Hardgrove Grindability Index

Hardgrove Grindability Index (short HGI) is a measure for the grindability of coal. Grindability is a index, therefore it has no unit. The smaller the HGI, the harder and less grindable is the coal.

Grindability is an important factor for the design a coal mill. As grindability depends on many unknown factors, HGI is determined empirically using a sample mill according to the following procedure: