What is Hardgrove grindability index?

The Hardgrove Grindability Index was developed in the 1930s from experimental work by R. Hardgrove to determine the relative difficulty of reducing various coals to a particle size required for efficient combustion in pulverized coal boiler furnaces. The HGI is now commonly applied to specifications for coal to be used in the iron making, cement and chemical processes.

There are several different "standard" methods for the determination of HGI (e.g. - AS1038 Part 20:2002, and ASTM D409-2006), which can lead to significantly different results. Further, HGI is an empirical measure and not a physical proprty of coal; as such the results of any standard test have relatively low repeatability for a given coal sample.

In general, the higher the HGI value, the more readily a coal can be reduced to smaller particle sizes.