Temperature of solution has an influence on ammonium paratungstate (APT) crystallization. Above room temperature, with the temperature increasing, APT crystal experience 5 steps: acicular→thin strips→schistose→cuboids→solid. The size of APT grain also gradually changes from few microns to 40~60μm. Temperature raises lead to the morphology and structure of APT crystal changing and crystal coarsened.

 

Both the process of ammonium paratungstate crystal growth and nucleation are controlled by chemical reaction steps. The APT crystal growth is an exothermic process, APT nucleation rate and crystal growth increased with solution temperature rising. Because APT crystallization process is rely on chemical reaction to produce saturated evaporation and exit a slow chemical reaction from paratungstate A→B. So, the new crystal growing up and the existed crystal increasing occurred simultaneously during the nucleation period.

 

Growth of nucleation and crystal are two processes with a trade-off relationship. When rising the temperature, on the one hand, nucleation rate constant and nucleation rate and the number of new grains increases; on the other hand temperature and crystal increasing continually make total area of solid phase enlarged, super-saturation△C decreased rapidly, which in turn inhibiting nucleation rate and crystal growth rate. Experiment shows that temperature grow up from 91.5℃ to 94.5℃, influence coefficient of the nucleation and the crystal growth rate are 0.827 and 0.933 with super-saturation falling down. At the same time, rate constant effect on influence coefficient of nucleation and the crystal growth rate are 1.616 and 1.291.

 

How the crystallization temperature affect the crystal grain size of APT ultimately depends on nucleation rate and crystal growth rate of the relative size ultimately.The experimental results show that the effect of crystallization temperature on the nucleation greater than that of crystal growth.