**4. Conclusions**

New phase equilibria information for the Na2O-SiO2-WO<sup>3</sup> system were determined experimentally. A phase diagram targeting a high silica and low tungsten region (40–100 wt% SiO2, 40–60 wt% Na2O, and 0–20 wt% WO3) was constructed with isotherms of 1073, 1173, and 1273 K. The solubilities of WO<sup>3</sup> in SiO2, Na2Si2O5, and Na2SiO3, and the solubility of SiO<sup>2</sup> in Na2WO<sup>4</sup> were accurately measured using EPMA. The newly established phase diagram was compared with the existing data and phase diagram. The present study provides valuable information for the future development of the thermodynamic database and development of tungsten extraction technology.

**Author Contributions:** Conceptualization, B.Z.; methodology, K.S. and X.M.; validation, K.S.; formal analysis, B.Z. and K.S.; investigation, K.S.; resources, X.M.; writing—original draft preparation, K.S.; writing—review and editing, B.Z., K.S. and X.M.; supervision, B.Z. and X.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors would like to thank the financial support from Jiangxi University of Science and Technology through the joint laboratory on high-temperature processing. The authors acknowledge the facilities, scientific, and technical assistance of the Australian Microscopy and Microanalysis Research Facility at the Centre for Microscopy and Microanalysis, The University of Queensland.

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

#### **References**

