The Hardgrove Grindability Index (HGI) is included in most thermal/PCI coal specifications as the indicator of mill performance. While HGI is adequate to characterise the grinding of most coals, some Australian coals can be disadvantaged as the HGI will indicate higher power requirements, lower throughputs and /or a coarser size distribution than actual. 

The evaluation of a coal's behaviour in the thermal or PCI coal market requires knowledge of the size distribution of the organic and inorganic components of the coal to enable the determination of performance parameters such as combustibility, slagging and fouling and handleability.

In a recent CoalTech report, the mill performance of maceral groups or microlithotypes was shown to be additive for most coals. That is, each maceral group or microlithotype behaved independently and a size fraction of the product PF was the mass weighted sum of the petrographic components of that size . It was possible to determine the size distribution of the product PF for a wide range of milling conditions based solely on petrographic analysis.  As microlithotypes were not determined directly they were estimated from the maceral analysis.  The size distribution of individual maceral groups or microlithotypes can also be estimated.  Size distribution based on petrographic analysis  proved to be a better estimate than that obtained based on the HGI.

Mill power can also be estimated from petrographic analysis, but the HGI is a better predictor of mill power.

Based on these findings an improved mill performance nomogram was proposed which extends the typical mill curve to account for the different maceral composition of coals.

In an ACARP project it was found that for most coals, a good estimate of a blend's size distribution can be made assuming that the size distribution of the individual coals, milled under the same conditions, are added together in the proportions of the blend. The exception is when a very soft coal (HGI 90) is blended with a very hard coal (HGI 35).  In this case preferential milling (more reporting to the smaller size fractions) of the softer coal occurs.

All coals studied in this project show some sign of preferential grinding of the softer maceral group when the coal was milled individually or in a blend.  This preferential grinding of macerals is due to differing strengths of the macerals which dictates how the size reduction of the maceral varies with energy used in the breakage of the particle. 

Breakage characteristic curves (change in size reduction per unit of energy) for vitrinite and inertinite were determined from the milling data of the coals and blends.  For these curves the mill specific power was proportioned to the maceral groups based on the petrographic analysis and blend composition. These curves have similar trends as those found for the breakage of lithotypes.  Generally, these trends are for the particle size to initially decrease rapidly then approach a constant size with increasing breakage energy. The results indicate that the breakage characteristic curves of maceral groups in individual coals do not change when they are blended with other coals.

It is only when the reduction in breakage energy proportioned to a maceral group of a coal in the blend moves to the steeper region of its breakage characteristic curve that the preferential milling of a coal in a blend is observed in the size distribution of the blend.  This would also explain the non- linearity of Hardgrove Grindability Index (HGI) determined on some blends when compared to their component coals as HGI is a measure of size reduction for a fixed energy.

The results show that the breakage of a coal particle can have three mechanisms, these are:

The results also explain why some coals, those with an inertinite dominated breakage mechanism, do not follow the generally trend between HGI and the maximum vitrinite reflectance.

It was shown that relationships between mill specific power and HGI, Rosin Rammler parameters and vitrinite reflectance and the breakage characteristic curve of vitrinite and inertinite allows one to determine the mill performance of a coal or a blend. Currently it is not possible to estimate the breakage characteristic curve from petrographic analysis.  Further milling testing, under fixed mill conditions, for wider range of coals will assist identifying methods to predict the breakage characteristic curves for the vitrinite and inertinite maceral groups.