Method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum system

Abstract

This invention discloses a method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum, belonging to the field of powder metallurgy technology. Molybdenum trioxide is loaded into a loading container and placed inside a reactor. The pressure inside the furnace is evacuated to an absolute pressure ≤1×10⁻⁶. ‑2 A high vacuum state of 400-1500 Pa was established. Hydrogen gas was introduced to maintain a low vacuum state within the reactor, with an absolute pressure of 400-1500 Pa. The temperature was raised to a low-temperature range of 450-550℃ and held at this temperature for 100-140 minutes for the reduction reaction. Then, the temperature was raised to a high-temperature range of 820-1000℃ and held at this temperature for 160-200 minutes for the reduction reaction. After heating was stopped, hydrogen gas was continuously introduced into the reactor to maintain the low vacuum state. After natural cooling, the hydrogen gas supply was stopped, and nitrogen gas was introduced to bring the pressure back to atmospheric pressure. This method, by establishing a dynamic high vacuum environment, effectively removes the reaction byproduct water vapor, significantly inhibiting the agglomeration and growth of the powder, and successfully obtains high-purity, low-agglomeration, small-sized, and uniformly distributed ultrafine molybdenum powder. It also achieves breakthroughs in ultra-high purity, ultrafine particle size, and high production efficiency.

Description

Technical Field

[0001] This invention belongs to the field of powder metallurgy technology, specifically relating to a method for preparing ultrafine molybdenum powder by hydrogen reduction under a vacuum system. Background Technology

[0002] Molybdenum powder, as an important metallic powder, is widely used in aerospace, electronic devices, high-temperature furnaces, surface coatings, and as an additive in high-performance alloys. Its performance largely depends on the particle size, morphology, purity, and dispersibility of the powder. Ultrafine molybdenum powder (typically referring to powders with a particle size less than 1 μm) has higher application value due to its high specific surface area and activity, which can significantly improve the densification and mechanical properties of sintered products.

[0003] Currently, the mainstream method for industrial production of molybdenum powder is the staged reduction of molybdenum oxides (such as MoO3 or MoO2) with hydrogen. Traditional processes are typically carried out in tubular furnaces or rotary kilns under atmospheric or slightly positive pressure. However, this method has some inherent drawbacks: 1. Incomplete reduction: Under normal pressure, the water vapor generated by the reaction is difficult to remove quickly and will accumulate in the gaps between powders or in the furnace, leading to "water vapor poisoning" phenomenon, inhibiting the further progress of the reduction reaction, easily producing intermediate product residues, and affecting product purity.

[0004] 2. Severe Agglomeration: The presence of water vapor promotes the sintering and growth of powder particles, leading to the formation of hard agglomerates. These agglomerates are difficult to break up in subsequent processing, affecting the pressing and sintering properties of the powder.

[0005] 3. Wide particle size distribution: Due to the inhomogeneity of temperature and atmosphere, the reduction degree and growth rate of the powder are inconsistent, resulting in a wide particle size distribution and poor uniformity of the final product.

[0006] 4. High energy consumption and low efficiency: To ensure complete reduction, it is often necessary to extend the holding time or increase the reduction temperature, which increases energy consumption and production costs.

[0007] To overcome the above problems, some studies have attempted to improve the situation, such as using multi-stage reduction and adding inert gas for dilution, but the effects have been limited and none of them have fundamentally solved the problems of agglomeration caused by water vapor retention and the obstruction of reduction kinetics. Summary of the Invention

[0008] The purpose of this invention is to provide a method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum.

[0009] The objective of this invention is achieved as follows: the method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum includes the following steps: Molybdenum trioxide was filled into a loading container and placed in a reactor. The pressure inside the reactor was then evacuated to an absolute pressure ≤ 1 × 10⁻⁶.-2 Pa high vacuum state; Hydrogen gas is introduced to maintain the reactor in a low vacuum state with an absolute pressure of 400~1500Pa; the temperature is first raised to a low temperature range of 450~550℃ and held at this temperature for 100~140min for reduction reaction; then the temperature is raised to a high temperature range of 820~1000℃ and held at this temperature for 160~200min for reduction reaction. After heating is stopped, hydrogen is continuously introduced into the reactor to maintain a low vacuum. After natural cooling, the hydrogen supply is stopped, and nitrogen is introduced to atmospheric pressure to obtain the target molybdenum powder.

