Tungstic acid can be roughly divided into yellow tungstic acid and white tungstic acid.
More and more practice has shown that tungstic acid not only has an important use in tungsten chemistry, but also has great value in tungsten metallurgy.
Powdery white tungstic acid can compose tungsten niobium and tungsten tantalum Heteropoly salt. Tungsten phosphoric acid and tungstic acid can also be composed. The heteropoly acid prepared from powdered white tungstic acid has better catalytic performance than that prepared with tungstic acid.
The preparation of white tungstic acid is to drop Na2WO4 solution into dilute acid. In the middle of the process, there is no acid anion, so the precipitation rate is high. In the Na2WO4 solution tungsten exists as monomeric WO42-form. The presence of dilute acid immediately results in protonation and formation of tungstic acid precipitation. Generally, the prepared white tungstic acid is a mixture of mono - tungstic acid and poly - tungstic acid. When the mono tungstic acid was converted into poly acid, the tungstic acid was converted into colloid, and its activity was similar to that of yellow tungstic acid.
Pour out method for impurity removal of white tungstic acid. The 0.5-0.8m / L Na2WO4 solution was dripped into H2SO4 or H N solution until the pH value <=1, the powdery white tungstic acid can be obtained with a precipitation rate of 100%. When the P, A, s or Si in Na2WO4 exist in less than 30 m g / L, the resulting white tungstic acid does not contain these three impurities, and these impurities are dissolved in water in the form of twelve tungsten heteropoly acid.
In the preparation of ammonium tungstate, ammonium tungstate, tungsten phosphorus heteropoly acid, white tungstic acid is much better than that of yellow tungstic acid, and the yield is almost 100%. Considering the low acidity and high yield of white tungstic acid, the advantage of white tungstic acid in tungsten metallurgy is more obvious.