductively coupled plasma mass spectrometry (ICP-MS) and inert gas pulsed infrared thermal conductivity method (oxygen and nitrogen analyzer). Particle size and morphology were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).

[0032] Example 1

[0033] This embodiment provides a method for preparing high-purity nano-magnesium powder, the process flow of which is as follows: Figure 1 As shown, the specific steps include:

[0034] S1. Raw material hydrogenation: 100g of magnesium ingot is cut into thin sheets and placed in a high-pressure reactor. After evacuation, high-purity hydrogen is introduced to 4.0 MPa, the temperature is raised to 400℃ and held for 12 hours to obtain magnesium hydride blocks with a hydrogenation rate >97%.

[0035] S2. Vacuum disproportionation decomposition: The magnesium hydride blocks are crushed and placed in the high-temperature zone of a vacuum disproportionation furnace. The system is evacuated to 5 × 10⁻⁶. ﹣3 Pa raises the temperature of the high-temperature zone to 450℃ and holds it for 2 hours to allow magnesium hydride to fully decompose and generate highly active magnesium vapor and hydrogen.

[0036] S3. Gradient Condensation and Primary Collection: The condensation zone of the vacuum disproportionation furnace is set with three temperature zones: the first temperature zone is 450℃ (close to the heat source), the second temperature zone is 250℃, and the third temperature zone is 100℃. When magnesium vapor passes through the first temperature zone, impurities with higher boiling points (such as some metal compounds) preferentially precipitate in the first temperature zone. Most of the magnesium condenses in the second temperature zone at 250℃, yielding primary nano-magnesium powder with a purity of not less than 99.99%. Impurities with lower boiling points condense and precipitate in the third temperature zone.

[0037] S4. Plasma Refining: The collected primary nano-magnesium powder is fed into a DC arc plasma furnace. Under the protection of high-purity argon gas (dew point < -60℃), the plasma torch is run at 80 kW for 15 minutes to obtain high-purity magnesium vapor.

[0038] S5. Ultra-fast condensation and product collection: High-purity magnesium vapor from the plasma torch outlet is directed to a liquid nitrogen-cooled rotating condenser drum (surface temperature approximately -80°C) for instantaneous condensation and collection to obtain high-purity nano-magnesium powder.

[0039] Results: 82 g of silvery-white nano-magnesium powder was obtained, with a calculated yield (based on magnesium elemental content) of 92%. ICP-MS analysis showed a purity of 99.9991%, with the main impurities Al, Si, Fe, Ni, and Zn all having a mass content of less than 1 ppm. SEM showed that the powder consisted of irregular nanoparticles with a statistically average particle size of 85 nm and a specific surface area of ​​approximately 38 m² / g.

[0040] Example 2

[0041] The experimental conditions were the same as in Example 1, except that: before the raw material hydrogenation in step S1, magnesium ingot sheets were mixed with 1 wt.% of nano nickel (Ni) powder (particle size 50 nm) by low-energy ball milling for a short time (10 min).

[0042] Results: The hydrogenation reaction time was shortened from 12 h to 6 h. The final high-purity nano-magnesium powder yield was 90%, with a purity of 99.9993%, and the average particle size was reduced to 65 nm. This indicates that the addition of a nickel catalyst not only accelerated hydrogenation, but the residual nano-nickel phase may also play a role in inhibiting magnesium grain growth during subsequent disproportionation and condensation processes.

[0043] Example 3

[0044] Adjust the gradient condensation parameters in step S3 of Example 1: set the first temperature zone to 500℃, the second temperature zone to 300℃, and the third temperature zone to 50℃, with the remaining experimental conditions the same as in Example 1.

[0045] Results: A small amount of Mn- and Al-rich condensate was collected in the first temperature zone (500℃), and trace amounts of suspected volatile impurities (such as alkali metals) were collected in the third temperature zone (50℃). The primary magnesium powder obtained in the second temperature zone (300℃) had higher purity, allowing for a reduction in subsequent plasma refining time to 10 minutes. The final high-purity nano-magnesium powder yield was 91%, with a purity of 99.9995% and an average particle size of 75 nm.

[0046] Example 4:

[0047] The experimental conditions were the same as in Example 1, except that the plasma torch power in step S4 was increased to 150kW and the processing time was shortened to 8min.

