2. Concentration of WO₃ will affect the precipitation rate of chromium in the crystallization process of APT

Set the crystallization conditions like follows: the concentration of NH₄OH is 2.76mol/L,195 g / L, 238 g / L, 258 g / L, 295 g / L; adding chromium acetate by the mass ratio of Cr/WO₃=0.5%; stirring speed for 300rad/min; temperature of 100℃; ending point pH crystallization equals to7.0. Making the figure that effect of Cr-doped content on chromium concentration in APT powder as below:

Remark: The horizontal coordinate is precipitation rate of chromium  (%), and the vertical coordinate is initial concentration of WO3(mol/L).

 

On the picture we can see that the initial concentration of WO3 increased from 170g/L to 295 g/L, and the precipitation rate of Cr decreased from 81% to 59%. Therefore, we can get that with the initial concentration of WO3 increased, the precipitation rate of Cr decreased. Furthermore, （NH₄）₂WO₄ will promote the dissolution of Cr(OH)₃, the balance of precipitation—dissolution of chromium in the higher concentration of WO₃, the higher Cr ion concentration, the lower rate of chromium precipitation.

 

3. Effect of stirring speed on precipitation rate of chromium is shown as following:

The speed increases from100rad/min to 500rad/min, the precipitation rate of chromium decreased from 84.3% to 37.9%. We will draw that increasing the speed of stirring leads chromium precipitation rate decreased. While stirring is conducive to the precipitation of chromium and promote formation and growth of Cr(OH)₃ crystal, but precipitation rate of chromium reduce significantly. Because in the process temperature reaching to 100℃, ammonia volatilizing rapidly with the temperature increasing, and the concentration of pH and OH^- has been greatly decreased.

 

Based on the analysis above, adding chromium in the crystallization of ammonium paratungstate can prepare apt powder containing chromium. Chromium in APT mainly exist in the form of Cr(OH)₃, and without saturation. Increasing the initial concentration of WO₃ and stirring speed will lead to rate of precipitation of chromium decreased.