Hardgrove grindability is a measure of the relative ease with which a coal sample can be pulverized or ground to smaller sizes. All mined coals need to be ground to very small sizes (often powder size) before being utilized. For example, most steam coals (used to generate electricity) are ground to 60 mesh (250 microns), which is approximately the size of talcum powder, before being put in boilers.
In a Hardgrove test, samples are crushed to a specified size, air-dried, and weighed, then run through a laboratory grinding (milling) machine for 60 grinds (rotations). The milled sample is then placed on a sieve of set specifications. The amount of sample that passes through the sieve is used to calculate the Hardgrove grindability index. The results are compared against a standard sample to provide the index and used to determine how much energy and expense will be needed to grind down a certain coal for a particular use (ASTM method D409-12; American Society for Testing and Materials, 2013, p. 397–411).
The HGI is on a scale from 30 to 100. High numbers are easy to grind; low numbers are difficult to grind. HGI is related to rank, moisture content, and ash (mineral) content (Sinha et al., 1962; Stach and others, 1982; Suárez-Ruiz and Crelling, 2008). In general, coals become more easily grindable through rank until they reach approximately 89 to 90 percent carbon in the semi-anthracite rank, at which point coal is increasingly harder to grind with increasing rank (Van Krevelen, 1992). Macerals and microlithotypes also influence grindability at specific ranks and moisture content (Stach and others, 1982; Hower and Wild, 1988; Trimble and Hower, 2003).