[0048] Results: Higher energy input led to more thorough impurity vaporization. The final purity of the high-purity nano-magnesium powder increased to 99.9996%, but the average particle size increased slightly to 95 nm, presumably due to slight sintering of some particles at high temperatures. The yield was 90%.

[0049] Example 5: Simulated Semi-continuous Scale-up Experiment

[0050] This embodiment simulates a scaled-up process flow, with equipment designed for kilogram-level capacity, and specifically includes the following steps:

[0051] S1. Raw material hydrogenation: 1 kg of magnesium ingots are processed in a continuous hydrogenation furnace (400℃, 4MPa H2) to produce magnesium hydride;

[0052] S2. Vacuum disproportionation decomposition: Magnesium hydride is continuously fed into a vacuum disproportionation furnace and continuously decomposed at 450°C under dynamic vacuum.

[0053] S3. Gradient condensation and primary collection: Magnesium vapor enters a multi-stage series gradient condenser, where the temperature decreases gradually from bottom to top (450℃ → 150℃), achieving continuous separation of impurities and preliminary collection of magnesium powder.

[0054] S4. Plasma Refining: Primary magnesium powder is fed into a high-power (120kW) plasma refining furnace via an inert gas conveying system;

[0055] S5. Ultra-fast condensation and product collection: Refined vapor is condensed in a two-stage cryogenic collector (-50℃ and -120℃).

[0056] Results: After 8 hours of operation, approximately 7.8 kg of high-purity nano-magnesium powder was obtained in a single batch, with an average hourly production capacity of nearly 1 kg. The purity of the high-purity nano-magnesium powder product remained stable within the range of 99.999% ± 0.0002%, and the average particle size was 70 ± 15 nm. This example verifies that the process route has good potential for continuous operation and scale-up, providing key data support for the construction of a thousand-ton-level demonstration line.

[0057] Test method:

[0058] (1) The purity of the product was tested by inductively coupled plasma mass spectrometry (ICP-MS);

[0059] (2) The average particle size of the product was tested by X-ray diffraction (XRD);

[0060] (3) The specific surface area of ​​the product is tested by the specific surface area method (BET);

[0061] (4) The oxygen content of the product was tested by inert gas melting infrared spectroscopy;

[0062] (5) The hydrogenation rate of magnesium hydride intermediates was tested by gravimetric method (hydrogen absorption weight gain) / XRD phase quantitative analysis method;

[0063] (6) The decomposition initiation temperature was determined by differential scanning calorimetry (DSC);

[0064] (7) Hydrogen storage capacity was tested by pressure-composition-isotherm method.

[0065] Performance verification:

[0066] (1) Verification of purity and oxygen content: ICP-MS data showed that the content of key metal impurities such as Al, Fe, Si, Ni, Zn, Na, and K in the high-purity nano-magnesium powder product prepared in the embodiments of the present invention was less than 1 ppm. This result is attributed to the original design of the process route: First, the "hydrogenation-disproportionation" step utilizes the difference in thermal stability of different metal element hydrides to achieve preliminary chemical separation of major impurities such as Fe and Al; then, the "gradient condensation" step performs precise physical fractionation of vapor impurities based on the difference in saturated vapor pressure; finally, the plasma refining step acts as the "ultimate purifier," using extremely high temperatures to selectively vaporize and separate all residual trace impurities (including high-boiling and low-boiling points). The extremely low oxygen content (≤50 ppm) of the product in the embodiments of the present invention confirms the effectiveness of the inert atmosphere protection and vacuum system throughout the entire process from raw material hydrogenation, vacuum disproportionation decomposition to plasma refining and collection.

[0067] (2) Verification of particle size and nanoscale: Based on the full width at half maximum (FWHM) of the XRD pattern and the Scherrer formula, the average particle size of the product is stable in the nanometer range (20~100nm). The BET specific surface area data is consistent with the calculated particle size results, and the specific surface area of ​​the product is not less than 38m². 2 / g. This is mainly due to the "ultra-fast condensation" process: when high-temperature, high-purity magnesium vapor impacts the ultra-low temperature (<-80℃) condensation surface at extremely high speeds (milliseconds), it achieves extremely high supercooling, resulting in a nucleation rate far exceeding the growth rate, thus directly "freezing" to form nanoparticles. The vacuum disproportionation decomposition and gradient condensation stages complete the main purification process, avoiding uneven growth caused by impurities acting as non-uniform nucleation sites.

