Theory & Method
Hardgrove Grindability Index Tester – QualiHGI™ 60A
The Hardgrove Grindability Index (HGI) is a dimensionless index that expresses the relative ease of grinding coal compared to a reference coal of defined grindability. A high HGI value indicates a soft, easily ground coal; a low HGI indicates a hard, difficult-to-grind coal. The index is widely used because it correlates directly with pulverizer mill capacity — a doubling of HGI corresponds approximately to a doubling of mill throughput at the same product fineness.
The test method specified in ASTM D409 and ISO 5074 uses a standardized ring-and-ball mill geometry: a rotating lower grinding ring driven by the spindle, eight 25.4 mm steel balls resting in a race on the lower ring, and a stationary upper grinding ring loaded vertically downward by a defined spring load of 284 ± 2 N (63.9 lbf). The upper ring constrains the balls in the race while the lower ring rotates, causing the balls to roll and slide against both rings and grind the coal sample trapped in the grinding zone.
The test procedure begins with coal sample preparation: the feed coal is crushed and sieved to produce a 50 g charge of the 0.63–1.25 mm (600–1,250 μm) particle size fraction, dried to air-dry basis. The prepared sample is loaded into the grinding bowl of the QualiHGI™ 60A. The microcomputer control system then drives the spindle at 20 ± 1 r/min for exactly 60 revolutions — automatically counting each revolution and stopping precisely at 60 turns. After grinding, the entire bowl contents are sieve-analyzed on a 75 μm (No. 200 mesh) sieve, and the mass of material passing the sieve (i.e. ground below 75 μm) is measured by weighing.
The Hardgrove Grindability Index is calculated from the mass of sub-75 μm product using the standard calibration equation established with ASTM reference coals of known HGI. The QualiHGI™ 60A performs this calculation automatically using the built-in linear regression correction curve. Periodic calibration of the instrument is performed using ASTM-certified reference coals spanning the grindability range of interest, with the calibration constants stored in the microcomputer and used for all subsequent HGI calculations — ensuring traceability to the ASTM D409 reference standard.