[0010] Compared with the prior art, the technical solution described in this invention has the following advantages: 1. In traditional processes, molybdenum trioxide and the water vapor and oxygen adsorbed on the furnace surface compete with hydrogen for a reducing environment during the initial heating stage, leading to localized oxidation and over-oxidation, resulting in material loss and uneven composition. The technical solution described in this invention evacuates the furnace to a high vacuum state before heating, allowing the adsorbed water vapor and oxygen to be fully desorbed, eliminating interference from oxidizing impurities at the source.

[0011] 2. Under dynamic vacuum conditions (400~1200Pa), the mean free path of gas molecules increases, allowing hydrogen to rapidly diffuse and penetrate the powder layer for reduction. Simultaneously, the water vapor generated during the reaction can be instantly removed from the reaction interface, breaking the MoO2 + H2 reaction. The reversible reaction equilibrium of Mo + H2O causes the reaction to proceed strongly to the right, greatly promoting the reduction kinetics and making the reduction more thorough. The resulting molybdenum powder has an extremely low oxygen content (below 500 ppm).

[0012] 3. Water vapor is removed in time, eliminating the key medium that causes interparticle migration and sintering, thereby effectively inhibiting the growth and sintering tendency of MoO2 and Mo particles at high temperatures. At the same time, due to the low total pressure, the surface migration energy barrier of Mo atoms increases, thereby preventing the agglomeration and coarsening of molybdenum powder particles, resulting in ultrafine molybdenum powder with excellent dispersibility. The primary particle size can be controlled within the range of 0.1~0.8μm.

[0013] 4. Under vacuum conditions, the atmosphere inside the furnace is uniform and the temperature field is stable. All powder particles are in a highly consistent reaction environment, so the reduction process is synchronized and the particle size distribution of the final product is very concentrated.

[0014] 5. Compared with the traditional purge mass transfer mode, the above technical solution improves the reduction kinetics by precisely adjusting the pressure and flow rate to control the reaction rate and nucleation process. It can complete the complete reduction at a relatively low temperature and in a short holding time, thereby reducing energy consumption and production costs.

[0015] 6. The entire reaction process is carried out in a closed system under a slightly positive pressure below atmospheric pressure, which avoids the possibility of large-scale mixing of hydrogen and air, making it safer than atmospheric pressure reduction processes.

[0016] In summary, the technical solution described in this invention establishes a dynamic high vacuum environment to remove water vapor, a byproduct of the reaction, in a timely and effective manner, thereby significantly inhibiting the agglomeration and growth of the powder. This results in the successful production of high-purity, low-agglomeration, small-sized, and uniformly distributed ultrafine molybdenum powder, while achieving breakthroughs in ultra-high purity, ultrafine particle size, and high production efficiency. Detailed Implementation

[0017] The present invention will be further described below, but this is not intended to limit the invention in any way. Any modifications or substitutions made based on the teachings of the present invention shall fall within the scope of protection of the present invention.

[0018] The method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum system according to the present invention includes the following steps: Molybdenum trioxide was filled into a loading container and placed in a reactor. The pressure inside the reactor was then evacuated to an absolute pressure ≤ 1 × 10⁻⁶. -2 The high vacuum state of Pa is used to completely remove air and adsorbed water vapor inside the furnace.

[0019] Hydrogen gas is introduced to maintain a low-vacuum dynamic equilibrium state within the reactor at an absolute pressure of 400-1200 Pa. The temperature is first raised to a low-temperature range of 450-550℃ and held at this temperature for 100-140 minutes to carry out the reduction reaction. The main reaction occurring during this stage is: MoO3 + H2 → MoO2 + H2O. The dynamic vacuum environment continuously removes the water vapor generated during the reaction. The temperature is then raised to a high-temperature range of 820-1000℃ and held at this temperature for 160-200 minutes to carry out the reduction reaction. The main reaction occurring during this stage is: MoO2 + 2H2 → Mo + 2H2O. Again, the dynamic vacuum environment ensures the rapid removal of water vapor.