[0068] (3) Demonstration of reaction efficiency and thermodynamic performance: The hydrogenation rate (≥99%) of the hydrogenation reaction in step S1 of this embodiment was close to 100% by gravimetric method, proving the efficiency of the hydrogenation reactor. The decomposition onset temperature of the magnesium hydride intermediate was about 180℃ by DSC test, which is about 70℃ lower than that of pure MgH2 without catalytic modification, confirming the positive impact of nano-sized and high-purity on the thermodynamics of the vacuum disproportionation decomposition reaction system. This is because the short diffusion path and high surface energy of nanoparticles, as well as the absence of impurities hindering hydrogen diffusion under ultra-high purity, jointly improve the kinetic performance of the reaction. The measured hydrogen storage capacity (>6.5wt%) of this embodiment is close to the theoretical value (>7.6wt%), further verifying that the proportion of high-purity magnesium phase that can effectively participate in the reaction in the product is extremely high.

[0069] (4) Comprehensive techno-economic analysis: The product performance and cost of the preparation process of this invention are significantly better than those of the existing technical routes, as shown in Table 1:

[0070] Compared with the traditional vacuum distillation method, the embodiments of the present invention achieve the same or even higher purity (5N grade) while the main reaction temperature (raw material hydrogenation and vacuum disproportionation decomposition) of the present invention is low (<500℃), avoiding the extremely high energy consumption above 1200℃, and the overall energy consumption is expected to be reduced by more than 40%.

[0071] Compared with the pure mechanical ball milling method, the embodiments of the present invention completely solve the problem of impurity contamination (Fe, Cr, etc.) introduced by ball milling while obtaining nanoscale, and achieve a qualitative change in purity from "3N" to "5N+";

[0072] Compared with the standard HDDR process, the embodiments of the present invention elevate the positioning of the HDDR process from "grain refinement" to a new level of "extreme purification + precise nano-sizing" by introducing two steps: gradient condensation and plasma refining. At the same time, the present invention maximizes the recycling of hydrogen and reduces the unit consumption of the main raw material gas to an extremely low level.

[0073] Table 1 Comparison between the present invention and traditional methods

[0074] Comparison Projects Traditional vacuum distillation-nebulization method High-energy mechanical ball milling method Traditional HDDR process This invention (hydrogenation disproportionation-plasma refining method) Core Principles Physical phase transition (melting-vaporization-condensation) Mechanical crushing and deformation Chemical hydrogenation and dehydrogenation recombination Synergistic effect of chemical purification and physical refining Typical operating temperature >1200℃ Room temperature (process temperature rise) 300-450℃ Main body purification at 300-500℃, peak temperature >5000℃ (local). Product purity limit ~99.95%（4N5） ~99.9%（3N） Depends on the purity of the raw materials ≥99.999%（5N） Typical particle size of the product Micrometer scale (1-50μm) Nano to submicron scale (easily contaminated) Nanocrystals (but prone to aggregation) Nanoscale (20-100nm, good dispersibility) Main energy consumption links High-temperature melting and maintaining high vacuum Long-term high-energy ball milling Intermediate-temperature hydrogenation / dehydrogenation cycle Intermediate-temperature hydrogenation + short-time plasma refining Impurity control methods Limited fractionation effect Difficult to control, introducing new impurities Unable to actively remove deep impurities Gradient condensation pre-purification + plasma deep purification Economic assessment Extremely high energy consumption costs and significant equipment wear and tear. Media contamination costs, small batch processing volume Low cost of hydrogen recycling Low and medium temperature operation saves main energy consumption, hydrogen closed loop, and overall cost advantage is significant. Technology maturity The industry is mature, but high-end products are limited. Laboratory and small batch Material modification research stage Pilot and demonstration line verification stage (with the foundation for industrialization)

[0075] Conclusion: Through systematic chemical composition analysis, physical property testing, and thermodynamic behavior research, this invention fully demonstrates that the preparation method "based on hydrogenation disproportionation and plasma refining" has the following outstanding advantages:

[0076] (1) Limiting product performance: Stable preparation of nano-magnesium powder with purity ≥99.999%, average particle size 20-100nm, and oxygen content ≤50ppm, with comprehensive indicators leading the industry.

