Preparation technology of ultrafine spherical tungsten powder
Metal tungsten with high melting point and high hardness is an important strategic material, which is widely used in tungsten carbide tools, electric filaments, tool steel additives, rockets, space vehicles, nuclear reactors and other fields. With the development of science and technology, new special requirements are continuously put forward for raw tungsten powder. For example, high-quality cemented carbide requires ultra-fine tungsten powder; electronic materials and filter materials require spherical tungsten powder.
Because fine-grained cemented carbide can significantly increase the hardness and strength of the alloy, the particle size of tungsten powder used in the production of cemented carbide has gradually changed in the direction of smaller in the past 20 years. At the same time, because the morphology of tungsten powder will also have a great impact on the properties of tungsten materials, the dense and regular spherical tungsten powder not only has good fluidity, but also has a large bulk density and small sintering shrinkage, which can obtain a tungsten material with an ideal microstructure. As a result, people's interest in the study of tungsten powder morphology control is growing, and the preparation of spherical tungsten powder has become an important direction of tungsten powder research.
Tungsten Halide Hydrogen Reduction Method
Generally, the process of reducing tungsten oxide with hydrogen to produce spherical tungsten powder requires a large amount of hydrogen, and the production cost is relatively high. Therefore, much attention has been paid to the preparation of spherical tungsten powder by the hydrogen reduction method of tungsten halide at home and abroad. There have been many studies. Tungsten halide generally uses WCl6, but also WF6. Zhao Qinsheng and others use tungsten and tungsten waste as raw materials to directly chlorinate tungsten hexachloride. After hydrogen reduction, ultrafine spherical tungsten powder with a purity of> 99.9% and a particle size of 0.02 to 0.1 μm is obtained.
The main characteristics of tungsten powder prepared by the halogenation method are high purity, fine particles, uniform particle size, spherical particles, and high thermal stability. However, due to the use of tungsten halide hydrogen reduction to prepare tungsten powder, the reaction involves strong corrosive hydrogen halide gas, and the harsh working conditions will not only pollute the environment, but also corrode production equipment, so this method is rarely used at present.
Tungstate hydrogen reduction
The molecular structure of self-reducing tungstate (ART) contains amine groups such as N2H5+, NH2CH2CH2NH3+, CH3NH3+, etc. During thermal decomposition, a large amount of reducing gas is generated and a large amount of heat is released. As a result, the amine tungstate has a self-reducing type. And self-pulverization, can obtain the reduction decomposition product with fine particle size and narrow particle size distribution. Tang Xinhe and others used hydrogen to reduce the blue tungsten oxide obtained from the thermal decomposition of reductive tungstate (ART) to prepare spherical tungsten powder with agglomerated particle size <0.5μm and a single particle of about 20nm.
This method has a significant effect on refining the particle size of tungsten powder, and can obtain spherical tungsten powder, but there are problems such as higher production costs, more procedures, lower metal recovery rate, and waste liquid need to be treated, which limit the The application of law in industry.
Tungsten powder secondary oxidation and reduction method
Domestic Peng Zhihui and Li Hanguang adopt tungsten powder secondary oxidation and re-reduction technology. By strictly controlling the parameters such as the reoxidation temperature and oxidation time of the polygonal tungsten powder, the more active angular parts and rough parts (protruding parts) of the polygonal tungsten powder particles are made. ) The preferred part is oxidized and then reduced with hydrogen to make the surface of the tungsten powder particles smoother and become spherical or quasi-spherical particles. Then through the fluidization-dynamic suspension sedimentation dry classification method, it is classified into spherical or quasi-spherical tungsten powder that meets the characteristics of the barium tungsten cathode material.
This process can obtain spherical or quasi-spherical tungsten powder with low cost. Its disadvantage is that the spheroidization is insufficient and the spheroidization rate is low.
