Hardgrove Grindability Index

The Hardgrove Index ( HGI short, from english "Hard Grove Grind Ability Index ") is a measure of the hardness of coal during grinding. The hardness degree is indicated in the unit (° H), for example, 40 ° or 55 ° H H. The smaller the Hardgrove index, the harder the coal.

The grindability is an important parameter for the workability of the coal and the sizing of the coal mills. Since the grindability of various factors depends that are not yet fully known in detail, it is determined empirically with a sample mill as described below.

The index is named after Ralph M. Hardgrove (* 1891 in Massillon, Ohio, † October 29, 1978 in Jacksonville, Florida), who developed the method of determination in the 1930s in the service of the company Babcock & Wilcox in the U.S..

The process has now been widely accepted internationally and is standardized by the American ASTM standard D 409, DIN 51742 and international standard ISO 5074.

Measurement methods

Fifty grams of air-dried coal with a grain size from 0.6 to 1.2 mm are introduced into the mill and the grinding media loaded with a weight. After sixty revolutions of the grinding media, the crushed coal is placed on a test sieve. From the rate of passage D, the Hardgrove index H is calculated as:

The method gives only relative values, since the setting of the mill test is performed with a reference carbon, Hardgrove index which is equal to 100.