Research On Preparing Ultrafine Tungsten Powders With Tungsten Trioxide

Self-made tungsten trioxide (WO₃) by calcining ammonium paratungstate (APT) was selected as the raw material to produce ultrafme and homogenous tungsten powder by oxidation-reduction method. The mechanics of the oxidation-reduction method and the reduction kinetics characteristic of tungsten trioxide were studied.The oxidation-reduction method in this article is a quite new processs for preparing ultrfme tungsten powder. The process is that as follows: tungsten trioxide is reduced to tungsten powder at first, then the tungsten powder will be oxidated to tungsten trioxide, and then the tungsten trioxide will be reduced to tungsten powder again, ultrafine and homogenous tungsten powder will be produced by repeating this cyclic process several times.The studies on kinetics of hydrogen reduction of tungsten trioxide were carried out by thermogravimetry. According to the phenomenon of the sample's weight will change if phase transformation occur during the reduction, the weight changing of tungsten trioxide during the reduction was investigated by balance. To draw the reduction kinetics curves of tungsten trioxide, two stages of tungsten trioxide being reducing to tungsten were studied respectively. From the data of weight loss, reaction speed and the factors affecting it could be determined.The main conclusions are as following:(1) Ultrafine tungsten powder can be produced from tungsten trioxide choosen in this research through the oxidation-reduction method (4 oxidations and 5 reductions) in right conditions; the average grain size of ultrafine tungsten powder is about 0.38μm(2) Through several circles of oxidation-reduction, tungsten powders in random grain sizes can be used as the starting material to produce ultrafine tungsten powders which can be used as raw material for high-performence cemented carbide.(3) The particle size of tungsten trioxide prepared from the oxidation of tungsten powder in right conditions has nothing to do with the starting material tungsten powder. By oxidation of tungsten powder at low temperature and slow speed, porous tungsten trioxide can be produced, and reduce this kind of tungsten trioxide will prepare ultrafine tungsten powder.(4) Tungsten trioxide can be completely reduced to TBO (WO_2.90) at 490～500℃after about 35 minutes. Furthermore, the complete conversion time will be shorten with the reduction temperature rising and the hydrogen flowrate increasing.(5) The lowest temperature of the TBO (WO_2.90) being reduced to tungsten is about 750℃. If the temperature below 750℃, the reaction time will delay or the reduction will be incomplete. In the process of TBO reduction to tungsten, tungsten dioxide (WO₂) formation may occurr simultaneously, so the tungsten dioxide volatilization and the quick-reaction of reducing WO₂ to W may be the possible reason why the reaction speed changing. Furthermore, at a higher reaction temperature (above 775℃), WO_2.72 phase may be formed in this reduction process, then the quick-reaction of reducing WO_2.72 to W may directly be occurred without the path of reducing WO_2.72 to WO₂, and those large numbers of new surfaces of produced W-nuclei have catalytic activity for the reducing reaction, so the reducing reaction have a quicker reaction speed from the beginning to the end.(6) The influence of hydrogen flowrate on reaction speed is more enormous than reaction temperature.