2018年8月11日星期六

Ultrafine Tungsten Powder Preparation -- Hydrogen Reduction Method (1)

Ultrafine tungsten powder is superior to common coarse grain material in mechanical, therefore, the mechanical properties, physical properties and chemical properties are better than those of ordinary coarse-grained materials. 

However, the preparation of ultrafine tungsten powder is more stringent than ordinary tungsten powder, not only have better quality raw materials, but also more suitable methods and equipment are required.

Since 1950s, a great deal of research work has been done on tungsten smelting industry both at home and abroad, and many years of systematic research have been carried out on the preparation of ultrafine tungsten powder, and some effective methods have been developed. Hydrogen reduction is the main method of preparing ultrafine tungsten powder. It is simple, advanced, reasonable, stable and reliable, low cost and so on.

With the development of tungsten powder manufacturing, great progress has been made in the selection of tungsten oxide raw materials, from the early tungstic acid and yellow tungsten into blue tungsten and purple tungsten as the main material. The structure, composition and physicochemical properties of the raw material have great influence on the particle size and uniformity of tungsten powder.

hydrogen reduction method image

The hydrogen reduction process of tungsten oxide is a rather complicated physical and chemical process, including chemical transformation, phase change process, solid phase diffusion and chemical vapor migration.

Tungsten trioxide hydrogen reduction reaction can be described as the following equation:

WO3+3H2→W+3H2O

The equation may seem simple, but the actual operation is a complex process, the process parameters of each link, such as reduction temperature, material thickness, hydrogen flow and ventilation direction are connected, the interaction of different parameters will lead to different results, a little deviation could affect the particle size of tungsten powder.

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Ultrafine Tungsten Powder Preparation -- Hydrogen Reduction Method (2)

Hydrogen reduction is the main method for preparing ultrafine tungsten powder.

It is a rather complicated physical and chemical process, including chemical transformation, phase change process, solid phase diffusion and chemical vapor transfer.

In W-O-H there are 6 solidification phase, which are: tungsten trioxide (WO3), beta tungsten oxide (WO2.9, W20O58, blue tungsten) gamma tungsten oxide (WO2.72, W18O49, Purple Tungsten) two tungsten oxide (WO2), -W alpha and beta -W. Therefore, the process of hydrogen reduction reaction of tungsten oxide is complex, and different reduction processes will occur with the change of temperature and humidity: In a high temperature and a humid reducing atmosphere, the reduction proceeds in distinct stages ( WO3 → WO2.9 → WO2.72 → WO2→ W ); In low temperature and dry hydrogen, it is possible to reduce tungsten oxide to tungsten powder directly from high valence tungsten oxide ( WO2.9 → W,WO2.72 → W ), the process is roughly: tungsten oxide → hydrogen reduction → screening → batch → tungsten powder.

hydrogen reduction method image

The reaction pathway of tungsten oxide reduction process is different, and the rate of reduction reaction is different, which leads to different particle size of tungsten powder after reduction. Some scholars have shown that tungsten powder obtained by WO2.9 → WO2 → W ( or WO2.72 → WO2 → W ) is different from tungsten powder obtained by WO2.9 → W ( or WO2.72 → W ).

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Ultrafine Tungsten Powder Preparation -- Hydrogen Reduction Method (3)

The hydrogen reduction process of tungsten oxide, is a very complex physical and chemical processes, including chemical conversion, phase transformation, solid diffusion and so on.

hydrogen reduction method image

The process generally is: tungsten oxide, hydrogen reduction, sifting, blending, tungsten powder. Different tungsten oxide reduction process of reaction pathways, the reaction speed is different, in general, the reduction reaction pathway and to avoid the simple single reaction generated in the process of WO2 is beneficial to preparing fine tungsten powder uniformly. That is to say, the tungsten powder obtained by route WO2.9 → W or WO2.72 → W would be more satisfactory.

Hydrogen reduction of tungsten oxide is usually carried out in a multiple tube furnace or a rotary furnace. Multiple tube reduction furnace has 3 to 5 containers used for loading, use artificial way in accordance with the order of loading and unloading material, excessive hydrogen or against the tungsten material movement direction into the tubular reduction furnace and material reaction; The rotary reduction furnace is heated uniformly by constant rotation so that the material is in a dynamic powder flow.

