Acknowledgements
This study was financially supported by the Postgraduate Innovation Fund Project of the Southwest University of Science and Technology (14ycx042), the Opening Project of the Key Laboratory of Vanadium-Titanium Magnetite Comprehensive Utilization, Ministry of Land and Resources (No. KLVMCU-2013-01), the Opening Project of the Key Laboratory of Solid Waste Treatment and Resource Recycle, Ministry of Education (13zxsk06), and the Applied Basic Research Programs of Science and Technology Department of Sichuan Province (2014JY0124). The authors appreciate the financial support and thank the editor and reviewers for their very useful suggestions and comments.