The Improved Grindability Test: Comparisons with HGI

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 property of coal and other commodities. It has, however, three deficiencies:

• it uses only the coarse (harder) part of the sample - producing biased (low) results
• it grinds a standard mass rather than a volume - producing low values for stone bands
• it reports the result as an arbitrary index - giving no direct indication of the physical properties of the coal tested.

In the initial project, C12063, a test was developed and demonstrated, which overcame all of the deficiencies of the Standard Hardgrove Grindability Index (HGI) 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 µm). The test is called the Improved Grindability Test (IGT).

In the project reported, the test was applied to a larger number of coals, covering a wide range of rank, coal type, and mineral matter content and type. These comprised eleven from NSW, ten from Queensland, one from West Australia, and four from ACARP Project C13063 Milling of Blends (plus six blends made from these four coals).

• A Standard procedure, based on modifications to the current HGI Standard, was prepared for consideration by the committee of Standards Australia.
• The results confirmed the IGT result i.e. the SMD after grinding, to be an excellent indicator of the grindability of a coal, and in good general agreement with the HGI.
• However, crushing pre-history was considered to affect the result and a modification to the method, to provide more initial crushing steps, and/or a slightly longer grind time, is needed to overcome this.
• Petrographic tests confirmed that the HGI test portion contains less of the soft components (mono-macerites) and more of the hard components (tri-macerites) than the discarded portion, which comprises about half of the prepared sample.
• An interrupted IGT, where the 0.75 µm material was removed twice during the test and recharged each time with an equivalent mass of feed material (-2+0 mm), showed that the overall sizing after grinding was little different to that from the Standard IGT. The IGT results correlated closely with the mass of recharge material, giving rise to confidence in the IGT as an indicator of power station mill capacity. The mass% of fines removed trended towards steady state after 4 to 5 minutes.
• The precision of the test, in an experienced laboratory, was found to be similar to that for the HGI, but unsatisfactory in another two laboratories doing the test for the first time.
• Sizings of blends prepared at 4.75 mm top size were coarser before and after the IGT than calculated from the individual components.