Preparation of Sodium Tungsten Bronze (cubic) by the Reduction of Tungstates with Hydrogen

Abstract

Preparation of sodium tungsten bronze (Na., W03: 0.4. ≤ x≤ 1, cubic) by the reduction of Na2W207 or Na., W, Ois with hydrogen was studied by gravimetry, X-ray diffraction analysi s and SEM observation. A hydrogen flowing system reactor was used and effects of temperature and hydrogen concentration on the solid products were examined. In the case of the reductuon of Na2W2O7; For hydrogen concentration with more than 50 vol%, excessive reduction took place even below 550 °C and unfavorable products, metallic tungsten and 9-tungsten (W30), were observed. On the other hand, for hydrogen concentration with les s than 20vol%, the extent of the reduction was easily controlled above 625°C and only two phases (NasWO, (cube-like crystals, the size: 5-20 pm) and Na.2W04) were observed as solid products, where x value (0.75< x< 0.90) was found to be mainly dependent on hydrogen concentration. Crystal growth with cube-like shape above 625 °C was considered to be attributable to the liquid phase related process since the eutectic point of the Na2W04-Na2W207system is 622 °C. In the case of the reduction of Na.2W4013; Productions of metallic tungsten, Na2W207 and Na2W6019 were observed. However, for temperature and hydrogen c oncentration with (675 ° C, 5 vol%) or (650 ° C, 10 vol%), final product was mostly single phase of NazW03 (x 0.50).