Technical Market Support » Thermal Coal
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.
Initial IGT Project C12063
A test was developed and demonstrated, which overcame all of the deficiencies of the Standard Hardgrove Grindability Index (HGI) Test. It used a standard volume of a representative 'by zero' sample, and expressed the results of the standard grind in terms of the Sauter mean diameter, SMD (in µm). The test was named the Improved Grindability Test (IGT).
Extension IGT Project C13065
The test was applied to a wide range of coals, comprising 22 from three States and four from ACARP Project C13063 'Milling of Blends' (plus six blends made from these four coals).
Petrographic tests confirmed that the HGI test portion, in all cases, contained less of the soft components (mono-macerites) and more of the hard components (tri-macerites) i.e. gave a result that was not representative of the whole coal.
A Standard procedure, based on modifications to the current HGI Standard, was prepared for consideration by the committee of Standards Australia.
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, with nonidentical sample measuring apparatus.
Sizings of blends prepared at 4.75 mm top size were coarser before and after the IGT than calculated from the individual components i.e. hard coals had a disproportionate influence on a blend.
A 'Steady State' IGT was developed, where the -75 µm material was removed twice during the test and replaced by an equivalent mass of feed material (-2+0 mm). Results from the Standard IGT 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 overall outcomes confirmed that the IGT result was an excellent indicator of the grindability of a coal. However, breakage pre-history was considered to affect the size distribution of the IGT test portion and hence the result. A modification to the method was needed to overcome the effect of prior breakage and this was the basis for this (third) project.
Current IGT Project C15068
Six coals of widely varying type were sampled at exposed coal faces i.e. with no prior breakage history. These were dropped repeatedly, crushed to pass 50 mm, and divided into A and B streams. A was 'gently' crushed to pass 8 mm (jaw crusher), while B was 'vigorously' crushed to pass 8 mm (swing hammer mill). Each -8 mm sample was then crushed to pass 2 mm by passing about ten times through a plate mill with progressively closer settings. The results have shown that the effects of prior breakage history can be substantially minimised, although not completely eliminated (especially for the softest coals), by increasing the starting sample top size from 4.75 to 8 mm and crushing all of the sample each time, for a large number of passes, to the 2 mm top size for the IGT test.
Testing outcomes, and the interpretation of results, have been significantly compromised by sample losses during preparation and testing.
Precision at a given laboratory seems acceptable, although agreement between laboratories is disappointing.
The Steady State version of the IGT shows great promise, with the mass% of -75 µm produced by grinding, under 'steady state' conditions (5 minutes) being a useful indicator of a coal's grindability.