Yellow Tungstic Acid Preparation Method
Yellow tungstic acid is a common tungstic acid, the pale orange yellow powder, the product generated in the treatment of tungsten ore by hydrochloric acid reacting with sodium tungstate. However, since it requires the concentrated hydrochloric acid and the heat which the corrosion is too strong, so this method is rarely used, but the researches on generation mechanism have played a promoting role on the preparation of other tungstic acid, like active powdery white tungstic acid.
Nowadays, the yellow tungstic acid is mostly prepared by homogeneous precipitation method. According to the standard analysis method of tungsten ore, yellow tungstic acid is precipitated on the solution surface after lots of concentrated hydrochloric acid is used and their concentration are reduced by evaporation, then it is sinking after stir. The homogeneous precipitation method is named because the product spontaneous precipitation in a clear solution, and will not stick to the wall after drying in the air. Calcium tungstate can dissolve in concentrated hydrochloric acid by heating or be placed under room temperature, the hydrogen chloride will disappear gradually, and the acidity reduce constantly, it will precipitate tungstic acid, and these are different from the traditional method of adding acid into sodium tungstate.
In the concentrated hydrochloric acid, tungsten will exist in the form of tungsten acid ion (WO2Cl42-) firstly, then after placing for some time or in the condition of heating, HCl will gradually escape until the concentration of hydrochloric acid is reduced to some degree, them coming out of generation of yellow tungstic acid (WO3•H2O).