An Improved Grindability Test

The Hardgrove Grindability Index test, developed in 1932, is universally used in specifications for coal purchase and power station pulveriser design, to indicate the grinding properties of coal and other commodities. It has, however, three deficiencies:

• it uses only the coarse part of the sample - producing biased (usually low) results when the coarse fraction is non-representative of the whole sample
• it grinds a standard mass rather than a volume - producing unrealistically low values for high density materials such as stone bands
• it reports the result as an arbitrary index - giving no direct indication of the physical properties of the coal tested. These deficiencies often lead to misleading results and disputation.

In this project a test has been developed and demonstrated, which overcomes all of the deficiencies of the Standard Hardgrove Grindability Test. It uses a standard volume of a representative 'by zero' sample, and expresses the results of the standard grind in terms of the Sauter mean diameter, SMD (in ?m) of the 'before' and 'after' samples. The test is called the Improved Grindability Test (IGT), and the Sauter Mean Diameter 'before' to 'after' ratio is called the Improved Grindability Test Factor (IGTF). A detailed method has been described as the basis for an Australian Standard.

It is recommended that the IGT be used in conjunction with the HGI wherever possible, to enable the results of the HGI test to be better understood and interpreted and to give greater precision to the relationship between IGT and HGI. At the same time, the bias in HGI results needs to be identified, by petrographic examination of the tested and discarded portions of the sample, so that correlations may be made between the IGTF and true (unbiased) HGI values only.