Method for Preparing Homogeneous Sodium Ammonium Paratungstate

According to criterion of grain grading by Sandvik, tungsten carbide made by coarse grained [grain size 3.5~4.9μm] has advantages that good toughness, high hardness, heat conduction and red-hardness [refers to tool steel can still remain high hardness (more than 60 HRC) in the external heated ], which is widely used in the field of mining tools, die stamping, drilling for oil, hard-facing materials.

The process of conventional mechanical mixing method for preparing alkali metal doped is as follows: adding a certain amount of sodium salt into APT or yellow tungsten oxide (WO₃), mixing by mixing equipment. But, because sodium salt added in mechanical mixing method is not too much, it is difficult to guarantee the uniformity of ammonium paratungstate or yellow tungsten powder, thus could easily lead to many problems like uneven granularity, incomplete crystallization, generating more fine particles and so on, and seriously affecting the properties of tungsten carbide.

In the view of the existed defects on doping sodium salt process, a simple process for preparing homogeneous sodium APT and yellow tungsten is proposed, and the preparation process is as follows:
1. Using alkali decomposition of tungsten concentrate which went through ion exchange and molybdenum removal as raw material, the concentration of WO₃ required 200~300g/L, mass concentration of Na⁺ for 10~40 ppm;
2. Tungstate solution and compounds contain sodium mixed according the mass percent of Na/W03 for 0.05~0. 25%;
3. Evaporation and crystallization, in the temperature 80 ~ 100℃, stirring speed 50~200 r/min, pH of final crystallization point among 6.4 ~ 7;
4. Stop heating, and cooling it down to 20~50℃;
5. Filter liquid by the way of vacuum filtration, drained, drying and getting APT powder with uniform sodium, or calcined APT in the calcining furnace in 680~740℃ to generate yellow tungsten oxide powder containing uniform sodium.

The advantages of this method lie in:
1. No need to adjust the concentration of ammonia, simply production process, easy to control;
2. Low impurities content because of the high requirements of materials selection, and introducing no addition impurities, thus do not need washing products.