Preparation Of Ultrafine Pyrochlore-type Tungsten Trioxide Powder From Sodium Tungstate Solution By Hydrothermal Method

Pyrochlore-type tungsten trioxide (WO3·0.5H₂O) has a layered structure with pore ring at the top corner formed by W-0 skeleton frame and the distorted octahedral WO6 is its structural unit. The researches on pyrochlore-type WO3 were concerned by scholars. It has wide application because of its special reticular formation and metastability. It could be used as the former body of WO3 in the field of tungsten smelting, this can shorten the technological process and achieve the utilization of the alkali. The hydrothermal method is hopeful to ensure high reaction rate, but it was limited because of the complex construction of the hydrothermal system with sodium tungstate solution. Therefore, it's significant to intensify the precipitation process aiming to prepare high quality powders of H₂W₂O₇ and increase precipitation yield.WO3 was choosed as additive to preparation of WO3·0.5H₂O because it can dissolve in sodium tungstate easily and change the pH of the hydrothermal. The influences on reaction rate and structural morphology of products prepared from sodium tungstate which was pretreated by CO2 or oxalic acid were investigated in detail. Meanwhile the influencing factors of reaction process, such as temperature, pH, concentration, and the preliminary explanation about the mechanism of hydrothermal synthesis of H₂W₂O₇ were presented.The results indicate that ultrafine pyrochlore-type WO3 can be synthesized from sodium tungstate solution by adding pure WO3 to the reaction system, and the precipitation rate of tungsten species in the tungstate solution could reach 80% because the additive can keep the pH of the hydrothermal system stable, but if the solution was pretreated by CO2 or oxalic acid, the reaction time can be shorten from 24h to 8h, and the reaction rate can be improved. The suitable pH range of solution for preparing H₂W₂O₇ could is 3.5-8.9. XRD analyses indicate that the structure of products is cubic crystal. EDS analyses reveal that a small amount of impurity of sodium presents in the precipitates and it could be completely removed by washing in the diluted hydrochloric acid. SEM micrographs show that the average diameter of the precipitates is about 1.0μm. Based on the FT-IR analyses, the growth unit of H₂W₂O₇ was W₂O₇^2-, and the multicomponent ionic balance in sodium tungstate solution under neutral or weak alkaline environment has been discussed, both of which help to determine the mechanism of hydrothermal synthesis of H₂W₂O₇.