In the reduction furnace, tungsten powder particle size reduction temperature can be roughly divided into two sections: in the low temperature (<800 °C), the rate of change of particle size with temperature (i.e., slope of tangent) is more gentle; at high temperature (>800 °C), particle size begins to fluctuate with temperature. When the temperature is high, the reduction reaction will go through the intermediate stage of WO2. WO2 can be easily sintered and recrystallized at high temperature, which results in the thickening of tungsten powder. So in the actual production, taking into account the low temperature reduction reaction speed is slow, affects the production efficiency, often use the method of ladder type heating. That is, the temperature of the latter temperature zone of the reduction furnace is higher than the previous one. If the temperature gradient is too large, the particle size distribution will become wider. This is also why the use of multiple tube equipment is more than the rotary furnace equipment.

different reduction temperatures image

In addition, tungsten oxides form WO2(OH)2 with water vapor, when the compound has a large vapor pressure at 500~700 °C, it will be reduced to H2 on the reduction product, thereby thickening the particles. In addition, the water vapor will oxidize the tungsten powder, and the oxidized products will be reduced on the surface of the tungsten oxide, which will thicken the particles. Therefore, to obtain tungsten powder with finer particle size, more H2 flow is needed than usual to remove water vapor to prevent the formation of WO2.

In the process of producing ultrafine tungsten powder, the key is to control the suitable temperature and reduce the formation of volatile hydrated tungsten oxide (WO2(OH)2), and restrain the process of chemical vapor migration.

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Tungsten Oxide Hydrogen Reduction Equipment -- Four Tube Reduction Furnace

Tungsten oxide hydrogen reduction process is generally carried out in a multiple tube furnace, in the tubular reduction furnace, the most used in China are the thirteen tube reduction furnace and the four tube reduction furnace.

four tube reduction furnace image

Four tube reduction furnace is commonly used as a tungsten oxide hydrogen reduction unit, the furnace shell is welded by ordinary steel plates, and the four tubes are placed at the same level on the same hearth. Compared with the old four tube muffle furnace up and down the structure, the temperature uniformity is much better. The automation level of the four tube reduction furnace is higher than the thirteen tube reduction furnace, and it is easier to realize mechanization and electrification, in addition, the capacity of the four tube reduction furnace is large, but the unit energy consumption is lower than the thirteen tube reduction furnace.

The furnace temperature of the four tube reduction furnace is between 600 °C and 1000 °C centigrade, and natural gas is used to heat the natural gas supply, generally speaking, the loading capacity of a four tube furnace is slightly larger than a thirteen tube reduction furnace, preparation of ultrafine tungsten powder temperature is set to the third gear. But theoretically, four tubes reduction furnace to make ordinary tungsten powder is not bad, the preparation of super tungsten powder is not as handy as the thirteen tube reduction furnace.

In industry, the production of tungsten powder is usually reduced by hydrogen reduction furnace. However, there are also documented that an enterprise adopts the two stage method to obtain fine particles of tungsten powder. When the two stage reduction is adopted, the first stage is carried out at a lower temperature (500 °C - 700 °C), the main function is to produce two suitable tungsten oxide for grain size, using four tube reduction furnace. The second stage was carried out at 600 °C and 1100 °C, using thirteen tube reduction furnace. But the author believes that the best scheme for preparing fine particles of tungsten powder should be to avoid the formation of WO2. In practical work, it is feasible to make a single reduction of hydrogen with blue tungsten or purple tungsten.

Whether it is a four tube reduction furnace or a thirteen tube reduction furnace, the capacity to produce ultrafine tungsten powders from them is not very high, in order to make the tungsten oxide powder uniformly heated, it is inevitable to reduce the amount of tungsten powder loaded into the container, only a thin layer, so the production capacity is obviously not to be seen. But fortunately the four tube reduction furnace is high, some scholars scholars suggest that one or two upper boats should be placed on the bottom of the reduction boat, and the hydrogen can flow in the boat, which can greatly improve the production capacity of ultrafine tungsten powder.

If you have any inquiry of tungsten, please feel free to contact us:
Tel.: +86 592 5129696/+86 592 5129595
Fax: +86 592 5129797

Scan QR code of Chinatungsten Wechat platform to follow up the daily latest price and market of tungsten, molybdenum and rare earth.