ACARP Project Number: C20010
Published: June 12
Lauren Johnson, Philip
Despite the simple chemical composition of
metallurgical coke, the structure is complex, consisting of a
network of variously sized pores and walls of varying thickness.
Adding to this complexity is the varying forms of the carbon in the
pore walls which have different microproperties and vary in the
optical anisotropy dependent upon the parent coal properties. It is
well known that the microscopic structure and texture of coke is of
intrinsic importance to both the hot and cold strength of coke.
Many papers have been written about coke characterisation, and the
relationship that the parent coal's maceral composition has with
the produced coke's microtexture and microstructure. As a result a
large number of classification systems have been developed to
differentiate the various microscopic features of cokes. Despite
the large variation in classification systems around the world coke
microtextures have been and are increasingly used to assist in coke
quality assessments manually and automatically by microscopic
imaging techniques. While it is recognised that microscopic coke
composition is directly related to coke quality parameters,
differences in classification are a limiting factor in the use of
microtextures in the international community.
For the purposes of this report, coke
microtexture describes the nature of the carbon in coke, the
crystalline development and the degree of anisotropy present.
Whereas coke microstructure describes the spatial relationships
present in the coke material for example porosity and pore wall
The objectives of this project were to review the
coke classification systems in existence to assist in the
development of a comprehensive and uniform classification that is
applicable to Australian and international coals. A reference
document developed as part of the project that outlines this
classification, including photomicrographs, can be used to assist
petrographers when applying the classification to the analysis of
metallurgical cokes. The application of the classification system
selected and the usefulness of the reference document produced will
be tested by a round robin by Australian petrographers using two
As a result of the review of the various
classification systems in existence a reference document was
produced detailing a method of microtextural classification
selected to be tested by a round robin of various petrographers
within Australia. The classification method selected is known to be
used within Australia and is able to be applied to cokes that have
been made from coals with a range of rank and coal types.
Additionally the method uses the results generated to give an
overall measurement of anisotropy known as the coke anisotropy
quotient (CAQ) which has been related to coke quality parameters
such as coke strength after reaction (CSR).
Two cokes were dispatched to a number of
petrographers for analysis, the cokes selected were manufactured
from two coals being tested at ALS Coal's Coking Research Centre,
the two coals had similar rank but different maceral
concentrations. Only two petrographers from one laboratory returned
results for the round robin. It was found that between
petrographers using the same classification system the fused carbon
was able to be determined with good reproducibility. Despite
differences in the classifications of the domains within the fused
carbon the overall anisotropy measured by the CAQ was very similar
between the two petrographers. It is suggested that it is most
important for the CAQ to be close between analysts as this is the
parameter that is used to relate the coke microtexture to other
coal quality parameters.
The round robin samples were also sent to two
commercial laboratories that conduct coke microtextural analysis.
One of which conducts manual coke microtextural analysis and the
other automated imaging analysis. The results obtained from these
laboratories were compared to the mean results from the round
robin. It was found that for one coke the total proportion of fused
carbon is similar between the three methods but for the other coke
the total proportion of fused carbon is quite different. It is
possible that this is due to the classification of partly fused
carbon. Additionally differences between the analytical methods and
classification methodologies used to differentiate the texture of
the fused material do not allow for an easy comparison of the
results between laboratories.
It is recommended that further dialogue between
coal producers and petrographers occurred at Australian and
International levels to determine the most suitable method for
characterising coke at a microscopic level with the aim to develop
an AS or ISO standard method.