1. Basic introduction

Tungsten is an important rare metal with high melting point (3380℃), high density, high hardness, low thermal expansion coefficient, as well as excellent anti-corrosion performance and high strength at elevated temperatures, becoming the main raw material for varieties of functional and structural, has a wide range in industrial applications. Ammonium paratungstate (APT) is an important raw material for preparing tungsten. At current industrial, APT is first calcined into yellow tungsten oxide or tblue tungsten oxide in week reducing atmosphere, and then turned into tungsten powder in hydrogen reduction. Tungsten powder prepared in this way will be polymorphs with very regular shape, powder size mostly controlled among 2 ~ 5 m, and becomes the main raw material of tungsten and tungsten carbide (WC) for the military and civilian fields. However, this process is the process with long time, easy to bring impurities, production costs high relatively. This paper introduces a new method –One-step reduction method which can produce high quality tungsten powder, meanwhile, shorten the process, and reduce the cost.

 

2. Basic principles for the preparation of tungsten powder

Usually, hydrogen reduction for tungsten trioxide (WO₃) is as below:

WO₃ (s) +3H₂ (g) =W (s) +3H₂O (g)

The whole reduction process relatives with temperature, when the temperature higher than 885K(about 611℃), the reaction was divided into 4 phase, which is  WO₃→WO_2.9→WO_2.72→WO₂→W; when the temperature lower than the 885K(about 611℃), the reaction was divided into 3 phase, that is WO₃→WO_2.9→WO₂→W. Hydrogen reduction is endothermic reaction, thus temperature increased benefic to the reaction. Under the environment of high temperature, water vapor, WOx and W will become WO2 (OH) 2 with higher vapor pressure, reaction like bellows:

WO_x (s) + (4-x) H₂O (g) =WO₂ (OH) ₂ (g) + (3-x) H₂ (g) 

W (s) +4H₂O (g) =WO₂ (OH) (g) +3H₂ (g)

The reaction between tungsten oxide and water vapor has a great influence on the particle size of tungsten powder. With different operating conditions, calcination and reduction of APT for processing, the size of tungsten powder will change significantly.