Cause of Coal Grindability. Correlation of coal rank, maceral composition, hardness, and brittleness.

The authors used 10 coal samples, which were vitrinite rich and of different coal rank (carbon content were from 70.0 % to 87.4 %), and the relation between maceral compositions in vitrinite group and grindability indicated by the Hardgrove grinding index (HGI) was elucidated.Brittleness of the coal structure rather than its hardness dominates coal grindability. The brittleness of collinite and the collinite content have a specific relation with HGI and coal rank. Higher rank coal includes a higher collinite content, and the collinite becomes increasingly brittle with increasing coal rank and collinite content; in the results, the HGI of the coal samples becomes higher.After grinding coal samples by a Hardgrove grinding tester, the collinite content in the fraction of -75μm increases with higher HGI. These results show selective grinding of collinite and indicate that only collinite in vitrinite group has a strong influence on coal grindability.