HGI is an empirical measure of the ease of grinding coal, a property that is vital for milling. It is an index showing the relative hardness of that particular coal compared to the standard soft coal with an index of 100. The parameters that add to the difficulty in grinding are
• Hardness • Fibrous nature of coal • Presence of sticky and plastic materials
ASTM D409 Standard Test Method for Grindability of Coal by the Hardgrove-Machine Method gives the procedure.