[0020] After heating is stopped, hydrogen is continuously introduced into the reactor to maintain a low vacuum. After natural cooling, the hydrogen supply is stopped, and nitrogen is introduced to atmospheric pressure to obtain the target molybdenum powder.

[0021] The molybdenum trioxide is 4N molybdenum trioxide (MoO3) powder.

[0022] The molybdenum trioxide layer thickness within the loading container is 10-18 mm to ensure effective penetration of the reducing atmosphere and effective escape of byproducts. The loading container is a high-temperature resistant ceramic boat.

[0023] The reactor is a vacuum tube furnace.

[0024] The vacuuming device is achieved through a vacuum unit consisting of a Roots pump and a molecular pump (or diffusion pump), which, together with a precision pressure controller and a hydrogen flow meter, achieves a dynamic balance between hydrogen intake and extraction, ensuring that the pressure inside the furnace remains stable within the set range.

[0025] The purity of the hydrogen gas is ≥99.999%. The flow rate of the hydrogen gas is 0.5~2.0L / min, which is adjusted according to the size of the reaction furnace, the amount of charge, and the pressure inside the furnace.

[0026] The absolute pressure of the low vacuum is preferably 500~1000 Pa.

[0027] The low-temperature range is preferably 480~520℃. The heating rate of the low-temperature range is 2~4℃ / min, preferably 3℃ / min.

[0028] The high-temperature zone is preferably 850~900℃. The heating rate of the high-temperature zone is 4~6℃ / min, preferably 5℃ / min.

[0029] The phrase "stop hydrogen supply after natural cooling" means that while continuously supplying hydrogen, the temperature is allowed to cool naturally to below 60°C, at which point the hydrogen supply is stopped.

[0030] Example 1

[0031] Weigh 100g of 4N molybdenum trioxide (MoO3) powder with an average particle size of 1.5μm, and evenly pack it into a high-temperature resistant ceramic boat with a layer thickness of 10mm. Place the boat in the isothermal zone of a vacuum tube furnace, seal the furnace, and start the molecular pump to evacuate the furnace pressure to an absolute pressure of 5×10⁻⁶. -3 Pa is a high vacuum state.

[0032] Hydrogen gas with a purity ≥99.999% is introduced at a flow rate of 0.5~2.0 L / min to maintain a low vacuum dynamic equilibrium state with an absolute pressure of 800 Pa inside the vacuum tube furnace. The temperature is first increased to a low temperature range of 500℃ at a rate of 3℃ / min, and held at this temperature for 120 min to carry out the reduction reaction. While keeping the pressure and hydrogen atmosphere constant, the temperature is then increased to a high temperature range of 880℃ at a rate of 4℃ / min, and held at this temperature for 180 min to carry out the reduction reaction. After heating is stopped, hydrogen is continuously introduced into the reactor to maintain a low vacuum state. The reactor is allowed to cool naturally to 50°C. The hydrogen supply is then stopped, and nitrogen gas with a purity of ≥99.999% is introduced to atmospheric pressure before the reactor is discharged, resulting in ultrafine molybdenum powder with an average particle size of <0.5μm.

[0033] Example 2

[0034] Weigh 100g of 4N molybdenum trioxide powder with an average particle size of 1.5μm and evenly fill it into a high-temperature resistant ceramic boat with a layer thickness of 15mm. Place the boat in the isothermal zone of a vacuum tube furnace, seal the furnace, and start the molecular pump to evacuate the furnace to an absolute pressure of 6×10⁻⁶. -3 Pa is a high vacuum state.

