Below is the producing method of yellow tungsten oxide:
1.Leaching out tungsten concentrate by sodium hydroxide, the chemical equation is:
Sodium hydroxide solution leaches out tungsten concentrate can accelerate the reacting by stirring and adding oxidizing agent such as saltpeter. Impurities in tungsten concentrate like silica, phosphorus, arsenic, fluorine, molybdenum and tin can react with sodium hydroxide and produce arsenate and silicate. The coarse sodium tungstate solution is obtained.
2. Producing tungsten compounds from coarse sodium tungstate solution has several methods like chemical purifying method, extraction of organic solvent method, ion exchange method. Ion exchange method has been widely applied in tungsten manufacture. The most characteristic and effective one is strong-base anion exchange resin to deal coarse sodium tungstate solution. 2/3 of our tungsten manufactures use the method to produce APT.
Ion exchange method dealing coarse sodium tungstate solution to prepare pure ammonium tungstate has the following steps:
After diluting, there are anion like WO42-、MoO42-、HAsO42- exist in coarse sodium tungstate solution. These anions are constantly absorbed to resin. With the absorbing process, sodium tungstate solution flows into exchange column. Some anions which have been absorbed will be exchanged to the down step of resin by high concentrate tungstate ion. Finally it would be exchanged from resin and separated with effluent.
In case of loss of tungstate ion from solution of some high impurities contained sodium tungstate solution, when there is tungstate ion goes through waste liquid, absorption stops. Impurities on resin haven’t been completely exchanged, so by elution the impurities will be removed further.
Using NaCl and NaOH as eluting agents, let it pass the resin layer. The Cl- and OH- can exchange impurities on the resin.
When resin absorbed with tungstate ion contacted with high concentrate chlorine ion solution, Cl- exchanges WO42- from resin and let it goes into solution, at the same time, resin reconverts into Cl- again and can be used again in the next cycle.
3. Adding ammonium hydroxide into purified sodium tungstate solution, ammonium tungstate solution is obtained. Producing APT by evaporation crystalline method. In this process, ammonium tungstate solution is heated, ammonia escapes and pH value lowers down. Ammonium tungstate will converts to ammonium paratungstate. With the escaping of water, APT is separated out.
4. By dry and calcine APT can produce tungsten trioxide. The temperature is controlled under 500℃.