**4. Conclusions**

The following conclusions were drawn on the basis of the results obtained in this work.

Aeration leaching is feasible at room temperature. For a reaction time of 4 h and adequate stirring speed of 800 rpm, the effectiveness of metallic iron removal differed for different solution systems. When using NH4Cl, the MFe content could be reduced to 1.98% in SR, but the TiO2 content only reached 69.16%. A higher NH4Cl concentation did not improve aertion leaching. Using 2% NH4Cl with hydrochloric acid, the presence of the acid helped to improve the leaching, but the SR was only upgraded to 75%, which indicated that the reaction needed more time to improve the purity. In the hydrochloric acid system, the MFe content was as low as 0.14% and TFe content was about 4%, indicating that the SR could be upgraded to about 90%.

**Author Contributions:** This is a joint work of the five authors; each author was in charge of their expertise and capability: Q.Z. for writing, formal analysis and original draft preparation, M.L. for data curation, L.Z. for experimental assistance, M.Z. for validation, T.Z. for methodology. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was supported by the National Natural Science Foundation of China (NSFC) (Grant No. 51204040); Fundamental Research Funds for the Central Universities (Grant No. N180725023).

**Acknowledgments:** The authors gratefully acknowledge the Laboratory Center of Northeastern University for chemical analysis. Guangxi Yueqiao New Material Technology Co., Ltd. for providing the reduced ilmenite samples.

**Conflicts of Interest:** On behalf of all authors, the corresponding author states that there is no conflict of interest.

## **References**


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