6. Conclusion
The measurements of coal hardness and strength are used to
assist in predicting the mechanical behaviour of coal for drilling and
gas extraction, mining and pillar support, crushing for mineral
liberation, grinding for power plant or coke oven feed. An established
knowledge about certain coal properties is essential regarding its
utilisation. It is indispensable to determine its chemical, physical and
mechanical properties in early stages of exploration. The SSRH test
can be examined during the exploration to assist and estimate
relationships to rank and type and to obtain empirical correlations
between other coal parameters (e.g. VR). The results of the uncon-
fined samples show that the hardness varies with lithotype along
with an increase of hardness with decreasing amount of bright
bands. This corroborates the notion that coal becomes weaker with
more telovitrinite, regardless of rank. Whereas the results of the
confined samples show that the hardness correlates with increasing
rank and changes at the border from medium volatile bituminous to
low volatile bituminous coal. The expected trend line for unconfined
samples indicates the same parabolic trend.
In conclusion the SSRH test is a fast and low budget method to
estimate the surface hardness of coal. Nonetheless a large number
of measurements need to be taken to yield a good arithmetic mean
for each sample due to the high variability of the results based on
the heterogeneous surface of coal (relatively high SD). However,
the SSRH responds to rank and type and controls its behaviour.
The images of the tested samples confirm that the impact is much
more intense on homogeneous (micropores) than on a heterogeneous
(meso- and macropores) surface.
Data generated in this study illustrates that the mechanical
behaviour relates to rank and can control fracture as well as
breakage behaviour of a given coal. However, further studies with
coal samples covering a wider range of rank are required to assist
in developing predictive models for coal behaviour during mining,
mineral liberation, washability, drilling or fracture stimulation for
CSG drainage.