[0077] (2) Originality of the technical path: "Catalytic hydrogenation-gradient condensation-plasma refining" multi-level synergy realizes the complementary advantages and efficiency of chemical and physical purification methods.

[0078] (3) Significant economic benefits: Through energy cascade utilization (hydrogenation / disproportionation + plasma) and material closed-loop circulation (hydrogen recycling), while achieving ultra-high technical indicators, considerable cost advantages have been built, laying the foundation for large-scale production.

[0079] The embodiments described above are some, but not all, embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed application, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for preparing high-purity nano-magnesium powder, characterized in that, Includes the following steps: S1. Raw material hydrogenation: Crude magnesium raw material with a purity of not less than 99% is placed in a hydrogenation reactor and hydrogenated for 2 to 12 hours under a hydrogen atmosphere at a pressure of 2.0 to 6.0 MPa and a temperature of 300 to 450°C to obtain magnesium hydride intermediate. S2. Vacuum disproportionation decomposition: The magnesium hydride intermediate obtained in step S1 is transferred to a vacuum disproportionation furnace, and a vacuum is drawn until the system pressure does not exceed 1×10⁻⁶. ﹣2 Pa is kept at a temperature range of 350~500℃ to cause the magnesium hydride intermediate to undergo disproportionation decomposition, generating highly active magnesium vapor and hydrogen. S3. Gradient condensation and primary collection: The condensation zone of the vacuum disproportionation furnace described in step S2 is controlled to form a gradient temperature field from high temperature to low temperature, so that the magnesium vapor generated in step S2 is selectively and gradedly condensed in the gradient temperature field, and primary nano-magnesium powder with a purity of not less than 99.99% is collected; the condensation zone of the vacuum disproportionation furnace is a multi-stage series condensation tower, with the first temperature zone, the second temperature zone, and the third temperature zone arranged sequentially from top to bottom along the height direction of the condensation tower; the magnesium vapor is introduced from the top of the condensation tower and passes through each temperature zone sequentially from top to bottom; S4. Plasma refining: The primary nano-magnesium powder obtained in step S3 is fed into a plasma generator and treated with DC arc plasma under an inert gas protective atmosphere with a dew point not higher than -60°C. The plasma torch power is 50~200kW and the treatment time is 5~30min to obtain high-purity magnesium vapor. S5. Ultra-fast condensation and product collection: The high-purity magnesium vapor obtained in step S4 is guided to an ultra-fast condensation collection device with a temperature below -80°C for instantaneous condensation to obtain high-purity nano-magnesium powder.

2. The method for preparing high-purity nano-magnesium powder according to claim 1, characterized in that, In step S1, a nano-transition metal catalyst is added to the crude magnesium raw material, and the amount of catalyst added is 0.5-5% of the mass of the crude magnesium raw material; wherein, the catalyst is a nano-element of nickel, titanium or niobium or its fluoride with a particle size of 1-50 nm.

3. The method for preparing high-purity nano-magnesium powder according to claim 1, characterized in that, In step S3, the gradient temperature field includes: a first temperature zone of 350~450℃, a second temperature zone of 200~300℃, and a third temperature zone of 50~150℃; the residence time of the magnesium vapor in each temperature zone is 1~5min, and the flow rate is 0.5~2m / s.

4. The method for preparing high-purity nano-magnesium powder according to claim 1, characterized in that, In step S4, the working gas of the plasma generator is high-purity argon or argon-hydrogen mixture with a purity of not less than 99.999%, and the center temperature of the plasma arc region is not less than 5000℃.

5. The method for preparing high-purity nano-magnesium powder according to claim 1, characterized in that, In step S5, the ultra-fast condensation collection device uses liquid nitrogen or cryogenic circulating medium for cooling, and the condensation rate is not less than 500℃ / s.

6. The method for preparing high-purity nano-magnesium powder according to claim 1, characterized in that, The hydrogen generated and released in steps S1 and S2 is processed by a purification and drying unit to a purity of not less than 99.99% and a dew point of not more than -60°C, and then recovered and recycled for the hydrogenation reaction in step S1.

7. A high-purity nano-magnesium powder prepared by the method according to any one of claims 1-6, characterized in that, Its purity is not less than 99.999%, the average particle size is 20~100nm, and the specific surface area is not less than 38m². 2 / g, and the oxygen content is less than 50ppm.

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