## 4 Discussion  
In order to study the effects of agitator types on recrystallization, PIV physical simulation technique was used to simulate the flow field distribution in the reactor. The velocity vector cloud diagrams of the flow field distribution with different agitator types at the same agitator speed and agitator position are shown in Figure 7. It can be seen that the flow field in the reactor was more evenly distributed when using the disc turbine impeller, and a relatively uniform velocity liquid flow area was formed in the whole reactor while the low-velocity liquid flow area was smaller, as shown in Figure 7a. When the Intermig impeller was used, the high-speed liquid flow area was mainly distributed at both sides of the agitator shaft and at the bottom of the reactor. However, the other areas of the reactor belonged to the low-speed liquid flow zone, and the flow field was not evenly distributed, as shown in Figure 7b. When the propelling propeller was used, two high-speed circulating liquid flow zones were formed on both sides of the agitator, and the high-speed liquid flow zone was mainly distributed on both sides of the bottom of the reactor. However, the other areas of the reactor belonged to the low-speed liquid flow zone, and the flow field was not evenly distributed, as shown in Figure 7c. In conclusion, the distribution of flow field in the reactor was more uniform when the disc turbine impeller was used, and the relatively low-speed flow area of the reactor was smaller. Combined with the SEM test results (Figure 3), this suggests that under the uniform flow field, the large particles collided and broke into the small particles of uniform size, and then these particles became nuclei again and grew into \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Re Powders** & **D50 (\(\mu\)m)** & **Specific Surface** & **Reduction** \\  & & **(m\({}^{2}\)/kg)** & **Temperature (\({}^{\circ}\)C)** \\ \hline Preparation of NH\({}_{4}\)ReO\({}_{4}\) without recrystallization & 42.16 & 105.80 & 1000 \\ Preparation of recrystallization NH\({}_{4}\)ReO\({}_{4}\) & 25.21 & 182.10 & 700 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Specific surfaces and D50s of the rhenium powders. Figure 6: Particle size distribution of the rhenium samples. **a–**Sample was prepared with the NH\({}_{4}\)ReO\({}_{4}\) crystals without recrystallization. **b–**Sample was prepared by the recrystallization NH\({}_{4}\)ReO\({}_{4}\) crystals at 200 rpm. The red line represents particle size (\(\mu\)m), and the blue line represents the accumulation. complete crystals under uniform flow. Therefore, the stirring effect of the disc turbine impeller is more suitable for the homogeneous recrystallization process. As discussed above, the recrystallization of ammonium perrhenate was accompanied by agglomeration, and appropriately increasing the stirring intensity reduced the agglomeration effect. The SEM and XRD analyses showed that an appropriate reduction of the agglomeration effect was conducive to increasing the growth rates of the ammonium perrhenate crystals. When the stirring intensity was 200 rpm, the growth rates of the (101) and (112) crystal planes corresponding to the characteristic peaks of the crystals were relatively fast, as shown in Table 3. Therefore, the crystal development was more complete at the same crystallization time. However, when the stirring intensity was 250 rpm, the agglomeration of the system was excessively reduced; the growth rates of the (101) and (112) crystal planes corresponding to the characteristic peaks of the crystals were subdued, resulting in a slow and incomplete crystal development. This suggested that the induction period for nucleation was reduced and the formation of crystal nuclei and growth of crystals reached an ideal equilibrium state when an appropriate reduction in the agglomeration was obtained. The high stirring intensity also made the large sedimentary particles collide, transition into the secondary crystallization stage, and maintain a uniform grain size in the crystal.