Fracture Toughness as a Replacement for Hardgrove Grindability Index

The Hardgrove Grindability test is used to determine how well a particular coal will perform in a coal pulverizer. It is also used commercially as a limiting value property in coal specifications. Inadequacies in the standard test have been recognized for some time and various modifications have been proposed to rectify these.

This project sought to determine if fracture toughness (FT), a fundamental property of coal could be measured effectively using micro indentation equipment and potentially replace HGI for one or both of the above duties.

FT may be measured directly or inferred from measurements of plastic energy consumed during indentation. Plastic energy (PE) measurement was selected for investigation as the simpler method capable of routine instrument measurement. It was also selected to complement another ACARP project attempting to measure fracture toughness of coke directly.

This project has identified a type of indenter, diameter of indenter and load that gave stable repeatable measurement of PE with coal samples using micro indentation techniques. Fracture toughness can be inferred from this measurement. This was developed from seven different combinations of indenter and loading tested on five coals.

Correlation of PE with HGI was relatively good over the range 30-50 HGI, the range for domestic coal marketing. Correlation with HGI was not as good over the range 50-70, the range for export coals.

Standardized methods were used in the test although these methods have not yet been accepted in part 4 of the ISO standard being developed for this equipment. The test is automatically repeated on new coal surface up to twenty times to get representative results. Operator intervention is required at this stage of procedure development to select an appropriate family of curves from which to calculate PE.

While the instrument has a significant capital cost the likely cost of commercial testing was low on a per sample basis, requiring 15 minutes sample preparation followed by automatic testing, calculation and printing of results.