Plasma method
Because the plasma has the characteristics of high temperature, high enthalpy, high chemical reaction activity, controllable reaction atmosphere and reaction temperature, etc., it is very suitable for preparing spherical powder with high purity, small particle size and uniform particle size distribution. In recent years, there have been new reports about the research in this area. For example, MORIYSOHI of Japan's Hosei University has conducted research on the production of ultrafine spherical tungsten powder by a mixed plasma composed of high-frequency plasma and DC plasma, and produced spherical tungsten powder with an average particle size of 10nm. Domestic Gu Zhongtao and others have carried out research on plasma spheroidization of tungsten powder, and the spheroidization rate can reach almost 100% by controlling the process conditions.
The advantages of plasma spheroidized tungsten powder are that the energy is highly concentrated, the temperature gradient is large, the energy input can be accurately controlled by controlling the process parameters, and the thermal energy utilization rate is as high as 75%. After plasma spheroidization, the fluidity of the tungsten powder is improved, and the loose packing density and tap density of the tungsten powder are increased.
The Institute of Process Engineering, Chinese Academy of Sciences has a high-frequency thermal plasma device with a power of 30kW, which has been used to carry out a series of powder spheroidization studies, and obtain spherical silica, spherical alumina, spherical nickel powder, etc. Spherical powder. In addition, the device is used to spheroidize tungsten powder. In plasma, irregular tungsten powder particles are melted and spheroidized on the surface to obtain dense, smooth spherical tungsten powder.
Other methods
Spherical tungsten powder can also be obtained by partial preferential oxidation alkaline washing method, ammonium paratungstate cyclic redox method, spray drying method, H2O2 oxidation hydrothermal crystallization method, granulation sintering method, ammonium tungstate ultrasonic stirring-drying-reduction method, etc., but In actual production, there are various drawbacks such as low spheroidization rate, low actual yield, and waste liquid need to be treated, and its application in actual industrial production is limited.
Li Jun adopts the microwave single-film cavity method to prepare spheroidized tungsten powder with simple technical procedures and equipment, and the tungsten powder is evenly heated without secondary pollution. However, the heat source of the microwave single-film cavity method is insufficient, resulting in unstable performance and poor consistency of the prepared tungsten powder. Therefore, it is urgent to find a heat source with high heat and high enthalpy to meet the requirements of tungsten powder spheroidization conditions.
to sum up
With the rapid development of 3D printing technology, porous materials, high-density powder spraying and injection molding, there is an increasing demand for high-quality spherical tungsten powder. High-quality spherical tungsten powder not only has good fluidity, good sphericity, high loose packing density and tap density, but also low oxygen content. In recent years, China's powder metallurgy technology has continuously improved, but there is still a big gap compared with foreign advanced technology. The production technology and process of high-quality spherical tungsten powder abroad is relatively complete, and industrial production has been realized. However, the domestic technology and process for preparing spherical tungsten powder has many drawbacks, and it is in the stage of small batch research and development.
(1) The energy consumption problem is serious. Most of the existing spherical tungsten powder preparation technologies require tungsten powder prepared by conventional reduction methods as raw materials, and then processed to obtain spherical tungsten powder, which makes the cost of spherical tungsten powder higher and production efficiency. low.
(2) Product quality problems. The existing tungsten powder spheroidization technology generally has a low spheroidization rate and difficulty in particle size control. For example, the tungsten powder particles become apparently coarse after high-temperature plasma treatment. In addition, for the preparation of high-purity spherical tungsten powder, how to spheroidize the tungsten powder at high temperature while avoiding oxidation is particularly important.
(3) The environmental protection situation is severe. There are problems in the preparation of spherical tungsten powder, such as pollution of land and water, and damage to the ecological environment. Although the development of the technology for preparing spherical tungsten powder by plasma method is not very mature, the spherical tungsten powder prepared by this method has good quality, high purity and uniform particle size distribution. Therefore, plasma method will be one of the indispensable processes for spherical tungsten powder. ,with broadly application foreground.
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Preparation technology of ultrafine spherical tungsten powder
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