1. China University of Mining & Technology, Key Laboratory of Coal Processing and Efficient Utilization (Ministry of Education), Xuzhou 221116, Jiangsu , China
2. China University of Mining & Technology, National Engineering Research Center of Coal Preparation and Purifi cation, Xuzhou 221116, Jiangsu , China
3. Gunma University, Division of Environmental Engineering Science, 1-5-1 Tenjin-cho, Kiryu 376-8515, Japan