Crystal growth is a process of heat and mass transfer, chemical and heat and mass transfer resistance mainly exists in the viscous flow near the crystal surface layer, thus increasing the speed of relative movement between solid and liquid can improve growth rate of the crystal. Increasing the relative speed between solid and liquid in an appropriate range can improve growth rate of crystal nucleus; however it will break the crystal when the speed reach a limit point. Stirring has a significant impact on evaporation and crystallization, because evaporation process is accompanied with ammonia loss, and stir ammonia could induce the nucleation.

 

Ammonium hydroxide solution is unstable, and the ammonium is easy to lose. When the heating and stirring tungstate solution, ammonium paratungstate come into being continuously with the volatilization of ammonia and water, at the same time, it makes the concentration of tungstate from un-saturation to super-saturation. Continue heating evaporation, crystal nucleus taking shape and growing into crystals in a certain size and shape with the ammonia atmosphere.

 

Stirring causes induction period shorten, fast stirring speed makes large whirlpool diameter and deep height, flows fast, increases the contact area of the solution and the pot wall and accelerate the heat exchange rate to promoting the reaction process. The super-saturation increases with the NH3 escaping out. At the same time, stirring causes diffusion velocity speeding up, and the width of metastable region narrower, △c smaller, thus inducing constant K decline. With stirring speed increasing, the control of crystal growth rate turns from diffusion transfer to surface reaction, the homogeneous and secondary nucleation of ammonium paratungstate affected by stirring enlarge continuously. Between them, effect on the secondary nucleation is particularly significant. Stirring can prevent aggregate formation, also it exacerbates crystal broken-up and secondary nucleation, affects ammonium paratungstate homogeneity from the opposite side.