Ultra-clean coal can be obtained for coal containing less clay minerals and in the contrary, clean coal with less than 2% ash can hardly be obtained for coal containing more clay minerals finely dissimulated in coal matrix; 煤中的无机矿物主要是黏土,黏土含量小的煤可以分选出超净煤,反之,黏土和黏土与有机物连生体含量高的煤,很难分选出灰分低于2%的超净煤;