*3.6. Elemental Mapping Analysis*

or may be incorporated into the BFSZ.

*3.6. Elemental Mapping Analysis*  Elemental mapping analysis was conducted to compare and study the regional (or phase) distribution characteristics of elements in the BFSZ sample obtained under various crystallization times. As seen in Figure 6, the BFSZ sample obtained under various crystallization time contains all BFS-derived metals. As shown in Figure 6A, Si, Al, O and Narich phase was formed and the Si, Al and Na were distributed in the same distribution area. From the morphological point of view, this is considered to be zeolite, which once again proves the successful preparation of BFSZ. At the same time, some components such as iron, calcium and magnesium also appear in the mapping image. However, no related crystal structure on iron, calcium and magnesium was discovered in the XRD pattern (see Figure 1). It is likely that those secondary metal ions either cannot form a crystalline phase Elemental mapping analysis was conducted to compare and study the regional (or phase) distribution characteristics of elements in the BFSZ sample obtained under various crystallization times. As seen in Figure 6, the BFSZ sample obtained under various crystallization time contains all BFS-derived metals. As shown in Figure 6A, Si, Al, O and Na-rich phase was formed and the Si, Al and Na were distributed in the same distribution area. From the morphological point of view, this is considered to be zeolite, which once again proves the successful preparation of BFSZ. At the same time, some components such as iron, calcium and magnesium also appear in the mapping image. However, no related crystal structure on iron, calcium and magnesium was discovered in the XRD pattern (see Figure 1). It is likely that those secondary metal ions either cannot form a crystalline phase or may be incorporated into the BFSZ.

*Minerals* **2021**, *11*, x 9 of 11

**Figure 6.** Elemental mapping of BFSZ synthesized at crystallization times of (**A**) 4 h (**B**) 6 h and (**C**) 8 h. **Figure 6.** Elemental mapping of BFSZ synthesized at crystallization times of (**A**) 4 h (**B**) 6 h and (**C**) 8 h.

## **4. Conclusions**

The effects of crystallization time on the alkaline fusion-hydrothermal synthesis of BFSZ were investigated. The BFSZ obtained under different crystallization times was measured by XRD, FT-IR, BET/BJH, XRF, FE-SEM and elemental mapping analysis. It was found that BFSZ crystallization was almost complete for 6 h. A further increase of crystallization time did not have a significant effect on the phase formation. Under 6 h aging, the main phases in BFSZ were NaA zeolite with the average SSA of 49.64 m<sup>2</sup> g −1 . Additionally, the cubic crystal with a developed surface in BFSZ crystals with particle size of about 2 µm could be clearly observed. Elemental mapping analysis showed that a Si, Al, O and Na-rich phase was formed and the Si, Al and Na were distributed in the same distribution area. Hence, it can be concluded that the optimal crystallization time for the synthesis of BFSZ using alkaline fusion-hydrothermal treatment is around 6 h. Research on the properties of the obtained BFSZ needs to be conducted through further studies.

**Author Contributions:** Methodology, C.L.; formal analysis, X.L. and L.L.; investigation, Q.Z.; writingoriginal draft preparation, C.L. and S.W.; writing—review and editing, C.L. and S.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** The authors acknowledge the support of the Postdoctoral Research Foundation of China (No. 2017M611799) and Basic Research Program of Jiangsu Province (No. BK20190690) for offering the research fund.

**Conflicts of Interest:** The authors declare no conflict of interest.
