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,
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
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 anACARP 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
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:
Vitrinite and Inertinite Breakage: The breakage of both vitrinite
and inertinite consumes energy in the milling process. .
Vitrinite Dominated Breakage: The breakage energy of the coal is
dominated by the breakage of the vitrinite.
Inertinite Dominated Breakage: The breakage energy of the coal is
dominated by the breakage of inertinite.
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
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.