[0035] Hydrogen gas with a purity ≥99.999% is introduced at a flow rate of 0.5~2.0 L / min to maintain a low vacuum dynamic equilibrium state with an absolute pressure of 500 Pa inside the vacuum tube furnace. The temperature is first increased to a low temperature range of 480℃ at a rate of 2℃ / min, and held at this temperature for 140 min to carry out the reduction reaction. While keeping the pressure and hydrogen atmosphere constant, the temperature is then increased to a high temperature range of 850℃ at a rate of 5℃ / min, and held at this temperature for 200 min to carry out the reduction reaction. After heating is stopped, hydrogen is continuously introduced into the reactor to maintain a low vacuum state. The reactor is allowed to cool naturally to 50°C. The hydrogen supply is then stopped, and nitrogen gas with a purity of ≥99.999% is introduced to atmospheric pressure before the reactor is discharged, resulting in ultrafine molybdenum powder with an average particle size of <0.6μm.

[0036] Example 3

[0037] Weigh 100g of 4N molybdenum trioxide powder with an average particle size of 1.5μm and evenly pack it into a high-temperature resistant ceramic boat, with a layer thickness of 18mm. Place the boat in the isothermal zone of a vacuum tube furnace, seal the furnace, and start the molecular pump to evacuate the furnace pressure to an absolute pressure of 5×10⁻⁶. -3 Pa is a high vacuum state.

[0038] Hydrogen gas with a purity ≥99.999% is introduced at a flow rate of 0.5~2.0 L / min to maintain a low vacuum dynamic equilibrium state with an absolute pressure of 1000 Pa inside the vacuum tube furnace. The temperature is first increased to a low temperature range of 520℃ at a rate of 4℃ / min and held at this temperature for 100 min to carry out the reduction reaction. While keeping the pressure and hydrogen atmosphere constant, the temperature is then increased to a high temperature range of 900℃ at a rate of 6℃ / min and held at this temperature for 160 min to carry out the reduction reaction. After heating is stopped, hydrogen is continuously introduced into the reactor to maintain a low vacuum state. The reactor is allowed to cool naturally to 55°C. The hydrogen supply is then stopped, and nitrogen gas with a purity of ≥99.999% is introduced to atmospheric pressure before the reactor is discharged, resulting in ultrafine molybdenum powder with an average particle size of <0.5μm.

Claims

1. A method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum, characterized in that, The process includes the following steps: Molybdenum trioxide was filled into a loading container and placed in a reactor. The pressure inside the reactor was then evacuated to an absolute pressure ≤ 1 × 10⁻⁶. -2 Pa high vacuum state; Hydrogen gas is introduced to maintain the reactor in a low vacuum state with an absolute pressure of 400~1500Pa; the temperature is first raised to a low temperature range of 450~550℃ and held at this temperature for 100~140min for reduction reaction; then the temperature is raised to a high temperature range of 820~1000℃ and held at this temperature for 160~200min for reduction reaction. After heating is stopped, hydrogen is continuously introduced into the reactor to maintain a low vacuum. After natural cooling, the hydrogen supply is stopped, and nitrogen is introduced to atmospheric pressure to obtain the target molybdenum powder.

2. The method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum system according to claim 1, characterized in that, The molybdenum trioxide layer thickness in the loading container is 10~18mm.

3. The method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum system according to claim 1, characterized in that, The hydrogen flow rate is 0.5~2.0 L / min.

4. The method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum system according to claim 1, characterized in that, The absolute pressure of the low vacuum is 500~1000 Pa.

5. The method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum system according to claim 1, characterized in that, The low-temperature range is 480~520℃.

6. The method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum system according to any one of claims 1 or 5, characterized in that, The heating rate of the low-temperature section is 3℃ / min.

7. The method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum system according to claim 1, characterized in that, The high-temperature range is 850~900℃.

8. The method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum system according to any one of claims 1 or 7, characterized in that, The heating rate of the high-temperature section is 5℃ / min.

9. The method for preparing ultrafine molybdenum powder by hydrogen reduction under vacuum system according to claim 1, characterized in that, The phrase "stop hydrogen supply after natural cooling" means that while continuously supplying hydrogen, the temperature is allowed to cool naturally to below 60°C, at which point the hydrogen supply is stopped.