Tungsten Trioxide Hydrogen Reduction Factors

The gas-solid multiphase reaction of Hydrogen (H₂) reduction of tungsten trioxide (WO₃) is:

WO₃+3H₂ ===W+3H₂O

Reduction Temperature
Volatile tungsten oxide increases with increasing temperature, the higher the reduction temperature, the more water vapor can promote the formation of tungsten oxide and volatile hydrate WOx • nH₂O and restored good and cheap tungsten powder particles of tungsten oxide surface that is reduced, which leaving the primary particles grow thick. Since WO₃ greater volatility than the WO₂, therefore, the production of fine particles of tungsten powder requires the use of a lower reduction temperature. Generally speaking, we use two-stage reduction, the first reduction (WO₃ - WO₂) should be adopted a lower reduction temperature, while the second reduction (WO₂ - W) used a high temperature reduction. Production of coarse particles of tungsten powder is used for some high-temperature reduction (direct reduction) method.

Hydrogen Humidity and Flow
Effect of humidity on hydrogen tungsten powder particle size is very significant. Hydrogen humidity, which is conducive to the formation of tungsten oxide and water vapor volatile hydrates and restored good tungsten powder and tungsten oxide surfaces, resulting in tungsten powder particles grow. In favor of increasing the hydrogen flow rate of water vapor reduction product discharged in time, it increases the reduction reaction speeds, it is easy to obtain fine particles of tungsten powder.

Boat Loading Capacity (Material Thickness)
More than the amount loaded boat or material layer is too thick, hydrogen is difficult to penetrate deep inside the material layer. The inner layer of tungsten oxide reduction is not complete, the steam generated is also difficult to discharge, causing the boat long thick inner layer of tungsten powder, and obtained tungsten powder particle size is uneven.

Push the Boat Speed
Push the boat speed, reducing short time. Tungsten trioxide will not enter the high-temperature region in stepwise reduction of chance to low temperature materials, which resulting in growth and high oxygen content of the tungsten powder. Pushing the boat speed is too slow, tungsten trioxide residence time in the high temperature zone too long; it is easy to grow thick and decreases productivity for tungsten powder.

Temperature Gradient
Large temperature gradient furnace is equivalent to push the boat too fast, which can lead to volatile metal tungsten oxide particles grow.

WO3 Original Size and Impurity
When the reduction process was controlled by hydrogen reduction of fine particulate WO₃ generally get fine particles of tungsten powder; but since the surface of the fine particles of WO₃ can be high, you may get coarser tungsten powder if reducing improper process control. WO₃material containing metal sodium impurities, which will lead to more long thick tungsten powder; molybdenum or chromium and other impurities can inhibit the role of long thick tungsten powder.

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