Preparation technology of ultra-fine spherical tungsten powder

Tungsten metal with high melting point and high hardness is an important strategic material and is widely used in tungsten carbide cutting tools, electric filaments, tool steel 

additives, rockets, spacecraft, nuclear reactors and other fields. With the development of science and technology, new special requirements are constantly put forward for raw 

material tungsten powder. For example, high-quality cemented carbide requires ultra-fine tungsten powder; electronic materials and filter materials require spherical tungsten

powder.

Since fine-grained cemented carbide can significantly improve the hardness and strength of the alloy, the particle size of tungsten powder used in the production of cemented

carbide has been increasingly smaller in the past 20 years. At the same time, since the morphology of tungsten powder will also have a great impact on the properties of 

tungsten materials, dense and regular spherical tungsten powder not only has good fluidity, but also has high packing density and small sintering shrinkage, and can obtain 

tungsten materials with ideal microstructure. As a result, people's research interest in tungsten powder morphology control is growing, and the preparation of spherical 

tungsten powder has become an important direction in 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 

high. Therefore, great attention has been paid at home and abroad to the preparation of spherical tungsten powder by the hydrogen reduction method of tungsten halide. 

There have been many studies. Tungsten halides generally use WCl6, but also use WF6. Zhao Qinsheng and others directly chlorinated tungsten and tungsten waste as raw 

materials to form tungsten hexachloride, and produced ultra-fine spherical tungsten powder with a purity of >99.9% and a particle size of 0.02 to 0.1 μm through hydrogen 

reduction.

The main characteristics of tungsten powder produced by the halogenation method are high purity, fine particles, uniform particle size, spherical particles, and high thermal 

stability. However, due to the use of hydrogen reduction of tungsten halide to prepare tungsten powder, the reaction involves highly corrosive hydrogen halide gas and poor 

working conditions, which not only pollutes the environment but also corrodes production equipment, so this method is rarely used at present.

Tungstate hydrogen reduction method

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, amine tungstates have both self-reducing forms. And self-pulverizing, reduction 

decomposition products with fine particle size and narrow particle size distribution can be obtained. Tang Xinhe et al. used hydrogen to reduce blue tungsten oxide obtained 

from the thermal decomposition of autoreducing tungstate (ART) to produce spherical tungsten powder with an agglomeration particle size of <0.5 μm and a single particle 

size of approximately 20 nm.

This method has a significant effect in refining the particle size of tungsten powder and can obtain spherical tungsten powder. However, there are problems such as higher 

production cost, more processes, lower metal yield and the need to process waste liquid, which limits the method. application of the method in industry.

Tungsten powder secondary oxidation and reduction method

Domestic Peng Zhihui and Li Hanguang use tungsten powder secondary oxidation and re-reduction technology. By strictly controlling the re-oxidation temperature, 

oxidation time and other parameters of polygonal tungsten powder, the more active angular parts and rough parts (protruding parts) of polygonal tungsten powder particles 

are ) is first partially oxidized and then reduced with hydrogen, thereby making the surface of the tungsten powder particles smoother and becoming 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 barium tungsten cathode material.

This process can produce spherical or quasi-spherical tungsten powder with low cost. Its disadvantage is insufficient spheroidization and low spheroidization rate.

plasma method

Because plasma has the characteristics of high temperature, high enthalpy, high chemical reaction activity, controllable reaction atmosphere and reaction temperature, it is 

very suitable for preparing spherical powders with high purity, small particle size and uniform particle size distribution. In recent years, there have been many new reports on 

research in this area. For example, MORIYSOHI of Hosei University in Japan conducted research on the production of ultra-fine spherical tungsten powder using 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 spheroidizing tungsten powder. By controlling the process conditions, the spheroidization rate can reach almost 100%.

The advantages of plasma spheroidizing tungsten powder include highly concentrated energy, large temperature gradient, precise control of energy input by controlling 

process parameters, and a thermal energy utilization rate of up to 75%. After plasma spheroidization, the fluidity of tungsten powder is improved, and the bulk density and 

tap density of tungsten powder are increased.

The Institute of Process Engineering of the Chinese Academy of Sciences has a high-frequency thermal plasma device with a power of 30kW. It has used this device to 

conduct research on the spheroidization of a series of powders, and obtained spherical silica, spherical alumina, spherical nickel powder, etc. Spherical powder. In addition, 

this device is used to spheroidize tungsten powder. In the plasma, irregular tungsten powder particles are melted and spheroidized on the surface to obtain dense, 

smooth-surface spherical tungsten powder.

Other methods

Spherical tungsten powder can also be obtained by partial preferential oxidation alkali 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 disadvantages such as low spheroidization rate, low actual yield, and waste liquid needs to be disposed, and its application in actual industrial 

production is limited.

Li Jun uses the microwave single-membrane cavity method to prepare spheroidized tungsten powder. The technical process and equipment are simple. The tungsten powder 

is heated evenly and does not produce secondary pollution. However, the heat source of the microwave single-film cavity method is insufficient, resulting in unstable 

performance and poor consistency of the tungsten powder produced. Therefore, there is an urgent need to find a heat source with high heat and high enthalpy to meet the 

needs of tungsten powder spheroidization conditions.

Summarize

With the rapid development of 3D printing technology, porous materials, high-density powder spraying and injection molding, the demand for high-quality spherical 

tungsten powder is increasing. High-quality spherical tungsten powder not only has good fluidity, good sphericity, high bulk density and tap density, but also has low oxygen 

content. In recent years, China's powder metallurgy technology has continued to improve, but there is still a big gap compared with foreign advanced technology. The 

production and preparation technology and process of high-quality spherical tungsten powder abroad are relatively complete, and industrial production has been achieved. 

However, the domestic technology and process for preparing spherical tungsten powder has many disadvantages and is still in the small-batch research and development 

stage.

(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. This makes the cost of spherical tungsten powder higher and the production 

efficiency Low.

(2) Product quality issues. The existing tungsten powder spheroidization technology generally has low spheroidization rate and difficult particle size control. For example, 

the tungsten powder particles become significantly coarser after high-temperature plasma treatment. In addition, for the preparation of high-purity spherical tungsten 

powder, how to spheroidize the tungsten powder at high temperatures while avoiding oxidation is particularly important.

(3) The environmental protection situation is severe. The preparation of spherical tungsten powder has problems such as contaminating land and water quality and damaging 

the ecological environment. Although the technology for preparing spherical tungsten powder by plasma method is not yet very mature, the spherical tungsten powder 

prepared by this method has good quality, high purity and uniform particle size distribution. Therefore, the plasma method will be one of the indispensable processes for 

spherical tungsten powder. ,have a broad vision of application.